Quiz - estimate the Cd

[Hersbird]

I can't believe that car was built by cartoons!

[Hersbird]

I guess it depends on how the drag is measured. Does the propulsion sucking in the front hole negate what that would show if you placed that car in the tunnel without it running and the air just stacked up against the turbine?

[kach22i]

VW XL1 = .189

I went one step lower than the above in a contextual comparison.

I think you guys are right about the air intake being a variable outside of our familiar norms.

I do not recall Cd's of aircraft being posted in the forum. However, isn't a body this close to the ground a doubling factor?

[aerohead]

Might have that at home, so I won't guess.
This car may have been in the shootout with another streamliner 'Thunderbolt,' which Alex Tremulis ( himself a world motorcycle speed record holder ) wrote about in the 1980s.
During the 'duel,' one of the teams took the risk of removing the rear stabilizing fin and radiator, attempting to shave enough drag for a world record run. They were both in 357-mph territory if memory serves me.

[Vman455]

The Bluebird CN7 originally had no fin, and crashed at 365 mph. The car was rebuilt and the fin added before its record run; it ran only one more time after that as a demonstration on an airstrip where it overshot the end of the runway and crashed at low speed. (According to an article in Motorsport magazine I found yesterday. That article, published in 2002, also reported the drag coefficient and drag area).

[freebeard]

Originally Posted by kach22i

I think you guys are right about the air intake being a variable outside of our familiar norms.

A turbojet > the Meredith Effect.

(According to an article in Motorsport magazine I found yesterday. That article, published in 2002, also reported the drag coefficient and drag area)

Enquiring minds....

[JulianEdgar]

And the answer is....


"near to 0.16" - the the most definite value I have.

[Vman455]

Here's the link to the Motorsport article. Of interest is this passage:

Whereas Goldenrod had a quoted frontal area of 9sq ft and drag coefficient of 0.117 to give a CdA of 1.05sq ft, Bluebird had a much larger frontal area (26sq ft) and a drag coefficient of 0.16, giving it a CdA of 4.16sq ft — getting on for four times greater.

But also interesting:

The other great restraining hand on an LSR car is tyre drag, and in this respect Bluebird was almost certainly superior to Goldenrod, as Ron Ayers — Thrust SS C’s aerodynamicist — has explained in these pages (October 2001, page 54). Quantifying tyre drag on salt has never been a simple matter, not least because the compression of the surface depends on its condition, which varies from season to season and year to year. But George Eyston’s aerodynamicist Jean Andreau had developed an equation to allow its estimation which, in addition to showing that tyre drag is much more highly speed-dependent on salt than on a hard surface like asphalt, established the crucial importance of tyre pressure. According to Andreau’s formula, tyre drag at 300mph could be reduced by three-quarters by doubling inflation pressure from 60 to 120psi. For this reason, Cobb’s tyres had run at a high 120psi; CN7’s were enigmatically specified as operating at ‘greater than 100psi’, but in truth ran substantially higher. During the Goodwood shakedown in July 1961, Dunlop technicians were spotted setting the pressures to 130psi. For the record attempts, Ken Norris recalls, 160psi was used. So, despite the fact that tyre drag didn’t feature in his calculations for Bluebird because of the difficulty in quantifying it, he could be confident that CN7 had unprecedentedly low rolling resistance, a factor that will have helped nullify, at the very least, its higher aerodynamic drag.

And here's the article mentioned there, by Ron Ayers, on Goldenrod (which you should read in its entirety, because it's fascinating):



This car has cD = 0.1165 and area A = 8.5 ft^2.

[JulianEdgar]

Interesting article. When we did some travelling a few years ago we had on our list to see as many LSR vehicles as possible. They're all glorious in their own ways.

[freebeard]

newatlas.com: Jessi Combs breaks 48-year old land speed record
By Angus MacKenzie October 15, 2013

Jessi Combs burnt her candle at both ends, but in 2013 she piloted a grounded F-104 on four wheels made of solid aluminum. On the Alvord Desert in Oregon.

The FIA rules require the vehicle to be considered a “car” sporting four wheels. So in order for the Eagle to blast across at the lake bed at subsonic speeds, solid billet aluminum wheels were chosen. Solid aluminum wheels not only reduce rolling weight but remove traditional concerns regarding centrifugal forces associated with rubber tires.

The 1000mph Bloodhound and others do the same.