I got a Li-Poly 2 cel 640 pack from Dave Thacker a while back, are these going to be too much for the Curtiss Junior?

From : Don Stackhouse

It wouldn't be my first choice. My first reaction was "No Way!", but then I thought:"The Junior does carry weight surprisingly well, maybe I should try it before passing judgement."

Just to be sure, I went out in the back yard just now and did a test flight. I don't have any Kokam 560 cells, but I cannibalized a 2-cell 700 mah E-tec pack (weight 1.2 ounces) out of one of the Ryan ST prototypes. I'm not sure what the exact weight of the Kokam 560 cells are (although the 700 E-tecs should be in the same ballpark) and the E-tecs do have a higher max current rating than the Kokam cells, but for the little MPS-1A in the Junior the difference should be fairly minor.

I pulled the 250 mah E-tec pack off the nose of the Junior and taped the 700 pack on as far aft as I could get it without re-mounting a bunch of other stuff like the receiver. It was still noseheavy, but the daylight was fading fast, so I decided to try it that way anyway.

I tried to ROG, but it nosed over as soon as I opened the throttle, even with full back stick. With the correct C/G it should work.

I gave it full "up" trim and tried a hand launch in the 60' x 120' space in my back yard that the surrounding 10' high corn has boxed me into. With full throttle and about 1/4"-3/8" of additional back stick it climbed out, not as well as it does with the little 250 pack, but adequately. Turning radius during climb was also not as tight as with the lighter battery, but I didn't have any significant trouble keeping it over the grass (I've lost airplanes in corn before, and it's NOT pretty - I avoid flying over it if at all possible). In about 30 seconds it was at about 100' and I throttled back.

With the little 250 pack the Junior will cruise comfortably at about 1/4 to 1/3 throttle. With the big 700 pack I needed more like 1/2 to 5/8 throttle, still not unreasonable. With the correct C/G and a little less trim drag it might do a little better, although I doubt it would be a big factor, the Junior already has a fairly large, efficient horizontal tail on the end of a fairly long moment arm (in fact the main deviation from the scale outline is a tail moment arm 1" longer than scale, to make it handle better for beginners).

Glide was a little steeper due to the increased induced drag (the plane was 0.7 ounces heavier than with the little battery, about a 23% increase) and the extra parasite drag from the higher gliding speed, and the landing speed was definitely higher, but otherwise it did OK. I didn't do a timed endurance test because: A. I'm not THAT much of a masochist, and... B. it was nearly dark already anyway.

Altogether not bad, although I like it better with the reduced weight. The flight time in still air with the 250 cells is already something in excess of 1/2 hour (the last time I clocked it I got 32.5 minutes), so it really does not need any more flight time. With the 560 cells (allowing for the higher throttle settings due to the weight increase, and the lower efficiency of the Kokam cells), I'd guess that flight times might be around 45-50 minutes, but at the price of a slightly more sluggish plane.

There would also theoretically be a reduction in motor brush life due to the higher throttle settings, but that should not be a significant issue. We have other airplanes in the series that cruise at higher throttle settings and still get excellent brush life.

My recommendation is that if the price of another battery is a show-stopper for you, yes, it will fly with what you have. However, it will fly significantly better with the 250 2-cell pack.

Don Stackhouse
DJ Aerotech