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Author Topic: Bungee Launch 36" Gliders  (Read 35597 times)
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Yak 52
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« Reply #275 on: September 04, 2017, 05:12:26 AM »

Pete sorry, just saw your edit - both too quick  Grin

More span is always good because the more air you can 'reach' and use to make lift with, the better. The bungee class has a maximum span of 36" so it makes sense to build a model with 35.99" wings  Cool

If the span is fixed at 36" then aspect ratio only changes the wing area. Lower aspect = more area.

The Prefect aspect ratio is about 12. Some of the most successful non scale models have an aspect ratio of about 6 so they have twice the wing area. Including the Veron Cirrosonic (https://outerzone.co.uk/plan_details.asp?ID=5146) and Mercury Gnome (https://outerzone.co.uk/plan_details.asp?ID=4478) and Corsair (https://outerzone.co.uk/plan_details.asp?ID=5735) when built at 36".

The low aspect ratio also gives a greater chord and so higher Reynolds number, which is a big factor in the drag and therefore sink rate.

There's obviously more to it but generally when span is limited low aspect is better.


On a related but OT subject it's worth noting that with some FF glider classes like Nordic A1 or F1A the span is not limited by the rules but wing area is. In that case high aspect ratio is better. You are accepting the penalty of lower chord/Reynolds number but maximising span.
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« Reply #276 on: September 04, 2017, 09:27:33 AM »

Rich Weber was flying a low aspect ratio Primary at the FAC non-nats this summer and it flew rather nicely...more importantly, I think, is a good moment arm for directional stability as they tend to wander a bit on the line going up...
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« Reply #277 on: September 04, 2017, 10:47:46 AM »

I think a 3 ft bungee glider might be a good way for me to get into a bit of gentle non-scale competition in a low key way. I might try and build one for next summer. There are also one or two opportunities (eg. Oxford Scalefest) to fly scale gliders in FF competition. I was wondering if it's possible to make an uncomplicated scale glider which is also at least vaguely able to compete with non-scale designs so that I can build a 'two birds with one stone' model. Is this a daft idea? If not, which real gliders most closely resemble good hi-start designs?

Pete,

There is a scale glider prize in the Peterborough Flying Aces 36" bungee launch competition - held last weekend (but I was on holiday unfortunately). Monique won this a few years back with a Slingsby Kirby Prefect enlarged to 36". There's a thread on here if I recall.

Anything with low aspect ratio and plenty of dihedral effect would be a good starting point. The dihedral makes all the difference in windy conditions and low aspect ratio maximises launch height.

Jon

I didn't win the glider comp, The Prefect was awarded model of the day or something like that.

Pete, if you're going to build one, it won't compete with the purpose built non scale gliders, so you're going to have to build two gliders now Wink
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« Reply #278 on: September 04, 2017, 11:24:47 AM »

Well I'll see. At least they're quick to build.
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« Reply #279 on: September 04, 2017, 01:08:41 PM »

Jon,
Shouldn't we be dialling in induced drag here too?
This is one thing that goes against low aspect ratio and is a factor to be considered I think?

I do agree that high aspect ratio wings would give some problems at 36" span though.

I opted for 9:1 with my design .... it got a 1st on the first outing at Peterborough ...
A 2nd on the second outing ....
and was trodden on the third time out!
The point I am trying to make (I think) is that you will struggle to find an efficient scale glider with an aspect ratio of 6:1,
but success can be had with something of a higher aspect ratio?
Add to this, Monique's glider flew very well and looked great in the air!
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Yak 52
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« Reply #280 on: September 04, 2017, 03:01:45 PM »

Jon,
Shouldn't we be dialling in induced drag here too?
This is one thing that goes against low aspect ratio and is a factor to be considered I think?

Russ, this is only the case when wing area is fixed and span is flexible. If the wing area is fixed, you can only increase aspect ratio by increasing span (which is beneficial as I said above.)

