Correct way for calculating area of edf?

[viper1990]

I'm not sure this question has been asked before or not,anyway here goes.

In order for me to predetermine my intake area and exhuast area, first i have to calculate my area of the edf, which is the net area of the air sucking through the fanblades.

So according to the diagram i drawn, which is the correct way of calculating the cross-sectional area of the edf (shaded in blue).

And it is best that the intake area is 100% or equal to the calculated cross-sectional area of the edf?

If too small, the edf will suck less and thus less thrust.
If too big, the extra area in the edf will creat unwanted drag.

Oh one more thing, i'm going to put a intake ring on my edf,even though it's going to be buried inside a ducting tube. I've done test and seems that it produce extra ~20% thrust with the intake ring.

Hope someone could anwser this so i could start on my next edf plane

[joe yap]

I'm not sure this question has been asked before or not,anyway here goes.

In order for me to predetermine my intake area and exhuast area, first i have to calculate my area of the edf, which is the net area of the air sucking through the fanblades.

So according to the diagram i drawn, which is the correct way of calculating the cross-sectional area of the edf (shaded in blue).

And it is best that the intake area is 100% or equal to the calculated cross-sectional area of the edf?

If too small, the edf will suck less and thus less thrust.
If too big, the extra area in the edf will creat unwanted drag.



Oh one more thing, i'm going to put a intake ring on my edf,even though it's going to be buried inside a ducting tube. I've done test and seems that it produce extra ~20% thrust with the intake ring.

Hope someone could anwser this so i could start on my next edf plane

'A' should be the way to determine the fan swerp area.

Intake ring is to improve the air flow into the fan in static condition, thus improving the static thust. Once the airplane is flying, the ram air will take over. Once you put an intake ring, anywhere other than the most forward portion of the ducting, it'll do nothing to improve the static thrust .
See this as well;

http://www.jethangar.com/

[viper1990]

Thanks for the extra info!

Anyway, interesting website you got there..

" On the bench, the system is pushing 14+lb thrust at 95 amps and 4500 watts. With our 6000 mah batteries, we are pushing our ARF A-7 at speeds well over 160+ mph and attaining 6 minute flight times"

So far the highest power motor is use is only rated at 300watts, talk about 4000watts and 14lb thrust, thats like the weight of a bowling ball.

I got a another question though.., is there a rule of thumb for calculating the intake area for turbine planes? because i notice the intake area is significantly smaller then the exhuast area.

From the picture, how come the turbine dosen't need a intake duct , won't that cause drag on the plane?

[joe yap]

Thanks for the extra info!

Anyway, interesting website you got there..

" On the bench, the system is pushing 14+lb thrust at 95 amps and 4500 watts. With our 6000 mah batteries, we are pushing our ARF A-7 at speeds well over 160+ mph and attaining 6 minute flight times"

So far the highest power motor is use is only rated at 300watts, talk about 4000watts and 14lb thrust, thats like the weight of a bowling ball.

I got a another question though.., is there a rule of thumb for calculating the intake area for turbine planes? because i notice the intake area is significantly smaller then the exhuast area.

From the picture, how come the turbine dosen't need a intake duct , won't that cause drag on the plane?

Ideally, it does. But it can get away with it to save the complications, since it produces so much power. Don't forget, gas turbine engines produces gasses by burning fuel, and they just need air to support combustion, not to be accelerated. As such, models just need to have small air intakes, and you can save some drag already.

[simonwoo]

Hi Joe, I bought a replacement EDF unit for my F18 and there isn't enough space for the intake ring. Actually come to think of it, I think the stock fan also doesn't come with an intake ring (I can't be sure now). If the ring usually used for 'exposed' EDF units? The way I see the fan is housed in the F18, I'm also not sure if an intake ring will make any difference??

[joe yap]

No need at all if your fan unit is mounted inside the fuselage. It'll do nothing at all, except giving you some hard time trying to fit it in. For a unit mounted within a fuselage like most modern fighter jets, the inlet lips of the air intake must have rounded edges instead of sharp edges. Ideally, those fans which are fed by 2 or more air intakes should have the plenum meet at just before the fan blades, instead of the spinner. As such, clever designers often omit the spinner and they only join the 2 intake plenums' centre divider just before the fan blade LE. This is to prevent the mixing of air from the 2 seperate plenum which creates unwanted swirl, and thus making sure that the air is really fed into the fan as directly perpendicular to the fan blades as possible, to achieve maximum velocity gain.

