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Asymmetrical aircraft have left- and right-hand sides which are not exact mirror images of each other. Although most aircraft are symmetrical, there is no fundamental reason why they must be, and design goals can sometimes be best achieved with an asymmetrical aircraft.
Asymmetry arises from a number of design decisions. Some are inherent in the type of aircraft, while others are consciously introduced.
On a powerful propeller-driven aircraft, the engine torque driving the propeller creates an equal and opposite torque on the engine itself. Because the engine is fixed to the airframe, this reactive torque is transmitted to the aircraft, causing a tendency to roll in the opposite direction to the propeller.
On some early types, especially during the pioneer years, a single engine drove twin propellers and the drive was arranged to turn the propellers in opposite directions to cancel their torque. Examples include the Wright Flyer and the early designs by J.W. Dunne.
In a rotary engine, common during World War I, the whole crankcase and cylinder assembly rotates with the propeller. This gives it an especially powerful torque reaction. Some aircraft, such as the Sopwith Camel with its relatively heavy Clerget 9B engine, were noted for having a faster turn to one side than the other, which influenced combat tactics both with it and against it. By contrast, the contra-rotary Siemens-Halske engines were more balanced.
On some single-engined types with more conventional engines, such as the Italian Ansaldo SVA engine torque was counteracted by lengthening one wing to create extra lift on that side, providing a counter-torque.
As engines became more powerful towards the end of World War II, some single-engined fighters used contra-rotating propellers, both to handle the high power without increasing diameter and to reduce the torque asymmetry.
Twin-engined aircraft with their propellers rotating in the same direction are also asymmetric. Counter-rotating propellers avoid this, either by building pairs of engines to rotate their crankshafts in opposite directions, or by using a reversing gear in one of the propeller reduction gearboxes. Handed engines have rarely been used, owing to cost, but were sometimes used for naval aircraft such as the Sea Hornet, to simplify their handling across a narrow carrier deck.
The wake of a propeller gains angular momentum, which can produce an asymmetric effect over the tail control surfaces, especially during takeoff when the engine power is at maximum but the aircraft speed is low. This does not affect the symmetry or otherwise of the aircraft itself.
While most aircraft have a central thrust line, it can sometimes be advantageous to break symmetry. For example, a single front-mounted tractor propeller may provide sufficient thrust, alongside a nose-mounted cockpit for good pilot visibility. In such cases the engine's thrust line is offset to one side, creating a turning moment. This moment is typically counteracted by making the tail fin work as a wing turned sideways, creating sideways lift to provide an equal and opposite turning force. The Northrop Grumman B-2 Spirit stealth bomber utilizes asymmetric thrust as a means to enhance yaw control. [1]
The mechanism for varying the angle of a swept wing is complicated, heavy and expensive. The two halves must be aligned with each other and each supported at one end. A single oblique wing may be supported in the middle and without needing a linking gear. The idea has been tried successfully on the NASA AD-1.
The first airplane to fly, the Wright Flyer had an asymmetrical arrangement of pilot and engine. Both needed to be close to the centre of gravity above the front of the wing, so each was moved to one side to make room for the other. The propellers were symmetrically placed, so one engine drive chain was longer than the other. The longer, port drive belt was also twisted across itself so that the propellers rotated in opposite directions.
During World War I, Swiss-born Hans Burkhard designed several asymmetrical aircraft. Burkhard obtained German Patent number 300 676 for his design on 22 September 1915. The Gotha G.VI first flew in 1918, but did not reach production before the war concluded. [2] [3]
During World War II, German designer Richard Vogt experimented with several asymmetrical aircraft, including:
The BV 141 was heralded by the Germans as the first asymmetric aircraft, [2] an evaluation batch was built, but it was never ordered into full-scale production. The BV 237 ground-attack aircraft was ordered but later cancelled.
