There has been a lot of interest in the ease of flying a helicopter recently, and questions have arisen on how easy it is to control a helicopter in less than ideal circumstances such as low visibility.
What are the main difficulties in controlling a helicopter, compared to controlling a fixed wing aeroplane in similar circumstances?
Controlling a helicopter is trickier than controlling a fixed wing aeroplane, because:
The hover. in a conventional helicopter with a single rotor on top, the hover is an unstable situation requiring constant correcting inputs with the cyclic stick, as shown in the above clip. The guest pilot on the left seat has a successful go at controlling the cyclic stick (the stick at the centre), while the impressively capable right seat pilot controls the pedals and the collective stick. The helicopter wants to fall away to the sides and forwards/aft all the time, from disturbances caused by the wake interference with the ground, wind etc. Random accelerations need to be corrected by providing control corrections, based on what the pilot sees in relation with the environment. Peripheral vision is pretty crucial in this, since it rapidly and accurately picks up movements. Four degrees-of-freedom need to be actively controlled: pitch, roll, yaw, vertical speed.
That is all to hover at constant altitude. Now hover and climb:
Horizontal rotor. Once the helicopter picks up speed, its vertical and horizontal stabiliser keep the nose more or less aligned into the airflow - more at higher speed, but the neutral angle also changes with speed. The picture above is also shown in this answer, and depicts what happens with relative blade speed in forward flight. The blade going forward creates more lift than the blade going aft (which is compensated by increasing the Angle of Attack (AoA) of the retreating blade), but there is an area of reverse flow to the side of the rotor axis which results in a sideways tilt of the rotor. Which must be compensated by a cyclic stick left input, more stick at higher speed.
Rotor torque. Installed engine power equates to torque applied to the rotor, times the rotor rotational speed. A propeller is smaller than a rotor and rotates faster: the propeller requires less torque application for a given engine power setting, and the ailerons at the end of the long fixed wings can easily compensate for that. The helicopter rotor requires a lot of torque, which requires a tail rotor to compensate. The faster the helicopter flies, the more torque, the more force required from the tail rotor - which thrusts sideways, creating a drift in the flight path, which must be trimmed by the lateral stick. Even with a vertical stabiliser, tail rotor thrust is a function of airspeed.
Humans can learn to control helicopters of course, but there is a lot more to it than controlling a fixed wing aeroplane. Our brains rely on inner ear and visual inputs for detecting accelerations. More brain stimulation by input detection via multiple channels, results in more precise control. Plus experience of course.But take away an input, like the peripheral vision when flying in reduced visibility, and the control of a helicopter becomes much harder.
In larger helicopters such as S76s, pilot control is helped by a Stability Augmentation System (SAS), an Inner Loop system with no feedback on the flight controls; and an Automatic Flight Control System (AFCS) which changes the trim point of the flight controls as a function of flight path and target point. But these systems are not flight critical: if they fail, the pilot must take over, and must be able to fully control the helicopter by manual input. From visual and inner ear cues, and this always remains harder than in the auto stabilising fixed wing.
Fixed wing aeroplanes can and must be designed to be aerodynamically stable. Helicopters cannot.
The first poster is spot on. Additionally, the most common response to emergencies is the exact opposite between helicopters and airplanes.
The first reaction ingrained in you for emergencies in an airplane is to push the yoke forward to lower the angle of attack, and to open the throttle to full (feather the inop engine and slightly retard the throttle of the good engine in the case of an engine failure in a multi-engine).
The first reaction ingrained in you for emergencies in a helicopter is to pull the cyclical (yoke) back, and to lower the collective.
I don’t have a lot of hours in helicopters. When people ask me what it feels like to me this is my response.
Flying an airplane is like driving a car. It is inherently stable. You can fly it straight, level and unaccelerated hands off as long as you pay attention.
Flying a helicopter is more like riding a motorcycle. It is inherently statically unstable. Your entire body is fully involved at all times. Your attention should also be fully involved.
