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convert flat bottom wing to semi elliptical

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convert flat bottom wing to semi elliptical

Old 07-15-2008, 12:34 PM
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rickswl
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Default convert flat bottom wing to semi elliptical

I have been flying my Eagle 2 for about two years, (when I want some gentile fun I pull it out) Unfortunately the wing got somewhat trashed in the garage. I would like to make the plane fly better upside down, and was thinking of making a new wing with semi elliptical instead of another flat bottom, without screwing up the flight mechanics very much. The cord is 11.5/8" with 63 inch wingspan and around 750 sq inches of wing area. the weight is about 5.5 lbs I read through the thread on airfoils and really got confused, I don't need a high tech wing. Any design would be concidered.

Thank you
Larry
Old 07-15-2008, 01:34 PM
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Tall Paul
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Default RE: convert flat bottom wing to semi elliptical

What you are looking for is a semi-symmetrical airfoil, for a straight chord wing.
An elliptical wing would be something like you would see on a Spitfire.
A Clark-Y airfoil will let you do what you want, without changing much of anything on your plane.
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Old 07-15-2008, 01:50 PM
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Craig-RCU
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Default RE: convert flat bottom wing to semi elliptical

I had a Goldberg Eagle 2 that I crashed and rebuilt the wing with a semi-symmetrical airfoil and the inverted flight performance and top speed were greatly improved. The top speed was so much improved that I had to balance the ailerons with counterweights to stop them from fluttering. I also rebuilt the wing without dihedral which, combined with the semi-symmetrical airfoil, improved the all-around aerobatic qualities of the plane so much so that someone once commented to me that my Eagle 2 flew like a fun-fly plane because of the extreme aerobatics that it would perform.

When I rebuilt the wing I didn't know much about aerodynamics. I just looked at other planes' airfoils that I wanted my Eagle to fly like and approximated their airfoils. Most of the wing was in tact, so my method of making a semi-symmetrical wing was to just add a bit of wood to the bottom of each rib as pictured below. At first I didn't want to mess with wing incidence issues, so I left the center section of the wing flat-bottomed where the wing met the fuse but later crashed and repaired again and made the wing-saddle fit the semi-symmetrical airfoil shape. I kept the new semi-symmetrical wing's incidence the same as the original Eagle 2 design and never had any incidence issues. The picture below shows 1/16" sheeting for the area at the root where the wing of the Eagle 2 is sheeted. Hope something here helps you out.
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Old 07-15-2008, 10:06 PM
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Default RE: convert flat bottom wing to semi elliptical

Craig, that airfoil you show would likely fly worse upside down than the stock Eagle airfoil. That little undercamber cusp on the lower rear adds a lot of effective camber to the airfoil design.

What you want if you want to make it fly better upside down is LESS camber. Something like the one shown here would be easy to build thanks to the flat lower rear section so the wing can be built on a flat board with all the ribs sitting on that flat segment. And with the curves on the upper and lower sides being fairly bold the overall camber of the airfoil will be lower and it'll require less elevator compensation to fly upside down.

Something else to consider as well. It's not JUST the airfoil. Moving the CG back so the model trims for level cruise with a lower amount of decalage is equally or more important to producing a good aerobatic sport model. A better airfoil is more just the icing on the cake instead of the moist chocolate core....
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Old 07-15-2008, 11:40 PM
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Craig-RCU
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Default RE: convert flat bottom wing to semi elliptical

Not that a Clark Y wouldn't offer even better performance than my airfoil but, I'll have to disagree with you Bruce about the performance of my airfoil as compared to the stock, flat bottomed airfoil of the Eagle 2. The inverted as well as speed performance was greatly improved. With the flat-bottom airfoil, inverted performance could be described as more like the wing was "plowing" through the air instead of "planing" through it. At high speed the monokote on the original flat bottom would rattle a lot indicating much turbulence. I had hoped that the modified airfoil was going to improve inverted performance but, I was really surprised to find that making the airfoil thicker with the added bit on the bottom made it faster too.

