Thread: Theory on lift
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Old 28th Jul 2018, 21:00
  #366 (permalink)  
Join Date: Jul 2007
Location: Germany
Posts: 0
Interesting. So, for me, I wish we didnt delude people with fudges to explain WHY aircraft fly. Instead, It should be explain the theory of lift is a highly complex topic, with the overall production of lift explained through a number of effects which by themselves do not explain fully the theory of lift, however, assist with the understanding of what is going on in terms of flow fields and on the 2D level.

In ,my mind, there are a few truths that can be stated which at a deeper level, assist understanding of lift. They are:

There can be no lift without Viscosity. Without viscosity, the flow will not remain adhered to the wing and work can therefore not be done on the flow. The wing slips through the flowfield and does not impact it. As there is no change in flow direction, there is no change in inertia and thus no downwash.
There can be no lift without considering the 3D flowfield.
There can be no lift without a circulatory flow, which by thought, can be tied to the requirement of viscosity in the production of lift. Circulatory flow is not a fudge to explain lift. It actually occurs (dont believe me, move your hand through water at an angle of attack. you can observe the reverse circulation depart the trailing edge as your hand starts to move).
The net effect of the complex flow around a wing is the generation of downwash, the effect of which due to Newtons third law exerts a force upon the wing and thus doing work. Without viscosity, we cannot have downwash as work cannot be done on the flow by the wing.
An ideal wing that is 100% efficient would convert 100% of downwash into lift with no drag, other than that induced by the rearwards deflected lift vector (what we call induced drag).
We dont live in the ideal world. Ignoring compressibility,, the effect of viscosity upon which lift depends is responsible for the generation of non-lift dependent drag.
In the compressible world (Re dependent, big planes, assume M >0.4), the impact of compressibility on the flow field (particularly pressure field) can both positively and negatively impact both lift and drag.
The 3D flowfield significantly impacts the properties of the wing, particularly the impact upon downwash across the span.

Just my musings. Please feel free to rip apart my mental model!
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