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Aviation History
1957
1957 - 1517.PDF
FLIGHT, 18 October 1957 607 VTOL Design Problems The Tilt-wing Formula Discussed in Lecture by Vertol Design Chief The prototype tilt-wing Vertol 76 with its propellers in the "rotor" position. Pitch control is by a horizontal fan in the tailplane and yaw by a vertical fan in the fin. INDICATIONS of widening spheres of interest were inevidence at the first meeting of the Helicopter Association's1957-58 lecture programme, held in the R.Ae.S. library on October 4, when a paper entitled "Some Design Problems ofTilt-wing VTOL Aircraft" was presented by Mr. W. Z. Stepniewski, chief of research and development, Vertol AircraftCorporation. The chairman was Professor J. A. J. Bennett. Mr. Stepniewski began by saying that the tilt-wing/propellerconcept represented the most promising VTOL configuration where the aim was to provide a machine with a speed range ofbetween 300 and 400 kt and five minutes of hovering time. Two important advantages of the configuration were the relativelyhigh rates of payload to gross weight and the relatively low fuel- consumption at cruising speed. If the required hovering timewas greater than five minutes, the tilt wing showed up even more favourably than other types of vertical-lift aircraft. This wasparticularly true in the case of comparison with turbojet-sup- ported machines, but where extended hovering was required forany particular operation the conventional helicopter provided the best answer. In fact, an important conclusion he had drawn fromhis investigations was that while the tilt-wing convertible offered numerous unique advantages for certain types of operation, theVTOL would never supersede the helicopter entirely. One of the most difficult design problems to resolve was thatof matching the power requirements for hovering flight with those for normal cruising speed. Much more power was requiredin the hovering condition and in multi-engined convertibles it might be found expedient to switch off one or more of the enginesin cruising flight. The most important design parameters to be considered wererotor-propeller disc-loading, rotor-blade geometry and tip speed, wing loading, and wing aspect-ratio. These could not be con-sidered independently, since it was desirable to have the entire wing area submerged in the rotor-propeller slipstream to preventstalling of the wing during transition from one flight condition to another. Where a cruising speed of approximately 300 kt wasrequired for shorter ranges (of, say 200 miles) a low-aspect-ratio wing having a high loading of about 100 lb/sq ft and two rotor-propellers seemed an attractive compromise . For longer ranges, a high-aspect-ratio wing with slightly lower loading and fourrotor-propellers appeared to become more advantageous. The lecturer then outlined in mathematical terms the factors whichhad led to the choice of values of these parameters in the design of the Vertol Model 76 flying test-bed. The rotor-propeller of the tilt-wing aircraft performed the dualtask of a rotor in hovering and a propeller in forward flight. It was necessary therefore to make the same thrust generator themost efficient lifting and propelling device. In addition, a margin had to be provided between operational and maximum-sectionlift-coefficient to allow for their use as a source of control forces and moments in the hovering and near-hovering conditions. Ingeneral, the desired results could be achieved by observing the following conditions: — (a) Solidities would be rather high, and in order to reduce them effortsshould be made to develop aerofoils that would permit operation at high average rotor lift coefficients and high tip speeds in hovering. (b) Large collective-pitch changes between hovering and forward flight had to be expected and should be allowed for in the design of pitch controls. " (c) The optimum twist-distribution of the rotor-propeller for the cruising flight condition should be selected in preference to the optimum twist-distribution for hovering. (d) Allowance should be made for rotor-propeller speed to be consider- ably greater in hovering flight than in the cruise condition. £ Obviously, the transition from hovering to forward flight andivice-versa were the most important manoeuvres to be considered from the design point of view. Transitions could be of twoforms: one was a straightforward tilting of the wing while the machine maintained constant altitude; the other was a pilot tech-nique involving variation of altitude during the transition to ensure that the wing angle-of-attack remained substantially con-stant throughout the operation. In other words, the transition from hovering to forward flight was made while gaining height andincreasing forward velocity, whereas the transition from forward flight to hovering was made whilst gaining height and decreasingairspeed. The flight path in the latter case took the form of an exaggerated flare-out, allowing height to increase. The machinewas then lowered vertically after forward speed had fallen off. There was still a considerable lack of theoretical and experi-mental data regarding the behaviour of wings and rotor-propellers in this flight regime, and many of the problems associated withthe prediction of forces and moments acting on the wing- propeller assembly still awaited solution. However, with the useof models it had been established that the rate of wing tilt was probably one of the most important parameters. It appeared thatwhereas fast rates of tilt were advantageous in the transition from hovering to forward flight, the reverse applied during the oppositemanoeuvre. In fact, even a complete tilt over a period as long as 45 sec might not be slow enough for transition from cruise tohovering at constant altitude if wing stalling problems were to be avoided. Stability considerations of the tilt-wing VTOL in hovering andslow flight were similar to those of the conventional helicopter. The VTOL problem was complicated, however, by the introduc-tion of forces acting on the wing itself which tended to accentuate unstable tendencies under certain conditions, particularly in thefore-and-aft sense. A good deal of work had been carried out with analogue computers, and pilots had been able to "fly" theanalogue in the early design stages to assist in the determination of acceptable stability characteristics. There would still be muchmore work to be done when flight-test results were available in sufficient quantity; but from present knowledge it appeared thatsatisfactory solutions were technically feasible. Mr. Stepniewski went on to describe the Vertol Model 76 fly-ing test-bed in detail. Originally, they had set out to build a much smaller machine but the more ready availability of the T53 free-shaft turbine had led them to design the machine around that engine. Consequently, its gross weight was in the region of3,000 lb. The machine was completed in eleven-and-a-half months from the date of the contract. To obtain uncoupled pitch and yaw control moments and atthe same time utilize a known control system (a complete Bell 47 helicopter cockpit with full controls is incorporated in the VertolModel 76), it was decided that two separate tail-rotor control fans would be used. One was mounted vertically in the lower fin foryaw control and the other horizontally in the tailplane for longi- tudinal pitch control. Lateral control in roll was achieved bymeans of differential collective-pitch control of the two rotor- propellers. Vertical control was by means of the conventionalcollective-pitch lever, which varied collective-pitch of the two rotor-propellers simultaneously. Orthdox aeroplane control sur-faces in the wing and empennage, actuated by the same stick and rudder pedals, were also incorporated for forward flight. Control runs were so arranged that change-over from hoveringto forward-flight controls was automatic as transition took place. The only controls which retained the same function in bothregimes were die collective-pitch lever and twist-grip throttle. The pilot's switch to operate the wing-tilting mechanism waslocated on the "cyclic pitch" stick. Since autorotation of the rotor-propellers was unlikely in theevent of engine failure while hovering, it would be necessary in such a case to tilt the wing back to the horizontal position and
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