Comet 1 & Comet 2
The first prototype had large, single main wheel (these being replaced by a four wheel bogie unit on production aircraft) together with four Ghost 50 Mk1 engines buried within the wings and a pressurised cabin to facilitate high altitude travel in absolute comfort. Designed by Ron Bishop (designer of the DH Mosquito) the Comet represented a new category of passenger travel and as such it was subjected to extremely rigorous testing including both pressure and water tank trials.
The configuration, of the Comet was not significantly different from that of contemporary long-range propeller-driven aircraft. A comparison of the characteristics of the Comet with those of the Lockheed Constellation indicates that the Comet was a somewhat lighter aircraft, had a lower wing loading and a wing of lower aspect ratio but had a cruising speed of 490 miles per hour at 35 000 feet as compared with 331 miles per hour at 23 000 feet for the Constellation. The range with maximum payload of 44 passengers was 1750 miles. At a much reduced payload, a range of slightly over 4000 miles was possible. By present-day standards, the Comet 1A was a small, relatively low performance aircraft. By comparison with other aircraft of the early 1950's, however, it was extremely fast.
The Comet 1A was powered with four DeHavilland Ghost turbojet engines of 5000 pounds thrust each. Engines were mounted in the wing root – this minimises yaw accompanying loss of engine on take-off, but poses a hazard in the event of engine fire/disintegration and does not allow for easy uprating of engines (cf. hanging engine pods under wing) – poor design for development. Fuel consumption of turbojets is lower at high altitude.
The takeoff thrust-to-weight ratio was a very low 0.17. As a consequence of this low thrust-to-weight ratio, very precise control over the aircraft attitude was required during the takeoff roll to prevent overrotation and subsequent high drag and loss of acceleration. At least one aircraft was lost as a result of overrotation during takeoff. The four engines were mounted in the wing roots, two on each side of the fuselage. This engine arrangement has the advantages of placing the engines near the longitudinal center-of-gravity position and of minimizing the asymmetrical yawing moment that accompanies loss of an engine during takeoff; at the time, it was also thought to be a low-drag arrangement. The proximity of the engines to each other and to the passenger cabin, however, posed a possibly hazardous situation in the event one of the engines disintegrated. Engine disintegration was a very real concern in 1950. Engine maintenance was also complicated by the wing-root mounting arrangement.
In comparison to noisy propellor-driven airliners of the same era, this new design offered a quiet and comfortable passenger cabin and consequently showed signs of already being a commercial successat its 1952 debut. The clean, low-drag design of the aircraft featured many new design elements including a swept-wingleading edge, integral wing fuel tanks, and four-wheel bogie main undercarriage units. Large picture windows and table seating accommodations for a row of passengers afforded a higher degree of comfort and luxury as compared to other airliners of the period. Amenities included a galley that could serve hot or cold food and drinks plus a bar along with separate men's and women's toilets. Emergency provisions included several life rafts stored in the wing roots near the engines along with individual life vests stowed under each seat.
The aerodynamic design of the wing was conventional except for the use of 20° of sweepback. The aspect ratio of 6.6 was low, as compared with contemporary long-range propeller-driven aircraft. The high-lift system consisted in a combination of simple plain and split trailing-edge flaps. Some aircraft employed fences on the wings. The passenger cabin was pressurized to maintain a cabin altitude of 8000 feet at an aircraft altitude of 40,000 feet.
The aerodynamic controls were hydraulically boosted. Several of the Comet's onboard systems were also new to civil aviation. One such feature wasirreversible, powered flight controls, which increased the pilot's ease of control and the safety of theaircraft by preventing aerodynamic forces from changing the directed positions and placement of theaircraft's control surfaces. Additionally, the primary control surfaces, such as the elevators, were equippedwith a complex gearing system as an extra safeguard against accidentally over-stressing the surfaces orairframe at higher speed ranges.
Operationally, the design of the cargo holds led to considerable difficulty for the ground crew, especiallybaggage handlers at the airports. The cargo hold had its doors located directly underneath the aircraft, soeach item of baggage or cargo had to be loaded vertically upwards from the top of the baggage truck,then slid along the hold floor in order to be stacked inside. The individual pieces of luggage and cargoalso had to be retrieved in a similar, slow manner at the arriving airport.
The de Havilland DH-106 Comet 1 was the first pressurized civilian jet transport. As such, the Comet was in many ways, new technology, so there were no precursor events. Much was still unknown about fatigue in pressurized flight. The Elba report even stated, "Enough is now known about the fundamental physics of fatigue for engineers to be aware that there is still much to be learnt." The Comet became the aircraft from which the whole aviation industry learned the fatigue behavior of pressurized fuselages.
Investigative testing, with concurrence from extensive examination of crash wreckage, revealed that the relatively squarish windows were creating stress concentrations much higher than anticipated. These stress concentrations fatigued the material around the window corners, which would quickly lead to a rupture of the fuselage.
The Comet design employed a general structural design philosophy referred to as "safe-life." Critical parts are assigned a safe amount of time, cycles, or flight-hours in which they will function properly. This "life" is determined from extensive designing, testing, and evaluating. The Comets' safe-life was targeted at 10,000 cycles. After this, significant repairs or replacements would be required in order that the aircraft could be safe for another calculated number of cycles.
Boeing, and other manufacturers of that era, on the other hand, designed their pressurized aircraft, such as the Boeing 707, under the structural design philosophy of "fail-safe." In fail-safe design, a critical part has redundancies, alternate load paths, and provisions for cracks to be stopped and turned, should they occur. Often, this may result in more complex, or heavier designs. Today, both the fail-safe and safe-life philosophies have evolved into "damage tolerant" standards, which integrates attributes of each approach into one objective standard.
Only a very limited number of Comet 1's were delivered before the model changed enough to get a new designation. The Comet 1A had increased fuselage strength, among other updates. It wasn't until the next model, the Comet 1XB, that the window shape was changed from squarish to the now-conventional oval shape.
One aircraft (G-ALYT) was also fitted with Avon 502 engines and larger intakes and appeared as the prototype Comet 2X. De Havilland DH106 Comet 2X De Havilland DH106 Comet 2X (G-ALYT) used for Crew Training at RAF Halton 1959-64. 16 Production Comet 2's were for commercial use but with confidence in the aircraft at an all-time low most were transferred to RAF service as the Comet C. Mk2 for Air Transport Command. Other examples meanwhile served the RAF in electronic intelligence (2R) and training roles (T2) and a single Comet 2E (XV144) was used by the RAE for blind landing, head-up display and autopilot trials.
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