Foot Forces Exerted at Various Aircraft Brake-Pedal Angles
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HUMAN FACTORS, 1971, 13(5), 445-456 Foot Forces Exerted at Various Aircraft Brake-Pedal Angles H. T. E. HERTZBERG and FRANCIS E. BURKEl, Anthropology Branch, Aerospace Medical Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio This study reports the forces (means and standards deviations) exerted by the foot at various angles of extension about its ankle. A sample of 100 rated pilots was selected by height and weigh t. The forces were measured in a cockpif mock-up constructed around a specially instrumented F-80 rudder·pedal assembly. The right rudder pedal (hinged on the rudder bar) could be rigidly set to any desired angle between vertical and 75° forward of vertical. Foot forces were measured at 11 positions of the instrumented brake pedal in both neutral and extended positions of the right leg, and in three cockpit sizes (37 in., 3914 in., and 41 in.)-66 measures on each man. In all three conditions, maximal forces were exerted within a 20° zone between 15 and 35° forward of vertical. Subjective comfort preferences, expressed by 86 pilots, closely paralleled the force findings. It is concluded that aircraft brake-pedal systems should be designed to maximize the effectiveness of the foot in that optimal zone, and that the same zone should be considered for other foot-operated controls, like automobile accelerator pedals, INTRODUCTION can exert its maximum torque about the ankle, as well as the magnitude of the forces them- Anyone who does work with his muscles selves. The data were gathered during 1949-50, knows that effort and fatigue can be minimized but could not then be written up_ As no similar by finding and using the position of optimal leg-strength study has appeared since then (but mechanical advantage of the limbs for any given compare Hunsicker [1955] for arm strength task. A shoveler, for instance, quickly learns the measured according to Air Force instructions) best grip location to minimize the effort of these results seem still timely as data assisting continued work. But when the worker on a optimal brake-pedal design. fixed machine must conform to the situation For the purpose of this study, it was built in by the machine designer, he may be at a assumed that pressures could be exerted equally considerable muscular disadvantage and con- by either foot. While this is not strictly true, sequently suffer undue fatigue. This is the only the right·foot pressures were measured, as condition encountered in some airplane brake these are generally the subject's maximal forces, pedals, and not infrequently in other types of and hence those needed for non-fail structural' pedals. Although rudder-and-brake controls design. have to be generally similar in location and adjustability, the design variation in angle of RUDDER AND BRAKE CONTROL: brake-pedal face among different airplanes is often a source of complaint among pilots. This THE GENERAL PROBLEM is because the specifications require only that the brake pedal shall travel 300 from full "off' It is a cardinal principle in vehicle design, to full "on". and especially in aircraft, that the control levers With that situation in view, the present must move in the direction in which the study was undertaken to ascertain the pedal operator's limbs naturally move and thereby angles in the cockpit through which the foot exert force. But this by itself is not enough as a design criterion; there must be considered also a I Now of the Federal Bureau of Investigation, U.S. zone in the arc of travel of any limb in which Department of Justice, Coeur d'Alene, Idaho. the operator can exert force with the least 445 446-0ctober, 1971 HUMAN FACTORS effort; i.e., with the greatest efficiency. All seat when he is flying in a straight line. This is control levers, therefore, should be designed so called the neutral position for both the rudder that they can move and transmit the operator's bars and the pilot's legs. His knees are com· force through just that zone. But such zones ,foltably bent, so that he can straighten one have received relatively little attention, and that knee toward what is called the "extended leg~' is the reason for this study of leg and foot 'position, and bend the other knee further to action in the cockpit. retract that leg as it conforms to the aft-mOVing To an investigator seeking to improve the rudder bar. As takeoffs and landings demand efficiency of foot controls for pilots, it may sensitive control but no great leg displacement, seem paradoxical that the greatest range of it is during taxiing that the pilot may have to leg-and-foot positions required, and of foot exert