Glider/ Ultra-Light personal Jet Aircraft DESIGN - Suggestions and help please!

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Mac790

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Hi Seb,

Just checked again, the 64(1)-412 is not documented. And the 6412 looks much too thin for a glider foil (IMO). Looks like I will have to forget about this one.

Why is it that you mention the earlier FX? Not long ago you suggested the more recent FX 79-K-144/17 and AH 93-K-132/15. Was this based upon the assumption that my glider is not aerobatic?

I have read the article that you posted. I can see why the Ribblet airfoils are good but actually it's difficult to find good info on aspects of aerobatics that influence airfoil selection. Would the criteria be relatively similar to picking out an airfoil for a "normal" glider?, I understand I will need high Max Cl to pull Gs and I also understand that I will pay the price by having high Cm and thus relatively big tail/ elevator? Perhaps not such a big problem for an aerobatic glider though?

Sorry for confusion, I'll try to clarify it.

First we should vary a conventinal glider, from a aerobatic jet glider.

For the conventional gliders at the beggining (I'm not talking here about "Flintstone" era gliders) they we using NACA 6x-xxx series airfoils, next they replaced NACA foils with Wortmann ones. Currently they use in most gliders flapped airfoils, DU ones. The point about using flapped airfoils is that, that by changing flap position you changing camber. Thanks to that flapped airfoils are good (high L/D) not only for slow flying (thermalls) but also for fast gliding. And this is probably the biggest advantage over non flapped airfoils. So when you asked about the best glider airfoils you can get for free, I mentioned those two.

But the question is, is it a good idea for your application to use flapped airfoils? (for this reason I posted that Swift movie, during aerobatics there is really no time for "playing" with flaps) Personally I have some doubts about it, but it's my personal opinion. When you mentioned NACA airfoils, my first idea was that maybe some earlier FX ones might be better option, as those airfoils replaced NACA ones, back in sixties.

I understand I will need high Max Cl to pull Gs

For a racing airplane our Orion was using Ribllet 37 series airfoil, when I asked him about it, this was his response.

Orion said:
The design point for the race is actually the turn, not the straight, which sort of forces you to use sections that have a better l/d at the higher lift coefficients.

But final decison belong to you, you have to study airfoil polars and decide which one suit better for your application.

Seb
 

Dana

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Hi Seb,

Just checked again, the 64(1)-412 is not documented. And the 6412 looks much too thin for a glider foil (IMO). Looks like I will have to forget about this one. ...it's difficult to find good info on aspects of aerobatics that influence airfoil selection. Would the criteria be relatively similar to picking out an airfoil for a "normal" glider? I understand I will need high Max Cl to pull Gs, and I also understand that I will pay the price by having high Cm and thus relatively big tail/ elevator?...

As somebody said above, the 6412 is not at all the same thing as the 64(1)-412. The thickness distributions and camber lines are completely different between the different series (4 digit, 5 digit, 6 digit) NACA airfoils.

I don't know that you necessarily need a high Clmax; you don't necessarily need to pull very high g for acro (especially in a glider); you want to keep it smooth, keep drag and thus Cl down to conserve energy, no? In power planes, low moment airfoils like the 23012 have proven good for aerobatics, and the sharp stall of that series may be desirable for stalled maneuvers like snap rolls, etc., but clearly that's not a sailplane airfoil... does the typical glider aerobatic envelope include snap maneuvers?

-Dana

Some people should thank their lucky stars that everything I wish for doesn't come true.
 

Bearcat F8F

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Seb, thanks for the clarification. Whats the best program for airfoil analysis? Xfoil doesn't seem to work on my PC for whatever reason. XFLR5 does but that's only for low Re's? Is Javafoil ok?

Dana, the S-1 and MDM-1 do great snap rolls. And I don't know of any other gliders with similar capabilities. So a "typical" glider envelope probably does not include this.
 

