Spaceplane/Aircraft Building for the Engineering Proficient

[Fortunateson1969]

So I\'ve been playing KSP for some time. I\'ve been using the c7 and other aerospace parts for considerably less time.

I have built spaceplanes, airplanes and the like and have had moderate success. There seem to be problems that plague my atmospherically bound craft, and they never seem to be the same. Planes and the like seem to be infinitely more complex to build than rockets (stack as much boosters and fuel on the beastie as you can and shoot for the moon you caveman). Perhaps it\'s the nature of the program itself, but I\'m wondering if it\'s my own personal inequities that cause the issues-namely my lack of an Aerospace Engineering Degree (I myself only possess a meager Computer Science/Applied Mathematics double major).

I know about statics, and lifting bodies, and how wings work (kind of), and I even know how to fly a plane (in simulations, I\'ve played too much x-plane for my own good) but I\'m wondering if there\'s some math I can do here to even the playing field.

So here\'s my questions in no particular order.

1.) Should I be balancing my lift constant of the craft with the thrust the engines can produce? I seem to either have planes that pitch down or up, and I\'m wondering if my upward force vector should be equal to my lateral force vector (overcoming forces, free body diagrams...etc)

2.) My planes seem super tetchy. How many is too many control surfaces? Should I be using a certain number for however heavy the craft is/the lift quotient?

3.) The air breathing engines in this game seem to have very little science-that I can see-associated with them. Are those nacelle things purely aestetic? They all seem to have intakes, and they seem to max out at some number (correlated with altitude and the thickness of the air there no doubt), I seem to want to get them to their optimal flow, but no matter what I do the efficiency goes down. What is the efficiency? The amount of fuel that is wasted due to insufficient 'oxygen'? The efficiency of the intakes in getting oxygen into the engines?

4.) Is there a point where my excessive lift surfaces generate more drag than lift?

5.) Is it possible to make a plane that is, forgive the pun, 'pitch perfect'? Meaning that at some thrust, it will sit on the 0 pitch marker and not translate vertically? It seems like most planes in the (real life) universe do that-or have I been lied to by flight sims, and the meatball is showing an adjusted value? I feel like I have to hold a pitch of 3.5 or some such to stop the plane from dropping into the sea below.

I appreciate any help you can give me-and don\'t be afraid to lay it on me mathmatically. I\'m a big boy and can take some scary numbers (or lack therof as engineering school taught me)

[ferram4]

Even though I have an Aerospace degree, I\'ve never actually done any math for KSP, I\'ve always just eyeballed my planes. As to your questions:

1) There is no reason that your horizontal force should be equal to your vertical force if constant stable flight is your goal. If you could elaborate it would be much better.

2) If you think your aircraft is too touchy, then it probably is. If you\'re placing the number of control surfaces based on what is necessary to get the plane to pitch up and take off, odds are there are too many. Try moving the main landing gear more rearwards and angle up some of the forward lifting surfaces, whether they are control surfaces or simple wings (use shift-WSADQE to do this, if you don\'t know how )

3) Nacelles and intakes are aesthetic, so far. All the code is based in the engine nozzles. Efficiency seems to be based not only on airflow, but on velocity, since it diminishes as velocity increases. I think that efficiency is current thrust divided by maximum thrust, though I haven\'t looked at the code.

4) Yes, if you fly at a very low angle of attack, that will happen, since no wing part generates lift at zero angle of attack, but I believe they do create drag.

5) You can kind of do that, but it requires angling every wing part. It\'s honestly better to design your plane to fly at ~3.5 degrees and angle the engines on the body.

Hope that helps. I haven\'t looked into the math of the aero stuff in the game, since the aerodynamics are wrong (drag force based on mass, lift proportional to velocity instead of velocity squared.)

[Clouds]

1.) Should I be balancing my lift constant of the craft with the thrust the engines can produce? I seem to either have planes that pitch down or up, and I\'m wondering if my upward force vector should be equal to my lateral force vector (overcoming forces, free body diagrams...etc)

I don\'t think it\'s quite like that. Lift will need to equal or exceed the force of gravity, and lift also depends on forward speed (which relates to thrust and drag). The specific balance will depend on what kind of craft you want (glider? jet fighter? cargo plane? long-distance?)

A tendency to turn could be due to imbalanced centers of weight, lift, and/or thrust. Those should all line up.

2.) My planes seem super tetchy. How many is too many control surfaces? Should I be using a certain number for however heavy the craft is/the lift quotient?

Again, not quite like that. A control surface\'s force depends on air speed, and its torque depends on leverage (if it\'s far out at the tip of a wing or tail, it can start and stop a turn much more quickly). So, if you want a compact yet heavy plane, you might need more. For a long, wide plane, only a few control surfaces at the ends will do, for a powerful but slow rate of control.

3.) The air breathing engines in this game seem to have very little science-that I can see-associated with them. Are those nacelle things purely aestetic? They all seem to have intakes, and they seem to max out at some number (correlated with altitude and the thickness of the air there no doubt), I seem to want to get them to their optimal flow, but no matter what I do the efficiency goes down.

Yes, separate intakes and nacelles do nothing for engines. That feature isn\'t in the game yet.

What is the efficiency? The amount of fuel that is wasted due to insufficient 'oxygen'? The efficiency of the intakes in getting oxygen into the engines?

I\'m not sure. My guess, it\'s the current maximum thrust (as if at full throttle) divided by the optimal maximum thrust, depending on air speed, altitude, and the stats of the engine.

4.) Is there a point where my excessive lift surfaces generate more drag than lift?

Probably. =P

5.) Is it possible to make a plane that is, forgive the pun, 'pitch perfect'? Meaning that at some thrust, it will sit on the 0 pitch marker and not translate vertically? It seems like most planes in the (real life) universe do that-or have I been lied to by flight sims, and the meatball is showing an adjusted value? I feel like I have to hold a pitch of 3.5 or some such to stop the plane from dropping into the sea below.

You can. However, you need to know that presently, wings in KSP do not actually lift like an airfoil - they\'re just like flat boards that resist sideways airflow. To make a plane that flies level yet maintains altitude, you need tilted wings. You can rotate parts in 5 degree increments when building by holding shift and using WASDQE. Getting the balance right is a matter of either trial and refinement, or great experience, or unknown calculations.

[TouhouTorpedo]

Well I\'m not an aircraft expert exactly, but with your 5th question I think the guy above is right. For an aircraft to fly at zero angle pitch it almost certainly has an angle of incidence on the wings even at zero degrees pitch (unless heavily cambered) However, I\'m reasonably sure no aircraft flies at 0 degrees pitch straight and level. Not even Jet airliners, though the angle they fly at is very slight to stop the carts escaping. Ever noted how it feels like a slight hill in an airliner? it is.

Also for pitching all real aircraft require trimming for stable flight. Some have 'Fixed Trim' which I\'ve seen on the rudder of a small stuntplane before to deal with the prop wash. Regardless though dont worry about the pitch movement, just trim for throttle level.

Far as control goes use big ones on the rudder and tail and if you must little ones for airelons. That usually works for me. It may help to reinforce your structure more, as wings flexing can really screw up the aircrafts behavior.