prop diameter vs pitch

Thanks Jim,

I think I know what I want to do now.

I view the pitch/diamter question much like I do selecting gear ratios on a multi-speed bicycle.

Much depends on the "terrain" to be traversed. "Straight and level" is one thing and lots of "ups and downs" another.

So the "objective" helps determine the selection.

In the immortral words of Igoramus, "RPM isn't everything."   

SD

Ron --
I doubt you'll be able to effectively run a 5.5x4 on any of the reedie models --- they just don't have the ponies to turn this load.

Obviously, for a given RPM, to increase speed, you need to increase pitch, but to maintain the RPM, you need the power to sustain it -- and therein lies the problem.

Another way to hold your RPM up is to reduce the diameter (lower the load), but smaller diameter props also come with shortcomings. First, as the plane speed increases, the smaller props cannot move the volume of air needed to overcome the increased parasitic drag --- it's like hitting a wall, so to speak. The second difficulty with small diameter, high pitch, high RPM props moving at a slow speed is blade stall --- until the plane gains enough speed to come onto the prop, it can wallow around as if the engine wasn't turning up sufficiently. Once the plane speed increases, blade stall is reduced and it flies much faster, until parasitic drag becomes great enough to block any further improvement in speed.

Static RPM on the ground can give you a ballpark idea about performance, but you really can't get good engine/prop performance without flying several diameter/pitch combinations and trying multiple vendors. Sometimes I'll take a prop and start cutting it down an 1/8" at a time until it starts to fall off. Then, for that plane/engine/prop, I've got a pretty good idea of what works best.

Hi Ron,

Many good replies here especially by Andrew. So...for our purpose (straight line speed), experience has shown me that a minimum pitch of 4" will be required.
The exact diameter is in question but 5" is the absolute maximum & my experience has shown 4.75" is much better. I have seen a 4.5"D X 4"P prop work OK, so even diameters of 4.5" can work under certain circumstances. It is almost non-existent than fine pitch props swung at higher RPM are better. They may well be better for combat or stunt...but not speed. An APC (make sure it's centered) 4.75D X 4P is a good place to start. Trimming the blade 1/32" at a time can yield some improvement until you go too far... The APC 4.5D X 4P is also another interesting looking prop which allows more rpm since it's a lighter load.
Many years ago, Rev-UP used to make funny 4"D X5,6,7,8" P props. Some people got them to work in the Open monoline 1/2A TeeDee speed class. However, the conditions of skinny line, very high nitro, & teeny tiny models allowed them to work...sometimes. Those kind of props fell out of favour many years ago.
There is one condition where larger diameter props will work. That is if you're contemplating using a single blade prop. These can work very well, but really ought not to be considered for sport events. Too much work & trouble for anything but specialty flying. Good luck with your efforts Ron. OH...BTW, for what it's worth, the very best of competitors rarely know how a prop will perform without having to do a test flight (or three). Some things just can't be figured out in the shop...

Cheers, Paul

Cribbs74 wrote:I am trying to select props for the reed speed plane. Ron

Hi Paul, thanks for posting!

Years ago I heard or read a easy rule of thumb: "Pitch for speed, diameter for RPM." and if you think about it, it make sense.

Many years ago several modeling magazines would write about engine tests with a variety of "then" available propellers so you could graphically see (from bench test) a table where peak RPM and Torque interacted with various diameters and pitches ...this was not a recipe for the best prop of a particular engine but gave a very good starting point

Knowing any particular engines Peak Torque for a given prop pitch and diameter is critical to help select a series of propellers

Of course the problem gets exponentially worse when you factor several different propeller designs

Blade shape, width, swept area, hub diameter, over all weight, tip weight, pitch at each station, flex, rigidity, and several factors of dynamic forces like laminar flow; Gortler Number; harmonic and dynamic balance and many factors I don't remember are all important

HOWEVER... Thanks to the internet all of us can ask what works best with XXX plane and XXX engine for XXX performance and usually the majority opinion will be very close and rational thus preventing trying to acquire a bunch of obscure propellers

I love the 21st century but do miss the simplicity of 1966....

Thanks everyone for the replies! I was hoping Paul would chime in although I value everyones opinion.

I think I know where I need to start now. I hope my plane survives the first landing so I can test as necessary.

Ron