Turnigy motor thrust stand

[Andy48]

Posted by Geoff Sleath on 09/02/2016 15:23:19:

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

Don't know, why are you?

This hobby would be boring if there were no new challenges around the corner.

Edited By Andy48 on 09/02/2016 16:52:15

[Tony K]

Posted by Andy48 on 09/02/2016 12:09:53:

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.

Andy, the formula, F=ma, only applies if the mass is rigid and constant, like a snooker ball or a planet.

Force is a rate of change of momentum. Momentum is mass times velocity - mv.

So F= d((mv)

If the mass is constant, the formula becomes F= m(dv).

dv is acceleration, so F=ma.

But when the mass is is a fluid (air) and the rate at which it flows through the propulsion system (propellor), which is almost constant, then the formula is F= (dm)v

So the force or thrust equals the rate of change of mass times velocity.

The rate of change of mass is the mass flow - kg/s

Therefore thrust equals mass flow times velocity.

[eflightray]

I ran foul of measuring 'static thrust' many years ago, and came to the conclusion that without measuring prop rpm as well, thrust can give you poor information.

Back to my original testing, (probably 30 years ago), it involved at strip of rubber hooked to the tail and I measured the stretch. Crude but it worked, (numbers such as ounces/pounds can be meaningless).

I tried a range of 'likely' props to get the best stretch, chose the best, (amps were crudely measured with a multimeter).

Off to the field. I could have probably thrown it faster, it just slowly sank to the ground from a gentle hand launch.

The 'light bulb moment', came later. The prop had the diameter for good thrust, but not enough pitch speed.

That's the crunch. Without enough pitch speed a plane wont fly how you want. So by all means use you static thrust stand, AND a tachometer for prop rpm, then do a rough calculation for prop pitch speed.

You may also find to get a good pitch speed you will need to drop the diameter, and the measured thrust to keep within the max amps.

Personally I go for pitch speed every time, I haven't bothered with thrust measurement since.

 

Ray.

 

Edited By eflightray on 09/02/2016 19:35:22

[Andy48]

Posted by Tony K on 09/02/2016 18:54:21:

Posted by Andy48 on 09/02/2016 12:09:53:

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.

Andy, the formula, F=ma, only applies if the mass is rigid and constant, like a snooker ball or a planet.

Force is a rate of change of momentum. Momentum is mass times velocity - mv.

So F= d((mv)

If the mass is constant, the formula becomes F= m(dv).

dv is acceleration, so F=ma.

But when the mass is is a fluid (air) and the rate at which it flows through the propulsion system (propellor), which is almost constant, then the formula is F= (dm)v

So the force or thrust equals the rate of change of mass times velocity.

The rate of change of mass is the mass flow - kg/s

Therefore thrust equals mass flow times velocity.

I appreciate that, I was looking from the model perspective where M is the mass of the model not at the airflow through the propeller. i.e. the net force on an object is equal to the mass of the object multiplied by the acceleration of the object.

[Geoff S]

When I first started this aeromodelling lark I had just retired early (1995) as an electronics design engineer so electric propulsion interested me. It was really no place for a rank beginner with generally underpowered models with heavy NiCads and equally heavy brushed motors so I tried to understand what it was all about.

My first efforts involved a spring balance fastened to the bench. I then tried to work out how much thrust you could get from a given prop at different speeds. To that end I made a table of propeller volume assuming 100% efficiency and then, knowing the mass of air I could determine how much thrust. I added a fiddle factor (I called it a propeller constant to be fancy) to allow for less than 100% efficiency. I even designed an optical tacho using a PIC which was also going to give thrust readings - the tacho worked but I never got round to programming the thrust calculations. I find the prop volume table quite useful in comparing the load imposed by different props eg a 12x6 prop has a volume of 678 cu ins/rev and an 11x7 665 cu ins/rev which means that they are both likely to be drawing similar current. I was strongly influenced by an article by Ken Nixon in IIRC Electric Flight back in 1997 or 8.

