RE: Why are ducted fans considered so inefficient?
 

You can only use what is available if you want a ALMOST real turbine sound.

If you add up the total lengths of all the fins. It will be much more than a 2 bladed prop absorbing the same horsepower & torque. also add in the flow straightner vanes.

PUTT PUTT or WHINNNEEE.

:D

RE: Why are ducted fans considered so inefficient?
 

There is a contradiction in this argument. In straight and level flight the maximum speed is set by the drag and propulsion force. With the same given weight and size and as a function of this drag, the propulsion force limit the speed. Then, with the same input power, the most efficient propulsion system gives the aeroplane the highest speed. At subsonic speed above 200km/h this is the propellor. In this speed range the propellor is not hampered with reverse flow at the tip's neiter compressebility.

RE: Why are ducted fans considered so inefficient?
 

With plastic impeller versus metal impeller.............the metal impeller should be thinner than the plastic impeller due to the higher strength of the metal. All else being equal.............I am guessing now......the thinner metal impeller would present less resistance in cutting through the air than the thicker plastic impeller. So the question is...........is the metal impeller really more efficient than the plastic impeller in our model jet EDF's?????
Thanks for your valuable reply.
larrysogla

RE: Why are ducted fans considered so inefficient?
 

It is so boring to watch a PRACTICAL posting turned into a bunch of theoritical test data, that was ordered to be biased in most cases.

Let see. All the WW II guys are almost gone. Therefore all external propellor uses are useless.

What is funny is both are props. One just has a bunch of fixed shape & pitch. That is called a TUNED pump. It has the highest efficiency at 1 ... 1..... speed only.
Why do you think we design electric motor powered water pumps that look EXACTLY LIKE a electric EDF ?? All the blades have the same pitch along the entire length. Right.
We do not know what we are designing. EDF design came HUNDREDS of years before all other pumps. Try that at pump design 101.

For the under $ 100 RTF planes, props rule at the flying field on a watts input / per pound of weight, to fly in a scale like manner. All that useless long twisted shape is just there to make it look neat. Sure it is.

RE: Why are ducted fans considered so inefficient?
 

What happens if you have a high torque outrunner brushless motor with a very long shaft that extends both at the front of the motor and also extends at the back of the motor. Then 2 impellers are mounted, one for the front of the motor and the other at the rear end of the motor. With a stator in between to straighten the airflow from spiral airflow to straight out the back airflow for increased efficiency and thrust. Would this dual impeller design allow for higher thrust per given watt. If so, is this higher thrust due to the increased exhaust velocity??? Is this a more efficient setup than a single impeller design???
larrysogla

RE: Why are ducted fans considered so inefficient?
 

This question has been asked before and the answer is that there will be no improvement in thrust levels over a single fan. Power in/power out and all that.The basic problem with DF is that, at the speeds we fly, shifting a large area of air at relatively low speed is much more efficient than a small area at relatively high speed. In the end, you can't change that. The 'exhaust velocity' thing is not part of the thrust process, T=MA (Thrust = Mass x Acceleration). Higher exhaust velocity is simply a result of increased Acceleration, and that occurs in the fan. Which is why short fans will give more 'T' than the same one with a long exhaust duct. You don't want to have any drag behind the fan, because it takes a bit of your 'T' to overcome it, and the faster the exhaust velocity, the greater the drag.

RE: Why are ducted fans considered so inefficient?
 

Pimminz,
Thanks for your valuable reply.............I guess, the question I should really ask is that at the very high RPM(50K) that an EDF has to spin to give a maximum flying speed of over 100 MPH, the motor is consuming a lot of watts because for a given motor, the watts consumed is NOT double from 25,000 RPM to 50,000 RPM..............but as we all know it is probably closer to 5X the watts consumed and NOT 2X the watts consumed. Therefore, for a given RPM.......which is for illustration purposes we will use 25,000 RPM as an example, the dual impeller axial flow design with a stator in-between redirecting the spiral airflow to straight out back airflow...............the exhaust velocity will be higher for the same 25,000 RPM than a single impeller design.........but because we are at 25,000 RPM the wattage consumed at 25,000 RPM is less than the wattage consumed at 50,000 RPM, even though we are putting in twice the load with the dual impeller design. With the dual impeller design, the dual load will probably result in wattage consumed at a little over 2X over the single impeller design at 25,000 RPM.........which is still less than the probable 5X wattage consumed at 50,000 RPM. We could theoretically gain an advantage by reducing the RPM but still produce the same amount of thrust by using axial flow dual impellers. The idea is to reduce the RPM to lower the watts consumed but hopefully gain the same amount of thrust by using dual axial flow impellers at a lower RPM. Remember that energy produced at 2X the RPM is increased by the square which is 4X, so using the rule that power in equals power out and accounting for the loss of efficiency at the higher(50K) RPM, the wattage consumed at 50,000 RPM is probably 5X the wattage consumed at 25,000 RPM. Hopefully some manufacturer will produce the dual impeller design so we can compare efficiency in the actual world.
larrysogla

RE: Why are ducted fans considered so inefficient?
 

