Difference in Prop Diameter/Pitch

[HighRoller]

I've got a question I can't seem to find an answer to. If you downsize a prop by one size and increase pitch one twist what would be the difference in performance? Also should an engine tach higher or lower or the same? I'd appreciate Any help. Thanks

[Vic3D]

It will fly faster, less noise, more thrust. You will fly faster downlines, and it's not gonna be as responsive in 3D because of the less diamater.

[HighRoller]

It seems to me that a smaller diameter should turn more RPM but I don't know if or how much the pitch increase would affect the high end RPM? What affect does pitch play in top end RPM?

[TheTank]

Total thrust is a combination of rpm, diameter, and pitch. Also, the type of prop or blade style comes in to play. You can find props of two different diameters, with different pitch, that turn the same RPM, that produce the same amount of thrust. And also any combination of those parameters. Vic is right about the braking, you will see a difference in breaking as you go to a smaller diameter. You will also see less top end speed with less pitch although you may have the same amount of thrust. There are a couple of thrust calculators out there that even list different props. Google for them, they arent hard to track down. Another aspect not mentioned here is how much the airframe comes in to play. Thrust figures and calculators are great, but are often affected dramatically by the airframe they are own, and its size, weight, and drag characterisitics.

[TheTank]

I believe, off the top of my head, that mathematically, thrust is

(pi)(R-squared)(pitch squared)= volume(pi)
pi cancels out
(Radius squared)(pitch squared)= volume

Per unit/revolution volume = thrust

My math may be wrong, I dont have the ability to check it here.... and the more I think about it, the more I am second guessing myself.

Other performance related things you run in to are as speed increases, so does parasitic drag on the airframe, affecting performance. Tons of variables go into figuring out the best performing prop for an airframe, it usually boils down to trying the various types/brands to find the best one because there are so many variables out there that its extremely hard to say how a prop will perform on a given airframe.

[thewrap]

Pitch is meant for Torque for speed planes.
Diamater is meant for RPM.
You have to find the right combination.

[Vic3D]

nevermind...

[DKjens]

	Quote: Originally Posted by thewrap
	Pitch is meant for Torque for speed planes. Diamater is meant for RPM. You have to find the right combination.

WHAT?
More pitch smaller diameter means higher air velocity, but less thrust.
Less pitch larger diameter means lower velocity and more thrust.

A prop with high pitch and low diameter will fly the plane faster but not accelerate very fast out of the hole.

A prop with low pitch and larger diameter will have lower top speed but will accelerate much better out of the hole (out of a hover).

If we look at the common 150cc size airplanes and compare a 32x10 to a 30x12, there may very well be more differences than just top speed and hoverability. The 30x12 will some times make the plane torque roll better and be softer in its vertical movements, i.e. not as likely to bob up and down with power input.

As has been mentioned, props are different, engines are different, airframes are different, it's all about testing, trying and matching. I like a 30x10 prop on a heavy airframe, because the engine accelerates very fast and you get much better vertical pull out than with a larger prop.

[thewrap]

	Quote: Originally Posted by DKjens
	WHAT? More pitch smaller diameter means higher air velocity, but less thrust. Less pitch larger diameter means lower velocity and more thrust. A prop with high pitch and low diameter will fly the plane faster but not accelerate very fast out of the hole. A prop with low pitch and larger diameter will have lower top speed but will accelerate much better out of the hole (out of a hover). If we look at the common 150cc size airplanes and compare a 32x10 to a 30x12, there may very well be more differences than just top speed and hoverability. The 30x12 will some times make the plane torque roll better and be softer in its vertical movements, i.e. not as likely to bob up and down with power input. As has been mentioned, props are different, engines are different, airframes are different, it's all about testing, trying and matching. I like a 30x10 prop on a heavy airframe, because the engine accelerates very fast and you get much better vertical pull out than with a larger prop.

Thanks, I get confused easily with it. I don't know why I just have always had a problem with it.