Motor / prop calculator?

[Peter Beeney]

PeterF @17/02/2019 23:36:01

Peter,

Just jumping in here out of interest, can I say that I simply check any rpm figures with a tacho. In the first instance this would be the unloaded speed the motor achieves. This is the starting point; in most cases the figures are fairly accurate, but sometimes they can be a bit adrift. Recently I checked an unlabelled motor, the model’s spec sheet gave it’s kV as 750, whereas the actual revs per volt was 600. Had I not known this it might have caused some head scratching later on.

Then checking out the motor’s specification sheet will invariable give me a clue on propeller size, I can then find the exact revolutions at full throttle again. If I compare this figure with the unloaded revs and also knowing the resistance I can make a very rough estimate of the current flow. But as I’m usually not that not sure anyway I simply check using a clip on power meter.

I’ve never used any of the online calculators, but I have no problems with these whatsoever. Anything to make life a bit easier.

I have to say that I’m not quite sure by the ‘as motor power is proportional to rpm cubed,’ statement. I’ve always though that the the mechanical power is the product of multiplying the rpm by the torque. The torque is at a max at the instant of start up and deceases down to a min at the unloaded revs per minute. Thus the power curve rises to a peak from zero at start up and back down to zero at the unloaded revs. My aim would generally be to get as close to the peak power output as possible.

Interesting stuff……maybe…

PB

[PeterF]

Peter,

Power being proportional to speed cubed is one of the basic fan laws, I have worked on large fans throughout my career. This strictly relates to a stationary fan and you are correct that a moving prop will have some unloading. Your view of the motor current being peak at zero revs due to peak torque is in our terms as model fliers somewhat academic in my view and is only of transient concern as the prop is accelerating to full rpm and not something we would be able to measure. We are interested in steady state performance, and once the prop has reached full rpm, its aerodynamic load from moving all the air around is at its peak.

Consider a plane on the runway with the motor off. The throttle is opened fully, the prop rapidly accelerates to maximum static rpm and takes a certain amount of power to do this, which is largely irrelevant. At the peak static rpm, there will by a high power delivered to the prop as it is now moving the maximum amount of air from zero velocity to lets say pitch speed. The model starts to accelerate, takes off and reaches it maximum airspeed. The power required to overcome the drag of the airplane increases rapidly with its airspeed, again by the cubic law. Therefore, the planes maximum power occurs at maximum airspeed. The propeller has to deliver this maximum forward thrust, therefore the motor can not fully unload. It will unload some amount but never a lot, because for us, the motor essentially has a load that increases with prop rpm.

The cubic law is involved in nearly all turbulent fluid flow phenomena. If you have a pipe and you blow a certain amount of air through it with a fan, you will have a certain pressure drop. If you double the air flow (by doubling the fan rpm) the pressure drop will increase by a factor of 4, in other words 2 squared. The power required is the pressure drop times the flowrate. Therefore, doubling flow requires 8 times as much power.

This is simplistic but hopefully useful.

[PatMc]

Posted by PeterF on 18/02/2019 21:44:26:

The model starts to accelerate, takes off and reaches it maximum airspeed. The power required to overcome the drag of the airplane increases rapidly with its airspeed, again by the cubic law. Therefore, the planes maximum power occurs at maximum airspeed. The propeller has to deliver this maximum forward thrust, therefore the motor can not fully unload. It will unload some amount but never a lot, because for us, the motor essentially has a load that increases with prop rpm.

 

Peter, surely the thrust is at it's max when the aeroplane is static then diminishes during the acceleration at the same time drag increases . These changes continue until thrust & drag are equal and the aeroplane is at a steady speed in level flight.

Edited By PatMc on 18/02/2019 22:15:46

[PeterF]

Pat,

Thanks for your point, I had alluded to the motor offloading somewhat but not explained it very well, I know it occurs, just didn't get it over very well. Yes, as the model speeds up, the propeller thrust does reduce as the drag increases, but I believe that the drag increases faster than the thrust reduces, so the motor remains under a high load.