Sorry to get techy Smiley but the following attempts to answer the query (forgive me if it's incomprehensible - it's complex and I'm shattered Smiley):

It's important not to confuse the formula for the total induced drag:


with the formula for induced drag coefficient:


It's easy to see the last bit of the second formula and assume that high AR means less drag. This is the case when area is fixed and high AR means you get more span. However, where span and weight remain fixed, actual induced drag remains constant. So in a fixed span class with no area limit, low aspect ratio has no downside unless it comes with more weight.

[Mathsy bit: the first formula for Di essentially shows that you can improve induced drag by reducing weight (L in the formula) or increasing ρ (rho = air density), V (speed) or b (span). We assume speed and air density to be constant for the sake of comparison and span is fixed by the rules so all we can do to reduce induced drag is reduce weight. You can also see in the Di formula that there is no mention of area or AR.]

Its a common misconception (that I've also fallen for in the past) based on the the induced drag coefficient that low aspect ratio is bad for drag. But where the span is fixed and area is flexible it's not so Smiley especially in FF with no need to fly fast (as RC gliders do) in fact low aspect, large area and maximum chord is better for all round drag reduction and improved sink rate - as long as the weight doesn't creep up too. You can see this low AR optimisation in the fixed span classes such as E36 and P30.

At model scales the dominant form of drag is actually profile drag from the airfoil (actually a much higher proportion than suggested by the classical theory that minimum drag occurs where profile and induced drag are equal.) And profile drag is the one that is so badly affected by low Reynolds number ie small chords. So high aspect ratio in a fixed span class comes with a considerable profile drag penalty.

As with all things a balance is needed and I would think 5-6 is a sensible lower limit for aspect ratio. Higher aspect will still work fine and obviously good air and good trimming is a major factor but the optimum for launch height and sink rate performance will be hard to find in a scale model, lovely as they are Smiley
Bungee Launch 36" Gliders
Bungee Launch 36" Gliders
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« Reply #281 on: September 04, 2017, 03:51:40 PM »

I am going from memory here but I seem to remember some data that showed sails aspect ratios as 9-1 being the best compromise to weight vs length.  Of course carbon and all sorts of new materials do away with that study from long ago.
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« Reply #282 on: September 04, 2017, 07:21:19 PM »

Quote
Russ, this is only the case when wing area is fixed and span is flexible. If the wing area is fixed, you can only increase aspect ratio by increasing span (which is beneficial as I said above.)
Jon,
I'm no expert ... and do not intend to be!

I think the discussion could get a bit cyclic (... and I'm tired too!), but I'm still not sure why the fixed span limit necessarily leads to low aspect ratio for peak efficiency?
I suppose my 'point of reference' is an A1 glider with a not massively different span still going the high aspect ratio route (I know the area rule applies).
I can imagine a 'mini me' 36" version of an A1 adopting a slightly lower aspect ratio ... but to go as far as 6:1? The 36" class has no minimum weight also ... something else in favour of a higher aspect ratio being viable?
As I say, no expert ... just an observer of the evolution of other classes.

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Yak 52
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« Reply #283 on: September 05, 2017, 05:07:21 AM »

I suppose my 'point of reference' is an A1 glider with a not massively different span still going the high aspect ratio route (I know the area rule applies).

The three lower aspect plans I linked to, Cirro-sonic, Corsair and Gnome have all been consistent winners in the hands of Geoff Stubbs, John Ashmole and Dave Rumball. Ian M tried a high aspect OD at one point but gave up on it in the end. He ended up with a very light P30 style wing.

This is not to say a higher aspect model can't work - the Rumball's Yard obviously did well Smiley But the theory and practice do point towards low aspect being optimal.


... just an observer of the evolution of other classes.

I agree it feels counter intuitive because high aspect ratio is associated with success in so many other disciplines. But the high aspect ratio is good for
- area limit with no span limit (A1, F1A)
- penetration (radio gliders, full size sailplanes)
neither of which are needed in a span limited FF class.

What is needed is a good high launch and a low sink rate (plus stability, etc)

Low aspect maximises launch height on a bungee, where a large wing area can convert the pull force to height better (high max CL - think of it like a payload competition.)

