Coffin corner (aerodynamics)
Coffin corner is the region of flight where a fast fixed-wing aircraft's stall speed is near the critical Mach number, at a given gross weight and G-force loading. In this region of flight, it is difficult to keep the airplane in stable flight; because the stall speed is the minimum speed required to maintain level flight, any reduction in speed will cause the airplane to stall and lose altitude. Because the critical Mach number is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves, any increase in speed will cause the airplane to lose lift, or to pitch nose-down, lose altitude; the "corner" refers to the triangular shape at the top of a flight envelope chart where the stall speed and critical Mach number are within a few knots of each other. The speed where they meet is the ceiling of the aircraft; this is distinct from the same term used for helicopters when outside the auto-rotation envelope as seen in the height-velocity diagram.
Consideration of statics shows that when a fixed-wing aircraft is in straight, level flight at constant-airspeed the lift on the main wing plus the force on the horizontal stabilizer is equal to the aircraft's weight. In most circumstances this equilibrium can occur at a range of airspeeds; the minimum such speed is the stall speed, or VSO. The indicated airspeed at which a fixed-wing aircraft stalls varies with the weight of the aircraft but does not vary with altitude. At speeds close to the stall speed the aircraft's wings are at a high angle of attack. At higher altitudes, the air density is lower than at sea level; because of the progressive reduction in air density, as the aircraft’s altitude increases its true airspeed is progressively greater than its indicated airspeed. For example, the indicated airspeed at which an aircraft stalls can be considered constant, but the true airspeed at which it stalls increases with altitude. Air conducts sound at a certain speed, the "speed of sound".
This becomes slower. Because the temperature of the atmosphere decreases with altitude, the speed of sound decreases with altitude. A given airspeed, divided by the speed of sound in that air, gives a ratio known as the Mach number. A Mach number of 1.0 indicates an airspeed equal to the speed of sound in that air. Because the speed of sound increases with air temperature, air temperature decreases with altitude, the true airspeed for a given Mach number decreases with altitude; as an airplane moves through the air faster, the airflow over parts of the wing will reach speeds that approach Mach 1.0. At such speeds, shock waves form in the air passing over the wings, drastically increasing the drag due to drag divergence, causing Mach buffet, or drastically changing the center of pressure, resulting in a nose-down moment called "mach tuck"; the aircraft Mach number at which these effects appear is known as its critical Mach number, or MCRIT. The true airspeed corresponding to the critical Mach number decreases with altitude.
The flight envelope is a plot of various curves representing the limits of the aircraft's true airspeed and altitude. The top-left boundary of the envelope is the curve representing stall speed, which increases as altitude increases; the top-right boundary of the envelope is the curve representing critical Mach number in true airspeed terms, which decreases as altitude increases. These curves intersect at some altitude; this intersection is or more formally the Q corner. The above explanation is based on level, constant speed, flight with a given gross weight and load factor of 1.0 G. The specific altitudes and speeds of the coffin corner will differ depending on weight, the load factor increases caused by banking and pitching maneuvers; the specific altitudes at which the stall speed meets the critical Mach number will differ depending on the actual atmospheric temperature. When an aircraft slows to below its stall speed, it is unable to generate enough lift in order to cancel out the forces that act on the aircraft.
This will cause the aircraft to drop in altitude. The drop in altitude may cause the pilot to increase the angle of attack by pulling up on the stick, because increasing the angle of attack puts the aircraft in a climb. However, when the wing exceeds its critical angle of attack, an increase in angle of attack will lead to a loss of lift and a further loss of airspeed - the wing stalls; the reason why the wing stalls when it exceeds its critical angle of attack is that the airflow over the top of the wing separates. When the airplane exceeds its critical Mach number drag increases or Mach tuck occurs, which can cause the aircraft to upset, lose control, lose altitude. In either case, as the airplane falls, it could gain speed and structural failure could occur due to excessive g forces during the pullout phase of the recovery; as an airplane approaches its coffin corner, the margin between stall speed and critical Mach number becomes smaller and smaller. Small changes could put the other above or below the limits.