The intake ring is only useful if the fan unit is mounted outside the fuselage within a nacelle, where there is no inlet plenum to guide the air into the fan. It'll smoothen the airflow from ambient into the fan and thus recover some efficiency when the plane is stationary. Once the the plane is travelling forward at certain speed, the ram air will do the same and the ring has much lesser effect.

I almost miss a point. If a fuselage mounted fan does not even have a proper inlet plenum to feed the air directly into the fan, then the intake ring will help to recover some losses. But this is much less effective than haveing a proper inlet plenum.

[viper1990]

Sry to interrupt, but i got some EDF 'graphs and diagrams' to share after my 3-4hours researched on the internet more about edf intake area ,exhuast area, inlet shapes, nosecone effects, ducting losses,static thrust vs efflux velocity, and i must say EDF is really complex stuffs

All these factors put together to give a really smooth , low drag, high efficient ducting.

makes me think that propellers are kindergarden stuffs

[viper1990]

Oh, i got a question and see anyone could anwser this 'chim chim question' (maybe to some its not haha ), Sry if its lenghty , so take your time to read

The following diagram/picture, I've made improvement to the edf ducting in my F-16 such as, i'm going to smoothen the surface inside by placing thin transparency plastic (from art friend) inside the entire duct.

Designed 1cm thick inlet instead of my previous 5mm thickness and sand it round.

But the major changes i made is the recalculated duct area for my f-16.
I came out with figures 113% FSA for intake and 93% for exhuast.Reason for changing it because a prop plane running on 11x4 prop 1000kv actually caught up with my F-16. Slow because due to excessively large intake..., How embarrsing

My fan specifications:
Total area= 38.4cmsq
Motor area= 8cmsq
Fan swept area = 30.4 cmsq

Now heres the thing, as the intake area is 34.4cmsq, as it proceed inwards through the duct, the net area increases to 38.4cmsq (just before the edf blades,catchment area), through the expansion of air,i am right to say velocity at intake> velocity at catchment area.

and because of the loss in velocity,It is true that:
A) am I actually causing the blades to stall easily?
B) Is there some ductloss due to energy used to expand the air?

Ok so overall it goes like this:
1--> Air enters intake 113%FSA
2--> Air flow inwards and expands/depressurize to a net area of 126% FSA right before the edf blades (losses here?)
3--> Air flow through the annulus which is 100% FSA, the other 26% must be also squeezed through along with the rest of the air (I think this is one of the huge losses)
4--> Air flow past the blades and round the slanted stators which spins the air inside the ducting like a vortex,why can't the stators be straight (whats the reason behind this?)
5-->Air flow around the motor and quickly expands to fill up the empty space behind the motor (another huge loss? I think thats why there is tailcone to smoothen the airflow behind the motor)
6--> Air flows to the back to the exhuast of 80% FSA, air is compressed and efflux velocity goes up (losses here too?)

So that is where all our power from our batteries went?! Compressing and decompressing air inside the duct.
Therefore i belive any edf planes needs a good ducting and somewhat a good performance EDF to cover up these losses.

These are my analogy,Comments from anyone?!

[joe yap]

Viper,

I'm wondering why do you use 113% FSA for your air intake in the first place. You probably should stay with 100% at the inlet lip area and try to keep the cross sectional area as constant as possible. The only position where 113% FSA is applicable is at where tip of the spinner is, which the area transit from the front to the ducted fan shroud itself.

In static conditions, the extra large inlet will allow the air to be fed easily into the fan, which will provide plenty of static thrust. This often makes you feel very good and happy that your setup looks like it is very efficient. However, once the model is travelling in air, the ram air will be scooped into the inlet, supplying too much air for the fan to process. The excessive air from the inlet will not be flowing into the duct system but being forced to flow away from the inlet, just like when you left the water running over a glass full of water. You are in fact creating a lot of parasitic drag which slows down the plane.

Now you are thinking of enlarging the nozzle area to 93% which further slows down the efflux velocity although it increases your static thrust. Don't forget, the trick of Ducted fan system is the get the right balance between static thrust and efflux velocity. The measures you are taking seems to be going opposite of what you want to acheive.

