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Xu Fei 

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26 June' 1981 



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^ - ^mirJrHangkong, Nr. 7, July 19 80, 

pp. 8-11 | 

Country of origin: £hina)^,7 p 1 i- i 

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Date 26 j uni9 81 


Xu Fel 

In the past 10 years there have already been quite a few art¬ 
icles which have reported that the blown flap technique was cap¬ 
able of obviously improving many types of aircraft characteristics 
under conditions in which the aircraft is operating at low speeds 
and with an attack angle of less than 30°. For example, if one 
uses a blown flap at the place where the taper of the forward edge 
of the wing thins the wing down, then it is possible to make an 
improvement in the non-linear lifting force. If one uses a blown 
flap at a place in the vicinity of the leading edge of the rear 
flaps, then it is not only possible to increase the effectiveness 
of the flaps themselves, but it is also possible to add to the 
effective lifting power of the main body of the wing itself. When 
the same technique is used on other lifting surfaces, such as the 
standing tail and the horizontal tail surfaces as well as the for¬ 
ward edges, it is also possible, in all these cases, to obtain rela¬ 
tively large improvements in the aerodynamic capabilities involved. 

In the last few years the agencies responsible for aviation 
research in several countries as well as several aircraft manufact¬ 
uring companies have shown an increasingly lively interest in this 
area of research. Worthy of particular attention are the following 
several directions in which the research has gone or will go: 1) 
the work of researching the technology of a blown flap has not been 
confined only to the area of examining the mechanisms involved; 
tnis research has already passed on to the concurrent study of prac¬ 
tical designs; 2) the range of the angles of attack which have been 
included in the research has been expanded to include angles as large 
as 90°; 3) the range of speeds which are included in the research 

has also been extended to Include the realm of transsonic speeds. 

The preliminary results of this research indicate clearly the fact 
that it is possible for the technology of the blown flap to introduce 


obvious improvements in the longitudinal and lateral aerodynamic 
characteristics of aircraft under conditions in which there is a 
large angle of attack. This research also indicates that this 
technology is capable of making significant reductions in the 
separation phenomena induced by the shock waves which are attend¬ 
ant on flights at transsonic speeds and in cases in which the 
vibrations which come from very large angles of attack are a factor. 
The fact that this technology has such a capability demonstrates 
that it has a potential for use in the designs of the fighter air¬ 
craft of the future. The purpose of this article is to give a 
short introduction to the subjects which have been mentioned above. 

1. The influence of the blown flap technique on the low speed 
capabilities of the F-5E 

Like the F-16, F-17 and F-l8 fighters and others, the F-5E also 
makes use of a compound wing arrangement with an added wing on the 
side edge. This is a type of arrangement which will not only cause 
the fighter to have advanced transsonic and supersonic speed capa¬ 
bilities, but also will make it possible for the same aircraft to 
have excellent low speed capabilities as well. This is a relatively 
good arrangement for the accomplishment of this dual purpose. If 
one is dealing with moderate angles of attack and larger, then the 
employment of the added strip of wing on the leading edge of the 
main wing and the detachea vortices which it produces makes it poss¬ 
ible to obtain relatively large amounts of vortical lift. Moreover, 
this sort of wing arrangement extends the vibration threshold of the 
wing as well as the stall threshold. However, when one is dealing 
with an angle of attack of approximately 24°, then the wing strip 
vortices become hard to maintain and they break up. As a result of 
this phenomenon, there is a limitation placed on the usable range 
of the aircraft capabilities in this area. The Northrup Aircraft 
Company during the water flow analysis which it did on the F-5E,dis¬ 
covered the fact that if one uses a simple tube or pipe as a blown 
flap at a sweepback angle of 55°, then one not only completely elim¬ 
inated the separation flow on the wing but also added to the edge 


vortex and induced a vortex on the leading edge of the wing. For 
an illustration of this, see Figure 1. This experience was suffi¬ 
cient to cause the Northrup company, on the basis of its use of the 
blown flap technique on the F-5E, to later make a complete evalua¬ 
tion of the aircraft and this new technique. 

