Causes of retreating blade stall:
- High airspeed
- higher collective angle
- Too much forward tilt to the tip path plane
- Low Rotor RPM
What is retreating blade stall?
Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the retreating rotor blade has a lower relative blade speed, combined with an increased angle of attack, causing a stall and loss of lift. Retreating blade stall is the primary limiting factor of a helicopter's never exceed speed, V NE.
What are the conditions that cause blade stall?
When operating at high forward airspeeds, the following conditions are most likely to produce blade stall: High blade loading (high gross weight) Low rotor RPM High density altitude Steep or abrupt turns Turbulent air
Why do helicopter rotor blades stall?
A tendency for the retreating blade to stall in forward flight is inherent in all present day helicopters and is a major factor in limiting their forward speed. Just as the stall of an airplane wing limits the low speed possibilities of the airplane, the stall of a rotor blade limits the high speed potential of a helicopter.
What are the warning signs of retreating blade stall?
The major warnings of approaching retreating blade stall conditions are: Abnormal vibration Pitchup of the nose Tendency for the helicopter to roll in the direction of the stalled side. When operating at high forward airspeeds, the following conditions are most likely to produce blade stall:
What are the three warning signs of an impending retreating blade stall?
The major warnings of approaching retreating blade stall conditions are: Abnormal vibration....When operating at high forward airspeeds, the following conditions are most likely to produce blade stall:High blade loading (high gross weight)Low rotor RPM.High density altitude.Steep or abrupt turns.Turbulent air.
How can you overcome or recover from a retreating blade stall?
Flight performance during a retreating blade stall Recovery involves lowering the collective pitch, relieving forward pressure on the cyclic or more commonly, both. Either of these control movements should restore the proper attached airflow over the retreating blade thus generating lift again.
What corrective action can a pilot take to prevent a retreating blade stall at its onset?
At the onset of blade stall vibration, the pilot should take the following corrective measures: 1 - Reduce collective pitch. 2 - Increase rotor RPM. 3 - Reduce forward airspeed.
How do you heal a blade stall?
7:209:42Retreating Blade Stall/VNE in Helicopters - YouTubeYouTubeStart of suggested clipEnd of suggested clipFirst you're reducing the angle of attack in the blades potentially affecting the stall conditionMoreFirst you're reducing the angle of attack in the blades potentially affecting the stall condition reducing that stalling effect - you're going to reduce your airspeed.
What causes loss of tail rotor effectiveness?
Loss of tail-rotor effectiveness (LTE) occurs when the tail rotor of a helicopter is exposed to wind forces that prevent it from carrying out its function—that of cancelling the torque of the engine and transmission. Any low-airspeed high-power environment provides an opportunity for it to occur.
What is the most difficult helicopter to fly?
As the most technically advanced helicopter in the world, the Apache AH Mk1 was also the hardest to fly…. To train each Apache pilot from scratch cost £3 million (each custom-made helmet alone had a price tag of £22,915).
What is blade flapping?
Blade Flapping is the up and down movement of a rotor blade, which, in conjunction with cyclic feathering, causes Dissymmetry of Lift to be eliminated.
How can ground resonance be prevented?
The wait-and-hope approach is only sometimes successful, so a better solution to ground resonance is to prevent it. Helicopters with multiple-blade rotors have shock-absorbing landing gear with powerful dampers that allow it to soak up the energy that would otherwise set the helicopter shaking.
How do soldiers not fall out of helicopters?
TLDR – Soldiers do not fall from the helicopter due to physics. When the helicopter turns, the centripetal force presses inward, keeping soldiers sitting on the cabin or the cabin's lip pressed to the floor. In today's military, soldiers are also typically harnessed or strapped.
How is blade tip stall prevented?
Blade stall normally occurs when airspeed is high. To prevent blade stall, the pilot must fly slower than normal when: The Density Altitude is much Higher than Standard. Carrying Maximum Weight Loads.
Can a helicopter recover from a 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.
What is Blade compressibility?
1:169:30Advancing Blade Compressibility in Helicopters - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo as the airflow approaches the airfoil. It has a relatively smooth laminar flow. So as the airMoreSo as the airflow approaches the airfoil. It has a relatively smooth laminar flow. So as the air gets closer to the wing. It almost hugs the shape of the airfoil as it passes around it.
Which is the best technique for minimizing the wing load factor when flying in severe turbulence?
Which is the best technique for minimizing the wing-load factor when flying in severe turbulence? Set power and trim to obtain an airspeed at or below maneuvering speed, maintain wings level, and accept variations of airspeed and altitude.
