Thiết kế kỹ thuật máy bay chiến đấu Episode 1 Part 6 doc

A small slat, called the a/u/a bastard wing lying like a thumb along the leading edge of the wing at the structural wrist, has opened to deflect high pressure air from beneath the leadi

versions with the same aerofoil sections Fig 3.10 shows the differences in lift and drag

coefficients that occur with Reynolds number with the same aerofoil section Thus, a

narrow chord can cause a ‘peaky’ lift curve, with a sudden hard stall; but increase the

chord and the stall characteristics are softened Furthermore, at a given angle of attack

a broader chord can generate a higher lift/drag, bringing with it better cruise

same aerofoil surface

The disadvantages of narrow chords can be alleviated somewhat by the introduction

of controlled turbulence, and vortices Fig 3.11a and c show vortex generators at a flap

knuckle, to delay separation, anda vibrating rubber band in front of the leading edge

of the wing of a model sailplane, to generate turbulence, so improving the flow

Roughened surfaces are used in Nature to improve flying qualities The barn owl, for

example, has feathers with vortex-generating spiked vanes forming a serrated leading

edge The wing trailing edges are soft downy surfaces The combination enables the

owl to fly silently by delaying transition noise in the boundary layer, a thing quite

impossible when the wings are lacquered The noise is caused by fluctuating changes of

pressure as the flow oscillates between laminar and turbulent at the point of transition

on the wing chord

THE DESIGN OF THE AEROPLANE

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Bats and many insects have thin membrane wings that are not efficient devices in

themselves But such wings contain networks of fine blood vessels, or are covered with

tiny hairs, which appear to cause favourable roughening at the Reynolds numbers

involved, enabling the creatures to fly better and more efficiently than would otherwise

be the case The bee, which has membrane wings, has to vibrate them before the first

flight of the day to warm them up, increasing blood flow The bumble bee, in spite

of a high and seemingly inefficient wing loading, nevertheless carries nearly half of its

weight as payload through such natural boundary layer control

Table 3-1 compares the lift coefficients of a number of wings (based upon ref 3.3)

with those of aeroplanes Birds flapping their wings generate lift more efficiently than

many aeroplanes employing advanced high-lift devices

TABLE 3-1

Maximum Lift Coefficients of Animal and Aeroplane Wings

(based upon ref 3.3)

The Frontispiece shows clearly a number of aerodynamic features discussed in this

chapter The gannet was photographed while flying slowly A reversal of flow by the

trailing edge vortex has lifted the downy feathers near the trailing edge of its wings to

delay further separation The process is automatic The body of the bird is well

cambered with tail and feet slowing the air underneath, increasing loca] pressure and

lift, while also trimming out the nose-up pitching caused by the wings being swept

forwards, which brings the lift ahead of the centre of gravity (a change of geometry

which delays tip-stall by causing the boundary layer to drift towards the root, which is

thus caused to stall first) A small slat, called the a/u/a (bastard wing) lying like a thumb

along the leading edge of the wing at the structural wrist, has opened to deflect high

pressure air from beneath the leading edge into the flow over the upper surface, again

delaying separation

Just as down ruffles the airflow over the wing of the bird, causing favourable

turbulence, so too are vortex generators attached to wings and other surfaces to draw

in free stream air and wash away stagnating air carried along with the aircraft A vortex

generator is a small plate aerofoil surface set at an angle of attack to the local flow, with

its tip far enough away from the skin to protrude through the boundary layer An

intense tip vortex is established with suction in its core, in an area of adversely rising

pressure Each suction source opposes the stagnation of the flow, marked by rising

static pressure, and acts like a broom which lifts air clinging to the skin to deposit it

away in the free stream

Vortex generators are used in areas of messy flows: bad junctions, flap and control

surface knuckles, and in areas just forward of quite sharp curves which the air would be

unable to follow, especially at large angles of attack Examples are wing leading edges,

humped canopy fairings, and some tail and rear fuselage surfaces

98 THE DESIGN OF THE AEROPLANE

—— s›t>¬:““t= m=m=.e=.=.=nsiiriiililElElaEaEEaSESSsSlaSSAa0A0SaSaaaIBH

Plate 3-1 Streamlined flow breaking down into eddies and

vortices: (Top) at zero angle of attack (note the stagnation

streamline at the leading edge): (Bottom) at the stall (note the'way

in which the stagnation streamline has migrated to a point below

and behind the leading edge) (National Maritime Institute, UK)

Related by function if not form are dorsal fillets, fuselage, tailplane and other

strakes Their quite highly swept leading edges, which must be relatively sharp-edged,

generate favourable vortices which increase tail and fuselage damping, or which re-

attach separating flows, at large angles of pitch and yaw (as when sideslipping and

spinning)

