Biggest gains are at the back???

[aerohead]

Originally Posted by jimepting

Found a Fit today over in the Lowes lot while looking for grill block parts NOW I see what you are talking about. Stand behind the Fit and the morphing into a better transom shape is very apparent. When I got home I stood behind my Gen 1 Insight and decided that the Insight had done a much better job, but of course they didn't make nearly as many compromises. Now you are making me want to put some tell tails on the corners of the rear to see which way the flow is going.

Do you think the Insight is cursed with the longitudial vortices at the upper corners of the rear (i.e. the corners of the tail lights)?

I measure a terminal exit tangent angle of 14-degrees on your Insight,which is knocking on 'ideal' contour.What I suspect,is that the yarn might orient itself towards the roof near the side/roof intersection but I don't think you'll find vorticity.Just strong downwash and inwash at near identical velocity.There was a brief moment in the wind tunnel video of the Insight when they went across the side mirror with the smoke and if memory serves me,that while perturbed by the mirror,the flow stabilized where you would suspect vorticity to be present ( can I trust my memory?)
In Hucho's 2nd Edition,in Chap.4,'Analysis of aerodynamic drag' he says only "surfaces must be matched to prevent high pressure variation along the flow path."This is where the wind tunnels pay for themselves!
In his Fig.4.14 he shows a drag maximum @ 30-degrees down-slope,with high vortex induced-drag.If the slope is increased,the downwash bursts the vortices and the wake changes over to a squareback type wake.
I think HONDA's done such a good job that you're not at risk.
Tell tails on the side of Kamm's lowest drag 1939 K-5 show precursor 'hunting' towards the roof.Hucho's wind tunnel photos and the ones VW sent me of the only surviving K-Car show zero vorticity,just turbulence.The Landsberg Castle K-Car and K-5 have similar roofs.

[aerohead]

Originally Posted by jimepting

I've been interested for a while with this Bonneville fascination, but I must say I don't fully understand. It is true I suppose that at"level" top speeds of well over 100 MPH, ALL the power is going to overcome aero resistance. So one can make some calculations of Cd with reasonable accuracy. Is that the point?

Bonneville is referred to as the 'Great White Dyno.' And along with the MPG/CdA relationship from GM Aero Lab,Hucho gives us the rule of thumb,that for a 30% drag reduction,we might expect a 10% increase in top speed.
Or,if no other variable exists,except the shape of a vehicle,then if you experience a 10% increase in top speed,then you must have reduced the CdA by 30%.
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If I achieve terminal velocity and I know the meteorological data during the run,along with engine rpm,I can deduce the available horsepower.
That horsepower will be the 'Road Load' at that velocity.
Since rolling resistance is already embedded within the relationship I should be able to reverse-engineer the new drag coefficient.
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BamZipPow's V-6 T-100 was tested by CAR and DRIVER and demonstrated a top speed of 96 mph,hitting an aerodynamic 'wall.'
I've had mine to an unofficial 115 mph with lots of throttle left.
From GM's relationship the Toyota is at around Cd 0.18 to achieve the mpg which it gets.
The top speed at Bonneville would allow me another back-door with which to figure Cd.We'll see.

[aerohead]

This is pretty obscure stuff but maybe something to ponder.
*When A.G.Eiffel was doing wind tunnel investigations of Maurice Farman struts,with struts of equal width,but with 'pointed' or 'blunt' forebodies,the blunt nosed struts had the lower drag.
The pointed strut had 50% aft-bodies.
The blunt struts had 60% aft-bodies.
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*After Daimler Benz's success with the 200 mph 'pointed-nose' C 111-III,in 1978,they came out with a higher horsepower 250 mph 'blunt-nosed' C-111 IV.
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*Porsche racing cars also saw an evolution in which they went to a flat nose.
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* Also,in Hucho's 2nd Edition,Chapter 8,'Reducing aerodynamic drag,page 327,Fig.8.52,a 'Streamlined front-end' registered Cd 0.23 vs 0.22 for a nominally radiused blunt 'Baseline' nose.
A drop-nose 'Streamlined' nose did come in at Cd 0.21,reminiscent of VW's 'ideal' nose of the 1970s.
These noses constitute 5.84% of the overall drag.
The base drag at the rear constitutes 72% of the total drag.

