Turnigy motor thrust stand

[Geoff S]

In an idle moment I was having a look at the Hobby King/Turnigy motor test stands like this one. I was wondering if anyone had any experience of them - do they work well? are they well made? and, most important, are they of any value in assessing a drive train's performance?

I know static thrust isn't necessarily of any great value as thing change once the model is flying but would a gadget like this make valid comparisons between different set ups eg different props and/or battery voltage?

In other words are they worth the money? This one is about £60, there's a cheaper version (the Mk1?) over on the EU warehouse here.

Geoff

[PatMc]

TBH I can't see any practical use for this gadget at all.
If you want to make comparisons between props you'd need to measure the speed of the air being moved at the same time as measuring the thrust & the power in would have to be identical, or nearly so, for the comparisons to be valid.
You can buy a lot of props for £60 if you just done the comparisons on a "suck it & see" basis.

[Martin Dance 1]

I have the mk1 type and provided you an use it in still air does provide a rough comparison of propeller performance. However you also need a tachometer and a watt meter if you are going to make any sensible performance comparisons. Clearly the aim would seem to be to achieve the highest thrust figure with the lowest current draw and have an eye on RPM. Or some combination of the parameters.

[PatMc]

Posted by PatMc on 01/02/2016 23:23:18:

TBH I can't see any practical use for this gadget at all.
If you want to make comparisons between props you'd need to measure the speed of the air being moved at the same time as measuring the thrust & the power in would have to be identical, or nearly so, for the comparisons to be valid.
You can buy a lot of props for £60 if you just done the comparisons on a "suck it & see" basis.

The crossed out bit should be : ...compare the thrust at identical current or identical rpm (doesn't matter which) or compare current/rpm at identical thrust...

However this is still only a static comparison, when moving the "worse" prop might well make more use of the power.

[Andy48]

In answer to Geoff's question, no, buy April's RCME instead, and save £57.

To challenge PatMc, how much testing have you actually done using static thrust?

Lets go back to basics. For the AVERAGE electric model we want three things:

1. There is enough power to get the plane off the ground and to fly well.

2. We need to know that the motor/prop/ESC combination is not going to draw more current than either the motor, ESC or battery can comfortably handle.

3. We would like the combination to be as efficient as possible, to get a longer flight time.

All we really need to be concerned about is static thrust and current draw. I use a very simple rule of thumb. If the static thrust is equal to the weight of the plane, then it is going to be a comfortable flying experience. The plane will take off easily from grass and have plenty of power for normal flight. If one wants unlimited vertical flight then the static thrust needs to be higher. High speed planes are a completely ballgame however.

A static thrust test jig allows us to see the actual thrust of a motor combination, to note the maximum current draw, to compare different propellers, to set up the ESC and even to check different battery specifications, and all this can be done on the bench. You can check for such things as overheating and vibration. One can test different sizes and makes of propeller quickly and easily. Once the model is built, everything can be installed, in the knowledge there is no further setup or fiddling required, other than perhaps setting up the ESC limits and failsafe when one first hooks up a receiver.

To come to Pat's point about it only being a static comparison, I've done tests in the air, using the full range of telemetry including airspeed, current draw, and motor RPM. As this is logged on the Taranis it is easy to look at the whole flight afterwards. I found very little difference between performance in the air and static testing.

If one is really pedantic, one might suggest we ought to test in a wind tunnel. That way we could check for propeller unloading. Actually a static test jig can do this too. Using either GPS or a pitot tube airspeed indicator, I know many of my models fly around the 50-75mph mark. Well, yesterday the wind speed was over 30mph as measured in the garden, so I did some testing, first with the test rig downwind, and then upwind, giving a windspeed difference of 60+ mph. Propeller unloading? Look into this and one will find much is concerned with ic engines, or real planes and then assumed to be the same with electric. In reality with the jig into the wind the thrust was 10% lower than downwind, not that big a difference really. The motor data suggested a thrust of 890g with a 4S, I actually achieved 670g using a 3S.

A static test jig is a really useful bit of kit. At the end of the day, as telemetry (and the test jig) shows, with electric there is a constantly varying situation as the voltage drops on the battery, with a large drop within the first 30 seconds. To use the test jig, I log the current, voltage and thrust every 30 seconds on full power. From that data one can quickly compare different setups.

 

Edited By Andy48 on 02/02/2016 12:28:12

[Geoff S]

Andy, do you mean next April's RCME? Or another year?

