rbjjr

Need help w/ airfoil choice for large scratch built airliner MD-90. Wingspan will be a little over 8 feet and built of foam. I have the airfoil number for the real thing but have no drawings. Should I use the actual airfoil or substitute another more suitable to a large scale model? I would like a well behaved airplane. In addition to the wings, what airfoil should be used for the tail surfaces? I don't have an airfoil number for these.

SKYBLAZER1

In no way should you use the scale airfoil unless you intend your jet airliner to fly scale speed. I dont think it is feasable even using reynolds effect calculations (please dont ask me for the formulas...I'd have to go back to the darn library and consider the calculated weight, aspect ratios, wing loadings, A bone connected to the fuse bone or something like that stuff). Also, if the scale airfoil were used one might consider employing every aspect of the scale wing which is highly detailed which would include slats, spoilers, and flaps...lots of work there. Sounds like a nightmare huh?
In my opinion I would settle for a nice tapered planform that would be semi symmetrical for good stability if I felt that the wing area was too small in scale outline to adequately support the usually huge fuselages that come with commercial aircraft. I would be better off advising you if I knew the approximate overall projected weight of the model before I threw an actual number at the wing area of your bird. Then we could try to figure it out. Also, since the outline should stay scale...the only thing you can do is make the wing fatter by changing or thickenning the airfoil at the root and tips. When you do build your ship dont forget to build in around 2 degrees of negative incidence into the tips so she doesnt to mushy on landings. I'm leaning toward semi-symmetrical here for good lift. A moderately thicker airfoil at the root tapering to a slightly thicker airfoil at the tip.

So, there you have it. I have been flying models for 30+ years.
Take my advise or leave it. Good luck.

BernieG

You might be extremely surprised to learn that Jets models of F16 and F104 fly actually very well with the scale airfoil.....And they have much less wing and much more speed than an airliner !

I would say go for it ! Make a mok model out of foam, and try it on a slope or towed, before doing the real thing, to see how it fly and verify the CG. An airliner is a very fat sailplane......So the mok model should have a nice glide. Once you get it to fly OK, build the final model with the exact same CG, and you should be safe.

Bernard

rbjjr

I appreciate the input very much. I am relatively new to this so please excuse some potentially uninformed comments.

1) Might anyone suggest an airfoil shape? It has been suggested to me that I use NACA 0008 or NACA 0010. What about these? Or can anyone provide one more suitable?

2) Don't yet know the weight of the model but if I follow the formula of 1"=1', the wing area would total approximately 1,200 sq inches.

3) This may sound crazy, but I was considering the use of leading edge slats, spoilers and flaps (flaps for sure) because it would be closer to scale and, dare I say, it would look so cool!

I have no idea, however, how to make the slats work, although I could probably figure out the spoilers. Anyone have any experience w/ slats on a model? And, if so, how in the world to keep them on the wing and make them work?

4) Skyblazer1: Could you explain "negative incidence into the tips" please?

5) I have also heard the term: "washout" being used when referring to wing design. Could anyone explain this term?

Thanks again and sorry for the dumb questions.

BMatthews

I think you have the right idea asking about using other types of airfoils. Some of the full size airfoils used on airliners are pretty strange looking. I've had good luck using an Eppler 201 on a relatively heavy loaded electric sailplane. This particular airfoil seems to handle the loading well and still shows a very good speed range. As a bonus this particular airfoil is 12% thick which would help with structural considerations. Another similar airfoil that I would consider is the Selig s4322 (or was it a 4233, it's the one that's about 12% thick also in any event). Both these airfoils are about 3% camber which would give a good speed range.

Also being swept it's wise to use a lot of washout in the wing (reduced angle of incidence at the tips which means the center line of the airfoil is pointed down at the tips relative to the airfoil centerline at the root. The angle of incidence is the angle between the arbitrary aircraft center line and the centerline of the airfoil.) to avoid tip stalling and the resulting deepening of the stall angle that apparently comes from the swept wing planform. Not sure how much to use but if you can't find the scale numbers then 4 or 5 degrees should be quite safe.

Flaps are certainly do-able but I'd think the LE slots would be more trouble than they are worth. You'd have to figure out how to carve the highly undercambered profile of the slot and then how to form the main airfoil to house the slot and finally how to make the tracks that move the slot into the correct position without jamming. And all in miniature.........<shudder>

Good luck.

