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fulfilling a unique and invaluable niche in remote area logistics, helicopter flight defies gravity and mechanical practicality and yet are surprisingly safe. This is how they work

Helicopter Flight – safe but not sure

Helicopter Flight – safe but not sure

Travelling through any medium, an aerofoil with a positive angle of attack ‘α’ with respect to the horizontal, will cause a medium deceleration below the aerofoil (thus increasing pressure) and an acceleration above it (thus decreasing pressure). The pressure differential generates a lift force. The amount of lift generated  depends on the size of the aerofoil, its speed through the medium and the density of that medium. When the lift generated by the aerofoil is greater than that of the body to which it is attached, that body will rise. Let us start by looking at each of these elements impacting lift generation.

Medium Density vs Aerofoil Size – the baseline properties of solids, liquids and gasses are very similar. Almost every substance has the three states. The change in state is a simple function of atomic vibration. The level of vibration is a dynamic of energy.  Available energy is a factor of temperature.  At Absolute Zero (-278°C), atomic vibration ceases altogether so (typically) everything is in a solid state. As the temperature rises, energy levels increase and with it atomic vibration, causing a decrease in substance density and the associated changes in state (solid > liquid > gas). In higher densities, aerofoils are more efficient. So in liquids it only requires a small aerofoil (of a couple of meters) to lift a 300 tonne hydrofoil out of the water while, in the lower density of air, to lift an equivalent aircraft, it requires an aerofoil (wing span) of more than 50 meters. Air density similarly varies with temperature. So, in cold climates aerofoils are more efficient than in tropical climates, which has a fundamental impact on aircraft operation.

As one would expect, the way aerofoils work on aeroplanes and helicopters is exactly the same: what differs is the way the causal airflow is generated. Aeroplanes have fixed wings and so need a very long straight road (runway) to get these very large and heavy trucks up to a speed of some 200 km/hr where the lift generated by its aerofoil equals its weight and it flies. Helicopters are more subtle, generating the airflow to realise the required lift by simply rotating the aerofoil blades at some 2-300 rpm. The larger (heavier) the helicopter, the larger the size and number of its aerofoils (blades). The biggest chopper in the world is the Russian Mi-26 with roughly 100 tonnes of Lift provided by 7 huge blades – and that must be somewhere near the limit of what is possible.

So, returning now to the impact of temperatures on aircraft operation. Increasing temperature increases atomic vibration thus reducing air density and decreasing the lift generated by aerofoils at any given airspeed.  The operational impact of this is significantly different for fixed and rotary wing types. For aircraft with fixed aerofoils (wings) increasing temperature is of little consequence as all one needs to do is go faster to generate the required lift, thus requiring a longer runway to achieve lift-off.  For rotary wing this is not always an option: where a runway is available they can mitigate the reduced lift with increasing temperature in the same way as aircraft with fixed aerofoils by doing a running take-off. But in the absence of a runway – operating from helipads – they have no option other than to reduce the aircraft weight to get it off the ground, typically by carrying less passengers and cargo (rule of thumb – a reduction of 100kg for every 10°C increase). Hence, in Fixed Wing operations, weight calculations tend to be more casual while for aircraft using rotary wings, every kilogram including fluids, fuel and even pilot bulk is accounted for to the point of paranoia!

And then there is the mechanics relating to the aerofoil types. By definition, aircraft with fixed wings are simple with aerofoil length and lift being (essentially) linear: not so for the rotary types.  For α start, airspeed at the inner end of a rotating aerofoil is very much less than at the tips, so the generated lift is anything but linear. This is complicated further by the fact that as a rotary wing aircraft moves forward, the advancing blade will generate a great deal more lift than the retreating blade. This is to the point that above some 60 km/hr, the difference is enough to flip the aircraft over. The only way to compensate this is to reduce the ‘α’ of the former and increase it on the latter. Given aerofoil (blade) weights of some 200kg and rotations of some 300 rpm this makes for complex and robust mechanics and explains why it took more than 50 years after the first fixed wing aircraft staggered off the ground, for an equivalent brief flight to be realized using rotating aerofoils.

Then there is the need to compensate the inconveniences engendered by Newton’s Laws and particularly that whereby action and reaction are equal and opposite. So significant is this that one ton of rotating aerofoil at 300 rpm will cause the 4-ton aircraft to which it is attached to rotate at some 75 rpm in the opposite direction – while good for training astronauts, such is unacceptable for normal folk. Hence the small fan put at arms length at the back of the aircraft (the tail rotor) to counter that effect. This all results in the need for multiple sets of complex mechanical devices on rotary wing aircraft which, although Safety Critical, are a simplex installation (typically a ‘no-no’ in electro-mechanics) – any failure is thus always catastrophic and fatal. Such risk is not further mitigated by the need to combine the nominal opposites of robust reliability with lightness of structure.  It is a very real measure of man’s ingenuity that this ‘mechanical oxymoron’ has, in the main, been achieved.

No sweat – with thanks to Captain Ed Cooke

No sweat – with thanks to Captain Ed Cooke

And pity the pilot…… Due to said Newtonian Laws, a change in any one rotary wing parameter – airspeed, ‘α’, direction or power – and all the others have to be corrected to maintain the stability in flight. Simply stated, while airplanes with fixed wings naturally want to fly, those with rotating wings are defying nature and do not. As such, any disturbance in the delicate balance of mechanical forces in the latter and it will cease flying, immediately and disastrously! So, while pilots in fixed aerofoil aircraft, once airborne, can relax, those in rotary types are typically brooding anticipators of impending doom.

So, does this mean helicopters are essentially unsafe ?  Certainly they are less forgiving. Users must take nothing for granted, be disciplined in following procedures and respect nature. While fixed wing aeroplanes can usually ‘punch through’ bad weather and get above it, Helicopters, in the main, have to live within it.  And with the complexity of its mechanics, there is so much more to go wrong in an helicopter, so they tend to be less reliable. A measure of this is that for every hour of flight of a helicopter, there needs to be some 12 technician man-hours on the ground, while typically this is only three for fixed wing. But that said, as a function of its fixed wings, an aeroplane needs to travel very fast to maintain controlled flight so, when things go badly wrong to the point that control of flight is no longer possible, they return to terra firma at fast and furious speeds leading to considerable destruction and high fatalities. Helicopters on the other hand, in the event of complete power loss will experience a slow and undramatic return to earth such that, where the terra is firma and not liquid, passengers may even be unaware that there has been a failure except for the fact that the arrival point is not that desired.

In short, while aircraft using rotating aerofoils are inherently less reliable than those with fixed aerofoils, bottom-line, they are inherently no less benign and even safer.