andy pattinson

You’d think that picking up the tail would at least put you on a level footing with nose-wheel pilots, but no - at least, not until you’ve dealt with the third consideration - torque gyroscopic effect. The force applied to move the axis of the prop’s rotation - that is, to tilt the aeroplane’s longitudinal axis - will be modified by precession so that it acts at a point displaced by 90° in the direction of rotation. Think of it as a finger pushing the top of the prop disc from behind; in a right-hand engine, the precessed force acts as a push on the right-hand side of the disc, compounding the leftwards yaw. The fourth and final factor is torque effect. The equal and opposite reaction to the force which rotates the propeller tries to rotate the airframe in the opposite direction. On take-off, this causes one wheel to be pressed more firmly onto the ground than the other, giving rise to more friction on that side and therefore another yawing force. On a right-hand engine, you’ve guessed it, it’s the left wheel that drags, compounding the leftward yaw even more. For all of these reasons, then, you can see why on the take-off run, the throttle should be advanced at a rate such that the reactions are never greater than the ability of the rudder’s increasing authority to control them.

Just found this as well which explains a lot. THE EFFECTS OF POWER In the single-engine prop-driven aircraft that many modellers fly, as many as four factors can add up to create a swing on take-off. The first of these is the slipstream of the prop, which takes the form of a vortex that runs around the fuselage in a helical path and eventually encounters one side of the fin, setting up a yawing moment which causes a tendency to swing. If you’re a tail-dragger pilot, you’ll also have to contend with the effects of blade asymmetry, which will be most marked when the prop axis is not in line with the flight direction of the aircraft, that is, at the start of the take-off run, when the tail’s down. In this condition, the length of the path of the blades through the air will vary as a result of two factors. One of these is the difference in the angle of attack of the blades: The down-going blade meets the relative airflow at a higher angle of attack than the up-going blade, and so generates more thrust. The other cause is the fact that, in their tilted state, the blades are effectively travelling forward through the air at different speeds. The easiest way to visualise how this comes about is to imagine an aircraft moving forward in a nose-high attitude. In the time taken for a blade starting at the top of the tilted disc (the rearmost position) to rotate to the bottom (the foremost position), it will have travelled further through the air (distance travelled by the aircraft + distance of forward rotation) than a blade starting at the bottom of the disc (the foremost position) and rotating to the top (the rearmost position), which amounts to distance travelled by the aircraft - distance of backward rotation. This gives rise to extra thrust on the long-path half of the disc, and so to more yaw. In a right-handed engine, then - one in which the top of the prop moves to the right when viewed from the cockpit - the extra thrust is on the down-going right-hand side, yawing the model left. The unfortunate thing, of course, is that blade asymmetry starts doing its thing from the start of the take-off run, when you’re winding on power - increasing slipstream effect, in other words - and when your rudder is at its least effective.

Hi Paul , I have just finished building the SFM J3 Cub and am also experiencing exactly the same problems as your self with the recommended electric set up, On its maiden flight it took off pretty level until throttle was applied and it rolled hard left and ended up in a hedge causing some damage to the left wing, wing mounting, and it managed to pull  the tail off, all of this has been repaired now, but i took it back to the fields after repairs and it almost did the same thing again, since then i have found the following a page which will not allow me to post the link, but if you type in left turning tendencies on your search engine and look for a site called boldmethod.com it explains it in good old plain English.   Does you cub plan show any side or down thrust on the motor mount, I have lost my plans and cant seem to get a copy any where, it was just something a club member mentioned the other day, I have the SFM Mustang as well , that has side thrust / down thrust built into the motor mount and that flies really well so far.   Edited By andy pattinson on 17/05/2019 09:39:08 Edited By andy pattinson on 17/05/2019 09:41:26

Hi Guys, I have just built one of the sfm Mustang kits and thoroughly enjoyed it so far, But i have a slight problem, i have lost the plans somehow and i am trying to get the thing balanced ready for flying, could any one tell me how far back from the leading edge the Cof G is please. Any help is much appreciated. Regards Andy