Propeller performance question - RC Groups
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Mar 17, 2012, 09:42 AM
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Propeller performance question

Been lurking around here for awhile now, but not sure I've seen this particular question discussed...

I have tested two propellers - one an APC 11x5.5, and one an APC 11x7.

The 11x5.5 puts out 320 watts. The 11x7 puts out 360 watts.

I expect the 11x7 to be the faster prop, but would it also give the best vertical performance, or could the 11x5.5 actually be the better vertical performer? I know I should just fly them both and find out which I like best, but I wondered if there was a general rule of thumb on this.

This is on a PZ T-28 with a Turnigy G10 motor, BTW.
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Mar 17, 2012, 10:26 AM
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whitecrest's Avatar
Theoretically, the 11x5.5 should give you better vertical performance. It would be like going to a lower gear while driving. The trade-off is more pull for lower speed. The lower-pitch prop should draw less current during static testing.
Mar 17, 2012, 10:35 AM
Registered User
That's what I thought, but wasn't sure with electric if watts told the whole story for both climb and speed.

And yes, the 5.5 only drew 27 amps vs. 31 amps for the 7.

I'll just try them both and decide which I like better. I like the idea of getting another roll or two in while going vertically, whereas all-out speed doesn't really do that much for me...
Mar 17, 2012, 11:28 AM
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Dr Kiwi's Avatar
The 11x5.5 puts out 320 watts. The 11x7 puts out 360 watts.

Just being pedantic... but motors and props don't "put out" watts... motors consume them (input-watts), props convert motor output into thrust and speed... how much prop output you get depends on the efficiency of the whole system.
Mar 17, 2012, 11:37 AM
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Piece's Avatar
Phil, you stole the post right out of my equally pedantic brain

For future reference, a better way to phrase it would have been "the motor consumes 320W with the 11x5.5 prop"... et al. For all we know, the motor could be 5% efficient, smoking hot, and putting just 16 of those 320W into the prop.

Since it's a not-unreasonable-but-not-excellent-quality Turnigy motor, it's proboably sitting around 70-75% efficiency, which would mean roughly 240W are actually going into the prop... Which really isn't super-useful information unless you want to get very picky about your setup.
Mar 17, 2012, 11:57 AM
Registered User
Ahh, ok.

Since you guys brought it up, what determines the efficiency of the motor? Weight vs. output? Timing? Bearing quality? Does the ESC play a role in motor efficiency? All of the above combined?

How would efficiency be measured? Is it a metric derived from running several different same-size motors against each other with a given prop/ESC/battery combination and logging the results?
Mar 17, 2012, 12:30 PM
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Piece's Avatar
Efficiency is essentially power out divided by power in. If you put in 1000W and you get 800W out at the shaft, you have 80% efficiency. To measure it, you just need to figure out how much power the motor is putting out, which can usually be determined with a tachometer and prop power-requirement data (how much power it takes to spin a prop at a given RPM).

Efficiency is mostly determined by the motor's physical and electromagnetic characteristics, as well as input power and load. Magnet size and strength, air gap between magnets and stator, wire gauge and quality, winding quality, type of wind, stator plate thickness, interplate insulation, flux ring/can characteristics, copper fill on the stator, copper/iron ratio, and a number of other factors play into efficiency. RPMs, voltage, and current will also be factors. A motor's efficiency can change broadly depending on the running conditions.
Mar 17, 2012, 01:31 PM
Suspended Account
Originally Posted by aero550
I expect the 11x7 to be the faster prop, but would it also give the best vertical performance, or could the 11x5.5 actually be the better vertical performer?
No real rule of thumb can answer this, I bet on the 11x7 being the best vertical performer, but with more amps, so probably not for a long time, but that's just a guess !

BTW: what's your field's elevation ASL?
Mar 17, 2012, 03:29 PM
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olmod's Avatar
Another old rule of thumb adjust prop size to get 80% of the unloaded rpm of a brushless motor and that is usually best efficiency of the motor.
electric watts in = mechanical watts out btw.
Mar 17, 2012, 04:25 PM
Registered User
Originally Posted by aero550
That's what I thought, but wasn't sure with electric if watts told the whole story for both climb and speed.
No power (watts) isn't anywhere near the whole story. In fact if you really want an idea of what the performance might be like it would be worth investing in a tachometer and measuring the prop speed.

It's the prop doing all the real work and prop size plus speed (rpm) tells you most about what's going on. E.g. if the 2 props are turning same speed the one with the higher pitch will be faster....but if the low pitch prop is actually turning faster, which is often the case, it may not only have more thrust (acceleration or vertical pull) but even more eventual speed. Nothing's ever as simple as you might hope .

Mar 17, 2012, 04:35 PM
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scirocco's Avatar
Assuming the Kv=1100 version of the G10, both props should be loading the motor within the motor's efficient operating region. The motor 'wants' to spin at its no load rpm regardless of prop fitted. The 11x7 won't get as close to that no load rpm as the 11x5.5, but neither is it overloading the motor.

