Seeking out Stability
Thank the Wright brothers for your trainer's forgiving nature
It took the better part of a century of aeronautical research and development to put a fleet of safe, stable general aviation airplanes at pilots' disposal. The Wright brothers' 1903 Flyer-forbearer of them all-is a fascinating place to begin any discussion of stability and control in modern general aviation aircraft. The Flyer launched a revolution, but it was nowhere near market-ready, and the Wrights knew this.
The U.S. Air Force's Test Pilot School at Edwards Air Force Base gave its students an opportunity to "fly" the 1903 Wright aircraft by computer. A replica of the Flyer had been built by the Los Angeles Section of the American Institute of Aeronautics and Astronautics. Extensive wind tunnel testing at the California Institute of Technology yielded data on its aerodynamic performance, which the school converted into computer models for flight simulators.
All the Air Force test pilots crashed the Flyer on their first attempt to fly it. John Hair, information officer for the Flight Test Center at Edwards and a high-time fixed- and rotary-wing pilot, said, "Once it starts-it goes! There's simply no delay that would give you a chance to recover, and maximum control input won't save it. All of the Air Force test pilots in that class gained great respect for the Wright brothers as pilots."
Because their understanding of stability was evolving with their designs, it took the brothers until 1904 to fly a complete circle, and it wasn't until 1905 that they were able to make continuous circles, figure eights, and long flights (38 minutes). Many Wright historians describe the 1905 Flyer as the world's first "practical" airplane.
Wilbur galvanized European audiences with an improved machine in 1908. It was still unstable in pitch, but marginally stable in yaw and roll. In 1910, the Wrights finally morphed their configuration into something more closely resembling what today is considered conventional aircraft design, with rudders and a horizontal stabilizer/elevator in the rear. They made an additional change that was critical to the survival of Wright aircraft: They finally moved the center of gravity (CG) ahead of the aerodynamic pivot or neutral point of the aircraft. No aircraft is stable unless the CG is ahead of this neutral point.
F.E.C. Culick, professor of aeronautics and astronautics at the California Institute of Technology, says the Wrights "made a mistake with their choice of canard configuration and got stuck with an unstable airplane." Culick has been involved with the Wright Flyer replica program of the AAIA from the beginning; it is one of several re-created Flyers scheduled to fly during this year's centennial of powered flight.
Canards and pitch control
The Wrights put the elevator out in front (they called it the "rudder") by design. Wilbur was concerned about pitch control after the death of Otto Lilienthal in 1896. After hundreds of successful glides in Germany, Lilienthal suffered a fatal injury after a strong gust of wind caused his monoplane glider to nose up, stall, and crash. Wilbur believed the front "rudder" would not only provide adequate control in pitch, but it could be seen by the pilot, who would know exactly what it was doing. In earlier gliding flights they had learned that pitch control was essential, in addition to roll control. They also thought the "tail in front" would provide some crash protection for the pilot.
Today, we call the Wrights' "rudder" a canard surface, and some aircraft flying today have a small wing ahead of the main wing; Burt Rutan's VariEze and related homebuilts, as well as the production Beech Starship, come to mind.
Although Wilbur thought of the canard only as a control surface, it also was a lifting surface that made it possible-barely-to fly the aircraft even with the CG badly misplaced. In conventional aircraft, the horizontal tail surface in the rear actually is a wing that "lifts" downward to counteract the tendency of main wing lift to rotate the nose down.
When well designed, a canard aircraft is stallproof. The canard surface contributes upward lift, and it is designed to stall before the main wing as the aircraft's angle of attack increases. Voil¿! The nose drops and the angle of attack decreases, avoiding a main-wing stall and restoring lift to the canard. Supporters like to say that a canard can't be stalled. Unfortunately, canard aircraft have stalled. When they do, they enter a flat "deep stall" that may be virtually unrecoverable because it is impossible to dump the lift of the canard surface and lower the nose to restore lift to the main wing. The location of the CG is more critical in canards than in conventional aircraft, and weight too far aft is usually responsible for the fatal stalls.
