How to improve your EDF efficiencies

[Babylon5]

Hi, after looking at some of the previous posting, it look like the right time to start this interesting topics.
In general EDF is not as efficient systems compare to open blade propeller driven power systems. However, EDF provide a more realistic setup for jet plane. The systems can be tweaked up to improve efficiencies.

Some typical questions are

1)Does all the electrical power you put into the plane turns into meaningful thrust or your systems is just using up all the batteries energy to warm up the coffee or your favourite tea?

2) Does one use high KV motor or lower KV motor?



3) More or lesser blade Paddle count is better?


4) Does one use light weight motor or heavy weight?


5)More or less number of cells (Lithium or NICAD) added to the power sytems?


6) What else can be done to get more static thrust for lesser electrical power systems?


7) How critical is the air intake ?

Some of this discussion is highly debatable but lets discuss the issues in
friendly and helpful ways?


Cheers

[edmond22]

Some things that I keep in mind:

1. KV not so impt, more important is how many cells you intend to use, I usually match my KV to that.

2. More cell count also means higher voltage and lesser current consumption. I usually opt for the max I can go to get the best power to current draw ratio.

3. Try reducing exhaust by 5-15% using a cone that tapers gently. This helps in increasing your top end speed but have to take note that static thrrust is also compromised so a balance is needed depending on whether you are hand launching or bungee launching.

4. Lip surrounding the intake is important, studies has shown marked differences in Static thrust with and without.

5. Propeller driven system will always outrun a EDF system. This seems to suggest for the same given powerplant, Propeller driven is simply more efficient than EDF.

[Hawkeye]

Stick a prop at the front of the plane

[renold]

6) What else can be done to get more static thrust for lesser electrical power systems?


7) How critical is the air intake ?

Some of this discussion is highly debatable but lets discuss the issues in
friendly and helpful ways?

From Continuity Equation (simpify verson) as follow;
D*A*V = Constant:
D-> fluid density (air in this case)
A-> area (A1 at intake and A2 at exhaust)
V-> velocity (V1 at intake and V2 at exhaust)

At static condition;
D1*A1* V1 = D2*A2*V2;
V1 is low due to plane at rest (all depending on suction driven from impeller), if temperature remain constant, the way to increase V2 is to reduce A2 (from continuity equation). The idea of variable nozzle at jet zone. If temperature does change, (the case of real jet engine); V2 will be high. Therefore large thrust.
Improvement points are:
- to increase air temperature, maybe from heat release from electric motor and possible battery.
- to reduce exhaust area (jet nozzle), but not too low that it would create large back pressure.
- increase V1, by additing turbo fan and switch off after take-off.

At dynamic (plane flying) condition;
V1 = free air velocity + plane velocity
V2 = V1 + power input (less efficiency) of impeller.
One will find the required thrust at plane flying is less than to take-off.

Area of intake and exhaust can be calculated. Air temperature can also be measure. Measuring of air velocity would not be easy. There are few ways to do it, but I don't have the apparatus.

Just my understanding.....

[Invisible Angel]

1)Does all the electrical power you put into the plane turns into meaningful thrust or your systems is just using up all the batteries energy to warm up the coffee or your favourite tea?
This really depends on the KV of the motor and DF u used on the EDF plane.

2) Does one use high KV motor or lower KV motor?
Normally, one that is between 3500KV - 4000KV is good enuff.


3) More or lesser blade Paddle count is better?
This depends on the recommendation. practically, i find that 6 blades DF provide better thrust. but as i say, i also depends on ur motor.


4) Does one use light weight motor or heavy weight?
try not to use too heavy motor if u r going for handlaunch. tendency is that plane will drop before it can take off or fly if ur motor doesn't have enuff thrust.


5)More or less number of cells (Lithium or NICAD) added to the power sytems?
recommendation is LIPO, not NICAD. NICAD is too heavy.


6) What else can be done to get more static thrust for lesser electrical power systems?
u must ensure that there is no air gap that will cause the suction to flow back to the intake, which will cause resistance and great power loss.


7) How critical is the air intake ?
What do u think?


e.g.
My setup
Interceptor, fr. JH
BL Motor: Align 4050KV,
LIPO: TP 3S2P (25C)
Max. Amp. drawn: 30A
BL ESC: 60A (Kia Su)


Cheers and Happy flying.

[joe yap]

Hi, after looking at some of the previous posting, it look like the right time to start this interesting topics.
In general EDF is not as efficient systems compare to open blade propeller driven power systems. However, EDF provide a more realistic setup for jet plane. The systems can be tweaked up to improve efficiencies.

Some typical questions are

1)Does all the electrical power you put into the plane turns into meaningful thrust or your systems is just using up all the batteries energy to warm up the coffee or your favourite tea?

2) Does one use high KV motor or lower KV motor?



3) More or lesser blade Paddle count is better?


4) Does one use light weight motor or heavy weight?


