Originally Posted by
Meikleour
Most people seem to agree that airspeed fluctuations due to windshear are the result of the aircraft having an inertial groundspeed which causes the changes to be apparent as it experiences differing air masses and takes a finite time to regain its momentum ......................now, when flying in a stable but moving air mass where does the effect of aircraft momentum go? (why is it always ignored?)
I have observed many times descending into NRT with a tailwind of 200kts (not uncommon at certain times of year) with the autopilot in IAS hold and smooth wind conditions. The arrival called for a 90 degree heading change and whilst the IAS remained constant the rate of descent increased dramatically once the turn was complete (ie. much lower groundspeed) before stabilising back to the normal rate. This behaviour always seemed at odds with the theory.
This is explained as follows: The aircraft turns due to a horizontal component of the lift force. Lift force is the equal and opposite effect of momentum given to the air. In a turn, part of that momentum is horizontal. As the aircraft turns from downwind to crosswind it loses momentum as it loses groundspeed. This acceleration is caused by the aero-forces acting on the aircraft modified by the angle of drift. The air gains horizontal momentum and part of that is in the same direction as the wind. So aircraft loses momentum and wind gains momentum.
At the same time, due to inertia, the airspeed is affected by the reducing tailwind component and there is a slight tendency for airspeed to increase. (Same as an increasing headwind)
If the aircraft is descending in IAS hold and constant thrust, the effect is that the auto pilot will hold the airspeed and the rate of descent will
decrease slightly but only during the turn.
When the aircraft rolls out on the crosswind heading, the thrust is insufficient to maintain the reduced rate of descent and the IAS hold pitches the nose down to maintain the airspeed. The rate of descent increases rapidly but settles back once equilibrium is restored at the original rate of descent.