Thrust vector: Single engine vs Multiple
 

I hate to pose a question that so nakedly exposes my ignorance of aerodynamic basics, but we have a problem with a B-17:

On a single engine plane with wing and stabilizer incidence set at 0 degrees, putting 2 degree down incidence on the engine will cause the whole plane to go down, correct?

On a similar plane with multiple engines (wing-mounted), what happens if the engines are given a down incidence of 2 degrees?

Will the plane still go down?

Or does the engine thrust line force the wing and stab into relative 2 degree positive position in flight and cause the plane to go up?

Or does it depend on the plane? B-17?

Thank You

mt

RE: Thrust vector: Single engine vs Multiple
 

It is called downthrust, not down incidence. Downthrust is used to prevent excessive climb under power that may cause the airplane to stall. In models, it is very common in trainers, which are normally set up to be rather nose heavy, in order to give them good pitch stability. Such a setup needs a bit of downthrust to avoid having to use "down" elevator input to prevent climbing too steeply and stalling when a lot of power is applied, when the airplane is trimmed for level flight at slow cruise.

A couple of degrees of downthrust will not cause the airplane to go down, unless the CG is far further aft than normal, in which case it will require little or no downthrust. If the airplane noses down under full power, it needs less downthrust... (much less).

RE: Thrust vector: Single engine vs Multiple
 

Thanks,

mt

RE: Thrust vector: Single engine vs Multiple
 

Since you're talking thrust on a multi-engine plane, you should also consider putting some right thrust on the right engine(s) and maybe a little bit of left thrust on the left engine(s). Reason: saving your plane in OEI condition.

If you loose an engine on a multi-engine plane, you will obviously have adverse thrust. Imagine, for example, losing the left engine on a twin. That right engine itself will have a natural torque-induced yaw to the left (as nearly all our engines do), and it is on the right wing, so it will pull the right wing forward- more left yaw. There may be too much yaw for the tail/rudder to counter, which would result in the tail stalling and your plane spinning to the ground.

The left engine would require a bit less outward thrust as its natural torque-induced left yaw will somewhat counter the right yaw caused by the engine being left of the CG.

With or without outboard thrust, throttle should be reduced if you loose an engine to avoid stalling the fin. Outboard thrust gives you a little margin to tolerate more throttle during OEI. I would recommend 2 deg. on the left, 3-4 deg. on the right.

Check the twin/multi forum for more.