A STUDY OF AERODYNAMIC DRAG ON FAIRED HPV'S

A Study Of Aerodynamic Drag On Faired HPV's
by John Lafford

I was inspired to run some comparison calculations following the publication of the data from Sam Whittingham in BHPC issue 66, particularly after his predictions proved to be quite accurate. The equations for the calculations are taken from my note in 'So You Want to Build an HPV' entitled "Drag Coefficients for All' on page 40, and inserted into an Excel spreadsheet.

The calculations have been grouped into sets to give interesting information in a number of areas and allow comparisons to be made between different venues (altitude) and slopes. Also information about several famous machines is included. The info about US machines is gathered from IHPVA magazines and the Bean data is from wind tunnel tests conducted at MIRA in 1992, courtesy of John Kingsbury, plus results from the successful hour record run.

All the machines are compared at sea level on level ground at 50 mph to show how the power requirements differ between them. Then all the machines make an attempt on the hour record using a known standard 1 hour power input , PKH, (Pat Kinch for 1 hour). This shows the Yellow Bean at 47 miles (as set) rising to 51.5 miles for the Varna Diablo. Finally, a selection is made of the best features from all the contenders, plus very good available tyres to show what could be achieved. This shows that there is still plenty of scope for designers and riders to make new records.

Set 1

Diablo as used to set the record at 4500 ft altitude with 0.006 slope and tyres with Crr = 0.006. The drag coefficient is selected as 0.11 to fit the information supplied by Sam Whittingham.

Machine	Weight kg	Rolling Resistance Coefficient Crr	Drag Coefficient Cd	Area sq.m	Cd.A sq.m	Air density factor	Slope	Speed mph	Speed km/h	Power Watts	Conditions (altitude / slope)
Diablo	100	0.0060	0.110	0.1830	0.0201	0.816	-0.006	20	32.2	7	At 4500 ft. altitude and 0.006 slope
								30	48.3	24
								40	64.4	58
								50	80.5	112
								60	96.5	194
								70	112.6	309
								80	128.7	461
								82.2	132.3	500

Set 1a

This shows that the slope was worth 216 Watts of extra effective power.

Set 2

As Set 1 but using better (available) tyres, showing that the record could go up to 88mph at the 500 W input power.

Machine Weight
kg Rolling
Resistance
Coefficient
Crr Drag
Coefficient
Cd Area
sq.m Cd.A
sq.m Air
density
factor Slope Speed
mph Speed
km/h Power
Watts Conditions
(altitude / slope)

Diablo 100 0.0030 0.110 0.1830 0.0201 0.816 -0.006 20 32.2 -19 At 4500 ft. altitude and 0.006 slope; better tyres

30 48.3 -15

40 64.4 5

50 80.5 47

60 96.5 116

70 112.6 217

80 128.7 356

88.1 141.8 500

Set 3

As set 1 but coming down to sea level, which would make a record at 77mph.

Set 4

As Set 3 but on level ground giving a 'true' record of 66.6mph.

Set 5

As Set 4 but using better tyres takes the record to 71.6mph.

Machine Weight
kg Rolling
Resistance
Coefficient
Crr Drag
Coefficient
Cd Area
sq.m Cd.A
sq.m Air
density
factor Slope Speed
mph Speed
km/h Power
Watts Conditions
(altitude / slope)

Diablo 100 0.0030 0.110 0.1830 0.0201 1 0 20 32.2 35 At sea level, level ground; better tyres

30 48.3 69

40 64.4 123

50 80.5 203

60 96.5 317

70 112.6 470

71.6 115.2 499

Set 6

As Set 3 at sea level and level ground, but using better tyres, showing the best legal record possible with the Diablo at sea level of 82.0mph. But compare with Set 1 at 50mph where it required 112 W, but in Set 6 only required 72 Watts.

Machine Weight
kg Rolling
Resistance
Coefficient
Crr Drag
Coefficient
Cd Area
sq.m Cd.A
sq.m Air
density
factor Slope Speed
mph Speed
km/h Power
Watts Conditions
(altitude / slope)

Diablo 100 0.0030 0.110 0.1830 0.0201 1 -0.006 20 32.2 -17 At sea level, 0.006 slope; better tyres

30 48.3 -10

40 64.4 18

50 80.5 72

60 96.5 159

70 112.6 286

80 128.7 459< /td>

82 131.9 500

Set 7

Yellow Bean using data from MIRA tests, 1 hour world record set at 47mph and tyre test data from the actual tyres tested by me [I think the tyres were Moulton slicks. This was the machine in which Pat Kinch set the Hour record of 46.96 miles / 75.6 km - DL].

