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Old Jan 29, 2016, 05:42 AM
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Iota 70" R/E Depron Glider

I've been working on a depron glider design. The idea is to create plans for an economical but refined design capable of thermal soaring. Control will be rudder and elevator so the components in the model should be under 20 quids worth.

It will have a sheet and rib wing giving a 'proper' airfoil. It will be bungee launched, but I may also make an e-glider fuselage version as I have an appropriate motor and folding prop kicking around.

Initial drawings have been about getting the biggest wing possible from a single sheet of 2/3mm. This works out at between 70-80" span and just over 500in^2 wing area.

I'll probably end up going with the five panel wing. It sacrifices a tiny bit of area but gives a refined planform at the right aspect ratio (10)
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Old Jan 29, 2016, 07:25 AM
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S’match, Just thinking we’d figure out a proper Spar system first , then drape the foam over that . Maybe a simple Jedelsky airfoil would be be more “refined” , yet easy to build from foam ( and Light) . ... just crumbs for thought ... ? You doing specific soaring tasks ?

We also found a simple KF airfoil worked great in thermals ... When the air is going up , everything goes up , right !!! hahahaha
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Old Jan 29, 2016, 07:48 AM
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Hi gpw, nothing specific in terms of tasks, just thermal duration for fun in the windy old UK
I'm looking for something with similar performance to a 2 metre woodie or something like the Parkzone Radian my mate flies. Just a compromise between cost, simplicity and easy building but with satisfying soaring ability. Cost is a factor just because I'd like to design something as a low price entry for people who'd like to get into gliders...

I don't think the Jedelsky wing is quite up to the job - I'm finding I can do built up wings in depron at a similar weight to a sheet wing once spars are added. The Jedelsky curved plate airfoil will give the float but would suffer poor penetration for coming back upwind.

I'm afraid I'm a skeptic when it comes to KF airfoils, great for simplicity but not, I fear going to be competitive with a 'proper' glider airfoil. They have their place but its not what I'm after on this one

The attachment shows an airfoil from another of my models, this one was just eyeballed but I think it would be possible to use this method for a reasonably good section - at least something as good as the Aquila or Bird of Time airfoils. I would hope for something that could at least match gliders from say the 80's
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Old Jan 29, 2016, 08:26 AM
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So for simplicity's sake I will probably go for this planform. Spars are yet to be decided on but it will be full span and capable of withstanding a bungee launch. I have some plywood hanging about but spruce or carbon wood do just as well. It will be one piece (no joiners) for simplicity and weight reasons.

The fuselage will be a 6mm depron box more or less, with doublers up forward. All covered in parcel tape.
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Old Jan 29, 2016, 09:40 AM
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Slowmatch,

I'm fond of the MH32 airfoil, and did a foamie build-out with very shallow steps that's flying very well, as far as glide performance and wind penetration capability. It's not a classic 'stacked layer' KFm3 foamie build; building it out to the MH32 profile with a well-shaped leading edge, well tapered trailing edge, max thickness in the proper location, etc. are all a part of ending up with the glide performance. You could likely do a similar build-out of any other airfoil profile for which you have a preference. (My build details are in the DANCER thread.)

Wishing you the best of results on your design project!

Viking60
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Old Jan 29, 2016, 05:46 PM
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Thanks Viking, MH32 is the airfoil used on a 1.5m javelin launched woodie I inherited. Sadly at over 400 grams it's rather obese and has never thermalled well, but it flies nicely.

By using the bungee launch method, this model won't need the speed as required by hand launched models but obviously penetration is still good to have.

I will probably use something like the AG35-38 series, perhaps the thicker foils anyway as any less than 8-9% starts to get a bit thin from a wing stiffness point of view. I don't expect to get the section perfect but its a good starting point.

I will check out the Dancer!
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Old Jan 31, 2016, 09:45 AM
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A bit more progress, the general design layout is now established. Wing span has ended up at 70". My weight target is something between 350-450 grams.

The fuselage is going to be a bit of an experiment in getting a structure thats stiff and light. And also the tow hook for launch will need some thinking out so as to spread the load properly.
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Old Jan 31, 2016, 10:37 AM
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Quite a challenge ... NICE looking drawing !!!
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Old Jan 31, 2016, 04:30 PM
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I'm subscribing to this, I learnt to fly 2 channel gliders in the early 90's, it was with an Aquilla and I loved it and I just recently found plans for it so I may end up with a modern take on an old classic!
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Old Feb 02, 2016, 08:16 AM
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I've been playing with this wing in XFLR5 to simulate various airfoils and it confirms that the Jedelsky wing has a decent float: it will thermal well, but poor penetration: you'll struggle to get between thermals and back upwind without losing a lot of height. The Aquila airfoil doesn't come out very well either.

