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Author Topic: Bungee Launch 36" Gliders  (Read 34525 times)
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skyrocket
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« Reply #100 on: October 13, 2013, 07:05:17 AM »

How about that Hugh O'Donnell lightweight from 1957 that is being kicked around in SAM-35 recently? it appeared in Zaic's 1957-1958 Yearbook and sports a MVA 301 airfoil....I've been changing my thinking on bungee  in that we shouldn't set a design/published date limit but stick to the idea of no auto surfaces and 36" span with the 25ft/75ft bungee...
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applehoney
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« Reply #101 on: October 13, 2013, 10:18:23 AM »

Agree with your suggestions.      Have tile-printed and assembled a plan for  a 36" version of my own 'Walkin' Shoes' for attention in relatively near future.

For anyone within reach of Geneseo, NY, I expect to have a bungee glider event listed for the Great Grape Gathering in September next.
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skyrocket
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« Reply #102 on: October 14, 2013, 08:22:23 AM »

I will be at the GGG in Sept and hope we have a good turnout for this event. I'm looking at several models right now to compete with. The Corsair I built behaves quite well on the line and I like it alot but I don;t think I'll have it much longer because as I fly it it gets better and better and I might as well build a backup now incorporating all the things I learned this year. As i mentioned earlier, the O'Donnell Lt. Wt. looks good as does the Yeabsley "Revenge". I really do think low A/R is the way to go and the only question I have is just how small of a stab can you go with low A/R wings when designing your own?
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Yak 52
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« Reply #103 on: October 14, 2013, 10:02:02 AM »

how small of a stab can you go with low A/R wings when designing your own?

The effectiveness of the tail (tail volume) is affected by aspect ratio. The formula is

Tail volume = Tail area x tail moment arm / wing area x average wing chord


A low aspect ratio - increasing area and widening the chord does reduce the tail volume. So you might need a bit bigger than what just 'looks right' on other higher aspect models.

A rough calculation based on a 36" span and 6" chord (aspect ratio of 6) suggests that a sensible minimum might be:
- a tail area 25% of wing area
- a minimum tail length (LE to tail LE) of 3 times the chord.

This would give a tail volume of 0.66 which is not over large for this type of model. I did compile a list of tail volumes for gliders (if I can find it.)


Jon
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applehoney
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« Reply #104 on: October 14, 2013, 10:22:06 AM »

I'm curious as to why one would want to go to the smallest stab consistent with stability, unless weight is a factor ?   We'd have no maximum total surface limitations, surely ?
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Yak 52
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« Reply #105 on: October 14, 2013, 12:12:32 PM »

Jim, it's just that a small tails will have less drag. There's a sweet spot somewhere between that and good trimming.

The tail volume of these towliners varies widely. Here is some data I compiled. (The tail volumes are worked out based on the true tail moment arm - CG to stab aero centre)


Model                       Aspect ratio           Vh

JB87 (Nordic A1)         14.75               0.83
Mercury Grebe             8.5                  1.36
Lulu                           10                   1.00
Veron Cirro-sonic          6                    0.75         (2 x winner at PMFC Aces)
Thermic Trooper           7.5                  0.64
May Morning                7.4                  0.83
Mercury Gnome (32")    6.3                  0.85         (another winner at PMFC Aces)
Corsair                        6                    0.94
Walkin' Shoes             8.5ish               1.16


I think I would be aiming for a tail volume (Vh) of 0.8 myself.

BTW the May Morning is an Andy Crisp plan that I haven't seen mentioned yet:
http://www.rcgroups.com/forums/showpost.php?p=20990553&postcount=273
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applehoney
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« Reply #106 on: October 14, 2013, 01:25:16 PM »

Hmmmm ....   the 'Shoes' close to the bottom of the desirability limit.

Drag of the stab has never before entered my thoughts in close to 70 years of modelling - not that I ever knew what drag was in the early days.

Still not over concerned about it.  WS originated from power model exposure of the time - powerful stab, made even more so by undercamber, and a CG further back than most gliders.  Lightly loaded, floats well ... good enuff for me
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Yak 52
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« Reply #107 on: October 14, 2013, 01:45:28 PM »

Well it's a teeny weeny advantage (a little less drag) versus a big old problem (untrimability), so I think you're right  Smiley

As I said, for my OD model I'll be plumping for around Vh0.8 which should be quite safe yet not overly large.