Other projects and design studies of the period included:
The Blohm & Voss BV 155 was a German high-altitude interceptor aircraft intended to be used by the Luftwaffe against raids by USAAF Boeing B-29 Superfortresses. Work started on the design as the Messerschmitt Me 155 in 1942, but the project went through a protracted development period and change of ownership, and prototypes were still under test and development when World War II ended.
Hamburger Flugzeugbau (HFB) was an aircraft manufacturer, located primarily in the Finkenwerder quarter of Hamburg, Germany. Established in 1933 as an offshoot of Blohm & Voss shipbuilders, it later became an operating division within its parent company and was known as Abteilung Flugzeugbau der Schiffswerft Blohm & Voss from 1937 until it ceased operation at the end of World War II. In the postwar period it was revived as an independent company under its original name and subsequently joined several consortia before being merged to form Messerschmitt-Bölkow-Blohm (MBB). It participates in the present day Airbus and European aerospace programs.
The Blohm & Voss BV 141 was a World War II German tactical reconnaissance aircraft, notable for its uncommon structural asymmetry. Although the Blohm & Voss BV 141 performed well, it was never ordered into full-scale production, for reasons that included the unavailability of the preferred engine and competition from another tactical reconnaissance aircraft, the Focke-Wulf Fw 189.
Aircraft equipped with contra-rotating propellers (CRP) coaxial contra-rotating propellers, or high-speed propellers, apply the maximum power of usually a single piston engine or turboprop engine to drive a pair of coaxial propellers in contra-rotation. Two propellers are arranged one behind the other, and power is transferred from the engine via a planetary gear or spur gear transmission. Contra-rotating propellers are also known as counter-rotating propellers, although the term counter-rotating propellers is much more widely used when referring to airscrews on separate non-coaxial shafts turning in opposite directions.
The critical engine of a multi-engine fixed-wing aircraft is the engine that, in the event of failure, would most adversely affect the performance or handling abilities of an aircraft. On propeller aircraft, there is a difference in the remaining yawing moments after failure of the left or the right (outboard) engine when all propellers rotate in the same direction due to the P-factor. On turbojet and turbofan twin-engine aircraft, there usually is no difference between the yawing moments after failure of a left or right engine in no-wind condition.
A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The International Civil Aviation Organization (ICAO) defines a rotorcraft as "supported in flight by the reactions of the air on one or more rotors".
P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, wherein the propeller's center of thrust moves off-center when the aircraft is at a high angle of attack. This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yaw slightly to one side. A rudder input is required to counteract the yawing tendency.
Counter-rotating propellers (CRP) are propellers which turn in opposite directions to each other. They are used on some twin- and multi-engine propeller-driven aircraft.
Richard Vogt was a military German aircraft designer who was known for his original airframes, including the asymmetrical BV 141 during World War II. After the war, he moved to the United States as part of Operation Paperclip, where he worked on American military aircraft design.
The Blohm & Voss P 194 was a German design for a mixed-power Stuka or ground-attack aircraft and tactical bomber, during World War II.
The Blohm & Voss Bv P 188 was a long-range, heavy jet bomber design project by the Blohm & Voss aircraft manufacturing division during the last years of the Third Reich. It featured a novel W-wing planform with variable incidence.
The Blohm & Voss BV 237 was a German proposed dive bomber with an unusual asymmetric design based on the Blohm & Voss BV 141.
The Blohm & Voss P 163 was a design project for an unconventional bomber during World War II. Constructed mainly from steel, its crew were accommodated in large wingtip nacelles, giving it a triple-fuselage appearance. Its propeller drive system was also unusual, with the central fuselage containing twin engines coupled to a front-mounted contra-prop.
The Blohm & Voss P 204 was one of several design studies by Blohm & Voss for asymmetric dive bombers during World War II. It was also unusual in having hybrid propulsion comprising both piston and jet engines.
The Blohm & Voss P 192 was a design study for a dive bomber/ground attack aircraft intended to replace the Junkers Ju 87.
The Blohm & Voss P 193 was a design study for a dive bomber/ground attack aircraft intended to replace the Junkers Ju 87.
