Systems Operation

THE PRINCIPLES OF THE DASH WEAPON SYSTEM’S OPERATION as it pertains to the QH-50D; the currently deployed variant.

     The QH-50D drone is controlled throughout its mission by the shipboard guidance system on the ship, and by the receiving set and AFC set on the drone. (See figures 1 and 2.) Refer to figure 3 in the "Description" Section for a listing of the components, which are included in these groups.

    Commands originating at the deck or CIC control are fed to the selected coder where they are translated into discrete binary coded pulses, which, in turn, are fed to the selected transmitter. The fm output of the transmitter is modulated by the coded pulses to produce the radio command signals to control the drone.

    The shipboard guidance system has the capability of coding and transmitting six proportional and nine on-off commands. In the present system, the command channels are:

1. Proportional:

  1. Fore-aft airspeed (pitch).

  2. Lateral airspeed (roll).

  3. Heading or yaw (tip brakes).

  4. Altitude (collective pitch).

  5. Spare 1.

  6. Spare 2.

2. On-Off:

  1. Cruise-maneuver.

  2. Memory-station.

  3. Engine off.

  4. Cable release.

  5. Weapon arm.

  6. Weapon release 1.

  7. Weapon release 2.

  8. Spare 1.

  9. Spare 2.

    The proportional channels are used to transmit commands containing magnitude and direction information and the on-off channels are used to transmit switching information.

    The airborne receiving set completes the radio data link. The modulated FM command signals are picked up by the airborne receiver, demodulated, and fed to the decoder as pulses similar to those fed to the transmitter by the coder. The decoder separates the decoded pulses into the appropriate channels and converts them to analog values in the form of AC voltages. The drone control command voltages from the digital-to-analog stage of the decoder are applied, at the appropriate points, to the airborne AFC set.

The AFC set provides the four servo loops required to convert the outputs of the decoder to movement of the drone control surfaces, and also includes sensors, which maintain drone stability in its commanded flight regime. Rate information for servo system damping is derived electrically. The AFC set also contains relays, which execute the on-off commands.

This section, in discussing stabilization and control of the drone in the azimuth axis, utilizes the terms "yaw" and "heading." While the two words are not synonymous, each one is valid when used in the proper sense. The word "yaw" is commonly used in discussions of (aircraft) stabilization systems, where it is taken to denote “.... angular motion about the vertical axis of an aircraft." In the present instance, in which a remote control command is superimposed on the stabilization system, the yaw axis reference is biased to produce a "heading" command. Heading is defined as "the (compass) direction in which the airplane (aircraft) is pointed...." Thus, the drone is automatically stabilized in "yaw" and its "heading" is remotely commanded.



The four axes in the drone stabilization and control system are listed below


    For purposes of clarity and conciseness, this section describes the four stabilization and control axes as an absolute system without regard for aerodynamic perturbations and electrical tolerances. It is intended only that principles of system operation be discussed in this section. Exact values relating drone response to given commands are not included as they are not appropriate to the basic understanding of operation.

1.     Fore-Aft Airspeed (Pitch). An airspeed command is interpreted by the drone as a demand for a longitudinal tilt of the swash plates to produce a longitudinal thrust vector proportional to the airspeed commanded. The steady state angle of swash plate tilt for a given airspeed command is related principally to true vertical (with respect to earth). Swash plate tilt with respect to true vertical is related mechanically to the pitch attitude of the rotor mast. This mechanical relationship is maintained by summing the measurement of pitch attitude (through the pitch pick-off in the vertical gyro) with continuous servo follow-up position information (through the pitch follow-up synchro in the servo actuator). In the pitch axis, the relationship of gyro displacement to swash plate tilt (with respect to the mast) is approximately 1.5 to 1. 

