If induced drag is perpendicular to the wing, then…

5 Answers

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   Kris Kortokrax on Oct 31, 2015

   Where did you get the idea that induced drag acts perpendicular to the wing?

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   Drew on Oct 31, 2015

   Oh, I typed that wrong. I meant if total lift is produced perpendicular to the wing.

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   Brian on Oct 31, 2015

   Your question asks what appear to be two scenarios. One, a decent and the other a 90 degree nose dive. The answers are two fold, but first let me say that anytime lift is being produced there is a vector component parallel too and opposite the flight path; induced drag.

   In a decent lift is still being produced, so induced drag still exists.

   In a perfect 90 degree vertical decent, which by the way is the same as a perfect 90 degree up line, the pilot must hold the AoA that gives him/her zero lift. If the pilot does not the lift produced will cause the aircraft to deviate from its 90 degree vertical path. Kris can probably better define this as I think he’s got quite a bit of aerobatic/combat type experience. The long and short of it is in this scenario no lift is being produced so no drag is produced.

   Reminder – lift is defined as a force that acts perpendicular the flight path.

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   Drew on Oct 31, 2015

   I incorrectly referred to the lift and drag vectors in the above question in the sense that lift is always a vector pointing straight up from the earth and drag as a vector pointing parallel the earth (opposite the flight path).

   In level flight, the wing is inclined at some AoA, so the total lift vector produces the upward lift vector and the rearward vector, which is induced drag. In a descent–let’s say the wing is perfectly parallel the earth (is this a normal descent scenario? please let me know)–however, there is still some AoA since the flight path is angled toward the ground and the relative wind is hitting under the wing. To make it simple, let’s say whatever AoA the wing was at in cruise is the same AoA in the descent–basically, just tilt the wing and flight path together until the wing is parallel the earth. In this scenario, the total lift vector could be broken up into a forward thrust vector and a vertical lift vector. A portion of the induced drag vector would add to the vertical lift vector.

   I was thinking it was interesting how producing induced drag would actually increase the vertical lift vector and decrease the drag vector, but I didn’t have my lift and drag definitions correct. I just wanted to see if the above was true. Thank you.

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   Brian on Nov 04, 2015

   It depends on how you wish to define normal. I opt to follow a pretty uniformly agreed upon norm, you will find it published in Aerodynamics for Naval Aviators (AFNA) and many engineering texts on the subject. That standard is that the four forces can be assumed equal whenever pitch attitude is at or less than 15 degrees nose up or down.

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   As for the rest of your description you seem to have a solid grasp on what you\’re trying to say, but your definitions and use of terminology is shaky. There are two common methods of vector analysis in this field:

   The first is a fixed coordinate system, which is less common. However in the pilot world it seems more common because the core FAA texts use this system in their definitions. The definition of the lift vector in this system is: Lift is the sum of all upward acting forces. The problem with this method is it that, while making analysis simpler, it makes a full 3d analysis quite cumbersome.

   For this reason most analytical approaches tend to stick with a moving coordinate system. In this case the axis of the aircraft make up the coordinate system and move with the aircraft. Lift is then defined as the force acting perpendicular to flight path. This is the definition I used in my first response to you.

   The point of all this is not to say any one part of your thought process is wrong. Instead I aim to encourage you to ensure you have a clear definition of the words you\’re using to describe these conditions. One of the largest errors in this area is lack of concrete definitions.

   —

   On an aside, \”total lift\” is purely an FAA made up word to describe \”Aerodynamic Force,\” which is defined as the force vector created by an airfoil. AF produces both an upward and reward component as you described.

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