>I'm not completely certain how to make sure the washout is correct
Listers, I've already corresponded direct with Dave about this, but
maybe I should repeat what I said here for everyone's benefit.
The dimensions given in my copy are in error because the trigonometry
appears to be based on the full width of the jig block. However, by
the time you are checking the washout, you will probably have cut
down the jig block to allow for LE wrapround, so smaller dimensions
are needed. When I discovered this, I wrote thus to Roger Bull at the
factory:
>Today I was preparing for the first layup on the starboard flap, and
>came across something else that I think needs at least
>clarification, if not correction, in the manual.
>
>What may affects other builders as well as me is the way the flap
>washout is specified to be set. (Incidentally, I couldn't follow the
>procedure in the manual, partly because the upper support blocks
>were cut away to a feather at the tips and the end of the TE support
>block was actually touching the table itself.) The manual specifies
>_removal_ of a shim of 8.4mm at the TE to give a 1.4deg change of
>angle. I left the TE alone, and _inserted_ a shim at the LE, and
>found that it needed something over 6mm to achieve a difference in
>angle of 1.4 deg between the root and the tip (measured with a MD
>Smart-Tool digital level, checked both ways round). At first I
>thought my thinner shim might just be because of the way the support
>block had been feathered off, but then I began to check the
>trigonometry.
>
>For the specified 8.4mm shim and an angle of 1.4deg, the baseline of
>the triangle (x) would have to be:
>
>x = 8.4 / tan(1.4)
>
>x = 8.4 / 0.0244
>
>x = 343.7mm
>
>That is about the chord of the whole flap at the tip. However, by
>the time the builder is checking the washout, about 75mm will have
>been cut off the jig block at the LE. The distance from the TE to
>the shortened LE edge of my jig block is 275mm. So, the shim (y)
>needed for that situation is:
>
>y = 275 * tan(1.4)
>
>y = 275 * 0.0244
>
>y = 6.72mm
>
>That is in the right ball-park for my measured value, and makes me
>feel that I've got it right.
>
>But the question is, has the manual been telling builders to use a
>shim appropriate for the jig _before_ cutting off the LE part when
>the procedure is much more likely to be done _after_ that operation?
>It appears the result would be to have washout of about 1.75 degrees
>instead of the specified 1.4 degress. Probably not a disaster, but
>not quite what was wanted.
And Roger responded:
>I am sure you must be right - as long as you end up with the correct
>washout, that's the main thing. I will suggest to Andy to put a
>note in the next Newsletter.
I'm not sure if anything ever appeared in the newsletter, though ...
Andy, Neville, care to comment, is the latest manual fixed on this
point?
regards
Rowland
--
| Rowland Carson PFA #16532 <http://home.clara.net/rowil/aviation/>
| 670 hours building Europa #435 G-ROWI e-mail <rowil@clara.net>
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