Experiment: tuft testing the underside of a variable-angle tail extension

[RobertSmalls]

Piwoslaw posted a graph from Piechna's book:


This agrees with my tuft testing as well. For minimum drag, 4° looks great. Even up to 7° looks fine. But things do not rapidly fall apart until 15°. From 15-30°, it looks like you'd be better off just not having an underside. I will avoid these angles.

Of course the actual numbers may be different because the wood block isn't shaped exactly like an Insight, but the trends should be the same.






SI2, I've left the exhaust and wheel wells wide open. Things don't look good except down the center of the car.

[aerohead]

Originally Posted by RobertSmalls

I was under some mistaken impressions about what the tufts would look like, hoping for a sudden and easily discernable transition from attached to separated.

I studied this video of an airfoil for a while. It's a more intuitive flow visualization than tufts. It looks to me like a very thin seperation bubble begins to form around 4s into the video. It becomes quite thick, with corresponding increase in drag until 6-7s, but the airfoil hasn't stalled until 8s.

Comparing this airfoil to my tufts, I have to agree, the higher angles I tested have higher drag. The seperation bubble moves farther forward the steeper I make the tail.

I guess the tuft test confirmed that the underside of the tail will be a compromise between drivability and drag (neither of which I have good data on). I'm leaning towards 7°. However, another idea is to just go with 12° in fiberglass, and have a clip-on tail extension extension for road trips and FE runs, to bring the underside angle to 7° or even 3°. Although, would the reduction in drag underneath the car make up for the increase in transom area?



Btw, Jim, thanks for the tip on the brake cables. Those things annoy me every time I see them dangling from my car.

Robert,I've gone back to 1922 and worked forward looking at tails 'n such.
It looks like everything 'helps' whether ideal or not.
The very existence of 'some' tail appears to offer,for lack of a better expression,a 'stuffing the wake' effect,which helps force where vorticity of the turbulence can go,keeping it closer to the outer,more energetic air rather than tumble 'deeply' into the void which would otherwise exist should the tail not be there.
Not very scientific,but it's my best take on something I have no specific data on.
Unless you can 'swing' the diffuser up and down,you've got to compromise or she'll get knocked off.
I'd just go for it and live with the difference.
Given more time,I might be able to accurately' predict' the drag difference,but I'm not there yet.
I can get away with murder with the trailer because of the inter-axle span,but that's not for everyone.

[COcyclist]

Originally Posted by silverinsight2

Originally Posted by aerohead

Robert,I don't have my book with me,but my failing memory is emphasizing that nobody would see attached flow with more than 4-degrees up-sweep on a diffuser.
The other thing mentioned,is that the diffuser is worthless without a complete and well-executed bellypan ahead of it.
I like the strakes.This is something Dr. Morelli used on his Cd 0.23 CNR car for directional stability in crosswinds.
I don't know about the racer stuff.I 'think' that in racing,that allowing the low pressure of the wake to be communicated forward under the car could help with downforce on the track,especially if spoiler dimensions were limited by the sanctioning body,and this would be an advantage during high-speed cornering.
So my intuition tells me that going over 4-degrees is a downforce issue.
And certainly,'departure' angles for the real world ( 10-degrees ) severely limit diffuser angles unless 'active.'

Originally Posted by RobertSmalls

Piwoslaw posted a graph from Piechna's book:


This agrees with my tuft testing as well. For minimum drag, 4° looks great. Even up to 7° looks fine. But things do not rapidly fall apart until 15°. From 15-30°, it looks like you'd be better off just not having an underside. I will avoid these angles.

This seems to match up with the Piechna graph posted earlier if I read it correctly. I always wondered why sports car diffusers were so much steeper than the oft quoted 4º ideal. A friend has a Lotus with a diffuser like the one in the photo above. It also has a factory belly pan and a huge (for the size of the car) rear spoiler. It would seem Lotus is willing to sacrifice low drag for downforce.

[Bicycle Bob]

The skin drag of the strakes adds energy to the wake away from the main surface, and prevents large-scale turbulence. The Bertone BATs used somewhat the same principle, as well as creating what a HVAC guy would call a split diffuser.

To study tuft behaviour, you could put a small streamlined shape on your hood, with a tuft on top for crosswind-induced angle of attack indication, and one on the side to indicate separation.