Prop pitch question

[john melia 1]

If the pitch of the prop is increased , will the motor/engine have to work harder ?

[Olly P]

Yes. Assuming the diameter is constant.

Thge prop is trying to 'screw' through the air faster, This means it is doing more work....

Edited By Olly P on 03/07/2013 09:46:40

[john melia 1]

thanks olly

[will -0]

Um, I'd sort of disagree - although it rather depends what you're actually asking:

If the pitch is increased without changing the diameter, the engine will have to work harder to turn the propeller at the same RPM.

In fact what usually happens is that the engine won't be able to turn as fast due to the increased load, (RPM will drop) so may not actually work any harder than before (ie the BHP doesn't change much). This is complicated by the fact that the BHP that an engine develops is dependent on the RPM, but ignore that for now.

[Martin Harris - Moderator]

Yes - in the way you have to pedal a bike harder in a higher gear, but it's a little more complex than the simple increase in load for an electric motor...

An IC engine will have a "power curve" which means for best efficiency, the rpm will need to be within a certain range. This will depend on the engine characteristics - a lowly tuned engine may have a wide power band and tolerent of a large range of propellers whereas a "racing" engine may deliver very little power outside a narrow rpm band where it is tuned to be extremely efficient.

The "science" is usually far less complex than selecting a cell count/motor wind/propeller combination for electric power and the manufacturer will give a suggested range of props - from which you would select one appropriate to the model/style of flying. Often, this means finding a prop which will limit the noise below your club's requirements and we very rarely achieve anything like peak power output from our engines unless flying in pylon racing competitions etc..

Edited By Martin Harris on 03/07/2013 10:12:51

[Mike Smith]

Thought I'd resurrect this thread rather than starting a new one. I thought I had generally got my head around electric setups, but have been experimenting today with a couple of different pitch props (same diameter and make) and am a bit confused by the results.

I ran the same test with each prop, using a fully charged 3s LiPo. The readings below are taken at full throttle.

9 x 6 prop

• Peak Current 46.08A
• Peak Power 528.99W

9 x 4.7 prop

• Peak Current 61.75A
• Peak Power 690.61W

The motor is a 3542/1250kv running on a 3s LiPo and is rated as a 500W motor with a max current of 50A

The results seem counterintuitive to me and suggest that an even coarser prop would actually draw less power.

Can anyone explain this (preferably in fairly simple terms)?

[GONZO]

RPM for each test would be very useful. Something looks wrong to me, check the prop dimension again. Are the props of the same make/type(slowfly or normal) etc as blade shape and area also have a large effect?

[Peter Beeney]

Yes, in a word Mike, this doesn’t entirely add up. With the greatest respect, increasing the pitch will make the motor run slower and thus use more current. For the moment I’d say there seems to be some sort of discrepancy between your propellers. Is it worth going back and having another look?

If these figures were inverted it would make more sense, although at a complete stab in the dark the 9 x 6 doesn’t seem to be that far out anyway.

A tacho is also invaluable, this will prove the lower rpm.

Hope this helps for starters.

PB

[Mike Smith]

Sorry, I wasn't able to measure the RPM. Yes, both props are the same make/range (EMP Electric) and diameter.

[Mike Smith]

Thanks Peter,

I will try measuring the RPM, but my tacho is built into my power meter so am unable to measure power consumption at the same time.

[Peter Beeney]

Sorry GONZO, didn’t mean to interrupt… great minds, and all that…

PB

[GONZO]

Peter, I would question your assesment of my mind. Sometimes, but not often, my two brain cells actually fire at the same time

[Phil Green]

When you say peak power, is that the instantaneous peak from the wattmeter? ie a one-off peak reading, not the steady state reading?

Are you slowly building to full rpm or is this full power from standstill?

Its just that I use quite a few 3542 -ish motors of similar kv, usually on an APCe 10x5, and they dont take anything like 62 amps!

Its not a 4S you've picked up by mistake is it?

