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Aviation History
1947
1947 - 1888.PDF
FLAPPING HINGE BLADE LINK "RfcHK* OCTOBER 30,1947 Detail of rotor head showing support, control connections, reduction gear, flapping and drag hinge articulation of blades, and the interesting tensile- cum-torsion anchorage of the blades. FLEXIBLE COUPLING TPANSMISSON BRISTOL HELICOPTER Before going on to describe in detail the structural design of the rotor, we might with advantage pause briefly to review the basic forms of blade motion and the reasons for such motions. We shall deal only with general principles, the design theory being far too complex a subject even for superficial treatment within the space at our disposal. The helicopter can be said to have two fundamental forms of flight, these being respectively vertical motion and translational motion, and although quite obviously these two forms can be superimposed one upon the other, they are, nevertheless, separate and distinct and entail quite separate control input motions on the part of the pilot. Vertical flight is achieved by collective change of blade pitch; that is to say, the pitch setting of all blades is simultaneously increased or decreased to provide greater or lesser lift and so cause the aircraft to rise or descend vertically. This is controlled by what is called the collective pitch lever placed between seats in the cockpit so that it is pulled upward to increase pitch and pushed downward to decrease it. The other form of blade motion is called cyclic pitch change which, as its name suggests, indicates that the angle of attack, or pitch, of each rotor blade will progressively increase and decrease during a cycle, or single rotation. Reference to the schematic diagram will render understanding of this principle quite simple. As an example, we will assume the aircraft to be flying in a due Northerly direction and the corresponding pitch change of one blade will then be as follows : when the blade is pointing North the pitch angle will be a minimum and, as the blade rotates through East its pitch will pro- gressively increase until, when pointing South, it will be at its maximum value; from South the pitch will progressively decrease through West until it is a minimum once more at North. This action is applicable to each blade, and we can summarize it by stating that the point of maximum blade pitch will always be at 180 deg. to the direction of flight. Again, for example, with the aircraft pointing Northward but flying sideways to the West, blade pitch will be at its maximum when the blade is pointing East and at a minimum when pointing West. These blade motions result in a relationship between the thrust axis of the rotor and the centre of gravity of the aircraft causing the machine to move in the appropriate £ direction. Thus, when all' blades maintain the same pitch throughout their rotation, the rotor thrust axis passes vertic- ally through the centre of gravity of the aircraft and thus the machine will hover or rise or fall vertically. When, how- ever, cyclic pitch change is introduced, although the rotor hub does not itself tilt, the blade articulations, in the form • of flapping hinges, allow the blade to rise as its pitch increases relative to its nominal position at minimum pitch, and thus the thrust axis of the rotor as a whole will be displaced from the aircraft's centre of gravity, so causing the aircraft to move in a direction opposite to the displacement. Referring back to our illustration of cyclic pitch control with the aircraft flying Northward, since the maximum blade pitch is achieved when each blade points Southward, the thrust axis of the rotor will be inclined rearward at the base so that it passes downward on the Southern side of the centre of gravity of the aircraft, so inducing the machine to move in a Northerly direction. The diagrams on the following page indicate this and also show the relative position of thrust axis to centre of gravity for other motions of flight. As stated previously, although collective pitch change and cyclic pitch change are separate entities, they can be simultaneously applied so that they aircraft can ROTATION Schematic illustration of cyclic pitch change showing how, when aircraft is flying Northward, in rotation of a blade from North via East, South and West back to North, the pitch angle progressively increases from a minimum at North to a maximum at South, and then progressively decreases back to its minimum at North. The pitch angles shown above are exaggerated.
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