Well, I guess that's why the idea of a motor test bed appeals. My professional life revolved around measurement and control, mostly on gas turbine test beds and rigs so I like the idea of transferring that to electric propulsion for models.

Geoff

[PatMc]

Posted by Andy48 on 09/02/2016 12:09:53:

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

Thrust is not power, it's part of the power equation.
Power = Force (thrust) * Velocity.

This is the reason eflight Ray & myself have experienced models that appeared to have plenty of power based on the thrust (measured in my case) but let down (literally) by lack of pitch not allowing the model to reach flying speed.

[Andy48]

Posted by eflightray on 09/02/2016 19:34:16:

I ran foul of measuring 'static thrust' many years ago, and came to the conclusion that without measuring prop rpm as well, thrust can give you poor information.

Back to my original testing, (probably 30 years ago), it involved at strip of rubber hooked to the tail and I measured the stretch. Crude but it worked, (numbers such as ounces/pounds can be meaningless).

I tried a range of 'likely' props to get the best stretch, chose the best, (amps were crudely measured with a multimeter).

Off to the field. I could have probably thrown it faster, it just slowly sank to the ground from a gentle hand launch.

The 'light bulb moment', came later. The prop had the diameter for good thrust, but not enough pitch speed.

That's the crunch. Without enough pitch speed a plane wont fly how you want. So by all means use you static thrust stand, AND a tachometer for prop rpm, then do a rough calculation for prop pitch speed.

You may also find to get a good pitch speed you will need to drop the diameter, and the measured thrust to keep within the max amps.

Personally I go for pitch speed every time, I haven't bothered with thrust measurement since.

Ray.

Edited By eflightray on 09/02/2016 19:35:22

Interesting, but seeing this was 30 years ago, I am assuming you were using a brushed motor.

As I said above, with modern brushless motors there is a very wide power band, and when including the improved efficiency and power to weight ratio there is little comparison to the older brushed motor. I can only say what I have tried on the ground and tested in the air.

Using the same motor and speed controller I've tried a number of propellers of different pitch and diameter, the limitation being the maximum current of the motor. With a fairly ordinary 3548 840kv motor drawing a maximum of 50 amps, it would take a whole range of props on 4S. In the end I tried from 10 inch props to 13 inch props (the latter being the limit for clearance with the ground in a plane), and tried also a range of pitch. Basically the static thrust curve was the same when measured against power input. Going up to the maximum current, the 13 inch prop was too large, while it got some improvement in power the current would have fried the motor after a short time. With a 13 x 8 at the 700 watts max, the static thrust was 2.31kg, with a 13 x 6.5 the static thrust was 2.25kg, with an 11 x 5.5 the static thrust was 2.2kg. At 300 watts, all these props gave a thrust of 1.33-1.39 kg.

I then tried the same props out in the air with full telemetry on a 2.2kg model. I measured all sorts of things from airspeed to current and voltage, gps position, motor rpm, motor temperature and ESC temperature. Each time I used a new battery, all the batteries being fairly new and from the same batch. Later on analysing the telemetry, there was very little to choose in flight. Top speed was within a few mph on level flight in the same direction, and indeed all the logs were similar.

What was quite noticeable with all the props was the drop in efficiency above half power, with the larger pitch props fairing marginally better.

This should have an impact on top speed, in theory without any losses a 5.5 ought to achieve 70mph with my setup, and a 8 pitch just over 100mph. In reality the top speed was around 55mph with all the props, i.e. the speed when the drag matched the thrust. Thus pitch will certainly have an influence on a fast low drag plane with plenty of thrust.

In the end I chose an 11 x7 which kept the current well within safe limits but still with plenty of power.

It would appear that both you and Pat are basing your judgement on brushed motors and nicad batteries. It may well be true what you say with this setup, after all those motors would have a much lower torque and be more susceptible to differing prop sizes. All I can say is that based on the testing I have done with low kv (high torque therefore) brushless motors and lipos, a static test is very useful.

