Aerial tow target

The invention relates to a high speed aerial tow bag target of textile maial having a reduced aerodynamic drag provided with a frontside air entrance orifice and at least one air exit orifice at its tail. In order to reduce the drag of the tow bag and to improve its stability under flight condition the tail is provided with a contour tapering to the end of the bag and the hole area of the air exit orifices is made smaller than the cross section of the air entrance orifice at the front side.

FIELD OF THE INVENTION 
The invention relates to a high speed aerial tow target of textile material 
having a reduced aerodynamic drag provided with a front side air entrance 
aperture and at the backside at least one air exit aperture. 
This kind of tow targets consisting generally of natural or artificial 
textile material are towed behind an aircraft and are used as aerial 
targets for the exercising shooting onto flying targets, wherein a shot 
receiving sensor means installed within the tow target recognizes the 
projectiles passing the target and indicating them in form of a shot sign 
on the earth. Besides the so called smooth tow target of textile materials 
also so called solid tow targets are used the last ones are configured as 
rigid aerial trailers. 
DESCRIPTION OF THE PRIOR ART 
The known smooth or soft tow targets are in general tow bags of the above 
mentioned kind which are pretensioned and kept stable in configurational 
aspects by the static pressure of the impinging air stream and which are 
advantageous in economical aspects in comparison to aerial trailers and 
deliver a greater security against damages on striking the ground, if 
these trailers are shot off or pulled down. 
A disadvantage of the tow bag targets in relation to the aerial trailers is 
manifested by the fact that the first ones cannot carry with them smoke 
cartridges marking the tow target after having been remotely ignited. 
Because of security reasons, however, the tow targets must be definitely 
recognized in order to avoid under all circumstances any confusion between 
such a tow target and the air-craft towing that target. Therefore, tow bag 
targets must be essentially greater in size than aerial trailers in order 
to provide a sufficient visibility and thus distinctiveness. The 
relatively great dimensions, however, have the drawback of an increased 
drag or air resistance which is for a definite tow speed in general ten to 
twenty times greater than that of aerial trailers. The drag, however, is 
the essential feature for the maximum speed of use of a tow target, 
because the tensile strength of the tow ropes and the reserve power of 
towing the aircraft are limited. 
The high drag of usual tow bag targets, however, is not only caused by 
their great dimensions, but also by an aerodynamically unfavorable 
configuration and flowing conditions. This configuration is characterized 
therein that the tow bags of in general circular cylindrical form are open 
at the front side and are closed at the back side by a flat cylinder. The 
impinging flow enters the front side aperture which is kept open 
perpendicularly to the impinging flow by numerous strings of even length, 
the so called spider. The air entering the front side aperture produces a 
static pressure inflating the tow bag target. The tensile strings are 
concentrated into a tow rope and are connected thereto by a rope coupling. 
This construction of the tow bag target which is usual in the practice 
causes essential air turbulences in the front area as well as at the tail 
which are the essential reason for the high tow resistance or drag. 
Especially at the rear wall extending nearly perpendicularly to the 
longitudinal axis of the bag the air flow is interrupted and turbulence is 
caused over the whole area. 
In case of the usual equipment provided with the acoustical shot sensor 
means in the front area of the tow bag it is necessary to diminish the 
front side air turbulence and thus the acoustical background noise. This 
is reached by means of a circle of relatively small radial orifices in the 
rear bag wall, through which the impinging air stream is partly flowing so 
that the front side turbulences are diminished. On the other hand the 
lateral blowing out of the air through these orifices increases the drag 
but it is favorable insofar as it stabilizes the flight condition, as such 
a kind of tow bags is usually rotating about its longitudinal axis because 
of unsymmetrical manufacturing tolerances. This rotation, however, is to 
be avoided, because it causes a changing orientation and a lateral 
impinging stream onto the acoustical shot sensors affecting the correct 
operation of the system. 
