SIZE OF OPERATING SITE
Since the area required for a landing from the hover is small, it is clearly easier to provide or find suitable locations for this as opposed to a short landing strip. However, it is sometimes suggested that, because a short takeoff strip has to be provided for a VSTOL aircraft to take off at max weight, then the existence of this takeoff strip means that it can be used for short landing and hence STOL is all that is necessary. This view takes no account of several reasons why an STO, even at maximum weight, can be safely carried out from a much shorter and narrower strip than is acceptable for a slow landing even at a lighter weight. First consider factors affecting strip length:
(a) An aircraft can be positioned for takeoff at the very beginning of the strip and the subsequent ground roll required to unstick can be very accurately predicted (as a function of weight and thrust), making it acceptable to plan to unstick close to the end of the strip. On the other hand, some distance will be needed for a landing at both ends of the strip to allow for scatter in touchdown position and stopping performance.
(b) The acceleration available for takeoff may well approach 1g and be unaffected by a wet or icy surface but it is difficult to design for a similar level of deceleration throughout the ground roll when landing, even in good conditions. Given slippery conditions, wheel braking effects can all but vanish.
(c) The use of full power on takeoff maximises the lift effect of thrust, reducing the speed needed to fly at a given weight. However, in order to have a go around capability when landing, some power margin must remain at touchdown. This results in a reduction of the lift element from thrust on a slow approach, compared to that available during a STO. Replacing this powered lift with V squared aerodynamic lift can need a surprisingly large increase in V at the lower approach speeds of short landing aircraft, countering the advantage to be expected from reduced weight at the end of the sortie.
A narrower strip is acceptable for takeoff compared to landing because:
(a) An aircraft can be lined up very accurately before a takeoff, whereas there is a need to allow for lateral scatter when landing.
(b) Direction can be controlled relatively easily during the slower first part of the takeoff ground run and then the quality of aerodynamic directional control improves as speed increases. The opposite applies when landing and the use of brakes, aerodynamic devices and reverse thrust all tend to degrade directional control and stability. This stems from the use of large forces to slow down quickly on the ground and so quite minor asymmetries in those forces can cause the aircraft to veer. Experience shows that such asymmetries can also result from crosswinds interacting with the complex flow patterns around an aircraft using high power or aerodynamic devices for deceleration.
Provision must be made at the end of a strip used for landing for the aircraft to turn round and backtrack or clear the strip at the side, whereas places for landing from the hover can be provided some way away from the short takeoff strip, thus easing flow control on the ground through the land/hide, replenish/takeoff sequence. When the same strip has to be used for both takeoff and landing, a larger and more complicated site layout becomes necessary. Even in the orderly peacetime world of civil aircraft operations, the advantages and smooth traffic flows that result from using different runways for takeoff and landing are apparent for all to see.