Propeller alternative chart

[Christopher Morris 2]

Is there a brushless prop alternative chart about. I am using a motor that recommends a 12x6 prop & this is a bit tight & would prefer a 10-11" alternative. Anyone seen a convertor about. Thanks

[kc]

When we used glow engines the rule of thumb was to go down a size on diameter and up one size on pitch. So instead of 12 x 6 we could use 11 x 7 or 10 x 8. But electric seems much more critical and needs use of a wattmeter to check the watts and amps. Also you need to consider the cowl diameter - a large dia radial style cowl might not work so well with a smaller dia prop. Also for certain styles of flying ( funfly etc) the larger dia prop has some effect when just idling which helps slow the model down ( according to some such as Peter Russell /Straight & Level column when I asked him in RCME decades ago )

You could even consider fitting larger wheels or bending the undercarriage to give more ground clearance  rather than changing the prop.

 

Edited By kc on 08/01/2020 13:19:59

[Chris Walby]

I don't think there is due to there being to many other variables to consider.

The two on pitch to one on diameter (or 1 to 1) seems a good rule of thumb e.g. 12x6 or 11x8 works out although considerations for high pitch props is a factor in overall flight performance

Alternatively there are some good motor calculators on the web that take account of the many factors that need to be considered.

Me, prop to suitable ground clearance, watt meter to determine pitch with a small safety factor and away you go. If you have a need for speed then start with a high pitch prop and determine the diameter.

[kc]

Obviously Chris W has different size thumbs to me as his rule of thumb differs to mine!

Edited By kc on 08/01/2020 13:25:39

[Nigel R]

In the 12" region, the 2" pitch for 1" diameter works out about right.

Might be different for other sizes.

You can easily use a prop power calculator to figure an equivalent load of different diameter. Plug in the original prop and rpm, note power, then play with prop size.

http://godolloairport.hu/calc/strc_eng/index.htm

[Andy Stephenson]

Given pitch has a roughly linear relation to the volume of air moved and the area of sweep of a prop is proportional the square of the diameter, the rule of thumb I use is 1 on diameter is worth 2 on pitch for the same shaft load at the motor/engine.

A.

[Brian Spearing]

You can’t beat measuring what’s actually happening. Just get a clip-on ammeter and try different props. If you can multiply measured current by the battery voltage you don’t need a wattmeter.

[Christopher Morris 2]

Thanks everyone, it gives me a rough guide to work to without buying a mass of props. I will try the rule of thumbs mentioned & fit my watt/amp meter. I am also making up a jig with a set of pull scales to measure the motors pulling/power..

[Bob Cotsford]

As a point of interest I used E-calc the other day to get predictions for a model I'm putting together (Eflite Deuces Wild, Emax BL2826, 60A esc, 4S) and found 12*6 and 11*8 APC-E showed similar power/current estimates (680/660W respectively) but of course the 8" pitch gives a higher S&L speed.

Edited By Bob Cotsford on 08/01/2020 16:59:28

[Christopher Morris 2]

Thanks Bob

[kc]

Well I seem to be in the minority here so I have to concede that the majority may be right - 2 pitch change per inch rather than the 1 I suggested. We are talking about electric though

With glow my 1 for 1 rule of thumb appears to be borne out by engine tests in RCME such as a test of an ASP FS70 in RCME April 2004 where an 11 x7 gave 11,600 max revs while a 12 x6 gave almost the same at 11,400 max revs. 1 up in dia and 1 down on pitch gave pretty similar results with same make of props. There is a difference with different makes of props of the same size. Electric seems much more critical though so practical tests are the only way- wattmeter tests then flight tests.