Claw

Yes, this! Yeah, thanks for the tip. I had never noticed that before in the SPH. I experienced it during flight but thought it was due to something else. I've done the delta-deluxe before. It works great for a lot of things. I just wish the bigger parts fit together a little better.

I forgot to ask, do you mean to say you made a plane that had nothing but canards? I'd be interested seeing that.

I think I get what you're saying about canards, but I'm still not sure that I would say they are unstable. It's all about how you use them. I can make a craft unstable with a stabilator also. That doesn't necessarily mean stabilators are unstable. EDIT: Okay, I see what you're saying now. I went to the SPH and rotated a plane around. I see that the shifting CoL isn't limited to canards but to all control surfaces. With the canards, it's an issue because the CoL shifts forward. Yes, in the SPH/VAB. My tutorial is for in flight, so you must turn on/off control surfaces on both sides. Should I edit the tutorial to make that more clear? Thanks again! I want this to be useful so comments and experience are appreciated.

- Realize there is no requirement to land on the KSP runway. So feel free to land in the clear grass zones around KSC (or elsewhere) until you get the hang of it, just watch out for small hills. If you are landing on/around KSC, also watch out for the mountains when approaching from the west as they reach pretty high (so aiming to be less than 5,000m more than 10km out is sometimes an issue). - I would not recommend landing with the nose 15 to 20 degrees nose high unless you've designed your craft for that. - The speed of your touchdown will really depend on your craft. You want to be as slow as possible, but your plane should still be controllable with no more than 5 degrees of AoA (the difference between the prograde marker and the yellow "V"). This should allow your back wheels to touch down first. As someone said, 100 m/s is a descent starting point. 40 m/s might be way to slow for your heavier craft. - When you get close to your landing area, you want to use your prograde marker to fly off of. When you're 50 to 100m up, move your prograde marker to the horizon and slowly zero out your throttle. This should slow you down and let you sink slowly to touchdown. You'll have to adjust your height and throttle based on how big your craft is. I find using throttle to land helps out a lot if you're using the keyboard to fly. - You can activate your brakes before you land and they will start braking right away. Watch out for the SAS to sneak up here and throw your airplane all over the place.

Could it be a docking port bug? You said it was docked to something on the way up. Perhaps it didn't actually get an "undocked" state? What does it say when you right click on it? It might be worth submitting a bug report.

Also know that RAPIERs have the same ISP as a TurboJet as you climb out in air breather mode, so they have the same fuel economy as a TurboJet. However, because of the slightly lower thrust than TurboJet, the RAPIERs do all those things that thereaverofdarkness and bsalis said (such as being a little more sluggish on takeoff, and requiring less air at altitude). Unfortunately, they also weight a little more than their TurboJet/Rocket alternative.

Hmm, I only find them unstable if I try to over control. They seem to have a lot of power to me, but that's different than being unstable. Maybe that's because I don't typically design to fly past 20 degrees. I'll add a note, but I want to experiment more first since I haven't seen this yet. Do you have a design where you run into this? In the SPH sure, but not airborne. Unless you're talking about something else? Thanks for the notes!

I'm not sure exactly what you're looking for, but perhaps another way of looking at it is to start from the other end. The fact that an object (like a ship) sweeps an equal area in equal time around a body (like a planet) is the result of how the object orbits. During an orbit: When the object is further out from the body, it's speed is slower. When the object is closer to the body, it's speed is faster. (Hopefully you've seen this while playing the game.) So, when the object is further away AND is traveling slower, it just so happens that the area covered is equal to when the object is closer AND going faster. The "area rule" is a way of using math to match those relationships for a given object.

