I have to say Islander's description of windshear does seem to sort of vindicate what Mike and John have been saying all along, doing low level circuits in a helicopter with abrupt turns seems likely to cause a sort of 'windshear' effect.
Absolutely, it does! But contrary to what Mike stated, in a steady wind that 'windshear' effect is identical whether turning upwind or downwind.
Although to me why Mike thinks groundspeed has anything to do with the plane's momentum in terms of the air is still unclear.
'Cos he's a 'downwind turn' flat earther!
I know I won't be able to persuade him otherwise, but just maybe I can give an illustration that will help others that are less entrenched.
For this, we need to engineer an absolutely steady wind. So we are going to embark on HMS Ark Royal and go boating. But bear with me, some aviating comes later!
Let's clear the ship's hangar of all those nasty whirly and V/STOLy things, and put a proper aeroplane in place. We're going to fly it in the hangar, so it's going to have to be small. A model aeroplane. And perlease, don't throw Reynolds Number at me, that's completely irrelevant for this discussion.
Okay, let's close all the doors to the hangar so it's air tight, and ask the ship's captain to give us 35 kts. To eliminate any confusion, btw, we've chosen a day and location where there's zero real wind ... that took some finding, I can assure you! Inside the hangar we have perfectly still air, yet that air is moving over the earth's surface at 35kts. We've just created the perfect, never seen in practice, absolutely steady 35 kt wind!
Now let's fly our model. As we fly upwind at 35KIAS towards the ships stern, we have a groundspeed (earth reference) of zero. Now let's cross our fingers and risk one of those terrible downwind turn thingies. On completion of the turn our model is now headed towards the ship's bow with a groundspeed (earth reference) of 70kts.
By 'eck, says Mike, look at all that extra momentum (or he may say look at all that extra kinetic energy). That needs an energy input from somewhere, so extra power is required, and until it takes effect airspeed will decrease.
Flying the model, meanwhile, we leave the power unchanged and, aside from a tiny decrease in airspeed in each of the turns, which is promptly restored upon levelling the wings, the airspeed as measured by our super-accurate instrumentation remains absolutely constant regardless of the direction of travel.
Which leaves Mike somewhat bemused!!