5)More or less number of cells (Lithium or NICAD) added to the power sytems?


6) What else can be done to get more static thrust for lesser electrical power systems?


7) How critical is the air intake ?

Some of this discussion is highly debatable but lets discuss the issues in
friendly and helpful ways?


Cheers

For your first question, unfortunately only a fraction of the power you burn up is turned into useful work, due to pipe losses, electrical and mechanical losses, but the biggest contributing factor is due to poor designs of the ducting system which result in aerodynamic losses. You can do very little to reduce the first few losses but they are not a important as the aerodynamic losses.
In order not to turn your favourite ducted fan unit into an expensice hairdryer, you need to know how the air behaves and thus producing useful work. First of all, unlike a propellor which the produce the thrust directly, a fun unit only produce thrust only after the air leaves the exhaust nozzle, and not the fan. So you have to make sure that the air inside the duct flows properly until it leaves the fan.

In bernoulli's theorem, subsonic air must be considered imcompressible within a duct, and any expansion and compression will result in losses. The amount of airflow moved in a ducted fan unit at a single point of time is equivalent to the fan sweep area only, and not the total fan area. As it passes beyond the exhaust cone, and if the exhaust nozzle does not taper down to the same area, air will expand is it has to fill the bigger area. The net result is that air slows down again. You may think "WHAT???!!! I just burnt so much power to accelerate the air and it has slowed down just before the exhaust???!!!" You are spot on!

A lot of modellors do not understand this and they try to recover the efflux velocity by spinning at even higher RPM. It will work if you pump in enough power, but doesn't it sound stupid that you are burning thousands of watt just to produce some useful power and fan noise? Just give a thought; is the older duct fan plane designs can fly well just on brushed motors and heavy nicad batteries, why are we still having problem with our brushless motors and Lipo packs?

Ideally, the exhaust nozzle throat area should be the same as the fan sweep area, which is the total fan area minus the centrebody area, which air doesn't flow. However, to recover the efflux velocity due to pipe loss, a usual practice of 80-90% of fan sweep area is adopted and accepted generally. Having a exhuast cone that align the airflow after the motor can reduce turbulence and recover some efflux velocity. For long exhaust ducts, keep it within the duct system. For ducted fan units with short or no exhaust duct, keep the cone straight and only start to taper off only after the air leaves the shroud.

[joe yap]

For question 2, generally higher rpm designs are more efficient than fan impellors designed for lower rpm. However, you have to understand what you are doing in the first place. After all, it's the efflux velocity that limits the planes speed and static thrust only means how soon the plane can accelerate to the commanded speed. So the thrust figure stated in the unit has very little use in the first place. Unfortunately, most people use it as a common benchmark and sometime still ended up an unflyable model.

Let's say I need the efflux velocity at at least 144km/hr or 40m/s to safely fly my model at around 60 km/hr. If I'm using a motor which drives the impellor at lower rpm, I need to have an impellor with higher pitch blades to achieve the efflux velocity. At lower airplane speeds or stationary, the fan can stall which explains the lack of thrust at lower airplane speeds. This is a common problem with older fan designs but they still fly planes, as long as the plane is accelerated by a bungee during launch or given a very good hand launch.

If I have a motor which drives the impellor at much higher rpm, I only need a shallow blade pitch impellor to do so. Although it produces less efflux velocity with the same rpm, the motor can spin faster at full throttle and still capable of producing good velocities. With shallow pitch blades, stall is less likely to occur at lower airplane speeds or even at stationary. So, you'll get the best of both worlds.

However, you can't go infinitive on the RPM as all impellor design has a mechanical limit. Branded units like Wemotec will tell you the design limits but the cheaper units will let you know, when you see a spinning gulotine flies out of the duct or burst into pieces.

So whatever KV motor you want to choose, get one that match the desired rpm and battery cell count.

[joe yap]

Question 3 :The number of blades is determined by how much power can you extract from the motor toproduce work. More blades means more static thrust and more amp draw. It does recover efflux velocity losses within the duct but do not increase the velocity, assuming that the pitch is constant.

If you motor is drawing much less current than what is capable of, you can afford to increase the blade count, but make sure that the rotor design is capable of handling the power output from the motor. Likewise, if your motor is drawing very high amps, reduce the blade counts.

Question 4: Personally I'm not so concerned with the weight of the motor. The centrebody of the unit already restrict the size of the motor and you can do very little here. The power and kv matching is more important. I'm sure that given a physical size limit there's very little motor choice in the market that varies very widenly in term of weight.

Question 5: To add or not to add? My answer is to match. If your motor and esc calls for higher cell count, you can use more cell. If you motor is design for 2-3 cell Lipo, I think you'll be nuts to use 4 cells. If you hit the electrical limits of you powerplant and still not producing enough power, do something to the duct system or airframe, or change the powerplant setup althougher. Burning your motor,esc and bloating your Lipo is the last choice.

