15
$\begingroup$

I missed a test question which asked that if an airplane was spinning to the left which wing was stalled. The supposed correct answer was that both wings are stalled (I had answered that the left wing only was stalled). However after looking at the this article on Wikipedia it seems to indicate that only one wing needs to be stalled to spin:

In a normal spin, the wing on the inside of the turn stalls while the outside wing remains flying. It is possible for both wings to stall, but the angle of attack of each wing, and consequently its lift and drag, are different. Either situation causes the aircraft to autorotate toward the stalled wing due to its higher drag and loss of lift.

So my question is was it fair for me to have missed that test question since according to Wikipedia a spin can occur with only one wing stalled?

$\endgroup$
  • 12
    $\begingroup$ I would like to note, since the question is tagged with "faa-knowledge-test", the answer is "yes" because the FAA is like the trivia guy at the Mexican restaurant on Saturday night... whether they're right or they're wrong, they're always right; that's just how the game is played. $\endgroup$ – Ryan Mortensen Apr 5 '19 at 2:39
11
$\begingroup$

No, one wing has at least partially attached flow. How else would there be a rolling and yawing moment which keeps the spin movement alive?

During a spin the aircraft experiences a linear variation in angle of attack over span. The pitch attitude is between 40° and 60° nose-down, and the local angle of attack is 90° minus the pitch angle, which is between 50° and 30°, at the center wing. Move outward from there and the angle of attack increases on the retarding side and decreases on the advancing side.

As a consequence, the outer advancing wing will experience a moderate angle of attack which can even become negative at the tip. Therefore, a sizeable portion of that wing side has attached flow with high lift and low drag. On the other side the angle of attack grows to 90° and beyond, so the wing is fully separated and the aerodynamic force is normal to the wing surface. See below for a diagram of the flow direction: The dark blue vector is from the falling motion and the red vector is the product of the yawing moment $\omega_z$ times the wing station y. Together they combine to the green vector which produces a resulting aerodynamic force R:

flow over a spinning wing

On the left is the retarding wing and on the right the advancing wing. Note that the aerodynamic force is in line with the flow vector on the retarding wing with its fully separated flow while the aerodynamic force is normal to the flow vector due to the attached flow on the advancing wing. The difference in the local forces produces a yawing and rolling moment which balances with the damping forces. If there would not be such an asymmetry, the motion would die down quickly.

Even in a flat spin, where the pitch attitude is around 0° (resulting in 90° angle of attack at the center wing), the advancing side of a moderate to high aspect ratio wing produces some nose thrust from partially attached flow. How else would the aircraft keep spinnig? Low aspect ratio designs produce a propelling nose vortex on the forward fuselage which keeps the motion alive.

| improve this answer | |
$\endgroup$
  • $\begingroup$ So this is kind of like a helicopter rotor retreating blade stall where the retreating side can stall due to higher angle of attack? $\endgroup$ – DLH Apr 4 '19 at 21:59
  • 1
    $\begingroup$ @MikeY: I believe that you are referencing washout which means the root of the wing will stall before the tip and this is used to keep some roll stability in the stall. However I believe the situation Peter presents here is that the tip stalls before the root. $\endgroup$ – DLH Apr 4 '19 at 22:38
  • 2
    $\begingroup$ "How else would there be a rolling and yawing moment which keeps the spin movement alive?" Rhetorical questions are unbecoming, Peter! :) $\endgroup$ – Fattie Apr 4 '19 at 22:51
  • 3
    $\begingroup$ @RyanMortensen: There is a lot of nonsense making the rounds in pilot circles. I have not encountered the "both wings are stalled" myth myself, but how false it is depends on what stalled means. Note that I (indirectly) say that the inner part of the advancing wing is stalled. But what counts is the outer part with the long lateral lever arm, and that is not stalled. $\endgroup$ – Peter Kämpf Apr 5 '19 at 7:09
  • 1
    $\begingroup$ @PeterKämpf "How else would the aircraft keep spinning?" That depends what the pilot is doing with the rudder. A flat spin doesn't mean zero forward airspeed! But you are right, most of this is a debate about terminology, not physics. It may be true (for some definition of "stall" ) that both wings are stalled in a "straight and level" full stall, but if that stall evolves into a spin one wing then becomes unstalled. But from the pilot's point of view, so what? Carrying out the standard recovery procedures don't depend on knowing the answer to that sort of trivia question. $\endgroup$ – alephzero Apr 5 '19 at 10:57
12
$\begingroup$

Yes, both are stalled.

