T E A M
Hello Everyone,
I thought of compiling some of the general information (thanks to the help of WeMoTec) on EDF in order to help new “EDF-Jet Fighter - Jocks”
How do I choose a fan?
The first essential step is to check the cross sections of your fuselage. Next Calculate the available intake and nozzle area. These should be equal to the swept area of the fan unit. This information is referred to as Ageo (It should be stated by the fan manufacturer and retailer).
If an intake or exhaust area is not big enough to suit the fan, the system will not work properly.
This however is not entirely true for fast models (such as the Super Sniper, speeds in excess of 200km/h) which may have a cross section that is smaller down up to a ration of 0.8:1.
Material:
As you will see below we are using big forces on our small fan units to get the desired output power. Material and quality plays an important role in your decision on your next fan unit. EDF Fans can rotate in excess of 65,000 RPM. In comparison a typical glow trainer spins the prop at speeds of 10,000 to 16,000 rpm.
The faster the rotor spins the higher are the centrifugal forces acting on the rotor. Weak materials will expand and in the best case touch the EDF housing and are sanded down to an extend that the gap between rotor and fan housing will be too big and result in a loss of efficiency and power.
Worst case the rotor will bind with the housing (melt together) which might damage your ESC or disintegrate and in some cases destroy the model.
Hence make sure your EDF Fans are of high quality and rigid design with strong, non flexing blades.
How much power do I need ?
As much as possible
!! Ok just kidding.
Once you have decided on a certain fan unit, you will need a suitable motor and battery.
First you should weigh or estimate the total weight of the model including radio and covering, but without the fan unit, motor and battery. To get your estimated flying weight, add another 60-100% of the model weight for the power train. 60% is more realistic if you intend to use LiPo packs, 80% if you choose NiMh and 100% if you go with NiCd.
This gives you the estimated total weight of the model. If you however have chosen a specific power setup already you may as well add the weight based on the data provided by the manufacturer (maybe add a small buffer).
For a slow model like an airliner, 150W of motor input power per kg should be sufficient, a normal model may ask for about 200-300 W/kg and really hot ships need up to 500 W/kg.
If you choose 200 W/kg for a normal model and 300W / kg for ROG from grass you should be on the right track.
Next in the list is thrust
How much thrust?
Once you finalized your power requirements you then check the readings for the fan of your choice for the amount of thrust (in grams or kg) you will get at certain power levels.
Rule of thumb for thrust to weight ration:
- 0.3-0.4 for slow models like airliners
- 0.4-0.6 for medium fast models and ROF of slow models
- 0.6-0.8 for fast jets and ROF of medium fast models
- >0.8 point of no return, you are hooked to EDF Jets J !!
Efflux velocity should match flying speed
These days we can get impressive amounts of thrust out of tiny fan units. But it is still wise to match the size of the fan unit to the expected flying speed.
Example:
If you build an airliner, take a big fan and run it at low power levels. If you go for speed, take a smaller fan and put in a lot of power.
In general you should aim for more static thrust with slow models and more efflux velocity with fast models.
Please take note: Due to the lower static thrust, high efflux velocity models without ROG capabilities should be bungee launched!!
Now that you know the basics of how to choose your next EDF Fan, a few additional information’s on Fan efficiency and design:
The airstream should be straight in flight !!
It is a common misconception that the airstream should be straight in static tests. For a high efficiency system the airstream should be straight at the expected FLYING SPPED (such is the case with Fans from WeMoTec).
So what does this mean?
Let’s take FAN A and FAN B. Both fans are of the same size but from different manufacturers. Both setups use the same motor, ESC and Battery.
FAN A shows a straight airstream in static tests and therefore probably a slightly higher static thrust then FAN B.
Does this mean FAN A is better and has a higher efficiency?
No. Once the model reaches a certain flying speed the airstream in FAN B becomes straight due to the pressure difference and FAN A efficiency begins to sink because of exactly the same reason. Hence it is important to choose a fan which has been tested extensively.
BTW, another rumor is that twisted stators are generally more efficient.
Also this is not true as test have shown. The Mini Fan 480 and WeMoTec Midi Fan for example show highest efficiency only with straight strators.
A clean intake lip is responsible for at least 20% of the total thrust!
An example: if one tests the Midi Fan without intake ring thrust decreases by about 20%. It is not the maximum size of the intake that counts, but the aerodynamic cleanness (bigger intake area with at the same time reduced turbulences due to intake ring). Additional intakes (cheater holes) should be resisted wherever possible.
Tailcones do not increase thrust!
