1、Introduction to Aircraft Design,Chapter 5 Whats under the skin? Structure and propulsion,5.1 General,There are many systems in an a/c, each of them is important, but each may have conflicting requirements. A good designer must weigh up the conflicts, use relevant analyses and synthesize the a/c into

2、 an efficient whole. Outside of the a/c has to be bigger than the space required inside the a/c. It is often necessary to modify the external shape to accommodate the interior, with consequent aerodynamic changes.,2020/6/25,2,隱身技術(shù),5.2 The structure,Low weight. Acceptable material and manufacturing c

3、osts. Adequate strength to meet the max. expected loads, with a suitable safety factor. Adequate stiffness so that distortions are kept within acceptable limits. Good in-service properties such as fatigue and corrosion resistance together with tolerance of expected temperatures and other atmospheric

4、 conditions.,2020/6/25,3,隱身技術(shù),5.2.1 Materials,Light alloys: Aluminum alloy. High strength and stiffness-weight ratios, relatively low cost, easy of handling. max. temp. for continuous operation is 130C，equivalent to a Mach no. 2.2 due to kinetic heating. Aircraft which do not use light alloys as the

5、 prime structural material are rare.,2020/6/25,4,隱身技術(shù),5.2.1 Materials,Titanium: Higher strength-weight,stiffness-weight ratio, density. More cost and difficult to handle in the process of manufacture. Titanium alloys are used when heat resistant properties are required, such as firewalls, main frame

6、s and structure close to engine exhaust and gun barrels. May be used in close proximity to CFCs. (thermal expansion and corrosion),2020/6/25,5,隱身技術(shù),5.2.1 Materials,Steel: 1st metal a/c used steel rather than light alloys. Even higher strength-weight and stiffness-weight ratios, density is as 3 times

7、 as the light alloys. Highly stressed components, such as landing gear and engine pylons, make extensive use of steel. Cases of many earlier solid rocket motors were welded steel units. Ti cost is high.,2020/6/25,6,隱身技術(shù),5.2.1 Materials,Magnesium alloys: The main use of which is large castings and th

8、e very low density results is a weight saving. Low resistance to corrosion and difficult of making joints without welding due to crack propagation. The biggest drawback is unfortunate past experiences with corrosion of the material. Its cost is not very high.,2020/6/25,7,隱身技術(shù),5.2.1 Materials,Non-met

9、allic materials: reinforced plastics, rubber, sealants and cockpit transparencies. Resin-bonded glass fiber, poorer stiffness, temperature limitation due to resin and difficulty of attachment to metal parts. Carbon fiber composite. Carbon-fiber honeycomb wing skins and fuselage panels give good stif

10、fness and minimize the number of ribs and frames.,2020/6/25,8,隱身技術(shù),5.2.2 Load-carrying methods,A structure is a device for transferring mechanical loads from one point to another using the following mechanisms. See Fig. 5.2. Tension: the simplest way of carrying a load is to use a tension member. It

11、 is inherently stable. A failure implies severance of the structure, normally with catastrophic results unless alternative load paths are provided. Compression: all compression members must be designed with consideration of the question of instability.,2020/6/25,9,隱身技術(shù),5.2.2 Load-carrying methods,Sh

12、ear: a shear member reacts load can be considered as a combination of diagonal tension and compression. Bending: In reality a beam is not a single type of load carrying member but a simple structure. The load are carried by a combination of tension, compression and shear. The strength of a beam vari

13、es according to the shape of the cross-section. Torsion: a special form of shear. Torsion usually occurs in structure when the load path changes direction.,2020/6/25,10,隱身技術(shù),5.2.3 The main structure members,Flying surfaces: The flying surfaces as typified by the wing are essentially beams. In additi

14、on to acting as a beam to resist spanwise airloads, the wing must also carry considerable torsion loads and provide sufficient stiffness to prevent excessive twist. The torsion load arises primarily from flaps and control surfaces, and the torsional stiffness is necessary to prevent divergence, flut

15、ter and control reversal.,2020/6/25,11,隱身技術(shù),5.2.3.1 Flying surfaces,A reduction of thickness/chord ratio directly reduces the enclosed area, and thus the effect on twist is varied as the square of the depth. Fig. 5.3 shows the spanwise and chordwise loads. Reduced aspect ratio reduces both bending m

