Acceleration—aircraft versus car

Sir,

I would like to take up the points raised by your correspondent, GB Carver, in connection with the relative acceleration of the car and the aircraft.

Two very important factors render such comparisons a trifle ambiguous. In the first place there is the question of wind speed : at all times an aircraft’s speed is considered as relative to the air mass in which it moves and not to the ground over which it moves. Thus acceleration figures for take-off can only really be compared when an aircraft takes off in a flat calm. Thus, for example, a Firefly Mark VI weighing around six tons and powered by a RR Griffon developing a little under 2,000 bhp will be airborne in the length of half a flight deck (say 100 yards plus) when embarked, whereas the same aircraft shore-based, in no wind conditions, will need about five to six times that distance to reach its “unsticking” speed of around 85 knots. This affects the acceleration figures as follows : During all the time that the wheels are on the deck (at sea or ashore) the airscrew has to pull against the normal drag of the aircraft—as in flight—plus the frictional drag of the tyres. The moment this drag ceases acceleration improves considerably and this moment depends on the ground speed at takeoff not the air speed.

The second important factor is the pitch of the propeller blades. The introduction of the variable pitch propeller has considerably increased aircraft acceleration at take-off because it permits a “fine” pitch giving a better grip in the air at very low speed which would be unacceptable for cruising. Readers of “Reach for the Sky” will recall that Group-Captain Bader failed to leave the ground in a Spitfire fitted with a variable pitch propeller because he had left the pitch in “coarse,” whereas the same aircraft would unstick very quickly in fully fine. One of the factors affecting aircraft acceleration at low speed is therefore the degree of fine pitch attainable. On the other hand if the pitch is too fine the propeller blades will not absorb enough power from the engine to remain at reasonably slow rpm and the tips of the blades may become supersonic—as anyone who has heard a Harvard or Grumman Avenger on take-off will testify. Again tip speed is obviously affected by blade length, but that is another question.

In applying this to a car propelled by an airscrew a fine pitch would obviously be desirable for sprints and hill-climba, whereas a slightly coarser pitch would give high-speed cruising, though it might take quite some time to reach cruising speed. Also for pure straight line sprinting a considerable lessening of road/tyre friction would be desirable, though not at the expense of a complete lack of steering.

In closing it is interesting to note that for all their screaming jets, the initial acceleration of the “kerosene kids” is poor by comparison with the propeller-driven aircraft. Perhaps the turbo propellers have the best of both worlds.

May I add my word of appreciation for your excellent magazine, it represents unbeatable value for money.

I am, Yours, etc,

Peter Graham. Huddersfield: