BlueMonster

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I am trying to choose a fan, but I am running into specifications I don't understand.

How do I gauge power output? All I can compare anything to is nitro engines. What would the pounds of thrust be that woud be equivelant to a nitro engine?

What is static thrust?

What is the difference in the number of fan blades? Is more blades equal to more air moved? Would more blades require a more powerful motor?

Larry Dudeck

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The first thing you must remember is this - $ for $, an EDF will never exceed the performance of a nitro driven prop. Yes, you can spend many dollars on an EDF system that will exceed the performance of the nitro driven prop but the cost will far exceed that of the nitro system.

Please bear in mind that I think EDF's are the best method for pushing a model around. But facts are facts.

How do you gauge the power output? With an EDF one must rely on the static thrust and efflux velocity. Both of these are dependent upon the amount of power you are using to drive the fan.

One rule of thumb is 100 watts per pound will provide adequate performance but probably not any verticals. For hot performnace, figure 200 w/lb. It all boils down to thrust vs. weight. Any thing less than 1:1 won't give you verticals whereas thrust/weight ratios greater than 1:1 will provide verticals. So on a 3 lb model, you can figure anywhere between 300 and 600 watts/lb, depending on your flying style.

Static thrust is just that. It is the thrust produced by the fan when the plane is stationary. Thrust is a function of the amount of air passing through the fan and the change in the air's velocity. The product of these two variables just happens to be at it's maximum when the plane is sitting still. As the plane's speed increases, the static thrust decreases. At the same time, the drag forces are increasing and very soon the plane's velocity induced drag just equals the thrust produced by the fan. At this point, thrust equals drag and no further increase in speed is possible (unless in a dive).

Efflux velocity is the speed at which the air exits the plane and the speed of the plane will not exceed the efflux velocity. Efflux velocity can be varied by changing the size of the outlet. By decreasing the outlet size, the velocity will increase. But you will soon reach a point where the decrease in area results in enough restriction that negatively effects the fans performance. The change in pressure of an EDF is miniscule, if even measurable. So it doesn't take much restriction to change the performance.

Compare this to a turbine. The turbine's thrust developed remains fairly constant so the maximum velocity will exceed that of the same model driven by an EDF.

The number of blades is a science unto itself. Consider a nitro driven prop, comparing a 2 blade with a 3 blade. The analogy should hold. If the fan is to push more air, it will require more power.

Perhaps others will jump in and add something.

Darryl Usher

Larry: I thought we were talking about DF vs EDF not prop. My BVM .91 DF gives me 9.5 lb pounds of thrust installed in an airframe. My EDF says 19+ pounds, so I am expecting about 15 pounds installed in same airframe. The weight will be about 1 pound higher with EDF.
Darryl

Larry Dudeck

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Daryl,

I must have misunderstood the question. Neither here or there, though.

At 19 lbs of thrust, you have to be talking about a Schubie. Even with the extra pound of weight, the additional thrust should provide a good performance increase.

What airframe are you using?

Darryl Usher

My F-106, about 90" long and 51" wing. 17 lb as an DF. About 1000 sq in of wing.
www.acsip.com/~dgu
Darryl

Larry Dudeck

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Daryl,

Nice web site.