Motor thrust measurement

[John Privett]

But any difference you detect will be relevant only for a static aircraft - ie. as it begins the takeoff run, and as soon as it starts to move then conditions deviate from your test conditions.

Without (further, different) tesing, how will you know that the prop that turns out to be best when static might not actually give the worst performance when moving? - ie. for the vast majority of the flight!

 

Edited By John Privett on 01/06/2013 22:36:17

[Erfolg]

I am not concerned in establishing what is the best propeller for a given situation.

Perhaps if i try a slightly different approach.

If we accept as a starting point that electric specific propellers, such as the APC electric is the benchmark, for converting a given voltage and ampage into thrust.

What if I take a Master electric/IC propeller of the same dia and pitch, how much less thrust will it provide? Is it 1%, 10%, 30% less? What if I take a JM polycarbonate, or a Tornado Nylon prop, would it matter in thrust delivered?

Another benchmark could be, could a Master carbon prop, operating at 20,000 revs be a better bet than a APC?

Please bare in mind, I do not know the answer. It could be using anything other than a APC is not a good move, or may be it does not really matter. It could be that at the extremes that using a APC is not appropriate and there is in principal a better solution.

I am hoping over some time to at least get a feel, if not a definitive answer.

In some ways it is similar to my snake experiment. Where some one had suggested that it did not matter if intermediate positions on the snake were anchored, as long as both ends were securely anchored. The experiment provided the answer, he was correct, even with extreme end loading, there was no additional movement to the inner, at the ends. Additional attachments not making a jot of difference. There was a postscript to that experiment, I tried a pushrod with the same end loading conditions. The pushrod crippled and broke long before I could reach the same end loading, even worse, there was significant effective shortening, as the load increased with the bicycle spoke bending, in addition to the side deflection/bowing of the assembly. I concluded the worst choice was a pushrod, even when done well..

 

Edited By Erfolg on 01/06/2013 23:32:58

[Simon Chaddock]

Erfolg

Such a test as you propose is quite logical as it will measure the difference in efficiency between two props of the same nominal characteristics, i.e. diameter and pitch allbeit at only one flight condition..

I would hope you will find that at the same revs the 'e' props are slightly more efficient but I expect they will also be subtle differences between the props from various manufacturers as well.

I for one would be very interested to hear the results.

[DH 82A]

"" I have often read that we should only use electric props on electric models. "

I think the reason for this statement is that E props are far lighter in construction, so if used on an I.C. engine, there is a danger of throwing a blade, etc. On the packaging of APC E props there is a warning not to be used on I.C. engines.

There is no reason not to use I.C. props on electric, some of my E models have them.

[Erfolg]

I think that this link to RC Groups may be of interest to many.

It seems some one is already doing something along the same lines as I intended doing (hoping to start making my equipment next week).

As far as i can tell, if we use the APC(e) as the base line, primarily they have been the touchstone/benchmark for many electric flyers. The indications are that other props are drawing more current, and producing more thrust for a given voltage.

My thinking then moves along the lines of efficiency, so is that gram thrust per volt or is gram thrust per watt, which seems more rational, a relationship between power in, to thrust out.

At the back of my mind is the notion, that as a modeller i tend not to worry about circa 5% differences, although I do sit up when differences are +20%.

As I have not taken in all the information yet, I personally have no opinion.

[Simon Chaddock]

Many EDF do give a thrust efficiency in g/watt.

Why the same for props?

[Tony K]

Posted by Simon Chaddock on 06/06/2013 00:10:49:

Many EDF do give a thrust efficiency in g/watt.

Why the same for props?

Simon, did you mean, "Why NOT the same for props"?

Erfolg, I am following this and I hope you can obtain some meaningful results.

You wrote,"The indications are that other props are drawing more current, and producing more thrust for a given voltage". This suggests (considering the characteristics of brushless motors) that the "other" prop must be a coarser pitch than the base line sample.

Obviously, if propellor B causes the motor to draw more current without increasing thrust, it is less efficient than propellor A, and vice versa.

I presume this is the aim of your proposed testing.

Your thinking along the lines of thrust per watt also seems more rational to me.

BTW, I do not go along with the idea that static testing is irrelevant.

[Simon Chaddock]

Erfolg

Ooops!

Without rather sophisticated in flight measurement static thrust testing is about the only way to get accurate comparison between props. Any difference may not translate directly into flight performance but its a good starting point.

I suspect it would be pretty difficult to really spot say a 5% difference in full power thrust when actually flying a plane.

Edited By Simon Chaddock on 07/06/2013 10:02:19

[Erfolg]

My main interest is in understanding what are the features which enable an individual to select a prop with some understanding with respect as to which features are significant.

Using a non modelling analogy, it is known that the addition of carbon to a steel determines the ability to heat treat. The addition of alloying elements can have a major impact on the heat treatment process, with respect to cooling velocity. Very high quantities of say carbon, and you no longer have a steel. Yet a knowledge of the role does enable engineers to make rational decisions.

From what I have read on RC Groups, weight is a factor, although no significant issue is made of the observation. I need to read this bit again.

Again, it is observed that pitches are not consistent, yet no comment has been made as to why. I do not believe that manufactures are making mistakes, I believe it is a conscious decision. From other publications, I remember that for example a 6" pitch, will not be 6" when measured, if the producer knows what they are doing. an allowance is made for slippage/fluid mechanics. I seem to remember that for control line speed, where motor revs are very high circa 20,000 rev/min, is very different to a cooking IC at 10,000 rev/min, for supposably the same pitch.