A low sink rate is achieved by
- low span loading for minimising induced drag
- low wing loading
- low profile drag from good airfoils (thin with plenty of camber)
- a reasonably large chord for higher Reynolds numbers

You are right about there being no weight limit. Obviously a low aspect ratio model will have more structure and be harder to build light. So for span loading higher aspect may permit a lower all up weight. But this has to be balanced against the wing loading and Reynolds number problem with shorter chords.

That said, Geoff's Cirrosonic is quite heavy (about 160g IIRC) and still won the Flying Aces a few times.


Please forgive me for getting too deep into theory, just trying to explain the direction my own research for the class has taken - my own 36 incher is still on the board and needs covering! It has an aspect ratio of 6  Grin but I need to turn theory into practice  Roll Eyes


Jon
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« Reply #284 on: September 05, 2017, 06:40:04 AM »

Don't apologise, Jon; I'm enjoying the theory this time because I can just about follow it. Very nicely explained!
With no experience of bungee gliders I shall probably just use someone else's design for my first go, but you've both given me good pointers on the sort of thing to choose.

Incidentally, my indoor Lilienthal glider goes up very easily on a running tow (never properly tried it on a bungee). Is its very low aspect ratio a factor in that? Can't say it's sink rate is overly impressive though!
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« Reply #285 on: September 05, 2017, 07:00:53 AM »

I look forward to seeing your theory put into practice Jon  Smiley (no sarcasm intended .... I do want to see it)

Please remember, I just thought other factors should be considered .. I'm not running up against what you are saying.
At the end of the day, a good launch is perhaps paramount.

I've been unable to enter for the last couple of years ... the numbers were rrelatively low in the glider comp this year.
If at least the three of us enter next year, then hopefully numbers will pick up again.
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« Reply #286 on: September 05, 2017, 08:52:23 AM »

No probs Russ, I must stress that it's only my current understanding of the problem, so I welcome any further ideas, corrections etc.

The model will get finished eventually (I've got Moley gently pushing me now too  Cool) the problem is that the airfoil I gave it could be better (my limited understanding at the time of designing) so it may need another wing.

One thing not mentioned is the tail surfaces - high aspect ratio needs less tail area and so there is a little less drag. It's definitely a balance of lot's of factors and matching the model weight to the bungee power is critical to the launch height. And of course something that releases high and trims nicely will do well even if on paper the sink rate is 'worse'.

Perhaps the Rumball's Yard needs a rebuild?  Wink

Thanks Pete, yes I think the low A of the Lilienthal glider helps with the good climb out. If you imagine making a high aspect model of the same span and weight that's what you'd want to compare it with.

The sink rate will be affected by parasite drag from scale struttery and bracing wires - another complication for a scale model. And perhaps the Lilienthal glider airfoil has a little too much camber IIRC. I do think a 36" Primary glider might do ok if it was built light enough?
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« Reply #287 on: September 05, 2017, 11:01:41 AM »

Thanks Pete, yes I think the low A of the Lilienthal glider helps with the good climb out. If you imagine making a high aspect model of the same span and weight that's what you'd want to compare it with.

The sink rate will be affected by parasite drag from scale struttery and bracing wires - another complication for a scale model. And perhaps the Lilienthal glider airfoil has a little too much camber IIRC. I do think a 36" Primary glider might do ok if it was built light enough?

You see the trouble is you've now got me thinking about building a 3ft span, bungee launched outdoor Lilienthal! Wouldn't it look great against a blue sky? That's my whole trouble- I can't ever seem to get going on sensible stuff, purposefully built to fly well, without getting side-tracked by another foolhardy scale project! Grin

(Btw, although my indoor Lilienthal has a couple of posts in the wing, it doesn't actually have any rigging. I left it off to reduce drag, but granted it does have a very cambered and unorthodox aerofoil.)
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« Reply #288 on: September 05, 2017, 11:11:40 AM »

Jon has covered the tech. stuff very well as usual so I have just a couple of graphs to contribute.  I did a paper on drag in the 2003 Free Flight Forum and at one point in the paper got around to discussing limited span classes and in particular the P30 class.   am sorry that you probably can't read any small stuff, the original paper wasn't printed all that well and my copy won't have improved things but I will try to explain.  I assumed a P30 with aspect ratio ranging from 4 to 12. A base weight is entered for the highest Aspect ratio and then an incrementof weight added per inch of chord. A base zero lift drag is entered and can be incremented and the induced drag is calculated as usual for AR.  The spreadsheet at the top of the page calculates the speed, L/D and sinking speed and calculates glide time from an assumed height of 120 feet.

