A helicopter does not want to fly. It is maintained in the air by a variety of forces and controls working in opposition to each other. And if there is any disturbance in this delicate balance the helicopter stops flying immediately and disastrously
One of the early lessons that a helicopter pilot learns is that nothing is simple and straightforward in helicopters.
Look at the phenomenon of stall for instance.
Most high school kids would be able to explain how airplanes stall when flown slower and slower till the wing crosses a critical angle of attack and loses lift rapidly. It is one of the earliest lessons of flight school. Recovery actions from incipient stall typically involves lowering the attitude and building up forward speed, which will entail a significant loss of height.
Helicopters are Different!
While stall and recovery is rather straightforward in aeroplanes, helicopters – true to their complicated nature – can stall at high speeds. This phenomenon called ‘Retreating Blade Stall’ (RBS) is one of the reasons why helicopters cannot compete with aeroplanes in the ‘need for speed’ department. RBS is one of the major factors in limiting a helicopter’s never-exceed speed (VNE).
Stalling at High Speeds
The helicopter is kept in air by rotating wings – rotor blades that individually and collectively provide the lift required for flight. As the helicopter moves through the air at higher and higher speeds, the retreating blade or the blade moving away from the direction of movement of helicopter experiences a continuous reduction in relative airflow. This is compensated by higher and higher angles of attack to maintain symmetry of lift across the rotating disc. At some speed, the retreating blade will reach a critical angle of attack and stall. The helicopter responds by pitching up and rolling towards the retreating side.
The Rotor’s Envelope
Many factors such mass, density altitude, speed, longitudinal and lateral centre of gravity, control margins, rotor speed, load factor, collective pitch (blade angle of attack) etc. define the flight envelope of the helicopter. Of these, speed, rotor RPM and load factor impact the flight envelope of a helicopter more than others. Individually or in combination of one or more of these factors, the rotor can be brought to stall. Ray Prouty once wrote “all parts of a helicopter are of equal importance but main rotors are more equal than others!”
Signs of Incipient Stall
At the point of stall, the rotor blades experience static instability and cannot flap to equality. To complicate matters further, blade position is dynamic and so it continues to move in and out of stall as it rotates around the azimuth. This increasing rotor imbalance may manifest as low frequency vibrations and rotor ‘roughness’ which usually warn the pilot of impending stall. However, with active vibration control systems and ‘smooth’ helicopters becoming the norm, you may not always receive such a forewarning. The manufacturer usually resorts to placards, VNE warnings on PFD, Rotor RPM high/low warnings, ‘g’ meters etc. to caution the pilot when approaching the flight envelope limits. Bash on regardless and you can expect hazardous situations and even loss-of-control (LoC) as recent instances have proved. Then there are cross coupling issues, fuselage response, rotor lead-lag modes, automatic flight control system inputs etc. which can collude in strange ways. In the extreme, the helicopter can even disintegrate in flight.
When to Expect RBS
The rotor’s envelope depends on a variety of factors that include a design maximum lift coefficient of the blade airfoil, helicopter mass, load factor, density altitude and Mach number. Higher and higher density altitudes can push the rotor into stall at a lower indicated airspeed than at lower altitudes (akin to the ‘coffin corner’ of aeroplanes). So can a lower rotor speed. Similarly, a heavily loaded helicopter cruising high on a hot day can experience the effects of RBS earlier. It is for this reason that the VNE placard is prominently displayed in the cockpit where it can be seen and followed by pilots. But here’s the catch. This placard is only valid for a calm, clear day without turbulence or other precipitate conditions. If you encounter a gust or decide to pull up to avoid a bird while cruising 5 knots below VNE, you could push the rotor into stall.
A Recent Incident
As per an Australian Transport Safety Board (ATSB) report, on 15 February 2013, an Emergency Medical Service BK 117 dropped almost 4000 feet from cruising altitude of 5000 feet after the helicopter violently pitched up and rolled left. The helicopter was on a trauma recovery flight and flying understandably fast. It is interesting that the pilots noticed pressure fluctuations in the servo hydraulic system indications just prior to the incident. Although unconfirmed, this could be indicative of a condition variously described as incipient ‘servo stall’, ‘jack stall’ or ‘servo transparency’ due to blade pitching moments being transmitted back into the servo system through the pitch links and swash plates. ATSB investigations found that the helicopter was being operated at a weight, density altitude and speed that predisposed it to RBS. The pilot instinctively (but erroneously) gave forward cyclic input to counter the pitch up and thereby delayed recovery from RBS. Thankfully, the helicopter was recovered by 800 feet above ground with no injuries and minor damage to stabilizers. The outcome could well have been more disastrous. The report mentions at least two more similar incidents on the same type of helicopter. It has happened on other helicopters too.
