Imagine that a helicopter raises its nose up and then there is a loss in speed. Can it enter a stall? If so, how do you recover from the stall?
Yes and No.
The definition of a stall is that the airfoil stops generating lift, which happens when the critical angle of attack is reached.
In an airplane, this happens during normal flight by pitching up until the critical angle of the airfoil is reached.
In a helicopter, aft cyclic ("pitching up" in an airplane) will only serve to make the helicopter climb and slow down until it reaches equilibrium in the new attitude.
Helicopters, however, can suffer from what is called "retreating blade stall," when the blade is at full pitch on the retreating side of the rotor:
This doesn't happen under normal circumstances, but there are four factors which contribute to retreating blade stall:
fast -- close to Vne
heavy -- at max gross weight
hot -- above 35C
high -- at high altitude
Retreating blade stall is considered an emergency, and can be recovered from by reducing power.
A stall, as in slowing down and then descending too fast - yes, in the following scenario:
- Helicopter flies with forward speed, above the hover ceiling.
- Pilot moves cyclic to aft, and pulls up collective to maintain height.
- When all airspeed is gone, the helicopter cannot maintain hover at this weight/altitude combination and starts to lose altitude.
- Descending vertically, the helicopter descends in its own downwash and enters a dangerous situation called Vortex Ring State. Even if more collective travel was available, it would not create more lift in this state.
There is a discussion possible about what stall exactly is. If it is flow separation, the rotor blades are technically not stalled as @Jan Hudec points out. If it is stopping the generation of aerodynamic lift, this is exactly what happens during both fixed wing stall and rotor wing vortex ring state.
Even the way to get out of vortex ring state is the same as with fixed wing stall: move the cyclic forward so the nose tips down. Or sideways alternatively left/right and descend like a leaf from a tree.
Forward motion is key to auto-rotation. Autorotation with proper blade pitch maintains lift.
In that sense powerless helicopters can be considered "gliders". Both pitch down to utilize gravity to maintain forward airspeed. Helicopters (and autogyros) indirectly maintain rotor speed by converting differences in drag from forward airspeed to angular rotor velocity, just like an anemometer.
If a helicopter loses power, it will begin to sink. Similar to a glider, pitching down is key maintaining controlled flight. The Bell UH-1 Iroquois has a horizontal stablilizer structure on its tail to assist this manuever.
Can a helicopter stall?
Lack of understanding and proper training can certainly lead to a very bad situation if power is lost. If one pulled up into a hover$^2$ with the blade at full angle of attack, lost rotation, and started sinking, the entire rotor could stall and may even start rotating backwards$^1$.
But even a helicopter without a horizontal stabilizer pitches down in a sink if the tail produces more drag torque around the center of gravity than the nose.
$^1$ unless blade pitch is brought to negative
$^2$ with sufficient rotor angular momentum, this manuever could be done for a limited amount of time - such as just prior to landing.
Hello I wanted to give this question an answer that is technically correct. The truth is no you cannot stall a helicopter. But yes you can recover from conditions that would normally stall a helicopter before it stalls. Not to plagiarize wikipedia. But lets start with the lift equation:
L=1/2 ρ×V$^2$×Sref×CLift where:
L denotes lift force.
V defines the velocity of aircraft expressed in m/s.
ρ is air density, affected by altitude.
Sref is the reference area or the wing area of an aircraft measured in square metres.
CL is the coefficient of lift, depending on the angle of attack and the type of aerofoil.
Or you could say Charlie Licked twice on a Pizza under the table and Vomited on Sunday.
Clift = 2 L / (p×V$^2$×Sref)
Lets see how lift is defined
"A fluid flowing around an object exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it can act in any direction at right angles to the flow."
Now lets see how an airfoil stalling is defined
"The critical angle of attack is the angle of attack which produces the maximum lift coefficient. This is also called the "stall angle of attack". Below the critical angle of attack, as the angle of attack decreases, the lift coefficient decreases. Conversely, above the critical angle of attack, as the angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface. On most airfoil shapes, as the angle of attack increases, the upper surface separation point of the flow moves from the trailing edge towards the leading edge. At the critical angle of attack, upper surface flow is more separated and the airfoil or wing is producing its maximum lift coefficient. As the angle of attack increases further, the upper surface flow becomes more fully separated and the lift coefficient reduces further."
"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 critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number."
So a stall is when an airfoil such as an airplane wing or helicopter blade which is measured from the chord line to the tip is no longer able to generate an airflow where there is less pressure above the airfoil and more pressure below the airfoil generating lift. Some people think of this as the wing has a buoyancy less then the air around it. Not an actually correct definition as to compare to a boat as a boat does not need to move in the water to displace the water less then itself and float above it. But for whatever practical purpose an aircraft moving in the air has less displacement then the air underneath it as long as it keeps moving.
When a wing or wing tip stalls it is no longer creating an airflow generating lift the other forces such as drag and weight are now greater then lift and thrust. In a plane if a plane stalls the plane can regain lift by regaining an airflow around the wings by creating a sufficient angle of attack. Basically when the nose is pointed downwards it regains the ability to generate lift around the wings as the aircraft has regained the ability to create a sufficient angle of attack.
For the angle of attack to generate lift around the blades of a helicopter the blades must be moving and nothing must disturb the airflow around the blade rotor system.
