Motor Specs: Why Does a Lower-Pitch Prop, at Lower Speed, Produce More Thrust?

[OkieDave]

https://store.flitetest.com/emax-gt2215-1100kv-motor/

Look at the specs on the 2215/09. When using a 10x4.7, the motor makes 8300 RPM and 1250g thrust. When using the same diameter, higher-pitched 10x6, it makes 8850 RPM and 1140g thrust. That doesn't make sense to me; first, a higher-pitched prop should result in lower, not higher, RPM, and secondly, if it can maintain even the same RPM with the higher-pitched prop, it should produce more thrust, to say nothing of it turning faster.

See also: https://store.flitetest.com/emax-cf2822-1200kv-motor-with-collet/ specifically the 2822; the 2812 looks right to my understanding.

I was a flight instructor for years, and have many hours in controllable-pitch prop aircraft; I can definitively say that increasing the pitch causes the engine to slow down, all else being equal. Yes, you use low pitch for maximum thrust (i.e. takeoff), but that's because it allows the engine to turn faster. Using minimum pitch at the same (or lower) speed produces less thrust, not more.

Please help me understand.

 

[m00ndancer]

Could be that the 10x4.7 is a slow-fly prop and the 10x6 is a APC-clone. Different types of props.
Different surface area. Don't forget that we are talking static thrust not in the air thrust. I would guess that the 10x6 is much more efficient at speed.

 

[Craftydan]

Could just be bad data . . . I wouldn't put it past EMAX to fat-finger something in and never catch it.

In that respect, Motor specs are a grain-of-salt thing -- the better the company, the more you pay, the more likely measured meets reality consistently. The cheaper, the less time is spent on the data. User measured data is far more reliable . . . assuming they've been careful in documenting both what and how they measured.

In the end, it's all a thin veneer over TLAR.

 

[OkieDave]

Could just be bad data . . . I wouldn't put it past EMAX to fat-finger something in and never catch it.

In that respect, Motor specs are a grain-of-salt thing -- the better the company, the more you pay, the more likely measured meets reality consistently. The cheaper, the less time is spent on the data. User measured data is far more reliable . . . assuming they've been careful in documenting both what and how they measured.

In the end, it's all a thin veneer over TLAR.

I kinda figured that to be the case, but I'm new enough that I'm willing to assume I don't understand rather than assuming I'm right. Egg on the face is remarkably tough to wash off.

 

[Snarls]

Could it be that the higher pitched prop is stalling slightly under the conditions of the bench test and as a result produces less thrust.

 

[Fuzzy Whumpkin]

Everything about aviation physics (in my understanding) comes down to air being nothing more than a VERY low density fluid, so the physics of boat propulsion should be analogous to those of airplane propulsion. under that assumption, i did a little digging and found a few helpful sites. the quote below doesn't directly answer your question, but is a nudge in the right direction, feel free to read more and let us know if you find your answer. the boat idea came because i know submarines experience "cavitation" when the pressure behind the propeller blades drops low enough to boil (in the chemical term, as in liquid to gas phase change) the water, and i wondered if something similar might be happening here.

Also fluid movement is either laminar or turbulent flow - laminar is much much more efficient, so maybe the reduced efficiency when you'd expect increased efficiency is because you've passed some threshold where the turbulent flow caused by the propeller reaches far enough back that the airflow over the wing surface is affected (goes from laminar to turbulent)

anyways, those are my shots in the dark, here are the pros words:

(from https://www.vicprop.com/propeller101.htm)

We have shown that the propeller, in order to generate thrust must accelerate or move a mass of water astern. Now the propeller, being shaped like a screw might conceivably, as it rotates, slide through the water as a machine screw would into a nut, without displacing any water aft. If this happened the propeller (and the boat) would, in one revolution of the shaft, advance an amount equal to the propeller pitch. This would be called zero slip. But in order to produce a thrust,
we must accelerate or move some water aft, and therefore it is apparent that the propeller will not advance the full
amount of its pitch in each revolution, but will advance some lesser amount, depending upon how much water it accelerates astern in the process of producing enough thrust to offset the resistance of the boat to being driven ahead.
If the boat were tied to a dock, the propeller would not advance at all but would generate maximum thrust because full engine power would go into accelerating water astern. This would be called operation at 100% slip.
The term "apparent slip" is used to indicate the difference between the theoretical speed that the boat would obtain on the propeller pitch and the rpm of the propeller shaft, and the actual speed of the boat.
Slip must not be confused with efficiency that is a measure of the percentage of engine power converted to thrust by the propeller. We have seen that we must have slip in order to generate thrust and the amount of slip will be proportional to the amount of thrust required by the boat.
High-speed runabouts and fast cruisers require relatively low thrust and therefore operate at low slip whereas tugs and other heavy vessels require high thrust and therefore operate most efficiently at high slip.

so it seems to me that you have a balance between thrust generated (max when the boat is tied to the dock, 100%slip) and efficiency of the motor(Max when all the energy of the motor goes into propelling the boat/plane forward, not propelling the fluid back 0% slip)

here are some other references for you

http://www.boattest.com/view-news/4226_do-your-props-have-the-right-cup-rake-slip-pitch-and-hub

http://bblades.com/props-101/

 

[Fuzzy Whumpkin]

Man o Man! you got me going on this one!! here's another link and some more ramblings
http://web.mit.edu/16.00/www/aec/flight.html

so, the term "thrust" is just a forward force in physics. Force = Mass x Acceleration. here's where things get tricky, though! What mass is being moved???? the propeller is pushing air, but is attached to the plane, so it will be moving both of them at all times. Note that to keep the boat/plane still, there had to be an artificial rearward force applied (tying the boat to the dock) - well the same would apply to the opposite end of the spectrum, to achieve a no slip scenario, you'd need an artificial forward force.

The problem with understanding may stem form the fact that you're comparing takeoff - when the mass of the plane is more or less at rest - to flight - when the mass of the plane is already moving in the direction of the thrust generated by the prop. also, those stats are comparing a mechanical measurement of just one of the forces acting on the plane in flight - thrust.

Also realize that forces come in pairs - the thrust of the engine at takeoff comes because the air being pushed backwards is pushing forward on the propeller with an equal and opposite reactive force. so the amount of thrust generated is equal to the ability of the propeller to push on the air -the more mass it can move, the higher the thrust. and the more it accelerates that mass, the higher the thrust. so assuming pitch affects the mass of air moved and rpm affects the amount the air is accelerated, the tested thrust either is not right, or there are significant other forces acting that haven't been taken into account (but since this isn't a common phenomenon, that's not happening) so after ALL THAT... my answer is no, the numbers are wrong, you're not.

as far as real world applications go, my physics 101 professor told us on the first day "you're all expert physicists, you just don't know how to explain it" what he meant was you've spent your whole life learning that, for instance, to make this ball land in that hoop, I need about this much force at this angle. the same goes for flight - your real world experience with the way things move and react under different conditions has more validity than an artificial lab test - and the physics of what's happening shouldn't do much more than explain why you're right.