Airfoil for an ultralight sailplane?

Hello everyone! Its great to be here! I usually dabble in remote control glider forums and was pleasantly surprised at the great attitude/awesome projects going on over here! Anyways, I'll cut to the chase.

I am planning on building an ultralight glider very similar to the Goat/Bug planes found here http://home.att.net/~m--sandlin/bug.htm

My plane will be somewhat different in a few aspects however. I plan on using composites throughout the wing and possibly fuselage. The wing construction will be that of a composite D box for the leading edge, and an open rib structure for the aft portion of the wing. The plane will a bit smaller than the goat plane and should weigh a good bit less due to both the composite structure, and the small size of the pilot (I am very light by todays standards at only 130lbs) Now, what I really need to know is what sort of an airfoil to use! The goat plane uses some very VERY simple airfoils that are essentially hand drawn. Should i go with something like that? Or is there a better, more efficient option for what I'm planing on doing? Thanks for your time!

Michael

 

Hi Michael and welcome to the group.

Selecting an airfoil section is sort of a balancing act between performance, structure, volume and flight characteristics. In your case though, since you're working toward an ultralight and a fabric covered structure, I'd say you need structural depth in order to minimize the weight and a good amount of efficient lift for low stalls and reasonably good performance in thermals or in updrafts.

The classic "turbulent" sections, the 2412 or 4412 should do well although you might benefit a bit more by going to the 15% counterparts. For a bit less pitching moment, you might also want to examine the five digit series such as the 23012 or its 15% family member, the 23015.

Another alternative is to pick up the book by Harry Riblett and look at some of his lower speed selections such as the 30- series.

The basic problem you'll have is that with fabric covering, the actual performance will most likely be measurably different from the published data, but you should be able to get close if you pay attention to the quality of the build.

Hope this at least heads you in a useful direction.

 

Orion: Thanks so much for the quick and detailed response! I think the 2412 naca series should do very well for my project. It comes in at a very similar thickness as that found in the goat plane which gives me additional hope. I now have one more question.

In the RC glider scene, a fairly common airfoil to use for "built up" gliders (d box/rib) is the AG 36 airfoil. It has a curved front section on both the top and bottom, where it differs from normal foils is that both the bottom and top rear sections are flat, this makes for better performance with covered models as the sections that have covering film are flat. Is there anything like the ag 36 that you might suggest/is commonly used for cloth craft? Thanks again!

Michael

 

I'm not familiar with that one but in general, the curved shape of the section should provide a better geometry for the fabric to stretch over, giving it more stability throughout the speed range. I would think that's to your benefit, despite the slight loss of section shape between the ribs. At the low Reynold's Number you'll be operating at, I wouldn't think it would be all that much an issue.

 

Michael, that kind of structure exactly are you planning to use? Just curious. I have been dreaming about an ultralight sailplane from composites for a long time.

Carbon rods for spars? What kind of shear web? What kind of D-tube - fiberglass? How do you plan to mold it? What kind of ribs - foam or wood? How will the ailerons and flaps be made? What kind of hinges? Sorry for too many questions.

Oleg.

 

Have you seen that page?
http://www.hpaircraft.com/hp-24/

If you have been dreaming for long time, that page should be for you.

Have you heard about AL12M?
[video=youtube;f5yjwvi2XeM]http://www.youtube.com/watch?v=f5yjwvi2XeM[/video]


The previous owner was selling plans (if I remember correctly) for that self launching sailplane unfortunately new owner sells only kits form.
http://www.flylight.co.uk/gliding/index.htm

Anyway sailplane its a nice idea for raising fuel prices.

 

Yep. That's not an ultralight sailplane though. Way above my league and probably most of the "normal" builders. Bob has been plugging away at his HP-24 project for many years now.

Yep. This one is very much what I would love to get (build?..). But even this is a very complex machine. My aspirations are a lot lower after several years of "dreaming". It has to be a lot simpler to build it in a reasonable amount of time.

As far as I know, it was never offered as a kit or in plans form. They only built and delivered 1-2 gliders, from what I gather. There is zero information about owners of Alatus (AL-12M).

Michael, tell us more about your project!

