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I was at a big conference for the first three days last week, and was on the road for the last two days, so that ate into my building time.

I temporarily mounted the elevators to check whether the elevator tips matched up with the horizontal stab tips. One is pretty much perfect, and the other elevator tip sticks out ever so slightly past the stab tip, but you can't see it unless you take a straight edge and hold it against them. So I am going to leave it as-is. I put some filler on the rudder tip to make it match the vertical stab tip.

I dimpled the rivet holes on the elevators, and started countersinking the holes in the tips. Then I decided to install the elevator counterweights, as they must be installed before the tips are riveted in place. And the counterweights are too heavy as delivered, so some of the lead must be removed so the elevators balance properly. I needed to shorten the trim tab pushrod and install it, so all the components were in place when I balanced the elevators.



I removed a bunch of lead from the aft end of the outboard counterweight (I'm not done yet, as it is still not quite balance).



I drilled a couple of small holes in the inboard counterweight. I'll paint the aircraft after I am flying, and that will move the CG of the control surfaces to the rear. I'll mix up some lead shot and epoxy and pour it in the holes to restore the balance.

Why must the elevators be balanced you ask? To prevent flutter. Read on for an over-simplified explanation of flutter



Flutter - Imagine an RV elevator without balance weights. Now imagine that you are flying along at high speed, and some kind of disturbance causes the HS tips to deflect downwards a tiny bit. The hinge line of the elevator is moved downward when the HS moves downwards. The CG of the elevator is aft of the hinge line, so its inertia will cause it to deflect upwards when the hinge line moves downwards. The upwards movement of the elevator puts a download on the HS, which causes it to deflect even further down. Eventually the stiffness of the HS structure will cause it to bounce back towards the neutral position, but it will overshoot neutral and deflect upwards a bit. Now the elevator will deflect downwards due to inertia, putting an up load on the HS, causing it to deflect even further upwards. Eventually the structural stiffness will cause the HS to move back the other way.

We will have one or more cycles of this motion. If we are slower than the flutter speed, the various motions will be damped, and everything is OK. If we are flying faster than the flutter speed, the amplitude will get higher every cycle until we have structural failure.

There are several things in play here: the structural flexing of the HS, the motion of the elevator, and the aerodynamic forces (and damping) caused by the movements of the elevator.

Each of these things has its own natural frequency, the natural frequency of the aerodynamic stuff will vary with airspeed. If the natural frequencies of the various things are far enough apart the various motions will probably not couple together, and no flutter should occur.

If the natural frequencies are similar, then the various motions can couple together, and the amplitude will increase on each cycle until failure of the VS or rudder occurs. This can happen very quickly, in a matter of seconds. This is flutter (or at least one type of it).

Generally, things are OK at some low enough speed (although if the design was poor enough this low speed might be less than take-off speed - I saw a video of flutter during a take -off roll of an ultralight). As you increase speed the natural frequencies start coming closer together, and at some speed you have a flutter problem.

Now, if we put a sufficiently heavy counter weight ahead of the hinge line, the center of gravity (CG) of the surface will be on or ahead of the hinge line. When we flex our HS downwards, the elevator will not deflect (if the elevator CG is on the hinge line) or deflect trailing edge down (if the elevator CG is ahead of the hinge line). In either case the motion of the elevator does not amplify the motion of the VS, and we have removed one of the factors which gives flutter.

The above description only covers one type of flutter - that of a fixed surface with a hinged surface behind it. Flutter can also occur on surfaces that don't have hinged portions - the whole surface twists, which changes the aerodynamic forces on it, which causes it to deflect, etc.

CAUTION - just because we have elevator and rudder counter weights it does not guarantee we are free from flutter at any airspeed. Flutter is a bit of a black science. It is not perfectly understood - Boeing lost half of a vertical tail during flutter testing of a modified Navy 707 about 12 years ago. They did a bunch more analysis and ground testing, went back up and lost half the tail again. If Boeing can't predict this stuff, we should be cautious until we have demonstrated via flight test that our aircraft is free from flutter (especially if we have modified something).

For more info on flutter, and flutter flight testing, see NASA Technical Memorandum 4720, A Historical Overview of Flight Flutter Testing and FAA AC 90-89A "Amateur-Built Aircraft and Ultralight Flight Testing Handbook". Also have a look at Flight Testing Homebuilt Aircraft", by Vaughan Askue, available from the Builder's Bookstore.

Flutter is a very dangerous event. The aircraft could shed control surfaces before you have a chance to slow down. But, a properly built RV, flown within Van's recommended envelope, should be free from flutter.

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