Kevin,
I don’t think we are far apart in what we said but I notice, on this forum, that If someone buys a child’s toy aeroplane, puts in a bit of old knickers elastic and manages to fly across the patio the responses will go on for days but if someone ventures something technical like a multiplication sign or a mention of pi, they will soon be warned not to clutter the forum with rubbish; hence I probably sacrifice clarity for brevity.
I was trying to say that the numerical values of the P/D and J would always be similar on a propeller that worked because the geometric values of the propeller, Diameter and Pitch, must be related to the flying speed and the rotational speed. Personally I tend to think of the blade angle (theta) at three quarter radius (r/R=0.75) instead of P/D ratio and then P/D = 0.75.pi.tan (theta).
To reinforce what I said before I append another graph made by the NACA in 1939. They took a variable pitch propeller, 10ft diameter, placed it in a wind tunnel, rotated it at a certain speed (n), varied the wind speed in the tunnel(V). So tests were at various values of J = V/nD. They also tested at different blade angles at three quarter radius from 15 degrees to 60 degrees in 5 degree steps, hence 10 graphs in one. If you take one of the blade angles marked you can calculate P/D from the equation above. (eg. At 40 degrees, tan40 is 0.839 which, times 0.75pi is 1.98 P/D). I you now regard the J scale as a P/D scale and run up from 1.98 you hit the 40 degree graph near the peak. This will always happen because the P/D ratio must be a near match for J for an efficient propeller.
Martin, This graph answers your query about efficiency at higher P/D ratios but it now has me worried because I find it hard to believe that a P/D around 4 could give such high values. Perhaps I should also remind people that twisting the blade to do these tests means that none of the propellers are true helical and it seems to make negligible difference to efficiency. I am sure my ‘knickers elastic’ friends will be only to happy to say, ‘I could have told you that’!
John
From: Indoor_Construction_at_yahoogroups.com [mailto:Indoor_Construction_at_yahoogroups.com]
Sent: 07 April 2015 13:20
To: Indoor_Construction_at_yahoogroups.com
Subject: Re: [Indoor_Construction] ECh Day 2
P/D is a geometric property, telling something about the shape of the prop. Plotting P/D on the x-axis shows performance for fixed rpm and fixed airspeed but with varying geometry (eg. pitch).
J is the advance ratio, it relates the airspeed to the rotational speed. Plotting J on the x-axis shows performance of the prop over a range of rpms with fixed airspeed and a fixed prop geometry (P/D).
Some time ago I programmed for a course I followed a program that calculates the various parameters of a propeller based on Blade Element Momentum Theory. It produces reasonably accurate results (about 90%) for faster model aircraft. At least for relative comparison, it might be a handy tool, even for F1D type propellers. Most difficult is probably to have reasonable airfoil data (Cl and Cd of an x % arc) over a range of Reynolds numbers. And also some actual test data would be needed to validate the output of the program, I do not know if such data is available?
Kevin
2015-04-07 12:20 GMT+02:00 'John Barker' john.barker783_at_ntlworld.com <mailto:john.barker783_at_ntlworld.com> [Indoor_Construction] <Indoor_Construction_at_yahoogroups.com <mailto:Indoor_Construction_at_yahoogroups.com> >:
[Attachment(s) from John Barker included below]
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> [mailto:Indoor_Construction_at_yahoogroups.com <mailto:Indoor_Construction_at_yahoogroups.com> ]
Sent: 07 April 2015 09:55
To: Indoor_Construction_at_yahoogroups.com <mailto: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 Wed Apr 08 2015 - 09:55:37 CEST