The results are plotted in the graph at bottom left.  The top line is with no extra weight or drag added and the lower lines as more weight and drag are added.  As the weight increases the high aspect ratio flight time gets nearer to the low aspect ratio time.  The left hand graph is with a lift coefficient of 0.8, the graph on the right assumes a lift coefficient of 1 but even if a higher Cl could be achieved  the low AR would still be best unless the increase in weight due to low AR was considerable.

Another point, not relevant to gliders but it is to a P30 is that the power required for level flight is drag x velocity.  With a low AR, large area wing the velocity will be less and probably less power required to climb.

The 0riginal paper is seven pages.  It is probably clear enough to read if anyone would like a copy and the spreadsheet pictured in Figure 4 is useable I think.

John       PLEASE SEE A LATER POST FOR THE ATTACHMENT

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Re: Bungee Launch 36" Gliders
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« Reply #289 on: September 05, 2017, 11:27:38 AM »

My last post got messed up and would not load the pic I wanted.  K hope I have it correct now.
John
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« Reply #290 on: September 06, 2017, 05:44:20 AM »

Thanks John, that makes sense.Your results also seem to suggest that low aspect ratio is better as long as weight remains reasonable.

Just for my own interest I did a little analysis in XFLR5 of an A=9 and an A=6 model with full tails. Both were given the same tail volume (Vh0.76) and the same tail moment (3 x chord)

The wing with an aspect ratio of 9 has a 4" chord, the A=6 has a 6" chord.

I gave the A=9 model a weight of 80g (similar to Russ's Rumball's Yard) and the A=6 model a weight of 120g (quite an increase really.)

I tried the models with two of my own modified airfoils
- 'Polymilder' which is a thin section and particularly good at low Reynolds numbers
- A modified BE50 which is a 9% undercambered foil and more realistic for light structures

As seen in the graph below, the low aspect model had a better sink rate in all cases. When using the better lower Re foil, the gap was narrower, showing that careful attention to low Re airfoils is needed with a higher aspect ratio. But with the thicker airfoil, the gap between the models was rather more, meaning that with more practical construction the 6" chord had a big Re number advantage.

The sink rates shown don't include fuselage drag so are not realistic but even so there is a difference of about 20 seconds from 120' if put in terms of John's results.

So XFoil analysis upholds the case for low A, more obviously than I expected.

Also the lighter the model, the slower and so the lower the Re number. So perhaps weight is less of an issue for that reason also.


Jon
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« Reply #291 on: September 06, 2017, 06:50:56 AM »

Ah, but yours DTs into the top of the Ferry Meadows trees whereas  mine flys into the lower branches and lives for the next flight! ;-)
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« Reply #292 on: September 06, 2017, 06:57:40 AM »

Too right Russ  Grin it's all about the flying in the end!
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« Reply #293 on: September 06, 2017, 07:11:10 AM »

On a more serious note ... can I provoke you into further calculations?!

Have you calculated the spectrum of input energy available from the bungee?
Also, with the heavier models needing more bungee extension ... does the consequent drag of the higher acceleration 'sap' this energy significantly.
Also (!) Does 'kiting' compensate for this?

The reason I ask is because my model seems to climb very well on the line ... usually a good launch includes  the model sinking a little before it will release. I have observed a greater sink rate compensated by a high launch height with my model when compared to other successful models.
Oooh, so many parameters, so many variables!
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« Reply #294 on: September 06, 2017, 09:39:14 AM »

Russ,

I don't have any calculations for rubber energy I'm afraid - sad to say I used to leave that sort of thing to Ian M who would no doubt have had a spreadsheet for it  Smiley

From my more recent experience with RC bungee and F3-RES, I would say that the key is to match the weight to the force of the bungee. The heavier models do really seem to suffer from lower launches and need more wind. The lighter the model, the higher it can get off available energy. The 36" hi-start is of the low power/long stretch variety so kiting is always going to be the best strategy and the key will be getting the model to release under line tension. My RC glider gets 80m height off a 60m total line and pings off under considerable tension - easier with RC of course.


