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.

How's it look at the rear spar attach? Guessing 0.7 would be pretty noticeable back there.

Jason, I'll try to get a picture of the rear spar attach on here tomorrow.

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.

Initial description changed

Withdraw comments.


Bob Axsom

Bob Axsom said:

....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

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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

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?

Have you contacted Van's yet?

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.

Juvat said:

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.

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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.

Get rid of the digital level and measure with a machinist's level. Notice how far up and down you can move a digital level before it registers. Most important thing is that they are exactly even. I just did the spar drill on a friend's 7 yesterday. My 7 was done with a machinist level also and it would fly wings level hands off from cruise right through the stall. Don

yakdriver said:

Get rid of the digital level and measure with a machinist's level. Notice how far up and down you can move a digital level before it registers. Most important thing is that they are exactly even. I just did the spar drill on a friend's 7 yesterday. My 7 was done with a machinist level also and it would fly wings level hands off from cruise right through the stall. Don

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I disagree, a digital level is fine and is easier to read. A resolution of 0.1 degree should be fine for our application.

What is required is to calibrate the level (RTFM) and then do a check on a known surface twice with the level pointing in opposite directions.

I bet your fine

The wings being equal is the most important thing, That is quite a bit of extra incidence, I set my wings .1-.2 degrees more positive than book specs for edge distance purposes and it flies true. A call to Vans couldn't hurt though.

Randy
8A

FWIW, countless RV's were rigged with a builder's bubble level and fly great.

Sometimes we outdo ourselves by overthinking things. I've seen RV's with one elevator an inch higher than the other, some with forward swept wings or rear sweep. It's a 50/50 deal....at 50 feet and 50 MPH, no one's gonna notice.

Best,

Sometimes we outdo ourselves by overthinking things. I've seen RV's with one elevator an inch higher than the other, some with forward swept wings or rear sweep. It's a 50/50 deal....at 50 feet and 50 MPH, no one's gonna notice.

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Thanks Pierre. One experienced RVer I spoke with yesterday shares your sentiments. He looked at the airplane "big picture" of how everything has been put together so far, and he says it's going to fly just fine. And FWIW, he said what I've heard now twice from others: simply make a slight adjustment in the HS incidence as needed and she's going to fly great. The best news here so far is that the wings have equal incidence.

I sure do appreciate everyone's comments and suggestions here. It's a huge help guys!

Edge distance

As others have noted above, the most critical measurement concerning the wing mounting is the edge distance on the rear spar attachment. Without the edge clearance shown in the plans, it is not a +6, -3 G airplane.

Yes, absolutely correct Brad. Minimum of 5/8" edge distance is required. No doubt about that.

Please forgive me cuz it's been a long while for me, but I remember that spar carry through and spar stub as being very narrow top to bottom. I can't really picture pulling the spar down a half inch and still being able to get the required edge distances.

pierre smith said:

FWIW, countless RV's were rigged with a builder's bubble level and fly great.

Sometimes we outdo ourselves by overthinking things. I've seen RV's with one elevator an inch higher than the other, some with forward swept wings or rear sweep. It's a 50/50 deal....at 50 feet and 50 MPH, no one's gonna notice.

Best,

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I agree, my comment was to keep the digital level if you have it and are happy using it - not to "throw it away" as the previous post said.

I didn't say to get a digital level to do the job....

A good bubble level is fine, but it also needs to be checked for zero by the two direction method.