Ducted fan theory and practice

[Richard Sharman]

Posted by Simon Chaddock on 19/08/2012 16:27:53:

"Considering its only foam......

This was just a (semi-) humerous jibe at modern construction methods ! Of course foam is a great material, in the right place at the right time. But it has been experience of recent fixed wing planes at our club that the smaller, cheaper, foamier, ready-builts tend to be less satisfying, less robust, and more expendable than the larger, conventionally built ARTF's especially from the more up-market companies. Perhaps you get what you pay for ? So it was a (pleasant) surprise that the Tornado was successful when it could have been, well, disappointing.

[Erfolg]

Have you any thoughts as to why the model is performing in excess of your expectations and I assume the norm for the size of model?

[Richard Sharman]

Posted by Swissflyer on 21/08/2012 11:47:31:

Richard, please can you give me the wing area of your GR4 Tornado in sq dm, then we can do some calculations for it.

The wing dimensions of the Tornado are different according to the variable geometry wing setting:

Extended: span 810mm, area 9.925sq dm, wing loading 94g/sq dm (=30oz/sq ft)

Closed: span 620mm, area 8.835sq dm, wing loading 105g/sq dm (=35 oz/sq ft)

The lighter wing loading clearly helps on take-off and landing, the heavier wing loading corresponds to higher flight speeds with less responsive controls. What else can you tell from this?

R

[Richard Sharman]

Posted by Tony K on 20/08/2012 11:52:42:

Posted by Richard Sharman on 18/08/2012 12:45:27:

The question is: why does it fly so well ?

The basic parameters must be understood; wing loading, aspect ratio, thrust line, tail moment, control surface size, etc.

Are there other properties which should be considered, eg. polar moment of inertia, or is it just the inter-reaction of the basics?

I don't know what the real answer is, it just seems to be a happy co-incidence of power, weight, balance and control. I've known quite a few models that seemed similar on the face of it, but were disappointing. In this case the designer seems to have got it the relationships just about right.

Just in case anyone is thinking of rushing out and getting one, I would add one item of caution: the swing mechanism leaves a lot to be desired, and I did spend quite a bit of time improving it.

[Erfolg]

From your comments, it appears that it is the flying characteristics which impress you, not particularly the DF performance.

[Richard Sharman]

Posted by Erfolg

With Richards background in Computational Fluid dynamics, I am a little surprised that he has not knocked up a programme which would permit the examination of various duct and fan arrangements. It certainly would be an interesting project, with his skills and toolbox.

Posted by Mark Powell 2 on 21/08/2012 17:23:12:

CFD/Richard/Knock up. Didn't know that. I was employed for many years at the same place as him, and he taught Computet Science at the local Tech, presumably as a 'sideline' to which I went.

Knock up a program? Maybe he wants to keep his work separate from his pleasure, or maybe he is holding back, just interjecting to put us straight now and again. (the Mark Powell 2 unauthorised biography) His planes fly well.

Yes, it's all true, I do have a Ph.d in CFD, and I did teach Comp.Sci as a sideline, and Mark was one of my students, and I do keep work separate from pleasure, and I am holding back. Tony K, John Olsen, Mark and Erfolg are doing a splendid job of keeping the discussion going, so please continue -- it's all very interesting, and just a little entertaining at times!

On a serious point, "knocking up" a serious program to make useful deductions about flows in ducts is just a little more work than it might seem, even if you have all the right toolkits available. And you still need serious hardware to do it properly. It's all been done in academia, high-tech start-ups, and the aerospace industry, so it's not worth even contemplating the effort here. Maybe someone can borrow time on a system somewhere ?

[Erfolg]

Maybe a spread sheet with mostly locked fields? maybe? Richard!

I have been a fan of Finite Element Analysis, having a piped system assessed using the technique, the analysis matching operational measurements. In addition heat transfer assessments were pretty much spot on.

Southampton Uni did some work for me on electro statics from fluid flow, which also was held in high regard, at least by me.

Quite a fan of the techniques.

[Richard Sharman]

Now, here's different kettle of fish (which I don't think Mark has seen yet):

The e-flite Habu. This is a very sophisticated piece of kit (mostly of fibreglass, wood, etc No foam in sight). Notice the ultra smooth finish of the exterior (the duct interiors are similarly beautifully finished). The duct entry lips are nicely smoothed, the ducts are nice and straight and not obscured in any way. The ducting tapers nicely as per theory. The model has retracts, flaps, full functions etc.

Main dimensions are: span 1030mm, wing area 25.3 sq dm, AUW 3.55 Kg, wing loading 137 g/sq dm (=45 oz/sq ft)

Operating figures: 6s 4000mAh lipo at 70Amps gives around 1600 Watts with Delta-V 32 80mm fan and 2150Kv DF32 brushless motor.

