Center Stick or Side Stick BYEDKOLANO Side stick or center stick? Simple question, right? Maybe. Simple answer? Not by a long shot. Recalling one of too many aviation adages, ask two pilots — get at least three opinions. Probably more in this case since it's not a complete question. Which side is the side stick on? Does it rotate or slide? Large displacements or small? Is it installed in a cruiser or an acrobat? Does the center stick pivot at the cockpit floor? For both axes? Can the pilot rest his forearm on his leg? Does his leg get in the way of stick displacements? How the stick is mechanized depends on several factors. Weight is always a consideration. Simplicity might be a concern in a homebuilt design. Available cockpit space could also drive the decision toward one installation over another. Because there are several consequences of each design, a single issue is rarely the determining factor.

CENTER STICK CONSIDERATIONS An airplane with high lateral (aileron) control forces might employ a long center stick for leverage with a lot of available displacement. The Mustang is a good example. Lateral stick forces are quite high and the maximum roll rate is fairly low by today's standards. Consequently, a full stick aileron roll approaches a maximum one-handed effort and can only be achieved, for some, if the pilot lifts his leg slightly to make room for the stick. One method around the leg interference problem is to have the lateral pivot above the longitudinal pivot as shown in Figure 1. With a tall enough upper stick, this arrangement can still allow large lateral displacements but keep the deflected stick above the pilot's thigh. Naturally, some leverage is sacrificed, because the lateral stick moment arm is less than it would be with its pivot on the floor. If the "length of the lateral stick" is considerably shorter than the "length of the longitudinal stick," the pilot can discern the difference tactilely. Some pilots object to 46 OCTOBER 1996

A longer stick also improves control precision. Using the above example where 2" is a full lateral deflection (measured at the top of the grip), doubling the length of the stick results in a 4" displacement for a full stick input. The improved precision comes from the increased range of stick travel available for the same angular displacement, as illustrated in Figure 3, and from the shallowed stick force gradient (fewer pounds/inch). FIGURE 1 These stick length effects can be readily observed in any airplane. Make a stick input holding the grip. Then such an operational incongruity. Airplanes with small (short) cockpits make the same input (same angular necessary have shorter center sticks. stick displacement) holding the stick There may be insufficient room for the below the grip. The force should be dual pivot solution in this case. The Lan- higher, and precision should suffer. Lengthening the stick re-introduces cair 320/360 is an example. Because the pilot's seated position is so low, the cen- the leg interference potential. It is also ter stick must be fairly short for likely to place the grip higher than is ergonomic reasons. The result is leg in- comfortable. Many pilots enjoy the terference when full lateral stick inputs "vernier" effect of resting their foreare applied. Fortunately in the case of arms on a thigh while holding the stick. the Lancairs, the roll rate is fast enough This technique is quite helpful when making small stick inputs, because it that full stick is not often needed. In cockpits big enough to accommo- provides a reference. That reference is date them, large displacements can lost when the grip is too high. Add airfacilitate precise control inputs. Dou- plane motion or turbulence or control bling the available stick throw by system friction to this scenario, and changing the gearing without changing precision suffers even more. The same discussion applies to the the maximum control surface throw cuts the stick displacement gradient in longitudinal (pitch) axis with some obhalf. If, in the former case, a 2" lateral vious differences. Instead of leg deflection produced a 100°/sec. roll interference, for example, a pilot's rate, that's a 50°/sec. per inch average belly might interfere with full aft stick gradient. Performing the suggested displacements, although this is genermodification has no effect on the maxi- ally not the case. In the push direction, mum roll rate, but the gradient shallows short-armed pilots might have a hard to 25°/sec. per inch (100/4) (see Figure time reaching full forward stick if the 2). The modification also increases available displacement is large. This leverage, reducing the stick force re- might be the case in an acrobatic airplane where certain maneuvers require quired for a particular roll rate in half. Leverage can also be improved the stick to be placed fully forward. merely by making the stick longer. A Since the acrobat has a long stick for longer stick means a longer moment arm precise inputs and since the pilot is resulting in less force required at the tightly strapped in with a shoulder hargrip for the same angular displacement. ness, reaching the forward corners of Since control surface deflection is deter- the control stick box becomes a valid mined by the stick displacement angle, consideration in the airplane's design. There's usually more latitude in the the stick can be any length. Doubling the stick length doubles the moment arm, longitudinal control force department cutting the stick force in half for the than in the lateral case. Pilots are naturally stronger in the push/pull direction same control surface deflection.

