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Question for Aeronautical Engineer?

Piper J3

Well Known Member
I have a Van’s RV-12 that I purchased that has a heavy wing. The RV-12 has the fuel tank mounted in the fuselage offset to the copilot’s side. In addition to the fuel tank being right of centerline I also fly with and without a passenger.

The original builder added a crude sheet metal trim tab mounted under the left flaperon at the wing tip. See top of drawing below... The current sheet metal tab is 1.5” x 12”L and is bent at 45 degrees. In my opinion it is not very efficient because it severely interrupts flow at trailing edge of the control surface. It does however work fairly well.

I want to install a proper servo-driven trim tab with piano hinge so I can compensate for various loading scenarios. See bottom of drawing below… The new inset trim tab will be same dimensions – 1.5” x 12”L.

My question is… will the new trim tab provide same counterforce as current sheet metal tab but with less trim tab deflection?

Aircraft cruising speed is 115 knots.

I’m looking for an educated opinion before I start cutting metal.

Thanks in advance…

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I am not an engineer, etc., etc. etc...


However, before you start adjusting anything, I'd lose the sheet metal trim tab and then start the adjusting. There's no sense in adjusting for something that doesn't belong.


Good luck!


Bob
 
As Mike states, there is reams of data on heavy wing.

I offer that most common issues are the aileron not properly rigged (to high/low). Check this first. Squeezing the trailing edge will do little if you don?t first correct the rigging.

Bob?s point is correct as well, pull off the installed tab so you have a clue on what is going on.

Carl
 
This is often easily cured by squeezing the trailing edge a bit.

Do a search for "heavy wing", you should find a lot of info.

Yes, I have done the "trailing edge squeeze" with sheet metal tab removed with little effect. I also looked at setup geometry and it all looks good. Existing sheet metal tab works well in both cruise and landing configuration with flaps extended fully. Flies straight with ball centered. I have a very early kit SN#58 so maybe things got better as design was refined.
 
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Perhaps a more basic test first? Check that your wings aren't twisted.

Use a digital level and measure the angle with blocks on the rear spar and main spar every 2 ft or so on each wing.
 
FWIW, here is what I am doing to address dynamic lateral trim on my RV-12:

1. Ensure that the aircraft is properly rigged i/a/w Van's plans -- Done.

2. Bias the lateral trim using squeezed TE adjustment and fixed rudder trim tab to result in hands-off lateral trim at my typical operating condition (i.e., most frequent power setting and fuel/pax loading) -- Done.

3. Design and install a spring-based lateral trim system similar to that used on a number of other RV models (i.e., a small lever or knob located in the cockpit that varies a weak spring's tension on the aileron portion of the flaperon circuit) to account for varying fuel/pax loadings -- In progress.

Until I install such a spring-based system I find that the AP does a nice job of keeping a slightly heavy wing level albeit at the cost of increased AP servo use/wear and the occasional Trim Roll or Roll Slip warnings.

I must admit that it would be nice, but not necessary, to have the AP Auto Trim the ailerons via a dedicated trim tab or other trim mechanism. I am not a big fan of cutting up a portion of the flaperon -- primarily due to weight/balance/strength/complexity concerns -- but that is just me. Let us know how your project proceeds.
 
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First, a heavy wing is not a rare occurrence in a built RV,
Second - fix it with a permanent adjustment, not tabs and servos. [unless Steve says so]
third- start with Vans, the 12 design might present different causes and solutions than the 6-7-8 or the 9-10-14's due to construction.

Read this - https://www.vansaircraft.com/pdf/Wing_Heavy.pdf then call the help line.

The ailerons provide lift, if that lift is not even, then stick force (to counter the roll) and cockeyed alignment during flight will result.

Good luck on the journey.

Edit: The advice from Steve Smith (below) is how I finally got mine solved!! The final adjustment to a hinge pivot was .030" to completely neutralize.
 
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All the comments here are good. It should be emphasized to check the vertical position of the aileron relative to the wing surface. Lay a straight edge on the wing over the last few inches before the hinge gap. slide the straight edge aft until it either hits the aileron skin, or slides over it. Note this condition. First and foremost, left and right should be the same. It is most desirable to have the aileron skin be slightly high of the tangent line extending off the wing skin, rather than below the tangent line. So with the ailerons in neutral position, a straight edge should hit the aileron as it is slid aft along the wing.

I doubt it is possible to build in any wing twist accidentally on a fully match-punched kit like the RV-12. You would have to really work at it to systematically take every bit of clearance in one direction on assembly to end up with twist.

Now, all that said, if you REALLY do want to put a trim tab on your aileron, what you have sketched should work well. The fixed trim tab you have now is rather poor, it is like a spoiler or split flap, it works some, but not as effective as it would be if it was an extension off the trailing edge. It could probably be deflected 10 degrees instead of 45 if it was mounted at the trailing edge.

I believe Paul Dye made a servo-controlled aileron trim tab on one of his airplanes. and I think he did something even simpler than what you sketched, so I would suggest you talk with him about it.
 
Some things you could check.
Is the ball perfectly in the center and you are sure there is no yaw? Roll due to yaw (basically diherdral effect/Clbeta)can be greater than you might expect.
Are the flaperons rigged correctly?
Have you checked the wing relative to the fuselage to make sure it is square?
Have you checked the tail relative to the wing?

A better fixed tab would be way easier and lighter than adding an actuated trim tab. Here's a random picture of a typical trim tab from Google.
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The simplest thing to do is adjust your fuel load to compensate. I weigh 250 and sit in the left seat, so I always have a "heavy left wing". Putting 5 more gallons in the right wing tank than in the left perfectly balances me out. (I do have a servo-controlled 1" x 6" trim tab on the left aileron which will also do the trick.)
 