For instance, a turn causes the inner wing to have a lower airspeed, the outer wing, a higher airspeed. The aircraft could exceed both limits at once. Or, turbulence could cause the airspeed to change to beyond the limits; some aircraft, such as the Lockheed U-2 operate in the "coffin corner". In the case of t
A jet airliner is an airliner powered by jet engines. Airliners have two or four jet engines. Airliners are classified as either the long-haul wide-body aircraft or narrow-body aircraft. Most airliners today are powered by jet engines, because they are capable of safely operating at high speeds and generate sufficient thrust to power large-capacity aircraft; the first jetliners, introduced in the 1950s, used the simpler turbojet engine. The first airliners with turbojet propulsion were experimental conversions of the Avro Lancastrian piston-engined airliner, which were flown with several types of early jet engine, including the de Havilland Ghost and the Rolls-Royce Nene, they retained the jets being housed in the outboard nacelles. The first airliner with jet power only was the Nene-powered Vickers VC.1 Viking G-AJPH, which first flew on 6 April 1948. The early jet airliners had much lower interior levels of noise and vibration than contemporary piston-engined aircraft, so much so that in 1947, after piloting a jet powered aircraft for the first time, Wing Commander Maurice A. Smith, editor of Flight magazine, said, "Piloting a jet aircraft has confirmed one opinion I had formed after flying as a passenger in the Lancastrian jet test beds, that few, if any, having flown in a jet-propelled transport, will wish to revert to the noise and attendant fatigue of an airscrew-propelled piston-engined aircraft" The first purpose-built jet airliner was the British de Havilland Comet which first flew in 1949 and entered service in 1952.
Developed in 1949 was the Avro Canada C102 Jetliner, which never reached production. These first jet airliners were followed some years by the Sud Aviation Caravelle from France, the Tupolev Tu-104 from the Soviet Union, the Boeing 707, Douglas DC-8 and Convair 880 from the United States. National prestige was attached to developing prototypes and bringing these early designs into service. There was a strong nationalism in purchasing policy, so that US Boeing and Douglas aircraft became associated with Pan Am, while BOAC ordered British Comets. Pan Am and BOAC, with the help of advertising agencies and their strong nautical traditions of command hierarchy and chain of command, were quick to link the "speed of jets" with the safety and security of the "luxury of ocean liners" in the public's perception. Aeroflot used Soviet Tupolevs. Commercial realities dictated exceptions, however, as few airlines could risk missing out on a superior product: American Airlines ordered the pioneering Comet, Canadian and European airlines could not ignore the better operating economics of the Boeing 707 and the DC-8, while some American airlines ordered the Caravelle.
Boeing became the most successful of the early manufacturers. The KC-135 Stratotanker and military versions of the 707 remain operational as tankers or freighters; the basic configuration of the Boeing and Douglas aircraft jet airliner designs, with spaced podded engines underslung on pylons beneath a swept wing, proved to be the most common arrangement and was most compatible with the large-diameter high-bypass turbofan engines that subsequently prevailed for reasons of quietness and fuel efficiency. The Pratt & Whitney JT3 turbojets powered the original Boeing DC-8 models; the de Havilland and Tupolev designs had engines incorporated within the wings next to the fuselage, a concept that endured only within military designs while the Caravelle pioneered engines mounted either side of the rear fuselage. The 1960s jet airliners include; the 1960s jet airliners were known for the advancement of turbofan technology, as well as the advent of the trijet design. Jet airliners that entered service in the 1960s were powered by slim, low-bypass turbofan engines, many aircraft used the rear-engined, T-tail configuration, such as the BAC One-Eleven, Douglas DC-9 twinjets.
