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-   -   Why are ducted fans considered so inefficient? (http://www.rcuniverse.com/forum/aerodynamics-76/9485782-why-ducted-fans-considered-so-inefficient.html)

sheik480 02-07-2010 10:55 PM

Why are ducted fans considered so inefficient?
 
High rpm, steep pitch, and low airspeed (such as when a model is flying slowly) would, in my mind, cause the fan blades of a ducted fan unit to stall, much like an airplane wing stalls when the relative angle of attack exceeds a certain limit. This, in turn, would create an extreme deficit in smooth aerodynamic flow and low efficiency. A static thrust test with just the fan shroud would recreate these conditions to the worst degree. On the other hand, however, if a fan was in-closed in a high aspect ratio duct (such as would be found in a scale military jet) flying at speed, the air would have plenty of chance to accelerate to optimum speed before reaching the fan itself. This would lend itself to an optimum situation to maximize the efficiency of the ducted fan. Even in a similar situation with lower airspeed relative to the actual fuselage, the air would be able to accelerate somewhat down the length of the duct before reaching the fan, and thus minimize losses due to fan blade stall. Anybody wanna input on this?

BTW, I also posted this thread in the edf forum

combatpigg 02-07-2010 11:40 PM

RE: Why are ducted fans considered so inefficient?
 
In "duct talk", high aspect is bad, low is good.

pimmnz 02-07-2010 11:46 PM

RE: Why are ducted fans considered so inefficient?
 
It's not that ducted fans are inefficient, of themselves, but that any duct behind the fan will reduce thrust, and the forward duct needs to be carefully designed to not reduce the mass flow, not to accelerate the air into the fan, it can't do that without using some of the fans energy to do that, and again that will reduce the thrust left for flight. There are other considerations too, but as the fan speeds increase with modern electrics the ducted fan can be a useful and cheap alternative to a turbine. BUT, with the provisio above. The way the thrust is developed is completely different between the two systems.
Evan, WB #12.

HighPlains 02-08-2010 09:55 AM

RE: Why are ducted fans considered so inefficient?
 
Not only is the power absorbed by the external surface drag, you have quite a bit of internal surface drag, so a prop driven airplane will be faster than one that is ducted fan driven given equal power, model size, and weight.

victorzamora 02-08-2010 10:56 PM

RE: Why are ducted fans considered so inefficient?
 

Quote:

ORIGINAL: HighPlains

Not only is the power absorbed by the external surface drag, you have quite a bit of internal surface drag, so a prop driven airplane will be faster than one that is ducted fan driven given equal power, model size, and weight.
But wouldn't you significantly decrease tip losses with a well-designed duct. Also, a propeller driven aircraft loses thrust as speed increases while an EDF can be maximized for a certain speed. There ARE a lot of reasons why EDF's are good for speed. The reason EDF's have poor static thrust is because they're geared for high speed. It's like a car that only has a fifth gear.

Tall Paul 02-08-2010 11:45 PM

RE: Why are ducted fans considered so inefficient?
 
Ducted fans appear to be quite popular on planes like Boeings and Airbuses... :)

da Rock 02-09-2010 06:28 AM

RE: Why are ducted fans considered so inefficient?
 

Quote:

ORIGINAL: Tall Paul

Ducted fans appear to be quite popular on planes like Boeings and Airbuses... :)

Those are actually high bypass jet engines. Quite a bit different animals.

One reason ducted fans are considered to be inefficient for models is how the vast majority of model ducted fans look when taking off. As mentioned, they are like a car stuck in high gear trying to get going after the light changes. They are inefficient in one of the most important parts of the envelope. When you want acceleration, you don't usually reduce diameter as drastically to what most DFs have.

Sport_Pilot 02-09-2010 09:18 AM

RE: Why are ducted fans considered so inefficient?
 
Quote:

Those are actually high bypass jet engines. Quite a bit different animals.
They are differant because they are made for high speed at high altitude. Otherwise they are a turbine powered ducted fan with jet assist from the engine exhaust.

Lnewqban 02-09-2010 11:35 AM

RE: Why are ducted fans considered so inefficient?
 
sheik480,

What do you mean by "so inefficient"?

What are you comparing the efficience with?