I love the quest for perpetual motion.

RE: Why are ducted fans considered so inefficient?
 

HighPlains,
It is obvious that for the same watt consumed, the propeller plane will have greater static thust than a ducted fan. As so many have explained over and over...........it is because the propeller has a larger swept area than the ducted fan............and because of this, the propeller is able to move huge amounts of air at a lower RPM(say at 10,000 RPM for a 9x6 electric prop), while the ducted fan(example is a 3" diameter impeller) has to spin at over 30,000 RPM to produce the same amount of thrust. Now the wattage consumed to spin at 30,000 RPM versus 10,000 RPM is NOT 3X, but rather it is 9X. So if you can have dual impellers to accelerate the same air mass at the same velocity at a lower RPM versus a single impeller EDF, then "theoretically" you are going to save on the watts consumed versus the single impeller EDF to produce the same thrust. With propellers, the thrust is produced at a very much lower RPM, saving huge amounts of watts consumed to produce the same thrust versus a high RPM single impeller EDF. The numbers on the watts consumption are already there in the published world and you can see the huge amounts of watts consumed by single impeller EDF to equal the same thrust as a propeller. For electric motors, the watts consumed increases with the RPM on an exponential scale and NOT on an arithmetic scale. Dual axial flow impellers can "theoretically" increase the efficiency by lowering the RPM for a given thrust. Again, I hope some manufacturer puts out a dual axial flow impeller EDF for us to see the actual, real world advantage.................if any. Thanks for your valuable reply.
larrysogla

RE: Why are ducted fans considered so inefficient?
 

Not quite so.
Adding another impeller is just another way of adding more blades to a single impeller. There are proos and cons from both structural and aerodynamic viewpoints, but the same air mass must pass trough both. And with a given air mass, the end result is pretty much the same.

Second, a ducted fan have better static thrust than a propellor as long as their swept area is the same. Given the same area, the the DF can even do this with less input power. The reason is that the duckt reduses the reverse flow loss considerably compared to a free propellor in the static case. Correct shaping of inlet and outlet will increase thrust even more compared to a propellor as the duct augument the air flow and the fan work with a "larger" air flow than the sweep area of the fan. The RPM of the fan is in this case much the same as a free propellor. As the speed rise the drag of the duct increases, but the reverse flow of a propellor decrease. At a certain speed the increasing drag of the duckt become larger than the decresing reverse loss of the propellor and the free propellor is most efficient.

Third, watt's consumed byan electric motor is proportional to the load. Eddy current losses rises a little with RPM, but that's when the motor is working outside it's design spesification. Low or high RPM, the motor efficiency is pretty much the same.

RE: Why are ducted fans considered so inefficient?
 

Power in at all speeds varies WILDLY with the EDF because it has a FIXED pitch in a FIXED tunnel. This is a TUNED engine. No if ands or buts about that.

The open propellor has VARIABLE pitch across the length of the blade. It also has NO inlet or outlet restrictions to make power.


Comparing the 2, is like comparing Marble & Limestone. Same but very different.

RE: Why are ducted fans considered so inefficient?
 

True, and as a TUNED engine, the DF is a one speed device.The ducted fan is efficient in just a narrow speed range around the design speed.

RE: Why are ducted fans considered so inefficient?
 

About a month ago, I posted on this thread about the unducted fan program from a long time ago. Looks like it might be coming back.

http://www.popsci.com/technology/art...leaner-flights

Maybe this time they will have their ducks in a row.

RE: Why are ducted fans considered so inefficient?
 

Cyclops2,
I have to respectfully differ from your suggestion and to quote you "The open propeller has VARIABLE pitch across the length of the blade." I have to differ because as an example......a 9x6 propeller has a pitch of 6" from close to the hub all the way to the tip.................the reason the curvature of the blade changes from close to the hub to the tip is because as the diameter increases towards the tip the angle of the blade has to decrease towards the tip to maintain the 6" pitch. The curvature of the blade is compensated for the increasing distance from the hub to the tip so that the 9x6 propeller will move forward ON THE DRAWING BOARD a distance of 6" for every full rotation ON ANY SPOT IN THE PROPELLER BLADE. The pitch stays the same in a 9x6 from hub to tip although the curvature is changing to compensate for the distance from the hub to maintain the 6" forward motion for every full rotation. The angle of the blade is changing so that at any spot on the propeller from close to the hub to the tip it will give you a forward motion of 6" on a 9x6 propeller. Pitch is the length of forward motion for every full rotation of the propeller as determined ON THE DRAWING BOARD. Anyway, it would be fun to have a manufacturer produce a dual axial flow impeller so us hobbyists can have another direction to experiment, dabble, investigate. As we all know, one of the fun in this hobby is that we can go in different directions without the risk of a full scale R&D gazillion dollar budget and EPA noise and pollution restrictions. Thanks for your very interesting discussion.
larrysogla

RE: Why are ducted fans considered so inefficient?
 