Rigid Rotor Helicopters
With advancements in technology and blade design, helicopters are becoming faster and faster, flying ever so closer to their limits. Add to that, low stiffness, hinge-less (rigid rotor) or semi-rigid rotors are becoming the norm as designers strive to improve performance, hub design and reduce maintenance loads. On helicopters such as the BK 117 with rigid rotor system, the effects of RBS can be more pronounced due to higher virtual hinge offset and capacity to generate high control moments at the hub. Since fuselage reactions to changes in disc attitude are quite instantaneous in rigid-rotor helicopters, response to RBS can be quite sudden and vicious.
Recovery from RBS
If your job involves flying close to Velocity Never Exceed (VNE) at high loading (think HEMS or air ambulance), high density altitude or if you are involved in heavy manoeuvres (flying displays), you have no choice but to remain aware of situations that can cause RBS and the correct recovery technique. As per the FAA Helicopter Flying Handbook, “correct recovery from retreating blade stall requires the collective to be lowered first, which reduces blade angles and thus angle of attack. Aft cyclic can then be used to slow the helicopter.”
In-Flight Resonance Modes
When flying at the limits of the helicopter’s speed envelope, disastrous chain of events can emanate from anywhere. In a tragic crash on 06 July 16, the experimental, fly-by-wire Bell 525 prototype Flight Test Vehicle 1 (FTV1) crashed while undertaking One Engine Inoperative (OEI) flight tests close to the VNE. As per a report in the Rotor & Wing International, “the twin-engine helicopter was flying at 183 kt with rotor power at 92% during a single-engine, never-exceed-speed test when a six-hertz appeared on recorded flight data that appeared to originate in the tail rotor system”. The main rotor RPM dropped off and what appears to be an in-flight resonance followed that broke up the helicopter in-flight, killing both test pilots. The detailed report from NTSB is awaited while Bell resumed flight tests of the 525 ‘Relentless’ a year later. The accident is a sober reminder that there is still so much unknown about helicopters and their behaviour.
Helicopters employing low-stiffness blades on a hinge-less hub are also known to be susceptible to in-flight rotor-fuselage ‘air resonance’ modes that can be excited under certain conditions. While we are familiar with ‘ground resonance’ its airborne equivalent ‘air resonance’ can occur if there are natural fuselage body frequencies involving horizontal hub motion that are close to the blades’ in-plane natural frequency minus the rotor speed. Conventional fully articulated rotors are not susceptible to this type of resonance and are more prone to ground resonance. A 1975 research paper by ARS Bramwell, The University of London, describes air resonance as a condition “in which instability or large lag plane amplitudes may occur due to coupling of blade lagging and flapping with the modes of body motion”. That we haven’t seen too many accidents to modes like these shows how hard designers have been working to exclude these modes through design, use of mechanical dampers etc.
Lateral Cyclic Control Saturation
Rigid rotor helicopters can generate high control moments at the hub due to their virtual hinge offset. During high power, high disc loading manoeuvring flight, this can make them susceptible to another condition called ‘Lateral Cyclic Control Saturation’. For example, on a helicopter with rotors turning clockwise (seen from above), during a high pitch, high ‘g’, high rate of bank turn to the left (advancing side), the retreating side experiences an increase in angle of attack due airflow from below causing the disc to flap up on the retreating side. In a ‘g’ loaded left turn therefore, the turn may tighten on its own and pilot could run out of right cyclic to arrest or upright the roll. Since the disc is already heavily loaded, any pitch up or down further aggravates the problem. Applying top rudder (in this case right rudder) and reducing collective pitch is the only safe recovery action which will incur a height loss – something that can be fatal at low heights. This was found to be the reason behind at least three crashes of the rigid rotor Advanced Light Helicopter (ALH) or the Dhruv, including the fatal crash of a Dhruv from the Indian Air Force helicopter display team ‘Sarang’ in Feb 2007. Two more Dhruvs sold to Ecuador crashed similarly leading to cancellation of the contract in 2015 and much embarrassment to the manufacturer Hindustan Aeronautics Limited (HAL). HAL has since incorporated a Control Saturation Warning System (CSWS) for the ALH fleet.
I hope this is motivation enough for you to read up the relevant sections in your helicopter’s flight manual pertaining to its flight envelope. Respect speed, angle of bank, rotor RPM and all such limits; they are there for a reason. If you violate them, even inadvertently, expect nasty surprises.
Now you know why Harry Reasoner said “in general, airplane pilots are open, clear eyed, buoyant extroverts, and helicopter pilots are brooders, introspective anticipators of trouble. They know that if something bad has not happened, it is about to.”
Be aware, be safe! Happy landing, folks!
©KP Sanjeev Kumar, 2017. All rights reserved.
Dhruv picture in cover photo courtesy www.hal-india.com. Red Bull MBB Bo-105 picture featured above courtesy www.arejaye.deviantart.com
Please consult your Rotorcraft Flight Manual for limitations and recovery procedures as applicable to the helicopter type.