Super Cub: Trim & AFT CG

I have a friend who purchased a certified Super Cub last year. Put a Firmin pod on. His SC does NOT have the up-gross kit.

He reports that with full fuel, min clothes, light passenger, camping gear, … , he is at or slightly over gross. Moreover, loaded, he is within a tenth of an inch of AFT C-of-G.

He reports his trim indicator is close to the middle. I asked him to confirm stab position matches trim indicator.

My question: I would think that when at or close to aft CG, trim would be at or close to full nose down. Why does this not seem to be the case?

My guess is that his weight and balance certificate or his calculations are in error. Not everyone preparing these documents, or using them correctly determines which Datum to use: Leading edge of wing, or some distance forward of LE. I'd bet that part of his W/B certificate, or his calculations, is where the error is. In other words, his mechanic used 60 inches fwd of LE for datum, but he's using the Leading Edge for datum for calculations.

You are absolutely correct: At a true far aft CG, that trim should be far nose down.

MTV

Also determine if the plane was level when weighed, and where the rear scale was placed relative to the datum. That can influence the arm length for the tail and resulting CG est. I think.

Gary

Trim is speed dependent. You indicate trim is neutral, but don’t say at what speed. Keep in mind the farther aft the CG is, the less the trim will do as you are approaching a condition of pitch stability where the trim is less effective (as cg moves aft stability reduces to the point where you can’t trim for speed).


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I haven't pulled out any paperwork yet and have a glass of whine in hand but I got to thinking. My cub with me, full fuel gear, and a fat guy in the back seat. Puts me at about 1,800 lbs. I still have nose down trim left, I have a 2,000 gw kit but it really has nothing to do with the trim. PA18 a had the same trim and was good to 2,070. So I don't think he should be close to full nose down with only 1750 lbs on board. I have only run out of trim a few times and I was very heavy with a lot of weight behind the rear seat. One thing to consider is his indicator may not be correct as Ted suggested. Tell him to figure out how many total turns he has then count his turns to determine the true elevator position. I would also recheck the math as MTV suggested. DENNY

I ran into this on an 18, be sure the jack screw yoke is installed properly and not upside down

A good review of Cub W&B: https://www.supercub.org/forum/showthread.php?43118-Super-Cub-Weight-and-Balance-Report

Gary