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Old Jul 02, 2016, 04:19 AM
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Hålo 2m RES

Hi all,

I've been developing my own design 2m glider based around the German F3B-RES concept. It's called the Hålo (ok so its just Halo - but the accented å looks cool )

The idea is to build a light weight high performance glider for bungee launch after the fashion of the PuRES, Slite etc but for scratch builders. This means using reasonably simple build techniques and available materials - and, importantly keeping the cost reasonable.

The wing construction will be a simplified Allegro Lite style spar using readily available pultruded carbon strip. The fact that it's for bungee and not winching means I can reduce the strength and weight a little. A sheeted d-box provides torsional stiffness.

The boom is a carbon fishing rod section. Using a fishing rod boom would have been seen as a poor substitute a few years back but they are catching up and I've found sections stiff and light enough for only a few quid. Perhaps not optimal but quite acceptable for this design.

I've spent a good deal of time optimising in XFLR5, but at some point you have to commit and get building I've decided to go for a V-tail and single spoiler. Airfoils will be AG36 through AG38. As to target weight - I'll be happy if it ends up under 500g.

This is a development thread, so I'd welcome input as I go! Pics show some layout sketches, XFLR5 model and a full size layout on my floor.


Jon
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Old Jul 02, 2016, 10:12 AM
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Hålo 2m RES

Great to see the next step inspired development of the RES movement.

My thought is the boom looks a little short? have you checked Drela's spreadsheet on basic sizing checks?

Also the issue with sweep back wing planform is that spar build becomes more complex to build defeating your intent of keeping it simple.

I have recently designed a 2m RES motorised woody for our F5J 2m comps and went through several of these.

PS do I see a mixture of metric and imperial measurement on your drawing? That's like writing both a left hand and right hand!
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Old Jul 02, 2016, 02:31 PM
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i prefer "RESonator"

with that much area 500g should be really nice but IMO you shouldn't have too much problem getting the weight down closer to 400.

i've also got one in the works but not going to attempt any kind of build thread until it's actually done.... shooting for 350g or less and using carbon tube spars with open bays. coming up with a decent name seems to be harder than the actual design itself
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Old Jul 02, 2016, 05:20 PM
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Thanks for the input Super Six - all opinions welcomed Yes I use Curtis Suters Sailplane Calc file which incorporates the tail sizing checks suggested by Mark Drela. I've also added a couple of my own including tail moment arm divided by mean chord. On this model it's 4 times the MAC, which is plenty. In fact the tail is half an inch longer than the PuRES. Its possible that the full size layout pic was taken before I extended it a bit though.

I have some ideas for the wing joiners which should make them a little easier than the Allegro method. FWIW this isn't meant to be ultra simple or a first build, just a little less involved than the Allegro. The sweep is there for looks mostly! I love optimising a design but occasionally struggle to live up to my principles

Regarding metric and imperial - I'm pretty ambidextrous and convert between the too. In this case the metric is there because the carbon spar material comes in 1m lengths. Balsa is still mostly imperial here in the UK though.

Quote:
Originally Posted by ZAGNUT View Post
i prefer "RESonator":
Ha that was the name of an earlier iteration - a bit of the guitarist me getting mixed up with the modelling me

Yeah, I'm hoping for closer to 400g in practice but the modest aspect ratio of 11.5ish allows a bit more. I've spent quite a lot of time optimising aspect ratio and whilst intuition says a high aspect super lightweight model would be best, it doesn't pay to go much less than 12, according to XFLR5, even when the smaller tail requirement is considered.

I'm hoping this thread will help me keep some momentum on the design/build


Jon
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Old Jul 03, 2016, 02:14 AM
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the newest version of saiplane calc on Curtis' site calculates the tail moment from 25% wing MAC to 25% tail MAC for you.

for EDA i like Martin Brungard's eda1 from the charlesriverrc site as it converts actual panel length to projected length and compensates for wing taper.
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Old Jul 03, 2016, 05:16 AM
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Zagnut, I'm using Sailplane Calc 2011, is there a newer one?

I calculate the tail moment from CG to 25% tail MAC. This is the actual physical length of the tail moment arm as the CG is the 'pivot' in flight. It's a fine point though because the 25% wing MAC is close to the CG for normal tail volumes in a model of this type. 1/4MAC is close enough for practical purposes. (Pedantic? Moi?)

I also use the Brungard eda file, but for practical reasons Sailplane Calc works I think.

I have also written various excel files for my own use. At some point I need to integrate them all into one big design package...

Montag DP's CGCalc is extremely good also. It gives multiple panels on the tail and the point mass calculator is excellent for laying out the component parts for minimum nose weight.

http://www.rcgroups.com/forums/showthread.php?t=1106300
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Old Jul 03, 2016, 05:32 AM
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25% MAC is the supposed aerodynamic center of the wing so i would think that moments should relate to it and not the CG...but yeah, they're close enough for either to work.

i'm good at calculating near exact weights of the frame but i still want to have the tail built before i finalize the nose length. nothing worse than building light and then having to add ballast to get the CG right....

oh, newest sailplane calc is from a month ago or so. has a neat converter added to the dihedral page as well as the moment arm addition.
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Old Jul 03, 2016, 09:46 AM
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Hålo 2m RES

So what is your root chord length? Also are you going to include the sub spar as per AG foil? and have you considered the BC foils.