The explanation of your post seems to match a high pitch propeller.

sheik480 02-09-2010 08:48 PM

RE: Why are ducted fans considered so inefficient?
 

Quote:

ORIGINAL: LNEWQBAN

sheik480,

What do you mean by ''so inefficient''?

What are you comparing the efficience with?

The explanation of your post seems to match a high pitch propeller.
So inefficient as in often being quoted as half as much thrust for watts consumed.

BMatthews 02-09-2010 09:00 PM

RE: Why are ducted fans considered so inefficient?
 
A big factor in ducted fans is the issues with the ducts leading to the fan inlet. There's skin losses that occur that are reynolds number related. This skin effect scales highly differently from our size to full size. In effect OUR ducts appear a lot smaller than they are to the airflow where on full sized stuff the duct suffers very little loss in effective area. This issue is why a lot of the old ducted fan designs used huge non scale inlet ducts and why many scale models use cheater holes in the belly in order to reduce the pressure drop at the fan inlet. Only the bigger non scale model inlets may actually provide a slight pressure boost at the inlet compared to atmospheric pressure.

I've often thought that if we could have two stage EDF's where one motor was running a lower pitch fan and served more to get the air accelerated and present it at the optimum speed for the separately powered second stage that we'd really get something. But to make it work this concept or any concept work requires careful inlet duct design. Outlet ducting isn't so bad. It should be tapered to help increase the exit speed of the air but not so tapered as to generate a back pressure of any significance or you risk slowing the airflow through the entire system. Mucho testing would be needed to find the optimum setup.

nmking09 02-10-2010 12:41 AM

RE: Why are ducted fans considered so inefficient?
 
Tip losses on props are very minimal, once you get past 8K rpm. Shrouding is very effective at increasing efficiency at low RPM's 1-5K. It makes a lot of sense too, you have to give the air time to begin span-wise flow before you need a shroud.

Because our stuff usually runs at 10-15k RPM we really can't take advantage of ducting. Now, when you start talking about turbines and bypass that is a whole new can of worms. You can't even come close to equating a EDF to a turbine, physics is completely different.

victorzamora 02-10-2010 09:22 AM

RE: Why are ducted fans considered so inefficient?
 
Yeah, I guess I didn't really take into account the incredible speed that our EDF's turn at.

However, I do think that the 2-stage EDF might be something to more seriously look into. I wonder if there's a free program to simulate such a thing for calculations and testing.

HighPlains 02-10-2010 10:37 AM

RE: Why are ducted fans considered so inefficient?
 
If you remember, there was a NASA progam where they built an unducted fan to see if it was more fuel efficient which is a strong measurement of performance. It has a 30% lower fuel burn than a normal ducted fan. Look up GE's UDF or the GE36 engine.

Lnewqban 02-10-2010 11:35 AM

RE: Why are ducted fans considered so inefficient?
 

Quote:

ORIGINAL: sheik480


Quote:

ORIGINAL: LNEWQBAN

sheik480,

What do you mean by ''so inefficient''?

What are you comparing the efficience with?

The explanation of your post seems to match a high pitch propeller.
So inefficient as in often being quoted as half as much thrust for watts consumed.
Nothing crazy about that, how much do you think the mechanical efficiency of the engine of your car is?

It is much harder to transform energy from thermal to mechanical than vice versa.
Why? Many little losses that waste energy.

The performance (and hence, the efficiency (how much gets out / how much gets in)) of all these energy transformers, varies with the conditions in which they work.
There is an optimum application for propellers, turboprops, turbofans, jets, etc.

http://en.wikipedia.org/wiki/Air_propulsion

CoosBayLumber 02-10-2010 06:17 PM

RE: Why are ducted fans considered so inefficient?
 
Quote:

ORIGINAL: BMatthews

A big factor in ducted fans is the issues with the ducts leading to the fan inlet. There's skin losses that occur that are reynolds number related. This skin effect scales highly differently from our size to full size. In effect OUR ducts appear a lot smaller than they are to the airflow where on full sized stuff the duct suffers very little loss in effective area. This issue is why a lot of the old ducted fan designs used huge non scale inlet ducts and why many scale models use cheater holes in the belly in order to reduce the pressure drop at the fan inlet. Only the bigger non scale model inlets may actually provide a slight pressure boost at the inlet compared to atmospheric pressure.

