航空專業(yè)英語：專英1教案第4章

1、Ch. 4 Flight Performance transport aircraft performance： take-off performance 起飛性能 climb performance 爬升性能 cruise performance 巡航性能 descending performance 下降性能 landing performance 著陸性能 turning performance 盤旋性能 engine-out performance 發(fā)動(dòng)機(jī)失效后性能 stalling performance 失速性能4.1 take-off performance Consider a

2、n airplane standing motionless at the end of a runway. The pilot releases the brakes and pushes the throttle to maximum take-off power，and the airplane accelerates down the runway. At some distance from its starting point，the airplane lifts into the air.4.1.1 Forces acting on an aircraft in take-off

3、 The forces acting on an aircraft in take-off are weight W、lift L、thrust T、drag D and a rolling resistance R caused by friction between the tires and the ground. This resistance force is given by the coefficient of rolling friction and the net normal force（W-L）exerted between the tires and the groun

4、d.4.1.2 take-off distance The sum of Sg and Sa is the total take-off distance for the airplane. The Sg is called the ground roll（or sometimes the ground run）which is covered along the runway before the airplane lifts into the air. The Sa is the take-off distance in airborne over the obstacle which i

5、s generally defined to be 50ft for military aircraft and 35ft for commercial aircraft. The ground roll Sg is further divided into intermediate segments. These segments are defined by various velocities，as follows：1,Vstall stalling speed 失速速度2,Vmcg minimum control speed on the ground 地面最小可操縱速度 This i

6、s the minimum speed at which enough aerodynamic force can be generated on the vertical fin with rudder deflection while the airplane is still rolling along the ground to produce a yawing moment sufficient to counteract that produced when there is an engine failure for a multiengine aircraft.3,Vmca m

7、inimum control speed in the air空中最小可操縱速度 If the airplane were in the air（without the landing gear in contact with the ground）, the minimum speed required for yaw control in case of engine failure is slightly greater than Vmcg. For the ground roll shown in Fig.，Vmca is essentia1ly a reference speed-t

8、he airplane is still on the ground when this speed is reached. 4,V1 decision speed 決斷速度 This is the speed at which the pilot can successfully continue the take-off even though an engine failure（in a multiengine aircraft）would occur at that point. This speed must be equal to or larger than Vmcg in or

9、der to maintain control of the airplane. If an engine fails before V1 is achieved，the take-off must be stopped. If an engine fails after V1 is reached，the take-off can be achieved.5,VR take-off rotational speed 起飛抬前輪速度 At this velocity, the pilot initiates by elevator deflection a rotation of the ai

10、rplane in order to increase the angle of attack, hence to increase CL. Clearly, the maximum angle of attack achieved during rotation should not exceed the stalling angle of attack. Actually, all this is needed is an angle of attack high enough to produce a lift at the given velocity larger than the

11、weight, so that the airplane will lift off the ground. However, even this angle of attack may not be achievable because the tail may drag by the ground.（Ground clearance for the tail after rotation is an important design feature for the airplane, imposed by take-off considerations.）6, Vmu minimum un

12、stick speed 最小松桿速度If the rotation of the airplane is limited by ground clearance for the tail, the airplane must continue to accelerate while rolling along the ground after rotation is achieved，until a higher speed is reached where indeed the lift becomes lager than weight. This speed is called the

13、minimum unstick speed.7,VLO liftoff speed 起飛速度，離地速度 For increased safety, the angle of attack after rotation is slightly less than the maximum allowable by tail clearance, and the airplane continues to accelerate to a slightly higher velocity, called the liftoff speed, denoted by VLO. This is point

14、at which the airplane actually lifts off the ground. The total distance covered along the ground to this point is the ground roll Sg. 4.1.3 Factors affecting take-off performance The design parameters have an important effect on take-off ground roll. Sg depends on wing load W/S, thrust-to-weight T/W

15、 and maximum lift coefficient（CL）max, the ambient density wind and ground conditions. Sg increases with an increase in W/S.Sg decreases with an increase in（CL）max.Sg decreases with an increase in T/W.Sg increases with a decrease in .Sg increases with a tailwind，and decreases with a headwind.4.2 Clim

