How do you select a fan?

[Erfolg]

For some time I have considered building a EDF model.

I even have a fan unit@ 75 dia, 40mm lg., weight 113g, motor 2409-2T, kv 2570 kv, cont, rating 43 amps, 3-4 cell Lipo, max watts 500. 3 bladed fan.

So how do i know if it is any good?

Given the large number of fans available, other than going by brand, how can you decide on a suitable unit. Does the number of blades matter, what does?

[Simon Chaddock]

Erfolg

Really the only thing you can actually measure is the static thrust and the current draw.

The thrust will give you a rough idea of the maximum size and weight of the plane and of course the current draw will tell you size of the battery which of course has a bearing on the planes total weight.

I try to find the claimed specification for the fan unit to give an idea of what to expect but I usually test it myself to ensure it is not overly optimistic.

As a rough guide I like to get as close as possible to an installed 1:1 static thrust to weight (but not less than 1:2 unless it is a glider type) along with a battery capacity capable of supporting a full power current for 3 minutes at a 20C discharge rate.

More blades will give more thrust and draw more current but like multi bladed props at the expense of overall efficiency although for some scale applications you have to accept this simply to get the required thrust in the available diameter.

EDFs tend to be closer to the limits of what is possible than prop planes so I suspect half the fun of scratch build one is finding out if you have got it anything like right.

 

Edited By Simon Chaddock on 21/08/2014 13:08:52

[Erfolg]

Any opinion with respect to if it matters if a inrunner is used or an outrunner. Particularly as many sold now appear to be outrunners.

How important are the revs, particularly as the stress seen by the units is clearly increased with higher revs?

I am a little surprised that as far as I can see, now one seems to indicate any thrust values for units, particularly when sold as a unit, it does seem to be an obvious value to check?

[Gary Manuel]

Not sure whether this will help. I've never used it myself, but it's worth a play:

**LINK**

[roger graves]

Quality,Quality,Quality. choose a branded fan/motor combo then there will be loads of info on the forums to help you.

I have seen a fairly powerful EDF which when run up produced sufficient heat to soften the plastic mount, this caused the motor to droop under it's own weight and let the fan start to eat the housing.

Sometimes some things cost more for a reason.

[Simon Chaddock]

Erfolg

HobbyKing do.

I have just bought this one. Cheap and cheerful but it does what I am looking for.

The problem is any thrust quoted will be the unit 'stand alone' in free air. By the time it is installed the thrust will be less but by how much will depend entirely on the nature of the ducting.

Originally In runners were used both to handle the revs and to keep the motor diameter down. Much cheaper out runners are now common in the lower power cheaper units.

[Erfolg]

Roger, you will certainly be correct in many instances, based just on cost. Yet it will not always be true.

If we were never to consider new products, or were just be guided by price, there would be no insensitive to manufacture or sell new products.

At the same time I appreciate your general thrust, that the duty is so severe, that well made, relative to duty products are a must.

It does seem odd, that for complete units where it would appear to be a selling point to state the performance as a differentiator that it seems to be missed.

[Erfolg]

Simon, that is very interesting, just the sort of info you would expect, to enable us punters to make considered purchases.

Just as Tony suggests on the TSR2 thread, there is not a lot of thrust, relative to the watts.

To get a good performance the airframe needs to weigh NOWT ! as they say, not sure where, although the weight is the issue, particularly, when considering the weight of Lipos of any capacity.