We’ve probably all experienced turbulence on a flight before, but what actually is it and what causes it? We asked our Head of Flight Sciences, Anastasios Kokkalis, to explain the science behind turbulence and how it affects Airlander.
What is turbulence?
Atmospheric turbulence is a type of flow characterised by chaotic, small-scale irregular air motions resulting in winds that vary in speed and direction with respect to space and time. Atmospheric turbulence that affects aviation can arise from three main sources:
- Vertical wind shear instabilities – areas of temperature inversions and around the jet stream
- Convection – thermal effects and thunderstorms
- Mountain waves – the effect of the terrain below
How does turbulence affect aircraft?
A g unit (equal to the acceleration of gravity) is often used to quantify the effects of atmospheric turbulence. A gust of 1 g – moderate turbulence – will cause significant aircraft motion but the aircraft remains in control at all times. In the upper part of violent thunderstorms, vertical accelerations of about 3 g – severe-to-extreme turbulence – have been reported. However, the severity of turbulence experienced by an aircraft does not just depend on the strength of the air disturbance but also on the size of the aircraft. Moderate turbulence in a large aircraft may appear severe in a small aircraft.
How will turbulence affect Airlander?
Airlander is significantly less affected by turbulence compared to a traditional passenger jet for two reasons. Firstly, Airlander has a substantial inertial mass due to the 44,000 cubic meters of gas held within its hull and this inertia helps reduce the effect of turbulence on the cabin.
Extensive experimental and theoretical studies in Europe and the USA, as well as considerable flight experience (our own aircraft and other airships), has shown that due to their large size, hybrid aircraft like Airlander act as a low-pass filter to small wavelength atmospheric turbulent eddies (the whirl of air that forms when wind encounters a solid object). This means Airlander does not respond to turbulence with eddies of a wavelength less than approximately 20% of the aircraft length. As a result, Airlander just rides over such eddies without these causing any discomfort to the crew and passengers. The larger the aircraft, the larger the eddies that they can ride without causing undue discomfort. Airlander’s size means the number of eddies encountered in the atmosphere that could cause discomfort are significantly reduced, and this should result in an improved ride quality for the crew and passengers when in flight in moderate turbulent air.
Secondly, Airlander’s hull is effectively the wing and due to its large size (it is nearly 100m long) the hull ‘bridges’ across the relatively short period turbulence so that it ‘averages’ the impact of turbulence out (several up eddies and down eddies may be acting on the hull at the same time thus causing the overall disturbance force acting on Airlander to be minimised).
So, while you will feel turbulence when you fly on Airlander, its effects are expected to be substantially reduced when compared to an aeroplane subject to the same level of turbulence.
How can the effects of turbulence be further mitigated?
An Airlander pilot has several strategies available to mitigate or avoid the adverse effects of atmospheric turbulence on the aircraft. The mitigations are very much dependent upon the initial cause of such turbulence. For instance, should the cause of the encountered turbulent flow be due to a thunderstorm, the pilot may choose to fly above it, below it, or around it. Modern weather radars and air-to-ground communications enable the pilot to have a clear picture of the weather situation ahead that allow the pilot to plan accordingly.
Although atmospheric turbulence can potentially impact Airlander, like it does traditional passenger aeroplanes, Airlander’s design should mitigate the experience. Additionally, extensive operational knowledge exists from past flight experience as well from recent theoretical and experimental studies. This knowledge allows an Airlander pilot to take appropriate action to mitigate or avoid the adverse effects when required.