garys
Posts: 721
Joined: 1/31/2002
From: San Diego,
CA, USA
Status: offline
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In knife edge, a tail heavy airplane will drift towards the bottom because of the down-elevator trim component that's being used to keep it from climbing during level flight. The opposite is true for nose heavy (up trim causes it to go towards the top). Although that's not to say that "any" well trimmed airplane will fly straight in knife edge, it's generally true with pylon racers and aerobatic airplanes.
I trim different than anybody has mentioned on this thread.
First I trim for straight and level flight, and then I put it on the course and see what it's doing. The first thing I watch is what it's doing in the turns, then I watch the straights.
If it's climbing or diving through the turn, you need to make sure it's a "roll" or a "yaw" that's causing the climb. 95% of time it's a roll, although some airplanes do have a yaw component (particularly if the effective vertical tail area is too small...or if the V tails don't have equal throws). If it's a roll, I use tip weight to correct it (change, golf club tape, whatever you have..but I prefer the tape as it's not as thick). I've only had a couple Q40's have a yaw, and I used a couple of percent of rudder mixed with elevator to correct it. The Proud Bird that Jim Allen and myself fly did it when we first started flying it. I added about half an inch to the top of the fin when we re-did the mold and corrected the yaw issue. It's now available from Fly Fast Composites (www.flyfastcomposites.com).
I fine-tune the CG based simply on flying the course. I don't worry about inverted flight, ect. Personally I don't like my airplanes to go "straight" on the course, I like them to come in slightly on the course, while rolled up about 60 degrees. If it's coming in too much for me, I move the CG back slightly, or I move it forward if it's flying too straight on the straights.
After any adjustments, I then go back and make sure the airplane will fly straight and level. You should get the airplane to fly the course correctly...AND be able to fly straight and level.
I find that if I fly the straights too straight, I'm in the turn too much and loose too much speed (you have to make a full 180 degree turn, where I'm probably in a turn 160 degrees or so)...The longer you're pulling elevator, the more you're slowing down. Hense, slower lap times, and losing heats. Another by-product of not going dead straight is it's easier to make corrections if you come out of a turn too early or too late. If I'm too late getting off the elevator in a turn, I roll out more, and go straighter down the straightaway, if I'm too early letting off elevator, I leave it banked up more and arc the straight more. I only have to adjust how much aileron I use, and minimize the total control movements. The more control movements you make, the slower you go, especially with elevator movements.
Something else I watch for is adverse yaw with aileron input, and adjust aileron differential to get it close. I don't worry about getting it perfect, but it's got to be close. I've found that most shoulder wing Q500's really should have about 30% more "up" aileron travel than "down". Q40's tend to take a bit less, but most of them can use some differential as well. Without the differential, they may be pulled off heading as you're rolling up for the turn. If the airplane is not rolling up correctly, you're going to be fighting it more.
I believe you need to have the airplane fly straight and level so that when you do have to make a correction (which everybody has to do), you're only trying to get the airplane back to where you need it, rather than fight an airplane that's not only out of position, but now trying to something else because it's only trimmed for on the course.
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GS
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Flying Characteristics: Viper Q-500 Pulling up in Turn ... - 