How does it fly? Amazing! Good take-off, very fast in flight, awesome in the air, very precise control behaviour, but quite difficult to land, even with flaps. So, despite a wing loading which would kill some models, and power that is not out of this world, the result is very satisfying: near turbine performance at a fraction of the cost. I think EDF has finally arrived.

[Mark Powell 2]

The Habu is lovely, I have thought of getting one. But... it is too heavy. The manufacturers say about 7.5 lbs. My 48 inch span RBC balsa kit Grumman Panther is a pound less, has much more wing area, a weight/thrust ratio MUCH better than the Habu. and still flies like a set of dumbells at times., so it has become a hangar queen.

So I am put off the Habu, though I am still tempted. E-Flite has gone from the small 70mm fan models to the 80mm Habu and Phantom. I am waiting for their next move up. Then I can stop trying to figure it out for myself. Shame, but I do like a little flying now and again.

The Dvorak El Bandito (balsa and glass kit and an unauthorised copy of the turbine BVM Bandit) is similar to the Habu, but lighter, with a bigger fan, and more power. But is is diffucult to obtain.

Incidentally, I took the time to look at several professional pilot (most of them airline pilots) sites. I found that IDENTICAL threads are going on. Even rockets, who cares? faster than exhaust velocity, etc. etc. etc. Nothing is missing, nothing is added. So we are not alone....

PS: landing speed it all. If you worry about getting it down you won't really enjoy the flying, no matter how good. My OS91  ATRF scale pylon race is a nightmare, even on our large concrete site, and there is no flat grass alternative. And I do know how to approch/land, as I fly fullsize. But it is no fun to land, so I don't fly it much. And what's the point of that?

Edited By Mark Powell 2 on 22/08/2012 07:44:47

Edited By Mark Powell 2 on 22/08/2012 08:08:33

[Tony K]

I have had a look at the Sidney university paper on Froude's theory (Swissflyer's link).

Quote, "As this air passes through the rotating blades...its speed increases"

Does it? Further down the page is the statement, "V2 = V3".

Quote, "Since the disc is thin and the area of cross section at (2) and (3) are equal...

If V2 =V3 and the cross sections are equal how can P2 be different than P3?

What is P anyway? I assume it is the dynamic pressure, 1/2 rho v^2. The density doesn't change at the speeds we are talking about and V2 =V3 so P2 must be equal to P3.

In my opinion the written equation F = A(P3 - P2) = mdot(V4 -V1) is meaningless because A(P3 -P2) = 0.

Perhaps one of you better educated chaps can tell me where I am going wrong.

[Tony K]

Posted by Erfolg on 21/08/2012 12:28:12:

Swiss Flyer

A quick look at the equations ( in your link) seems to suggest that the air stream ratio of V1 to V4 should be in the region of 1 to 3, at max efficiency (for the fan duct system, although that ignores duct losses, as opposed to the free air equations used).

Erfolg, how do you know the values of V1 and V4?

UoS quote "The pressures at (1) and (4) are equal to the free stream value."

That suggests to me that the velocities are also equal to the free stream value.

 

Edited By Tony K on 22/08/2012 14:47:18

[Erfolg]

Tony

You do not necessarily know the velocity of V1 & 2. They represent the geometry of the diagram.

V1 could represent a position upstream of the entry to the system, V4 that at the boundary of the airframe. As has been written though, the Froude relationship, is a free system.

The concept of V2 & 3 being essentially identical, is not unique, it the basis of integration.

I am sure that you are not suggesting that it is not the fan which accelerates the air, or does something else do it?

[John Olsen 1]

"In this household, we obey the laws of thermodynamics" - Homer Simpson

Tony, you said " As thrust equals drag there must be a thrust being produced although there is no acceleration of the air before, through, or after the system."

I'm sorry but this is a breach of Newtons laws of motion..."For every action there must be an equal and opposite reaction" For the aircraft to experience a thrust, there must be a thrust on something else, and presuming the ground is out of reach, that must be a thrust on the air, which being a fluid, must move in response to the thrust. Actually even if we did thrust against something solid, like the ground, that too must move, although the ratio of the masses tends to be such as to make the movement unmeasureable.

What will actually happen at higher speeds is that the air arriving at the fan already moving will reduce the angle of attack of the fan blades, which will reduce the pressure difference across the fan, which will reduce the thrust. Eventually we will reach a speed where the thrust equals the drag (which is of course rising rapidly as speed increases) and we will be at maximum speed.

The last paragraph does not consider any possible changes due to the changing load on the power plant. The reduced angle of attack of the fan blades will reduce the drag of the fan, reducing the load on the power plant, which will therefore typically speed up. If the torque curve is fairly flat, this can mean that more power is actually available. The fan could also be stalled under static conditions although I guess we would try to avoid that. So the fall off of thrust as speed increases need not be linear, but will certainly be the overall trend as we approach the maximum speed.

