While the post war years of the early 1930s saw the introduction of civilian aviation and the transition from the wooden bi-plane to the new composite and all-metal monoplane designs, it was the outbreak of WWII which introduced some of the biggest innovations in advance propeller driven flight and aircraft design.
In 1918, the fastest WWI British biplane fighter, the Sopwith Dragon could reach a speed of just over 149mph.
Just prior to the outbreak of WWII, the USAAF two seater trainer, the North American T-6 Texan, could reach 209 mph, while the British Mk I Hawker Hurricane and Supermarine Spitfire were exceeding 342mph and 367mph respectively.
By the close of WWII, the US carrier-launched Grumman F8F Bearcat could reach speeds of 455mph with the British Hawker Tempest and Spitfire Mk 24 also averaging 450mph each.
The real performance breakthrough however was the introduction of the first production jet fighter, the Luftwaffe’s Messerschmitt Me 262, which ripped across the skies of Europe at a staggering 560mph.
Britain quickly responded with their revolutionary turbo jet, the Gloster Meteor at 600mph. The age of the jet fighter had arrived and so too, the race to break the theoretical and elusive Sound Barrier of 767 mph or Mach 1.
Breaking the Sound Barrier...
No one quite knew what such an airspeed would do to an aircraft or the effects on its flight characteristics although many pilots during WWII had reported severe vibration, shaking and almost complete loss of control in hi speed dives when approaching such speeds.
On the morning of October 14 1947, test pilot Chuck Yeager squeezed into the cramped cockpit of the Bell X-1 slung under the bomb bay of a B-29 Strato cruiser which then took his tiny craft up to 29,000 ft before releasing it over the Mojave Desert.
Firing its experimental XLR-11 liquid rocket engine, the tiny swept-wing jet blasted across the sky at a staggering Mach 1.06 or 700 mph.
The arrival of the subsonic fighter...
Yeager reported that when approaching the Sound Barrier, the X-1 started to shake and bounce with a noticeable loss of aircraft control but once the aircraft had passed Mach I, the vibration eased and control was regained. With fuel depleted, the X-1 glided to a landing on a dry lake bed. The sound barrier had been finally broken
Within less than six years, other experimental US Airforce jets had reached new airspeed records of over Mach 2.44 or 1620mph.
Advances in aerodynamics and lessons learnt from those experiments radically changed aircraft design and introduced the ‘swept-wing’ to many of the world’s airforces. This then saw the introduction of jet fighters and bombers capable of subsonic speeds above 620mph, such as the US F-86 Sabre and British De Havilland Sea Venom.
Even though these new subsonic aircraft were far more technically advanced aerodynamically than their propeller and turbo prop predecessors, all experienced the same severe buffeting and partial loss of flight controls Chuck Yeager experienced in the Bell X-1 on approaching the Sound Barrier.
The reason for this is that the airflow around an aircraft is not always the same as the airspeed of the aircraft’s actual travel.
This is mainly due to the fact that the airflow speeds up and slows down as it travels around the aircraft's often complex structure.
Local airflow in some areas near the airframe may in fact reach the speed of sound even though the actual speed of the aircraft is considerably lower. This will often interrupt the wing’s airflow causing it to separate and create a weak shock wave that can result in catastrophic failure if ignored.
To take into account the huge variations in aircraft design and construction, aircraft designers and engineers assign a ‘Mcriut’ (Critical Mach Number) to all subsonic aircraft. This is the lowest Mach number at which the airflow over any given point on the aircraft reaches the speed of sound - a number of Mach 1.0.
The Mcruit number can differ widely between aircraft. Older aircraft with a thicker wing construction such as the USAAF P-38 Lightning has a critical Mach number of 0.69. While P-38 pilots could occasionally broke this speed in a steep dive, many lost control and ploughed straight into the ground.
The Supermarine Spitfire with its significantly thinner elliptical wing profile, had a Mcruit number of 0.89 allowing the pilot to push the aircraft just that little bit harder.
A supersonic jump...
In 1953, military aviation took another major leap forward with the introduction of the world’s the first production jet fighter able to reach supersonic speeds of more than Mach 1.4 or 924 mph, the F100 Super Sabre.
Since the critical Mach number is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves, any further increase in speed will cause the airplane to lose lift and fall out off the sky.
In aeronautical circles, this critical point is known as the ‘Coffin Corner’ .
The “corner” refers to the triangular shape at the top of a flight envelope chart where the stall speed and critical Mach number are within a few knots of each other. The “coffin” refers to the possible death in these kinds of stalls.
A pilot alert…
While a pilot’s Airspeed Indicator will display the aircraft’s true speed by the flow of air through the primary pitot tube, it cannot determine just how close sections of the wings and fuselage are to their critical Mach number. For this the pilot relies on the aircraft’s Mach-meter.
Installed in all subsonic and supersonic aircraft, the Mach-meter takes its readings from a dual set of pitot tubes strategically positioned on both the Port and Starboard side of the aircraft and averages out the readings to display the ratio of the true airspeed to the speed of sound.
Shown in decimal fractions it provides a constant visual warning to the pilot of the aircraft's proximity to it’s critical flight area.
All subsonic and supersonic aircraft capable of flying close to or beyond their critical Mach number have a Mach-meter installed usually located next to the main flying panel and Airspeed Indicator. Each can be preset to indicate the speed limit for that particular aircraft which can also vary depending on altitude and ambient air temperature.
This Mach-meter with a decimal reading of .5 to Mach 1.0 is a beautiful example of a Cold War-era Mach speed Indicator Mk 1A and bears the British Air Ministry ‘Broad Arrow’ marking on the rear of its bakelite housing. The two rear import tubes are marked Port and Starboard.
The display glass is clear and unmarked and a small adjustment screw at the 6 o'clock position below the dials can be rotated to move the white indexing bracket around the parameter of the dial to indicate the maximum acceptable range for that specific aircraft and the flying conditions.
Hand painted and varnished numbers across the top of the Indicator’s frame display part and serial numbers in the absence of a manufactures plate which was quite common on instruments of this period.
This is an extremely rare example of one of the first Mach-meters installed in the De Havilland Vampire and was located directly beneath the aircraft's gunsight in the Vampire two-seater trainer and at the top left of the Port Instrument Panel in the single-seat fighter version.
This De Havilland Vampire jet Instrument comes complete with detailed Scale Model, Mango Wood Stand & Plaque plus Printed Fact Sheet featuring photo of instrument in aircraft cockpit.
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