PURPOSE: (If you don’t want all the theory and practice flying, just skip to the summary for basic rules of thumb.) This tutorial will cover different types of flight controls and how you can adjust your flight controls so that your airplane / space plane flies how you want. We will also see the effects of inertia on an aircraft’s ability to roll, pitch, and yaw and how you can adjust your flight controls to compensate. As with my other tutorial(s), I will try to demonstrate the basics of design as opposed to telling you how to make a specific aircraft. This tutorial does use an aircraft to demonstrate the principles in flight. This is a bit of a long article, but some of the airborne steps sort of repeat themselves (you’ll see what I mean). So after you do the steps a couple times, you should get the hang of modifying the test aircraft to see the different effects. Flown in Stock KSP v0.23 TOPICS: For easy reference, here’s what you’ll find below: BACKGROUND Section 1: Pitch Control (Elevon, Stabilator, Canards, Torque, Thrust Vectoring) Section 2: Roll Control (Aileron, Differential Stabilator, Torque) Section 3: Yaw Control (Rudder, Torque, Thrust Vectoring) Section 4: Inertia (Pitch, Roll, Yaw) SUMMARY – (Includes basic rules of thumb if you get tired of reading the rest) This information might be basic or slightly advanced based on your level of knowledge. You should be able to jump into any section you wish. BACKGROUND: We will cover two areas in this tutorial: Flight Controls and Rotational Moments of Inertia. I’ll cover some of the background theory before we dive into flying. Also, while there are some Kerbalisms that you can use to optimize your design, I will try and stick to basics so that you can get the idea, then you can tweak your own aircraft to do what you want. For this tutorial, we will use the aircraft pictured below. This airplane is stable enough and has enough gas that you can (hopefully) fly through the whole tutorial. Feel free to try and copy the design, or just steal the .craft from here. It is a mid-mounted, delta wing design with no dihedral to keep aero effects from confusing our experimentation. Of note, only the cylindrical monopropellant tanks at the root of the wings (one of the left side and one of the right side of the fuselage) are full. The wing tip cylindrical tanks and all spherical tanks are empty. Also, all flight control surfaces start out disabled except for the large ailerons. So, what the heck is all that stuff?... Control Surfaces: If you are unfamiliar with the types of control surfaces, check out this article by Keptin. It has a lot of other useful information in it as well, but you can just read up on the control surface section for this tutorial. Also, I tried to give some short definitions below as a refresher for our basic test airplane. One general thing to note before we dive into controls: The further your control surface is from the Center of Mass, the more effective it will be. - Aileron – Controls roll. Typically placed on the wings. - Elevator – Controls pitch. Typically placed on the trailing end of the fuselage (behind the wing), but can also be found in “T-Tail†and other configurations. In the case of the test craft, the elevator is actually called a vertical stabilator because the whole thing moves. - Canard – Controls pitch. Typically placed on the forward part of the fuselage (in front of a wing). - Rudder – Controls yaw. Typically placed on vertical tails. - Elevon – A combined Elevator & Aileron which controls roll and pitch. Typically placed on the trailing end of a delta wing. - Trim – Removes forces from the flight controls during flight. If you fly with SAS on, this is pretty much taken care of for you. It is not something you exactly design into your Kerbal airplane / space plane, but certainly affects the flight characteristics. I mention it here because we will use the Yaw/Pitch/Roll (YPR) indicators in flight to show us what kind of “trim†the SAS is providing, thus telling you if you need to adjust your design. If you want to fly without SAS, then you’ll need to use [Alt + WASD] to adjust your trim. [Alt + X] resets the trim to zero for all three axes. To demonstrate all of these, we will turn on/off the control surfaces in flight. To turn on/off the control surface, right click and disable each axis (yaw/pitch/roll). In flight, unlike the SPH, you have turn on/off the flight control on both sides. It does not automatically do symmetry (KSP v0.23). Inertia: Inertia is the resistance an object has to being moved. Translational Inertia is usually pretty obvious to most people in how hard it is to push something around. For example: it is harder to push a large rocket with an LV-45 than to push a small rocket. Once that large rocket is moving, it’s harder to slow down. Airplanes/spacecraft not only has translational inertia, but also rotational inertia. If you have already flown large rockets in Kerbal, you probably already know what I’m talking about. If you haven’t, we will do a little inflight demonstration for how this can affect your airplane / space plane design. Another note: an airplane has three rotational moments of inertia. For simplicity, we will call them “Rolling Inertia,†“Pitch Inertia,†and “Yaw Inertia.†I will spare you the math and details. Just know that roll inertia is how resistant an aircraft is to roll, pitch inertia’s resistance to pitch, and yaw inertia’s resistance to yaw. Pitch and Roll Inertia are usually much more obvious in aircraft / space plane design because you don’t usually fly in a lot of yaw. Simply put, the more mass an aircraft has out on the wings, the more resistant it is to roll (more roll inertia). The more mass an aircraft has on the nose and tail, the more resistant it is to pitch (more pitch inertia). More mass on the wings and/or nose/tail tends to make the aircraft more resistant to yaw (more yaw inertia). To demonstrate all this, we will use the monopropellant tanks to transfer mass around, much like an ice skater moving his/her hands in and out while spinning. LET’S GET STARTED: Section 1: Pitch Control (Elevon, Stabilator, Canards, Torque, Thrust Vectoring) Sooo, pitch control… When I’m designing Kerbal planes I find that I spend a lot more of my flight control tweaks on pitch than anything else. I think that’s because when flying a space plane you do a lot of pitch work so changes are more noticeable. Plus, it takes a lot of tweaking to get the feel you want when the CoM is moving around, lift is changing, and you get a sudden kick of thrust when trying to leave the atmosphere. Fortunately there are a lot of options available to help with pitch control. Unfortunately most work on the same principle so you don’t necessarily get magical perfection using a canard vs. elevon vs. stabilator. On the plus side, you can use any of these in your design if you know how and where to use them which makes it possible to create effective designs. 1.1 Elevon Elevons are ailerons that can also act as elevators (brilliant!). So they are attached to the trailing edge of a wing, and that wing is traditionally a delta wing (although it doesn’t have to be and in our case, isn’t). Because wings are generally not as far back on the airplane as say, a tail, the pitch control surface is not quite as far away from the CoM as what we get with a tail. This means it is (generally) not quite as effective as a stabilator or canard. (Lift rating also factors in.) Also, in reality an elevon often becomes an important lift contributor since it is an extension of the wing. This means that when you use the elevon to pitch up, you are also giving up some of your wing’s lift. Depending on your Kerbal design, this can cause the aircraft to want to descend or slow down while you’re trying to pitch up (because you are losing lift and creating drag). So, let’s actually go fly! Flight: 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). To disable a flight control, right click on it and deactivate the yaw/pitch/roll. Remember to do the other side. All flight controls (except the large elevon) are disabled by default for the supplied .craft. 3) Activate the yaw/pitch/roll authority for large control surface on the wings (remember to do both sides). This control is active by default for the supplied .craft. Starting around 2000m, fly the airplane through a loop (just hold back until the plane flies through straight up, straight down, then ends up back facing where you started). When you fly the loop, notice how fast and far your nose pitches up, then sort of springs back. Also pay attention to the maximum and minimum altitudes. You should reach a max altitude about 3500 m and end the loop at about 900 m. After that, notice how it isn’t very “springy†and bouncy it is as you fly the loop. It’s slow, but nice and smooth. You can play around with this for a while to get the feel of it. When you’re done, fly on to the next part. Modifications: 1) Fly level at about 2000m, full throttle. 2) Disable the large elevon’s yaw/pitch/roll control. 3) Activate the yaw/pitch/roll for the small control surface on the wings (remember both sides). Starting around 2000m, fly this airplane through a loop just like before. Notice how springy this one is and how slow it pitches. You should reach a max altitude about 4000 m and end the loop at about 600 m. It still flies nice and smooth. The small control surface is about as far behind the CoM as the large control surface, but has a slightly lower lift rating. The end result is that it takes longer to pitch through a loop than with the large surface. 1.2 Horizontal Stabilator This is the classic pitch control surface for high performance fighter type airplanes. In the example airplane, this is further away from the CoM than the elevons but has slightly less lift rating (KSP v0.23). Stabilator effect on the wing’s lift is not the same as for elevons, but can still be a bit of a factor if your plane is maxed out on weight. You might end up in a spot where you don’t have enough pitch authority, but it won’t steal lift from your wings since it pitches by pushing down on the tail. So you can generally keep the nose up until you simply run out of lift or your stabilator can’t turn any further. Modifications: 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). 3) Activate the yaw/pitch/roll for the horizontal stabilator (remember both sides). Starting around 2000m, fly this airplane through a loop just like before. Again, compare the feel in springiness and how fast it turns with the elevon. You should reach a max altitude about 3400 m and end the loop at about 1200 m. Even though it has a slightly lower lift rating than the large elevon, the stabilator is further behind the CoM than the elevons and pitches a little faster. Again, feel free to fly around a bit with this and go on to the canards when you’re ready. 1.3 Canards This is the forward set of winglets. It has been used as a pitch control surface for a few high performance fighter type airplanes and many private airplanes. In the example airplane, the canard is further away from the CoM (forward) than the stabilator and has the same lift rating. Canard effect on the airplanes lift is different than for a stabilator or elevons. Because the canard is in front of the CoM and the CoL is behind the CoM, canards actually provide lift upward instead of downward (as with a stabilator). So this means if your airplane or space plane is at maximum lift angle of attack (around 25 degrees in KSP v0.23) and you try to pitch up, your canard may actually lose lift and the nose will drop. This is because the canard’s AoA goes above 25 degrees causing the canard’s lift to decrease. This is slightly different than how it works for an elevon, but the effect look similar. Modifications: 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). 3) Activate the yaw/pitch/roll for the canards (remember both sides). Starting around 2000m, fly a loop just like before. Again, compare the feel in springiness and how fast it turns with the stabilator. You should reach a max altitude about 3200 m and end the loop at about 1500 m. Because the canard is further away from the CoM than the elevons and stabilator, it pitches faster. Also notice how far you can get the nose to move initially, then it sort of bounces around. KSP tends to like canards because of the above factors. In reality, the equipment needed to put canards on the front end of an airplane is complicated because that pesky cockpit is in the way. KSP doesn’t suffer from the same limitations so you can put canards wherever you want. However, that doesn’t guarantee that your plane will fly like you want. Plus, once you get into space, canards turn into more inertia that you have to rotate around with RCS. (See Section 4 for inertia.) 1.4 Torque When I mention torque here, I’m referring to the gyros available in your command module. In the case of this test airplane, it’s the venerable Mk-1 cockpit. Up till now we have left it on all the time. We’re still going to leave it on here, but fly with everything else turned off. Modifications: 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). That’s it. Fly around and see how it does. Do a loop if you want, but it probably won’t go very well. This airplane is stable enough that torque alone is enough for the SAS to maintain stability. In fact, we have been relying on the torque when switching around the flight controls. You probably wouldn’t want to fly an airplane or space plane this way, but there’s no reason you can’t! If you disable torque to control your space plane in the atmosphere, remember to turn it on (or your RCS) at high altitude because you will need it when on the fringes of space. 1.5 Thrust Vectoring The TurboJet engine we are using has up to 1 degree of thrust vectoring. It isn’t much, but like torque it can help out in controlling your plane. Modifications: 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave thrust vector on). 3) Disable torque control for the Mk-1 cockpit (right click on the cockpit and select “toggle torqueâ€Â). Make sure you’re nearly level. The plane is stable enough to maintain level flight, but there isn’t much thrust vector authority to pitch up since the engine is about maxed out with trim to maintain level flight. Realize the nozzle is pointing up to provide a down force (like a stabilator). This is because the Center of Lift is behind the CoM. If your craft is neutrally stable, thrust vectoring will point up less. When you’re done, make sure you turn the torque back on. Also, feel free to turn on several control surfaces and see how it flies. Section 2: Roll Control (Aileron, Differential Stabilator, Differential Canards, Torque) I think roll control is a little more straight forward, but some people might not realize that you can control roll with more than just ailerons. There aren’t quite as many options as with pitch, but you can still design different effects to suit the style you want. 2.1 Ailerons On this plane we have big inboard ailerons, and small outboard ailerons. The lift rating doesn’t really scale well with the physical size (in KSP v0.23 anyway), but we’ll still get the idea from this demo. Generally speaking, ailerons out near the wing tip will be better at rolling the aircraft than ailerons near the fuselage. However, as with canards, ailerons further out on the wings creates more inertia that uses more RCS to rotate in space. (See Section 4 for Inertia.) Modifications: 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). 3) Activate the yaw/pitch/roll for the large ailerons (remember both sides). 4) Make sure the Mk-1 torque is back on if you did Section 2. Fly it around, note how fast the airplane rolls. Do 3 or 4 full rolls left or right and see how it does. Modifications: 5) Disable the yaw/pitch/roll for the large ailerons (remember both sides). 6) Turn on the yaw/pitch/roll for the small ailerons (remember both sides). Fly it around, note how fast the airplane rolls. Do 3 or 4 full rolls left or right and see how it does. Even though the control surface is smaller, it is much more effective out on the wingtip. In addition to the inertia I mentioned above, if you get to aggressive with aileron placement or your lift is maxed out, using ailerons can cause your wings to twist asymmetrically. This will give you all kinds of yaw/roll/pitch problems that might not make sense. Strutting the wings can help here, especially if you have large or funny angles on your wings. 2.2 Differential Stabilator / Canards The concept of differential stabilator is the same as canards, so for the sake of shortness (which this article already isn’t) we’ll discuss both here. Since the stabilator and canards are closer to the fuselage, you can (hopefully) imagine that they are less effective than the ailerons as demonstrated with the inboard/outboard ailerons above. So let’s try it out. Modifications (hopefully you’re getting the idea): 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). 3) Activate the yaw/pitch/roll for the stabilator (remember both sides). Fly it around, note how fast the airplane rolls. Do 3 or 4 full rolls left or right and see how it does. Compare to the ailerons. Modifications: 4) Disable the yaw/pitch/roll for the stabilator (remember both sides). 