Bearcat F8F

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Seb, one more thing. Flapped airfoils - does the FX 79 and AH 93 fall into that category? I thought you can make any airfoil flapped? I mean by adding a flap at the trailing edge of the wing and changing the camber of the foil. Why are the modern DU foils flapped and the old ones aren't/ can't be?
 

Mac790

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Bearcat F8F said:
does the FX 79 and AH 93 fall into that category?
Yes both FX 79 and AH 93 are flapped sections.

I was looking at renderings of that jet glider posted a couple pages before, and I would hazard to say, that it has also NACA 6x-xxx series on it. Take a look at airfoil shape and compare it with NACA plots eventually FX series but it's probably NACA airfoil. The problem is, that there are only two professional aerobatic gliders, Fox and Swift, it seems that both use NACA 64-xxx airfoils...

It would be nice to get in touch with one of those aerobatic glider pilots, and ask him a few basic questions, I don't know any personally. You could try to write to Maciej Pospieszynski and ask him a couple basic questions, how it flies, how many G's he usually pulls during show, etc. Tell him that you are a student doing a research for your final project, etc. Those people are usaully "mad" about aviation and might answer you. If you want I can tranlate it into Polish, so it will be easier for them to answer you.

Bearcat F8F said:
Why are the modern DU foils flapped and the old ones aren't/ can't be?
The new one are designed to be flapped the old one weren't. It's really nicely written in the "Fundamentals of sailplane design" from page 90.

I don't know how far you are with your project, but for the first conceptual calculations you could "try" NACA 6x-xxx series and check it out. Without knowing envelope, cl that you need to pull "some g's" it's really hard to say, which airfoil do you really need. I don't know how is it in England but here during our finals, tutors expected some reports, some every week other every month etc. During those "meetings" we have an opportunity to ask them about some things, get some advices, etc.

Seb
 

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Bearcat F8F

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On their website they have a whole section devoted to the development of NLF(1)-215M airfoil. Perhaps that is what they are using? Looks similar enough to a glider foil, here's their pdf on it. http://adaexp.com/files/NC00.00.00.003.pdf It's on page 3.

I too was looking at their design for quite a while as it's pretty much exactly what I'm designing. The whole tail section is really similar to the Fox. And I really like their wing design although it looks like very complex geometry. On the other hand, not a huge fan of the jet exhaust location. I am going to try and redesign my original twin-boom into a single boom with a similar layout that I could later use for some comparison.
It would be nice to get in touch with one of those aerobatic glider pilots, and ask him a few basic questions, I don't know any personally. You could try to write to Maciej Pospieszynski and ask him a couple basic questions, how it flies, how many G's he usually pulls during show, etc. Tell him that you are a student doing a research for your final project, etc. Those people are usaully "mad" about aviation and might answer you. If you want I can tranlate it into Polish, so it will be easier for them to answer you.
To be honest there's enough that I can gather from the videos on how it flys and what Gs they pull. Unless they can tell me the geometry of the NACA foil they use it wont be that much use.

I don't know how far you are with your project, but for the first conceptual calculations you could "try" NACA 6x-xxx series and check it out. Without knowing envelope, cl that you need to pull "some g's" it's really hard to say, which airfoil do you really need. I don't know how is it in England but here during our finals, tutors expected some reports, some every week other every month etc. During those "meetings" we have an opportunity to ask them about some things, get some advices, etc.
Yes it's the same here. We only need to hand in 1 interim report (this will be after Christmas) but we do have regular meetings. However I feel that given how specific this airfoil subject is, my project adviser will not be able to help. Although I will ask his opinion on it for sure.

I have not started any conceptual calculations as of yet, entirely because I want to have a better idea of what wing I will have as it's obviously very important. Today I've spent the whole day going over from scratch over all the parameters important to airfoil design, and to some extent wing planform and geometry. I would say I have a decent understanding of the subject but not good enough to be able to know exactly what kind of laminar drag bucket I need for example... although I understand some aspects which would be preferable in my case. Not sure if a "deep" bucket will be better though in order to do snap-rolls, or stick with a smooth, shallower bucket for better stall characteristics?

