Ultralight Trikes
From Wikipedia, the free encyclopedia
Ultralight Trikes, also known as Flexwing Trikes, are a type of powered hang glider, a form of propeller-powered aircraft that merges a rigid tube-and-fabric wing with a three-wheeled undercarriage. While most powered aircraft have three-wheeled landing gear, the term "trike" refers specifically to the form of aircraft described here.
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[edit] Control
Flight control in a trike is similar to controlling a hang glider, in which the aviator or pilot is suspended from a lightweight wing made from high-strength tubes and fabric. The pilot controls the attitude of the wing by holding onto a triangular control bar that is rigidly attached to the wing. Pushing, pulling, and turning the control bar causes a corresponding shift in the aircraft's center of gravity.
For instance, pushing the bar forward causes the center of gravity to shift back. This, in turn, causes the nose of the aircraft to pitch up, causing the angle of attack to increase which causes the aircraft to climb. In contrast, pushing forward on the control stick of a traditional aircraft would cause that aircraft to dive.
Turns are accomplished by moving the control bar sideways opposite from the direction of the intended turn. This causes the center of gravity- represented primarily by the weight of the undercarriage and pilot- to shift in the direction of the intended turn. This in itself does not cause the aircraft to turn, but it does cause the aircraft to bank, or tip, to the side.
A banking maneuver becomes a turn because of another feature of the trike's wing called dihedral. Dihedral refers to the a shallow V-shape to the wing when viewed from straight ahead. In other words, the wing tips are higher than the center of the wing. Dihedral makes it possible for a two-axis aircraft to have fully three-dimensional flight. For example, if the pilot wishes to turn left, he or she simply banks to the left by pushing the control bar away to the right. By holding the aircraft in that position and resisting the wing's natural tendency to return to straight-and-level flight, the pilot is causing the left wing to be level relative to the ground. At the same time, the right wing is being raised at an angle which allows wind to slip upward past it with less resistance or drag. Thus, the right wing actually travels faster through the air, turning the whole aircraft to the left. In contrast, a traditional aircraft with three-axis control is able to turn more directly using its rudder.
This similarity to the way in which a hang glider is controlled is no coincidence. In fact, trikes are essentially propeller-powered hang gliders with seats and wheels. Trikes have often employed wings originally designed for hang gliding. As weight and performance goals have increased purpose-built wings have become more commonplace.
[edit] Stability and Equilibrium
Because trikes are most often used for recreational flying by part time pilots, a premium is placed on gentle behavior, natural stability, and ease-of-operation.
Unlike a traditional aircraft with an extended fuselage and empennage for maintaining stability, trikes rely on the natural stability of their wings to return to equilibrium in each of the three axis of motion: roll, yaw, and pitch. In calm air, a properly designed wing will act almost as if it's being controlled by an autopilot.
For roll stability the trike's wing relies on dihedral to naturally return to the equilibrium of level flight. This is because departures from level flight cause the higher wing to lose lift and fall and the low wing to have more lift and rise.
The yaw axis, which represents the direction that the aircraft is facing relative to the wind, is stabilized through the sweep of the wings. Instead of having wings that extend almost straight out side-to-side as in many types of traditional light aircraft, trikes are provided a V-shaped wing when viewed from overhead. Thus, if one wing advances ahead of the other it presents more area to the wind and causes more drag on that side. This causes the advancing wing to go slower and to fall back. The wing is at equilibrium when the aircraft is traveling straight and both wings present the same amount of area to the wind.
The third axis, represented by pitch, is also stabilized by the sweep of the wings. However, it is not the effect of drag so much as the location of the lifting effect that allows the aircraft to maintain level flight. The undercarriage is always hung from the wing's natural center of lift. Thus, pitching down causes the aircraft to speed up. This, in turn, causes the wing to increase its life and rise. Conversely, pitching up causes the aircraft to slow down which, in turn, causes the nose to drop back down again. Due to the sweep of the wings, lift is being generated both in front of and behind the center of lift from which the undercarriage is hung. Thus, the need for a traditional tail is eliminated.
Furthermore, the fact that the wing is designed to bend and flex in the wind provides favorable dynamics analogous to a spring suspension. This allows the wing to be less susceptible to turbulence and provides a gentler flying experience than a similarly sized rigid-winded aircraft.
[edit] Engine Placement
Because trikes don't require an empennage, the space directly behind the pilot is used to mount the engine and propeller. Engines typically range from between 25-40 hp for single-seaters and 60-100 hp for two-seaters. An aft engine placement allows exhaust to stay behind the pilot(s) and enhances visibility. It also means that the turbulent vortex of air behind the propeller is not coming in contact with the wing.
[edit] History and Regulation
Trikes are referred to as "microlights" in Europe and have been extremely popular since the 1980's. In the United States trikes are often referred to as "ultralight trikes" and are designed to operate under the designation of the Federal Aviation Regulations (FAR 103) that define an ultralight as a single seat vehicle with under 5 US gallons (19 L) of fuel capacity, an empty weight of less than 254 pounds (115 kg), a top speed of 55 knots (102 km/h), and a maximum stall speed that doesn't exceed 24 knots (45 km/h). Furthermore, ultralights are only allowed to operate during daylight hours. FAR 103 makes further weight allowances for two-seat trainers(in which both persons are able to control the craft and thus definable as pilots), amphibious landing gear, and ballistic parachute systems.
[edit] Popularity
Due to their relatively low cost, low fuel consumption, light weight, ability to take off and land in very short distances, and ability to fly in somewhat turbulent conditions, trikes have become very popular with recreational pilots. In particular, trikes have been used to fly across oceans, frozen tundra, barren deserts, and even into backyard landing strips. As with all types of aviation, regulations in the United States dictate where and how these aircraft are allowed to fly, especially with regard to avoiding other air traffic and populated areas. Notwithstanding, trikes continue to grow in popularity with those living in urban areas due to their extremely portable nature and rapid assembly time. Most trikes may be transported on a very small trailer, stored in a standard garage, and assembled for flight in less than thirty minutes.
There are currently around 56 manufacturers from all over the world that build trikes [reference?]. Some started out making hang glider wings and now provide both wing and undercarriage. Many produce only the undercarriage and procure their wings elsewhere. The majority of these companies are found in Europe with a fair number appearing in the United States. Manufactures can also be found in India, South Africa, Australia, and elsewhere. Many websites can be found featuring videos of trikes in flight, along with photos, how-to articles, and discussion forums.
