Microwave Helicopter

AN EXPERIAmNTAL MICROWAVE -POWERED KELICOPTER

W. C. Brown J. R. Mims N. I. Heenan

RAYTHEON COMPANY Microwave and Power Tube Division

Super Power Operation Spencer Laboratory

Burlington, Massachusetts

Summary

Microwave power transmission and helicopter technologies have been successfully combined to pro- duce a hovering vehicle which is held aloft solely by power derived from a microwave beam. New, efficient, and lightweight antenna and rectifier technologies allow the helicopter to support its energy-capture system and a substantial payload in addition to its own weight. Projections for further development of the device in terms of technological improvements a re made.

I. Introduction

A microwave-powered helicopter may be defined as a vehicle which rides on a microwave beam and extracts from the beam all the energy needed for its propulsion.

The purpose of this paper is three-fold: first, to describe successful microwave-powered helicopter experiments performed at the Spencer Laboratory of the Raytheon Company under contract with the Rome A i r Development Center of the Air Force Systems Command*; secondly, to discuss significant design parameters of the microwave-powered helicopter; and finally, to project the impact of available, but as yet, unused technology upon the payload fraction of the microwave-powered helicopter.

The motivation for the study and development effort leading to the microwave-powered helicopter flights described in this paper is the concept of a high altitude platform which can be kept on station for per- iods of weeks o r even months by means of a microwave beam, performing communication and other useful functions that could be associated with an extraordin- arily high transmitting tower. The near-term objectives, however, have been to demonstrate that a heav- ier-than-air vehicle of modest size could be kept aloft

solely by means of microwave energy and to gather data and perform analyses which would be of benefit in pre- dicting the practicability of such a device at high altitudes and in programming its further development. A s a result of the successful experimental flights and the favorable nature of other data that has been collected, it is reasonable to expect that high altitude microwave- powered helicopters will be developed within the near future and that they will accomplish practically useful missions.

The significance of the microwave-powered helicopter development, however, extends considerably be- yond its potential to provide a new and useful aerospace capability. It represents the first application of power transfer by microwave beam, an emerging technology more basic in nature and with many potential applications, but needing an initial application to speed its growth and acceptance. The microwave-powered helicopter also represents an interdisciplinary development of unusual scope which cuts across the established charters of many professional, corporate, and govern- ment organizations. It represents for the first time the penetration of microwaves into the propulsion aspects of aircraft design and into the area of aerospace component activity generally referred to a s '?energy sources'f.

Historically, the development of many new technologies after the first modest experiments have been slow because of the need for the development of better components. Fortunately, in the case of the microwave- powered helicopter, the component technology is in an advanced state and there is "on-the-shelf" technology in microwave power generators, antenna technology,

*The microwave-powered helicopter was demonstrated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

to the press and other communication media on 28 October 1964.

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and microwave power rectification which will permit a rapid escalation of the microwave-powered helicopter development.

In the following material, the experimental microwave -powered helicopter and its successful flights are described in Part II. This is followed in Part III by a discussion of some of the major design parameters and components of the microwave-powered helicopter, leading to an evaluation of the payload potentialities of the microwave-powered helicopter in Par t l37.

II. Summary of the Experiments

The microwave-powered helicopter experiments to be described in this section were performed at the Spencer Laboratory of the Raytheon Company, a t Burlington, Massachusetts, under a contract with the Rome Air Development Center. They are an outgrowth of internal studies within Raytheon Company in the areas of power transmission by microwave beam1* 2s and in microwave-powered vehicles that have been con- ducted over a period of years.

A microwave-powered helicopter system consists essentially of two major parts, (1) a microwave beam system from which the microwave-powered helicopter obtains power for its support, and (2) the microwave-powered helicopter itself. The microwave beam system consists of a source of microwave power and an optical system whereby this power is focused into a narrow beam. The microwave-powered helicopter consists of a means of extracting rf energy from the microwave beam, a means of converting this energy into dc power, a dc motor, and a helicopter rotor. These various elements are s h o p to comprise the overall system as shown in Figure 1.