Designers have been trying to squeeze out every bit of output power by optimising the duct system to reduce waste. The constant area rule is the golden rule of thumb they have been using since day one. Any expansion and compression of the air will reduce performance in the duct system and thus, the ideal change of duct diameter from the 'transition areas' like the spinners and motor cone has to be determined by calculating the annular areas of the the sub-stations where the spinner and motor cone are. However, a little deviations will not create significant losses, unlike drastic area changes like poorly design ducts, especially straight cylindrical ductings.

The reason the nozzle area is usually reduced is that some efflux velocity will be lost due to skin friction, even with well designed, constant area thrust tube or exhaust duct. Thus reducing the throat area of the nozzle will recover or increase the efflux velocity will a relatively small penalty. Afterall, it is tha efflux velocity that allows the plane to accelerate. Static thrust only makes accelertion quicker, that's all.

You mentioned about the increase in cross sectional area of 126% at the fan inlet, did you forget to minus the area taken by the spinner cone?

Most modern EDF units designed for brushless motors at very high RPM, have relatively lower pitch than older generation EDF units, which makes the fan blades less prone to blade stall, even at static conditions. It may happen at the point you just started to turn the rotor, but as soon as the air is being sucked through the inlet, the initial velocity of the air will unstall them.

Ideally, the air after being accelerated by the fan, should flow parallel to the duct system to have the maximum efflux velocity. However, the fan also impart rotational forces to the air. As such, the exhaust fan air actually travels in a helical motion. If you resolve the exhaust air, a small part of the thrust actually tends to rotate the entire aircraft, rather than pushing it forward. So, for best results, the fan exit guide vanes, or sometime referred as the stator vanes or flow straighteners are supposed to do the job of taking out the swirl. The big question is, "how"? At different aircraft and rotor speeds, the swirling force changes. If you try to take out all the swirl at static conditions, the reverse swirl will happen when the aircraft starts to fly. If you do not have swirl on the fan exit guide vanes, there will be very little swirl when the aircraft is travelling at high speeds, but it will suffer at static conditions. So, designers only try to take out some of it as a compromise. How much is enough? This will depends on the designed aircraft speeds.

This is the perhaps the main reason, why EDF starts to lose thier efficiencies if you drive the rotor at much higher rpm than the designed rotor speeds. Add one more cell? You get more power output, but with even more power consumed.

After talking so much c*ck, my take is that the main reason your F-16 flies so slowly, is that it has a very big drag chute under its belly.

[Vortices]

Dear viper,

Firstly glad to see someone going the distance to understand and improve EDF efficiency.

Edf is not that new anymore, plenty of material on the internet nowadays even for brushless set up. Just spend sometime looking.

I have experimented with various edfs, initially 55mm now have progress thru 70mm. I have come to conclude EDF efficiency have gone up so much in the last year. Now there are ARF that runs on 1300 mah 3s batt 64mm fan hand launcheble and respectable high speed performance with about 4 minutes of endurance. With 1800 batt performance stays about the same and endurance incresase repectable amount.

For your F-16, if your motor is not too small (lesser than 250w-electrically) it should be quite fast. The trick is to find out where all the power have gone.

After all that is mentioned of efflux velocity, do becareful not to concentrate too much on high speed exhaust as this will cause a drop in propulsive efficiency. Particularly at the speed that our model flies. In short, too much energy is spent accelerating air and too little is use to accelerate the plane.
This is the same reason prop planes flies more efficienly than edfs at low speed. However the efficincy will increase when the planes accelerate to higher speeds. Unfortunately for practical rc flying, we almost never reach those speeds. Therefore better solidity (more fan blade area) on a lower kv motor will yeild more thrust than a thin bladed fan at higher rpm. This is apparent when you compare higher end fan units. Of course, dont over do this,otherwise the plane will have a very narrow speed margin.

To increase your f-16 speed range, I would recommand you choke the exhaust to 85% FSA and maintain inlet to 100%. FSA.

The rule of thumb I have discovered from my edfs:
1)Good high speed performance: intake 100%fsa, 80%fsa for exhaust. However this combnation give poor low speed and static performance, making T/O ROG very long and Very difficult hand launch . A bungee is recommanded.