5) Turn on the yaw/pitch/roll for the canards (remember both sides). Fly it around, and compare to the stabilator. Is there much difference? Realize that at higher Angles of Attack, your craft may respond differently to rolling with canards versus stabilator, especially at high altitude. So it’s good to check that out too if you plan on using them to help your craft roll. KSP (v0.23) allows you to selectively cut out controls to tweak it the way you want. 2.3 Torque As with pitch, you can roll your aircraft with torque only. Torque tends to be more effective in roll than pitch because of inertia (see Section 4 for inertia), but this can vary dramatically between designs. Modifications (bet you can guess): 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). How well does the roll compare now? Section 3: Yaw Control (Rudder, Torque, Thrust Vectoring) We finally made it to yaw. There isn’t a whole lot to mention here because for the most part in KSP (v0.23) yaw doesn’t play a huge factor in most (smaller and flat) symmetric designs. There are certainly designs you can make where it’s a bigger factor, but the limited options make it more straight forward. As your craft grows in overall size or is really long/wide, control placement becomes more important. 3.1 Rudder Ahh, the mighty rudder. It can mess up your aircraft in a big way if you get too crazy with it. But it can also help a lot to recover from that bothersome asymmetric thrust. By the way, putting a rudder really high up on your airplane can cause it to act like an aileron. Imagine sticking another wing straight out the top and plunking an aileron on it. So rudders tend to be close to the body and near the tail, so that it acts in an intuitive way. It certainly doesn’t have to be there, and you can use stabilator/canard type designs for a tail. It all depends on what you need and want. If you have a really wide aircraft, you might need rudder further away from the body to make it effective. Modifications (hmm, is this new?): 1) Fly level at about 2000m, full throttle. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). 3) Activate the yaw/pitch/roll for the rudder (remember both sides…oh wait, there’s only one). Yaw that thing around. Loads of fun and useful for steering a little if you want to stay flat. High AoA rudder control isn’t very realistic yet (KSP v0.23), but you can still use it. Concern about yaw is hard to demonstrate with this airplane because it has a pretty basic shape. If your airplane is really wide, you may need to move rudders out away from the body (and away from the CoM) to make them more effective. 3.2 Torque And just like pitch/roll, you can yaw with torque only. Give it a try! (I’ll let you figure out the steps…) For small airplanes and space planes that aren’t too unusual, the torque provided by the command module is usually plenty of authority to deal with yaw, unless you end up with asymmetric thrust. If you make a flat, flying wing design with no vertical tail, you can use torque to change your heading slightly with yaw so you don’t have to roll the airplane. Section 4: Inertia Okay, so we talked in the background section about what inertia IS, but why should you care about pitch/roll/yaw inertia? When designing your aircraft flight controls, overcoming (and stopping) the inertia is a big factor. If an airplane or space plane has most of its mass (or most of its parts) concentrated near the Center of Mass (a lower roll/pitch inertia), then you won’t need as much flight control area to get the control authority you want. In fact, if you over do it with the amount of control, you risk putting your aircraft / space craft out of control. In the case of an aircraft where the concentration of mass is more spread out in the wings or along the fuselage (a higher roll/pitch inertia), you will need more effective flight controls to get the authority you want. Also, this type of design will be slower to START turning and slower to STOP turning. 4.1 Pitch Inertia If you recall, pitch inertia is how resistant the airplane or space plane is resistant to changes in pitch. So what parts are we going to use for the pitch inertia? Since we have already explored pitch flight control systems, we will start off with the test airplane in a controllable state and move monopropellant from the middle of the plane to the nose/tail to increase the pitch inertia. Modifications: 1) Fly level at about 2000m. 2) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). 3) Activate the yaw/pitch/roll authority for the canards and the horizontal stabilator (remember to do both sides). Before we do any more modifications, let’s fly this around again to see how it works out. Start out at 2000 m and do a loop (just hold ). You should reach a max altitude about 3000 m and end the loop at about 1800 m. Note how “springy†the nose is right after you start pulling. It initially pitches up to 60 degrees nose high, comes back down to 45, then back up to 90 degrees. No we’ll move some weight around. So we can see what is happening to the SAS pitch control, we’re going to first disable the canard/stabilator pitch controls and watch the Y/P/R trim indicators for changes in pitch. Disabling the flight controls makes the trim indicators more sensitive because only thrust and torque are giving input. 4) Disable the yaw/pitch/roll authority for the canards/stabilator (both sides). 5) Take a look at the pitch trim. It should be about 4 notches up, depending on your fuel. 6) Transfer monopropellant from the LEFT fuselage monopropellant tank to the monopropellant tank on the NOSE. Make sure you fill the forward tank completely. (Right click on the left fuselage cylindrical monopro tank, then hold [Alt] and right click on the nose sphere tank. Transfer fuel in.) 7) Take a look at the pitch trim, it should now be about 7 notches up. You might also notice the plane is flying with about 3 notches of yaw. If you’re not sure why, it’s because we have shifted the Center of Mass slightly right. The Center of Thrust no longer lines up with the Center of Mass which results in flying a little sideways. 8) Transfer monopro from the LEFT fuselage tank to the tank on the TOP of the aircraft. (We’re putting monopro up here to make sure the fuselage tanks end up balanced left and right.) 9) The pitch trim should hardly change, but the airplane will be flying with a little more yaw. 10) Transfer monopropellant from the RIGHT fuselage tank to the REAR tanks on both sides. (Using the right wing tank will keep the plane balanced left/right.) 11) Take a look at the pitch trim, it should now be back to about 1 notch up, better than before we started. Also, there should be no yaw. 12) Activate the yaw/pitch/roll authority for the canards/stabilator (both sides). Start out at 2000 m and do another loop. You should reach a max altitude about 3000 m and end the loop at about 1800 m. So it basically pitches at the same rate. However, note how the “springiness†of the nose is different. This time it jumps to 70 degrees nose high, back down to 55, then up past 90. This is because the airplane has enough control authority to get the plane pitching, then the higher inertia makes it want to keep pitching. This can make it easier to lose control, and harder to get it back if things start moving fast. Note that there are two spherical monopro tanks on the tail and only one on the front. If you recall from the SPH, the tail monopro tanks are about half as far from the CoM as the nose tank is. Filling one in front and two in back helps leave the CoM unchanged between the lower and higher moment of inertia configurations. Basically this means when designing a plane, the CoM and Inertia are affected more by weight placed further from the CoM. More Modifications (if you want): 1) Leave the canards/stabilator active, turn on the large ailerons (both sides) and fly around. 2) Activate all the surfaces (small and large elevons, stabilator, and canards – both sides) and fly around. In this configuration, when the aircraft pitches up hard, it gets close to going out of control. If you pitch down hard, it goes out of control. Why? Because gravity helps in the pull down and it’s just enough to go past the point of stability. This is harder to do when the pitch inertia is lower (monopro in the cylinder tanks). When you are done flying around, make sure you transfer the monopropellant back into the cylindrical tanks on the fuselage body. 4.2 Roll Inertia So roll inertia is how resistant the airplane or space plane is to changes in…(wait for it) roll! Since we just did this in pitch, hopefully you can guess what happens with roll. For this exercise, we will use the inboard (large) ailerons plus the monopropellant tanks on the tips of the wings. Similar to the pitch inertia experiment, we will move some of the monopropellant from the middle of the plane to the wingtips to change the roll inertia. Modifications: 1) Fly level at about 2000m. 2) If you haven’t already, make sure all the monopropellant is transferred back into the cylindrical tanks on the fuselage body. 3) Disable all flight controls in yaw/pitch/roll (leave torque and thrust vector on). 4) Activate the yaw/pitch/roll authority for the large aileron (remember to do both sides). Before we do any more modifications, let’s fly this around again to check out how it rolls. Start out at 2000 m and do a few aileron rolls. Note how quick it is to start rolling, and how long it takes to stop rolling. No we’ll move some weight around. While we transfer monopro this time, we will watch the Y/P/R trim indicators for Yaw changes. There is no need to disable the flight controls (like we did in pitch), since the ailerons are not providing yaw assistance. 5) Transfer monopro from the LEFT fuselage tank to the LEFT wingtip tank. Yes, transfer all of the monopro. The aircraft will yaw a lot. The aircraft will fly stable if you started out level and don’t mess with the pitch too much. Without rudder, the SAS can’t keep up with that much mismatch between the Center of Mass and the Center of Thrust so the airplane skids around in a turn. (If you are having problems with the aircraft going out of control, transfer half of the monopro for the left side, then all the monopro on the right, then return and finish the left side.) 6) Transfer monopro from the RIGHT fuselage tank to the RIGHT wingtip tank. Make sure you get it all. The aircraft should no longer be yawing. Start out at 2000 m and do more aileron rolls. The airplane rolls a bit slower, but not dramatically. But if you start and top several rolls, you will hopefully see a bigger difference in how long it takes to start and stop rolling. You can mess with the aileron (small/large) configuration and see how things change. When you are done flying around, make sure you transfer the monopropellant back into the cylindrical tanks on the fuselage body. 4.3 Yaw Inertia I bet you can guess what yaw inertia is by now (resistance to yaw maybe…). Since we just did pitch and roll, you are hopefully getting the idea. Yaw inertia is usually not a big impact to smaller airplanes or space planes (flying in atmosphere) because they are symmetric left/right. Still, if you want to see how it can affect your craft (especially when encountering things like asymmetric thrust), then you can mess with the yaw inertia. Realize that yaw inertia for an airplane or space plane is basically affected by the weights of pitch and roll that we just talked about, but at the same time. Why? The reality is that pitch and yaw are relatively unaffected by vertical weight in the airplane because smaller airplanes tend to be flat. So in our discussions of pitch/roll inertia, we basically ignored the up/down weight distribution. In yaw, you can’t simply ignore the left/right or nose/tail weights unless your craft is very short or very long. And in these cases, where you put your yaw control can be very important. Modifications: Hopefully you’ve gotten the idea from the Pitch and Roll Inertia sections on how to work through this. I’m going to leave it as an exercise for you to try out. Activate the rudder/canards/stabilator in various orders and move monopro out to the wings (or nose/tail) and see how it goes! Good luck! SUMMARY: This is such a long article, that I tried to pull together some short reminder points here. -- Flight Controls in General -- - Flight controls placed further from the Center of Mass (CoM) are more effective. - Flight controls with a higher lift rating, at the same distance from the CoM, are more effective. - Putting on heavier control surfaces or placing them further from the CoM adds to rotational inertia, which costs more fuel to rotate in space. - More flight control authority is not automatically better. It can cause over-controlling in the atmosphere and may lead to an out of control aircraft. - You can use flight controls as an added lifting surface instead of adding another wing, which can keep your overall design size smaller/lighter. As of KSP v0.23, just right click and deactivate the surface. It will not move and becomes part of your wing/tail/etc. - Flight control chatter (SAS): Also, I didn’t dive into it because this aircraft doesn’t chatter. But if you have an airplane or space plane that chatters around (where the flight controls go crazy because of the SAS) you can selectively cut out some of the control surface functions to try and make it stop. This tends to happen to me when I have too many flight control surfaces that end up fighting with each other to stabilize the craft. -- Pitch Control -- - Elevon – Can give plenty of pitch ability for craft that don’t need to be highly maneuverable. If your airplane or space plane is heavy with little wing area, using elevons can cause lift problems. - Horizontal Stabilator – Allows you to pull the nose up by pushing down on the tail. Generally you can keep the nose up until the wing runs out of lift or the stabilator can’t turn any further. Typically less effective than canards due to being closer to the CoM. - Canards – Are generally more effective than an elevon or horizontal stabilator because it is further from the CoM. You can still run into pitch control issues if your craft relies on the canards for lift, or if it flies at high angles of attack. - Torque & Thrust Vectoring – Generally not strong enough to give you all your pitch control needs from just the command pod. If you stack up a few control wheels they can have a big effect on small craft. The benefit here is that all the torque is available for use in space but doesn’t add to drag due to lift. Also, they could add less rotational inertia than flight control surfaces, depending on where you place them. The downside is they have a lot higher mass than most control surfaces (KSP v0.23). -- Roll Control -- - Ailerons – The primary roll control source. If you have a strangely shaped wing or it carries a lot of weight, ailerons may cause the wing to warp causing yaw/roll/pitch problems. Strutting or moving flight controls can help this problem. - Differential Stabilator / Canards – Work as well as ailerons, but add less inertia since they are typically attached to the fuselage and are closer to the CoM (although they don’t have to be). - Torque – Might be enough to control your airplane or space plane, depending on size and how maneuverable you want it to be. You generally need a canard/stabilator/elevon for pitch control too, so why not just go ahead and use those for roll control also? Well, if torque gives you enough control, not using the canard/stabilator/elevon for roll can minimize asymmetric lift issues (making yaw and pitch easier to control). -- Yaw Control -- - Rudders – Not horribly important on smaller aircraft, but placement becomes more significant for larger or oddly shaped airplane and space planes. If your aircraft is really wide, you might need to move the rudders out to the wings and away from the body (away from the CoM) to make them more effective. - Torque – For smaller airplanes and space planes, torque from the command module can be enough to provide yaw control unless you have asymmetric thrust problems. You may still need a vertical tail to provide stability, but you don’t necessarily have to attach a rudder to it. -- Inertia -- - Rotational inertia is a measure of the airplanes resistance to rotation (yaw, pitch, or roll). - More mass located near the CoM gives a lower rotational inertia. - Moving/adding mass out on the wings increases roll and yaw inertia. - Moving/adding mass to the nose or tail increases pitch and yaw inertia. - Moving mass out to the wings/nose/tail increases RCS fuel consumption for rotation, but doesn’t change fuel consumption for translation. One example of how to combine all this: If you have a space plane doing a lot of translation but very little rotation, lower mass control surfaces that are placed in more effective places (i.e. small ailerons out on the wingtips) are better. - Lower mass parts (generally) have lower drag factors, helpful in atmospheric flight. - Lower mass parts consume less Delta-V. - Placing them out further increases pitch/roll/yaw inertia. This needs more RCS fuel for rotation, but we aren’t planning on doing a lot of rotation so the drawbacks are lower. Good luck and happy Designing!

