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RE: Cool - 9/17/2003 6:37:50 PM   
Elwyn



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Joined: 12/22/2002
From: Broken Arrow, OK, USA
Status: offline 		I think there is a small counterweight on the opposite side of the hub.

(in reply to encan)
       Post #: 26

RE: Cool - 9/17/2003 7:09:51 PM   
William Robison



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Joined: 11/10/2002
From: Mary Esther, Florida, FL, USA
Status: offline 		A propellor blade is far more efficient traveling in undisturbed air. This is why a helicopter requires more power to hover that to climb straight up. This relative inefficiency is caused by the trailing blade operating in the air disturbed by the leading blade. And of course, every blade of a prop with two or more blades is both a leading and a trailing blade depending on your point of view.

This traveling in undisturbed air is one of three definite advantages of a single bladed prop. The second advantage is being able to use a larger diameter, the larger disc area also conveys greater efficiency.

The third advantage is one people don't think about until they start flying speed, with belly landings on the paved runway. No matter how carefully you position the prop on the engine, you are going to drag one on the ground every now and then, and this will ruin the prop for max speed runs. Might still be OK for practice flights after you rebalance it. With the single blade set to come on compression in a vertical position you'll never sand your prop on landing.

The two major disadvantages are the counterweight, it has to be a lot heavier than the second blade would be since it's very close to the hub, and all the thrust exerts a side load on the crankshaft bearings, increasing the wear..

Hope this satisfies your curiosity about the props with one blade missing.

Bill.

PS: For my general flying I think a two blade prop has a blade missing - I like three-bladed props. wr.

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(in reply to mgreb)
       Post #: 27

RE: Cool - 9/17/2003 8:08:21 PM   
Sport_Pilot



Posts: 7565
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline 		Hate to say this cause usually I agree with Bill, but the effeciencies of a single blade prop has nothing or little to do with disturbed air. As the aircraft is moving foward or even on the ground the air that the prop has disturbed is moving behind it, that is one reason why you have to add right rudder on take off, the disturbed air is spiriling around and hits the top left side of the rudder turning the plane to the left. Since the disturbed air is behind the plane the next prop blade is in clean air no matter how many blades are used. The reason less blades are more efficient are two fold. The drag of the blade tips are very large, more blades more loss, the area where the prop goes into the spinner is another smaller loss that is affected by the number of blades. In fact if you use a ducted fan this loss is greatly reduced and you can increase thrust by adding blades right on top of each other such as used on jet turbine engines. The other reason is as downunder described, the more blades the smaller the prop arc, the area of the prop arc is more important than the blade area for generating thrust. More area is more efficient. However sometimes when going really fast, such as around 400 hp you have to reduce the disc diameter to gain more air velocity and add more blades, though most multi blade props are used simply for prop clearance.

(in reply to William Robison)
       Post #: 28

RE: Cool - 9/17/2003 8:38:32 PM   
William Robison



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Status: offline 		Hugh/Sport:

Your points are valid. I tried to stay as simple as I could. But I did address the radius/diameter in talking about the propellor's disc area.

And with sufficient airspeed even a 30 blade prop would have each blade entering undisturbed air. You will notice please, my counter example was a hovering helicopter with essentially zero airspeed. And when pitched for max air speed the inefficiency has a very bad effect on the airplane's accelleration, and running out of fuel before comong "On the pipe" isn't uncommon, all because the prop doesn't work very well at low speeds.

A good example is my old "Royal Rodent." On a paved or smooth clay circle it will take off, and run 140 mph in the air, using a 9x8 "Rev-Up" prop. On grass it wont take off with that prop. I have to switch to a 10x6 just to get off the ground.

Bill.

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       Post #: 29

RE: Cool - 9/18/2003 2:23:25 AM   
Jim Thomerson



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From: Austin, TX, USA
Status: offline 		Interesting to note that the top four finishers in Open CL Precision Aerobatics at the last Nats used three-bladed props. Is there something about disc density which hasn't been mentioned here?

(in reply to William Robison)
       Post #: 30

RE: Cool - 9/18/2003 3:09:26 PM   
gcb



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From: Port Ewen, NY, USA
Status: offline 		Do you suppose anyone has tried a ducted fan CL speed model? I know they probably couldn't enter a contest but I would think someone has experimented with them.
I remember someone using a fan to supercharge their air intake. This was back when speed flyers were experimenting with tuned intakes (late 50's?).

George

(in reply to Sport_Pilot)
       Post #: 31

RE: Cool - 9/18/2003 4:17:19 PM   
downunder



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Status: offline 		Jim...that's something that's been bugging me for some time now. I seem to recall there was some esoteric advantage to using a 3 blade in stunt (other than ground clearance). I think it had something to do with when turning a hard corner the thrust distribution wasn't affected by the slightly different blade angles presented to the air by the upward travelling blades compared to the downward travelling blades due to the angle of attack of the model during the turn. I tried a search on SSW some time ago but I'm guessing I didn't put in the right search term

(in reply to mgreb)
       Post #: 32

RE: Cool - 9/18/2003 7:29:57 PM   
William Robison



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Status: offline 		DU:

What comes to mind for me is the decrease in the precessive (gyroscopic) forces exerted, and this is due as much to the smaller diameter as anything else.