BernieG

Don't know enough Airfoilds by their name to help you there.


Bob Violett F-100 has working slats. They are molded in Carbon and actionned via a servo.

both refer to the same thing. relatively less incidence at the tip compared to the root. On a real airliner, this exist, I would say 3 degres or more would not surprise me. The real Me 163 had 6 (six!) degres. If you find a good plan - 3 view of your subject, I am pretty sure thsi washout has been taken into account by the designer.

Quote:

Thanks again and sorry for the dumb questions.

The dumbest questions are those not being asked....

Bernard

Roderick-RCU

Hello, read the post SCALE WINGS I posted by mistake as a new THREAD



Roderick

probligo

rbjjr,

There is an age-old question that everyone gets asked around these here parts when an enquiry starts with "I don't know much about this but..."

The question is "How much experience have you really got?"

This is the honesty point. Because without in any way trying to rain on your good idea, we are talking some serious technicalities here. Put it another way, you would not go out and build a GT40 without having a crack at a go-cart and a Lotus 7 first. Or would you?

OK Down to the questions.

I think personally that you are being extremely ambitious to take on an opening and shutting wing like this without trying out the simple things first. I note Bob Violett's name cropping up in the discussion there. How many TOC's has that guy been to? Use that as a judgement of his skill level.

So, for an airfoil. Someone has suggested a simple Eppler, or a cambered (puh-leeze not "semi-symmetrical" ) S- or SD foil. Nothing wrong with those at all. Select one about 9% up to 11% thickness to give enough room for the engineering (like stress distribution).

Second, a swept wing like the one you are thinking of will need minimal dihedral, but stick some in (scale has anyhoos). Take a look at some of the flying wing web pages as well (I can oblige there if necessary) because with the amount of sweep this model will have, and the very small tail surfaces, I think that you need to give some serious account to treating it like a "sorta" flying wing - i.e. a ton of tip washout to start with...

Third leave off the fancy bits like flaps, le flaps, spoilers, etc etc until you have a really good measure of the bird you are developing.

Fourth, my major rule number one - SIMPLICATE EVERYTHING AT EVERY STEP.

Finally, give serious thought to building a simple, even profile, model no bigger than 40" span. Think about building some simple panel gliders at 20" span, and even 40" span to sort out the detail of wing form, washout, CG position, stability margins, etc etc etc.

Don't lose sight of the dream.

rbjjr

Thanks to all for the input.

Main concern at this point is to locate a good flying airfoil and agree that after some comments and research on my part, a SS airfoil is the best bet. Smaller "prototype" model as suggested by a couple of you is a great idea too. LE flaps or slats will be difficult I'm sure and certainly are not a requirement to make the project fulfilling.

Primary objective now is to pick the wing airfoil. There are so many "simple" SS designs and I have had a couple of suggestions. Any input on actual airfoil choice would be appreciated. I am looking for good manners at this point more than high speed scale flight.

Tail surfaces are a concern too but it has been suggested that I could use NACA 0010 here with no problem.

Comments appreciated.

Best regards

BernieG

You know, Probligo, sometimes it's pretty good to say nothing when you don't know what you are talking about...

the TOC is a great turnament, but it judge a certain category of pilots flying a certain category of airplanes. If you don't know anything else, it just show your lack of experience and narrow focus. Bob Violett is in the hobby since something like 40 years, has won many pylon racing competition before he started into jets. Now his experience is about 10 different kits that he produce under his name, and he won this year Top Gun competition with his F-100 Sabre that he produce. For more references, look there http://www.bvmjets.com/ And if you have never seen him fly (my guess is that he is about 65 years old), you really miss something. Do you think that says enough about his skill level ?

Bernard
PS- I have seen many "3d" guys fly, who are TOTALLY UNABLE to land a jet properly.....

Ollie

There are many good airfoils to choose from but none of them will work well on a wing which has one part of the wing working against another part. To get good results the washout along the wing has to be coordinated with the taper and sweep to get an efficient lift distribution. This is far more important than which good airfoil you select. See:
http://aero.stanford.edu/WingCalc.html
Put your planform into this online program and try various washout angles to see which gives the best results over a range of angles of attack and lift coefficients. The closer the "e" number is to 1.0, the more efficient the lift distribution. The airfoil you choose has to be capable of the coefficient of lift without stalling.