I was able to get up to about 22k and 1100ish m/s. Although the nose feels heavy and the pitch trim was working hard at the end. Your CoM is nearly in the center of your ship, which is actually pretty good, and you have a LOT of wing and engine mass behind your CoM. But your CoL is so far forward because you're using wing sections to create the forward hull. This is going to be a problem with this design and you might have to find something useful to use as ballast up front. EDIT: By the way, I flew in stock Kerbal. - I recommend more intakes. Four intakes for eight engines is not enough. I'm not saying you need to cover your jet with intakes. I find three per engine to usually be enough, but some people use a lot more. You should probably have at least one per engine though. You could slide a couple radials in between the wings or in that little pocket between the main body and the inboard wing engines. Remember, they don't have to be directly connected to your engines. - I don't think you have enough pitch control. You have a lot of elevons on the wings, and they are what is causing your wings to flex so much during takeoff. However, they are closer to your CoM and aren't giving you nearly as much pitch as the tail. - I locked the outboard most elevon on the top and bottom wing and the wings didn't flex nearly as much on takeoff. If you offload some of the pitch control from your wings, they won't flex as much and you might be able to delete some struts. - Try adding some inline reaction wheels somewhere (maybe in the nose?). They may help your pitch trim keep up. - You could consider adding a canard design and that would do wonders for your pitch, but I think this is a bad idea because it will shift your CoL forward a lot. However, depending on how you do it, it might also shift your CoM forward. - Your CoL is sitting a little high compared to your CoM. I think for a ship this heavy and draggy, having a high CoL is causing what little pitch control you have to work extra hard. Try shifting the wing (or tail, because it's probably easier) down to lower your CoL and see how that works. - Recess your cluster of four engines forward into the main body some. This will move the CoM forward. - Your ship is so large, you actually don't need as much ballast as you have up front. You can fly it mostly neutrally stable (with the CoM and CoL in the same spot). Your ship has so much inertia that if you improve your pitch control, you will be able to hand fly and keep it from going out of control. - Make a sandwich tail (similar to your wings) or add more tail area (with control surfaces). - Replace the large control surface on the tail with 4 or 5 small control surfaces. This will boost your tail lift from 1.4 to 5.0, which will move your CoL back some and increase your pitch authority at the same time. Anyway. I don't know if any of that will be helpful, but you have a beautiful ship and it flies pretty good below about 18,000 m. I tried adding intakes and messing with the tail a little and managed to get it up to 28,000 m at something like 1450 m/s. So I think you have a solid design that you can work off of. Good luck!!