This experimentation was carried out in the Northrup company's 
7x10 foot wind tunnel. For a view of the external form of the sur¬ 
face of the model, see Figure 2; the interior diameter of the jet 
intake was 0.132 inches. At the point of contact, between the lead¬ 
ing edge strip and wing, the M number used during the test was 0.18; 
the average aerodynamic arc length Re number was 1.02x10^; and, with 
differing coefficients for the amount of blowing (Cp = 0.02^0.12) 
and for the sweepback angle (A = 55° and 70°), observations were 
made of the longitudinal and lateral characteristics of the entire 

The results of experimentation demonstrate that when one is 
dealing with angles of attack larger than 10°, the use of the blown 
flap technique induces even larger amounts of vortical lift. More¬ 
over, the use of this technique under such circumstances causes an 
angle of attack for the aircraft as a whole which results in the 
stall speed being extended from the original 24° to 32 ° or even 
more and it causes a very clear improvement in the lift-to-drag 
characteristics of the aircraft when one is dealing with large 
angles of attack for the aircraft as a whole. Besides this, the use 
of the blown flap technique causes a very small moment of lift on 
the forward or nose section of the aircraft. In fact, a moment of 
lift is almost equal to zero. This causes the linear portion of the 
force moment curve to be extended to even larger angles of attack 
and this extension has no influence on the stability of the air¬ 
craft as a whole. One thing which particularly needs to be pointed 
out is that there is an influence on the lateral performance of the 
aircraft as a whole which is exerted by the use of the blown flap 
technique on the F-5E. From Figure 3> it can be seen that when one 


Figure i. An examination of the 
water tunnel flow spectrum for a 
1/40 scale model of the F-5E 
(a = 24°) 

Key: 1—no blast; 2—with the 
presence of blast; 3—separation 
flow; 4—edge vortex; 5—Jet 
nozzle; 6—leading edge vortex 

is dealing with a situation in which the angle of sideslip is -10° 
and the angle of attack is larger than 8°, then the use of this 
blown flap technique induces even larger increases in the amount 
of asymmetrical lift. This results in an obvious and advantageous 
increase in the rolling moment. Besides this, in the case of 
another aspect of the problem, when the angle of attack is increased 
to as much as 20°, if the blown flap technique is not used, the air 
flow along the surface of the wing will give rise to severe separ¬ 
ation phenomenon. When this phenomenon takes place the edge vortex 
has already broken up and the wake from this type of edge vortex 
is capable of causing the effectiveness or efficiency of the tail 
to go into a very steep decline. Due to these conditions, the 
curve for the moments of yaw (the curves for the lateral forces are 
also similar) experience a strong increase. As for the effects 
which appear after blowing, the stall speed for the wing involved 
is postponed or pushed back. On the other hand, there is an in¬ 
crease in the edge vortices. Due to these developments, there is a 
consequent increase in the blowing magnitude; the efficiency of the 
tail increases and the moments of yaw and the lateral force charac¬ 
teristics also experience an improvement. 

In order to investigate the results of using different trim 
set-ups on the flaps of the aircraft and the interaction between 
these trims and the blown flap technique, we also carried out a set 
cf comparative experiments with the forward or leading flaps and the 
trail flaps set at respectively 24° and 20°. We obtained results by 

using the blown flap technique and other results when the tech¬ 
nique was not used. The results of these tests demonstrated that 
the use of ohe blown flap technique is capable of improving the 
longitudinal characteristics of the aircraft as a whole. However, 
the tests also revealed that the effects of this technique on the 
lateral characteristics of the aircraft are not great. Besides this, 
it was discovered that when the leading edge flaps had a trim, then 
the advantageous influence of the blown flap technique on the wing 
of the aircraft was diminished. In such a situation, the use of 
the technique was primarily responsible for the strengthening of the 
edge vortices and that is all. 

During these tests, the coefficients which were used for the 
amounts of blowing were Cy = 0.02^0.12 and these values corresponded 
respectively to amounts of engine flow in the F-5E of 7%-33%- 
After going through the process, it was found that the blown flap 
technique actually reduced the residual power which is stipulated 
for use during evasive maneuvers. If one is speaking in terms of 
the sort of fighter aircraft which are currently in use, this reduc¬ 
tion of the amount of flow through the engine is unacceptable. Be¬ 
cause of this problem the blown flap technique can only be used in 
cases in which one is designing engines which are not too sensitive 
to thrust losses and have high bypass ratios. Only in the case of 
such engines will it be possible to use this technology in the design 
of the fighter aircraft of the future. 

