Aerodynamics questions

[Kosmic319]

Hello, we’ve run a Piper DF-5 for a year with a big fiberglass floor behind the side pods and under the transmission. I haven’t really seen any of the pro teams running one all the way out to the gearbox like ours. Just wondering if it’s something that we should leave off for the upcoming season. Also, would it help cooling if we were make a big aluminum plate to block air from hitting the outer tanks of the radiator and therefore only allowing it through the fins?

[t walgamuth]

Hello, we’ve run a Piper DF-5 for a year with a big fiberglass floor behind the side pods and under the transmission. I haven’t really seen any of the pro teams running one all the way out to the gearbox like ours. Just wondering if it’s something that we should leave off for the upcoming season. Also, would it help cooling if we were make a big aluminum plate to block air from hitting the outer tanks of the radiator and therefore only allowing it through the fins?

I am just a guy sitting at my computer with nothing except my own head.

The floor under the chassis would seem to me to help flow through that area. I have a lip where one used to be on my 98 VD but at autocross speeds it seems ineffective to add it in....though i would if I had the parts etc. to do it. I also think your dam idea would help flow therefore make less drag and more speed. It should also help cooling though perhaps not enough to measure.

[DaveW]

Lacking any test data, I would keep the undertray on. Typically an undertray, even a flat one like yours, will add some downforce in the rear and clean up the airflow, adding almost no drag. And smoothing the air path to the radiator(s) will reduce turbulence and drag. I assume you are under weight enough that adding the plates will not be a weight penalty.

[Rick Kirchner]

If I had to guess the floor is giving you a little bit of drag reduction, and under the right conditions, maybe some downforce.

A nice fairing that goes from the side to the edge of the tank will no doubt decrease drag from your cooling system. Great place for a 3d printed part!

[DaveW]

An empirical data point:
Before I ran the Shrike S2000 (1986-87) we did a bit of testing at Grattan which included the effect of an aluminum sheet ~30" wide x 40" long that closed off the bottom of the engine bay. This had almost zero rake, i.e., pretty much parallel to the ground. We started the test with it on, and to determine if it helped, we removed it. I immediately spun going over the RH rise just before the LH "Bowl" due to losing the downforce it provided.

Obviously, we put it back on and continued to run it as long as I had the car.

[cjsmith]

I have tested 2 FF cars in a fixed floor wind tunnel (A2 tunnel in North Carolina). Both VD cars tested with rear under trays. With out divulging too much, we found the drag penalty wasn’t worth the very small reduction in lift it provided. We also saw an increase in mass flow through the side pods without the tray. That being said, a fixed floor wind tunnels don’t provide as accurate results on the bottom of the car as a rolling floor test. Keep in mind, this is how my car responded to the rear trays, your results may vary. Which is what a race engineer told me when I was setting up a test plan. No 2 cars are the same, you have to try it and see what the numbers tell you. As Dave mentioned, try a test session with your current state and one with out the tray and see what happens, you may or may not be surprised. If you are close to min weight and you remove the tray remember to add something back in or have a couple extra burgers before the race. Removing the tray may require other changes to fine tune the cars handling. If you look at the fast US pro 1600 cars or the cars running in the UK, none of them have rear trays.

[problemchild]

The "Pro" guys with Honda engines are more concerned about engine cooling and ease of maintaining cars on busy weekends. While downforce and drag may have some significance on cool days at Road America, VIR, and Daytona ...... it is the furthest from our minds while "pack racing" in the heat of summer at most race tracks. I removed the rear undertrays on the 2 DF-5s that I worked on, primarily because of the weight. I would probably leave it on for a gentlemen racer requiring ballast.

The act of generating downforce on a US Formula F/1600 may have been a painful political tool, but it has no use while on track. Winning races is all about being able to complete passes in the pack-racing environment.

My cars don't have undertrays or long exhausts. It has nothing to do with performance. I just want to minimize the cost of parts, and repair time, when repairing a crashed corner.

[cliff]

I have the same Piper model and have run it both ways. I thought it was beneficial on tracks like Mid Ohio and it gave me a bit more confidence in the kink at Road America. Mental I am sure. As Greg stated, there is a trade off...you are adding some weight and it does hinder cooling a bit. And you can add about an hour to the assembly and dis-assembly messing with it. YMMV!