Formula 1 cars have wings to help keep them on the road during cornering, they don't need them whilst moving in a straight line. And that's ok , they "only" travel at 220 mph.

In a different speed regime, at 1,000 mph, the Bloodhound SSC has small canard type wings about a quarter way down from the front. Reduction of drag is the aim here obviously, so keeping the wings as small as possible is important.

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I am well aware I am second guessing all of the BloodhoundSSC.com experts here, but at 1,000 mph, the slightest bump has the potential to turn a small upward displacement into a complete flip over.

My questions are:

  • Are the Bloodhound designers depending on the speed, from say 300 mph upwards, to create sufficient down force from what looks like quite small (in surface area) wings?

  • How does downforce scale with speed? I have no experience of the fluid dynamics involved. Which set of equations is involved in the computer simulations?

I have very little knowledge of this area of physics, apart from basic aviation based practical knowledge and I would really appreciate knowing how you go about determining the correct aerodynamic tradeoffs that are involved.

The references cited above don't go into much detail, otherwise I would not ask this question, and I certainly don't expect a fluid dynamics course, just a short outline of how this situation would be dealt with.

  • 2
    $\begingroup$ In general, force scales with velocity squared -- $L = 1/2 \rho C_l V^2 A$ where $L$ is lift (or downforce), $\rho$ is density, $C_l$ is the lift coefficient, $V$ is velocity and $A$ is surface area of the lifting body. I'll do some more digging into this particular design, but it could just be that velocity squared and assumption of very, very small bumps are all that it requires. But going over the speed of sound complicates things quite a bit regarding what the properties are like on the actual body. $\endgroup$ – tpg2114 Sep 15 '15 at 22:25
  • $\begingroup$ @tpg2114 thanks, all I could think of was a vague connection with the N-S equations. I know they are making big efforts to get the "runway" as smooth as possible, and that also I would not drive it, for sure. $\endgroup$ – user81619 Sep 15 '15 at 22:29
  • $\begingroup$ I've always wondered: what's the difference between a supersonic car, and a supersonic plane that happens to be flying extremely low and dragging an idler wheel across the ground? $\endgroup$ – Daniel Griscom Sep 16 '15 at 3:01
  • $\begingroup$ @DanielGriscom probably not very much, if the car hits a bump :) seriously not very much in the situation you describe. Obviously no wings but just as important, the weight, the car has a limited range, keeping the fuel load just enough to achieve the run, and then the return run. The Bloodhound is being "driven" by an ex fighter pilot who has virtually no steering, just a throttle and a brake. Brave guy... $\endgroup$ – user81619 Sep 16 '15 at 8:25
  • $\begingroup$ @tpg2114: at mach 1.4, aerodynamic forces far exceed weight. As far as bumps, I assume the designers know they are designing a low-altitude aircraft. So the center of mass has to be forward of the aerodynamic center (as the tail shows), so that when there are lateral or vertical disturbances, the natural tendency would be to damp them out. $\endgroup$ – Mike Dunlavey Sep 16 '15 at 19:01

First, you should be aware that while normal racers need a good deal of downforce, it is used to increase tire friction and allow tighter turns. The Bloodhound, by contrast, is not intended to turn at all when at speed - ideally the driver would never turn the steering wheel. It just gets out on the track, gets up to speed, clocks the necessary distance, then slows down, all in a straight line.

Second, aerodynamics has been figured into the vehicle design, but in a sense the vehicle makes the problem easier than it might be. At 7 tons and a fairly small area it really doesn't want to fly, although admittedly supersonic operation will give it some opportunity. However, consider that the center of thrust of the jet engine is, from pictures on the website at least 4 1/2 feet off the ground. While this would cause real stability problems if the beast really were trying to turn at speed, it has the effect of producing a torque that pushes the nose down as long as thrust is being produced. Presumably braking is performed mainly by the rear wheels to produce similar torque at the end of the run. And the vehicle doesn't have the problem of a large hull to generate the sort of lift that kills hydroplane racers.

Third, operation will be on a playa in S. Africa. Like the Bonneville Salt Flats in the US, a playa is extremely smooth and flat, so the likelihood of the nose pitching up is small.

With all of that said, I wouldn't want to be providing life insurance to the driver.

  • $\begingroup$ Thanks very much for your answer, I accept your first paragraph 100 percent, but I wouldn't drive this car with a gun to my head. They broke the sound barrier with an earlier version and I worry they are a touch overconfident. Still they know much more about the car and how to achieve their goal than I ever will. Best of luck to them. $\endgroup$ – user81619 Sep 16 '15 at 0:41

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