Prop pitch question

[Martin Harris - Moderator]

Sorry, but the most likely explanation in my opinion is, as suggested by TheFlyingCrust, that the higher pitched prop is stalled and producing less thrust that the lower pitched one.

As Phil suggested but didn't expand on, a stalled prop tends to produce a lot of harsh noise - is this noticeable Mike? I often hear a clubmate's F5b model which is incredibly noisy at launch but much quieter on subsequent 6kW bursts - unless he allows the airspeed to decay more than usual when the racket returns!

[Mike Smith]

Thanks Peter (et al) for your input,

I will see if I can get hold of a clamp meter at some point... certainly a puzzle with the results I have so far.

The motor does seem to get very hot at full power which is one of the reasons I wanted to "lighten the load" with a finer pitch prop.

Will post an update if I ever get to the bottom of it.

Mike

[Mike Smith]

Posted by Martin Harris on 19/07/2014 21:02:50:

Sorry, but the most likely explanation in my opinion is, as suggested by TheFlyingCrust, that the higher pitched prop is stalled and producing less thrust that the lower pitched one.

As Phil suggested but didn't expand on, a stalled prop tends to produce a lot of harsh noise - is this noticeable Mike? I often hear a clubmate's F5b model which is incredibly noisy at launch but much quieter on subsequent 6kW bursts - unless he allows the airspeed to decay more than usual when the racket returns!

It's a possibility Martin, but not something I noticed with my untrained ear. Excuse my ignorance, but does a stalled prop effectively mean that it's unable to cope with the revs?

[Martin Harris - Moderator]

It means that the airflow has detached from the propeller's aerofoil when it reaches the critical angle of attack (it works just like a wing) and the lift produced(which we think of as thrust) drops dramatically. At the same time, the turbulent air gets chopped about producing a rasping noise - although on smaller props this won't be as dramatic as the F5b's 20 x 16 or something similar example I quoted.

Once in the air, the forward movement of the aircraft reduces the angle of attack of the airflow over the propeller which can then produce lift more effectively - the load actually reduces as the airspeed increases.

The lower pitched prop should draw less current in the air compared to the higher pitched one but to prove it you'd either need telemetry or data recording - or some comparative test flights to establish capacity used from the battery on similar flights.

 

Edited By Martin Harris on 19/07/2014 21:26:55

[Martin Harris - Moderator]

...if you're interested, take a look at these logs:

If you look at the current drawn at the early part of the climb (the pressure sensor lags slightly) it drops much faster than the voltage which shows how the power reduces as airspeed increases. You can also see a levelling in current drawn although the voltage still reduces steadily as I dropped the nose slightly at @ 205 seconds - thus accelerating the model due to the reduction in AoA.

[Peter Beeney]

Martin,

With the greatest respect but how does a stalled prop explain the fact that the 9 x 4.7 apparently turns faster than the 9 x 6 and yet uses more current? The revs and the current are surely linked together, simply a case of the effect any given load has on the motor?

I think the lower pitched prop should also draw less current on the ground, which Mike’s revolutions figures would seem to corroborate, but the increased amps certainly do not. For me, that is something of a main mystery…

All props, electric and i/c, fine or coarse, will unload to some degree in the air I guess, so in the case of electric motors the current will invariably reduce; it’s that load effect again.

Hopefully Mike will come up with an answer…

PB

[Martin Harris - Moderator]

Sorry Peter, I have to admit to missing the RPM figures earlier when catching up with the thread which had moved on a lot while I was out flying today and was commenting on the basis of the original current readings after flicking through to check whether anyone had considered a stalled prop...

I can only blame too much sun - despite the gloomy forecasts, it was a cracking day in my part of the country!

Edited By Martin Harris on 19/07/2014 22:50:51

[John Olsen 1]

This all seems very odd, and I would point out that if the higher pitch prop is stalled it will actually take more torque to turn it, which should be reflected in more current. When an airfoil stalls the lift (thrust) reduces but the drag increases. Even if it is not stalled, it will be operating at a higher angle of attack, and hence will have higher drag.

