Kevin,
Some days ago I thanked Kang for the information on the smaller propellers and said this: “Obviously with a smaller diameter the Froude efficiency will drop slightly but the efficiency related to P/D ratio would be improved, certainly in the later stages of the flight if the VP lowers the blade angle. Has this been discussed at all?” Kang did not reply but Tapio asked about my claim that efficiency varied with P/D. I responded to Tapio and appended two graphs. [What has me completely puzzled is that no one has replied to my post but the two graphs have appeared at the bottom of posts by about three other people]
This was my note to Tapio, and the graphs are appended again below:
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 (perhaps too much?) for now but ask if necessary.
John Barker - England
From: Indoor_Construction_at_yahoogroups.com [mailto:Indoor_Construction_at_yahoogroups.com]
Sent: 07 April 2015 09:55
To: Indoor_Construction_at_yahoogroups.com
Subject: Re: [Indoor_Construction] ECh Day 2
I am wondering what makes models with smaller props do better. Since generally, a prop has higher efficiency with larger diameter. Is the small prop really the way to go, or is it a coincidence that the best times were flew with small props? Furthermore, the effective size of a prop can also be reduced by less chord or a smaller pitch, how would those measures relate to a smaller diameter?
Kevin
2015-04-05 16:49 GMT+02:00 mkirda_at_sbcglobal.net <mailto:mkirda_at_sbcglobal.net> [Indoor_Construction] <Indoor_Construction_at_yahoogroups.com <mailto:Indoor_Construction_at_yahoogroups.com> >:
Hi Aki-san.
I didn't take into account the knot, but this is what I calculated:
grams/meter
0.4 grams
Length
Loop
1.13
0.353982
13.93628
6.968142
1.23
0.325203
12.80325
6.401626
6.5-7" loops are what I've been flying with this season.
Still have no idea how you'd get that number of turns into 5/99.
Regards.
Mike Kirda
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Received on Tue Apr 07 2015 - 03:20:46 CEST