I think that this may be a major factor, for us electric modellers, as there is a very wide range of revs compared with IC, from geared props, sometimes just a few hundred revs a min, the mid range Kv being nearer the IC @ 10, 000 rev/min and many inrunners doing circa 20, 000 rev, min.

At present I have done nothing as having house cavities insulated is diverting my attention and today has prevented me going out flying. Thanks to God, it seems to have been done now.

As I have said, some IC props are very similar in weight to IC props. I have just cancelled some props on order, as the ones I had ordered are very much heavier than the APC type weight, which I am using as a benchmark, for the time being. Just another cock up, by me.

[Andy48]

I built myself a simple test rig to the one shown above to measure static thrust some time ago. I would love to know just how you could ever measure in-flight thrust, so it would appear that static thrust is better than nothing. On Ebay, the luggage balances are just a couple of pounds each. My cheapo one gave exactly the same reading as the airport scales, so they seem pretty accurate.

I find it an invaluable aid to getting the motor/ESC/prop/battery combination right before flight. I use it to program the ESC before use too. I have found that different makes of prop seem to make little difference, neither does the make of ESC. Some of the ESC settings do make quite a difference to performance. However the biggest effect seems to come from the motor itself. Try swapping similar spec motors over using the same props for instance and you get some quite big differences. For instance I can get nothing like the power out of Emax GT motors compared to their BL equivalent, although the former are supposed to be more efficient.

You can also test the performance of different sized props at different power levels too.

[Erfolg]

You are correct Martian

The aim is to accept that the in air performance will be different.

There is another way we could consider the issues. We are aware that unlike IC engines, there is a massive variation in Kv of motors, also that there is a wide variation on voltage from 2/3/4s and more. So that for any given revs, it is appropriate to some circumstances. We just know that the revs of electric set ups are far more varied than IC, that is in general. So any one set actually replicates some hypothetical revs for a flying model.

What is different is the air vector due to the lack of forward trajectory. Which also varies with air speed within a range from 0 - max flying speed.

I am of out now flying

[Erfolg]

I have struck the plant pot idea on the head,

I concluded there were two issues, the first that I needed a substantial clearance between the propeller tip and the plant pot side, The reason being is that propellers draw air in from a region substantially greater diameter than the prop. So even with a large clearance, I could not be sure that this issue was having a substantial effect on the results.

The second was that which ever way I ran the motor, the slots to allow the air ti discharge would also have some impact.

Which has decided that it will be the conventional "L".

Although i did toy with a "I" with the prop on the top. I decided stability could be an issue if sufficiently high to keep the prop out of my way.

A simple beam, pivoted in the middle seem attractive in that it could be run on the garden table quite safely.

[John Privett]

Posted by Erfolg on 07/06/2013 13:33:52:

I seem to remember that for control line speed, where motor revs are very high circa 20,000 rev/min,

Actually, you'd be closer with double that! Analysis - by other people - of the video I shot of Paul Eisner breaking his own c/l speed record 10 years ago confirmed Paul's estimated figure of around 38,000rpm.

[PatMc]

Posted by Tony K on 07/06/2013 08:46:15:

BTW, I do not go along with the idea that static testing is irrelevant.

What useful info does static thrust give ?

[Erfolg]

Hi John

It would not surprise me that the figure I have used is not anything like a current value. I would have been recalling values possibly from around the late 60s.

I have been reading a paper written by Selig, that one of the contributors has pasted a link to. It arouses my suspicions that many of the electric propellers were operated at low revs. Selig only gets into the 5,000 region. I am mainly aiming for the 10,000 revs, to enable reasonable pitches to be used and mid sized propeller diameters to be used to improve efficiency by propeller diameter.

I will shortly, I hope start on the recording of actual results.

I have been surprised at the largish variation in propeller weights with IC props. Some are very similar to electric props, yet others are circa 25% heavier.

I can see that I will have to record a lot more than I envisaged even with a limited selection of propellers. All of which will take time.

I have tried tentatively to measure the thickness of a couple of propellers, at various stations. Which is harder than i envisaged. Micrometers are not well suited to the process, due to the none flat surface. Outside callipers give the feel, yet to get any measurement I would need slip gauges, which I do not have. Plus it would be a slow process. The question then would be is the thickness ratio comparable, and is this where the extra mass is. Or is it that a denser material is in use.

The more I think about the issues, the more complex I can make the analysis. This is not the way forward, it has to be simple questions, which give some useful data.

[Erfolg]

Hi Pat Mac

I am not concerned with obtaining specific propeller useful information.

Without a wind tunnel and a lot of data collection, I am not sure that any specifically relevant information. Even then I would suggest that it would essentially be comparative data for specific sized and manufactured propellers.

What i am trying to establish is limited in its objective, although it could be hard to determine with the certainty wanted. That is

1. Is propeller mass a major factor in the thrust achieved, for a specific diameter and pitch.
2. Does the revs have an impact with respect to so called electric props and IC props.

Selig seems to hint that revs do matter, in that the higher the Reynolds numbers improve efficiency. I will undertake a few calcs to establish which region props are operating in. Again I suspect that at low revs, the numbers could be in on the cusp of one region relative to the other. The higher revs defiantly putting the prop in the turbulent region.

It is noted by Selig that his props all had flash, which was left. This alone is enough to ensure that any laminar flow, is almost certainly not.

If I can answer the two questions then we could have useful parameters in selecting a propeller. Not a complete answer, just a bit more insight.

[Erfolg]

Allthough not strictly on subject, here is a good value KC scale.