This thing looks gorgeous! I'm downloading it now and can't wait to check it out.

Sure, I hope I was able to help. By the way, here's the likely reason your plane is so hard to control in pitch. Here is a test flight in level flight at about 8,000m and 160 m/s (where I assumed you were having problems). The red line is the airflow (represented by the prograde marker). If you look at your forward wings (the "structural wing" element), it is still at negative AoA. Your forward-top section is angled down to far and is providing a downward lift force, while your lower half is probably getting close to max lift (and will soon start to decrease lift). Essentially your forward angled sections are canceling each others lift out (sort of). Also, a side effect of using an elevon is that you put a lot more force into your wings, causing them to flex. I went back to my first design and put a single strut inside the wings, on the main delta part. Also, I went and locked out the roll and yaw function of the lower elevon, leaving it for pitch only. This helped minimize pitch losses when rolling and pitching the craft. Plus, activating your elevon (to pitch up) causes your wing to lose lift, making your heavily laden/low lift design slow down further. That's why minimal pitch changes (till you're higher and faster) are important for flying this design. Don't hesitate to use the aerospike in little bursts if you get too slow in that 8,000 to 14,000 ish altitude range. In my (limited) experience, MechJeb has a hard time controlling these kind of characteristics. It worked well for dealing with throttling the air breathing RAPIERS though. If you move your gear a little forward, you won't need to use the aerospike. EDIT: Ahh, I misread what you said. I mean to say if you move the gear forward, takeoffs shouldn't be a problem at KSC. Yes, essentially. Because of what I said above.

To be honest, I don't remember what my speed was, but I know it was slow. It was slow because of how the plane flies. I guess I should have also said I wasn't using MechJeb to throttle modulate for me. I was flying by hand since I assumed SRAS was also and I didn't fully milk the engines since that's not what I was worried about. That being said, I went back and flew the profile again to see if I could match your numbers. The plane is already unstable because of the angled wings so I figured I wouldn't get to 25 km and 1000 m/s. So I first ran a profile with the same craft using straightened wings. I was able to reach about 1050 m/s at 25,000m altitude. However, when I ran the same test with the original craft, I maxed out at about 23,500 m and 920 m/s. I could give it a little boost with the aerospike, but the speed/altitude bled off. This was with a vertical climb rate of about 10 m/s through 21 km (and lower as I climbed). I'm not sure if your calculations account for the forward, angled wing. Plus it's a little less stable.

I think if you just pull off the aerospike, you might not have enough thrust to overcome the slow speed during climbout since you have to fight the pitch so much. Your aircraft is pretty heavy right now and the RAPIERs just aren't pushing it fast enough. You can swap out the RAPIERs for TurboJets, but you might end up with the same problem. Can't hurt to try. Good luck!

Oh, why thank you Specialist! I feel honored. I will likely create more tutorials assuming this one is well received. I've already created another aircraft to demonstrate the effects of flight control surfaces and moments of inertia.

Since it's hard to see everything in your pictures, I spent a little time trying to replicate your ship. I was a bit thrown off because you said the mass is 28.802 and the picture shows 25.802 so I tried to match both of those weights with the CoM and CoL picture you put up. I also noticed you said it "flat stall" and not "flat spin" so I will address the "stall." By the way, I love the way the wings look. So... Design: The only way I could get that much weight was by using all fuel/oxidizer tanks. I don't know if that's what you did, but if you're going to use the RAPIER in air breather mode, you don't need excess oxidizer. Unless you have RCS/monopro hidden in there somewhere. I gotta ask: Are you flying SAS on or SAS off? SAS off it sure wants to pitch down. Takeoff: I noticed this thing is a beast to takeoff. I had to wait till I ran off the end of the runway, but it flew. Climbout: If I get the nose way up then let it droop, the airplane caught itself and climbed out slowly. If I tried to fight and hold a specific pitch angle, or tried to climb too aggressively, the nose eventually fell and stalled out. So if this is what you're running into, try climbing out very shallow until you can get high enough to get some speed. I found my pitch to be around 30 degrees with the prograde at 5 to 10 degrees. Around 20k (when the RAPIERS start to go asymmetric), you can throttle back and hit ignite the aerospike. Your pitch is already high enough for the aerospike to put you in the right direction, and throttling down allows the RAPIERS to air breath a bit longer. Soon after, switch RAPIER's mode and go full throttle. Now you just have to play out orbital mechanics as for a rocket. With a big HOWEVER... Orbital insertion: My airplane wanted very badly to pitch up. I'm guessing it's because you have the tail fins, ladder, and parachutes on and your CoM is so far back. Your plane is so neutrally stable that when you burn off fuel, the CoM shifts back and up (behind your CoL but that doesn't matter in space). When it's time to ignite the rockets, all that trust makes it buck like a bronco because your CoM is above the Center of Thrust. 80 km So I tried a few tweaks if you want to give something similar a try. I ditched about 1/4 to 1/3 of the oxidizer the RAPIERS were carrying. I moved the rear landing gear forward and outboard from the engines to wing connector (closer to the CoM), and removed the ones from the wings. I also added a canard to the tail (so it's actually a horizontal slab) in between the wings, but just in front of the RAPIERs, to move the CoL back a little and give some more flight control authority (which the SAS made full use of.) I also placed it slightly low on the fuselage to offset the weight of the tails but it wasn't quite enough. If you're going to add a canard to the back, you might want to put a strut or two in between the sandwich of the wings because the extra force caused them to flex. If you put it inside, I doubt you'll be able to see it. That being said, the thing still flew with about 20 degrees AoA but took off and climbed out a bit easier. I just parked the pitch at 30 degrees and let it ride till 18km and lit the aerospike. If you adjust pitch AT ALL at this point, it will fall due to lack of speed. During orbital insertion, the plane still wanted to pitch up. So if you want all those chutes and ladders (no pun intended), then you'll have to figure out how you want to shift your CoM down some. You might also want to check your CoM location after you burned off your fuel. Right now it's neutrally stable but will probably be unstable when you want to reenter the atmosphere. (By the way: Make sure you remove your gear while you play with the CoM. They mess up the CoM in the SPH and don't really contribute to it in flight.) Good Luck!!

