Aerodynamics & Diverting Wind Resistance ?

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318willrun

318willrun

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Wind "through" or Wind "around" ??? How could/should a guy go for a gain ?? I have a couple ideas, seeing what else is out there......
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And at what "speed" or MPH before it matters ??
And even then, how MUCH does it matter ???
And, is there a small change to a '80 D150 front end that would be beneficial - behind the grill ??? .... keep'n it stock appearing
 
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The square front end actually creates an area of high pressure in front making it sort of a wedge but nothing like a birds beak. Square front end is still square but it could actually develop an over pressure area for a carb intake, ie cowl induction.
 
pishta said:
The square front end actually creates an area of high pressure in front making it sort of a wedge but nothing like a birds beak. Square front end is still square but it could actually develop an over pressure area for a carb intake, ie cowl induction.
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But we know vans and square body trucks are harder on gas than a car of same weight/gearing, and most give it up to wind resistance. Also, in my former years, pickups mph is always down compared to cars of equal weight, and gearing and such in the 1/4 mile. I'm of the belief that wind resistance plays a big part on the big end.
 
Do you see many hollow airplanes? Wind around is the only way... wind 'through' still has to hit 'stuff', 'stuff' causes drag. That's why despite the number of piston engined small aircraft, few have liquid cooling systems, and most of the liquid cooled stuff winds up breaking even compared to air cooled - the added drag offsets the added power.

That said, you can reduce the speed of the air through the engine compartment or under the vehicle. Air dams, and divergent radiator ducts (like used on late model stock cars) can help.

The equation for aerodynamic drag is Cd*Q*A. Where Cd is the coefficient of drag, Q is the dynamic pressure, and A is the cross sectional area. Reduce any of them, you reduce drag. Q is 1/2*rho*V^2, where rho is density. So Q goes up with the square of speed. Then there's induced drag, which is drag caused by creating lift, or downforce. This is also linearly related to dynamic pressure, which is related to the square of velocity. So, this is why reducing the speed of the air through the innards of a vehicle can help - and with enough speed, you can still keep up with the requisite mass flow required to keep the engine cooled off.

Regarding shape: subsonic should be blunt end forward (think small airplane), supersonic you want pointy end forward (think Sr71). Dynamic pressure is highest at the stagnation point - or a surface which is completely perpendicular to the direction of flow. This is why a truck or van will have more drag than a something with the face slanted back some. But, that really only counts in clean air. The turbulence caused at the back is a whole other issue. Pointy end to the rear applies here (subsonic) and can have much greater gains than rounding off the front end.

If we were to take a fake 'Cd' number of say .50 for a truck, with a frontal area of 42 sq ft (7' wide, 6' tall - assumed), and a standard sea level density of 0.0765lbs/cubic foot, you'd have about 134lbs of drag at 50mph, 537 lbs at 100 mph, and 2150lbs at 200mph.

Lets look at 537lbs of drag at 100mph. With a 30" tall tire, that's a radius of 1.25 ft - which would require 430ft-lbs of torque in order to create 537lbs of thrust at the road surface. Assuming 1:1 at the trans, and let's say 4.10 at the rear end that's 105 ft-lbs of engine torque (110 ft-lbs with 3.91's).

You can find online calculators to play with and see what you can see. Obviously, the effect will be most pronounced with higher trap speeds, and the higher your average speed is. At the top end of the track, many drag cars aren't adding a lot more MPH, and usually because aerodynamic drag is becoming a bigger and bigger factor.

You choices to reduce drag are streamlining, and reducing frontal area. Both add up to lower your Cd. Lower roofline, smoother flow, less wetted area (which includes ANYTHING in the airstream - even under the vehicle if air flows there). Air dams, diffusers, louvers, etc can all help. Even turbulators on the surface can help energize flow and keep it from separating at areas near your mirrors, fender flares, side marker lights, etc.
 
Phreakish said:
Do you see many hollow airplanes? Wind around is the only way... wind 'through' still has to hit 'stuff', 'stuff' causes drag. That's why despite the number of piston engined small aircraft, few have liquid cooling systems, and most of the liquid cooled stuff winds up breaking even compared to air cooled - the added drag offsets the added power.

That said, you can reduce the speed of the air through the engine compartment or under the vehicle. Air dams, and divergent radiator ducts (like used on late model stock cars) can help.

The equation for aerodynamic drag is Cd*Q*A. Where Cd is the coefficient of drag, Q is the dynamic pressure, and A is the cross sectional area. Reduce any of them, you reduce drag. Q is 1/2*rho*V^2, where rho is density. So Q goes up with the square of speed. Then there's induced drag, which is drag caused by creating lift, or downforce. This is also linearly related to dynamic pressure, which is related to the square of velocity. So, this is why reducing the speed of the air through the innards of a vehicle can help - and with enough speed, you can still keep up with the requisite mass flow required to keep the engine cooled off.