Cheers
Phil

 

Edited By Phil Green on 19/07/2014 17:31:15

[Mike Smith]

Phil, I've used the peak reading (which is stored on the meter) as it's not easy for me to read the display during the full power test, whilst holding onto the model and the TX, etc... Do I detect from your question that this may not be a valid method of measurement?

I increased the power smoothly over a few seconds.

Perhaps I should try again and see if I can get a "live" reading of power consumption?

Definitely a 3s LiPo (I don't have any 4s)

Since last posting, I measured the following (max) RPM for each prop:

9 x 6: 12,960

9 x 4.7: 13,850

[Peter Beeney]

Exactly so, GONZO, my two behave in a similar manner. Although I sometimes think nowadays, due to it’s ever increasing longevity, the compression is well down on one …

Mike, with those rev counts, I think it’s very unlikely those power readings are right…

PB

[GONZO]

Think you may have it there Phil with the 4S. Otherwise their v/good lipo's to give over 11v when suplying nearly 62A (690.61W / 61.75A = 11.184V). But, still doesn't answer the prop anomally.

[Mike Smith]

OK, have just run another comparison, this time taking the steady state readings rather than the peak readings. Difficult as the battery power drops away when running at full power, so the readings are constantly on the move, but the results are broadly similar in terms of the finer prop pulling more power than the coarser.

9 x 6 prop

45A - 500W

9 x 4.7 prop

55A - 580W

[Peter Beeney]

It might be some sort of an answer because these are peak instantaneous readings, so they would be at startup, I guess. I don’t have a wattmeter so I’m not very familiar with these anyway. I think I’d be tempted to check it out with a steady state reading using a partly discharged pack, this must surely be a more accurate picture of the general performance?

Although why the 9 x 4.7 is the greater of the two is still confusing, could this now just be a quirk in the way the meter records the instantaneous readings?

PB

[Peter Beeney]

Mike,

I think there has to be a cross up somewhere, if the 4.7 is turning faster than the 6 then it will use less current. Of this I have no doubt, so I’m wondering if you might be able to perhaps borrow another meter to check it out?

PB

[TheFlyingCrust]

Somewhere on this site theres a thread discussing whether a windmilling prop produces more drag than a static one. And the answer was yes, because the static one was stalled.

So by inference, could it be that the finer pitched prop you're using is not stalled where the coarse one is (at least in part) thus providing less load on the motor? That would all change once there's some airspeed involved.

Just a thought. I can't find the original thread.

Ian

[Phil Green]

I think you would hear it cavitating if it was stalled

[Simon Chaddock]

John

As Peter pointed out the way a brushed or our type brush less electric motors work is that the for the same applied voltage the faster it goes then the less current it takes.

So as expected it turns faster with the 9x4.7 so it is taking less current then the 9x6 but this is only true if the applied voltage is the same.

As you say the readings vary when under full power I wonder if you are overloading the battery. You would expect an 'adequate' battery and ESC(?) to provide a virtually constant reading for at least a minute even under full power.

What battery are you using and Is your ESC rated for at least 60A continuous?

[Mike Smith]

I'm using a fully charged (and balanced) Dynamic 3s 3200mAh LiPo with an Overlander XP2 80A ESC (80A continuous, 100A max).

[Mike Smith]

...I meant to say that the LiPo is rated at 30C continuous, so should be good for 96Amps.

[Peter Beeney]

Mike,

If the revolutions figures are correct, and that does look to be favourite at the moment, then the current figures are most unlikely to both be correct. This would be apparently defying some standard electrical principles. Unless it’s possible some current is somehow going somewhere else, which sounds highly unlikely but it might mean that something is getting a trifle warm, perhaps. Ideally a clamp meter would be one answer, then you can at least verify what’s going where. That’s all I use anyway, with a tacho, but if the amps go down the revs are going up anyway.

I for one will be very interested if you do find a solution to this, it all does appear to be a bit unusual, to say the least.

Good Luck!

PB