An understanding of the market and the way it is likely to change with time is central to the planning of any business enterprise. Forecasting in air transport is particularly difficult because of the number of factors which influence the potential market (political, economic, social, environmental and technological) and because demand itself is very much influenced by the range of prices and qualities offered to the consumer. The lecture will review the purpose of forecasting and then examine the techniques of forecasting that are available in order to assess their usefulness in airline planning.

The preparation of a forecast is not an end in itself: rather it should be an essential and central element of the business activity. It should be part of the systematic search for opportunities and an aid to giving an overall direction to a business enterprise. For these reasons, forecasting should be seen as part of the mainstream of the planning activity and should not be left to isolated individuals in ‘backwater’ departments. Part of its value is in the formal evaluation of market intelligence and the understanding of the forces which are at work in the market place. Much of the benefit of a forecasting exercise will be lost if the ‘intelligence’ it generates is not diffused into the rest of the organisation. Similarly, line departments need to be involved in the forecasting process to help forecasters avoid ‘wishful thinking’ and pure ‘star gazing'.

Fleet planning is a complex process. Undertaking it is time consuming as, properly conducted, it involves virtually every part of the airline organisation in a series of sub-analyses all of which impact on each other in some way. Parts of the process are deeply technical but others may be purely subjective relying on the intuition or judgement of the airline’s one expert in the particular field.

Obviously in fleet planning we have to investigate suitable types of aircraft. Most of the factors involved are fairly well known. Their ability, or otherwise, to operate our routes economically and without significant restriction is vitally important as is our ability to maintain them in service. We need to understand our markets and the factors which influence passengers to travel in the first place, and with us rather than our competitors in the second.

Cabin capacity is the number of seats available. If there are too few seats too many potential passengers are turned away, potential revenue is not realised and the proper service is not provided to the travelling public. If there are too many seats a lot of them will be unused when demand is low and costs will be high in relationship to revenue.

Cabin capacity is a consideration when aircraft are being procured but at that stage it need not be calculated very precisely. Once the aircraft is in service quite small adjustments may make the difference between profit and loss, a particular problem being where to divide the cabin between the different classes.

The TiltRotor concept combines the wing of an aircraft with the rotors of a helicopter. The result is a VTOL aircraft which is fuel-efficient in both cruise and hover. It can be thought of as a turboprop aircraft which can take off and land vertically, but it is also an advanced rotorcraft which has twice the speed and range of present-day helicopters.

When presented with yet another VTOL aircraft proposition, many people are quick to look for its shortcomings. The most obvious is the empty weight penalty. As an aircraft, the TiltRotor is burdened with a helicopter’s transmission, rotor, and controls. As a helicopter, the TiltRotor carries along an aircraft wing and controls, with a small hover efficiency penalty imposed by the high disc loading.

The purpose of this note is to bring to attention a result which seems to have been long forgotten — indeed it may never have been widely known — and to present a parallel new result.

The ‘long forgotten’ result is that, for a simple cantilever thin walled torsion tube of area A(y) the torsional deflection θ at the tip when subjected to a torque T(y) is a minimum, for constant tube weight, if the skin thickness t(y) is proportional to , where y is the spanwise co-ordinate.

The problem of potential flow about two-dimensional aerofoils undergoing general unsteady motion at low speeds has been studied by Giesing and by Basu and Hancock using the method of surface singularities, both source and vorticity, distributed on the aerofoil surface, satisfying the boundary condition of zero normal flow on the instantaneous aerofoil surface. These two methods differ in the application of the Kutta condition.