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Given the similarities between Lift and drag and their very similar relationships to the density, surface area, and airflow velocity, could one, in theory, describe aerodynamic lift as a kind of drag where most of the force from lift is just directed in the vertical direction where this vertical component of the force opposes gravity?

Romeo_4808N
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6 Answers6

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An 'aerodynamic force' (just one force...) appears when a body is immersed in a fluid stream. By convention, two components are chosen, one of them parallel to the stream direction, called 'drag', and the other one, perpendicular to that 'drag' is termed 'lift'.

xxavier
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At least as the terms are normally used, lift is not drag, and drag is not lift.

What I suspect you're looking for is: "induced drag", which is the part of the drag that's caused by an airfoil generating lift.

Induced drag is opposed to "parasitic drag", which includes things like friction drag (friction between the airfoil's skin and the air) and interference drag (e.g., there's some extra drag where the wing mounts to the fuselage) and so on.

Jerry Coffin
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Using the wing as a frame of reference, then the aerodynamic force is related to diversion (change in direction) of the relative flow. In a normal situation, the relative flow in front of the wing is horizontal, and the wing curves the flow downwards a bit. Assuming an ideal case where the flow speed isn't changed, then the horizontal component of the initial velocity is decreased somewhat (induced drag), and the vertical component is increased from zero to some non-zero value (lift). In a real world situation, the flow speed is reduced due to friction, viscosity, profile drag, ..., which further reduces the horizontal component of initial velocity (other components of drag).

Using the air as a frame of reference, drag is related to the forward acceleration of air, while lift is related to the vertical acceleration of air. Consider the case of a bus on a highway, the air is accelerated forwards, most of this due to the low pressure area behind the bus, some due to the air being pushed forward and around the front of the bus, while there is zero lift.

On a side note, lift calculations are simpler than drag calculations because lift calculations can be based on flow field calculations that calculate speeds (relative to the wing) and the coexistent pressures related to those speeds. Drag calculations are more complicated because they need to take turbulent and boundary layer effects into account.

rcgldr
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One might ask what application would considering lift as a type of drag have? They are compared as L/D ratio as a parameter of design performance, however, especially as pilots, knowing the distinction may be extremely important as they apply to flight.

The amount of lift one can generate is dependent on velocity (for a given aircraft). Why this is critically important is that velocity is the kinetic energy state, which is created by adding energy to the system from the thrust. A little like filling a barrel. One spigot is altitude (potential energy) that you can pour back in as needed. The other is drag. Thrust keeps filling the barrel, but it must be filled to a certain minimum level or the plane does not have enough velocity to fly.

Empty the barrel too fast (steep climb), you can grab your potential energy (altitude) and pour it back in to get flying speed back. The thrust can only be added at a certain rate, as we know on our take off roll.

Drag finds application as how much energy is being taken out (as related to fuel consumption or max velocity) and can be compared with lift for a given configuration, AoA, airfoil type, etc.in engineering theory, but as far as piloting, not related IMHO.

Bob

Robert DiGiovanni
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Maybe the best answer here is that, while lift and drag are separate quantities, their behavior is governed by the same physics ie conservation of momentum and Newton’s Third Law of Motion.

Romeo_4808N
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    Physics laws are universal, but lift and drag are 'separate quantities' only by convention. The aerodynamic force is decomposed in lift and drag just because it's useful to do it so. Other de-compositions are possible, if probably not as useful, but that is the only 'reality' of lift and drag, an instrumental reality... – xxavier Sep 08 '18 at 16:03
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I think that's the wrong way to look at it. The lift force is 90 degrees to the airstream. When you're gliding, the lift force is producing thrust, not drag, or else you would stop dead and fall from the sky.

I think of the lift-related drag component as the energy consumed in inducing the air to move from point A to point B, creating the pressure differentials and down thrust from action/reaction, as the wing passes through it.

John K
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    No a component of the gravitational force acts as thrust in a glide, not lift. – Romeo_4808N Sep 07 '18 at 13:27
  • But the gravitational force is straight down, not forward. – John K Sep 07 '18 at 14:28
  • In a glide, a component of the gravitational force now acts parallel to and opposite the direction of, the relative wind, causing the airplane to move forward. – Romeo_4808N Sep 07 '18 at 15:12
  • So when a helicopter tilts its rotor and angles its lift vector forward, the forward thrust component is generated by gravity? – John K Sep 07 '18 at 15:39
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    No. That’s not the same thing. You need to do a free body diagram of that taking into account the relative wind as well. Both Lift and thrust there are being generated by engine power. Now if the helicopter loses engine power and begins an autorotative descent, then the forward thrust is being provided by gravity. – Romeo_4808N Sep 07 '18 at 15:49
  • A forces & velocities diagram of a gliding condition here https://imgur.com/DUJqHqo – xxavier Sep 07 '18 at 16:50
  • They're actually just opposite sides of the same coin. This is like arguing how many angels fit on the head of a pin. – John K Sep 07 '18 at 17:17
  • No it is not. By definition lift cannot point forward as thrust (lift must be at 90deg to flight path). Claiming otherwise only disseminates confusion in this world of my-ignorance-is-just-as-good-as-your-phd – Radu094 Sep 07 '18 at 19:47
  • Sooo.. if lift is 90 deg to the flight path, and the flight path gradient is down, say 10 degrees, as in a glide, the lift vector isn't tilted forward 10 degrees and doesn't this tilt represent a forward component? – John K Sep 07 '18 at 21:35
  • i can see why this is disorienting: if the flight path is down 10 deg, then yes: the lift is also tilted 10deg. It is not pointing “forward” (even though it is now skewed 10 deg to the left) because your “forward” is now not left, but downwards-left... therefore NONE of the lift is contributing to moving you “downwards-left”. Mind you, the gravity has NOT been tilted 10 deg, so now a part of it IS actually pulling you “forwards” ... as it is pulling you “down”( which is a part of “downwards-left” that is your flight path) ... ok, maybe this is not clearer: just draw a picture – Radu094 Sep 08 '18 at 11:12
  • better yet, use the picture from Pilohead’s (also wrong) answer. See how none of the L is pulling you towards V?(they are perpendicular) But some of the W is pulling towards D? That picture is for a climb, where weight adds on to the Drag to pull you backwards. In a descent, part of the weight (W sin y) will add forward with thrust (if any) to pull you “forward” from the plane’s point of view – Radu094 Sep 08 '18 at 13:56
  • Ok I get it. Thanks. – John K Sep 09 '18 at 00:11
  • Why don't you ask a question? – Muze Apr 28 '19 at 03:14