Building a VTOL Space Plane that Maintains Perfect Balance

[StevenLawyer]

Fellow KSPers:

For those having trouble building Space Planes that maintain balance throughout their fuel burn, I put together a quick tutorial:

[Aethon]

Revolutionary. Granted I'm not much of a plane guy, but ... this is outside my box. Thanks Steven.

[Claw]

Moved to the tutorial section.

Cheers,

~Claw

[Kasuha]

There are many inaccuracies and misconceptions in this video. WAY too many for me to correct them all. What you built is not balanced craft.

To learn about how fuel is drawn from tanks, I suggest you to read this thread. Not everything you say about it in the video is accurate.

I wonder which tank you're referring to has center of mass at different point when empty than when full. As far as I know, all parts only apply their mass to single point. Turning that rear tank around in my opinion changes nothing.

To build a balanced plane/VTOL, you most importantly need to build it so both its wet and its dry CoM are at the same place. That itself is not simple as it will not happen if you slap parts together in any way. Only after that you need to make all the pipework to drain fuel evenly all around the ship to prevent it going out of balance.

It is not necessary to use the plate to block the engine drawing fuel from the tank to which it is attached. If you send a pipe to that engine, it will scan the plane through that pipe first.

Also the gap between the engine and the plate is just geometric artefact, it would not block the fuel from flowing if the plate was crossfeed capable.

[StevenLawyer]

Kasuha:

Thanks for your reply and the link to your article on fuel flow. I commend you for your testing. It's a very informative article that has expanded my understanding of fuel flow. I have bookmarked it. I do stand corrected on the CG of larger tanks shifting as they drain fuel. I had learned this from another poster and thought I had encountered it in a ship I built in 0.21, but some quick tests in 0.242 did not reveal any tanks in which the CG shifted with change in the fuel. So, I am uncertain whether this has changed or my understanding was incorrect. In any event, I agree that CG does not shift within a particular tank as it drains fuel--In other words, I agree that fuel tanks are modeled as single points of mass in the dimensional center of the tank.

As to the your comments regarding using metal plates, they are not crossfeed capable, so I'm not sure what you are saying. In my experience, if you don't use them, then even if you run the fuel line, your engine will not draw from the tanks evenly. However, I will experiment further with this. Perhaps this has changed in 0.242 such that the plate is no longer necessary.

I respectfully must correct your statement that my ship is not a balanced craft. I have built numerous VTOL ships using the techniques in this video and have flown them to fuel exhaustion--they have remained in perfect balance. I hold a degree in physics and am a semi-professional pilot who flies high performance turboprops in which a clear understanding of the change in COM wet to dry is important. The way you make sure that the COM of the vessel wet (i.e., full of fuel) and the COM of the vessel dry (i.e., out of fuel) are in the same place (i.e., the COM doesn't move), is to make sure the COM of the fuel by itself is exactly on top of the COM of the vessel as a whole (whether wet or dry). It is impossible to have a stationary COM from wet to dry if the starting COM of the fuel alone is fore or aft of the COM of the vessel as a whole. If you need me to provide you the mathematical proof of this fact, I would be glad to do so. So, while what you say is true--namely that the COM wet and dry must be in the same place--what I say is also true and is the practical way in which you accomplish a matching wet and dry COM. The COM of the fuel alone must be centered on the COM of the craft (wet or dry), or else the overall COM will move as fuel is burned.

Again, thanks for your comments and for your research on fuel burn!

[O-Doc]

Nice tutorial. I feel inspired to make another VTOL.

Just couple of points about the vid. Your CoM is off because landing gears are mass-less in the physics engine but not the VAB/SPH. Same as a number of parts, notably batteries. So keep those parts off when doing your aerodynamics. Second, docking ports are better than those adapters you use to attach engines to. They let you add fuel drop pods on top for extended range variants plus, they look better.

One more thing. Instead of fuel lines and the engine plate for that turbo, get inside the last fuel tank and attach a line of cubic struts to attach the engine to. I haven't heard of wings being able to convey fuel, must test this. I use docking ports to attach VTOL engines to wings and that blocks the fuel so no need to use a plate.

Good job.

[Kasuha]

As to the your comments regarding using metal plates, they are not crossfeed capable, so I'm not sure what you are saying.

What I'm saying is that a fuel pipe attached directly to the engine has precedence over any fuel tank that is attached to that engine axially. Your plane would draw the fuel exactly the same way if you removed the plate.

I respectfully must correct your statement that my ship is not a balanced craft.

Okay I take my comment partially back, you actually did put the CoM at the right place in SPH, I must have skipped it somehow when I was watching the video for the first time. The only thing is, at least as far as I know, the landing gear still has no mass in flight. So you need to set up your CoM before you place it. That applies to all other massless parts as well.

Of course the SAS module you used can compensate for a lot more inaccuracy.

I am also fairly certain that putting CoL behind CoM has deeper reasons than movement of CoM. My experience is that planes with CoL over CoM are rather unstable even with full fuel. Maybe you compensated for that by putting your wings higher? I'm not sure. What I know, though, is that my balanced plane has CoL great deal behind CoM and it flies great.

