Trim drift
12-04-2004 | 11:59 AM
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From: Eglin AFB, FL
Trim drift
Hi guys,
Can anyone explain to me why I always have to adjust my elevator trim at the beginning of each day at the field? I am not changing anything on the airplane, and servos are not loose. CG is the same with full fuel, and no large changes in atmospheric conditions. Do you guys have to do this as well? Thanks,
Steve
Can anyone explain to me why I always have to adjust my elevator trim at the beginning of each day at the field? I am not changing anything on the airplane, and servos are not loose. CG is the same with full fuel, and no large changes in atmospheric conditions. Do you guys have to do this as well? Thanks,
Steve
12-04-2004 | 12:37 PM
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RE: Trim drift
There are lots of factors that can cause this.
--Temp changes will result in expansion and contraction of the model components. Fiberglass pushrod vs CF fuse or CF pushrod vs fiberglass fuse. Because the two materials are different the Coefficient of thermal expansion will be be different. Example CF will tend to not change with temps or will change very very small amounts on the order of 10^-5 or 10^-6. However fiberglass will change a bunch 10^-2 or 10^-3. It could be the pushrod is expanding or contracting or it could be the entire fuse is expanding or contracting. Especially if the model sets in the sun.
--Wood tends to change with humidity
--Lift will change depending on air density, and temperature.
These things can all change on a day to day basis, also on a flight to flight basis. Remember you are talking about a couple clicks either way...The resulting angle on the elevator is extremely small. Today our servos and radios are extremely accurate and precise...This means you can feel these changes. Its common.
To avoid this try to make the pushrod material out of the same material as the fuse. A CF Fuse and CF pushrod assembly will change at about the same rate. This is not always easy to do. The CF pushords are extremely stiff and light. That's why they are a good choice. Another idea is cables are in my opinion a nightmare in this area. Yes they will expand and contract at about the same rate but the tension on them is always changes and they change and the fuse they are attached too changes as well. For me this make the elevator feel spongy. Just know that you will need a few clicks of elevator on your downwind trim pass...That is why you have a downwind trim pass.
A Procedure turn on T.O will give you the best possible run at a trim pass. This is why many non-FAI flyers do it. A nice wide sweeping turn gives an opportunity to check elevator trim and to feel the wind conditions. If properly setup with trim resolution at a min value its common to need 3-5 clicks of elevator depending on the day. When looking at the model this amount of movement is so small you can barely see it. ITS "IN the Noise" as Tony F would say.
Troy Newman
--Temp changes will result in expansion and contraction of the model components. Fiberglass pushrod vs CF fuse or CF pushrod vs fiberglass fuse. Because the two materials are different the Coefficient of thermal expansion will be be different. Example CF will tend to not change with temps or will change very very small amounts on the order of 10^-5 or 10^-6. However fiberglass will change a bunch 10^-2 or 10^-3. It could be the pushrod is expanding or contracting or it could be the entire fuse is expanding or contracting. Especially if the model sets in the sun.
--Wood tends to change with humidity
--Lift will change depending on air density, and temperature.
These things can all change on a day to day basis, also on a flight to flight basis. Remember you are talking about a couple clicks either way...The resulting angle on the elevator is extremely small. Today our servos and radios are extremely accurate and precise...This means you can feel these changes. Its common.
To avoid this try to make the pushrod material out of the same material as the fuse. A CF Fuse and CF pushrod assembly will change at about the same rate. This is not always easy to do. The CF pushords are extremely stiff and light. That's why they are a good choice. Another idea is cables are in my opinion a nightmare in this area. Yes they will expand and contract at about the same rate but the tension on them is always changes and they change and the fuse they are attached too changes as well. For me this make the elevator feel spongy. Just know that you will need a few clicks of elevator on your downwind trim pass...That is why you have a downwind trim pass.
A Procedure turn on T.O will give you the best possible run at a trim pass. This is why many non-FAI flyers do it. A nice wide sweeping turn gives an opportunity to check elevator trim and to feel the wind conditions. If properly setup with trim resolution at a min value its common to need 3-5 clicks of elevator depending on the day. When looking at the model this amount of movement is so small you can barely see it. ITS "IN the Noise" as Tony F would say.
Troy Newman
12-05-2004 | 12:00 PM
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From: Eglin AFB, FL
RE: Trim drift
Thanks Troy,
Makes sense...wood airplane, metal pushrods, all expanding and contracting at different rates. Now that I have practicing pattern I have started to notice these small things.....thats a good thing!
Steve
Makes sense...wood airplane, metal pushrods, all expanding and contracting at different rates. Now that I have practicing pattern I have started to notice these small things.....thats a good thing!
Steve
