Teknisk Tidskrift haft.31, 4 aug.1934
Technical Magazine iss.31, Aug.4 1934
PRACTICAL DEVELOPMENT OF VERTICAL TAKE-OFF AIRCRAFTS
by K. G. MOLIN, engineer
A pair of machines of so called cyclogiro type — or more right to say — proposals of this — have came from more public data during last years, and it is motivated here to look through in brief some attributes of this type.
Its principles are not some news itself, because of projects of cyclogiros — or "paddle machines", if to use Rohrbach's term — were existing already in end of 18th century. Also, during 19th century paddle systems have occured from time to time, and beginning from 1920 at least seven different types can be found in patent documents. The two types, which looks to be the most promising: one builded by Swedish-French engineer Strandgren with "Liore & Olivier" and one designed by worldwide-known German aero technician Rohrbach.
Strandgren's patent was granted in 1924, and one machine seems to be ready in construction and being tested now in factory. Some details about principles are however not published, and it try therefore near in hands to it brief retell Rohrbach's calculations for his machine, which is also under construction. Already Rohrbach's name which is one of this skillful aeroengineers guarantees for to his work has correct basis — promises that this machine which is being constructed maybe not immediately become in all details perfect. His calculations have however except for that are checked in German Laboratory of Flight and National Advisory Committee for Aeronautics in USA and is recognized as reliable.
How Rohrbach's cyclogiro must look follows from fig.11, which indeed is not correct in every detail picture but however gives good notion about principles. In approximately same position as wings of a high-wing monoplane, is located a shaft on both sides of fuselage, and rotates around this shaft by means of set of arms like the paddle wheels in a wheelboat by means of set of arms three long, narrow wings. Shaft is driven via differential transmission of a motor — or some motors if a man from a point of view of safety will desire it — placed inside of fuselage. Every little wing is attached to the central shaft by means of set of arms.
In cyclogiros as well as in autogiros must occur a variation of angles of attack for to compencate the different speeds and accordingly to airflows. This variation of angles of attack is calculated so, that both lift and thrust are developed during a most part of revolution. Principles of this follows from fig.12. Direction of rotation is those, that wings in upper part of revolution are moving forward in the direction of flight. Wings' speed of rotation is 1.5 times more than machine's maximal speed and about 2 times more than a normal speed. Maximal rotation speed of rotors is 420 rpm, that corresponds to peripherial speed of 80 m/s. Minimal allowable speed of rotation at which machine is still kept in air is 270 rpm.
Of fig.12 which explains four positions of one of the blades, following variations of angle of attack, same as that occurs at horisontal flight with normal speed. In all positions blade hence produces forward force and during practically full revolution — except for in back position — vertical force component. Rotating wings gives hence both lift and dfaft; propeller for movement in direction of flight thus is not necessary.
Design of organs for variations of angle of attack is still not known, but probably the method is used where inside rotor shaft camshaft is placed, which in its turn affects pushing bars placed inside the streamlined arms connecting the wings and rotor shaft. In any case this adaptation is adjusted by steering levers, such that a change of rotor system is being achieved in result. Cyclogiros can thus move itself in any direction. To move inclinely up and inclinely down, directly up and directly down, and even backward. If motors has reduced rotation speed or stop, rotors automatically disconnects and autorotates, and a machine, just like the autogiro, can perform gliding or vertical descent. All these maneuvers are carried out with help of an ordinary lever of "high-rudder" type. Lateral movement is achieved by means of that due to differential transmission different speed of rotation is given to rotors, and simultaneously with it oscillatory movements of blades change in appropriate way. Then one of rotors creates more draft than another. Rudder thus is not needed, but maybe it is first type of machines suitable to facilitate course stabilization with one rudder. — The control over the machine roll turns out by means of entering of appropriate difference in lift of two rotors.
Rotors' movement will, indeed, infuence on fuselage stability, but according to calculations variations in this respect will not achieve more than 12°. Considering fuselage position in horizontal flight with maximal speed as neutral position, at maximal vertical climb nose will rise on 6°; behaviour at vertical descend will be opposite. It is considered that somebody in fuselage will not be afraid during flight.
As to sensitivity to aerodynamic stress, the cyclogiros like autogiros are relatively favorable. Normally, aerodynamic forces reach only 12% of centrifugal forces, maximal, as counted, no more than 18-20 %. Thus it is considered that risk to overload a wing system by abrupt maneuver is absent. — But cyclogiros can't be so tolerant to puffs, gusts etc. as autogiros, you see, in cyclogiros whole wing system is rigidly managed and it can't avoid of transmission of shocks to fuselage (one/near). — From other side, one can overload cyclogiro without a risk, but indeed its achievements will become worse. The calculated flying properties are represented below.
Rohrbach's cyclogiro has a lenght 8.6 m, height 4.3 m, span 10 m. Blades has proportions 14:1 and length 4.4 m. Power is 240 hp, possibly from two motors. Calculated empty weight is 680 kg, useful load (including 3 persons) is 270 kg; gross weight hence is 950 kg.
With this weight maximal speed is 200 km/h, travelling speed at RPM 75% is 170 km/h, minimal speed is 0 km/h, maximal backing speed is 30 km/h. Ceiling is 4500 m in forward flight and 500 m in vertical climb. Range of flight is 400 km with passengers and 700 km without passengers.
With an overloading of 250 kg, that is, with 1200 kg take-off weight, one has to refuse vertical climb and stationar hover without loss of height. Minimal speed becomes 21 km/h, maximal speed — 190 km/h, ceiling — 2700 m. Range of flight is 1050 km with 2 passengers and 1550 km without passengers.
Power loading for a possibility of vertical climb is 4 kg/hp, for minimal speed 27 km/h — 8 kg/hp, and for minimal speed 54 km/h — 10.8 kg/hp. — Efficiency of rotor system has appeared sufficiently higher than of a propeller; despite of losses in transmission from motor to rotor shaft its efficiency is about 85%, which is 15% more than one of propeller.
During researches which NACA made with rotor according to Platt's system, was made a sketch of a machine with gross weight of 1360 kg (3000 lbs) and motor power 300 hp, also were calculated rate of vertical climb 210 m/min, and maximal rate of climb with simultaneous forward movement 460 m/min; this last value is about 30% more than what airplane with same power loading can show.
About Srandgren's machine few calculations concerning the aerodynamic quality are still known. Principles of wingrotors functioning are the same as in Rohrbach's cyclogiro, though Strandgren probably began experiments sufficiently earlier. However, only in last year he at last began construction of the machine.
Experiments with models were carried out since 1924, first in aerodynamic laboratory in Saint-Cyr and subsequently with "Liore & Olivier", where even real-size experiments were undertaken. The most part of charges was paid by Societe d'Expansion Franco-Scandinave.
Experimental machine exterior is represented on fig.13; it seems can hardly be called streamlined. Rotors has a diameter of 6 m and consists of five wings everyone. Every wing has a span of 245 cm, depth of 40.8 cm and thickness of 3.8 cm. It is made from duralumin; its weight is 5 kg. — Motor, 130 hp Clerget, is placed in a fuselage. Calculated gross weight is about 600 kg.
At a rotation speed of 120 rpm rotors produces a lift of 800 kg. Rotors have passed a test at 180 rpm despite of some difficulties.
Strandgren's rotor functioning seems to be in all sufficient corresponding to what is said in reports of Rohrbach's project.