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Flight Envelope

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Article Information
Category: Theory of Flight Theory of Flight
Content source: Skybrary skybrary
Content control: Eurocontrol Eurocontrol


Description

For any aircraft the Flight Envelope will be found in the Aircraft Flight Manual (AFM) – and reproduced in the AOM. It will describe the boundaries of altitude and airspeed within which normal flight manouevring can be safely conducted. This envelope will be a restricted version of the design "level flight envelope" which takes account of the allowable manoeuvring load or "g" limits also defined in the Aircraft Flight Manual (AFM).


Jet Engined Aircraft

The efficiency of lift, drag and thrust all vary with speed and altitude and this collective variation therefore influences performance. The flight envelope will show the maximum speed allowable and at what altitude it can be reached and the maximum altitude and at what speed it can be reached as well as the stall speeds at different altitudes. It is possible to consider a flight envelope as having three edges - the ‘slow’ edge, the ‘fast’ edge and the ground. The last of these is self evident!


Wing Stall Speeds

The ‘slow’ edge of the flight envelope is determined by the stall speed and in practice by the slightly higher speed of the associated pre-stall buffet. A wing will stall more readily at higher altitudes because air is less dense. The effect of reduced air density affects the onset of wing stall in two ways:

  • Since lower density air is less viscous, a wing will stall more easily in it and does so at higher airspeeds for any given angle of attack.
  • Since the angle of attack necessary to generate lift increases with decreasing air density, the angle of attack needed to fly safely at higher altitudes must also increase, which raises the stall speed.

The moderating factor on this definition of the ‘slow’ edge is that aircraft weight affects the point at which a given wing stalls. Given that a finite proportion or total weight is fuel which will be used during a flight, the flight envelope will be at its most restrictive on this edge when the aircraft begins to climb at or near MTWA after take off and will slightly expand as fuel is used and weight reduces.


Thrust and Drag

The ‘fast’ edge of the flight envelope is primarily determined by how thrust and drag change with altitude. Because the air is less dense at higher altitudes, drag reduces, but for the same reason thrust reduces. The balance of these two effects in any particular case determines the altitude at which the maximum envelope airspeed will be reached.

In the denser air at low altitudes, both drag and thermal heating are much greater than at high altitudes. High airspeeds in dense air are limited by aeroplane structure considerations. Engines taking in large quantities of dense air and further compressing it will get very hot which imposes another potential airspeed limit. Engines have a much easier time working with less dense high altitude air which is also colder, since this allows compression without limiting temperatures being reached. The engine can therefore operate at higher rotational speed and at higher compression ratios, which make it more efficient. It also means that the engine intake can take in a greater volume of air per second and compress it properly so that the fuel used can generate more thrust.

However, these dual advantages of increased thrust, reduced drag and higher airspeed only work up to a point. Since engines work by taking whatever flows into the intake and accelerating it backwards, as this air becomes less dense, there is less air to be accelerated backwards, so the thrust force decreases. A direct analogy is that humans can use oars to row a boat in water but could not do this to move an object in air. Thrust is the product of the rate of airflow through the engine per second and added speed which the engine gives to that air. When air is less dense, the engine can accelerate it more, but with very thin air at high altitudes, the volume of air put through the engine per second is small and so the thrust decreases even though the air is being ejected from the engine as fast as the engine can push it.

To summarise, as air density reduces with increasing altitude, air changes from being ‘too dense to be compressed at full power’ to ‘just dense enough to compress as hard as the engine can without getting too hot’ to ‘so thin that even though it’s being compressed and used to burn fuel there’s not much exhaust created’.

Other factors also limit achievable thrust when the air is very thin:

  • the ratio of fuel to air burnt in an engine needs to be within a certain range and with a reducing supply of air, less fuel can be burnt.
  • the compressor blades may stall when the air is too thin for the same reason that a wing at a given angle of attack will stall more easily at altitude. If the blades do stall, then the airflow disturbance which results means that there is no steady airflow to be compressed and so the engine flames out.


Maximum Altitude

The top of the flight envelope - maximum altitude – will be defined by either:

  • an altitude where the engine is working as hard as it can and the wings are approaching the stall or
  • an altitude where the engine is on the verge of flaming out.

This ‘fast’ edge of the flight envelope is therefore a complex blend of the speed at which maximum altitude (as described above) is reached and the altitude at which maximum speed (where the air density and speed lead to the optimum relationship between drag and engine thrust) is reached. Maximum altitude derived as described above for a typical transport aircraft, even when operating at maximum weight, will normally be well below the altitude at which high mach number buffet (rather than conventional wing stall buffet) would occur and introduce a new limit on maximum altitude.

The practical effect of this is that the available speed range for aircraft at high weights and at typical cruise altitudes will be quite small compared to the surface case.

Piston Engined Aircraft

This is much simpler. Piston engines have no problem taking in the dense air so their efficiency at low altitudes is not limited by high air density. The lower altitudes are their best for both speed and propulsive efficiency. Decreasing air density as altitude increases means a rapid loss of engine efficiency unless ways of increasing the airflow through it - turbochargers or superchargers - are employed. Even these do not get a piston engine high enough for the aircraft which it is powering to gain real benefit from the reduced airframe drag as air density reduces.

Further constraints derive from the propeller blades themselves. Propeller blades, like compressor blades, will stall more readily in lower air density. Propellers are also fully exposed to the airflow instead of the air being slowed down by passage through an intake, so they reach a stalled condition more easily. Finally, the slow speed of a piston-engined propeller-powered aircraft at higher altitude also leads to the wing reaching a stalled condition more easily.

Turboprop Aircraft

The first gas turbine engines employed in civil aviation employed the thrust generated to power a propeller. It soon became apparent that the propulsive efficiency of a turbo-propeller aircraft could be exceeded by relying on the simple acceleration of air when flying at speeds above about 380 knots. This occurs because although the engine itself is able to function well in lower air density, at the higher aircraft speeds which can be developed mean that the propeller tips begin to approach ‘mach number buffet’ as air density reduces. The overall effect of this is that the flight envelope of a turboprop aircraft is simply a slightly more extensive one than that of the piston-engined aircraft.