IAS, Drag , Density, TAS

[Keith.Williams.]

DRAG AND IAS

The Airspeed Indicator (ASI) measures the dynamic pressure and produces an Indicated Airspeed output (IAS) that is determined by the value of that dynamic pressure. Every time the ASI senses a particular value of dynamic pressure it indicates the same IAS.

This means that if we climb at constant IAS, we must be climbing at a constant dynamic pressure.

Lift = CL 1/2ρ Vsquared S

Drag = CD 1/2ρ Vsquared S

And both CD and CL are determined by angle of attack.

If we assume that the weight of our aircraft remains constant, then as we climb at constant IAS (which also means constant dynamic pressure) we must keep the angle of attack constant to maintain constant lift to match the constant weight. This in turn means that the CD will remain constant.

So as we climb we have a situation in which CD and 1/2ρ Vsquared are both constant, so (unless our surface area (S) changes), we must have constant drag.

So climbing at constant IAS produces constant drag.



POWER REQUIRED

Dynamic pressure is equal to 1/2ρ Vsquared, where ρ is the air density and V is the TAS.

So if the IAS remains constant, then any change in air density must be balanced by an equal and opposite change in TASsquared.

If for example an aircraft climbs at constant IAS then the TAS must gradually increase so that increasing TASsquared compensates for the reducing air density to give constant dynamic pressure.

The important point here is that if IAS is constant, then as we climb, the TAS must gradually increase.

Power Required = Drag x TAS

So in our constant IAS climb we have constant drag multiplied by increasing TAS, so the power required increases with the TAS.

Note that none of the above is absolutely true. We should for example be talking about EAS not IAS and at high altitude things like compressibility effects will complicate matters a bit. But for the purposes of the early stages of the JAR ATPL POF syllabus, the above is accurate enough.