I have plotted the BC foil for my next build with the sub spar full depth to give the covering a little more sticking surface to create the undercamber.


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Old Jul 04, 2016, 03:37 AM
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Looks good

Slowmatch, your design appeals to me, I like it a lot! Good luck with the rest of the development and build. I do a lot of "development" in my head and by building small balsa chuckies with scaled down dimensions to test the basics.

I designed the below wing using Winghelper from www.winghelper.com, it took less than half an hour to design it, and this is for someone with VERY limited CAD ability - this program is great! The foils are AG38/36 and similar spar to the Allegro using the standard carbon lengths bought at my local hobby shop. The F5j version has more traditional spruce spars as they are not under the same stresses for launch.

The winch/bungee version is 590g - all balsa fuselage and the F5J is 660g AUW - I can get this lighter using composite fuz etc, although I have found that when the wind picks up the planes get thrown around a bit so will need ballast.

Again, enjoy the build and I am following!
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Old Jul 04, 2016, 06:07 AM
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Ok, thanks Zagnut, that had passed me by but I'll check it out.

Quote:
Originally Posted by ZAGNUT View Post
25% MAC is the supposed aerodynamic center of the wing so i would think that moments should relate to it and not the CG...but yeah, they're close enough for either to work.
In danger of hijacking my own thread here but in the interests of clarity: The physical property approximately assessed by tail volume is a force (lift at the tail aerodynamic centre ie 25% tail MAC) applied at a lever, ie some distance from a point of rotation, which is the centre of gravity.

Using 25% wing MAC is an approximation and assumes the CG will be at 25% MAC - near enough for tail volumes around 0.4 that you find on radio gliders. On free flight models you can find tail volumes of 1 or more and CGs back at 80% so the simplified tail moment will underestimate tail volume.

This pdf from Mark Drela clarifies the definitions nicely: http://ocw.mit.edu/courses/aeronauti...bs-06/spl8.pdf

Sailplane calc uses the 1/4MAC method and I happily use it - as we've said, its close enough. But it's good to understand where the fudge is even so

The wing aerodynamic centre becomes relevant when you start assessed the actual moments involved in static stability calculations because the lift can be assumed to act at that point. If the CG (the pivot!) is behind 25% you get a small nose up moment as a result. But this is a bit beyond the scope of tail volume.
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Old Jul 04, 2016, 06:17 AM
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Super Six, the root chord is 8" (or possibly a bit more depending on how I decide to taper the trailing edge stock.)

I've looked at the BC profiles in the wind tunnel test thread but decided to go for the AG36-38 for ease of building. I will probably omit the sub spar and smooth the profile accordingly, and I'm not intending to sheet as far back as Drela suggests. I'm aware that this is a fudge but for me this is a 'get me flying' model so I'm happy with the compromise. I do have some airfoil ideas for the next model, in fact in terms of the aero design I have already done much better than this but the sections are thin (6%) and so the engineering may be tricky.

The sweet spot for this model is to be 'good but practical'. I generally build much smaller models so I'm feeling my way with the structural design on this one. If all goes well I will continue with further iterations but you have to start somewhere. I tend to let the perfect be the enemy of the good (in design terms) but the perfect never gets built


Skinnyone, nice planes! I've not tried Winghelper but it looks cool. This model will be built without access to a laser or CNC cutter but I might go that option in the future.


Jon
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Old Jul 04, 2016, 07:23 AM
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on the AG35-38 keep in mind that Drela was going for a balls-to-the-wall winch proof spar AND he wanted sheeting above it....with these lightweight planes i really think we can thin things down by quite a bit and still get enough strength.
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Old Jul 04, 2016, 07:58 AM
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Yeah, I agree. Hard to know how much without some practical experience though. I've got the AG36 at the root so slightly thinner than the Alegro Lite on the centre section.

I've been reading a lot of the rc-network threads - it looks like the Baba Competition has a similar spar to what I have in mind - but it appears to have no Kevlar wrapping. That would save quite bit of weight, but I'm not sure how much stiffness you'd lose?
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Old Jul 04, 2016, 08:53 AM
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the kevlar wrap is just to keep things from delaminating. can't see it doing much at all for stiffness unless way, way overdone. would probably still be wise to wrap high stress areas like joiners and the center. really light thread and just a wipe of epoxy to hold it should be enough and not add any significant weight.

edit: stiffness in torsion might be another matter.....wide caps with a full web and wrapped in both directions would be pretty stiff. sheeted D box also works but only so much it can do with a really thin wing profile.

gotta compromise...i like to remind myself of the gentle lady or wanderer wing construction whenever i start worrying about strength and stiffness.....
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Old Jul 04, 2016, 04:02 PM
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That's true... But then they do have big fat airfoils
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