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Author Topic: Another Gowen LPP  (Read 6739 times)
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piecost
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« Reply #50 on: October 12, 2016, 06:34:02 PM »

Hi Fred,

Thanks for your compliment. I consider the major contribution to my performance was that the venue for the competition was my home site and thus I could practice every month, concentrating on LPP to the exclusion of other classes. This gave me ample time to match the rubber to my propeller, relative to those who could only attend the competition or made ocasional visits.

I enjoyed designing a new propeller but have to admit that the assumptions for the inputs were not well defined and much care was needed to give an output that looked similar to well proven designs. I used Hepcat's Prop Picker with a selected pitch and chord to give the power and rpm and a MIL spreadsheet to derrive the chord and pitch distribution.

Whilst I have a method for determining the MIL propeller shape; I must admit to not seeing the underlying the maths. Can you provide a copy of Dr Larabee's original paper? I would be interested in playing with your propeller design code and seeing what it made of my propeller. Can you prove a more explicit instructions on how to download the Lazarus Pascal? I am rather a dummy at this sort of thing.

I found your May 1999 Rubber Turns, Torque Calculator very interesting and adopted it for my own motors. So, thanks for taking the time to publish such tools.

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Hepcat
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« Reply #51 on: October 13, 2016, 08:14:50 PM »

Piecost,
Belated congratulation on your double Nationals win.  That took some dedication.  Also, as so many others have said, thank you for the detailed information you pass onto the rest of us.

Do you have all the Larrabee information that you want?  I have the 1979 NFFS Sympo (16 pages covering the subject in English and French!) which has several descriptive pages and an appendix with 21 numbered calculation steps.
I also have the 1977 Sympo in which Larrabee had an 8 page paper talking about minimum loss propellers in a more general way and, finally: 'Propeller Design for Motor-soarers' which he gave as a NASA Conference report in 1979.  This is an 18 page report covering the subject in a more technical manner.  As a matter of interest I do have a copy of Sydney Goldsteins famous 1929 paper which proved Betz and Prandle were correct but I don't recommend it, I was lost by the end of the first page.

I was very interested when you said that you had a method for designing an MIL propeller but did not see a clear connexion with the underlying maths because that is exactly how I am.  I know my mathematical abilities are slipping away with age.  Not that they were ever more than was needed for basic design engineering but I can look at an expression nowadays and it just appears as a jumble of symbols. It can be very upsetting at times.

Strangely I cannot pass much comment on Larrabee's two Symposium papers because I have never used his approach.  Once, when I changed jobs, I had a long break from modelling, and during that time the Sympos started.  When I eventually saw one I wanted all the others but it took some years to get them second hand.  By the time I saw the Larrabee ones I had other methods I did not want to change.
John

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frash
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« Reply #52 on: October 14, 2016, 08:20:53 AM »

Thanks Hepcat and piecost,

I already had the Larrabee paper scanned from the 1979 NFFS International Symposium so I emailed it to piecost yesterday. I have a paper copy of the 1977 Sympo paper somewhere but have not yet found a scanned copy here. Perhaps for John to send the 1977 paper to piecost would help the most. However, this is really not my call.

Interest in Larrabee and most other methods from you two and others is very welcome.

Thanks to you both and to many others for making HPA so strong, particularly to the guys that run HPA. Ratz again properly sorted out my attempts to post corrections and delete old versions.

Fred Rash
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piecost
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« Reply #53 on: October 14, 2016, 10:08:06 AM »

Hello John,

Thanks for your congratulations.  By, the way; I also came third with your Bar Fly design in Legal Eagle, see link:

http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=16906.0

Fred kindly sent me the 1979 NFFS Symposium Report, but I will appreciate it if you can send me any others.  I still read your propeller articles from time to time and appreciate their clarity. It is very likely that I will get bogged down with Larrabee's maths as it is a bit too much like work and I'd rather spend my modelling time using my hands rather than my brain. I'd like to have a play with Fred's programme if I can get over the slight hurdle of installing the code and getting it running.