Edit: seems to me that planes with CoL over CoM might be unstable in gliding.

Edited September 8, 2014 by Kasuha

[O-Doc]

I am also fairly certain that putting CoL behind CoM has deeper reasons than movement of CoM. My experience is that planes with CoL over CoM are rather unstable even with full fuel. Maybe you compensated for that by putting your wings higher? I'm not sure. What I know, though, is that my balanced plane has CoL great deal behind CoM and it flies great.

Edit: seems to me that planes with CoL over CoM might be unstable in gliding.

Correct. This is due to the location and direction of the CoT. Think of the craft being suspended by a string at the CoL. The thrust will push the craft's nose up unless CoL is level with CoM. The CoM is moved forward to offset the CoT's position, direction and strength keeping the craft "suspended" correctly. This analogy is stupidly simplistic as other factors play a role but, keeping that in your head when designing is a good way to visualise while you design. The reason why most aircraft have a higher CoL than their CoM is due to the keel effect, not to be confused with the dihedral effect, to make the craft more stable in flight and liftoff/landing in relation to wind gusts.

[StevenLawyer]

What I'm saying is that a fuel pipe attached directly to the engine has precedence over any fuel tank that is attached to that engine axially. Your plane would draw the fuel exactly the same way if you removed the plate.

I agree. I was unaware of that until I saw your excellent study on fuel flow. Thanks again for that.

Okay I take my comment partially back, you actually did put the CoM at the right place in SPH, I must have skipped it somehow when I was watching the video for the first time. The only thing is, at least as far as I know, the landing gear still has no mass in flight. So you need to set up your CoM before you place it. That applies to all other massless parts as well.

Yes. I knew about massless parts, but I keep designing with them in hoping that eventually the devs will make the physics in play match the SPH and VAB.

I am also fairly certain that putting CoL behind CoM has deeper reasons than movement of CoM. My experience is that planes with CoL over CoM are rather unstable even with full fuel. Maybe you compensated for that by putting your wings higher? I'm not sure. What I know, though, is that my balanced plane has CoL great deal behind CoM and it flies great.

Edit: seems to me that planes with CoL over CoM might be unstable in gliding.

Correct. This is due to the location and direction of the CoT. Think of the craft being suspended by a string at the CoL. The thrust will push the craft's nose up unless CoL is level with CoM. The CoM is moved forward to offset the CoT's position, direction and strength keeping the craft "suspended" correctly. This analogy is stupidly simplistic as other factors play a role but, keeping that in your head when designing is a good way to visualise while you design. The reason why most aircraft have a higher CoL than their CoM is due to the keel effect, not to be confused with the dihedral effect, to make the craft more stable in flight and liftoff/landing in relation to wind gusts.

In the real world, in any airplane, while the airplane is in level flight the center of total lift is directly above the center of mass. Otherwise, the plane would begin to either nose up or nose down. That said, traditional (i.e. non-canard) airplanes put the center of lift of the main wings behind the center of mass to induce static stability. By doing so, a traditional airplane's horizontal stabilizer (the horizontal tail) actually has to generate negative lift (downward lift) in order to maintain level flight. It is this downward lift of the tail (which is just a small wing that can generate lift in either direction) that, when combined with the upward lift of the main wings moves the total center of lift forward to match the center of mass. The reason why this set up promotes stability has to do with the relationship between lift and airspeed. Imagine a plane in level flight with the autopilot off (i.e., the plane is trimmed for level flight with no control input). The pilot now momentarily pushes the yoke forward to induce a nose down pitch and lets go. As the airplane accelerates, the tail produces an increased amount of downward lift due to the increased airspeed. As it does so, it pivots the nose upward back toward level flight. If the airplane starts to overshoot to a nose high position, the reverse happens--less airspeed over the tail means less downward force on the tail, which means the nose pitches down. I've flown lots of airplanes, from small Cessna's to corporate aircraft. They all exhibit this behavior (by design)--otherwise, the plane would be dangerous. In a canard airplane, it is just the opposite--you have the center of lift of the main wings forward of the center of mass and then the canard out front provides additional lift. for analogous reasons, this configuration promotes the same type of stability in a canard airplane.

Now to KSP. As to Kasuha's comment: I agree that a center of lift exactly on top of a center of mass (i.e., in all three axes) makes an unstable plane. You're correct that it is the putting of the center of lift in line (fore/aft) with the center of mass, but ABOVE IT (higher) that makes the plane stable. I find when I do this that I can fly my planes level completely hands off. In other words, without ASAS engaged or constantly having to use WASD. In my experience, when the COL is aft of the COM, that's not the case.

As to Doc's comment: Yes, in the real world, the center of thrust being below the COL will tend to ever so slightly drive the nose up (this is not necessarily a bad thing). I'm not sure that is accurately modeled in KSP--it may be that rotation on any axis due to thrust may only relate to the COM, not the COL. I'm finding no problem with having the COL higher (vertically) than the COT.

