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Old Apr 09, 2004, 01:27 AM
Danger! Danger!
FrankW's Avatar
Portland Intl, Oregon, United States
Joined Dec 2002
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Recommended Bob-e-cat airfoil for 40" wingspan

Okay, I've picked out my next project for when I'm done with my Minx 300. I want to build a Bob-E-Cat for an old Kress Jets fan unit I purchased a few years back (for a Ziroli designed HE-162). It's an RK-720E I believe.

I want to do a Bob-e-cat with a 40" wingspan and the EJF versions are either too big or too small. I found plans for the plane online. I want A/E/T control, with possible retracts. I plan on having a built up balsa wing (never cut a foam wing and don't really want to). I would like to keep the weight of the aircraft to about 35 ounces.

Anyways, to the point, what would be the best airfoil(s) to use on this project? Any help would be apresiated, thanks.

-Frank
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Old Apr 09, 2004, 12:41 PM
Carbon fiber is our friend
Steve C's Avatar
United States, CA, Lodi
Joined Feb 2001
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Airfoil choice depends on a lot of factors.

Will it fly upside down a lot? How fast do you expect it to go? How slow do you want it to go? What is the chord length? What will the wing loading be? Should it glide really well (it would fly on minimal power, but this makes a plane harder to land in a tight spot), or would you prefer to have it slow down easily? There could be structural concerns that will dictate thickness, or landing gear that needs to fit in the wing.

There is not one right answer for sport models. It's easier to chose when there's a specific goal in mind, like pylon racing, but even then it is a compromise between turning ability and speed.

You could look at some of the old NACA 5 digit sections. Something like the 23012 or 24010 have reasonably low drag and lift to the moon. Like I said though, it depends on what you want.

Steve C
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Last edited by Steve C; Apr 09, 2004 at 01:03 PM.
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Old Apr 09, 2004, 12:59 PM
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FrankW's Avatar
Portland Intl, Oregon, United States
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-It will fly mostly right side up (maybe a little upside down)

-I expect it to go fairly fast, somewhere it the 60 mph range (I hope)

-I want it to be able to slow down to about 15-20 mph for landing (although even slower wouldn't be a problem either)

-The wing loading will be about 13 oz/sq. ft

-A good glide ratio isn't of a high concern.

-I would like the option of being able to put in Robart 1/2A micro retracts, but it's not nessesary.

Frank
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Old Apr 09, 2004, 01:13 PM
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Steve C's Avatar
United States, CA, Lodi
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That puts your average chord at about 9.7 inches. Is that right?

Steve C
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Old Apr 09, 2004, 01:20 PM
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FrankW's Avatar
Portland Intl, Oregon, United States
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Yeah, that's about right.

-Frank
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Old Apr 09, 2004, 01:27 PM
Carbon fiber is our friend
Steve C's Avatar
United States, CA, Lodi
Joined Feb 2001
4,176 Posts
If a decent choice is made and you have that low a weight, your minimum speed should be easily between 15 and 20 mph.

Some good choices would be Selig 3016 and Selig 5010, which works on flying wings by the way The bad part is that they have thin trailing edges, which is tough to built in wood. That's why the NACA sections may be a better choice.

Steve C
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Last edited by Steve C; Apr 09, 2004 at 01:29 PM.
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