Explaining Induced Drag??
Joined: Sep 1998
Posts: 1,615
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From: wherever
Pipertommy,
To make the lift Bernoulli accelerated the airflow and coanda has thrown it downwards a little in response Newton has done the equal and opposite to us, The air behind the wing is a little lower than that in front so the blue line is the free stream air relative to us but the green is the air that we have influenced. The angular difference is the effect we have had on the air and that can be expressed as lift normal to the free stream and drag parallel to it.
To make the lift Bernoulli accelerated the airflow and coanda has thrown it downwards a little in response Newton has done the equal and opposite to us, The air behind the wing is a little lower than that in front so the blue line is the free stream air relative to us but the green is the air that we have influenced. The angular difference is the effect we have had on the air and that can be expressed as lift normal to the free stream and drag parallel to it.
Joined: Jun 2009
Posts: 1,344
Likes: 80
From: Bedford, UK
Posted this on another thread....
Your question started me wondering too. The explanations on the web aren't very convincing. The idea that wing tip vortices are responsible sounds a bit odd and the argument that the efficiency of longer wings proves it, would require the effects of wing loading (reduced circulation around the wing) to be considered.
The rotation of the total force vector implies that the axial force increases more quickly than the normal force but for an inviscid fluid I am unclear as to the physical source of the axial force given that pressure can only act normal to the surface. So the origin of induced drag relies on the viscosity of the fluid (well that it is not zero at least).
Perhaps using momentum theory where the force on the wing is the reaction to the force applied to deflect and slow the air, the separation of the boundary layer at the trailing edge, and the path this opens for air at the edge to be less deflected as it moves upward.
Wouldn't buy the wing tip theory except as an extra inefficiency but if that's the right book answer...........
Your question started me wondering too. The explanations on the web aren't very convincing. The idea that wing tip vortices are responsible sounds a bit odd and the argument that the efficiency of longer wings proves it, would require the effects of wing loading (reduced circulation around the wing) to be considered.
The rotation of the total force vector implies that the axial force increases more quickly than the normal force but for an inviscid fluid I am unclear as to the physical source of the axial force given that pressure can only act normal to the surface. So the origin of induced drag relies on the viscosity of the fluid (well that it is not zero at least).
Perhaps using momentum theory where the force on the wing is the reaction to the force applied to deflect and slow the air, the separation of the boundary layer at the trailing edge, and the path this opens for air at the edge to be less deflected as it moves upward.
Wouldn't buy the wing tip theory except as an extra inefficiency but if that's the right book answer...........
Joined: Mar 2005
Posts: 1,338
Likes: 1
From: Uh... Where was I?
I think it is all related: wingtip (finite wing and subsequent "leakage"), vortex, induced drag.
We can consider induced drag as an inefficiency, just as friction drag is, only it occurs when there is lift. In fact, it is similar to form drag (indeed due to viscosity). Form drag is related to "eddies". The wingtip vortex is a big eddie.
If we could paint a vortex for just a couple of seconds (not just the core but the whole vortex) would we see it moving forward?
Kutta and Koukowski play in the Manchester United or in teh CIty?
We can consider induced drag as an inefficiency, just as friction drag is, only it occurs when there is lift. In fact, it is similar to form drag (indeed due to viscosity). Form drag is related to "eddies". The wingtip vortex is a big eddie.
If we could paint a vortex for just a couple of seconds (not just the core but the whole vortex) would we see it moving forward?
Kutta and Koukowski play in the Manchester United or in teh CIty?
Joined: Feb 2009
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From: Virginia
Things kind of break down for me when I think about what would happen if it were somehow possible to build a wing with infinite aspect ratio. Induced drag would be zero. But would that mean that drag was independent of lift?
No. To get greater lift at a given airspeed (for example because of a higher total weight), you'd need a higher angle of attack. A higher angle of attack means more parasitic drag. Because there's no induced drag, I guess you'd say that the higher drag was caused by the higher angle of attack, not the higher lift. But can you really separate the two?
No. To get greater lift at a given airspeed (for example because of a higher total weight), you'd need a higher angle of attack. A higher angle of attack means more parasitic drag. Because there's no induced drag, I guess you'd say that the higher drag was caused by the higher angle of attack, not the higher lift. But can you really separate the two?
Joined: Mar 2005
Posts: 1,338
Likes: 1
From: Uh... Where was I?
I know what you mean
The downwash is not due to the vortex. If there is lift there has to be downwash, according to Newton. If there is angle of attack, then the air is pushed down and the wing pushed up.
In an infinite wing there would be downwash but no vortex.
If the wing is finite some air leaks from lower surface to upper surface, thus creating the spanwise flow and the vortex. The less the aspect ratio, the more the spanwise flow, hence the more intense the vortex is. (less aspect ratio means that a larger proportion of the air around the wing will leak and create vortex).
The downwash is not due to the vortex. If there is lift there has to be downwash, according to Newton. If there is angle of attack, then the air is pushed down and the wing pushed up.
In an infinite wing there would be downwash but no vortex.
If the wing is finite some air leaks from lower surface to upper surface, thus creating the spanwise flow and the vortex. The less the aspect ratio, the more the spanwise flow, hence the more intense the vortex is. (less aspect ratio means that a larger proportion of the air around the wing will leak and create vortex).
