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Peter Kämpf
Peter Kämpf
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The difference is the change in Mach number over altitude.

And it is more than just the compressibility error in the IAS indication.

The maximum lift coefficient of a wing goes down with Mach number. While at sea level and 140 KIAS you fly at 21% of the speed of sound (Mach 0.21), at cruise altitude (I guess that means 30,000 ft) the true speed is already 360 KTAS which -- together with the decline of the speed of sound at lower temperature -- translates to Mach 0.63.

In order to estimate the change in maximum lift coefficient, look at the factor maximum lift coefficient times Mach squared: Above maybe Mach 0.4 to 0.5, this is what should stay (roughly) constant. A typical value for a modern wing would be 0.4, so we divide this by 0.63² = 0.397. SoThus your maximum lift coefficient at Mach 0.63 has dropped to about 1.0. At lower altitude the maximum lift coefficient of the clean wing is closer to 1.6.

Technically, the wing might even be able to create higher lift coefficients at Mach 0.63, but buffeting will make this intolerable. The stall speed at cruise level, therefore, is the buffet speed and cannot be directly compared with the (real) stall speed at sea level.

The difference is the change in Mach number over altitude.

And it is more than just the compressibility error in the IAS indication.

The maximum lift coefficient of a wing goes down with Mach number. While at sea level and 140 KIAS you fly at 21% of the speed of sound (Mach 0.21), at cruise altitude (I guess that means 30,000 ft) the true speed is already 360 KTAS which -- together with the decline of the speed of sound at lower temperature -- translates to Mach 0.63.

In order to estimate the change in maximum lift coefficient, look at the factor maximum lift coefficient times Mach squared: Above maybe Mach 0.4 to 0.5, this is what should stay (roughly) constant. A typical value for a modern wing would be 0.4, so we divide this by 0.63² = 0.397. So your maximum lift coefficient at Mach 0.63 has dropped to about 1.0. At lower altitude the maximum lift coefficient of the clean wing is closer to 1.6.

The difference is the change in Mach number over altitude.

And it is more than just the compressibility error in the IAS indication.

The maximum lift coefficient of a wing goes down with Mach number. While at sea level and 140 KIAS you fly at 21% of the speed of sound (Mach 0.21), at cruise altitude (I guess that means 30,000 ft) the true speed is already 360 KTAS which -- together with the decline of the speed of sound at lower temperature -- translates to Mach 0.63.

In order to estimate the change in maximum lift coefficient, look at the factor maximum lift coefficient times Mach squared: Above maybe Mach 0.4 to 0.5, this is what should stay (roughly) constant. A typical value for a modern wing would be 0.4, so we divide this by 0.63² = 0.397. Thus your maximum lift coefficient at Mach 0.63 has dropped to about 1.0. At lower altitude the maximum lift coefficient of the clean wing is closer to 1.6.

Technically, the wing might even be able to create higher lift coefficients at Mach 0.63, but buffeting will make this intolerable. The stall speed at cruise level, therefore, is the buffet speed and cannot be directly compared with the (real) stall speed at sea level.

Source Link
Peter Kämpf
Peter Kämpf
  • 237.3k
  • 17
  • 601
  • 944

The difference is the change in Mach number over altitude.

And it is more than just the compressibility error in the IAS indication.

The maximum lift coefficient of a wing goes down with Mach number. While at sea level and 140 KIAS you fly at 21% of the speed of sound (Mach 0.21), at cruise altitude (I guess that means 30,000 ft) the true speed is already 360 KTAS which -- together with the decline of the speed of sound at lower temperature -- translates to Mach 0.63.

In order to estimate the change in maximum lift coefficient, look at the factor maximum lift coefficient times Mach squared: Above maybe Mach 0.4 to 0.5, this is what should stay (roughly) constant. A typical value for a modern wing would be 0.4, so we divide this by 0.63² = 0.397. So your maximum lift coefficient at Mach 0.63 has dropped to about 1.0. At lower altitude the maximum lift coefficient of the clean wing is closer to 1.6.