SAFETY OF VAMPIRE JET FIGHTERS

Press, Volume LXXXIX, Issue 27055, 2 June 1953, Page 12

SAFETY OF VAMPIRE JET FIGHTERS

Pilots’ Confidence In Their Aircraft

ADVANTAGES OF GAS TURBINE POWER

BARKER and A. P. McVEIGH, two reporters of "The Press,” who Ohakea} SPent several days Wlth 75 s <iuadron at the R.N.Z.A.F. Station/ (in> Any one of the pilots using the Vampire at Ohakea will, in discussing his impressions of the aircraft, make as his first point the complete safety of the aircraft. The Vampire, he will say, is easy, perhaps deceptively easy, to fly. It is docile, tractable, and has no vices. To the pilot it represents something approaching the ideal aircraft—a machine in which he may place his trust and may fly with confidence.

The Vampire, the pilot says, has effortless speed, a very satisfactory rate of climb and manoeuvrability combined with lightness of control. Pilot comfort, a factor sometimes sacrificed in a fighting plane, has not been forgotten in the Vampire, and the pilot’s confidence is further bolstered by a pressurised cabin, a sound oxygen system, and a cockpit heating and ventilating system.

Compared with the piston-engined aircraft flown during World War II the cockpit lay-out of the Vampire is simplicity itself. The pilot is left free to concentrate on pure flying rather than on the mechanics of his aircraft, a convenience of some importance to a fighter pilot who is his own flight engineer, navigator, gunner, bombaimer, and radio operator. One of the officers of 75 Squadron remarked that given the range he could fly a Vampire from one end of New Zealand to the other and be sure that all his navigation was “spot on.”

But lavish though they are in their praise of the aircraft, all the pilots realise its limitations. First, the Vampire was not designed to penetrate the sonic barrier (the single-seater has a limiting Mach number of approximately .78 and the dual of .82), and any attempt to force the machine beyond its limit would induce compressibility effects (the loss of control, porpoising. caused when the speed of the air over the wings rises above that of sound and disturbs the flow over the tail surfaces] for which the aircraft was not stressed.

At high altitudes the Vampire flies at a speed not too far from the sonic barrier, and the pilots are told, and soon realise, that it is not difficult, when diving or manoeuvring, to lose control in compressibility. But with the correct recovery techniques control of the aircraft is immediately regained. Vampire v. Mustang

There has arisen in this country the rumour that pilots prefer the pistonengined Mustang, a top line fighter in the last war, to the Vampire. Two of the American-built aircraft are attached to 75 Squadron. Pilots who have flown both types state emphatically their preference for the jet. “The Vampire’s performance is infinitely superior to the Mustang’s,” one officer said. “There is no comparison.” There are many reasons for this choice, but primarily because, pilots say, the Vampire “feels nicer to fly.” Without becoming technical it can be said that it would be almost impossible to get from a piston-engined aircraft a performance to equal that of the Vampire. Another advantage from the pilot’s point of view is the very reliable. uncomplicated engine—failure of the Goblin is almost unknown. Both aircraft were, of course, designed for different jobs. Power Outputs Compared

A comparison of the power output of jet engines and piston types is interesting. A jet engine’s output is measured as pounds static thrust, and a piston engine is rated in terms of shaft horsepower delivered by the engine to the propeller. With the piston engine power is lost in flight because the propeller cannot transmit all the available power of the engine into thrust.

As thrust horsepower is a measure of the rate of doing work, it is possible to compare the two types of engine at a certain aircraft speed. At 375 miles an hour, a jet engine producing 33001 b thrust can be said to be producing 3300 horsepower. Conversely, if 33001 b thrust was required to propel a theoretical aircraft at 375 miles an hour, then 3300 horsepower would be required from the propeller of the piston type. Allowing a maximum efficiency of 80 per cent, from the propeller it can now be calculated that an engine delivering more than 4100 shaft horsepower would be required. At speeds of over 400 miles an hour the piston engine-propeller combination loses efficiency, and at speeds near the sub-sonic area the propeller efficiency drops to approximately 50 P£, r S en L With such poor transmitting efficiency the piston engine so required would be of such a size and weight that it would be a designer’s nightmare.

With the jet engine it is a different tale: as designed aircraft speed increases there is a marked rise in propulsive efficiency and shaft horsepower developed. It is worthy to note that when a Vampire is cruising at 500 miles an hour the thrust horsepower developed is well over 4000. The power-weight ratio of the two types of engines further demonstrates the effectiveness of the gas turbine. The Goblin engine used in the Vampire weighs about 17501 b, giving a ratio at high speeds of over 2 h.p. per lb. The engine in the Mustang produces at best little more than 1 h.p. per lb at any speed, a ratio never exceeded by any great margin in piston engines. One of the disadvantages of the jet is its high fuel consumption at medium speeds and altitudes. However, as both these increase the disadvantage is lost. If a Vampire were to fly at 300 feet and at a fairly low speed its maximum endurance, including take-off and landing, would be only 65 minutes. If it flew most of the time at 30,000 feet the endurance would be 115 minutes or more and it would cover

twice the distance in miles.

Essentially the jet engine is for high speeds at high altitudes. Because of the influence of ram effect and the very low air temperature engine efficiency is improved and it is possible to provide the designer with high power outputs at a height where decrease in airframe drag allows faster cruising speeds. A piston engine, being relatively unaffected by ram effect, rapidly loses power with altitude owing to a decrease in air density. Though this power loss can be restored by supercharging, the overall effect is a decrease in power because the supercharger itself absorbs power and propeller efficiency falls off with altitude and high speeds. On the piston-engined aircraft’s side is its superior take-off performance. At low speeds when the ram effect is not so great the jet engine appears to lack power. , The Problem of Drag One of the many headaches which have become the lot of the modern aircraft designer is the problem created by the drag of air on the aircraft. If an aircraft, theoretically, flying at 300 miles an hour is held back'by 10001 b of drag it will require 10001 b of thrust horsepower to propel it at 300 miles an hour. As the speed increases the drag increases by the square law and the thrust required to balance the drag becomes very great. When the aircraft gets beyond .7 Mach it enters a zone of disproportionately high drag, which demands an enormous amount of thrust to overcome. This area is known as the sonic barrier. After Mach 1 has been passed the drag falls off, but it never resumes its original proportion to the speed. Another Barrier: Thermal

Beyond the sonic barrier is another “wall,” perhaps even more difficult to pierce. This is the thermal barrier. The rise in temperature caused by speed in an aircraft travelling at 400 miles an hour is approximately 28 degrees Centigrade; and. as the speed increases, the temperature climbs rapidly until at 800 miles an hour it is about 100 degrees. Even in the yampire there is a considerable rise in temperature, and in a faster aircraft a refrigeration plant would be required to keep the pilot from roasting. Vampire’s Manoeuvrability

Because of its relatively high speed, manoeuvres in the Vampire occupy a a very large amount of space. -A turn made at 400 miles an hour covers a circle nearly 10 miles in diameter, and a high speed loop needs about 7000 ft. These are fairly gentle moves and the figures given must be counted as maximums rather than usual. The limiting factor in manoeuvres and aerobatics is the force known to pilots as “G”—the centrifugal pressure exerted on the pilot and the machine m a x£ n ’ J n a Particularly violent action the blood can be drained from the pilot’s brain by this force, causing a black-out. The greater the pilot’s Physical fitness the greater number of Gs he can withstand without becoming unconscious. Fortunately, the pilot regains consciousness soon after the aircraft straightens out, and the force is removed.

From the pilot’s angle the Vampire has delightful handling qualities, requiring very little effort to complete steep turns, rolls, and other aerobatics. It stalls at a low speed (90 miles an hour) and is stable over a varying fuel load. This latter feature is important, for many World War II fighters were inclined to be difficult to fly to operational limits with’a full load of fuel. (To Be Continued.)