To choose an airfoil, go to:
http://soaring.cnde.iastate.edu/calcs/frames.shtml
There you can compare the polars of up to five airfoils at once to find an airfoil that matches the coefficient of lift range for your wing design and the flaps up speed range you are looking for.

Working back and forth between these two programs can allow you to refine your design.

probligo

Bernard <sigh> I know Bob Violett is a highly skilled modeller.

I know that he has competed and with distinction at more TOC's than one or even ten. (When did the first of the Vio-jets come on the market?)

Please go back and read the words again. Perhaps rather than pulling your six-gun when I drop a match, you will see that I was responding to BernieG's comment which made (for me at least) leading edge slats sound like a very easy thing to do.

What do the words "I think you are being extremely ambitious" mean to you? They are in the quote...Then juxtapose those words with the comment about Bob Violett...Am I trying to make him sound like a beginner? No. I am trying to tell a beginner that he is taking on something that an expert - viz Bob Violett - might do but which is not a first model idea.

Loud and clear enough for you???

Sheesh!!

Johng

My Feedback: (4)

Another fine airfoil site is at:

http://www.nasg.com/afdb/list-airfoil-e.phtml

You can select airfoils, and plot cl vs cd AND cl vs alpha for a range of tested reynolds numbers.

It's just a little slow, but very valuable.

TexasAirBoss

My Feedback: (22)

Wow, what a project!

A freind of mine once built a Boeng 737. He used the scale wing. The real airliner had a lot of positive incedence at the root and then it washed out toward the tips. He built his plane just like the real one. His plane flew with a nose down attitude!! The real airplanes generally fly with there foil at a higher angle of attack then do the models. Thats something to watch out for!

BernieG

like most planes ! may seem strange, but that's just because we rarely have the opportunity to hover at 33000 feet to see an airliner pass close by at speed.....Now I agree that, as our models don't have to lift as much weight as those big things, a slight reduction of the incidence might help.

But for other planes, if you look at their profiles at speed, they all (Spitfire, Mustang, 109, Corsair, P-40) have a slightly nose down attitude, as it's better for both drag and pilot visiblity. The only exception that I know of is the Hellcat, which always seems to fly a little tail low, I guess it was made to help diminish the risks of nose over while "landing" on sea, gear up.

Bernard

Jeff Thompson

I'd like to butt in on this subject:

Why reinvent the wheel? It looks like a whole bunch of people have been making large-scale airliners. After seeing a post somewhere on RC Universe yesterday with a link to some Australian who made a really big sloper (photos below--I haven't gotten a reply from them yet) I'd like to build one myself out of EPP for the slope. Instead of going scratch, however, I want to pick the brain of someone who has done it successfully themselves.

I've seen a lot of gorgeous German fiberglass jets too, but they're fiberglass and powered. Anyone here know who to ask or where to look for information on my quest?

I do have a crazy idea that may or may not work which could keep the weight of a huge EPP sloper down while keeping the fuselage stiff; I'll run this up the flagpole & see who shoots it down: Find a very large, cylindrical balloon or bladder(s) which would make a tight fit inside a hollowed-out EPP fuselage. Inflated, it would stiffen up the fuselage without adding tons of weight. Of course, longerons would also be needed. Light winds at the slope? No problem, fill the bladder with helium!

Here's the photos of the plane that caught my eye:

Tall Paul

I've seen an large electric B-717 which uses fluted corrugated cardboard for the fuselage.. With a few formers and maybe some longerons, it's about as stiiff and light as needed.
The large diameter all by itself results in a stiff fuselage.

Jeff Thompson

Stiff maybe, but slope aircraft also have to be tough and resilient for when they hit trees, rocks or other planes. Still, corrugated cardboard (or even better, corrugated plastic - Coroplast) might be just the material I should use. I'm surprised I didn't think of that myself, as I used to fly a SPAD Dart before a marsh swallowed it up. It would be pretty easy to form an airliner fuselage with, too. Thanks for the idea! Now I just need to buy up all the Coroplast in my area.

