Used airplane- Heavy Wing, suprising solution...
 

I bought the RV7 and had my 'test' pilot (insurance pilot) flew it back from LA (lower Alabama) and he commented on the heavy wing. He said there was about 10lbs of pressure required at cruise to maintain level flight and the roll was VERY pronounced without the correction.

We got her back into the hangar and started all the usual inspections to work it out.
1. Electric Aileron trim- the previous owner 'tightened' the springs to increase the forces, all it did was stiffen the controls. (undid springs for testing)
2. check aileron hinges, alignment- (less than a 1/4" difference)
3. check fairings, noticeably wingtip rise/droop (ok- not enought to cause 10lbs of force on the stick)

measure, measure, measure, test test test

Finally, I was speaking to a RV8 builder on a hangar saturday and recounted my experience and he spoke of a similar problem.

NOW BEFORE THERE IS A LOT OF FLACK;
1. I consulted multiple builders, AP, tech advisors
2. Multiple qualified Eyes visually inspected
3. CFI, tech advisor, AP- rode with me on the test flight

Ok- on with the show;

The RV8 builder suggested RV's are still homebuilt airplanes despite the quality of the kits there will be variation from one to another in construction. There can be slight 'inequalities' that are not noticeable and immeassurable (we hate to use the word 'twist') that can affect the subtle characteristics without adversely affecting the quality of construction.

Or more coarsely, As this gentleman said- All RV's are 'off' in some way or another- you just don't know it. After going through all the same dignostics- his fix was to adjust the leading edge of the vertical stabilizer in the opposite direction of the heavy wing.

After inspection/consultation- I moved the leading edge of my vert. stab on my RV7 1/4" to the right (heavy left wing) which 'in theory' placed it on the centerline (there is a 1/4" joggle built in). We re-attached the fairing with 4 screws temporarily (some work required to re-do that fairing permanently, didn't want to screw it up if it didn't have any effect).

We took a test flight and got her up to cruise and the ball was centered and almost ALL the heavy wing was removed. To the point that, some Aileron alignment and subtle tweaking can fix the rest of the issue.

My theory- there's a LOT of variables on the plane.

Could there be 1/4" twist in the fuse over the entire lentgh, creating an inability to measure a true centerline without significantly affecting quality- sure.

Could the specific prop and engine combo affect the amount of P force and contribute- sure

In summary, if you have a heavy wing- in level flight check the ball. If you are flying level (counteracting the heavy wing) and it's like walking around with a 10lb weight outstretched and the ball is out, A significant culprit may lie in the vertical stabilizer leading edge.

It took less than an hour to get the fairing off, move the leading edge, install some temporary bolts (similar to the permanent bolts specified) and temporarily re-install (not beautiful but on) the fairing for a test flight.

A permanent fix is to make a new doubler plate that is riveted to the fore support and bolted to the horizontal stab. Use the same holes in the horizontal stab angles and use the same holes in the vertical stab (back drilling). You're left with a few extra AN-3 lightening holes in the flat stock of the fore spar of the horizontal stab (don't temporary drill through angle stock- it's structural).

Very nice
 

I think this one qualifies as an elegant solution. IMHO.

Interesting. I often wonder whether the two planes of the horizontal stabilizer shouldn't be rigged at slightly different angles to ensure they are at the same effective angle of attack for optimum effiiciency in cruise flight.

...or is this actually the case?

What about gear leg fairings & wheel pants? Is it possible you just cancelled out another effect? Did you check TAS before & after?

Check the twist in the horizontal stabilizer. Somehow, even building on an H frame, I ended up with a twist in my horizontal stabilizer. As anal as I was at that point in the build I still can't figure how it got that way. I won't be flying for about six months so I won't know the full impact of the condition until then. Van's says fly it and not to worry. A twisted stab could give the same effect. You might try putting a digital level on each stab tip to see what you get. I think the corrective action would still be to change the offset of the vert. stab..

Steve Eberhart
RV-7A, O-360-A1A, Catto 3 blade, slider. Working on the engine systems.

