Fear of flying Bernoulli-Style
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Several things are of importance.
(a) any discussion must address the reality of what is observed in flight, windtunnels, or wherever
(b) if one looks at windtunnel smoketrail traces, there is a clear momentum (mass flow if you prefer) change associated with the flow downwash. The force balance (Newton if you like) provides the vertical entity we term lift. If you like the idea of throwing air down to get lift, then that is fine.
(c) however, if one considers how this momentum change is going to transfer a force to the aircraft, one has to consider how forces are transferred in any fluid flow - and that is by the effect of pressure differences acting on surfaces of the aircraft. Hence we measure the pressure patterns at the aircraft surface by means of pressure tappings and do some sums to figure out how much force and moment gets involved. In this activity we look at the Euler or Bernoulli relationships to predict numbers.
Trailing vortex flow is conveniently visualised using the ideas of spanwise tip flows.
Flow separation is conveniently described in terms of pressure gradient problems.
For the needs of pilots, the pressure pattern approach is most useful.
If you like pretty pictures of pressure tapping plots to explain lift and drag forces, then fine.
(d) Vortex flow exists. We can see starting and trailing vortices. If a whirling cylinder is trialled in the windtunnel, then a very significant lift force is observed. These observations lead to the ideas of circulation theory which are of use to the aerodynamicist's calculations if not the pilot's immediate needs.
(e) These approaches are just different ways of looking at what is happening - use whichever is most convenient and/or appropriate for your needs at the time.
(f) impact considerations are fine for waterskis on the TOP of the fluid (water) under consideration. However an aircraft is immersed in the fluid - not planing on top of it. In addition, smoketrace observations clearly show that the real world fluid flow is not similar at all to a planing waterski - and, in any case, at the low densities of air, the forces predicted by particle impact calculations are far too small to account for observed forces.
(g) a lot of people get confused by talk of pressures. The various plots you might see are talking in terms of gauge pressures, not absolute, so that when one speaks of a negative pressure or vacuum, one means a slight reduction from ambient. Similarly, a positive pressure means a slight increase above ambient. We are talking about quite small pressure differences overall.
(h) windtunnels are not mystical entities from the world of JRR Tolkien. There are some things which have to be taken into account, such as wall errors and similitude effects, but the flow in the wind tunnel pretty much matches the flow in the real world.
Have I confused the issue ?
(a) any discussion must address the reality of what is observed in flight, windtunnels, or wherever
(b) if one looks at windtunnel smoketrail traces, there is a clear momentum (mass flow if you prefer) change associated with the flow downwash. The force balance (Newton if you like) provides the vertical entity we term lift. If you like the idea of throwing air down to get lift, then that is fine.
(c) however, if one considers how this momentum change is going to transfer a force to the aircraft, one has to consider how forces are transferred in any fluid flow - and that is by the effect of pressure differences acting on surfaces of the aircraft. Hence we measure the pressure patterns at the aircraft surface by means of pressure tappings and do some sums to figure out how much force and moment gets involved. In this activity we look at the Euler or Bernoulli relationships to predict numbers.
Trailing vortex flow is conveniently visualised using the ideas of spanwise tip flows.
Flow separation is conveniently described in terms of pressure gradient problems.
For the needs of pilots, the pressure pattern approach is most useful.
If you like pretty pictures of pressure tapping plots to explain lift and drag forces, then fine.
(d) Vortex flow exists. We can see starting and trailing vortices. If a whirling cylinder is trialled in the windtunnel, then a very significant lift force is observed. These observations lead to the ideas of circulation theory which are of use to the aerodynamicist's calculations if not the pilot's immediate needs.
(e) These approaches are just different ways of looking at what is happening - use whichever is most convenient and/or appropriate for your needs at the time.
(f) impact considerations are fine for waterskis on the TOP of the fluid (water) under consideration. However an aircraft is immersed in the fluid - not planing on top of it. In addition, smoketrace observations clearly show that the real world fluid flow is not similar at all to a planing waterski - and, in any case, at the low densities of air, the forces predicted by particle impact calculations are far too small to account for observed forces.
(g) a lot of people get confused by talk of pressures. The various plots you might see are talking in terms of gauge pressures, not absolute, so that when one speaks of a negative pressure or vacuum, one means a slight reduction from ambient. Similarly, a positive pressure means a slight increase above ambient. We are talking about quite small pressure differences overall.
(h) windtunnels are not mystical entities from the world of JRR Tolkien. There are some things which have to be taken into account, such as wall errors and similitude effects, but the flow in the wind tunnel pretty much matches the flow in the real world.
Have I confused the issue ?
