The Airplane - Jay Spenser [66]
To understand what the Wrights and their contemporaries grappled with, it is helpful to take a brief look at the airplane as we know it today. All airplanes have three axes of motion: pitch, roll, and yaw. Mutually perpendicular, these axes intersect at the airplane’s center of gravity, which is a point within the fuselage between the forward part of the wings. The center of gravity can shift fore or aft depending on how the airplane’s load is distributed. As long as it remains within defined limits, however, the airplane will fly properly.
The pitch axis can be pictured as a side-to-side horizontal line running more or less in line with the wings. When an airplane rotates about this axis, its nose tilts up or down. In contrast, the roll axis runs fore and aft through the fuselage. When an airplane rotates around this axis, its wings dip to the left or right. As for the yaw axis, it runs vertically. When the airplane rotates around this axis, its nose swings sideways left or right.
All airplanes have control surfaces that deflect in the slipstream to alter the course of flight. The elevators are horizontal surfaces at the rear that control pitch. Mounted outboard on the trailing edges of the wings are the ailerons, which control roll. The rudder, which controls yaw, is the tail’s vertical control surface.
On a typical airplane, the control wheel (or stick in the earlier days) controls pitch and roll. Fore and aft inputs make the airplane descend or climb, while left or right inputs cause it to bank. As for yaw, the rudder pedals control that.
As all pilots know, trying to turn a typical light airplane using just the rudder pedals (yaw only) causes the airplane to respond very sloppily. After a lag, the wing will drop because of a coupled rolling moment. Thereafter, constant rudder corrections are needed to hold the wing at the desired angle of bank.
Trying to turn using just the wheel (roll only) also results in sloppy behavior. The nose initially slides to the wrong side before coming around and banking in the desired direction. This phenomenon is called adverse yaw.
The secret to smooth, coordinated turns in the air is using both wheel and rudder at the same time. This banks the plane smoothly without adverse yaw. It also keeps the plane flying directly into the slipstream instead of inefficiently skidding or slipping.
Lastly, all pilots understand the distinction between stability and control. That they are separate subjects, albeit closely related, is today self-evident.
All of this sounds perfectly reasonable and logical today. At the turn of the last century, however, it was a different story. Nobody knew enough even to be able to agree on the definitions of key concepts, let alone formalize the requirements for achieving control in the air.
Of all the challenges inherent in inventing the airplane, controllability was the most baffling. Solving it was pure detective work.
George Cayley thought he understood controllability, but his designs made provision for pitch and yaw control only. Being analogous to a whale’s flukes and a ship’s rudder, those flight controls were the obvious ones. Flying his model gliders delighted the Yorkshire baronet. At times, however, they probably also brought a puzzled frown to his face. This would have happened when he launched a glider with its cruciform tail canted left or right. Instead of turning like a ship, the model would have swung to the desired side, fallen off in a bank, and crashed. Why did this happen? What did it augur for a future form of transport that Cayley alone imagined at the time of the Napoleonic wars?
Based on Cayley’s thinking, the Henson Aerial Steam Carriage also featured control around two axes. William Henson’s machine—designed in detail as a full-size airplane but built only as a model—lacked any mechanism in the wings for roll control.
Otto Lilienthal likewise had two-axis control, although his two were pitch and roll (with fixed vertical tails, his gliders had no provision for yaw control). But pitch and yaw were the most