EDF Ducting and EDF Efficiency

[MDKramer]

I apologize for resurrecting an old thread, however, with a little luck the people who participated in it are still around and could help bring what's bouncing around in my head into a bit better focus.

What I'm thinking of is possibly converting a Byron F-16 to EDF. Perhaps using a JP 120EDF as a fan.

I don't have a kit in front of me at the moment, so I don't have clear measurements to work from (yet). I've seen several F-16 EDF conversions, and it's all the same. Remove Rossi, Byro-Fan and tuned pipe, scrape all the castor sludge off the interior of the tail section, bolt EDF to existing fan mount former, wire and fly.

Looking up the tail-pipe of the Byron, it's nothing like smooth. Formers, contours of the fuselage, etc... I cannot help but think that the addition of an exhaust duct that would be centered in the existing outlet by retrofitting a more closed former system would eliminate turbulence, making the thrust more efficient.

Then I look at that massive cheater hole in the bottom of the fuselage, and the massive scale air intake, and I cannot help but wonder if a smooth, consistently tapered duct from the intake to the front side of the fan would actually provide adequate smooth air flow while eliminating the need for that huge, loud, turbulent hole in the belly.

Is this a feasible enough concept to warrant sitting down at the calculator, mocking up some test ducts and taking measurements on the bench before sliding them into a fuselage? Or is it just a pipe dream? (pun fully intended)

Please be kind, I've just begun digging into this idea, and this was the first source I found with people who actually sound like they know what they're talking about. LOL

Thanks in adavance!

Mike

[dusmmdb]

Gents, can anyone point me in the direction of a tutorial on "building" inlet ducts?. The layman in me just tells me to cut foam to size and mother it in fiberglass and then dig out the foam. BUTTTTTT......... im sure there is a professional way to do it

Mike

[MrVoyagerx]

Follow the noise and watts levels.

[fhhuber]

Sorry for chiming in so late. I was wondering about the sonic cloud produced on a fast moving jet. If it the turbulance being built up around the fans.

The visible coning mist is the interface of the shockwave with moist air.

This can occur while the aircraft is slightly under Mach 1 as the shock wave starts forming around some parts of the aircraft earlier.

[Skyboom]

Sorry for chiming in so late. I was wondering about the sonic cloud produced on a fast moving jet. If it the turbulance being built up around the fans.

[fhhuber]

You can get an assortment of EDF rotors to put in your fan. I don't know of adjustable blade rotors, but you can get many different rotors of any diameter. Different blade counts and different pitches.

You can "cheat" and use a higher power fan rotor and motor in a lower rated fan housing. I did that to the Hobby King BAE Hawk - Red Arrow 70mm EDF 990mm (discontinued), using the motor and rotor from a Dynam Me262. LOTS more power than the included rotor and recommended motor.

When you add power you may have to add more cheater hole to get adequate air to the fan.

You can try a simple test without buying or changing anything... struggle for altitude and get it high. Point the nose down 45 deg keeping power on. See if the plane having accelerated is letting enough air enter to keep up with what the fan can push out. Smooth WIDE turns so you don't lose speed. If that helps, then altering the inlets to improve airflow will help.

sometimes the molds don't have the right taper for the proper 85% FSA outlet area. You can alter the outlet (usually needs some squeezing) with a section of styro or thin wall plastic drink cup. Cut the bottom off for the proper outlet and fit the cup inside the duct. You may want to make a ring to keep the new outlet centered in the original.

It just happens that the taper of a common fast food drink cup is a good taper for EDF outlet cones.

[xviper]

I fly at 7000 ft. alt. After buying several EDF's that don't fly for squat, I realized it wasn't the plane, but thin air. Are fan blades available with a higher pitch and would this help the problem? Thanks, Doc

HobbyKing, years ago, sold empty fans and blades (individual) that you could pick separately that had user selectable pitch. You then balanced and assembled them at home. I don't know if they or anyone else does this now.

[doctormike]

I fly at 7000 ft. alt. After buying several EDF's that don't fly for squat, I realized it wasn't the plane, but thin air. Are fan blades available with a higher pitch and would this help the problem? Thanks, Doc

[PaulZ]

Thanks Alpha! Very helpful and as an engineer this discussions gets my "propellers spinning". Gives me an appreciation for the work that goes into making these models fly so amazingly. Thank you!

[xviper]

I don't have enough brain cells to take in what's been said in this thread. :Confused:

[air-jon]

I read these dissertations on aerodynamics and fans and such and of course I can really only absorb/understand about 30% because I don't speak equations...but I love to glean what I can from the little bits of english in between the numbers. Then you find the references In each paper and go down those rabbit holes. It's amazing the amount of work that has gone into ducted fan design alone!!!