The rear-engined T-tail arrangement is still used for jetliners with a maximum takeoff weight of less than 50 tons. Other 1960s developments, such as rocket assisted takeoff, water-injection, afterburners used on supersonic jetliners such as Concorde and the Tupolev Tu-144, have been superseded; the 1970s jet airliners introduced wide-body craft and high-bypass turbofan engines. Pan Am and Boeing "again opened a new era in commercial aviation" when the first Boeing 747 entered service in January 1970, marking the debut of the high-bypass turbofan which lowered operating costs, the initial models which could seat up to 400 passengers which earned it the nickname "Jumbo Jet". Other wide-body designs included the McDonnell Douglas DC-10 and Lockheed L-1011 TriStar trijets, smaller than the Boeing 747 but capable of flying similar long-range routes from airports with shorter runways. There was the market debut of the European consortium Airbus, whose first aircraft wa
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are indicated airspeed, calibrated airspeed, equivalent airspeed, true airspeed, density airspeed. Indicated airspeed is what is read off of an airspeed gauge connected to a pitot static system, calibrated airspeed is indicated airspeed adjusted for pitot system position and installation error, equivalent airspeed is calibrated airspeed adjusted for compressibility effects. True airspeed is equivalent airspeed adjusted for air density, is the speed of the aircraft through the air in which it is flying. Calibrated airspeed is within a few knots of indicated airspeed, while equivalent airspeed decreases from CAS as aircraft altitude increases or at high speeds. With EAS constant, true airspeed increases as aircraft altitude increases; this is because air density decreases with higher altitude, but an aircraft's wing requires the same amount of air particles flowing around it to produce the same amount of lift for a given AOA.
The measurement and indication of airspeed is ordinarily accomplished on board an aircraft by an airspeed indicator connected to a pitot-static system. The pitot-static system comprises one or more pitot probes facing the on-coming air flow to measure pitot pressure and one or more static ports to measure the static pressure in the air flow; these two pressures are compared by the ASI to give an IAS reading. Indicated airspeed is the airspeed indicator reading uncorrected for instrument and other errors. From current EASA definitions: Indicated airspeed means the speed of an aircraft as shown on its pitot static airspeed indicator calibrated to reflect standard atmosphere adiabatic compressible flow at sea level uncorrected for airspeed system errors. Outside the former Soviet bloc, most airspeed indicators show the speed in knots; some light aircraft have airspeed indicators showing speed in statute miles per hour or kilometers per hour. An airspeed indicator is a differential pressure gauge with the pressure reading expressed in units of speed, rather than pressure.
The airspeed is derived from the difference between the ram air pressure from the pitot tube, or stagnation pressure, the static pressure. The pitot tube is mounted facing forward. Sometimes both pressure sources are combined in a pitot-static tube; the static pressure measurement is subject to error due to inability to place the static ports at positions where the pressure is true static pressure at all airspeeds and attitudes. The correction for this error is the position error correction and varies for different aircraft and airspeeds. Further errors of 10 % or more are common. Calibrated airspeed is indicated airspeed corrected for instrument errors, position error and installation errors. Calibrated airspeed values less than the speed of sound at standard sea level are calculated as follows: V c = A 0 5 minus position and installation error correction. Where V c is the calibrated airspeed, q c is the impact pressure: the difference between total pressure and static pressure, P 0 is 29.92126 inches Hg.
Units other than knots and inches of mercury can be used. This expression is based on the form of Bernoulli's equation applicable to isentropic compressible flow; the values for P 0 and A 0 are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated. Equivalent airspeed is defined as the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed and altitude at which the aircraft is flying; that is, it is defined by the equation 1 2 ρ 0 2 = 1 2 ρ 2 where ρ is the density of air at the altitude at which the aircraft is flying. EAS is a measure of airspeed, a function of incompressible dynamic pressure. Structural analysis is in terms of incompressible dynamic pressure
In aeronautics, a propeller called an airscrew, converts rotary motion from an engine or other power source into a swirling slipstream which pushes the propeller forwards or backwards. It comprises a rotating power-driven hub, to which are attached several radial airfoil-section blades such that the whole assembly rotates about a longitudinal axis; the blade pitch may be fixed, manually variable to a few set positions, or of the automatically-variable "constant-speed" type. The propeller attaches to the power source's driveshaft either directly or through reduction gearing. Propellers can be made from metal or composite materials. Propellers are only suitable for use at subsonic airspeeds below about 480 mph, as above this speed the blade tip speed approaches the speed of sound and local supersonic flow causes high drag and propeller structural problems; the earliest references for vertical flight came from China. Since around 400 BC, Chinese children have played with bamboo flying toys; this bamboo-copter is spun by rolling a stick attached to a rotor between ones hands.