Cfelton

Jeff,
Why not give corrugated cardboard a try? It's inexpensive and very strong & resilient. It doesn't crack or shatter like balsa and fiberglass, but just dents locally, and can easily be repaired. I have been designing and building cardboard models for over 30 years, so I know what it is capable of producing, like my 70" wingspan FW 190 and 60" wingspan P-51 control line models in the pics. Visit http://home.earthlink.net/~charlesfelton to see pics of construction techniques and models designs. One of the biggest advantages of cardboard is that it can be scored and folded allowing each wing half to be built from one piece of cardboard. Hows that for simple and fast construction? Check out the site to see both R/C and CL models and let me know what you think.

Jeff Thompson

Cfelton, you make some very good points about cardboard. I can see where it would have advantages where building is concerned. For the slope, however, toughness and abrasion resistance are very important, especially where I fly. Many slope sites have rocks and trees downslope, and inevitably the planes land down there from time to time. Wings made from corrugated plastic are also made from one piece of material. Different bending methods are used, and as I do heating & A/C work (I am a technician) I have metal bending and working tools available at our shop & can bend full sheets of coroplast, should I need to, on our large brake. Should I make a smaller model before building a large one, however, I'll use cardboard, as it's easier to work with.

By the way, you make some great looking models with cardboard. Do you normally paint them, or cover them with poly films?

Cfelton

Jeff, All my models are finished with 2 coats of clear dope, sanded lightly after each one, fillowed by 2 coats of color. Any kind of poly film or contact paper can also be used, but in that case I would skip the dope as the film adheres to the raw cardboard much better. Send me a photo if you do build with cardboard.

Jeff Thompson

RBJJR, did you ever find an airfoil for your airliner project? A member of the soaring club I'm joining built a C5 for the slope, and I'll ask him lots of questions when I meet up with him. I'll keep you posted. Meanwhile, here's a photo and some links to movies of the C5's first flights. One thing about the C5 I would have done different: Dave's C5 is completely made of pink foam, and sustained damage every time it landed downslope. While pink foam might be fine for such a large fuselage, I'd at least make the wings and tail out of EPP if not the whole plane.



Here's an exerpt from the MRCSS (Minnesota RC Soaring Society) website, mrcss.org, followed by two movies of the plane's first flights:

"The highlight (of the club's May '04 trip to Chamberlain, SD) was Dave Engelson's giant C5 transport plane built from pink foam. Dave waited till Saturday to fly this huge plane because he was finishing up the radio installation in the motel.

Larry grabbed the huge fuselage and made a couple of runs without letting go to see if the plane would lift. Looked like it would, so he took another run and launched the C5 off the hill. Everyone had stopped flying to watch and take pictures.

As soon as the plane was airborne, everyone cheered. The flight began as a slow decent down but the plane began to wobble because of rearward CG. Dave picked a spot to land down the hill and landed with slight damage to tail assembly.

The plane was brought up the hill and repairs made. Nose-weight and a tow hook were added. Next flight was a bungee launch that started much better. The plane flew stable, made a couple of turns and even gained altitude for a time. Lift was not strong enough to keep the giant bird up for long and it made another emergency landing at the bottom of the hill. More damage ended the test flight for the day."

MOVIES
[link=http://www.maxconrad.com/miscimages/C5Flight1.mpg]Flight One:[/link] [link=http://www.maxconrad.com/miscimages/C5Flight2.mpg]Flight Two:[/link]

Johng

My Feedback: (4)

That C-5 is sweet!! Except for the missing engines, the profile is exact. Looks like it'll be a good one when it's tweaked out.

I'd love to be able to get any info about this plane (CAD files, anything else). Not many opportunities for slope flying here in FL, but that would make an excellent starting point for an electric conversion.

Please let us (me) know any more you find about that plane. If you can get me the builder's contact info, I'd really appreciate it. [sm=thumbup.gif][sm=thumbup.gif]

Jeff Thompson

I've been flying with some of the MRCSS club members since late winter, and just sent my membership in today. On the 17th of June they're having another South Dakota trip which I'll be attending, and hopefully I'll see him there. Otherwise I'll ask some club members for info.

You might want to check out the club website at [link=http://mrcss.org]MRCSS.org[/link]

--Jeff

Johng

My Feedback: (4)

I found the builder's e-mail info on the web site. Now to harrass him.