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Russ Erb's N6786E

On June 7, 2008, Russ Erb wrote:

Bearhawk #164 N6786E "Three Sigma" also took to the air today at 0926 PDT.

This flight, however, was a little more fast paced and potentially exciting than the Mudbug's. Log a 0.1, assuming you can stretch 3 minutes liftoff to touchdown to be a 0.1.

After spending a couple hours troubleshooting a bad EGT probe (not a show

stopper) and a bad plug (show stopper) with 3 aborts (you must be spring loaded to abort, flying only if you can't find a reason to abort), I fed in the throttle slowly concentrating on keeping the nose straight and waiting for the tail to come up as it had in the Citabria. I was doing so well keeping the nose straight and waiting for the tail to come up (it didn't) that I momentarily forgot about controlling roll. Observers claim the wind shifted to a direct right crosswind just as I was taking off, so the right wing started lifting without the left wing. After a "what the heck" and a "oh krap, a heavy left wing" I suddenly remembered that I could move the stick to the right and fix that little problem.

Climbing out, about the time I sorted out my lateral control issue, I had accelerated to 80 knots when the real fun started. I got the big "F" word

-- that's right -- FLUTTER! (There will be a brief pause while bd cleans his screen from the coffee he just spit all over it) That's right--I've got a stick in my hand that is oscillating at about 5 Hz with an amplitude of about +/-1 inch. It's strong enough that I can't stop it by holding the stick. Meanwhile, I'm looking at the other stick grip and thinking "Why the

h*** is it doing that?" Then exercising the time honored axiom of "If you do something and something goes bad, back up one step to what you were doing before". Standard abort procedure for flutter is to pull power and slow down.

I pulled the throttle and slowed down about 10-15 knots and the flutter went away. At this point I'm about 200 feet AGL thinking that I should probably be higher. To climb I need more throttle, but if I push in the throttle, I'll speed up and the flutter will come back. Then I remember that I can add power AND pitch up to control airspeed, so I did that and started a climb to pattern altitude.

One observer saw the flutter and made a radio call, but his radio call was after I had solved the problem. Another observer didn't see the flutter but heard the power reduction and thought the engine had failed.

At this point I knew what the problem was and decided that this would be once around the pattern for a full stop. All of those carefully planned test cards for a gentle buildup to the landing task went figuratively out the window. Now it's a risk elevation and just hope everything works as expected. I put down two notches of flap (the first before checking to see if I was slow enough, but I was) and later remembered the plan was to use three. I don't remember much about the landing other than it bounced once.

My instructor/observer said it looked like a good landing. I guess I fooled him.

I'm saying the flutter problem was brought on by the trim tab pushrod being too flimsy, allowing the tab to easily move up and down, which then drives the elevator up and down, which drives the control stick back and forth, which drives the pilot's excitement level over the limit. My pushrod is built per the plans (#164) from T1 tube (1/4" x 0.035). Before flying again I will replace these with stiffer pushrods. That's also with two full size trim tabs.

Does anyone out there know if the specification for the trim tab push rod was increased to a larger tube in later iterations of the plans? (While proofreading I found on the Mudbug site that this was addressed in an Engineering Change in Jan 2004 directing a change of tube size to 5/16 x

.028 tube. While the EC says it is non-mandatory, I would consider it mandatory now. You don't want the excitement I had.)

Other issues:

Lateral balance: After the buffooned takeoff and flutter excitement, I wasn't paying too much attention to lateral stick forces. However, I do not remember noticing anything odd about the lateral forces, so my guess is that I don't have a heavy wing.

Engine: I need to replace an EGT probe and possibly another spark plug. I also have oil appearing from unknown locations around the exhaust pipes for cylinders 3 and 4. Not sure why--needs more investigating.

I figured out that the nose didn't come up like the Citabria because the Citabria's elevator is not balanced, so it flops to the down position, which helps the nose come up. The Bearhawk elevator is balanced, so it tends to the neutral position. Next time I'll think more about lifting the tail.

On a poetic note:

Besides being the first (and possibly last for a long time) time that two Bearhawks, especially plans built Bearhawks, have taken to the air for the first time on the same day, there was another interesting twist. Some of you know that my N-number (N6786E) was derived from my wedding date (7 June 1986). Well, look at the date on this message. Yes, Three Sigma took to the air on the 22nd anniversary of the date that it was intended to honor.

Didn't plan it that way. Just worked out that way. Also, I'll always remember the date of the Miss'ippi Mudbug first flight.

It might be a few days before I get to flying again. Besides making new parts and parts to order, I've got anniversary duties tonight and an EAA Chapter Newsletter to get out this weekend.

Meanwhile Eric needs to get out an fly so that he'll be able to join me at OSH.

Russ Erb

Bearhawk #164 "Three Sigma", Rosamond CA Bearhawk Reference CD http://bhcd.erbman.org

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