The MH32 has good penetration but the AG35 really has it all, good thermal performance but also a good glide ratio at high speed. In other words you will be able to speed up without losing too much height.

So I hope to have reasonable performance by approximating AG35 as well as I can with this construction style.
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Old Feb 08, 2016, 09:31 AM
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Got a bit distracted building a depron DLG
This is the wing, 30" span, just 6mm sheet with a carbon spar and tape covering.
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Old Feb 08, 2016, 03:06 PM
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Not familiar with the AG35 airfoil, but googling it, it looks a lot like the Selig SA7035 airfoil. I have done a couple gliders with that airfoil (my cobbled attempt at it anyway) One was a third wing for my OSG, 60"span and the other was an enlarged OSG using MPF to get a 2meter wing. On both planes, the wing has way better penetration, and about as much thermal performance as the KFM2 wing that has been my favorite wing of the original OSG. In fact, having been used to the draggier KFM wing I found I had trouble slowing down to stay in thermal action with the conventional airfoils. (which may sound crazy). I built them with a spar at high point of chord, and an leading edge "spar" both of wood I had around (pine/poplar). The construction was similar to the Experimental airlines folded foil, with a center foam "spar" surrounding the wood spar and foam rolled over it to achieve the airfoil shape. It was kinda fiddly to get the shape right, probably would have been better with ribs! In any case, I find it to be a nice airfoil for sport soaring (I'm not a competitor). I trust your's will be as well!
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Old Feb 10, 2016, 05:23 AM
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Thanks Springer, your OSG looks nice. Yeah, I think the experimental airlines type foil looks quite good, in that it could be used to create a smooth airfoil. The trick is going to be making it accurate.

I've finished the DLG... 68 grams but will need some nose weight to balance. It utilises a special TA airfoil (truly awful!) just lightly sanded flat 6mm depron. But it should "make a good sloper" tail surfaces are b*lsa... but don't tell anyone
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Old Feb 10, 2016, 09:06 AM
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Yup, achieving an "accurate" airfoil in foam is the holy grail of foamie gilders. I wonder whether balsa rib monokote construction is "accurate" either. The actual airfoil in the area between ribs can't be the same as the section at that ribs. I suspect that the only really accurate airfoils are the hyper expensive molded CF jobs, but then the accuracy of the molds still has to be perfect. On the other hand, I recall a thread where jet plane flyer, who had been criticising our use of draggy, inefficient KFM airfoils took us up on our "encouragement" to try one; on his Elf DLG. (The upshot was a depron KFM2 wing got about 60% of the performance of the Elf's wing in his well designed tests. So performance less yes, but cost waaaaaay less - funperbuck ratio very high!). In the discussion it was noted that airfoil may account for only about 10% of total performance for contest gliders. Important to the competitors, but not so much for sport flyers.

As for me, I build the wings up, then use an 18" long sanding block and section templates to get the airfoil as close as I can (or until I get bored with sanding!) I cover them with coffee filter/wbpu varnish or packing tape which compensates for thinning of the foam when I sand it down. Crazy, maybe but it works for me.
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Old Feb 10, 2016, 12:29 PM
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Mark Drela actually designed the AG35-38 series specifically for built up balsa construction so they have flat sections where the covering sag would be. I expect to get a little sag of the depron sheet between foam ribs, but I'm really not taking to that level, I'll just be happy to get close. The thickness, camber and high point are the main thing.

Yeah I read that thread, kudos to JPF for making the comparison. I've flown an Elf and its very nice! 60% is a massive difference though, especially in a glider where efficiency is all. I agree that for fun soaring rather than competition, a few percent difference in performance isn't a big deal, but I do like to think a good design can be optimised for the job. In flat field thermal gliding a few percent can be the difference between elation and frustration.

I don't believe its just a cost issue, you can design a balsa glider that will perform very nicely, like the Mimi DLG for example. The Elf is a more refined design, especially where weight is concerned and that pushes up the complexity and cost. And it's an ARTF of course.

It depends on ones viewpoint doesn't it? I enjoy the design side of things and I have designed and flown competively in the past where airfoil improvements have made all the difference. Fundamentally I just want something that flies really well!

I think for many people it's a combination of is cost AND simple construction for minimum bother factor? I don't have an issue with KF airfoils if thats what people want to do, I have after all used a simple flat section on this DLG, but in a glider where you can't just open the throttle, aerodynamic refinement has to be up there on the list of design goals.

The aim for this model is to provide plans for an entry level glider at minimum cost and simple construction BUT optimum aerodynamics to give satisfying flying. It's also just a personal experiment for me to see if depron can match some of the woodie stuff I have. If this model can improve on the aforementioned 1.5m HiLite I'll be quite happy


Jon
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