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If I were standing outside on a windy day holding a flat rectangular object such as a food tray up in the wind, orthogonal to the direction of flow, the drag created would be classified, mostly, as form drag; a differential in pressure from the higher pressure upwind side of the tray and its lower pressure downwind side.

If an airplane's wing began at a 0 degree angle of attack and were slowly rotated to a 90 degree angle of attack, lift would increase until the critical angle of attack is exceeded, but the coefficient of drag would continue to increase up to 90 degrees.

I assume the drag created by the wing at 90 degrees is much like the form drag created by the tray in the wind.

Is induced drag nothing more than form drag with a different classification due to its usual nature of discussion in regards to lift?

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No, by definition it isn't.

There is drag. Drag is caused by different physical phenomena. According to the cause, it is classified to:

Induced drag

Induced drag is side-effect of generating lift over finite wing span.

Lift is an upward force that the air exerts on the wing. By principle of action and reaction (Newton's third law of motion), the wing exerts downward force of equal magnitude on the air, and since the air is free to move, that force accelerates the air downwards.

Now there are two equivalent arguments why this causes drag:

  • In the frame of reference of the wing, the wing is not doing any work (because it is not moving), so it can't change kinetic energy of the air, and therefore can't change its total velocity. Since the vertical component of that velocity increases, the horizontal component has to decrease and this requires forward force from the wing. The backward reaction to that force is the induced drag.

  • In the frame of reference of the oncoming air, the kinetic energy does increase. Therefore the wing has to do some work on the air, which means applying force in direction of its motion. The backward reaction to that force is the induced drag.

The explanations are both equally true, just using different frame of reference.

Induced drag decreases with (square of) speed (for constant lift), because at higher speed there is more air to accelerate, so it only needs to be accelerated by less.

Induced drag is also independent of cross-section. If you cancel lift by moving elevator or aileron up, the cross-section may not change at all (the flap might still be behind the wing), but the induced drag will vanish.

By the way, it follows from this that induced drag of wing at 90 degrees is zero, because it is not producing any lift. The form drag is, of course, huge.

Form drag

Form drag is caused by imperfect pressure recovery behind the body. At the front of the object moving through fluid the fluid slows down, which comes with increase of pressure. In ideal laminar flow the fluid closes behind the object with the same slow speed and same high pressure. But as the object moves faster, the inertia causes the flow to separate and form a turbulent area behind the object where the pressure remains low. This pressure difference cause the form drag.

The form drag depends on cross-section and shape of the object. It always increases with speed.

Interference drag

When complex structure, like an aircraft, is analysed, the form drag of each component (wing, fuselage, empennage etc.) is first analysed independently. But as the components are assembled together, the pressure fields around the parts negatively affect each other and this increase in drag is then called interference drag.

Skin drag

Skin drag is caused by the friction between the surface of the object and the fluid. The skin drag depends mainly on wetted surface of the object. It increases with speed in similar way to the form drag, so for practical purposes it is usually combined with the form drag into one term, the parasitic drag.

Wave drag

As the object moving through fluid approaches the speed of sound, the flow exceeds it as it tries to avoid the object and forms a shock wave. Behind the shock wave the pressure is much higher and this increase is not balanced by any increase on the aft part of the object, therefore causing large amount of drag.

This kind of drag only appears as the flow speed approaches speed of sound, but then it quickly grows larger than the other forms.

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  • $\begingroup$ Induced drag is different as it decreases with speed, but so does the cross-section of the object striking the air and creating the drag. I wonder: if a specific angle of attack were forced to remain constant upon a wing in a controlled setting, would the drag not increase with speed? If it would, that would also lead me to think it is just a type of form drag. $\endgroup$ – Ryan Mortensen May 30 '16 at 15:25
  • $\begingroup$ @RyanMortensen, if you kept the same angle of attack and increased speed, the lift would increase and the induced drag would increase with it. But e.g. by changing camber, you can keep the cross-section the same while changing induced drag significantly. But ultimately, the argument is that the definition is different. I have rewritten the answer to include more detailed definition of the other forms. $\endgroup$ – Jan Hudec May 30 '16 at 18:30
  • $\begingroup$ Your paragraph on interference drag is absolutely the best, clear, concise explanation I have ever read for it. I really like your answer the way it is written now, but I will give some time for others before concluding the Q&A process. $\endgroup$ – Ryan Mortensen May 30 '16 at 18:42
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    $\begingroup$ Your two bullet points are really confusing me. The way I've come to understand it is that the vector of the resultant force (the complete aerodynamic force on the wing due to AoA) is not perpendicular to the airflow but angled slightly backward because the air is being forced downward and forward. If you break the vector angle down into vertical and horizontal components (with respect to the airflow) the vertical is lift and the horizontal is induced drag. We might be saying the same thing, just different ways, but I'm not grasping your explanation. $\endgroup$ – TomMcW Jun 4 '16 at 20:46
  • $\begingroup$ @TomMcW, well, the force is angled, but you need to know why it is angled. There might be some direct argument from the lift theory, but that is rather complicated. However, simple argument can be made from just laws of motion and conservation of energy and that is what I am trying to do here (conservation of energy is a funny physical law; it holds in all reference frames, but the energies look very different in each, so we have different, equally valid, descriptions depending on which reference frame we use). $\endgroup$ – Jan Hudec Jun 6 '16 at 13:06
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Induced drag is really a way aerodynamicists account for the local flow direction change (net downward) in the immediate proximity of a finite wing with sharp trailing edge as it moves thru a fluid at a positive angle of attack. Induced drag is a term that is convenient as it implies a drag-due-to-lift origin and it does contribute to the total force buildup opposing motion. But induced drag does not arise as a surface normal force nor skin traction force. It is thus unlike form drag, skin friction' and excrescence drag which arise due to viscous shearing interaction between fluid and wing surface. These drag forces, measurable in the wind tunnel, act on the wing skin as either a pressure-area normal force or tangential traction forces. Form drag requires a robust internal structure to resist it; skin friction peels paint; and excrescence drag requires strong mounts for antennae and other protuberances. Induced drag is not measured in the wind tunnel; it is calculated after the fact. It and does not require structural accommodation to resist a force because it is not a force.

Of what physical nature, then, is it? It is actually the result of local distortion of the horizontal-vertical rectilinear reference framework that engineers have adopted for force accounting in airplane performance work. By acclaim, the reference used to account for motion-retarding effects is parallel to the velocity vector at infinity, and all lifting effects are reconciled perpendicular to that vector. Because of the downward flow component at the wing itself, the actual local velocity vector is inclined at a slight downward angle to the vector at infinity; the lift vector which is always normal to the actual local airflow, is thus angled backwards at the same angle. When this 'bent-backwards' lift vector is reconciled into the force reference system (velocity st infinity), there will thus be a component of lift (its sine) aligned in the drag direction. This we call induced drag.

It is important to understand the physics of this conveniently-named, but mis-informing term, to grasp what it means to airplane design and operation.

An English physicist-engineer-mathematician named Lanchester figured this out and published it in his textbook, 'Aerodonetics', before the Wright Brothers even flew at Kitty Hawk.

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In short, no. Induced drag is a byproduct of moving an airfoil through a fluid to create lift. It is the sum of both the form drag of the airfoil as well as the unit vector of the lift line opposing the thrust vector of the propellor or jet exhaust.

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