[jime57]

Originally Posted by aerohead

If I achieve terminal velocity and I know the meteorological data during the run,along with engine rpm,I can deduce the available horsepower.
That horsepower will be the 'Road Load' at that velocity.
Since rolling resistance is already embedded within the relationship I should be able to reverse-engineer the new drag coefficient.

I'm beginning to see the logic of all this, but I would approach it a bit differently, particularly as regards available wheel horsepower. Most speed events these days have portable chassis dynos available in the immediate area. I'd be very tempted to figure out when my runs were going to occure and have a dyno run done either before or after my top speed testing. Deducing power is a lot less accurate than measuring power.

Perhaps I've been spoiled by having a devoted old race buddy who tests my race car for free, but it certainly would eliminate a lot of guessing(i.e. deducing).

[aerohead]

Originally Posted by jimepting

I'm beginning to see the logic of all this, but I would approach it a bit differently, particularly as regards available wheel horsepower. Most speed events these days have portable chassis dynos available in the immediate area. I'd be very tempted to figure out when my runs were going to occure and have a dyno run done either before or after my top speed testing. Deducing power is a lot less accurate than measuring power.

Perhaps I've been spoiled by having a devoted old race buddy who tests my race car for free, but it certainly would eliminate a lot of guessing(i.e. deducing).

The thing about dynos is that one needs a priori rolling resistance data for the track you're running on,as well as the CdA info,to properly load the dyno.If the loads aren't represented up through the driveline to the flywheel,the engine can go to a place which won't reflect the real world.
To do an end run,you'd have to pull the engine and use a engine-dyno and you'd need to know your exact powertrain mechanical efficiency near your anticipated velocity,as it varies as a function of rpm.Ditto for tires.
As an air-pump,a very close power estimation can be figured from terminal velocity rpm vs rated power rpm.
If say,I top out at 4500 rpm (300 rpm short of rated power rpm) then under
'standard 'atmospheric conditions my Road Load bhp at top speed would be 140.625 bhp.
With a driveline mechanical efficiency of 92%(SAE),the load at the drive wheel would be 129.375 horsepower.Of course,this would be adjusted for ambient air density which can be had at the local airport after 'un-correcting' the barometric pressure.
Hucho's relationship would tell me my Cd,and I'd know if there was any wiggle room left with gearing or tire diameters to optimize performance.
"Air drag,that poison of speed!" HOT ROD Magazine,1962

[jime57]

Originally Posted by aerohead

The thing about dynos is that one needs a priori rolling resistance data for the track you're running on,as well as the CdA info,to properly load the dyno.

I would have thought the rolling resistance data would be available from the folks who provide speed trials tires. There must be some source of this data at an event like the Bonneville Trials.

[drmiller100]

Lots of theories in the world which disappear faster then an ice cube at Bonneville.

No one needs a dyno at bonneville other then the big white ones.

[jime57]

Originally Posted by jimepting

I would have thought the rolling resistance data would be available from the folks who provide speed trials tires. There must be some source of this data at an event like the Bonneville Trials.

I should have thought this through a bit more. Of course the rolling resistance varies, so there aren't likely to be any standard values. After all it isn't pavement

[aerohead]

Originally Posted by jimepting

I would have thought the rolling resistance data would be available from the folks who provide speed trials tires. There must be some source of this data at an event like the Bonneville Trials.

I like to think that at some level,power absorption data would be available over the speed range of a particular tire at 'normal' load.
Nobody in N.Texas can tell me anything about Goodyear racing tires.
Cal Tech was provided with calibrated tires for coastdown tests at Edwards AFB,California.Those numbers are produced on a steel-drum roller though.During tests,the pressure and temperature had to be factored.
Walter Korff of Lockheed Aircraft Corp.,designer of the Summer's Bros.'Goldenrod',in a 1963 SAE Paper did include a power curve (probably for Firestone bias-ply speedway tires) up to 150 mph.It's the only thing I've seen like it.
I've been told about a curve which went to 250 mph but have never seen a copy.
If any 'spare time' comes along I'll root around some more.
By looking at drag-limited top speeds for cars in which their Cd and Af are provided,and test weight can be closely estimated,from the gearing and engine rpm at that speed,we can kind of reverse-engineer brake-horsepower available.Modern dual-clutch automatics are as efficient as manuals,so 92% mechanical efficiency for the powertrain can be used.
With the power known,and subtracting out the air drag horsepower,the R-R horsepower is left,and using the test weight,a R-R force coefficient falls out.