[wolpertinger]

Wow, reading the conversations make me realize why I have 2 ears, 2 eyes and 1 mouth. Twice as much listening and reading before talking so keep the lessons coming.... :Cool:

[Elbee]

Guys, I want to thank all of you who contributed to this topic, personally. So, I thank you! I hope this thread continues as is reasonable as the information is great and the discussion relevant to me, anyway. One can never have too much knowledge and I am woefully behind in the tacit. Gratefully, LB

[Alpha]

I love conversations like these.

You're right, Air-Jon, length absolutely matters, both before the EDF and behind the EDF. It's easy to imagine all of these factors operating in extremes; I think most of us can easily envision that a 90mm F-16 with a ten foot long intake and a five foot long outlet tube would be less efficient at ingesting, transiting, accelerating, and expelling air. Logically there must be a sweet spot. Refer to my checklists above about resistance and pressure and the conclusion is foregone.

A lot of our prototype testing involves flight simulation modeling and in-flight measurement tools, because moving airflow is what we're most interested in optimizing. With each model aircraft, the "sweet spot" differs, because different aircraft bring different numbers to the equation (drag coefficient, etc). Astute fliers will notice that in general most of our modern generation Freewing jets are configured such that they have good acceleration when near stall speed. This is to help them "power out" of bad spots, such as when low and slow immediately after a waved off landing. To achieve this, we sacrifice portions of pure static thrust (on the tarmac at a standstill) and max dynamic thrust (full throttle diving).

[air-jon]

All kinds of Nope! Ducted fan/shrouded propeller....all the same thing. I promise you if that were true, we would all be flying on turboprops across the oceans. Turbofan is a ducted fan, powered by a gas-turbine. Ducted fans can be much more efficient than open props in certain circumstances.

I'm the case of "deeply ducted fans" as we deal with in fighter jet models....the efficiency goes way down due to limited diameter and long ducting losses.

I never said you can't stall a ducted fan, only that it much more stall resistant than a propeller if the fan has proper intake and exhaust area.

https://massflow.archivale.com/ductbook.htm

https://www.grc.nasa.gov/www/k-12/airplane/inleth.html

https://www.researchgate.net/publica...n_VTOL_Systems

https://www.researchgate.net/publica...FD_Simulations

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700006092.pdf

[fhhuber]

Sorry, but EDFs are really easy to stall.

Just try the fish scale test... It is almost guaranteed that 100% of the EDFs you own will be proven to stall

Also it takes 150% as much power to get the same performance from an EDF as from an open prop... if the rest f the airplane (including weight) is the same. But the EDF model will weigh more...

Don't confuse a shrouded fan with an EDF

A shrouded fan the prop is in a ring that acts to reduce tip losses. The prop is still turning in the same rpm range as a conventional prop without the ring.
The shroud is typically less than 1 radius in length. Most of the length is for structural rigidity. Only the portion equal to 2 times blade chord is really needed to get the benefits of the shroud.

[air-jon]

Fans are much more difficult to stall than a prop. this is another advantage to ducted fans, (and fan stall another common misconception) a fan will not be fully stalled in a a stationary position...unless the intake is greatly restricted, or there is significant turbulence. a non-ideal intake lip will certainly be stalled at takeoff. This is easily tested on the bench by running a fan (with a proper bell intake, like the ones provided by WeMoTec) and observe the thrust and current consumption. Now, if you devise a slightly longer intake bell you can add a remotely actuated shutter...and you start to restrict the intake airflow, you can induce a fan stall (this can be done by reducing the exhaust as well). you will initially see a rise in power consumption, and then a sudden drop in consumption and increase when the fan stalls and is no longer moving air through the system with a closed intake...its just swirling like a blender (warning!...fan motors overheat very quickly in this state..ask me how I know...and this is a potently dangerous experiment..dont try at home ) This is why your home vacuum cleaner actually increases rpm when you block off the airflow.

Open propellers operate in a free air mass and the effect of the low pressure in front of the blade is quickly dissipated in all directions. there is also lots of reversion at the tips of an open propeller which promotes the tips to stall even sooner. there is very little or no reversion at the tips of a properly designed ducted fan. With an ducted fan, the parcel of air the fan disk sees is uniformly accelerated onto the fan disc. most of the pressure differential in front of the fan blades is distributed forward all the way to the intake lip. this creates a very different aero environment for the fan to live in tan an open propeller and is they keystone of ducted fan efficiency.

I only know what I do about ducted fan theory from reading academic papers from the interweb. there are hundreds dating back to the 1930s...written by guys and gals much smarter than me! you just have to dig a bit to mine the good ducted fan papers. once you lear the academic lingo,...you can search the terms like "deeply ducted fans" yielded this: also try fan tip reversion, fan pressure recovery..etc… good stuff to read when you cant sleep on a long flight...on a ducted fan powered airliner;)I promise you those fans are not stalled when an airliner releases the brakes at full throttle from Orange County Airport, ballistic noise abatement procedure...(they actually warn you before takeoff!)

https://www.eucass.eu/doi/EUCASS2017-651.pdf

[fhhuber]

the fan can not pull in air fast enough to prevent stalling the fan blades at full throttle if the model is not moving forward. Thus there is a ram air effect of a sort.

