Which Ducted fan plane to recommend?

[peary]

wow...
can the last 2 post be put into 'general information on edf'?

[Warbird]

Hello Yap,

Very well put . However I would like to note 2 points:

1. As you already mentioned efflux and static thrust are two different factors to be considered when choosing your fan and power train. However which should have higher importance heavily depend on the individual plane type.
You will be looking for high efflux on speed planes whereas you will trade high exhaust velocity for static thrust for slower flying planes. Fighter Jet vs. Airliner.
For DT8666s Saber a tradeoff between efflux and static thrust has to be considered to securely hand launch the plane as a bungee launch would require significant changes to the airframe due to the foam construction.

2. Nozzle cone behind the motor. Due to the small motor size of fans up to the 480 class fans it has been proven that a fairing cone does not improve performance significantly. Usually the opposite is the case as our modern brushless motors generate heat, which if a fairing cone is used, can not disperse their heat properly anymore. Hot motors run less efficient, hence a cone could reduce the overall performance. Fans of the 90mm or bigger size however can benefit from a cone as you described in your post.

BTW:
"Using diameter and angle is just a very lay man rule of the thumb."

Not correct if you consider the fact that it was referring to a very specific fan. Also the angle in which the duct expands or decreases will influence the losses in velocity you will experience. The airflow will not be able to follow the duct anymore if a certain angle is reached (Similar to your fist-hit-face example). The laminar flow along the duct wall will become turbulent (due to the created vacuum) and result in velocity loss if the ducting opens up to quickly. The other way around pressure will build up if the ducting diameter drops to sudden (very short nozzle). The result is the same.
As you already mentioned the full length of the ducting should be used to gradually change the diameter (if really required). However in case the ducting is to short and the angle therefore is above the mentioned, other methods have to be applied.

[joe yap]

Yes, it's very true. Static thrust is still important in a certain way. No planes will ever even move if there is zero static thrust. For every kind of powerplant setup, with limited power available, designers are constantly trying to strike a balance between static and dynamic thrust. My point is, too often a modellor will go full force trying to achieve as much static thrust as they can but forgot about the importance of the efflux velocity.

Even comparing a jetliner and a jet fighter design, you can't escape the importance of the 2 factors mentioned above. It's still the same theory. It's just that due to the different flight characteristics, they require a different kind of fan unit. The same goes for the full size, for example, a fullsize jetliner uses high bypass turbofan whereas a jet fighter will usually be fitted with low-bypass turbofan or even turbojets.

You have the very practical point regarding the issue of motor cooling, which requires the tailcones to be removed, thus compromising the aerodynamic efficiencies for electrical efficiencies. However, a better design will be heat sink to be installed on the motor can with the fins in line with the airflow, and of course a tail cone fairing. This will also have a little penalty but is considerable smaller than the aerodynamic losses without a tail cone. I'm not too sure how much decrease in heat dissipation will be incurred with a fairing installed, but it has also been claimed by some modellors who use it on his stock Kyosho T-33, together with the 'area rule' theory incorporated, that the plane improve significantly without modifying the powerplant setup. This also bring us to a point. If you were to use the available power wisely and efficiently, you don't have to push your motor to that kind of limit. Duct fan motors are already more or less exposed to the airstream. For our pylon racers, we only have small NACA scoops which should see more of this problem than a duct fan setup, isn't it?

Your point of angle is also valid, but since no one who has the right of the mind will purposely angle the ducts to extremes, the issue is relatively minor compare to the 'area rule'. As long as the area of the air remains the same throughout every millimeter of the duct, that's also to say that they remains uncompressed or unexpanded, they will follow the duct smoothly. Don't get mistaken, vectoring fluid flow within a pipe or duct will not cause turbulence, just frictional losses, assuming fluid is incompressible.

I mentioned that the diameter is a layman rule of thumb, simply because it's determined soley on the fan sweep area. I've been mentioning that the area throughout the duct, under ideal condition, should be kept constant. That also applies to the exit. So, you should calculate the total fan sweep area, ,divide it by Pi,square root this figure, and times 2. You'll get the diameter of the exit nozzle.

I'm not very sure of your point of changing the diameter of the duct abruptly causes the air pressure to buildup or becoming a vacuum. The air pressure will only change if the AREA it has to move through has changed. If you follow the 'area rule', this will not happen, since every single air molecule in every millimeter of the duct has the equal amount of space to occupy in the next millimeter. Keep it that way, every molecule is happy, no need to expand or compress, no losses in velocity.