Set 8

Bean II with data from MIRA, and using the same tyres as the Yellow Bean. This shows about 6% improvement over the Yellow Bean [The brown one - slightly improved shape and full monocoque construction. Set assorted sprint records at RAF Fairford in 1992 - DL].

Set 9

This shows what the Bean II could have achieved in the sprint event at Battle Mountain where it could have given Diablo [and Kyle Edge - DL] a good challenge at 77mph.

Machine Weight
kg Rolling
Resistance
Coefficient
Crr Drag
Coefficient
Cd Area
sq.m Cd.A
sq.m Air
density
factor Slope Speed
mph Speed
km/h Power
Watts Conditions
(altitude / slope)

Bean II 98 0.0030 0.065 0.4450 0.0289 0.816 -0.006 20 32.2 -15 At 4500 ft. altitude and 0.006 slope; better tyres

30 48.3 -4

40 64.4 31

50 80.5 97

60 96.5 202

70 112.6 354

77.4 124.5 500

Set 10

Data for Cheetah at the 8000 ft altitude used to record 68mph. No slope is allowed for, though there probably was one.

Set 11

Data for Gold Rush at 8000 ft and 0.004 slope as used in winning the Du Pont prize at 65.45mph in 1986. It shows that a whopping 672 Watts was required from Fred Markham to achieve this.

Set 12

Data for Cutting Edge as quoted by Matt Weaver.

Set 13

Comparison of all the vehicles at 50mph, which approximates to the present world hour record, where the slope is zero, and at sea level. The machines are listed in descending order of power required showing Gold Rush to be least efficient and Diablo the most efficient with the other machines quite similar.

Machine Weight
kg Rolling
Resistance
Coefficient
Crr Drag
Coefficient
Cd Area
sq.m Cd.A
sq.m Air
density
factor Slope Speed
mph Speed
km/h Power
Watts Conditions
(altitude / slope)

Gold Rush 80 0.0100 0.1000 0.4645 0.0465 1 0 50 80.5 493 At sea level, level ground

Yellow Bean 98 0.0054 0.0750 0.4153 0.0311 50 80.5 329

Cutting Edge 96 0.0060 0.1100 0.2601 0.0286 50 80.5 322

Bean II 98 0.0054 0.0650 0.4450 0.0289 50 80.5 313

Cheetah 93 0.0060 0.0550 0.5063 0.0278 50 80.5 313

Diablo 100 0.0060 0.1100 0.1830 0.0201 50 80.5 269

Set 14

All the machines make an attempt on the hour record using a known standard 1 hour power input , the PKH, (Pat Kinch for 1 hour). This shows the Yellow Bean at 47 miles (as set) rising to 51.5 miles for the Varna Diablo.

Machine Weight
kg Rolling
Resistance
Coefficient
Crr Drag
Coefficient
Cd Area
sq.m Cd.A
sq.m Air
density
factor Slope Speed
mph Speed
km/h Power
Watts Conditions
(altitude / slope)

Gold Rush 80 0.0100 0.100 0.4645 0.0465 1 0 38.9 62.6 286 At sea level, level ground

Yellow Bean 98 0.0054 0.075 0.4153 0.0311 47.0 75.6 286

Cutting Edge 96 0.0060 0.110 0.2601 0.0286 47.4 76.3 286

Bean II 98 0.0054 0.065 0.4450 0.0289 48.0 77.2 286

Cheetah 93 0.0060 0.055 0.5063 0.0278 48.0 77.2 286

Diablo 100 0.0060 0.110 0.1830 0.0201 51.5 82.9 286

Set 15

A selection is made of the best features from all the contenders, plus very good available tyres to show what could be achieved. This shows that the hour record could be raised to 72.5 miles with existing technology, showing that there is still plenty of scope for designers and riders to set new records.

Comment

If we take the parameters of the Choice Machine and run it at Battle Mountain with 500 Watts power input, the speed possible is a staggering 111mph. It is of course not quite valid to do that, as it would take a longer distance to get up to speed, which may not be available, and the rider would not have so much power left at the end of the acceleration phase. However, it would still be frighteningly quick. The results call into question the validity of allowing a small slope to be used on the course as it has such an enormous effect on the results. It is clear that location, location, location is all-important. Ideally we would use courses that are dead flat. In the real world this is unlikely, but I venture to suggest consideration that start and end points of courses should have the same altitude which might make the results more credible.

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