Slats, slots and flaps

A slat is a small, cambered, aerofoil surface near the leading edge of a bigger one,

arranged so as to form a slot Air flows through the slot from a region of higher

pressure to one of lower, infusing kinetic energy and delaying separation The slat

provides a high local C, because of favourable suction from the main surface behind it

which keeps the flow over it attached However, suction and lift of the main surface are

reduced somewhat by the presence of the slat at the leading edge The Handley Page

slot is one of the best known The lift curve is extended to larger angles of attack, while

the increment gain in lift depends upon slat chord/ wing chord (one of 30 per cent more

or less doubles Cumax)

All flaps are camber-changing devices of one form or another, although some also

increase wing area Camber change is harder to visualise with a split flap, but it serves

the purpose by pushing air downwards, increasing the pressure underneath,

intensifying the circulatory component of flow around the wing leading edge The nose

flap behaves very much like a slat, by introducing a strong camber near the leading

edge The leading edge suction peak is reduced at large angles of attack, boundary

layer is thinned and stall angle of attack is increased

Trailing edge flaps are the most obvious camber-increasing devices The increased

slope of the top surface, towards the trailing edge (seen most clearly with the geometry of

the plain flap in table 3-2) increases the potential swept volume to be filled by the

surrounding air Large lift coefficients are achieved, but separation is hastened The

flap increases the angle of attack of the basic section (an approximate truth which can

be seen by drawing a straight line between the leading edge of the wing and the trailing

edge of the flap) Many and varied flap forms are to be found, some employing slats

and slots, so as to delay separation

Normal flying control surfaces are plain flaps (see chapter 12)

Plate 3-2 Wind-tunnel model of author’s Warren-winged S37-3, which needed roughened leading edges to cause representative attached flow at test Reynolds numbers It was impossible to induce a conventional stall and g-break with this wing, based on the work of Norman Hall-Warren (Shell Aviation News, UK)

Aerofoil section families

There have been many aerofoil sections designed in many countries Some of the more

important have been grouped in rational families and series (usually having common

thickness distributions, but with variations of their mean lines and relative

———

100 THE DESIGN OF THE AEROPLANE

TABLE 3-2 Flap and slat characteristics

Basic aecsfoil Ctark Y C———_ 1.29] 15 7.5 |-o.o85|TN4S4

|

Naca

aA SloE

deflected 45°

NACA

THE NATURE OF AIR 101

a Basic Seckion Stalled

b Section with leading edge droop (orc flap) ak Same qugle of attack AS A,

ant C Section With leading

edge slak and trailing

Fig 3.12 The effect of leading edge slat, flap and trailing edge flap upon lift and

angle of attack of basic wing section

Administration)) in the United States NACA sections are perhaps the most

universally known and well used

Table 3-3 shows the salient characteristics of a number of useful sections, including

the relatively new GA(W)-1 and GA(W)-2 profiles developed by NASA These show

very good lift/drag, but they tend to suffer from powerful pitching moments which

tend to offset some other qualities It is necessary to balance requirements for deep,

light structures and good locations for spars; with good lift /drag; with ample lift at low

speed and good stall characteristics; and with the size and weight of tail needed to cope

with the pitching moments

Air displaced faster than sound (supersonic)

At low airspeeds, around 100 knots at sea level (51.4 m/s), the dynamic pressure is only about 34 Ibf/ft? (1630 N/m?) compared with the sea level static pressure of 2116 lbf/ft2 (1.0132 X 10° N/ m2), ISA But at speeds faster than M 0.3 or so, displacement of the air

is accompanied by quite dramatic changes Molecules are squeezed together Their pressure and density increase Calculations of dynamic pressure by means of eqs (2-9) and (3-4) cannot be carried out by simply inserting whatever value is found from tables for air density, p The change in density with compressibility causes us to add a percentage increment:

percentage increase in dynamic pressure, gq = 25 per cent M? + 2.5 per cent M4

which is shown in fig 3.13 Thus, at 66 knots the incompressible dynamic pressure is about the same as the compressible value, 10 Ibf/ft? (480 N/m2) But at 400 knots, the incompressible value would be 542 Ibf/ ft? (26 016 N/ m2), and the actual compressible value nearly 10 per cent more, at 596 Ibf/ ft? (28 608 N/ m2) A change of this magnitude causes marked changes in lift, drag and pitching moment

Very few light aeroplanes have been troubled by compressibility, because of inadequate engines But modern materials which enable designers to achieve advanced, high quality, low drag profiles; automotive technology which provides cheap flap, gear and other systems; and the appearance of several small jet engines, may lead té a potent new breed of aircraft within reach of the amateur builder and private owner

