I've got an HP 21 and one thing you'll have to watch out for is the fact that they're *heavy* -- over 9 oz from memory.

They're also pretty weak.

Mine is presently in a small Piper cub that is desiged for .10 to .15 two-strokers. It turned out *very* nose-heavy with the HP in it so I had to put a 4-cell AA-sized battery pack as far towards the tail as I could get it (and the CG is still a little too far forward for comfort). It also flies *very* scale-like -- which means that it has only just got enough power to climb gently at full throttle.

These are *very* sweet motors but they are incredibly expensive, as heavy as a plain-bearing .40 2-stroker and put out less power than a good 0.10 2-stroker.

You'll love the noise (there's almost none) but you probably won't be impressed with the effect of the extra weight on your glider.