Thanks to both of you for the dialogue. I love the intelligence of the members of this community!

[O-Doc]

Yes. I knew about massless parts, but I keep designing with them in hoping that eventually the devs will make the physics in play match the SPH and VAB.

These parts have gone from having mass to being mass-less. It's unlikely they are going to go back to having mass anytime soon.

In the real world, in any airplane, while the airplane is in level flight the center of total lift is directly above the center of mass. Otherwise, the plane would begin to either nose up or nose down.

That's simply not true. Only sports planes and fighter jets have this type of configuration to increase turning performance. The further away from the CoL the CoM(CoG) is the more force is required(lift, hence drag you create) to pull that CoM through a turn. Then, the rest of your understanding about how aerodynamic stability works seems a bit muddled. So, I won't quote the rest of it. Let's instead, have a look at a classic design configuration, widely regarded as one of the most aerodynamically stable.

You can immediately notice the relationship between the CoT to the CoM and the CoL to the CoM. Why is this relationship so obvious? Because the more thrust you put into the prop the more speed you will generate and hence more lift to counteract the thrust. When thrust is high and lift is low a nose up moment is induced providing obvious safety benefits. This and the natural keel effect, is part of the reason why this configuration is a classic. The other being that, the prop wash provides lift so, as long as the prop is powered you never get a complete stall condition on the craft. This is also why it can STOL and is used almost exclusively in hard to access terrain.

In terms of aerodynamic stability, this like all aerodynamically stable aircraft will nose down during stall. An aircraft that noses down during stall will pick up speed and start generating lift again by doing so. Once you lose power in an aerodynamically unstable craft you will eventually lose enough speed to induce a stall. At which point the craft will tumble and crash. It's always best to bail out prior to the stall once you lose power. Of course, fighter pilots already know this.

This is a really badly shot videa but, you can see at the end when the guy loses power on his RC Piper Cub the plane slows, stalls, noses down and unstalls a number of times before he safely lands it.

Tailplanes and nose planes work during Y-axis sideslip. During Y-axis sideslip tailplanes will sit in a condition of negative AoA as you rightly pointed out. This AoA generates lift well behind the CoM towards to direction of travel. Fighters jets have stabilaters instead of the tail plane and elevon combination so there is no lifting surface in a negative AoA generating aerodynamic stability. Foreplanes perform the opposite function. In Y-axis sideslip they produce a positive AoA thereby decreasing aerodynamic stability. If they are not balanced by other lift or control surfaces they will rotate the airframe away from the direction of travel to an angle at which they stall. Aircraft with foreplanes must have enough lift to maintain a turn to the angle of the foreplane stall or it will tumble. The exceptions being, computer controlled aircraft like the Eurofighter which uses canards that have controllable AoA. Canards are to foreplanes as stabilators are to tailplanes and the principles are the same. Canards can provide even more positive AoA than foreplanes, or less. They are usually forward swept because forward swept wings can achieve higher AoA before stall condition is produced. It's usually between 60-75 degrees AoA before complete stall.

Most civilian aircraft you see with canards have their main wings well behind the CoM which act like tailplanes. The reason why you would choose this configuration in a civilian craft is to do with payload considerations. In all aircraft the wing root is the primary structure of the plane and takes up alot of space. To make room for larger payloads you need to move the wing root away from the CoM. This can be achieved with foreplanes and forward swept wngs, or a combination of the two. Another reason for canard configurations are due to improving visibility for the pilot.

Thanks to both of you for the dialogue. I love the intelligence of the members of this community!

Me too.

[Aethon]

Well I've never flown a Cessna ( Piper Warrior/Archer Pa-151, 161, for me ), as to the stability of the Cessna, I had always heard that they would do a hammer stall pretty easily, where it will fall toward the left or right wing tip. Is this not correct?

Edited September 11, 2014 by Aethon

[O-Doc]

Well I've never flown a Cessna ( Piper Warrior/Archer Pa-151, 161), as to the stability of the Cessna, I had always heard that they would do a hammer stall pretty easily, where it will fall toward the left or right wing tip. Is this not correct?

Here's an excellent example of this:

The pilot has put too much power into take-off and the nose has pitched up into a stall. The speed is too low for the tailplane to counteract the difference between thrust and lift forces on the plane.

[Aethon]

Good god. What was he doing??? Makes my skin crawl. Gonna be a white knuckle drive to work.

"Go around ya bunch a crazies!" Kramer- Seinfeld.

[O-Doc]

Hehe. I think the pilot got a bit of a shock when he lost airflow into the engine. There's that moment right after where an experienced pilot would have kept the dive unpowered and set it down. But that wouldn't have been as "fun".

[Aethon]

I'd like to comment on the plane crash there but I'm trying to remove it from my consciousness (nightmares).

Stephen, I sense you will continue to experiment on these issues and would be interested to hear results from further testing. Please PM me if you make a video update.