Retreating blade stall
Encyclopedia
Retreating blade stall is a hazardous flight condition in helicopter
s and other rotary wing aircraft
, where the rotor blade rotating away from the direction of flight stalls. The stall
is due to low relative airspeed
and/or excessive angle of attack
(or AOA
). Retreating blade stall is the primary limiting factor of a helicopter's airspeed, and the reason even the fastest helicopters can only fly slightly faster than 200 knots.
A rotor blade that is moving in the same direction as the aircraft is called the advancing blade and the blade moving in the opposite direction is called the retreating blade.
Balancing lift across the rotor disc is important to a helicopter's stability. The amount of lift generated by an airfoil is proportional to its airspeed. In a zero airspeed hover the rotor blades, regardless of their position in rotation, have equal airspeeds and therefore equal lift. In forward flight the advancing blade has a higher airspeed than the retreating blade, creating unequal lift across the rotor disc.
A fuller treatment is provided in dissymmetry of lift
.
The pilot may compensate the induced roll with left or right cyclic control input (as determined by the rotation of the rotor) up to a degree. However, the rapid rate of change of blade flex and angle of attack causes uncontrolled longitudinal twist and severe vibration in later stages, resulting in total loss of cyclic control if left unchecked.
Assuming no rotor damage, recovery from the condition is possible by using the procedure described below under Flight performance during a retreating blade stall.
to compensate for reduced blade airspeed has the effect of maintaining lift only until the point where critical angle of attack is reached, after this point lift sharply decreases. This situation is called retreating blade stall.
All airfoils have a critical angle of attack (also called a stall angle of attack) which is the angle of attack that produces most lift. Above this angle flow over the airfoil becomes detached and lift decreases, this is commonly called a stall
.
When a fixed-wing aircraft exceeds its critical angle of attack the entire aircraft loses lift and enters a condition called a stall
. The usual results of a fixed-wing stall are a sharp drop in aircraft altitude and a dive. Stalls in fixed-wing aircraft are often a recoverable event.
In a retreating-blade stall, however, only the retreating half of the helicopter's rotor disc experiences a stall. The advancing blade continues to generate lift, but the retreating blade enters a stall condition, usually resulting in an uncommanded increase in pitch of the nose and a roll in the direction of the retreating side of the rotor disc. In counter-clockwise rotating rotor systems (as in most American-made types) this is the left side. In clockwise rotating systems it is a roll to the right.
Recovery involves decreasing the angle of attack and allowing the retreating blade to recover from its stalled condition. This is done by lowering the collective pitch.
The stall will not happen due to speed, but a higher pitch on the retreating side. If you are at speed and immediately reduce collective you have potentially put the blade back in a non-stalled state, but you will now begin to slow down, descend and level out.
Retreating blade stall starts at the tip region and develops inboard (one of the exam questions in Helicopter Principles of Flights).
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
s and other rotary wing aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
, where the rotor blade rotating away from the direction of flight stalls. The stall
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
is due to low relative airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
and/or excessive angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
(or AOA
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
). Retreating blade stall is the primary limiting factor of a helicopter's airspeed, and the reason even the fastest helicopters can only fly slightly faster than 200 knots.
Advancing vs. retreating blades
 |
|
| retreating blade side | advancing blade side |
A rotor blade that is moving in the same direction as the aircraft is called the advancing blade and the blade moving in the opposite direction is called the retreating blade.
Balancing lift across the rotor disc is important to a helicopter's stability. The amount of lift generated by an airfoil is proportional to its airspeed. In a zero airspeed hover the rotor blades, regardless of their position in rotation, have equal airspeeds and therefore equal lift. In forward flight the advancing blade has a higher airspeed than the retreating blade, creating unequal lift across the rotor disc.
A fuller treatment is provided in dissymmetry of lift
Dissymmetry of lift
Dissymmetry of lift in rotorcraft aerodynamics refers to an uneven amount of lift on opposite sides of the rotor disc. It is a phenomenon that affects single-rotor helicopters in lateral flight, whether the direction of flight be forwards, sideways or in reverse.The dissymmetry is caused by...
.
Compensation
Most helicopter designs compensate for this by incorporating a certain degree of "flap" of the rotor blades through articulation of the rotor head and/or individual rotor blades, depending on the type of rotor system. When flapping, a rotor blade will travel upward during its advance, creating a lesser Angle of Attack (AOA) and therefore lesser lift. When the blade retreats, the blade travels downward to flap down, increasing the AOA and therefore generating greater lift. Semi-rigid and Fully articulated rotors are two common designs which compensate for the effects of dissymmetry of lift through mechanical means in the rotor head. Semi-rigid rotor systems flap both blades in equal and opposite motion as the rotor head teeters on a teetering hinge. Fully articulated rotor systems accomplish flapping motion primarily with independent mechanical movement about a flapping hinge and/or span-wise flexing of the rotor blades. In the case of the less common Rigid rotor system, the only method used for articulating flap is to allow the rotor blades to independently have a certain degree of span-wise flex and not at their attachment point to the rotor head.The pilot may compensate the induced roll with left or right cyclic control input (as determined by the rotation of the rotor) up to a degree. However, the rapid rate of change of blade flex and angle of attack causes uncontrolled longitudinal twist and severe vibration in later stages, resulting in total loss of cyclic control if left unchecked.
Assuming no rotor damage, recovery from the condition is possible by using the procedure described below under Flight performance during a retreating blade stall.
Failure
These compensations can only do so much. Increasing angle of attackAngle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
to compensate for reduced blade airspeed has the effect of maintaining lift only until the point where critical angle of attack is reached, after this point lift sharply decreases. This situation is called retreating blade stall.
All airfoils have a critical angle of attack (also called a stall angle of attack) which is the angle of attack that produces most lift. Above this angle flow over the airfoil becomes detached and lift decreases, this is commonly called a stall
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
.
When a fixed-wing aircraft exceeds its critical angle of attack the entire aircraft loses lift and enters a condition called a stall
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
. The usual results of a fixed-wing stall are a sharp drop in aircraft altitude and a dive. Stalls in fixed-wing aircraft are often a recoverable event.
In a retreating-blade stall, however, only the retreating half of the helicopter's rotor disc experiences a stall. The advancing blade continues to generate lift, but the retreating blade enters a stall condition, usually resulting in an uncommanded increase in pitch of the nose and a roll in the direction of the retreating side of the rotor disc. In counter-clockwise rotating rotor systems (as in most American-made types) this is the left side. In clockwise rotating systems it is a roll to the right.
Flight performance during a retreating blade stall
As the aircraft approaches the airspeed at which it will encounter retreating blade stall, the aircraft will shudder and the nose will begin to pitch up. The resultant upward pitching of the aircraft's nose will begin to correct the situation as it results in slowing the aircraft. However, if uncorrected, and if the aircraft continues to accelerate, the aircraft may roll in the direction of the retreating blade.Recovery involves decreasing the angle of attack and allowing the retreating blade to recover from its stalled condition. This is done by lowering the collective pitch.
The stall will not happen due to speed, but a higher pitch on the retreating side. If you are at speed and immediately reduce collective you have potentially put the blade back in a non-stalled state, but you will now begin to slow down, descend and level out.
Causes of retreating blade stall
Retreating blade stall is more likely to occur when the following conditions exist either alone or in combination:- High gross weight
- High airspeed
- Low rotor RPMRevolutions per minuteRevolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
- High density altitudeDensity altitudeDensity altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground...
- Steep or abrupt turns
- TurbulentTurbulenceIn fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
ambient air
Retreating blade stall starts at the tip region and develops inboard (one of the exam questions in Helicopter Principles of Flights).