In fact the camber of my airfoil actually is less than the original up to the point of the spar while the camber of my airfoil is greater than the original aft of that point. So, I gather from that fact and my testing that the camber of the forward part of the airfoil has more influence over the inverted performance than does the camber of the aft part of the wing in my situation of modifying the original Eagle 2 airfoil. I should also point out that airfoils designed for flying wings have reflexed aft cambers too, so the good inverted performance of my airfoil shouldn't be too shocking from that perspective.
Old 07-16-2008, 07:32 AM
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da Rock
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Default RE: convert flat bottom wing to semi elliptical

The overall camber of the airfoil is really a major contributor to inverted as well as upright performance. The less camber while inverted (would could be considered as negative camber) the better. So the more semi-symmetrical you have the better. Matter of fact, symmetrical will do great rightside up or upside down.

In practice, you don't need camber in our models to any real degree, as it's primary benefit shows up in the upper CL range, and with our wing loadings, we don't need more max CL to do much of anything. Unless your Eagle 2 has gained a lot of weight from repairs, you can use any profile and it'll work as well rightside up as it ever did. But if you want to improve inverted, the "rounder" the underside of the profile the better. And that's basically the same as saying, the less cambered the better.

To keep the same decalage is worthwhile for upright flight, but becomes less important as the flight time inverted increases.
Old 07-16-2008, 09:20 AM
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Default RE: convert flat bottom wing to semi elliptical

I'd just start with a fully symmetrical airfoil with zero camber and be done with it. The airfoil can be adjusted from benign to aggressive with thickness ratio and wing load (span). Everything else on a powered model seems to me a compromise to performance with no benefits to show for your trouble (scale planes being a huge exception!). Craig-RCU, the definition of camber is the departure of the mean chord line over the entire chord.
Old 07-16-2008, 12:53 PM
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Default RE: convert flat bottom wing to semi elliptical

Have I been misusing the term camber? My airfoil doesn't have a reflexed camber line like the first picture below, but it does have "that little undercamber cusp on the lower rear" that Bruce mentioned that is similar to the undercamber cusp on the upper rear of the first picture below. FWIW I've added a picture of the mean camber line of the original flat-bottomed Eagle 2's airfoil (red line and airfoil shape) and the mean camber line (blue) of my modified airfoil by adding the blue bit on the bottom of the red airfoil.

edit: Maybe I should have said that my camber line has a tighter radius aft of the spar than the original Eagle 2 airfoil instead of saying that it has more camber aft of the spar.
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Old 07-16-2008, 03:59 PM
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Default RE: convert flat bottom wing to semi elliptical

Ok, now I understand. You were comparing the difference between before and after. I misinterpreted you as saying there was a forward camber and a rear camber in a given wing. Just so we are all on the same page with terminology heres a diagram that shows everything in the most common terms.
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Old 07-18-2008, 08:36 PM
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Default RE: convert flat bottom wing to semi elliptical

I'm just about finished with a semi-symetrical wing I'm going to use to replace the flat bottom airfoil one on an old Hangar 9 trainer. I'm using an NACA 2415 airfoil with the same chord and lengthened the span a couple of inches. I reduced the dihedral from an incredible 6" to 2" also. I should have the wing covered and the new wing saddle cut on the fuselage in a day or two. I'll post a flight report ASAP.[8D]
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Old 07-18-2008, 10:32 PM
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Default RE: convert flat bottom wing to semi elliptical

Craig, you answered your own question about reflex vs camber.

So you actually added some material to the bottom of the stock Eagle airfoil? If this is the case and your pink and blue sketch is what you did then yeah, despite appearances you both thickened and reduced the camber all in one go. That would explain why you noticed the improvement. Even better would have been to make that curved part run all the way to the trailing edge and then reflex the ailerons slightly. But what you did is still a change in the right direction. That would explain your great results. It also points out just how bad it is in most cases to use airfoils that come from the side curves of shoes.
Old 07-23-2008, 08:01 AM
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rickswl
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Default RE: convert flat bottom wing to semi elliptical

Thanks for all of your suggestions, but where can I find the dimensions for the 2415 airfoil? I like the idea of just adding a piece onto the bottom of the existing airfoil. I have the shape of the original flat bottom rib. However I am still open to other airfoils.

Thanks again
Larry
Old 07-23-2008, 09:08 AM
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Default RE: convert flat bottom wing to semi elliptical

There is a free airfoil program called Profili for download. Hundreds of airfoils to plot right off your printer. The advanced features are extra but, it works great to just print out airfoils as is.[8D]
Old 07-26-2008, 08:29 AM
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rickswl
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Default RE: convert flat bottom wing to semi elliptical

Thanks Propbuster

Thanks for your information I will try to find the program

Larry

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