Stall (Aerodynamics)


STALLING

 

The nature of the boundary layer determines the stalling characteristics of a wing. In particular the phenomenon of boundary layer separation is extremely important.

Boundary Layer Separation. Boundary layer separation is produced as a result of the adverse pressure gradient developed around the body. The low energy air close to the surface is unable to move in the opposite direction to the pressure gradient and the flow close to the surface starts to flows in the reverse direction to the free stream. The development of separation is shown in Fig below. A typical velocity profile is shown corresponding to point A, while a little further down the surface, at point B, the adverse pressure gradient will have modified the velocity profile as shown. At point C the velocity profile has been modified to such an extent that at the surface flow has ceased. Further down the surface, at point D, the flow close to the surface has reversed and the flow is said to have separated. Point C is defined as the separation point. In the reversed flow region, aft of this point, the flow is eddying and turbulent, with a mean velocity of motion in the opposite  direction to the free stream.

Boundary Layer Separation

Boundary Layer Separation

 

 

 

 

 

 

 

 

 

Critical Angle of Attack.

The marked reduction in the lift coefficient, which accompanies the breakdown of airflow over the wing, occurs at the critical angle of attack for a particular wing. Weight, bank angle, load factor, density altitude and airspeed have no direct effect on the stalling angle of attack. In subsonic flight an aircraft will always stall at the same critical angle of attack except at high Reynolds Numbers, as explained in the chapter on Viscosity and Boundary Layer. A typical lift curve showing the critical angle of attack is shown in Fig below It should be noted that not all lift is lost at the critical angle of attack. In fact, the aerofoil will give a certain amount of lift up to 90°.

Critical Angle of Attack

Critical Angle of Attack

 

 

 

 

 

 

 

 

 

 

 

 

 

Aerodynamic Symptoms of Stall. The most consistent symptom of stall warning arises from the separated flow behind the wing passing over the tail surfaces. The turbulent wake causes buffeting of the control surfaces, which can usually be felt at the control column and rudder pedals. As the separation point starts to move forward, to within a few degrees of the critical angle of attack, the buffeting will usually give adequate warning of the stall. On some aircraft separation may also occur over the cockpit canopy to give additional audible warning. The amount of pre-stall buffet depends on the position of the tail surfaces with respect to the turbulent wake. When the trailing edge flaps are lowered the increased downwash angle behind the (inboard) flaps might reduce the amount of buffet warning of the stall.

Pitching Moments. As the angle of attack is increased through the critical angle of attack the wing pitching moment changes. Changes in the downwash angle behind the wing also cause the tail pitching moment to change. The overall effect varies with aircraft type and may be masked by the rate at which the elevator is deflected to increase the angle of attack. Most aircraft, however, are designed to produce a nose-down pitching moment at the critical angle of attack.

Factors Affecting Aircraft Behaviour during Stall

The Effect of Wing Section.   Stalling is due to the effects of flow separation, and is characterized by loss of lift as well as increase in drag. Thus, if stalling occurs in flight, the aircraft will lose height, unless some action is taken to prevent it. Additionally there are aspects of aircraft behaviour and handling at and near the stall which depend on the design of the wing. The shape of the wing section affects the behaviour of the ac near stall. With some sections, the stall occurs very suddenly and the reduction in lift is very marked. With others, the approach to the stall is more gradual, and the reduction in lift is less pronounced.

It is desirable that the stall does not occur too suddenly, and the pilot should have adequate warning, in terms of the handling qualities of the aircraft, of the approach of stall. This warning generally takes the form of buffeting and lack of response to control inputs. If a particular wing is such that it stalls too suddenly then it may be necessary to provide some artificial pre-stall warning device.

Features of wing section design, which affect the behaviour near stall, are:

  • Leading edge radius of curvature.
  • Thickness/chord ratio.
  • Camber, and particularly the amount of camber near the leading edge.
  • Chord wise location of the points of maximum thickness and maximum camber.

Generally, sharper the nose, thinner the wing, or further aft the positions of maximum thickness and camber, the more sudden will be the stall.

The Effect of Protrusions      Any protrusions on the wing, or indeed elsewhere on the aircraft, may significantly affect the stalling pattern by causing local flow separations. The main cause of such affects is the positioning of the engines, externally carried weapon stores, fuel tanks or configuration changes, since separation may occur near engine nacelles, or spring from the intakes of jet engines etc.

Other protrusions may be deliberately placed on the wing with a view to control stalling behaviour. These include:

  • Vortex Generators. Vortex generators can either take the form of metal projections from the wing surface or of small jets of air issuing normal to the surface. Both types work on the same principle of creating vortices which entrain the faster moving air near the top of the boundary layer down into the more stagnant layer near the surface thus transferring momentum which keeps the boundary layer attached further back on the wing and therefore delay stall.
  • Boundary Layer Fences. These are simply small plates intended to prevent the outward drift of the boundary layer, which is a factor in causing the tip stall. Notches in the wing leading edges tend to produce a similar effect.

