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Lift, Friction of 18-Wheelers
Name: Kurt
Status: student
Grade: 9-12
Location: FL
Country: USA
Date: Winter 2011-2012
Question:
I am a 9th grade student working on a science fair project. In my project, my idea is to design airfoils that can be fitted to the top of 18 wheelers to create lift on the vehicle and thus reduce rolling friction and hopefully give 18 wheelers a higher miles per gallon ratio. I have been testing my designs in a small scale wind tunnel that I have created and I am unsure how to scale my results to a full sized 18 wheeler with a full scale airfoil. For example, if a 36 square inch surface gives me 0.4 ounce lift at 12 miles per hour wind speed, what life could I expect for a full sized 480 sqare foot surface at 70 miles per hour (where the attack angle is the same in both cases)? Thank you for your time. I hope that my work can result in more energy efficient transportation of goods.
Replies:
Kurt -
I think wings at modest attack angles usually operate with roughly fixed lift-to-drag ratio,
as speed varies, and as Reynolds number or scale varies but stays high.
You know aerodynamic drag goes as the square of airspeed,
so with proportionality, so does lift.
Your idea is innovative, but there are
two things you might want weigh against it, right on your poster.
Those are:
- the lift will cost some air-drag, according to the lift-to-drag ratio of your wings, which will be between 10 and 20.
10 is easy, but I do not think there is any way one could do better than 20.
So the lift will reduce rolling friction, but increase air drag by 5% to 10% of the lift generated,
and I would like to know if it is a net gain or loss in vehicle drag.
I think maybe you could estimate that.
It would look unusually smart to show that consideration on a poster,
even if it means the wings do not save energy.
- truck drivers greatly value traction to retain steering and braking ability.
It can be very dangerous to reduce the weight on the tires while carrying a heavy load.
I am not sure how to weigh this and get to a scientific answer.
It is sort of a matter of practicality, rather than science.
http://en.wikipedia.org/wiki/Lift-to-drag_ratio
(Gloss past that first drag-vs-airspeed plot, I find it and its implications confusing.)
http://www.grc.nasa.gov/WWW/k-12/airplane/ldrat.html (more equations for lift and drag)
(Cl can be a mess, depends on angle of attack and maybe other things,
but it's fair at your level to assume it's a constant equal to "max Cl", just like Cd is a constant.)
http://www.hq.nasa.gov/pao/History/SP-468/ch7-6.htm (lift to drag over aircraft history)
(and two chapters before that, which plot Cl and Cd separately...)
Jim Swenson
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Update: June 2012
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