[ SPEEDS ] IAS, TAS and Mach Number

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- Climb with a constant IAS (Dynamic Pressure)

- TAS Increases

Dynamic Pressure = 1/2 x Density x V^2

When climbing, the density decreases. To keep the Dynamic pressure constant the TAS (V^2) needs to increase. The opposite is true in a descent.

- Mach Number Increases

MN = SQRT(Dynamic pressure / Static pressure)

When climbing, the static pressure decreases. If we keep the Dynamic pressure (IAS) constant, the only factor in the equation that we can increase to compensate is the Mach Number. The opposite is true in a descent.

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- Climb with a constant Mach Number

- TAS Decreases

MN = TAS / LSS

Local speed of sound (LSS) is only dependent on temperature. When climbing, the temperature will decrease, so will the LSS and looking at the equation, to keep the ratio TAS/LSS a constant we also need to decrease the TAS. The opposite is true when descending.

In the isothermal layer (temperature stays constant), the TAS will also stay constant.

- IAS Decreases

MN = SQRT(Dynamic pressure / Static pressure)

During a climb the static pressure drops. The ratio needs to remain a constant so the Dynamic pressure (IAS) has to decrease too.

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- IAS and Cl when climbing/descending at constant MN

When we climb at a constant Mach Number, the IAS will decrease as we saw above. This also means that the lift coefficient increases. In a descent, the opposite is true and the lift coefficient can be reduced due to the increase in IAS in a constant Mach descent.

L = Cl x 1/2 x density x V^2 x S

We saw already that during a constant Mach descent, the TAS will increase. When altitude decreases, density will also increase.

Looking at the equation for lift we can see that when density and V^2 increase, we can reduce the Cl (reduce the AoA) to maintain the same lift.

The opposite is true for a constant Mach climb, where the Cl is increased.

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- Mmo and Vmo

Mmo = Maximum operating Mach Number
Vmo = Maximum operating Speed

When we are climbing at a constant IAS, our Mach Number will keep increases so there is the danger that we might exceed the Mmo (as a Mach Number). That is why at a cross-over altitude, an aircraft switches from constant IAS to constant Mach Number.

The opposite is true for a descent. When we are descending at a constant Mach Number, our IAS keeps increases and there is the danger that we might exceed Vmo. That is why at a cross-over altitude, an aircraft switches from constant Mach Number to Constant IAS.

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- The "Coffin Corner"

As the altitude increases, the Vs (stall speed) will increase because the air density and therefore lift decrease.

As the altitude increases, the Mach speed will decrease which will lower the speed at which Mach buffet starts.

So at a certain altitude, both the stall speed and the Mach Buffet speed will meet each other which creates a triangle in the speed envelope called the "coffin corner". If the aircraft slows down it will stall (under Vs) and if it speeds up it will exceed the buffet speed (high speed stall).