Lightly loaded, floats well ...good enuff for me

And very pretty  Smiley




Jon
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applehoney
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« Reply #108 on: October 14, 2013, 02:05:25 PM »

Sheesh ...    Now I have to live with a 'Shoes' that thinks it has a big butt .......    thanks a lot, fellas   Angry   
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skyrocket
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« Reply #109 on: October 14, 2013, 03:51:56 PM »

sheesh is right...I'm drawing up a 36" er right now with a
36" x 5" chord wing
18" x 3 1/2" chord stab
and 20" wing LE to stab LE.......so I may have to re-think it before I cut wood and if what you say is true (and I don't doubt your math) maybe it should have a smaller stab...nez pax?
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Yak 52
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« Reply #110 on: October 14, 2013, 04:23:13 PM »

That gives you a tail volume of about 1.25 (assuming the CG to tail AC is 18") and an aspect ratio of 7.2

On the big side perhaps but still within bounds...?

I must stress again - I've only watched these 36"ers fly so far (although I have looked into the design side of them quite a bit.) So feel free to ignore  Grin
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skyrocket
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« Reply #111 on: October 14, 2013, 04:41:09 PM »

So I did some playing around with numbers and came up with 18" x 3" chord stab and 15" between wing TE and stab LE and came
up with about Tv = .9 which is a little better and I get it because the moment arm is a little longer needing a smaller stab and that checks out with the rubber jobs I fly with smaller stabs now...I still don't understand where you find the tail moment arm so I used the wing TE to tail LE dimension instead...any help guys?
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Yak 52
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« Reply #112 on: October 14, 2013, 05:17:40 PM »

I would be inclined to keep the tail longish (better damping) and cut the stab area a little?

I've been working on a moment arm 3 times the wing chord.


I still don't understand where you find the tail moment arm

There are a few fudged versions but the most accurate is from the CG (the pivot point around which the model rotates) to the stabilizer's aerodynamic centre (the point at which the force of the stab acts)

The final CG may not be known yet but with these larger tail volumes it's probably going to be something like 50%-60% of the chord.

The stabs' Aerodynamic Centre is at 25% of the stab mean chord.

With no sweep or taper its very simple (it gets awkward when you have to calculate the mean chords on nice curvy wings and stabs)

For the list I gave, I measured the CG to the tail AC. Where the CG was not shown on the plan I assumed it was 50%. As long as you are consistent it doesn't matter if you fudge a bit.


So for your new design the moment arm would be

15" minus 50% of the chord plus 25% of the stab chord
15 - 2.5 + 0.75 = 13.25"

Giving Vh = 0.795
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lincoln
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« Reply #113 on: October 14, 2013, 06:06:47 PM »

Shouldn't the measure of tail volume be from 25 percent MAC of the wing to 25 percent MAC of the tail? If you do it from the c.g, I should think you'd need to include the contributions of both surfaces?
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Yak 52
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« Reply #114 on: October 14, 2013, 06:42:14 PM »

Nope. That's one of the quick fudges (works fine in RC where the CG more like 25% anyway).
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lincoln
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« Reply #115 on: October 14, 2013, 10:43:50 PM »

Not sure I believe that. What's the tail volume on a tandem wing?

I don't understand how the wing's contribution can be ignored, which is what you're doing if you don't run that additional number and use the c.g. instead of the wing AC. If you included a term accounting for the wing's distance from the c.g. as well, I might have an easier time with it.

Check out page 282 and 283 here: http://faculty.dwc.edu/sadraey/Chapter%206.%20Tail%20Design.pdf

I'll admit right off that you can find it the other way too, in some reputable on line sources.

I checked Raymer's Simplified Aircraft Design for Homebuilders, and he uses leading edge to leading edge!!

I checked Hoerner's Fluid Dynamic Lift, but couldn't  find a Vh defined. Some of his equations use the surface AC to the c.g., though.

I suppose that you can just say that it's called "tail" volume, and so by definition the wing doesn't count, but no one told the air that.

Thinking about it, seems to me the only really valid way to go about it is to find the aircraft's AC and then compute a Vh for the tail, and then another one for the wing, using the distance between each surface's AC and the aircraft's as the pertinent length. This has the further benefit of treating weird configurations, extra wings, etc. fairly. If the c.g. is not on the AC, seems to me that considering the static margin kinda automatically takes that into consideration.

