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Aviation History
1953
1953 - 0122.PDF
120 FLIGHT The HELICOPTER in PRACTICE An Exploration of Fundamentals from the Pilots Point of View By B. H. ARKELL, A.R.Ae.S. THERE can be few associated with the aircraft industry who have not at some time had an opportunity of examining a helicopter at close quarters and, indeed, many are able to lay claim to having flown in one. However short such a flight may have been, or however cursory the examination, it will have given at least some insight into the problems associated with this branch of aeronautical science. The basic thought underlying all helicopter flight is that the pilot sits in a fuselage and "flies" the rotor above him. In the fixed-wing aircraft he sits in a machine proceeding along a flight- path determined by a line of thrust fixed in relation to the fuselage; by changing the attitude of the fuselage through the controls any particular flight-path may be selected. In the helicopter the opposite takes place : any movement of the controls to select a desired flight-path changes the line of thrust of the rotor by tilting it in relation to the fuselage, which thereafter follows the rotor into the new path. To the fixed-wing pilot accustomed to an immediate response in change of fuselage attitude, there seems to be a lag in response to control movements, and this effect is accentuated by a further lag in the time taken for the angle of tilt of the rotor to respond to a cyclic change of blade-incidence from the control column. The controls of the helicopter are comparable with those of the fixed-wing aircraft in that they include a control column, rudder pedals, and throttle; but the added degree of freedom in the vertical sense calls for an additional manual control, which is known as the collective-pitch lever. It is largely the co-ordination of this new control with the others which introduces the difference in flying technique, though there are some differences in response to the other controls. A pilot of average experience could expect to feel reasonably at home after about 15 minutes with the stick and rudder controls in straight and level flight—provided he was not called upon to make collective-pitch changes. Control column and rudder pedals are positioned normally. The collective-pitch lever is usually at the side of the pilot's seat, moving up and down through an arc in the vertical sense. The throttle is at the end of this lever and in most types of helicopter today it takes the form of a twist-grip control. There is little standardization as yet in cockpit layout and, although an interim measure of agreement has been reached, some controversy does still exist as to which side of the seat it is better to have the collective pitch lever. The difference of opinion arises in the choice of which hand is better to operate the collective pitch and which the control column. Due to some mechanical design-features it does also introduce the new question of whether the captain of the rotor- craft should sit in the left-hand or right-hand seat, where side-by- side seating is fitted. Although there are examples of both the two-bladed and three- bladed rotor on present-day machines (the latter probably being the more usual), it is simpler for theoretical purposes to consider the rotor as having two blades : then, whatever one blade is doing, the other is doing exactly the opposite on the other side of the cycle. The flapping-hinges through which the blades are attached to the rotor head, form, in effect, a universal joint giving the rotor freedom of tilt in any direction with relation to the fuselage. The blades are also given freedom of movement about their longi tudinal axes through a thrust bearing at each blade-root. The blade is controlled about this axis by a system of levers on the head coupled to a universally mounted swashplate which revolves with the head and immediately below it. The control-column linkage tilts this swashplate, and the angle of the rotor disc follows the tilt of the plate as it varies cyclically the pitch of the individual blades. If a blade receives an increase of incidence at one point in the cycle the other will receive an equal decrease on the opposite side. The blades are continually changing pitch as they pass through the cycle of rotation, and by the time they have passed through 180 deg the conditions are reversed. The cyclic-pitch TOTAL FORCE HORIZONTAL COMPONENT QUITE a number of people are confident that they are aware of the basic operating principles of the helicopter—until they try to explain them to somebody else: then they discover gaps in their knowledge. In this article an attempt is made—success fully, we feel—to clarify many less obvious aspects of the subject, and to show what the novice helicopter-pilot may expect to meet. Mr. Arkell has had long Service-experience as a pilot of rotorcraft, and after the war was with Fairey's helicopter branch. He is a founder-member of the Helicopter Association. range which can be applied by this means varies with the different types of helicopter, but is of the order of plus or minus 5 to 7 deg in the fore-and-aft sense, and plus or minus 4 to 5 deg laterally. Movement of the control column tilts the rotor in the direction of that movement; there is a time-lag in the response to this control because the swashplate applies minimum incidence to the blade at a point 90 deg before the effect of minimum tilt (or minimum flapping, as it is correctly termed) is desired. The blade has to move through this 90 deg arc of its travel before the minimum incidence takes effect as minimum flapping. On the opposite side of the rotor (and simultaneously, of course, to provide the rotor tilt) maximum flapping takes place 90 deg after maximum incidence, and 180 deg after minimum flapping. With a rotor speed of say 240 r.p.m. this time-lag is only about ^ of a second, but, added to the time which the fuselage takes in assum ing the attitude dictated by the angle of the rotor, it is sufficient to give the novice helicopter pilot a feeling that nothing is happening when he first moves the controls. The result is that in the early stages he will always over-correct in control movements until the time-lag is picked up. This lag in response to cyclic-pitch control is common to all helicopters. The helicopter is maintained in the hovering attitude with the rotor substantially horizontal and the control column in the central position. Any tilt imparted to the rotor by movement of the control column tilts the total lift vector out of the vertical and introduces a horizontal component to move the machine. This tilting of the total lift vector also, however, reduces the vertical component of lift, which will cause the machine to sink, and the pilot's other hand must compensate for this loss by increasing collective pitch and power with the collective-pitch lever. The mechanism which this lever controls is superimposed on to the cyclic-pitch mechanism from the control column in such a manner that, by raising or lowering the lever, an equal increase or decrease of incidence is imparted to the blades collectively (as on a variable-pitch airscrew), and quite separately from any cyclic- pitch variations from the control column. On a machine using a universally mounted swashplate for cyclic-pitch control, this may be effected by raising or lowering the swashplate mounting without interfering with its angle of tilt. The collective-pitch range through which the blades may operate is usually from about plus The diagram on the right shows that minimum and maximum flapping lie 90 deg behind the positions of minimum and maximum incidence. The two diagrams below illustrate, from the pilot's point of view, the sequence of events required for side ways fight. When the total rotor-force is tilted out of the vertical, the horizontal com ponent causes the helicopter to move sideways. At the same time, a coupie acts about the e.g. to tilt the fuselage; the time taken for this causes the fuselage tilt to lag behind the rotor tilt. ROTOR CENTRE- OF-PRESSURE EQUIVALENT POSITION Or MINIMUM FLAPPINC (LOWEST POINT REACHED BY BLADE) i_ STICK HELD TO LEFT POSITION OF MAXIMUM FLAPPINC (HIGHEST POINT REACHED BY BLADE)
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