Aeroplane Fun Times

[Vigelius]

Firstly, a confession: pretty much every successful plane I have made in KSP, space or otherwise, has been huge. Success has been more or less inversely proportional to mass. Oddly.

That's not a status quo I was happy to let slide, so I've been experimenting with small purely atmospheric planes and having some success. But I have questions for you aerodynamic wizards out there. For reference, the craft I've been playing with are in the album below.

1. You'll see from the twin-engine plane that the engines are very low relative to the CoM. This causes absolutely no problem at all! I've tried them fully on top of the wing, fully beneath the wing and some different embedded setups, makes no difference. What's that about??

2. On the other hand, the height of the CoL relative to the CoM makes a big difference but in the opposite direction to what I would expect. Every plane design runs into the 'lawn dart' concept at some point, whereby the CoL tries to align itself directly behind the CoM relative to the direction of travel. With that in mind, I would expect a plane with high CoL to be trying to pitch up the whole time, and vice versa. In practice, I find the opposite to be true and the balance very sensitive. Why??

3. Anhedral and dihedral angle and high or low wing don't actually seem to have a very dramatic effect on roll stability. They do however seem to have a massive effect on pitch as per question 2. That is to say, in order to make a dramatic change to roll, pitch will be made totally uncontrollable. In which case, does the angle of the wing really matter??

4. Yaw control escapes me completely. Example, true for both single and twin designs:

i. apply starboard yaw input

ii. vessel behaves as though fixed through an axis from cockpit roof to rudder tip, with force applied to port at rudder base

iii. dramatic roll to starboard around this axis, involving a slight pitch drop and smaller yaw to starboard.

I've tried bigger and smaller rudder, bigger and smaller vertical fin, vertical elements on the wingtips, vertical elements on the tail horizontals. So far nothing has worked at all. Any clues???

Selection of mods, FAR aerodynamics.

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[capi3101]

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

keptin pretty much answers all four of your questions in one fell swoop. Since you're using FAR, the principles are all the more accurate.

[Specialist290]

I'm going to second capi's recommendation. I still have trouble with building a proper spaceplane, but I've had some success with building traditional airplanes using those guidelines in the past.

[BudgetHedgehog]

1. You'll see from the twin-engine plane that the engines are very low relative to the CoM. This causes absolutely no problem at all! I've tried them fully on top of the wing, fully beneath the wing and some different embedded setups, makes no difference. What's that about??

Well, that's easy - the aerodynamic forces stabilise your plane and the thrust, while applied off-axis to the CoM, isn't providing enough torque to unbalance the plane. There's just not enough thrust - replace the props with say, aerospikes, and you'll notice a difference. One spaceplane I made used RAPIERs that were very slightly below the CoM (it had the forward cockpit section radially mounted to the top). Worked fine in atmo and all the way to the top of the flight envelope. Switched to rocket mode and fwoomp, it flipped up and around and I was facing backwards before I had a chance to stop it. I learnt that day that angling engines is a viable tactic.

[cantab]

1) Engines low will tend to pitch the nose up when more thrust is applied, engines high tend to pitch it down. As Obsessed says, the horizontal stabiliser will control this. If you fly without SAS you'll notice you need to change pitch trim as you change power, but then you sort of need to do that anyway because changing power means you need to change AoA to maintain level flight. If you fly with SAS you might not even notice.

2) I'm not sure about this. If the wing is low that in theory will make the plane less stable in pitch, but I don't think it's a big effect.

3) For me a slight dihedral does help roll stability. Usually I use the rotate gizmo with angle snap, then hold down shift for 5 degree increments. Wing height seems to have less impact.

4) This sounds like normal behaviour

If rudder is continuously applied in level flight the aircraft will yaw initially in the direction of the applied rudder – the primary effect of rudder. After a few seconds the aircraft will tend to bank in the direction of yaw.

For almost all planes, turning is primarily done by rolling, and the rudder just used for adjustment. (A plane with a strong dihedral effect may be able to turn using the rudder primarily, some model planes do that so they don't need ailerons at all.)