How do Aircraft Engineers manage CoP/CoM/CoT in their designs?

[Volt]

I think we can all agree that the biggest pain in the behind in KSP's aircraft section, especially for those of us using FAR and trying to build VTOLs, is balancing the Centers of Pressure, Thrust and Mass to achieve stable, hassle-free flight. Most of the time ingame, we cope with it by moving wings, moving masses, or pumping fuel around, only one of which can realistically be done in the flight scene. My question to you people-smarter-than-myself is, how do aircraft engineers do it? Particularly in the cases of aircraft such as the Harrier and, to a lesser extent, the Lightning II? (which can mostly be explained by 'lol flight computer')

[Camacha]

Most of the time ingame, we cope with it by moving wings, moving masses, or pumping fuel around, only one of which can realistically be done in the flight scene.

What do you mean by this? Designers have the choice whatever to do what they like with the first, quite a bit of legroom with the second and the third is also something that is done a lot.

[K^2]

These are very important factors in aircraft design, but you also have a lot more tools for dealing with them. For starters, these things don't have to align quite as well as they do in KSP. You still want CoM ahead of your wing CoP for stable flight. But the trick is that the airplane's horizontal stabilizers provide negative lift. So as the airplane speeds up, they push the tail down, balancing against airplane's CoM. In addition to that, you have trim tabs, which allows you to adjust how much negative lift you get with neutral input.

These things are in KSP, but because aerodynamics is so much simplified, they don't work quite right. Specifically, you have one, fixed CoP. On a real aircraft, CoP shifts dynamically. That means that KSP aircraft have a very narrow envelope. CoM can only be a hair ahead of CoP, and CoT has to be on the same line. For a real airplane, CoM has an envelope of good locations. You do still have to check to make sure that CoM is within the envelope. (weight and balance) But it's far, far more forgiving than KSP.

[ratchet freak]

CoM is by adding all the materials that go into the plane.

All the rest is by Computer simulations and windtunnel tests and gut feeling on the ground.

If you suspend a scale model in a windtunnel you can measure the various forces of the parts and decide whether to extend the wings another inch or not; do the modification and do the test again.

[magnemoe]

They can shift center of mass with pumping fuel, its also important then storing cargo, even passengers, I was on a flight where we was moved backward, this is mostly relevant on small planes.

[shynung]

...the trick is that the airplane's horizontal stabilizers provide negative lift.

How did they manage that? Negative AoA? Or just flipping the aerofoil around - flat on top, curved below?

If using something like FAR, is it possible to simulate it in KSP?

[Fuzzy Dunlop]

It's possible (indeed necessary) to simulate in stock KSP. Honestly all this "centre of lift must be behind centre of mass" stuff is a gross oversimplification.

If your CoL is behind your CoM your plane's nose will drop. To raise it you must shift the CoL in front of the CoM, you can do this by increasing the angle of attack of cannards (which adds lift to the front of the plane) or by decreasing the angle of attack of your elevators (adding negative lift to the back of the plane).

Edited December 15, 2014 by Fuzzy Dunlop

[PakledHostage]

How did they manage that? Negative AoA? Or just flipping the aerofoil around - flat on top, curved below?

The horizontal stabilizer's angle of incidence (angle relative to the wing) is often negative (nose down), but it is important to be aware that the geometric angle of attack isn't necessarily the aerodynamic angle of attack. The wing produces downwash that changes the apparent angle of attack that the horizontal stabilizer experiences. The amount of downwash changes with the circulation (not to be confused with lift) produced by the wing. On large aircraft like airliners, the horizontal stabilizer's angle of incidence can even be changed in flight. There is a large screw jack (or two) at the front of the trimmable horizontal stab that pivots the stabilizer as required.

Also, symmetrical airfoils are perfectly capable of producing lift. You don't need camber. What you do need is a sharp trailing edge. All airfoils have one, regardless of whether they are symmetrical, cambered or are composed of a single membrane that has no relevant thickness (like a sail). This is because the kutta condition occurs when an aerodynamic object has a sharp trailing edge. A consequence of the Kutta condition is circulation about the wing which is fundamental to the production of lift (and downwash, see above).

[Volt]

Useful information, guys. I wasn't aware of the negative lift trick. So the balancing of the CoP so carefully using only positive lift is something only us KSP designers do? TIL.