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  #1  
Old 08-22-2012, 04:26 PM
Gash's Avatar
Gash Gash is offline
 
Join Date: Aug 2009
Location: Goodyear, Arizona
Posts: 877
Default My Wing Incidence is 0.7 Degrees Too Low--What Now?

I am the second owner of my RV-8 kit, and it looks like we inherited a mistake from the last builder that wasn't caught earlier. This week we discovered that the wing incidence is 0.7 degrees less than Van's recommendation. <<CORRECTION: the wing incidence is 0.7 degrees more than recommended. Long story, but it comes down to me getting help with the wings and a miscommunication led me to believe it was negative as opposed to positive incidence error. Sorry for the confusion!>> Both wings have equal incidence. The horizontal stab incidence is perfect though.

So here are my questions:

1. How big of a deal is this? Is this going to be something I can trim out in flight and live with, or will it be unstable or possibly even dangerous?

2. Has anybody else had this problem before, and what should I do to fix it? I've done an extensive forum search and read of a few guys with too much wing incidence, but nobody with too little wing incidence.

Thank you in advance for any ideas from people who can help with this. As you can imagine, I'm working hard to suppress the panic that something like this causes.
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Karl, Goodyear, Arizona (KGYR) ATP, CFII
RV-14A, Flying
Extra 330LX, Flying
RV-8, Sold
RV-7, Sold
Bearhawk 4-Place, Sold
=VAF= donor 2020

Last edited by Gash : 08-22-2012 at 08:57 PM.
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  #2  
Old 08-22-2012, 04:34 PM
Jason Krause Jason Krause is offline
 
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Location: Glendale, AZ
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How's it look at the rear spar attach? Guessing 0.7 would be pretty noticeable back there.
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  #3  
Old 08-22-2012, 04:43 PM
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Gash Gash is offline
 
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Jason, I'll try to get a picture of the rear spar attach on here tomorrow.
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Karl, Goodyear, Arizona (KGYR) ATP, CFII
RV-14A, Flying
Extra 330LX, Flying
RV-8, Sold
RV-7, Sold
Bearhawk 4-Place, Sold
=VAF= donor 2020
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  #4  
Old 08-22-2012, 04:49 PM
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Bob Kuykendall Bob Kuykendall is offline
 
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Location: Douglas Flat, CA
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Working the math, it looks like it would take an error of about half an inch at the drag spar to yield a 0.7-degree error in incidence. The things I would worry about are whether there is adequate edge distance at the drag spar tang and fork, and also the fit of the flaps and lower skin against the bottom of the fuselage.

My suspicion is that it would not cause any noticeable change in flight characteristics.
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  #5  
Old 08-22-2012, 05:03 PM
Bob Axsom Bob Axsom is offline
 
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Default Initial description changed

Withdraw comments.


Bob Axsom

Last edited by Bob Axsom : 08-22-2012 at 09:14 PM.
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  #6  
Old 08-22-2012, 05:13 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
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Quote:
Originally Posted by Bob Axsom View Post
....I have considered raising my HS leading edge to reduce drag but I have differing reports on speed gains (3 kts to 0 kts) and I'm not yet ready to redo the tail fairing....

Bob Axsom
If the elevator and trim tab are fair to the stabilizer at the desired flight condition, then your stabilizer is already set correctly. If they are not, then that would be a means of correcting the out-of-trim case.

If you do make that change, it might be worth somehow making it ground-adjustable.

Dave
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  #7  
Old 08-22-2012, 05:26 PM
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Alan Carroll Alan Carroll is offline
 
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Location: Madison, Wisconsin
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I've got the same question as Bob Axsom, how did you measure the incidence? It seems to me it would hard to be that far off and still have the rear spar tang mate to the fuselage carry-through structure, while maintaining the appropriate (and important) edge-distance on the rear spar bolt holes.

What exactly are you measuring, and how?
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  #8  
Old 08-22-2012, 07:36 PM
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curtis curtis is offline
 
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Default My Wing Incidence is 0.7 Degrees Too Low--What Now?

Have you contacted Van's yet?
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  #9  
Old 08-22-2012, 08:54 PM
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Gash Gash is offline
 
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Thanks for the replies guys. First off, I need to make a (major) correction to the original facts in the thread here. The incidence error is positive 0.7 degrees, not negative. Long story short, I am getting help with the rigging and was not there for the measuring, and learned about the problem first in a conversation without looking at the airplane myself. I misunderstood and thought that it was a negative 0.7 degree error, but it's just the opposite: 0.7 degrees positive error. Sorry guys, I feel a little dumb for getting the facts opposite when starting this thread.

We found this out by looking at the drag spar and saw that "hey, there's plenty of room there! There's not going to be any problem at all with edge distance." Well duh, that's because it's up error instead of down error.

Anyway, to answer your questions. First off, this was measured digitally and with a laser level using the Vans recommended method. Measurement was done by an experienced builder who knows what he's doing, and confidence is high in the accuracy (aside from my original plus/minus mistake!)

On the second question, I did call Vans later on this afternoon. I was told that first of all, this will not affect flight safety at all. It may change the handling qualities of the airplane somewhat, and if it does, adjustments should be made by shimming the horizontal stab...exactly what you guys are already saying here.

I am relieved that this isn't going to be a big deal. The airplane will fly just fine, and the HS can be tweaked after test flying to fine tune the handling. No big deal.
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Karl, Goodyear, Arizona (KGYR) ATP, CFII
RV-14A, Flying
Extra 330LX, Flying
RV-8, Sold
RV-7, Sold
Bearhawk 4-Place, Sold
=VAF= donor 2020
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  #10  
Old 08-22-2012, 09:18 PM
Kyle Boatright Kyle Boatright is offline
 
Join Date: Aug 2005
Location: Atlanta, GA
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Quote:
Originally Posted by Juvat View Post

We found this out by looking at the drag spar and saw that "hey, there's plenty of room there! There's not going to be any problem at all with edge distance." Well duh, that's because it's up error instead of down error.
Being off by .7 degrees either way on incidence is enough that an edge distance might be compromised.

If you can, post a picture of all 4 of the drilled fittings - the fitting on the inboard end of each wing and the fitting on each side of the fuselage. You have to have maintain edge distances in all 4 places.
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