Read the OP

The simplest thing to do is adjust your fuel load to compensate. I weigh 250 and sit in the left seat, so I always have a "heavy left wing". Putting 5 more gallons in the right wing tank than in the left perfectly balances me out. (I do have a servo-controlled 1" x 6" trim tab on the left aileron which will also do the trick.)

RV 12, no wing tanks........
 
Update... This afternoon we went out to plane with a long straight edge to determine if flaperons were installed at correct pivot height with respect to the top wing surface. We ended up adjusting pivot heights to raise the left flaperon and lower the right flaperon in small successive steps. It took six flights but now the plane flies hands-off with no trim tab. Flap deployment is hands-off straight also.

Thanks to all for providing valuable information and for boosting my confidence to alter flaperon pivot heights slightly outside of the spec dimension.

VAF is the best source of information. This flying fraternity is the greatest?
 
Flaperon settings

Go back to basics, under the flap arm there is a mix box and a 3 to 4 " bolt that sets the mix box correctly before drilling the 4 bolts under the baggage floor to set the flaps. Check the build manual for the instruction. Good luck
 
Go back to basics, under the flap arm there is a mix box and a 3 to 4 " bolt that sets the mix box correctly before drilling the 4 bolts under the baggage floor to set the flaps. Check the build manual for the instruction. Good luck
Having just finished doing some work in that area... It is quite possible to have the torque tubes drilled incorrectly, the pushrods different lengths, and still have the plane fly straight hands-off.

We had noticed a little "something" at full right aileron deflection with flaps up. Of course the only time most people normally ever do that in an RV-12 is during pre-flight checks. Anyway, it took is a while to track down the source of the odd feeling/sound we had noticed, but something was obviously not right with the flaperon rigging. The right torque arm was at the wrong angle, rotated forward (downward) compared to the left. With flaps up and full right aileron, the pushrod would bump against the top of the fuel pump.

I just finished replacing the right torque tube a couple days ago. We set everything up per plans, drilled the new torque tube and set everything dead to spec (though now I need to go back and check the pivot height -- thanks scsmith and Piper J3 for that!) I was at a loss as to how it could have been done so wrong. What we figured out along the way was apparently the setup was originally done while the flap lever was UP rather than down as specified in the plans.

But -- my long delayed point is, the airplane flew just fine. The stick was centered, ball centered, no heavy wing that wasn't explained by fuel and pilot/passenger loading. And fortunately, the interference was not enough to cause any damage to the pushrod or the fuel pump.
 
I have seen this a couple of times on the 12. First make sure the rod end dimension is correct on the flaperons. If you decide to squeeze the trailing edge it's the opposite the heavy wing that you squeeze. Tourque tubes are the last check. If none of this works a minor half turn to the rod ends of the heavy wing will make changes as it sounds like you found.
 
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Yes, rod end adjustment to raise/lower vertical flaperon position with respect to upper wing surface is very effective. I had a very pronounced heavy wing which I countered by raising flaperon on one wing and lowering flaperon on the other wing.

I don't know the aerodynamic theory but I suspect that changing relative height must change induced drag. Maybe just magic, I've been flying for more than 50 years and I still marvel at how airplanes fly through the air...
 
Hey Jim .... Just another thought that may work well for you so you don't need to fly with the airplane rigged slightly off to compensate for your heavy wing.

When I first flew my RV-12 I had a heavy left wing and tried to tweak the flaperons a tad to make things better .. but before getting draconian I ran across a post here in the forums that solved my issue.

Long story short, the aluminum that overhangs the rear spar by 2" or so was not flat with the skin on the wing. Some places were high and some places were low. I lowered the flapperons and using a thin metal 12" ruler on edge made sure the skin extending aft of the rear spar was totally even with the skin on the wing forward of the rear spar. A thin block of wood was used to tweak the aluminum up or down as necessary. After tweaking both wings the RV-12 now flies straight. The amount the skin was actually off was not that much seemingly ... but guess the airflow over the flapperons was affected just enough to make a difference.

Not saying that is your issue, but it sure is an easy fix if it is.

Happy flying,
 
John

Yes, I knew about the cantilever top skin behind the rear spar and already tried that. What I didn't know about, until this thread, was the effect of raising/lowering the flaperon pivots. Worked a charm...
 
Yes, rod end adjustment to raise/lower vertical flaperon position with respect to upper wing surface is very effective. I had a very pronounced heavy wing which I countered by raising flaperon on one wing and lowering flaperon on the other wing.

I don't know the aerodynamic theory but I suspect that changing relative height must change induced drag. Maybe just magic, I've been flying for more than 50 years and I still marvel at how airplanes fly through the air...

What causes the perception of a heavy wing is an asymmetry in the natural float position of the ailerons. When you fly hands-off, the ailerons seek a compromise position between where the two ailerons separately would like to float to, a position where the hinge moments (pushrod forces) are balanced. But that position may not be aerodynamically balanced in terms of the whole wing. You can end up with the natural float position such that one aileron is slightly trailing-edge up compared to the other one, making that wing seem heavy.

When you raise the aileron hinge axis slightly, the raising the aileron surface with respect to the wing skin at the hinge gap, you create slightly more suction on the upper surface of that aileron as the flow goes over the slight bump or curve in transitioning from the wing to the aileron. That slight increase in suction causes that aileron to want to float a little bit more trailing-edge high. The two ailerons together find a new natural float position with that aileron slightly higher. With that new balance position, that wing gets a little bit 'heavier'. So you want to raise the hinge axis slightly on the "light wing" side.
 
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