Thin Blade Propeller Profile Effeciency

Knowing that a long skinny wing is more efficient than a short fat one, I throw this question up for discussion:

Measure the surface area of a proven 2 blade rubber propeller.

Construct a 3 or 4 blade propeller of the same diameter, with the blades cut town to the same total surface area as the 2 blade.
(Propellers would also weigh about the same.)

Would the thinner profile blades move air more efficiently?
(requireing less rubber & longer run time)

Or would the surface area being the same, create the same drag even though the aspect ratios are different?
(Same amount of torque required for both... = no increase in efficiency)

How would you calculate pitch and blade profile for such a propeller?

What do you think, and why?

Note that this is asked knowing that in the past, single blade propellers with a counter weight were popular for Wakefield type models......

Steve

Hi Steve
Very briefly and very, very simply:
First part of any prop discussion must include a spec of what power you're using. Rubber power runs a slow prop and the rules are different from a gas or electric job.
Assuming a rubber prop, the next challenge is to get enough air moving in the right direction to provide thrust, given that relatively slow prop; at the same time, rubber props under 8"-9" go WAY down in efficiency as they go smaller, because they are dealing with less air. That's oversimplified, but I'll avoid the kinky details here--just remember, the smaller the model is, the crankier it gets because the air is less like compressible fluid and more like massless cottage cheese.
SO, by thinning down your blades to make a four-blade prop, you're effectively dropping into a smaller air regime too. You may not be moving the air that your two-blade prop did.
The other part: as your prop spins, part of its work goes into spinning a disc of air along with it. Hopefully the prop will be climbing out of this disc as it spins (thereby creating some lift, or thrust if you like), but you may need the wide paddles to get anything happening at all. Try it and see.
Michael

Thanks for the reply Michael,

Since this was asked in Rubber Scale, I assumed that rubber power was therefore naturally implied.

It was the effect of narrower baldes at low Reynolds numbers that I was inquireing about.

The interest is in the efficiency. Both 2 & 4 blade propellers would present the same surface area and diameter to the on comming air.

However, the narrow blades should have less drag for the same lift like the difference between high and low aspect ratio wings.

What I am reading in your resonse is that at these low numbers, the effeciency of the aspect ratios favors the Paddle blade.

By the way, I'm a retired high time helicopter instructor pilot.

The concept of flying forward out of rotor turbulance into clean air is the same with the rotor blades of a helicopter just like it is with a propeller. (We call it entering into translational lift)

Naturally, my interest is in a model in forward flight, not stationary.

I guess I'll have to set up a test rig where I can play with this under controlled conditions.

Any suggestions?

Thanks again,

Steve

Hi Steve
Helecopter question:
On helecopter blades, two to four, what effeciency rateing would there be in given same power settings? Or, does airfoil play more in blade effeciency?

Rubber scale,
So it's safe to say that scale prop blades would in fact work just as well on scale rubber models?

Denie

Hi Denie,

A discussion of Roterywing Aerodynamics can get quite complicated.

Briefly, you will notice that generally rotor blades are very long and thin, just like a glider wing (or U-2 wing). This is a very efficient lift profile.

There is a limit to how long they can be because of blade tip speed.

Higher speed helicopters tend to have shorter wider blades (To provide the same realitive blade surface)

A good example is the UH-1D Huey and the AH-1G Cobra. The Cobra blades are shorter and much wider than the UH-1.

This means nothing at a hover, but once you introduce forward speed, the aircraft airspeed is added to the speed of the advancing blade and subtracted from the retreating blade.

If both aircraft's blades turn at about the same RPM, the blade tip speed of the Cobra is less and allows it to have higher airspeed before the tip of the advancing blade goes supersonic and shakes the helicopter to pieces.

The Cobra is built for speed, while the Huey if a troop lifter. The narrower blades of the Huey can lift more weight than the wide blades of the Cobra with the same power. But it is not as fast.

2Blades vs 4Blades:
As engines evolved producing more power, they could transfer more torque to the rotor system. In Bell helicopters like the 206 & 212, the rotor diameter was already optimized for the aircraft, so to take advantage of the more powerful engines and keep the more efficient blade aspect ratio, Bell added more blade surface area by adding two more blades.

So, in this case the 2 bladed system couldn't take efficient advantage of the increase in engine power.... The 4 blade system can more efficiently use the higher power now available.

Helicopter airfoils:
Rotary wing airfoils are near symetrical so that they can autorotate when called to do so. Also, there is much less blade twist over the length of the blade than a propeller.

This is a very short explanation. There are several aerodynamic forces that I have not explained that also figure into things, but this is basically the gist of it. If unclear, I can go into more detail, but don't want to take up too much band space here.

Scale props????
Do you mean actual scale props with much less surface area to interact with the air, or a more "scale looking" multibladed ( ie 5 bladed late model Spitfire) propeller with the same diameter and total surface area of the 2 blade rubber prop???

You tell me. That is the question at hand.



Steve

Michael, how do you feel about two 4-blade 5 inch Henn props on a 22.5" scale job? Motor length could be 7 inches, and prop diameter is non-negotiable. The prototype flew with 4-blade plastic cup props of rather broad chord (at least it looks that way to Backstrom and me), so this should be better, no?
a.

Art wrote:Michael, how do you feel about two 4-blade 5 inch Henn props on a 22.5" scale job? Motor length could be 7 inches, and prop diameter is non-negotiable. The prototype flew with 4-blade plastic cup props of rather broad chord (at least it looks that way to Backstrom and me), so this should be better, no?
a.

It's way past pumpkin time for me, sorry--my life isn't conducive to having a hobby right now (and not much of a life either); so, brielfy again:
There's a good reason that the duration guys all use two-blade props, and occasionally one-blade even today.
I had a three-blade prop on my dime V14 Bf109, it flew but mostly limped around--while Art Collard's build from the same plan was a serious contender in Dime with a trimmed 6" Peck. And I broke my heart with a five-blade one on my Fairey Spearfish, & when I switched to a Tern Aero it just jumped into the sky. That was enough empirical evidence to convert me.
Your mileage may vary. Someone arguably can get a three- or four-blade prop to fly (Iron Mike swears by his spliced Czech Yellow ones), but it's pretty widely agreed that our models will generally perform better with what's been hammered out over the years. Hey, try stuff out. Maybe you'll like it.
Michael

Thanks Michael,

That is good information.

A counterballanced single blade propeller would only have it's own wake to fly through and be very efficient.

Also, all of the sufrace on one blade would allow a much larger diameter propeller to use with the same rubber motor as a smaller two blade. (1/3rd span of Wakefield??)

However, this would eliminate the use of scale landing gear, due to propeller size.

I'm looking to get the best performance with a scale diameter propeller.

Looks Like I'll be using a two blade paddle rather than a multi blade high aspect ratio prop.

Thanks for your input.

Steve