Roll

Pitch

than in the side-to-side direction. The FARs (Part 23) reflect this difference, citing the maximum allowable pitch control force for normal maneuvers as double the maximum allowable roll control force. This relationship applies for temporary and prolonged control force applications. How much force a particular pilot is capable of applying longitudinally is a function of arm position. One government study reported male pilots capable of exerting more than twice the pull force on the stick when the stick was approximately between his knees than when it was almost touching his stomach. The results were less dramatic in the push direction with the maximum push force capability occurring when the p i l o t ' s arm was fully extended straight out in front of him (of course, this would make for a very tall stick). Too little longitudinal control force is not good either. The FARs base the minimum pull force needed to achieve the positive limit maneuvering load factor on aircraft weight, but specify an absolute minimum of 15 pounds. This is a sampling of some of the physical considerations for center control sticks. Not addressed are the intangible ones. Some pilots simply prefer center sticks, case closed. Perhaps they learned to fly in a center stick airplane, or maybe they own one. It could be those pilots just like having the stick in their right hand and throttle in their left, finally, there's the ambidextrous offering of the center stick. A right-handed pilot can fly temporarily with his left hand while writing a clearance or frequency with his right.

SIDE STICK CONSIDERATIONS Side sticks provide an entirely different feel. Merely replacing a center stick with a side stick without changing anything else in the airplane is likely to result in a very different pilot opinion of the airplane's longitudinal and lateral handling qualities. The differences arise from several aspects of the overall airplane feel. Following are several academic explorations of some of the effects. As in the preceding center stick discussion, what follows looks at the effects in isolation for a hypo-

FIGURE2

-2D——H

FIGURE 2

thetical case. That is, each issue is addressed relative to the same airplane with a center stick and not to the other side stick issues. Since the side stick is probably a lot shorter than the center stick it replaced, the moment arm is reduced, lessening the mechanical advantage, resulting in higher stick forces. The shorter stick provides less throw available than the center stick, necessitating a gearing change to achieve the same control surface deflection. The consequence is a steeper stick force per stick displacement gradient, and that means more work for the pilot and less precision than the center stick case. Most side sticks are designed to allow the pilot to rest his forearm on a sill or console while holding the stick grip. The same "vernier" advantages mentioned for the arm-on-thigh center stick case apply here, except it's probably more significant in regaining some of the precision lost due to the smaller displacements available. How significant depends on many factors — forces involved, displacements involved, pilot's sitting position relative to the stick, airplane responsiveness, and a host of others. High stick forces, particularly laterally, are generally undesirable in side stick arrangements. Side sticks encourage a racquetball technique when making roll control inputs. Pilots tend to expect to use wrist articulation to roll the plane as opposed to the tennis technique (elbow articulation) associated with center sticks. The longer the side stick moment arm, the more the pilot is forced to play tennis from a body position better suited to racquetball.

The lateral orientation of the side stick can affect the pilot's opinion also. Should the neutral (zero roll rate) position be vertical? Canted inboard or outboard? How many degrees? The Lancair ES uses dual side sticks mounted on the outboard cockpit walls. The neutral roll positions are canted inboard slightly. This canted arrangement is now only comfortable, but allows extra outboard displacement before contacting the cockpit wall. Now, the question of left side stick for the left sealer, right side stick for the right scatcr is raised. While this is no different than traditional side-by-side airplanes with dual center sticks and center-panel-mounted engine controls, some pilots raise their eyebrows at such an arrangement. Perhaps the facial contortions stem from one of those intangibles — "I might go for a side stick, but certainly not in my left hand." Universal advice: Try it first, then judge. How about airplanes with a single side stick? The Pulsar, Jabiru, and Velocity are examples of planes with a single control stick located between the seats. In these, it's a right side stick for left seatcr, left side stick for right sealer situation. Better, worse, or just differenl? In co/y cockpils leg interference can be an issue, excepl il might be the other pilot's leg interfering when rolling in his direction (also true in both directions for dual center stick cockpits with leg interference problems). Control transfer from one pilot to the other may be more cumbersome than in a dual stick airplane. While side sticks may generally be too shorl for Ihe dual pivot arrangement discussed in the center stick SPORT AVIATION 47

