Spiraling Slipstream?
 

I have a Modeltech me-109 with an 11 X 6 prop on it. No matter what I do, I can't counter the left-yaw as I power up. Full rudder deflection doesn't help, neither does my addition of a few degrees of right thrust to the motor mount.

Would a different prop help me? Anything else I might try to do? Anyone's input would be appreciated.

Spiraling Slipstream?
 

You can try a 10x7 and see if that works, but it probably won't do a whole lot. How much rudder throw do you have? Have you checked to see if the left wheel is not rolling as freely as the right?

Spiraling Slipstream?
 

Could it be? Assymetric thrust in a model size? It sure sounds like it. The prop on the right hand side has more Angle of Attack as it sweeps down then the left side has. As a result it pulls forward more on that side until the tail comes up and the prop disc is more in line with the direction of travel. Common handling problem on the warbirds but I didn't think we'd see it on a model.

How do the real planes handle it? More rudder throw I'd think?

Spiraling Slipstream?
 

That's actually known as P-factor, and yeah with larger GA planes it can take a lot of right rudder to overcome. In fact, in the Beaver I'm often using full right rudder while trying to get 'on the step' just to hold the nose straight. With a left crosswind sometimes it is impossible to keep the plane going straight until you're in a planing position.

Spiraling Slipstream?
 

UAflyer,

I think it is most likely the spiral propwash, as you suggest. One experiment you could use to find this out is to let the tail go up as soon as it can, so that the plane is taxiing with almost no angle of attack. If this does not get rid of the left yaw, then the yaw is not being caused by the 'P-factor'. My understanding is that the spiral propwash is the strongest of the forces that can cause left yaw. Since your plane is a scale model, you may not want to mess with the looks, but there is one modification that can help, if you don't mind the appearance. If you can even out the fin area above and below the thrust line, then the yaw effect will be reduced or eliminated. You could do this by removing some fin area from the top, and/or adding some on the bottom of the fuselage. If you want to make a prop change, you need to go to a lower pitch, rather than a higher pitch. In any case, I am a little surprised that you don't have enough rudder authority to overcome the yaw. Make sure you have at lease the recommended throw. Another thing you can do is to throttle up gradually, so that you don't have the full-strength propwash until you have some airspeed to give stronger yaw stability. I think you will notice that this problem goes away as the airspeed goes up ( I use the term airspeed, even though the plane is still on the ground, because it is the relative speed of the air moving past the fin that gives yaw stability ).

Good luck,

banktoturn

Spiraling Slipstream?
 

Though I did think of p-factor as a possibility, I forgot about the angle of attack being a player in the extent of the p-factor (I installed longer landing gear because the plane has a tendency to nose over to quickly and dig the prop into the ground; that certainly increased the angle of attack some). Still, I think that the effect is the result of the spiraling prop wash.

I was also thinking it might also be the engine putting torque on the airframe, placing more force on the left wheel than the right. That would be like a brake of sorts, also causing the plane to turn left.

More rudder throw isn't really possible. I had the sucker pegged and it still happened.

I think my first move is going to be putting a 10X5 prop on and going from there.

Thanks for all of you suggestions guys!

Spiraling Slipstream?
 

Let me take a guess.

I think that those "longer" gear that you installed have accentuated the "P" factor. The descending blade is chomping away and the other is not. Maybe you should shorten the gear back a little. Plus, I think that 109 rudder is pretty small to begin with. I wouldn't change it mind you. But in order to give it half a chance, I would throttle up very gradually.

I generally have my tail wheels cocked out to the left a little so that right rudder is required for straight taxi. It sort of gives me the jump on things as I throttle up.

Spiraling Slipstream?
 

Hi UAflyer

If the right thrust didn't improve the situation, it
could just be wheel alignment. try a tiny bit of toe in.

I had one of those 109's for a while. Behaved pretty well for me. the only thing i did was to bend the gear forward a bit so it wouldn't tip over on me.

where do you fly?

Spiraling Slipstream?
 

I have the Global 25 sized Me-109 with a 40 FP on it. It requires near full right rudder on take off and if I advance the throttle too quickly it will pull to the left no matter how much rudder input there is. The plane has as much rudder throw as I can possibly get on it. Once in the air it is a fine flying ship. A little Toe-in did help.

Just don't hold full up elevator and hold the tail on the ground until take-off like I see a lot of the people at our field do. They often cartwheel the plane down the runway after tip-stalling it just after breaking the ground and blame it on the wind.

A little off the subject.....I use it for combat. I had a midair with a Sig Wonder and chopped it in about 5 pieces. I T-boned the wonder and went right through it. When it hit there was a loud whack. The wonder looked as if it had really exploded and pieces slowly rained down. I thought my plane was a gonner until it flew out of the Wonder debris. I landed with the motor still running. About an inch of one of the prop blades was missing and the cowl was cracked. This also happened infront of a crowd of probably 500 (?)people. I felt really bad about it but couldn't stop laughing. The guy with the Wonder, who I know, didn't look too happy when he saw me chuckling. Those Germans could sure build tough planes......

Here is a pic. It has seen better days.