20.9.1 Roll-Ratcheting

Roll-ratcheting bears a resemblance to the aileron buffeting that occurs on sharp­nosed Frise ailerons. The limit cycle oscillations occur at about 3 cycles per second when the ailerons are hard over, and the flight records even look the same (compare Figure 20.4 with Figure 5.6). However, the two phenomena could hardly be more different.

Roll-ratcheting arises from interactions among a variety of mechanisms. These include arm neuromuscular effects, limb and stick mass effective stick bobweights, force-sensing side stick gains, and roll command prefiltering. At 2 to 3 cycles per second, pilot voluntary efforts are not involved, so that roll-ratcheting is not a form of the pilot-induced oscillations discussed in Chapter 21.

A major effort was made to pin down roll-ratcheting parameters, using a fixed based simulator (Johnston and McRuer, 1977). The progress of that investigation, which brought in flight test data from the NT-33 variable-stability research airplane as well as the F-16, is given in fascinating detail by Irving L. Ashkenas in a summary paper (1988). There is a convincing correlation involving the stick sensing force gradient (degrees per second of roll rate per pound of stick force) and roll time constant TR, in seconds. A single line divides roll command augmentation systems into ratcheting and nonratcheting cases. However, this particular correlation is thought to hold only for nonmoving or force-type side sticks, such as installed in the F-16 airplane.

The role of arm neuromuscular effects as a prime component of roll-ratcheting is ques­tioned by Gibson (1999). In Delft TU studies, a simple assumption of a lateral bobweight

loop was found to produce roll-ratcheting. A later paper from DVL, Braunschweig (Koehler, 1999) returns to the neuromuscular model with refinements, adding torso and hip dynamics to that of the arm. The Koehler paper claimsgood correlation with an F-16 XL roll-ratcheting incident. Gibson notes that a spectacular roll-racheting incident involving the F-18 is de­scribed by Klyde (1995). A mild rachet occurred on the BAe FBW (fly-by-wire) Jaguar airplane, which was cured by adding a stick damper and by changes to high-frequency control dynamics.

A prudent design approach to avoid roll-ratcheting might be to adhere initially to the Ashkenas 1988 force gradient/roll time constant criterion, supplementing this with detailed stability analyses that account for both neuromuscular and bobweight effects.