Control Forces

The feel of the primary flight controls (collective, cyclic, and pedals) play an important role in the perceived handling qualities of a helicopter.  Stiff, hard to move controls may enhance stability but can fatigue a pilot and inhibit precise maneuvering.  Loose, floppy controls are easier to maneuver, but increase pilot workload and allow inadvertent control inputs.  In this article we discuss such properties for a helicopter.

Reversible Vs Irreversible Control Systems

Primary flight controls are grouped into two general categories:  irreversible and reversible.  Irreversible systems have hydraulic or electromechanical power servos between the cockpit controls and the swashplate, which ultimately pitches the blades.  The aerodynamic forces on the blades have no impact on the feel of the controls.  The pilot can push/pull the control with the same force independent of the flight condition.  Most helicopters over about 8,500 pounds have irreversible control systems.

Reversible control systems do not isolate blade dynamics from the pilot’s control feel.  Blade pitch inertia and aerodynamic pitching moments feed back to and affect the pilot.  These systems often include mechanical advantage or dynamic balancing to reduce the load on the pilot’s arm/leg.  They may also include a spring to help hold the control around a desired “trim position.”  The advantage of these systems is simplicity: they are more reliable and easier to maintain.

Trim Systems

Pilots have other tasks, in addition to controlling the flight path.  They may need to check a map, talk to people or look for targets on the ground.  Hence, anything the control system can do to lessen pilot control workload is beneficial.  This is what trim systems are made for.   When a pilot achieves a desired flight state, say 80 knots cruise, he can engage a trim system to help maintain that state.  The trim system will apply forces to hold the controls in the required position.  They will apply an increasing force as any control deviates from the position.  These forces are designed to reduce pilot workload without harming controllability.

A pilot can deactivate the trim system at any time by pressing a force trim release (FTR) button, typically located on the control.  Upon releasing the FTR button, the trim system reactivates at the new control position.  The system can typically be deactivated indefinitely by pressing another button in the cockpit.

Static and quasistatic control forces

When a trim system is engaged, the force a pilot must provide to hold a control at a position is called the static control force.  The force required to move the control very slowly and steadily is the quasistatic control force, which amounts to the static force plus friction.  This excludes damping forces that resist quicker stick motions.  Quasistatic forces are characterized by the following properties.  All the properties are shown in an example force versus displacement plot (Figure 1).  A real-world plot is provided in Figure 2.

1. Trim system freeplay. This is the distance the control may be moved away from the trim center without generating an opposing force.  This is exaggerated in Figure 1 for clarity.  Freeplay should be very small or negligible.
2. Breakout force. The is a sharp increase in force required to move the control out of the trim position (freeplay region).
3. Force gradients. As the name implies, force gradients are proportional to the distance of the control from its trim center.  There are often two gradients: a larger one near the trim position (sometimes called the breakout force) and smaller one over the entire control range.  The larger one will have a maximum force $$F_M$$ obtained with a small displacement from the trim center and will hold $$F_M$$ outside that range.
4. Each control has a limited range, which may change as a function of other control positions or the flight condition.  When nearing these limits, called stops, the force required quickly increases beyond the pilots strength, no further movement is feasible.
5. A certain amount of force will resist the motion of a control.  This is called friction.  This is typically too small to hold the control fixed if the pilot releases it.  The force gradients will overwhelm friction to pull an off-center control back to trim.  In some aircraft, a knob near the base of the cyclic allows the pilot to adjust the friction.  When collecting data (e.g. for the plots below) it’s standard to turn the knob to minimum friction.  The solid black line in Figure 1 excludes friction.  The dashed black lines include friction: one line for each direction the control is moved.

Figure 1.  Static and quasi-static control forces.  The solid line shows the static forces that would be required to hold the control fixed at the specified displacement.  The dashed lines show the quasistatic forces which include additional friction acting against the direction of control motion.

Figure 2.  Real-world quasistatic control forces.   Notice the freeplay is negligible or non-existant.

Although not present in the above plots, an additional quasistatic force is present at extreme aircraft attitudes in flight.  The controls have mass and hence gravity may pull this mass noticeably, effectively biasing the control forces in a particular direction.   For example, consider a helicopter in a steep dive.  The nose down pitch angle causes gravity to pull the cyclic forward, increasing the force required to pull up out of the dive.

Dynamic control forces

When controls are moved faster, two more forces become relevant: damping and inertia.  Control damping increases with the speed of control movement.  For example, if the longitudinal cyclic is pressed forward very quickly, a damping force will push back against the direction of motion.  This is different from friction because friction does not increase with speed, while damping does.  Damping is added to heighten pilot awareness of control motion, prevent problematic pilot-induced oscillations, and reduce pilot control “jerkiness.”  For example, damping in the pedals can reduce yaw oscillations and tail boom stress.  Inertia, associated with control mass, acts against control acceleration

These forces are typically measured by releasing a control after displacing it a fixed distance from trim center.  The position of the control as a function of time is recorded until the control ceases to move.  The control will typically settle back in the trim center position but may have a few overshoots.  Figure 3 is a theoretical plot of a control position after releasing it from a 14deg off-center position.  Figure 4 is a plot of a real control release showing how, in reality, the damping often prevents overshoots.

Figure 3.  Theoretical displacement of a control after being released from a position away from trim center.

Figure 4.  Real-world plot of the displacement of a cyclic control after being released from a position away from trim center

Beep trim

When a pilot wants to maneuver to a new flight condition, he may disengage the trim system or “beep” the controls to the new flight condition.  A beep button is typically found on the control sticks that.  When "clicked," this button moves the trim system to center about a new position.  For example, the longitudinal cyclic may be “beeped” forward to transition from 80 knots to 90 knots cruise.  Important metrics for the beep system include the rate at which it moves sticks (the so-called trim rate) and the delay from when the beep is pressed until the control starts moving (trim lag).

Irreversible control systems, trim off

The trim system may be turned off, either indefinitely by a cockpit switch or temporarily by holding a force trim release (FTR) button.  This effectively removes force gradients (including breakout force) by releasing the fixed end of a spring so that it doesn’t stretch or compress with control movement.  This leaves the friction and stops as the only quasistatic forces.  A plot of the quasistatic forces Vs displacement with the trim off is shown in Figure 5.

Figure 5.  Quasistatic forces on an irreversible cyclic control with the trim system turned off.

Measurement equipment

Control force measurement (CFM) equipment is used to measure the forces and positions of flight controls, and even generate the types of plots shown above.  Such systems include calibrated force and position transducers, inclinometers, and other hardware to attach to control grips.  These systems also include software to read and store sensor outputs.  The most commonly used kits are made by Moog.

A common use of this equipment is to calibrate controls in a simulator.  Simulators incorporate control loading systems (CLS) to mimic the feel of real aircraft controls.  The data discussed above is captured in a real aircraft using a CFM kit.  This data will then be used to design the control loading system.  A CFM kit may then be used to measure the simulator’s CLS and compare the results to the real aircraft data.  The design may then be tweaked until the simulator data matches the real aircraft within an allowable tolerance.