Without flaps or spoilers, canard designs tend to approach landings flat. Adding flaps to the main wing for normal approach paths may require greater control authority from the canard surface to counter pitching moments from the flaps. This canard control authority, in turn, may make it possible to stall the main wing with the flaps up, eliminating the "stallproof" advantage of good canard design. A "translating" or variable-sweep canard can provide the best of both worlds. The Beech Starship has a variable-sweep canard connected to the flaps.
Wing warping and roll control
Roll control was the Wrights' greatest contribution to human flight, with their recognition that coordinated rudder was essential to counteract adverse yaw in rolls. Other active early experimenters didn't understand how an aircraft turned. They believed a rudder would work like a boat's, and that an aircraft could turn without roll. The aircraft they were building were designed to be inherently stable in roll and pitch. One of these was the Aerodrome designed by Samuel P. Langley, the secretary of the Smithsonian Institution. His models flew well, so he enlarged his design to full scale and watched the Aerodrome crash twice in 1903 immediately following its "launch" from a catapult track. The "pilot"-his mechanic, Charles Manley-had absolutely no control over roll and very little over pitch. A small V-shaped wedge behind the cockpit was supposed to give him yaw control. Langley's second failure occurred only nine days before the Wrights' first successful flight (see "The Wright Stuff," April AOPA Flight Training).
The Wrights understood that roll control was essential to making efficient, coordinated turns. Without the ability to control flight direction with some precision, they reasoned, you don't have a practical airplane. Wilbur spent hours watching birds, and he saw that they turned by warping their wingtips. Hence the wing warping mechanism of the 1903 Flyer and subsequent Wright designs until they converted to ailerons in 1915. The effect of wing warping is the same as that of ailerons. Warping the wing tip's trailing edge down causes it to rise, and warping the tip's trailing edge up causes it to drop. In the early Wright designs, the warping cables were controlled with a hip cradle, with the pilot prone. Later, they developed hand controls for sitting upright.
Wilbur was sure about one thing. Control was more important to him than stability. He later wrote that he and Orville had selected a "fundamentally different principle" than other would-be aviators. "We would arrange the machine so that it would not tend to right itself."
Because of their concern that side gusts would drive the 1903 Flyer into the sand, the Wrights opted for anhedral-wing droop-which further contributes to dynamic instability. They reasoned that anhedral would help to counter side gusts that could otherwise force the gust-facing wings upward and turn the down wings into sand shovels. At the altitudes at which they were flying, this was a real possibility. However, anhedral tends to force a low wing father down in a sideslip. Orville pointed out that most of their gliding was from a dome-shaped dune and the anhedral of the wing conformed somewhat to the shape of the dune.
Dihedral -wing tips higher than roots-in most modern aircraft does the opposite. It tends to raise a wing moving into a sideslip. In 1904, the Wrights removed anhedral, but put it back in 1905. At that time they also added "blinders," or small vertical vanes, to the "rudder" (horizontal stabilizer/elevator), which somewhat reduced directional stability. Stability and control are opposites. A stable airplane resists changes (good for a bomber or a transport). Control provides maneuverability at the expense of stability. Fighters and aerobatic aircraft swap stability for control.
Rudders in the rear
By the 1902 gliding season, the Wrights had added fixed rudders in the rear. During the last week of gliding in 1901, Wilbur wrote in his diary: "The upturned wing seems to fall behind, but at first rises." What he had discovered-the first aeronautical experimenter known to do so-was adverse yaw. It's the force that tends to move a rising wing backward, because more lift creates more drag and yaws an airplane against the intended direction of turn. Without rudders to counteract adverse yaw, the early Wright gliders tended to sideslip their low wing into the sand dune. They called this "well-digging."
The rigidly attached rudders of the 1902 glider usually worked to convert sideslips into a turning motion, as the airflow from the down wing side of the glider impacted the rudder surfaces. However, when the glider was hit by a side gust from the other side, it had a tendency to yaw the wrong way. Sometimes the anti-yaw correction with fixed rudders was too great. The effect was to initiate a spiral, which would have been fatal from substantial altitude.