 

I'm guilty my fault I didnt read correctly. I understand that you are talking about something similar to goat but with composite wings. I included few pictures (please note that these 2 gliders were produced in 80's so no "fancy" carbon just ordinary glass).
Almost forgot they were using Naca 4415

 

Regarding your mention of "flat" sections top and bottom aft of spar ; if these are fabric covered in flight they will be no longer "flat".Unless the tension in the covering material is incredibly high, the top side sections will "balloon" up, ie gently bulge, and the bottom sides will become slightly concave, the effect for both differing with airspeeds. These effects are vanishingly small for even large radio controlled glider models, where the covering is proportionally thick and stiff, and the unsupported areas are relatively small, but there will be significant effects at larger scales.
There was in fact a foot-launched glider with a wing section which actually took advantage of the distortion described, resulting in the "Princeton Wing."

 

Glider design.

thanks for the inspiration mac790, the glass glider looks pretty sweet! It does look really beefy and maybe heavy though : ). Do you know if its all glass? Us there maybe some kevlar in there too (im wondering about the brown color)?

Macboffin/Orion: Thanks for explaining the ballooning effects etc. It is clearly very different in full scale vs small scale.

Olgol: Your enthusiasm is inspirational, so thanks for the questions! I have put together some rough 2d drawings for you guys (I can neither draw well in 3d, nor have a program to do it in).

When I originally set out to build this craft over a year ago I figured I would use hand laid up carbon caps with foam cores. After testing a section of spar I had made to destruction, I quickly realized that was not the way to go. My hand made caps failed at around 40ksi instead of the 80 I was hoping for! That got me thinking then about using graphlite carbon rods. As im sure many of you already know, graphlite rods are very precisely made and thus have a compression rating of around 220ksi! They are also very cheap because, being so strong, you dont need much of them. For my plane I will probably need around 150' which is about 150$. When I decided to use graphlite, I had to change my spar design dramatically, no longer would 60 psi pink foam work for either a compression web, or a shear web. Now I am planning on using a vertical grain balsa web. Balsa is far stronger then foam per unit weight in all the directions that matter (compression/shear), thus I figure I can use a much smaller shear web and have a lighter spar.

I am planning on making foam ribs that are laminated with obechi veneer on top bottom/sides. This should make for very strong and light ribs. I would use glass on the sides of the ribs, however, because of how thin the glass would be, it would most likely just buckle under compression/be very delicate.

The last major part of my wing design is the D-Box. I am planning on making a large female mold, sort of in the style of a skate boarding ramp (if any of you know how those are made). Inside the mold I plan on laying up a layer of glass, then a thin (3-6mm) foam core, then another layer of glass. This one piece d-box will then be glued onto the ribs/cover the spar.

Im not all to sure what I will do for the trailing edge, if any of you have ideas that would be great!

As for the body, I think I will model it off of the goat fus. I would love to make a carbon boom, Im not sure if that will be practical/possible though, we shall see.

I haven't yet decided whether my plane will have any form of flaps on the wings. Because of my construction technique, I would love to keep them flap less, however I'm not all to sure how common that is in full scale.

To much writing for one post! If you have any questions feel free to ask!

Michael

P.s. Im planning on doing 2 joiners.

 

Re: Glider design.

I found some description in English so you can read a little bit more about it. By the way these gliders were designed by people from Warsaw University of Technology. Few years later these same people designed PW-5 "SMYK" world class winner glider in 1994.
http://jarek24.w.interia.pl/index2e.htm

The answer is really easy they kept that glider in museum outside so now its brown from UV. From my knowledge they didnt use kevlar you need to remember that in 80's it was impossible to get stuff like carbon or kevlar in Poland (you know communism, iron curtain etc)

 

Re: Glider design.

Do you have any pictures? (with test result) Could you tell us something more like:
- wet layup or vacuum
- type of resin MGS 335/285, West Systems etc
- room temp cure or "baking"
- type of carbon tape
- type of foam
- etc

Thanks
Seb

 

Re: Glider design.

It is obvious that you are coming from the RC modelling world . Your design looks just like an RC model.

Looks like it is based on Archeopteryx ( http://www.ruppert-composite.ch ) more than on GOAT You hardly have anything left from the GOAT designs.

Your tail boom looks way too long. Your nose is a bit too short. But that will be apparent when you do the CG calcs.

Changing the spar cap material to rods does not necessarily change the shear web design. If you can provide adequate means of attaching spar caps to the foam shear web, it can be done. Contrary to Orion's belief that styrofoam should not be used in airplanes, it has been and will be used successfully in ultralight designs. I am not convinced that very thick balsa shear web is the right approach. A sandwich of 1/4 or 1/2" foam and 2 layers of carbon cloth has a tremendous shear load capacity and very low weight. Fiberglass would require more layers and slightly higher weight, but probably not a big penalty.