What I was trying to explain was the conundrum Peter B had raised between my assertion that motor power rises with the cube of the rpm and his view that motor power rose then diminished. I did not clearly answer that, but your point has got me to the answer in my head.

To Peter B's

You describe power from the motor rising from zero to a peak and then declining to maximum rpm fully unloaded. However, to get a motor to follow this curve requires a variable load with rpm. With a propeller, this is a fixed device whereby the power demand can only increase as the rpm increases. Therefore, what you describe is the maximum potential power that the motor can supply at any rpm and I describe the load placed on the motor bythe propeller, these are different. The motor can only supply what is demanded of it.

Graphing both of these illustrates the difference.

I have plotted the maximum power deliverable by the motor over the rpm range (red) as the torque (blue) falls per your description. I have plotted the power required by a static (green) and a flying (purple) propeller. In our case, as the propeller is fixed, the motor may be able to deliver more power at 5,000 rpm than the propeller demands from moving air, and if the throttle is fully open, the difference in power not required to move air will be used to accelerate the prop to its peak rpm. The peak rpm achieved is less than the unloaded rpm and occurs where the potential power delivery from the motor matches the power demand from the prop. When the prop unloads in flight a small increase in rpm occurs. Typically for our models, we operate with the motor rpm around 80 to 90% of the unloaded rpm. Thus, any higher power demands during propeller acceleration are transient and have little effect on motor temperature. What motor rating is all about for our models is the steady state condition because this is where they spend a lot of their time.

Hopefully, I have better explained the differences in our thoughts.

Once again, this is simplistic.

[Martin Harris - Moderator]

Posted by PatMc on 18/02/2019 22:05:54:

 

Peter, surely the thrust is at it's max when the aeroplane is static then diminishes during the acceleration at the same time drag increases . These changes continue until thrust & drag are equal and the aeroplane is at a steady speed in level flight.

Perhaps I'm missing the point but I understand that at maximum speed in level flight, thrust = drag and drag is at its maximum. Throttling back will cause a reduction in airspeed (and consequently lift but this is irrelevant to this point). Doesn't this equate to maximum power being delivered at the maximum speed? Isn't the "reduction in thrust" with increasing speed a measure of the differential between the power required for any instantaneous speed and the power available for further acceleration?

Edited By Martin Harris on 19/02/2019 00:32:48

[Nigel R]

We also have the airspeed going into the prop to contend with.

If airspeed into prop is equal to pitch speed then we have no thrust at all. And consequently no power consumed. And the motor would be able to unload to max rpm.

This couldn't be steady state flight of course as drag would then slow the airframe down.

As I understand it etc.

[Former Member]

[This posting has been removed]

[Dickw]

Posted by PeterF on 18/02/2019 23:39:48:

........................................

Graphing both of these illustrates the difference.

I have plotted the maximum power deliverable by the motor over the rpm range (red) as the torque (blue) falls per your description. I have plotted the power required by a static (green) and a flying (purple) propeller. In our case, as the propeller is fixed, the motor may be able to deliver more power at 5,000 rpm than the propeller demands from moving air, and if the throttle is fully open, the difference in power not required to move air will be used to accelerate the prop to its peak rpm.................................

Once again, this is simplistic.

That is a very useful graph to liiustrate what is happening, but before people rush off to get max motor power by propping for 50% unloaded rpm, perhaps we should add that the motor efficiency will be about 50% at that point. That's why we generally prop for the 70 - 90% unloaded rpm range where the efficiency will be more like 70 - 90%.

Dick

[Stearman65]

Posted by David Mellor on 19/02/2019 10:42:59:

Just to lighten the mood a touch......

I'm up in Scotland, anti-fouling our boat before she goes back in the water tomorrow.

One spect that is important in propeller efficiency is....... barnacles. Here's my nice bronze 14" x 8.5" prop after cleaning the little blighters off .

That brings back memories, on the hard in Aguadulce Spain 2003, 42 degrees +, using black antifouling for med' use.

Happy days

[Martin Harris - Moderator]

Got no barnacles on any of my props but a clubmate had a very interesting experience on a cold damp day when doing touch and goes with a Moki 180 powered 3DD (no lack of grunt). After a couple without incident, he tried another, the engine responded normally but the model failed to accelerate.