It needs a good few more people out there to do some serious testing both on the ground and in the air with different configurations and motors to really understand what is actually happening.

[Andy48]

Posted by PatMc on 09/02/2016 20:54:50:

Posted by Andy48 on 09/02/2016 12:09:53:

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

Thrust is not power, it's part of the power equation.
Power = Force (thrust) * Velocity.

This is the reason eflight Ray & myself have experienced models that appeared to have plenty of power based on the thrust (measured in my case) but let down (literally) by lack of pitch not allowing the model to reach flying speed.

er no.

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system.

A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine.

[Andy48]

Can I put this in very simple terms. The motor/lipo prop combination on the front of the plane provides a push forward. We can feel that when we hold a plane and apply full throttle. That is the thrust. That is all that moves the plane forward. No throttle, prop not turning, no thrust.

If that thrust is greater than the resistance to the model from the wheels when stationary then the model will move forward. One can crudely measure the thrust by placing the model on a smooth surface held back by a spring balance. Here we measure the thrust in gms or oz or whatever. Then we can use that same balance with the model on the flying field (into the wind) to see how much force it takes to move the plane over the grass or whatever.

If the thrust is greater than the force required to move the model then it will move forward. As it does so, the ground resistance/ rolling resistance call it what you like becomes less as some of the weight of the plane is now reduced by lift under the wings. However, the drag on the plane starts to increase. Eventually the plane will lift off. If, however the thrust is only marginally greater than the rolling resistance then we could have other problems. The drag could increase more than the reduction in rolling resistance, and we could reach a situation where the drag and rolling resistance now balance out the forward thrust and the plane will not gain enough speed to take off. Remember too we also have to contend taking off into the wind which will affect the drag. In days of heavy nicads, and lower battery power this could easily be a reality.

However, by measuring the static thrust we can ensure that there is always a safe margin of reserve power to overcome this, either by changing props, changing the motor or by changing the battery. As I said, I use a very rough rule of thumb that for an ordinary sports model, if the thrust is about the same as the weight of the plane, on a reasonable grass runway with reasonable wheels, a plane should lift off easily. The weight of the plane will affect the rolling resistance and the acceleration.

[PatMc]

Posted by Andy48 on 09/02/2016 21:04:15:

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system.

A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine.

Brushed motor & nicads or brushless & lipos, it doesn't matter what is turning the prop the same physics apply.

The point you are missing is that a prop driven aircraft does not produce a reactive force in the same way as a jet or a rocket. To produce thrust the prop must in effect "grab" the air in front and push it back faster than the engine attached to the airframe is moving.
If an prop driven aircraft could reach pitch*rpm speed the velocity of the mass being expelled would be equal to the velocity of the aircraft. I.e. thrust = zero.
In practice when an aircraft is travelling at a steady rate neither climbing or sinking then thrust = drag. The difference between thrust at this point and static thrust depends on the lift to drag ratio of the aircraft.

Jets & rockets expel an expanding mass at high velocity from within the airframe to produce thrust independent of the airspeed.

[Andy48]

Posted by PatMc on 09/02/2016 22:01:43:

In practice when an aircraft is travelling at a steady rate neither climbing or sinking then thrust = drag. The difference between thrust at this point and static thrust depends on the lift to drag ratio of the aircraft.

Thereby pointing out that indeed the static thrust is a very useful measure of how the plane will perform.

[PatMc]

Posted by Andy48 on 09/02/2016 22:16:56:

Posted by PatMc on 09/02/2016 22:01:43:

In practice when an aircraft is travelling at a steady rate neither climbing or sinking then thrust = drag. The difference between thrust at this point and static thrust depends on the lift to drag ratio of the aircraft.

Thereby pointing out that indeed the static thrust is a very useful measure of how the plane will perform.

How so ?