section, they arc not without innovation. The Sky Arrow is a tandem airplane with both pilots having rightconsole-mounted side sticks. The stick pivots for roll control and slides for pitch control. This sliding pitch mechanization does not i n h i b i t the wrist-articulation technique for small inputs. Of course, the stick doesn't pivot, but the pilot's pivot-like wrist motion still moves the stick fore and aft. Larger inputs require the pilot to slide his arm, losing some of the "vernier" advantage offered by the fixed arm-on-sill reference.

PERCEPTIONS Step onto a Piper Warrior's right wing. Push the yoke forward, step on the floor, and sit in the right seat. Then "slide" over to the left seat, and pull it forward to where everything is reachable. Hold the yoke in one hand and the throttle in the other. Look around. Visualize t h i s . Now, walk up to a Berkut. Hop up so as to be sitting on the canopy sill. Awkwardly balance

and rotate so you can "feed" your feet It should; it has a different mission. into the tunnels which house the rudder The choice of center or side stick pedals. Slide down the reclined, fixed can provide an illusion of enhanced or seat back until you're seated. Hold the degraded aerodynamic performance. throttle in your left hand, the side stick The Sky Arrow's maneuvering perforin your right. Look around. Visualize mance is anything but eye-watering, this. Feel any different? Expect anyyet its side stick, cozy cockpit, and thing different? panoramic view make it feel like more Sometimes the environment can in- of a little fighter than it is. The result is fluence a pilot's opinion and therefore an airplane that's more fun to fly than his preference. Sure, comparing a Warone with similar performance but with rior to a Berkut isn't fair, but suppose a different cockpit layout. Of course, the pilot hadn't flown either one. Supthe stick placement is just one of sevpose he just tried them both on for size eral synergistic influences affecting the at an airshow. It's probably safe to say pilot's overall perception of the cockthat he'd feel a lot more excited in the pit environment. Berkut. He'd probably expect a much While most pilots don't have a faster, more mancuvcrable, perhaps choice of control stick arrangements in even more fun to fly airplane nestled a specific airplane, their opinions are into the snug cockpit with the fighter heavily influenced by the location and feel. How much of the environmentally mechanization of the control stick. Piinfluenced perception of this hypothet- lots can weigh the success of the design ical scenario carries over to the based on the airplane's intended usage, experienced pilot is difficult to say. It's control forces and displacements and a good bet, however, that the Berkut gradients, available cockpit space, and experience will conjure more visions a dozen other factors, but ultimately it of aerobatics and enemy-cloud-gun- just boils down to what makes the pilot ning patriotism than the Warrior will. feel good. +

1995 Skybolt • Steen Aero Lab, Inc • Hale Wallace - Owner/Operator.

Flight Proven® aircraft paint manufactured by Randolph Products Co. for over 60 years. • • • •

Rand-O-Poly Polyurethane system for Ceconite® fabric Ranthane Polyurethane for metal aircraft Randacryl Acrylic lacquer for metal aircraft Butyrate Dope Right Proven® top selling preeminent dope for fabric

Randolph m

PRODUCTS^CO.

Nitrate & Butyrate Dopes • Polyurethane, Enamel & Acrylic Finishes • Epoxy & Wash Primers • Reducers • Aerosols For information, use SPORT AVIATION'S Reader Service Card

48 OCTOBER 1996

Carlstadt, New Jersey 07072-0830

201/438-3700 • Fax 201/438-4231