I think in my case it may be a good idea to go with something proven and tested, except no data for the Swift and Fox exist on their NCA 6-series. So I could gamble with an FX and use it. The DA-40 uses an FX 63 foil I think. Of course it is not jet-powered but none-the less a good relatively new a/c to use a glider foil that works.
 

Bearcat F8F

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Sorry, one more thing.

I can't seem to get a realistic-looking laminar drag bucket for any airfoil in Javafoil. Is this something to do with Javafoil specifically, do you know? Re of 3mil or 9mil makes no difference either. I'm a bit confused why it doesn't look right.
 

Matt G.

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The problem is, that there are only two professional aerobatic gliders, Fox and Swift, it seems that both use NACA 64-xxx airfoils...

What about the Grob Twin II Acro and Salto? Neither uses a NACA 6-series airfoil.

Few, if any, gliders in the past 40 or so years have those airfoils. There have been many, many better airfoils developed since then. The Schempp-Hirth Austria originally had a 6-series airfoil, and it was eventually changed to an Eppler 266 (I think) airfoil, dramatically improving stall and low-speed performance. I'm not sure why anyone would want to use a 6-series airfoil on a glider this day in age.

Also, I saw Ribblett's airfoils mentioned somewhere...he states in his book that none of his airfoils are a good fit for a glider, so I wouldn't go down that path, either.
 

Bearcat F8F

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Matt, the only reason to go for the NACA 6-series is because of the MDM-1 and S-1. Both are unrivaled in capability. There probably are much better new alternatives but there has not been any new glider capable of equal aerobatic performance than the Fox and Swift.

Although the Salto looks highly capable. What airfoil does it use, do you know?
 

Mac790

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Bearcat F8F said:
On their website they have a whole section devoted to the development of NLF(1)-215M airfoil.

Ok, I took a look at that paper and it seems that they de-cambered NLF foil (de-cambered 215 is really similar in shape to the NACA 6x-xxx, so it mislead me). The problem is that you don't know, did they make any other changes, beside it. If you start palying with it you might be in same position in the next couple months.

Matt G said:
Few, if any, gliders in the past 40 or so years have those airfoils. There have been many, many better airfoils developed since then. The Schempp-Hirth Austria originally had a 6-series airfoil, and it was eventually changed to an Eppler 266 (I think) airfoil, dramatically improving stall and low-speed performance. I'm not sure why anyone would want to use a 6-series airfoil on a glider this day in age.

The only real solution for it, is to use XFLR, Xfoil, etc do some test, check out which airfoil will suit his misssion. After that get some official papers for his final section and compare XFLR results with it. It's easier to get papers for "well known" sections.

edit
Bearcat, I'm a little bit confused now, are you going to build pure aerobatic plane? or something that can do some rolls occasionally. Almost every plane can perform basic aerobatics, and you don't need any "special" airfoils for it, unless of course you want to design a pure aerobatic plane. If not stay with what I wrote in post 151, and forget about all that stuff after that.

Seb
 
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Matt G.

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The Salto uses a Wortmann airfoil, although I am not sure which one. Another aerobatics-capable glider is the IS-29(?) Lark, which also uses a Wortmann airfoil. The Grob Twin II acro uses an Eppler airfoil if I'm not mistaken.

I don't know much about the MDM-1, but in 5 minutes of reading I have learned that the MDM-1 is a derivative of the S-1, which is probably why they have the same airfoil. The acro-capable glider market is pretty small, which is probably why not many new ones have been developed. The two examples you are using as the 'gold standard' are nearly 20 years old already. Since you don't know why the designers of those two gliders picked that airfoil, I'd be leery of choosing it for yours based on that. Pick the right airfoil for your design and mission.

I have missed some of the middle of this thread; I thought at the beginning you wanted cross-country capability as well. If this is still true, it will not be competitive in that respect with any current offerings with a 60-year-old airfoil. You also mentioned earlier something about a 6412 being 'too thin' for a glider. Modern gliders have pretty thin airfoils; 50 years ago thicker airfoils were needed so that their wooden wings could have the necessary strength without being ridiculously heavy, but with modern carbon fiber construction this is no longer a problem.
 