Numerobis, thanks for the code. I haven't seen it yet so it's nice to know what the current programming is.

Thanks for the tip. I will add it, but I imagine landing gear can just about be it's own thread.

Your plane must be getting lift or it wouldn't fly at all. If you're using ailerons to control pitch, then they're actually called elevons. And they are a legitimate design "feature." There is additional basic aero info in the tutorial section. Keptin has a good article (Basic Aircraft Design) that touches on a lot of basics. Although it doesn't quite cover rotating pieces and trim, it talks about Angle of Attack and a lot of other considerations. I've written a tutorial to go along with Keptin's work. Edit: There is also this thread, which links to a lot of info. Most of it is not about airplanes, but there is some air/space plane info in there.

Oh yeah. I read another tutorial where someone mentioned NOT using landing gear on small space planes, VTOLs, etc to save weight. So after messing around with different kinds of skids and rail type landing gear, I bit the bullet and launched this behemoth. Around 31 tons with 53 landing gear, and I only lost about 8 m/s in max speed. I'm pretty sure it was all due to trim drag since I made no effort to keep the exact same Center of Mass.

Thanks! I hope it helps someone and I had fun making it. For the landing gear, do you mean I should put more info in here about strengthening the landing gear? Or replacing the landing gear with cubic struts? I was afraid of diving too much into landing gear design on this one since it was pretty long already and I was trying to focus mostly on aero.

This isn't a physics engine issue. All airfoils in this game are (currently) symmetric. You must have Angle of Attack on your airfoil to produce lift. This means there will always be a difference between your prograde vector and the aircraft body pitch (the yellow "V"). This is also how it works for real airplanes. You said "that means I'm flying level, no pitch." In aerodynamics, these are two different things. "Flying level" means you are not gaining or losing altitude. "No pitch" can mean a variety of things, but I'm gathering you mean to say your yellow "V" is level with the horizon (or maybe coincident with your prograde vector?). I haven't used FAR, but if it is an accurate representation of physics, you would still have the same result if you use a symmetric airfoil. That being said, even if you have an asymmetric airfoil (like camber used on light civil airplanes), that asymmetry is only good for one aircraft weight and speed. That means if you go faster/slower or weigh more/less, you will have some Angle of Attack on the airplane. (Technically it's the Angle of Attack relative to the wing that is important.) Sooo, if you want your yellow "V" to be in the center of your prograde marker and level to the horizon, then you need to tilt the front end of your lifting surfaces upward so they have angle of attack when flying straight ahead. Then you need to figure out what the airspeed is that corresponds to your aircraft weight to achieve zero pitch. That airspeed will change as you burn fuel (due to weight and Center of Mass changes), and will be different if you add a new component to your airplane. Also, as far as losing trust to gravity, this is also how it works in real life. You're either going to lose thrust directly to gravity, or you lose thrust to overcome the drag that is produced from creating lift. And creation of lift is required to overcome gravity. Without gravity, you wouldn't need lift to begin with.

If you're considering an update for empennage/control-surface layout, canards, etc... I would be interested. Here is a picture of the aircraft I'm toying with for a new tutorial. Right now it's about control surfaces and moments of inertia. I was thinking about adding a T-tail but I'm not sure I want to complicate this design with one. Perhaps that would be best done with another aircraft.