16、oment and twist and this is thus structurally advantageous. Increased taper has the effect of moving the airload towards the root, thus reducing the bending moment. Sweepback is thus a disadvantage in this point.,2020/6/25,12,隱身技術(shù),5.2.3.1 Flying surfaces,Recent advances in manoeuvre load control (ML

17、C) and gust load alleviation (GLA) actively modify the airload distribution. In GLA, the flight control system senses an up-coming symmetric gust and sends a signal to the ailerons. These rapidly deflect symmetrically to reduce wing-tip camber, then reduce wing-tip lift and the spanwise center of pr

18、essure moves inboard. Thus the wing shear force and bending moment are reduced.,2020/6/25,13,隱身技術(shù),5.2.3.1 Flying surfaces,The Airbus Industries A320 used GLA to reduce the fatigue damage to the wing, thus leading to reduced material and weight. The wing functions as a beam and a torsion box (with sk

19、ins). Fig. 5.4 shows different wing constructions and Fig. 5.5 shows how the wing components fit together. The other main components shown are ribs which have several functions.,2020/6/25,14,隱身技術(shù),5.2.3.1 Flying surfaces,Wing ribs support the skin to form the aerofoil shape, and stabilize the stringe

20、rs to stop them buckling. Flaps, ailerons and engines introduce concentrated loads into the wing, which must be carried by heavier ribs. Most aircraft use the area between the spars as a fuel tank. Special ribs are used to seal the tanks and some intermediate baffle ribs may be used to prevent fuel

21、sloshing.,2020/6/25,15,隱身技術(shù),5.2.3.2 Fuselage,The loads carried by the fuselage are basically similar to those imposed upon a wing. Bending can take place either in a vertical or horizontal plane, due to tailplane and fin loads respectively. Fin loads also impose a torque as do asymmetric tailplane l

22、oads. Undercarriage loads impose vertical bending shear and torque loads. Unlike the wing, fuselage normally has ample depth for bending and enclosed area for torque, and is thus a more nearly ideal type of structure.,2020/6/25,16,隱身技術(shù),5.2.3.2 Fuselage,Fuselage construction is usually one of two for

23、ms. A fully effective beam with stringers stabilizing the skins (桁條式), or a number of longitudinal, discrete members with shear carrying skins (桁梁式). These members are known as longerons. Cross-sectional frames give section shape, transmit local loads, and supply edge supports for the stringers and

24、longerons.,2020/6/25,17,隱身技術(shù),5.2.3.2 Fuselage,Pressure cabins are normally integral with the fuselage structure. Passenger and freight floors usually contribute to the resisting of bending loads, as well as transmitting local loads to the shell. Wing bending loads are sometimes transmitted round the

25、 fuselage frames, but where possible, taking the loads directly across the fuselage leads to a lighter structure.,2020/6/25,18,隱身技術(shù),5.3 Propulsion,All a/c need a propulsion system for sustained flight. A vast array of engines is on offer. An obvious distinction can be made between the air breathing

26、engines and rockets. The former have a relatively high installed engine weight but low fuel consumption, while the opposite is true of the latter. The former is used where long time operation is required and the latter where the operation can be completed in a short time or where the air is too thin

27、 to support combustion. See Fig. 5.8 and 5.9.,2020/6/25,19,隱身技術(shù),5.3.1 Air breathing engines,Piston engine, driving a propeller: used for small, light aircraft. Installed weight for a given power is relatively high. Cooling is the important consideration with piston engine. The vast majority of curre

28、nt engines are air-cooled, simple but with considerable drag penalty. Liquid-cooled engines were used extensively in WWII fighters and bombers, the radiator used to cool the liquid also produces drag.,2020/6/25,20,隱身技術(shù),5.3.1.2 Turbo-prop and prop-fan engines,The turbo-prop engine is the most suitabl

29、e powerplant for subsonic speeds up to about M 0.65. A turbine engine drives a propeller through a reduction gearbox. It has lighter weight than a piston engine despite the gearbox. Low SFC, smooth running and secondary advantages such as source of compressed air. Fig. 5.11 shows a turbo-prop engine

30、 installation from the Cranfield F-93B project.,2020/6/25,21,隱身技術(shù),5.3.1.2 Turbo-prop and prop-fan engines,The early 1980s saw a sharp rise in fuel prices and this lead to a new variety of turbo-prop engine. Variously called prop-fan, unducted fan, or open rotor, such engines have a conventional turb