I've mentioned before that the laminar and turbulent flow is not to be explained in this way. The terms are not used to describe the way the main air stream flows. It's simply used to describe the very thin layer of air, just adjacent to the duct wall, or an airfoil, which is also known as the boundary layer.

The significance is that laminar boundary airflow has very low skin drag, but also very low in energy and surface tension.Thus, streamlines of air flow in the main stream can easily be detached from the boundary layer of air on the skin surface, which creates the real turbulence and tremendous increase in drag. For an airfoil, this condition is known as stall. A surface moving through the air with very low Reynolds number is known to have laminar flow conditions.

The turbulent airflow, which also only happens within the few microns above the object's surface, happens at higher Reynolds numbers. The very thin layer of air has very small but agitated eddies running along the surface, creating a lot of energy and surface tension. Although the skin drag is higher, this condition is desireable, as it makes the air 'sticky', resisting the main airstream from breaking up. Thus, wings can fly at higher angle of attack before stalling. Almost all full size planes has turbulent airflow throughout the entire wing surfaces. If not, designers purposely install gadgets like vortex generators and turbulators to induce this conditions. Golf balls has dimples for the same reason.

Having explaining the boundary airflows, the airflow through the duct is relatively fast, and as such it has high Reynolds number. The boundary airflow WILL BE TURBULENT anyway, and so it's not a point of concern. Give you an example. If you turn on your tap slowly, the water flow will be smoothly as it's having laminar flow. Turn on the tap wider, the flow will be turbulent no matter how much you try to control. Your ducted fan is never used like the former.

I hope I'm not burning you out further. But at least this is what I can understand from my academic years.

[Warbird]

No worries. No one is burned out so far
I think we are talking past each other on the turbulent/laminar part as we mean the same but describe it differently.

[Babylon5]

50 mm micro EDF

Hi warbird thanks for the info. Can you tell me with 2025-5300 with 3 cells lipo setup , how much amps is it drawing?

I am looking at 350 to 450 gms thrust with 3 cell lipo targetting at 8-10 amps.
Is it Possible to achieve this efficiencies.
Reason is the 180 size motor limited amps. At most is 10Amps.
They will probably fries if I put in more amps.

Cheers

Babylon5

Hi Babylon,

The micro fan peaks at around 550grams with a very hot setup.
the 2025-5300 at 3S and the 2025-4200 at 4S generate around 440-470grams of thrust.

Hi DT8666,

You could try the micro fan with a 2025-4200 at 4S or a 2025-5300 on 3S but you would need to reduce your intake and exhaust ducting. A two big ducting will reduce efficiency the same way as a small ducting as the air flow is not laminar but very turbulent.

If you reduce your ducting down you can still use the same diameter at the intake but slowly and smoothly reduce it towards the fan (0.3mm foil could be used). The reduction angel should not exceed 10 degree to keep the airflow laminar.

Exhaust ducting should have the diameter of the fan at the fan and drop down to a 40mm diameter at the exhaust for best efficiency. Also here drop it gradually from fan -> exhaust

Last but not least make sure your intake has a nice round lip to increase the effective area (either sand the foam or use micro balloons and epoxy to shape and later sand).

Alternatively try to get either the replacement fan from Alfa or a similar sized fan from one of the hobby shops.
Try and add some cooling holes to the motor mount and reinforce the front plate with metal. You could also wrap the motor in a thin copper shield that extends out behind the motor to increase the cooling area. Together with the cooling holes this might be enough to keep the mount from melting.

Sorry that I can’t help with the right size of fan for the Alfa.

[Babylon5]

EDF mathematics

Hi Joe, thank you for enlightening me. When you mentioned the science of fluid dynamics, you brought me back memories where I have to study Hydrostatics, Fluid dynamics, , calculating the centre of pressure ...those weren't my cup of tea...just study to past the exams. Now I learn to appreciate these subjects at least I know what you are talking.
Just to enlighten RC flyer, air behave like fluid. All calculation relating to fluid are also valid for objects in air, including lift, drag, centre of pressure. This also happen to submarine...same equation applies only difference is co-efficient applies.

Anyway come back to RC EDF, static thrust is still important especially if you intend to hand launch the Jet with high wing loading and the need to take care of drag and some of the jet inlet and exhaust arrangement for optimise efficiencies.

Agree with Joe that once the jet is airborne, the dynamic thrust pre-dominates and the jet accelerate untill equilibrium or terminal velocity is attain. In short the faster the jet flies the more efficient is the dynamics of the turbine systems.


Thanks Joe, and keep up the good work.