Tailcones are not thought to increase thrust. Their task is rather to increase the efflux velocity so that the flying speed of the model can be maximized. The tail area corresponds to 90% of the ducted fan swept area.
Hint: A tailcone should be kept as short as possible. If the thrust tube between strators and outlet is longer then approximately 100mm, the tube should be the same diameter up to the outlet pipe.
Do I need a spinner?
Although it sound surprising, the spinner is not that important. So if you experience trouble, leave it off. If you have a bifurcated intake (example Super Sniper) it makes more sense to place a tube in the ducting the same diameter as the rotor hub and mount this in front of the rotor instead of a spinner. This is aerodynamically much cleaner (example Super Sniper).
Do I need a streamlined cone?
Not necessarily. Their influence is rather limited and with such a cone a motor runs hotter. Hot motors run less efficiently.
Rule of thumb: do not use a cone on small fans. On bigger fans it may make sense but do not expect miracles.
Any internal ducting should have a constant cross-sectional area over its whole length!
All cross-sectional changes in the internal ducting mean pressure changes, and with it acceleration and deceleration of the air. This should be avoided.
As much as possible the duct through the fuselage should be the same size (uniform cross sectional area) as the intake.
If a ducting needs to be widened, this should happen always smoothly at an angle of less than 4 degrees.
Position of the fan in the ducting
Aerodynamically there are not many differences whether you put the fan at the font or back of the ducting. But be aware of the following aspects:
- Critical wire length for brushless setups (in case the battery needs to be in the nose)
- CG
- Also in general fans runs smoother if they are placed towards the rear
A gap at the blade tip up to approx. 1.5mm is not harmful.
The airflow around the blade tips is always turbulent. Therefore a gap of up to 1.5mm for the big fans (>92mm, example Midi Fan) and 1mm for smaller fans (<92mm, example Mini Fan 480) does not influence the performance of the EDF. Only if the gap is bigger, you may realize losses in static thrust. Aeff, mentioned as part of the product specification, reflects this phenomenon.
This summarizes it more or less.
I will add some additional information later.
Do you guys find this useful? Should we make this a Sticky?
I thought of compiling some of the general information (thanks to the help of WeMoTec) on EDF in order to help new “EDF-Jet Fighter - Jocks”
How do I choose a fan?
The first essential step is to check the cross sections of your fuselage. Next Calculate the available intake and nozzle area. These should be equal to the swept area of the fan unit. This information is referred to as Ageo (It should be stated by the fan manufacturer and retailer).
If an intake or exhaust area is not big enough to suit the fan, the system will not work properly.
This however is not entirely true for fast models (such as the Super Sniper, speeds in excess of 200km/h) which may have a cross section that is smaller down up to a ration of 0.8:1.
Material:
As you will see below we are using big forces on our small fan units to get the desired output power. Material and quality plays an important role in your decision on your next fan unit. EDF Fans can rotate in excess of 65,000 RPM. In comparison a typical glow trainer spins the prop at speeds of 10,000 to 16,000 rpm.
The faster the rotor spins the higher are the centrifugal forces acting on the rotor. Weak materials will expand and in the best case touch the EDF housing and are sanded down to an extend that the gap between rotor and fan housing will be too big and result in a loss of efficiency and power.
Worst case the rotor will bind with the housing (melt together) which might damage your ESC or disintegrate and in some cases destroy the model.
Hence make sure your EDF Fans are of high quality and rigid design with strong, non flexing blades.
How much power do I need ?
As much as possible
!! Ok just kidding.Once you have decided on a certain fan unit, you will need a suitable motor and battery.
First you should weigh or estimate the total weight of the model including radio and covering, but without the fan unit, motor and battery. To get your estimated flying weight, add another 60-100% of the model weight for the power train. 60% is more realistic if you intend to use LiPo packs, 80% if you choose NiMh and 100% if you go with NiCd.
This gives you the estimated total weight of the model. If you however have chosen a specific power setup already you may as well add the weight based on the data provided by the manufacturer (maybe add a small buffer).
For a slow model like an airliner, 150W of motor input power per kg should be sufficient, a normal model may ask for about 200-300 W/kg and really hot ships need up to 500 W/kg.
If you choose 200 W/kg for a normal model and 300W / kg for ROG from grass you should be on the right track.
Next in the list is thrust
How much thrust?
Once you finalized your power requirements you then check the readings for the fan of your choice for the amount of thrust (in grams or kg) you will get at certain power levels.