The experimental system made use of all these elements and in addition employed a tethering system which kept the helicopter positioned on the beam at all times while providing no vertical support. This system is shown in Figure 2.

The microwave beam system, shown in Figure 3, made substantial use of experimental equipment1 that had been used for earlier investigations of efficient microwave power transfer. A magnetron oscillator generating from three to five kilowatts of power at a frequency of 2450 mc/sec was used as the power source. A 9.5 foot diameter ellipsoidal reflector with a focal length of 52 inches was used as the beam-forming re - flector. It was illuminated by means of a diagonal horn.

The microwave-powered helicopter is shown in Figure 4. The six-foot diameter rotor is driven by a universal fractional horsepower motor which is adapted from an ordinary electric drill. A special gear box em- ploying ball-bearing construction throughout is used to

reduce the high shaft speed of the drill to the 400 rpm. of the helicopter rotor. The rotor power for helicopter takeoff is 0.105 horsepower. The weight of the helicopter without any payload is 5.25 lbs. At the saturation level of its antenna and rectifier system, it can support a payload of 1 . 5 Ibs.

The most novel feature of the helicopter is the combination receiving antenna and rectifier which is made up of over 4000 point-contact silicon rectifier diodes. The antenna is designed with great consideration given to keeping its weight a s light as possible. The dc power output of the combination antenna and rectifier is 280 watts; 180 watts is the minimum power at which the helicopter will fly.

The microwave helicopter is shown in typical flights in Figures 2 and 5. The helicopter typically took off from its support which was located twenty-five feet from the transmitting antenna and climbed to an altitude of fifty feet. The special three-wire tethering system which was used to keep the helicopter over the beamand to supply the counter torque to prevent the body of the helicopter from spinning was held in tension between con- crete posts in the ground and a heavy steel beam sup- ported from the top of the building.

Many flights of the microwave-powered helicopter using microwave energy a s the sole means of support were made, including one of ten continuous hours at an altitude of fifty feet. This flight was a required objective of the contract. There was no observable deterioration in performance after this period of operation.

III. Description and Performance of the Major Components of the .Microwave-Powered Helicopter

The microwave-powered vehicle may best be analyzed by a consideration of the major parameters that must be considered in its design. As shown in Figure 6, thelift provided by the rotor of the helicopter in steady flight is just equalled by the weight of the motor, rotor, antenna and rectifier, control system, counter torque provision, and payload. Conspicuously missing is the weight of any fuel. In most cases, it will be desirable to maximize the payload by getting a s much lift as possible from a given amount of shaft horsepower, and to reduce the weight of the various components which a re required to supply that power and the weight of the control system and the counter torque mechanism. It will be found in Section Tv that very high ratios of payload to total weight may be obtained by taking advantage of the best available technology.

The following material concerns itself with adis- cussion of qualitative and quantitative aspects of vertical lift of the helicopter rotor, the electrical motor, andthe combination antenna and rectifier, with particular

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emphasis on their applications to the experimental microwave-powered helicopter discussed in the previous section.

Helicopter Rotor Design

The rotor of the helicopter provides upward lift by increasing the velocity and therefore the momentum of a mass of air that Bows downward through the rotor. In the ideal case where the rotor is operating with 100% efficiency, the relationship of the upward thrust to the shaft horsepower, the rotor diameter, and the air density is given by the equation --

L = 124 (PR)2/3 P ~ / ~ * d e r e ( 1) L = upward lift, pounds P = power supplied to rotor, horsepower R = rotor radius, feet P = mass density of air, slugs per cubic foot.

It will be noted in this expression that the lift for a given power input is proportional to the two-thirds power of the rotor radius. The implications of this may be made more evident by expressing Equation (1) in terms of ''power loading" and "disc loadingT1. The "power loading" is defined as the lift divided by the shaft horsepower while the 'Idisc loading" is defined a s the lift divided by the disc area, aR2.

