User manual

Part B SectionAppendix A.2
A.2 MECHANICS OF WATER FLYING
01 FORCES
Forces acting on the aircraft weight (Gravitational force)
Weight acts vertically downwards from the centre of gravity. It acts the same when on
water as when in the air and CofG is movable depending on loading. On water it is
possible for the aircraft hull to be supported by two wave crests but be unsupported near
the CofG (sagging), or to rest on one wave crest but be unsupported at bow and stern
(hogging). The hull strength required to cope with these situations is greater than that
required for impact with the surface and imposes a weight penalty.
02 BUOYANCY (HYDROSTATIC FORCE)
Water exerts a force on all surfaces of the hull it is in contact with. The resultant force is
buoyancy and this can be considered to act vertically upwards from the Centre of
Buoyancy. It has intensity equal to the weight of water displaced by the aircraft which is in
turn equal to the weight of the aircraft C.of.B is also a movable point depending on how the
aircraft sits in the water, whether or not the water is moving and how the aircraft is loaded.
03 THRUST / DRAG COUPLE
The thrust line is that of the engine propeller shaft but is set higher than the drag line
resulting in pitch changes with power change on the water This produces a nose down
pitch with increasing power from the start of the take-off and is one of the reasons for full
up elevator at the start of the run.
04 HYDRODYNAMIC FORCES
The planing hulls of the flying boat or floatplane are designed for maximum hydrodynamic
lift and minimum hydrodynamic drag at typical take-off speed. At low speed, as at the start
of the take-off run the hydrodynamic drag is great and the hydrodynamic lift is poor, and
the drag operates below the CofG and C.of.B while the thrust is above both. The hull will
want to plow through the water not plane over it. Up elevator helps to keep the thrust line
developing some lift, keeps the bow from plowing and encourages an increase in speed
and improved planing. The step in the hull helps separate the water from the hull to reduce
drag and surface tension.
05 AERODYNAMIC FORCES
An increase in speed over the water allows the normal aerodynamic force couples of lift /
weight and thrust / drag to operate as for any other aircraft and the hydrodynamic forces
decrease. As the aircraft leaves the water it changes from being supported by water at the
C.of.B to being supported by lift at the wing Centre of Pressure. The C.of.P is well forward
of the C.of.B and so the aircraft will tend to pitch up at take-off. This is a slow attitude
transition for some aircraft and a rapid change for others. It explains in part, the need to
check forward slightly on the controls of PBY to maintain correct attitude in the climb after
take-off. This effect is also caused by the aerodynamic lift provided by the exposed hull
after this clears the water.
L = W
L = F aerodynamic +F hydrodynamic+ F Hydrostatic
W = F gravity
Stichting Catalina PH-PBY Rev No: 02
01 April, 2006 Page: 1