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Ps76
Topic Author
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Difference Between Propellor & Electric Hand Fan?

Thu Jul 08, 2004 2:30 pm

Am currently trying to figure out propellors  Insane Insane

If I stuck a small battery operated mini-fan (eg. http://www.ahkea.org/content/busmart/Photo/Robert%20Li/Mini%20Fan.%204.23.jpg) facing backwards on the front of a small model airplane it would push it forward and if the weight-power-aerodynamics etc were ok I guess it might even fly.

I guess my question is what's the difference between a propellor and a fan pointing backwards?? I see ga propellors seem to have much thinner blades than a fan but are they basically the same

Also, trying to get an idea of just how much power these things have, any idea how many hand fans I'd need to get the same push as a lycoming engine? or 4 turbofans on a 747 on takeoff Smile Smile??

Thanks for any help

P


 
QantasA332
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RE: Difference Between Propellor & Electric Hand Fan?

Thu Jul 08, 2004 4:54 pm

There really isn't a difference between your average fan and a propeller. They both 'push' air, one simply with the purpose of creating moving air and the other with the intent to produce a lifting force (thrust). To answer your question then, yes, a fan's operation and a propeller's operation are basically the same, though the blade design and other smaller details obviously differ a lot between the two. So, while you can theoretically propel a model airplane with a little fan and get it to fly, in reality the propulsion would be very inefficient and the thrust to weight ratio would be horrible enough that in most cases it wouldn't be sufficient for flight.

As for how many little fans would produce as much thrust as a small reciprocating engine or even a turbofan? Let's not go there...I doubt there have even been enough of those little buggers made to equal, at the very least, the latter engine type!  Laugh out loud

Cheers,
QantasA332
 
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mighluss
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RE: Difference Between Propellor & Electric Hand Fan?

Thu Jul 08, 2004 6:33 pm

Just a guess...

the narrower blades in a prop, are because they run at much higher rpm?

(so if you put a very large ventilator fan in a C152, may be you will have blades flying away in a very short time... Big grin )
Miquel.
 
SlamClick
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RE: Difference Between Propellor & Electric Hand Fan?

Fri Jul 09, 2004 12:53 am

A useful generality to keep in the back of your mind: Airfoils, like a fan or a propeller or a wing are more efficient when they impart a small movement to a large mass of air than when they impart a large movement to a smaller mass of air. Thus a helicopter uses less power in forward flight than it does at a hover.

Airplane propellers are thinner than your fan, both in chord and camber because their design favors the normal mode they are intended to be operated in. Their design is not biased to blow air around while the plane is stationary, it is intended to screw its way through the air at cruise speed. (since you spend most air time cruising) or, some favor climb modes, but always with some forward speed.

Really simpleminded generalities here, easily shot up, but not shot down, by aerodynamicists.
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QantasA332
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RE: Difference Between Propellor & Electric Hand Fan?

Fri Jul 09, 2004 8:44 am

A useful generality to keep in the back of your mind: Airfoils, like a fan or a propeller or a wing are more efficient when they impart a small movement to a large mass of air than when they impart a large movement to a smaller mass of air.

The main reason behind that has to do with the air movement itself. When a propeller-driven vehicle passes you, you feel a breeze (usually referred to as the prop's "slipstream"). That breeze is air that the prop set in translational and rotational motion, and associated with that motion is some kinetic energy. Where does the energy come from? It has to come from somewhere...

As it turns out, the energy originates right back at the engine/propeller, meaning some energy is lost in propulsion - and that means a decrease in efficiency (this is the underlying principle behind why propeller propulsion is generally more efficient than jet propulsion). So, while that roiled air is what you want from a fan, it certainly isn't what you want from a propeller. Accordingly, prop blades are designed with that increase-in-roiled-air decrease-in-efficiency phenomenon in mind to try to keep the roiled air to a minimum, while fan blades take advantage of, and want to maximize, that same roiled air. Am I making sense?  Nuts

Thanks for bringing that up, SlamClick, as it's an important point - which I completely forgot to mention.

Airplane propellers are thinner than your fan, both in chord and camber because their design favors the normal mode they are intended to be operated in.

...and also to reduce transonic flow effects at the tips.

Cheers,
QantasA332
 
Ps76
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RE: Difference Between Propellor & Electric Hand Fan?

Fri Jul 09, 2004 9:11 am

Hi,

Thanks for the help

I guess I can think simplistically as a ga prop as being a fan that's very directional, operates at high rpm and is efficient at "grabbing" and pushing air when moving at 100kts??

Now all I have to do is figure out this whole prop rpm / torque thing and what if anything it has to do with the gears on my mountain-bike!!

Thanks

P
 
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Starlionblue
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RE: Difference Between Propellor & Electric Hand Fan?

Fri Jul 09, 2004 11:27 am

I've done some electrical model plane flying. Less power and endurance than combustion but also less noisy and smelly, which means you can fly around in more places.

Anyway, electrical engines for model planes are a bit different from the fan in your pic. First of all, they are cylindrical and small so that they can fit in the fuse with a small cross section. See here for a pic of a typical engine: http://www.kyosho.com/accys/kyog1922.html

Their RPM is also typically higher than fans. As far as I know, most desk fans are well under 5000 rpm, while here we are in the 16000-28000 rpm range. Granted, you sometimes use reduction gears (hey, just like in a real plane  Smile) but still.

Finally, the props are more like airplane props. See here for pics http://www.kyosho.com/accys/kyoq0104.html. Thin blades with tapered edges for the reason stated by Slamclick and QantasA332. You want to push the plane, not the air.


Electric model planes tend to be underpowered (or battery life would go under the already low 10-20 minutes you get now), and the best application is motor assisted gliders such as the lovely Stratus http://www.kyosho.com/airplanes/kyoa1080.html. Note the foldable prop so you can land. Basically there is a tradeoff between power and battery life which hand held fans need not bother with.

So my advice is, buy real model airplane parts. They are built for this. Your fan seems much too small and heavy. As for the pusher prop idea, I don't see why not.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
lowrider
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RE: Difference Between Propellor & Electric Hand Fan?

Sat Jul 10, 2004 2:10 am

Price, unless it is an aviation approved fan.
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