[PatMc]

Posted by Andy48 on 09/02/2016 21:58:12:

Remember too we also have to contend taking off into the wind which will affect the drag.

Yes, but only when the model's on the ground. It will reduce the rolling resistance & the model will have to contend (?) with an easier model take off run.

[Tony K]

Posted by PatMc on 09/02/2016 22:01:43:

If an prop driven aircraft could reach pitch*rpm speed the velocity of the mass being expelled would be equal to the velocity of the aircraft. I.e. thrust = zero.

So, if thrust = zero, where has the drag gone.

When your aircraft is flying at maximum speed it has accelerated to the point where the available thrust is no longer able to overcome the drag. That does not mean the thrust is zero, if it was then the drag must also be zero.

At maximum speed you must have maximum thrust.

[Dickw]

He said “If” indicating a theoretical example, but if you want a practical example:-

If you put a model into a dive so that the force of gravity adds to the prop thrust it can accelerate to the point where its speed is equal to that defined by prop pitch and rpm. At that speed the prop thrust will be zero.

If the plane accelerates any more through gravity the prop will be providing a negative thrust i.e. a braking force.

A spinning prop can be used as an airbrake when descending – I have used it myself.

It is not true that at maximum speed you must have maximum thrust.

Wind tunnel examples quoted earlier showed the thrust dropping as speed increased. Maximum thrust will give you maximum acceleration, but the maximum speed will usually be at a lesser thrust.

Dick

[PatMc]

Thank's Dick, that's pretty well what I'd have posted as an answer.
I often use the spinning prop airbrake method to steepen the approach with E-glider that aren't equipped with spoilers or airbrakes.

[Andy48]

If you put a model into a dive so that the force of gravity adds to the prop thrust it can accelerate to the point where its speed is equal to that defined by prop pitch and rpm. At that speed the prop thrust will be zero.

You've contradicted yourself here. First you said "the force of gravity adds to the prop thrust" then "at that speed the prop thrust will be zero"

What you should have said is "at that speed the drag will equal the prop thrust".

True, a prop can be used as an air brake, provided there is no/reduced power to the motor, just like lifting your foot off the accelerator in a car. I'm not at all sure about your thoughts that you would get a negative thrust.

I doubt the speed will ever equal the prop pitch times RPM, there are always losses. Indeed the prop pitch alone is not constant across the radius of the propeller.

Point taken about maximum speed/thrust.

Maximum thrust will give maximum acceleration, but as the drag increases so the acceleration decreased until you reach the situation of balanced forces when acceleration is zero and top speed is reached. As the speed increased so the thrust decreases somewhat so it would seem, though I've not quite got my brain around exactly why. Can someone explain?

It is also about semantics. One could perhaps state that maximum speed will usually be at a lesser thrust than static thrust.

From what I can see, the drop in thrust from static to about 50-60mph with a low torque motor (ie low kv) would appear to be about 10%.

[Andy48]

I think I have worked out why Ray and Pat got poor results from static testing.

Firstly it is clear that both were using brushed motors and nicad batteries. From this we know that the power to weight ratio was poor compared to brushless. It is likely that the torque curve and speed range for such a system was quite limited. Thus in a static test, a particular propeller might give the best thrust, but once the propeller is in a moving stream of air, the prop will now be operating well off the peak of the torque curve and best operating speed, hence disappointing performance once launched. The analogy of a car would appear to be similar, which is why one needs gears for different speeds. Perhaps in such a system the prop is acting in just the same way as a gear, and at different speeds, a different prop is needed to keep within the useful part of the torque curve. Surely this is why real planes use variable pitch propellers as they have to operate over a wide range of speeds.

Brushless motors/lipos are a completely different kettle of fish. Firstly there is ample power available, secondly power to weight ratio is much improved, and thirdly and most important a low kv brushless motor has a wide torque curve, starting at very low speeds. In this situation, while a pitch/RPM calculation will provide good results, it is no longer critical. Going back to our car analogy, this is why electric cars do not need/use gearboxes. Pitch /RPM becomes important again on a fast model where the possible max speed approaches the theoretical max speed derived from the prop/RPM calculation. No doubt if a model was capable of anything like the speed of a real plane then we would need variable pitch propellers, which I know do exist, but then that is a whole new ballgame.

