Single engine propeller aircraft issues

[Koogus]

First post here so sorry if I am posting this in the wrong place

When ever I try and build single engine propeller aircraft it rolls to what ever direction the engine is spinning and I also notice that they tend to slowly pitch down on there own.

My question is that is there a way to mitigate these issues?

Edited August 15, 2019 by Koogus
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[Snark]

Hello, and welcome to the forums! 

3 hours ago, Koogus said:

When ever I try and build single engine propeller aircraft it rolls to what ever direction the engine is spinning and I also notice that they tend to slowly pitch down on there own.

My question is that is there a way to mitigate these issues?

The usual mitigation is to put two propellers on, spinning in opposite directions.

There's basically no way to make a single prop not torque your plane around-- perhaps other folks may have some suggestions for how to compensate, but speaking for myself, I just always use props in counter-rotating pairs.

[Kaerbanogue]

Is your propeller mounted on the top of the craft like an helicopter or on the front like a plane? 

On the top you can add a second propeller spinning in the opposite direction or a tail rotor or even add motion wheels I guess. If it's on the front maybe your propeller and craft's weight ratio aren't balanced. Try different spinning speeds and blades orientations, and make sure you're using the turbine engine. 

Hope it'll help

[ajburges]

Have you already reduced engine mass? In builder, tweak the engine to 2-10%. You don't need much torque to drive a rotor to make max rpm and the torque is what causes the undesired roll. Reduced engine mass will reduce (but not eliminate) the magnitude of the undesired forces.

Also, throttle up gradually to reduce the impulse.

Edited August 16, 2019 by ajburges

[AHHans]

21 hours ago, Koogus said:

When ever I try and build single engine propeller aircraft it rolls to what ever direction the engine is spinning and I also notice that they tend to slowly pitch down on there own.

The roll (torque from the propeller) is actually something that real aircraft also have to deal with, but I believe that the effect is much stronger in KSP (in relation to the other forces) than in RL. While it is possible to fly a single-prop plane in KSP it's way more fun to have the torque countered by a second prop (or have multiple pairs of props). The propellers can be on the same axis, or you can have the symmetrically to the left and right of your plane, both methods work.

One simple way to get counter-rotating props on a single axis is to have a single, un-powered servo or rotor (to act as a bearing) attached to the fuselage and a powered rotor attached to that un-powered one. Then attach the blades to the base (one direction) and the front (other direction) of the powered rotor. When the rotor is powered up, the two sets of blades will spin in opposite directions.

The pitching down is probably because you have designed your your plane to be aerodynamically stable (tending to fly nose forward), with the center of mass in front of the center of lift. Being pulled down (by gravity) at the COM and pushed up (by the lift-forces) at the COL, this means that there is a net torque that will pitch the nose of the plane down. AFAIK you'll just have to correct for that with control inputs. You can either have SAS set this input, trim the plane with the manual trim, or correct it manually. (And, yes, this is also an effect that real life pilots need to deal with.)