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Old Oct 25, 2010, 01:09 PM
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nhogberg's Avatar
Texas
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Airfoil for 48" minimoa

Hi Guys,

I'm scratching a 48" wingspan Minimoa. The chord is just shy of 4" at the root. I would like to build two wings, one for thermal and one for slope.

This will be a small ship with the fuse about 17"'s.

What airfoil for slope and thermal?

2 channels, should they be rudder and elevator or aileron and elevator?


Thank you in advance!

Nels

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Old Oct 25, 2010, 03:32 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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For such a small chord just go with a Clark Y. It'll work fine for both situations. No need for two wings.

There's no point in building it with anything super fancy since I'm assuming you will be using tissue or plastic covering over an open rib structure. The sag in the covering between the ribs will render using any airfoil that requires a faithful shape to achieve any advantages null and void. So we come back to the basics. And the Clark Y shape has shown itself to work well over a very wide variety of sizes and speeds for such things even when it has covering sag between the ribs.
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Old Oct 25, 2010, 05:02 PM
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biber's Avatar
Aachen, very western Germany
Joined Dec 2004
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I'd go with a very thin section, certainly less than 7%, and around 3% camber.
Look, what freeflight models of similar size and weight use.
Even a moderately cambered plate will get very close to the best you can hope for at these tiny Reynolds numbers.

biber
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Old Oct 25, 2010, 05:21 PM
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For thermalling, definitely thinner IMHO.

If you have a 4" root chord then you'll be looking at very low Reynolds at the tips!

Have a look at MA409 (smoothed) it's around 7% thick, 3% camber

http://www.profili2.com/eng/dett_profilo.asp?Id=1105


EDIT
Bruce is right though - I've used Clark Y quite successfully on open frame models at around Re 15,000. At this size, wingloading and low drag will be more important than the airfoil specifics.
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Old Oct 25, 2010, 05:28 PM
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Here's a plot (computed) for MA409 at Re 40,000

(from this paper: http://icas-proceedings.net/ICAS2004/PAPERS/192.PDF )

It's considered quite a success on some FF and micro RC models.


EDIT
Apologies - It's not computed it's a tunnel test!
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Old Oct 25, 2010, 07:00 PM
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Thank you guys!

Also should I do 2 ch aileron and elevator or rudder and elevator?

I guess I will use the clark Y and when I get to the outer wing panel just cut every 3rd rib or so and sand the rest. (I know it's cheating, but something this small it doesn't sound like it will make much difference.

You guys are so smart!

Thank you,
Nels
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Old Oct 26, 2010, 12:49 AM
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The size and service of the model is very close to what contest freeflight models are built for. So something similar to what they use will work well. The Ma409 looks very good and might give a bit more penetration than some of the more cambered freeflight airfoils..

BUT.. Vey thin airfoils are hard or impossible to accuratly build from balsa stick and tissue. First as mentioned previously it's very hard to reproduce the airfoil at anything other than the rib stations. Second it's very hard to build wings that are both light and strong with such thin airfoils, wafer thin TE's are another no-no for stick and tissue builds.

For strength/weight and building simplicity something close to a Clark Y but maybe a shade thinner is often a good compromise. I've used a simple flat bottom airfoil called 'Neelmeyer' (after it's designer K. Neelmeyer) with good effect. It's about 9% thick (attached)
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Old Oct 26, 2010, 02:14 AM
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Thank you Jet,

I pulled up the Neelmyer and Clark Y on my old profili.

After I get done sanding I will prolly be closer to the Neelmyer, lol!

Here it is with the inboard panels done. I had some huge trailing edge, I cut it up and used part for the LE and part for the TE.

I already started with the Clark Y, you can see my "ole" 98mm plywood template in the pic. Geesh, still another 30" of wing to go!

I really Appreciate! everyones input, I like to build but the airfoil stuff is a mystery to me...



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Old Oct 26, 2010, 03:55 AM
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Looks good!

Make sure the LE doesn't end up too fat.

I would think rudder and elevator should be ok - with a small fin and dihedral you should have control and spiral stability. Plus the gull wing would make aileron linkages more complicated.
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Old Oct 26, 2010, 06:30 PM
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Only the outer right wing left. Man, this thing is whooping me...

Thank you for all the info. Keep it coming.

Regards,
Nels


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Old Oct 26, 2010, 06:40 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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I see that you're using ailerons. A wise choice. The problem with gull dihedral is that all the angle is at the wrong part of the wing. The angle is what makes the model roll but the higher angled section is in by the fuselage.

Someone built a gull wing Cleveland Condor over in the Vintage forum and found that it really didn't do well with rudder at all. He cut into the wings and retrofitted ailerons and is far happier with the model now.

Another hint. Cut some arc shaped extra stringers for the nose. With the number of keels and stringers you have now the nose will look very segmented when you cover it. Double the number of stringsers just to the leading edge of the wing will make a huge difference in how it looks after covering.
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Old Oct 27, 2010, 12:30 AM
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As an alternative to Bruce's extra stringers on the nose.. Simply fill in between the existing stringers with soft balsa block or sheet and carve/sand to shape. this gives the nose a nice smooth contour and the weight doesn't matter because she will need noseweight anyway. Might as well add noseweight in a form that adds strength and improves the looks. Solid nose also prevents covering punctures when landing.

Have you built washot intio the wings?.. On a sharply tapered wing like this you will need plenty of washout to prevent wing dropping... about 5 degrees of twist i'd guess. on an open structure like this you can pull in washout during covering.

Steve
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Old Oct 27, 2010, 02:57 AM
B for Bruce
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I'm not seeing any stubs from the wing spar that will carry through the fuselage either. You'll want to arrange something to do that. Just glueing the root of the wing to some stringers and bulkheads is a recipe for a quickly needed repair.
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Old Oct 27, 2010, 08:03 AM
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Hi Guys, thank you for the information.

The nose, I haven't finished it yet, I am waiting until I make a canopy and then I will finish fairing it in with scraps of block.

The wings will actually be joined together in the center. I will then cut the top stringers loose and fashion a cradle for it to sit in and glue the top stringer contour to the wing when it is done. then rubber band the wing/top fuse, into the cradle.

Washout: I dont have any yet so I will do that before and during the tissue-dope stage. Thank you for that bit.

Stringer, I think I will add 1 or 2 more stringers (3/32) to the section between the leading edge and top stringer, thank you for that Idea. The will make the tissue covering easier. (the stringer there now is (1/8).

Keep the crits and info coming, thank you.

Regards,
Nels


It is a scratch build so I usually add gussets and fill in stringers with sheet in the areas I think need it.

So far, when calculating the half wing weight by 2 and the fuse and stabs, the weight is 1.4 oz's.
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Old Oct 28, 2010, 11:15 PM
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Well, it's ready for reinforcement and radio gear to go in, then I can start the dope and tissue.

I am going to put some 1/16 sheet and 1/2 oz cloth and epoxy on the dihedral sections.
I made a balsa plug for the canopy. I will try and pull a coke bottle or some such over it.

It's at 2.25 oz's right now.

TWO QUESTIONS:

1) I did pick up some 1/8 square spruce. If I add that should I run it on the bottom about mid-way of the chord???

2) Right now the horizontal stab is in line with the wing. Other gliders seem to have them set at a negative incidence to the wing. Should I do that also?

Thank you for all your information.



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