If we set p equal to sea level conditions, the relationship between power loading and disc loading for a 100% efficient rotor is --

Power Loading = 38 (2) Disc Loading

This ideal relationship is shown as the solid curve in Figure 7. It will be noted that for low values of disc loading, a great deal of lift may be obtained for a small value of horsepower. However, there are practical reasons such as weight of the rotor, the lower lift to drag ratio represented by the smaller Reynold's number of the slowly moving rotor, and the tendency of the blade to stall a s it moves away from the Rind, which prevents very high power loading.

Helicopter rotors never attain the ideal relationship shown in Figure 7 and given by Equation (2). It is the general practice to describe the quality of a helicopter rotor by rewriting Equation (2) to the equation below:

Power Loading = 38 M (3) Disc Loading

where the factor M is the figure of merit of the rotor. Very good rotors sometimes approach an M of .75 by using a tapered and twisted rotor and by operating at maximum lift to drag ratio. The simple rotors used on

the microwave-powered helicopter exhibited an RI factor of about .55, as represented by the actual data on the rotor as shown in Figure 8. Liftoff occurred in the region of power loading of 50 lbs. per horsepower and disc loading of . 168 lbs. per square foot.

I t may also be observed from Equation (1) that the lift of a given rotor varies as the two-thirds power of the shaft horsepower, assuming that the value of M remains constant. A plot of the lift a s a function of the shaft horsepower delivered to the rotor for the experimental microwave-powered helicopter is shown in Fig. 9, together with a plot of the theoretical two-thirds power characteristic. Although the data taken on this helicopter blade was limited to a lift of 8 lbs. by the available drive power, other similar rotors have been tested up to thirteen pounds of lift without failure and it is probable that such blades would absorb a full horsepower and give a lift of about twenty pounds.

The rotor used on the experimental microwave- powered helicopter utilized an NACA 0015 airfoil, was 6.3 feet in diameter, and had a chord of 2.5 inches. The blade was constructed by laminating balsa around an aluminum spar. The total blade weight was .84 lbs. The coning angle of the blade under takeoff conditions was approximately 80.

The Combination Receiving Antenna and Rectifier

The key to a successful application of the transmission of microwave power to aerospace vehicles lies in the interception and rectification2 of microwave energy with devices that have a high ratio of power- handling capability to their weight, are reasonably efficient, and can be cooled easily without adding weight or other complication. An additional practical need is for the capture antenna to have nondirectional characteris- tics in order to dispense with weight problems con- cerned with maintaining dimensional tolerances and proper pointing of a directional antenna.

The first approach to the solution of this problem was the concept of breaking up the receiving aperture into a number of smaller horns o r parabolic reflectors, each terminated with an efficient rectifier of sizeable power- handling capability. Such a concept still left considerable directivity in the array and introduced a problem of dis- posing of the dissipated power in the rectifier. Neither one of these problems is a severe one with land-based installations but both of them represent severe problems in an aerospace environment. Nevertheless, the first attempts to fly a microwave-powered vehicle were made with this approach and the practical difficulties encountered emphasized the superiority of a different approach not as far advanced but with the inherent capability of providing all of the requirements of an aerospace receiving antenna and rectifier.

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The basis for the new approach was an outgrowth of work on the use of the point contact silicon semiconductor diode at Purdue University2. In this program of work it was determined that the silicon point-contact diode made an efficient rectifier but that its power- handling capability was so low that it would require twenty thousand diodes, representing a weight of four pounds without any supporting structure included, to supply a kilowatt of power. This was entirely too much weight for the longer-range objectives of microwave- powered helicopters, but there was substantial hope that the ratio of power-handling capability to weight could be improved by a diode development program.

