Helmet Fire has got the idea of the effect I'm talking about; for clarity, I'll rephrase my initial question:
When established in a constant AoB turn at say, 90 kts, in balance (in trim), as the helicopter turns from a downwind to an into-wind heading, a nose-down pitch is required to maintain IAS. Conversely, when it turns from an into-wind heading to a downwind heading, a nose-up pitch is required to maintain IAS. Why?
The effect is only apparent with wind. When the wind is calm, the same pitch attitude can be maintained throughout the turn to maintain IAS.
Also HF, it's not a visual 'grounspeed' effect. I first noticed this when instrument flying!
Ideas? J