Ptarmigan

This might sound like a stupid question. Do EDF require an electric motor that is specialized for it or any electric motor will do well with EDF? The reason is, I saw a Speed 600 motor for $10 (Cermark), which is a good deal and will like to use it. I know higher the speed (200, 280, 300, 400, 540, 600, etc.), the stronger the motor is. I plan to make my next RC airplane and likely EDF foamie.

Steve Lewin

Sorry but you have it wrong. Higher Speed xxx numbers just mean the motor is larger and heavier. To be successful EDF usually needs small, light and very powerful motors. $10 Speed 600s are none of those. I'm afraid cheap motors and EDF don't really go together.

Anyway the shafts on these motors are all different. A fan will only fit the type of motor it's designed for.

Steve

Ptarmigan

Steve, you answered my question. At least I will not make a stupid mistake later.

bratcher

Actually, I have been looking at a lot of EDF lately, and it seems the real trick to EDF is high RPM. For this reason, it seems that most EDF use a modified version of some other motor to acheive this (usually fewer windings - that gives higher RPM). To that end, there are fans available that use SPEED 600 size motors - look at www.eam.net or www.hobby-lobby.com and search for GR1374 - that is a unit for a 600 size motor. I haven't seen one, but it looks cool - seems to use both impellers and stators, like a turbo jet. Anyway, there are fans available for 600 size motors. I would also suspect that Wemotec makes a 600 size EDF, and Vasa might make one as well...

Steve Lewin

Sure there are plenty of fans that use 600 *size* motors. You can get plenty of expensive 600 size brushless motors and some fairly expensive ballraced, modified car motors etc. I've just never seen a fan that will work with the very low cost $10 S600 size motors.

Steve

Ptarmigan

Steve, ever seen a cheap motor with a ducted fan? The reason I asked of cheap motors for EDF was that I noticed many EDF kits are quite expensive. I noticed that nacelle adds to the price of the EDF kit.

Steve Lewin

The problem is that ducted fans are by their nature pretty inefficient. That tends to mean you need as much power as possible to make them fly well. If you run a cheap motor hard enough to get enough power it has a very very short life.

If you have some idea of exactly what EDF you want there is probably a relatively inexpensive power setup for it but I don't think you'll get it down to $10. Probably the only cheap way to get into EDF is to get hold of a used all-inclusive foamy kit like the old Kyosho T33 or F16. With the stock motor they fly but not all that well.

Steve

Ptarmigan

I did some looking around for EDF. I have saw one EDF set for $30 from Wattage. They are Speed 400. You can also get the EDF blade and motor separately. The Wattage EDF motor cost $11. I could just build the component for the EDF out of foam. I know that is the case the other Wattage EDF airplane kits. The kits you mentioned I have read that the motor is weak. The Kyosho F-16 uses a OS .15 engine for the ducted fan.

bratcher

While it may be true that you can't slap any cheap motor into any EDF, you can get cheap fans from GWS, Wattage, and Wemotec (by cheap, I mean $50 or less). 7.41 ounces of thrust from a GWS EDF-75 (Speed 400 size motor) on 9.6V / 9.3A, for $20 or so, looks pretty reasonable to me.

Also, ducted fans ARE efficient - moreso than an open prop for the same size blade. GWS has published specs on their EDF units with and without the fan, and wihle the gain is not huge, there is definitely an increase in thrust from a ducted unit than from a non-ducted unit. I think there is a misconception that they are inefficient, due to the fact that you can get quite a bit more thrust out of (for example) a geared 400 size motor pushing an 11" prop than you can out of a direct drive 400 size motor turning a 3" fan. Going back to motors, ducted fans typically use motors that trade torque for RPMs, so they can spin the smaller fans faster. Either way, if you want a realistic looking "jet" for less than $2,000, ducted fan is probably your only choice.

Steve Lewin

I'm interested that you call that a misconception. Getting a lot less thrust for the same input power is precisely what I call inefficiency. What do you regard as inefficient if that isn't it ?

I'm talking about the amount of power required to fly a plane of a given size and weight. Prop beats ducted fan every time. That''s why you see so many jet look-a-likes with pusher props. They're a lot more efficient .

Still you're right that small EDFs and their motors and batteries are getting better and better. It's certainly now possible to get a jet flying well for reasonable money. Just don't try using a big heavy old $10 Speed 600 .

Steve

bratcher

Steve, you are correct - a large geared prop is more efficient than a small ducted fan. The caveat was that for the same size prop / fan, an EDF is (or should be) more efficient. Sorry if I wasn't clear there

Also, it's worth pointing out that not all planes can use larger geared props - some designs work better with smaller props, and some (like a Mig 15 or F-86) just beg to be ducted fans

St. Martin

Comparing props to ducted fans, I'm sorry to say, is like comparing apples to oranges.
The larger prop is only more efficient in lower speeds, then e.g., in a dive it's effeciency is lost as it is now acting as a brake.
Edf's come in different intended uses. The mini-fan is a high efflux fan, reaching it's highest efficiency w/ high power systems.
This makes good use of clean airframe design. In the proper installation, for it's intended purpose it can be a dog or a screamer.
The KY fan is larger, being a high thrust, med efflux, fan. Good for "dirty" type of jets.
The jet can be more effecient in the high speed portion but stinks in low speed accel. The prop will always win here.
In edf we can modify the intakes and outlets to achieve a very high efflux velocity. It would be a achievement to have a jet that's airspeed matches it's power efflux, in level flight. The prop could win here, also.
High speed props make it much easier to approach the eff. of the edf for high speed. But as in full scale, the prop eventually becomes a brake as in a dive the edf would be cleaner. The prop loses.
In a battle between the two, how could the sides be judged equal, in power etc.
I guess what I'm saying is that every thing said is right. Which taste better, apples or oranges?
Thanx Steve