2)Good TO/LDG performance: Intake 110% with a good intake lip, 95% fsa exhaust. This give very good low speed performance. Easy launches and short ROG. However beyond a certain speed after airbourne, the plane just doesnt want to accelerate even with full throttle.

3) A good compromise that I have use on 3 model successfully, is 100% fsa intake with 85% to 90% fsa exhaust. This give shorter ROG and still good high speed performance.

However after saying all that a good motor and fan combination is of paramount importance to start with. This would vary in efficiency depending on Fan type, motor KV and intended flying speed. The best combination would yeild the highest thrust/amp.(gm/amp). -it should well be thrust/watt but since volt is more or less fix by the use of no. of cell, thrust/amp will work the same for practical measurement.

Following are the long ez that I have converted to edf. this plane is meant to fly slow for edf training to/ldg. It carries 4300 kv motor on 56mm hyperflow fan.100% fsa intake and 90% exhaust FSA. It is a fun plane to practise t/o and ldg and all aerobatics that doesnt need rudder.

The j-10 is design to fly fast, 3500kv on 70mm fan. >100%fsa as it has a giant cheater hole, 85% fsa exhaust area. She take forever to take off at full throttle, with recent improvement it still takes about 40m to 60m to lift off. However after airbourne it continues to accelerate to a respectable speed at 3/4 throttle. Runs out of space very fast.

The f-16 was build to fly at a very wide speed range. I wanted to fly extremely slow, it has droop leading edge. But it was design for 4s. 3500kv on 64mm fan. Unfortunately I lost the plane on maiden due ESC BEC failure.
But the short time that I had control, this plane rocks! cant wait to build another.

Hope this helps.

cheers
Kevin

[Vortices]

....and the f-16

[viper1990]

Thanks for the info again guys.

It has been a real headache to choose the intake and exhuast areas... or probably i'm not decisive enough to just go with the standered "edf rule of thumb" thats why i go to the extent of serious in-depth reading on edf calculations.

Anyway, i'll go with your choice of 100% intake 85% exhuast. Once i'm done building the plane i'll post my test results.

when i started flying rc plane, my goal was to madien my very first plane myself...and getting the plane up flying for more then 30seconds was everything. Now i've developed this hunger to design even better planes/ducting for performace and speed,can't help it.

You mentioned about the increase in cross sectional area of 126% at the fan inlet, did you forget to minus the area taken by the spinner cone?.

Yep, i forgot to minus the area taken by the spinner cone. My 68mm edf spinner cone takes up 26% of the total intake area or 126% of the FSA.


Lastly, So sry if i have been very long winded in this subject.

[Raven]

Thanks for the info again guys.

It has been a real headache to choose the intake and exhuast areas... or probably i'm not decisive enough to just go with the standered "edf rule of thumb" thats why i go to the extent of serious in-depth reading on edf calculations.

Anyway, i'll go with your choice of 100% intake 85% exhuast. Once i'm done building the plane i'll post my test results.

when i started flying rc plane, my goal was to madien my very first plane myself...and getting the plane up flying for more then 30seconds was everything. Now i've developed this hunger to design even better planes/ducting for performace and speed,can't help it.



Yep, i forgot to minus the area taken by the spinner cone. My 68mm edf spinner cone takes up 26% of the total intake area or 126% of the FSA.


Lastly, So sry if i have been very long winded in this subject.

lol... thats why i ask you to take a look at the WM400 EDF. Solve all your exhaust cone problem.

http://www.jethobby.com.sg/ezsite/galery/41/1.pdf

[joe yap]

Actually, why not leave out the exhaust nozzle cone during the construction and made yourself a few nozzles with different throat areas. In this case, you can try out each of them to find out which one will give you the best result.

[viper1990]

I read through the wm400 edf long time ago already. Not a bad edf unit actually,but never seen anyone flown with it with my own eyes so i don't really know it's performace until i try it out myself.

Actually, i did see videos of it running in rcgroups, just that,those ppl are dumping in 1.4kW of power inside it , my motor is only at 400watts.

Now deciding between wemotec fans and wm400. Both have good reviews over at rcgroups!

And yep..., the variable nozzle area thingy is a good idea.., i can test out different areas and see how it affects the speed and i'll come down to a perfect area with good efflux velocity and static thrust and stick with it.