Motor orientation

[poulsbobill]

I have several two engine small electric planes like the Aeroace, Sky buddy and now the tyco Skyforce. Skyforce ( http://www.rcgroups.com/forums/showthread.php?t=617816)

The skyforce has adjustable motors in terms of orientation. I can set them straight ahead, inward, outward or even askew. (see drawings attatched). Yaw is a function of differential thrust motor speed by the Tx rudder input.

My question is to achieve the best flight and turning response...which is best? ( and why).

AS always, thanks

Bill

[yoyoML]

Answer me first, where can I buy these cute bipes!!???

My guess is, for best turning response you don't want the motors to point inward. Because in a turn the outer motor produces much more thrust, but if the airflow then goes inward it will hit the vertical stab and push the tail inward, opposite of your intended direction.

edit: The Assault Plane is a ME-262 not-look-alike... :(

[poulsbobill]

Got mine from target, and toys r us have them for 29.99, two planes and a motor pack.

Bill

[Sparky Paul]

Any offsetting of the thrust line would be important if one motor failed, and there was a rudder.
As there's no rudder, only differential thrust anyway, set the motors straight.

[Brandano]

To expand a bit on the topic, I suppose it depends on where your CG is. To maximize the turning efficiency you want the longest possible arm between each motor and the CG, and to achieve that you must aim them in such a way that the line perpendicular to the motor axis passing through the CG is the longest possible. So you should have them perpendicular to an imaginary line between the CG and the motor pivot point. On the other hand if you are happy with the turning performance then pointing them straight will give you the highest efficiency in forward flight. Actually, if the engines aren't counter rotating then you might want to offset them both slightly to one side to counteract any "p" effect... but apart than for the sake of argument I doubt that any tuning will cause major differences in flight behaviour.

[poulsbobill]

To expand a bit on the topic, I suppose it depends on where your CG is. To maximize the turning efficiency you want the longest possible arm between each motor and the CG, and to achieve that you must aim them in such a way that the line perpendicular to the motor axis passing through the CG is the longest possible. So you should have them perpendicular to an imaginary line between the CG and the motor pivot point. On the other hand if you are happy with the turning performance then pointing them straight will give you the highest efficiency in forward flight. Actually, if the engines aren't counter rotating then you might want to offset them both slightly to one side to counteract any "p" effect... but apart than for the sake of argument I doubt that any tuning will cause major differences in flight behaviour.

Brandano,

I have too similar planes (see pics) from the kit i linked with different lengths. The shorter squatter one turns sharply and the longer one does lazy turns. The cg on the squatter one is closer to the motors than the longer one. Too much fuse to turn sharpley? (ie larger moment arm). The longer one has the battery/rx way up front to get the cg right. The shorter one has all this near the cg.

Bill

[Brandano]

Well, the shorter one will probably have a shorter tail, which in turn will make it less effective. By the way, thinking about it a bit more, another thing to consider when turning a plane is what amount of sideslip it will have. Sideslip will mean that one wing is more exposed to the airflow in the case of a swept wing, or one wing will have higher AOA in case of a diedralled wing, meaning that the aircraft will want to turn. Does one of the planes have a higher diedral? In that case whatever promotes more sideslip will help the turn, and that would go in favor of thrust lines pointing outwards

[poulsbobill]

Well, the shorter one will probably have a shorter tail, which in turn will make it less effective. By the way, thinking about it a bit more, another thing to consider when turning a plane is what amount of sideslip it will have. Sideslip will mean that one wing is more exposed to the airflow in the case of a swept wing, or one wing will have higher AOA in case of a diedralled wing, meaning that the aircraft will want to turn. Does one of the planes have a higher diedral? In that case whatever promotes more sideslip will help the turn, and that would go in favor of thrust lines pointing outwards
Dihedral about the same. But the layout of "weight" around the cg is way different. The shorter one has all the weight centered arounbd the cg and the bigger one is all spread out.

Bill

[Brandano]

You are suggesting that you might be fighting against inertia rather than against the tendency of the plane to go straight? But in that case you'd also have trouble stopping a turn, not only starting it. By the way, probably I should have said the center of drag rather than center of gravity in my earlier post. They are usually fairly close, but don't always match. In any case, a plane with the mass centered in a smaller area will usually be more responsive than a planewhere the mass is spread in the structure. Also, localized mass distribution might cause other issues, like in the case of the X3 and F104 and the phenomenom of inertial coupling, where jaw inputs would translate into roll inputs instead. I don't think these effect would be too noticeable in a small indoor plane, though