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Old Sep 07, 2015, 11:12 PM
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Question
EDF augmentor tube design

I'm working up a EDF plan that in real life is a huge turbofan. The goal is to bury an EDF pod inside of a larger duct and using the bypass air as part of the overall thrust. My question is what should the relationship be of the end of the EDF to the end of the duct. Obviously there can be three answers with parameters on two of the answers. The simple answer is the ends should be the same. The more complex answers would be ending the EDF forward or aft of the bypass duct AND by how much? The other question is will the thrust change and by how much?

Obviously, duct shape has to be controlled by fan cross sectional area/volume.
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Old Sep 08, 2015, 11:14 AM
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You're going to find that this is mostly a cut and try sort of deal. It'll be much like tuning a music instrument.

I suspect that you're likely to find that the exit nozzle of the EDF unit should be located at some point in or slightly ahead of a venturi shaped augmenter tube. You'll want that sort of shape in order to make the exit blast energy do the work of pulling along the standing air in the feed tube ahead of and around the EDF unit.

Skin effect or boundary layer effects at our sizes and speeds also means that we need the duct leading to the EDF and the bypass gap around the EDF unit to be larger than we may think at first glance. So don't be surprised if you can make an augmenter venturi but you find that you can't fit the fuselage with the required size of the bypass duct.

Because so much of this is based on tuning to achieve any gain at all I think I'd start with the EDF and rear cone or venturi test shapes on a rail that allows you to easily alter the position of the EDF to the rear duct. Try different shapes and sizes of entry to exit and vary the insertion or spacing until the rear duct is actually pulling anything in at all and you see a change in thrust.

Once you get a setup which gives you a positive change THEN play with how to add the bypass duct and how large it has to be to avoid the gap between the EDF and the duct causing so much drag that you don't get any increase at all.

On the actual augmenter duct you hopefully find that you can use a straight taper and get some usable increase. But it's going to be tough to find out how much taper you can get away with using and get a pressure build without the pressure coming back up to the EDF and inlet gap and simply stalling out the fan.

Likely as not what you'll get by doing this is a change of the thrust to a higher volume but slower speed. So it'll accelerate faster but probably won't have as high a top speed in level flight. This is because you're adding mass by drawing in the air from the bypass but you've only put so much energy into the air so the result will be that in a tuned up system that the speed of the exhaust will drop while the mass increases so you have the same power but at a different speed.

Inlet to outlet size will depend on what the EDF can tolerate for back pressure without stalling out the blades. Once that happens you'll end up with a net loss in airflow power. So that's why it's so important to work the bypass air into the EDF exhaust carefully and to tune it. I can't see any way to say "make it this big at the front and that big at the back" because much of the sizing will depend on the EDF fan in terms of blade pitch and the tolerance of the fan to back pressure from being in a tube. I would suggest that the exit will want to be at least the size of the EDF exit area. But how much more to allow the bypass air to exit with the EDF air I don't know. Maybe for giggles start with a rolled cone which can be reshaped between test runs at each end and try a few sizes. And this is where a rail to hold the EDF and cone which might even have a dial knob to allow movement of the EDF or cone during a run would be nice. The whole works can be on a thrust test stand and you can alter the spacing of the EDF to cone while watching the thrust meter.

Keep in mind also that most EDF units run better when the inlet has it's own venturi entry cone. You'll want to start with that along with some sort of outer smoothing or you WILL end up with lots of flow turbulence robbing you of thrust. One loss from the EDF trying to swallow the turbulence off the sharp entry lip located too close to the fan and the other being the turbulence off the outer side of the EDF when the bypass air is flowed past the EDF unit and into the exit cone.

Here again you'll want to play with a rolled up bypass tube that allows you to vary the size so you can find the smallest size which does not produce too much drag. The air being drawn in by the lower pressure around the outer portion of the augmenter cone and into the exhaust blast is not responding to all that much of a drop in pressure. So almost any drag from a rougher sort of surface producing boundary layer turbulence, such as off a rough outside EDF ring molded surface with stiffeners and such, is going to act like an air brake to the bypass air flow.

Once you get a simple cone working I'd suggest trying to make it a little more like a proper venturi by trying a double or even triple truncated cone setup for the augmenter tube and see if that raises the efficiency at all. The closer it can get to a proper venturi shape the better the effect should be. But making a true venturi is a LOT of work. But perhaps a triple cone lashup might give you a nice middle ground between a simple cone and a full on venturi.

And yes, this isn't the first time I thought about this. I got the idea many years back and worked on it long enough to come up with the questions but never got around to trying to find the answers to the questions. The biggest issue is to avoid a backpressure on the EDF that results in it not producing a good air blast. And I think you'll find that this occurs more easily than you/we'd like. So it's important to not go too crazy with reducing the exit of the augmenter tube and creating that back pressure. You'll also find that the taper angle plays a part. We want as short a tube as we can. But if the taper from inlet to outlet is too sharp you'll get the buildup in pressure we're trying to avoid. So a longer angle is needed or you can't get the exit size you want.
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Old Sep 08, 2015, 12:07 PM
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Interesting. Given the requirement for more simplicity in models than full size, since Bruce is spot on that the result at least should be to slow the exit velocity to better match the plane's airspeed [physics definition of power: reaction engines produce zero power when standing still. disregarding heat generation] while preserving flow momentum.

As a simple designer, I suggest just using a fan with larger and/or more blades with appropriate pitch. Fullsize turbofans do this through gearing and ducting, but the losses in model size lead me to another conclusion.
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Old Sep 08, 2015, 12:14 PM
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If using a bigger EDF with larger and/or more blades at appropriate pitch is not feasible .... how about a ducted propeller? Same result, less work.
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Old Sep 08, 2015, 01:36 PM
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Big holes, small props.