16、b performance zooming 急躍升 power required 需用功率 thrust required 需用推力 power available 可用功率 thrust available 可用推力excess power 富裕功率 excess thrust 富裕推力A climb is an ascending flight path at constant speed.It is usually a straight path but, sometimes, a climbing turn is necessary. a zoom, which exchanges s

17、peed for height.An aircraft flying at high speed /low altitude can rapidly convert the kinetic energy to potential energy by raising the nose and “zooming”.In a normal climb to cruise altitude, such techniques are inefficient. Fuel burnt in the engine is converted into increasing potential or kineti

18、c energy. A zoom is an increase in altitude that exchanging the kinetic energy of motion for potential energy. i.e. by converting excess velocity V to increased altitude. A zoom is only a transient process.a climb where the aircraft is driven upward by excess thrust from the engine and sustains its

19、speed throughout. ceiling 升限 absolute ceiling 絕對(duì)升限，理論升限 service ceiling 實(shí)際升限，使用升限A steady climb is achieved by converting propulsive energy（thrust）to give altitude. A climb can be maintained until the reduction in thrust, due to increasing altitude, eventually reduces the rate of climb to zero. This

20、 is known as the absolute ceiling of the aircraft. The service ceiling is the altitude where the maximum rate of climb is reduced to 100 feet per minute. types of climb There are three（or four）types of climb：maximum angle climbmaximum rate climbnormal climbcruise climb4.2.1 Force acting on an aircra

21、ft in climbIf you maintain a steady climb at a constant indicated airspeed, the engine-propeller must supply sufficient thrust to overcome：the drag at the climb airspeed; and the component of the weight that acts negatively along the climb path. In a climb there is no acceleration. The forces are in

22、 equilibrium and consequently the resultant force is zero. In a climb thrust is greater than total drag; total lift is slightly less than weight. T= D + Wsin, L = Wcos As the climb becomes steeper, more and more thrust is required and less and less lift. In a vertical climb, which some fighters can

23、achieve, the thrust balances the drag and all of the weight, there is no lift at all. We normally climb at 50 or less, so the difference between lift and weight is small. （cos 50=0.996）4.2.2 Maximum rate climb Vertical speed is called rate of climb. Rate of climb is shown in the cockpit on the verti

24、cal speed indicator（VSI）. Power =work done/ per second=force distance/ per second= force velocity=thrust TAS V sin =Vy=V（TD）W=（Pa-Pr）/W=富裕功率/WPa power available（PaTV）, Pr power required （PrDV） Pa-Pr excess power If W=const.，（Vy）max=（Pa-Pr）max /WClimbing for maximum rate is achieved at a speed where

25、there is maximum excess power. The greater the excess power，the lower the weight, the lower the drag，and the greater rate of climb.4.2.3 Maximum angle climb sin = （T-D）/W T-D：excess thrust 富裕推力 The angle of climb depends directly on the amount of excess thrust and the weight.the lower the weight, th

26、e greater the angle of climb.the greater the thrust, the greater the angle of climb.the lower the drag, the greater the angle of climb.For maximum climb gradient, the aeroplane should generally be kept in a low-drag configuration. The speed for the maximum rate of climb（Vy）usually occurs at a speed

27、somewhere near that for the best lift/drag ratio, and is therefore faster than the speed for the maximum angle of climb. dh = Vy dt = V sin dt dL = V cos dt4.2.4 Factors affecting climb performance Performance in the climb, either angle or rate of climb, will reduce due to reduced thrust and power;i

28、ncreased weight or drag;low air density due to high temperature or higher altitude;inaccurate airspeed（either too fast or too slow）. 1，temperature High ambient temperatures decrease climb performance. If the temperature is high, then the air density（）is less. The engine-propeller and the airframe wi

29、ll both be less efficient, so the performance capability of the aeroplane is less on a hot day.2，altitude Increasing altitude decreases climb performance. Power available from the engine/propeller decreases with altitude. The climb performance, both the rate and the angle of climb, will decrease at