The use of the blown flap technique under conditions in which 
the angle of attack is very large 

Figure 2. A planar view of a 1/10 
scale model of the F-5E (dimensions 
are all in inches) 

1—jet nozzle or mouth; 2 —inches 


Figure 3- The influence of the blown 
flap technique on the lateral charac¬ 
teristics of the aircraft involved 

1—sideslip; 2—horizontal stabilizer; 
3—jet angle; 4—the overall coeffi¬ 
cient of roll moment; 5—the overall 
coefficient of lateral force; 6—the 
overall coefficient of the moment of 

If one is considering measures which can be applied to today's 
advanced aircraft in order to improve their flight characteristics 
when they are operating with large angles of attack (measures such 
as large leading edges on the wings or the operation of the flaps 
in particular ways as has already been discussed earlier in this 
article), then one must accept the fact that the range of angles 
of attack with which these methods can be employed is limited (it 
is limited to within a range of approximately 30°). When the 
angles of attack are increased to values larger than this, counter¬ 
productive effects occur. Unlike these other methods of improving 
operational characteristics, the blown flap technique is certainly 
not limited to any range of angle of attack. This special charac¬ 
teristic of this technology presents the possibility of making the 
operational ranges within which aircraft can fly even larger than 
they are now. The aerodynamic mechanism which forms the founda¬ 
tion for the blown flap technique is similar to the theoretical 
foundations of leading edges on wings. It is not difficult to come 
up with the idea of combining this new technique and large leading 
edges on wings. The use of these techniques together would cer¬ 
tainly raise the operational characteristics of fighter aircraft to 
an even higher level. Below we will present a short introduction 
to the results of experiments into the possibility of using this 
type of arrangement. 

Figure 4 is a diagram of the model which was used in these 
experiments. The aspect ratio of the wings is 3.2; the sweepback 


Figure 4. A diagram of the model 
which combines the blown flap 
technique and the placement of 
large leading edges on the wings 

angle of the leading edges is 32 ° and the tip-to-base ratio is 
0.3; moreover, the wings have a twist. The sweepback angle of 
the leading edge extension at the front of the wing is 75°; its 
area is 115 of the area of the wing and there is a jet nozzle at 
the place where the aspect ratio is 105; the sweepback angle of 
the blowing flow is also 75'; the experiments were carried out in 
the eight meter wind tunnel at ONERA and the range of angles of 
attack which was used was 0°~90°, Cy = 0^0.2. 

The results of these experiments demonstrate that the blown 
flap technique employed with the very large angles of attack above 
30° has the following two effects: 

1. It increases the total lift of the aircraft; moreover, it 
results in relatively large improvements in the moment of lift on 
the nose of the aircraft as well as in the trim characteristics of 
the aircraft. Figure 5 gives measurement results for the lift char¬ 
acteristics for various coefficients of amounts of blowing. The 
broken lines in the figure are lines of continuity for the points 
of trim which correspond to various types of configurations. It can 
be seen that when the angles of attack are greater than 70°, all 
blowing cause relatively obvious increases in the amounts of lift. 
For this type of arrangement then, due to the fact that the posi¬ 
tion of the increase in lift is produced by the interference between 
the jet flow and the vortical flow is forward, the trim character¬ 
istics of the aircraft improved; see Figure 6. 

2. Let us consider the problem of improving the vibration 
characteristics of an aircraft. Based on results of tests which 
were run on certain fighter aircraft both in the air and in wind 
tunnels, discontinuous detached vortices on the wings can cause 
vibrations in the aircraft. If it is possible, from a certain 
appropriate location, to blow out a set of small but relatively 
high speed air flows in order to convert the detached vortices into 
one stable vortex and reduce the pressure pulsations of the air 
flow on the wing surface it becomes possible to delay the occurrence 
of vibration. Of course, if one is doing a precise determination 
of the vibration characteristics of a wing, it is still necessary 
to do a detailed analysis of the aerodynamic structures involved. 
However, from several analyses of aerodynamic qualities of objects, 
it is also possible to obtain some indirect but reliable informa¬ 
tion on vibration characteristics. 

Figure 5• 

1— various trims which 
correspond overall to a 
5% static stability; 

2— i trim is the yaw trim 
angle of the horizontal 

Figure 6. The influence of 
angle of attack and flap 
blowing on the equilibrium of 
the degree of yaw for the 
horizontal stabilizer. 


Figure 7 is the root-mean-square curve for the bending moment 
which is measured by placing strain gauges in the base section of 
the wing of the aircraft. From the wave-like movements of this 
curve, it is possible to derive the vibrations in the wing at 
various times. It can be seen from the results that the peak 
values of the curve which records the results for the case in which 
the blown flap technique is used are shifted toward even larger 
angles of attack; moreover, the order of magnitude of these peak 
values has also decreased. The significance of this simply is that 
the angles of attack which correspond to the dissipation of diver¬ 
gence of the vibration have been increased and the strength of the 
vibration has also been decreased. What is even more important 
than these aspects, however, is the very large increase in lift 
which corresponds to the occurrence of vibration (see Figure 5). 

C.-o,C\2«C.-oi. Key: (1) Vib. 

Ct<N> ** 1 . 08 , 

This is an increase of almost 40$. 