PURPOSE: This tutorial is aimed at those who want to build a basic airplane/space plane, but find themselves overwhelmed when looking at some of the pretty amazing space planes other people have built. I don’t want to show you how to assemble a specific airplane or space plane that I thought up (though we will use an example). I want to show you how to design your own plane that flies how you want, so you can experiment and learn. (Don’t feel overwhelmed now, but the airplane we build/experiment on in this tutorial can be modified to fly in a 200km orbit and return to Kerbal Space Center.) BACKGROUND (basics of aeronautics): I was going to write an information topic about basic aeronautics because the ones I read were very simplistic. What I mean by that was there was little depth beyond “put your center of gravity in front of your center of lift.†This IS an incredibly important tip. However, I kept seeing comments from people unable to design or fly their own aircraft for various reasons. Fortunately, before I started writing I found this article "Basic Aircraft Design", written by Keptin, and I think it is a very good starter. I tip my hat to the author for spending time making the illustrations and breaking the terms down. Do not feel like you have to fully grasp the concepts, as the point of this tutorial is to actually SEE how the different factors affect flying in stock Kerbal. Keptin’s article covers these topics, in this order: Center of Mass (Is mentioned, but not described as he assumes you already know what this is.) Center of Lift (and its relationship to Center of Mass) Center of Thrust (and its relationship to Center of Mass) Control Surfaces (Ailerons, Rudders, Canards/Elevators, Elevons) Wing Shape (High, Moderate, Low Aspect Ratios) and Wing Sweep Wing Placement (High, Mid, Low Wing, Dihedral, Anhedral) Angle of Attack Landing Gear (and effects on takeoff/landing angle of attack), landing gear width, overweight I recommend you read Keptin’s article and have a very basic understanding of the above terms. You can either read his whole article and come back here, or read his article and mine at the same time. I decided to try and mirror my tutorial with his (with a few exceptions since we have to design). Also, I will not cover all of Keptin’s topics in this first tutorial. Some of the topics are pretty big, but if people want more, I will write another. Plus I found this tutorial became pretty long as it is (about 10 Microsoft Word pages without pictures). So if you want to shorten your reading in Keptin’s article, see my topics below. TOPICS: For easy reference, here’s what you’ll find below: Section 1: Creating a Test Aircraft Section 2: Landing Gear Placement Basics Section 3: Horizontal Center of Lift Design Section 4: Horizontal Center of Mass Design During Flight Section 5: Vertical Center of Lift Design – aka Wing Placement (High, Dihedral, Low, Anhedral) Again, I believe this information will all be fairly basic and meant to be hands on. So if you’re looking for advanced tips this may not be the tutorial for you. LET’S GET STARTED: Section 1: Creating a test aircraft. So you read in Keptin’s article (or already know) about the center of gravity, lift, and thrust. So now it is time to put these three together into an aircraft that is reasonably stable so we can test out different designs. I’m going to assume you know the basics of the Kerbal interface and how to find the parts, although if you haven’t rotated and flipped parts in the Space Plane Hangar, I have included those keys. Let’s build our airplane: (see the picture below for help) 1) Go to the Space Plane Hangar and start a new ship. (Select “New†if you have any parts or previous ships up in the hangar to clear it out. You might also want to start a new save game if you are squeamish about killing your kerbals, as you will probably crash a few times.) 2) Select the Mk1 cockpit, add two Mk1 fuselage sections and one TurboJet engine. 3) Turn on angle snap. 4) Place one tail fin on the top of the rear fuselage, as far back as you can get it. 5) Landing gear: Place one “small gear bay†on the underside of the nose so that the back edge of the gear lines up with the seam between the Mk-1 and the forward fuselage section. 6) Place the rear gear (with symmetry on) on the underside of the rear fuselage section. Line it up so that the back edge of the gear bay lines up with the seam between the rear fuselage and the turbojet engine. Place the rear gear slightly up the sides of the fuselage. Make sure angle snap is on and you will have to rotate the gear 45 degrees [shift-Q, nine times]. Ensure the gear points straight down. 7) Turn on the Center of Gravity (yellow & black bubble) and the Center of Lift (blue & black bubble) indicators. 8) Select the Delta wing, turn on symmetry, and mount the wings in the middle of the fuselage side. Align the center of lift (blue & black) bubble just behind the center of gravity (yellow & black) bubble. You can use the little “spikes†that stick out of the side of the blue lift bubble to help with alignment. 9) Attach a “Standard Control Surface†to the back edge of the wing, with symmetry on. 10) Add “XM-G50 Radial Air Intakesâ€Â, with symmetry on, and align the intake’s connecting point with the flame symbol near the back end of the airplane fuselage (left side). 11) Give it a name and save if you want to. Overall it should look like this… Go Fly – But wait! Before you actually go fly I want to describe what we will do. We will use the same basic profile for takeoff and flying the plane. That way the changes we make during the tutorial will be a bit more obvious, and the results repeatable. Read through this next part before you do your first takeoff. Here’s our procedure… 1) Rotate the camera around to a comfortable viewing angle. Personally, I like a view that allows me to see the control surfaces and landing gear. 2) Ignite the engines [space Bar], run the throttle up [shift] to full power, then activate the SAS [T]. 3) At 120 m/s, pull the nose into the air (or pull back on your joystick) to get the nose a few degrees up. Holding 5 to 10 degrees nose high is fine. 4) After the aircraft climbs away from the runway, retract the landing gear [G]. 5) Continue to accelerate till you reach around 160 m/s. Pull the nose into the air again [s, or Joystick] and try to get it straight up. Work hard to get the dot in the ‘V’ right on the straight vertical dot. 6) When you reach around 2000 m, push forward [W] till you get the nose back to the horizon. 7) Turn off the SAS [T] and see what happens. This is where our generic procedure will end. Feel free to play with and fly the airplane around more after this point. However, this flight profile will serve as a baseline for my comments on aircraft reactions you’re looking for (listed as “Test Reportâ€Â) in each tutorial section. It’s up to you if you want to fly before reading the Test Report, or read the Test Report and then go experience it. Now, actually GO FLY! – Test Report: What you’ll notice during the flight is the aircraft won’t actually lift off the ground at 120 m/s and will run off the end of the runway if you let it. We will discuss why that is in the next section (if you haven’t figured it out already). Go ahead and run off the end of the runway and make sure you’re still pulling back. The aircraft will fly away easily and is controllable all the way up. The nose sort of bounces a little during the pull, but it’s not too hard to pull the nose up and push it back down. SAS off, it flies about the same except it doesn’t snap to a stop during rolls and pulls. Depending on your exact placement, it might want to pitch slowly forward. This will be our starter aircraft for Section 2. Section 2: Landing Gear Placement Basics Landing gear placement is one of the basic considerations for your takeoff roll. I’m not going to delve into all the types of landing gear or problems in this tutorial (maybe on another tutorial) because I want to focus on small aircraft aero basics. However, this basic tricycle gear can cause problems leading to wasted aerodynamic tweaks that can mess up airborne performance. For example, one way to fix the takeoff of our basic airplane is to put on canards, or we could tip the wings up. However, we can adjust the landing gear first without increasing part count (and weight and drag) and without changing the airborne aircraft characteristics… Let’s rotate the landing gear around. 1) Select the rear landing gear (make sure symmetry is on) and reset the rotations you did earlier (Press [space]). 2) Rotate the gear so the wheels are on the front of the gear bay instead of in back (Press [D, 2 times]). 3) Now, rotate the gear 45 degrees down like earlier [shift + Q, 9 times] and attach to the rear fuselage with the back end of the landing gear again lined up with the seam between the TurboJet engine and the rear fuselage. This leaves the center of gravity unchanged, but moves the wheels very far forward. Go Fly! Test Report: Oh no! What happened? The aircraft tips back on its tail because the wheels are so far forward. That's okay, go ahead and activate the engine and take off anyway to watch what happens. Wait to activate the SAS until after the nose wheel is back on the ground. At 120 m/s, rotate the nose off the ground. You'll find it takes very little to get the nose up, and the aircraft is very stable after that. The rest of the profile looks like it did before. So we fixed the rotation problem (sort of) without changing flight performance or part count. Obviously starting out with an airplane tipped up and the engine on the ground isn't ideal. So let’s refine this a little. 1) Go back to the hangar and select the gear, undo rotations, and rotate it back down 45 degrees (remember: symmetry on, [space] to reset, [shift + Q] to rotate). Except this time when you align the gear on the rear fuselage, place it so that about 1/3 of the gear bay is in front of the Center of Gravity. 2) Adjust the wings so the center of the Center of Lift bubble is aligned with the back end of the Center of Gravity bubble. (The bubble will move some, it's okay.) Go Fly! Test Report: The aircraft takes a little more to pull it off the ground than when the wheels were flipped around, but it also doesn't sit on the engine. The rest of the flight profile is unaffected. Go ahead and play with gear placement more if you want. Also note that even though the landing gear moves the Center of Mass bubble in the SPH, it doesn't actually change the flight characteristics. Despite having weight and drag in the SPH, the landing gear currently have no effect on in flight mechanics (KSP v0.23). This isn't too big of a deal with most aircraft, but if you're working on touchy stability with strange airplanes, it might be important. Because of this, you may want to consider adding the landing gear last during construction so that it doesn't throw off the Center of Mass bubble when placing the Center of Lift. I will possibly cover more landing gear in another tutorial if people want it. Personally I'm a fan of having the gear all swing forward when they retract, but you can place them in either direction. We'll use this most recent configuration as our baseline airplane for the next sections. You might want to save a copy to save you time later after making modifications. (By the way, this configuration is capable of flying up to about 64km if you let it keep going straight up.) Section 3: Horizontal Center of Lift Design Okay, so when talking about Center of Lift, what's actually important in basic design is the relationship between the location of the Center of Lift, and the location of the Center of Mass (at least for now...). So what we're going to do move the Center of Lift around a bit in relation to the Center of Mass (horizontally) and see how the airplane flies. We will discuss vertical changes in Center of Lift in a later section. 