The movement of the mass being thrusted on is why the efficiency is higher if we can act against a larger mass. The thust is the same (but opposite direction) for both our vehicle and the object being acted on, so the work done on each will be proportional to the distance that the force acts through. If our reaction mass is large it will accelerate less for a given force, so the force will act through a smaller distance and we will do less work on the reaction mass for the same work done on the vehicle.

John

[Mark Powell 2]

'Ground is out of reach....There must be a thrust on the air'

Uh, no. If there is anything whatsoever getting in the way of the exhaust it will iimpair the thrust. It works best in a vacuum. Difficult for an air breathing jet, of course, but the combustion chamber and exhaust outlet does not 'know' where the air has come from, could be a big sac in the fuselage.

The outgoing gas, regardless of what makes it outgoing, being it combustion of fuel in a supplied oyxgen atmosphere (rocket), being shoved out by a fan, or being heated (turbine) and the resulting expansion causing it to go out of the exhaust as it cannot expand anywhere else, does not 'push' against anything. The outgoing exhaust does not at any rate. And Newtons law is not broken. (There are other things beside the ougoing exhaust.)

Suggestion: Let's simplify things. Let's assume that the air that our EDF fan uses is supplied from a sac at at the same pressure as its surrounding. That's why I say 'sac' rather than 'balloon' Lets forget it won't last very long. Then we can forget the intakes and intake ducting, and only work on the fan, the outlet tube, and the outlet. Remember that what happens to the air once it has left the aircraft is of no importance. That is what we are told by every full size expert., with no disagreement at all. I am prepared to believe it.

I have removed half the problem and all the red herrings, but I don't have the maths to go further. Up to you.

Edited By Mark Powell 2 on 22/08/2012 21:45:15

[Myron Beaumont]

Just to stir things up maybe but I was just thinking about an "augmenter tube" I fitted to a jetex 100 powered Hawker Hunter many moons ago & how it increased the thrust compared with a model without one using the same "motor" It collected atmospheric air ,rather like todays by-pass fan gas turbines in a way, through the scale inlets .I know its not really relevant but is it? I'll get mi coat .Perhaps the inlet configuration and exhaust for that matter is more important than we think on EDFs Food for thought or not?

[Mark Powell 2]

Myron, I had one of those. Mine was built from the Jetex 'Tailored' kit, priced at 25 shillings. They did the Swift as well. Designed for tthe Jetex 150 Jetmaster. The others, all at ten shillings and sixpence were much smaller and designed for the Jetex 50.

Mine flew quite well, but the fuel pellets for the Jetmaster were very expensive. You pu three pellets in.

I think (guess) th the augmenter tube worked (and it did work) by letting the air in it get tangled up with the exhaust and thus dragged out with it.

Not irrelevant, the thread title is 'Theory and Practice' Not seen much practice, other than the OP, myself, and Simon's Skyray.

[Tony K]

Posted by Erfolg on 22/08/2012 15:28:18:

The concept of V2 & 3 being essentially identical, is not unique, it the basis of integration.

Yes, but are P2 and P3 the same? If not, why not?

[Tony K]

Posted by John Olsen 1 on 22/08/2012 21:03:54:

Tony, you said " As thrust equals drag there must be a thrust being produced although there is no acceleration of the air before, through, or after the system."

I'm sorry but this is a breach of Newtons laws of motion..."For every action there must be an equal and opposite reaction" For the aircraft to experience a thrust, there must be a thrust on something else, and presuming the ground is out of reach, that must be a thrust on the air, which being a fluid, must move in response to the thrust. Actually even if we did thrust against something solid, like the ground, that too must move, although the ratio of the masses tends to be such as to make the movement unmeasureable.

John, I am not sure if I completely understand your point here. Isn't it the air moving which causes the thrust?

I didn't say the air was not moving, I said it was not accelerating.

Let me try to visualise it another way. Imagine the column of air which passes through the duct is a rope. With the model held stationary the rope is driven through the duct and its weight times the speed it moves creates a force. That is static thrust.

Now take the two ends of the rope and tie them to fixed objects. The model will now move along the rope at the speed at which aerodynamic drag is equal to the force pushing it along.

To the ouside spectator the rope is not moving but from the models point of view it is (but not accelerating) and, as before, the weight of the rope times the speed it moves gives you the force acting in the opposite direction. That is dynamic thrust.

 

Edited By Tony K on 23/08/2012 01:33:23

Edited By Tony K on 23/08/2012 01:34:23

[Mark Powell 2]

"What will actually happen at higher speeds is that the air arriving at the fan already moving will reduce the angle of attack of the fan blades, which will reduce the pressure difference across the fan, WHICH WILL REDUCE THE THRUST. Eventually we will reach a speed where the thrust equals the drag (which is of course rising rapidly as speed increases) and we will be at maximum speed."

