Quote:
Originally Posted by MetroMPG
Re CAD...
Sorry, should have said "CAD", in quotes. With a nod to Craig Vetter, I was talking about "Cardboard Aided Design". 
Thanks for the input, Phil.
I'm a little nervous about those nervous looking side tufts. It's clear I could increase the top taper - those tufts are laying almost still right to the transom.
What I don't know - and maybe you or Bob can tell me - is this: should my goal be to taper as much as possible until I actually see evidence of separation through circulation/reverse flow, and then back off a bit?
In other words, will we achieve favorable drag results even as we increase the amount of "nervousness" in the tufts (evidence of tubulence and a thickening boundary layer) through further tapering?
It's clear I can taper the top more. Not sure about the sides though. Maybe you can decide when I post some video this week.
Given that Cd is supposedly constant in the range of speeds where road vehicles operate, why would a vehicle want to actively adjust its angle of attack? Is the nose up/tail down attitude a concession to low speed driving for clearance, making the top angle aerodynamically sub-optimal until the nose goes down? |
Darin,I apologize,as I'm trying to catch up line by line.
With respect to the longitudinal centerline curvature,I enlarged my image of the Ford Probe-IV and put a protractor to the back,for the 45-mph full-down suspension orientation.
With the nose dropped and hiney sticking higher into the air,a straightedge laid across roof to trunk lip reads 17-degrees.
So it looks like your 15-degree final exit angle is "conservative." However,looking at the EV1 and it's boat tail,the straightedge also yields 15-degrees.Maybe conservative is good!
Kamm advocated no more than 10-degrees although his K-cars violate that rule,and the K-3 car which we have a photo of clearly shows the boundary layer separation and emergence of turbulent wake at the back of the roofline where the curvature begins to get "fast".
As far as the sides go that's a bit more tricky.Kamm started "narrowing" his roof at same time as it was curving down.
How the front of the car divides the airstream,and the forward architecture determines the local velocity and available kinetic energy of the flow at any given point.
Typically,the nose scoops more over the car and the energy there is greater than the sides and can sustain the most "bending."
I don't have Hucho's later book and I don't know if he goes deeper into side flow.
Jaray's/Klemperer's/Fachsenfeld/etc. work suggest that if you respect the teardrop form,as a half body of revolution,we can curve the sides just as we would the top.
But the teardrop doesn't have a "roof." And I'm more conservative,allowing only half what might work over the roof.No good science there,sorry!
I know that for any given length,that if I take the sin of 7-degrees,multiply that by the length of the run,and then "bend" a line off center that measurement,at that length,I get a curve the air will follow.
I'm going to do the full 22-degree Mair' maximum on the trailer,top and sides.And it's an unknown quantity.The VW worked at 20-degrees and NASA chose 20-degrees and you can see from their tufts that it's pretty clean.
I have extremely gentle curve transitions on the boat hull up to 22-degrees,then the rest of the body just shoots back at the constant 22-degrees.We'll see in a few weeks what happens.
I can get away with murder underneath because of the extra axle of the trailer.It's belly is a continuation of the T-100's 2.5-degree diffuser.
All the under-belly fairings and strakes are shaped to respect the SAE 10-degree departure angle.
By the way,your wheel fairings should have your rear tires below Cd 0.10 now for what you've done.They mimic the "chute" style underwing fuel tank and rear faired aero wheels which are at Cd0.10 (and do not have the lead fairing as your's).
I'll shut up and read more of your posts.Your boat tail is awesome!