The advantages offered by the variable-pitch propeller are not nearly utilized to their fullest extent in military and civil aeronautics at the present time. It is quite true that current types make possible a more or less satisfactory adaption dependent on construction and operating principles of the propeller to the wide speed and performance ranges of modern airplanes under takeoff, cruising and maximum speed conditions. For these purposes between 15° and 30° of pitch angle change are adequate. However, future developments will go far beyond these requirements.
By setting the blades at high pitch, for example, extraordinarily high flying speeds can be reached in accelerated glide descents without risking engine racing.
The feathering of the blades enables the rotation of the engine to be stopped in the event of engine failure, concurrently with the reduction of propeller drag to a minimum, a maneuver that has been found to be of particular value in the case of damage to the power plant of multi-engined airplanes.
By setting the blades at negative pitch angles, on the other hand, the propeller assumes the functions of an efficient air brake in dives and during the landing run without necessitating the additional weight of special brakes. Dives can thus be carried out at much reduced velocities which is important especially for military purposes. The normal landing run may be reduced by as much as two-thirds in some cases, a feature that is highly desirable when fairly small airfields are utilized.
Furthermore, the favorable effect of constant-speed operation on the different flight maneuvers, resulting from the maintenance under all conditions of constant propeller engine rpm should be noted. 1
However, for the materialization of these demands, the creation of a new and harmonized design of propeller hub was found necessary as well as of the control and operating mechanisms. Such a type, developed systematically and capable not only of meeting the present but no doubt also the future requirements mentioned above, is in production at the Escher Wyss works. Development work on the propeller was much facilitated by studies and suggestions made by the Institute of Dynamics of the Swiss Federal School of Technology (Prof Dr J Ackeret) and by the Technical Section of the Federal Military Department.
The firm of Escher Wyss is probably the only undertaking to have gained extensive engineering experience in the entire field of high-speed rotating machines (steam and water turbines, axial and radial compressors, blowers and pumps as well as variable-pitch marine propellers). The high speeds of modern machines of these types led to the adoption already years ago of blade designs resembling airfoil sections and in order to obtain the highest degree of efficiency under different operating conditions of blades controllable, mostly automatically, in actual operation.
The company's long activity in the development of such machines and the application of the experience gained in one specialized field to another offered a solid basis for the creation of a hub suitable for operation under the stress of high centrifugal forces, for the controllable mechanism and the governor apparatus of the new Escher Wyss variable-pitch propeller.
The Escher Wyss variable-pitch propeller 2 is operated purely hydraulically and permits, in principle, the execution of the maneuvers mentioned above and this with a small number of simple and sturdy parts. The entire mechanism consists of the actual control hub, the engine-driven governor and oil pressure generating unit, the latter two combined in a single casing, as well as the conduits leading from the governor to the propeller shaft.
In its design the pitch-control mechanism strongly resembles our thoroughly proven Kaplan turbines and screw pumps as well as our variable-pitch ship propeller types. As in their case, the pitch-changing forces in both directions, ie, for high- or low-pitch setting, are generated by positive oil pressure. The only source of pressure oil supply is the engine oil. The utilization of mass centrifugal forces for turning the blades has deliberately been refrained from. For propellers with a wide range of pitch adjustment and the speed and reliability they require in the blade angle change, only such adjustment methods are admittedly eligible as can be controlled from the exterior in every case, since blade positions must be passed through in which the forces tending to cause the blades to go into low pitch are very small or eliminated altogether due to the centrifugal forces.
An important new element in the control hub is the ingenious automatic locking device for all blades which is built into the control cylinder. Should the oil pressure fall below a determined value owing to damage to the oil supply system or also to deliberate release by the pilot, the locking apparatus instantaneously and in any position prevents any further axial motion of the control cylinder built into the hub and, thus, of the propeller blades.
The propeller is then ready to operate in the manner of fixed-pitch types. This provides the desired freedom from blade hunting around the longitudinal blade axis and makes possible in the case of even loads, eg, under cruising conditions, an automatic relief of the oil and servomotor systems and a corresponding economy in energy. The locking device operates so accurately and instantaneously that it responds also to the slightest load variations. When the propeller is employed as a brake producing negative thrust, such a safety device, which simultaneously prevents the blades from inadvertently returning to negative pitch settings, is highly important.
In a certain normal pitch range, which can be adjusted to take care of individual requirements, the Escher Wyss propeller operates as a constant-speed propeller. The pitch limit of this range which is automatically controlled by the governor can be passed only by deliberate action on the part of the pilot. As a result, the blades are set at the brake or feathering positions only when they are fully under control of the pilot, If not required, the extreme positions of the present type can be eliminated altogether by suitable adjustments to the control cylinder, whereupon the propeller operates as a normal, modern constant-speed unit.
