14.4. Interference of the engine nacelles with parts of an airplane

14.4.1. Nacelles location on the fuselage lateral area in its tail part

Fig. 14.6.

There is formed a convergent-dilative channel between nacelle and fuselage promoted separation of the boundary layer and growth of profile drag of the system nacelle-fuselage (Fig. 14.6). At subsonic speeds with in a channel it is possible to expect the appearance of shock waves, that causes the further growth of drag. Obviously, the additional drag caused by an interference is conveniently to take into account into nacelle drag with the help of introductions of the correction factor.

In particular, the nacelle profile drag in the system with fuselage is determined with taking into account an interference factor

,

where - nacelle quantity, - profile drag of one isolated nacelle with midsection ( , - characteristic area).

The factor takes into account an interference effect

, ; , .

It is necessary to point another one effect - decreasing of nacelle lift in the airplane system because of its falling in the wing downwash. This effect is increased at lifting devices deflection.

14.4.2. Nacelle installation onto wing.

As well as in the previous case a source of additional drag is the formation of a channel between nacelle and wing with increased flow rate in a channel and reduced speed in the outgoing area of nacelle. Here increasing of the boundary layer happens and the flow stalling is possible. In the channel at can occur shock waves. All haven said concerns to nacelles located on pylons or directly under the wing.

Fig. 14.7.

Other reason is connected to features of flow about swept wing at engine nacelle installation on it. Disturbance of flow on the isolated wing takes place in this case, the additional chamber of streamlines by nacelle walls (Fig. 14.7) occurs. In area nearby the wing leading edge an additional rarefaction will occur due to increasing of speed. In area , on the contrary, decreasing speed occurs nearby the wing leading edge with further opportunity of increasing at the nacelle tip. Critical Mach number of the nacelle-wing system decreases. Thus, the flow disturbance on the wing connected to the nacelle installation (or pylon) causes an increase of drag.

Most unfavorable nacelle location is directly under a wing without offsetting. In this case areas of minimum pressure on the wing surface and nacelle coincide, due to that the positive pressure gradients grow and the conditions for earlier stalling of the boundary layer are created.

Nacelle displacement forward or back, its location on wing axis or on pylon causes a decreasing of the interference drag. The least interference is received at nacelle location wing chordwise.

As well as in case of the nacelle installation on the fuselage, an additional drag is accounted by a factor in nacelle drag, which depends on nacelle location relatively to the wing:

,

, , .

Here the factor takes into account nacelle displacement along perpendicular to the wing plane ; - mutual influence two nacelles located on one wing cantilever ; - nacelle displacement along the wing chord . If one nacelle is located on the wing cantilever, then . The geometric parameters , , are shown in fig. 14.8.

Fig. 14.8. Engine nacelle location on a wing

The mutual influence of two nacelles installed on one wing cantilever causes growth of drag, mainly, in an outcome of increase of flow rate between them and growth of pressure gradient on the nacelles surface.

It is possible to reduce the interference drag of the wing-nacelle system if nacelle axis is disposed with taking into account the direction of local flow rate.

The positive effect of nacelle interference with the wing can be received at . In this case nacelles should be placed behind the line of wing maximum thickness. The increased pressure induced by a shock wave from the nacelle creates a component force of pressure directed forward (negative drag); there small lift appears.