Actually when I first played with electric flight I'd only just begun to be interested in aeromodelling and it was in the days of 400/600 brushed motors and NiCads. I did rig up a few static test experiments by having a fuselage on the bench restrained with a spring balance. I also spent bit of time trying to work out how much air was moved by a propeller and how much thrust it would generate based on its density. My excuse is that I'd only just retired as an electronics designer concerned with measurement and control of gas turbine test rigs and I was trying to pretend I was still interested

I'm sure I could design and make a rig similar to the Turnigy one but it would take a lot of time, would be a secondary activity and I don't have a whole lifetime left. I'd rather build and fly models (as well as go on the occasional bike ride) and there are no pockets in shrouds

Your experiments are interesting on the differences in thrust when the air is moving. I have current measuring telemetry on my Tiger Moth. My intention is to use it on new models for the first few flights to get a better idea of what happens in the air. It was only a 6 minute maiden in a weather window in December and it revealed I was drawing only about 20 amps in level flight and a maximum of 50 flat out.

I'm a sucker for measuring instruments ( I bought an Astroflight Wattmeter when they were £50) so I admit I'm tempted to get the Turnigy device which does look quite well made.

Geoff

[Andy48]

April 2016. - takes a couple of hours to build at most.

[Dickw]

Posted by Geoff Sleath on 02/02/2016 13:09:51:.........................

Your experiments are interesting on the differences in thrust when the air is moving. I have current measuring telemetry on my Tiger Moth. My intention is to use it on new models for the first few flights to get a better idea of what happens in the air. It was only a 6 minute maiden in a weather window in December and it revealed I was drawing only about 20 amps in level flight and a maximum of 50 flat out.

I'm a sucker for measuring instruments .......................Geoff

I guess your Tiger moth data would look something like the attached log from one of my models. Around 10A for much of the flight with peaks up to 50A. That was using an on board logger, but these days I get most of that data via telemetry as well.

Full throttle static thrust is useful for initial take off/launch or prop hanging, but I have never found it to be of much additional use on most of my models.

Dick

[Geoff S]

I didn't log the data on my Moth test flight because I couldn't remember how to do it I have altitude logging on my Phoenix 2k but I'd had a a brain fade so I just listened to the current logging in real time during the flight and relied on brain logs (which are brief and short term in my case). I've now rediscovered how to do it so subsequent flights will have data to look back on. Just waiting for some warmer, calmer weather.

My 6 minute maiden was deliberately ended early to give me chance to check on energy consumption. I had 50% left in the 4 AH 4S LiPo if the battery checker is anything to go on.

Geoff

[Dickw]

Posted by Geoff Sleath on 02/02/2016 16:35:58:

I didn't log the data on my Moth test flight because I couldn't remember how to do it I.............

Geoff

At least you got some useful information, which is the important bit.

Perhaps I should add to my earlier post that while I have never found static thrust particularly relevant or useful, any data has to be better than none provided you can figure out how best to use it.

Dick

[Andy48]

Why isn't static thrust useful for electric flight?

[Geoff S]

I was just thinking about static thrust when I realised all the test beds at my former employer (and pension provider ) measure static thrust so I suppose those that design and develop gas turbines must find some use for it. Royce's certainly spend a lot of money measuring it, amongst other things. Not being a mechanical or aeronautical engineer I don't know much about the details I was merely their servant.

Geoff

[Dickw]

Posted by Andy48 on 02/02/2016 17:32:54:

Why isn't static thrust useful for electric flight?

I didn't say it wasn't useful, I said that I had never found it particulary useful to me.

Perhaps I fly faster planes, but as the speed of flight increases thrust drops off rapidly (particularly with lower pitch props) and the drag increases so I can't go any faster than when the falling thrust value equals the rising drag value. Static thrust doesn't help me at all.

My slower models mostly fly at part throttle (as shown in the data log I attached earlier) so what throttle setting should I use to test static thrust. (that log dates back to about 2010, ands I have been logging data since 2008).

Each to his own though, so if it works for you stick with it. I get on well with performance simulation software and find it matches my real world results closely, but I know many people can't make it work.

Dick

[AJ]

Bruce on the rcmodelreviews youtube channel has one of these. He has done a number of videos comparing diferrent props and motors. Worth a look

A.

[Simon Feather]

Personally, I do find a static thrust measurement a useful guide as to whether a given model is likely to have enough power or not. Yes, there are many other factors at play and it won't give an accurate representation of what actually happens in flight, but I find it helps.

