Chute failures and F/C body aerodynamics

I have been watching the problem of chutes wrapping under the rear axle of TA/FC for several years. While I don't have numbers on this I do have a sense of what is going on...and what's wrong.

Two of the heavy hitters several years ago decided the whale tail/shark fin single spill plate was a great idea. One of them mentioned that spill plates were not necessary and the only reason he had the fin in the center of the deck was so he would have a place to stick some contingency decals.

These two guys also started having the same problem. The chutes were suddenly being sucked under their cars more frequently. In an effort to stop the problem, one of them started running chute launchers. But the problem persisted.

After some exciting top end excursions, one of them decided it was time to look at the back of the car to see if the cure was in the wind. And it was.

This person decided to keep the whale tail/shark fin and install spill plates that start even with the deck and extend down to the bottom of the back bumper. The other person followed suit. This slight addition to the back of their cars practically eliminated any further problems.

Splill plates above the deck contain and guide the air over the deck. Adding or subtracting spoiler behind the deck acts like a wall to keep air on the deck like the sides of a bath tub work to hold water in the tub. The higher the spoiler, the more air it contains creating more downforce. The lower the spoiler, the reverse is true. But what about the spill plate area that extends below the deck to the bottom of the bumper? I see a lot of cars running without this short piece of material.

At speed the air that collects behind the body is very turbulant. I remember the 1960 Buick station wagon my folks had when I was a kid. Buick added a couple of stainless steel pieces on the "D" post of the wagons to keep the exhaust from coming in the tailgate window if it was down while driving. It also did another thing. Those two pieces kept the dirt and soot from accumulating on the back of the car, too. They "cleaned up" the air behind that big wagon. Hmmm.

Extending the spill plates from the deck surfaces to the bottom of the rear bumpers on each side of your TA/FC acts as an air deflector to keep the air from rotating (as violently) around the back of the car and sucking the chutes under your car. This one single change to your car if you don't already have it is way more effective than adding any chute launchers.

I'm not saying this will end all your problems, but it will reduce the chances significantly. Like I said, I have been tracking this issue for a long time, and I am convinced the lack of spill plates from the deck surface to the bottom of the back bumper is a significant contributing factor.

Think of this visual. Have you ever ridden in the back of a slow moving boat and watched the wake action behind it? The water swirls around the back of the boat and actually holds and drags foam in the water behind it. Same thing with the chutes behind your car.

I'm posting my opinion on this because I don't think we need knee jerk rules revisions. The racers should be able to look at their cars and solve this issue themselves without governing bodies doing it for us. You might also want to read what I posted concerning chute releases and procedures in Will Hanna's Two Chutes topic.

I don't think Jay will mind me posting this picture if it helps out other racers. Jay's a pretty smart guy and very weight conscious when it comes to his car, and I doubt he puts anything on it without a legitmate reason. Check out the back. He has the shark fin...and he has spill plates that start at the deck and extend down to the bottom of the back bumper:



Photo courtesy of Auto Imagery, Inc.

 

There is one more thing that really helps also. If you put a wickerbill on the end of the spoiler, aprox. 1/2 to 3/4 inches tall the problem seems to go away. Fuel cars with the big spoilers don't seem to have this problem. Problem is nobody wants to do this as the car will lose 3 to 5 mph.

As a parachute manufacture I have been looking at this issue for some time now. I have spoken with some areodynamic engineers including Tim Gibson. There is one common denomenator in all this: Tail wind. Any comination of tail wind or cross wind seems to magnify the problem.

As a driver I always pull both chutes. If they have to come get you off the track, so what. I have had NHRA officals tell us at places like Las Vegas and Sonoma to pull one chute so we can get off the track. This is a recipe for disaster!

DT

 

DT:

We installed a tow hook on the face of the weight bar to make it easier for the safety crew to hook up and tow our car. I would rather be towed 1,000 times than be pulled out of the net...again.

RG

 

You are absolutely correct, Will. Sometimes you make your own luck.