Dang, Prototypes are different...somehow... - RC Groups
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Apr 26, 2006, 06:30 AM
Needs brain lubrication
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Dang, Prototypes are different...somehow...


During the second powered testflight we suddenly got an idea
of why it would have been wise to attach the fuselage pod more thoroughly to the wing.
It went of in flight and we are lucky to have only little damage on wing, pod and the gear.
Since only minor disintegration of the model occured, we came to the conclusion that,
except for the pod attachment, it's a tough model.

However, there was still this mysterious pitching up, with all controls neutral.
We already made some airfoil gauges to check, if the airfoil came out as accurate as we wanted.
And it seems quite ok.
That means, we didn't sand an unwanted reflex into the TE for example
wich could otherwise have lead to a pitching up moment... *sound of scraping his head*

So we will have to measure the exact washout too,
in order to find out why that thing has still this pitch up attitude.

Note, that it has only 1.2 degrees washout and an airfoil with negative pitching moment.
Normally that'd make anyone think, it should want to pich down.

Another thing we will try, is to get the thrust line a bit higher and tilt it,
so that any pitching up moment it could generate would be decreased or eliminated.

Finally one could blame an insufficient torsional rigity of the wing.
I don't really want to do stuff like areo-elastic tayloring on a model, but...
If the cm of the airfoil is indeed negativ (as calculated),
its torque may increase the washout in flight and lead to the pitching up.
But the wing is not exactly wobbly.

A method to check that out would be to set up all controls to have some reflex.
Then, with a cm set to zero, there shouldn't be any in flight torsion left, except for the intended original washout.

Further I'd guess, the influence of wing bending can be neglected,
since the integrated spar with carbon caps and GF shear web is doing a quite good job.
There seems to be no tendency to flutter, either, so we are well happy with that part.
(@ Polyhedral & Ed Soars: No pun intended )

And besides all that it really is fast already.

biber
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