2. Lateral Airspeed (Roll). A lateral airspeed command (available in the maneuver mode only) is interpreted by the drone as a demand for lateral tilt of the swash plates to produce a lateral thrust vector proportional to the command. In contrast to the pitch axis, the steady state angle of lateral swash plate tilt for a given command is related purely to true vertical. The measurement of roll attitude (through the roll pick-off in the vertical gyro) is summed with servo follow-up position information (through the roll follow-up synchro in the servo actuator) at a ratio, under steady state conditions, of 1 to 1. Thus, for each degree of roll attitude of the rotor mast, the swash plate is tilted an equal number of degrees with respect to the mast. During command transients or drone deviations from the commanded flight regime, this I to I ratio is temporarily altered, through a washout network, to enhance drone stability on the short-term basis.

In the cruise mode, a heading change command causes a fixed bank angle signal to be cross fed to the lateral axis to produce a coordinated turn.

3. Heading or Yaw (Tip Brakes). A heading command alters the heading reference within the directional gyro system, causing extension of the appropriate upper or lower rotor tip brakes. As the heading error signal approaches zero, the tip brakes are retracted and the directional gyro stabilizes the drone on its new heading. In the cruise mode, the heading change command introduces a cross feed signal to the roll axis to produce a coordinated turn.

4. Altitude (Collective Pitch). The commanded drone altitude is maintained by the barometric altitude control through the collective pitch system. An altitude command change introduces an error signal in the collective pitch servo loop. When the drone has reached the commanded altitude, the pick-off connected to the barometric pressure-sensing capsule produces a voltage, which bucks out the command, bringing the error to zero. The collective pitch system thus is readjusted until the ratio of lift versus weight is equalized.

    Electrical power for the airborne system is provided by an AC generator, which is driven, through gearing, by the rotor drive system. The generator provides the following nominal voltages:

115 volts, 400 cps, phase A, B, C.

22 volts, 400 cps, phase 1, 2, 3.

26 volts, 400 cps, phase A, phase A-90 degrees.

    A three-phase rectifier circuit in the relay assembly, operating from the 22 volt, 400 cps supply, provides unfiltered 28 vdc to operate relays, solenoids, and other equipment. The 22-volt generator winding also supplies power to two three-phase rectifier circuits in the electronic control amplifier. One of these circuits provides a regulated 28-vdc supply for internal use within the unit itself. The second rectifier circuit is used in conjunction with the 28-volt supply in a voltage doubler circuit to produce a 39-vdc supply for transistor biases.

    The weapon system has the capability of simultaneous deployment of more than one drone from each ship. This is accomplished by presetting three variable bits of the digital data link message structure. It is not necessary to change carrier frequency on the controlling ship. An eight-position address selector, which is set externally, is provided on the decoder. Corresponding selectors are provided on the deck and CIC controls. Prior to launching, a different address is selected on the decoder of each drone aboard the destroyer. The first drone may be launched and placed in the memory mode while the next one is launched. Only one drone at a time may be under station mode control.


    Although the address feature permits more than one drone to be operated simultaneously from a single ship or station, it is still possible that a second station, simultaneously radiating on the same frequency even though on a different address, can cause radio frequency interference and cause the drone to assume carrier loss or cause the passage of spurious commands. The likelihood of such an occurrence is dependent upon the relative signal strength of the two stations.

    When the drone is placed in the memory mode, it continues to fly at its last commanded altitude, airspeed, and heading until the controller switches back to the station mode and introduces a new command. If carrier is lost while the drone is on memory, it remains on memory. If carrier is lost while the drone is under station control, the cyclic pitch controls move to neutral, and the drone assumes a "No wind" hover at its last commanded altitude and heading. In the cruise mode, a time delay circuit (carrier loss smoothing module) in the pitch axis prevents severe drone pitch up when carrier is lost or severe pitch down when carrier is restored.

    Within the flight envelope set forth in Section V, a collective pitch limiting system functions to prevent excessive drop in rotor rpm in the event of excessive aerodynamic loading. This limiting system, which operates as a function of rotor rpm error, includes a frequency sensor, which provides a varying de voltage output proportional to frequency variations of the input obtained from the airborne generator. If the rotor rpm drops below approximately 98 percent of rated rpm, due to a rotor overload resulting from excessive command, a command-bucking signal is applied to reduce the magnitude of the command. This bucking signal is applied to the altitude axis to reduce rotor loading through the collective pitch system.