[Dickw]

Posted by Andy48 on 10/02/2016 16:02:11:

If you put a model into a dive so that the force of gravity adds to the prop thrust it can accelerate to the point where its speed is equal to that defined by prop pitch and rpm. At that speed the prop thrust will be zero.

You've contradicted yourself here. First you said "the force of gravity adds to the prop thrust" then "at that speed the prop thrust will be zero"

What you should have said is "at that speed the drag will equal the prop thrust".

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

From what I can see, the drop in thrust from static to about 50-60mph with a low torque motor (ie low kv) would appear to be about 10%.

No, you are trying to put words into my mouth, but perhaps I didn’t express myself clearly enough.

What I was pointing out is that:- yes thrust equals drag at max at speed in level flight, BUT in a dive you are adding the force of gravity so that the model can accelerate beyond that normal max to a point where prop thrust drops to zero and gravity is providing the force (drag = gravity).

"From what I can see, the drop in thrust from static to about 50-60mph with a low torque motor (ie low kv) would appear to be about 10%."

You accept that thrust drops with speed, so all I am adding is that the actual drop depends on many factors including prop pitch, rpm, and model speed. Your example of 10% is probably correct for the props you use at the speeds you fly at, but is not a universal value and may be considerably more in other circumstances.

Dick

[PatMc]

Posted by Andy48 on 10/02/2016 16:02:11:

True, a prop can be used as an air brake, provided there is no/reduced power to the motor, just like lifting your foot off the accelerator in a car. I'm not at all sure about your thoughts that you would get a negative thrust.

I doubt the speed will ever equal the prop pitch times RPM, there are always losses. Indeed the prop pitch alone is not constant across the radius of the propeller.

There has to be some power to the motor in the manner I & many other E-glider fliers use the prop as a drag producer. If power was cut the dynamic brake of the ESC would stop the prop which would fold. The drag produced by the prop in this condition is effectively "negative thrust".

If the prop didn't reach pitch times RPM the airbrake effect wouldn't work but it demonstrably does.

Edited By PatMc on 10/02/2016 16:46:45

[PatMc]

Posted by Andy48 on 10/02/2016 16:27:50:

I think I have worked out why Ray and Pat got poor results from static testing.

Firstly it is clear that both were using brushed motors and nicad batteries. From this we know that the power to weight ratio was poor compared to brushless. It is likely that the torque curve and speed range for such a system was quite limited. Thus in a static test, a particular propeller might give the best thrust, but once the propeller is in a moving stream of air, the prop will now be operating well off the peak of the torque curve and best operating speed, hence disappointing performance once launched. The analogy of a car would appear to be similar, which is why one needs gears for different speeds. Perhaps in such a system the prop is acting in just the same way as a gear, and at different speeds, a different prop is needed to keep within the useful part of the torque curve. Surely this is why real planes use variable pitch propellers as they have to operate over a wide range of speeds.

Brushless motors/lipos are a completely different kettle of fish. Firstly there is ample power available, secondly power to weight ratio is much improved, and thirdly and most important a low kv brushless motor has a wide torque curve, starting at very low speeds. In this situation, while a pitch/RPM calculation will provide good results, it is no longer critical. Going back to our car analogy, this is why electric cars do not need/use gearboxes. Pitch /RPM becomes important again on a fast model where the possible max speed approaches the theoretical max speed derived from the prop/RPM calculation. No doubt if a model was capable of anything like the speed of a real plane then we would need variable pitch propellers, which I know do exist, but then that is a whole new ballgame.

As I've previously stated the type of motor/battery that turns the prop is irrelevant, the physics are identical.