I guess a nit-pick is on "what is stalled"? I adopt that you are at or beyond the point that an increase in AOA results in an increase in lift (critical AOA). That's the top of the blue curve in the plot below.

Also, a stalled wing does not mean every point on the wing has unattached flow. It means the wing is operating at an AOA where an increase in AOA results in a decrease in lift.

At low angles of attack (AOA) planes are naturally stable in roll. The downgoing wing sees a higher AOA which results in more lift, and a restoring force. The upgoing wings sees a lower AOA, and less lift, so it is stabilizing too.

At a high AOA, though, you are operating on the backside of the wing lift diagram. In the picture below, this would be at and beyond 20 degrees AOA.

enter image description here

Now, the upgoing wing sees more lift, which leads to positively reinforcing going up. Same but opposite on the downgoing. It sees less lift.

Also, the red line shows drag. That downgoing wing (on inside of the spin) sees a great increase in drag, which will lead to a yaw towards that wing, i.e., pro-spin.

So to get in the situation where a roll/yaw movement is positively reinforcing, you need to be in stalled AOA. You might start with just one wing, you'll get to both.

JMHO!

EDIT

Here’s a NASA video of a wing with tufts when it is both on the inside of the spin and outside. Stalled in both (but different airflow).

| improve this answer | |
$\endgroup$
  • 3
    $\begingroup$ I think this is a good answer. I've been reading up on spins. While I think that maybe only one wing will stall during the departure phase of the spin, by the time the spin is in the developed stage both wings will be stalled. I think the test question could have been better worded though. $\endgroup$ – DLH Apr 4 '19 at 21:35
  • 3
    $\begingroup$ @DLH I think this is a poor answer because it is wrong. $\endgroup$ – Peter Kämpf Apr 4 '19 at 21:44
  • $\begingroup$ @PeterKämpf: Oh man I wish you had answered sooner, I was persuaded. $\endgroup$ – DLH Apr 4 '19 at 21:53
  • 2
    $\begingroup$ In your answer, you ask, "What is stalled?" Good question. So have you defined what you are considering stalled to mean for the purpose of the answer? Some people will think of stalled to mean something like, "not generating enough lift to oppose the force of weight". The technical answer is exceeding the critical angle of attack. $\endgroup$ – Ryan Mortensen Apr 5 '19 at 2:29
  • 4
    $\begingroup$ @PeterKämpf Why is he wrong the flight test shown in the video clearly shows both wings stalled during the spin. $\endgroup$ – DJ319 Apr 5 '19 at 12:27
4
$\begingroup$

A spin is an autorotation that requires an asymmetric thrust force to sustain. This requires the wing span to be anchored at one end by drag, with the other end developing enough thrust to overcome the (rather weak) stabilizing force of the vertical fin and drive that end forward, rotating the plane. The AOA is highest at the inboard end and decreases as you move outboard due to the higher forward velocity. At some point along the span, the outer end is unstalled or only semi-stalled and is making at least some amount of lift/thrust.

| improve this answer | |
$\endgroup$
0
$\begingroup$

This "test" question may apply to a certain type of aircraft and spin procedure (Cessna 172) that has to be stalled straightforward (there for both wings stalled), followed by the "wrong" inputs (rudder into spin, ailerons away) to make it spin. The important concept is that differences in drag and lift between the 2 wings, whether one is stalled or not, keeps the plane in a self sustaining yaw/slip.

Important is the role of the V stab/rudder in maintaining or ending the spin. Looking at a simple cup shaped anomometer helps visualize the effects of pro-spin rudder into the spin and anti-spin rudder away. Stopping yaw with opposite rudder is key to breaking a spin, and controlling yaw is key to not entering one.

Also key is how uncoordinated aileron input, trying to roll away from a turn, can cause a spin. (The "inside" slower wing is now compounded with the down aileron creating a higher AOA). Although aileron roll effect can reverse in the stall AOA regime, rudder will not. But this also means that applying opposite ailerons in a spin can be explored! (Qualified instructor recommended).

But for the 172, just letting go of the yoke, power to idle, and opposite rudder would break the spin if CG was correct.

Every plane and situation is different (as seen by these answers), it is advisable to find out how your plane handles and how to control it, no matter what the "correct" test answer is.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.