Modern aileron and control linkage design has virtually eliminated adverse yaw. Good rudder technique, however, is still a clue to good pilot skills, and it's not at all instinctive. Aeronautical engineer John Foster, who works for the NASA Langley Research Center in Hampton, Virginia, says, "My experience as a flight instructor is that learning to use the rudder can be a challenge for some student pilots, but it is an essential skill for good airplane control. Many students try to use ailerons alone instead of rudder with aileron."
For the 1903 powered flights the Wrights linked their rudder control to the wing-warp wires. They had done this first in the 1902 glider. Orville had suggested independent control, but the difficulty of providing a separate rudder control mechanism for the prone pilot ruled it out. Wilbur thought of linking the rudder wires to the wing-warp hip cradle, which was an immediate success in their glider flights. This was the world's first (and nearly last) coupled system, where the rudder turned at the same time that the wing was warped. It worked better than fixed rudders, but the pilot had no way to adjust rudder movement independently for gusts, or for intentional slips, and the Wrights still had difficulty turning.
It wasn't until 1905 that they disconnected the rudders from wing warping for separate pilot control and began flying repeated circles and figure eights. In addition, they increased the area of the vertical tail and the canard, moved them farther from the wings, and gave the wings slight dihedral. At last, they had full control of pitch, roll, and yaw, and marginal stability in yaw and roll. The Flyer was still unstable in pitch.
The Wrights were having trouble with incipient spins until the fall of 1905, when Wilbur realized that "being unable to stop turning" was the result of insufficient speed and lower wing stall brought on by the increased centrifugal load of circling. He wrote: "We discovered the real nature of the trouble and knew that it could always be remedied by tilting the machine forward a little, so that its flying speed would be restored."
An unlinked rudder is also useful at high angles of attack in lifting a dropping wing without using aileron. In aircraft without wing flaps or spoilers/drag brakes, a cross-controlled rudder (rudder opposite to wheel or stick deflection) may be used to steepen a landing descent with a sideslip.
In 1940, the Wrights' coupled linkage made a comeback, in the Ercoupe-a two-place, all-aluminum craft with twin vertical tail surfaces and tricycle landing gear. The nosewheel was steerable. A brake pedal was on the floor. The idea of coupling rudder, ailerons, and a steerable nosewheel was to make it easy to taxi and fly-like an automobile. Control, CG, and engine power were such that it couldn't be spun and was hard to stall. Obviously it couldn't be slipped to lose altitude or align with the runway on landing. Castering wheels allowed you to land it in a crab. More than 5,600 Ercoupes were built by various manufacturers until Mooney dropped the design in 1970. Alas, nearly all of the remaining Ercoupes have been modified with conventional rudder pedals, losing their coupled control linkage.
The center of gravity
The Wrights had control, but they were still fighting pitch instability. Early Wright airplanes had to be flown all the time. By 1908, most other successful aircraft manufacturers had moved beyond the Wrights in the stability department, with the CG moved forward of the aerodynamic pivot point of the aircraft.
For aeronautical engineers, a balanced aircraft is one where the lift of the wing (behind the CG) tending to force the aircraft nose down is balanced by the nose-up force from the horizontal tail. This balance acts around the CG. Engineers refer to a critical "neutral point" when discussing pitch stability. In conventional aircraft, the neutral point is usually about 30 to 40 percent of the wing's chord (the distance from the leading edge to the trailing edge). In stable designs, the CG is always ahead of the neutral point. When the CG is moved behind the neutral point in GA airplanes, you can get into trouble. That's why you need weight and balance calculations for any aircraft that you fly. The 1903 Wright Flyer's neutral point was about 10 percent of the wing chord, while the CG was about 30 percent of chord!
A rear CG is most critical on landing as speed diminishes and the elevator loses effectiveness. If the aircraft is unstable in pitch, the nose rises on its own, beyond the pilot's control, until the aircraft stalls. On takeoff it can cause a sudden uncontrollable pitching up of the nose.
After seven years of powered flight, with design competition moving ahead of their airplane, the Wrights gave up their basic configuration. The 1910-1911 Model B was the first Wright aircraft to move the CG ahead of the neutral point. It was the first to move the tail feathers to the rear.
