Elevator counterweights

[Tom Martin]

On other aircraft that I have built each elevator is balanced independently with a slightly forward bias that should help to account for paint.
With the RV14 the instructions simply call for the weights to be cut and fixed to the elevators.
This puts the right elevator in what I would call the correct balance, slightly forward.
The same weights on the left elevator do not produce enough weight to balance the heavier trim tab and trim servo.
Have others added extra weight, I think that they should be balanced individually as other models are.

[DanH]

Originally Posted by Tom Martin

I think that they should be balanced individually as other models are.

Tom, I agree, but can't claim it as fact. I've simply settled on a belief that a builder can't go wrong with 100% balance, done as individual panels, after paint.

Anyone care to comment on that belief?

[Tom Martin]

With the weights on both elevators, and the elevators tied together, the "elevator system" appears to be balanced correctly. And thus it appears that the Van's engineers have decided that it is the total balance of the two elevators that is required rather then balancing each individual.
Set alone the left elevator trailing edge ends up full down, while the right one is full up. There would not be a great deal of weight added or removed from either to fully balance them.
This is another example of how the engineers have simplified the construction of the RV14. I guess it bothers me a bit because the previous models have always emphasized individual balance. It could be that was done so that gross differences in weights between the surfaces would not be allowed. With the RV14 I guess they feel a designed small difference between the surfaces is an acceptable trade off for a more uniform fleet and ease of construction. In theory if you cut the weights according to plans elevators could be swapped on different RV14s directly with no adjustments!

[DanH]

Originally Posted by Tom Martin

With the weights on both elevators, and the elevators tied together, the "elevator system" appears to be balanced correctly. And thus it appears that the Van's engineers have decided that it is the total balance of the two elevators that is required rather then balancing each individual.

The pitfall to balancing elevators as an assembly is limited torsional stiffness.

Imagine an extreme example: balance a pair of elevators by placing all the counterweight mass in one tip, and none in the other. This experimental pair of elevators would have static balance equal to another pair with one half the CW mass in each tip. However, with all the mass on one end, dynamic force applied at the other end would twist the connecting spar and tube assembly, possibly so much that the counterweight mass would remain stationary as the light end fluttered. Doubling the length between mass and force makes the assembly half as stiff.

Obviously the effect is lessened as the balance weights become more equal. It's entirely possible that all RV elevator assemblies exhibit more than adequate flutter margin with one elevator overbalanced and the other under-balanced, as Tom describes above. However, without an "Amen" from somebody who actually knows that margin, I'd argue that balancing as individual panels is the safe bet.

[JHartline]

I don't have anything to add to the discussion but I'm hoping someone from Vans might comment on this. It's very timely for me since I'm currently hanging elevators and it would be simple to adjust counterweights at this stage. I'll also try calling and report back if I find out anything.

[Logan]

Hi, all,

After seeing this thread, and having just completed the empennage kit for a -14A, I asked Van's. Their response was:

The rudder, ailerons and flaps are item that you simply built as the plans show. Elevators on the other hand are different in that they do need to be balanced to hang in trail with the horizontal stabilizer. At this point the engineers have not created a document specific to the RV-14 (not guaranteeing they will) so we treat this the same as you would an RV-9. Balancing needs to be done to each elevator without control push rods attached and separated from the opposite elevator.

So, looks like we can go ahead and balance them.

Regards,
Logan

[Tom Martin]

Logan
Thank you for clarifying that.

[JHartline]

I got pretty much the same, very prompt, reply from Vans. They said to balance the elevators individually - after paint they should be neutral balance.

[cczarnik]

Even though I'm a 7, I'm at the same stage and found the servo side heaver than the counter weight. To balance individually I'm going to need to add.

Question is how ya'll accomplish that? Right now my plan is to glass in some lead shot when doing the tips on the left side. Unless somebody has a better approach?

[czechsix]

Originally Posted by cczarnik

Even though I'm a 7, I'm at the same stage and found the servo side heaver than the counter weight. To balance individually I'm going to need to add.

Question is how ya'll accomplish that? Right now my plan is to glass in some lead shot when doing the tips on the left side. Unless somebody has a better approach?

I had this same issue on my RV-8A years ago. I pop riveted an aluminum z-bracket about 1.5 inchs aft of the lead counterweight (with the top flange of the Z pointing forward toward the weight), then mixed some lead shot with epoxy and filled in the space between the z-bracket and the lead counterweight. I put a thin layer of epoxy/micro slurry over the top to smooth it out. After curing I touched up with sandpaper and after painting it looked pretty good. The z-bracket has helped hold it in place and it has never shown any signs of coming loose.

It's a good idea to make sure the elevators are a bit overbalanced before painting. Too much is better than not enough, because after painting it's easier to drill out a bit of lead than add more.