Don't be suprised that there are ducted fan designs that still use Nicad.

[joe yap]

Question 6: With given available power, you can only practically increase thrust by recovering from losses. You can improve the duct design, smoothen the airflow by using proper exhaust cones and smooth dust surfaces. Think of the Bernoulli's theorem and adopt the 'constant area' rule thoughout the duct. Faired the offensive motor wires and leave the ESC out of the duct.

By increasing the exhaust nozzle throat area can increase thrust, but causes a drop in velocity. Unless you have remotely controlled variable exhaust duct, which allows the nozzle to open during take off and gradually close up for level flight, like the full size fighters do, this choice is pretty useless.

Question 7: The amount of air in the intake determines how much mass airflow is available to produce useful work in the first place. Again, the area should be the same as the fan sweep area in theory, but pipe losses in a long duct will reduce the airflow and so you need to compensate by increasing the area a little. The intake lip should be rounded and not sharp as you don't want to create and turbulent airflow into the duct.

For split intake ducts like the BAE Hawk has, the point where they meet should be just infront of the fan blades, and not the spinner. When the plan yaws, the differential airspeeds flowing through the ducts can create turbulence when they meet and you certainly don't want it to happen before they got ingested into the fan. Just let the fan accelerate and mix them before discharging them behind the fan exit guide vanes is better. In this case, the spinner is pretty useless and some designer simply leave it off and put a fixed fairing at the wedge fairing.

Do note that scale jet fighter designs usually have longer intake ducts which restrict static thrust at static. Some designers enlarge the intake or put cheater hole to recover this but these will cause some compensation at higher airspeeds. If you have a long runway, you can get away without all these features. For the same reason, full size jet flighter has auxiliary intake ports to draw more air during take-off which close up when speed builds up.

[Babylon5]

More thoughts for the EDF

Hi, thank you guys for your wonderful feedback and discussion. Looks like this discussion is getting interesting.

I was surfing the internet for more information on jet engine design.If you notice carefully full size jet engine has lots of blade arrangement with little gaps between each blade.
On the otherhand some of the very good RC EDF like the *schubeler has low blade count. Have you thought about this difference. I have mentioned this in other thread but would like to hear from any experts opinion.


* Incase some of you have not heard about this fan or miss my other thread
go to this link

http://www.shredair.com/vector.html

This EDF set up is propelling the plane vectorII at 225mpH in 2003.
At this speed my leg will be shaking so hard and my nose will bleed.... ...just kidding.


Cheers

[edmond22]

Hey Babylon,

the Jet is very different, it compresses air, consists of stages and actually puts in a combustion process later to achieve the high thrust velocity output. See http://travel.howstuffworks.com/turbine3.htm

The EDF to me (IMHO) is mostly a asthetic factor for RC modelling, which works well to power planes but it is in no way close to a Jet engine. For all out speed, I think propeller is still the way to go, so using EDF is more for scale reason.

[$h@d0w]

Hey Babylon,

the Jet is very different, it compresses air, consists of stages and actually puts in a combustion process later to achieve the high thrust velocity output. See http://travel.howstuffworks.com/turbine3.htm

The EDF to me (IMHO) is mostly a asthetic factor for RC modelling, which works well to power planes but it is in no way close to a Jet engine. For all out speed, I think propeller is still the way to go, so using EDF is more for scale reason.

Or no more broken props in a bad landing.

[edmond22]

I thought that as well! But it worked well for edf mounted on top like a wing etc. But for those scale ones with proper intake at the bottom below the canopy kind keep eating grass! Always a headache cleaning those up esp during wet weather.

[joe yap]

Yes, I agree too. Duct fan only accelerates air whereas a pure gas turbine engine only draws air for combustion. The produced gasses is the main thrust producer. However, our modern turbofan works like a ducted fan, but driven by a pure turbojet.

Gas turbine engine compressor do not accelerate air, rather they actually compress air for combustion. In practical, each stage of blades (rotor) and vanes (stator) builds up a certain pressure before going to the next, which eventually builds up to the design pressure. However, this can only work at the designed engine speeds, airspeed air density and temperature. At different conditions, they compressor stages will be unbalanced and will surge. So designer usually over design the compressor by having it building up enough pressure for high power settings and to dump off the excess air at lower engine speeds.

After combustion, the expanded gasses are made to turn the turbine blades to drive the compressor. If it's a turbofan,turboshaft or turboprop engine, the exhaust gasses will also have to drive a power turbine, which drives the fan, prop etc. The balance will be exhausted as secondary thrust producer. For turbojets, there'll be no power turbine to be driven and the accelerated exhaust gasses will be pushed out to produce thrust.

[$h@d0w]

I thought that as well! But it worked well for edf mounted on top like a wing etc. But for those scale ones with proper intake at the bottom below the canopy kind keep eating grass! Always a headache cleaning those up esp during wet weather.

Lots of grass in my F16 after it "dropped from the sky" during maiden.