Go Back   EcoModder Forum > EcoModding > Aerodynamics
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 02-17-2010, 10:13 PM   #21 (permalink)
Recreation Engineer
 
KamperBob's Avatar
 
Join Date: Dec 2009
Location: Somewhere USA
Posts: 521

Black Stallion - '02 Toyota Tundra 4WD xCab

Half Pint - '06 Yamaha XT225
Thanks: 311
Thanked 136 Times in 101 Posts
Magic number

Quote:
Originally Posted by MetroMPG View Post
Frontal area can be approximated by multiplying your vehicles width times its height,times 0.84.
I missed the basis for 16% reduction. Is height corrected for ground clearance, sides corrected for tapers, or what?

Cheers
KB

  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 02-18-2010, 12:26 AM   #22 (permalink)
(:
 
Frank Lee's Avatar
 
Join Date: Jan 2008
Location: up north
Posts: 12,594

Blue - '93 Ford Tempo
Last 3: 27.29 mpg (US)

F150 - '94 Ford F150 XLT 4x4
90 day: 18.5 mpg (US)

Sport Coupe - '92 Ford Tempo GL
Last 3: 69.62 mpg (US)

ShWing! - '82 honda gold wing Interstate
90 day: 33.65 mpg (US)

Moon Unit - '98 Mercury Sable LX Wagon
90 day: 21.24 mpg (US)
Thanks: 1,533
Thanked 3,377 Times in 2,126 Posts
I think it was statistically derived from comparing w x h and known frontal area values.
__________________


  Reply With Quote
The Following User Says Thank You to Frank Lee For This Useful Post:
aerohead (02-20-2010)
Old 08-08-2012, 07:12 PM   #23 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 10,468
Thanks: 16,113
Thanked 5,560 Times in 3,323 Posts
375

Quote:
Originally Posted by kennybobby View Post
The 375 in the denominator should be 349. (348.66)

That term comes from 0.5 times the conversion factor for mph^3 to (ft/sec)^3 divided by the definition factor for horsepower = 550 lbs-ft/sec.

conversion factor:
mph * 5280 ft/mile / 3600 seconds/hr = ft/sec.

The 375 value assumes a mile is only 5153 ft.
Three of my reference texts use the 375 value.I made a leap of faith that the Ph.Ds had checked their own work.Apologize for the trouble.
  Reply With Quote
Old 08-09-2012, 06:41 PM   #24 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 10,468
Thanks: 16,113
Thanked 5,560 Times in 3,323 Posts
375 seems okay

Quote:
Originally Posted by kennybobby View Post
The 375 in the denominator should be 349. (348.66)

That term comes from 0.5 times the conversion factor for mph^3 to (ft/sec)^3 divided by the definition factor for horsepower = 550 lbs-ft/sec.

conversion factor:
mph * 5280 ft/mile / 3600 seconds/hr = ft/sec.

The 375 value assumes a mile is only 5153 ft.
I re-visited my books.And it looks like the 375 value is valid.
Working with the 550 lb'ft/sec value for horsepower calculation requires mph to be multiplied by 5,280 to get feet,then division by 3,600 sec/hr to get it into feet/sec..
If you already have a drag force at a given mph value,to get power,you can use this shortcut,multiplying the force by mph,then dividing by 375.
550/375 yields a constant percentage = to( feet/sec)/(mph) at any given velocity.It's always 1.466X.
Hope that helps!
  Reply With Quote
Old 09-09-2012, 12:18 AM   #25 (permalink)
Southern Squidbillie
 
Join Date: Aug 2012
Location: Heart of Dixie
Posts: 97
Thanks: 50
Thanked 25 Times in 21 Posts
Yes, your clever formula is quite correct...

Quote:
Originally Posted by aerohead View Post
I re-visited my books.And it looks like the 375 value is valid.
Working with the 550 lb'ft/sec value for horsepower calculation requires mph to be multiplied by 5,280 to get feet,then division by 3,600 sec/hr to get it into feet/sec..
If you already have a drag force at a given mph value,to get power,you can use this shortcut,multiplying the force by mph,then dividing by 375.
550/375 yields a constant percentage = to( feet/sec)/(mph) at any given velocity.It's always 1.466X.
Hope that helps!
Okay yes that helps and now i see where my confusion arose, and i agree that if you already have the Aero drag Force in unit of lbs, then you can get the Power in Horsepower by multiplying by speed in mph and dividing by 375:

1 mph x 5280 / 3600 = 1.466 ft/sec [the conversion from mph to ft/sec is 1.466].

power (in units of lb-ft/sec) / 550 = power (in units of Horsepower, HP).

So 1.466 / 550 = 1 / 375 does indeed convert mph to ft/sec and power to HP when multiplying Force in lbs times Speed in mph.

My confusion with your HP formula was three-fold: there was a factor of 1/2 missing inside the square brackets in your equation for the Aero drag "Force", and there was no conversion factor inside the brackets to convert speed in mph to ft/sec, and the constant term inside the brackets was so close to the value for the density of air.

Quote:
Originally Posted by aerohead View Post
...The formulas,once posted,will allow everyone to calculate aerodynamic loads ...
The horsepower it takes at the drivewheel of your vehicle to overcome aerodynamic drag can be estimated by the formula

HP =V/375 [ 0.00256 X Cd X A X (V squared)]

where V= speed in miles per hour, Cd is your drag coefficient,A= frontal area of your vehicle,and (V squared) is your speed times itself.
So now i see how your formula is correct, and please allow me to expand on the derivation of this formula:

Aero drag Force = 1/2 x (air density) x V^2 x Cd x A ; air density = 0.00237 lbm/ft^3

Power = Force x Speed , so the fully expanded version of the formula for the Aero drag in Horsepower would be

HP = (V x 1.466/550) x [ 0.5 x 0.00237 x Cd x A x (V squared) x (1.466 squared)]

So the constant term inside the square brackets of your formula is really NOT the air density, but is the product of 0.5 x 0.00237 x (1.466^2) = 0.00256 , and this all simplifies down to the clever formula that you provided:

HP = (V/375) x [ 0.00256 x Cd x A x (V squared)]

  Reply With Quote
Reply  Post New Thread


Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
Aerodynamics Seminar #2 - by Phil Knox MetroMPG Aerodynamics 2 05-10-2013 06:34 PM
Aerodynamics Seminar #1 - by Phil Knox MetroMPG Aerodynamics 7 08-08-2012 07:00 PM
Aerodynamics Seminar #3 - by Phil Knox MetroMPG Aerodynamics 4 02-16-2009 08:15 PM
Aerodynamics Seminar # 5 - by Phil Knox MetroMPG Aerodynamics 4 01-29-2008 02:41 PM
Aerodynamics Seminar # 4 - by Phil Knox MetroMPG Aerodynamics 0 01-28-2008 09:36 PM



Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2019, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com