Basic Airplane / Space Plane Aero Tutorial

[Claw]

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.

Edited January 13, 2014 by Claw
Added landing gear mass/drag note.

[BigFatStupidHead]

Very nice tutorial; I'm certain this will help a lot of people. I'd like to mention aircraft landing gear are massless and dragless while in flight.

Edited January 12, 2014 by BigFatStupidHead
Clarity!

[Claw]

Very nice tutorial; I'm certain this will help a lot of people. I'd like to mention aircraft landing gear, like the small cubic strut, are massless and dragless while in flight.

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.

[BigFatStupidHead]

That was a little cryptic, wasn't it? I mean to say that the small gear bay landing gear do not have physics while in flight; they only appear to have physics while in the VAB.

[Claw]

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.

[numerobis]

I recommend not placing landing gear until the very last stage of construction, because it throws off the CoM indicator, being massless in flight but massive in the SPH/VAB.

[Claw]

I recommend not placing landing gear until the very last stage of construction, because it throws off the CoM indicator, being massless in flight but massive in the SPH/VAB.

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

[Specialist290]

It's always nice to see new content in the Tutorials forum, especially when it's this informative I''ll have to use this as a guide for my own forays into spaceplane building -- I've never really been that good at it, but referencing this should definitely help me with troubleshooting my designs.

[Claw]

It's always nice to see new content in the Tutorials forum, especially when it's this informative I''ll have to use this as a guide for my own forays into spaceplane building -- I've never really been that good at it, but referencing this should definitely help me with troubleshooting my designs.

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.

[sal_vager]

Lots of useful info here Claw, stickied for Feb

[Arugela]

Can this be redone a bit(or refixed). Think something messed up the formatting. Some nice stuff to seperate and indicate which problem it's demonstrating(besides the pics obviously) might be nice to help it easy to sort through. 8)

Or am I the only one seeing all the funky characters and stuff. This is still very helpful potentially.

Just found this also:

http://www.boldmethod.com/learn-to-fly/aircraft-systems/canards/

Edited January 24, 2016 by Arugela