A slip is an aerodynamic state where an aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow or relative wind. In other words, for a conventional aircraft, the nose will be pointing in the opposite direction to the bank of the wing; the aircraft therefore is flying inefficiently. Flying in a slip is aerodynamically inefficient. More drag is at play consuming energy but not producing lift. Inexperienced or inattentive pilots will enter slips unintentionally during turns by failing to coordinate the aircraft with the rudder. Airplanes can enter into a slip climbing out from take-off on a windy day. If left unchecked, climb performance will suffer; this is dangerous if there are nearby obstructions under the climb path and the aircraft is underpowered or loaded. A slip can be a piloting maneuver where the pilot deliberately enters one type of slip or another. Slips are useful in performing a short field landing over an obstacle, or to avoid an obstacle, may be practiced as part of emergency landing procedures.
These methods are commonly employed when flying into farmstead or rough country airstrips where the landing strip is short. Pilots need to touch down with ample runway remaining to stop. There are common situations where a pilot may deliberately enter a slip by using opposite rudder and aileron inputs, most in a landing approach at low power. Without flaps or spoilers it is difficult to increase the steepness of the glide without adding significant speed; this excess speed can cause the aircraft to fly in ground effect for an extended period running out of runway. In a forward slip much more drag is created, allowing the pilot to dissipate altitude without increasing airspeed, increasing the angle of descent. Forward slips are useful when operating pre-1950s training aircraft, aerobatic aircraft such as the Pitts Special or any aircraft with inoperative flaps or spoilers. If an airplane in a slip is made to stall, it displays little of the yawing tendency that causes a skidding stall to develop into a spin.
A stalling airplane in a slip may do little more. In fact, in some airplanes stall characteristics may be improved. Aerodynamically these are identical once established, but they are entered for different reasons and will create different ground tracks and headings relative to those prior to entry. Forward-slip is used to steepen an approach without gaining much airspeed, benefiting from the increased drag; the sideslip moves the aircraft sideways where executing a turn would be inadvisable, drag is considered a byproduct. Most pilots like to enter sideslip just before touching down during a crosswind landing; the forward slip changes the heading of the aircraft away from the down wing, while retaining the original track of the aircraft. To execute a forward slip, the pilot banks into the wind and applies opposing rudder in order to keep moving towards the target. If you were the target you would see the plane's nose off to one side, a wing off to the other side and tilted down toward you; the pilot must make sure.
However, airframe speed limits such as VA and VFE must be observed. A forward-slip is useful when a pilot has set up for a landing approach with excessive height or must descend steeply beyond a tree line to touchdown near the runway threshold. Assuming that the plane is properly lined up for the runway, the forward slip will allow the aircraft track to be maintained while steepening the descent without adding excessive airspeed. Since the heading is not aligned with the runway, forward-slip must be removed before touchdown to avoid excessive side loading on the landing gear, if a cross wind is present an appropriate sideslip may be necessary at touchdown as described below. In the United States, student pilots are required to know how to do forward slips before embarking on their first solo flight; the logic is that in the event of an engine failure, the pilot will have to land on the first attempt and will not have a chance to go around if the aircraft is too high or too fast. The sideslip uses aileron and opposite rudder.
In this case it is entered by lowering a wing and applying enough opposite rudder so the airplane does not turn, while maintaining safe airspeed with pitch or power. Compared to Forward-slip, less rudder is used: just enough to stop the change in the heading. In the sideslip condition, the airplane's longitudinal axis remains parallel to the original flightpath, but the airplane no longer flies along that track; the horizontal component of lift is directed toward the low wing. This is the headwind or tailwind scenario. In case of crosswind, the wing is lowered into the wind, so that the airplane flies the original track; this is the sideslip approach technique used by many pilots in crosswind conditions. The other method of maintaining the desired track is the crab technique: the wings are kept level, but the nose is pointed into the crosswind, resulting drift keeps the airplane on track. A sideslip may be used to remain lined up with a runway centerline while on approach in a crosswind or be employed in the final moments of a crosswind landing.