Regarding shape: subsonic should be blunt end forward (think small airplane), supersonic you want pointy end forward (think Sr71). Dynamic pressure is highest at the stagnation point - or a surface which is completely perpendicular to the direction of flow. This is why a truck or van will have more drag than a something with the face slanted back some. But, that really only counts in clean air. The turbulence caused at the back is a whole other issue. Pointy end to the rear applies here (subsonic) and can have much greater gains than rounding off the front end.

If we were to take a fake 'Cd' number of say .50 for a truck, with a frontal area of 42 sq ft (7' wide, 6' tall - assumed), and a standard sea level density of 0.0765lbs/cubic foot, you'd have about 134lbs of drag at 50mph, 537 lbs at 100 mph, and 2150lbs at 200mph.

Lets look at 537lbs of drag at 100mph. With a 30" tall tire, that's a radius of 1.25 ft - which would require 430ft-lbs of torque in order to create 537lbs of thrust at the road surface. Assuming 1:1 at the trans, and let's say 4.10 at the rear end that's 105 ft-lbs of engine torque (110 ft-lbs with 3.91's).

You can find online calculators to play with and see what you can see. Obviously, the effect will be most pronounced with higher trap speeds, and the higher your average speed is. At the top end of the track, many drag cars aren't adding a lot more MPH, and usually because aerodynamic drag is becoming a bigger and bigger factor.

You choices to reduce drag are streamlining, and reducing frontal area. Both add up to lower your Cd. Lower roofline, smoother flow, less wetted area (which includes ANYTHING in the airstream - even under the vehicle if air flows there). Air dams, diffusers, louvers, etc can all help. Even turbulators on the surface can help energize flow and keep it from separating at areas near your mirrors, fender flares, side marker lights, etc.
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This was a very interesting read. Thank you. I've thought of things such as air dams behind the grill but in front of the radiator supports, I've thought of adding the "old school bug shields" to get the air up over the front windshield, etc etc.
 
There was a legitimate article a while back about Warren Johnson where he said that one of his previous blunt-nosed Oldsmobile cars (Achieva(?) was actually more aerodynamic than the pointy-nosed Firebird that he had to run because of his GM sponsorship.
 
Locomotion said:
There was a legitimate article a while back about Warren Johnson where he said that one of his previous blunt-nosed Oldsmobile cars (Achieva(?) was actually more aerodynamic than the pointy-nosed Firebird that he had to run because of his GM sponsorship.
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Interesting...... Wind does crazy things... But so hard to test at home. Almost need to make changes, then compare ET slips. MPH would be the huge indicator
 
Oldiron440 said:
Get out the saws awl and start cutting.....
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yeah, weight reduction helps. But for wind resistance, I'd like to work behind the scenes (grill) to keep "stock appearing" and perhaps work under the truck. I know most new cars have a large plastic cover underneath covering steering and engine components.
 
Likely the best you'll do is a tonneau cover on the bed, close up the front openings you don't absolutely need, and a front spoiler
 
318willrun said:
Interesting...... Wind does crazy things... But so hard to test at home. Almost need to make changes, then compare ET slips. MPH would be the huge indicator
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Then you have to make sure conditions are the same between the runs/days. A light breeze and its direction would throw off the numbers, not to mention compensating for temp and humidity differences. Also have to consider the weight of any mods and what that takes away from any aerodynamic benefits they may provide.

Yea, aerodynamics does weird things. A good example is what the turbulence does in the bed of a pick-up.

I don't know what they were looking at or for, but here is an interesting example of what Chrysler did searching for aerodynamic advantages in the early '70's. They also drooped, chopped & channeled the front ends later trying to keep up with the smaller brand X cars. But you can probably find more tips that could be applicable from researching vehicles in Nascar, Bonneville Salt Flats, Can-Am, etc.

(4) Aero Testing the 1973 Mopar Missile Drag Racing Car (Plymouth Duster) | Allpar Forums
 
Locomotion said:
Then you have to make sure conditions are the same between the runs/days. A light breeze and its direction would throw off the numbers, not to mention compensating for temp and humidity differences. Also have to consider the weight of any mods and what that takes away from any aerodynamic benefits they may provide.

Yea, aerodynamics does weird things. A good example is what the turbulence does in the bed of a pick-up.

I don't know what they were looking at or for, but here is an interesting example of what Chrysler did searching for aerodynamic advantages in the early '70's. They also drooped, chopped & channeled the front ends later trying to keep up with the smaller brand X cars. But you can probably find more tips that could be applicable from researching vehicles in Nascar, Bonneville Salt Flats, Can-Am, etc.