Cheers

Babylon5

Yes, it's very true. Static thrust is still important in a certain way. No planes will ever even move if there is zero static thrust. For every kind of powerplant setup, with limited power available, designers are constantly trying to strike a balance between static and dynamic thrust. My point is, too often a modellor will go full force trying to achieve as much static thrust as they can but forgot about the importance of the efflux velocity.

Even comparing a jetliner and a jet fighter design, you can't escape the importance of the 2 factors mentioned above. It's still the same theory. It's just that due to the different flight characteristics, they require a different kind of fan unit. The same goes for the full size, for example, a fullsize jetliner uses high bypass turbofan whereas a jet fighter will usually be fitted with low-bypass turbofan or even turbojets.

You have the very practical point regarding the issue of motor cooling, which requires the tailcones to be removed, thus compromising the aerodynamic efficiencies for electrical efficiencies. However, a better design will be heat sink to be installed on the motor can with the fins in line with the airflow, and of course a tail cone fairing. This will also have a little penalty but is considerable smaller than the aerodynamic losses without a tail cone. I'm not too sure how much decrease in heat dissipation will be incurred with a fairing installed, but it has also been claimed by some modellors who use it on his stock Kyosho T-33, together with the 'area rule' theory incorporated, that the plane improve significantly without modifying the powerplant setup. This also bring us to a point. If you were to use the available power wisely and efficiently, you don't have to push your motor to that kind of limit. Duct fan motors are already more or less exposed to the airstream. For our pylon racers, we only have small NACA scoops which should see more of this problem than a duct fan setup, isn't it?

Your point of angle is also valid, but since no one who has the right of the mind will purposely angle the ducts to extremes, the issue is relatively minor compare to the 'area rule'. As long as the area of the air remains the same throughout every millimeter of the duct, that's also to say that they remains uncompressed or unexpanded, they will follow the duct smoothly. Don't get mistaken, vectoring fluid flow within a pipe or duct will not cause turbulence, just frictional losses, assuming fluid is incompressible.

I mentioned that the diameter is a layman rule of thumb, simply because it's determined soley on the fan sweep area. I've been mentioning that the area throughout the duct, under ideal condition, should be kept constant. That also applies to the exit. So, you should calculate the total fan sweep area, ,divide it by Pi,square root this figure, and times 2. You'll get the diameter of the exit nozzle.

I'm not very sure of your point of changing the diameter of the duct abruptly causes the air pressure to buildup or becoming a vacuum. The air pressure will only change if the AREA it has to move through has changed. If you follow the 'area rule', this will not happen, since every single air molecule in every millimeter of the duct has the equal amount of space to occupy in the next millimeter. Keep it that way, every molecule is happy, no need to expand or compress, no losses in velocity.

I've mentioned before that the laminar and turbulent flow is not to be explained in this way. The terms are not used to describe the way the main air stream flows. It's simply used to describe the very thin layer of air, just adjacent to the duct wall, or an airfoil, which is also known as the boundary layer.

The significance is that laminar boundary airflow has very low skin drag, but also very low in energy and surface tension.Thus, streamlines of air flow in the main stream can easily be detached from the boundary layer of air on the skin surface, which creates the real turbulence and tremendous increase in drag. For an airfoil, this condition is known as stall. A surface moving through the air with very low Reynolds number is known to have laminar flow conditions.

The turbulent airflow, which also only happens within the few microns above the object's surface, happens at higher Reynolds numbers. The very thin layer of air has very small but agitated eddies running along the surface, creating a lot of energy and surface tension. Although the skin drag is higher, this condition is desireable, as it makes the air 'sticky', resisting the main airstream from breaking up. Thus, wings can fly at higher angle of attack before stalling. Almost all full size planes has turbulent airflow throughout the entire wing surfaces. If not, designers purposely install gadgets like vortex generators and turbulators to induce this conditions. Golf balls has dimples for the same reason.

Having explaining the boundary airflows, the airflow through the duct is relatively fast, and as such it has high Reynolds number. The boundary airflow WILL BE TURBULENT anyway, and so it's not a point of concern. Give you an example. If you turn on your tap slowly, the water flow will be smoothly as it's having laminar flow. Turn on the tap wider, the flow will be turbulent no matter how much you try to control. Your ducted fan is never used like the former.

I hope I'm not burning you out further. But at least this is what I can understand from my academic years.

