Prop selection

[Chris Walby]

Some insight or opinion would be warmly welcomed on the question of propeller performance and its effect on an electric set up (assuming motor & ESC are easily capable of driving either of props). Is there a difference in low, medium and high air speed performance between say either a 8 (dia) x 10 (pitch) or a 10 (dia) x 8 (pitch) beside the theoretical higher speed of the former? Second question, what effect does running high pitch props do for low air speed (landings) performance on throttle response at the TX stick, if anything with something more extreme like 7 x 10 pitch?

[Martin Harris - Moderator]

Hi Chris,

There will be differences in acceleration - the higher pitched prop being less efficient at low air speeds (due to it being in a stalled/semi-stalled condition - think of the racket Dick W's F5b props make at launch and how much quieter they are when operated from flying speed) and less efficient in the climb at lower airspeeds. There may also be subtle differences in power output as the 10 x 8, 8 x 10 correlation is only an approximation.

I would think that there will be significantly less braking effect (drag) from the smaller diameter prop disk.

Edited By Martin Harris on 31/07/2016 19:34:55

[Don Fry]

There be experts on the site, but I would've expect these props to exist on different motors, the 8/10 is on a racing screamer, a small 35 size pylon machine perhaps. The 10/8 belongs on a 46 fast jobbie, but still a lot more moter to turn the prop. An inch in prop size does not equate to an inch less pitch. Diameter loads engines more than pitch. But I wait for experts to sort the technicalities.

[eflightray]

The bigger diameter will also mean a higher current draw, it will be quite significant between 8" and 10" diameter.

- a bigger diameter prop will give more initial thrust, but will naturally need less pitch, so will run out of speed sooner,

- a smaller diameter higher pitch prop will give lower thrust and lower initial acceleration, but will give a higher top speed.

Example, larger diameter lower pitch for say 3D flying, small diameter high pitch for sheer speed.

Hope you have a wattmeter if playing with prop sizes.

Ray.

[Chris Walby]

Thanks to all who have replied, I'll digest your advice....To answer Ray, yes the watt meter has been employed, but I am not seeing much as its a 320KV 1600W 80A motor on 5S lipo pulling 9A & 415W on a 2 bladed 16 x 10 at 1.7kg thrust...and there is my dilemma...should have got a 580KV motor! I have a plan...3 blades and then failing that 6S lipo. I was interested to see comments regarding the low/mid air speed effect...

[Geoff S]

The load a propeller imposes on a motor is directly proportional to the pitch but to the diameter it's proportional to the square. Some time ago I made myself a table which listed the volume of different props ie the theoretical volume of air it would move with one revolution assuming 100% efficiency.

It's simply the area of its arc (0.5 dia squared) multiplied by the pitch. So for example, 12x6 prop would be 6x6 = 36 multiplied by 6 which = 216 cu ins. Upping the diameter an inch to 13 gives a value of 253 cu ins.

I know it's a very crude method but it gives a feel for the effect of changing a prop and the order of change in current that might result.

So if you want high speed then you use a a high pitch low diameter prop and the reverse for a slow flying scale model. By bearing in mind the props volume the current draw will be similar.

Geoff

[eflightray]

Posted by Chris Walby on 31/07/2016 21:29:16:

Thanks to all who have replied, I'll digest your advice....To answer Ray, yes the watt meter has been employed, but I am not seeing much as its a 320KV 1600W 80A motor on 5S lipo pulling 9A & 415W on a 2 bladed 16 x 10 at 1.7kg thrust...and there is my dilemma...should have got a 580KV motor! I have a plan...3 blades and then failing that 6S lipo. I was interested to see comments regarding the low/mid air speed effect...

A 320kv motor would normally be driven by an 8s to 10s Lipo, or a very big diameter prop on lower count cells to get the best out of it.

Air speed obviously depend on how draggy the airframe is, and the props pitch speed.

A simple, (very simple) idea of pitch speed is to multiply the prop rpm by pitch (in inches). There will be other losses but you could use the above as max speed.

This is why it's called electrickery

Ray.

[robert chamberlain]

Just for comparison purposes for the load the prop puts on the engine/motor, I have sometimes used,----P X D X D X D to give me a raw number. Truthfully, I have never correlated this to anything like amps or RPM. Just a thought to put out there. Maybe I drink too much coffee.------Bob C

PS--I usually just follow the manufacture's advice