Electric VS IC Propellers

[SkynetAir]

I am fairly new to flying RC planes, my first two have been the Wattage Piper Cub and the MPX PicoCub. After quite a few crashes and repairs, I seem to have got the hang of it. Now I was thinking of getting either the Multiplex Twinstar, or even the Cargo for my next plane. I noticed that these planes have 2 bladed propellers, and in my opinion, they would look better with a 3 or 4 blade prop. The problem is, I cant find any so called electric props, however there are some nice IC ones. So my question is, what is the difference!? They are both made of plastic, maybe the IC ones are a tad heavier, but surely if ballanced, that wouldn't matter??

Thanks

Andy Peaple.

My flying experience to date:
http://www.peaps.btinternet.co.uk/flight/flightdiary.htm

[Martin Irvine]

The fewer the blades the better. Each blade disturbs the one behind it so the best prop would be a single bladed version, (often done in free flight competition). Full scale aircraft use them to load down the engine without having to resort to enormous props.

Gas props are heavier to deal with the shocks involved. They are also built for higher RPMs - which is why the purpose built electric props usually look different as they are designed for lower speeds.

Cheers,

Martin

[Jim Finn]

Hi.........I use a three blade electric I got from Hobby Lobby. it is a 9X8 I think and is on a speed 400 geared.

[Ben74]

Originally posted by Martin Irvine
The fewer the blades the better. Each blade disturbs the one behind it

i have also read that two bladed props are more efficient than multi bladed. however, my understanding is that the larger blade size is what accounts for the efficiency, due to some complex physics having to do with the density of air relative to blade size. a two bladed prop has larger blades than an equivalent multi bladed prop.

the blades disturbing the air for the ones behind it doesn't quite make sense to me. when a prop is moving forward through the air, it is corkscrewing through the air. the blades never encounter turbulence from each other, unless the model is not moving. is this not correct?

sorry, didn't mean to get off topic. i think some of the differences between gas and electric props are weight, rpm limts, and fuel resistant plastic.

[SkynetAir]

Ok. If 2 bladed props are better than multi-bladed ones, then why do most real planes have 4 bladed ones?

So, it wouldn't matter too much if I used an IC prop on a speed 400 gearbox setup then?

Andy P.

[Andy W]

Because most full-scale airplanes don't have an option of extending the landing gear to get enough prop clearance to swing a 2-blade. Ground stability would be a problem.
A great many full scale airplanes have 2-bladed props (all of the low-end Cessna line, for example)..
..a

[Jim Finn]

Yes more than 2 bladed props are used to use available power without loosing gound clearance

[stuartaw]

Originally posted by APSystems
So, it wouldn't matter too much if I used an IC prop on a speed 400 gearbox setup then?
Andy P.

It won't matter much. The gas prop will be heavier, and slightly less efficient (it's designed to survive some nasty vibration from a single cylinder engine)

Small 2-blade masters and Graupners work perfectly OK. I wouldn't really recommend a 3-blader. IIRC the prop on the cargo gear-sets (suitable for cargo for twinstar) is a 7*6.5 In three blades this would be about 6*6. I don't recall seeing any gas 3-bladers this small.

What scale people often do is make a static prop for looks on the ground and a separate flying prop for flight.

Stuart @ Slough RCM

PS - just read your web-page. Interesting! One comment - the batteries you are using for the Pico-cub are the AAA pencells, yes?
You said it needed more power even with 8 cells. The plane will fly OK on 6 Nicads, so I suspect your pencells are not up to the job. I've had very mixed results with these 700AAA cells, but even the best (Sanyo ones with a plain green sleeve) struggle to produce the 7A or so that's probably needed by the Pico cub. It sounds like you can solder OK, so how about rearranging your Nicad pack into the correct shape to fit into the Pico-cub battery box? 7*600 Nicad should easily power the Pico-cub.

[Ron van Sommeren]

Originally posted by APSystems
... they would look better with a 3 or 4 blade prop. The problem is, I cant find any so called electric props, however there are some nice IC ones.
David Radford at www.aircraft-world.com stocks 'Varioprop' propellors (http://www.aircraft-world.com/prod_datasheets/varioprop.htm). They come in a lot of flavours: 2, 3 and 4 bladed, 2 shaft diameters, 3 blade types. And they all have adjustable pitch!

