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Aviation History
1944
1944 - 1016.PDF
FLIGHT X S& Rhombic Ruminations The Designer of the WjwrgsPtoung Tithdftn Explains Why \*** the Maehme Should he peeress -v..- 'By NORMAN THE Warren-Young tandem monoplane to which refer ence was made in Flight of NoveVnber 18th, 1943, is designed for the owner-pilot, andv»U|h this object^ in view a design has been evolved in which safenTIfTgTHSr^ acteristics are primary considerations. Freedom from vice at high angles of attack was the aim of the design. With this aircraft the sudden stall is 1/ HALL WA I J planes rfe no^ret at a greater angle of incidence than tie front plangfli^ias a stable travel. This is due to the down^ uch causes the rear planes to operate at a smaller angle of attack than the front planes. As the angle of attack of the combination is increased, however, the rate of change of lift is higher for the rear planes. This results in a rearward, and therefore eliminated, and stability in pitch and roll is achieved by use of the particular wing formation, and with out the aid of mechanical devices. The principles upon which these properties de pend derive from the char acteristics of sweptsbacC" T and swept-forward aerofoils, and also from the effects of two aerofoils operating in tandem. These effects are well known, having been observed in wind-tunnel tests. Specifically they are as follows: Sweep-back or sweep-forward of about 20 deg. produce a very marked flattening of the peak of the lift curve. A characteristic curve for such aerofoils shows CL varying linearly with angle of attack up to the usual angle of maximum lift; thereafter the lift falls very gradually and smoothly, to rise to a second peak at about double the angle. In some tests, particularly for the swept-forward aerofoil, CL reinafns sub stantially constant, a high lift coefficient being maintained up to an angle of over 30 deg. Such wings show no critical angle, and the stall is a gradual process, spreading slowly over the surface and distributed over a wide range of angles of attack. This sweep effect can be ascribed to a lateral flow, due to the pressure gradient along a line normal to the axis of symmetry of the aircraft. The lateral component ejects the stagnated stream outwards in the case of the swept-back wing and inwards for the swept-forwjrfawing. The manner in which the stall occurs for the swept-back and swept-forward wing is therefore dis similar. With the swept-back wing, the turbulence spreads from the trailing edge and eventually affects the tip region, leaving the centre part of the wing relatively unaffected. An exactly opposite condition obtains for the wing with forward sweep, and here the tip remains unstalled at high angles. Stable C.P. The principal characteristic of a tandem aerofoil combination con cerns the behaviour of the centre of pressure. This, when the rear IN our issue of November 18th, 1943, we published a note by Mr. T. R. Young on the tandem monoplane planned by him and Mr. Warren. That note was, in effect, a reply to the Editor's article of August 12th, 1943, in which was asked the question, " Does the tandem still have a future 1 " Since then Mr. George Miles has contributed his views (April 27th, 1944) and some photographs of the Miles ' Libellula " (May 4th, 1944). In this article the designer of the Warren-Young states the basic principles upon which he worked in arriving at the unorthodox layout. To us it still remains open to question whether the Warren- Young can truly be classified as a tandem. It seems to be that and something more. It partakes of the tandem principle, but also of the Cedric Lee circular plane built around 1912. And even before that there was the " Rhomboidal " of early Brooklands days. Then there was a little American light plane which appeared after the first world war. In that machine (a biplane) the top wing had a pronounced sweep-back and the bottom wing a sweep-forward and a dihedral. The Warren- Young differs, however, in having the wing tips common to both wings, and it is this feature which distinguishes it from others. stable, overall movement of the centre of pressure. Such tandem formations are con sequently stable longitudin ally and do not require a tailplane. The influence of the lift characteristics of the tip surfaces in relation to those of the front and rear planes is important in modifying the overall centre of-pres- sure behaviour. The pre cise adjustment of thi^rela tion, by suitable choice of aerofoil section and taper ratio for the tip surfaces, will affect the stability characteristics of the air craft. Obviously the quali ties desired are: a smooth resultant lift variation, with lift maintained up to a high angle of attack; a steady rear ward travel of the centre of pressure with increasing inci dence ; and no excessive diving moments at the high-speed end of the range. Docile Behaviour From a consideration of the fore going principles the behaviour of the Warren-Young tandem may be estimated. The front planes will reach maximum lift art the usual angle of attack. With increase of angle, the lift of the rear planes will continue to rise due to the downwash effect, whilst the CL of the front planes will fall gradually. The resultant lift coefficient for the combination will therefore continue to rise, though with a reduced slope. Eventually, with increasing incidence, the rear planes will reach maximum lift and, due to the for ward sweep, the lift will be main tained, showing no discontinuity at very high angles. This history, which is dependent upon the choice of aerofoil sections and the degree of decalage, reveals a smooth and gradual variation of lift^with no critical angle, and a steady back ward movement of the centre of pressure. Stability in roll arises directly from the differential stalling, since rolling instability is associated with the stall, and here the instability of a stalled part of the formation is damped by the stability of sri. f* -ifc>-*
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