The one-piece hub casing forms the blade-mounting structure and encloses and protects all the remaining moving parts. On a central, hollow guide tube which offers the possibility of firing through the hub a fixed piston is mounted. Above the piston and guide tube is attached the actual control cylinder. This cylinder is displaced along the guide tube by the boosted oil supplied and automatically controlled by the constant-speed governor pump; it reaches either face of the piston through corresponding bores in the central guide tube. At its forward end the control cylinder carries a ring with articulated connecting rods which transmit the pitch-adjusting forces to the blade sockets and thus cause the axial rotation of the blades. The simple construction employing connecting rods for the transmission of the forces gives a connection which operates without play for practically unlimited periods. The connecting rod principle has always been found to be a safe, simple and easy solution of the problem also in the other machines produced by Escher Wyss (turbines, pumps and blowers).
The entire locking mechanism is attached to the fixed piston in the interior of the hub and completely submerged in oil. Before the pressure oil reaches one or the other face of the piston it operates small spring-loaded auxiliary pistons whose motion actuates locking parts which can mesh with corresponding parts in the interior of the control cylinder. When the pressure oil arrives it first releases the spring loaded locking devices between piston and cylinder: only then can it flow into the lines leading to the forward or the rear chamber of the control cylinder and axially displace the cylinder. The entire apparatus is automatically operated by the governor oil but can be switched over to manual operation by the pilot at any time.
Although the Escher Wyss hub contains this additional apparatus which is not featured in current types of propeller, its exterior shape is not clumsy or bulky. Special care has been taken to provide facilities for the easy assembly and removal of the individual parts. The blades are readily screwed in or out from the outside without necessitating the dismantling of the controllable mechanism located in the center. On the other hand, the pitch-adjustment and locking apparatus may be taken out of the hub and replaced after the removal of the hub front inspection plate. This special feature of the Escher Wyss variable-pitch propeller offers considerable advantages in practical operation.
The propeller is basically designed for fitting on flanged or grooved shafts. In both cases the main components are identical, As a result of the methodical development and in spite of the wide pitch angle range the weight of the propeller is remarkably low. (The complete three-blade propeller for 1,000 to 1,200 hp weighs about 270 lb; the complete auxiliary unit about 9.5 lb) In the event of engine failure, or on the ground, the pitch angle can be adjusted by means of a small hand pump or by compressed air.
A special testing plant has been developed to permit of the stresses, which occur during flying under the most widely differing conditions. being exactly studied and measured and their influence on the various parts determined.
For load tests on the hub a measuring frame is used. At the three corners of the sturdy frame. freely movable hydraulic cylinders are supported. the piston rods of which are screwed into the hub under test in exactly the same manner as the blades of the propeller. By pumping oil under pressure into the hydraulic cylinders arranged in the corners of this frame, stresses corresponding to the centrifugal forces are exerted on the blades. For test purposes same can be increased manifold. The hydraulic loading cylinder and the rods are designed in such a manner that the adjusting mechanism in the hub also can be operated and tested under load. This check of the hub body primarily relates to measurements of the stresses occurring in the various parts of the casing. The stresses on the hub casing are as uniform as possible, being ascertained with the aid of a soap-skin model: the supporting surfaces of the blade thrust bearings are represented by wire rings between which the soap-skin hangs.
Apart from such static testing methods, overspeed tests can also be carried out in a special pit. With the aid of a small steam turbine, the hub can be rotated at high speed. In place of blades, steel stumps are screwed into the blade holders, the centrifugal force of which corresponds to that of the relative propeller blades. In this manner, the power required for such overspeed tests can be considerably reduced. For normal shop tests special test beds have been installed on which the propeller can be operated together with motor drive and governor under conditions approximating service operation.
Following the completion of this exhaustive test program, the Escher Wyss propeller has been adopted by the Swiss Air Force and is now produced in series for installation on fighter planes.
Notes:(1) Attention has been drawn to the great importance of all these possibilities, of which hardly any are yet exploited in practice, by;
Ackeret: "Probleme des Flugzeugantriebes in Gegenwart und Zukunft," (Present and future problems of airplane propulsion), Schweiz Bauzeitung, Vol 112, No 1 1938, Page 1
and "Verstell-propeller," Special number published on the occasion of the Centenary of the SIA, Swiss Federal School of Technology 1937, Page 161,
further "Constant speed-Luftischrauben". Flug-Wehr und -Technik No. 1 and 2. 1939.
(2) C Keller: "The Escher Wyss variable-pitch Airscrew" Escher Wyss News No 4/1938, Page 75,
or in Flug-Wehr und -Technik, Volume 1 No 2, February 1939.
"The Escher Wyss wide-range C P Airscrew" Interavia Supplement to No 717-718 dated 15. 7.40.
This article was originally published in the April, 1942, issue of Aviation magazine, vol 41, no 4, pp 100-101, 209-210.
The original article includes 7 photos showing the completed propeller and details of its construction,and 2 graphs showing stress distributions in the hub.
Photos are not credited, but are certainly from Escher Wyss.