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Elevator Fit Woes

S

skelrad

Well Known Member
Friend
I am at the stage of fitting my horizontal stab and elevators to the fuselage. I had already drilled the elevator control horns to each other by clamping the counterbalance arms to the horizontal stab to align everything. That all went fine. The problem is when I put it on the fuselage and bolted it to the pushrod, it became very obvious that even though the counterbalance arms are both still lined up with the horizontal stab, the elevator trailing edges are not even with each other (the left trailing edge looks like it sets about 1/2" lower than the right). I know a lot of people have misaligned trailing edges, but I'm not sure what is too much and will show up in flight. I'm trying to figure out if I should just leave it as-is, rebuild the entire left elevator, drill out some rivets and reset existing parts (although not sure that would help), etc. The right elevator is perfectly flat and straight. The left is harder to judge with the big cutout for the trim tab.

I'm not sure if it's related, but the right counterbalance arm sits 1/8" away from the horizontal stab and is nice and even. The left counterbalance arm is closer to 1/4" and not perfectly even with the tip of the horizontal stab.
 
I can't offer you any advice but I do sympathize with your plight as I had some similar happen to me with my RV-14. When I got to the place in the plans where you clamp the counterbalance arms to the HS before drilling the elevator horns, I looked at the trailing edges of the elevators and noticed one was about 1/2" lower than the other. It seemed more important to me that the trailing edges were aligned so I clamped them together before drilling the rudder horns. My counterbalance arms are now mismatched. One is 3/16" higher than the HS. The other is 3/16" lower. It REALLY bothers me but I've decided to leave it alone.

Since the plans for the RV-14 do NOT mention checking alignment of the elevator trailing edges before drilling the rudder horns, I can only assume there are a lot of flying airplanes with mis-matched elevators. IOW, we're not alone.
 
If the trailing edges are out of alignment by 1/2" that equates to about 2 degrees of deflection. It will affect setting the travel limits.

I don't understand why builders or the plans call for aligning the counterbalance arms. That is a cosmetic issue, whereas alignment of the trailing edges is an aerodynamic issue.

The fix that I would apply is to take one of the elevators to a TIG welder and weld closed the hole in the control horn and then re-drill it with the trailing edges aligned. While checking the alignment I would also use a digital angle finder at various stations along the elevators to test for possible twist and if there is any then use the mid-point position for drilling.
 
I think this is a by product of the way RVs are built now. Cleco together, rivet, done. Builders aren’t conditioned to think about building things straight because “it can only go together one way- so it must be right”
The mitigation for twist with 9,14,10s with no taper in the elevator is to just do this on a flat surface. This is not foolproof. If you start riveting at one end and rivet in one direction only for example- you will get twist.
With older and tapered surfaces you needed to jig the surface vertically to ensure no twist. It was always front of mind.
I think that there should probably be more guidance to pay attention to possible twist. Especially seeing as it’s early in the build for first time builders.
All that said, I agree there are probably many out there like this and it likely makes little difference in a practical sense. But a straight airplane is a fast airplane.
 
Richard Connell said:
I think that there should probably be more guidance to pay attention to possible twist...
Click to expand...
I think the plans should mention clamping the trailing edges together to check for alignment. This would at least give new builders an opportunity to decide how they want to proceed with drilling the elevator horns. As a new builder, I found it strange that there is no mention of this.

I was really careful not to start riveting at one end and work to other - on any surface. Pretty OCD about it really. I actual add a row of tape to trailing edges with the rivet sequence numbers on it so I make sure to rivet in the sequence I want. I've looked at my elevators every which way with long straight edges and can find no discernable twist anywhere. I don't see anything obvious with my HS but haven't inspected it in detail because I'm not rebuilding it even if I find twist. I just know the problem isn't with my elevators. Either way, it's really demoralizing to have the elevators not perfectly aligned with the HS. The only good news is nobody else will notice it unless they look for it.
 
I wouldn't assume that your elevators are twisted. I built reasonably straight elevators and had the some problem with the trailing edges not aligning when the counterbalances were lined up with the stab. Even the amount of error I saw is about what you're seeing. It made the most sense to me to ensure the trailing edges were aligned and accept that the counterbalances wouldn't be flush with the stab when drilling the horns. I'm not flying yet but I stand by that decision.

If I were in your shoes, I'd consider leaving things as they are instead of rushing to weld up the hole and redrill. It may not be perfect, but you may find in phase 1 that it has zero effect on the handling qualities. When I look at my old cherokee and see the imprecision of a still airworthy airplane, I'm reminded that we're not building the space shuttle but rather a tractor with hand tools in our garages.
 
I heard back from Van's. The response was pretty non-committal about what to do. They said that while the 1/2" difference is not ideal, it will not likely be all that great in flight. Options given were to leave it as-is or weld the horn hole shut and redrill in more of a middle ground location. Obviously, another option is to rebuild that elevator completely. I'm inclined to leave it for now and fly first, then rebuild that side at that point if the flight characteristics are actually noticeably impacted.
 
Another thing to contemplate is just how much it will twist in flight and in what direction. Given that the tab is only on one side it’ll twist that elevator. At cruise - if it turns out you have significant nose down trim (tab up) then that’ll force the TE down on that side. It may well bring it closer into alignment- which may be what the factory is implying by saying it’ll be less in flight.
It could be the other way around though if your LH elevator is low to begin with it’ll make it worse.
There’s been some discussion on RV10s that exhibit elevator twisting with split trim tabs (different direction) when misrigged. So it seems that there’s enough force in the system to move things a fair bit. Which I found surprising - a bit scary even.
 
My tail was destroyed by a BT-13 pilot who taxied into my aircraft while it was sitting on the ramp (I was not in the aircraft) This was rebuilt by a professional airframe shop (other pilot’s insurance )and I flew it home (that is the short story after 6 months) . Later when updating the autopilot it was noted that my elevators were not aligned- during preflight I always just checked each individually, not looking at the alignment of both together. Back to the airframe shop, where they drilled a new hole in one of the elevator horns to align the elevators, although the horns are now not aligned. I cannot discern any difference flying it now compared to when the elevators were not aligned. Now have about 500+ hours on it since realignment of the elevators with the horns remaining unaligned, don’t see why one would need to consider rebuilding the entire elevator.
 
This is a common problem and one that I had myself, the solution is to weld up the hole in one of the elevators, then refit to aircraft, clamp the training edged in line and then re-drill the hole in horns. The elevator tip may be out of alignment but the main elevator will be aligned and fly straight. Two years on my RV9A is still flying straight as an arrow despite the tips being slightly misaligned.
 
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