D-tube molds - you are brave if you think you can do it yourself in any reasonable amount of time. You will need 3 huge molds for your design. Did you think about how to make the molds?

I was planning making D-box also and using foam positives (plugs) and bypassing the molds. But even that is a huge undertaking. I reduced my aspirations to just a very small rigid leading edge. Torsion strength will have to be provided by a large box spar. The spar does not touch the surface of the airfoil and can be as ugly as needed to be able to make it without (almost) any tooling. The airfoil accuracy suffers a little, but that is the price for being able to actually make the wing instead of dream about it. So far it dies not help my progress though I postponed my glider project for now.

 

Re: Glider design.

Way too complicated and heavy! A rib made of 1" or 1.5" styrofoam capped with wood or plywood (!) strips is very adequate, very easy to make, and has been used before (Affordaplane and other designs). No need for laminating the sides! The foam has enough shear load capacity. You can use 40psi or 60psi foam if you want to increase the strength, but then you may have to slice the foam into 1" sheets yourself, since high density foam usually (always?) comes in 2"x24"x96" sheets only.

 

Hi,
If you are familiar with Graphlite rods, you are also, presumably, familiar with how Jim Marske makes his spars. I've attached sketches from Jim's site just in case.

It seems to me that you're wasting productive brain cells in reinventing the wheel with this. Jim's method is simple, easy and effective.

Cheers,
Duncan

 

Och, ye beat me to it! Here's the link to the page where the pics came from:
http://www.continuo.com/marske/ARTICLES/Carbon rods/carbon.htm

You may also want to wander around here:
http://www.continuo.com/marske/index.htm
since the other Marske sailplanes may meet your needs - why design something new if you don't have to?

Last, but not least, poke around HomebuiltAirplanes for posts by Topaz - he's designing a sailplane from scratch and his thoughts would likely be of value to your project.

 

Great responses guys! I only have a little time right now to reply, so I will only say what is immediately on my mind for now.

I have indeed studies both the monarch page, and the graphlite page posted bellow, along with the spar making technique shown in said pages. Now from the outside perspective, one may wonder, why is this guy trying to re invent the spar when there are perfectly good designs already out there? This is perfectly reasonable, however I have two reasons that you may or may not have considered.

1) Have a look at this:
http://www.boatdesign.net/articles/foam-core-properties/index.htm
Balsa wood is a clear winner over different types of foam in both compression and shear for its weight. Because of this, and its low cost, I feel it would make a better web then foam. (another thing is, because you cant hot wire pvc, balsa would be no harder to work with)

2) While the posted I beam does look really nice/easy to make, my spar wouldn't be very much like it... My spar only has 6 small rods, or 3 decent sized rectangular rods! I feel like that lay up would be somewhat over kill dont you, especially for only 3 rods! My lay up is essentially the same in that it has some form of rectangular shear web with a bundle of rods on top and bottom glued both to the outer glass or carbon shear web and the inner balsa shear web.

I feel like those are two pretty decent reasons for going with my simpler build, however I'm sure you guys have thought of something I haven't. Again, if I was using 12 rods, I wouldn't hesitate with using that method, the thing is, Im thinking only 3.

More info tomorrow/replies tomorrow!

Michael

p.s. Olog: You are right, my body design is far from perfect (I would have to have lead feet to make that body work...) Its a work in progress :gig:

 

You mentioned making a carbon tail boom, have you looked at the building techniques of the Carbon Dragon ultralight glider. I bought a set with the intention of building one but after studying them decided I did not have the time to invest. And it was so much easier to buy a partnership in an HP-11, Schreder was a great designer. Anyway, the CD tail boom is two halves built like you described, in a half pipe, and then attached over a few spruce bulkheads attached with four stringers. The mold is supposed to be one of the easiest parts of the plane to fabricate using just a few wooden C shaped blocks and a piece of Masonite or Formica. I would highly recommend researching the Carbon Dragon if you are going to design your own UL glider, it has a lot of great ideas and uses carbon and carbon reinforced spruce through out to make a foot launchable sailplane.

 

Hello Mac790,

Can you tell me what glider is pictured in your post above? Thanks

Sorry, found it in the next post.

 

https://picasaweb.google.com/100929808852675212367/ProjektAxelSzybowiecUltralekki?noredirect=1#

=AXEL1 \glass\,

https://picasaweb.google.com/108109095271638092325/Axel2

=AXEL2 \carbon+kevlar\...

and AXEL0 + RACKET120! \55 +27 kg !!!\