An EDF Thrust tube (Tutorial)

About the EDF:
Let’s talk a little about the EDF (Electric Ducted Fan) itself. And EDF uses a higher KV motor, this means it does its work at a higher RPM. It takes longer for the motor to wind up, so it has a slower acceleration. The plane has to be built around the EDF, more so than the Prop motor combo. An EDF system is also harder to set up and it cost more. In front of the EDF, you need an intake ring and a sufficaint supply of air. Behind it, you need a thrust tube that is tuned to your needs.

Fans with a higher number of blades:

• They have more power
• Are better suited to doing stunts
• Due to the pitch of the blades, they are a bit slower.
• They comparatively use a lower KV motor (the 2700 to 1000 KV range)
• The amp draw is much higher
• They use a more powerful motor
• The 10 and 12 blade ones sound like a jet turbine

A word on efficiency:
When compared to a Prop motor combo, the EDF is faster but generally less efficient. It is designed to do a specific RPM range very well. At a fast cruising speed, in a straight line or a slow turn, an EDF is more efficient; everywhere else, it’s not. The EDF seems to be less efficient than a prop because most of us are stunt pilots, an EDF is designed more for speed. EDFs have sluggish acceleration and can not maintain momentum like a prop-motor system of the same amperage.

Why do I need a Thrust tube:
Because it’s a ducted fan, controlling the air going into it and out of it are both important. An EDF is made to install in a plane, not on it. The ducted fan blade is designed toward efficiently moving the air, not wasting energy focusing the air as a prop does. With the thrust tube containing and directing the trust, and the fan efficiently moving the air; you have a very fast combination. The EDF has greater than 20% more power with the trust organized and directed by the tube. Because of the Thrust Tube's diameter reduction at the exit, the air exits at a higher velocity than it entered the system. If the thrust is organized and directed, the air velocity increases because you are funneling it through a smaller, tighter thrust path. Unorganized and undirected thrust moves slower because it’s the same amount of air moving through a much larger, loser thrust path. A thrust tube will make an EDF look sound and perform a lot more like jet engine.

Aerodynamics:
The area between the back edge of the duct, and the thrust’s airflow has very poor aerodynamics. Due to the low pressure area the duct's trail edge creates, it has a destabilizing effect on the thrust. A thrust tube not only improves the aerodynamics of the thrust’s airflow, it improves the aerodynamics of the whole Electronic Ducked Fan System. Any sharp edges like the ones on the trail edge of the duct, degrades the EDF's performance. The trail edge of a thrust tube is thin and aerodynamic. This makes for a much smoother thrust exit.

Thrust tube length:
There are three things to look at.

• The fan causes a swirling motion in the air behind it. The trust tube should long enough to get past this. The buffer the tube's back pressure creates helps calm the swirling motion faster .
• If the tube is too short and the angle of the graduation on the thrust tube is too steep, The air siding down the inside wall is committed to that direction, this causes a fight at the exit of the thrust tube. We need a smooth flow, not a fight.
• The Thrust tube exit needs to be far enough away from the duct to where the thrust airflow can have a smooth aerodynamic exit.

Weather the thrust is a spray, or a well formed column of air is important. The straighter the air exits the tube, the fewer losses you will see. A reasonably long tube with some graduation is the best. I have heard the best tube length is about 4 times the diameter of the fan; for a 70mm EDF, it's around 10 to 12 inches. If you are confined by space and you must use a short tube, that's much better than no thrust tube at all.

The exit diameter is adjustable:
A thrust tube is used to organize and focus an EDFs thrust. It can also control the density of the air escaping out the back of the ducted fan jet. By changing the diameter of the exit on a ducted fan jet, a couple things that can happen.

• By decreasing the exhaust exit diameter the airspeed rushing out will increase. This may also increase the maximum speed of your model, the heat and the load on the motor, and of course the amp draw. If you want a faster top speed, you will need a smaller exhaust diameter.
• With a large exhaust exit diameter, additional power may be gained. The motor generally does not work as hard so it pulls fewer amps. If you want to do a lot of stunts, this torque setting has more acceleration, and the power system will rarely need any heat management. If you want better vertical performance, you will need a larger exit diameter.

