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Apr 04, 2011, 03:15 PM
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Discussion

Prop to big? Prop to small?


I'm still not sure I have my head wrapped around this yet.
If I have a motor that is designed for a 5" or 6" prop and I put an 8" prop on... The motor will fry.... right?
If I have have a motor that is designed for a 9" or 10" prop and I put a 6" prop on... What happens?

Also, watts is a measure of the power a given motor can produce, that then converts to oz. of thrust depending on the size and shape of the prop. Is that right?

If it's not specifically listed in the specs, how can I determine what size prop a motor can take?

Thanks
Last edited by SquireM; Apr 04, 2011 at 03:23 PM. Reason: additional question
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Apr 04, 2011, 07:18 PM
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Dr Kiwi's Avatar
If I have a motor that is designed for a 5" or 6" prop and I put an 8" prop on... The motor will fry.... right?
Probably, unless you lower the input voltage

If I have have a motor that is designed for a 9" or 10" prop and I put a 6" prop on... What happens?
Nothing of concern... it will simply draw lower current.

For the same watts-in, large lower pitched props produce lots of thrust... smaller higher pitched props produce lots of speed , but less thrust.

The "appropriate prop size" is dependent on Kv and applied voltage... you must keep the current below the recommended maximum for the motor.
Apr 05, 2011, 02:43 AM
Registered User
Usually when you see a manufacturer recommend a range of props it's because their motor is also good for a range of voltages. If so, smaller prop will be for highest recommended battery volts.

Your aim is normally to get as many watts as you can out of your motor. Watts = volts x amps, so you do this by (a) applying as many volts as the motor is specced for and (b) putting a prop on that will make it draw as many amps as it is specced for. Even though the manufacturer may give recommendations, you still need to measure the actual amps when you try different props, because different brands and different models will result in different amps for the same nominal size
Apr 05, 2011, 03:22 AM
Registered User
You've basically got it:

Prop bigger than recommended - too much current, lots of power but only for a short time then, as you say, the motor fries.

Prop smaller than recommended - low current, not much power, perfectly safe but probably not enough grunt to get the plane in the air

You can use one of the many motor calculators (Motocalc, Drivecalc etc) to "test" various sizes of prop to find a reasonably suitable size. Or perhaps just look at what people are using on similar motors - for this purpose it's same Kv and battery cells you need to look at. Basically all 1200Kv motors running on 3S lipos will use roughly similar size props. Higher Kv or more cells, smaller prop, lower Kv or less cells, bigger prop.

Steve
Apr 05, 2011, 03:25 AM
Registered User
meshyx's Avatar
Quote:
Originally Posted by SquireM
If it's not specifically listed in the specs, how can I determine what size prop a motor can take?
The two most important factors are motor weight and motor KV. The easiest way would be to find another motor that is similar in weight and KV, and see what props it uses.

Another way is to hook up a watt meter, and follow the rule of thumb that 3 watts per gram is a comfortable power level.

A third way is to measure the RPM of the motor at full speed with a prop.
With a small prop the motor will manage maybe 80% of the theoretical RPM (KV times voltage). Then increase prop sizes until you see RPM dropping toward 70%, which means the motor is starting to be overloaded.
These are not exact numbers, and I'm sure I've bungled something here enough that dr. kiwi will correct me
Apr 05, 2011, 07:07 AM
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peterangus's Avatar
Quote:
Originally Posted by meshyx
A third way is to measure the RPM of the motor at full speed with a prop.
With a small prop the motor will manage maybe 80% of the theoretical RPM (KV times voltage). Then increase prop sizes until you see RPM dropping toward 70%, which means the motor is starting to be overloaded.
As meshyx suggests, the slowdown from no-load speed will tell you whether the prop suits the motor.

For practically all satisfactory power installations, the slowdown speed lies between 85%NLS and 70%NLS.

For the typical flyer, the ideal is about 74%NLS.

More info at https://www.rcgroups.com/forums/member.php?u=24765
Apr 05, 2011, 07:20 AM
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Dr Kiwi's Avatar
Hi Peter,

I've bookmarked your blog! 74%NLS seems right on the money.

Just wondered whether you had a applied the same calculations to EDF impellers...from my EDF testing I'd think 74%NLS would be extreme over-loading... 85%-90% might be a better target????

Cheers, Phil
Apr 06, 2011, 07:22 AM
Registered User
peterangus's Avatar
Phil

I've never dabbled with DFs, but I will hazard a response.

The value of the optimum %NLS [or target %NLS] does not depend on the type of load [prop, DF, whatever]. It depends primarily on the run duration to battery cut-off. [eg: 70%NLS for a 5 minute run......74%NLS for a 10 minute run......78%NLS for a 20 minute run. "Optimum" means lightest combined weight of motor+battery+etc.]

Whether the target %NLS can be used satisfactorily [ie; with no overheat], depends on two completely independent factors. Namely, duty cycle and motor cooling arrangements.

Poor cooling, or a severe duty cycle, can each compel the choice of a higher %NLS.

This applies regardless of the type of load. But the constraints are more-difficult with DFs than with propellers [eg; lots of props to choose from; few,if any, choice of fans].

In this discussion, the motor choice is not pre-determined. A change to a higher %NLS implies a change to a larger motor, to maintain the same motor output power.
Last edited by peterangus; Apr 07, 2011 at 11:14 AM.


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