Tip stalling. The desirable stall pattern of any wing is a stall which begins on the root section of the wing first. Therefore the wings of an aircraft are designed to stall progressively from the root to the tip. The reasons for this are threefold:

  • To induce early buffet symptoms over the tail surface.
  • To retain aileron effectiveness up to the critical angle of attack.
  • To avoid a large rolling moment, which would arise if the tip of one wing stalled before the other (wing drop).

A rectangular straight wing will usually stall from the root because of the reduction in effective angle of attack at the tips caused by the wing tip vortex. If washout is incorporated to reduce vortex drag, it also assists in delaying tip stall. A tapered wing on the other hand will aggravate the tip stall due to the lower Reynolds Number (smaller chord) at the wing tip.

The most common features designed to prevent wing tip stalling are:

 

  • Wash Out      A progressive reduction of angle of incidence from wing root to the wing tip, called the washout, will result in the wing root reaching its critical angle of attack before the wing tip.
  • Root Spoilers. By making the leading edge of the root sharper, the airflow has more difficulty in following the contour of the leading edge and an early stall at the wing root is induced.
  • Change of Section. An aerofoil section with more gradual stalling characteristics may be employed towards the wing tips (increased camber).
  • Slats and Slots. The use of slats and/or slots on the outer portion of the wing increases the stalling angle of that part of the wing.

 

STALLING SPEED

 

In level flight the weight of the aircraft is balanced by the lift, and from the lift formula it can be seen that lift is reduced whenever any of the other factors in the formula are reduced. For all practical purposes density (ρ) and wing area (S) can be considered constant (for a particular altitude and configuration). If the engine is throttled back the drag will reduce the speed and from the formula lift will be reduced. To keep the lift constant and so maintain level flight, the only factor that is readily variable is the lift coefficient (CL).

As has been shown, the CL can be made larger by increasing the angle of attack, and by doing so the lift can be restored to its original value so that level flight is maintained at the reduced speed. Any further reduction in speed necessitates a further increase in the angle of attack, each succeeding lower IAS corresponding to each succeeding higher angle of attack. Eventually, at a certain IAS, the wing reaches its stalling angle, beyond which point any further increase in angle of attack, in an attempt to maintain the lift, will precipitate a stall.

 

V  (stall)  ∝  √ Lift

This relationship is readily demonstrated by the following examples:

  • Steep Dive. When pulling out of a dive, if the angle of attack is increased to the critical angle, separation will occur and buffet will be felt at a higher air speed.
  • Vertical Climb. In true vertical flight lift is zero and no buffet symptoms will be produced even at zero IAS.

Basic Stalling Speed. The most useful stalling speed to remember is the stalling speed corresponding to the critical angle of attack in straight and level flight. Basic stalling speed may be defined as the speed below which a clean aircraft of stated weight, with the engines throttled back, can no longer maintain straight and level flight. This speed is listed in the Aircrew Manual for different weights.

The factors, which change VB, therefore are:

  • Change in weight.
  • Manoeuvre (load factor).
  • Configuration (changes in CL max).
  • Power and slipstream

Weight Change

 

V (stalling speed with weight change)   =  V (Basic stall speed) x  √ w2/ √ w1

Manoeuvre     The relationship between the basic stalling speed and the stalling speed in any other manoeuvre (VM) can be obtained in a similar way by comparing i.e. the ratio VM : VB as:

VM : VB  =  √ L : √ W  

or   VM    =   VB x   √ L/ √ W

The relationship (L / W) is the load factor, n, and is indicated on the accelerometer (if fitted). Thus VM = VB√n and in a 4g manoeuvre, the stalling speed is twice the basic stalling speed.

Also   VM   =   VB x 1/ √ Cos Θ  where Θ is the bank angle

 

Effects of High Lift Devices. The primary purpose of high lift devices (flaps, slots, slats, etc.) is to increase the CL max of the aeroplane and reduce the stalling speed. The take-off and landing speeds are consequently reduced. The effects of a typical high lift device is summarized here:

Configuration CL max Stalling AOA
Clean (flaps up). 1.5 20°
Flaps down. 2.0 18.5°

 

The contribution of high lift devices must be considerable to cause large reduction in stalling speed. . So we can summarize that any change in CL max due to operation of high lift devices or due to compressibility effects will affect the stalling speed. In particular, the lowering of flaps or extending the slats will result in a new stalling speed. These changes are usually listed in the Aircrew Manual.

Summary

Boundary layer separation is produced as a result of the adverse pressure gradient developed round the body.

In subsonic flight an aircraft will always stall at the same critical angle of attack.

The wing of an aircraft is designed to stall progressively from the root to the tip. The reasons for this are:

  • To induce early buffet symptoms over the tail surface.
  • To retain aileron effectiveness up to the critical angle of attack.
  • To avoid a large rolling moment, which would arise if the tip of one wing stalled before the other.

The most common design features for preventing tip stalling are:

  • Root spoilers.
  • Change of section.
  • Slats and slots.

Basic stalling speed is the speed below which a clean aircraft of stated weight, with engines throttled back, can no longer maintain straight and level flight.

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