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Author Topic: Bungee Launch 36" Gliders  (Read 34928 times)
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Hepcat
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« Reply #300 on: September 07, 2017, 05:30:04 PM »

Response to #298.

Jon,
I was interested in your reference to EDA.  I did a lot of work on it at one time so, just keeping things simple for the moment, do you treat an even chord wing with square tips as an even chord wing with square tips or do you assume an elliptical lift distribution from tip to tip?
Hepcat John.
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« Reply #301 on: September 08, 2017, 06:38:32 AM »

I am following very closely this topic, trying to apply the lesson to E-36 which has the same span limit but a somewhat different speed envelope.

Jon, can you give us the 5 wing cords for your Ellipse 36 wing other than the root cord of 6,75"? And the length of the single panels.

I would like to play/use with XFLR5, but did not have the courage to open it till today  Sad  How about a tutorial on a new topic???

Urs

Urs,

Attached is a first attempt at a 5 panel wing at A=6. This is not yet optimised in XFLR5 in any way but it does 'perform' better that the A=6 constant chord wing on sink rate. It's drawn in metric but chords are essentially 7" root, 6" mid and 4" tip. Rib centres at 47mm (near enough 1 and 7/8".)

When you mention the different envelope for E36 are you talking about a faster climb? In that case I think there would be potential benefit in a more elliptical planform.

Happy to attempt an XFLR5 thread if theres some interest... must admit I'm just learning it myself too though!


Response to #298.

Jon,
I was interested in your reference to EDA.  I did a lot of work on it at one time so, just keeping things simple for the moment, do you treat an even chord wing with square tips as an even chord wing with square tips or do you assume an elliptical lift distribution from tip to tip?
Hepcat John.

John, I did a bit of research on the methods a few years ago but these days I just use one of two calculators:

Martin Brungard's found here: http://www.charlesriverrc.org/articles_modeldesign.htm

And the one in Curtis Suter's 'Sailplane Calc' here: http://tailwindgliders.com/Files.html

They come up with slightly different answers. If I recall correctly the Brungard one takes account of panel areas but the Suter file assumes an elliptical lift distribution.


Jon
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USch
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« Reply #302 on: September 08, 2017, 01:51:08 PM »

When you mention the different envelope for E36 are you talking about a faster climb?

Jon, exactly. The best E36 go to 180m height in 10 seconds, not my ones (yet). Assuming a spiral climb gives about 240-250m to travel, so actual climb speed would be around the 25m/sec mark.

Urs
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« Reply #303 on: December 12, 2017, 03:00:15 AM »

How strong should the tow hook mounting point be, to keep the tow hook from being ripped off the glider? A piece of balsa? Reinforced with fiberglass cloth? A piece of basswood? Plywood?

Most plans only have the tow hook position marked out, but not the specifics of the mount.

Thanks!
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« Reply #304 on: December 12, 2017, 09:31:26 AM »

My FROG WREN has the hook inserted in just regular balsa.  Seems to work O.K.

However, it does penetrate the keel piece and wrap over the top of it.

Remember that the bungee is only a 1/8th" piece of rubber. Not much strain to it.

It goes up so stately that I love it.

Gotta build some more designs soon.  Like a LU LU, Gnome, etc.  Kinda wish i could include a WALKING SHOES like Jim has.  Maybe some day. But, 1st I'm building a THERMIC 'C by JASCO. I already have the JETCO model of the 'C'. I'm kinda stuck on building just LASER CUT models except for this one model 'C' which is taking forever.

Might sideline it to get started on something faster to build.

A while back I got a DRIFTY kit for a much smaller glider and had it enlarged to 36" W.S.  Have it framed up, ready to cover.  GottA 'GET 'ER DONE'.

 Van...
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Pete Fardell
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« Reply #305 on: May 21, 2018, 12:37:37 PM »

Just reviving this thread because I'm thinking of building a glider so I can participate in the occasional bungee launch/hi-start event. Haven't done this before at all, so have been looking around for something simple. I came across the FROG Petrel, which is in the plans gallery and looks pretty and straightforward. Anyone made one? It's only 33" span so I also wondered about enlarging it to the 36" max for these bungee glider comps.
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« Reply #306 on: May 21, 2018, 03:26:04 PM »

Pete

Looks OK to me. I would probably enlarge it to 36".


Peter
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« Reply #307 on: May 21, 2018, 04:25:24 PM »

Whatever you build, it's a very enjoyable event Pete  Smiley
SAM35 'legal' gives access to the most events .... I would  guess the Petrel qualifies?
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« Reply #308 on: May 21, 2018, 05:22:10 PM »

Thanks chaps! I shall do this then (once my Fokker's finished) and enlarge it to 36". Some more glider newbie questions...
Do I need to incorporate an auto rudder and/or DT, or are they generally unecessary in this event? 

The tip dihedral looks quite extreme to me. Am I better to reduce it a bit?

Russ, will changes in size or messing with dihedral etc. make it SAM35 illegal?

 (Screen shot of the Petrel plan attached to save anyone finding it.)
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billdennis747
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« Reply #309 on: May 21, 2018, 05:29:24 PM »

Thanks chaps! I shall do this then (once my Fokker's finished) and enlarge it to 36". Some more glider newbie questions...
Do I need to incorporate an auto rudder and/or DT, or are they generally unecessary in this event? 
Definitely A/R and a Tomy DT which you can start when you let it go
I always enjoy my Dolphin - you get a different view of the model
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« Reply #310 on: May 21, 2018, 06:45:50 PM »

Quote
The tip dihedral looks quite extreme to me. Am I better to reduce it a bit?
My recommendation would be to leave it as it is Pete. It has a flat centre section so it will need it.

