Any aeronautical engineers on here? Lift (downforce) question...

Discussion in 'PSI Superchargers Tech Questions' started by Scouder, Nov 9, 2010.

  1. Scouder

    Scouder New Member

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    Simple question, not so simple for me.

    We have really poor air density here, 8400 is really good for us, 9400ish is common, 10,200 from time to time. On the average we have about 20% less air than the sea-level guys. I am wondering how this affects the downforce on my wing.

    If air density drops 20%, does downforce drop 20%? Or, putting it another way if air density drops 20%, would I need 20% more surface area on the wing to compensate?

    We already know that less density equals less force, the root of the question is whether or not it is a strait-line effect. To me it seems that it would be. Downforce is created by the air molecules passing over the slow side of the airfoil. If you reduce the number of molecules by any given percentage the associated downforce should be reduced by the same percentage. At least that's the way it appears to this country boy.

    Any input?

    -Brian
     
    #1
    Scouder, Nov 9, 2010
  2. jim phillips

    jim phillips ta/fc

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    you can always move to a better climate
     
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    jim phillips, Nov 9, 2010
  3. Woodchip

    Woodchip Top Alcohol Dragster

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    #3
    Woodchip, Nov 9, 2010
  4. Scouder

    Scouder New Member

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    Thanks Jim, I might have never thought of that!


    The NASA formula confirmed what I thought:

    A reduction in air density of any given percentage equals a reduction of lift (downforce) of the same percentage.

    A reduction in air density of any given percentage can be compensated by an increase in wing area of the same percentage.

    Thanks for the link.

    -Brian
     
    #4
    Scouder, Nov 9, 2010
  5. Mike Canter

    Mike Canter Top Dragster
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    To add:

    The density numbers are actually worse than you think. At 9400 ft. and about 70 deg. F, air density is approximately 30% lower.

    Back to your question - your wing downforce is proportional to air density, wing area, and the square of your speed. Wing downforce is also a function of wing angle. The function is somewhat linear, but only over a limited range, just like an airplane experiences when it stalls at too high of an angle of attack. For a typical full body racecar, the range is more limited than an airplane because the racecar wing is seeing air at a variety of angles of attack coming off the car body, whereas an airplane wing is often in "clean" air. A typical dragster wing performance is closer to an airplane wing because it sits up high.

    So your country boy analysis, is leading you to the right conclusions.

    If you have a rear suspension sensor and a data logger, use them to determine if you have the right downforce.
     
    #5
    Mike Canter, Nov 10, 2010
  6. Scouder

    Scouder New Member

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    Mike,

    Thanks for the input. The NHRA rule for TD says 750" limit. For me to get the same downforce here that my wing would get at sea level at 750" I would have to make it 960 inches! Hmmmm.

    -Brian
     
    #6
    Scouder, Nov 10, 2010
  7. Mike Canter

    Mike Canter Top Dragster
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    The other solution is the wing angle BUT if too much angle then the wing will stall and you will loose all lift and it will be like a boat anchor. Depending on the air flow over the race car zero degrees in reference to the ground may not be zero degrees to the air coming over the car then over the wing. If it is a full bodied door slammer then the position of the wing on the car is critical. Too far forward and not high enough can actually create lift when the air comes over and around the roof. I have watched more than one car's rear tires become airborne because of this. In a door slammer the wing needs to be as far back and as high as it is allowed. A dragster is easier because the air is cleaner. The other thing is some "wings" I have seen are not designed as true upsidedown wings and really do nothing but slow the car down.
     
    #7
    Mike Canter, Nov 10, 2010
  8. Scouder

    Scouder New Member

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    Mike,

    It's funny you should mention a wing that does nothing. The reason I am building a new wing is because the wing profile that came on my dragster looked like it was creating a lot more drag than downforce. It looked very deep through the belly and on top of that it was only about 625 inches. I bought some wing ribs from Frank and Scott Parks that look much better as far as the profile goes. I'm gonna go ahead and make it 960 inches.

    -Brian
     
    #8
    Scouder, Nov 10, 2010
  9. Charles Stewart

    Charles Stewart Member

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    You can also, use a "wicker bill" (try different heights. (Beware, some class rules limit height of a wicker bill)) to add some downforce and keep drag at minimum instead of an aggressive angle in your wing, that will bring drag.
    Charles Stewart
     
    #9
    Charles Stewart, Nov 15, 2010

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