What does static thrust tell us.

Yesterday at the flying field, one of our mature, competition, well known and lastly accomplished modellers was showing me his "Cloud Models" Me 163 Komet.

This model was originally designed and flown using a 480 motor, with Nicads, using a 6" *3" prop. The 163 shown to me had a -200w motor turning a 9" *6" prop, using an outrunner.

My own uses an inrunner @7"*4" turning at 23,700, pulling 302w. My model will not fly to date.

Our discussion discussed, bungee launching, and his Mig 15 DF, which then lead to static force. Nothing was resolved, other than taking CG dimensions and weight. Which turned out to be almost identical with my own model.

As with so much with me, I woke up thinking about the relevance of static thrust. I remembered that in fan engineering (H&V), the definition given was the amount of water that could be supported, in a tray of zero weight (or equated to water), by the force of a fan discharging vertically up to the tray.

I thought this has nothing to do with speed, it is all about a force.

I then thought what determines the speed of the model?Here the DF Mig 15 rang a bell I think. It is all about the efflux velocity of the duct and the mass of air. It seems pretty much a momentum issue. It struck me that it is the velocity of the air leaving the propeller/duct which determines maximum speed of the model. The model in at least horizontal  flight will always have velocity that will be less than this velocity of the fluid stream. I do recognise it will be an average velocity across the fluid stream. It cannot fly faster than this velocity (without gravites help or hinderence)

So I though what does static force actually tell us (or me)? I thought well Isaac told us that F= M*a. Well I do know the force, and it would be possible to get an average velocity across the fluid stream and air density is pretty standard at low levels. What static force is pointing to, is the max acceleration that the model would be capable of. Although there are a lot of other variables reducing the value, propeller characteristics, airframe, wing drag and so on. Lastly it tells us can the model prop hang or even climb vertically.

I came to the conclusion, that on its own static force tells me little and may not answer what is wrong with my model.

Although my club mates help, is pointing towards, piloting issues as much as set up issues.

Now awaiting BEB's verdict and the brick bats thrown by others, Ohhh, that hurts, be careful.

Edited By Erfolg on 21/06/2012 11:44:57

as a start point, static thrust should give you a guide to relative acceleration but takes no account of top speed, which as you say, depends on the velocity of the air from the prop rather than it's mass. Also, with high pitch setups the blades may actually be stalled when the model is static, so thrust may increase when the model is moving.

Ignoring the case when the blades might be stalled; static thrust tells us whether or not a model can prop hang then not much more.

Once a model's moving the thrust decreases at the same time as drag increases once they are equal the model ceases to accelerate. If the velocity it has reached is sufficient for the lift generated to overcome it's weight it will fly if not it will sink.

Static thrust gives information about the amount of thrust available when the plane isn't moving so it's only accurate when prop-hanging

It obviously gives some hints towards acceleration but the actual acceleration is going to depend on drag. Ditto pitch speed can hint towards top speed but drag is going to have a big impact (when you get into high speeds particularly).

I think of it like this: static thrust and pitch speed are a bit like knowing a car engines torque/BHP and gear ratios. It's going to give you a hint towards performance but if you put this combination in a lightweight spaceframe car like an Ariel Atom it's going to be somewhat different than if you put it in a HGV.

This is unusual, we all seem to agree.

We now await BEB's intervention, hopefully not to tell us we are mistaken

I was led to believe that "static" meant, Still ( not moving) e.g 0 pound of thrust will leave a 10 pound model static. will 1 pound of thrust move the 10 pound model or do we need 10 pounds of thrust to move it and if so once the model is moving do we need the 10 pounds or 1 pound of thrust to keep it moving?

Erfolg

23,000 rpm (static?) from a 7x4 does make me wonder about the efficiency of the motor/prop combination and just how much of your 302W is being used to overcome the drag of turning the prop blades at that speed rather than creating thrust.

Lower kV motor, less revs and more pitch?

This is static thrust

Simon, it's all being used to overcome the drag of the prop blades but a large proportion of that "drag" is being converted into thrust in exactly the same way that the drag generated by a wing is converted into lift.

thrust is pushing or pulling power created from another force eg the force from the jet engine is thrusting the plane forward and "reverse thrust" is is slowing the momentum down. birds create thrust by flapping their wings. thrust is force, thrust is the force needed to escape drag. so in effect, static thrust = not enough force to break the drag.

Ben

I am pleased to see that others have thought along similar lines to myself. The only difference was I was thinking of using a large plant pot as a protective shroud, with the majority of the rest cut away. I like it, never liked the leer approach as the stand would take some storing, the shroud could be hidden in the garden shed.

Simon/Patmac

I do not know the answer to your question, I had thought of hanging the model from the cloths line on a spring balance to get a static thrust, as an indicator. Have you any ideas?

Erfolg, may I refer you to Richard Sharman's "Theory and Practise" thread on the EDF board, to which you also contributed, and which unfortunately fizzled out without confirming any theory or practise.

Top speed is achieved when thrust equals drag. As far as aircraft are concerned, drag has two components. There is parasite drag, which depends on how slippery the model is, and there is induced drag, which depends on how efficient the lifting surface (wing) is.

The interesting thing is that induced drag is higher at low speed (greater aoa) and reduces as the aircraft flies faster whereas parasite drag is lowest at low speed and increases with airspeed.

An aircraft designed to fly at high speed will have a relatively much higher induced drag at low speed, hence the need to get the thing moving quickly with a bungee.