It is far easier to use lift, drag and pitching moment coefficients to describe the changes that occur with a wide range of operating speeds, altitudes and Reynolds numbers By doing so units largely disappear and arithmetic becomes easier We may add pressure coefficient, Cp, to this vocabulary, which enables us to describe the way in which pressures behave, in non-dimensional terms, with variations in airspeed and angle of attack

Pressure coefficient is defined as:

C» = pressure difference/dynamic pressure = (p — p ,,)/q oo

= (g~ đ,)/4 œ — (G/¢,,)—1 (in terms of dynamic pressures) (3-17a) and, knowing too that qg varies as V2, eq (2-9), then at speeds too low for compressibility:

» = (V/V,,)? — 1 (in terms of airspeeds) (3-17b) Now, look back to fig 3.6b (or fig 3.12) which shows suction and compression lobes around a wing The same picture can be drawn more usefully by representing the chord as a straight line, and then plotting suction upwards and compression downwards, in terms of C,, as shown in fig 3.14 Eq (3-17b) demonstrates that peak suction marks maximum local airspeed; peak compression, minimum local airspeed

If the local speed of flow reaches the speed of sound, the molecules of air are unable

to adjust in time to changes in pressure brought about by the surface contours Their behaviour is affected critically We speak, therefore, of subcritical and supercritical flows and their associated aerodynamics

Supercritical flows begin to occur when an aeroplane is flying at a speed around M

M2 M4

(derived from ref 3.5)

0.6 Although the aeroplane is flying subsonically the airflow has then reached sonic speed somewhere on the airframe (where C, is most negative) As the aircraft approaches the speed of sound progressively large regions of local flow achieve supersonic speed Other regions in which airflow is gathered up and carried along with the aircraft to a certain extent (making C, positive) are subcritical, and these may still

be found when the aeroplane has reached M 1.2 or even 1.4, depending upon the magnitude of the positive C, peak

We say, therefore, that the aeroplane is flying transonically as long as there is a jumble of mixed subcritical and supercritical flows somewhere on its skin

When air is forced to supersonic speeds it objects, and tries to readjust to subsonic motion as quickly as possible If able, it does so with a sharp deceleration, characterised by a shock wave which, under normal flight conditions, shows as a violent rise in static pressure taking place in a distance of a few thousandths of an inch

This is accompanied by an equally sharp rise in temperature Shock waves can be heard

on the skin of an aircraft by microphones inside the wing Many readers will have heard the sonic bang of a supersonic aeroplane, when standing at a point on the ground where coalescing shock waves pass in the form of a strong pressure front

As long as the air is prevented from decelerating after reaching the speed of sound locally on the airframe, Bernoulli’s law holds and it can be caused to accelerate to even higher supersonic speeds through a series of expansion waves (expansion, because molecules are forced to move apart and their static pressure and density fall) An expansion wave is the opposite of a shock wave

at various Stations along

the pressure Aistributrons are Uniquue Eo one augle

of aktack , And Indeprtndent

of Erue airspeed Ambient Static pressure ana density

Stag nat tow Pernts occur where Cp =O

of less Erianguiac, whieh Suggests that Lhe centre

of pressure Ciks cenlroid)

lies about Ya chord aft

of the leading edge

Fig 3.14 Conversion of pressure diagrams into lift distribution across chord (needed

for structural design and stressing) These diagrams apply to an essentially slow-flying

aeroplane with subcritical flow everywhere

To see how it all works, imagine a streamlined solid of revolution like that shown in

fig 3.15, moving through the air like a javelin At very low speeds the sharp point

merely nudges molecules gently, so that they are able to transmit their warning pulses

at the local speed of sound in all directions Pressure waves radiate outwards in

spherical ripples

106 THE DESIGN OF THE AEROPLANE

iks OWN pressuce Waves

Zone of

bo < P

Fig 3.15a Generation of a normal shockwave at high speed after von Karman (ref 2.1)

But as the solid body moves faster and faster, it gradually catches up with the pulses, crowding them together ahead of itself When the body travels at the speed of sound (M 1.0) warning of its advance cannot be transmitted ahead, because it is travelling with its waves, which coalesce to form a normal shock wave at right angles to the line of flight Ahead of the normal shock is an undisturbed region that von Karman calls the

‘zone of silence’ (ref 2.1) Behind it is his ‘zone of action’ In front of the shock are the ambient conditions of the free stream Behind the shock there has been a rise in pressure and temperature (kinetic heating), caused adiabatically by displacement, such that:

temperature rise AT = (true airspeed, mph/ 100)?°C (3-18)

to within three parts in 1000 Alternatively, the temperature rise of the boundary layer

is given by:

under conditions found at high altitude