To commence sideslipping, the pilot rolls the airplane toward the wind to maintain runway centerlin
A short take-off and vertical landing aircraft is a fixed-wing aircraft, able to take off from a short runway and land vertically. The formal NATO definition is: A Short Take-Off and Vertical Landing aircraft is a fixed-wing aircraft capable of clearing a 15 m obstacle within 450 m of commencing take-off run, capable of landing vertically. On aircraft carriers, non-catapult-assisted, fixed-wing short takeoffs are accomplished with the use of thrust vectoring, which may be used in conjunction with a runway "ski-jump". Use of STOVL tends to allow aircraft to carry a larger payload as compared to during VTOL use, while still only requiring a short runway; the most famous examples are the Sea Harrier. Although technically VTOL aircraft, they are operationally STOVL aircraft due to the extra weight carried at take-off for fuel and armaments; the same is true of the F-35B Lightning II, which demonstrated VTOL capability in test flights but is operationally STOVL. In 1951, the Lockheed XFV and the Convair XFY Pogo tailsitters were both designed around the Allison YT40 turboprop engine driving contra-rotating propellers.
The British Hawker P.1127 took off vertically in 1960, demonstrated conventional take-off in 1961. It was developed into the Hawker Siddeley Harrier which flew in 1967. In 1962, Lockheed built the XV-4 Hummingbird for the U. S. Army, it sought to "augment" available thrust by injecting the engine exhaust into an ejector pump in the fuselage. First flying vertically in 1963, it suffered a fatal crash in 1964, it was converted into the XV-4B Hummingbird for the U. S. Air Force as a testbed for separate, vertically mounted lift engines, similar to those used in the Yak-38 Forger; that plane flew and crashed in 1969. The Ryan XV-5 Vertifan, built for the U. S. Army at the same time as the Hummingbird, experimented with gas-driven lift fans; that plane used fans in the nose and each wing, covered by doors which resembled half garbage can lids when raised. However, it crashed twice, proved to generate a disappointing amount of lift, was difficult to transition to horizontal flight. Of dozens of VTOL and V/STOL designs tried from the 1950s to 1980s, only the subsonic Hawker Siddeley Harrier and Yak-38 Forger reached operational status, with the Forger being withdrawn after the fall of the Soviet Union.
Rockwell International built, abandoned, the Rockwell XFV-12 supersonic fighter which had an unusual wing which opened up like window blinds to create an ejector pump for vertical flight. It never generated enough lift to get off the ground despite developing 20,000 lbf of thrust; the French had a nominally Mach 2 Dassault Mirage IIIV fitted with no less than 8 lift engines that flew, but did not have enough space for fuel or payload for combat missions. The German EWR VJ 101 used swiveling engines mounted on the wingtips with fuselage mounted lift engines, the VJ 101C X1 reached supersonic flight on 29 July 1964; the supersonic Hawker Siddeley P.1154, which competed with the Mirage IIIV for use in NATO, was cancelled as the aircraft were being built. NASA uses the abbreviation SSTOVL for Supersonic Short Take-Off / Vertical Landing, as of 2012, the X-35B/F-35B are the only aircraft to conform with this combination within one flight; the experimental Mach 1.7 Yakovlev Yak-141 did not find an operational customer, but similar rotating rear nozzle technology is used on the F-35B.
The F-35B Lightning II entered service on July 31, 2015. Larger STOVL designs were considered, the Armstrong Whitworth AW.681 cargo aircraft was under development when cancelled in 1965. The Dornier Do 31 got as far as three experimental aircraft before cancellation in 1970. Although a VTOL design, the V-22 Osprey has increased payload when taking off from a short runway
Takeoff and landing
Aircraft can have different ways to take off and land. Conventional airplanes accelerate along the ground until sufficient lift is generated for takeoff, reverse the process to land; some airplanes can take off at this being a short takeoff. Some aircraft such as helicopters and Harrier Jump Jets can land vertically. Rockets usually take off vertically, but some designs can land horizontally. Takeoff is the phase of flight in which an aircraft goes through a transition from moving along the ground to flying in the air starting on a runway. For balloons and some specialized fixed-wing aircraft, no runway is needed. Takeoff is the opposite of landing. Landing is the last part of a flight, where spacecraft returns to the ground; when the flying object returns to water, the process is called alighting, although it is called "landing" and "touchdown" as well. A normal aircraft flight would include several parts of flight including taxi, climb, cruise and landing. STOL is an acronym for short take-off and landing, aircraft with short runway requirements.