[joe yap]

Hi Babylon,

Yes, 'fluid' doesn't mean just liquid in terms of engineering, it also include gasses. Provided air is moving in subsonic state, it's assumed imcompressible and will behave the same way. However, once it has gone supersonic, the story is totally different, as air starts to be compressed. In fact, the reverse of the Bernoulli's theorem is true.

Static thrust is also important. The purpose of the statement I've mentioned in the first post, not to go over zealuos over static thrust, is to remind us not to forget dynamic thrust. If you were to realise, most of the commercial ducted fan unit only specify thrust ratings but does not give any figure on the efflux velocity, which I feel is equally important. You'll need to match the efflux velocity to the aircraft flying speed to make sure it can fly, then you talk about static thrust to determine how much runway you'll need to get it off the ground.

Having said all these, my objective of the posts is to help people understand how the useful work is being produced in a duct fan system, which hopefully can help enthusiast to extract more useful power from the system, which has previously gone to waste. Improving the duct design does improve both efflux velocity and thrust, as it minimise losses.

[joe yap]

Just another pointer. Some planes may have a little problem being hand-launched. For these cases, check the condition of the wings and the Centre of Gravity. It's unwise to assume that you do not have enough static thrust right away. When launching, keep the wings level while pointing headwind and give all out of what your arms can offer, but not enough to rip the wings apart. Most mistakes done are people launching ducted fan planes like parkflyers, as the plane does not get the chance to accelerate to flying speed before hitting the ground.

My speed 400 pylon racers has very small wings and very high wing loading sfor thier size. The prop is very high pitch and is always in stalled condition when the motor is running on ground. Static thrust is very very pathetic. But that's where the difference is, when you javelin it into the air. Let the aircraft sink a little to let it accelerate itself to flying speed. Once it gets on the step, you'll see the magic of dynamic thrust, even for these very low power setup. Pylon racers are not very different from ducted fan designs.

[DT8666]

[QUOTE=joe yap]Just another pointer. Some planes may have a little problem being hand-launched. For these cases, check the condition of the wings and the Centre of Gravity. It's unwise to assume that you do not have enough static thrust right away. When launching, keep the wings level while pointing headwind and give all out of what your arms can offer, but not enough to rip the wings apart. Most mistakes done are people launching ducted fan planes like parkflyers, as the plane does not get the chance to accelerate to flying speed before hitting the ground.

QUOTE]

Just sharing my 2cents on testing EDF hand launch. goto Bedok Reservior. the slope provides a very good error margin and time for the speed to pick up in case it does not goes level flight at launch.

Thsi way, you will know if it ok to do hand launch at normal flat ground. I hv done many test in the past and it works well so far. including the T-33 few years ago. at least after that, I know for sure I don't need to use maximum strenght to launch the T-33.

Do note that if you test it now where head wind is strong, it may not be the same if you launch it at flat ground.

I just done another test today on another e-plane to find out the right CG and handle level. work well for this as well this time.

[newbie]

I have a RTF Super Sniper for sale.

http://www.daddyhobby.com/forum/showthread.php?goto=newpost&t=12649

[Ryan]

ernest... if we follow the diagrams Joe showed us... souldn't the F18 be poorly designed? or is the exhaust of the plane's ducting already reduced significantly enough for a proper ducting exhaust?

[ER69]

ernest... if we follow the diagrams Joe showed us... souldn't the F18 be poorly designed? or is the exhaust of the plane's ducting already reduced significantly enough for a proper ducting exhaust?

Yo Ryan, Sorry for the late reply.
Theoretically, I agreed with wat Joe had posted, It does affect the EDF perforamce if we dont do it right, like having a proper ducting and exhaust .
Infact I have doubt on the design of the Het F-18 exhaust for a start as well, however, after fixing it up and tested it, I personally think that it works just as good as its look.
I personally think that it will not affect much on a small EDF jet but you will see the different if you have a poor ducting and exhast on a bigger EDF jets.
Hey, go ahead and build it mann... so we can do some formation flying again..

[joe yap]

The HET exhaust design is okay.The exhaust need not to be round, just the the area should be correct. Only thing is that the tail cone fairing is being omitted. With such massive power input, the plane flies well. But if more effort is done on ducting designs, you can extract more useful work out of the power input.

Having said all these, my butt is also getting itchy. Now I'm seriously thinking of building my next model as a ducted fan plane.

[ER69]

Having said all these, my butt is also getting itchy. Now I'm seriously thinking of building my next model as a ducted fan plane.

Joe, Just do it... A380??

[joe yap]

Nope. I bought 8 cheapo micro EDFs from Jet hobby that come with 180 motor. Time to put those theory to reality.