Thrust
I would have never guessed I was only getting 4 lb thrust with a 9X10 turning 16K, felt like way more than that.
I figure for a 190 mph + airplane you will need a 10 pitch prop to unload to around 20K or an 8 pitch to 25K. The best I ever saw with 10 pitch on the Jett FIRE 50 was 16.5 peak on the ground, and so far the best I have done with the 8 pitch was 20.6.
The 9X10 seemed to have a lot more pull with those big sport blades. To get the 20.6K on the 8 pitch I've had to cut the NN prop way down to around 7.5 inches diameter and thrust has really suffered.
When the engine was new it certainly seemed to unload the 3.5K in the air with the 9X10 but something happened where I couldn't get it to unload any more.
Turning the 8 pitch now, judging from the enormous increase in pitch (sound) it sure sounds like is unloading at least 3 or 4 thousand in the air.
Considering that I've had to cut a 8.75 X 8NN all the way down to 7.5 inches I'm not sure if you can turn a sport APC 9X8 at enough rpm with a Jett 50 to get close to 200.
It really sounds like your Nelson is doing what my Jett was doing, failing to unload. If you are turning 21.3 on the ground I would think the Nelson would unload several thousand minimum and put you into the 200 mph zone.
Now here's something to think about: a buddy of mine, (the legendary and incomparable Jr. RC Psycho) has a Dust with a YS 45 turning a 9X8 that is brutally fast. It may just be a more powerful engine than the Jett 50.