CATOBAR is a system used for the launch and recovery of aircraft from the deck of an aircraft carrier. Under this technique, aircraft are launched using a catapult and land on the ship using arrestor wires. Although this system is more costly than alternative methods, it provides greater flexibility in carrier operations, since it allows the vessel to support conventional aircraft. Alternate methods of launch and recovery can only use aircraft with STOBAR capability. STOBAR is a system used for the launch and recovery of aircraft from the deck of an aircraft carrier, combining elements of both STOVL and CATOBAR. Horizontal takeoff, horizontal landing — is the mode of operation for the first private commercial spaceplane, the two-stage-to-space Scaled Composites Tier One from the Ansari X-Prize SpaceShipOne/WhiteKnightOne combination, it is used for the upcoming Tier 1b SpaceShipTwo/WhiteKnightTwo combination. A prominent example of its use was the North American X-15 program. In these examples the space craft are carried to altitude on a "mother ship" before launch.
The failed proposals for NASA Space Shuttle replacements, Rockwell X-30 NASP used this mode of operation but were conceived as single stage to orbit. The Lynx rocketplane is a suborbital HTHL spaceplane, slated to begin atmospheric flight testing in late 2011, yet has not as of late 2015. Reaction Engines Skylon, a design descendant of the 1980s British HOTOL design project, is an HTHL spaceplane in the early stages of development in the United Kingdom. Both the Lynx rocketplane and SpaceShipTwo have been proffered to NASA to carry suborbital research payloads in response to NASA's suborbital reusable launch vehicle solicitation under the NASA Flight Operations Program. An early example was the 1960s Northrop HL-10 atmospheric test aircraft where the HL stands for "Horizontal Lander". Different terms are used for landing depending on the source of thrust used. VTVL uses. VTOL is an acronym for vertical landing aircraft; this classification includes fixed-wing aircraft that can hover, take off and land vertically as well as helicopters and other aircraft with powered rotors, such as tiltrotors.
The terminology for spacecraft and rockets is VTVL. Some VTOL aircraft can operate in other modes as well, such as CTOL, STOL, and/or STOVL. Others, such as some helicopters, can only operate by VTOL, due to the aircraft lacking landing gear that can handle horizontal motion. VTOL is a subset of V/STOL. Besides the ubiquitous helicopter, there are two types of VTOL aircraft in military service: craft using a tiltrotor, such as the Bell Boeing V-22 Osprey, aircraft using directed jet thrust such as the Harrier family. Vertical takeoff, vertical landing is a form of takeoff and landing proposed for expendable spacecraft. Multiple VTVL rocket craft have flown. In aviation the term VTOHL as well as several VTOHL aviation-specific subtypes: VTOCL, VTOSL, VTOBAR exist; the zero length launch system or zero length take-off system was a system whereby jet fighters and attack aircraft were intended to be placed upon rockets attached to mobile launch platforms. Most zero length launch experiments took place during the Cold War.