    The engine-off command, through the data link, removes the gyro output signals from their respective servo axes, so that the rotor system will not be affected by destroyer pitch, roll, and yaw while the rotors are slowing down.




    Proportional channel commands are generated by the positioning of the manual controls, which, in turn, are mechanically coupled to digital shaft encoders. The digital shaft encoders translate shaft positions into various coded switch closures. The coded switch closure information is then fed through a relay assembly to the audio frequency coder. The coder converts these switch closures into a series of pulses. A pulse represents a closed switch, and the lack of a pulse represents an open switch. In the binary digital system, a pulse is indicative of “1”, and the lack of a pulse signifies "0". Each switch is assigned a position in the binary numbering system and is referred to as a binary digit or bit.


    Thus, when a proportional channel command is applied, the associated digital shaft encoders generate a series of pulses, which are then translated into a frequency shift keyed (FSK) signal. The FSK signal can have one of two frequencies at a given time. One frequency represents the binary "1" and the other frequency represents the binary "0". The FSK signal becomes a constant amplitude signal, which switches from one frequency to another responding to the coded pulses. This frequency shift is referred to as a pulse code modulation of a frequency modulated subcarrier (pcm/fm). The FSK signal from the coder is introduced into the transmitter.

    The radio transmitter is a frequency-modulated (fm) transmitter containing a network, which transforms the FSK signal into a signal to frequency, modulate the RF carrier. When the FSK signal is applied to the radio transmitter, it is coupled to the internal power supply. The signal is then fed to the amplifier modulator stage to modulate the RF carrier. The output of the amplifier modulator is then raised to the transmitting frequency and the proper power requirements by the frequency-multiplier-amplifier stage. The final output signal is transmitted through the antenna to the drone.




    Movement of the aerodynamic control surfaces (rotor blades and tip brakes) of the drone is achieved by the introduction of an error signal into the input to the appropriate servo amplifier module in the electronic control amplifier. The output of the servo amplifier module is applied to one or the other of the two oppositely rotating electromagnetic clutches in the related axis of the servo actuator, and the output arm is driven in a direction characteristic of the polarity of the error signal. A velocity generator, internal to the servo actuator, and geared to the output arm, provides rate information to the minor loop for purposes of damping and loop stability.

    A synchro, also geared to the output arm, provides servo actuator output position (follow-up) information required for minor loop stability. This information is utilized in several ways in the various axes. Depending on the operating regime (on deck or in normal flight), all or part of the follow-up signal may be applied to a washout network. The washout network permits the follow-up signal to pass through on a transient basis; on a long term basis (depending on the washout time constant) the follow-up signal is decayed essentially to zero. The manner in which the follow-up washout it utilized is described as follows for each of the four axes. When the drone is operating on the deck, it is necessary to eliminate the effect of the washout circuitry in order to avoid undesirable control positions. The landing gear skid switches are utilized to short out, or disconnect, the washout circuits.


    The fore and aft airspeed command output of the decoder is an ac voltage with a phase characteristic of the direction commanded and a magnitude proportional to the speed commanded. This signal is applied, through summing networks, to the servo amplifier to cause the servo actuator to move the swash plates in the longitudinal cyclic axis. (See figure 3.) The summing networks in the command path also accept information from the vertical gyro pitch pick-off as well as servo actuator follow-up position information. When the magnitude of the combined gyro and follow-up inputs equals the magnitude of the command input the servo loop is nulled with the controls positioned to cause the drone to fly at the commanded airspeed.

    The flight characteristics of the drone are such that the pitch attitude versus airspeed curve is relatively flat in the region of 0 to 50 knots. When pitch attitude alone is measured (through the gyro pitch pick-off) and used for position information, a low gain system results and precise airspeed control in the low speed range is difficult to achieve. 

    The curve of swash plate tilt (with respect to the mast) versus airspeed is steep in the 0 to 50 knot range and levels off in the upper range. When the two functions (pitch attitude and swash plate tilt) are measured and summed algebraically, the curve representing airspeed command versus the sum of gyro and follow-up becomes very nearly linear, and more precise control in the low speed range is achieved. This system is mechanized by applying the follow-up synchro output to the input leg on a continuous basis (not washed out). The resultant signal [command - (gyro + follow-up)] is amplified in the appropriate module in the electronic control amplifier. The output of the servo amplifier module is applied to the servo actuator to position the swash plates at the angle required to produce the commanded airspeed. 


