IAS with tail wind or head wind

[Chris Scott]

Windsheer and IAS; and GS

Hi aslan1982,

I shall assume that you are thinking of sudden changes in wind?

Let's say a light aircraft is cruising straight and level at sea-level in calm conditions (no wind), nose heading north (HDG 360) at a steady IAS of 100kts. Because the wind is calm, the aircraft's GS (speed over the ground) is also 100kts, and its track over the ground (TRK) is also north.

(1) Now, say a wind of 20kts from the north (360/20) springs up instantly [we call this wind sheer]. The IAS will rise instantly to 120, increasing the lift over the wing by about 40%. The GS will still be 100. However, unless the power is increased, the extra drag created will slow the aircraft down to 100kts IAS again, at which speed the GS will have fallen to 80kts. The track of the aircraft (TRK) will not have changed.

2) Instead of (1), let's say the wind springs up finstantly from calm to 20kts from the south (180/20). In this case, the IAS will drop instantly to 80kts, resulting in a loss of lift of about 35%. The GS will still be 100. The reduction in drag will allow the aircraft to accelerate gradually to IAS 100 again, at which point the GS will be 120.

If the above is of any use, let me know and I can have a go (later) at the crosswind case if necessary.

Chris

PS (by EDIT): Sorry Rainboe, it's all yours!