For the second occasion during the 1951/52 session of lectures, this Association has been honoured by the presence of an eminent member of the United States aircraft industry, since this evening we have Mr FRANK PIASECKI as our lecturer

You all know Mr PIASECKI very well by name In 1940 he founded an engineering group known as the P V Engineering Forum This later became the Piasecki Helicopter Corporation, and today he is Chairman of the Board of Directors In 1943 the first single rotor helicopter designed by that small band of people emerged, it was the PV2, and was flown successfully by Mr PIASECKI himself Quick to appreciate the military possibilities of a helicopter capable of carrying a large load and with ample cabin space, he initiated and led the development of the big PV-3 tandemrotor helicopter, and the United States Navy, recognising the merits of the design, awarded a contract for the supply of a quantity of machines of this type

In studying the aerodynamics of rotor blades we are often uncertain how far we may apply quantitatively the data obtained from wind tunnel tests of aerofoils Of course, the rotor blade behaves qualitatively like an aerofoil, but I sometimes feel doubtful about calculations of the effect of detailed changes in blade design, say for example, the effect of taper on performance, and wish to stress the desirability of wind tunnel tests by which some at least of these questions could be settled more easily than by flight tests

I have found in preparing this lecture that covering the whole field would be impracticable I intend to concentrate mainly on the compressibility problem which, closely associated with stalling, seems to me to be the principal aerodynamic problem facing the blade designer today

In designing rotor blades their strength on the ground, their torsional stiffness, their aerodynamic shape and smoothness, etc, have all to be considered, but the primary purpose of rotor blades is to sustain helicopters in controlled flight in the air, and one of the most difficult problems arising is that of assessing the bending stresses to which rotor blades are then subjected The present treatment is, therefore, largely concerned with the estimation of the bending stresses to which rotor blades are subject in flight

The loads tending to bend blades are first taken as being identical with those to which an idealised inflexible blade is subjected These loads are analysed along and perpendicular to the blade length, and equations for the bending equilibrium of flexible blades in the lift and drag planes are developed Difficulties associated with the solution of these equations are overcome by the construction of special solutions, called “Type Solutions,” which satisfy these equations and the end support conditions of the blade These type solutions give a physical picture of the problem and help in the calculation of blade stresses

The differences between the loads on an idealised inflexible blade and flexible blades are discussed In the lift plane it is demonstrated that they are, in general, small, but the possibility of dynamic amplification of higher harmonics in the loading system is brought out, and it is suggested that there may be a need for further work on this aspect of the problem, particularly for high speed helicopters

In presenting this paper an attempt has been made to outline some of the additional problems which are connected with the design and construction of rotor blades driven by jets at the tips

The complications which arise when ram or pulse jets are fitted are not discussed in detail as it is considered that these problems are also included among those which occur when pressure jets are used as the means of propulsion

This short paper will therefore be confined to a discussion on the engineering of rotor blades driven by pressure jets at the tips

This short talk describes the manufacture of the metal rotor blades now going into quantity production for the S 51 and S 55 Helicopters at the Westland Aircraft Works These blades are a Sikorsky development so that there are many details both of design and construction which I am not at liberty to discuss On the other hand I hope to give a general indication of the way in which they are made, and of the scale of facilities which are available They are the first all metal helicopter main rotor blades to go into quantity production outside the United States

The backbone of the blade is an extruded light alloy spar which begins life as a billet in the extrusion presses at Messrs Birmetals Since the blades of the S 55 Helicopter use the same type of extrusion as the smaller S 51 blades, in future all extrusions will be delivered to us in the longer length and will be cut down for the manufacture of S 51 spars Each extrusion contains two shear webs inside the main contour and these run the entire length of the spars

In some ways I am an intruder into this discussion on the Design and Construction of Rotor Blades, because my paper deals with what happens to rotors long after they have been designed and constructed, when, that is to say, they are passing through the hands of the test pilot, the operating pilot and the maintenance engineer I shall try to confine myself to those parts of the rotor’s career which have repercussions on design and construction

Tuning, whether of an engine or a musical instrument, consists of adjusting all the variables available in a systematic manner calculated to make the device as a whole work in the best way possible If the device is a piano, this may mean doing what the piano tuner does on his three-monthly visit, but it may perhaps involve a bit of what happened when the earlier types of piano were developed into what J S BACH called his “well-tempered clavier”

Our Company has been making wooden helicopter blades of different types since 1937 I want to tell you something of their materials and manufacture and would welcome questions in the course of my talk

Fig 1 shows a cross-section of the big Cierva blade as used on the W 11 This is monocoque construction Notice the centre of gravity of the blade and the incidence of the tip superimposed on that at its widest section

The main spar is built up at one-inch thick blocks of compressed wood laminated of 1/32 inch Canadian birch veneers bonded with resin film and pressed to about 2,000 lbs per square inch at 300°F The density is about 1 2

As a matter of interest, by laminated wood we mean that the grain in all the laminations runs parallel, whereas in plywood the grain of adjoining laminations is at right angles

The advantages of constructing resin-bonded composite metal/wood or metal/fibrous helicopter blades, according to the design and by the “cold” assembly process developed by the author, may be summarised as follows

It is easy to demonstrate from first principles and also from the results of many tests that metal-clad sandwiches, suitable for helicopter rotor blade faces, fuselage shells, and many other aircraft components, may be designed with a bending stiffness from 10 to 40 times the stiffness of a simple sheet of metal of the same specification, size and weight This means, of course, that for an aerodynamically loaded surface, the spacing of ribs, stringers or other substructural members may be up to about three times that for the simple sheet with equal flexural deformation A simple calculation will give the weight-saving which can be effected by the elimination of such supporting members—and this saving would be available for further stiffening of the surface if desired