30、altitude.（mixture leaned 貧油）3，airspeed1）flying too fast If you fly faster than the recommended speed, there is less excess thrust and less excess power. 2）flying too slow If you fly slower than the recommended speed, the excess power is reduced and, therefore, the rate of climb is directly reduced.4

31、，the effect of wind on climb performanceWhen air moves, the aircraft will travel a lesser or greater horizontal distance（headwind and tailwind respectively）in that time.The angle of climb relative to the terrain, is steeper in a headwind and is reduced by a tailwind.4.3 Cruise performance4.3.1 Force

32、s acting on an aircraft in cruise lift produced by the main plane（wing） CP weight of the total aircraft CG drag which acts rearward opposite to the flight path thrust produced by the engines In steady（unaccelerated）, straight and level flight the aircraft is traveling in a straight line at a constan

33、t altitude and at a constant airspeed，total lift equals total weight, and thrust equals total drag. The aircraft is in equilibrium. thrust= drag, lift= weight. This means that all forces and moments are in balance there is no resultant force to accelerate or decelerate it- nor resultant moment to ch

34、ange its attitude in pitch, roll, or yaw.1, pitching moments of the four forces couple 力偶（thrust/ drag, lift/ weight）Where the CP is behind the CG, the lift- weight pair produces a nose-down pitching moment.The thrust- drag pair produces a nose-up or nose-down pitching moment. The size of this momen

35、t is significantly less than the lift- weight moment because of a small moment arm. nose-down pitching moment 低頭俯仰力矩（-）nose-up pitching moment 抬頭俯仰力矩（+）2, balancing moment-the tailplanehow balancing the four forces to achieve pitching equilibrium The conventional aircraft is fitted with a tailplane

36、that provides a means of balancing the results of the four main forces.The large distance of the tailplane from the centre of gravity（large tail moment arm）allows the use of relatively small tailplane forces to achieve the necessary restoring moments. The tailplane may be fixed or varied.Most large

37、aircrafts have a variable-incidence tailplane with an attached elevator which provides a much wider range of trim capability and greater control power. Variable-incidence tailplane are commonly found on aircrafts that have a large CG range, a large speed range and varying pitch moments. variable-inc

38、idence 變安裝角, trim 配平3, variation in the balance of forces and moments1）variation of speed in the cruise A reduction in cruise speed will require an increase in angle of attack to maintain lift equal to weight, which will result in a forward movement of the CP. A forward movement of CP position for a

39、 constant lift will reduce the magnitude of the nose-down pitching moment. Since the drag that results from a lower speed will also change, then the total balance of forces will change, requiring a new balancing moment from the tailplane. The result of changing speed in the cruise is to vary the mag

40、nitude of the tailplane force.2）variation of weight in the cruise In a normal flight, the weight gradually reduces as fuel is burned-off. If the aeroplane is to fly level, the lift produced must gradually decrease as the weight decreases. If there is a sudden decrease in weight，say，by half a dozen p

41、arachutists leaping out, then to maintain straight and level flight, the lift must reduce by a corresponding amount. The CL（angle of attack）or the airspeed must be reduced so that less lift is generated. Suppose that the aeroplane is cruising at a particular angle of attack, say, for the best L/D ra

42、tio. To maintain this most efficient angle of attack as the weight reduces, the airspeed must be reduced to reduce the lift so that is still balance the weight. So, if the height and the angle of attack are to be keep constant, then the airspeed will have to be reduced. The power（thrust）will be adju

43、st to balance the reduced drag. a dozen 一打,12亇 parachutist 降落傘 leaping out 放出 If you wish to keep the power constant and you want to maintain height as the weight decreases, the lift must be decreased by reducing the angle of attack（decreasing the CL）. Therefore the speed will increase. If you want

44、to keep the speed constant and maintain height, then as the weight reduces you must reduce the lift produced, and you do this decreasing the angle of attack. H=const. WL W=L=（1/2）V2SCL const. =const.CL=const.V（DTD=T） T=const.（CL）V（D=T）V=const. （CL） Sometimes the weight increases in flight by, for in

45、stance, the formation of ice on the structure. An increased weight will mean that increased lift is required to maintain level flight- and once again the above discussion applies, but in reverse. Ice on the wings, especially on the upper surface near the leading edge will cause a drastic（急劇的）decreas

