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fin

 
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chenoweth(at)gwi.net
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PostPosted: Sun Apr 29, 2007 3:22 pm    Post subject: fin Reply with quote

Let me start with the questions first and then the story.

The question: Is the rear spar of the fin straight enough when a straight edge laid along the hinge brackets is aft of the center one by about 1/32 inch?
How about if it's out of line by more than 5/64 but less than 3/32?
These are apparently my two choices.

The story:
If I mount the fin per the drawing, front spar ahead of hold-down bracket and fin pushed forward so that the hold-down bracket just contacts the flanges on the fin's bottom ribs, the rear fin spar is firmly against the 712 bulkhead but the out of true is the over 5/64 amount I mentioned above. Which, by the way, is the amount it turned out to be out of true when I built it. At that point the rudder seemed to work ok with it.

If, however, I mount the fin with the forward spar behind the hold-down bracket and with the bracket just touching the flange of the fin's bottom rib, the rear fin spar does not seat against the 712 - it's about 1/32 aft of it at its top. But I get the less than 1/32 out-of-true measurement in the hinge line. Based on the way I'm making the up elevator stop having the rear fin spar not contact the 712 is not a problem because I can position the stop slightly aft to make good contact with the spar and still have it function properly with the elevator horns.

Any answers/comments you'd care to make would be very much appreciated.

Bill
RV9
Albion, Maine
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kayce33(at)earthlink.net
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PostPosted: Sun Apr 29, 2007 6:01 pm    Post subject: fin Reply with quote

Hi Bill,
I checked mine with a drilled acorn/cap nut, string thru the drilled hole with a weight.
Mine came out with about a 1/64th" off center error.....seems easier than a straight edge & worked with flaps too.
Harold, RV9A Fuselage wiring
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chenoweth(at)gwi.net
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PostPosted: Wed May 02, 2007 3:17 am    Post subject: fin Reply with quote

Harold,
Sorry for the delay in thanking you for the advice. I ended up discussing the situation with Van's. Their position was that it had to be straight. The suggestion was to raise the rear spar (since mine is a tailwheel version I could only do that about 1/16 due to edge margin issues) and/or to trim some off the 981 front bracket. I ended up doing both plus filing some off the top hinge brackets so they were the same dimension from hole to aft edge as the two lower ones so that the straight edge technique would produce accurate results. After all that I'm between .005 off to absolutely straight. Van's said the .005 was close enough. Haven't drilled yet.
Bill
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kayce33(at)earthlink.net
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PostPosted: Wed May 02, 2007 5:09 am    Post subject: fin Reply with quote

Hi Bill,
Sounds as if your accuracy was superior to mine. You must do good work.
Harold
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bill(at)repucci.com
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PostPosted: Wed May 02, 2007 5:59 pm    Post subject: fin Reply with quote

Bill,

Don't forget that you have to trim the front spar before installing it.

For some reason Van's doesn't mention this until after you have built the VS. It is anyone's guess as to why then but it would be easier to do before riveting that thing together.

Bill


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chenoweth(at)gwi.net
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PostPosted: Thu May 03, 2007 4:58 am    Post subject: fin Reply with quote

Bill,
No issue there - I had it trimmed. The biggest problem seemed to be that the 981 was bottoming on the fin rib flanges before the rear spar was straight. Also, the fact that this is a tailwheel version severely limits the amount that I could move the rear spar up. Taken together I didn't have a lot of slack to get the amount of forwardness I needed to get the rear spar straight. As you surely have noticed the instructions re this are hamburger. My primary concern was not knowing just how straight the hinge line had to be. Ken at Van's generally dismisses my concerns about tolerances but in this case was adamant that it needed to be straight so I took it seriously.

Reacting to Harold's comment I must say that I have pretty high expectations but only moderate skill. This combination can produce a fair amount of anxiety in situations like the current one. But, sometimes I luck out - often I don't.

Bill
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