VTHL—vertical takeoff, horizontal landing—is the mode of operation for all current and operational orbital spaceplanes, such as the Boeing X-37, the NASA Space Shuttle, the 1988 Soviet Buran space shuttle, as well as the circa-1960 USAF Boeing X-20 Dyna-Soar project. For launch vehicles an advantage of VTHL over HTHL is that the wing can be smaller, since it only has to carry the landing weight of the vehicle, rather than the takeoff weight. There have been several other VTHL proposals that never flew including NASA Space Shuttle proposed replacements Lockheed Martin X-33 and VentureStar; the 1990s NASA concept spaceplane, the HL-20 Personnel Launch System, was VTHL, as was a circa-2003 derivative of the HL-20, the Orbital Space Plane concept. As of March 2011, two VTHL commercial spaceplanes were in various st
JATO, is a type of assisted take-off for helping overloaded aircraft into the air by providing additional thrust in the form of small rockets. The term JATO is used interchangeably for rocket-assisted take-off. Early experiments using rockets to boost gliders into the air were conducted in Germany in the 1920s, both the Royal Air Force and the Luftwaffe introduced such systems in World War II; the British system used large solid fuel rockets to shoot planes off a small ramp fitted to the fronts of merchant ships, known in service as Catapult armed merchantmen, in order to provide some cover against German maritime patrol planes. After firing, the rocket was released from the back of the plane to sink; the task done, the pilot would fly to friendly territory if possible or parachute from the plane to be picked up by one of the escort vessels. Over two years the system was only employed nine times to attack German aircraft with eight kills recorded for the loss of a single pilot; the Luftwaffe used the technique with both liquid-fueled units made by the Walter firm and BMW – and solid fuel, themselves made both by the Schmidding and WASAG firms – as both attached and jettisonable rocket motors, to get airborne more and with shorter takeoff runs.
These were used to boost the takeoff performance of their medium bombers, the enormous 55-meter wingspan Gigant, Messerschmitt Me 321 glider, conceived in 1940 for the invasion of Britain, used to supply the Russian front. The enormous Me 321s had air tow assistance from up to three Messerschmitt Bf 110 heavy fighters in a so-called Troika-Schlepp arrangement into the air with loads that would have made the takeoff run too long otherwise, but with much attendant risk of aerial collision from the trio of vee-formation Bf 110s involved in a simultaneous towplane function, meant to be eased with the substitution of the trio of Bf 110s with a single example of the unusual, twin-fuselage Heinkel He 111Z purpose-designed five-engined towplane; the use of reaction-assisted takeoff methods became important late in the war when the lengths of usable runways were curtailed due to the results of Allied bombing. Their system used jettisonable, self-contained Walter HWK 109-500 Starthilfe known as "Rauchgerät" - smoke generator, unitized liquid-fuel monopropellant rocket booster units whose engines driven by chemical decomposition of "T-Stoff" almost pure hydrogen peroxide, with a Z-Stoff catalytic compound.
A parachute pack at the blunt-contour front of the motor's exterior housing was used to slow its fall after being released from the plane, so the system could be re-used. First experiments were held in 1937 on a Heinkel He 111, piloted by test-pilot Erich Warsitz at Neuhardenberg, a large field about 70 kilometres east of Berlin, listed as a reserve airfield in the event of war. Other German experiments with JATO were aimed at assisting the launch of interceptor aircraft such as the Messerschmitt Me 262C, as the Heimatschützer special versions fitted with either a version of the Walter HWK 109-509 liquid fuelled rocket engine from the Me 163 Komet program either in the extreme rear of the fuselage or semi-"podded" beneath it just behind the wing's trailing edge, to assist its Junkers Jumo 004 turbojets, or a pair of specially rocket-boosted BMW 003R combination jet-rocket powerplants in place of the Jumo 004s, so that the Me 262C Heimatschützer interceptors could reach enemy bomber formations sooner.
Two prototypes of the Heimatschützer versions of the Me 262 were built and test flown, of the three designs proposed. In contrast to the wide variety of aircraft types that the HWK-designed Starthilfe modular liquid monopropellant booster designs were tested with, seeing some degree of front-line use; the experimental, HWK 109-501 Starthilfe RATO system used a similar bi-propellant "hot" motor to that on the Me 163B Komet rocket fighter - adding a 20 kg mass of a combination of B-stoff hydrazine, mixed with "Br-stoff" for a main "fuel" to the T-Stoff monopropellant still destabilized with the Z-Stoff permanganate for ignition as the oxidizer, tripling the 109-500's thrust figure of 4.95 kN with a burn of 30 second duration — due to the "hot" system's similar risks demanding similar special fueling and handling procedures to that of the Komet's 509A rocket motor, the 109-501 seems to have remained a experimental design, only being used for the test flights of the Junkers Ju 287 V1 prototype jet bomber.