The development which precipitated serious consideration of the use of the semiconductor diode for an airborne vehicle was the determination by a Raytheon- sponsored program that the diode could be successfully incorporated into an antenna array consisting of half- wavelength dipoles spaced a half-wavelength from each other, a quarter-wavelength in front of a metallic reflector, and each terminated by four diodes in a bridge- rectifier configuration as shown in Figure lo2. Such an array was found to have somewhat better directivity than a single-half-wave dipole, as shown in Figure 11, and the combined capture and rectification efficiency of such an assembly was found to be close to 50%.

While a combination antenna rectifier of the half- wave dipole configuration should have some direct application, the power-handling capability of such anarray using the diodes then available was only five watts per square foot which was entirely too small for an experimental microwave-powered helicopter. Attention was therefore directed to packing many more diodes into a given area, leading to the arrangement of bridge rectifiers shown in Figure 12, and the general concept of the diodes representing an admittance sheet in space. The construction of a typical six-inch square module of 280 diodes which gave a power output in excess of 14 watts when inserted in the antenna is shown in Figure 13.

The admittance sheets represented by these strings of diodes are not well matched to space, however, nor are they self-supporting in a large antenna array. The conductance represented by these sheets is about ten times that of space so an inordinately large amount of power is reflected from them. On the other hand, very little power is transmitted through such an admittance sheet.

Fortunately, a solution was found to both the matching problem and the mechanical support problem by placing a grating consisting of a plane of parallel rods in front of the plane of diodes and adjusting it to achieve a maximum saturable power output from the diodes. Spacers placed between the rods and the diode modules provided support for the plane of diodes. As a result of this program, the total weight of the two-foot square, 4480 diode, self-supporting antenna array with

an operating level of 250 watts was just slightly over two pounds, representing a specific combined weight of antenna and rectifier of eight pounds per kilowatt.

Another approach to the combined antenna-rectifier problem would have been to terminate each dipole of a half-wave dipole array with a large number of rectifier diodes to make the power-handling capability comp- arable to the approach that was used. This approach was unattractive because of the cooling problem that might arise with such a high packing density of diodes. The antenna is so placed that the rectifier diodes are auto- matically cooled by the downwash of the helicopter rotor, but since the antenna is located toward the center of the rotor where the downwash velocity is low, it is desirable to deploy the diodes in such a manner as to maximize the exposed area of each diode and its leads.

The combined antenna rectifier approach that was used was not only a key element in making the first microwave-powered helicopter flights a success but because of a recent breakthrough in the specific weight of semiconductor diodes, it will undoubtedly be the con- tinuing approach for aerospace applications. The specific weights of the new Schottky barrier diodes are less than one-half pound per kilowatt making it possible to think in terms of specific weights of the combined antenna and rectifier of less than two pounds per kilowatt. Another aspect of these new diodes is their higher efficiency and their much smaller size in relationshipto their power rating which -will reduce their drag in wind and in the slipstream of higher-velocity rotor down- washes.

Instead of being a source of unreliability, the large numbers of diodes employed actually provide a high degree of reliability through the redundant nature of the parallel series connections within each module. A s shown in Figures 12 and 13, each leg of one of the twenty-eight element bridge rectifiers contains seven diodes. If one of these shorts out, the normal voltage drop across this diode is divided among the six remain- ing diodes. If one of the diodes opens up, the strings are in such close proximity to each other that adjacent strings of other bridge rectifier assemblies take the additional load. It is possible to have a score of open connections or shorted diodes in the individual strings, located at random throughout the antenna, without a severe degradation in performance.

The module concept that was employed in the construction of the microwave-powered helicopter antenna provided a high degree of flexibility in matching the antenna to the motor load. The performance of the individual modules themselves a re highly insensitive to a wide range of loads applied to them, as shown in Fig. 14, in which rslative dc power output for fixed rf input is plotted as a function of load resistance. A very non- critical optimum performance of about thirty volts and half an ampere was indicated for each module. It was

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therefore convenient to form sub-groups by combining four modules in parallel and then to connect four of these sub-groups in series to provide two amperes of current at a nominal potential of 120 volts.

