For example, in the days before jets had enough power to exceed the speed of sound in level flight and the attempt had to be made at an altitude just above the ground, the venue for the record attempt was increasingly someplace hot. The limit on speed had become the abrupt change in drag with Mach number. Mach number varied with air temperature. Higher temperatures meant a higher Mach number and therefore the airplane could achieve a higher speed over the ground.
However, the record aircraft were almost never stock. Engines vary more than you might think in terms of horsepower and thrust even before being tweaked for more. Engine and other limits were routinely exceeded at the expense of durability and safety. Removal of nonessential stuff, e.g. military equipment, to reduce drag and weight was a standard practice. (If guns and ammunition were carried, the fact was always worthy of note in the press release announcing the record.)
I was recently reminded of a couple of examples of somewhat more extensive modifications. The thrust provided by the J79 engines in the F4H-1 that then LtCol Robert Robinson (USMC) used to set a speed record of 1,606 mph in December 1961 was augmented by water injection.
Robert F. Dorr CollectionIf you look closely at the aft cockpit of the record setting airplane, you'll see what appears to be the water tank.
Needless to say, Phantoms in the fleet were not capable of anywhere near 1,600 mph.
Even more extensive modifications were made to a Navy HSS-2 in pursuit of a helicopter speed record. In February 1962, it was used to set a record of 210 mph versus a fleet Sea King's top speed of not much more than 150 mph.
Unofficial speed records were even more likely to approach bogus. One example is the well- publicized report that the XF4U-1 had demonstrated a speed of 400 mph. For a discussion of the unlikelihood of that, at least in late 1940, see http://thanlont.blogspot.com/2008/10/400-mph.html.
Sometimes the false claim was at least somewhat inadvertent. On 17 June 1980 during envelope expansion with a full load of research instrumentation and no particular attention paid to drag reduction, the Bell XV-15 tiltrotor research aircraft achieved a measured true air speed in level flight of 301 knots as first calculated by engineering.
Much celebration ensured and a press release immediately issued by marketing. As it turned out, the initial calculation failed to take into account the correction required on the outside air temperature measurement, understandably because it is negligible at the speeds that helicopters normally achieve. When recomputed correctly, the actual speed was a few knots less than 300. Bell management chose not to update the number since it had already been promulgated worldwide. In any event, the milestone did not have the expected impact because 301 knots—while roughly twice that of a helicopter not tricked out like the HSS-2—was half that achievable by a jet-powered VTOL like the Harrier. Not until the XV-15 flew at the Paris Air Show in 1981 and it was subsequently flown by guest pilots did the operational benefit of the tiltrotor concept become apparent to prospective customers.