In early 1939, the United States National Academy of Sciences provided $1,000 to Theodore von Kármán and the Rocket Research Group at the Guggenheim Aeronautical Laboratory to research rocket-assisted take-off of aircraft. This JATO research was the first rocket research to receive financial assistance from the U. S. government since World War I when Robert Goddard had an Army contract to develop solid fuel rocket weapons. In late 1941 von Kármán and his team attached several 50-pound thrust, solid fuel Aerojet JATOs to a light Ercoupe plane, Army Captain Homer Boushey took off
A vertical take-off and landing aircraft is one that can hover, take off, land vertically. This classification can include a variety of types of aircraft including fixed-wing aircraft as well as helicopters and other aircraft with powered rotors, such as cyclogyros/cyclocopters and tiltrotors; some VTOL aircraft can operate in other modes as well, such as CTOL, STOL, and/or STOVL. Others, such as some helicopters, can only operate by VTOL, due to the aircraft lacking landing gear that can handle horizontal motion. VTOL is a subset of V/STOL; some lighter-than-air aircraft qualify as VTOL aircraft, as they can hover and land with vertical approach/departure profiles. Besides the ubiquitous helicopter, there are two types of VTOL aircraft in military service: craft using a tiltrotor, such as the Bell Boeing V-22 Osprey, another using directed jet thrust, such as the Harrier family and new F-35B Lightning II Joint strike Fighter. In the civilian sector only helicopters are in general use. Speaking, VTOL aircraft capable of STOVL use it wherever possible, since it significantly increases takeoff weight, range or payload compared to pure VTOL.
The idea of vertical flight has been around for thousands of years and sketches for a VTOL shows up in Leonardo da Vinci's sketch book. Manned VTOL aircraft, in the form of primitive helicopters, first flew in 1907 but would take until after World War Two to perfect. In addition to helicopter development, many approaches have been tried to develop practical aircraft with vertical take-off and landing capabilities including Henry Berliner's 1922–1925 experimental horizontal rotor fixed wing aircraft, Nikola Tesla's 1928 patent and George Lehberger's 1930 patent for impractical VTOL fixed wing airplanes with a tilting engines. In the late 1930s British aircraft designer Leslie Everett Baynes was issued a patent for the Baynes Heliplane, another tilt rotor aircraft. In 1941 German designer Heinrich Focke's began work on the Focke-Achgelis Fa 269, which had two rotors that tilted downward for vertical takeoff, but wartime bombing halted development. In May 1951, both Lockheed and Convair were awarded contracts in the attempt to design and test two experimental VTOL fighters.
Lockheed produced the XFV, Convair producing the Convair XFY Pogo. Both experimental programs proceeded to flight status and completed test flights 1954–1955, when the contracts were cancelled; the X-13 flew a series of test flights between 1955 and 1957, but suffered the same fate. The use of vertical fans driven by engines was investigated in the 1950s; the US built an aircraft where the jet exhaust drove the fans, while British projects not built included fans driven by mechanical drives from the jet engines. NASA has flown other VTOL craft such as the Bell XV-15 research craft, as have the Soviet Navy and Luftwaffe. Sikorsky tested an aircraft dubbed the X-Wing, which took off in the manner of a helicopter; the rotors would become stationary in mid-flight, function as wings, providing lift in addition to the static wings. Boeing X-50 is a Canard Rotor/Wing prototype. A different British VTOL project was the gyrodyne, where a rotor is powered during take-off and landing but which freewheels during flight, with separate propulsion engines providing forward thrust.