My emphasis.

Again, no. Take a aircraft powered by high bypass turbofans. As any passenger will tell you, the aircraft's rate of acceleration INCREASES as it proceeds along the runway from a standing start. (The take off speed is between a quarter and a thid of the aircraft's maximum speed so is not insignificant)

Where does this increase in the rate of acceleration come from? From the engines.

What is their throttle setting? Maximum.

So with the same power applied the thrust musr have increased as the speed increased, simply because the rate of acceleration has increased and the only thing that can make that happen is an increase in thrust.

Been there, done that. Sat at the front, watched the dials (including the 'G' meter). 160 knots, pulled the stick back. 180 knots we all leave the ground. Magic. And the rate of acceleration is increasing all the time.

Froude (or Sidney University) says "as this air passes through the rotating blades its speed increases"

A poster here questions that. Sometimes a little practical experience, as in (1) flying a 747 or (2) building a model with a propellor on it gives a clearer perspective.

[Tony K]

Posted by Mark Powell 2 on 23/08/2012 05:54:35:

Froude (or Sidney University) says "as this air passes through the rotating blades its speed increases"

A poster here questions that. Sometimes a little practical experience, as in (1) flying a 747 or (2) building a model with a propellor on it gives a clearer perspective.

Yes, I question that, particularly as further down the page it is clearly stated that V2 = V3. As it is also stated that section 2 is "the front of the propellor disc" and section 3 is "just behind the disc", how can speed increase from V2 to V3?

Also, if the velocity is the same at V2 and V3 how does the pressure change?

[Mark Powell 2]

Tony,

I have not read it. Your quote from it only mentions the speed, not the pressure.

You question that with 'Does it?'

No Ps, Vs, or anything else needed. Put your hand six inches in front of a propellor and then six inches behind it. You will be left in no doubt that the speed does increase.

John says the thrust recuces as the speed increases. It does eventually, particularly with an EDF or turbofan. But if he flew a 747 off the ground he would know it was wrong much of the time. a little practical experience is somtimes useful.

I note that my 'simplify' post has been ignored by the main three players.

Edited By Mark Powell 2 on 23/08/2012 10:34:11

[Tony K]

Posted by Mark Powell 2 on 23/08/2012 10:33:14:

No Ps, Vs, or anything else needed. Put your hand six inches in front of a propellor and then six inches behind it. You will be left in no doubt that the speed does increase.

So the UoS writer is not correct in stating that V2 = V3.

That would mess up the momentum equation on the third page and how do you apply the continuity equation?

[Erfolg]

Tony

Note the mention of 6". Where as V2 &V3 can have a difference approaching zero.

What our good friend (Froude) realised, is that across the velocity would essentially be exactly the same, that is immediately in front and immediately behind. That does not say that the pressure either side would be identical. It is the pressure difference that causes the airflow and everything that flows from it.

I personally would take the work of Froude, Hook, Newton, Euler, and so many more very seriously. Even if you know their are limitations to their work, the self same people often recognised/acknowledged the fact themselves, yet also knew that for most peoples understanding, and practical application, they worked well in our world.

[Mark Powell 2]

Tony, I just sometime think some of these 'academics' are nitwits. A while ago there was a progamme about making a full size replica of one of gliders. But they 'improved' it. A proffessor of aeronautics, plus a cuople of guys studying for doctorates (so thet had degrees already) decideded that the main spar would be better if the main spar was one solid vertical piece of spruce,, with oval lightening holes in it, rather than the original top and bottom with a light vertical grain web. So it instantly broke.

And these idiots get a degree?

[Tony K]

Posted by Erfolg on 23/08/2012 11:39:50:

Tony

Note the mention of 6". Where as V2 &V3 can have a difference approaching zero.

What our good friend (Froude) realised, is that across the velocity would essentially be exactly the same, that is immediately in front and immediately behind. That does not say that the pressure either side would be identical. It is the pressure difference that causes the airflow and everything that flows from it.

I personally would take the work of Froude, Hook, Newton, Euler, and so many more very seriously. Even if you know their are limitations to their work, the self same people often recognised/acknowledged the fact themselves, yet also knew that for most peoples understanding, and practical application, they worked well in our world.

Erfolg, I will add Boyle to the list, the relationship between pressure and volume.

The only way there can be a total pressure difference is if the density of the air changes. i think you will agree that doesn't happen.

We are, of course, discussing V2 and V3 not some arbitrary distance outside of those boundaries. It is obvious that a particle of air will accelerate from free stream at V1 to V2 and will negatively accelerate back to free stream from V3 to V4.

So, at V1 you have static pressure, at V2 you have static plus dynamic which is equal to static and dynamic at V3 (because V2 = V3 and the cross section area is the same), then at V4 you have static again.

Where is the pressure difference?