Experiments have also been carried out in which there was no 
use made of the leading edges on the wing and the jets were blown 
out from the main wing. The results from these tests were similar 
to the ones which we have already discussed. The only discrepancies 
were a certain variation in the magnitude of the results. 

Figure 7. The influence of 
flap blowing the vibration 


3• The employment of the blown flap technique at supersonic speeds 

The blown flap technique can be used to control various types 
of separation formations when aircraft are in low speed flight. 

As a result of this, the technique is also capable of raising the 


threshold of the onset of vibration. This point cannot easily be 
doubted. If this is the case at low speeds, is it possible to use 
this technique to improve the vibration characteristics of the 
same kinds of aircraft in subsonic flight? As a result of this 
improvement, can the coefficients of lift at high subsonic speeds 
be improved? Indeed yes. At present, there are already several 
aerodynamic designs which have given relatively good results. 

These designs include such features as wings whose leading edges 
have cracks or slits in them, secondary extensions forward of the 
edge of the main wing and next to the fuselage as we have already 
seen and the wing flaps. Wing twist is also included. However, 
all of these designs, to one degree or another, either add to the 
weight of the structures involved or increase their complexity. If 
we compare these designs to what will be described below, the sim¬ 
plicity of the structure involved in the use of the blown flap tech 
nique will appear even more outstanding. Below we will give a 
simple introduction of the results of wind tunnel experimentation 
of a transsonic application of the blown flap technique. 

Figure 8 is an illustration of the model which was used in 
these tests. The extensions on the leading edge of the wing can be 
folded up; the sweepback angle of the leading edge of the wing is 
40°, and the experiments were carried out in the ONERA S3MA inter¬ 
mittent type wind tunnel (experimental section 0.78x0.56 m). 

When a double jet flap nozzle for blowing which has the same over¬ 
all Cy value as a corresponding single nozzle version was used, 
the experiments proved that this type of complex jet nozzle is 
capable of causing the effective range of the Jet flow to be very 
greatly extended. The axis lines of these jet nozzles and the 
sweepback lines of the corresponding wings included angles of 10°. 
This is done because the jet flow has an enlarged range. 

The results of these tests showed that flap blowing in a trar.s 
sonic flow field has the ability to improve the lift and drag 

capabilities and the force moment characteristics of a wing (the 
results in these areas were similar to those which were obtained 
during the low speed flights). Also it is capable of improving 
the shock-induced separation phenomenon and lessening the vibra¬ 
tion of the wing. 

In a transsonic flow field when the surface of the wing gives 
rise to shock waves, due to the fact that there is a very strong 
negative pressure gradient in front and behind the shock wave, it 
is very easy to induce boundary layer separation behind the shock 
wave. Changes in the configuration of the boundary layer can also 
cause irregular movements of the position of the shock wave, both 
forward and backward. This type of unstable, pulsating separating 
flow develops to a certain stage and then it gives rise to a 
vibration in the structure of the wing. As far as this phenomenon 
is concerned, excepting the fact that it is possible to observe it 
directly by the use of a flow spectrum from a wind tunnel, it is 
also possible to get an analysis of the spectral characteristics of 
this type of phenomenon by means of sensors installed on the wing in 
order to transmit data on air flow pulsations. It is also possible 
to install on the base of the wing strain gauges through which one 
can measure the periodic oscillations of the bending moment of the 
wing. There is also an extremely useful engineering method. One 
finds the point at which the curve which represents the changes in 
the axial force as a function of changes in the angle of attack 
a bends. This point is taken as an indicator for the beginning of 
separation and "the onset of vibration" due to the shock wave see 
Figure 9* 

Figure 8. An illustration of 
the complex jet nozzle on the 

1— pressure pulsation sensor; 

2— complex jet nozzles 


267 cm 



When the angle of attack is small, the axial force corres¬ 
ponds to the drag. When a is increased, due to the fact that the 
contribution of the lifting forces to the axial force is increased, 
the overall axial force must gradually decrease in size; this pro¬ 
cess can even continue to the point where negative values can occur. 
After the appearance of separation on a wing as a result of the 
action of the shock wave, the increase in the lifting force slows 
down and the increase in the drag forces speeds up; as a result of 
this, the overall axial forces do not decrease again. When the 
point of boundary layer separation moves forward to shock wave, the 
separation layer suddenly bursts like bubbles the area of the 
separation is suddenly enlarged and the drag forces are suddenly 
increased. As a result of this, the axial forces suddenly increase 
in magnitude. In the wake of such a situation there are violent and 
unstable aerodynamic phenomena, like rapid motion of the shock wave 
and violent pulsations in the air flow. When such things occur, the 
wings involved will begin to vibrate obviously. 

