Matching prop size to motor.

[Basil]

When using a motor/prop match in the past I was either lead by the plan or relied on advise from a supplier( 4Max). I would like to try a given prop size, dictated by the space available  for a given motor. I have a Watt meter, if I was to make a jerry rig and include the relevant test meter  what am I actually looking for?.For a given motor, lipo and prop I would get a given watt/amp reading. What am I looking for?  What parameters am I looking for that says that combo is ok to use or not?

If anyone answers please keep it simple , thanks in advance.Bas

Edited April 20 by Basil

[Philip Lewis 3]

You are looking to not exceed the current rating of the motor.

[Allan Bennett]

Generally speaking, you're trying to get a prop that will cause the motor to draw as many amps or watts as it (and the ESC) is rated for.  Also, depending on the model, you can trade off speed for thrust by altering the pitch and diameter -- extra diameter will require less pitch to achieve the same amps or watts but should give you extra thrust at the expense of outright speed.

[Basil]

I was just about to refer to a request for info' re the KV etc . Can we go back to basic understanding og what the figures quoted for a given motor actually mean Say a 5045-500 motor . I know that the '500' refers to the KV but am not able to connect every thing together. Can some one explain it in simple terms so that I can tie it all together and see if a given prop/motor combo is ok. Please.

[Basil]

Philip, how do you know what that figure is?

 

[Shaun Walsh]

You need the motor manufacturer's specification. This will tell you the max current the motor can accept  without smoking. There may be 2 ratings, one constant and a higher one, time limited.

The 5045 refers to the diameter and length of the motor can (although some manufacturers quote the diameter and length of the wire wound stator inside).

Do not exceed the motor's current capacity.  Choose an esc with a higher capacity than the motor, for example if the max amps for the motor is 40 amps, use a 50 amp esc or higher. 

The motor manufacturer may recommend different size props for different battery  voltages, e.g. 10x6 for 3s, 8x6 for 4s, generally the higher the battery voltage the smaller the prop for a given motor/kv value.

I normally do the following:

Determine what prop size I require. 

Define the battery voltage. 

These 2 parameters will define the approximate kv value I need.

Determine the power/weight ratio required for the model, this (together with the battery voltage) will give the max current draw the motor must accept. 

I then look for an appropriate motor with the correct voltage, current and kv rating. The esc is then chosen to give some amp headroom.

eCalc allows you to play with motor/prop/battery combinations to be explored without the expense of buying different combinations. 

 

Edited April 20 by Shaun Walsh

[Philip Lewis 3]

30 minutes ago, Basil said:

Philip, how do you know what that figure is?

 

You need either a watt meter or the appropriate telemetry sensors, a 500kv motor is probably suitable for 4 to 6 cell LiPo's, 500 watts at 6 cell 24 volts (roughly) is around 20 amps. Most motor makers quote max current rather than max wattage because it's the current that will destroy the motor if exceeded. 

[Basil]

I need to digest this info, in the meantime ..............

I dont want to muddy the water but I need to run a three bladed prop!!! Is there a calc for it or will it need to be measured?.

[Shaun Walsh]

3 minutes ago, Basil said:

I need to digest this info, in the meantime ..............

I dont want to muddy the water but I need to run a three bladed prop!!! Is there a calc for it or will 

www.ecalc.ch

Edited April 20 by Shaun Walsh

[Philip Lewis 3]

Or you can put the prop on and run the motor up slowly, if you sstart to approach the max current rating before full throttle is reached then it's to big, as said above you can go down in either pitch or diameter it all depends on type of model it's in, an aerobat might use something like a 14X10 something scale would use more diameter and less pitch. 

[SIMON CRAGG]

Basil, I can feel your pain!.

 

Having been down the same road many years ago, the best solution is to to download e.calc and pay a few dollars for the full version.

 

Like most of these things, it looks a bit daunting at first, but if I can master it sure you can.

 

It will answer all of the questions you have, and more without having to purchase endless different prop etc. etc.

 

Have a play with it.......fantastic bit of kit!

[Simon Chaddock]

Thee are many variables to answer this question.

Obviously keeping within the specification limits of the motor, ESC and battery is paramount but thereafter even with the same prop type (IC, electric or slow fly) and the same diameter and pitch but of different manufacture may not give exactly the same result.

 

A basic Wattmeter measurement should prevent wrecking the motor but like most things related to aerodynamics there is no simple "best" prop/motor combination as the choice also depends on the design of the plane and to some degree on how the individual pilot wants it to perform.