Ducted Fan

[cncdoc]

Ordinarily I would have put this under the last topic where Marbleturtle alluded to a Ducted Fan being used on a Cozy. I searched for the topic in the forum search tool and ended up with very little.

 

So, has anyone (besides me) researched the PSRU-less alternative to a prop?

 

From what the research says, it is pretty awesome up to about 160 mph where intrinsic design encumberances take over.

 

So now at least we'll have a seperate thread for it anyway.

 

Twin ducted fans? Hmmmm. One in front and one in back or nacelles on the wings?

 

Makes for interesting pictures in one's mind.

 

Did I mention they don't use PSRUs?

[dust]

are the fans variable pitched?

 

can you reduce the exit opening to increase velocity?

 

can you go to 25000 feet

 

he he he

 

enjoy the build

 

mike

[No4]

There is a Long EZ flying with a ducted fan, (or is it a shrouded propeller??)

 

http://www.bridgingworlds.com/duckt.htm

 

To quote our own Mr Slade from the www.homebuiltairplanes.com forum,

 

"IMHO Perry's experiments are a wonderful example of the true experimental movement, but I think he has a long way to go. The duct concept isn't very efficient at low speeds (eg less than 300 kts or so), and he's not getting a lot of power from his 13B. The result is that his cruise speed is way below a lycoming powered EZ. A redrive weighs about 35lb. I bet that duct weighs a lot more, so he's not gaining anything net weight wise."

 

I totaly agree, but if I may add....

 

from my basic gas turbine rating notes

 

@ 250 mph; prop efficiency 80%; Turbo fan 60%, and Jet 40%

@ 450 mph; prop efficiency 60%; Turbo fan 70%, and jet 55%

@ 600 mph; prop efficiency 40%; Turbo fan 80%, and jet 70%

 

of course jets are the only engines that can go faster than 600 mph.

 

From A.C. Kermode "Principles of Flight" (my bible)

 

.." the amount of thrust provided will be equal to the rate at which momentum is given to the air."

 

Thus Thrust = mass times velocity, or T=mv

SO!

Accelerating 1 kg of air to 10 metres per second will give you the same thrust as accelerating 10 kg of air to 1 metre per second.

 

Kinetic Energy is 1/2MV2

 

1kg@10m/s = 0.5 * 100 which equals 50 watts

10kg@1m/s = 5 * 1 which equals 5 watts

 

So really you want to accelerate as much air as possible, rather than less air as fast as possible.

I will be trying a big 5 blader which will spin between 1000 rpm and 1750 rpm, depending on airspeed.

[marbleturtle]

Twin ducted fans? Hmmmm. One in front and one in back or nacelles on the wings?

Side by side in the back... but not on nacelles. Jets need nacelles to get out of the boundary layer of air to reduce induction turbulence. Ducted fans can use the slower boundary layer for greater thrust generation. I was thinking of blending the shroud into the top of the strakes... this way only about 1/2 of ducting adds to the frontal area. Perry Mick's entire single duct (save a few small patches blocked by the strakes) is added frontal area. Keep in mind this is just theory I'm considering.

 

Accelerating 1 kg of air to 10 metres per second will give you the same thrust as accelerating 10 kg of air to 1 metre per second.

 

Kinetic Energy is 1/2MV2...

 

...So really you want to accelerate as much air as possible, rather than less air as fast as possible.

 

Well... maybe. If you take a rotary spinning at high RPM (7500?), normally you attatch a PSRU and a large prop to move a large amount of air. I'm wondering if I can simplify things by using a direct shaft to a high RPM fan (7500?). This smaller fan would be accelerating less air faster.

 

But I have now been thinking two fans! Can 2 small fans move as much air as one large prop? Our props are partially blocked by the fuselage. Fans on the strakes would have full front exposure. (?) Ducting blended into the strakes reduces some of the disadvantage of duct fans, namely the added frontal area of the duct. (?) This is starting to keep me up at night!

 

The $64,000 question I'm working on is, can the small fans create as much or more Kinetic Energy by moving "less air faster" than the PSRU system moving "more air slower".

 

 

I guess I've been looking at boats too long. Big giant paddle wheeler, pushes lots of water slow and more efficient. Direct drive little outboard propeller... pushes small amounts of water fast!

 

 

are the fans variable pitched?

can you reduce the exit opening to increase velocity?

can you go to 25000 feet?

 

no.

no.

Only when I reduce power and drop from 30,000 feet.