46、e in the efficiency（CL）of the wing and a significant increase in drag.To maintain the recommended angle of attack for the required cruise mode, a lower speed must be flown for the lower weight-alternatively, the aircraft may be allowed to slowly climb（cruise climb）.3）variation of centre of gravity p

47、ositionDifferent CG position will result in variation to the balance of moments about the CG. A change in CG position results a different tailplane force.4）attitude in level flight To obtain the required lift at low speed, a high angle of attack（high CL）is required, while at high speed, only a small

48、 angle of attack（low CL）is needed.4.3.2 Performance in level flight The thrust required for steady, straight and level flight is equal to the drag（D=T）,and so the thrust- required curve is identical to the drag curve. High thrust is required at high speeds to overcome mainly parasite drag（low angle

49、of attack, low induced drag）.Minimum thrust is required at minimum drag speed（which is also the best L/D ratio speed, since L is constant in straight and level flight and D is at its minimum）.Increased thrust is required at low speeds（high angle of attack）to overcome mainly induced drag. Power= thru

50、st true airspeed We can develop a power required curve from thrust required curve by multiplying the thrust required, at any point on the curve, by TAS any that point and then plotting the result against the same speed scale. This will give us the power required to maintain level flight at that spee

51、d.1，what does the graph show ？ maximum level flight speedMaximum level flight speed occurs when the power available from the engine-propeller matches the power required to produce enough thrust to balance the drag at the high speed.minimum level flight speedThe minimum level flight speed is usually

52、not determined by the power capability of the powerplant, but the aerodynamic capability of the aeroplane.In steady straight and level flight, the lift must balance the weight.W = L = 1/2V2SCLAs altitude is increased, air density（）decreases. One way to generate the required lift and compensate for t

53、he decreased density （）is for the pilot to increase the true airspeed V so that the value of 1/2V2 remains the same as before.2, speed stability and the drag curve1) higher speeds Above minimum drag speed, any minor speed fluctuation is self correcting. This is called speed stability. In the regime,

54、 an increase in airspeed increases the total drag, due to an increase in parasite drag causing the aircraft to slow down. A decrease in airspeed decreases the total drag, due mainly to an decrease in parasite drag causing the aeroplane to accelerate back to its original speed. In the normal airspeed

55、 regime, above the minimum drag speed, the pilot does not have to be active with the throttle since the aeroplane is speed stable and, following any disturbance, will tend to return to its original trimmed airspeed without pilot action.2) lower speedsAt lower speeds（below minimum drag speed）it is a

56、different matter. If a gust causes airspeed to decrease, the total drag increases. Total drag now exceeds thrust, causing the aeroplane to slow down further.If a gust causes airspeed to increase, the total drag decreases. Drag is now less than thrust, causing the aeroplane to accelerate further away

57、 from the original speed. This is called instability. 4.3.3 Maximum range cruiseMaximum range is the greatest distance that can be achieved from the fuel available. It shows how far the aircraft can travel irrespective of the time taken. irrespective 與無關(guān) 比航程 specific air range （SAR）Specific air rang

58、e（SAR）is a ratio of distance to fuel consumption.SAR = distance/ fuel used=（distance/time）/（fuel used/time）= TAS/ fuel flow耗油率 specific fuel consumption（SFC）Specific fuel consumption（SFC）is a ratio of fuel flow to thrust.SFC= fuel flow/ thrust fuel flow=SFC thrust SAR= TAS/ (thrust SFC）（SAR）max= （TA

59、S/drag）max/（SFC）min SAR of an aircraft is dependent on two separate factors, an airframe consideration and a separate engine consideration.1, airframe consideration（max. TAS/ drag） TAS/ drag=（TAS/IAS）（IAS/drag） For maximum value of TAS/ drag, we need a maximum value of TAS/ IAS, together with the ma

60、ximum value of IAS/drag. The maximum IAS/ drag ratio is achieved, when a line from the origin in drawn at a tangent to the drag IAS curve，this IAS being 1.32 times the IAS for minimum drag（VIMD）. This speed is valid at all altitude.（for the jet aircraft）Power required=thrust required TASPower requir