Bearcat F8F

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Seb, depends what you mean by "pure". Yes I want it be "unlimited" if possible. i.e. be able to do any stalls/ spins/ snap rolls etc. However, I would like it to perform decently enough in gliding flight. I guess a glide ratio in the high 30s would be something to aim for. Whether or not I can incorporate any decent x-country capability remains to be seen. It's not a priority for now. I would like something that will perform similarly to the Salto, S-1 and MDM-1 (if possible) whilst combing the whizz factor of a jet.

Matt, thanks for the info. I will see what non-flapped FX foils are available in the public domain and could suite my application.


I have fine-tuned my original crude CAD drawing. I've now stuck the Jet Beetle H250 engine inside the fuselage. Added a little more aerodynamic shaping to the booms and added a more realistic tail profile. However it's still a basic drawing with lots of details missing. I was just trying to fine-tune the whole aerodynamic shape a little bit. Once I start calculating things hopefully things will begin to fall into shape better.

The wing for this drawing is still the FX 79 11m wingspan. All subject to change.

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Himat

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It do look like a nice aeroplane from the images!
But one question, why have you chosen to have inverse taper on the tailplane?

As for where different airfoils have been used, there is a list. The Incomplete Guide to Airfoil Usage: The Incomplete Guide to Airfoil Usage
If not mentioned before, the Pilatus B4 used a six digit NACA airfoil, the NACA 64(3)-618 acording to Wikipedia. If you study "the incomplete list" you will find that Slingsby used NACA 63-XXX on their sailplanes. Later or it look like Slingsby instead chosed Worthman FX airfoils.
 

StarJar

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Wow Bearcat, those are really nice renderings. You should give a course on how to do that!
It do look like a nice aeroplane from the images!
But one question, why have you chosen to have inverse taper on the tailplane?

I think Himat has a point here, though. The "dorsil fin" on, say, a Cessna 172's vertical stabiliser, keeps air from the fuselage side from slipping over the top. This in effect turns the rear fusalage into a virtual vertical stabiliser, to add to the real vertical stabilizer's area and force.
On your glider, the tapered section next to the boom, will not have the same effect as that. The air on that section can slip around the end, with only the thin boom to block it. In other words a non tapering surface would be slightly more efficient, IMO.
But overall, the rendering looks really cool. Wish I could do such a rendering, but ya, I know, it just comes from a lot of hard work and practice.
 

PaulS

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Your rendering doesn't show the intake for the jet... Is that an oversight?
I like the overall appearance but I have to agree that using a non-tapered tail will increase effectiveness and reduce drag - even if it were as wide as the base of the tapered portion shown.
 

Aircar

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This thread seems to have become hung up on airfoil section theoreticals, like so many others, and since Xfoil and the other airfoil design programs became available --I have seen people dissappear into their computers for years obsessed with some fifth decimal place drag count or similar fetish and never get off the ground.

Mark Drela had a real 'need' to find a very much better airfoil for the man powered aircraft challenge (if you can call the creation of such a use-free aircraft a need...) --and splitting hairs really did make the difference between go or no go.

There are many aerobatic and semi aerobatic gliders with all sorts of airfoils and they have bugger all influence on the handling --my old Sagitta was one of very few fully aerobatic gliders ever built and at least had the right sort of cockpit environment for it - a nice bubble canopy with mid wing and an unrestricted field of view (great for watching the horizon re appear . I used to fly it with the rear sliding part of the canopy off and in this case you really FEEL aerobatics which is what exuberant rather than deathly serious competition aerobatics are supposed to be about. Only ONE person wins a competition --no one loses if you are flying for the love of it.