Starting with the Fairey Gyrodyne, this type of aircraft evolved into the much larger twin-engined Fairey Rotodyne, that used tipjets to power the rotor on take-off and landing but which used two Napier Eland turboprops driving conventional propellers mounted on substantial wings to provide propulsion, the wings serving to unload the rotor during horizontal flight. The Rotodyne was developed to combine the efficiency of a fixed-wing aircraft at cruise with the VTOL capability of a helicopter to provide short haul airliner service from city centres to airports; the CL-84 was a Canadian V/STOL turbine tilt-wing monoplane designed and manufactured by Canadair between 1964 and 1972. The Canadian government ordered three updated CL-84s for military evaluation in 1968, designated the CL-84-1. From 1972 to 1974, this version was demonstrated and evaluated in the United States aboard the aircraft carriers USS Guam and USS Guadalcanal, at various other centres; these trials involved military pilots from the United Kingdom and Canada.
During testing, two of the CL-84s crashed due to mechanical failures, but no loss of life occurred as a result of these accidents. No production contracts resulted. Although tiltrotors such as the Focke-Achgelis Fa 269 of the mid-1940s and the Centro Técnico Aeroespacial "Convertiplano" of the 1950s reached testing or mock-up stages, the V-22 Osprey is considered the world's first production tiltrotor aircraft, it has one three-bladed proprotor, turboprop engine, transmission nacelle mounted on each wingtip. The Osprey is a multi-mission aircraft with both a vertical takeoff and landing and short takeoff and landing capability, it is designed to perform missions like a conventional helicopter with the long-range, high-speed cruise performance of a turboprop aircraft. The FAA classifies the Osprey as a model of powered lift aircraft. Attempts were made in the 1960s to develop a commercial passenger aircraft with VTOL capability; the Hawker Siddeley Inter-City Vertical-Lift proposal had two rows of lifting fans on either side.
However, none of these aircraft made it to production after they were dismissed as too heavy and expensive
CATOBAR is a system used for the launch and recovery of aircraft from the deck of an aircraft carrier. Under this technique, aircraft launch using a catapult-assisted take-off and land on the ship using arrestor wires. Although this system is costlier than alternative methods, it provides greater flexibility in carrier operations, since it imposes less onerous design elements on fixed wing aircraft than alternative methods of launch and recovery such as STOVL or STOBAR, allowing for a greater payload for more ordnance and/or fuel. CATOBAR can launch aircraft that lack a high thrust to weight ratio, including heavier non-fighter aircraft such as the E-2 Hawkeye and Grumman C-2 Greyhound; the catapult system in use in modern CATOBAR carriers is the steam. Its primary advantage is the amount of control it can provide. During World War II the US Navy used a hydraulic catapult; the United States Navy is developing a system to launch carrier-based aircraft from catapults using a linear motor drive instead of steam, called the EMALS.
Only two states operate carriers that use the CATOBAR system following the decommissioning of Brazil's NAe São Paulo in February 2017. S. with its Nimitz-class and Gerald R. Ford-class and France with its Charles De Gaulle. U. S. Navy Gerald R. Ford-class carriers will use the EMALS electromagnetic aircraft launch system in place of steam catapults; the Chinese Type 002 aircraft carrier under construction at the Jiangnan Shipyard, will feature an integrated electric propulsion system that will allow the operation of electromagnetic launch catapults, similar to the Electromagnetic Aircraft Launch System used by the United States Navy. INS Vishal, India's second indigenous aircraft carrier of the Vikrant-class, is planned to be of 65,000 ton displacement and to utilize the EMALS electromagnetic aircraft launch system developed by General Atomics as it supports heavier fighters, AEW aircraft and UCAVs that cannot launch using a STOBAR ski jump ramps. List of all aircraft carriers
A rudder is a primary control surface used to steer a ship, submarine, aircraft, or other conveyance that moves through a fluid medium. On an aircraft the rudder is used to counter adverse yaw and p-factor and is not the primary control used to turn the airplane. A rudder operates by redirecting the fluid past the hull or fuselage, thus imparting a turning or yawing motion to the craft. In basic form, a rudder is a flat plane or sheet of material attached with hinges to the craft's stern, tail, or after end. Rudders are shaped so as to minimize hydrodynamic or aerodynamic drag. On simple watercraft, a tiller—essentially, a stick or pole acting as a lever arm—may be attached to the top of the rudder to allow it to be turned by a helmsman. In larger vessels, pushrods, or hydraulics may be used to link rudders to steering wheels. In typical aircraft, the rudder is operated by pedals via mechanical hydraulics. A rudder is "part of the steering apparatus of a boat or ship, fastened outside the hull", denoting all different types of oars and rudders.
