13.1.2. Wave drag of the rear part
The
fuselage rear parts have tapering in many cases. The reduced pressure
is established on tapered rear parts at supersonic speeds. The factor
of wave drag of the fuselage rear part depends on the shapes of
outlines, its tapering and aspect ratio, number
,
and also from aspect ratio of the fuselage cylindrical part
.
It is more less, than its aspect ratio and number
are more.
We
shall mark, that wave drag
will depend on aspect ratio of a cylindrical part
:
for
fuselage with a short cylindrical part (
), because, in this case, flow before the rear part will not have
time to become uniform and to accept values of undisturbed flow.
At
it is possible to consider, that the rear part is streamlined by
undisturbed flow and calculate of the wave drag
irrespectively of what body it is located.
Calculation of the wave drag factor for conical rear part is performed by the formula
.
(13.12)
For the rear part with any generative lines (close by shape to parabola)
,
.
(13.13)
In
case of the pointed rear part it is necessary to accept
in the formulae (13.12) and (13.13).
If
the engine is installed in the fuselage rear part, the factors
will depend on the shape and parameters of outflowing jet. The jet
extending at
causes pressure increase near the rear part due to flow deceleration
in rear shock waves. It promotes decreasing of
.
13.2. Fuselage profile drag
Fuselage profile drag is considered as drag of an equivalent body of revolution. The amendments are entered for the account of fuselage design features which distinguish it from the body of revolution.
Factor of fuselage profile drag
(13.14)
where
- profile drag of an equivalent body of revolution, which is
determined as follows:
,
(13.15)
where
- friction drag coefficient of one side of a flat plate in an
incompressible fluid flow at identical with the specified fuselage
Reynolds number
and coordinate of point in which laminar boundary layer becomes
turbulent
.
If
to accept, that the fuselage is streamlined by completely turbulent
flow (
),
that a little bit overestimate the drag, then
.
(13.16)
The
number
is calculated on fuselage length
and flight parameters
and
:
,
,
,
.
The
factors
and
in the formula (13.15) define the contribution of pressure forces and
compressibility effect in fuselage profile drag:
or
,
.
(13.17)
The ratio of the area of the lateral (wetted) fuselage surface to the area of midsection can be approximately calculated by the formula
.
(13.18)
The
account of fuselage design features is carried out by summing of
additional drag factors
.
The increment of the factor of fuselage profile drag caused by tail fairing or tapered rear part at subsonic speeds of flight is calculated by the formula
,
(13.19)
If
the jet stream outflows from the blunt base then
,
at absence of a jet stream
.
The factor
depends on the shape of rear part generative line: for an ellipse
,
for other curves (particular case - hemisphere)
.
If tapering of the rear part
then value of the factor
.
Beveled or bended rear part causes an additional drag
(13.20)
where
- angle of deflection of the rear part mean line,
at
,
at
.
The influence of canopy is estimated by such values:
-
for a passenger or transport airplane
;
-
for a manoeuvrable airplane
;
-
the fairings of main landing gears located on the lateral fuselage
surface increase drag on size
;
-
side or ventral air intakes increase drag on size
,
where
- summarized area of all air intakes.