considerably higher forces with more forces to be exerted, is usually encountered on complex limb movements. In flight, the pilot the ground rather than in the air. Taxiing the does not actuate his brakes, and seldom needs aircraft to and from the runway often calls for full leg extension except perhaps',during violent extreme displacements of the control levers combat maneuvers; but on the ground, turning because of the low efficiency of the rudder and the aircraft in a strong cross-wind; he often has aileron surfaces at slow air speeds. At such to exert a maXimal braking effort by one foot times, braking forces often are very important while that leg is fully exfended. Thus the in maneuvering the aircraft. In terms of body aircraft rolling at low speeds on the ground may action, the shift from control in the air to present problems of control more critical than control on the ground can be rather complex, those in flight, when considered in terms of the yet it is all a part of the pilot's responsibility. muscle groups and limb positions needed to For the reader unfamiliar with the cockpit exert the controlling forces. rudder-and-brake situation, a brief analysis may It is, of course, clear that the two adjust- be desirable. ments noted above-seat height and rudder-bar In any given aircraft, canopy height and distance-affect the angle of the pilot's knees, overall cockpit size are fixed, but various and thus also the forces he can exert. To obtain adjustments, both vertical and horizontal, are some estimates of the magnitude of knee angle provided. On entering the cockpit, the pilot at both neutral and extended positions, and for first adjusts his seat up or down according to both small and large men, a layout drawing was his sitting height, so that his eyes reach the prepared showing the configuration of the point from which he can see out satisfactorily, standard USAF fighter seat (seat pan, 6.5°, seat and move his head freely. This eye location is back, 103° from horizontal), with a horizontal an important design point, from which many vision line 39~ in. above the floor (heel-rest) cockpit dimensions start. In Air Force aircraft, level. Two plastic manikins, one dimensioned the ranges of those dimensions have been according to the 5th percentile of USAF body anthropometrically chosen to accommodate at sizes and the other to the 95th percentile least the central 90% of body sizes in our pilot (Hertzberg, Daniels, and Churchill, 1954) were population. Next, the pilot adjusts the rudder- fitted to the eye line, seat angles, and heel bar location horizontally forward or aft for his positions. For the neutral leg position (thighs own leg ·length, so that the balls of his feet rest on the seat-pan line, heels on the floor, balls of comfortably but positively on the bars, with his feet as if on the rudder bars) the angular range heels usually on the cockpit floor. As the was 135° ± 10°. The angles were taken between rudder and its controls (the rudder bars-one for the centerlines running through the segment each foot) form a tight operational loop, it is hinges, not on the upper knee surfaces. For the obvious that when the pilot pushes the right extended leg position, the range was rudder bar away from him to turn the aircraft 160° ± 10°. In both cases, the smaller manikin to the right, the left rudder bar must come had the smaller angle. These figures, of course, closer to him by the same displacement. Thus are not experimental results; they are only the rudder bars are equidistant from the pilot's approximations measured from plastic man- H. T. E. HERTZBERG AND FRANCIS E. BURKE October, 1971-447 ikins; nevertheless they do give some notion of exerted upon it. In the sketch the pedal is the ranges of angular magnitude. In living shown in the vertical position, herein desig- 0 persons, the angles would probably be meas- nated as 0 , and the utmost position of flexion ured from the femoral trochanter to the mid- of the pilot's foot about his ankle. The pedal point of the knee to the ankle protuberance. could rotate through the arc of foot extension Two other factors affect the magnitude of to an angle of 700 past vertical (forward of the the forces the pilot can exert on the brake seated pilot), a range greater than that shown pedal: one is the size (primarily the length) of by any of our subjects. Thus the forces meas- his foot; and the other is the position of the ured in this study were exerted at successive foot on the rudder bar. The foot position used fixed angles of the pilot's foot. The operation in this research (heel on the rudder bar) was of the various parts of the apparatus is dis- chosen because the greater natural leverage cussed in greater detail below.