For prop hanging (which doesn't interest me at all) the thrust must, of course, equal the weight and a vertical climb is possible when thrust is greater than weight.

It is true that Newton said F=m*a. You can rearrange that to say a=F/m, acceleration equals force divided by mass.

In the equation F=m*a, the m refers to an object with a mass. In the case of moving air (a fluid stream), mass flow is used. So the equation becomes F= mass flow * velocity.

In the model's frame of reference, the propellor or fan is moving the same amount of air just as quickly when it is standing still as when it is at maximum speed.

I suggest, therefore, that static thrust is a useful indicator of the power units performance.

Hmmm Cocked things up while looking at Bens video and the linked test tower.

I think I need one of these anemometers, although not up to the standards of true experimental data capture, good enough to give an indicator how much air is being shifted, and the price is not bad, I wonder if HK, UK depot will stock this type of thing me bob. It seems good up to about 60 miles per hour, is that enough?

I know we are drifting from static thrust, yet, it the 163 issue that started the thread.

Edited By Erfolg on 21/06/2012 14:14:25

Although, I was not thinking about DF, the thought process, with respect to how fast can a plane go, has lead me to a thought which I have not previously had, and a answer, perhaps, a question which arises from others observations.

It seems that many if not all DF benefit from a modest reduction in the outlet, normally (apparently) via a tapered cone.

From the observation that a plane can fly no faster than the velocity of the air from the propeller, or in this case out of the back of the DF. I can see (I think) why benefit can be gained.

The next thought is why not taper down to a pin prick of a efflux, with now bags of velocity. I think we are back to the momentum issue, we need mass and velocity.

Which also explains why not have a very large efflux, now we have low velocity and higher mass.

I do not know if my thoughts with respect to DF's are correct, although it would answer why tapered cones seem to work. It seems to be a similar momentum effect that firemen have with there fire hoses, where constricting the hose outlet, allows a jet to be produced, that the fireman braces himself against, which is the equivalent of the force available to propel a DF.

It now seems obvious, to the extent some one else must have thought of this before, or I am wrong.

Are you sure about that? When a plane is moving forward at maximum speed the effective AoA of the prop / fan is very different to that when the plane is stationary. A high pitch prop can easily be stalled when static and only really start "hauling air" when the plane is moving forwards. In those situations they'll be moving very different amounts of air.

Erfolg- An anenometer will only tell you the efflux velocity of the fan / prop when the plane isn't moving forward. That's effectively you're theoretical pitch speed. Drag will be totally ignored, as will thrust. And it's not really an indicator of how much air is being shifted, just how fast. Don't forget in a venturi the amount of air flowing is constant but if you measure the speed of airflow throughout it will vary with bore diameter. If efflux velocity was all that mattered EDFs would have pin-hole exhausts...

My take: Vmax = when forward force equals drag (as static). Drag increases massively with speed. Static thrust vs pitch speed will only tell you whether your motor system is setup for top end speed or thrust (ie which way it's skewed) BUT if you don't have enough thrust you'll never get to your theoretical top end speed because drag will get in the way.

So what you really want is a set-up where you have just enough thrust (not of the static variety) to overcome the drag present at your theoretical top speed.

Although you'll never get there because pitch speed ignores prop/fan slip, inefficiences blah blah blah.

Ben

You have now opened a can of worms with respect to propellers.

I know that even BEB seems to compare a propeller to a screw. Yet when I look t a prop, I see ann aerofoil, which is wizzing in circular direction. I have all sorts of questions with respect to AOA, and why the air movement is principally, apparently along the axis of the airframe.

Is this a consequence of pressure differential either side of the blade and downwash from the aerofoil?

To be honest I cannot comprehend at all what is happening, particularly as the velocity of the model changes (AOA etc).

taking the screw analogy, I have wondered if the pitch can be considered to be as high 70 percent, or as low as say 10 percent, or is it constantly changing. What would be the best value and a average value.

We have to be careful not to mix props and ducted fans up as they work somewhat differently- after all a prop is unshrouded.

I got myself into a right muddle initially when I first started thinking about how to get one of electric flying wings to go faster. I couldn't work out why my "airscrew" wasn't working like I thought it should. As you say it's not a screw action. In a screw all rotation is converted into forward movement (even if not all the power being used is turned into rotation).

The AoA is like this:

The other thing to realise is that a propeller disc does not move a circular tube of air from in front of the plane to behind it. A lot of the air comes in from the tips of the prop.

Are you sure about these figures ?

Bearing in mind that 302W = 0.4 hp, have a look at the graph below, you'll see that an APC (ic) prop absorbs about 0.63 HP at 20,000 rpm. I've checked this against another graph that records from 0.5 HP to 0.65 HP for other brands of 7x4 ic props & between 0.8 HP to 1.1 HP at 23,500 rpm.

The watt meter was showing 302 Watts, 30 amps, I use an optical tachometer which indicated a max of 23,700 revs. The prop is 7 * 4 Graupner Cam, cut down to 6 * 4.

The motor does not seem to be manufactured now. It is a HXT 2835 (380s), Kv 2700, max load 35 amps.

The real question is why doesn't Erfolg's Me163 fly?

If it is a Cloud Models version it should weigh about 34oz (does it?) so should manage quite well on 300W, never mind the actual rpm, given that the prototype managed with a brushed 480 and a 6x3 folding prop.

So if is not a 'pilot or setup' problem then for some reason that motor/prop combination must be particularly inefficient.

It would be very interesting to know (even roughlly) what the static thrust is. For 300W on a 7" prop I would expect more than 20oz.