Hello PDR,
First, if I might steal a line from Shakespeare’s Mark Anthony: “I speak not to disprove what PDR has spoke but to say what I do know.” I am enjoying the intellectual exchange and not looking for a fight.
What I know from many years of observation is this: if I fly my ac at sea level at 70 KIAS my TAS will be 70KTAS. If I apply full power and maintain 70KIAS in the climb when I pass 10000 ft (as if) my TAS will be 80KTAS or thereabouts.
This is what I know. The why bit has been of no practical use to me as a pilot but, perforce, I have been a flying instructor a number of times and been required to offer a plausible explanation to the moe inquisitive Tyros that I have had the pleasure to teach.
You ask why I bang on about IAS/TAS.
Well, to begin with Indicated Air Speed is not actually a Speed except at sea level in ISA conditions. Better to remember that what you see on the ASI is Dynamic Pressure delivered via the pitot tube. The ASI could equally be gauged in inches of Mercury or Hecto Pascals. But, I’m quite pleased that they chose MPH/Kts.
Dynamic Pressure, in simple terms, is composed of the speed of the air and the density of the air. This speed is indeed a speed. It is the speed of the air in our ‘frame of reference’ – Planet Earth. It is the speed that the ac is moving in the surrounding air and, by convention, we call it True Air Speed (TAS). It is the speed used in Kinetic Energy, Momentum, acceleration and Newton’s Laws.
So for we pilots IAS (dynamic pressure) depends on TAS (speed through the air) and air density. If the density reduces (unavoidable in a climb) IAS will reduce.
But it doesn’t, we know that. But what we forget (because, for most it is of no practical value) while we maintain IAS, TAS is increasing (The ac is accelerating ) to balance the drop in air density.
The ac is not accelerating under the influence of air density. In accordance with Newton’s second Law the ac can only accelerate if acted upon by an external force – density is not a force.
As to vector diagrams:
In level balanced un-accelerated flight, Forces on the ac are not overcoming weight (gravity) they are balancing weight.
In a constant IAS climb the Forces on the ac are not balancing weight, they are overcoming weight, otherwise the ac would not move away from the centre of the Earth – climb that is. And, the Forces on the ac are accelerating the ac in the surrounding air – TAS increasing.
Some force is raising the ac in relation to the Earth’s surface; some force is accelerating the ac. If, a chosen vector diagram does not show this then I guess, as many suspect, it is just FM - Flipping Magic.
Which, when I have been in the mood, I have, sometimes, given as an explanation to many of the mysteries of flight.