[No4]

There is always the "un-ducted fan"

 

[mjgundry]

I am looking into a ducted fan for the Cozy Mark IV, powered by a 13b engine. If you are interested, I have a site with some *very* preliminary design data up at:

 

http://www.faa-engineers.com/~mjgundry/ductedfan/

 

This very same topic was recently discussed on the Cozy mailing list...

 

To recap:

 

A perfectly point designed prop will probably just beat out the perfectly point designed fan at the cruise speed of a Cozy. I'd like to see if the difference can be made as small as <10% in real life (not my playing with numbers).

 

Perry Mick's shrouded prop weighs 30 lbs, 5 lbs less than a PSRU. He says it is overbuilt, and the next one could probably weigh in at 20-25 lbs. The fan also weighs less than a prop.

 

Venturing a guess, I would say that Perry's design is running into a problem with compressibility at the fan. Doing the math, his blades are moving near Mach 1, and the limit of the airfoils is probably closer to Mach 0.7. The loud fan noise that he speaks of seems to bear this out. He is currently planning to try a smaller diameter fan which would help alleviate that problem. I am anxious to hear/see his results.

 

True enough, the inlet diffusers on high bypass turbofans are not efficient at low speeds. However, anyone trying that sort of geometry for an IC engine powered fan probably hasn't worked out the math. A properly designed ducted fan can actually outperform a prop at low speeds because it is capable of producing far more static thrust.

 

There is no reason that you can't have high mass flows with a ducted fan. In fact, one of the first stages in my preliminary design was maximizing the mass flow by adjusting various duct dimensions. As opposed to an inlet diffuser, low speed ducted fans accelerate the air into the fan, then diffuse the flow afterwards for pressure recovery. Using this technique, mass flow and efficiency of the fan can approach that of a larger prop. If you want to compare diameters, you should probably be comparing the diameter of the "capture area" of the duct with the diameter of the prop, and that is a tenuous comparison at best.

 

Yes, you can have variable exit geometries. I'm not sure than variable fan pitch has much utility. Variable exit geometry could help optimize cruise speed efficiency without adversly affecting low speed thrust, but I'm not sure that it is worth the complexity, given the speed range of a Cozy.

 

I am currently at the point where I have a duct geometry I'd like to try, and so I am modeling it for CFD analysis using NASA's ADPAC. I expect a some refinement as I adjust the design to accomodate off-design performance. I'd really appreciate informed feedback on my site, if anyone is so inclined.

 

Matt

[Aaron]

Originally posted by marbleturtle

[

 

Well... maybe.

 

No maybe about it. Accelerating less air faster takes more power for the same thrust.

[No4]

Hello Matt and all,

Well hats of to you my friend! You certainly sound like you have done your homework.

I had a good look at your site. I can't read the Mathcad document, and the html document came up with a lot of boxes with the dreaded red cross. Anyway, my simple brain would probably be unable to compute all that information!

 

I undestand the idea of accelerating the air into the fan, and then allowing it to expand, but isn't that drag? If you say the fan is 10% less efficient than a prop, and a psru only taps 5% power, then isn't the psru ahead? Also the weights seem almost identical. How do you compare the drag of this fan to a propeller?

 

Could I ask are you comparing a fan to a fixed pitch prop, or a variable pitch prop. Just off the top of my head I imagine the csu might match or better the static thrust of your fan. The fixed pitch prop will of course suffer inefficiency at any speed other than it's designed propulsion velocity.

 

Not really to do with the actual fan or prop, but I'm lead to believe that the psru will remove the gyroscopic, asymetric torque, and vibration problems inherant to propulsion. Direct drive will load the rotary's main bearings in a way it was not designed.

 

Modern turbo props reduce their shaft speeds somewhere in the region of 50:1, and use multiple long carbon fibre blades with scimitar tips.

For example the ATR-72, with twin 2500hp engines uses seven scimitar blades with a disc diameter of 14 feet, which spin at 900 rpm in cruise.

Which begs for me the question, why then are they not using a ducted/unducted fan?

I understand of course that just because everyone isn't doing it doesn't mean it is not a good idea.

 

Furthermore, I suggest that most props you are comparing to will be spinning at around 2500 rpm. I believe this magic number only comes from the Lycoming benchmark. To my knowledge no Rutan design has been fitted with a slower spinning prop.

I propose a race, my 1000 rpm thumper versus your 7000 rpm screamer.

How does September 2007 suit you?