11x7 is not an overly high pitch:diameter ratio, so the prop won't be stalled ; neither is pitch speed at ~60 mph excessive. If the battery can deliver the required current, expect about 10-15% more static thrust from the 11x7. For vertical and in fact all aspects of performance, my money is on the 11x7. In this case, I'd expect the 11x7 to turn about 400 rpm less than the 11x5.5, but that is not enough difference for the 11x5.5 to outperform the 11x7 in any part of the flight envelope.

I disagree with the lower gear analogy, because in this case with these 2 props, neither is overloading/bogging down the motor. Going down in pitch does reduce the load on the motor, but it's more like changing down AND taking your foot off the gas. The 11x7 is simply able to extract more power from the battery. The tradeoff is that if you want to use that power, the battery won't last as long.

Now if you were propped right up against the limits of the motor or esc or battery with a given prop and wanted to change emphasis from a wider speed range to optimising thrust at low speed while maintaining the same maximum power demand, then increasing diameter and reducing pitch while consuming the same power allows you to shift the way the acft behaves at the limits of the power system.

Speculation here, but I suspect some of the confusion in this space comes from trying to fly 3D models on hot 2 stroke IC engines that are capable of very high power outputs, but only at very high rpm, resulting in demand for props like 13x4. To me that would be like having too kigh Kv or too many cells.
Mar 17, 2012, 05:13 PM
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Dr Kiwi's Avatar
I hunted around to find a direct comparison from static testing an 11x5.5 APC E directly against an 11x7 APC E.. the voltage difference (0.1V) is close enough for government work.

Suppo A2820/6 (131g, ~980Kv):

11x5.5 APC E: 11.0v, 24.80A, 272W, 8670rpm, 45.2mph, 1478g, 52.04oz, 5.43g/W, RPM as % of Kv x V = 80.4%

11x7 APC E: 10.9v, 29.10A, 314W, 8100rpm, 53.7mph, 1466g, 51.62oz, 4.67g/W, RPM as % of Kv x V = 75.8%

11x7 draws more, runs slower, but pitch speed is higher, static thrust is a wash, "efficiency g/W" is a bit lower, prop loads motor down a bit more than the 11x5.5.
Mar 17, 2012, 05:31 PM
Suspended Account
I have nothing against rules of thumb !!

This case is just hard to resolve.

Some things:

At the same rpm, the 11x7 will give more thrust than the 11x5.5.
From the data I have, it should be around 1500 g for the 11x5.5 and 1850 g for the 11x7.
But torque (which is proportional to amps on an electric motor) will be around 340 for the 11x7 and 245 for the 11x5.5.

BTW, this is far more thrust than what the T-28 weights ! so I guess in both cases, the plane should do vertical unlimited vertical climb, unless batteries are a bit discharged.

Next point is what will happen with a real motor, knowing that the rpm will decrease as the load increase. Will the rpm be so low with the 11x7 that it will give less thrust than the 11x5.5 (static case)?

I doubt that the turnigy G10 won't be able to handle with the 11x7, but have no proof... so the best is to try.

I'm sure the 11x7 will also show a better efficiency in cruise fligth, which should compensate for the slightly worst motor+ESC efficiency
Mar 17, 2012, 06:32 PM
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Piece's Avatar
Looking at Phil's data, the 11x7 is producing virtually the same thrust as the 11x5.5 while running at 570 RPMs less in static testing. I know it's a different motor, but the props themselves aren't changing. Once both props unload in the air, I suspect the 11x7 will make up for the loss a fair amount and have superior performance both in level flight and when climbing. Naturally it'll cost a bit in current and flight time, but that's a given.
Mar 17, 2012, 09:35 PM
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scirocco's Avatar
eCalc suggests that for motors where the 11x7 isn't too far beyond peak efficiency, it will always do better for static thrust and pitch speed than the 11x5.5.

Even on smaller motors where the 11x7 is right at the rated max current, eg Park 480, it only gives up about 4-5% in predicted rpm, but is 27% more pitch. The bigger the motor, the smaller the rpm difference and the greater the advantage of the 11x7, eg a Hacker A30-10L predicts only 3.5% difference in rpm.

But Dr Kiwi's real data for the Suppo is interesting - around 7% difference in rpm, and rather low rpm in either case as a percentage of Kv x V. I'd have expected a 130g motor to do a bit better than that.

My empirical experience has been with a Hacker A40-12L (kv=410), well underpropped on 6S with 12x6, 12x8 and 12x10. I've recorded rpm somewhere, but the inflight experience is clearly that the rpm drop with the higher pitched props is more than compensated by the higher thrust from the higher pitch - much stronger vertical performance, and better cruise at part power too. Note though that even 12x10 is below peak efficiency current for the motor on 6S.

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