Factoids: PA18 with a new wide deck 0320 (170+hp) & 41" Borer on EDO floats. After rebuild the tail is a bit too light.

My horiz.stab (trimmed to cruise) is a fair bit below center & I think that’s causing more drag, (pulling the tail down) right? A light tail is good for hauling loads, but 95% of my flying is just me, full tanks & permanent baggage. I made a 4.5# lead weight to bolt onto the tailwheel bolts & went up (without it) and marked speeds @ each 100 rpm diff.

Then I hung the 4.5# on the tail & it made NO diff. in speeds. Damn. Trim is a bit higher, but still below center. I wonder about the droopy tips, ya think they would make the AOA higher? I’ve heard that they cause more drag, & I think higher AOA in that they’d trap more air under the back of the wing @ the tip. I dunno, but my tail is trying to dive.

Fuel burn goes up about .5 gph for each 100 rpm, except from 2200-2300 & from 2600-2700 it jumps about 1 gph.
Speed goes up about 5 mph for each 100 rpm. 2200 rpm = 80 mph, 2800 rpm = 110
2200, 80 mph it burns 5.6 (US) gph. 2500, 95 mph it burns just over 7. 2600, 100 mph it burns over 8.
What I didn’t do was adjust the mixture each time but I don’t think that would change it much @ a fairly constant altitude. A bit I guess.
Anything you can think I could do/try or just leave it & be happy?

Is the actual stabilizer itself (when trimmed) not centered, or is it just the indicator that is below center? I might trim to cruise and then land without making adjustments and get out and look where the stabilizer is positioned.

Did the theoretical CG of the plane change with the engine mods and/or floats? Does the math match the results your seeing in flight?

Nimpo,

Are you running out of elevator when you flare to land? Are you HAVING to carry some power to flare?

If not, stop worrying about it and go fly. Why are you worried about where the trim is anyway?

MTV

Cub271, Yep the stab. position & the indicator were centered during the rebuild. The stab. position is about 2/3 of the way down the jackscrew when trimmed for level flight. I never change the trim unless I change the load.
The whole CG changed when I rebuilt (natch) but when I did the math pre-changes it looked like it was gonna be good. My CG is about 16" behind the LE, which is right in the middle of what's "allowed".
I put in a different engine, light starter & alternator, inboarded the flaps, & replaced the heavy wet battery with an Odessey under the front seat. That made the biggest diff, but still good.

Mike, I have lots of elevator! If I had ANY control issues, I'd have been whining long ago! It stalls on touchdown just fine. I've just been thinking that if the tail wasn't trying to dive it would be a lot more efficient. NOT looking for speed (it's a Cub!) looking for efficiency. I think that my CG is not in line with my CL (lift). I was JUST under gross last summer on the trip to Whitehorse, & it was balanced & performed great when heavy.
I'm glad I didn't pursue aeronautics, this stuff makes me dizzy thinking about it!

And you noticed the change after your rebuild, correct? You weren't tail light before?

You might be right about the CL, and a light tail is generally going to be a CG forward of the CL. BTW, what's the weight difference between the new engine / prop combo and the old?

is it only on floats that you have to use a lot more up trim?

NimpoCub wrote:Mike, I have lots of elevator! If I had ANY control issues, I'd have been whining long ago! It stalls on touchdown just fine. I've just been thinking that if the tail wasn't trying to dive it would be a lot more efficient. NOT looking for speed (it's a Cub!) looking for efficiency. I think that my CG is not in line with my CL (lift). I was JUST under gross last summer on the trip to Whitehorse, & it was balanced & performed great when heavy.
I'm glad I didn't pursue aeronautics, this stuff makes me dizzy thinking about it!

Sounds like it's right about where it should be. Stop whining and go fly it

MTV

Stop whining? Then what? Ask Rolly or Nick, they'll have an opinion for sure. Have the Chilcotin hummingbirds shown up yet??