Did what I wanted.
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Old Sep 08, 2015, 11:07 PM
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OK, we have some interesting ideas here. Thanks! I'll work up a cad dwg to give some of my thoughts - might take a day or so.
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Old Sep 08, 2015, 11:27 PM
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I'm thinking that Lee may be onto something with the idea of a more or less full size prop in a scale like or fully scale duct ring assuming that this will be an airliner style model. Props work at model speeds for a whole lot of reasons. And likely as not they would be the model size version of a turbofan that suits our needs the best.

On the other hand if this will be a fuselage "bound" EDF and augmenter tube then I'd carry on with some investigation. On the other hand you may well find that Ray's fuselage tubes with props is your best option if you're talking the sort of size needed for a bypass size which will work decently well. After all EDF's don't have the best sort of thrust per amp or even the best speed per amp going. The one thing they ARE best at is allowing the models to look like real jets.
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Old Sep 09, 2015, 06:48 AM
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Remember jetex ?

http://sceptreflight.net/Model%20Eng...r%20Tubes.html
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Old Sep 09, 2015, 11:47 AM
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Sure, Ray! I had many Jetex 50's and a few of the larger ones, also a balsa molded-skin Douglas Skyrocket, some balsa flying missles, even a 2-engine heli that autorotated after burn-out, plus many self-built, incl an X-15 that wouldn't fly with the Scorpion but finally did with an Estes rocket engine.

Several used the augmentor tube. I don't know what the exhaust velocity was, but I am sure it was much faster than the planes' airspeeds. Max efficiency and power for such reaction engines comes when the exhaust velocity is equal and opposite to the airspeed. So efflux velocities of EDF's and props are already reasonably close to the plane's airspeed and likely not profiting from such augmentor tubes as with the Jetex's.

But that's just a reasonable guess: "your results may vary ...."
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Old Sep 09, 2015, 12:52 PM
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Here are some photos showing my experience with large turbofans and augmenter tubes!

Flow in the tube is established via momentum transfers; and as would be expected there are losses – thus you get a bit less F = m dot v out than put in.

If the big turbofan is buried in the fuselage who cares if it a fan or a prop in the model – go w/prop!

A nacelle system doesn’t give you much length – again go with a prop.

As Bruce indicated the exit/entrance spacing mostly influences the acoustic coupling; has very little effect on augmenter flow.

A bell certainly helps the inlet flow to air machine.

Wrote a spreadsheet more than 25 years ago that calculates the test cell parameters to show the performance with various engine: Hi * Low by pass turbofans, jets, and with or without re-heat.

Did rockets too!

Jim
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Old Sep 09, 2015, 01:51 PM
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Quote:
Originally Posted by Bob Reynolds View Post
and using the bypass air as part of the overall thrust
Has this been experimentally demonstrated? (I mean with measurements, not eyeballs). I haven't worked with EDFs much and I'd be curious to find out.
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Old Sep 09, 2015, 01:58 PM
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Something I've always been a bit surprised about with EDF's is that there isn't a lot of work put into pinching in the duct of the exit. Usually it's just a tube that ends about where the motor tail cone starts. I've taken this to mean that EDF's won't tolerate much in the way of back pressure before the blades of the fan become stalled out. Otherwise it seems like there would be some speed gains from a longer shaped exit cone with a shallow taper angle which speeds up the exhaust a little over what is found coming off the fan itself. Or perhaps any such gains are eaten away by the increase in surface area and boundary layer effects for a net loss? Again I've studied enough to understand the questions but never got around to playing with the fans themselves to try to optimize how they are used.
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Old Sep 09, 2015, 02:25 PM
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Quote:
Originally Posted by BMatthews View Post
Otherwise it seems like there would be some speed gains from a longer shaped exit cone with a shallow taper angle which speeds up the exhaust a little over what is found coming off the fan itself.
I suspect that sometimes people consider EDFs and jet engines to be more similar than they really are. Jet engine nozzles are designed with regard to the high temperature compressible flow. It's better to think of an EDF as a propeller with a shroud that helps reduce tip losses, like winglets.

The streamtube passing through a propeller exibits a natural contraction that depends on the freestream velocity and thrust:
http://web.mit.edu/16.unified/www/FA...00000000000000
I suspect that any nozzle that forces it to deviate from this contraction will hurt performance.
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Old Sep 09, 2015, 03:51 PM
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DPATE, I suspect you're right....

I know that the exhaust stream from an EDF would only tolerate minimal loading before the whole flow dynamic collapses. And that's why I assumed that nothing has been successful in terms of extending the exhaust duct.

And as you say it's got more in common with a ducted prop than any sort of jet engine. And given how prop efficiency drops with size in many ways it's a wonder we get the sort of performance that we do from them. Mind you we DO give up a lot based on the idea that if the same number of watts per lb were put into an open prop of the right diameter and pitch that we could achieve even higher level flight speeds than we get from an EDF.
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Old Sep 09, 2015, 05:29 PM
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As Bruce indicated the exit/entrance spacing mostly influences the acoustic coupling; has very little effect on augmenter flow. Jim
Model EDFs have a realistic "compressor scream" but significantly lack the roaring low overtones of a fullsize turbine. So i wonder if some energy could be put into driving an efflux resonance chamber instead of a thrust augmentor. [Toucans' beaks are primarily resonators ... and they are LOUD]. And if that would be lighter and/or more energy-efficient than an onboard speaker system.

Assuming it doesn't shake the plane apart, of course ....
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