Profile - Flying Slow

"an aeronautical experiment"

Reproduced from :
THE AEROPLANE

No: 688
JUNE 15, 1945

DURING 1937 requirements were specified by the Royal Navy for a shipboard aeroplane capable of an extremely slow cruising speed combined with long duration for the purpose of maintaining contact with enemy fleets during the hours of darkness. That was in the period when the lessons of the Battle of Jutland were still in mind and when radio-location was still in its infancy. Radar development eventually made the need for a "fleet shadower" redundant, but two prototype aircraft to specification S.23/37 were designed and constructed. The General Aircraft G.A.L. 38 and the Airspeed A.S. 39 emerged as a result of the designers' individual investigations into the aerodynamic problems involved. They proved to be amazingly similar in general layout and performance. No doubt the influence of reference to the same research reports on slow-speed flying had some bearing on the ultimate conclusions reached by the design staffs. These reports included some R.A.E. tests on a "flapped" Airspeed Envoy and some full-scale flight tests in the U.S.A. on slipstream influence on lift. The chief demands of this unusual specification were:

(1) A cruising speed of 38 knots for a duration of six hours;
(2) An exceptional field of view for the crew at cruising attitude;
(3) Dimensional restriction for shipboard stowage; and
(4) Minimum noise at low cruising speed.



(above) The G.A.L. 38 Fleet Shadower prototype with triple vertical tail surfaces. Later it developed a typical "Crocombe" tail as shown below.

In these enlightened days aeronautical engineers would immediately select some form of helicopter, but in 1937 even the Autogiro was regarded somewhat with awe and with doubts as to its reliability, neither of which suppositions were justified. So a straightforward solution was sought, but even then the resultant aeroplane induced quite a considerable flutter in official quarters. Even today the multi-motored aeroplane has yet to make its bow as a first-line type in naval aeronautics. Within the span limitation of 56 ft. the initial design problem was to arrange for as large an effective wing area, with a reasonably high aspect ratio, as could he obtained. The second was to equip the aerofoil with high-lift devices which worked. The third essential was that, in accordance with the research reports already mentioned, as much of the lifting surface as possible should be bathed in the slipstream from the airscrews, thereby giving a further increment of lift.



(above) The G.A.L. 38 in its final form with single fin and rudder and with sponson fin fairing and tail wheel removed.


With a reasonable loading for a naval aeroplane of about 15 lb./sq. ft., an essential factor in handling and landing in gusty conditions, and in the absence of special lift devices, difficulty would have arisen in developing sufficient lift at the very low cruising speed required. Both design teams selected a high-wing four-motor tractor scheme with large-diameter airscrews driven from geared Pobjoy motors. In the A.S.39 the thrust axes were inclined at a slight angle to the wing incidence in order to take full advantage of the airscrew slipstream over the wing? A negative angle of incidence between the airscrew thrust axis and the wing lift line (the airscrew axis being inclined downwards) in combination with the large amount of slipstream over the wing resulted in a measurable increase of lift. In theory, at low speed this slipstream, projected at a positive angle of incidence on to the wing, replaced to a large degree the normal air-flow over the wing due to the forward speed of the aeroplane. The net result of the designs was that lift coefficient under power of the Airspeed and General Aircraft Fleet Shadowers was about three and a half, the attainment of which in itself justified the experiment.

(below) G.A.L. 38 with wings folded.



A minimum speed of under 35 knots was attained and the aeroplanes cruised at about 38 knots slowly and sedately. Under moderate wind conditions they flew backwards relative to the ground, with ease. To maintain this cruising speed the aircraft had to fly at a large angle of incidence, but stability on all axes was quite positive. The ailerons were covered by the outboard airscrew slipstream and remained responsive at low speeds while lateral stability was good because at low speeds this steady slipstream rendered the aeroplanes less sensitive to wind gusts. Full-scale tests also showed that the tail surfaces maintained efficiency in slow-speed flight - a desirable feature obtained by a combination of slipstream from the inclined airscrews and downwash from the high wing layout. The reasons for the adoption of a high wing layout were to satisfy the demand for an exceptional field of view for the crew, to enable large-diameter airscrews to be used with adequate ground clearance and to arrange for a wing-folding system of simplicity and sturdiness allowing the wing to fold over the top of the fuselage. In both designs the wing, complete with the power units, folded back on each side, for the wing hinge points were close to the fuselage - a new feature for four-motor layout at the time.



