Flight, December 1935
THE BREGUET GYROPLANE
Large Payload Claimed: Some Technical Considerations: Test Flights to be Continued.
By R C. WOOD.
M . LOUIS BREGUET, the well-known French aeroplane constructor, has recently been making trials of a new machine
which he has been developing, and which he terms a gyroplane. In this design the engine drives two rotors, mounted on a pylon one above the other and turning in opposite directions.
Recently, M. Breguet pointed out that the speed of commercial aeroplanes is limited at present by the necessity of low landing speeds, and that for this reason they cannot be loaded at more than 150 kg./sq. metre (approximately 31 lb./sq. ft.) of wing surface at the maximum, notwithstanding that they may be equipped with the best-known aeronautical devices to aid their lifting power.
In order to obtain increased speeds, together with suitable landing qualities, for commercial machines, M. Breguet calls attention to the formula represented by the gyroplane, which he believes can furnish an interesting solution to these questions. He points out that the gyroplane gives promise of not only attaining considerably higher speeds, but also of sustaining a much greater load per unit of lifting surface, which would furnish a substantial economy in the commercial operation. Furthermore, owing to the fact that the forward speed of the gyroplane is relatively independent of the rate at which the blades revolve, the machine can take off and land vertically.
The machine, as can be seen from the illustration, is an improved helicopter, in which the rotating blades, instead of the traditional airscrew, furnish the forward propulsion. As these blades are mounted on universal joints, they can also be used to ensure the directional and lateral control. The air pressures acting on them depend at each instant on the relative speed of the blades and on the incidence at which the latter are adjusted. These elements vary during each revolution of the blades, the speed of rotation being added at each half turn to the forward speed at which the gyroplane is moving. Unless precautions were taken, this fact would cause loss of stability. M. Breguet has overcome the difficulty by attaching the rotor blades to their central hubs by means of universal joints mounted on two orthogonal (right-angled) axes. Each blade is therefore free to adjust itself during each revolution according to the direction of the resultant forces acting on it, and can rise and fall independently, with an out-of-phase movement corresponding to the angle at which it is mounted. When the blades advance against the relative wind pressure created by the forward movement of the machine they are raised, and a contrary movement takes place during the succeeding half-turn when the blades recede before the same wind pressure.
M. Breguet has evolved the following formula for the lift of rotating blades of this description:-
where P is the weight lifted,
W is the power consumed,
D is the diameter of the rotor.
Taking into account the "solidity" of the rotor, i.e., the ratio of the actual supporting surface to the surface of the circle swept by the blades, the maximum theoretical value of q, when stationary and at ground level, is expressed, in metric units, by the very simple formula q = 0.628 pv, where pv is the efficiency of the rotor considered as a ventilating helicoidal fan.
Thus, for example, if the efficiency attains the value pv = 0.7, this maximum is 0,428.
It should also be noted that, owing to the forward movement of the gyroplane, the blades of the rotors are continually reaching virgin zones of air, which greatly improves their efficiency, so that one may hope for a maximum value of q approaching unity.
With the above facts in mind, M. Breguet makes a comparison between a monoplane and a gyroplane having the same maximum weight of 10,000 kg. (22,000 lb.) and both having a speed of 360 km/hr. (223 m.p.h.).
Taking a monoplane with a wing loading of 100 kg./sq. metre (20 lb./sq. ft.), an aspect ratio of 8, a wing surface of 100 sq. metres (1,076 sq. ft.) and a wing span of 29.3 m. (96 ft.), and allowing a 75 per cent, efficiency for the airscrew, it is found that in order to obtain the speed of 360 km/hr., allowing for the usual commercial custom of utilising but 60 per cent, of the rated power of the engines, the monoplane would require: -
3,150 h.p. at an altitude of 2,000 m. (6,560 ft.).
2,500 h.p. at an altitude of 4,500 m. (14,750 ft.).
2,000 h.p. at an altitude of 7,000 m. (23,000 ft.).
The power furnished at these altitudes is respectively 1,880, 1,500, and 1,200 h.p.
On the other hand, M. Breguet finds, according to a series of formulae which he has prepared from tests he has made himself and also had checked at the Eiffel Laboratory, that a gyroplane would require rotating blades with spans 17.50 m. or of 19.65 m., according whether it is desired to fly at a speed of 360 km/hr. at altitudes of 2,000 m. or of 4,500 m. respectively.
A rotor span of 17.50 m. (57.4 ft.) would thus require an actual blade surface of 14.3 sq. m. (154 sq. ft.), which, taking a "solidity" coefficient of 0.06, would correspond to a wing loading of 700 kg./sq. in. This calculation is based on the supposition that the maximum cross section of the parasitic drag surfaces of the gyroplane would be properly streamlined and not exceed 6 sq. m. in area.
Under the foregoing conditions, the axis of the revolving wing surfaces - constituted in reality by two airscrews each with hinged blades, mounted one over the other and each turning in a contrary sense to the other - should be inclined at an angle of about 9 deg. from the vertical, in order to furnish the propulsive force.
The gyroplane thus equipped and flying at a speed of 360 km/hr. (225 m.p.h.) at an altitude of 2,000 m., would require 1,470 h.p., which would be equivalent to 60 per cent, of a rated power of 2,450 h.p. This would be but 78 per cent of the propulsive power necessary for an aeroplane under similar conditions of speed and altitude.
M. Breguet further calculates that the disposable load available in the foregoing monoplane and gyroplane respectively, at an altitude of 2,000 m. and flying at a speed of 360 km/hr., would be as follows: -
Useful load available 3,120 kg. 4,390 kg.
For a still-air range of 2,000 km. (1,240 miles) these loads could be apportioned as follows :-
Fuel and oil 2,300 kg. 1,850 kg.
Fuel tanks 150 kg. 100 kg.
Available for payload 700 kg. 2,440 kg.
Total 3,150 kg. 4,390 kg.
These figures show a ratio of about 3 1/2 to 1 in favour of the gyroplane.