what is the relationship between RPM and speed? Also what part does the prop size play in this?

if i have a 11x4 spinning at say 15000 Rpm and a 11x6 spinning at 13500, which would give me more speed, which would give more pull?

I think the bigger pitch would yield more pull, and the bigger the diameter would yield more RPM's.

I was asking questions about propellers awhile back and somebody (sorry, don't remember the name of who sent it to me) sent me this program. It's great for doing what if with your props. It has really helped me in getting the right prop for the type of flying I want to do.

I've posted a copy of it on one of my web servers. you can download it at

http://www.infinityok.com/thrusthp/thrusthp.zip


hope this helps

11-4 would give more pull. 11.6 more speed.

this might help



formula pitch x rpm x effency (1 would be a perfect airframe with no drag a .5 would be something like a cub for jet style prop planes a .85 would be close to
a real figure) x .000947 = mph

Lower pitch = more RPM. Higher pitch = more speed given almost = RPM. Take off in full scale is low pitch, cruise is in higher pitch. Unfortunately we do not have
manifold pressure gauges, mixture controls and adjustable pitch adjustments in flight for RC aircraft. With us usually more RPM = more speed.

at 100% efficiency (1.0 in the above formula) a 6" pitch (10X6, 11x6, etc( would move 6 inches forward every full revolution. 10000rpm=60000 inches/minute,or 5000 feet/minute or about a mile/minute, or 56.8 MPH (Which is what that formula gives). Obviously not the case, as most of my 40 size Big Stik will do well over 60 MPH flat out, and certainly better than a .85 efficiency will give ( which is 48 MPH-assuming a slick model, which the stik is certainly NOT). but then, the airplane moving thru the air is NOT a static case, as the mere "pitch" would indicate. Although a prop is called an "airscrew", its action is considerably more complex than that.
2 props,of the same diameter turning THE SAME RPM (nit pickers, note the caveats!) will generate two forces, which go by the term "Thrust". The prop with higher pitch will generate more "thrust", and accelerate a given (Again with the caveats) airframe faster than the low pitch prop will.
F=m*a.
going to a bigger diameter increases efficiency.

Basically a plane being restrained will have a lower RPM compared to one that has unloaded in the air which will have a higher Rpm. I would consider that unless you are hovering, you will get higher RPM's and speed in the air.

there you go. I put the caveats in, and they went right over your head...put a governor on the engines, so they will only turn 10000 rpm
I will bet you that a 12x6 turning 10000rpm will pull a given (that means the exact same airplane) airplane at a higher speed than a 10x6 turning 10000 rpm. They will both get that given (same) airplane up to any spped you chose faster than a 10x4 turning the same rpm (10000)
the pitch helps you decide, and does have some significance. Bernouli gets a say-so in the equation, however...
ask a heli guy who uses a governor what happens to his pitch curves when he changes the disk diameter...

I take it you are disagreeing with what I said, but I really don't understand what you just said. I will challange that a 12x6 at 10000rpm and a 10x6 at 10,000rpm must be on two seperate engines if at full throttle, or two different engines if at full throttle. The loading is much different.

12x6 will load at 124416
10x6 will load at 6000

AS you can see, the loading of a 12x6 is twice the 10x6.

Eugoogoly,

It is not quite valid to distiguish between speed and pull. The distinction that makes more sense is between pull at low airspeed and pull at high airspeed. What we frequently think of as pull ( thrust ) is pull at low airspeed, and what we frequently think of as speed is pull at high airspeed. Going fast requires high thrust just at hovering requires high thrust. The difference is that hovering requires thrust with virtually no airspeed, while speed requires as much thrust as you can get at high airspeed. Generally, thrust at low airspeed requires a low pitch prop, and thrust at high airspeed requires a high pitch prop. Often, the quest for maximum airspeed requires that the high pitch prop have a fairly small diameter, so that the engine can turn it fast enough.

banktoturn