The spinning creates lift, the toy flies when released. The 4th-century AD Daoist book Baopuzi by Ge Hong describes some of the ideas inherent to rotary wing aircraft. Designs similar to the Chinese helicopter toy appeared in other works, it was not until the early 1480s, when Leonardo da Vinci created a design for a machine that could be described as an "aerial screw", that any recorded advancement was made towards vertical flight. His notes suggested that he built small flying models, but there were no indications for any provision to stop the rotor from making the craft rotate; as scientific knowledge increased and became more accepted, man continued to pursue the idea of vertical flight. Many of these models and machines would more resemble the ancient bamboo flying top with spinning wings, rather than Leonardo's screw. In July 1754, Russian Mikhail Lomonosov had developed a small coaxial modeled after the Chinese top but powered by a wound-up spring device and demonstrated it to the Russian Academy of Sciences.
It was powered by a spring, was suggested as a method to lift meteorological instruments. In 1783, Christian de Launoy, his mechanic, used a coaxial version of the Chinese top in a model consisting of contrarotating turkey flight feathers as rotor blades, in 1784, demonstrated it to the French Academy of Sciences. A dirigible airship was described by Jean Baptiste Marie Meusnier presented in 1783; the drawings depict a 260-foot-long streamlined envelope with internal ballonets that could be used for regulating lift. The airship was designed to be driven by three propellers. In 1784 Jean-Pierre Blanchard fitted a hand-powered propeller to a balloon, the first recorded means of propulsion carried aloft. Sir George Cayley, influenced by a childhood fascination with the Chinese flying top, developed a model of feathers, similar to that of Launoy and Bienvenu, but powered by rubber bands. By the end of the century, he had progressed to using sheets of tin for rotor blades and springs for power, his writings on his experiments and models would become influential on future aviation pioneers.
William Bland sent designs for his "Atmotic Airship" to the Great Exhibition held in London in 1851, where a model was displayed. This was an elongated balloon with a steam engine driving twin propellers suspended underneath. Alphonse Pénaud developed coaxial rotor model helicopter toys in 1870 powered by rubber bands. In 1872 Dupuy de Lome launched a large navigable balloon, driven by a large propeller turned by eight men. Hiram Maxim built a craft that weighed 3.5 tons, with a 110-foot wingspan, powered by two 360-horsepower steam engines driving two propellers. In 1894, his machine was tested with overhead rails to prevent it from rising; the test showed. One of Pénaud's toys, given as a gift by their father, inspired the Wright brothers to pursue the dream of flight; the twisted airfoil shape of an aircraft propeller was pioneered by the Wright Brothers. While some earlier engineers had attempted to model air propellers on marine propellers, the Wright Brothers realized that a propeller is the same as a wing, were able to use data from their earlier wind tunnel experiments on wings, introducing a twist along the length of the blades.
This was necessary to maintain a more uniform angle of attack of the blade along its length. Their original propeller blades had an efficiency of about 82%, compared to 90% for a modern small general aviation propeller, the 3-blade McCauley used on a Bonanza aircraft. Roper quotes 90% for a propeller for a human-powered aircraft. Mahogany was the wood preferred for propellers through World War I, but wartime shortages encouraged use of walnut, oak and ash. Alberto Santos Dumont was another early pioneer, having designed propellers before the Wright Brothers for his airships, he applied the knowledge he gained from experiences with airships to make a propeller with a steel shaft and aluminium blades for his 14 bis biplane in 1906. Some of his designs used a bent aluminium sheet for blades, they were undercambered, this plus the absence of lengthwise twist made them less efficient than the Wright propellers. So, this was the first use of aluminium in the construction of an airscrew. A rotating airfoil behind the aircraft, which pushes it, was called a propeller, while one which pulled from the front was a tractor.
The term'pusher' became adopted for the rear-mounted device in contrast to the tractor configurat
Stall (fluid dynamics)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs; the critical angle of attack is about 15 degrees, but it may vary depending on the fluid and Reynolds number. Stalls in fixed-wing flight are experienced as a sudden reduction in lift as the pilot increases the wing's angle of attack and exceeds its critical angle of attack. A stall does not mean that the engine have stopped working, or that the aircraft has stopped moving—the effect is the same in an unpowered glider aircraft. Vectored thrust in manned and unmanned aircraft is used to maintain altitude or controlled flight with wings stalled by replacing lost wing lift with engine or propeller thrust, thereby giving rise to post-stall technology; because stalls are most discussed in connection with aviation, this article discusses stalls as they relate to aircraft, in particular fixed-wing aircraft. The principles of stall discussed here translate to foils in other fluids as well.