3.1 Center of Lift Aft of Center of Mass (Positive Stability, or Stable) This is how we've been flying the plane around up to this point, with the blue lift bubble behind the yellow mass bubble. Now we're going to move it much further back and see how it handles. Modifications: 1) Grab the wings and slide them aft just so there is a slight gap between the blue lift bubble and the yellow mass bubble. (Blue bubble closer to the engine than the yellow.) Now go fly! Test Report: The airplane is a little harder to takeoff than it was before and the nose is slower to get up and down. It still flies pretty smooth though, and I find it a little easier to fine tune the straight up part. If you paid attention to the altitudes during pull up and pull down, you’ll see it takes more room to turn. This is kind of important to note: SAS off, the airplane wants to pitch down slowly. If you design your airplane with the Center of Lift too far back, the SAS might not be able to compensate and the aircraft will slowly pitch over. 3.2 Center of Lift with the Center of Mass (Neutral Stability) Modifications: 1) Grab the wings and slide them forward so the blue lift line is coming out of the top center of the yellow mass bubble. Go Fly! Test Report: The airplane lifts off the ground really easily. The nose is quick to get toward straight up and is still a little "bouncy." If you manage to get really aggressive with the pull up (or your Center of Lift is a little too far forward), the airplane might go out of control a bit. SAS off, the plane might want to pitch up or down a bit depending on your exact Center of Lift placement. Advantages here are that the plane is a lot more responsive. However, it sits on the edge of being out of control. Bear in mind that the Center of Mass will shift during flight (aft for this airplane), so if you start out neutrally stable on takeoff, you may end up unstable during flight. 3.3 Center of Lift forward of the Center of Mass (Negative Stability, or Unstable) Unstable during flight you say?! Let’s try that out too. Modifications: 1) Grab your wings and slide that lift bubble forward so that there is a slight gap between the blue lift bubble and the yellow mass bubble. (With the blue ball closer to the cockpit!) Go Fly! (Or try to anyway.) Test Report: How was that liftoff? This thing is flyable if you know how, but you can see how much work it is and how easy it goes out of control. Usually you don't want an airplane like this for (hopefully) obvious reasons. It's actually kind of fun to watch it fly around. The SAS actually does a lot of work here trying to make the airplane stable. If you manage to not crash for a while, turn the SAS off and see how it goes. If you want to play around with a negatively stable airplane, adjust the Center of Lift so that it is only slightly forward of the Center of Mass (blue bubble inside the yellow bubble, but forward of center). You can slide that around a bit and see that an airplane with a forward Center of Lift is flyable but takes some work. It's sometimes easier to get the airplane under control without the SAS, then turn it back on when you're nearly flying right again. This might give you some confidence if you find yourself in a bad situation and you can rely on some piloting skill to save it without abandoning or reverting right away. 3.4 Revert the airplane to the baseline from the start of this section. This means putting the center of the Center of Lift bubble at the back edge of the Center of Mass bubble. Section 4: Center of Mass Design During Flight Okay, so now that we know a bit about the relationship between the Center of Lift and Center of Mass, how can we affect this during flight? Well, for our basic airplane, the Center of Lift isn't going to move around but the Center of Mass will because of fuel burn (and maybe because you leave or pickup a payload in orbit). For now, we'll just talk about planning a Center of Mass change due to burning fuel. 4.1 Center of Mass due to Fuel Burn for our basic airplane (horizontal Center of Mass) As we saw in Section 3, the airplane is most unstable when the Center of Mass is at the most aft position. So in this section we will adjust fuels to simulate the change in Center of Mass during flight. In this way we can find out if our aircraft will end up neutrally stable (Section 3.2) or unstable (Section 3.3). Then we can design the airplane to have the desired stability and maneuverability at all times in flight. In our basic airplane, normal Kerbal fuel feeding will use fuel from front to back. Let's see if we can figure out when the Center of Mass will be furthest aft. Modifications: 1) Right click on the front fuselage tank and run it completely empty by clicking and dragging the green bar down to zero. Watch how the Center of Mass yellow bubble moves aft, toward the engine. It will also move slightly down, but let’s not worry about that for now. 2) Now right click on the rear fuselage tank and run it completely empty. Watch the yellow bubble. It should move forward (and slightly down). So the Center of Mass is furthest aft when the forward fuselage tank is empty, and the rear one is full. 3) Refill the rear fuselage tank and make sure the forward fuselage tank is empty. Notice how the center of the blue bubble is still behind the center of the yellow bubble (although it is now inside the yellow bubble). Go Fly! Note: If your plane tips back on the tail, see if you can think how to fix it. (Hint: Think about Section 2: Landing Gear. This is part of the iterative process of designing.) Test Report: The plane is fairly maneuverable but still controllable at the most aft fuel condition. Not much else to say here other than, if your plane tips back on the engine and you didn’t figure it out, you’ll need to move the landing gear slightly back. (With the fuel cut in half, this configuration is capable of flying up to about 85km if you let it keep going straight up. Recall when fully loaded it was about 64km, a 33% increase.) NOTE: Make sure you refill the forward fuselage tank when you’re done!! Section 5: Vertical Center of Lift Design – aka Wing Placement (High, Dihedral, Low, Anhedral) Vertically moving the Center of Lift can be caused by two basic design choices. If you recall Keptin’s discussion of vertical wing placement (high, mid, or low) and wing up/down angle (also known as dihedral and anhedral), these affect an aircraft’s stability similar to having a forward/aft Center of Lift. In the case of our basic airplane, we have been flying with the delta wing mounted mid fuselage, so the Center of Lift has been (nearly) at the same vertical height as the Center of Mass. Moving the Center of Lift above Center of Mass (High Wing and Dihedral) tends to stabilize an aircraft. Moving the Center of Lift below Center of Mass (Low Wing and Anhedral) tends to destabilize an aircraft. Realize the concepts of wing placement and dihedral/anhedral are two different concepts, and we will explore both. 5.1 High Wing – Center of Lift Above the Center of Mass (Positive Stability) High wings tend to stabilize an airplane. Modifications: 1) Grab the wings (symmetry and angle snap on) and place the wing root 30 degrees above the fuselage center line and angle the wings so they are back to level [shift-Q, 6 times]. (30 degrees above fuselage center is two steps up with angle snap on. Hopefully you know what I mean. The wings should be level after you rotated with [shift-Q] six times.) 2) Place the Center of Lift blue bubble above the Center of Mass yellow bubble. No doubt, it’s definitely above! Go fly this one. Test Report: Very maneuverable in the pull up to vertical, and still fairly maneuverable and bounces a little. Great but what’s the downside? When you push forward, the airplane will likely go out of control if you push too hard/far. However, with a high Center of Lift (over the Center of Mass), the aircraft tends to “right†itself sort of like a curved leaf falling through the air. SAS off, it wants to pitch up (due to Center of Thrust issues.) 5.2 Dihedral Wing – Center of Lift Above the Center of Mass (Positive Stability) Just like high wings, dihedral tends to stabilize an aircraft. Modifications: 1) Grab the wings (symmetry and angle snap on) and reset rotations [space]. 2) Tip the wings up 10 degrees [shift + E, 2 times]. 3) Attach the wings mid fuselage so the blue lift vector goes up through the center of the Center of Mass yellow bubble. The blue lift bubble won’t be as high as last time. Go Fly! Test Report: You’ll see that it flies a lot like the high wing airplane. Fairly maneuverable to straight up with a little bit of bouncing. It suffers from the same instability as a high wing if you drive up to about 2000m and push forward hard [W]. And just like high wing, it pitches up with SAS off. 5.3 Low Wing – Center of Lift Below the Center of Mass (Negative Stability) Low wings tend to destabilize an airplane. Modifications: 1) Grab the wings (symmetry and angle snap on) and reset rotations [space]. 2) Place the wing root 30 degrees below the fuselage center line and angle the wings so they are back to level [shift + E, 6 times]. (Hopefully you know what this means now that you’ve suffered through high wing. Again, the wings should be level after you rotated with [shift + E] six times.) 3) Place the Center of Lift blue lift bubble below the center of the Center of Mass yellow bubble. Go fly it. It’s not too uncontrollable… Really! Test Report: Pulls up nice and quick to straight up, then suddenly spins around and goes crazy for a second. You can do some pretty crazy maneuvers with this and it sort of stabilizes with a little bit of pilot input. SAS off, it wants to pitch up. Although this time it’s due to instability, not Center of Thrust. 5.4 Anhedral Wing – Center of Lift Below the Center of Mass (Negative Stability) Just like low wings, dihedral tends to destabilize an aircraft. Modifications: 1) Grab the wings (symmetry and angle snap on) and reset rotations [space]. 2) Tip the wings down 10 degrees [shift + Q, 2 times]. 3) Attach the wings mid fuselage. Again, make the lift vector go up through the center of the Center of Mass yellow bubble. Notice how the center of the blue bubble is nearly centered in the yellow bubble. So it’s not actually below the Center of Mass (unstable), but it’s about as neutral as you can make it. ([shift+Q] 1 more time and you can make it neutral if you want.) Go fly. I bet you can guess how it will react, based on our dihedral experiment. Test Report: Flies a lot like the low wing aircraft, but maybe not quite as quick to go out of control and a little easier to recover. SAS off, it will want to pitch away from your prograde marker. Like the low wing, this is due to instability. 5.5 Vertical Lift Placement Summary Personally, to me anhedral looks awesome on an airplane. Strangely, I don’t really like it on a space plane (maybe because it isn’t the classic shape). If you place the wing slightly high, you can give them anhedral and it will still be reasonably stable (looks like a Harrier). If you place the wings low and give them dihedral, it looks more like a space plane (to me anyway). Try this and see how it flies: Move the wings down from the midpoint one angle snap (15 degrees) and give them dihedral [shift + E, 4 times]. Align the back edge of the blue lift bubble at the back edge of the yellow mass bubble. SUMMARY: I hope you have fun with this basic plane. It isn’t much, but I think it’s effective for playing and learning. Placement of the Center of Lift with respect to the Center of Mass is a big design crux of your airplane. Generally speaking, you DO want the Center of Lift behind your Center of Mass. However, you need not be afraid of having it neutral if you know what you want. Hopefully I’ve given you a bit of hands on knowledge and courage to try out different designs. Don’t forget to consider fuel burn in your design. The design choice of high/low/mid wing and dihedral/anhedral/neutral wings is really up to you. Just like the Center of Lift/Mass, it depends on how you want your airplane to fly and what you want it to do. You can also combine these in various options to get the look you want, but with stability. If you have a hard time flying a plane, you’ll probably want it more stable. If you find flying airplanes easy, then you can opt for a more maneuverable design. Realize the aircraft reacts differently as you get fast and higher up in altitude. And for space planes, you’ll need good controllability and stability as you leave/reenter the atmosphere. If you guys want, I can continue the tutorial to explain how design and test this thing to get into orbit with only two more parts.

Thanks! I'm actually almost done. The pictures took much less time than I expected. Although I do feel a bit weird throwing a tutorial up into the forums already. Wish me luck! (HA)