Roll Not Yaw.
 

Jim,
Something isn?t right here. You are trying to fix a ROLL problem with the YAW control; the Rudder. Secondary effect of rudder- Roll.
Nowhere can I see that you or the test pilot reported any out-off balance. Only a tendency to roll.
Sorting out whether you have the right offset on the fin is fraught with complexity, as you want zero rudder input on the cruise, with cruise RPM. Anything else and you will need left or right rudder and have to live with it. ie. Climb and descent.

Dan is right, as always, it could be the gear fairings.

But, what caught my eye was,
Clearly, you have made every effort to find the problem and a solution.
Admirable effort. And you have sought the best advice available.

If you had built the aircraft, and this is not a criticism (but as I would say to my wife, merely an observation), you would have picked up some useful bits knowledge along the way, which might have led you to question some opinions that you were offered. And please question mine, as I hope others will; and offer more comments. The great thing about DR?s forum, is that everyone is intent on helping.

One of the first things you learn when building, is the accuracy and stiffness of the kit.
When you first fit a skin to the fuselage (eg. On top of the fuse, immediately behind the cockpit) if you cleco it on the TOP and wrap it down the sides of the fuse there is no way you can get a cleco in the bottom. It is simply too short. But, as you cleco it progressively from the top down, the skin gradually pulls hard against the ribs and stringers and right at the end, the rivet holes in bottom edge lines up and the clecos slip in.
The kit is designed on a computer that calculates the radius of the bend of the skin and the position of every rivet hole. The data is sent to a bloody accurate CNC machine (http://en.wikipedia.org/wiki/CNC) that punches the holes. RVs are very accurate. And straight.
Most of the spamcans out there were built before the advent of CNC machines. They are the ones likely to be twisted and bent.
Once you have built a component, say ?YOUR? vertical Stab there is no way you can twist it. The stiffness is huge. Go and find a builder and ask him if you can pick up the fin or the rudder or what ever and try to twist it. You won?t budge it.
I am sure your RV-7 is straight.

Now, heavy wings. I had a heavy wing. I also had the Vans Construction Manual, (which you may not have) which tells you in the flight-testing section, how to rectify a heavy wing.
Following the manual, I found I had a 3/16? difference in the HEIGHT of the Ailerons, as determined by the aileron hinges.
I assume that this is what you are referring, to accept yours are a ?? out.
The fix Vans offered me, over the phone, was to slot the boltholes in the hinges on one aileron, bolt it on tight in the new position and go a do a test flight. If it flies straight, buy a new set of hinges and drill them to the modified position.
So I slotted the ailerons 3/16? and test flew it only to find I had over done it. So, I bought some new hinges and drilled the holes offset 1/8? from the originals.
It still wasn?t perfect and checking the hinges, I found I was 1/32? under. So, I slotted the hinge 1/32? and it flew straight as a die.
1/32? is enough to upset the balance.

So, with the ?designed? ?? left offset in the fin, mine flies dead straight, with no rudder or aileron required in the cruise.

Hope this helps.

Pete.

Experience in construction, Fairings...
 

I built the RV9a up to ordering a finish kit, so I'm familiar with the kits. That fact in general has given me the confidence to buy a PRE-built RV. At Falcon Field at any one time there may be half a dozen RV drivers/builders hanging out on a weekend and a fair number of them CFI's and mechanics.

That being said, measure, measure, measure, check, check, check- from the plans to the airplane all the dims check out within reason. The heavy wing situation is not abnormal to RV's and was clearly present when the builder finished the airplane (evidenced by the aileron trim springs).

I was building a standard built RV9a with a QB fuse. It came to me nice and pretty. I can easily believe that when building a standard built fuse there is a possibility of introducing a slight unnoticeable twist along the centerline. Over 14' of the fuse 1/4" clockwise twist will negate the built in "joggle" (actual word in vans plans). On the other hand, 1/4" twist (off centering the reference point) counterclockwise DOUBLES the P factor angle in the vertical stab.