It was found out that tow bags having the above mentioned lateral rear 
circle of orifices rotate slower than those which are not provided with 
such a circle of orifices, because the laterally emerging air flows have 
an effect retarding rotation. The problem, however, that the air 
resistance or drag is too high cannot be solved by such a kind of tow 
bags, because the dimensions of the tow bags must be maintained so great 
as in case of bags without a circle of orifices at the tail. 
It is certainly true that a rotation retarding effect can be provided by 
making the configuration of the tow bag from the front side to the rear in 
a slight conically expanding manner. By such a configuration, however, the 
drag is increased and manufacturing of such a conically configured tow bag 
requires a special accuracy and symmetry, because such a kind of tow bags 
are combined by seaming trapezoidal cuts of textile material so that 
not-parallel weaving threads are extending generally along the seams. 
SUMMARY OF THE INVENTION 
One of the objects underlying the invention is therefore to provide a tow 
bag target which is especially useful for high speed purposes. 
According to another object such a tow bag target should have an improved 
configuration resulting in a reduced drag or resistance on towing it 
through the air. 
According to a still further object of the invention the tow bag target of 
the above mentioned kind is to be constructed such that under flight 
condition the position of the tow bag target with respect to its 
longitudinal axis is maintained stable. 
DETAILED DESCRIPTION OF THE INVENTION 
These and other objects are advantageously solved by the invention by the 
fact that the tail of the tow bag target is provided with a tapering 
contour in the direction to the bag end and that the whole area of the air 
outlet orifices at the tail end is smaller than the cross section of the 
air entrance orifice at the front side. 
The solution of these objects is based on the consideration that in case 
the tow bag target is manufactured in a highly precise manner it should be 
possible to maintain the target generally in a stable position by means of 
a mass pendulum, for instance the arrangement of a shot sensor electronic 
device within the tow bag wall and thus in an eccentric position, in order 
to completely or partly avoid the usual radial arrangement of orifices at 
the tail positioned on a circular line. Instead thereof one or several 
axially directed throughput orifices are arranged in the tail of the tow 
bag in order to essentially diminish the increasing resistance caused by 
blowing out the air radially and laterally or to completely avoiding it, 
if possible. 
A precondition for the stability of the configuration of the tow bag target 
is that the whole area of the air outlet orifice at the tail is smaller 
than the air entrance orifice of the tow bag target at its front side. If 
this condition is not fulfilled, flutter movements of the bag walls are 
caused disturbing the operation and increasing extremely the resistance of 
moving or towing as well as the acoustical background noise. These 
drawbacks can because of local mechanical overstresses destroy the textile 
material of the tow bag target. 
By a converging, i.e. tapering configuration of the tail of the tow bag 
target break-off of the flow increasing the flow resistance is avoided. In 
this connection, the flow through of the axially directed air exit 
orifices is used to maintain the conical configuration of the tail of the 
tow bag in a stable condition by means of the developing static pressure. 
The straight direction of flow through the bag with only few turbulences 
make it possible in this connection to concentrate the emerging air in a 
manner without raising fluid break-off and turbulences during flow around 
the tail area of the bag. 
The required effect by using the characterized construction of the tow air 
target by means of a special configuration of the cross section of the 
through flow orifices at the tail of the bag can be gained by means of a 
single axial hole of circular configuration as well as by means of other 
configurations. Moreover, an arrangement of several orifices may be 
suitable. Moreover, a larger area of orifices may be divided by a netlike 
web or fabric into several single orifices. The converging contour of the 
tow bag in the direction of its tail may be advantagously realized by a 
conical or rotational-elliptical or spherical configuration. The 
progressive tapering of the tail of the tow bag target increase, however, 
the danger of flutter movements of the textile material at the tail end as 
a result of the decreasing difference between a local inner pressure and 
the outer pressure. As a result of such flutter movements periodical 
changes of the through flow cross section are caused which are 
disadvantageously affecting the stability of the flight, the tow 
resistance and the acoustical background noise level. In order to avoid 
such flutter movements the air exit or section of the orifices can be 
stabilized in configurational aspects by means of a solid annulus or ring 
fixed to the textile tow bag tail, for instance sewed into the textile 
material and tensioned at the tow bag wall by means of tensioning strings 
in order to avoid a lateral oscillation. 