The Turnigy device looks well made, but is it worth £60? My home made guesstimator may not be terribly accurate but it gives me confidence that I've got enough thrust! Proven several times, a couple of "that'll never fly" (one later proven at the field, thought I'd give it a go anyway, hey ho), and one "whooooaa!!"

I bought a cheap set of fishing/luggage scales off Ebay (about £4 if I remember rightly); use the webbing strap off a laptop case to go from the weighing hook, under the tailplane, over the fuz in front of the fin and back under the tailplane on the other side, fasten back to the weighing hook. Fasten the hanging hook to a sturdy object on the floor (I used more laptop bag webbing looped around the piano stool full of books!). Hook up the watt meter and let it rip. Make very sure though there is nothing loose in the room that can get sucked into the prop! I now know what happens when a children's mylar helium balloon meets a spinning prop ... we're still finding bits of mylar film in odd places!!! The bits were VERY small!

Of course this version requires the motor to be mounted to the model already, but for just four notes there have to be some tradeoffs.

You can also do something fairly quick and easy with a simple lever platform pressing on some kitchen scales... but the fishing scales work for me. (There's a pun there somewhere).

Simon

[Andy48]

...or buy April's RCME for a really simple system to use before you mount the motor in the plane.. and pretty accurate too, and £1 cheaper. he he

[PatMc]

Posted by Andy48 on 02/02/2016 12:26:29:

In answer to Geoff's question, no, buy April's RCME instead, and save £57.

To challenge PatMc, how much testing have you actually done using static thrust?

Enough to realise that measuring it achieved no useful results.

Lets go back to basics. For the AVERAGE electric model we want three things:

1. There is enough power to get the plane off the ground and to fly well.

2. We need to know that the motor/prop/ESC combination is not going to draw more current than either the motor, ESC or battery can comfortably handle.

3. We would like the combination to be as efficient as possible, to get a longer flight time.

Actually that’s 4 things, none of which are predicted by measuring the static thrust of the power combo.

All we really need to be concerned about is static thrust and current draw. I use a very simple rule of thumb.

No disagreement regarding current draw.

If the static thrust is equal to the weight of the plane, then it is going to be a comfortable flying experience. The plane will take off easily from grass and have plenty of power for normal flight.

Neither is guaranteed by 1:1 static thrust/weight ratio.

If one wants unlimited vertical flight then the static thrust needs to be higher. High speed planes are a completely ballgame however.

Agreed vertical climb requires ST>W, the max speed of climb will be reached when ST-W = form drag.
I don’t understand why you consider high speed model flight is in a different “ball game” – AFAIK same physics apply throughout our practical speed range.

A static thrust test jig allows us to see the actual thrust of a motor combination, to note the maximum current draw, to compare different propellers, to set up the ESC and even to check different battery specifications, and all this can be done on the bench.

But you’re still only measuring a static force, not power. When used to fly a model thrust diminishes as soon as the model moves forward. It continues to diminish as the model accelerates, at the same time the model’s drag increases. When thrust & drag are equal he model will cease to accelerate. The interesting parameter to measure would be the dynamic thrust over the whole period of acceleration until equilibrium.
An ammeter will do the rest either on the bench or in a model.

You can check for such things as overheating and vibration. One can test different sizes and makes of propeller quickly and easily.

I fail to see where a thrust meter is any particular use here.

Once the model is built, everything can be installed, in the knowledge there is no further setup or fiddling required, other than perhaps setting up the ESC limits and failsafe when one first hooks up a receiver.

You may feel it necessary to try different props over the course of the first few flights whether you have run static thrust test or not. But if you haven’t done static thrust tests at least you won’t have wasted time in a pointless exercise before going to the flying field.

To come to Pat's point about it only being a static comparison, I've done tests in the air, using the full range of telemetry including airspeed, current draw, and motor RPM. As this is logged on the Taranis it is easy to look at the whole flight afterwards. I found very little difference between performance in the air and static testing.

You haven’t compared any parameters other than current & rpm neither of which are measures of the model’s performance.

If one is really pedantic, one might suggest we ought to test in a wind tunnel. That way we could check for propeller unloading. Actually a static test jig can do this too. Using either GPS or a pitot tube airspeed indicator, I know many of my models fly around the 50-75mph mark. Well, yesterday the wind speed was over 30mph as measured in the garden, so I did some testing, first with the test rig downwind, and then upwind, giving a windspeed difference of 60+ mph. Propeller unloading? Look into this and one will find much is concerned with ic engines, or real planes and then assumed to be the same with electric. In reality with the jig into the wind the thrust was 10% lower than downwind, not that big a difference really. The motor data suggested a thrust of 890g with a 4S, I actually achieved 670g using a 3S.

