I did a search, but the only comments I could find regarding thrust angle on the Cub is that Cubs like a couple degrees of right thrust.

Anyway, here's the scoop...........this is the GP .40 Cub with a .48 4-stroke. I finally decided to take the gear off and bend them back into shape after all the rough landings and other "incidents" the plane has experienced over its career. Bent in some toe-in, bent the gear back out so they're not sprung looking, and tweaked the tailwheel a bit. Due to rain I only managed to get one flight in today (and won't be able to fly until next week), but it looks like this has improved ground handling (though maybe not as much as I thought it would). However, I'm very skeptical about all the right thrust the kit has built-in. The plane was built by someone else (it was a first kit for them, as well), and after the first few flights, I thought they'd built in too much right thrust, so I dropped part of a pushrod in between the mount and firewall on the right side to straighten the engine a bit. I seem to recall having somewhat better luck with the plane after that, but I was so inexperienced at the time and the gear was so bent, I couldn't really tell if the decreased right-thrust was helping things, or if I was just getting better at it.

Now that I've got the gear worked out, the plane is a little more consistent in ground handling and it's easier to observe what it's doing. Here's what I've noticed: on takeoff, I need (mostly) LEFT rudder to keep the plane tracking straight. On landing, I had to hold some RIGHT rudder to keep the plane from veering, with power at idle and the tail in the air. Also, I noticed before the gear adjustments and after, the plane turns to the right very easily on the ground, but barely turns left. It really seems like I need less right thrust, but then I read comments in various posts here that the built-in thrust is appropriate for a Cub. The left turning tendency on landing kinda threw me off. I did have very slight left rudder trim by that time, and the field is rough so I might have hit a bump with the left main, but I remember the plane veering left on landing before I adjusted the gear too, numerous times.

On my first takeoff today, the plane got away from me and ground-looped to the left. This makes me think that if I do indeed have too much right thrust, I've still got a problem with the gear somehow.........but I dunno how, as I've tried swapping the mains, and I just totally changed the angle of the wheels last night when I put in the extra toe-in.

I did have some crosswind, which might help explain the left rudder on takeoff (and perhaps the right rudder on landing, as I was landing the opposite direction.......though I noticed the same problem before adjusting the gear, which makes me wonder), but doesn't explain the consistent inability to turn left on the ground.

So, to all you GP Cub fliers out there............have you noticed any trouble with the built in right thrust? I'm thinking maybe since my model is using an engine at the low end of the power range, that maybe GP's judgement on the firewall angle is too much for this engine.

On a positive note, I seem to have finally got the 4-stroke singing! I had a lot of fun practicing approaches, sure helps when you can throttle waaaay back and not have to worry about killing the engine. Today I made a nice smooth scale-like wheel landing with the engine still running...........just a little rudder to keep it straight, ease the tail down, and taxi back to the pits. Great fun!!!!! Had to do some right-hand donuts to get turned back to the pits though, due to the aforementioned left turn problem. But I guess it did look pretty cool, come to think of it........ [8D]

-Matt Bailey

You need to check a few things. How does the model trim in the air? If it pulls to the right under high power, but to the left under low power, then you have too much right thrust. If the rudder is neutral in flight, and there's not much yaw when you change power, then the thrust line is correct.

Ground handling is not corrected by changing the thrust line. You have to work with the landing-gear geometry. When you push the model along the ground with the engine off, the model should track dead straight. The main wheels should have zbout one degree or so of toe-in. Toe-out will make ground handling a nightmare!

When set-up correctly, the model should need slight right rudder on takeoff to keep it straight. It may need none, depending upon how the thrust line works out.

The fact that you need right rudder on the landing rollout suggests that there's a problem with the landing gear geometry. You should need no rudder input as long as the model's tracking straight. Rudder input would be used to keep it straight. Holding rudder during rollout means that the wheels aren't aligned correctly.

Up until now I've chalked most of the odd behavior on the ground to the gear being all bent up, but that's fixed now (or should be..........). I did the ground test and it rolls pretty straight, although the problems I'm seeing on takeoff are with the tail up...........hard to ground-roll test the model with the tail up.

I suppose since there was a crosswind yesterday, it's possible that that's what caused the plane to yaw left on landing, and that it's just coincidence that this is the direction it tended to yaw before fixing the gear...........I'll have to wait til next week to try it some more.

Airborne, it's hard to say. Seems to track reasonably straight as far as coordination I guess, but it does require a fair amount of left aileron (perhaps due to something the novice builder did to the model). For good measure I also put in a few clicks of left rudder after flying around a bit to go along with this (this could be why it yawed left on landing but I'm more inclined to go with the crosswind theory). Jamming power in the air definitely pulls left...noticeable left yaw and slight left roll, as I expect, especially with the torque of a 4-stroke. Incidentally, I'm usually at full power throughout the takeoff roll, due to the location I'm flying at only having a short strip with tall
grass........can't really get by with slowly advancing throttle.

Thanks for the comments.

-Matt Bailey

I've had two 40-size gp cubs that I built myself. The only time I ever had ground-handling problems with them was after a few hard landings that bent the gear. With toe-out, it's a challenge. Otherwise, not a problem.