Then there's flying wings, with and without sweep!


Anyway, I suppose that's getting O.T.

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Yak 52
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« Reply #116 on: October 15, 2013, 07:20:29 AM »

Lincoln,


Anyway, I suppose that's getting O.T.

Yup! I will reply briefly but perhaps this is worth a thread of it's own?


I think either way is valid. And as long as you pick one and stick to it you will get a reasonable comparison for design purposes.

Martin Simons, Andy Lennon, Don Stackhouse and Darryl Stinton all give tail moment arm as the '25% MAC to tail AC'. But Mark Drela gives it as 'CG to tail AC'.

http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-01-unified-engineering-i-ii-iii-iv-fall-2005-spring-2006/systems-labs-06/spl8.pdf

as does this EEA article:

http://www.eaa62.org/technotes/tail.htm

(This dimension is given as lh in your previous linked article)



I suppose that you can just say that it's called "tail" volume, and so by definition the wing doesn't count, but no one told the air that.

This is the point. It depends what you are looking for. Tail volume is just a simple comparison of the tails effectiveness, not a full examination of the actual forces required for balance/trim (which does require looking at the wing's contribution, pitching coefficients, it's position ahead of the CG etc.) In practice the camber of the wing section (pitching forces) would need to be at least considered.

Tail volume is simply an area and a moment arm or lever. The true physical force acts as a lever at the CG, that's the 'pivot point' or fulcrum for the model in flight.

If you are looking at longitudinal stability calculations you can resolve forces and moments to any theoretical fulcrum mathematically and for convention we use the wings aero center (25% mean chord or MAC) because this is convenient for the pitching coefficients. You could just as easily resolve moments to the CG, it's just convention.

I suppose this is the reason you find '25% MAC to tail AC' in use. But the true physical property is 'CG to tail AC'.


I would argue that for free flight models with large tails and CG's as far back as 80% then the 'CG to tail AC' is more appropriate. You could have a much shorter tail moment arm than would be apparent from 'wing AC to tail AC' and this has implications for reduced tail effectiveness - especially for dynamic stability.



All this has got me doodling 36"ers again anyway Smiley
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skyrocket
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« Reply #117 on: October 15, 2013, 02:47:10 PM »

found this in Model Aircraft, June 1953...
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skyrocket
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« Reply #118 on: October 15, 2013, 07:35:24 PM »

Here's the start of a new 36" bungee glider...
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Yak 52
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« Reply #119 on: October 16, 2013, 06:27:25 AM »

This is your OD model? It looks gorgeous  Cool
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skyrocket
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« Reply #120 on: October 16, 2013, 08:07:20 AM »

Yes...OD...there is a A2 from the late 50's - early 60's by someone from Europe who flew a similiar looking model and I remember the nose being made from "sugar pine", or so the Aeromuddler or Model Aircraft said...I'm thinking it might be Nordic...Nuumanuka ?(sic)..but I just dig the old designs evern though these Bungees are the closest I've ever come to flying them...so many models, so little time...but I'm having FUN
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« Reply #121 on: October 16, 2013, 08:25:21 AM »

Well I really hope you scan it - it looks very attractive. I would seriously have a go at building a beta version. Is the dihedral 3 or 4 panels?
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skyrocket
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« Reply #122 on: October 16, 2013, 03:56:53 PM »

Let me build it first and see what the weights, fits and finish looks like and then I'll put something up. I chose tip dihedral because the Corsair flew so well with it and it simplfies building and I can put some spruce/basswood spars in the middle panel easily. Thanks for the kind words. Actually, I based everything on the Corsair because I like it so much. I also like geodetic construction to keep everything straight right out of the box. It's a confidence thing with me.  If you haven't built a 36" Corsair yet, it is really forgiving on the bungee and has a nice slooooow glide. I decided to build a 5" chord figuring it would hold in the wind better on the bungee. Also, I'll stick with no auto rudder to keep it simple and a fuse D/T. I'll try and find that A2 I was talking about and post a pic of it.
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skyrocket
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« Reply #123 on: October 17, 2013, 07:02:14 PM »

I located the model I was thinking of when I started to draw this new 36" bungee up...It's by Tahkapaa from 1959...
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« Reply #124 on: October 19, 2013, 12:38:25 AM »

Hi Peter,

Good luck with the building of the CB 34, you are almost finished.  Smiley

-Paul
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