Wind tunnel testing of thrust wouldn’t help in actually predicting a suitable prop for a model.

A static test jig is a really useful bit of kit. At the end of the day, as telemetry (and the test jig) shows, with electric there is a constantly varying situation as the voltage drops on the battery, with a large drop within the first 30 seconds. To use the test jig, I log the current, voltage and thrust every 30 seconds on full power. From that data one can quickly compare different setups.

You will have inferred my opinion of a static thrust test rig from my previous comments.
Voltage drop after a few seconds run time is common knowledge.

[PatMc]

I've had to add this post script as a seperate post as the reply with included quotes was too long for a single post.

Due to the software of this forum it's difficult to reply to so many points in a single post. I've therefore taken the liberty of italicizing the points quoted & embolding my replies.

Sorry if my comments spoil a forthcoming article but I'm expressing an honest opinion even if it does seem brutally blunt.

[PatMc]

Posted by Geoff Sleath on 02/02/2016 17:33:38:

I was just thinking about static thrust when I realised all the test beds at my former employer (and pension provider ) measure static thrust so I suppose those that design and develop gas turbines must find some use for it. Royce's certainly spend a lot of money measuring it, amongst other things. Not being a mechanical or aeronautical engineer I don't know much about the details I was merely their servant.

Geoff

This link explains why static thrust of a gas turbine engine is a useful measure but the static thrust of a prop driven source isn't.

[Andy48]

I have to say, I'm rather puzzled by most of your responses. I think you have your theory completely back to front.

But you’re still only measuring a static force, not power.  (your posts in bold)

Firstly lets take your link, it explains why static thrust of a prop driven source is not a useful measure of its power. On a model plane, how well if flies is not directly linked to the power of the engine or motor, but to the thrust. You could put a tiny prop on a huge motor and it will not fly well. You are making the assumption all through your responses that it is the raw power of the motor that is important rather than how that power is transformed into useful thrust by the propeller.

When used to fly a model thrust diminishes as soon as the model moves forward. It continues to diminish as the model accelerates, at the same time the model’s drag increases. When thrust & drag are equal he model will cease to accelerate. The interesting parameter to measure would be the dynamic thrust over the whole period of acceleration until equilibrium.

The thrust does not diminish significantly as the model moves forward. Yes it changes slightly but not by a great deal and that change will be gradual from rest to max speed. As you say, drag will increase, and when the drag equals the thrust the plane will go no faster.

Once the model is built, everything can be installed, in the knowledge there is no further setup or fiddling required, other than perhaps setting up the ESC limits and failsafe when one first hooks up a receiver.

You may feel it necessary to try different props over the course of the first few flights whether you have run static thrust test or not. But if you haven’t done static thrust tests at least you won’t have wasted time in a pointless exercise before going to the flying field.

Testing down at the field with different props showed exactly the same telemetry as static testing with different props, therefore the exact opposite to what you claim is true. Contrary to one of your comments above I was also measuring airspeed and using GPS data.

 

Edited By Andy48 on 07/02/2016 22:14:06

[Andy48]

You may want to have a look at this paper.... um they use wind tunnel testing though.

**LINK**

Look at the chart at the top of page 24. It shows thrust against airspeed.

25m/s is about 55mph, and as can be seen from the top two graphs, the drop in thrust is less than 10% from 0mph to 55mph.

[David perry 1]

Im still looking for the answer to whats in April 16

D

[PatMc]

Posted by Andy48 on 07/02/2016 22:00:02:

But you’re still only measuring a static force, not power. (your posts in bold)

You are making the assumption all through your responses that it is the raw power of the motor that is important rather than how that power is transformed into useful thrust by the propeller.

No, you are assuming that I'm making that assumption.
In my sentence above I only refer to the prop, no mention of motor power there or anywhere else in my responses.
What I am saying is that measuring static thrust is not measuring power - i.e. the power exerted by the prop to move the model.

A suitable prop needs to be able to convert the motor's power into sufficient thrust and be able to move the model at a flying speed.

A large, low pitch prop could produce a static thrust greater than the model's weight but not be able to move the model fast enough to take off. Whereas a smaller dia, higher prop converting the same power but producing static thrust well below the model's weight might well move the model fast enough for satisfactory flight.
I had an experience of this, many years ago in the era of brushed motor & nicads, that was my "eureka" moment. It made me to realise that measuring static thrust just wasn't worth the effort.

