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Rudder Position/Strobe

L

lllewis45

Member
My (I didn't build it), RV 7A has Aero Flash wing tip strobes mounted recessed in the wing tip fairing, under plexi-glass and so aren't visable from directly behind the airplane. My rudder has only the Aero Flash white position light (PN 153-0011). I would like to replace the rudder position light with the Aero Flash combination Position/ Strobe unit (PN 153-0011-12) to make it legal for night flight ( I have no tail anti-collision light). They fit in the same hole. My question is: The position light weighs 2.5 ounces and the Combination Pos. light/ Strobe weighs 3.5 ounces, Will I need to remove and re-balance the rudder? Or is the rudder typically balance in the 7?

Bernie
 
I doubt you will notice any difference with the small amount of add'l weight. If it were my plane, I'd just mount the new light and fly it.
 
There's nothing anywhere in the build sequence about balancing the rudder, nor do I know any RV builders who did such a thing. Balance it how? ... and in what axis? The rudder is cantilevered off the bearings/pivots ... I submit it is "unbalanceable."

(The elevators are a different story, however.)
 
There is a lead weight on the rudder top and yes, it is balanced. Rudder flutter is as real/dangerous as elevator flutter but I don't think that 1 ounce will matter. However, for peace of mind, either call Van's on Monday, or Scott may chime in before that.

Best,
 
Rupester said:
There's nothing anywhere in the build sequence about balancing the rudder, nor do I know any RV builders who did such a thing. Balance it how? ... and in what axis? The rudder is cantilevered off the bearings/pivots ... I submit it is "unbalanceable."

(The elevators are a different story, however.)
Click to expand...

It is true there are no instructions to balance the rudder however it not correct to intimate that rudders cannot be balanced. Your post indicates a dependence on orientation with gravity. While gravity is a factor, it is small compared to inertia and non-gravitational acceleration. The rudder has inertia just like the elevators and ailerons and it can be excited by an assymetrical loading just like the other surfaces.

The OPs question is valid but the scale of the one oz difference is small so I personally would not be too concerned about it.

HOWEVER, Some people like me have used Nova quickstrobes or other single bulb power supplies for the tail. I mounted mine IN THE TAIL, close to the front. It is HEAVY compared to the OPs situation. It is commensurate with the elevator trim servo. I calculated the moment arm and added the appropriate weight in the stock rudder counterweight. Even doing that, there is a risk because making something heavier, even balanced, reduces the natural frequency which is a part of the flutter physics.
 
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balance rudder?

grayforge said:
Balance the rudder? Are we supposed to do this?

Maybe I'm not to this point yet.
Click to expand...

+1
How does one balance the rudder?
Seems like it should be done after painting as the elevator.
Or is the balance weight intended to counter the size of the control surface?
 
lllewis45 said:
My (I didn't build it), RV 7A has Aero Flash wing tip strobes mounted recessed in the wing tip fairing, under plexi-glass and so aren't visable from directly behind the airplane. My rudder has only the Aero Flash white position light (PN 153-0011). I would like to replace the rudder position light with the Aero Flash combination Position/ Strobe unit (PN 153-0011-12) to make it legal for night flight ( I have no tail anti-collision light). They fit in the same hole. My question is: The position light weighs 2.5 ounces and the Combination Pos. light/ Strobe weighs 3.5 ounces, Will I need to remove and re-balance the rudder? Or is the rudder typically balance in the 7?

Bernie
Click to expand...

Plug and Play... then fly on. There are many RV's flying with NO rudder counter weights. :)
 
Rudder is only partly balanced ...

A few year's ago, I had a talk with Vans Help Line, they told that the rudder is only "part-balanced". This to move the fludder speed past Vne and create a good margin.

There is no information in the instruction of how much this part balance should be. Just build normal and paint normal.

A well documented crash show, that a rudder was starting to fludder with catastropic result. This was way beyond Vne, at about 250kt. The investigation report show also huge amount of paint/filler/putty.

So if you respect the designers Vne, paint normal, all is fine. Otherwise you should contact a fludder examiner. In germany you must test/calculate this.

By the way, early RV models did not have counterbalance at all build in the rudder, RV 3 and/or 4 (unsure)
 
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At least with the -10, the final package from Vans does include control surface balance numbers.
This is unfortunate terminology; these surfaces are not in balance, but rather, are within specified moments.
For the rudder, you have to build a horizontal jig to mount the rudder to, by its bearings. You then measure the moment (trailing edge heavy) by attaching a fish scale and measuring the force it takes to hold up the trailing edge, and multiply by the distance from the edge where you measure to the pivot point, and multiply the two together. Vans specifies the limits this moment must fall within.
 
Interesting... Is there something like this specified for the 7/7A or is it assumed that it'll be within spec after a normal build?
 
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