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Old 05-14-2017, 08:32 PM   #1 (permalink)
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Radio Antenna Drag

I have not removed my radio antenna, which was commented on in at least one thread. Now I know why because I finally did the calculation for drag from my radio antenna. Here's the calculation:

INPUTS
Antenna diameter 5/32"
Antenna length 29"
Air density 0.075 lb/ft3
Viscosity of air 3.8E-7 lb-sec/ft2
Speed 80 ft/sec (just under 55 MPH)

CALCULATIONS
Reynolds number = 0.156" / 12 in/ft X 0.075 lbs/ft3 / 32.2 ft/sec2 / X 80 ft/sec / 3.8E-7 lb-sec/ft2 = 6400

Cd for a cylinder at Re 6400 = 1.0

Drag = 0.5 X Cd X A X rho X V2 =
= 0.5 X 1.0 X 0.156" / 12 in/ft X 29" / 12 in/ft X 0.075 lb/ft3 / 32.2 ft/sec2 X 80^2 ft2/sec2 = 0.234 lbs drag from the radio antenna

At 80 ft/sec, this is 0.234 lbs X 80 ft/sec = 19 ft-lbs/sec = 0.034 hp (1 hp = 550 ft-lbs/second). The radio antenna air drag requires 0.034 hp (0.025 kw)

I averaged 35 MPG last year. If we simplify by assuming all driving was at that speed, then fuel flow was 55 MPH / 35 MPG = 1.57 gallons/hour = 9.4 lbs/hour. Power is estimated by assuming 0.5 lbs/hp-hr, so 9.4 lbs/hour / 0.5 lbs/hp-hr = 18.857 hp. Removing the radio antenna would reduce this to 18.857 - 0.034 = 18.823 hp. The resulting estimated gas mileage would be 35 MPG X 18.857 / 18.823 = 35.06 MPG.

I would gain an estimated 0.06 MPG. I drove 10,588.6 miles last year. That would be 302.53 gallons at 35 MPG, and 302.01 gallons at 35.06 MPG. The radio antenna cost me 0.52 gallons of fuel last year. The actual savings would be less because part of my driving is at slower speeds. I will pay that cost in order to listen to the radio once in a while.

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Old 05-14-2017, 10:20 PM   #2 (permalink)
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Im actually surprised it was that much.
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Old 05-15-2017, 12:07 AM   #3 (permalink)
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I think some older cars would be worse as they had brick shaped antenna bases. But I would not remove it either, gotta have my tunes!
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Old 05-15-2017, 01:30 AM   #4 (permalink)
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Wonder what the flags and Mascots add to the drag. Also here in Utah when the FANatics go to athletic events they have two flags fluttering on both sides of the car.
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Old 05-15-2017, 01:43 AM   #5 (permalink)
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Quote:
Originally Posted by MorphDaCivic View Post
Wonder what the flags and Mascots add to the drag. Also here in Utah when the FANatics go to athletic events they have two flags fluttering on both sides of the car.
Or the Jack Balls! -lol

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Old 05-15-2017, 12:44 PM   #6 (permalink)
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Just think of all the gas that radio can save you when avoiding traffic jambs and road construction.
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Old 05-16-2017, 01:51 AM   #7 (permalink)
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Thanks for doing the calculations.....now I won't feel too bad about adding my wilson 5000 CR antenna to the roof..... .06 MPG is hardly anything! (graned the wilson 5000 is taller but even double the mpg loss is still not too much).

As an aside, with your great mathematical skills, would it be possible to calculate how many MPG's one would gain by removal of side mirrors or conversely how much one would lose if one were to add vent visors (to allow one to keep the side windows open without fear of rain getting in).

Thanks in advance for any and all help.

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Old 05-16-2017, 12:24 PM   #8 (permalink)
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You donut always have to remove the antenna. A more efficient design might be possible, otherwise on my saturn I was able to move the antenna inside the a pillar or fender. You just need to get creative
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Old 05-16-2017, 12:53 PM   #9 (permalink)
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1600 ways to save an ounce.

Plus, not everything I do needs to have a positive ROI.







EDIT: Only problem now is, I'm going 3-4 months between tanks of gas.
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Old 05-16-2017, 01:06 PM   #10 (permalink)
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Quote:
As an aside, with your great mathematical skills, would it be possible to calculate how many MPG's one would gain by removal of side mirrors or conversely how much one would lose if one were to add vent visors (to allow one to keep the side windows open without fear of rain getting in).
Not saying he shouldn't give it a shot, but mirrors and visors ride within the turbulent boundary layer, while the antenna stands out. The situation is much more complex. That's why the Nissan Leaf has little glass fins on the headlight, to set up airflow across the rear-view mirrors a foot or two further back.

One might solder a 1/8" by 1/4" Isosceles triangle on the back side and hand file a Template profile.

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