Miles M.77 Sparrowjet
With his desire for more speed, Fred Miles conceived the idea of replac-ing the M.5 140 hp high-compression Gipsy Major with two Turbomeca Palas turbojets, each developing 330 lb maximum thrust (equivalent to about 550 hp in total).
The M.5 Sparrowhawk prototype survived the war, was flown to Redhill, and in 1953 was modified considerably by the installation to become the M.77 Sparrowjet.
With a length of 35in and a diameter of 16in, for an individual weight of 160 lb, the Palas units required new twin metal-sparred inner-wing attachments to the fuselage centre section for their upper wing-root housing. Designer, aerodynamicist, stress engineer and flight-test observer Grahame Gates, who was brought in to assist George Miles, recalls that the Sparrowjet centre-section attachments and engine mountings required some complex engineering. This included curved compression members with a pin-joint above each engine centreline, and the rear spar straddling the hot tailpipe. Being completely buried within the wing chord, the little Palas engines also needed short stainless-steel exhaust channels to protect the adjacent wing structure.
A completely new single-seat cockpit and forward-fuselage section was also required, resulting in an increase in length of nearly 7ft to 29ft 7in, including a new and bigger tail assembly to balance the additional side area of the longer nose. Two small fins near the tailplane tips were also added solely as mass-balance fairings.
While the original Hawk wooden wings with their ancient Clark YH aerofoil sections were retained, extensive modifications were required to incorporate a 38gal fuel tank in each for the turbojets. The wings' twin-sparred structure also had to be reinforced and re-covered with thicker ply skins to cope with the increased airspeeds, and their stiffness was further improved when 18in were clipped from each wingtip during initial flight development.
Squared-off tips, with no decrease in aileron span, had been schemed as a means of achieving an increase in lateral control effectiveness, and of reducing associated stick forces. Their planned removal, however, was delayed until after initial flight trials, to retain lighter wing loading and docile handling in the early stages of development. Overall wingspan was still 7in more than the Sparrowhawk's original 28ft, because of the additional wing-root engine bays.
With no propeller clearance problems, shortened, spatted Magister-type mainwheel undercarriage oleos with stub axles instead of alloy forks, and Goodyear tyres and brakes, could be used. Although these brought the Palas tailpipes close to the ground, no problems were reported from surface erosion. The two engines increased empty weight by more than 500 lb to 1,578 lb, although the maximum take-off weight went up by only 200 lb, to 2,400 lb.
All of these changes took time, and were further complicated by accompa-nying budget overruns, as well as the transfer of F.G. Miles's activities from Redhill to Shoreham in 1952. It was not until December 14, 1953, after three year's work, that G.H. Miles made the first flight of the redesignated M.77 Sparrowjet (c/n FGM 77/1006) at Shoreham. Originally flown under B conditions as G-35-2, it soon reverted to its original civil registration, G-ADNL, plus racing number 92 on the new broad-chord fin and rudder.
Few problems were encountered dur-ing Sparrowjet flight development, which was shared by G.H. and Miles test/sales pilot Ian Forbes. The Sparrowjet's inaugural flight also marked its initial pilot's first jet experience, a distinction which Fred Dunkerley shared when he had his baptism on type on Whit Monday, 1954. It was evident that, while the cruising and maximum speeds of the Sparrowhawk had been increased by over 50 mph, its pleasant flying qualities and docile low-speed handling had been retained.
In the C of A, the Ministry of Civil Aviation and the Air Registration Board (ARB) had imposed a restriction of 31,400 rpm maximum continuous power on the Palas turbojets, instead of their rated 33,800 rpm, which was permitted only for take-off or emergencies. This had the effect of reducing the Sparrowjet's design maximum speed performance of 240 mph to a revised handicap estimate of 227 mph.
Engine starting was normally via high-pressure air from a large ground cylinder to spin the centrifugal compressor up to light-up rpm. This system saved the weight and complication of integral electric starters, but effectively ruled out off-base operation without the necessary ground equipment.
G-ADNL usually operated from the airfield Barton, participating in many races, including:
SBAC Challenge Cup in 1956 at an average speed of 197 mph [316 km / h]
King's Cup Race July 13, 1957, at an average speed of 228 mph [366 km / h]
The M.77 SparrowJet was destroyed in a fire at a hangar at RAF Upavon in 1964.
Engines: two Turbomeca Palas turbojets, 330 lb max thrust each
Wingspan: 8.49 m
Total length: 9.40 m
Wing area: 14,50 m²
Empty weight: 717 Kg
Maximum weight: 1090 Kg
Max speed: 368 Km / h
Range: 432 Km
Climb rate: 640 m / min
Wing loading: 75 Kg / m²