    The manner in which a lateral airspeed command is applied to the drone control system is similar to that of the longitudinal axis. (See figure 4.) The command output of the decoder is applied, through summing networks, to the servo amplifier in the lateral cyclic axis. The output of the servo amplifier is applied to the electromagnetic clutches in the servo actuator to cause the swash plates to tilt in the appropriate direction. The follow-up signal from the synchro in the servo actuator is summed with the roll attitude information from the roll pick-off in the vertical gyro and applied to the input leg of the servo amplifier to buck the command signal. 

    In flight, the follow-up signal passes through a washout network and a fixed gain follow-up network. The washout network has a time constant of 4 to 6.2 seconds, after which time any signal initially passing through it is decayed essentially to zero. The washout network provides short term loop stability during transients. The fixed gain follow-up network allows 50 percent of the total follow-up signal to remain in the loop on a steady state basis. This “50 percent washout" system permits the lateral cyclic axis gains to be determined such that, for steady state conditions in a hover, each one degree of roll attitude (gyro signal) is equal to one degree of swash plate tilt relative to the rotor mast (follow-up signal).


    When one of two side-by-side weapons is dropped, the center of gravity of the drone shifts laterally toward the remaining weapon. This CG shift causes the drone rotor mast to assume an approximate 3-degree roll attitude. This angle is measured by the roll pick-off in the vertical gyro and the resultant signal is applied to the input to the servo amplifier and servo actuator output results. During the time the drone is rolling into its new attitude the follow-up signal is fed back to the input to the servo amplifier through the wash out network to maintain drone stability during the roll transient.

    When the drone attains its new steady state roll attitude, only the 50 percent fixed gain follow-up signal remains in the loop. Under this condition the follow-up signal equals the gyro signal and the swash plates remain tilted, with respect to the mast, by an amount equivalent to the roll attitude of the mast. Thus the swash plates remain normal to true vertical, the thrust vector passes through the new CG, and no lateral flight of the drone results. 

      When the second weapon is dropped, the CG shifts back to center and the rotor mast returns to a zero roll attitude. Following the roll attitude and follow-up washout transients, the gyro and follow-up signals are reduced to zero and swash plate tilt is maintained at zero with respect to both the rotor mast and true vertical.

    When the drone is on the deck, the washout network is disconnected by the skid switches. Under this condition, true and continuous follow-up information is fed back to the input to the servo amplifier to buck the command signals applied for system checkout purposes. Thus, actual swash plate tilt for given command inputs can be verified.

In flight, in the cruise mode, a signal originating in the heading axis is applied to the input to the roll axis to produce coordinated turns on the application of a heading change command. The magnitude of the turn coordination signal is such that a fixed roll angle displacement of approximately 20 degrees results. Turn rate is automatically adjusted as a computed function of airspeed to satisfy the aerodynamic requirements.




    The heading axis servo system functions in a manner similar to the pitch and roll axes. Differences exist in the servo follow-up circuitry and in the manner of the application of the command to the servo system. (See figure 5.)

    Heading commands are applied to the servo system through a heading data converter, which includes a motor-driven synchro transmitter, velocity generator, and two follow-up potentiometers whose wipers are positioned 180 degrees apart. The pulse coding of the heading command is discrete for each of two sectors: 0 to 178.6 degrees, and 180 to 358.6 degrees (Each of the 1.4 degree gaps is the equivalent of one bit of the coded message. This is the smallest increment that can be resolved in the digital heading command system.) Each of the two potentiometers bears a fixed relationship to a given heading sector. As the heading pointer is turned past 0 or 180 degrees on the heading card, the pulse code changes, and a relay in the decoder switches the system electrical connections from one follow-up potentiometer to the other. 


