Electric Thrust

[Joe Beavis]

When describing the power system of their model, most people give details of the battery, ESC, motor and prop. All very important - but the really interesting figure is the static thrust, which is what it is all about, and hardly anyone mentions. It is not hard to measure (but be careful!), and can be very illuminating when choosing props.

[DDH]

Thrust calculator

The results are almost as good as you'll get in practice. Check it out.

[eflightray]

But what does static thrust tell you about how a plane will actually fly ?

I will declare right at the start that I feel static thrust measurement is not that important, and have for a very long time.

It may be handy for a 3D model that is expected to hover, and climb vertical, but then you will be selecting a suitable prop for that type of model.

More important, in my opinion, is 'pitch speed'. Prop rpm x pitch.

You can increase static thrust by increasing prop diameter, and dropping the pitch to keep within the motors current rating. But you can easily lose out on the required speed the model may require.

Most important is the right prop doing the right rpm to fly a model how you want it. Different type of models require different power requirements. You may get the similar thrust measurement from a small high pitch prop or a larger diameter low pitch prop, but they wont fly the plane the same.

I know a lot of people make/buy thrust rigs and do tests, but unless you know all the parameters, plus why and how you use that information, I would still say - get a tachometer, (you should already have a wattmeter), and look at what types and sizes of prop are generally used on various types of model to make them fly that way.

Ray.

[Bob Cotsford]

How well does the static thrust predict a model's performance in the air? I ask as we often talk of props being stalled when running full chat on the ground, particularly when getting a model noise tested, as we all know that a stalled prop causes at least a couple of extra dBa (at least it's my excuse!). I can see it having relevance for slower flying models where there is less difference between ground and flying prop loads, but what about faster stuff?

ok, Eflightray beat me to it!

Edited By Bob Cotsford on 22/10/2019 13:08:55

[DDH]

Well said.

You may care to note with the thrust calculator that it also demonstrates how changes to the values of thrust impact on the estimated air speed. e.g. varying the prop size and (especially) the pitch can significantly alter the air speed for the approx same magnitude of thrust. That said, if you want loadsa thrust you need loadsa HP/kW. There again, if you want to swing bigger or multi-bladed props then it's loadsa torque that's the key.

Variable pitch props are even more interesting!

[Stuphedd]

15 years or so I built a thrust rig using strain gauges and linear frictionless bearings to try and understand this new brushless power system that was becoming popular but info was very limited .

It worked , and I got lots of data , re current , speed , prop size etc but all static figs and was of use only as a comparator , not actual lbs of thrust.

Last week I dug it out to test some exhaust /jet tubes I had made for a EDF Mig 29 . The fan assy alone , a short jet tube , a long jet tube , and various convergent nozzles .

I was very surprised to find out the fan alone gave the greatest static thrust , So the rig went back in the cupboard !

I flew the aircraft with the various jet pipes and it actually flew best without one !!!

cheers

[Andy48]

Posted by Bob Cotsford on 22/10/2019 13:08:08:

How well does the static thrust predict a model's performance in the air? I ask as we often talk of props being stalled when running full chat on the ground, particularly when getting a model noise tested, as we all know that a stalled prop causes at least a couple of extra dBa (at least it's my excuse!). I can see it having relevance for slower flying models where there is less difference between ground and flying prop loads, but what about faster stuff?

ok, Eflightray beat me to it!

Edited By Bob Cotsford on 22/10/2019 13:08:55

The only thing I use is a static trust rig coupled with the tx telemetry to measure such things as current. This has never let me down. It is surprising just how a slightly different sized prop can make quite a difference. Its also interesting to see the thrust at various throttle settings. Put all this together and it works well. Clearly such a rig would show the prop stalling. In the air, telemetry shows very similar results, and now being able to measure RPM, air speed current, battery voltage etc., I'm pretty confident with my electric systems.

However, for me, all out speed is not the most important criteria.

[Andy48]

Posted by Stuphedd on 22/10/2019 13:58:40:

15 years or so I built a thrust rig using strain gauges and linear frictionless bearings to try and understand this new brushless power system that was becoming popular but info was very limited .

It worked , and I got lots of data , re current , speed , prop size etc but all static figs and was of use only as a comparator , not actual lbs of thrust.

Much simpler, just use a simple electronic scale for weighing suitcases. Many are accurate to 50gm.

[Frank Skilbeck]

Ecalc gives static thrust and pitch speed too, it also predicts if the propeller pitch is adequate, based on model weight and wing area etc.

[Nigel R]

"static thrust, which is what it is all about"

Indeed it is, for a helicopter or drone.