John,
Jep, your explanation seems to hold. Makes indeed sense when you think
about it.
Here some data I generated with my program.
I think the efficiencies shown are still a bit higher than in reality
because the airfoil data I used was for a Reynolds number of 5k, which
should actually be closer to 2k.
Kevin
2015-04-10 19:13 GMT+02:00 'John Barker' john.barker783_at_ntlworld.com
[Indoor_Construction] <Indoor_Construction_at_yahoogroups.com>:
>
> [Attachment(s) <#14ca45286dc678df_TopText> from John Barker included
> below]
>
> Nick and Leopold,
>
> Thank you for your interest. I have changed the Subject line to something
> more appropriate. I don’t think the blade shape of the test propeller is
> very interesting, Nick, but I give two attachments. You will notice that
> the section is quoted as Clark Y but if so the thickness to chord ratio is
> altered throughout the blade.
>
> I have to be away tomorrow but I will say more later.
>
> John
>
>
>
> *From:* Indoor_Construction_at_yahoogroups.com [mailto:
> Indoor_Construction_at_yahoogroups.com]
> *Sent:* 08 April 2015 20:11
> *To:* Indoor_Construction_at_yahoogroups.com
> *Subject:* Re: [Indoor_Construction] ECh Day 2
>
>
>
>
>
> Hi John,
>
>
>
> I think technical discussions are welcomed here. Perhaps, most people
> don't feel confident enough in their opinions to contribute them though, so
> it would appear as though they are disinterested.
>
>
>
> I would be interested in seeing the planform of the prop that they tested.
> I think this as implications for off-design performance for modern F1Ds.
>
>
>
> Regards,
>
>
>
> Nick
>
>
>
>
>
>
>
>
>
> On Wed, Apr 8, 2015 at 12:55 PM, 'John Barker' john.barker783_at_ntlworld.com
> [Indoor_Construction] <Indoor_Construction_at_yahoogroups.com> wrote:
>
>
>
> *[Attachment(s) <#14ca45286dc678df_14c99f56f6e96ca4_TopText> from John
> Barker included below]*
>
> 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
> [Indoor_Construction] <Indoor_Construction_at_yahoogroups.com>:
>
>
>
> *[Attachment(s)
> <#14ca45286dc678df_14c99f56f6e96ca4_14c936596d634f1e_TopTe> 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]
> *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 [Indoor_Construction] <
> 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
>
>
>
>
>
>
>
>
>
- application/x-ygp-stripped attachment: stored
Received on Sat Apr 11 2015 - 03:36:03 CEST