Some vehicle aero flow visualizations with some nerdy scoop stuff

[Richard Hudgins]

Folks,

This video may help in everyone's understanding the flows around the Cobra. I know the vehicles are quite different from the Cobra but the video is still applicable.

[ame="https://www.youtube.com/watch?v=quDLzxmJl5I"]Flow Visualizations of MR2, RX7, Supra, FRS [/ame]

And just to rile things up a bit, a treatise on scoops:

NACA Duct vs. Scoop

[OnyxCobra]

very dry but great info
M@

[Ron61]

I will see if Morris will post his video of the air flow around his Cobra that they did when he was racing. I think the Kirkhams helped him with it and he has the best build thread I have ever seen. We have tried to get him to write a book with all of the illustrations and videos in it.

Ron

[Tommy]

Thanks for sharing. My undergraduate degree is in Aerospace Engineering so this all felt very familiar to me. What the flow pictures reminded me is that even minor breaks or obstructions in the smooth flow of the air can have a huge effect. Thus, a car as fundamentally un-aerodynamic as the Cobra is not well suited for very high speeds. .... Morris mitigated a lot of the Cobra's aero shortcomings in his car, but at the cost of considerably higher weight. I suppose that if you want to run on a track where top end speed is critical to success, that is the way to go.

[vector1]

I've seen cars with holes in the Lower rear fender with a rubber piece to let air out of the fender wells, must be something at highway speeds.

[vector1]

So if a scoop is within the boundary layer, and I'm guessing that can be seen in the videos, is there not any associated drag.

[Tommy]

vector1,
First, it would be wrong to assume that anything about fluid flow could be reduced to a simple rule of thumb. Second, there are several types of boundary layers, and the forces associated with them vary (i.e., drag, lift or down-force). In general, a smooth shape that allows air to stay very close to the surface of the object allows what is called laminar flow and produces the least drag and most lift/down force. Any obstruction or sharp turn in the shape of the object is likely to cause separation of the air flow from the object, destroying the laminar flow and increasing the drag. .... If the air scoop is placed in an area where the air flow has already separated and is mostly flowing above the scoop, the scoop will not add much to the drag. But the body shape that caused the separation will. Said another way, if the scoop is on a body that doesn't have a good aerodynamic shape, it won't hurt much. But if it is on a sleek, aero body, it will.

[Ralphy]

Tommy,

I don't know if you ever saw what the NASCAR guys are doing this year at their first pit stops. I understand they (NASCAR) plan to put an end to it next year.

Crew chiefs skirt the issue at Kansas

Ralphy

[Tommy]

Ralphy,
I don't follow NASCAR closely, so that's the first I've heard of that. But it makes sense. The faster you go, the more effect the little aero changes have. And when the difference between first place and tenth place in a three hour race is less than a second, little things matter a lot.

[Ralphy]

I'll keep relating to (sorry) NASCAR. I think it's the main reasoning (drag) for cowl induction. Bill Elliot was thought to have the greatest motors in his day. However I saw him interviewed once recently. He said that's not what they had. They were building their bodies to as exact minimal dimensions at every point on their cars. With the simple thought the less cubic inches the body had, the easier it would knife through the air. He was known to be a super speedway demon. Makes sense and they also sold a lot of motors! LOL

I'm halfway through the video and thinking. Putting an elongated hardtop on your Cobra has got to be very beneficial? Back to the vid!

Ralphy

[Tommy]

Speaking of cubic inches knifing through the air - my favorite airplane used an aero concept from the 1950s that resulted in the Coke bottle shaped fuselage. Seen from above, the fuselage narrows as the wings widen. The idea was for the cross sectional area of the airplane to stay close to the same as the air moved from the front to the rear. You don't want the cross section pushing through the air to be any bigger than necessary (think Ford GT40), and you also don't want it abruptly changing size along its length.



Here's one more thought about air flow that many people don't realize. What we think of as air is mostly empty space with free floating molecules of nitrogen, oxygen, carbon dioxide, water and other things drifting through it. These molecules are packed closely enough that they bounce off each other sort of like balls on a pool table. That is why a disturbance 1,000 feet away will be relayed to our ears as sound in about a second by the bouncing molecules. Thus, the speed of sound is roughly 650 MPH on a typical day on the ground. This matters in car design because it means that as long as we are moving at less than the speed of sound, the air hitting the front of the car is being influenced by the air hitting the rear, and vice versa. You can see that in a wind tunnel where a change at the back of the car will cause the air flow well forward of that change to move. So, you can't think of the aero design of a car as a group of unconnected features (e.g., air dam, hood scoop, windshield rake, tail wing). They all work together, making the job of getting it right much more challenging.

[vector1]

That's a pic of two jet engines with fins that someone rides on, nothing but respect for those guys.

[J.Jensen]

The reduction of the F5 / T38s cross section over the wing station increased the Mcrit speed and produced less drag at the very high speeds.

Its really simple, its the fattest part of the aircraft that is the limiting factor for maximum speed. For the same reason racers had flatter windscreens or just the small wind-deflector for the driver.

[Morris]

Richard....and Ron......

I'm enjoying reading this thread.....now maybe some folks will understand some of the modifications we made to KMP259.....

And how we were able to safely go as fast as we did at Road America....

Morris

BTW......excellent video.....

[Morris]

Here's a photo that told us where to put the wing.....

Can't seem to attach photo....

[G-Pete]

We should tell ALL vehicle manufactures and race teams to abandon full size wind tunnel testing and just get a 1:24 model and invest less than $100 dollars to get conclusions.

Pretty cheap compare to the $10K an hour in a wind tunnel.


Most important, we have to tell them their argument about the monocular structure is not valid - never was and never will.


Come on, give me a break!

[G-Pete]

Hmmm.....

[AL427SBF]

Originally Posted by G-Pete

... Most important, we have to tell them their argument about the monocular structure is not valid ...

Col. Klink begs to differ ...