I would also recommend using 1.5mm dia. screw in cup hooks (straighten to a normal hook ) and screw them into a piece of ply(1.5mm should be enough) along the bottom instead of binding wire hooks to the fuse. This makes it very easy to adjust the position for the best tow by just having  series of drilled holes.

It's a nice looking glider and should go well at 36"WS.

John
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« Reply #311 on: May 21, 2018, 07:19:03 PM »

Thanks, Bill. I'll have to look properly at all the AR and Tomy DT stuff that I normally skip over! John, thanks too- I'll leave the dihedral alone then. I need to check the rules to see if the the 36" limit is measured as if the wing was flat. (I'm guessing 'yes'.) I like your moveable screw-in hook idea too.
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« Reply #312 on: May 21, 2018, 07:36:10 PM »

Pete,
Couldn't see the rules on the SAM35 website but the SAM1066 has a combined rules document linked on the left hand column.
http://sam1066.org/

Scaling is allowed.
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« Reply #313 on: May 22, 2018, 03:46:35 AM »

Cheers Russ. Turns out it's a little too late to be 'classic'. 1964 according to the wonderful 'House of Frog' website  http://www.houseoffrog.co.uk where I found the attached pic. It's a year older than me in fact, which means I'm not 'classic' yet either I suppose.

No matter, I shall press on with it regardless...
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« Reply #314 on: May 22, 2018, 07:07:36 AM »

I really like the look of it too, Pete ... look forward to seeing it.

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Pete Fardell
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« Reply #315 on: May 22, 2018, 11:41:43 AM »

Although I have been mulling over what you said, Russ, about SAM35 legal models having the most opportunities.
If I do build this one though, which events over the summer have an anything goes 36" hi-start comp, preferably events with a bit of scale FF going on too. Peterborough Flying Aces, yes. I think Dreaming Spires too?
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« Reply #316 on: May 22, 2018, 12:53:05 PM »

I would go for any old 36" glider myself! The contest at Port Meadow is for any 36" span glider and the Peterborough ones are likewise - go for it!

Peter
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« Reply #317 on: May 22, 2018, 01:52:44 PM »

The Buckminster FF gala Sept 2nd has 36" and scale
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« Reply #318 on: May 22, 2018, 01:59:29 PM »

Excellent- that's three times I can fly it then, and all at things I was planning on being at anyway. Better crack on with it!
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« Reply #319 on: May 22, 2018, 02:28:35 PM »

...things I was planning on being at anyway.
EDIT: Actually, the Sept event at Buckminster is welcome news to me. Don't think it's on the BMFA website yet.
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« Reply #320 on: May 22, 2018, 07:04:26 PM »

I thought that for the last 100 years or so if rules were being set for FF competition, duration models that it had usually been found that a top limit on the total surface area and a minimum structure weight has been the way to go. After all the years of experience I am surprised that a new class should have a major rule that the wingspan shall be a yard long.

I speak as someone who enjoys 'bunjee' launching and probably did it before anyone else who is reading this. Does anyone else remember? In the war years we could not buy rubber strip but round about the end of the war some model shops got hold of some bits of 'bunjee' cord about half an inch diameter and perhaps 12 inches long.  I got one of these and with care it could be taken apart and inside were lots of strands of 1/32" square rubber. I tied enough together to make a decent 'hi-start'.
   
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« Reply #321 on: May 22, 2018, 07:36:56 PM »

I would definitely agree John that this gives those who wish to design new models more scope, however I believe this class is well established now and possibly originated as a fun event with Peterborough MFC.

That's an interesting bit of history - lots of time, patience and careful knot tying. I was not quite up to modelling activities at that time - still crawling, not walking let alone model building Smiley

John
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« Reply #322 on: May 23, 2018, 05:56:00 AM »

Quote
I am surprised that a new class should have a major rule that the wingspan shall be a yard long

John
E36 and P30 are pretty popular, as well!
Don
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« Reply #323 on: May 25, 2018, 04:59:00 AM »

Bit late to the party on this one - been away on a weeks climbing/sea kayaking/surfing in Cornwall Smiley

Don't reduce the dihedral Pete - you need all you can get. It gives you the best chance of getting up the line to the top rather than veering off and diving in  Undecided

I would suggest you do use a D/T. You just need one good flight without... Roll Eyes the maxes tend to be low anyway and when you're flying in a small field on a windy day a d/t is really handy.

Autorudder does make life a lot easier. Getting a straight climb and enough turn in the glide is a challenge. You could try an offset hook though?

John's idea of cup hooks is a good one but I would say you can go much smaller than 1.5mm if necessary, the pull force of the bungee (1/8" rubber) is very low. Another simple idea is to bend a piano wire 'paperclip' and attach it with two screws for and aft. You can then slide the hook forward or back to suit. The attached pic shows the idea although its a bit bent!

The span limit on the class tends to favour models with low aspect ratio, in other words large wing area.
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« Reply #324 on: May 25, 2018, 11:03:57 AM »

Thanks for all that, Jon. Whilst looking for a subject, I did remember you recommending a low aspect ratio in previous discussions, so was avoiding anything with very skinny wings. With that in mind, before settling (probably) on the Frog Petrel, I was wondering about this 'Doofa' too. It's not as pretty, and the undercambered ribs and pointy tips make it slightly more complicated for my first effort in this field, but the write up with the plan in Aeromodeller sounds quite encouraging. One for next time maybe?
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