davparlr
Senior Master Sergeant
Ground speed, as measured between two points on the ground, can be very accurate. It is the way world speed records were measured. It can also be very inaccurate, all the way down to WAG territory.
World speed records require cameras and clocks that are synchronized at both the start and finish lines and flights to be made both ways within a short period of time to average the wind conditions.
Flights from airport A to airport B certainly introduce a host of variables. Synchronized clocks at each airport? Pilots wrist watch?
When was the time started? Did the plane loose altitude over the course of the flight? like drop several thousand feet over 50 miles? Tail winds were what speed at what altitude?
However even test flights are not 100% accurate until/unless correction factors are figured into the instrument readings.
Sometimes extra instruments were fitted to aircraft including things like recording barometers and temperature recorders (they recorded on paper drums)
So the proper corrections could be made to the instrument readings, like the normal altimeter and air speed indicator.
Often a new/different pitot tube was fitted to test aircraft in order to minimize errors due to pitot tube location or shape until a general correction could be worked out for production examples. Also the correction factor for the pitot tube/airspeed indicator varied a bit with the speed of the aircraft and in some cases gave higher readings than true instead of lower.
Both the P-38 and F4U were designed with the intention of being 400mph aircraft. Both wound up being 400mph aircraft, just not in their original form or using their original engines. Since due to crashes of prototypes and delays in programs they over lapped considerably the only real value in which was first would be to settle a bar bet.
Precisely!