 

Best of luck with your design, I'm sure many people will be fascinated if it comes into reality.

Cheers

Adam

[mjgundry]

Originally posted by No4

Hello Matt and all,

Well hats of to you my friend! You certainly sound like you have done your homework.

I had a good look at your site. I can't read the Mathcad document, and the html document came up with a lot of boxes with the dreaded red cross. Anyway, my simple brain would probably be unable to compute all that information!

 

Bummer about the html sheet not apearing correctly. I checked, and I imagine it is because they are all little gif images. Unisys would have a field day. I'll convert them.

 

The math really isn't very deep. If you remember Bernoulli, it's actually quite simple.

 

Could I ask are you comparing a fan to a fixed pitch prop, or a variable pitch prop. Just off the top of my head I imagine the csu might match or better the static thrust of your fan. The fixed pitch prop will of course suffer inefficiency at any speed other than it's designed propulsion velocity.

 

Yes, I was thinking of a fixed pitch prop. A csu will do better than a fixed pitch prop, but I don't think it will match up to the duct.

 

August Raspet at U. of Miss. was getting 560 lbs of static thrust with 90 hp, and 1800 lbs with 250 hp. Pretty impressive.

 

Granted, this was an acedemic excercize. I wouldn't expect to get near that performance on the back of a Cozy. Thrust/horsepower falls off as disk loading increases, and I am working with a higher disk loading than Raspet's experiments.

 

The duct accomplishes the high static thrust by vastly reducing tip losses, and by dropping the pressure on the front face of the duct, and increasing the pressure on the exit. Because of this pressure diffential, the duct itself can be providing over half the total thrust. As speed increases, duct drag starts to overwhelm this effect.

 

Not really to do with the actual fan or prop, but I'm lead to believe that the psru will remove the gyroscopic, asymetric torque, and vibration problems inherant to propulsion. Direct drive will load the rotary's main bearings in a way it was not designed.

 

True. Perry has upgraded his thrust bearings and I imagine I would do the same. Also, a direct drive fan wouldn't be as bad as a direct drive prop in this situation. Smaller diameter - less gyroscopic loads. Lower total disk force - less shaft thrust (remember, the pressure is recovered aft of the fan and the duct actually does some of the pushing).

 

Modern turbo props reduce their shaft speeds somewhere in the region of 50:1, and use multiple long carbon fibre blades with scimitar tips.

For example the ATR-72, with twin 2500hp engines uses seven scimitar blades with a disc diameter of 14 feet, which spin at 900 rpm in cruise.

Which begs for me the question, why then are they not using a ducted/unducted fan?

 

There is a point where you can't accelerate (low speed duct) the air any more because you are already so fast as to cause problems with compressibility. Nor are you fast enough to get gains by having an inlet diffuser (high speed duct) in front of the turbine. So you just have a duct that does nothing but add drag. That point is right about where the turbo props operate.

 

IIRC, Marc de Piolenc, the author of the book I cite on my page, has said that speeds above 300 Kts are purely the realm of the turboprop and turbofan.

 

I understand of course that just because everyone isn't doing it doesn't mean it is not a good idea.

 

Best of luck with your design, I'm sure many people will be fascinated if it comes into reality.

Cheers

Adam

Thanks. Obviously it's still very early in my Cozy quest. It's as likely as anything that I will drive to nearby Wentworth Aircraft and pick out a Lycoming, just as the plans say.

 

Cheers,

Matt

[marbleturtle]

Well maybe... again!

 

I have no quarrel with the fact that a large prop is going to be more efficient than one ducted fan with a fully exposed frontal area duct... roughly 5-10% in favor of the large prop in cruise conditions if each are fully optimized.

 

What I am putting to paper to run through the numbers (eventually) is how 250HP driving a single pusher prop compares to 250HP driving two blended shrouded ducted fans (comparable surface area? I dunno yet). I'll see if I can draw a picture to show you what I mean.

 

The whole idea started after looking at Mick Perry's site. Mick is quick to admit that his ducted fan design is slower than most long EZ's, but many of his problems are identified and curable: low engine power, duct to large, fan slightly too large, etc. The idea of forgoing a PSRU is attractive if I can get close to 95% the efficiency of a standard prop. Now if I just find my slide rule.

[Jim Sower]

Thrust of a propeller IIRC is M(V2-V1) so, unlike an energy function that would vary exponentially with velocity, it doesn't much matter if you accelerate a little bit of air a lot or a lot of air a little bit so long as M * delta-V is the same.

