Re: prop efficiency [2 Attachments]
John,
Thanks for the detailed response. I appreciate it. It will take a while for it all to sink in and I do not have specific reason for the question, it just seemed like something I should understand.
Gary
-----Original Message-----
From: John Barker <john.barker783_at_ntlworld.com>
To: Indoor_Construction <Indoor_Construction_at_yahoogroups.com>
Sent: Wed, Feb 12, 2014 11:14
Subject: RE: [Indoor_Construction] prop efficiency [2 Attachments]
[Attachment(s) from John Barker included below]
Gary,
As regards question one there is no direct relationship between rotational speed and efficiency but perhaps there is a little light. One expression for propeller efficiency is( J/2π)(kT/kQ). The kT and kQ are the thrust and torque coefficients which I won’t go into here. The term J is common in propeller literature and is known as the Advance Ratio and is equal to V/nD where V is the forward speed of the aeroplane, n is the rotational speed of the propeller and D is the propeller diameter. You will see that V divided by n gives the distance travelled forward for each revolution of the propeller, hence the term ‘Advance Ratio’. A little thought will shew that if the rotational speed is high relative to the forward speed then J will be less and efficiency will be lower.
Turning now to question 2 and variation of efficiency with pitch. Blade angle and Pitch are really just two ways of describing the same thing; low blade angle=low pitch, high blade angle=high pitch. Just to set some limits on efficiency, imagine setting the blades in the hub at zero degrees (zero pitch). The prop will spin round at high speed generating no thrust whatsoever so the efficiency will be zero. Now insert the blades in the hub at 90 degrees (infinite pitch). Now the prop will struggle round giving a lot of torque but no thrust at all so again zero efficiency. So as the extremes give zero efficiency it is a good bet that the best pitch should be somewhere in between, and such is the case.
All that needs to be done is to set the blades at an angle such that the relative wind from the forward speed of the aeroplane and the rotational speed of the propeller meet the blades at a reasonable angle of attack. In the diagram below the bottom line of the triangle represents to rotational speed, the vertical line at the right, the forward speed and the sloping line combines the other two to give the speed and direction of the air approaching the propeller.
Now without a lot of experimentation or calculation you will not know the forward speed or the rotational speed so here is some guidance from people who have done it all before. P/D ratios below 1 are usually inefficient. Efficiency increases with P/D ratio up to a P/D of 2 or even a little more. Another advantage of a high P/D is that as the P/D increases the peak of the efficiency curve becomes less sharp so that the propeller is more efficient over a wider operating range.
Finally I don’t know of any good reason for washing out the tips of propellers and indeed, on VP propellers I would have thought it could be a problem unless checked carefully.
John Barker - England
From: Indoor_Construction_at_yahoogroups.com [mailto:Indoor_Construction_at_yahoogroups.com] On Behalf Of Warthodson_at_aol.com
Sent: 11 February 2014 19:54
To: Indoor_Construction_at_yahoogroups.com
Subject: [Indoor_Construction] prop efficiency
Assuming a helical fixed pitch indoor free flight prop:
1. How does prop efficiency vary as a function of prop RPM?
I.E. Is a prop more efficient during high torque launch, climb, cruise or decent?
2. Is possible to explain in simple terms how prop efficiency is affected by pitch?
I.E. At a given RPM is there an optimum pitch? Or do I need to be more specific?
3. Has it become generally accepted practice to "wash out" a typical indoor prop?
Gary Hodson
Received on Wed Feb 12 2014 - 11:15:11 CET
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