In 1909, Orville wrote to Wilbur about pitch stability: "The difficulty in handling our machine is due to the rudder (elevator) being in front...I do not think it necessary to lengthen the machine, but to simply put the rudder (elevator) behind instead of before."
Static stability is simply the ability, when trimmed, to continue flight without change in direction in glass-smooth air. Absent disturbances, a stable airplane doesn't tend to diverge from its trimmed condition. There are no aerodynamic forces trying to push the nose up or down, yaw the aircraft, or roll it. This is only so when the CG is located at the neutral point or ahead of it.
Dynamic stability in an aircraft refers to its ability to return to the desired flight path when disturbed by gusts. The tail structure corrects yaw (the vertical fin) and pitch (the horizontal stabilizer) like the feathers on an arrow. The process takes a few diminishing oscillations, and the amount of time necessary for an aircraft to do this is mandated in the federal aviation regulations.
Investigate your aircraft's stability yourself. In trimmed straight-and-level cruise, pull back smoothly on the yoke/stick, take your feet off the rudder pedals, kick a rudder, and release the yoke/stick. The aircraft will alternately trade yaw for roll. The action is produced by a combination of dihedral and the placement of the fin's center of area above and behind the aircraft's CG. These oscillations will probably take a little longer, but they will settle down.
The most potentially dangerous flight mode, as the Wrights discovered early in their flying program, is the stall/spin. In a spin the wing is stalled at a high angle of attack, and nose-down pitch is needed to recover. "A common scenario for stall/spin entry is simply getting too slow and not recognizing that you are getting close to loss of control," Foster says. "At high angles of attack, airplanes typically exhibit a loss of roll/yaw stability where the pilot can't maintain control. This is not something you wish to happen too close to the ground." Modern trainer aircraft typically take about 500 to 1,000 feet for a one-turn spin and recovery. Unintentional stall/spin entry is estimated to be responsible for loss of control in the majority of general aviation fatal accidents. Stall/spin accidents represented 10 percent of all accidents of fixed-wing aircraft weighing under 12,500 pounds from 1993 to 2001, but accounted for 13.7 percent of all fatal accidents, according to a new study by the AOPA Air Safety Foundation.
Fighters are designed to meet much different stability criteria and are far less stable by design. Some military aircraft, as we know, are so unstable to meet mission requirements that a pilot can't fly one without computer assistance. Computers continually sense aerodynamic forces acting on the airframe, make minute adjustments to the control surfaces, and make the aircraft "stable." The pilot "talks" to the computers with stick or yoke, and the computers fly the airplane. The stealth-technology B-2 bomber and F-117 fighter are two examples. The F-18 was the first line aircraft design to be completely flown by digital computers. Thanks to computer assistance and aerodynamic design, modern military fighters can be flown in stable flight at angles of attack as high as 60 degrees.
In 2003, pilots rely on a century of aeronautical research and development that puts fleets of safe, stable GA airplanes into the air. "Today's aircraft are stable and easy to fly for a wide range of pilots in the normal flight envelope," says Foster. "The industry has had many years and spent many dollars to develop design criteria, and these are well-accepted and well-proven."
The cheapest insurance you can buy against getting caught by the least stable flight mode is expressed in one word: Practice.
It's been said many times that pilots learn little from hours of straight-and-level cruise flight. They learn a lot from minutes of stall experience, and the most fortunate students will have learned even more per minute from spin training. Once you've soloed, you don't need your instructor beside you to practice stall recovery. Make it a point to do so regularly.
Photo courtesy AIAA
Don Byers has logged about 1,500 hours in aircraft from military T-33 jets and C-47 cargo aircraft to Piper Cherokees and a 1939 Piper J-4. He is a docent at the Virginia Air and Space Center in Hampton.
Want to know more?
If you enjoyed this article or the subject matter, you may find the following to be of interest:
- AOPA's Centennial of Flight resource page, including articles on the subject from AOPA Pilot.
- Additional information on the Wright brothers and their development of the Wright Flyer.
- Information on the various organizations building re-creations of the Flyer.
Links to these resources are available on AOPA Online.