These two variables are tradeoffs. It is a balance between thrust and speed that must be achieved according to your goals. Increasing exhaust exit speeds will decrease thrust and vice versa. The EDF's Thrust Tube needs between around 5 to 20% diameter reduction at its exit. The air should exit the thrust tube at a higher velocity than it entered the system at. If you reduce the tubes exit area by 20%, you will have 20% higher air velocity at the system exit. It actually will be a little less than that because of back pressure and friction. In a real world application, F1 tested a 70mm ducted fan. On this system, He proved the thrust tube exit size should be between 59mm to 62mm for the best performance. That is a 5% to 16% exit size reduction. At 59mm (16%), it will give you the best speed, and at 62mm (5%), it will give the most power. With exit sizes smaller than 58mm, you will have a very little thrust velocity speed increase, but the system will have a massive power loss. With exit sizes larger than 62mm, the system will have less power and speed. For a graph and more information showing this real world application test, see THIS post.

Installation:
Installing the intake ring and the thrust tube correctly will improve the EDF's performance. The intake ring should be the correct size and a tight fit. At the beginning of the thrust tube, you cannot have an expansion area between the EDF and the tube. The EDF’s duct’s diameter needs the same as the beginning diameter of the thrust tube. You want a smooth airflow between the two. The fit between the intake ring and the EDF, and the thrust tube and the EDF, should all be airtight.

This was hard to write, so I needed help:
Grey, ScottLott, MoTheG, and F1..Thank You !!!

 

So my 22 inch thrust tubes on my B-45 just might work as long as there 20% smaller or 56mm in diameter. I really need to put it on my thrust scale

 

It should work OK, the 20% diameter reduction will give better acceleration.
Let us know what you think after you do it.

 

ok well I will be home the weekend of the monster planes and if you don't mind I'd like to meet you in person... then I can just tell you then as well as post it here. maybe we will have to figure out how to find each other there. I will be there Saturday

 

While researching, I found a great article about EDF's. Unfortunately, it doesn't say anything about the length of the thrust tube, but it does give some ideas about how to optimize the diameter.

http://southamptonmac.org.uk/doc/EDFpart1.pdf

Donerug

 

Also, I talked to an aeronautical engineering friend of mine who sent this email after our discussion:

"well, i went home and refreshed all that crap i tried to forget years ago and i stand by my skepticism of ducted fan "nozzles". the purpose of a subsonic convergent nozzle is to adiabatically expand exhaust gases to match ambient pressure. so, a true nozzle is only warranted for compressible flow. the general rule of thumb is airspeed of <Mach .3 is incompressible. the standard speed out sound at sea level is ~1126 fps. a 4inch dia fan spinning at 20,000 rpm has a tip velocity of about 333fps. that means that the rotor itself is barely knocking on the door of compressibility - much less the velocity of the jet exhaust. therefore it's assumed the static pressure within the duct is equal to the ambient pressure. sooooooo i can't see any reason for decreasing the exhaust area except to maintain a constant area around the fan motor and its fairing. so in my humble opinion, the fan duct should only reflect the shape of the inner fairing. for incompressible flows, a fineness ratio of 3 is the rule of thumb for the most efficient aerodynamic body. that means that if the motor is 1 inch in diameter, then the entire length of the rotor spinner, motor body, and rear motor fairing should ideally be only 3 inches long. i know it seems stubby, but it's true. i have my intro to aerodynamics and jet engine design books in the car if you'd like to delve into the equations. one final note: even if some other artifact warrants the choking of the exhaust area, the design of the inlet in all cases is faaaaaaaaaar more important to efficient operation. one more final note: what's with the emphasis of thrust efficiency in ducted fans? at r/c speeds a propeller, or at the very least, a ducted propeller will produce much more thrust more efficiently than any fan ever will."

Donerug

 

You can not look at an EDF like a prop, or a jet engine. The big problem is the EDF's fan does not progressively change the pitch angle like a prop does; the EDF wildly sprays air out of the back. You need enough thrust tube to organize the thrust into a well formed column of air. Otherwize it will run like you put an APC prop on backwards.

And you do need to decreasing the exhaust exit diameter because an EDF needs a little back pressure to work well, the best reason is discussed in the above paragraph. Think of it as an air flow buffer.

What is a "ducted propeller" ??

 

So, assuming a 70mm fan is 3-4" long, then a total length of thrust tube and all should be around 10-12", which is what people have been saying... so i need to shoot for a 10-12" total length in order to get that magic 3/1 ratio the engineer hinted at. Sounds like everyone's data is about the same on this...good stuff to know when I start on Boxermad's edf eurofighter.

 

Evetything I've seen says a 70mm EDF needs a 11 to 12 inch tube. 4 x 70mm = 280mm = 11.02 inches. 4 times the diameter of the fan sounds better to me.