I've often thought that if we could have two stage EDF's where one motor was running a lower pitch fan and served more to get the air accelerated and present it at the optimum speed for the separately powered second stage that we'd really get something. But to make it work this concept or any concept work requires careful inlet duct design. Outlet ducting isn't so bad. It should be tapered to help increase the exit speed of the air but not so tapered as to generate a back pressure of any significance or you risk slowing the airflow through the entire system. Mucho testing would be needed to find the optimum setup.

Back a couple of years ago, I did some plans for Kress Jets. They have on the bench a few different DF models and engines. Most of the revision to plans was for additional air inlets "Up Front" of the propeller. They could get the thrust, but not the intake. The friction loss in the short air canal was dramatic.

You then need to run Reynolds numbers on near everything.


Wm.

Himat 02-11-2010 05:51 PM

RE: Why are ducted fans considered so inefficient?
 


Ducted fans in model aeroplanes is inefficient due to the way they most often are applied in model aeroplanes. Maybe a little stark statement, but a starting point.

As far as I have read in aerodynamic litterature a ducted fan is a good low speed device say, 0 to 200km/h. A propellor is a better at speed between 200 and 600km/h and then the turbojet exel untill exotic poverplants like ramjets and scramjets kick in beyond mach 3. High bypass turbofan are used, but they are kind of a compromise.

The duct in a ducted fan do a good job in minimizing tip losses and reverse flow at low speed. Accelerating the air into the duct creates a low pressure on the duct that can account for a third of the thrust. Deaccelrating the air aft of the fan improves low speed thrust further.This is god for static and low speed, but as the speed increase, the duct add drag. On the other side, with a proppellor the reverse flow is no problem at speed and nothing is to be gained from a duct. Tipp losses is still a issue, butthe tiploss is less than the drag from a duct.Have a look at ship propulsion and you will see that tugboats often have shrouded propellors and fast ships ordinary propellors. Don't get confused with waterjests, the physics of them is different. (They have more in common with rocket propulsion.)

Designing a ducted fan for speed and scale appearance is possible, but it's like stated above stuck in fifth gear.

High bypass turbofans do deaccelrate the air entering the duct. This make the static pressure rise which is very good for the core turbojet efficiency and it make it possible to have the fan at all, if not the fan blades would go supersonic and that is no good for the efficiency.Note that modern jet airliners with high bypass turbofans are slower than the old turbojet airliners.</p>

Tall Paul 02-11-2010 08:03 PM

RE: Why are ducted fans considered so inefficient?
 

Quote:

ORIGINAL: Himat
...

High bypass turbofans do deaccelrate the air entering the duct. This make the static pressure rise which is very good for the core turbojet efficiency and it make it possible to have the fan at all, if not the fan blades would go supersonic and that is no good for the efficiency. Note that modern jet airliners with high bypass turbofans are slower than the old turbojet airliners.</p>
.
Fuel costs dictate airliner speeds more than anything.
The L-1011 was designed to cruise at .92 Mach.
The "fuel crunch" dictated a reduction in cruise speed to .85 Mach.
There was an ancillary effect of this speed reduction.. The pitch angle at the lower cruising speed was higher than intended, so the girlies pushing the food carts around the cabin had an uphill push from the galley area to the passengers ahead of the wing. :)

CoosBayLumber 02-11-2010 09:02 PM

RE: Why are ducted fans considered so inefficient?
 
Quote:

ORIGINAL: Tall Paul
There was an ancillary effect of this speed reduction.. The pitch angle at the lower cruising speed was higher than intended, so the girlies pushing the food carts around the cabin had an uphill push from the galley area to the passengers ahead of the wing.
I can understand that on a real full-sized A/C, but how does this apply when some mean kid monster just opens it up, and then runs full blast much of the time? "If" there is a Pitch angle, I never see it, due to their violent manuevers.

Or, am I missing something? Going by too fast to see.



Wm.

cyclops2 02-27-2010 07:50 PM

RE: Why are ducted fans considered so inefficient?
 

