Plane stability

[TheDestroyer111]

I have designed many planes which are slightly unstable or barely stable in order to replicate the aerodynamic properties of an F-16, and most of them were barely statically stable (with no control input) in all flight conditions and flew relatively well at low altitudes and medium/low speeds, but for some reason whenever I went low&fast or just high altitude, the planes would become hyper-sensitive to control surface deflection needed for more than one g (or to engine thrust, if they used asymmetrical thrust), and, when I maneuvered them they either went into spins, or started temporarily flying at a high angle of attack, stabilising out once they bled off enough speed. So why does that happen? Is there any explanation, and can this theoretically happen IRL too?

Edited February 3, 2017 by TheDestroyer111

[AeroGav]

Well, in high dynamic pressure situations the controls will be over-sensitive, which can cause trouble with SAS causing pilot induced oscillation .  Also at high speed pulling up generates a lot of G force which could make the structure flex some.

I wouldn't go building neutrally stable or unstable aircraft  in ksp,  unlike the real f16 you don't have fly-by-wire to tame it.

I think you'd be better served trying to combine positive stability with powerful control surfaces.

If you want post-stall manuvering,  reaction wheels will allow you to pitch up beyond stall aoa at low speed and low altitude (and at medium speed and high altitude) - basically any "low dynamic pressure" situation.

eg. this tiny fighter is dominated by reaction wheel torque, 

https://kerbalx.com/AeroGav/Ghoul

if you give it full nose up on takeoff you can reach 45 degrees AoA, but at higher speed it's limited to more like 20,  and it returns to level flight when you let go of the controls no matter what.

Also btw even a very stable fighter like this one, which only lets you have 10 deg, AoA, can have fearsome manuverability if it's got a lot of lift.  In fact in a turning contest that lasts longer than half a second, it's likely to win because of the lower drag it enounters at moderate AoA

.

https://kerbalx.com/AeroGav/Silver-Gryphon

probably flies nothing like an f-16, but then again the f-16 was supposed to be good at sustained turn as well.

[bewing]

3 hours ago, TheDestroyer111 said:

Is there any explanation, and can this theoretically happen IRL too?

Absolutely.

Have you watched F16's at low speed? Their avionics system rapidly adjusts the aerodynamic control surfaces to create dynamic stability. Note that these are aerodynamic feedback systems, and they have maximum rates of changing deflection, and noise levels.

So under what circumstances would they not work to maintain craft stability? Well, first would be a vacuum. Or a near-vacuum, where the aerodynamics can't deflect far enough, fast enough to counteract an attitude deviation. ie. not enough aerodynamic control to maintain stability. When would that be? When you fly really high.

Second would be flying when you have too much aerodynamic control. When the surfaces have too much lift -- when the needed aerodynamic adjustments become microscopic, and fall toward the size of the mechanical noise of the control system. When would that be? When you are going low and fast.