(above) A head-on view of the A.S. 39, which gives some idea of the large wing area swept by the slipstream.

High Lift Devices: In the Airspeed design two slotted flaps occupied the entire trailing edge of each half-wing. The inner parallel sections were 30 per cent. chord lift flaps, while the outer sections tapered in chord and functioned as ailerons. During the slow-speed flight the incidence of the wing was about 16 degrees and to prevent the portions of the wing outside the slipstream arc from becoming stalled, short leading-edge automatic slots were fitted at the wing tips opposite the aileron flaps. These slots assisted to maintain lateral control and stability. Also fitted with tip slots, the G.A.L. 38 had slotted flaps operating through 40 degrees and slotted ailerons which drooped 15 degrees as the flaps were lowered. In addition, the lower wing sponsons - the G.A.L. 38 might be described as a " sesquiplane " - were equipped with 15 per cent. chord split flaps operating through 60 degrees and continuous across the underside of the fuselage. The entire General Aircraft system of "flappery" was operated simultaneously by a servo- pneumatic-hydraulic unit which had the merit of being infinitely selective. Feltham's experience with this slow-flying experiment must have been useful when the design of the Hamilcar freight glider was being projected and, similarly, the Airspeed Company must have found their experience with high-lift devices on the Fleet Shadower useful during the inception of the Horsa military glider. Flap control on the A.S. 39 was by a pneumatic ram using the same air pressure as the main wheel brakes, while a flap-aileron interconnecting mechanism was mounted in the leading edge of the wings opposite the point where the flap and aileron abutted.


(below) A side view of the Airspeed Fleet Shadower, which depicts clearly the special characteristics of its undercarriage.



Consisting of pilot, observer, and radio-operator, the crew of three in each aeroplane was accommodated to produce the greatest efficiency for their special operational work. The cabin-like noses accommodated the observer and gave an unrestricted view forwards and downwards and also a very good sight to the rear. Opening window panels were arranged, which deflected the airflow past the openings and not into the cockpit. In the G.A.L. 38 the front panels were arranged to slide on a circular arc track. Airspeed's, on the other hand, experimented with the rotating glass disc type of clear-vision windscreen, well known in marine practice but a new feature of aircraft design. Wing folding was accomplished on the Airspeed machine manually and a jury strut supported the root ends of the front spars when the wings were folded. The effort required was little more than that caused by the friction of the pivots. General Aircraft used a hydraulic wing-folding system, and the stout bracing strut which extended from the root of the sponson to the inner motor nacelle took care of the weight of each wing when folded. Universal joints allowed for the movement of the strut. The noise problem was helped fortuitously by the adoption of four small power units for each design, for the exhaust systems were more easily silenced than that of one large power plant of equivalent power. The 130 h.p. Pobjoy Niagara V aero-motor had a reputation for relatively quiet operation, a feature considerably helped by the low tip speed of its geared airscrew.



(Above) ...view of the A.S. 39 with the wings folded.

Several detail differences between the two aircraft have already been mentioned, particularly in the use by General Aircraft of a lower wing sponson which accommodated the two main wheels of the nose-wheel tricycle-undercarriage". The special conditions of landing on the deck of a carrier were night operation and the elimination of all possibility of the aeroplane nosing over on touching down. Airspeed's obtained the same effect as a nosewheel tricycle layout by arranging the main landing wheels much farther forward from the centre of gravity than was usual and by correcting the directional: instability incidental to such a system by a self-centring mechanism incorporated in the shock absorber of the tail wheel.