John

[Martin Harris - Moderator]

I certainly agree with that in the case of a wing but is it as simple as that?

Perhaps BEB will step in with some reasoned clarification but my purely empirical guess is that although the aerofoil principle is similar to a wing the stalled prop is rotating in turbulent air as it is static and new air is only drawn in to replace displaced air which would be dramatically reduced once the prop stalls and produces less lift/thrust. Hence Phil Green's reference to cavitation.

But I stand to be corrected as it is only my simple reasoning...

[John Olsen 1]

If the prop is drawing in less air then the angle of attack will be increased. In the limiting case, if the prop was not drawing in any air then the angle of attack would be the same as the geometrical helix angle at that radius, but then the air would have to go somewhere when the blade comes past wouldn't it. Generally creating turbulence is going to absorb more energy than a smooth flow, which is why we usually try to avoid it. The drag of an airfoil is not going to reduce with an increase of angle of attack, it is going to increase until it reaches the maximum at 90 degrees angle of attack, eg effectively a flat plate.

John

[PatMc]

I don't see that the stalled prop theory is anything other than a red herring. There's a contradiction between the current & rpm figures which can only be due to some error in measurement.

[Braddock, VC]

Going back to the original post and using data collected along the way of the thread and feeding this data into the following programme http://personal.osi.hu/fuzesisz/strc_eng/index.htm, assuming the prop is an apc copy the output of the motor comes out as 453 watts for the 9x6 and 433 watts for the 9x4.7. Overall efficiencies OF THE MOTOR are 85.6% and 62.7% respectively. (obtained by using your figures for watts)

I'm not advocating this programme is 100% accurate, for comparing like with like it gives a working approximation.

The reason for this mismatch is down to the physics of the way losses are incurred in three phase motors.

Answering the OP, there is a higher load on the 9x6 prop. Also the motor is probably overloaded by both props and affecting the results significantly.

[TheFlyingCrust]

The thread I was referring to discussed if a static prop in flight produced more drag to the aircraft than a windmilling one. The thread concluded that the windmilling prop produced more overall drag than a static one, which for me was counter-intuitive.

The only way I could rationalise this was with kites, which I also fly. A stunt kite flies aerodynamically whereas a single line kite flies in a stalled state. Of the 2 kites, given similar size, the single line one has much less string tension than a stunter.

I know the link is tenuous but it helped me understand the theory.

Yes, I reckon BEB's the man to give analysis.

Ian

[John Olsen 1]

I used to wonder why the competition rubber jobs used folding props instead of just windmilling with a ratchet like the simple ones did. You would think a freely windmilling prop with little friction on the shaft should not produce a lot of drag. However, even when freely windmilling, the airfoil section of the prop is moving through the air so will have form drag and skin friction. Because it is following the helix angle of the airscrew, it is moving through the air at a relative speed that is higher than that of the plane. Where the helix angle is 45 degrees the airspeed relative to the prop will be 1.4 times the plane speed, so the drag will be doubled. As the helix angle gets less, out towards the tips, the relative airspeed will be even higher...at the 30 degrees point the speed will be close to double, so the drag will be quadrupled. Any friction in the shaft will make things worse, by putting a negative angle of attack on the airscrew and thus increasing the drag, although it will reduce the airspeed a little by slowing the prop down. (The airspeed at any point on the prop is the vector sum of the components due to forward velocity and rotation.)

So is it better to lock the prop in place? Full size practice is to feather the blades so that (apart from the twist) they are edge on to the airflow. This will give the least drag, and also means the airflow is not trying to turn over the engine, possibly doing even more damage if it is a bit broken. Model practice with rubber models has always been to fold the prop away completely, or else to use a simple ratchet to allow the prop to freewheel. If the prop is locked, the blades will be stalled and I suspect will provide a fair bit of drag, probably more than the free wheeling case. Otherwise why did we bother with the ratchet...if you leave it out the rubber motor will eventually halt the prop.

