Quote:
Originally Posted by ChazInMT
Shadetree, I don't know whether to thank you or not.....what do you mean by the last paragraph? "If you want to read it from the experts..."
The spoiler improved mileage on that hideously shaped Pinto because it had a gigantic Vortex Generator for a rear end. (If you research this shape you'll see what I mean.) You could put a 12 inch diameter log across the area above the tail lights and it would improve the airflow off that perfectly shaped, worst possible way to design a rear end. So I dunno why you bring a spoiler mounted there to the table.
I understand that boat tailing the VW Bug would help a lot. There is no denying that. You can keep posting about it as a solution to the problem Morgan has put before us here, but you do so at the risk of violating his "relatively easy/simple modification" request I seem to remember him saying. I think he's looking to avoid having the "cut his car in two, throw away most of the rear half, then, rebuild it so its 10 feet longer and tapered" modification here, tempting as it may sound.
Why do you not feel a roof spoiler mounted at the back of his roof above the rear glass as I suggested would work brilliantly???
Truth be told, I'm seriously contemplating trying to improve the aerodynamics on my new car by doing the same thing. The only thing I see as an issue is the air coming off the c pillar wrapping into the area of the rear windshield. But I suppose in theory, if the rear air is creating a more dramatic low due to the lift, the stagnant are created by the roof spoiler would be higher pressure.
I'll close the door on long trips. |
The science tells us that the Koenig-Kamm roofline is the hot ticket,whether chopped at any convenient length for 'practicality'.or do the full monty,running it out to 80% ( the physical limit due the ground clearance ).
Short of that,things can be done as palliatives to deal the flow separation and keep construction more simplified.
With respect to the Pinto,the spoiler affected a place for the flow to re-attach to,and in so doing created a locked-vortex circulating above it,which is known to allow the outer flow to 'skip' over,almost as if it were a solid surface.
Had Car and Driver filled the area in as Chrysler did with the Airflow they would have achieved even better economy.Although,for a single piece of aluminum they did okay.Which I think is the spirit behind the 'simple' spoiler.
Your idea for the roof-mounted spoiler is money in the bank.I'll bet you a meal on it.
Mercedes has used them to good effect on their Autobahn stormers.
Ford's patented bi-wing spoiler would be even better,with Ernie's wing down below it.
Going back to the chopped tails,you can see from the table that there is a 28% difference in drag between 50% and 80% tail.
At 55 mph that's 14% mpg.
At 70 mph that's 16% mpg.
At 80 mph that's 28% mpg.
I'd never try it with a cantilever tail and it's the reason I settled on the trailer solution.
Kamm and Korff recommended chopping at 50% frontal area,which is not the same as 50% of the roofline slope.
Unless the sides of the car begin their inward taper at the same point as the roof begins to drop,the roof might have to be extended further in order to reach 50% frontal area.
Kamm's 'best ' car was only Cd 0.23 ( same as the modified Airflow ),so you can see that half of the drag remained even in his car.
I'd build that roof spoiler!