Tapio,
I mentioned this because of the many full size propeller tests that shew slightly higher efficiency and broader efficiency peaks as the P/D ratio is increased. I have always thought that trying to look at this on rubber models with varying torques and flying speeds would be daunting. I suppose that nowadays with the amount of rubber, prop speed and flying speed information that is available it could at least be tried at certain phases of the flight.
I give a couple of typical graphs below. Quite old but they were in a loose leaf folder, not the middle of a text book and therefore convenient to scan.
I have just noticed that one graph below (from Piercy) has J on the x axis and not P/D. (This is usual in technical books on propellers, the word Pitch is rarely used). Most of you will be familiar with the similarity of J and P/D but the following may help .
If you think of the Blade Angle triangle on a propeller in terms of distances moved then:
Tan of blade angle = P/pi D (r/R) where P is the distance travelled forward (pitch) and the bottom line is the distance rotated.
If you think of the triangle in terms of forward speed and rotational speed then:
Tan of blade angle = V/pi D n (r/R) (and as J=V/nd) = J/ pi(r/R)
Then equating the two expressions for blade angle P/D = J.
That is near enough for the present purpose but there are differences caused by blade angle of attack and inflow factors.
I am sorry but I also notice the other graph, by Glauert, uses his favourite lamda for the x axis which he terms the speed ratio (forward speed to rotational speed) and the relationship to J is obvious.
I think that is enough (too much?) for now.
John
From: Indoor_Construction_at_yahoogroups.com [mailto:Indoor_Construction_at_yahoogroups.com]
Sent: 03 April 2015 11:24
To: Indoor_Construction_at_yahoogroups.com
Subject: Re: [Indoor_Construction] ECh Day 2
On 3/4/2015 13:01, 'John Barker' john.barker783_at_ntlworld.com <mailto:john.barker783_at_ntlworld.com> [Indoor_Construction] wrote:
Thank you for the interesting information. Obviously with a smaller diameter the Froude efficiency will drop slightly but the efficiency related to P/D ratio could be improved, certainly in the later stages of the flight if the VP lowers the blade angle. Has this been discussed at all?
Interesting subject, and I have not seen any discussion on this. You are suggesting, that a smaller diameter prop would have a larger range of good efficiency (range in torque / prop RPM / output power that is). Could you explain how you drew this conclusion? (Not trying to criticize, but it is not obvious to me how this conclusion is drawn...)
-Tapio-
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Received on Fri Apr 03 2015 - 06:17:46 CEST