clark Y, zero lift angle?  
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All Forums >> RC Airplanes >> Aerodynamics >> clark Y, zero lift angle?
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clark Y, zero lift angle? - 3/8/2008 9:02:27 PM   
timothyb


 

Posts: 5
Joined: 9/29/2007
From: lindenhurst, IL, USA
Status: offline 		I am designing/building an electric biplane that has a clark Y airfoil for both wings. I used Compufoil to generate the airfoil. Compufoil shows shows the cord line for the airfoil. Is the cord line the same as the zero lift angle? If I set the cord line parallel with the thrust line it looks like the airfoil has a negative angle of attack. I have designed and built two other models that have a Clark X airfoil. I never paid much attention to the chord line and had the bottom of the airfoil sit paralell with the horizontal stab. One of the planes requires slight up elevator for level flight. The other one requires slight down elevator. I was planning on having the bottom wing at a zero angle of incidence and the top wing at about 3 degrees. I was also hoping to be able to adjust the angle of incidence for the horizontal stab, or maybe just set it at about -1 degree to offset the climb. Any feedback would be helpful as the next step in the building process will be to align the center section of the bottom wing to with the fusealage.
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RE: clark Y, zero lift angle? - 3/8/2008 11:01:06 PM   
Mike Connor



Posts: 1381
Joined: 1/10/2002
From: Tulsa, OK, USA
Status: offline 		The Clark Y airfoil will start generating lift at -3+ degrees. You could use Foilsim at
http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html
to help figure your angle of incidence needed to fly level at a given weight and speed.

_____________________________

Mike
http://www.mikesrc.us/

(in reply to timothyb)
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RE: clark Y, zero lift angle? - 3/9/2008 12:32:30 AM   
R8893


 

Posts: 405
Joined: 10/19/2002
From: Cincinnati, OH,
Status: offline 		Timothyb,
I have a Handley Page O-400 with Clark Y airfoil on the wings. Actually it is modified to be 1/16" thicker to allow a reasonable construction of the trailing edge. Flat bottom of both wings, engine thrust lines, and flat stabs are all parallel. Elevator trim has varied from a tiny bit of up at original CG and now trims a little down as I have moved the CG back. A smooth stable flier as I had hoped. 22# and 17 square foot wing area. Have not tried to fly it inverted yet, but it does loop. Every variation of wing vs wing vs stab incidence has been tried by somebody. I think I have had models with most of them. Probably depends on what flight performance you want. You might try checking out the setups on the Balsa USA WWI models, they have flat bottom airfoils.

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RE: clark Y, zero lift angle? - 3/23/2008 3:44:30 AM   
feihu



Posts: 178
Joined: 11/17/2002
From: Sun City, AZ, USA
Status: offline 		Timothyb:
I'm not sure I understand your wing and tail incidence settings.
If the top wing is set at a HIGHER incidence angle than the bottom wing, then the top wing will stall first.
The net incidence of the Clark Y wings as per your setup might be +4 degrees or more and setting the stab at -1 degree will create a further pitch up.
Perhaps someone else can chime in and suggest your wing and tail incidence settings.
feihu

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RE: clark Y, zero lift angle? - 3/23/2008 4:27:34 AM   
timothyb


 

Posts: 5
Joined: 9/29/2007
From: lindenhurst, IL, USA
Status: offline 		Thanks for the info. Actaully, I was planning on having the horiz. stab have a +1 angle of incidence (typo), giving the effect of a little down elevator to offset the anticipated extra lift. After looking at the designs of some park flyers in Model Avaition mag, I noticed the top wing usually had a higher incidence than the bottom wing. Do I want the bottom wing to stall fisrt, thus giving it a higher incidence than the top wing? Let me know if you have any experince with this. I just built the bottom wing center section with the airfoil set at a zero angle of incidence. Luckily I havn't glued it yet. Also, there are ailerons on the top and bottom if that makes any difference as far as the incidence goes.

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RE: clark Y, zero lift angle? - 3/23/2008 10:11:48 PM   
feihu



Posts: 178
Joined: 11/17/2002
From: Sun City, AZ, USA
Status: offline 		timothyb:

Here is a pic of the Tigermoth that I built years ago. Has a 4cycle 40 for power, wings are 11% Clark Y.
Both top and bottom wings have the flat bottom parallel to the horizontal axis of the airplane, and the stabilizer is also parallel to the horizontal axis. The airplane was a superb flyer and I sold it after a year of flying.

Ailerons on top and/or bottom wings have no affect on incidence so long as they are not deflected.

feihu



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RE: clark Y, zero lift angle? - 3/23/2008 10:46:56 PM   
da Rock



Posts: 6654
Joined: 10/11/2005
From: western, NC, USA
Status: online
quote:

ORIGINAL: timothyb

Thanks for the info. Actaully, I was planning on having the horiz. stab have a +1 angle of incidence (typo), giving the effect of a little down elevator to offset the anticipated extra lift.


The tail actually experiences an airflow that is heading down. It in the downwash of the wing. And there are lots of fullscale where the designers worked out the downwash angle and then set the tail incidence to suit. And they often wound up with negative incidence. One reason is because cambered wings generate pitching moments under load that need download from the tail.

There are really lots of things that need to be considered that aren't real simple.

You basically figure the tail incidence based on the downwash at cruise, and whatever moments the wing are going to generate at cruise, and plan to trim whenever you're not in that cruise mode. You wind up with an airplane that's less effort to keep trimmed and less work to fly.

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