 

The flow into and out of a prop disc (that doesn't have a lot of structure around it) is kind of funnel shaped - it tapers from an area larger than the disc ahead of the disc to an area smaller than the disc aft of the prop. A look at the cross section of a duct tells us that it is replicating (and perhaps enhancing) this natural phenomenon. Props have tip losses and that, basically, is what ducted fans aim to address. Sadly, they don't eliminate them altogether and the solution tends to outweigh the problem.

 

Perry is a very bright fellow who has done his homework really well. It is extremely unlikely that he would miss something as fundamental as critical flow around his fan blades. Ducted fans work well when lashed to a turbojet, but I tend to believe that if they provided any real improvement elsewhere, we would find them elsewhere, which we don't. Perry is far from plowing new ground here. He has discovered that he can reduce tip losses but not eliminate them by any means, and the inefficiencies of the fan itself cost him more than the increased tip efficiency buys him back. Furthermore, the duct is heavy and draggy and I doubt any of us will be able to produce a fan so much more efficient than a prop that it overcomes the intrinsic overhead of the duct.

 

But then again, the object of the exercise is experimentation and education, so go for it!! Hope I can be forgiven if I don't hold my breath until the big breakthrough.

[mjgundry]

Originally posted by No4

I undestand the idea of accelerating the air into the fan, and then allowing it to expand, but isn't that drag? If you say the fan is 10% less efficient than a prop, and a psru only taps 5% power, then isn't the psru ahead? Also the weights seem almost identical. How do you compare the drag of this fan to a propeller?

There is definitely more drag with the duct. However, the fan desn't have tip losses that the prop does. If you try to expand the flow too fast in a duct exit, you get flow separation, and even more drag, but we'll try to avoid that.

 

I expect that the PSRU is more efficient, at least when comparing as we are here. Perhaps other factors will enter in, and the fan will make par on efficiency. I don't know, and I'm a long way from feeling comfortable making a guess.

 

Regardless, efficiency isn't my only goal. Mechanical simplicity and cost are also major factors. We have a saying here that goes "When cost becomes a factor, it becomes the only factor". I can fly a long way at a 10% disadvantage for the cost of a PSRU!

 

Furthermore, I suggest that most props you are comparing to will be spinning at around 2500 rpm. I believe this magic number only comes from the Lycoming benchmark. To my knowledge no Rutan design has been fitted with a slower spinning prop.

I propose a race, my 1000 rpm thumper versus your 7000 rpm screamer.

How does September 2007 suit you?

You'll be way ahead of me if you finish in 2007, but I'm game.

 

Matt

[mjgundry]

Originally posted by Jim Sower

Perry is a very bright fellow who has done his homework really well. It is extremely unlikely that he would miss something as fundamental as critical flow around his fan blades.

Agreed about Perry, I find no end to the enjoyment of pouring over the data at his site. At the same time, I don't think he has missed the critical flow thing either. His older logs spell it out that his blade tips are nearing 1000 ft/sec. Then in the latest log he discusses eventually trying a smaller diameter fan. There are tradeoffs, and accepting the higher blade tip velocity to get higher mass flow may result in better efficiency overall, but I think his move to a smaller fan is worth a try.

 

Matt

[marbleturtle]

Perry is a very bright fellow...

Agreed!

...who has done his homework really well. It is extremely unlikely that he would miss something as fundamental as critical flow around his fan blades.

Whoops... Perry himself has written through the Unofficial Cozy e-mail list that he knows that parts of his design are wrong and at some point he is going to change them.

 

Let me say again that my goal would be, not to outperform a standard prop, but to reach close to the cruise performance of a standard prop while eliminating the complexity of driving a standard prop with an auto conversion. Of course, at static (0 mph) speed, the ducted fan IS already more efficient!

 

Also as I said before, if half of the "ducting" is actually the streamlined body of the aircraft, the penalty of the ductwork is reduced.

[mjgundry]

For those interested,

 

I've fixed the above mentioned mathcad->html file. It wasn't the gifs, but all the extraneous (and incorrect) information put there by the tool I used to convert rtf to html (Word 2000). Time to upgrade to the latest version of MathCAD which supports html output directly, I suppose.

 

http://www.faa-engineers.com/~mjgundry/ductedfan/design.html

 

I've also cleaned up the sketch of the duct cross section.

 

N joy,

 

Matt

[Joe Patterson]

That is an impressive Add to the Cozy.

 

Truely gives it a Space age appearance.