WOT: aerodynamics and object on cop cars, Who's right, me or co-worker? Options

[914rrr]

Do you happen to work for G2 Tactics?

No, but I am the Business Development Manager for one of their competitors...

Since this is a 'prototype/development product' I can't disclose very much, as I might incurr the wrath of the "Sales Prevention Department" AKA Contracts staff.

[fin]

Interfering bodies. (the proper name is eluding me at this moment) Door mirrors, roof lights, etc. When two objects are close to each other in the same airstream, they create a tremendous amount of drag. When Ordnance is loaded up externally on say F/A-18's, F-16's, Strike Eagles, the Flight Crew has to recompute increased fuel burn rates due to the increased drag of not only the shape of the ordnance, but also the turbulence created by the interference of the airflow about each shape. The increase in weight is a minor factor in the increased fuel burn. Another example. When the Air Force introduced the F-15, they stated that it had less drag than three bicyclists. Arms, legs, spokes and such cause all that drag.

Ask your coworker to look at the roof racks on the latest SUV's. They have almost all gone to side rails that are integrated into the roof structure. Oh, and why Yakima roof racks have that front "wedge", to lower drag.

I hate roof racks, which is one of the reasons I bought my BMW 530i Touring (a wagon) as the roof has integral t-slots in the roof to mount a rack. Keeps the profile clean. I always smirk when I see an obviously environmentally friendly driver (as stated by the bumper stickers) with an aftermarket rack for bikes that never gets taken off. A big increase in gas usage.

In summary, mount it as a bump, not on a stand-off mounting. I don't suppose that you would like to cut a hole in the roof of the vehicles so that the electronics are inside the cool air conditioned car and only the working piece is outside? Tough to sell the car when it's older, but the cabbies might just put lexan over it and call it a sunroof!

Trying to help,

FIN

[Cap'n Krusty]

Hard to hide a chain gun in a little aerodynamic package. And whaddya do with all the spent brass? The Cap'n

[914rrr]

Hard to hide a chain gun in a little aerodynamic package. And whaddya do with all the spent brass? The Cap'n

LOL (IMG:style_emoticons/default/av-943.gif) ...that would be the "Enforcement" option. (IMG:style_emoticons/default/ar15.gif) Kidding.

[fiid]

4)fuel economy? I don’t know, do cops pay it out of their on pocket??? Not a selling point.

You're not selling to the cop though. You're selling to the police force - which is a government entity. Whilst having the cop on your side is a good thing - if you're trying to get something bolted to the entire fleet, stuff like fuel economy start to make a big difference - think about having any number of V8 powered large sedans rolling around a city/district 24/7 (and making donut runs!) and you're talking about a lot of fuel. Adding another 5% on top of that bill isn't going to look good to a publicly funded entity.

[914rrr]

4)fuel economy? I don’t know, do cops pay it out of their on pocket??? Not a selling point.

You're not selling to the cop though. You're selling to the police force - which is a government entity. Whilst having the cop on your side is a good thing - if you're trying to get something bolted to the entire fleet, stuff like fuel economy start to make a big difference - think about having any number of V8 powered large sedans rolling around a city/district 24/7 (and making donut runs!) and you're talking about a lot of fuel. Adding another 5% on top of that bill isn't going to look good to a publicly funded entity.

My point exactly. There are loads of articles telling how recent gas price hikes have affected police departments: less patrols, curtailing or eliminating 'drive home' priveleges, etc.

What I need is calculation that shows how different device profiles / designs affect either top speed and/or fuel economy. I found some weblinks that give me pieces of it
(Cd's for different profiles)

and a drag equation from insideracingtechnology.com (gotta love racers!):

For real body shapes, air at standard conditions, V in mph, and drag in pounds of force, this equation becomes:

Drag = 1/391 x Cd x A x Vsquared

Sooo... if my math is correct, at 70 MPH I come up with about 10.7 lbs of force for a cube vs 7.1 lbs for a cube turned 45 degrees vs 4.5 lbs for a half circle vs .355 for a streamlined shape. How would this force expressed in lbs. equate to loss of MPG, extra HP needed to maintain 70MPH, etc? Also, how do you determine the Cd for combined shapes (a half circle on the front edge of a long cylinder)?

[maf914]

Look at the chart; The lowest drag coefficient was 0.04 for the airfoil shape. Make them mount their lights in an airfoil shaped pod raised above the roof. Or use an airfoil shape placed at a right angle to the roof, like the dorsal fin of a shark.

Have you noticed how in-car cameras are mounted on racing cars? In F1 they use a horizontal airfoil shaped enclosure at the top of the roll-over structure while CART, IRL, Sportscars, and stock cars use an airfoil shaped enclosure in a dorsal fin configuration.

[davep]

Don't forget that drag alone is not the only consideration. If the shape, even areodynamic, is not mounted correctly it can induce high speed instability. Does the device add lift or downforce at various speeds, and how does it affect handling.

It doesn't matter that a cop car seldom sees top speed, just one, once, is enough.

I would hazzard a guess that the increase in drag (cd x area) over the original drag figure would relate closely to the increase in fuel consumption.

[914 dror]

4)fuel economy? I don’t know, do cops pay it out of their on pocket??? Not a selling point.

You're not selling to the cop though. You're selling to the police force - which is a government entity. Whilst having the cop on your side is a good thing - if you're trying to get something bolted to the entire fleet, stuff like fuel economy start to make a big difference - think about having any number of V8 powered large sedans rolling around a city/district 24/7 (and making donut runs!) and you're talking about a lot of fuel. Adding another 5% on top of that bill isn't going to look good to a publicly funded entity.

any way they will have a fild test with cops of coarse and what they have to say about it will be concidered much more than any 5% that will or will not be even calculated.
as far as i know wind factor is not real high below 60 mph.

[914 dror]

4)fuel economy? I don’t know, do cops pay it out of their on pocket??? Not a selling point.

You're not selling to the cop though. You're selling to the police force - which is a government entity. Whilst having the cop on your side is a good thing - if you're trying to get something bolted to the entire fleet, stuff like fuel economy start to make a big difference - think about having any number of V8 powered large sedans rolling around a city/district 24/7 (and making donut runs!) and you're talking about a lot of fuel. Adding another 5% on top of that bill isn't going to look good to a publicly funded entity.

My point exactly. There are loads of articles telling how recent gas price hikes have affected police departments: less patrols, curtailing or eliminating 'drive home' priveleges, etc.

What I need is calculation that shows how different device profiles / designs affect either top speed and/or fuel economy. I found some weblinks that give me pieces of it
(Cd's for different profiles)

and a drag equation from insideracingtechnology.com (gotta love racers!):

For real body shapes, air at standard conditions, V in mph, and drag in pounds of force, this equation becomes:

Drag = 1/391 x Cd x A x Vsquared

Sooo... if my math is correct, at 70 MPH I come up with about 10.7 lbs of force for a cube vs 7.1 lbs for a cube turned 45 degrees vs 4.5 lbs for a half circle vs .355 for a streamlined shape. How would this force expressed in lbs. equate to loss of MPG, extra HP needed to maintain 70MPH, etc? Also, how do you determine the Cd for combined shapes (a half circle on the front edge of a long cylinder)?

Real simple - no way your gonna calculate it. This is the reason they invented the wind tunnel (IMG:style_emoticons/default/burnout.gif)
Got a friend in the Air force? NASA is good too (IMG:style_emoticons/default/burnout.gif)