<< Page 16 Page 17: Page 18 >>   Unusual Airplane Controls Although the aileron, elevator and rudder are the standard ways to get a plane to roll, pitch and yaw, they are by no means the only way. Unless you fly in a jet fighter or experimental airplane, though, you are not likely to experience alternative mechanisms first hand. The most common exception, found in many military aircraft and some small aircraft, are all-moving tailplanes. Instead of having a hinged plate that goes up or down to be the elevator, the whole horizontal tail tilts up and down, in effect a huge elevator. Very few airplanes have all-moving rudders (the SR-71 is the only one that comes to mind), but many have all-moving elevators (most jet fighters do, and even some smaller aerobatic airplanes). Above, a Canadian F-18 pilot tests his controls before takeoff. Here, he tests “roll left”. By the wingtips, the left aileron is up and the right aileron (like the flaps) is down. Similarly, the entire horizontal tail on the left deflected down, and the entire horizontal tail on the right deflected up (see “Elevons”). The next two exceptions were actually the standard during the early days of aviation, and can be found on the Wright Flier and its contemporaries. One is the Canard. The Canard is essentially an inverse-elevator mounted near the nose. If you want the nose to go up (pitch up), tilt your canards to a higher angle of attack, and they generate lift and push the nose up. They can also be tilted down to pull the nose down. Many modern canard airplanes can tilt one canard up and one canard down, for roll (more lift on one side, less lift on the other). Most European jet fighters have canards, as do some American kitplanes (mostly those designed by Burt Rutan), some Russian planes (like the SuperFlanker fighters and the Tu144), and some American experimental planes (like the F-15 ACTIVE, the X-31, X-36, etc). As a side note, “canard” means “duck” in French, and the term was used because canard airplanes usually had long necks with the wings at the back and the little canards at the front, so they looked like a duck in flight. Above, the F-15 ACTIVE, X-36, JA37 Viggen and JAS39 Gripen Some planes have non-moving canards just to generate lift near the front, or for more lift during landing. These canards are not used in maneuvering the plane, they just provide more lift at high angles of attack, and better balance (for stability – see page 100). The Tu144, Piaggio Avanti, VariViggen, Long EZ and B-1B are some examples: The other exception that was standard during the dawn of aviation but quickly abandoned as wings became stronger, is wing-warping. This involves twisting an entire wing in order to change its angle of incidence, to make it generate more or less lift. The wing becomes a huge aileron. In most early airplanes, wires were attached to the front and back of the wingtips. To roll left, the right wingtip would be twisted up (front goes up, back goes down) and the left wingtip would twist down. As wings became stiffer so airplanes could be heavier and faster, this became quite impossible... Above, the cables that warp and twist the wings of a Bleriot 9 are clearly visible, originating at the mast at the top, and at the wooden frame by the landing gear.Below, an Hanriot, its wings being twisted by the pilot: Recently, NASA engineers modified an F-18’s wings and control system with flexible but durable materials, allowing for its wings to be twisted by the ailerons during flight. Roll rates were higher than before the mod. Control surfaces form vortices at their tips, so induced drag is lower in the wing-warping F-18 while it is rolling than in a regular F/A-18 while it is rolling. One cannot say right now how popular this technology will become, especially with larger aircraft, but airliners are always looking for a way to save fuel... Above; Draggy vortices always form at the edges of control surfaces, as can be seen on the edge of a flap of a landing 737. This is the main argument for modern wing-warping. Other exceptions to the aileron-elevator-rudder system: Most fighters (and almost all tail-less airplanes like delta-winged planes such as the Concorde, Mirage, and Blackbird, and like flying wings such as the B-2 and N9M) have elevons. These are surfaces at the back of the airplane that work as ailerons and elevators. To pitch up, they both deflect up. To roll right, the right one goes up and the left one goes down. To do both, the right one goes up and the left one stays in place. In other words, their average deflection controls pitch, and the difference between their deflections controls roll. Above; Elevons at the back of a remote-control flying wing (same as on a B-2). The elevators in airliners can, if the wheel/stick is turned all the way in one direction, deflect differentially and work as elevons. “Tail-less airplanes” refers to planes without a horizontal tail (the place where the elevators would go). The Me163 Komet is a classic example, as are its American (X-4), British (Swallow) and Japanese (J8M) imitations. But some airplanes, like many flying wings, and new stealth prototypes like the X-47, do not have vertical tails either. This means either the pilot has no rudder controls (true for some airplanes, which makes them slightly harder to fly and much harder to land), or that the plane has drag-rudder air brakes. Drag-rudder air brakes are pairs of plates at the trailing edge of the wings, by the wingtips. They “open” – the top one goes up, the bottom one goes down – to create separation, and a big low pressure area behind themselves, like a parachute. If the right one opens, for example, the right wing will have a large wake behind it and will be sucked back, turning the plane to the right. So drag brakes can work (separately) as drag rudders. Of course, if both are opened at the same time by the same amount, both wings are sucked back equally and the plane simply slows down. AboveB-2 drag rudders slow it down for landing. See page 128 for more examples. Paragliders and powered parachutes (like the ones in The World Is Not Enough) use a control system very similar to drag rudders. They generate lift by having air go through a row of curved “pipes” (remember Euler-N?), channels that go from the front of the parachute to the back. By pulling on a cable, the pilot pulls a piece of fabric into the channel which blocks the channel on one side (to turn to that side) or both sides (to brake) - this works like drag rudders. Hang-gliders and Trikes use yet another method, one similar to that used by many small helicopters (like the homebuilt ramjet and pulsejet helicopters you saw in the Thrust section): The line where the center of lift acts must go through the center of gravity of the aircraft for straight-line flight (1). By tilting the entire wing (or rotor, in a helicopter) with respect to the center of gravity, the line where the center of lift acts no longer goes through the CG (2), which is usually roughly where the pilot is. More lift to one side means that side gets pulled up (3), and less lift on the other side means that side drops, and the aircraft banks or pitches. Another exception to conventional control is Thrust Vectoring: Thrust Vectoring is simply the engine’s ability to release air at angles other than forwards. If at the back of a fighter, air is released upwards, this is like the elevators being deflected upwards – the tail goes down, the nose goes up. If the exhaust is instead deflected to either side, the plane yaws. If two engines can deflect air, one up and one down, then the plane rolls. Notice that, except for thrust vectoring, all the other forms of control rely on air flowing over the control surfaces (all those different moving plates) at the right angle and at some significant speed. This means that if a plane is falling backwards like during a tail slide, or if it stalls, etc, the control surfaces might not do their job, and only thrust vectoring and some of the surfaces would work. Thrust-vectoring allows a pilot to control the attitude of his aircraft very precisely. The plane can be rolled, pitched, or yawed in any way the pilot desires, at any speed, at any angle. A thrust-vectoring jet can fly at 90 degrees angle-of-attack, do backflips, or even hover in place with the nose pointed straight up. Below is a video showing a lot of what Thrust Vectoring allows for: The only other system that always works (regardless of airflow) is RCS, or response-control system. This involves ejecting high-pressure gas (sometimes bled from a jet engine’s compressor, sometimes from a high-pressure container) at the wingtips (up and down for roll) and at the nose and tail (up and down for pitch, to the sides for yaw). This is how the Space Shuttle and other spacecraft are controlled in orbit, how the Harrier and Joint Strike Fighter and other VTOL jets are controlled during hover, and how that NF-104 and the X-15 were controlled in space. Above: the shuttle yaws right, as the RCS jet pushes left on the right side of the tail. Below: A Harrier’s tail. Openings for the yaw RCS are visible.