But I don't know if this was/is a consideration.

Bill.

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       Post #: 33

RE: Speed Record Video - 9/19/2003 2:25:31 AM   
Elwyn



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From: Broken Arrow, OK, USA
Status: offline 		Finally got around to viewing the video using the computer at work. Holy Cow!! One lap every 1.15 seconds or so. I've seen a little racing before but never have been at a big contest where they were flying speed. Hope to see jet speed flown one of these days although I don't think they are as fast as this little speedster. The sound is supposed to be unbelievable though.

(in reply to Bud Morrison)
       Post #: 34

RE: Cool - 9/19/2003 7:00:27 PM   
Clean



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Joined: 4/19/2002
From: Parkville, MO, USA
Status: offline 		Woweee that sucker really jumps up when it hits the pipe doesn't it? Last time I heard such a difference was when a certain dirt bike came up on the pipe when I wasn't expecting it, and I have the plate in my hand to prove it!

Aspect Ratio is why one bladers work better than multibladers, just like in Sailplanes, High aspect ratios deliver maximum lift for minimum drag.

3 bladers on PAMPA ships? Lets remember what they are dealing with, and I am going on speculation here but. They are dealing with a power band, delivered at a certain rpm on a certain pipe with certain fuel. This combination probably dictates a maximum diameter, multiple bladed prop to absorb and deliver the power at the correct rpm so that the engine can get up and down on the pipe at the ideal flying speed. Things evolve and everythgin is a compromise. I've no doubt that if the proper power band could be hit with a 2 blade prop, that'd be the way everyone would be going, it's cheaper and easier to install, trim and balance than a 3 blader. But I bet the wash from a multi blade prop is also more constant instead of the choppyness of a lesser bladed prop.

Or it could just be that a 3 blader looks cooler.

Aspect Ratio, that's the key though. Next is the Tip Vortices that was mentioned before, only one set on a one blader.

(in reply to gcb)
       Post #: 35

RE: Cool - 9/19/2003 7:06:33 PM   
Sport_Pilot



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quote:

Aspect Ratio, that's the key though.


Most 3 blade props have the same or nearly the same aspect ratio as the two blade props.

(in reply to Clean)
       Post #: 36

RE: Cool - 9/19/2003 7:58:53 PM   
Clean



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From: Parkville, MO, USA
Status: offline 		Yes, but a 3 blader with the same aspect ratio as a 2 blader loses more, whats more, if you swap from 2 to 3 blades the standard practice is to go down in diameter, lower aspect ratio effectively. Remember, air molecules do not get any smaller. With the same power, with less props, you get to have a longer prop, higher aspect ratio. Oh sure, you could have the same diameter prop and make it one wide blade, but they don't work quite as well due to the tip vortices covering a larger portion of the blade. I suppose it's a chicken and egg thing. Induced drag is what you are trying to get rid of, you do that by going to a more efficient wing, higher AR wings, as long as you can get on your airplane for the other aspects of the design, are what you go for. This is why the Corsair has those bent wings, trying to get as big a diameter prop on it while keeping it out of the dirt, and there is just a bunch of horsepower in that big ole Radial, more than when it started out. Happened on the Mustangs too, that's why with the Allisons you saw the skinny 3 bladders and on the later models with more power they went to those big ole paddle sized 4 bladders, gotta suck up the power.

Least that's what the videos and text said when I was in college, but, it was Tech college so I'm no aerodynamicists, they could've be dumbin things down fer us simple folk.

(in reply to Sport_Pilot)
       Post #: 37

RE: Cool - 9/24/2003 8:17:58 PM   
big max 1935


 

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From: huron s.d.
Status: offline 		Back in the 30's there was a single bladed propeller used on J-3's & other light planes , manufactured by the Everell Propeller Co. Increased speed , climb & fuel economy . It was quite unique as it was pivoted on an angle that automatically changed pitch in relation to power & speed . Be a real mind bender at a scale meet ! MAX H.

(in reply to mgreb)
       Post #: 38

RE: Cool - 9/25/2003 4:00:27 AM   
Clean



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Joined: 4/19/2002
From: Parkville, MO, USA
Status: offline 		No doubt you've all seen this single blade prop on this World Record FAI ship. The link has been posted everywhere, but if you haven't seen it, it's a pretty cool video.

http://www.flyrc.btinternet.co.uk/record/control_line.htm

(in reply to big max 1935)
       Post #: 39

RE: Cool - 9/27/2003 6:13:48 AM   
littlefly



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Joined: 1/20/2002
From: Crawfordville, FL, USA
Status: offline 		Hey Plane:
Your avatar looks like most of the guys around here after they've lost one!
Haw.
littlefly

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littlefly
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