John

[Peter Beeney]

I’ve just had enough time to do a quick power check for comparison on an electric model, an E-flite Corsair. Unfortunately I couldn’t do a rev check at the same time, my tacho needs a new battery; I may be able to fix this later.

This has a 450 size 960 kV motor, the supplied prop is a 2 bladed very flexible plastic 9.5 x 7.5. This is prop A. I’ve been unable to find any useful information on this motor, all the specs. on the website pages seem to be blank. I used a 9 x 6 APC i/c prop for a second opinion, aka Prop B. The ESC is a 30 amp.

I used 2 3S batteries throughout, 2,200mAh Overland, keeping them recharged between each test, thus they’re fully charged all the time. I have flown this model quite a bit, it goes very well but I’ve been considering changing the propeller anyway. I fly it mostly at full throttle to get the big wide open manoeuvers.

The first check was with one battery, battery E, and the two props. Prop A revealed a max or peak current of 64.4A, prop B 61.5A. This peak happens very fast indeed, you can’t see it on the ammeter, it’s the ‘inrush’ current into the motor before it starts to turn; and there’s not a very big gap here… Then a steady state reading, running up to speed for 15 seconds and then taking a ganders. Prop A gave a reading of 34.7 amps, prop B 22.6 amps. Assuming an average battery voltage of 12 then the watts are 416 and 271 respectively. This would appear to be in keeping with what I would expect to happen; a smaller prop is a lighter load, the motor therefore turns faster and uses less current.

Then I thought I’d try another battery, battery F. This is exactly the same Overland pack, except that’s it’s a fair bit older and has done considerably more work; same conditions, fully charged before each test. I ignored the peak readings for this one and so using prop A the steady state reading was 40.3 amps and for prop B 24.9A; this represents an average increase of about 12% in performance over the newer battery. This was something I really didn’t quite expect! I do have a number of identical packs, plus some from a different manufacturer, if I get a chance I’ll try a few others. But pack F was the oldest I’ve got! Watts now equal 483 and 298.

One or two points arising - this is in direct conflict with Mike’s observations relating to his motor, and I’m still not convinced that any stalled props are anything to do with his query. So the jury is still well outside on that one, until at least we get some more information,

Also I don’t much care for the plastic props, they appear to use an excess of power to do the same job. Handy for any belly landing models, as is mine, I’ve not bothered with retracts or flaps, the lighter weight helps the wing loading. I’ll try the i/c APC props in the air, they’ve always made an improvement in the past, flying wise.

Lastly this is driven by a 30 amp ESC. On these figures this would seem to be working at least right up to it’s operational limit, if not on downright overtime! So far I’ve not noticed that it’s winked, flinched, wobbled or even glitched, but for sure it’s, like, the makers ain’t not left a lot of headroom anywhere… there is a provision for a good through draught, though, the battery only ever gets warm, not hot. But I will also be checking the ESC as well now. Also I’ll be trying the model on the 9 x 6 ACP.

PB

[Peter Beeney]

Just to bring this to a logical conclusion, I now have some figures for the revolutions. These are also in keeping what I would be expecting to see.

So, first check, the no load result - 13,480 rpm, 4.1 amps.

9 x 6 APC prop - 10,390 rpm, 24.7 amps.

9.5 x 7.5 plastic prop - 9,210 rpm, 40.1 amps.

On this showing it certainly behaves in a proper manner, but the first discrepancy is the fact for a 960 kV motor to get up to these no loaded figures around 14 + volts would have to be applied, and I used my oldest but bestest 3S pack. If the figures are correct then this has to be at least an approx. 1100 kV motor, but it will all stand re-checking. Also even at the lower rev count the model is still theoretically a bit faster on the plastic prop; however, just how much more inefficient it is is in the air is anyone’s guess. A careful visual check on performance is going to be the order of the day. I’ll have to try and obtain a few of the more intermediate sizes, too, APC do seem to make a vast range. The decrease in the current consumption will make a welcome duration difference, though.

All that remains now is for Mike to find out why he was getting the seemingly strange back-to-front readings on his model.

PB