Rule of thumb for thrust to weight ration:
- 0.3-0.4 for slow models like airliners
- 0.4-0.6 for medium fast models and ROF of slow models
- 0.6-0.8 for fast jets and ROF of medium fast models
- >0.8 point of no return, you are hooked to EDF Jets J !!
Efflux velocity should match flying speed
These days we can get impressive amounts of thrust out of tiny fan units. But it is still wise to match the size of the fan unit to the expected flying speed.
Example:
If you build an airliner, take a big fan and run it at low power levels. If you go for speed, take a smaller fan and put in a lot of power.
In general you should aim for more static thrust with slow models and more efflux velocity with fast models.
Please take note: Due to the lower static thrust, high efflux velocity models without ROG capabilities should be bungee launched!!
Now that you know the basics of how to choose your next EDF Fan, a few additional information’s on Fan efficiency and design:
The airstream should be straight in flight !!
It is a common misconception that the airstream should be straight in static tests. For a high efficiency system the airstream should be straight at the expected FLYING SPPED (such is the case with Fans from WeMoTec).
So what does this mean?
Let’s take FAN A and FAN B. Both fans are of the same size but from different manufacturers. Both setups use the same motor, ESC and Battery.
FAN A shows a straight airstream in static tests and therefore probably a slightly higher static thrust then FAN B.
Does this mean FAN A is better and has a higher efficiency?
No. Once the model reaches a certain flying speed the airstream in FAN B becomes straight due to the pressure difference and FAN A efficiency begins to sink because of exactly the same reason. Hence it is important to choose a fan which has been tested extensively.
BTW, another rumor is that twisted stators are generally more efficient.
Also this is not true as test have shown. The Mini Fan 480 and WeMoTec Midi Fan for example show highest efficiency only with straight strators.
A clean intake lip is responsible for at least 20% of the total thrust!
An example: if one tests the Midi Fan without intake ring thrust decreases by about 20%. It is not the maximum size of the intake that counts, but the aerodynamic cleanness (bigger intake area with at the same time reduced turbulences due to intake ring). Additional intakes (cheater holes) should be resisted wherever possible.
Tailcones do not increase thrust!
Tailcones are not thought to increase thrust. Their task is rather to increase the efflux velocity so that the flying speed of the model can be maximized. The tail area corresponds to 90% of the ducted fan swept area.
Hint: A tailcone should be kept as short as possible. If the thrust tube between strators and outlet is longer then approximately 100mm, the tube should be the same diameter up to the outlet pipe.
Do I need a spinner?
Although it sound surprising, the spinner is not that important. So if you experience trouble, leave it off. If you have a bifurcated intake (example Super Sniper) it makes more sense to place a tube in the ducting the same diameter as the rotor hub and mount this in front of the rotor instead of a spinner. This is aerodynamically much cleaner (example Super Sniper).
Do I need a streamlined cone?
Not necessarily. Their influence is rather limited and with such a cone a motor runs hotter. Hot motors run less efficiently.
Rule of thumb: do not use a cone on small fans. On bigger fans it may make sense but do not expect miracles.
Any internal ducting should have a constant cross-sectional area over its whole length!
All cross-sectional changes in the internal ducting mean pressure changes, and with it acceleration and deceleration of the air. This should be avoided.
As much as possible the duct through the fuselage should be the same size (uniform cross sectional area) as the intake.
If a ducting needs to be widened, this should happen always smoothly at an angle of less than 4 degrees.
Position of the fan in the ducting
Aerodynamically there are not many differences whether you put the fan at the font or back of the ducting. But be aware of the following aspects:
- Critical wire length for brushless setups (in case the battery needs to be in the nose)
- CG
- Also in general fans runs smoother if they are placed towards the rear
A gap at the blade tip up to approx. 1.5mm is not harmful.
The airflow around the blade tips is always turbulent. Therefore a gap of up to 1.5mm for the big fans (>92mm, example Midi Fan) and 1mm for smaller fans (<92mm, example Mini Fan 480) does not influence the performance of the EDF. Only if the gap is bigger, you may realize losses in static thrust. Aeff, mentioned as part of the product specification, reflects this phenomenon.
This summarizes it more or less.
I will add some additional information later.
Do you guys find this useful? Should we make this a Sticky?
Hello Warbird! Can you give me your suggestions on powering this project. The plane is the Mr. Ming 380 Airliner. It was built as a slope glider and many guys are converting it into a EDF. I kind of got lost by looking at all the threads on other forums, still not quite sure what to run this Bird on. Here are some #'s from Mr. Ming:
there is still the option for a future upgrade of 2 additional fans if you want to break the sound barrier 
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