[Jim Finn]

Electric props are undercambered unlike the "back flat" ones for the slimers. This makes them work better at the lower rev's we use in electric powered flight.

[Tom Hunt]

Originally posted by Jim Finn
Electric props are undercambered unlike the "back flat" ones for the slimers. This makes them work better at the lower rev's we use in electric powered flight.

Too generalized! The "BEST" electric props (100+ watts) are the APC Electric series, They are only very slightly undercambered at the root and are basically flat bottomed out where all the work is done. The important thing is they are not not baseball club thick like a "slimer" prop! They have relatively thin "true" airfoil sections on them.

Early "undercambered" props like the master airscrew wood series (and now the glass filled series) are fine for less than 200 watts input and rpms below 6000.

I have many "direct drive" electrics that turn props faster than glow engines will. You'd NEVER want to run a significantly undercambered prop at those sorts of rpms!

Pick the right prop for the right job.... but when it comes to electric models.... stay away from anything you'd put on a glow model! You'd get surprisingly better performance from a prop designed for electric flight!

Regards
Tom Hunt

[vintage1]

Originally posted by Martin Irvine
The fewer the blades the better. Each blade disturbs the one behind it so the best prop would be a single bladed version, (often done in free flight competition). Full scale aircraft use them to load down the engine without having to resort to enormous props.

Gas props are heavier to deal with the shocks involved. They are also built for higher RPMs - which is why the purpose built electric props usually look different as they are designed for lower speeds.

Cheers,

Martin

I don't think its that simple. Use of multibladed props can be not only a way to get the power down with a short undercart, it can also be a way to absorb massive amounts of horsepower without the blade tips reaching transonic speeds. Modern turboprops use 4 balded props when they could easily use two. I suspect that there is some optimal number of blades and gearing that reflects what they need to do. Of course they are variable pitch.

I was very surprised on going for a joyride in a lightplane, to see that the RPM almost never varied from no power at all (single engine climbout simulation) to full takeoff power - all the throttles seemed to do was allow a coarser pitch to be used!

I also suspect that the effects of multi-balde interference is much less at lower RPM - i.e. geared setups.- and at sensible flying speeds where 'fresh' air is hitting the prop blades rather than 'stale' air from the preceding blade.

This may explain why rubber models used a single large blade - very low flying speeds.

My giut feeling is that for high RPM direct drive and slimer type applications, multiblade props will not work as well as twin, but for low RPM geared setups, they should not be at very much disadvantage.

Has anyone actually done any real tests?

PS, I can confirm that slo fly APC props do - as others have reported elsewhere - generate more thrust and draw more current than the equivalent staright 'E' props - at least at modest RPM.

[steve lewin]

Originally posted by vintage1
I also suspect that the effects of multi-balde interference is much less at lower RPM - i.e. geared setups.- and at sensible flying speeds where 'fresh' air is hitting the prop blades rather than 'stale' air from the preceding blade.

This may explain why rubber models used a single large blade - very low flying speeds.

Ah but what is it that explains why C/L speed models also use single blade props at 36,000 rpm (not a typo) and 200+mph ;) ?

Steve

[Earthchain]

For what its worth. Here are some very very general things I have learned about propellers.

1. The propeller, power plant and airframe should be designed as a system. (Does this make sense)

2. A higher aspect ratio blade (think sail plane wing) will produce the same amount of lift while generating less drag (induced drag) than a lower aspect ratio blade.

3. The actual airfoil used on a blade will affect the induced drag and efficiency.

4. Each blade will also create parasite drag. A three bladed prop airplane will have more parasite
drag than a two bladed plane. A single blade even less.

5. Propeller design is more an art form than a science.

6. Never get in a debate over propeller design.

These are very general, but they help me understand the two blades are better than three concept.

Maurice

[DeaninMilwaukee]

I have used an apc 7x7 gas prop on several of my sp400 geared planes with very good results. The biggest liability is that it is indeed a bit heavy.