Is it better to put engines on the back or under the wings of an aircraft?

[eeveelution]

Hey! I am using BDArmory, BDynamics, Skillful (only the dll for switching when in flight) and SP+ (and some other mods) and Im building a Low-Medium (10km max or so) altitude Ground-Attacker airplane. It's quite large, and without weapons weighs 14.44 tons/tonnes. Here's a pic of it:

It's center of mass is just a bit in front of the center of lift, and it's very manouverable as is. It flies 200-250 m/s at low altitude (<1km) and turns very well.

Now, here's my question:

Should I leave the engines where they are, or move them udner the wings? What difference will it make, if any? As long as it keeps it's manouverability and speed, and wont flip when taking off, im going to change the engine locations. The only fear I have is that if I do, it will be harder to take off.

[Jovus]

The main difference is that putting the engines under the wings will change the angle between your center of mass and your center of thrust. If the CoT is underneath the CoM, you'll create a lever arm that tries to push the nose up, with force proportional to the distance between them.

Such planes are still flyable with proper trim settings or SAS, and there might in fact be situations wherein you want to do this. (For example, it makes your loops tighter.)

[neamerjell]

As long as the center of thrust is pointing directly through the center of mass, either one. However, I suspect that if you have two engines mounted on the wings, and you have a flame-out, the spin will be much faster and harder to recover from.

[eeveelution]

Ok, I've tested it with the engines on the siides. It takes off easier, lands better, Is way less manouverable and a bit slower. The turning circle is giant, and n general it reacts slowly. Did I do anything wrong, or is it just btter with back mounted engines?

[Jovus]

That's strange. In order to turn, are you yawing the aircraft, or rolling and pitching?

[eeveelution]

That's strange. In order to turn, are you yawing the aircraft, or rolling and pitching?

Im rolling and pitching, like IRL.

[Moonfrog]

i suspect its because when u roll the aircraft, your torque has to lift the weight of the engine as well. So the position of the engine on the wing changes how the different forces affect the aircraft. Not sure why, but I think it has something to do with that. Also thrust force is different. when flying straight there even. But when u roll one up and then bank to turn. I imagine the two engines would create two separate circles for turning, the one in the middle would want to go faster, pull against your own turn rate. don't know that much about aircraft, just guessing, always a chance like Jeb I might be right.

[Melfice]

Assuming Moonfrog's theory is correct (which I think it is), then it may help to throttle back when doing turns.

[Kryxal]

Moving the weight of the engines on to the wings WILL increase the moment of inertia, so slow down your rolls, but that wouldn't directly affect the pull-up component. What WOULD affect it, though, is moving engines with thrust-vectoring closer to the CoM ... you don't have the leverage to make your turn faster with thrust vectoring anymore.

[Red Iron Crown]

Ok, I've tested it with the engines on the siides. It takes off easier, lands better, Is way less manouverable and a bit slower. The turning circle is giant, and n general it reacts slowly. Did I do anything wrong, or is it just btter with back mounted engines?

Two things happening:

1. Greater polar moment of inertia (fancy physics speak for resistance to changing direction) caused by decentralized mass.

2. The vectored thrust from the gimballed engine has less leverage to change the plane's attitude. The further aft of the CoM your engines are, the more effective the gimballing becomes.

[Wanderfound]

Im building a Low-Medium (10km max or so) altitude Ground-Attacker airplane. It's quite large, and without weapons weighs 14.44 tons/tonnes. Here's a pic of it:

http://s13.postimg.org/cmidoxlfr/screenshot34.png

I know that this doesn't always always apply in KSP, but: look to the real world for inspiration.

Yes, you'll see plenty of planes with big engines way out on the wings, but they're usually things intended for long-haul high-altitude level flight while carrying bulk cargo (whether that be boxes or people or bombs). They're trading off manoeuvrability for the sake of interior space.

It can work for a fast intruder (e.g. de Havilland Mosquito), but it's very easy to get it wrong (e.g. Messerschmitt Bf 110).

What you've built looks like a cool plane, but the shape of it is more high-speed interceptor/penetrator rather than low-altitude ground attack. You've got a MiG-25, but it sounds like you're after an A-10.

[cantab]

Por que no los dos?

[GoSlash27]

Truthfully, it doesn't matter much for what you're trying to do. Your plane will be more maneuverable with the engines tucked in, but you don't want an attack plane to be too maneuverable or it will make an unstable weapons platform.

Best,

-Slashy

[Razz]

doesnt matter where you put them - as long as they are facing in the right direction...

[ratchet freak]

If you put them under the wing then the CoM shift as the tanks drain won't be as big as the static mass of the engines will be closer to the center.

[Aethon]

That's strange. In order to turn, are you yawing the aircraft, or rolling and pitching?

As a former pilot ( but super dumb when it comes to aerodynamics [ makings of an 'ilarious cockpit preflight announcement ] it is standard procedure to use rudder (yaw) and ailerons (roll) for every turn. The only times I ever pitched ( pulled back on the 'stick' [ used the 's' key in ksp ]) while yawing and rolling IRL was during training. Can't say I've ever 'stepped on the ball' ( yawed while rolling and pitching ) in KSP. Does that make any sense??

If so why have I never found myself yawing during a standard KSP flight. Is it the aerodynamics of stock?

Sorry. Maybe shuda started my own thread.

[cantab]

Probably. The stock aerodynamics make drag not depend on orientation, meaning sideslip doesn't increase drag like it does in real life, so it's less of a problem if your turns aren't fully co-ordinated.