AS309 Unit Three Review



AS309 UNIT THREE REVIEW

These notes summarize important ideas in the class notes, but are not a substitute for the class notes.

1. Induced Drag

• Caused when wingtip vortices create a downwash angle at the trailing edge of the wing. The downwash angle is the angle formed by intersection of the relative wind vectors at the leading and trailing edges of the wing.

• Induced angle of attack (i is ½ the downwash angle.

• The drag portion of the aerodynamic force (AF) for a finite span wing consists of both induced and parasite drag components, whereas an infinite span wing creates only parasite drag.

• Wingtip vortices from large aircraft are very dangerous.

• Induced drag decreases as airspeed increases, while parasite drag increases.

• Aspect Ratio AR = b (span) / cave (average chord) = b2 / S (planform area).

• Other factors being equal, reducing the AR of a wing increases induced drag.

• The sweepback angle ( is the angle formed by the 25% chord line and a line perpendicular to the longitudinal axis.

• High ( is associated with high induced drag, especially at high angles of attack, as in slow flight for landing.

[pic]

2. L/D Ratio and L/D Curve

• L / D = CL / CD

• The L / D curve for a wing or an airplane is computed from the CL and CD curves.

• (L / D)max is the highest point on the L / D curve.

• In steady state level flight at 1G, the following identities approximately hold:

W / D = L / D, and Dmin = W (L / D)max.

• Best glide ratio occurs at (L / D) max.

• The airspeed for best glide varies with gross weight and altitude, but angle of attack for best glide is constant for a given aircraft configuration.

• If (L / D)max = k, then an airplane can glide k nm (feet, statute miles, &c.) for each nm (ft, statute mile, &c.) of altitude above the ground.

|[pic] |[pic] |

3. Total Drag Curve

• As airspeed increases, induced drag decreases and parasite drag increases.

• Total drag curve is constructed from scalar sum of parasite and induced drag for different airspeeds.

• Total drag curve is for a specific airplane in a specific configuration at a specific gross weight and specific altitude.

• Since T = D (approximately) in steady state 1G level flight, the total drag curve is also the thrust required curve.

• Except for airspeed associated with (L/D)max and very high airspeeds, there are two airspeeds that correspond to the same total drag. This leads to the region of reverse command in the thrust required curve.

|[pic] |[pic] |

4. Slow Flight

• Sweepback decreases AR, increases drag, decreases lift, and flattens CL curve.

• Wing stalls first where Cl / CL (wing loading ratio) is highest.

• Elliptical wing stalls evenly on aft edge from root to tip.

• Decreasing taper ratio λ makes wing loading higher at tips than roots, causing stall toward tips

• Sweepback has same effect as decreasing taper ratio.

• Stall warning more effective if wing stalls first at root due to increased aileron effectiveness (if ailerons are outboard on wing) and stall turbulence contacting body of aircraft and giving better stall warning.

• In region of reverse command, more power is required to fly slower. To slow, increase AOA and add power; opposite adjustment for speeding up. Throttle controls altitude and yoke (AOA) airspeed on the glide slope.

• Ground effect "neutralizes" wingtip vortices, decreases downwash angle, increases lift, and decreases drag. Starts within one wingspan of ground; pronounced when much closer (e.g. 50% drag reduction at 10% of span). With low horizontal tail, may encounter nosedown pitch entering ground effect and nose up pitch leaving it. Low pitot static system static port may cause low a/s reading.

5. Wind Shear

• 180o and 50 kt wind shifts have been observed.

• Downbursts can drive an a/c into the terrain.

• Headwind burst increases IAS; a/c pitches up; opposite is true for tailwind bursts.

• Crosswind bursts create lineup difficulties.

• Overreaction in correcting for these effects can cause dangerous situations on landing (see class notes for details).

• Best approach is to avoid wind shear conditions (cause: thunderstorms, frontal systems, low level jet streams).

6. A/C Caused Air Turbulence

• Wingtip vortices more dangerous than thrust stream blast or profile wake turbulence.

• Problem is most acute at high AOAs during t/o and landing.

• Worst downflow is at a/c centerline.

• Observe proper intervals (2-4 minutes for light/heavy aircraft).

• If not possible, takeoff downwind of departing heavy a/c; land upwind of arriving heavy a/c.

• Vortices move outward laterally at about 5 knots; crosswinds can exacerbate this effect on parallel runways.

• Winglets and splines used to diminish wingtip vortices.

7. Compressible Flow

• Speed of sound a = a0[pic], with ao approximately 661 kts at SL in standard atmosphere (dependent on temperature only).

