Section 3. Aerodynamics of the aircraft Topic 16. Aerodynamic characteristics of the airplane
The aerodynamic characteristics of an airplane are determined as a sum of the same name aerodynamic characteristics of the airplane isolated parts and additional items which occur as a result of an interference of the airplane various parts. As it was mentioned above, in some cases the interference addends are included in the isolated parts characteristics. In this case it is necessary to consider the aerodynamic characteristics of the airplane given part in the airplane system
Let's pass to the general formulas of the airplane aerodynamic characteristics calculation.
16.1. An airplane drag at zero lift
The airplane drag occurs as a result of tangent and normal aerodynamic forces components action onto each element of streamlined surface. First component determines friction drag, second - pressure drag.
The friction drag is stipulated by air viscosity. The pressure drag occurs as a result of pressure distribution changing along body surface in comparison with pressure distribution in ideal fluid at attached steady vortex-free flow.
Pressure drag together with friction drag
determine the profile drag. At Mach numbers
shock waves appear on the body surface, it results in pressure
redistribution and occurrence of the wave drag. The lift presence and
vortex sheet formation are the reasons of one more component
occurrence - induced drag.
Therefore, it is possible to present the drag coefficient in the following form:
,
(16.1)
where
- airplane drag coefficient at zero lift (
),
- factor of induced drag.
Factor of induced drag at will be considered separately. We have
,
at
;
,
at
,
(16.2)
where
- airplane profile drag factor,
-
airplane wave drag factor,
- sum of drag from additional sources (wing - fuselage interference,
horizontal tail-fuselage interference, interference from rivets,
welding, slot between the wing and high-lift devices or control
surfaces, small-sized design superstructures - pilot-static tubes,
brackets, suspension units etc.).
In some cases, drag from additional sources is
included in
and
,
increasing their values in
times (according to statistics, the additional drag is approximately
from airplane basic elements drag; thus the airplanes are known which
additional drag reaches up to
from
).
In this case note
,
at
;
,
at
.
16.1.1. Airplane profile drag.
The airplane profile drag is determined as a sum of its isolated parts drags taking into account an interference between them.
The computational formula for the profile drag factor looks like
(16.3)
where
,
,
,
,
- profile drag factors of the airplane isolated parts;
and
- factors of additional profile drag caused by interference of the
wing, horizontal tail and fuselage;
- quantity of engines nacelle;
- interference factor of engine nacelle with a fuselage or wing,
and
- flow deceleration coefficients;
- ratio of the areas of airplane isolated parts to the characteristic
area
.
Profile drags of the wing and horizontal tail are
determined at Mach numbers
,
with taking into account flow deceleration.