Well in fairness to Cyclops, only the most simplistic props have a True Helical Pitch from the hub to the tip. I'm not sure if Cyclops was referring to this or was just looking at the fact the angle changes down the blade. His post could be read either way.

The reason the THP props are not a good design is due to flow considerations around the fuselage as well as other many factors.

RE: Why are ducted fans considered so inefficient?
 

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

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

RE: Why are ducted fans considered so inefficient?
 

We are supposed to be discussing MODEL props & fans. Not $ 6,000,000 rocket pumps or REAL turbines.

The original post was simple enough.

Which type can fly the same weight & speed more efficiently ? [X(]

RE: Why are ducted fans considered so inefficient?
 

Larry

You know that the angle of the prop blade changes along the length of the blade. You stated that & you are correct.

If the angle changes. Then the pitch is changing. The design reasons for changing width & pitch along a blade are many & as usual, a compromise is reached.

My APC E 6" x 4" look like a idiot designed it. But it really is one of the most efficient that I have found.

RE: Why are ducted fans considered so inefficient?
 

What if someone put a ring around a 10" prop, like a moving shroud?

RE: Why are ducted fans considered so inefficient?
 

Huh?

RE: Why are ducted fans considered so inefficient?
 

Cyclops, you mis worded that bit about the pitch and angle. A constant pitch prop HAS to alter the angle along it's length to stay at the same pitch. I suspect that's what you meant but your fingers typed differently... :D

However there isn't a prop for electrics or glow engines out there that uses a true pitch from tip to tip. They ALL reduce the pitch greatly in the hub area to accomadate the realities of structure and mounting requirements.

A TRUE helical pitch prop is pretth easy to make. Take a toungue depressor or similar size strip of card stock. Mark the middle line and twist it about120 degrees from one end to the other. Then alter your grip so the middle line is directly vertical. That middle line is the prop axis and the "blades" will now have a true helical pitch from tip to tip.

RE: Why are ducted fans considered so inefficient?
 

Cute to picture, but not correct. The change in angle is not linear in proportion to position down the length of the blade. Angle is arc tan of pitch/circumference at each station.

RE: Why are ducted fans considered so inefficient?
 

You would probably get some difficult structural problems to solve and a plane serious affected by gyroscopic forces.Easier to make that ring stationary and live with the loss the gap between prop and shroud make.

But considering a 10" prop in a 2" to 5" long shroud you would improve the static and low speed thrust compared to a free propellor. University of Lousiana i think, had a full sizeprototype aeroplanewith a large diameter prop in a short shroud. The goal was to make a short take of and landing aeroplane.

As a side note, it have been done wiht ship thrusters, with the electric motor incorporated in the shroud.

RE: Why are ducted fans considered so inefficient?
 

That is a perfectly natural and excellent question!!

Isn't that the equivalent to install winglets on the tips of a wing?

Yes, because the ring will reduce the by-pass of pressure or tip vortex of the propeller, which is nothing more than a wing turning in circles.

As Mathews explained very well above, the negative result is that the surface of the duct or ring introduces surface friction and, with that, a reduction of the vane of flow that reaches the propeller.
This problem is more relevant for smaller than for bigger diameters, because the negative influence of the reduction of the vane (depends of surface and air viscosity) is inversally proportional to the ring diameter.

Huge propellers (or fan rotors) and rings would be better, but they present structural problems, as well as bigger drag of the airplane body surrounding it.

Any regular propeller is a ducted fan operating without the duct or ring.

Any axial rotating pump or fan is good (or more efficient) for big volumes of fluid to be moved between low pressure differentials.

RE: Why are ducted fans considered so inefficient?
 

If you could find time to read thru those links, you could see that those machines operate on the same principles, regardless the cost, size or materials.

Regarding the OP question, I believe it would be good to define "more efficiently".

For me, that is, the comparable machine able to transform the same amount of electrical or chemical energy in reserve into more useful kinetic and potential energy.

As I see it, the real problem is to make a fair comparison between both types of machines (or energy transformers), because they don't look or operate in a comparable way.