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Chapter 6

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Chapter 6

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  1. Chapter 6 Elements of Airplane Performance Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  2. Un-accelerated level flight Simple Mission Profile for an Airplane 1 Switch on + Worming + Taxi (Cruising flight) 4 3 Descent Altitude Climb Landing Takeoff 5 6 1 2 Simple mission profile Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  3. Airplane Performance Equations of Motions Static Performance (Zero acceleration Dynamic Performance (Finite acceleration) Thrust required Thrust available Maximum velocity Takeoff Power required Power available Landing Maximumvelocity Rate of climb Gliding flight Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  4. Time to climb Maximum altitude Service ceiling Absolute ceiling Range and endurance Road map for Chapter 6 Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  5. Study the airplane performance requires the derivation of the airplane equations of motion • As we know the airplane is a rigid body has six degrees of freedom • But in case of airplane performance we are deal with the calculation of velocities ( e.g.Vmax,Vmin..etc),distances (e.g. range, takeoff distance, landing distance, ceilings …etc), times (e.g. endurance, time to climb,…etc), angles (e.g.climb angle…etc) Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  6. So, the rotation of the airplane about its axes during flight in case of performance study is not necessary. • Therefore, we can assume that the airplane is a point mass concentrated at its c.g. • Also, the derivation of the airplane’s equations of motion requires the knowledge of the forces acting on the airplane • The forces acting on an airplane are: Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  7. Components of the resultant aerodynamic force R • 1- Lift force L • 2- Drag force D • 3- Thrust force T Propulsive force • 4- Weight W Gravity force • Thrust T and weight W will be given • But what about L and D? • We are in the position that we can’t calculate L and D with our limited knowledge of the airplane aerodynamics Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  8. So, the relation between L and D will be given in the form of the so called drag polar • But before write down the equation of the airplane drag polar it is necessary to know the airplane drag types Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  9. Total Drag ■ Drag Types [ Kinds of Drag ] Skin Friction Drag Pressure Drag Form Drag (Drag Due to Flow separation) Induced Drag Wave Drag Note : Profile Drag = Skin Friction Drag + Form Drag Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  10. ►Skin friction drag This is the drag due to shear stress at the surface. ►Pressure drag This is the drag that is generated by the resolved components of the forces due to pressure acting normal to the surface at all points and consists of [ form drag + induced drag + wave drag ]. ►Form drag This can be defined as the difference between profile drag and the skin-friction drag or the drag due to flow separation. Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  11. ►Profile Drag ● Profile drag is the sum of skin-friction and form drags. ● It is called profile drag because both skin-friction and form drag [ or drag due to flow separation ] are ramifications of the shape and size of the body, the “profile” of the body. ● It is the total drag on an aerodynamic shape due to viscous effects Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  12. Skin-friction Form drag Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  13. ►Induced drag ( or vortex drag ) This is the drag generated due to the wing tip vortices , depends on lift, does not depend on viscous effects , and can be estimated by assuming inviscid flow. Finite wing flow tendencies Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  14. Formation of wing tip vortices Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  15. Complete wing-vortex system Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  16. Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  17. The origin of downwash The origin of induced drag Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  18. ►Wave Drag This is the drag associated with the formation of shock waves in high-speed flight . Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  19. ■ Total DragofAirplane ● An airplane is composed of many components and each will contribute to the total drag of its own. ● Possible airplane components drag include : 1. Drag of wing, wing flaps = Dw 2. Drag of fuselage = Df 3. Drag of tail surfaces = Dt 4. Drag of nacelles = Dn 5. Drag of engines = De 6. Drag of landing gear = Dlg 7. Drag of wing fuel tanks and external stores = Dwt 8. Drag of miscellaneous parts = Dms Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  20. ● Total dragof an airplane is not simply the sum of the drag of the components. ● This is because when the components are combined into a complete airplane, one component can affect the flow field, and hence, the drag of another. ● these effects are called interference effects, and the change in the sum of the component drags is called interference drag. ● Thus, (Drag)1+2 = (Drag)1 + (Drag)2 + (Drag)interference Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  21. ■ Buid-up Technique of Airplae Drag D ● Using the build-up technique, the airplane total drag D is expressed as: D = Dw + Df + Dt + Dn +De + Dlg + Dwt + Dms + Dinterference ► Interference Drag ● An additional pressure drag caused by the mutual interaction of the flow fields around each component of the airplane. ● Interference drag can be minimized by proper fairing and filleting which induces smooth mixing of air past the components. ● The Figure shows an airplane with large degree of wing filleting. Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  22. Wing fillets Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  23. ● Notheoretical method can predict interference drag, thus, it is obtained from wind-tunnel or flight-test measurements. ● For rough drag calculations a figure of 5% to 10% can be attributed to interference drag on a total drag, i.e, Dinterference ≈ [ 5% – 10% ] of components total drag ■ The Airplane Drag Polar ● For every airplane, there is a relation between CD and CL that can be expressed as an equation or plotted on a graph. ● The equation and the graph are called the drag polar. Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  24. For the complete airplane, the drag coefficient is written as CD = CDo + K CL2 This equation is the drag polar for an airplane. Where: CDo drag coefficient at zero lift ( or parasite drag coefficient ) K CL2 = drag coefficient due to lift ( or induced drag coefficient CDi ) K = 1/π e AR Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  25. e Oswald efficiency factor = 0.75 – 0.9 (sometimes known as the airplane efficiency factor) AR wing aspect ratio = b2/S , b wing span and S wing planform area Schematic of the drag polar Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  26. Airplane Equations of Motion Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  27. Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  28. Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  29. Apply Newton’s 2nd low of motion: In the direction of the flight path Perpendicular to the flight path Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  30. Un-accelerated Level Flight Performance (Cruising Flight) Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  31. Thrust Required for Level Un-accelerated Flight (Drag) Thrust required TR for a given airplane to fly at V∞ is given as : TR = D Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  32. ● TR as a function of V∞ can be obtained by tow methods or approaches graphical/analytical ■Graphical Approach Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  33. 1- Choose a value of V∞ 2 - For the chosen V∞ calculate CL L = W = ½ρ∞V2∞S CL CL = 2W/ ρ∞V2∞S 3- Calculate CD from the drag polar CD = CDo = K CL2 4- Calculate drag, hence TR, from TR = D = ½ρ∞V2∞S CD 5- Repeat for different values of V∞ Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  34. 6- Tabulate the results Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  35. (TR)min occurs at (CL/CD)max Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  36. ■ Analytical Approach • It is required to obtain an equation for TR as a function of V∞ • TR = D Required equation Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  37. Parasite and induced drag TR/D CDo=CDi V∞ Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  38. Note that TR is minimum at the point of intersection of the parasite drag Do and induced drag Di • Thus Do = Di at [TR]min • or CDo = CDi • = KCL2 • Then [CL](TR)min = √CDo/K • And [CDo](TR)min = 2CDo Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  39. Finally, (L/D)max = (CL/CD)max • = √CDo/K /2CDo • (CL/CD)max = 1/√4KCDo • Also,[V∞](TR)min =[V∞](CL/CD)maxisobtained from: W = L • = ½ρ∞[V]2(TR)minS [CL](TR)min • Thus: • [V](TR)min= {2(W/S)(√K/CDo)/ρ∞}½ Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  40. L/D as function of angle of attack α L/D as function of velocity V∞ Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  41. L/D as function of V∞ : • Since, • But L=W • Then • or Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  42. Flight Velocity for a Given TR • TR = D • In terms of q∞ = ½ρ∞V2∞we obtain • Multiplying by q∞and rearranging, we have • This is quadratic equation in q∞ Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  43. Solving for q∞ • By replacing q∞ = ½ρ∞V2∞we get Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  44. Let • Where (TR/W) is the thrust-to-weight-ratio • (W/S) is the wing loading • The final expression for velocity is • This equation has two roots as shown in figure corresponding to point 1 an 2 Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  45. ●When the discriminant equals zero ,then only one solution for V∞ is obtained ●This corresponds to point 3 in the figure, namely at (TR)min Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  46. Or, (TR/W)min = √4CDoK • Then the velocity V3 =V(TR)min is • Substituting for (TR/W)min = √4CDoK we have Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  47. Effect of Altitude on (TR)min • We know that • (TR/W)min = √4CDoK • This means that (TR)min is independent of altitude as show in Figure Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  48. Thrust Available TA Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  49. Sonic speed Prof. Galal Bahgat Salem Aerospace Dept. Cairo University

  50. Thrust Available TA and Maximum Velocity Vmax Prof. Galal Bahgat Salem Aerospace Dept. Cairo University