The vibration problems of such an antenna are severe, especially when the helicopter with its rigid rotor is tied into a tethering system and there is a brisk wind. A moment is applied to the shaft each time one of the rotor blades comes into the wind, and the flexibility of the tethering system is such a s to allowa substantial deflection of the helicopter around one of its axes. The result is a whipping action which is transmitted to the antenna. Slow motion pictures of some of the original structures showed large amplitude vibra- tions of sections of the diode assemblies within the modules. Eventually, the strong welded joints between the diodes would break. This problem was eventually solved by better support of the diodes within the antenna, and a bracing of the antenna with either balsa strips or nylon line. These materials also introduce a high damp- ing coefficient to help damp out higher frequency modes of oscillation. The effectiveness of this approach was demonstrated by the ten-hour continuous operation of the microwave-powered helicopter with only one unim- portant weld break occurring even though brisk winds and moderate rainfall were encountered during a substantial portion of the run.

The Motor

The motor that was employed for the microwave- powered helicopter experiments was adapted from the electric drill that is commonlyused as a household tool and that can be bought in any hardware or department store. These motors can be operated on either alternating or direct current. They have brushes, and while this is one of their disadvantages for any high altitude, long duration flight application, it did not represent a handicap in the experiments described in this paper. The motor frame was reoperated to cut as much weight from it a s possible. The gear reducing arrangement was discarded because of its relatively heavy weight and because it did not provide the proper speed reduction. In its place was substituted a specially designed and constructed lightweight unit with hardened gears and ball bearings which operated at high efficiency and gave a speed reduction by a factor of 40. This permitted the motor to operate at its rated voltage and somewhat below its rated current for most flight conditions.

Even though as much weight as possible was saved by trimming weight from the motor, the specific weight of the motor itself was 13.5 pounds per horsepower. However, this is far from the state-of-the-art for a motor, particularly for one of the alternating current type. By designing an induction motor to oper - ate at a speed of between thirty and forty thousand rpm and taking advantage of the best core materials and high temperature insulation, it should be possible to have specific weights of about a pound per horsepower for

motors of a few horsepower and about half of this for larger-sized motors. By taking advantage of higher speeds and the same improved materials, dc motors could be made with specific weight about twice those for ac motors. The efficiencies of these motors will be in the 60% class, whichis somewhat below the efficiencies of lower speed, more conservatively rated motors, but still better than the 45%efficiency of the universal motor used for the microwave-powered helicopter experiment.

IV. Analyses of the Payload Fraction of the Microwave-Powered Helicopter

Many new technologies a re introducedbyan experimental demonstration which taxes the component technologies to their maximum. Later development of the major technology is then gated for many years while improvements are being made in component technologies. The development of the aeroplane is a classic example of this situation.

Fortunately, this is not the situation with respect to the component technoIogies of the microwave-powered helicopter. Four kilowatts of microwave power were used whiIe 400 kilowatts were available on the sheIf. Rectifier diodes with a power-output-to-weight ratio of 250 watts per pound were used while diodes with a ratio of 2500 watts per pound and higher efficiency are now available. A commercial drill motor and gear box with a power-to-weight ratio of 0 . 1 horsepower per poundor less and an efficiency of 50% was used while "on-the- shelf" technology could engineer a motor and gear box with 0.5 horsepower per pound and an overall efficiency of 70%.

The use of this 'fon-the-shelftf technology would immediately improve the performance of the helicopter and provide a payload fraction of close to 50%evenafter making allowances for increased weight of the vehicle by having to add a servo-mechanism system andaprovision for counter-torque.

With the use of Figure 1, the payload is seen to be as follows:

%bayload = -(WRotor fWhIotor + WRectenna (4)

' nbervo+WTail) where

%bayload = Wt. of Payload

L = Lift of Vehicle

WRotor

Motor

%ectenna

"servo

"Tail

= Wt. of Rotor and Swash Plate

w = Wt. of Motor and Gear Box

= Wt. of Receiving Antenna, Rectifiers, and Power Conditioning

= m7t. of Position Sensing, Servo-mechan- isms, and Telemetering

= Wt. of Tail Rotor and Boom.

