Chapter 7 - Performance

Objectives

The car must be fast enough to break the Record.

Drag will inevitably be more than estimated and conditions far from ideal. Bearing this in mind it is always best to have as much power in hand as possible. It is easy to throttle back, but next to impossible to wring more power out of a fully extended engine, especially at short notice.

Top speed is determined by thrust and drag, the drag being the total of aerodynamic and rolling resistance. When the drag builds up to equal the thrust, the car has reached its terminal velocity, more speed can only be obtained by increasing power or reducing drag. Acceleration is relative to the thrust/weight ratio. The greater the acceleration the shorter the run-up distance required to get up speed. This dictates that for maximum performance, drag and weight should be kept to a minimum in the design stage, and that maximum thrust should be extracted from the engine.

To get the maximum power available, the latest engine "Mark" of Avon was fitted to "Thrust 2". This engine was set up on the test bed to produce maximum power by increasing rpm, opening the nozzle as wide as it would go, and then restoring the reheat pressure by increasing the fuel flow to the burner ring. The resultant thrust was measured at 16,800 lb in the car.

With the engine set, three of the major variables were now `frozen'; thrust aerodynamic shape and weight. If extra speed was required we would have to look elsewhere. Two possibilities were - to reduce transonic drag by running in higher temperature, and to reduce wheel drag by running on a harder surface, or reducing wheel load by increasing the angle of attack.

Mach number

The speed of sound (Mach 1) increases with temperature. Transonic drag builds up rapidly as the speed of sound is approached. This means that for a given Mach number speed will increase with temperature.

Run No.8 was made in cold dawn air on 29th September. A speed of 623 mph was reached which represented a Mach number of .834 at the temperature of 43°F. From the table below it can be seen that for the same critical Mach number at 73°F the speed would be 643 mph a gain of 20 mph.

Mach No.	Temp.°F	Speed mph
.834	43°	623
.834	53°	629
.834	63°	636
.834	73°	643
.834	83°	649

This effect was used to our advantage on 4th October when the car ran at the hottest time of day with a temperature of 75°F and achieved a peak speed of 650 mph representing Mach .84.

Thrust 2: Speed of Sound/Temp Effect

Angle of attack

The surface at Black Rock is not ideal; parts of it are soft and crumbly and the wheels tend to 'plough' rather than 'plane'. This is costly in terms of wheel drag and hence speed. To reduce the load on the wheels and hence the drag, it was decided to reverse the aerodynamic downthrust. This was effected by increasing the angle of attack from zero to nose up incidence of . 17' (10 minutes). The change in load can be seen clearly on the recorder trace of run 10 which shows a maximum downthrust of 1200 lb at 450 mph reduced to zero at 6(X) mph and, in reverse direction, to become 2040 lb lift at 650 mph. This reduced the weight on the front wheels from 6360 lb to 3680 lb. The transition from downthrust to lift was due to shocks off the front wheels damming the underbody airflow and negating the venturi effect. The resulting drop in wheel drag must have contributed to our 'Record'speeds.

Results

It had been planned to build up to the Record with a minimum of 8 runs. In the event it took 10 and the results of these are tabulated below:

Date	Run No.	Acceleration	Run Up	Measured Mile Speed
17.9.83	1	460 in 27 s.	3/4 mile	394.477 mph
20.9.83	2	570 in 36 s.	1½ miles	488.465 mph
20.9.83	3	590 in 39 s.	3 miles	582.712 mph
21.9.83	4	Aborted
21.9.83	5	615 in 49 s.	4½ miles	608.416 {Av. 606.607}
21.9.83	6	617 in 50 s.	5 miles	604.534 mph {Av. 606.607}
22.9.83	7	617 in 50 s.	6 miles	607.903 mph
29.9.83	8	622 in 48 s.	5½ miles	622.837 mph
4.10.83	9	632 in 55 s.	5½ miles	624.241 {Av. 633.468} W. L. S. R.*
4.10.83	10	650 in 59 s.	6 1/8 miles	642.971 mph {Av. 633.468} W. L. S. R.*
4.10.83	11	-	6½ miles	620.555 mph