    The heading command output from the decoder is an AC voltage which increases in amplitude through the same range as the pointer is turned from 0 to 178.6 degrees and from 180 to 358.6 degrees. The command voltage is amplified and applied to the motor in the heading data converter. The motor drives until the related potentiometer nulls the closed loop. The motor positions the rotor of the synchro transmitter whose stator is connected to the stator of a differential synchro in the directional gyro control unit. The rotor of the differential synchro is connected to the stator of a synchro control transformer whose rotor is positioned by the gyro gimbal. The error signal from the gyro pick-off is shaped, amplified, and applied to the tip brake axis clutches in the servo actuator.

    When the drone is airborne and a heading change command is applied to the servo amplifier, the follow-up signal is fed back to the servo amplifier through the washout network. As the drone begins to turn to the new heading the follow-up signal is washed out and the heading command error remains as the primary input to the servo amplifier. As the drone approaches the new commanded heading, the error signal is reduced to zero, the servo actuator retracts the tip brakes, and the drone is stabilized on its new commanded heading.

Aerodynamic forces in the rotor system when the drone is in forward flight tend to create a torque imbalance in the rotor drive system. This causes the drone fuselage to tend to deviate from its commanded heading. The resultant error signals from the directional gyro are applied to the servo amplifier to provide corrective servo actuator output. As the drone returns to its commanded heading, the gyro error signal is reduced to zero and the servo follow- up signal is washed out with the tip brakes on one of the rotors extended by the amount required to remove the torque imbalance.

    When the drone is on the deck with rotors turning and a heading change command is applied, the servo actuator output arm is displaced in the appropriate direction. Since the washout network is shorted out through the skid switches, follow-up position information is fed back directly to the servo amplifier input to reduce the heading error. Servo actuator output arm displacement is proportional to heading error from 0 to approximately 34 degrees of heading error. At that point the servo actuator output arm is displaced to its limit and follow-up position information is at its maximum value. A heading change command of more than 34 degrees causes an error signal proportional to the difference between command and maximum follow-up to remain applied to the input to the servo amplifier.

    Airspeed data is cross fed to the heading axis to regulate turn rate as a function of airspeed for coordinated turns. The airspeed data consists of pitch attitude plus follow-up position information modified so that the signal applied to the heading axis represents swash plate tilt with respect to true vertical (true airspeed). The velocity generator in the heading data converter also supplies a signal to the roll axis to create a bank angle for turn coordination in the cruise mode.

      Prior to launching the drone, the directional gyro is slaved to (aligned with) the ship gyro system so that the drone heading in flight always is related to true north.



      Altitude information is obtained from the barometric altitude control, which includes an inductive pick-off mechanically connected to a static pressure capsule. The internal mechanism is automatically adjusted, prior to launch, to null the electrical output at the prevailing ambient barometric pressure.

    The altitude command from the decoder is summed with the output of the barometric altitude control, amplified, and applied to the collective pitch axis clutches in the servo actuator. (See figure 6.) A liner loop damping, follow-up, and washout are mechanized in a manner identical to the heading axis. 

    When an altitude or change of altitude command is applied, an error signal is introduced into the input to the servo amplifier and servo actuator output results. Collective pitch is increased or decreased to cause the drone to ascend or descend as commanded. As the drone changes altitude, the barometric altitude control senses the change in barometric pressure and causes a signal to be applied to the summing network in opposition to the command signal. When the commanded altitude is attained the error is reduced to zero and the servo actuator maintains the collective pitch setting at that required to maintain lift. Since the barometric altitude control senses only barometric pressure (as a function of altitude) the collective pitch system is automatically adjusted as required regardless of ambient temperature and drone gross weight.

    When the drone is airborne, the follow-up signal is washed out to remove the collective pitch position information from the servo loop. Thus the input to the servo amplifier is the sum of command and barometric altitude control output. When the drone is on the deck, the washout network is shorted out by the skid switches so that position information remains in the loop. This permits a check- out of control position versus command input on the deck, without drift of the control linkage, which would otherwise result. 


End of Systems Operation Section

Home Up Description Normal Procedures Emergency Procedures Auxiliary Equipment Operating Limitations Flight Characteristics Systems Operation Crew Duties All Weather Ops Performance Data


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