The hobby is about having fun flying, building & model aeroplanes & personal achievement, not taking and comparing numerous measurements.
It doesn't take long to gain the experience to know what range of prop sizes are going to be suitable for a motor/battery combo in a particular style/size/weight model. If a few props within the range are included in the flight box then if it's felt necessary after the first flight or two to change the prop for another that's no great deal. A lot more enjoyable than spending flying time poring over recorded data in the hope of not having to change a prop at the field.

I'm not saying that no measurements should ever be taken &/or recorded, they are often essential but shouldn't become the tail wagging the dog.

[Andy48]

Sorry but your theory really is incorrect. The thrust IS the power of the prop to move the plane forward.

A large, low pitch prop could produce a static thrust greater than the model's weight but not be able to move the model fast enough to take off.

This is really the point you have not understood. The static thrust is the push, force whatever you want to call it that wants to move the model forward. So long as the static thrust is greater than the rolling resistance of the plane, then it will move forward - it is accelerating. The rolling resistance is usually much less than the weight of the plane. As the rolling resistance decreases, so the drag will gradually increase . The problem you highlight above is not about the relationship between static thrust and weight but the rolling resistance which is greater than the static thrust.

You appear to be thinking that static thrust and dynamic thrust are two completely different forces, whereas they are not. At 0mph we have static thrust, at 1mph we have dynamic thrust, and the two are to all intents and purposes identical. There is plenty of evidence around to show that by the time we get to about 60mph, there has been a gradual change in the dynamic thrust from being the same as static thrust to differing by about 10%. At the end of the day Newtons second law of motion will always apply, F=M*A, F being the net force, be it thrust minus rolling resistance, or thrust minus drag.

Your practical experience of static testing relates to Nicads and brushed motors. Nowadays we are dealing with a whole new ballgame. Outrunners are significantly more efficient, and coupled with lipo batteries have completely different characteristics. For instance, testing shows that in reality changing props from one size to the next with an outrunner has remarkably little difference in the thrust curve, but does change the top end performance, ie the maximum thrust, and more important the maximum current.

Again I think you are taking a wholly simplistic view about static testing. Before a model even starts on the building board, you can test for an appropriate sized motor, see what current draw is like, look at the size of battery needed, even test different quality batteries. What are the best setting for the ESC? It takes but 5 minutes to get the best settings programmed on the ESC. How much better is the £30 super spec battery than the cheapo £12 battery. Experience coupled with an element of suck it and see, or pretesting - surely that's down to personal preference.

Its not about changing the prop down at the field, although these days I never need to do it, its about setting up the motor, ESC and prop for best performance, and KNOW that when it gets built into the model, there is no faffing about but that it will perform as expected. - even simple things like knowing the motor will run the right way.

It does not take hours of pouring over data, normally half an hour in the workshop with the static test jig clamped in the vice and the whole powertrain can be sorted, and the ESC programmed. However, if you do want to understand more about the powertrain characteristics, then it is ideal for that too.

Finally, every modeller is different, don't condemn something in the way you just have, because that is not to your liking. The hobby is about whatever aspect of it each of us finds interesting. Personally the time down at the flying field is precious to me, there are far too many days when its not suitable to fly for one reason or another, and the last thing I want to do is spend most of the session trying to get the model sorted. As a physicist I do like to understand what the plane is doing, as an engineer, I like to build a model that will be sorted before it ever takes to the sky.

[Geoff S]

Why am I regretting asking the question in the first place?

Having said that, I wonder why static thrust tests are always performed on full size gas turbines (I know they are because I've seen lots and I assume the designers at RR know what they are doing) and is therefore a valid measurement yet the same test when the load is a propeller isn't. How about turboprops? Or turbofans both of which are run on test beds with load cells measuring the static thrust.

How does an aeroplane 'know' what is providing the thrust - gas turbine, ducted fan or conventional airscrew? All generate the thrust by moving air mass. It seems to me the only difference between static thrust and that experienced in the air is that aircraft is moving forward and Andy has shown that, using telemetry, there is very little difference at the speeds we are interested in.

My cousin's lad makes a living tuning engines for drag racing. He has a dynamometer as well as a rolling road. In neither case is either engine or the complete car moving. Not sure how the load is applied on the dynamometer but it could be an airscrew.

Geoff

Edited By Geoff Sleath on 09/02/2016 15:24:48