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- Apr 17, 2012
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This isn't a biplane question, but you guys seem to appreciate a good mystery - or a good opportunity to explain nicely how what seems like a mystery is really a complete lack of understanding of how things work. 
1. The Jodel seems to take a lot of right rudder - enough that at cruise, the right rudder pedal is about 1/2 way to the floor. The wings are level, as near as I can tell, with the ailerons centered (and a lot of right rudder). However, does that really mean that the only reason for the ball wanting to be out to the right is needing more right rudder? How can I troubleshoot between a roll trim problem and a rudder trim problem?
2. I am stumped by an engine/prop problem. Again, I can't figure out how to tell which is at fault. I have a C-85 and a ground-adjustable prop. The first few flights, it seemed like climb was a bit slow, and the engine wouldn't turn more than about 2300 RPM, so I reduced the pitch setting a couple times, to the point that I think it is quite a bit flatter than it should need to be, based on cruise speed being a good 20 MPH slower than I expect. Now it climbs pretty well, and will turn up 2500 RPM in level flight. The odd thing is that I have to pull the throttle WAY back - like 2/3 back - before the RPM comes down to 2300. And when climbing, I can pull the throttle back half way without seeing a noticeable reduction in climb rate. It seems like if I can climb at half throttle, I should be easily able to overspeed the engine in level flight. I can't think of any situation short of a constant speed prop that should result in essentially constant RPM over half the range of the throttle. (And yes, I have checked, and the butterfly movement corresponds to the throttle movement.) The prop manufacturer thinks maybe the prop diameter is one inch too much, but while too much prop diameter will restrict max RPM, it doesn't seem like it would result in this "constant speed" effect.
I had started thinking maybe it was a carb problem - it has the Stromberg carb, and I have read that the venturis for 65 hp and 85 hp are interchangeable, so I was thinking maybe the small venturi was installed and was restricting airflow downstream of the butterfly. But when the airplane was wrecked, it had a 70/47 McCaluley on it - I don't think it would have turned that if it had the wrong venturi. So I'm really back to thinking it is the prop, but I just can't rationalize how a prop can exhibit this sort of non-linear behavior. Any ideas?
Thanks!
Warren
1. The Jodel seems to take a lot of right rudder - enough that at cruise, the right rudder pedal is about 1/2 way to the floor. The wings are level, as near as I can tell, with the ailerons centered (and a lot of right rudder). However, does that really mean that the only reason for the ball wanting to be out to the right is needing more right rudder? How can I troubleshoot between a roll trim problem and a rudder trim problem?
2. I am stumped by an engine/prop problem. Again, I can't figure out how to tell which is at fault. I have a C-85 and a ground-adjustable prop. The first few flights, it seemed like climb was a bit slow, and the engine wouldn't turn more than about 2300 RPM, so I reduced the pitch setting a couple times, to the point that I think it is quite a bit flatter than it should need to be, based on cruise speed being a good 20 MPH slower than I expect. Now it climbs pretty well, and will turn up 2500 RPM in level flight. The odd thing is that I have to pull the throttle WAY back - like 2/3 back - before the RPM comes down to 2300. And when climbing, I can pull the throttle back half way without seeing a noticeable reduction in climb rate. It seems like if I can climb at half throttle, I should be easily able to overspeed the engine in level flight. I can't think of any situation short of a constant speed prop that should result in essentially constant RPM over half the range of the throttle. (And yes, I have checked, and the butterfly movement corresponds to the throttle movement.) The prop manufacturer thinks maybe the prop diameter is one inch too much, but while too much prop diameter will restrict max RPM, it doesn't seem like it would result in this "constant speed" effect.
I had started thinking maybe it was a carb problem - it has the Stromberg carb, and I have read that the venturis for 65 hp and 85 hp are interchangeable, so I was thinking maybe the small venturi was installed and was restricting airflow downstream of the butterfly. But when the airplane was wrecked, it had a 70/47 McCaluley on it - I don't think it would have turned that if it had the wrong venturi. So I'm really back to thinking it is the prop, but I just can't rationalize how a prop can exhibit this sort of non-linear behavior. Any ideas?
Thanks!
Warren