What procedure should be used to correct for slight ballooning during landing?
Ballooning can be dangerous because the height above the ground is increasing and the airplane may be rapidly approaching a stalled condition. When ballooning is slight, a constant landing attitude should be held and the airplane allowed to gradually decelerate and settle onto the runway.
Which of the following is true concerning stall spin awareness training?
Which of the following is true concerning stall/spin awareness training? In-flight stall/spin awareness training should include "demonstration" maneuvers such as cross-control stalls, even if not required by practical test standards.
What regulation allows a private pilot to perform preventive maintenance?
The correct answer is C. Both FAR 43.7(f) and 43.3(g) permit a private pilot to perform preventive maintenance on any aircraft either owned or operated under Part 91 rules by that pilot.
What is a retreating blade stall?
Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the retreating rotor blade has a lower relative blade speed, combined with an increased angle of attack , causing a stall and loss of lift. Retreating blade stall is the primary limiting factor of a helicopter's never exceed speed, V NE.
What is a stall in a fixed wing aircraft?
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 virtually always a recoverable event.
What is a stall in a helicopter?
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.
How does a fully articulated rotor work?
Fully articulated rotor systems use a combination of flapping and a horizontal motion that moves the retreating blades forward slightly and moves them back again on the advancing side , thus creating more relative airflow and lift on the retreating side at the expense of the advancing side.
How does a helicopter's rotor work?
Most helicopter designs compensate for this by incorporating a certain degree of vertical "flap" movement of the rotor blades. 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 falls downward again, increasing the AOA and therefore generating greater lift.
What is a semi rigid rotor?
Semi-rigid rotor systems have a horizontal hinge at the base of the blades that allow flap as they rotate.
What is recovery in a blade?
Recovery includes lowering the collective to reduce the blade angle of attack, followed by application of aft cyclic to reduce airspeed.
Stall Recovery
Reducing collective pitch. This reduces the angle of attack in the plane of rotation and should be done first.
Stall Avoidance
Pilots should maintain an awareness of V ne taking into account the expected altitudes in flight, potential turbulence, and the weight of the aircraft.
Design Aspects
Developed in the 1980’s, the BERP (British Experimental Rotor Programme) rotor blade design has, by design, altered the area of effect of vortices and delayed the onset of retreating blade stall, allowing greater V ne and other performance improvements.
Accidents and Incidents
Bell 47 pilot encountered retreating blade stall when flying at V ne and subsequently could not maintain height. The out of balance feeling was probably one blade stalling before the other due to slight differences in manufacturing.
What causes a helicopter to stall?
A helicopter will never have more airspeed than it's retreating blade until a helicopter is designed that will travel faster than 400 kts (the tipspeed of the blades.) Stalling of an airfoil is when that airfoil reaches its "critical angle of attack." This is when the airflow over the airfoil no longer is smooth and laminar, instead causing "burbles" and turbulent air which decreases its ability to produce lift. Retreating blade stall is commonly associated with high forward airspeed. This is because of a phenomenon known as dysymetry of lift. The retreating blade has a lower velocity airflow while the advancing blade has a higher velocity airflow. This causes a greater amount of lift on the advancing side of the disk. In order to balance out the lift produce, the blades "flap." The advancing blade flaps up and the retreating blade flaps down. A blade flapping up, reduces its angle of attack and a blade flapping down increases its angle of attack. When combined with the increased angle of attack required to fly at high forward speeds, high density altitude or high weight; this extra angle of attack generated by the downward flapping blade can place this retreating blade past its critical angle and thus stall the blade.
How fast is the KD-1 stalling?
Here’s retreating blade stall of the Kellett KD-1 as measured by NACA. At mu = .35; ~100 mph, 70% of the retreating blade is stalled.#N#Unlike a helicopter, a gyroplane rotor stalls from the root outward. At mu ~ 0.35, the stick will be hard up against the forward stop and the machine will go no faster. It simply climbs with an increase of power.
How does a gyro rotor work?
A gyro rotor begins stalling at the root end and works its way outboard as forward airspeed increases. Most people are totally unaware of retreating blade stall in a gyro; nothing unusual happens. At some forward speed, nominally at 35% of rotor peripheral speed, the stick will reach the forward stop, but that’s the only indication.
Do helos and gyros stall the same?
Helos and gyros do not stall the same!
Can a gyro rotor stall?
Retreating blade stall can easily be demonstrated in a gyro that has an adjustable hub bar along with flat bottomed blade s. Crank up the pitch as far as the adjustment allows; with highly cambered blades, the rotor can still be started.