I understand that the Larrabee propellers revolutionised human powered flight, enabling the Gossamer Albatross and the MIT Daedalus to make long distance flights. But, I don't have a feel for its impact on rubber powered duration models. Was it a flash in the pan, fashonable for a while and then only considered by aero/programming enthusiasts? Or has it continued to influenced propeller shapes, even if poeple don't remembering it doing so. A successful concept may be adopted long after the oringal rational is forgotton?

Or do other constraints render it impractical for indoor duration? Such as:
  Only working fully at a design point, not over the motor run; is it still good off-design?
  Useful for variable pitch propellers?
  even such things as a propeller tip radius being too tight to bend the propeller outline around (not a problem for sheet LPP props)
  contraints on pitch distribution such as tip washout to prevent seperation at lauch

I suppose that human powered flight is the ideal application of MIL propellers since the flying speed and power requirements are constant and efficiency is all important.
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piecost
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« Reply #54 on: September 19, 2018, 04:22:04 PM »

I am pleased to report that I won gold in LPP in the 2018 UK nationals. After a break of 2 years both models were still in trim! In the single practice session before the competition I employed the same long, heavy motor that I had used in the Brabazon hanger under 56" beams in the Eton sports hall with ─░ts 28' beams:

Jun 16, 2.54g/m x 22" & 2.88g

This was combined with a similar propeller as employed before, but with a 2 Rod Gowen hub and the pitch reduced from 22' to 17.5. The practice session yeilded a near 8 minute flight so I was confident. This was the first time I had steered a model using a roach pole. This was a skill I
not developed in the 300' by 300' floor of the Brabazon hanger.

I spent the first day of the competition becoming very frustrated. I found that the motor bunching badly yielding fluctuating torque levels. More importantly, my digital torque meter was drifting by 0.10g. Since it measures 10 oz.in per gram; the torque levels were varying by a massive 1 oz.in. This was causing my model to have alternate flights refusing to climb or bashing into the roof. Indeed; I lost my best model on top of a roof beam. My trimming flights put up good times but the first two competition flights yielded under 5 minute tines. My third of 6m0s minutes was half way down the field of 11 entries with the best two models reaching around 7 minutes. On the second day I resorted to sharing a torsion wire torque meter and things became more.orderly. It was frustrating having to adjust the length of the winder to meter to match my model prior to each flight and not being able to stretch wind the motor hard tor fear of breaking the shared meter. But I found the 1g.cm scale more conducive to fine adjustments and crucially the reading did not drift.

I changed to a thinner and shorter motor to eliminate the bunching problems:

Super Sport, 2.34g'm x 16 1.97g

This worked well so made up 18', then 20 motors without problems. My third flight gave an improved but uncompetitive 6m0s, whilst the 4th suffered a beam hit,
ollowed by scraping down the net that divided the hall, landing in under 4 minutes.

With 2 lights remaining everything came right. The details are:

Super Sport of unknown vintage 2.12gmx 20 loop & 2.20g
Wind to 2050 turns, 41g.cm
Back off 410 turns, 13.9 g.cm
23 max height no touch
Average pm 161
8m315

I tound a broken slrand on landing. Whilst I had not been winding hard I had nol been resting the motor.

The sum of my best two flights was 14:41 which put me into the lead with 14:01 in second place. I was prepared to use my last competition flight if nessesary, but this was not.needed.

Reviewing the best flight I concluded that even though I had employed a.lighter and shorter motor than usual; I still had not wound it hard. The launch torque was half that I had used with the heavier motor in practice.

I had made a 20" pitch propeller but had not had time to tey it. It will certainly lower the rpm and require a.high launch torque so may yeild higher times.
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OZPAF
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« Reply #55 on: September 19, 2018, 07:18:20 PM »

Congratulations and thanks for the report.

John
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Olbill
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« Reply #56 on: September 19, 2018, 11:24:33 PM »

Wow! Good job!
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