31、ine core driving advanced propellers. They use supercritical section blades with swept back tips. Most examples have 2 rows of contra-rotating blades with 5 or 6 blades per row.,2020/6/25,22,隱身技術(shù),5.3.1.2 Turbo-prop and prop-fan engines,They have been designed to cruise at high efficiency at M 0.8 an

32、d have a claimed fuel consumption reduction of some 30% relative to current turbo fans. Blade-tip noise, vibration and possible blade shedding usually leads to the choice of aft fuselage engine mounting as on the A-85 project with Rolls Royce RB-509 project engines as shown in Fig. 5.12.,2020/6/25,2

33、3,隱身技術(shù),5.3.1.3 Turbo-jet engines,Above about M 0.65, blade compressibility effects cause a reduction in the efficiency of conventional propeller engines. Turbo-jet engine is most satisfactory for use in the M 0.75 to M 3.0 speed range at altitudes up to about 18 km. Fuel consumption and noise are hi

34、gh on the turbo-jet relative to the turbo-fan and thus few commercial a/c currently use them.,2020/6/25,24,隱身技術(shù),5.3.1.4 Turbo-fan engines,Fig. 5.13 shows a high bypass ratio engine which bypasses some of the compressed air round the hot section of the engine. These engines use a large fan to provide

35、 much of the thrust and in some case this implies a 3-shaft engine. Bypass has the advantage of reducing both fuel consumption and noise.,2020/6/25,25,隱身技術(shù),5.3.1.5 Reheat Turbo-jet engines,Reheat is also called after-burning. The addition of a second combustion stage to the turbo-jet engine, by inje

36、ction of fuel into the exhaust, extends the useful speed regime up to M 3.0. Fuel consumption is dramatically increased, but a smaller core engine may be used for the whole flight than if a large engine were designed to cater for a short term high thrust requirement. Burning extra fuel in the cold a

37、irstream of a bypass engine appears to be a profitable way of increasing thrust.,2020/6/25,26,隱身技術(shù),5.3.1.6 Ramjet,Ramjet is a simple, light-weight engine which basically dispenses with all moving parts. Since its operation relies upon the compression effect of forward speed and expansion through a n

38、ozzle, the engine is not suitable for flight below about M 2.0. Upper limit is around M 7.0. New space-launcher concepts have led to the project design of turbo ramjets. The lower stage of the 2-stage SL-86 would use turbo ramjet fuelled by liquid hydrogen, the upper stage would be powered by a liqu

39、id rocket.,2020/6/25,27,隱身技術(shù),5.3.2 Rockets,Rockets are more efficient that air breathing engines at speeds above about M 3.0. Although propellant consumption is inevitable high, due to the need to carry oxidant as well as fuel, in this lies one of their great advantages of independence of altitude.

40、Chemical rockets are normally classified into two types: solid fuel rockets and liquid fuel rockets.,2020/6/25,28,隱身技術(shù),5.3.3 Location of Powerplant,The powerplant must be located to ensure that there is adequate ground clearance during taxiing, take off and landing. In many cases there are also rest

41、rictions imposed to ensure that there is adequate airframe clearance. Fig. 5.15 shows a number of single and twin-engined propeller installation. Early jet transport had engines buried in the wing and most current military aircraft have engines mounted in the fuselage. These installations have reduc

42、ed drag, but complex and inflexible.,2020/6/25,29,隱身技術(shù),5.3.3.2 Overall propulsion efficiency,The overall propulsion efficiency is determined by the individual efficiencies of the air intake and exhaust nozzle, or propeller design. Intake efficiency is critically dependent upon Mach number. Up to M 1

43、.6 a simple pitot intake is adequate and it should be as short as is feasible. At higher Mach numbers more complex arrangements are necessary to deal with the shock waves, and variable geometry is desirable to maintain high efficiency over the whole flight speed range.,2020/6/25,30,隱身技術(shù),5.3.3.2 Over

44、all propulsion efficiency,The location of the intake has an important effect upon its efficiency. Nose intakes of minimum necessary length are best. Intakes mounted alongside a structure give rise to boundary layer and shock problems although the latter may be favorable in certain circumstances. Intakes located below a wing or fuselage usually have a h