7KCAB elevator position in level flight?

C

cspencer

New member
In level flight, the elevator of my 1976 7KCAB is in a markedly nose down position. Placing the elevator level results in a 20 degree climb.
I was advised by an A&P not to fly the airplane until it is properly rigged so I wrote to American Champion to get the fix. They say this configuration is normal since the 7KCAB was derived from the 7AC but has a heavier engine and thus the elevator flies in this position. They say that the 7AC is the only model that has an elevator that flies straight. My concern is, that if the above is true, why would the elevator fly in a nose down position? I would expect it to fly in a nose UP position if the weight is forward. Even so, I would think that the elevator should be level unless there was a severe CG problem. American Champion has so far ignored my second question. Perhaps I am missing something very elementary.
Does anyone have the straight story on 7KCAB elevator position?

Thanks and regards
Crspin
 
Crspin-

Amazing that the manufacturer told you it was because of the heavier engine, your reasoning is correct that a heavier engine would require nose up elevator.

The actual cause is that the stabilizer angle is the same as the 7AC which cruised at 80-85 MPH and your 7KCAB cruises at 120+MPH, if you streamline the stabilzer/elevator you will find yourself stabilized at 7AC cruise speed, if the power remains at cruise you will be in a climb, if you reduce power it will be possible to cruie with the tail streamlined at 7AC cruise.

Tom-
 
Tom,

Thanks, that makes much more sense!
It would seem that there would be a large penalty in drag with the elevator positioned as it is in level flight, but I understand that the added horsepower overcomes it as best it can. Is the stabilizer position corrected in later ACA offerings or is it type certificate issue?

Crispin
 
Crspin-

My personal guess is that 5-10 MPH is lost due to drag from the unstreamlined elevator/stabilzer.

All 7 AND 8 series aircraft are built in the same jig the 7AC's, and the 11AC aft fuselages, were built in. So changing the stabilizer incidence would require engineering, flight test to get the change approved, and the jigs would have to be changed, this is a big, expensive, project. While the jig used is the same one Aeronca used for the first 7AC both it and the, size, and placement of some structural members has been changed over the years.

Tom-
 
Cspencer:

We have the same situation with our '71, 7KCAB and have always wondered what is going on. The airplane would obviously fly cleaner with the stablilizer and elevator evened up a bit. Champions rational about weight does not make sense because we get the same thing with one or two people in the plane with the guy in the back weighing up to 220.

CH
 
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