In an auto rotation the engine has failed and the helicopter no longer is producing RPMs from the engine. But the helicopter blades are still moving from the airflow underneath the helicopter. The blades are still producing an airflow allowing the helicopter to slowly float to the ground as though it is doing a vertical descent. As long as the blades continue to provide a correct pitched angle of attack collectively then the helicopter will maintain enough lift to descend the helicopter to the ground safely. Lets see what wikipedia says about an auto rotation:
"Autorotation is a state of flight in which the main rotor system of a helicopter or other rotary-wing aircraft turns by the action of air moving up through the rotor, as with an autogyro, rather than engine power driving the rotor. The term autorotation dates to a period of early helicopter development between 1915 and 1920, and refers to the rotors turning without the engine. It is analogous to the gliding flight of a fixed-wing aircraft. Autorotation has also evolved to be used by certain trees as a means of disseminating their seeds further.
Airflow through a helicopter rotor. Above, the rotor is powered and pushing air downward, generating lift and thrust. Below, the helicopter rotor has lost power, and the craft is making an emergency landing, The most common use of autorotation in helicopters is to safely land the aircraft in the event of an engine failure or tail-rotor failure. It is a common emergency procedure taught to helicopter pilots as part of their training.
In normal powered helicopter flight, air is drawn into the main rotor system from above and exhausted downward, but during autorotation, air moves up into the rotor system from below as the helicopter descends. Autorotation is permitted mechanically because of both a freewheeling unit, which allows the main rotor to continue turning even if the engine is not running, as well as aerodynamic forces of relative wind maintaining rotor speed. It is the means by which a helicopter can land safely in the event of complete engine failure. Consequently, all single-engine helicopters must demonstrate this capability to obtain a type certificate."
Now if the blades were to stop moving the helicopter would stall and drop like a brick to the ground. Once the blades stall completely the helicopter cannot recover.
Another way in which a helicopter can stall is if it descends in its own vortices and these vortices disrupt the ability of the helicopter rotor system blades to create an airflow around the blades generating lift. Lets see what wikipedia has to say about vortex ring state
"The vortex ring state (VRS) is a dangerous aerodynamic condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor, causing severe loss of lift. The vortex ring state is sometimes referred to as settling with power. The Federal Aviation Administration (FAA) sees these terms as synonymous, whereas Transport Canada sees them as two different phenomena.
Vortex ring state, in which airflow is upward on the inner blade section, producing a secondary vortex in addition to the normal wingtip vortices. Turbulent airflow results in loss of rotor efficiency. If allowed to continue, uncommanded pitch and roll oscillations may occur, with a large descent rate. A vortex ring state sets in when the airflow around a helicopter's main rotor assumes a rotationally symmetrical form over the tips of the blades, supported by a laminar flow over the blade tips, and a countering upflow of air outside and away from the rotor. In this condition, the rotor falls into a new topological state of the surrounding flow field, induced by its own downwash, and suddenly loses lift. Since vortex rings are surprisingly stable fluid dynamical phenomena (a form of topological soliton), the best way to recover from them is to laterally steer clear of them, in order to re-establish lift, and to break them up using maximum engine power, in order to establish turbulence."
The fact is this. A well trained helicopter pilot can recognize situations such as engine failure and vortex ring state before a stall occurs and perform an auto rotation to recover from engine failure or a vuichard maneuver to recover from vortex ring state or to redu collective pitch (to reduce downwash), lower the nose to fly forward out of the downwash, and then apply recovery power to recover from vortex ring state.
But lets be clear. A helicopter can regain from engine failure with an autorotation landing provided the right landscape below and recover from vortex ring state with vortices increasing up the wing tips into the rotor system which will disrupt the airflow around the blades until eventually a total blade stall occurs. A plane can recover from an engine failure by gliding to a landing spot and staying trim. So both a plane and helicopter can recover from a condition which would lead to total stall.
But can a helicopter recover from a total stall the way an airplane can and regain a sufficient angle of attack to regain airflow around the wings? No. A helicopter cannot recover from a critical angle of attack where the entire airflow system around the blades is unable to produce lift. A helicopter is a lot harder to stall then an airplane. It is difficult for an experienced helicopter pilot to mess up a failed engine autorotation recovery or recovery from vortex ring state if he/she is experienced enough then you can recover from these conditions easily by recognizing the situation quickly as the helicopter rotor system RPMs do not decay past the point of no recovery in just a second or so and maintaining RPMs is still possible in an emergency situation.
But if a critical angle of attack is sustained in the entirety of the helicopter rotor blade system and lift is entirely disrupted across the entire blade system and the helicopter drops like a brick like a plane totally nose down in a stalled condition diving to the ground can a helicopter recover? No. The answer is a helicopter is harder to stall then a plane and for an experienced pilot is easier to recover from an emergency with correct maneuvering. But can a helicopter recover from a total stall?
Absolutely not. The lift equation is a theoretical viewpoint based on the principals of flight where of the 4 forces of thrust, lift, drag and weight drag and weight do not overpower lift and thrust and as the 4 forces are equal lift can be sustained. It is very practical. But much like the formula E=Mc2 the lift equation is not a formula that can solve a problem by plugging in numbers for variables. It is not a number cruncher. You have lift or you don't.
Can an airplane recover from total wing stall? Yes it can recover as the correct angle of attack can be reapplied to generate lift around the wings. Can a helicopter recover from total blade stall? Absolutely not.