One of the threads a few days ago captured my attention span for a few
moments. It was thatthread that discussed the timing of runs during the day
a B'ville. I did a small mind experiment (I have a small mind) and then
chatted with my Excel Spreadsheet. The most significant factor in speed is
aerodynamic drag on the body followed by losses derived from tires on soft
surfaces, and rolling friction. Aero drag has density as a factor in the
equation. Basic fluid mechanics gives aero drag as:
D = (1/2)*(density)*(Vel)*(Vel)*(A)*(Cd) where * is multiplication
density = rho = P(lbs/ft sq)/ (g)*(R)*(T)
P = barometric pressure in lbs/square foot
g = gravity constant = 32/174 ft/sec squared
R = Universal gas constant = 53.3
T = Temperature, degrees Rankine (460 + ambient)
At an altitude of approximately 4000 ft, the standard pressure is 1827. 69
lbs/ft^2. Assuming that the pressure remains the same during the day (this
seems to be a good assumption, but is it? I am no weatherman), then the
only variable needed to determine density is temperature. And once density
is found then aero drag and horsepower are easily found.
HP = D * Vel(ft/sec)/550
For my little mind experiment, I used a vehicle frontal area, A, of 5.0 sq
ft and a drag coefficient, Cd, of 0.3. For comparison purposes I used 200
mph as the reference speed. When I run the numbers, I get the results in
the following table.
If a run is made in the early dawn when the temperature is at say 70
degrees the the drag and hp are 130.02 lbs and 69.34 hp needed. If the
backup run was made at at time of day when the temp was at 100 F, then drag
and hp are 123.04 lbs and 65.62 hp. Or less hp is needed to move the
vehicle when the temperature is highest! In this simple case 9.46 % less
hp. Now the question is does the motor output reduce less that 9.46%
between those temps? If it does, it would be bet to run when it is cool, if
it does not, then run when the temp is up. Cars that are boosted and or use
nitrous probably would be better off waiting until the temperature is is up
to make their backup runs.
My $0.02 worth and worth every bit of it! If some one has a engine design
software code that lets you vary the inlet temp during performance calcs,
then it might be interesting to see how normally aspirated engine looses
power with temp, then a boosted motor with the same inlet conditions. This
is relative so it wouldn't matter what the engine is, just the percentage
changes. Have to be careful of the software tho, most reference back to
standard conditions.
What does it all mean? That I prolly have too much time on my hands! On
Sundays I take off from working on my car and I get tied of watching the
roundy round guys and football.
Table I
g=32.174 A=5.0 R=53.3 V=200mph Cd=0.3
T (F) T (R) P(lb/ft^2) rho(slugs) drag(Lbs) hp reqd
44.73 504.423 1827.69 0.00211 136.35 72.72
60.00 519.000 1827.69 0.00205 132.52 70.68
61.00 520.000 1827.69 0.00205 132.27 70.54
62.00 521.000 1827.69 0.00205 132.01 70.41
63.00 522.000 1827.69 0.00204 131.76 70.27
64.00 523.000 1827.69 0.00204 131.51 70.14
65.00 524.000 1827.69 0.00203 131.26 70.00
66.00 525.000 1827.69 0.00203 131.01 69.87
67.00 526.000 1827.69 0.00203 130.76 69.74
68.00 527.000 1827.69 0.00202 130.51 69.61
69.00 528.000 1827.69 0.00202 130.26 69.47
70.00 529.000 1827.69 0.00201 130.02 69.34
71.00 530.000 1827.69 0.00201 129.77 69.21
72.00 531.000 1827.69 0.00201 129.53 69.08
73.00 532.000 1827.69 0.00200 129.28 68.95
74.00 533.000 1827.69 0.00200 129.04 68.82
75.00 534.000 1827.69 0.00200 128.80 68.69
76.00 535.000 1827.69 0.00199 128.56 68.56
77.00 536.000 1827.69 0.00199 128.32 68.44
78.00 537.000 1827.69 0.00198 128.08 68.31
79.00 538.000 1827.69 0.00198 127.84 68.18
80.00 539.000 1827.69 0.00198 127.60 68.06
81.00 540.000 1827.69 0.00197 127.37 67.93
82.00 541.000 1827.69 0.00197 127.13 67.80
83.00 542.000 1827.69 0.00197 126.90 67.68
84.00 543.000 1827.69 0.00196 126.66 67.55
85.00 544.000 1827.69 0.00196 126.43 67.43
86.00 545.000 1827.69 0.00196 126.20 67.31
87.00 546.000 1827.69 0.00195 125.97 67.18
88.00 547.000 1827.69 0.00195 125.74 67.06
89.00 548.000 1827.69 0.00194 125.51 66.94
90.00 549.000 1827.69 0.00194 125.28 66.82
91.00 550.000 1827.69 0.00194 125.05 66.69
92.00 551.000 1827.69 0.00193 124.83 66.57
93.00 552.000 1827.69 0.00193 124.60 66.45
94.00 553.000 1827.69 0.00193 124.37 66.33
95.00 554.000 1827.69 0.00192 124.15 66.21
96.00 555.000 1827.69 0.00192 123.93 66.09
97.00 556.000 1827.69 0.00192 123.70 65.97
98.00 557.000 1827.69 0.00191 123.48 65.86
99.00 558.000 1827.69 0.00191 123.26 65.74
100.00 559.000 1827.69 0.00191 123.04 65.62
101.00 560.000 1827.69 0.00190 122.82 65.50
102.00 561.000 1827.69 0.00190 122.60 65.39
103.00 562.000 1827.69 0.00190 122.38 65.27
104.00 563.000 1827.69 0.00189 122.16 65.15
105.00 564.000 1827.69 0.00189 121.95 65.04
106.00 565.000 1827.69 0.00189 121.73 64.92
107.00 566.000 1827.69 0.00188 121.52 64.81
108.00 567.000 1827.69 0.00188 121.30 64.69
109.00 568.000 1827.69 0.00188 121.09 64.58
110.00 569.000 1827.69 0.00187 120.88 64.47
L.E. Mayfield
124 Maximillion Drive
Madison, Al. 35758-8171
1-256-837-1051
old >>>>>>>> http://www.hsv.tis.net/~mayfield
new >>>>>>>> http://home.hiwaay.net/~lemayf
DrMayf@AOL.com
lemay@hiwaay.net <<<<preferred
Sunbeam Tiger, B9471136
Sunbeam Alpine Bonneville Land Speed Racer,
'66 Hydroplane Drag Boat (390 FE)
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