(4) Aero Testing the 1973 Mopar Missile Drag Racing Car (Plymouth Duster) | Allpar Forums
Click to expand...
Interesting article. I've given the bed some thought as well. I don't think dropping the end gate is all there is to it. In fact, some have said the tailgate being up actually helps turbulence. And for sure, a tenth can be due to weather per day, but the MPH should be the real story. Ideal would be have something pre-fit to try on the same day.
 
Again, there is the "well known" - adjust the back of the hood up 1/2 inch so the air "passes through".
 
Phreakish said:
Do you see many hollow airplanes? Wind around is the only way... wind 'through' still has to hit 'stuff', 'stuff' causes drag. That's why despite the number of piston engined small aircraft, few have liquid cooling systems, and most of the liquid cooled stuff winds up breaking even compared to air cooled - the added drag offsets the added power.

That said, you can reduce the speed of the air through the engine compartment or under the vehicle. Air dams, and divergent radiator ducts (like used on late model stock cars) can help.

The equation for aerodynamic drag is Cd*Q*A. Where Cd is the coefficient of drag, Q is the dynamic pressure, and A is the cross sectional area. Reduce any of them, you reduce drag. Q is 1/2*rho*V^2, where rho is density. So Q goes up with the square of speed. Then there's induced drag, which is drag caused by creating lift, or downforce. This is also linearly related to dynamic pressure, which is related to the square of velocity. So, this is why reducing the speed of the air through the innards of a vehicle can help - and with enough speed, you can still keep up with the requisite mass flow required to keep the engine cooled off.

Regarding shape: subsonic should be blunt end forward (think small airplane), supersonic you want pointy end forward (think Sr71). Dynamic pressure is highest at the stagnation point - or a surface which is completely perpendicular to the direction of flow. This is why a truck or van will have more drag than a something with the face slanted back some. But, that really only counts in clean air. The turbulence caused at the back is a whole other issue. Pointy end to the rear applies here (subsonic) and can have much greater gains than rounding off the front end.

If we were to take a fake 'Cd' number of say .50 for a truck, with a frontal area of 42 sq ft (7' wide, 6' tall - assumed), and a standard sea level density of 0.0765lbs/cubic foot, you'd have about 134lbs of drag at 50mph, 537 lbs at 100 mph, and 2150lbs at 200mph.

Lets look at 537lbs of drag at 100mph. With a 30" tall tire, that's a radius of 1.25 ft - which would require 430ft-lbs of torque in order to create 537lbs of thrust at the road surface. Assuming 1:1 at the trans, and let's say 4.10 at the rear end that's 105 ft-lbs of engine torque (110 ft-lbs with 3.91's).

You can find online calculators to play with and see what you can see. Obviously, the effect will be most pronounced with higher trap speeds, and the higher your average speed is. At the top end of the track, many drag cars aren't adding a lot more MPH, and usually because aerodynamic drag is becoming a bigger and bigger factor.

You choices to reduce drag are streamlining, and reducing frontal area. Both add up to lower your Cd. Lower roofline, smoother flow, less wetted area (which includes ANYTHING in the airstream - even under the vehicle if air flows there). Air dams, diffusers, louvers, etc can all help. Even turbulators on the surface can help energize flow and keep it from separating at areas near your mirrors, fender flares, side marker lights, etc.
Click to expand...
Wow! Math! I don’t know many people that would bring up those equations! Now your asking for brain power!
 
How far do you wanna go?

On the KISS method, drop the front, air damn down low, but IDK about covering the bed or loosing the gate. After that, it gets expensive I’d guess. Shaving door handles, louver the hood, etc…
 
rumblefish360 said:
How far do you wanna go?

On the KISS method, drop the front, air damn down low, but IDK about covering the bed or loosing the gate. After that, it gets expensive I’d guess. Shaving door handles, louver the hood, etc…
Click to expand...
well, of course I complicate things and this will be no exception, but we are trying to keep it "stock appearing". Yes, air dam down low is a good idea.
 
I have heard that aerodynamic drag is better on a pickup with the tailgate up, than with it down.
My 428 cj Ranchero needed the heavy tailgate down to help the one-legger rearend.
The other trucks I have raced were much too slow for Aero to matter!
 
318willrun said:
well, of course I complicate things and this will be no exception, but we are trying to keep it "stock appearing". Yes, air dam down low is a good idea.
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You mentioned newer vehicles with the plastic under engine covering. See what you find on super cars via YouTube or something. They have what I would call “Belly Pans” going from front to rear. The bottom line in the lesson is, less air under the car is a good thing.

Sometimes this goes pretty far and additional help at the rear is needed in a rear exit air splitter.
 
I'd get a shorter, and thicker radiator. Cut the body mounts off the front, and drop the nose down as low as you can, or dare to go.

You get brownie points if you do that to the cab as well
 
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