For real all out aerobatics you want a symmetrical airfoil and an airfoil with a savage stall break -IF you want to do snap rolls on command (which is a horizontal autorotation or -spin- ) the Schweizer 1-23 was the most snappable glider I have flown and did nice predictable sustained snap rolls at about a forty five degree nose down attitude without increasing speed --so what ? Snap rolls in a Pitts or something are neck snapping violence and are temporary out of control and do not reflect the essence of sailplane behaviour --check out Manfred Radius's aerobatic air show routine for some idea of grace in aerobatic flight.

The Salto has a cut down Libelle wing and uses the Hutter airfoil .
 

Bearcat F8F

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Thanks guys.

I see what you mean with regards to the dorsal fins. Am I right in saying they create a problem in my situation due to the narrow (thin) tail booms, thus making little difference if any?

It do look like a nice aeroplane from the images!
Wow Bearcat, those are really nice renderings. You should give a course on how to do that!
Thanks for the kind word guys but I am still a novice! I've only started using Solidworks specifically for this project so I could do a bit more professional sketching that just paper and pencil. Having said that I am still only using basic functions of the program. I will get better.

Your rendering doesn't show the intake for the jet... Is that an oversight?
There are lots of things which are not there including control surfaces, landing gear etc etc. The intake should not be a problem. But I will finalise this aspect of the design a bit later once I have a fuselage that I am more or less happy with.
There are many aerobatic and semi aerobatic gliders with all sorts of airfoils and they have bugger all influence on the handling --my old Sagitta was one of very few fully aerobatic gliders ever built and at least had the right sort of cockpit environment for it - a nice bubble canopy with mid wing and an unrestricted field of view (great for watching the horizon re appear . I used to fly it with the rear sliding part of the canopy off and in this case you really FEEL aerobatics which is what exuberant rather than deathly serious competition aerobatics are supposed to be about. Only ONE person wins a competition --no one loses if you are flying for the love of it.
Aircar, thanks for the helpful insight! I completely agree. a) I do not want to get hung up on airfoil choice and b) I agree that it should be all about the experience and pleasure of flying and centering the aircraft around that basis as opposed to the ultimate aerobatic competition machine.

My biggest issue is justifying the choice of my airfoil in a realistic way for my final report and presentation in spring. I can't just say "most airfoils are more or less good enough and won't make much difference and hence I chose this one...". So this is my main concern and that's where I am stuck now. How do I justify a choice for my mission.
 

Bearcat F8F

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Had an idea for aerobatics:

Would it be beneficial to develop automatic deployments of leading edge slats (sort of like what fighters have for high AoA maneuvering)? And also perhaps using an FX 79 (or similar) which has a flap (camber change) that could also be automated depending on the flight regime? i.e. for high AoA 1 profile, for glide/ cruise another profile?
 

Mac790

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Aircar said:
...I have seen people dissappear into their computers for years obsessed with some fifth decimal place drag count or similar fetish and never get off the ground.
Hahahh so true.:) I gave you like for it.

Bearcat, the best idea for you is probably to make a table similar to those guys above. In the first row put requirements for your airfoil, next test some foils, just for comparison two flapped FX79, AH 93, one or two NACA 6x-xxx series, next some fixed geometry airfoils, for potential candidates take a look at page 91 in "Fundamentals of Sailplane Design". Compare them, and choose that one which is the closest to your requirements, from first row in your table. I don't see any other option. But of course you need to know, what to put into first row. When your PHD ask you , "why did you chose this particular airfoil?" show him the table, and say because, this particular airfoil was the closest to my requirements.

Just two additional papers, first on double/triple taper wings design, second (just to put more oil into fire:)) about aerodynamic wing twist.

Seb
 

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Bearcat F8F

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Hi guys, sorry for the long absence. I have got on with the calculations somewhat but need a little help.

If I know the area and dimensions of a conventional tail (i.e. the horizontal and vertical tail), can I convert this in some way to an equivalent inverted V-tail?

I know a formula for converting a normal tail to a V-tail, however was unsure whether the inverted V is just the same as an upright V in terms of dimensions and angle (except flipped upside down)?
 
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