What is Dissymmetry of Lift?
Unfortunately, unlike most fixed wing aircraft, a change in forward airspeed does not affect most of the aircraft equally. Helicopters have a giant rotating disc attached to them, which complicates things a little. To start out, imagine a counter-clockwise rotating disc in still air with a tip speed of 400 kts.
How do helicopters compensate for Dissymmetry of Lift?
We basically want the disc to correct for dissymmetry of lift by letting the blades flap (or sometimes slightly bend) up and down. Flapping is the vertical movement of a blade relative to the plane of rotation (the rotor disc).
What is Airflow Reversal?
Yea unfortunately we’re not done with all the aerodynamic shenanigans just yet. Airflow reversal, also called Reverse Flow, is another effect that needs dealing with that happens at higher airspeeds. Remember that we had to add the aircraft airspeed to the advancing side and subtract the aircraft airspeed form the retreating side?
What is Retreating Blade Stall?
So we have this retreating blade with a root area that doesn’t do much for us, reducing the total lift for this blade a lot. So what can be done to compensate for this?
Conclusion
Dissymmetry of lift has quite a few effects on helicopter and rotor disc design. It will always be one of the main downsides of having a rotor disc vs fixed wings. However, designs are still getting better and better every day, with higher Vne values across the board. Thank you for joining us today, and we’ll see you in the next article!
Overview
Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the retreating rotor blade has a lower relative blade speed, combined with an increased angle of attack, causing a stall and loss of lift. Retreating blade stall is the primary limiting factor of a helicopter's never exceed speed, VNE.
Retreating blade stall occurs at high forward speeds, and should not be confused with rotor stall…
Advancing vs. retreating blades
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 the square of its airspeed (velocity). In a zero airspeed hover the rotor blades, regardless of their position in rotation, have equal airspeeds and therefor…
Compensation
Most helicopter designs compensate for this by incorporating a certain degree of vertical "flap" movement of the rotor blades. 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 falls downward again, increasing the AOA and therefore generating greater lift.
There are three general designs. The earliest, and by far, least common design today, is the fully …
Failure
These compensations can only do so much. Increasing angle of attack 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.
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 d…
Flight performance during a retreating blade stall
As the aircraft approaches retreating blade stall conditions, it will shudder and the nose will begin to pitch up. The resultant upward pitching of the nose will naturally begin to correct the situation as it results in slowing the aircraft. If forced to continue the acceleration via flight controls (forward cyclic + collective), it may roll to the side of the retreating blade. Recovery involves lowering the collective pitch, relieving forward pressure on the cyclic or more commonly, both. Ei…
Causes of retreating blade stall
Retreating blade stall is more likely to occur in a helicopter when the following conditions exist either alone or in combination:
• High gross weight
• High airspeed
• Low rotor RPM
Recovery
Recovery includes lowering the collective to reduce the blade angle of attack, followed by application of aft cyclic to reduce airspeed.
Description
- Retreating blade stall is a hazardous and damaging flight condition in helicopters and other rotary wing aircraft, where the rotor blade on the retreating side of the rotor disc in forward flight and therefore with the smaller resultant relative wind exceeds the critical angle of attack. Retreating blade stall is one of the primary limiting factors in a helicopter's airspeed, and the reason even th…
Aerodynamics of Retreating Blade Stall
- In forward flight, the relative airflow through the main rotor disc is different on the advancing and retreating side. The relative airflow over the advancing side is higher due to the forward speed of the helicopter, while the relative airflow on the retreating side is lower. The rotor blades incorporate the ability to flap and this allows the adv...
Stall Recovery
- Reducing collective pitch. This reduces the angle of attack in the plane of rotation and should be done first.
- Gently Applying aft cyclic to slow the aircraft.Application of aft cyclic without a reduction in collective pitch will only worsen the stall. An attempt to roll away from the stall can also make th...
Stall Avoidance
- Pilots should maintain an awareness of Vnetaking into account the expected altitudes in flight, potential turbulence, and the weight of the aircraft. Warning: Sustained flight in retreating blade stall conditions may cause reduction in the fatigue life of components through vibrations.
Design Aspects
- Developed in the 1980’s, the BERP (British Experimental Rotor Programme) rotor blade design has, by design, altered the area of effect of vortices and delayed the onset of retreating blade stall, allowing greater Vneand other performance improvements.
Accidents and Incidents
- Bell 47 pilot encountered retreating blade stall when flying at Vneand subsequently could not maintain height. The out of balance feeling was probably one blade stalling before the other due to sli...
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