RE: Props-More Myths Busted

RE: Props-More Myths Busted
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RE: Props-More Myths Busted - 8/4/2007 5:37:13 PM

blw

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I would counter that airfoil shape and angle of attack are far more important over blade length. Things are very different in rotary wing aerodynamics. Propellor pitch and diameter is a much easier subject. Tail rotors are more similar to propellors than main rotor systems.

Almost all blade systems use weights, not just Bell systems. This is a part of tracking and balancing the system.

The way that weights make autorotation easier is by retaining blade energy once pitch is applied to halt descent in the final phases of an autorotation, usually in two steps. All helicopters are rigged to allow a specified RPM for the rotor system during autorotational descent, regardless of tip weights. You have less stored energy with a low inertia system, but it will autorotate the same as a high inertia system. You really have only one shot at it with low inertia if you did everything right on the way down.

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Post #: 351

RE: Props-More Myths Busted - 8/4/2007 6:28:05 PM

Flyboy Dave

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....lets keep the discussion here in this thread to airplane propellors, shall we ?

There are other Forums that discuss helicopters at RCU.

FBD.

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Post #: 352

RE: Props-More Myths Busted - 8/4/2007 11:21:55 PM

blw

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I think we are covering a wide subject here, but we have strayed enough about the fringe relationships. There is a relationship between props and rotors.

< Message edited by blw -- 8/5/2007 1:39:57 AM >

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Post #: 353

RE: Props-More Myths Busted - 8/5/2007 8:29:32 AM

proptop

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Let's see...a "tie-in" might be a variable pitch prop like the small electric "foamy" profile jobs are using?
It's basically a rotor head from one of those small electric helicopters...but the prop blades are pretty much flat?
They also hover and transition to level flight...even fly backwards...

I was looking the the Osprey the other day...there's another tie-in...right?

(or are we done with all that?)

I'm just sayin' "it's all good"...a lot of good info...(I have learned a lot reading through all of this )

< Message edited by proptop -- 8/5/2007 8:35:18 AM >

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Post #: 354

RE: Props-More Myths Busted - 8/5/2007 6:57:19 PM

Sport_Pilot

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quote:

ORIGINAL: DarZeelon

quote:

ORIGINAL: Red B.

Efficiency = (propeller thrust * velocity) / engine power

With this definition, as airspeed approaches zero so does efficiency. At zero airspeed the propeller may provide a lot of thrust, but because it produces no useful work (moving the aircraft forward) all the energy output of the engine is wasted and thus efficiency is zero.

Red,

I was never saying propeller efficiency is a function of speed; just that it is stated at a speed.

Prop-fans of current designs reach their optimal efficiency of just over 80%, when they are traveling at a forward speed of 0.8 Mach... The advance angle, on which the efficiency is dependent, is not stated - just the speed.

Also, because engine power output at 100% efficiency can be stated as calculated force * velocity and dividing the actual thrust * velocity by this number would equal efficiency, what you wrote and what I wrote are exactly the same thing.

Because the velocity is the same below and above the dividing line, one could just divide the actual thrust by the calculated thrust and get the same ratio.

And it is true. When the plane is stationary, efficiency is indeed zero... you cannot calculate thrust from HP at zero speed.
The calcuated thrust would then be infinity; and when any finite number is divided by infinity, the result is zero.

Thrust is dependent on the diameter of the prop and on its advance angle... One of the reasons a helicopter rotor makes more thrust than a propeller, using the same amount of HP...

I think velocity is actually delta velocity, the differance in the velocity of the air going into the prop and leaving the prop. For a static run on the ground the velocity is the same as the prop wash velocity, but in the air it is the propwash velocity minus the aircraft velocity.

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Post #: 355

RE: Props-More Myths Busted - 8/5/2007 9:16:53 PM

DarZeelon

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quote:

ORIGINAL: Sport_Pilot

I think velocity is actually delta velocity, the differance in the velocity of the air going into the prop and leaving the prop. For a static run on the ground the velocity is the same as the prop wash velocity, but in the air it is the propwash velocity minus the aircraft velocity.

Hugh,

Where, going into the prop?

Air from the whole forward section is going into the prop...

Only air thrown back is going in a general rearward direction.

There is only one type of velocity in this equation and it is the forward moving speed of the object pulled (or pushed) by the prop.

And since output is force * speed, efficiency at a speed is the percentage of the calculated force, the propeller managed to convert into actual driving force.

< Message edited by DarZeelon -- 8/5/2007 9:17:59 PM >

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Post #: 356

RE: Props-More Myths Busted - 8/5/2007 11:36:24 PM

gkamysz

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What is the current topic of discussion? I'm trying to follow along but I don't even know who it trying to prove what.

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Post #: 357

RE: Props-More Myths Busted - 8/5/2007 11:55:43 PM

Flyboy Dave

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....right now it's a battle to see who can drag the biggest, fanciest words out of Google search....

....with extra points given for fancy footwork, and degree of difficulty.

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Post #: 358

RE: Props-More Myths Busted - 8/17/2007 4:59:03 PM

jmead18h

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I hate to bust in and be off subject, but I'm looking for a scale, classic look on my 46 size P-51. I'm putting an OS 91 4 stroke on it and I really want to put a 4 blade "climb" prop on it. Any suggestions?

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Post #: 359

RE: Props-More Myths Busted - 8/17/2007 6:12:42 PM

blw

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Here is a good APC 4 bladed prop: http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LX1636&P=7

This one may be a good choice. I couldn't find any Graupner 4 bladed props. The APC props usually outperform Master Airscrews and I didn't find any of those either in 4 blades.

Here is a Bolly: http://www.bolly.com.au/images/12x6-4Bprop.GIF. Those are generally accepted as being good props, but these get expensive.

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Post #: 360

RE: Props-More Myths Busted - 8/17/2007 7:36:56 PM

jmead18h

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thanks a billion

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RE: Props-More Myths Busted - 8/17/2007 9:44:23 PM

XJet

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quote:

ORIGINAL: blw

Here is a good APC 4 bladed prop: http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LX1636&P=7

Not very scale-looking for a P51 though is it? :-(

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RE: Props-More Myths Busted - 8/17/2007 11:48:06 PM

blw

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I didn't find any others. Dying them black may help appearances. The Graupner shape would have looked better if in 4 blades.

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Post #: 363

RE: Props-More Myths Busted - 8/18/2007 2:09:36 AM

Flyboy Dave

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quote:

ORIGINAL: jmead18h

I hate to bust in and be off subject, but I'm looking for a scale, classic look on my 46 size P-51.
I'm putting an OS 91 4 stroke on it and I really want to put a 4 blade "climb" prop on it. Any suggestions?

....here's a nice lookin' prop, and only 10 bucks.