You can also stall the blades of conventional 2 blade prop without the duct. This depends on prop pitch and rpm. A high pitch prop at high rpm is easier to stall.

The EDF is a very high pitch prop at very high rpm.

Tie a fish scale to the back of the airplane and slowly advance throttle. You will find a point where the pull of the EDF drops off with increased throttle. You stalled the fan. More throttle may continue to decrease the pull n the fish scale. Its not likely to increase without somehow improving air intake. this is ne reason we have cheater holes. Better intake means the fan is at a higher rpm before it stalls.
A good intake shape can be a major improvement, especially at low speed. (trying to gain speed for takeoff)

The airplane moving forward has the air speed going into the duct inlet higher, so it takes a higher rpm to stall the blades. When the airplane is moving fast enough, the air is coming in fast enough that full throttle does not stall the blades.

[air-jon]

The length of ducting is fundamentally very important. every mm increases the thickness of the boundary layer and increases intake/efflux drag. the ideal fan is one with a short nacelle. Google

"Airbus E-Fan" to see what type of fan is created by lots of engineers, computers, and many 10s of millions of Euros. I assure you this is the most efficient EDF in existence.

There is no" ram air" with EDFs in level flight at constant throttle setting with a properly designed fan. In a properly designed ducted fan, the intake and exhaust velocity will always exceed the local external airspeed for a given fixed thrust setting, or an EDF would not accelerate. this is the most common misconception in ducted fan design. for instance, the efflux velocity of a typical 90mm EDF is around 180 to 200 MPH. Based on the delta between the intake and exhaust area, the intake velocity will be proportional to the delta. so intake velocity will be about 5 to 20% less than efflux velocity in the typical EDF (based on type of fan). For typical 90mm foamies, an intake velocity of 150 MPH would be expected, which is well in excess of the 100-120MPH max level speed. High performance composite EDF like a BVM Electra have higher efflux and intake velocities nearing 300 MPH and they still never experience "ram air" even when flying 220 MPH in level flight. The only way you could experience "ram air" is if the intake is vastly oversized, with very low intake velocity as Alpha pointed out earlier. This will kill the intake efficiency and reduce the top speed of the aircraft.

What does happen with speed is a reduction in intake turbulence, which effectively makes the intake larger when it goes laminar. depending on how sharp the intake is, this will happen at different speeds for different models. a stalled intake lip can make the intake lip function like one half its size. They do sound cool though!!!

Intake ducts have been compared by scholars to cylindrical wings with a fan inside (which is just a bunch more little wings) and all the standard wing principals still apply. Deeply ducted fans cause all the same problems as very low aspect ratio wings with higher drag and a thick boundary layer. this is why gliders, airliners have very thin wings (and short fan nacelles). Fan ducting requires a pressure differential as well with lower pressure at the intake and slightly higher pressure in the exhaust. This gets into pressure recovery and i'm not smart enough to talk on that..

You are correct about a slowly reducing diameter from intake lip to fan. this keeps the boundary layer minimal and creates the minimum drag with the air reaching maximum velocity just prior to the fan plane.

I think intakes are cool, others think they suck...;)

[fhhuber]

A bare fan is less efficient than one on a well designed duct.

Length of duct is not as important as getting the inside smooth and having the cross section area change slowly. Abrupt changes are bad.
Taper of a common styro drink cup is about as rapid as you want to try.
If length does not allow the styro cup taper to get the outlet down to 85% FSA you're probably better off with the wider outlet.

Inlet having a funnel effect and having a rounded leading edge is better then a straight tube.

Outlet being 85% of FSA (Fan Swept Area) is important for best results.
Pi * R^2 (Radius of fan) - Pi * R^2 (Radius of fan spinner) = FSA.
SQRT((.85 * FSA) / Pi) = desired R of outlet
For bifurcated, SQRT((0.5 * .85 * FSA) / Pi) = R of each outlet

I'll assume the ducting molded into the EPO foam is designed for the OEM "default" fan installation. Different fans of the same nominal diameter have different FSA.
Its easier to put in a cone to squeeze down an outlet that is too large than it is to open the outlet if its too small.

The most common issue I find with foam EDF models is inadequate inlet until the aircraft has built up speed to give a "ram air inlet" effect. (really bad lack of inlet area by some other brands)

I haven't done the math on the Freewing F-14 but I feel it needs some more cheater hole area. Once its up to speed it desn't need it. This one will be easy to alter just by changing the cheater hole grates.

Its easier to cut more opening than to close it off... But it can also be very hard to find a good place for the cheater hole, so you accept the longer take off run.