The particular model I mentioned was made using the wings & tailplane of an otherwise redundant 38" span F/F glider I'd made my son years before. It was intended to be used indoors in the pioneer days of indoor R/C. I had bought a motor two gearboxes of different ratios, a selection of GWS slow fly props, some suitable small nicad or nimh batteries (can't remember which) mini Rx etc. RTF was around 5oz - 6oz.
A few tests showed that using the highest gear down ratio with largest dia prop produced more thrust than the model's weight so seemed the best choice.
First flight was tried on a near windless day at my club's field. The model performed just as Ray described is his post. After a few abortive attempts & some battery re-charging I tried a vertical launch which proved quite amusing as well as educational.
The model climbed verticaly at a snails pace whilst slowly counter rotating then gradually run out of puff at about 50 or 60ft. I cut power & let it glide down with the prop freewheeling. I repeated the performance a number of times & if power was applied during the glide barely improved.
Next time out gearbox was changed to the lower ratio with a smaller prop pulling roughly the same current but the thrust was about 1/3rd the previous. Again tested at the field in near windless conditions. The model flew perfectly at a nice sedate pace exactly as wanted for indoor flying where it later proved to be a success.

[Andy48]

Posted by Dickw on 10/02/2016 16:27:58:

What I was pointing out is that:- yes thrust equals drag at max at speed in level flight, BUT in a dive you are adding the force of gravity so that the model can accelerate beyond that normal max to a point where prop thrust drops to zero and gravity is providing the force (drag = gravity).

I really doubt you could get to a situation in a dive where the powered prop thrust is zero. As you accelerate due to gravity so you continue to add drag so again you get to a balanced force state where the plane does not accelerate more, but it is travelling faster than in level flight.

Will the prop thrust drop to zero?. The prop will still be rotating under power, and thus there will be a flow of air through the prop. However, the flow of air past the prop will be higher, so that in effect the propeller will induce drag by restricting airflow through the prop. Effectively you've added another drag component to the force equation. However, it would be wrong to call it negative thrust. You can get negative thrust, but you have to set the propeller blade to a negative angle as you do in a real plane.

Taking Pat's point about a folding prop on a e-glider, this extra drag obviously acts a brake even though the prop is still generating thrust. Consider, if the thrust did go negative, at some point it would have to go through zero thrust and the prop would fold at that point.

Edited By Andy48 on 10/02/2016 19:15:20

[Andy48]

Posted by PatMc on 10/02/2016 17:42:05:

As I've previously stated the type of motor/battery that turns the prop is irrelevant, the physics are identical.

The physics of the plane is identical; the physics of the power train is not. There is a considerable difference between a brushed/nicad setup and a brushless/lipo setup.

 

Edited By Andy48 on 10/02/2016 19:18:55

[Andy48]

Posted by PatMc on 02/02/2016 10:48:01:

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.

To come back to the point, it seems you have answered it yourself. A simple test rig will enable you to do exactly what you suggest above:

compare thrust at identical current

compare thrust at identical rpm

compare current/rpm at identical thrust.

It can also be used in a moving situation too by testing outside on a windy day into the wind and against the wind.

Furthermore it can be used to test the whole combination. What difference do the different ESC settings make? How close does the motor come to the advertised power/kv. How do different motors compare, different ESCs.

It can also test the linearity of the thrust against the joystick position. Some of us are just curious. I got an interesting result on a limited test. It would be useful to see what others get.

[PatMc]

Posted by Andy48 on 10/02/2016 19:15:03:

Posted by PatMc on 10/02/2016 17:42:05:

As I've previously stated the type of motor/battery that turns the prop is irrelevant, the physics are identical.

The physics of the plane is identical; the physics of the power train is not. There is a considerable difference between a brushed/nicad setup and a brushless/lipo setup.

Edited By Andy48 on 10/02/2016 19:18:55

We're measuring the thrust not the power. The prop is providing the thrust. What turns the prop is irrelevent.