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Author Topic: Airfoils and Taper  (Read 5366 times)

Online Motorman

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Airfoils and Taper
« on: September 27, 2016, 04:42:02 PM »
I have a modern stunt wing that is tapered in span and thickness. If I want to use that airfoil on a wing with no taper and constant thickness do I take the average chord and thickness to make the new wing?

MM
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Offline Tim Wescott

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Re: Airfoils and Taper
« Reply #1 on: September 27, 2016, 05:16:15 PM »
Yes.
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Offline RogerGreene

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Re: Airfoils and Taper
« Reply #2 on: September 27, 2016, 08:51:07 PM »
It all depends on the engine and the amount of square inches the engine can pull around the circle. Then what span and chord will make that desired square inch wing. Your modern stunt wing will give you several typical chords and airfoils to get the desired square inches and span.

You gota know your end result and then design the plane to achieve your end fine plane.

Sounds simple but a lot is involved, just take it one step at a time and you can make it a winner.
 
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Online Motorman

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Re: Airfoils and Taper
« Reply #3 on: September 27, 2016, 09:27:40 PM »
My theory is to have the same wing area, wing span and the same lift/drag. I guess one would have more lift at the root and the other more lift at the tips. My goal is to have a wing that's easier to build but will perform just as well.

MM 
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Offline Mark Scarborough

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Re: Airfoils and Taper
« Reply #4 on: September 27, 2016, 09:43:09 PM »
My theory is to have the same wing area, wing span and the same lift/drag. I guess one would have more lift at the root and the other more lift at the tips. My goal is to have a wing that's easier to build but will perform just as well.

MM 
  well keep in mind, that wing you are modeling from, there is a reason it has taper,, and its not just cosmetics..
so to have the impression that it will fly "just as well" is a mistake, now that said, wil you be able to tell the difference? who can say, but it wont fly as well.....
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Offline Tim Wescott

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Re: Airfoils and Taper
« Reply #5 on: September 27, 2016, 10:27:08 PM »
  well keep in mind, that wing you are modeling from, there is a reason it has taper,, and its not just cosmetics..
so to have the impression that it will fly "just as well" is a mistake, now that said, wil you be able to tell the difference? who can say, but it wont fly as well.....

OTOH, if you can't fly well enough to know the difference, you may as well go for ease of construction.  Fancherized Twisters do pretty nice with a constant-chord section that has flaps attached.
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Offline Russell Shaffer

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Re: Airfoils and Taper
« Reply #6 on: September 27, 2016, 10:54:28 PM »
Or a Medic with no flaps at all.  I guess a lot depends on the pilot skills.  There is a fine video of David Fitzgerald flying a 500 plus pattern with a Ringmaster.  So just build the airplane you want and go have some fun. 
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Offline Howard Rush

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Re: Airfoils and Taper
« Reply #7 on: September 27, 2016, 10:57:51 PM »
I have a modern stunt wing that is tapered in span and thickness. If I want to use that airfoil on a wing with no taper and constant thickness do I take the average chord and thickness to make the new wing?

You could use the mean aerodynamic chord, rather than the average chord.  This Web site gives both a formula and a mathless way to calculate MAC: http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/formulas/mean_aerodynamic_chord.htm .  I didn't check the formula, but it's probably OK.  Locate the wing on the fuselage such that 1/4 of the way back from the leading edge is at the same point on the fuselage as 1/4 of the way back from the leading edge of the mean aerodynamic chord of the tapered wing.  There are some other considerations, but this should get you close to matching the characteristics of the original airplane.   Putting the wing at the right place on the fuselage is probably more important than matching the chord.

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Offline Chuck_Smith

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Re: Airfoils and Taper
« Reply #8 on: November 12, 2016, 06:50:59 AM »
My theory is to have the same wing area, wing span and the same lift/drag. I guess one would have more lift at the root and the other more lift at the tips. My goal is to have a wing that's easier to build but will perform just as well.

MM  

The wing's contribution to the airplane's L/D is more a function of aspect ratio than anything else.

The taper makes the wing a bit more efficient, but given the way we torture aerodynamics in stunt I wouldn't worry too much about it. A tapered flap on a constant chord wing is fine. Just remember that if you taper the flap and not the wing thickness towards the tip you add aerodynamic twist into the wing as you have a constant thickness and varying chord. Still, it works just fine. A Magician or an Oriental can tear up the pattern. Sneekers had constant thickness tapered wings and flew just fine..and fast too!

In the end, in stunt I believe a lot of conventional/mainstream aircraft design is really not applicable. (IMHO) We're not looking for speed and efficiency. We're ( well, at least I am) looking for the best balance of maneuverability and smoothness.

I guess one can argue that the tapered wing will have less drag in maneuvers...

Anyway, take the span of the original tapered wing, square it, and then divide by the wing area. That's your aspect ratio.  If you keep the same wing area then you can rearrange the same formula to replace wing area with chord times span... This will allow you to solve for the span or chord you need for the constant chord wing. You'll now have a constant chord wing with the same area and AR of the tapered wing which should have roughly the same drag due to lift as the tapered wing. If it were me I'd add 10% to the AR for the constant chord wing vs the tapered wing to make up for the loss of efficiency (as in Oswald for all you engineers out there).

If you really want to get into it, since the MAC is not (usually) the same as the average chord on a tapered wing you'll need to know the MAC for the original wing and then place the constant chord wing on the fuselage to maintain the same tail volume coefficient.


But the main thing is don't get too hung up on formulae and airfoils and all that. If it looks "right" it will probably be just fine. I've always believed the best stunt ships were eyeball engineered and then refined.

All IMHO and HOMV (Howard's Opinion May Vary)


Chuck



« Last Edit: November 12, 2016, 12:10:39 PM by Chuck_Smith »
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Offline Matt Spencer

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Re: Airfoils and Taper
« Reply #9 on: November 12, 2016, 07:15:23 PM »
One can always get cunning . The Eliptical tips seem to do something . This ( The Freebird ) won stunt at the Blitish Nats a few times .
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Offline RogerGreene

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Re: Airfoils and Taper
« Reply #10 on: November 17, 2016, 05:30:45 PM »
And It's a diesel.
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Offline Paul Smith

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Re: Airfoils and Taper
« Reply #11 on: November 24, 2016, 08:16:10 AM »
Stress is the other factor.  A constant chord wing has the center of lift farther out and therefore more stress at the center.  Thus heavier structure.
Paul Smith


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