Airspeed Envelop for Air Transport Airplanes Jet transports cruise near mcrit

Airspeed Envelop for Air Transport Airplanes
Jet transports cruise near M_CRIT,

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  Pilot must control airspeed within specified limits
  
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  M_CRIT—Mach number where airflow on the airframe first reaches Mach 1 (beginning of transonic flight regime)
  
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  Force Divergence Mach number—about 5% above M_CRIT
  
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    transonic aerodynamic effects begin
    
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    nose tuck due to aft movement of Mach wave on the wing
    
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    high speed stall—boundary layer detaches aft of the Mach wave
    
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  Must not allow airspeed to reach M_CRIT in an air transport aircraft
  

Definitions

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  V_S—1 G stall speed
  
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  V_MO—max operating airspeed to prevent ram-air damage
  
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  V_S and V_MO in KEAS are invariant with altitude
  
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  M_MO—max operating Mach number to avoid approaching M_CRIT
  
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  V_MMO—airspeed corresponding to M_MO
  
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    decreases as altitude increase in SA because speed of sound decreases while M_MO is constant
    
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    plotted in Figure 3.11, but conventionally labeled M_MO
    
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  M_DF—maximum demonstrated Mach number: Mach number at which aircraft has been tested and shown to be controllable
  
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  M_DF > M_MO because M_MO has a built-in airspeed safety margin
  

Figure 3.11 Airspeed Envelop for Boeing 767 at 1 G and 300,000# Gross



Figure 3.13

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  V_S(156 KEAS)— minimum airspeed to avoid stalling the wing (typically not V_S in performance manuals, which provide a safety margin above the 1 G stall speed) INVARIANT WITH ALTITUDE
  
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  V_MO (390 KEAS) —maximum airspeed to avoid ram air damage (also provides a safety margin) INVARIANT WITH ALTITUDE
  
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  V_MMO (KEAS; labeled M_MO)—maximum airspeed to avoid close approach to transonic flight (monitored on the Mach meter) DECREASES AS ALTITUDE INCREASES
  
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  M_MO (not specified in figure) INVARIANT WITH ALTITUDE
  
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  Crossover altitude (20,000’)—altitude where V_MO = V_MMO
  
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    Below crossover altitude, airspeed limited by V_MO
    
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    Above crossover altitude, airspeed limited by M_MO / V_MMO
    
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  Coffin Corner (60,000’)—altitude where V_S = V_MMO (near the coffin corner, it is difficult to distinguish a high speed stall from a low speed stall)
  
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  Allowable Airspeed Range—airspeed_MAX - airspeed_MIN
  
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    Below crossover altitude: V_MO - V_S
    
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    Above crossover altitude: V_MMO - V_S
    
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    At 20,000’, 390 – 156 = 234 KEAS.
    
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    At 30,000’, 325 – 156 = 169 KEAS.
    
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    At 40,000’, 260 – 156 = 104 KEAS
    

Figure 3.12: Airspeed Envelop for Boeing 767 at 2 G and 300,000# Gross

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  G force affects stall speed: 1 G V_S = 156 KEAS
  
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  2 G V₂ = V_S G = 156 2  221 KEAS
  

Figure 3.12

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  V_S = 221 (2 G stall speed)
  
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  V_MO = 390 KEAS,
  
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  Crossover altitude = 20,000’
  
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    V_MO = airspeed corresponding to M_MO = 390 KEAS
    
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    Below 20,000’, airspeed limited by V_MO
    
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    Above 20,000’, airspeed limited by M_MO / V_MMO
    
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  Allowable airspeed range
  
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    At 20,000’, 390 – 221 = 169 KEAS.
    
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    At 30,000’, 325– 221= 104 KEAS.
    
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    At 40,000’, 260 – 221= 39 KEAS
    
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  Coffin corner = 45,000’
  

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  Airspeed corresponding to M_MO =V_S
  
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  Allowable airspeed range = 221 – 221= 0.
  

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Figure 3.13. Maximum and Minimum B737 Airspeeds by Gross Weight

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  Pilots use a performance table to determine airspeeds operating range
  
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  105,000# and FL300: V_MAX = 311 KIAS; V_MIN = 196 KIAS; Allowable airspeed range = 311- 196 = 115 KIAS
  
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  130,000# and FL340: V_MAX = 259 KIAS; V_MIN = 241 KIAS; Allowable airspeed range = 259- 241 = 18 KIAS
  

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  Weight W affect stall speed V_S and V_MMO but not V_MO or M_MO [V₂ = V₁ (W₂/W₁)]
  
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  Allowable airspeed range gets smaller as
  
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    weight increases: V_S W
    

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  altitude increases
  
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    larger safety margins desirable
    
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    compressibility causes IAS to read higher than EAS