13.3. Base drag
The base drag is caused by flow stall behind the blunt base. Thus the value of rarefaction in the stagnant zone behind the blunt base depends on some factors: the rear part shape, presence or absence of a jet stream, geometric characteristics, flow mode, boundary layer status etc. Friction between an external flow and flow behind the blunt base causes pressure reducing. The level of pressure reducing depends on structure and thickness of the boundary layer. Increase of the boundary layer thickness reduces gas ejection in stagnant area, reduces rarefaction and increases the factor of base drag.
In
the subsonic flow the base drag occurs as a result of air ejection
properties streamlining the blunt base, in the supersonic flow (
)
the additional rarefaction happens from expansion of the supersonic
flow.
Fig. 13.3. Subsonic and supersonic flow about the blunt base
The
greatest size of base drag
will be at
(vacuum). Then the factor of pressure on the blunt base will be equal
(13.21)
and
as
,
then
.
(13.22)
In
the supersonic flow (
)
for calculation of factor of base drag which is distinct from
,
the following formula is offered
,
(13.23)
where
the factor
takes into account influence of the boundary layer,
- tapering of the rear part:
,
.
In
the subsonic flow (
)
the factor of base drag can be defined by the formula
.
(13.24)
The
ejection effect depends on the boundary layer status, in particular
from its thickness
.
Obviously, as more thickness of the boundary layer
,
then suction is less and
is less.
Fig. 13.4. Function of base drag on Mach numbers
The fuselage shape is determined by airplane assignment, type and weight of transported freight, requirements of aerodynamics and operation etc.
The body of revolution of the perfectly streamlined shape should be chosen for fuselage. The changes of surface chamber should be small and smooth, as the fractures increase drag. The smoothness of the shape should not be break by design juts, as the drag is increased also due to mutual influence of body parts.
It
is possible to decrease fuselage drag and relay appearance of shock
waves having given the laminar shape to fuselage, at which the
maximum thickness displaces on
shares of chord lengths, having created smooth contours, ideal smooth
surface, at that
increase up to values
.
The
main part of fuselages drag of subsonic planes is the friction drag,
therefore designers try to give them the shape with minimal surface.
The modern subsonic planes (
)
have fuselages with optimum aspect ratio
and rounded nose. At transonic speeds (
)there
is a wave drag on a fuselage, therefore it is more expedient to use
fuselages with high aspect ratio
and more pointed nose.
The
tail unit of passenger plane fuselages is usually a little elevated
for provision of required angles of attack of an airplane while
takeoff and landing. For transport airplanes the tail unit is beveled
and is even more elevated for freights loading. Therefore, the
fuselage aerodynamic characteristics of transport plane usually are
worse, than of passenger one. Special ribs installed along fuselage
near the back doorway are used for drag decreasing of the rear part.
These ribs allow to reduce fuselage drag on
and to increase lift-to-drag ratio approximately by unit during
cruising mode of flight.