Yes- rudder controls Yaw. In theory- a centered ball is only good for a "range" of flight; otherwise we would need a ruder trim tab. Makes sense to set that range for cruise. Yaw is indicated by the ball. What is yaw? In essence, drag.

Which came first- chicken or the egg. Does the yaw cause drag- Yes. Is the drag causing yaw- maybe. The vertical stab acts like a big sail- if it was angled 6 degrees to the left at 180mph, that would certainly induce yaw, how about causing the plane to roll- maybe. If wheel pant fairing can cause roll, couldn't the vertical stab?

Start big, work small. talking talking talking, measuring measuring measuring, reading reading reading- After seeking advice from the folks that had a similar problem, I determined that the vert stab might result in the most benefit (for my specific problem) for the least amount of effort (to test anyways). It took me less than an hour to be able to test. Huge difference, no evidence of speed loss, plane flying ball centered at cruise. If it didn't work - I have two AN-3 lightening holes in the span material of the fore horiz. stab spar. But my plane was not out of commission while I took off the ailerons.

NB: the builder filled in the ends of the flaps,ailerons, elevators with expanding foam then glassed over them. Good idea in theory, looks good, less drag. The expanding foam- expanded! I have noticed a slight flair on the outward side of the last ribs where he filled them in- maybe a 1/16". This makes finite measurements tough at the trailing edges (and a pain to maintain in the future).

If you want to fill the ends- maybe consider cutting foam then glassing. NOT expanding foam.

Still planning on fine tuning ailerons, fine tuning fairings- there is no ONE magical answer.

flying and FLYING

As with ALL these homebuilt experimental airplanes, you don't know what you don't know when you start building. When you're done, there's a lot of- "my next plane will be SOOOO much better". Now that the plane is assembled and flying, we have discrete components that can be adjusted, but not necessarily in need of REBUILDING. There are so many variables at play, you work from big to small result to get acceptable results to go FLYING (zoom zoom) not in my case to build perfection (I don't have the time nor the budget for perfection).

Now, I say that with a grain of salt, because I love to tinker and tweak. :)

It's possible-
 

Two fellows here went through the entire "heavy wing adjustment". One was in a RV8, and the other in a RV7a. The RV8 had a more pronounced heavy wing (like mine). Both took off fairings, flew, adjusted, flew, adjusted, flew, optimized, repainted.

The RV8 moved his vertical stab to correct the problem after several months of investigation. The RV7a installed a 2" aileron trim tab to correct the problem. Neither felt that the fairings played a significant factor in the heavy wing. The RV7a driver did say- it had a big impact on YAW at cruise.

The RV8 driver also adjusted wingtips, bought new aileron hinges- but could find no obvious aileron discrepancy. He's offered to give me his aileron hinge blanks if I wanted to use them, but first suggested the vertical stab test.

TAS- a bunch of the instruments are used on this plane (like used on another plane before this plane used) and I/We (I have a total of less than 2 hrs in this plane, my CFI/AP/EAA techie has about 5hrs) have a rough range of data to work with.

I plan of continuing adjusting before finalizing the vert stab. If ball moves the other direction after adjusting ailerons and fairings- yes, you would be correct.

No way of knowing until I try. This way, I can get my 35 HOURS of tailwheel time off without developing a massive case of carpal tunnel syndrome or overdeveloped left forearm. :eek:

Yawning Yaw
 

Jim,
Thanks for not taking offence by my assumption that you were not a builder. My sincere apologies.

I'm still not convinced it is the Vert. Stab. But, I accept it could be.

Tell me if this logic is wrong.

Fly staight and level.
1. Accept what ever aileron force you need to hold the wings level.
2. Apply rudder until the ball is centred. Better still (I think, watch the heading over 30 seconds.)
3. Now, if the aircraft is maintaining heading (wings level) there can be NO YAW.

Therefore, IF the fin is off, or the fuselage is twisted, the rudder input must be cancelling it out.

So, what ever aileron force you are applying MUST be to counter assymetric LIFT, whatever the reason. Wing twist, tip twist, Aileron misalign!

Tell me where my logic fails; If you think it does.

Pete.