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Oct 26, 2014, 12:17 AM
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conclusions on trimming and decalage


conclusions on trimming and decalage
the data that i have found the most precise on the issue was mentioned by mikeruth in his post 3919 of the forum of the radian: https://www.rcgroups.com/forums/show....php?t=2089760
he said: Page 20 of this article is a very good explanation of Setting decalage and CG.
I have been using this for years. http://www.rcsoaringdigest.com/pdfs/...SD-1992-03.pdf

the article was re-issued on 1992, and the copyright by Frank Deis is from 1990. it is the oldest i have ever seen on the subject, so the credit goes to him, and the flight test goes as far as 1973.
(by the way, the drawing has a mistake: in the rectangle at the left it says that cg is too far "forward" but should be "aft").
it shows how to do it and also shows tests on fixed stab and moving stabilator.
i will make a condensation of all that matters in that article. the goal is to have a decalage of about 1 to 3 degrees.
(decalage is the angular difference between the wing airfoil and the stabilizer).
this is the way i interpret it:
1.-start with a 33% balance point (called center of gravity);
test fly the plane:
2.-for a fixed stab (with moving elevator), trim the elevator until the plane glides well:
if the elevator is not parallel to the fixed stab, move the cg accordingly:
a.-if the elevator is up, move the cg back a little and try again until the elevator is parallel to the stab.
b.-if the elevator is down, move the cg forward a little and try again until the elevator is parallel to the stab.
3.- for a stabilator, balancing at 33%, trim it until the plane glides well.
measure the angle and if it is:
a.-more than 3 degrees, move the cg aft,
b.-and if close to zero, move it forward.
>>> now these are my notes:
1.-this depends what you want from the plane, as 1 with near zero is unstable, and 1 with too much, sinks and needs lots of downthrust.
2.-the article considers the wing incidence measured from the center of the leading edge radius to the trailing edge, but there is an easier way by measuring the bottom line of the wing airfoil. that is the usual way we do it.
3.-the thing that is not mentioned is that by moving the cg we alter the stability: moving it back it becomes unstable; forward too slow and erratic to response<<<

the dive test
the author says that he learned about it from Lemond Payne in 1973.
1.- climb to altitude and set the elevator trim for level flight;
2.-dive to about 45 degrees and release the controls for about 5 to 10 seconds, then
3.-resume flight (don't let it crash!).
if the plane pulls out of the dive and into a stall, the cg is too far forward (this happens because it has some 'up' trim). if the dive steepens, cg is too far back (because it has some 'down' trim).
4.-move the cg accordingly to correct the dive or stall; re-trim the elevator and re-try the dive test.
the goal is to reach the cg position where the plane starts to pull out and then goes into a steeper dive.
if the plane has a stabilator, it is trimmed. if has a fixed stab, check if the elevator is parallel to the stab. if not, adjust the decalage by shimming the wing or stab a little (some 1/32") at a time, and re-run the dive test until the elevator is parallel to the stab.

>>>again, my notes: notice that he adjusts by moving the cg and then the decalage in that order. but in some cases it is not only hard, if not practically impossible, to do so, and/or makes the plane unstable or too slow to respond, and still ends up changing the decalage. that is why i favor the idea of getting results by changing the decalage (and not the cg) from the beginning if the plane does not come out right of the dive test. still, what matters is that at the end of the trimming on the dive test, the elevator has to be parallel to the stab.<<<
Last edited by phil alvirez; Oct 26, 2014 at 03:38 PM.
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