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The relationship between payload and other parameters is more useful if i t is expressed in terms of the specific lift and weights, that is, lift per horsepower, weight of rotor per horsepower, etc. We rewrite the formula with lower case letters, noting that the specific weights a re not completely independent ofabsolute horsepower and that the lift of a given rotor varies as the two- thirds power of the shaft horsepower, making it necessary to specify the rotor for the particular value of horsepower being considered.

Proceeding to do so: (5)

W = P - ( w +w rotor motor rectenna servo+ hi$ + W +W

payload where the P and w's are specific lift or weight per horsepower.

For a small helicoptzr P can be about 20 Ibs/ hp for one horsepower input. In the Raytheon helicopter demonstration an P of 54 lbs/hp was obtained for a power input of 0.15 horsepower as shown in Figure 9.

Other typical "on-the-shelf'' values of specific weights that are realistic in the one horsepower region are:

W - - rotor 2 lbs.

W - - motor

2 lbs.

W - - rectenna

servo

3 lbs.

2 lbs. W - -

W tail - - 2 lbs.

Inserting these values into Equation (2) and solving for W payload

--

W = 20 - (2+2+3+2+2) = 9 lbs/hp. payload

This represents a payload fraction of 45%. For larger vehicles this fraction could well increase because of the more desirable specific weights that may be obtained for many of the components with larger power ratings.

Hence, a communications payload of about five pounds could be provided by a one horsepower helicopter. Such a helicopter could operate at altitudes up to ten thousand feet, since lift does not vary greatly with density, -- as the cube root of the air density a s indicated in Equation (1). A five pound communications payload could represent considerable equipment in this day of microminiaturization, particularly in view of the fact that the dc power input at any reasonable voltage maybe obtained directly from a small section of the rectenna.

References

W. C. Brown, "Experiments in the Transportation of Energy by Microwive Beam", 1964 IEEE Inter- national Convention Record, Vol. XII, Pt. 2, pp. 8-17.

Okress, et al, "Microwave Power Engineering", IEEE Spectrum, October 1964, pp. 76-100.

W. C. Brown, "A Survey of the Elements of Power Transmission by Microwave Beam", 1961 IRE International Convention Record, Vol. M, Pt. 3, pp. 93-106.

HELICOPTER

I

I , I I

i \ I I I 1 I 1 I I I I

MCROWAVE BEAM / ,

I I I I

Fig. 1-The basic elements of a microwave- powered helicopter system.

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Fig. 2-Experimental system used at the Spencer laboratory of the Raytheon Company for flight testing of the microwave-powered helicopter.

Fig. 4-Close-up view of the microwave-powered helicopter showing combination antenna & rectifier, rotor, & motor.

Fig. 3-Antenna feed and ellipsoidal reflector which formed the microwave beam for the experiment.

Fig. 5-View of the microwave-powered helicopter in flight at an altitude of fifty feet.

23 1

rl

fi

11 L

232

I I 1 I I 1 I

I I I I I L I .06 .12 .18 .24 .30 .36 .42

Horsepouer

Fig. 9-Rotor lift a s a function of rotor horsepower. Solid line shows experimental data, dotted line the theoretical relationship for constant &I.

Fig. 10-An array of half-wave dipoles terminated in a bridge-rectifier array of point-contact silicon diodes. Reflecting plate is located at a h/4 distance behind the diodes.

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.L

Fig. 11-Directivity of the half-wave dipole a r ray shown in Figure 10. Directivity was essentially the same about both axes of rotation.

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Fig. 12- -Schematic drawing

f

LOAD

showing arrangement of dipoles and interconnections within a diode module.

Fig. 13-Photograph of one of the sixteen 280 element diode modules which made up the rectenna.

Fig. 14-Experimental data showing the non-critical dependence of the DC power output of a typical rectenna module upon the re- sistive load connected to its terminals.

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Microwave Helicopter