Real world RPM

[rdb127]

""1.27 prop constant to be good""

So what do you plug in for the pitch on the A?

[soarrich]

	Quote: Originally Posted by rdb127
	""1.27 prop constant to be good"" So what do you plug in for the pitch on the A?

I used 8 pitch. Sorry I should have said that.

[BTerry]

	Quote: Originally Posted by soarrich
	BT Can you explain that to some one that was socially promoted out of high school like me? Like a formula for figuring it out. I've always known that extra 200 rpm was hard to get, but a formula would be appreciated. Pe Reivers, on his website, has a Excel spreadsheet that works well, but I'd like to see the math.

I don't know what formula Pe Reivers uses, but the following formula is fairly accurate for power calculations (which is easier to determine with electric motors).
There is a commonly used formula by Bob Boucher in his book "The Electric Motor Handbook". It is:

Power = k * P * D^4 * N^3 where:

Power is in Watts
P = prop pitch in feet
D = prop diameter in feet
N = RPM in thousands
K = Propellor coefficient, which depends on the propeller type. The K value from Pe's worksheet works well.

Basically, if you double the diameter you will increase the power required by the 4th power of the increase, or 2^4, or 16 times as much power. If you double the rpms you will increase the power required by the 3rd power of the increase, or 2^3.

Say we have a prop turning at 5000 rpm and absorbing 1000 watts (1.34 hp). If we double the rpm to 10k, the total power will increase by a factor of 2^3 to 8000, or 8 times as much power required to double the rpm.

Plugging this info into Pe's spreadsheet, using a 20x10 Mejzlik Standard prop (K = 1.18) bears this out.

Of course this formula doesn't account for density altitude, but the general principle is sound.

Now for Dick's example, if we increase rpm by 200 on a tiny glow engine is a smaller percentage of the total RPM, so the increase is relatively smaller say 14,000 to 14,200, 200/14000 = 1.4% increase in rpm which yields a 4% increase in power from 1.02 to 1.06 hp).

On a 150cc engine spinning a 32x10 prop at 5800 rpm, a 200 rpm increase in RPM is a greater percentage of the total (200/5800 = 3.4% increase in rpm which yields a 10.7% increase in power from 13.5 to 14.9 hp).

Does this help at all, or is it clear as mud?

Brett

[dick hanson]

	Quote: Originally Posted by BTerry
	I don't know what formula Pe Reivers uses, but the following formula is fairly accurate for power calculations (which is easier to determine with electric motors). There is a commonly used formula by Bob Boucher in his book "The Electric Motor Handbook". It is: Power = k * P * D^4 * N^3 where: Power is in Watts P = prop pitch in feet D = prop diameter in feet N = RPM in thousands K = Propellor coefficient, which depends on the propeller type. The K value from Pe's worksheet works well. Basically, if you double the diameter you will increase the power required by the 4th power of the increase, or 2^4, or 16 times as much power. If you double the rpms you will increase the power required by the 3rd power of the increase, or 2^3. Say we have a prop turning at 5000 rpm and absorbing 1000 watts (1.34 hp). If we double the rpm to 10k, the total power will increase by a factor of 2^3 to 8000, or 8 times as much power required to double the rpm. Plugging this info into Pe's spreadsheet, using a 20x10 Mejzlik Standard prop (K = 1.18) bears this out. Of course this formula doesn't account for density altitude, but the general principle is sound. Now for Dick's example, if we increase rpm by 200 on a tiny glow engine is a smaller percentage of the total RPM, so the increase is relatively smaller say 14,000 to 14,200, 200/14000 = 1.4% increase in rpm which yields a 4% increase in power from 1.02 to 1.06 hp). On a 150cc engine spinning a 32x10 prop at 5800 rpm, a 200 rpm increase in RPM is a greater percentage of the total (200/5800 = 3.4% increase in rpm which yields a 10.7% increase in power from 13.5 to 14.9 hp). Does this help at all, or is it clear as mud? Brett

clear enough for our purposes!

My little electric thrust stand was a "short course" setup on power /thrust etc..
hee is a pic (tho you have seen it)
I changed props and noted thrust (read on gram scale) to watts consumed (read on Whattmeter .)
Onthistype setup --watts directly correlated (college word) with thrust . The part which really was interesting was the prop correlation.
It did not matter what the prop looked like - -- If it produced Xamount of thrust -- the watts were the same - 2 blades -4 blades wide blade narrow blade did not matter
this flys in the face of what some assume is prop knowledge - but it is whut it is .

[sprice50]

Bear in mind this is all static testing. Friend of my ran and on board current logger and the current actually increased during forward flight when the prop wasn't stalled.

[rcign]

Zenoah G62..32-10 3W prop...3800 rpm....Torquemaster belt drive...

[closetguy]

MT57
11" header - MTW pipe
900 ft.ASL
pennzoil 32:1 two gals.threw engine
Vess 23-A 7550
runs as smooth as a swiss watch!

[raven7]

DA 50R on first gallon
Stock DA side dump muffler
32/1 oil mix
22-8 Wild Hare prop
650' ASL
7590 rpm

[mdgyro]

Fuji 64EI
pitts style wrap around muffler
23x10 nx prop
amsoil 100:1
7550 rpm

[RCEXTREMEPOWER]

RC Extreme Power 102cc twin by JC engines
New out of box 32-1 mix
27-A vess pro 6250 RPM
Video to come later

Milton

[2robinhood]

	Quote: Originally Posted by raven7
	DA 50R on first gallon Stock DA side dump muffler 32/1 oil mix 22-8 Wild Hare prop 650' ASL 7590 rpm

Did you mean 6590 rpm ?
Man, if you are at almost 7600rpm on the first gallon, your gonna have the best DA-50 I've heard of.
The best numbers in this whole thread of guys with DA-50's on cans are in the low 7's.
Highest I saw as 7300 with K&S cans.

[Flatlandman]

	Quote: Originally Posted by 2robinhood
	Did you mean 6590 rpm ? Man, if you are at almost 7600rpm on the first gallon, your gonna have the best DA-50 I've heard of. The best numbers in this whole thread of guys with DA-50's on cans are in the low 7's. Highest I saw as 7300 with K&S cans.

Nah on the break in on my da50 the menz 22x8 was hitting near 7600. Its the larger load or larger props that are getting around the 7 k mark with the da50.

[Alexzzz]

3W157Xi, 3W mufflers, cfk 30*12 three-blade Elster prop: 5550 rpm, brandnew engine.

Greetz

[Aerobat]

DA100, compact standard mufflers, Mej 27x10 6660
DA150, standard mufflers, Mej 32x10, 5700

[Jamieman]

DL50 Brand New on mineral oil
MTW75 canister
23 x 8 Mejlik
6500rpm