Help! Prop question

[chickenhawk]

We need a question answerd about props. Have a freind who crashed his Apprentice and broke the prop ( 15 size motor with 11 x 8 prop ) The only spare he has is a Eflite Air tractor ( BL10 motor with a 11 x 7.5 3 blade) prop. I told him not to use this 3 blade prop, he says that it will be ok.
Whos right?

 

[quorneng]

The 11 x 7.5 three blade may absorb a similar power at similar RPM to the 11 x 8 but the air flow from the prop will be going slower so the maximum speed of the Apprentice will be reduced however at slower flying speeds the thrust will be greater.
How well this thrust/speed combination suites the Apprentice or how you fly it you will have to find out.

 

[joelspangler]

I have to agree that the 3 blade prop will be fine. Lower numbers of props are typically associated with higher efficiency and higher top speed. Higher numbers of blades will be less efficient but thrust will be increased. With the 3 bladed prop, he'll be able to accelerate faster and get through higher grass on the ground with more thrust, but overall top speed will be slower, and flight times may be (slightly) shorter.

 

[FoamyDM]

I third this.

 

[Foamforce]

I’m surprised by the replies, but I don’t have much experience with 3 blade props. I thought that they would pull a lot more amps, so that you would need to step down more than just a half inch of pitch to use the same diameter prop. You all don’t think that this could overload his ESC?

 

[HVB79]

A three blade prop, even a slightly smaller one, will draw more amps. This could overheat the ESC or burn out the motor. But without knowing exactly what motor and ESC are in both planes it is impossible to say. Some motors and ESCs have their rating in watts or amps printed on them. With others you have to look up. The best way to do this is to find what the components are rated for then measure the amps & watts with a meter like this: https://www.amazon.com/dp/B06XPVKY13/?tag=lstir-20

Your friend is gambling that the power system is a bit oversize for the stock prop and has some extra head room. In the specific case he is probably right but you can't be sure without measuring or just test flying to see what happens.

 

[Merv]

. ... I told him not to use this 3 blade prop, he says that it will be ok.
Whos right?

Im in the do a prop test camp.

Run it on the ground at full throttle for 10 seconds or so. Stop the motor & take the temperature of the motor, ESC & battery by touching them. Warm is OK, but if any of these are so hot that you don't want to handle them, STOP, that is too much prop. If it passes, try a 30 second run & take the temps. If it passes, try a 60 second run & take the teps. If it passes, go for a fly.

Keep in mind that what works when it's mild out, 80's, may over heat when it gets hot outside, 95+. It's always a good practice to take the temperatures of everything when you land.

 

[Tench745]

Typical rule of thumb when switching from a 2-blade to a 3-blade is to drop the diameter by an inch or two. A 10x8 or 10x7.5 3-blade would put a similar load on the motor. 11x7.5x3 isn't that far different, so there might be enough headroom in the motor and ESC specs to handle it, but I would also recommend doing a test with a watt meter.