• M = V / a, where M is Mach number and a is speed of sound at operating altitude.

• Mcrit is Mach number where airflow on a/c first reaches Mach 1.

• Definitions: subsonic, transonic, and supersonic. See notes for details.

• In a shock (compression) wave, mach (flow velocity) decreases, while static pressure, density, and temperature increase, creating high drag.

• Normal shock waves differ from oblique waves (flow into a corner) in that downflow is subsonic, as opposed to supersonic.

• The wave angle of an oblique wave is the wedge angle + sin-1 (1/M) (decreases as m increases).

• As airspeed increases in transonic range, normal wave moves to trailing edge of wing, and bow wave forms.

• In an expansion wave (flow around corner), air velocity increases and pressure, temperature, and density drop (opposite of shock or compression wave).

• The force divergence Mach number is about 1.05 Mcrit. A pitching moment (tuckunder) often occurs due to shock wave stall, which decreases downwash. Other potential effects are buffet, control surface buzz, and diminished control effectiveness.

• In general, flight in transonic region is transitory.

8. Stability and Control

• A body has positive, neutral, or negative static stability.

• If and only if a body has positive static stability, then it has positive, neutral, or negative dynamic stability.

• Longitudinal (pitch) stability: rotation about lateral axis.

• Lateral (roll) stability: rotation about longitudinal axis.

• Directional (yaw) stability: rotation about vertical axis.

• Stability and controllability are to some extent tradeoffs.

• Positive dynamic stability desirable about all three axes.

• Different aircraft parts (wing, fuselage, engine nacelles or intakes, tail assembly) can be either stabilizing or destabilizing.

• The notes distributed in class summarize information which cannot be presented here in a more condensed form.

• In essence, the class notes are the review notes for the unit on stability and control. Tables reproduced below contain information that you must know if you want to get a good grade on the third test.

• Other terms to understand: Phugoid oscillations; short period oscillations; dorsal fin; lateral fin; dihedral; anhedral.

9. Longitudinal Stability

|Wings |Stabilizing if CG is forward of AC; |Nose up displacement increases lift, creating a moment in the direction|

| |otherwise destabilizing. |of displacement (Figure 8.13, text p. 251). |

|Fuselage |Destabilizing. |Ram air on upper or lower forward fuselage creates lift in the |

| | |direction of displacement (Figure 8.17, text p. 253). |

|Engine |Destabilizing if engine is forward of CG,|Ram air on nacelle after nose displacement creates force in direction |

|Nacelles |and otherwise stabilizing. |of displacement only when nacelle is forward of CG (Figure 8.18, text |

| | |p. 253). |

|Horizontal |Stabilizing. |AOA change of horizontal tail creates pitching moment opposite |

|Tail | |displacement direction (Figure 8.19, text p. 254). |

10. Directional Stability

|Wings |Sweepback gives a small stabilizing |The wing away from the yaw direction has less spanwise flow and more drag |

| |moment. |than opposing wing (Figure 8.35, text p. 265). |

|Fuselage |Destabilizing. |Ram air on forward fuselage side creates a force in the direction of |

| | |displacement. (Figure 8.36, text p. 266) |

|Engine |Destabilizing if engine forward of CG,|Ram air on nacelle creates force indirection of displacement only when |

|Nacelles |otherwise stabilizing. |nacelle is forward of CG. |

|Vertical |Stabilizing. |AOA change of vertical tail creates pitching moment opposite displacement |

|Tail | |direction (Figure 8.37, text p. 267). |

11. Lateral Stability

|Wings |Dihedral, high wing, and |With dihedral, wing into sideslip has higher AOA. CG below high wing creates |

| |sweepback are |pendulum effect. Sweptback wing into sideslip has less spanwise flow, giving more|

| |stabilizing; anhedral is |lift. (Figures 8.48-8.50, text pp. 275-276). |

| |destabilizing. | |

|Vertical Tail |Stabilizing. |Sideslip force creates moment against roll (Figure 8.51, text p. 267). |

12. Yaw-Roll Coupling

|Spiral |Static directional stability exceeds static |Lowered wing will not return to equilibrium position; aircraft |

|Divergence |lateral stability. |enters dive with tightening spiral. |

|Directional |Poor static directional stability. |Yaw (or roll-induced yaw) causes increased yaw and loss of |

|Divergence | |control |

|Dutch Roll |Lateral and directional stability more |Aircraft will oscillate in both yaw and roll. Very |

| |balanced than in spiral divergence. |aggravating, so modest spiral divergence is tolerated. |

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download