AIRCRAFT DESIGN.

New Zealand Herald, Volume LXVIII, Issue 20886, 30 May 1931, Page 19

AIRCRAFT DESIGN.

FLYING laboratories. The phenomenon known as "scale effect" plays a vital and frequently an annoying part in the design of new aircraft. Results obtained with the greatest care with accurately built models of the new craft, subjected in a wind-tunnel to air forces similar to those encountered in flight, show discrepancies with the behaviour of the full-sizo machine in the a ir and, though much of this discrepancy can bo calculated with some acciirafy- there invariably remain details of flying behaviour on which the model tests throw no exact light. Sometimes, for example, first flights of a new aeroplane reveal defects or "vices" of the control surfaces which the most careful model tests have failed to indicate. An obvious need, therefore, was an aeroplane so constructed that the forces operntiilg on the lifting surfaces could be ' measured directly in flight, and two monoplanes, the first of their kind in the world, which permit of these important measurements being taken in the air, are now in the possession of the British Directorate of Scientific Research. In recent months they have made many successful flights. Two Novel Machines. These two "flying laboratories" were constructed by Messrs. George Parnall and Company, a firm which has built a variety of service land and seaplanes to 'Air Ministry order. * They are two-seator "Parasol" monoplanes, the single pair of lyings being held above the fuselage on *n arrangement of struts, enabling changes from one kind of wing to another to be made easily and swiftly. The central design feature of these novel planes is that the wing bracing struts, certain'wing members and a main fuselage member, constituto botwoen them a kind of swinging cradle which allows relative movement between the wings and / the fuselage under the influence of the aerodvnnmical forces on the wings. This relative movement, however, is checked by a momber connecting tho rear spars of the wings with the lever of a dynamometer situated centrally in the fuselage, and controlled from the front, or observer's cockpit. The load thus communicated to the recording instrument, due partly to the "lift" and partly to the '"drag," or air resistance, of the wings, is indicated directly on the dynamometer dial. A second reading is needed to enable the "lift" and the "drag" components to be separated. This is obtained bv altering tho points of attachment of the wing bracing struts with the fuselage and getting further readings with the wing in the new position. Accurate Eesults Obtained. In addition to the usual flying controls, the pilot's cockpit contains a lever actuating a hydraulic brake on the airscrew, thus enabling tho wing-load test reelings to bo got without interference from tho slipstream thrown back by the 1 airscrew. After gliding with the airscrew stopped,/ tho engine may ba restarted by means of a gas-starter operated from tho pilot's seat. These rcseArch craft are of moderate size, each measuring about 42ft. from wing-tip to wing-tip. Powered with a single supercharged "Lynx" air-cooled / 226 h.p. motor, tho "Parasol" flies at a maximum speed, at 8000 ft., of 119 miles an hour and can climb to an absolute "ceiling" of no less than 29.000 ft. Not only docs the craft give more accurate results than the costlier wind-tunnel, but the pilot is a,ble to discover "vicious" tendencies in flight of a new wing arrangement, to pet first-hand information about tho effect of the wings on flying control, and to assign a value to loss easily tangible qualities, such as tho "feel" of art aircraft in flight with a given wing arrangement.