More the steering gear of ancient vessels can be classified into side-rudders and stern-mounted rudders, depending on their location on the ship. A third term, steering oar, can denote both types. In a Mediterranean context, side-rudders are more called quarter-rudders as the term designates more the place where the rudder was mounted. Stern-mounted rudders are uniformly suspended at the back of the ship in a central position. Although some classify a steering oar as a rudder, others argue that the steering oar used in ancient Egypt and Rome was not a true rudder and define only the stern-mounted rudder used in ancient Han China as a true rudder; the steering oar has the capacity to interfere with handling of the sails while it was fit more for small vessels on narrow, rapid-water transport. In regards to the ancient Phoenician use of the steering oar without a rudder in the Mediterranean, Leo Block writes: A single sail tends to turn a vessel in an upwind or downwind direction, rudder action is required to steer a straight course.
A steering oar was used at this time. With a single sail, a frequent movement of the steering oar was required to steer a straight course; the second sail, located forward, could be trimmed to offset the turning tendency of the main sail and minimize the need for course corrections by the steering oar, which would have improved sail performance. The steering oar or steering board is an oversized oar or board to control the direction of a ship or other watercraft prior to the invention of the rudder, it is attached to the starboard side in larger vessels, though in smaller ones it is if attached. Rowing oars set aside for steering appeared on large Egyptian vessels long before the time of Menes. In the Old Kingdom as many as five steering oars are found on each side of passenger boats; the tiller, at first a small pin run through the stock of the steering oar, can be traced to the fifth dynasty. Both the tiller and the introduction of an upright steering post abaft reduced the usual number of necessary steering oars to one each side.
Single steering oars put on the stern can be found in a number of tomb models of the time during the Middle Kingdom when tomb reliefs suggests them employed in Nile navigation. The first literary reference appears in the works of the Greek historian Herodotus, who had spent several months in Egypt: "They make one rudder, this is thrust through the keel" meaning the crotch at the end of the keel. In Iran, oars mounted on the side of ships for steering are documented from the 3rd millennium BCE in artwork, wooden models, remnants of actual boats. Roman navigation used sexillie quarter steering oars that went in the Mediterranean through a long period of constant refinement and improvement, so that by Roman times ancient vessels reached extraordinary sizes; the strength of the steering oar lay in its combination of effectiveness and simpleness. Roman quarter steering oar mounting systems survived intact through the medieval period. By the first half of the 1st century AD, steering gear mounted on the stern were quite common in Roman river and harbour craft as proved from reliefs and archaeological finds.
A tomb plaque of Hadrianic age shows a harbour tug boat in Ostia with a long stern-mounted oar for better leverage. The boat featured a spritsail, adding to the mobility of the harbour vessel. Further attested Roman uses of stern-mounted steering oars includes barges under tow, transport ships for wine casks, diverse other ship types; the well-known Zwammerdam find, a large river barge at the mouth of the Rhine, featured a large steering gear mounted on the stern. According to new research, the advanced Nemi ships, the palace barges of emperor Caligula, may have featured 14 m long rudders; the world's oldest known depiction of a sternpost-mounted rudder can be seen on a pottery model of a Chinese junk dating from the 1st century AD during the Han Dynasty, predating their appearance in the West by a thousand years. In China, miniature models of ships t