 

Should the end taper of the thrust tube be round? I saw someone on this forum simply "squish" the end of a pringles can down to an oval shape which is reducing the outlet hole size and adding thrust. The shape of the end of the tube was like the crevice cleaning tool on my old vacuum cleaner. Is this as efficient as a round outlet like using a cup?

 

Someone also mentioned the Venturi effect... If anyone is into hot rods and knows what a velocity stack is, you may also know what the venturi effect is...I could see this making a huge difference on inlet efficiency...by having the air flow in over a specifically curved surface, air volume intake can be increased...We need to be putting a "velocity stack " type shape in our inlets in order to capitalize on the venturi effect right? I see this shape added into the "intake ring" on a lot of EDF units, but I cannot help but think the ring is a bit small to capitalize on much of the venturi effect. If we extended the intake and put a larger intake ring on the end, we could add a velocity stack type force to the induction of air on our EDF units...food for thought

 

• I do not see an oval shape as being a big problem. It is less than perfect.
• A creviced tube??? Depending on the size and depth of the roughness or the crevices, the benefits are velocity sensitive (see Planes with shark skin ). If it interfairs with a well formed column of air exiting the tube, I don't like it.
• The Venturi effect: a fluid's velocity must increase as it passes through a constriction to satisfy the principle of continuity, while its pressure must decrease to satisfy the principle of conservation of mechanical energy. Plainly put: If you tap into a restriction in flow, you will see suction there. I don't see how that could be useful with an EDF, the suction is not free. It will use energy, and the losses will be higher then the gains.

 

I just was comparing the end shape of the tube to the shape of crevice tool included with many vacuum cleaners...lol...did not mean to imply the tube was crevised, it just looked like something I would use to clean in between couch cushions or something.

 

If I needed to do an oval shape with an EDF trust tube to make the jet look good, I'd do it in a heartbeat.

We talk about how to get the most out of something, but all of us have to make compromises all of the time. We shoot for the moon knowing we are going to come up short. A Styrofoam cup for a thrust tube is quick, easy , cheap, and looks good; It's less than perfect, but not by much.

 

Good talk! Well, I'm pulling the trigger on a 70mm EDF combo from hobbypartz this week to begin my build of the Boxermad84 EDF Eurofighter. Any thoughts on this combo? It has good reviews on the site, but I know most places obscure bad reviews so they are getting harder to trust.

This will be my first EDF, so I hope the combo works out well. I really liked the looks of the Rcpowers EF, but I really didn't want to mod it to get the performance I want out of the prop drive like cutting holes in the side and such...with the BM Euro, I can hide all the stuff inside and keep the outside really purdy and unmolested.

I plan on using a cup for the thrust tube and doing a custom intake with Dollar foam and a soda bottle neck to try and get a velocity stack shape at the front. I am planning to document the entire build of this jet in video form and eventually post it here in the RCP forums.

 

Boxermad84’s Eurofighter-EDF is a good looking jet.

On Boxermad84's jet, I do not see a thrust tube. Sticking an EDF in a square tube is not the same as a properly fitted thrust tube. It would do a lot better with the tube.

The front of his EDF was fine, the intakes looked sufficient.

 

there was a video of daves maybe a year ago where he was intrigued by the f117 that is similiar to the the one at banana hobby that makes use of a 70mm edf, but the thrust was routed through 2 rectangular outlets to keep the same look of the original plane. he had a buddy of his construct a rectangular thrust tube and tested it against a circular one. in the end, he felt that there was little if no restriction in thrust between the two. needless to say his buddy was a engineer of sorts and designed it to do that. i consider myself a shadetree engineer meaning i get the big picture and do nothing beyond looking it up and doing what looks good, and would probably make things worse.

 

very very helpful info. especially for what im doing now.

 

Here is a shot of my F-117 befor i closed it up

I added some plastic to transition from round to square on the exit of the EDF like the inlet side.
The exit area was about 85% of the area of the FAN. worked out OK...

 

I've been working on a scratchbuilt F-16 EDF for several weeks now. I used (with permission) boxermad's profile F-16 for general shape. After weeks of reading, researching and experimenting I finally decided to just 'dive in' and build the damn thing. Plan to maiden it tonight, wind permitting. Forcast shows ~10knots, so hopefully I'll have some news later tonight. I took build pics all along the way, and used a long, straight pringles type can for the tube. No reduction.

Here's a pic of her as is ready for flight tests..