On reaching the model in the adjoining field (happily there was no fence/hedge) we found that a considerable amount of lumpy ice had built up on the prop!

Edited By Martin Harris on 19/02/2019 14:50:42

[PatMc]

Posted by Martin Harris on 19/02/2019 00:29:18:

Posted by PatMc on 18/02/2019 22:05:54:

Peter, surely the thrust is at it's max when the aeroplane is static then diminishes during the acceleration at the same time drag increases . These changes continue until thrust & drag are equal and the aeroplane is at a steady speed in level flight.

Perhaps I'm missing the point but I understand that at maximum speed in level flight, thrust = drag and drag is at its maximum. Throttling back will cause a reduction in airspeed (and consequently lift but this is irrelevant to this point). Doesn't this equate to maximum power being delivered at the maximum speed? Isn't the "reduction in thrust" with increasing speed a measure of the differential between the power required for any instantaneous speed and the power available for further acceleration?

Edited By Martin Harris on 19/02/2019 00:32:48

I only referred to thrust not power which was a reference to Peter's "The propeller has to deliver this maximum forward thrust,"

[PeterF]

Posted by Dickw on 19/02/2019 14:26:51:

That is a very useful graph to liiustrate what is happening, but before people rush off to get max motor power by propping for 50% unloaded rpm, perhaps we should add that the motor efficiency will be about 50% at that point. That's why we generally prop for the 70 - 90% unloaded rpm range where the efficiency will be more like 70 - 90%.

Dick

Dick,

Thanks for this input, I wondered why we always run out motors at the 80 to 90% of unloaded rpm and putting that graph did raise it in my mind again.

Peter.

[Peter Beeney]

I think I’d be a bit reluctant to try propping, or loading, a motor to run at 50% of it’s unloaded rpm, not least because the performance might be pretty underwhelming; for me anyway. Also I might be a trifle concerned about the wisps of smoke emanating from around the nose area too, it’s most likely the paint blistering on the motor casing. I think I’ll stick to the tried and trusted method of finding the best prop by simple checks before I get airborne and then making any necessary small adjustments after. I aim to get as close to the unloaded revs as possible whilst still having the max performance I’d want to see from that particular model. For me this would still apply to whatever sort of model I’m flying, not making full use of the available energy all the time is perhaps a bit of a waste; and as another forumite recently remarked, “Wasted Watts are akin to carrying a lead weight around for nothing”, …I’d certainly have an affinity with that!

In fact, I suspect that motors when correctly propped do run fairly close to their unloaded speed. Depends to some extent on their resistance I think, but if they don’t the current flow might soon become a bit of a torrent…

I’m sure the various apps used for calculating prop size etc. are very good; but if some incorrect information is unintentionally fed in then that may change everything. All of our toys are really only consumer items I guess, and as such I suspect the testing and specification labelling might not always quite be up to scratch; but now having said that I still think today’s motors, ESCs and ancillary bits are remarkable examples of engineering. Now all we want is a battery boasting the same exacting standards. Unfortunately I reckon batteries must probably be one of the ultimate consumer items therefore a continuing turnover has to be continually maintained…

Just my ancient cynical view again.

PB

[Nigel R]

"I still think today’s motors, ESCs and ancillary bits are remarkable examples of engineering. Now all we want is a battery boasting the same exacting standards."

Definitely debatable. I've yet to see any software involved in our sort of kit meeting any particular exacting engineering standards.

"Unfortunately I reckon batteries must probably be one of the ultimate consumer items therefore a continuing turnover has to be continually maintained…"

If you can figure a way to make batteries last longer you will be a very rich man.

[Stearman65]

Posted by Stearman65 on 18/02/2019 11:48:23:

Posted by David Mellor on 18/02/2019 11:43:46:

Well, it was the balance method in general that I was referring to........

Just bought one of these on Ebay.

**LINK**

Edited By Stearman65 on 18/02/2019 11:50:11

My digital spring balance arrived this morning. Looks OK for the money.