A stall is a condition in aerodynamics and aviation such that if the angle of attack increases beyond a certain point lift begins to decrease. The angle at which this occurs is called the critical angle of attack; this critical angle is dependent upon the airfoil section or profile of the wing, its planform, its aspect ratio, other factors, but is in the range of 8 to 20 degrees relative to the incoming wind for most subsonic airfoils. The critical angle of attack is the angle of attack on the lift coefficient versus angle-of-attack curve at which the maximum lift coefficient occurs. Stalling is caused by flow separation which, in turn, is caused by the air flowing against a rising pressure. Whitford describes three types of stall, trailing-edge, leading-edge and thin-aerofoil, each with distinctive Cl~alpha features. For the trailing-edge stall separation begins at small angles of attack near the trailing edge of the wing while the rest of the flow over the wing remains attached; as angle of attack increases, the separated regions on the top of the wing increase in size as the flow separation moves forwards and this hinders the ability of the wing to create lift.
This is shown by the reduction in lift-slope on a Cl~alpha curve as the lift nears its maximum value. The separated flow causes buffeting. Beyond the critical angle of attack, separated flow is so dominant that additional increases in angle of attack cause the lift to fall from its peak value. Piston-engined and early jet transports had good stall behaviour with pre-stall buffet warning and, if ignored, a straight nose-drop for a natural recovery. Wing developments that came with the introduction of turbo-prop engines introduced unacceptable stall behaviour. Leading-edge developments on high-lift wings and the introduction of rear-mounted engines and high-set tailplanes on the next generation of jet transports introduced unacceptable stall behaviour; the probability of achieving the stall speed inadvertently, a hazardous event, had been calculated, in 1965, at about once in every 100,000 flights enough to justify the cost of development and incorporation of warning devices, such as stick shakers, devices to automatically provide an adequate nose-down pitch, such as stick pushers.
When the mean angle of attack of the wings is beyond the stall a spin, an autorotation of a stalled wing, may develop. A spin follows departures in roll and pitch from balanced flight. For example, a roll is damped with an unstalled wing but with wings stalled the damping moment is replaced with a propelling moment; the graph shows that the greatest amount of lift is produced as the critical angle of attack is reached. This angle is 17.5 degrees in this case. In particular, for aerodynamically thick airfoils, the critical angle is higher than with a thin airfoil of the same camber. Symmetric airfoils have lower critical angles; the graph shows that, as the angle of attack exceeds the critical angle, the lift produced by the airfoil decreases. The information in a graph of this kind is gathered using a model of the airfoil in a wind tunnel; because aircraft models are used, rather than full-size machines, special care is needed to make sure that data is taken in the same Reynolds number regime as in free flight.
The separation of flow from the upper wing surface at high angles of attack is quite different at low Reynolds number from that at the high Reynolds numbers of real aircraft. High-pressure wind tunnels are one solution to this problem. In general, steady operation of an aircraft at an angle of attack above the critical angle is not possible because, after exceeding the critical angle, the loss of lift from the wing causes the nose of the aircraft to fall, reducing the angle of attack again; this nose drop, independent of control inputs, indicates the pilot has stalled the aircraft. This graph shows the stall angle, yet in practice most pilot operating handbooks or generic flight manuals describe stalling in terms of airspeed; this is because all aircraft are equipped with an airspeed indicator, but fewer aircraft have an angle of attack indicator. An aircraft's stalling speed is published by the manufacturer for a range of weights and flap positions, but the stalling angle of attack is not published.
As speed reduces, angle of attack has to increase to keep lift
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system; the force applied on a surface in a direction perpendicular or normal to the surface is called thrust. Force, thus thrust, is measured using the International System of Units in newtons, represents the amount needed to accelerate 1 kilogram of mass at the rate of 1 meter per second per second. In mechanical engineering, force orthogonal to the main load is referred to as thrust. A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight; this can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the difference in velocity of the airstream.
Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a thrust reverser on a jet engine. Rotary wing aircraft and thrust vectoring V/STOL aircraft use engine thrust to support the weight of the aircraft, vector sum of this thrust fore and aft to control forward speed. A motorboat generates thrust; the resulting thrust pushes the boat in the opposite direction to the sum of the momentum change in the water flowing through the propeller. A rocket is propelled forward by a thrust force equal in magnitude, but opposite in direction, to the time-rate of momentum change of the exhaust gas accelerated from the combustion chamber through the rocket engine nozzle; this is the exhaust velocity with respect to the rocket, times the time-rate at which the mass is expelled, or in mathematical terms: T = v d m d t Where T is the thrust generated, d m d t is the rate of change of mass with respect to time, v is the speed of the exhaust gases measured relative to the rocket.
For vertical launch of a rocket the initial thrust at liftoff must be more than the weight. Each of the three Space Shuttle Main Engines could produce a thrust of 1.8 MN, each of the Space Shuttle's two Solid Rocket Boosters 14.7 MN, together 29.4 MN. By contrast, the simplified Aid For EVA Rescue has 24 thrusters of 3.56 N each. In the air-breathing category, the AMT-USA AT-180 jet engine developed for radio-controlled aircraft produce 90 N of thrust; the GE90-115B engine fitted on the Boeing 777-300ER, recognized by the Guinness Book of World Records as the "World's Most Powerful Commercial Jet Engine," has a thrust of 569 kN. The power needed to generate thrust and the force of the thrust can be related in a non-linear way. In general, P 2 ∝ T 3; the proportionality constant varies, can be solved for a uniform flow: d m d t = ρ A v T = d m d t v, P = 1 2 d m d t v 2 T = ρ A v 2, P = 1 2 ρ A v 3 P 2 = T 3 4 ρ A Note that these calculations are only valid for when the incoming air is accelerated from a standstill – for example when hovering.
The inverse of the proportionality constant, the "efficiency" of an otherwise-perfect thruster, is proportional to the area of the cross section of the propelled volume of fluid and the density of the fluid. This helps to explain why moving through water is easier and why aircraft have much larger propellers than watercraft. A common question is how to contrast the thrust rating of a jet engine with the power rating of a piston engine; such comparison is difficult. A piston engine does not move the aircraft by itself, so piston engines are rated by how much power they deliver to the propeller. Except for changes in temperature and air pressure, this quantity depends on the throttle setting. A jet engine has no propeller, so the propulsive power of a jet engine is determined from its thrust as follows. Power is the force it takes to move something over some distance divided by the time it takes to move that distance: P = F d t In case of
Rate of climb
In aeronautics, the rate of climb is an aircraft's vertical speed – the positive or negative rate of altitude change with respect to time. In most ICAO member countries in otherwise metric countries, this is expressed in feet per minute; the RoC in an aircraft is indicated with a vertical speed indicator or instantaneous vertical speed indicator. The temporal rate of decrease in altitude is referred to as the rate of sink rate. A negative rate of climb corresponds to a positive rate of descent: RoD = -RoC. There are a number of designated airspeeds relating to optimum rates of ascent, the two most important of these are VX and VY. VX is the indicated forward airspeed for best angle of climb; this is the speed at which an aircraft gains the most altitude in a given horizontal distance used to avoid a collision with an object a short distance away. By contrast, VY is the indicated airspeed for best rate of climb, a rate which allows the aircraft to climb to a specified altitude in the minimum amount of time regardless of the horizontal distance required.
Except at the aircraft’s ceiling, where they are equal, VX is always lower than VY. Climbing at VX allows pilots to maximize altitude gain per horizontal distance; this occurs at the speed for which the difference between drag is the greatest. In a jet airplane, this is minimum drag speed, occurring at the bottom of the drag vs. speed curve. Climbing at VY allows pilots to maximize altitude gain per time; this occurs at the speed where the difference between engine power and the power required to overcome the aircraft's drag is greatest. Vx increases with altitude and VY decreases with altitude until they converge at the airplane's absolute ceiling, the altitude above which the airplane cannot climb in steady flight; the Cessna 172 is a four-seat aircraft. At maximum weight it has a VY of 75 knots indicated airspeed providing a rate of climb of 721 ft/min. Rate of climb at maximum power for a small aircraft is specified in its normal operating procedures but for large jet airliners it is mentioned in emergency operating procedures.
Climb V speeds Variometer