RE: Props-More Myths Busted

RE: Props-More Myths Busted
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RE: Props-More Myths Busted - 8/18/2007 6:53:22 PM

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From: Rosh-Ha'Ayin, ISRAEL
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Hugh,

There is a certain eventual pressure difference, between the air directly behind the prop and directly in front of it...
This is essentially thrust...

We may be oversimplifying it, but any way you look at it, efficiency is essentially the eventual thrust divided by the calculated thrust and the 'plug number' is the actual flight speed of the plane; or if measured in a wind-tunnel, the speed of the air flow inside it...

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RE: Props-More Myths Busted - 8/19/2007 4:25:28 AM

blw

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That is a good way to put it, Dar.

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RE: Props-More Myths Busted - 8/19/2007 3:20:47 PM

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quote:

'plug number' is the actual flight speed of the plane;

They test props in a wind tunnel with no plane attached. They also test fans, including propeller fans, for efficiency and they never move a plane.

I think your statement is essentially correct, however it may give a slightly differant percentage if calculated that way. The actual equasion is THP/BHP or the theroritical power to move a propellor, over the actual HP. However the theroritical HP will change for a propellor that is static VS one with incoming air, thus the combined equasion with air velocity. If using advance ratio the TAS is devided by the RPM and diameter. But others simply use the differance in incomeing air or TAS VS propblast speed.

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RE: Props-More Myths Busted - 8/19/2007 5:28:01 PM

jaka

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Hi
Lot's of fysic theory here...but down to the nitty gritty...a small 10" fourblade prop isn't going to work as good as correct size two blade one. That's fact...won from experience. Every one that has been around this hobby long enough knows that!
So don't give the guy expectations that isn't going to be fullfilled. A too small 4 blade prop is also looking rediculous on any fighter.

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RE: Props-More Myths Busted - 8/19/2007 9:14:52 PM

RVM

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If nothing else, this is 100% true.

quote:

ORIGINAL: jaka

A too small 4 blade prop is also looking rediculous on any fighter.

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RE: Props-More Myths Busted - 8/20/2007 1:35:00 AM

blw

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The APC prop that I gave links to is an 11 inch prop for a .46 sized P-51.

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RE: Props-More Myths Busted - 8/20/2007 6:35:00 AM

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From: Rosh-Ha'Ayin, ISRAEL
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Barry,

This is what Tower shows for four-blade APC props.

The 11x6 size states (on the package) it is for .61 two-strokes, however.

The Saito .72 spun it pretty well.

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RE: Props-More Myths Busted - 8/20/2007 10:30:34 AM

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My mule engine and plane and thrust scale are now fixed. I have more props to test. Now I need to look for a small 4 blade to test on a .45. You wise guys that tout the great efficentcy of 2 blade need to tell full size aircraft manufactures to knock it off, using those bad multiblades. Take the 8 blade counter rotating turbo props. Shame on them--They shouldn't even leave the ground. Jaka where is your big prop test?

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RE: Props-More Myths Busted - 8/20/2007 12:50:01 PM

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Who said the full scale manufacturers are wrong? I don't think a single person has said that full scale is wrong in their prop choice. However most of the do a little better than picking a prop off the hobby shop shelf. Chances are that whatever they choose it will be a better than what 99% of modellers choose. I'm pretty sure I posted a link to Martin Hepperle's analysis of three blade props for F3D several pages back.

Maybe a long time full scale prop manufacturer can shed some light on the topic.

http://www.hartzellprop.com/engineering/engineering_faqs.htm

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RE: Props-More Myths Busted - 8/20/2007 7:03:50 PM

w8ye

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Was this interesting prop measuring tool mentioned on here before?

http://www.go-cl.se/propg.html

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RE: Props-More Myths Busted - 8/20/2007 8:05:38 PM

jaka

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Hi!
Speedster.... I know that fullscale props arn't two blade because then those Merlins and Alisons powered Mustangs and P-40 fighters must have very long gear legs and the prop diameter would put very large forces on the blades and hub.
The problem for enginers in those (WW2) days (and today in full scale racing) was/is to have a prop that could absorb the power from the engine and still not be too big in diameter.
That's why you see fullscale Sea Furys and Spitfire Mk 24 using 5 or more blades...and modern turbo-prop airplanes using even more prop blades. But in our model world every one that has been in this hobby long enough knows that for performance ...and speed (like Pylonracing) you got to have a two blade prop.
In Cl speed even one blade carbon props are used.

< Message edited by jaka -- 8/20/2007 8:07:01 PM >

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RE: Props-More Myths Busted - 8/20/2007 11:19:37 PM

blw

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The advantage of a single blade as in CL does not apply to RC or free flight as the P factor forces on the fuselage and vertical fin would cause problems. You don't really see this when on a control line, but you probably would in flight. A propeller with more than 2 blades would produce a smoother effect and dampen gyroscopic precession effects if they were felt in the first place. Vortices from the 'advancing blade' would not be a factor. There would be better vibration control. 3 and 4 bladed props are making a bit of a comeback in RC pattern flight today.

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RE: Props-More Myths Busted - 8/20/2007 11:24:21 PM

djlyon

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Lotsa single bladed props in free flight and they seem to work on F5D Pylon

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RE: Props-More Myths Busted - 8/20/2007 11:34:56 PM

blw

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I didn't know that. Interesting.

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RE: Props-More Myths Busted - 8/21/2007 12:58:31 AM

djlyon

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Look at page 130 Aug issue model aviation. See a pic of Gil Morris. Note the prop. Even though I know they are being used successfully in F5D and Free Flight I don't know if they are getting in the winners circle. I don't believe any have gotten there in F5D although I've seen them used.

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RE: Props-More Myths Busted - 8/21/2007 2:26:23 AM

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quote:

ORIGINAL: blw

The advantage of a single blade as in CL does not apply to RC or free flight as the P factor forces on the fuselage and vertical fin would cause problems. You don't really see this when on a control line, but you probably would in flight. A propeller with more than 2 blades would produce a smoother effect and dampen gyroscopic precession effects if they were felt in the first place. Vortices from the 'advancing blade' would not be a factor. There would be better vibration control. 3 and 4 bladed props are making a bit of a comeback in RC pattern flight today.

Number of blades has nothing to do with gyroscopic precession, it's all about mass. I'd like to see your explanation of the P factor theory of single blade props. Props with more than two blades do eliminate vibration during yaw and pitch movements. I can't recall the specifics, but have felt it several times test running electric motors in my hands. Has to do with the difference in inertia of the prop in the different positions through the yaw or pitch axis.

Three and four blade props are making headway in pattern for the very reasons full scale uses them.

We had plenty of logical discussions of multi blade props on the "other" forum.

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RE: Props-More Myths Busted - 8/21/2007 3:02:38 AM

blw