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7 Answers

Why does induced drag increase in a spin entry?

Asked by: 404 views Aerodynamics

The explanation I've heard for why rotation begins in a spin is that even after a stall, induced drag will continue to increase with angle of attack. But this doesn't make sense. Induced drag is proportional to angle of attack because angle of attack causes the pressure differential between the upper and lower surfaces of the wing. But in a stall, the boundary layer separates which increases pressure above the wing, which reduces the pressure differential. So it seems like induced drag should also decrease. Where am I going wrong here?

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7 Answers

  1. Brian on Mar 21, 2017

    Induced drag changes anytime a rolling motion occurs. The wing going downward will experience an upward component of relative wind that increases it’s angle off attack. In normal flight this simply results in dampening the rolling moment produced. However, if the wing is stalled when it begins this downward motion then this upward component of relative wind causes the wing to go deeper into stall, increasing the rolling motion, and repeating the process.

    If you search through Aerodynamics For Naval Aviators on spin dynamics you will find a 2-3 page explanation of this. The short answer is that induce drag changes anytime a rolling motion occurs; in normal flight it dampens the roll and in stalled flight it works with the roll, making it worse and resulting in a spin.

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  2. Brian on Mar 21, 2017

    Should have read the rest of your question so I wouldn’t need two answer; sorry!

    Induced drag proportional to the lift coefficient. The Cl curve peaks and falls off, as you’ve seen in many graphical representations I’m sure. However, it can also peak again at a point much higher than the first peak. In other words, it would depend on how deeply stalled you were as to whether or not the induced drag would be greater than it were when the spin was first entered.

    I’m curious, where did you read that induced drag increases, as a whole, on spin entry? When analyzing a spin it is more or less useless to discuss the aircraft as a whole since it is the relationship between the wings that results in the spin developing.

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  3. rmdomeni on Mar 21, 2017

    Thanks for the help but I am looking for a little more understanding. I’ll go a bit more in depth. In a spin entry, the wing that is more deeply stalled looses more airspeed than the less stalled wing, which causes the beginning of the rotation. The explanation for this in the PHAK as well as the airplane flying handbook and aerodynamics for naval aviators is that the more deeply stalled wing developes more drag. Granted it doesn’t specifically say induced drag, but since parasite is directly proportional to airspeed, it would follow logically that a wing with LESS airspeed would not see an increase in Dp. So it must be an increase in Di. This induced drag results from downwash and wing tip vortices, which are both the result of the differential pressure between the upper and lower surfaces of the wing. When the boundary layer separates, it is replaced by turbulent air which increases pressure above the wing (according to naval aviators). This decreases the pressure differential, which should cause Di to decrease. So it seems to me the more stalled wing should create less induced drag, and therefore less total drag.
    Also, what is the reason for this 2nd peak in the coefficient of lift curve? What causes it? Are you saying that this second peak and its proportional rise in Di is what causes the spin entry?

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  4. Brian on Mar 22, 2017

    Let’s focus on my first reply, but first evaluate the question. You ask, “why does induced drag increase in a spin entry.” Your discussion that follows focuses on the effects on an aircraft, as a whole, when transitioning from unstalled to stalled flight. You are correct, a stalled aircraft will experience a reduction in induced drag. Since Di is directly proportional to CL, a reduction in CL will result in an overal reduction in Di.

    For now, please ignore my mention of the CL curve’s potential for a second, higher, peak during deep stall. It is beyond the scope of your initial question and truthfully it’s good for jeapordy, but pointless in practicality.

    Let’s move on to your last post where you expand by saying, “The explanation for this … is that the more deeply stalled wing…” Notice our focus here is on the difference between each wing, which is what we should focus on. What your question should be is, “how does induced drag differ between each wing during a spin entry?” This is answered in my first reply.

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  5. rmdomeni on Mar 22, 2017

    Here’s why I basically repeated my question but made it longer. Your original answer makes the assertion that “induced drag changes any time a rolling moment occurs.” This is another way of saying induced drag varies directly with angle of attack. I agree. My point is that these two factors vary proportionally because of the pressure differential the AOA creates, and in the stall, that pressure differential is mostly gone. My question is why this direct relationship would continue after the stall. And I am talking about the wings. The diagram always shows the induced drag increasing on the more deeply stalled wing. This is my concern.
    Put another way, you’ve rephrased a brute fact about aerodynamics. I know this fact. I’m looking to move from rote knowledge to understanding. WHY does induced drag increase ANY time a rolling moment occurs? WHY does this relationship continue into a stall, above the critical AOA, when the factor that caused the relationship is mostly gone?

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  6. Brian on Mar 22, 2017

    I miss spoke with this statement: “Since Di is directly proportional to CL, a reduction in CL will result in an overal reduction in Di.”

    It would seem from the plots on page 310 of AFNA that the relationship between CL and Di differ in stalled vs high speed flight. The beginning of page 309 explains figure 4.32. Here is a link to the PDF: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

    Let me know if that helps make sense of it.

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  7. rmdomeni on Mar 22, 2017

    Unfortunately it simply presents the fact the drag continues to increase as Cl falls off. From looking at some stuff on nasa.gov I found, “However, once the wing stalls, the flow becomes highly unsteady and the value of the drag changes rapidly with time. Because it is so hard to measure such flow conditions, engineers usually leave the plot blank beyond wing stall.”
    So perhaps it’s all conjecture.
    I am still interested in why there is a second peak in the Cl curve. If you have a source for this, it might offer some clues to my original question too. Who knows. Thanks for the help anyway.

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