stek79,

I read the article by George Hicks on the Horizon site you posted. Basically he says the points I made just about verbatim. So I am not clear where the disconnect is? I did find 2 statements from George's writing that may be causing confusion??

Your original question was about high thrust line possibly reducing yaw from spiral slip stream. To which I basically answered right idea but wrong distribution. Raising thrust line to get equal side area above and below is not enough. As I wrote above "The imbalanced vertical area in the back is what causes the yaw problems." Equal area above and below is needed at the tail - back where a significant lever arm exists to force a yaw. Area below centerline located forward will not balance the vertical fin above located rearward. So basically I'm saying since fuselage area is near and about the CG it has relatively little moment arm to produce yaw. Vertical fin area located way in the back is at a large arm and side force here will create more significant yaw. I'm saying the vertical fin area is what must be balanced off to eliminate yaw from slipstream spiraling, and raising the thrust line won't accomplish this. Adding a sub fin, at the tail, of equal area, will.

Regarding Right thrust and P effect. I think what threw you off is my statement above: "The larger reason for right thrust is P-factor at slow speed." George’s states "The spiral slipstream is also the ONLY reason we need to put right thrust in our engines." George's statement is a little strong and needs to be taken in context. He goes on to say regarding spiral slipstream "This effect is pretty much the same whether the airplane is flying upright, inverted or on its side. It always tends to make the nose yaw to the left and to compensate for this we offset or angle the engine's centerline." He is speaking here from the perspective of a compensation that works equally for both upright and inverted. When he says it is the ONLY reason Right thrust is NEEDED, he is referring to "needed" for an "aerobatic" type design.

I am saying that “additional” right thrust, beyond the amount to compensate for spiral slipstream, can be added to compensate for P factor, but in the positive direction only. In this regard I am speaking of “additional” benefit for right side up flying only. Regarding P effect George writes: "This uneven disc loading produces a nose-left yawing moment when the disc is at a positive angle of attack and a nose right yawing moment when the disc is at a negative angle of attack (for a clockwise rotating propeller as seen from the cockpit). Since the direction of the yawing moment changes from upright to inverted flight we should not correct for this effect with right thrust." Obviously George is speaking here to the design objective of preserving equal upright and inverted behavior.

My point is that altering the thrust line further, can be used to compensate for P effect also, or any other effect for that matter. But this type of correction is specific to a certain flight condition only. The condition to which I referred is High (positive) AOA. The biggest example that we all must deal with is slow flight. I'm saying adding "additional" right thrust is a good compromise for right side up flying only to compensate for P effect during slow flight, strong pull ups, and steep climbs. Above I wrote "P effect...Basically offsets the center of thrust to the right causing a yaw to the left. Right thrust compensates. As speed builds and nose lowers the (P) effect reduces and the right thrust eventually becomes a nuisance, but aerodynamic (directional) stability is much greater now. So, adding more right thrust is a good compromise” – but for flying right side up only. During inverted flight the (additional) right thrust becomes left thrust but the P effect is still the same.

When I wrote that P effect is the larger reason for right thrust. I might more clearly have phrased that compensating for P effect also requires a larger "application" of right thrust. I mean to say that the majority use of "extra" right thrust, for all types of models, is to make them handle better from slow to cruise speeds, flying right side up only, since this is where the majority of flying happens, by all pilots of all skill levels. An aerobatic design is a specific application and the more appropriate amount of right thrust there is enough to compensate for the spiral slipstream only and no (or little) more.

Bottom line is this. A larger amount of right thrust improves upright handling at the expense of inverted handling. A smaller amount of right thrust benefits upright and inverted handling equally, only for designs that have imbalanced vertical fin area. An equal sized sub vertical fin will eliminate the need for any right thrust compensation of spiral slip stream. Right thrust remains useful to compensate for positive P factor, at the expense of all situations that generate negative P factor. Because our planes don’t have large sub fins at the tail, about 1 or 2 degrees of right thrust is optimum for aerobatic/pattern design in order to preserve upright and inverted handling equality. When around 3 degrees or more is used, the up lines become easier, but a tracking difference between inside and outside loops starts to develop. Also a tracking difference between upright and inverted 45 degree climb lines begins to develop. Airplanes that will fly right side up only benefit from larger amounts of right thrust. With lesser right thrust, planes yaw left during steep climb and straighten out at faster speed. With larger right thrust, planes can be made to fly straight during steep climbs and yaw right at faster speeds. About 4 to 6 degrees of right thrust produces a good compromise.

Last thing to note, since yaw from spiral slip stream increases as speed decreases, it acts the same as positive P effect at slow speeds and during steep climbs. The magnitude of the P effect by itself is made clear to multiengine pilots experiencing the difference in engine out performance between left verses right engine out condition (non counter rotating props).

Does this close the gap?

Multiflyer