Figure 10 shows experimental results for M = 0.9 and the curve 
covers the range C ^ a. From this figure it can be seen that the 
employment of the blown flap technique causes the angle of attack 
which corresponds to the point where the axial force curve bends to 
be very greatly pushed back. The value of the axial force in the 
are.a of the bend in the curve will also be much smaller. The sig¬ 
nificance of this is simply that the use of the blown flap technique 
has put off the occurrence of separation which is induced by the 
shock wave on the surface of the wing. At the same time, this raises 
the angle of attack and the lifting force values for the onset of 
vibration. The flow spectrum observations which correspond to these 
results are as follows. The blowing causes the area of separation 
caused by the shock wave to disappear. The reflection of this in 
the frequency spectrum analysis is to cause the mean-root-square 
value for the pulsations in air flow pressure levels to be reduced 
as are the peak values of the oscillating bending moment measured 
by the strain gauges in the base of the wing. Moreover, there is a 


shift toward even larger angles of attack. All of these perfor¬ 
mance characteristics were verified at the same time time during 
these tests. 

Figure 9. An illustration 
of basic wing vibration 
(M - 0.9) 

1—lift; 2—drag; 

3-r-oncoming flow; 4—axis 
line of the fuselage 

Figure 11 presents the influence of the flow coefficient on 
the vibration characteristics of wings equipped with the leading 
edges which we have already seen and the wings which are not 
equipped with this type of leading edge. These results are ob¬ 
tained for flow coefficients which apply when the blown flap tech¬ 
nique is applied and M = 0.9. 

Figure 10. Axial force 
characteristics of 
basic wings. 

The vertical coordinates are the coefficients of lift which 
correspond to the bending point of the axial force involved (the 
areas involved are the same). The curves show that the use of the 
blown flap technique is very effective when used to increase the 
usable lift produced by an aircraft in supersonic flight. At the 
same time, the fact that the two curves shown are nearly parallel 
explains why the blown flap technique has the capability of improv¬ 
ing the effectiveness of the leading edge of the wings themselves 
in supersonic flight. 


Figure 11. The influence of 
blowing on the coefficients 
of lift which correspond to 
the bending point in the 
axial force 

1—(illegible); 2—with 
additions on the front of 
the bases of the main wings; 

3— the main wing by itself; 

4— overall 

Finally, it needs to be pointed out that the blowing momentum 
coefficients used during these experiments were all very small 
and they were all completely within practical ranges. The improve¬ 
ment which was achieved in the vibration characteristics also means 
that there was a reduction in the drag losses which were caused by 
aerodynamic separation; this improvement was only capable of par¬ 
tially compensating for the engine thrust which was lost due to 
generation of the blowing stream. 


It can be seen from the results of the research discussed 
above that the blown flap technique has already reached the stage 
where it is not only a technology which can be applied to aerodyna¬ 
mic designs in order to improve individual characteristics of air¬ 
craft. For multi-purpose, advanced fighter aircraft of the future, 
which will travel at speeds three times the speed of sound, it is 
a technique which is capable of being applied, either by itself or 
in coordination with other aerodynamic measures, in order to improve 
many types of aircraft characteristics for different flight speeds 
and flight configurations. These sorts of improvements will also 
make it possible to reduce funds required for these designs by 
making use of these improved'characteristics. 

The special characteristics or advantages of the blown flap 
technique include: 

1. Its structure is simple and very well suited for appli¬ 
cations. When one makes use of tue blown flap technique, there 
will be no excessive Increase in the weight of the aircraft as a 
whole. It is possible, with relative ease, to install a blowing 
nozzle in any location on the aircraft where there is a need for 
blowing to be introduced and it is possible to use either single 
blowing nozzles or complex blowing nozzles. 

2. It is possible to exploit changes in the blowing coeffi¬ 
cients from various nozzles for longitudinal control and it is also 
possible to make use of asymmetrical blowing to exercise a lateral 

3. The influence of using the blown flap technique on other 
characteristics of the aircraft as a whole is very small. The 
blown flap technique is capable of being employed entirely on the 
basis of a need for such a capability; when no such need exists, it 
is only necessary to close up the nozzles. On the other hand, such 
techniques as the use of the leading edge additions to the base of 
the main wings are measures which are always present during flight. 
Because of this, in certain types of flight configurations, these 
sorts of permanent features on the airframe of the aircraft can have 

Finally, the key to whether or not the blown flap technique can 
be used is whether it "fits" on a high branching ratio engine.