Actually the bypass fan jet engines are using pure jet thrust & a multi blade exterior fan blade pumping COLD DENSE AIR. Cold dense air is always going to produce more thrust than super heated exhaust air.

So it is a win for the 32 bladed prop in the bypass fan section...The jet portion drives the much more efficient cold air multi bladed prop. :)

For what it is worth. My GWS A-10 runs on GWS 40 mm EDFs . I set the fans 3/4 of the way back in the engine housing. I cut the intake lip off the shroud part and did a long spackel filler blending to the inlet. It has the highest thrust of any EDF 40 fan I own. The motors are all Hyperion 12 mm 6000 kv on 12vdc.

intlpilot767 02-27-2010 11:31 PM

RE: Why are ducted fans considered so inefficient?
 
High Plains, The propeller driven acft. will always be more efficient !! But the Ducted fan acft will ALWAYS be Faster !! For the same given size and weight and power.
The propeller driven models prop drag caused by the spinning prop (Disc Drag ) will always limit the propeller driven acft"s speed. IP

intlpilot767 02-27-2010 11:40 PM

RE: Why are ducted fans considered so inefficient?
 
HighPlains, you are right on with efficiency !! However the UDF'S failed for a number of reasons. Including noise,drag,and a limited flight envelope.IP

grael 02-28-2010 04:18 AM

RE: Why are ducted fans considered so inefficient?
 
I'm usually an RC jetboat designer, but I've achieved my goals on that lately, and am working on gliders now. Very early days.

Anyway, I'm responding on this thread, because the issues with poor intake on air DFs, have some similiarities to jet boat jet units, in that the easiest way to reduce cavitation, is to reduce the outlet nozzle radius. In both air, and water pumping applications, the issue is that low intake pressure reduces the mass available to be screwed out by the impeller.

Nozzle diameter restrictions increase the exit speed of your working fluid in relation to the boat/plane's speed, so that you maintain more thrust at top speed.

When the EDF calculated screw speed is much faster than the plane's speed (i.e. stalled, at rest), then it makes sense to give assistance to the atmospheric pressure by giving a bigger opening, whether by expanding the intake, or by the mentioned "cheat holes". If you can put these near the top of the aircraft, all the better, you don't really want suction UNDER the aircraft. Best is probably like a sliding fuel engine throttle, so the servos don't have to work against the suction of the fan. So long as the paired openings go from fully matching, to fully blocked with servo movement, then it ought to give full control. An on board vacuume sensor  would simplify operation, and a partial tube inside a tube, rotated to align upper cheat holes is one option for easy construction.

EDF motors need to be securely fixed to the duct, with water jet units, the motor size is often greater diameter than the impeller, but in air pump designs, the motor can easily fit within a hub. I mould my water jet stators in one piece with the outlet, the key is to use the support structure for the hub to catch the flow spinning off and with the impeller direction, and to curve it back to straight out, and no longer twisting. This is critical with high pitched impellers, and clearing the working fluid will reduce stalled back flows.

cyclops2 02-28-2010 08:21 AM

RE: Why are ducted fans considered so inefficient?
 


How about.

The more blades on a prop, The less efficient it will always be.

We are not talking about torque or horsepower or speed.


Efficiency. 1 blade always is bitting into none turbulent air. It is the most efficient.......:)

da Rock 02-28-2010 09:16 AM

RE: Why are ducted fans considered so inefficient?
 
Quote:

ORIGINAL: cyclops2



How about.

The more blades on a prop, The less efficient it will always be.

We are not talking about torque or horsepower or speed.


Efficiency. 1 blade always is bitting into none turbulent air. It is the most efficient.......:)

Yeah, that's been known and accepted for a very long time.

As for the turbulence being the cause, as with everything aerodynamic, the number of tips is a primary cause as are differences in aspect ratio, blade areas, etc etc.

In spite of the emphasis many place on efficiency having significance, it has never stopped full scale designers from increasing blade count. They have had the benefit of better selection than modelers. Matter of fact, they haven't been limited by selection as many applications have designed exactly what was called for, thereby not having a problem with selection at all.

Also with models, the actual difference in efficiency is a complete unknown. And certainly is not something that should stop any modeler from trying a fan or prop with more blades.


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