Electric vs Glow props
 

Yesterday I mounted and tested a Master Airscrew glow 9x6 prop on my Tower RxR Corsair with the supplied 1000 kV motor and it was turning a little over 9,000 rpms with a fully charged 3S 2200 mah 30C. The recommended Great Planes electric 9x4.5 turned about 8,600 rpms. Heavier prop, more aggressive pitch and it was faster? I didn't fly the MAS prop, just tested it but it felt like it had more thrust. Can I fly that prop? Thank you for the advice and tips.

Bill in Pa.

Bill
"but it felt like it had more thrust"
is not exactly a scientific assessment in the same class as your rpm measurement.
I would not be surprised if a 9x6 glow prop turned a bit faster than an true electric as it is likely to be less efficient so will absorb a bit less power and thus create less thrust but it might well make more noise doing it though. ;)

Don't make any assumptions about props.

Judging static thrust by hand is a double dribble. To begin with, static thrust is measuring something that isn't really important overall. And then, anything done by hand is about like licking your finger, sticking it up and guessing how fast the wind is. It's maybe good for judging wind direction but not much else.

Either prop could be so far out of it's envelope on the ground that it's thrust is off. What it does in the air is what matters.

And don't put much faith in the mfg's pitch numbers. They are often just a guess. Props have varying pitch from root to tip. They also have varying airfoils from root to tip. In fact, what we think of as pitch is really the AOI (angle of incidence) and not actually AOA (angle of attack) which is what most think pitch is. And it's actually not that simple.

Using a tach gives you some info, but it's still not anywhere enough. Flying the two, one right after the other, will give you way better "info", and it's not numbers at all, is it.

If you were to measure those two with a prop pitch gauge, which was once a popular tool of pylon racers many years ago) it would be clear just how complex pitch is. And why the number printed on the prop is unreliable. I used to record prop gauge measurements of commercial props to learn what worked. It was astounding. I only do it now to see how uniform the blades are to each other. They seldom are. But it tells me if the mfg is improving their mass production. There aren't any magic props, nor any really reliable mfgs.

Funniest thing is that about all most modelers judge when comparing props is speed in level flight. And when was the last time you saw another modeler actually comparing props? And most humans can't judge speed at all. Keep in mind if you try, you're actually seeing ground speed, not airspeed. So make passes both ways. And since wind speed isn't very steady, back to back comparison makes a lot of sense.

BTW, you saw that the prop with the "slower" pitch also gave "slower" rpm. The mfg's advertised pitch is what we use to guess how fast the prop will pull our planes. They guess how fast it should be and slap a number on it. They might even test them some way. From what you noticed with the tach, would it seem the 8,600rpm prop would probably be slower than the 9k prop?

Heck, that slower prop just might be the faster one with twisted blades that are stalled more. It isn't of course. (or is it ;)

There are a couple of differences that can be noticeable. A glow prop is usually a bit heavier than the equivalent electric prop as it need a bit more strength to withstand the input pulses of power (each engine stroke stresses the prop a great deal). Also, if you do prop strike with a glow prop on an electric, good chance of bending the prop shaft where, if using the lighter electric prop, you would only have broken the prop. I often use a glow prop on my electrics with no noticeable problems. Yes, I've bent a few motor shafts too.