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Negative lift airfoil demensions

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Old 04-22-2008, 12:17 PM
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cheese_puddin
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Default Negative lift airfoil demensions

Hello,

I know that this is mostly for making things fly but I am making a wing for a car and I am using a NACA 23012 airfoil. I am usually trying to produce positive lift at semi high renynolds numbers but If i want a chord of about 1 ft how could i achieve a lift of -250 lbf if my car is going 100ft/s. I guess im confused because I am looking at the NACA specs and I am noticing data for high Reynolds numbers and im at 636,011 .......the only way i can achieve this type of lift is at some crazy planform area 23 square ft. and an angle of attack of 4 degrees. Ugh if im not making sense here then disregard the post but if anyone can help id appreciate it.

thanks brad
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Old 04-22-2008, 12:53 PM
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Default RE: Negative lift airfoil demensions

You're making fine sense.

It's still a wing. It doesn't matter if it's right side up and lifting something upwards or if it's turned over to push something down. Lift is lift and the same rules apply.

Keep in mind that on a car the airflow around the car's body will greatly alter the flow over the wing. Some area will be blanketed and at the very least the flow up and over the car body will affect the angle of attack of the wing. You can't assume that because you set the wing at a -4 degree angle that the air will be hitting it at a -4 degree angle of attack. If it's a sedan the air will be flowing down off the roof and you may find that to set the wing at a -4* angle to the airflow that the physical angle will be closer to 0*.

For a high lift coefficient you'll likely want to use a section with more camber and possibly even add a bent up lip otherwise knows as a Gurney flap. Something more like a NACA 6409 or 7509 or something similarly "banana like" would be more appropriate.

Also you're only talking 70 mph. It may well be that you need that big a wing to do that.

You can test for all this using Foilsim. Don't bother making the camber and stuff negative. Just work with positive values and figure on just flipping it all around. Try using the airfoil it gives you with a camber of up around 6 to 8% and a thickness of 8 or 9% and figure on an angle(of attack) of around 4 to 6 degrees. Then set the speed and alter the wing size until it's got the lift you want. If you don't like the size it gives back then you're going to need to alter your expectations. There's no miracle airfoil that's going to give you much more downlift for less drag. At most the airfoil choice at that point will help by maybe 5 to 10% one way or the other.

You'll also need to add some wool tufts on suction cup stalks onto the car around the wing area and examine the tuft alignment at speed to determine the local airflow angles in the area the wing will be mounted. If the tufts are highly turbulent or even pointed in odd manners then that isn't a good spot to mount the wing as there's a turbulent back flow bubble. Sort of like a back eddy in a white water river. You need to put the wing up and out where the airflow is relatively clean. Perhaps tape some long wool strands to the back lip of your roof just before the back window area. If you have an antenna up there then use that to tape the tufts to.
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