Jon
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« Reply #295 on: September 06, 2017, 10:45:29 AM »

No worries Jon .... it does interest me though.
In particular,  I wonder what the 'intersection point' of the data provided by John and yourself, and the maximum weight for a full height launch is ... if indeed there is one. Clumsy explanation,  but I hope you know what I mean?!
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« Reply #296 on: September 06, 2017, 06:47:28 PM »

The question of fixed span and optimum AR is an interesting discussion and Jon has covered it well. a point that Hepcat John indicated with his graphs is the effect of wing loading - or more accurately weight.

As the AR is lowered and thus the area increased then it's possible that the wing loading will reduce (even with a slight increase in weight) and the lowered flying speed and thus Reynold's numbers effect on the airfoil may then offset the gains in sinking speed.

Jon's graph high lights this as the best sinking speed is achieved with the lower AR at the higher weight of 120 gms with the best airfoil.

At the weights and sizes chosen - this would indicate that Reynold's number effect/airfoil is most important.
thus the optimum may not be the slowest model but a combination of speed, chord and airfoil.

There is another aspect of low AR which is not really relative to bungee launched gliders and that is that the actual nose pitching moment from the airfoil increases with chord. Now for a slow flying FF model at high CL that may not be a problem(it would increase the trim loads slightly) but it was most definitely when tried on 2M gliders with AR's as low as 4.5. Huge tail volumes to control the pitching moment were required when penetrating and thus were not a success due to the increased drag and extra weight.

John
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« Reply #297 on: September 07, 2017, 06:12:56 AM »

As the AR is lowered and thus the area increased then it's possible that the wing loading will reduce (even with a slight increase in weight) and the lowered flying speed and thus Reynold's numbers effect on the airfoil may then offset the gains in sinking speed.

John, yes the XFLR5 results bear this out. The attached graph shows that reducing weight of the A9 glider doesn't help the sink rate much.

Russ, I'm not too clear on how John arrived at his results (spreadsheet is hard to read for me) and in particular if he adjusted for Reynolds number effects. If not then the differences would be even more exaggerated. XFLR5 is very good for comparison in that it models actual planforms and airfoils very accurately. It just excludes fuselages so isn't a 'real world scenario' for calculating actual sink rate.

It just struck me that since the bungee is free flight rubber (7.5m of 1/8") that John (Hepcat) may be able to use the energy density from torque calculations to give an idea of the pull force and max stretch? I'm guessing the pull force would be around 800g-1kg? I will try to remember to measure it at the next Ferry session.

I think that a max weight figure would be a bit meaningless because it would depend on the wing area of each model and because wind is such a significant variable. However there is some merit in comparing the model weight to the pull force. With RC bungees the launch begins to suffer when you start getting to pull force less than 5 x model weight. For F3-RES most models are around 1/10 the pull and my Skidoo when up very well at around 1/15th.

With the 36" there is no limit on stretch in the rules so there's an argument for stretching something like 90% of what is possible. As in motor winding, the last 10% to max stretch bumps up the pull force but actually it's the force of the long consistent pull from 90% on that matters.

There's no doubt that lighter launches higher, but this does mean finding a way to get the model off a stretched bungee at the apex of the climb.

Another factor not yet mentioned is that stiffer structures launch higher as wing bending and torsion waste some energy in the climb. So weight is not the only thing to consider. Somewhere there is a balance between launch height and sink rate.

I've also been looking into 5 panel wings with tapered planforms. There does seem to be an improvement in sink rate in XFLR5.
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« Reply #298 on: September 07, 2017, 06:28:58 AM »

On another subject, I'm also keen to work out what the best dihedral is for 36" bungee launch. Anyone want to give details of successful models?

If you can provide the basic geometry I can work out the EDA.
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« Reply #299 on: September 07, 2017, 02:14:08 PM »

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
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