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Has wings, won’t fly

Cars are like flightless birds — they can run fast but not get off the ground

india Updated: Aug 27, 2009 16:04 IST
Grease Monkey

Last week, you learned that air can have an effect on how your vehicle behaves at speed. The shape of your car can suggest its aerodynamic properties as well. A car like the Wagon R or Santro has an upright stance, which presents a big frontal area.

This makes it difficult for the car to move against the wind — if you’ve ever walked into a stiff sea breeze, you’ll know what I’m talking about. Cars like the first-generation City or Esteem have a low bonnet and relatively small front area in proportion to the size of the car – this helps them move through the air better.

Wings to stay grounded
Appearances can be deceptive, however — the Mercedes-Benz E-Class may not have the most svelte design, but it slips through the air better than most cars, with its drag coefficient of a mere 0.27. The Esteem, in comparison, has a drag coefficient of 0.32.

This, however, is due more to attention to detail than the car’s overall shape.
Wings are what you see on the nose and tail of most formula racers. They help shape the airflow so the faster the car goes, the more it’s pressed into the ground for better grip. Wings also create drag, which lowers the car’s top speed.

This is why racing teams have to find a compromise between ‘downforce’ and drag — on a circuit with long straights like the Monza racetrack, very little downforce is preferred. On a twisty track, high downforce is preferred, as greater speed through the corners will offer a better lap time than good top speed on the straights.

A long nose to defeat drag
Purpose-built drag racers show both the characteristics described above — their nose starts off as a point, and there’s an enormous rear wing to keep the car on the ground. It may be hard to believe, but top fuel dragsters pass 300 kph with ease at the end of the drag strip. Thrust SSC, the supersonic car that currently holds the landspeed record, had another drawback to combat: the sonic boom that is generated when it goes supersonic.

Which is not the same reason the Concorde or the Tupolev TU-144 weren’t allowed to go supersonic over crowded cities — that was for reasons of noise pollution — but the sonic boom adds extra drag to the vehicle. The nose therefore originates from a point and is shaped as a long cone to minimise this drag — some early supersonic military aircraft like the Bell X2 possessed a rod-like projection in front of the nose to aid at supersonic speeds.

Is it a bird? Is it a plane?
Back on earth, though, cars use underbodies sculpted to channel air as fast as possible underneath the car — this reduces pressure under the car, sucking it down to the road.

This is what ‘diffusers’ do. Active aerodynamics, which means electronically controlled wings, are also present on some road cars.
The Porsche 911 Turbo, Audi TT and Bugatti Veyron all have rear wings that pop out of the body when the cars reach a particular speed to aid stability. Most notably, the McLaren F1 had a rear wing that would act as an ‘air-brake’ like the ones in aircraft — under hard braking, it would pop up at a greater angle than if it were under acceleration, to help slow the car down swiftly.

Ford’s ECOnetic cars as well as VW’s Bluemotion vehicles have much-improved aerodynamics to help maximise fuel efficiency at highway cruising speeds. Who knows, your daily commuter might have electronically-controlled wings as an option some day.