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EDF Power Rating

Old 04-02-2008, 08:01 PM
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Default EDF Power Rating

Trying to figure out how big a fan/motor set up to buy is making me crazy. Does any EDF manufacturer or web site publish the data like the engine manufacturers do that tell you this engine is suitable for a 35-40 oz airplane or a 3 to 4 pound airplane. I can't correlate the thrust/size of an EDF vs a engine size/Prop.

What I want to know in a nut shell is. What is the difference in thrust between a known size motor turning a prop vs the same size electric motor turning a fan. I know I didn't mention the prop pitch or number of fan blades, but I don't know how else to ask it.

Thanks

Paul
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Old 04-03-2008, 07:19 AM
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Larry3215
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Hi Paul,

(howdy neighbor!)

Unfortunately, its not that simple, so dont feel bad for not finding it easy to figure out - you're not the only one!

Thrust is a direct function of two things - the diameter of the fan or prop and rpm its turning at.

Thrust increases (or decreases) as the 3rd power of the rpm. In other words, if you double the rpm, the thrust will increase by 2x2x2 = a factor of 8 times. You will also need 8 times the power OUTput to double the rpm. If you increase the rpm by 1.5 times you get 1.5x1.5x1.5 = 3.375 times the thrust and you will need 3.375 times as much power to do that.

However, thrust increases (or decreases) as the 4th power of the diameter. So if you double the diameter of a prop or fan (keeping the rpm the same) the thrust will increase by 2x2x2x2 = a factor of 16. Again, you will need 16 times as much power to spin a prop that is twice the diameter at the exact same rpm.

Im ignoring pitch and tip losses and blade flattening and a host of other details that can effect those numbers to some degree, but thats the general relationship.

The point is that diameter is far more important - effective - at generating thrust than rpm.

For ANY given size of motor you care to choose - if you put a prop on it - you can get more thrust than if you spin a fan at the same power level. The reason is that you can always spin a larger diameter prop than a fan - less drag.

Ducted fans are just not a good at generating thrust. They are generally smaller in diameter for a give power absorbsion and therefore have to spin at much higher speeds to even come close to a prop of slightly larger diameter.

There are tip losses at the shroud interface and on top of that you generally have inlet and exaust ducting adding drag and reducing performance.

Add in that the cheepo fan units do not have well designed/efficient blades and the losses keep adding up.

On top of all that - most jets dont have enough room inside to fit in battery packs large enough to put out the power needed for truley hi performance.

All of that is why guys are usually extatic if they can get an edf with a 1/1 thrust to weight ratio. If they can get more than 1/1 they start having visions of world domination, Nobel Prizes etc

As far as selecting a fan and motor combo to power your plane - how heavy is it? Find a similar sized existing model and search for power options that are already in use and see what you come up with.

As far as mfg data - the GWS fan data is supprisingly accurate. Other than that, you have to move into the larger more expensive name brand fans before you start seeing good data.

Most modelers are not set up to record good data on edf's. Mostly all you see is something like "it draws 500 watts and screams!". "Data" points like that are useless unless they also include an rpm.

Lots of guys will publish thrust data from thrust stands, but that information isnt all that usefull either. Once you get a fan in the plane its a whole different story and once again they often leave off the most important data point - rpm.

Just because a motor/fan is sucking up 500 watts doesnt tell you anything usefull.

If one motor fan setup is drawing 700 watts and turning at 45,000 rpm and another is only drawing 600 watts, but is still turning at 45000 rpm, the lower power setup is far better - more efficient.

If a given setup is not getting the rpm, then its turning those watts into heated air.

You want to fly a jet not build a hair dryer

Good luck!

Larry
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Old 04-03-2008, 07:26 PM
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Larry,

Thanks, that was a very useful message, that helps lot. I have much to learn.
Here is the problem I still have no answer for. I have a good set of plans for a P-59. The wingspan is 45 inches. I would like to build it somewhere between 54 and 60 inches. I have not settled on the size yet because till I can figure out the physical size of the motor, fan and ducting I won't how much to enlarge the plans.

If I can identify two motors, fans and a suitable battery combination that will push a 64oz model (4 lbs)around the sky at a reasonable speed. Then I can subtract the weight of the propulsion system and that will tell my what by building margin is for the aircraft it self.

At this point even the 64oz is a guess. It will probably be much lighter, but I have to have a place for planning purposes. In my gas powered days with props I would just keep adding bigger motors till I got the performance I wanted. That's not going to happen here.

Again thanks for the info. A couple of us will be flying at the Eflaps field in Gig Harbor tomorrow morning about 10am are you available?

Paul
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Old 04-03-2008, 08:29 PM
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Hi Paul,

Is that at the new site in Gig Harbor? Id love to go but....

Saturday I was planning on going to the Maytown lake float fly down south of Olympia.

If the weather looks too iffy I'll stay home and see you out there....

On your plane - at 60+ ounces all up weight - Id guess your going to need 2ea aprox 70mm fans at a minimum and some hot motors. Your going to want around 800-1000 watts tottal from the two motor system/fan units.

Id check out the Executive Jet thread for power options and contact The Don for the hot ticket in motors.

I was thinking the Exec jets are coming in around 50 ounces?

If you need more - the next step up is 90 mm fan units like Im running in my 56" span F86. Its coming in around 6 1/2 - 7 pounds and I'll be running it at 1400+ watts on a single fan. Im expectiong around 6 pounds of thrust at that power level and around the 100 mph mark - I hope!

Larry

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Old 04-04-2008, 07:55 AM
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I just posted some thrust vrs RPM and power vrs RPM charts for the 90mm midi fan in my F86 thread if your interested in seeing what those numbers look like.

Dont know if you could fit 2 90mm fans in the plane or not, but thats the next step up from the 65-70mm stuff.

http://www.wattflyer.com/forums/showthread.php?t=26790

Larry
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Old 04-04-2008, 10:15 PM
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Larry,

I would like to see your F86 and discuss the P59 plans I have with you. Please give me a call 225-0780.

Paul
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Old 04-04-2008, 11:55 PM
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Larry,

I just spent half an hour reading through your F-86 build. On the theory that one EDF is simpler than two and seeing how you have already done the project R&D and I haven't started gathering material for the P-59 yet, it seemed logical to buy an F-86. So I did.

Paul
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Old 04-05-2008, 05:37 AM
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LOL Thats cool!

I have several recomendations I can pass along depending on what you want to do for a power system, retracts etc.

The weather looks too iffy for me to want to drive way down south to that float fly, so I'll see you guys tomorrow morning at the new flying site. I may not make it till 10:30 or so.

Larry
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Old 05-24-2008, 09:02 PM
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Where is the EFLAPS field in Gig Harbor? I am ready to start flying again...

Thanks for any help.
Dale
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