In order to gain a high accuracy and symmetry on manufacturing the tow bag 
target it is preferably provided in precise cylindrical configuration. On 
doing so it is possible to maintain constantly a parallel main weaving 
thread direction of the textile material over the whole circumference of 
the tow bag target and to minimize seam deformations because of a sloping 
run of the thread along the seams of the tow bag. 
Such a tow bag target has the advantage that the flight condition can be 
made stable by means of a pendulum, i.e. or the arrangement of the shot 
sensor electronic device within the tow bag wall out of the axial position 
and it has nevertheless a reduced drag or tow resistance because the air 
stream is largely free of turns and turbulences. In case an accurate 
cylindrical construction is wanted the additional advantage of simpler 
manufacture may be gained.

The cylindrical tow bag target 1 shown in FIG. 1 is provided with a conical 
tail 2 at the end thereof an air exit orifice 9 is mechanically reinforced 
by a solid ring or annulus 12 the arrangement of which is shown in FIG. 2. 
The tow bag cylinder 1 is pulled by means of pulling strings 3 fastened at 
its front side and provided with the air entrance orifice and forming the 
so-called spider. These pulling strings 3 run to a rope coupling 4 where 
they are concentrated, and this coupling provides the connection to the 
tow rope 5 pulled by a towing air-craft. Because of the static pressure 
the tow bag is inflated by the air flow 6 and forms the front side air 
entrance orifice 7. 
At the tail of the tow bag target the greatest portion of the impinging air 
flow is streaming in a cycling flow 8 about the outer wall of the tow bag 
and is mixed without raising great turbulences with the axial parallel 
through flow 10 leaving the air exit orifice 9. For the pendulumlike 
stabilisation of a constant flight condition about the longitudinal axis 
the shot sensor electronic device is not positioned in an axial location 
11. 
FIG. 2 shows the tensioning equipment of a solid annulus 12 in the air exit 
orifice by which oscillations can be avoided. This tensioning equipment 
consists of tension ropes or bands 13 which are fixed to the cylindrical 
wall of the tow bag target by fastening elements 14, for instance eyelets. 
Position and configuration of the conical tail 2 are stabilized by that 
tensioning equipment. 
FIG. 3 shows another embodiment of the tow bag target which is provided 
with a conical configuration 17 instead of a cylindrical one and consists 
of a practically airtight textile material. The tail 15 has not a conical 
but a rotational-elliptical tapering contour 16. The air entering the bag 
through the frontside air entrance orifice 19 accumulates within the bag 
and leaves the bag through several axial-symmetrically positioned air exit 
orifices having the configuration of holes 18 located in the wall of the 
bag in the rear third of the tapering contour 16. In this area the tow bag 
is provided with a semi-airtight rear wall 20. 
At least in case of the embodiment according to FIG. 1 the tow bag target 
consists of a textile material the main direction of its web threads 
extends axially. 
The two embodiments according to FIG. 1 as well as FIG. 3 fulfill, 
moreover, the condition that the hole area of the air exit orifices 9, 18 
is smaller than the cross section of the frontside air entrance orifices 
7, 19. 
Because of the converging tapering contour 15 of the tail of the tow bag 
target, the turbulence of the air flow in the range of the tail and thus 
the air resistance or drag are essentially reduced so that the usefulness 
of the tow bag target is essentially improved. This usefulness depends 
substantially on the tensile strength of the tow ropes and the power 
reserve of the engines or fuel consumption of the towing air-craft, i.e. 
factors depending on the drag.