Spins, Flat Spins, and Stalls


The field below us was spinning like Auntie Em’s house in the Wizard of Oz. The Cessna 152 spun in an almost vertical, nose-not-quite-straight-down attitude. By sitting up in our seats we could see the point that the world was spinning around. We had finished our third or fourth rotation when that point started to disappear below the engine nacelle. Not good. That could only mean one thing.

Our spin was starting to flatten. I would like to say that I calmly applied full right rudder, forward elevator, right aileron and full power, knowing that it would stop the spin. But the truth is that I stomped, slammed and hoped. I had never been in a flat spin. We spun for another two rotations before we recovered. We never completely ‘flattened out.’ I seriously doubt that I would be writing this if we had.

I had done spin demonstrations for students in Cessna 150's several times. What was different?

Airplanes are designed for the air to flow smoothly across its control surfaces. Since an airplane cannot spin without first stalling, we know that in a tailspin, the wings aren’t ‘flying’ any more. So the air is not flowing smoothly across the wings or the ailerons. That means that the ailerons have marginal effectiveness; but they do have some, albeit weak, influence on the aircraft’s behavior.

With the nose pointed almost straight down and the airplane going straight down, the horizontal stabilizer and the vertical stabilizer are well below their critical angles of attack*. The wind blows over the tail the way that it is supposed to. That means that it still works. As a result, the rudder and elevator can still exert substantial force on the airplane.

When a spin flattens out, the vertical part of the tail is moving sideways through the air and the wind is blowing over the horizontal surfaces edge-on rather than from front to back rendering both yaw and pitch controls ineffective.

With the wings and vertical tail surface exceeding their critical angles of attack and the wind blowing sideways across the horizontal tail, there aren’t many flight controls left for the pilot.

Many airplanes are unrecoverable from flat spins.

Perhaps I have convinced you that it is worth understanding what happens when any aerodynamic surface (wing, vertical or horizontal tail) exceeds its critical angle of attack. I would like to use this example as a way of understanding boundary layer separation.

In previous articles, we discussed the effects of wind changing direction; how centrifugal force could either raise or lower the pressure on the surface of a wing. When the wind follows a wing’s curve, it lowers the pressure on the wing and when it is forced away from the wing it raises the pressure on the surface. Since centrifugal force is proportional to the velocity squared, just a 10% increase in velocity results in a 21% increase in lift. Finally, since the wind travels over the top of the wing much faster than below it, over two thirds of the lift is generated by the top of the wing.

Now let me generalize a bit. These principles apply to air moving past any surface whether that surface was designed to generate lift continuously, such as wings and propellers, or only occasionally such as the horizontal and vertical surfaces of the tail.
What brought us close to being killed was that those principles apply equally to all surfaces, even those that were never intended to be lift-producing surfaces – in this case, a nacelle.

Before I explain how the nacelle started producing lift, it is important to understand why a surface quits producing lift. I had mentioned that when a wing exceeds its critical angle of attack, that its boundary layer separates from the wing and the wing quits producing lift. This, of course, is true of any surface regardless of what the aeronautical engineer had in mind.

Atmospheric pressure holds air against the wing forcing it to follow its curve. Centrifugal force tries to pull it away from the top and pushes it against the bottom. As the angle of attack increases, the pressure on the top decreases and the pressure on the bottom increases. Eventually, the difference between the air pressure above the wing and below it is so great that atmospheric pressure is too weak to overcome the differential and the layer of air flowing past the upper surface is peeled away from the wing starting at the trailing edge and very rapidly – almost instantaneously – away from most of the upper surface. This is the moment when everything starts to unravel!

The wing starts to experience the infamous stall. High pressure air curls around the trailing edge of the wing, moving forward filling in the low pressure area. Now the air in contact with the upper surface is moving along with the airplane. No relative velocity means no centrifugal force; no centrifugal force means no lift (in this case on the upper surface.) The wind is still pushing against the bottom of the wing, so it only loses about two-thirds of its lift even though it feels like free fall.

Airplanes are designed so that the horizontal stabilizer and its elevator reach their critical angle of attack after the wing. Airplanes pitch down when they stall because the wings lose most of their lift but the tail is still doing its job. (Can you imagine what would happen to an airplane if the tail stalled first?)

One last idea to ponder then I’ll bet you will figure out why I had a different experience in a C150 than I did in a C152.

If you were to put a short fence along the top of a wing at its thickest point, you would prevent the air from flowing over the rest of the wing. This would immediately destroy the lift on the top of the wing and generate a tremendous amount of turbulence and consequently, drag. This device I just described would be called a ‘spoiler’. (Virtually all modern jets have spoilers that they can deploy when they want more drag and less lift.)
Here is my explanation of why I encountered the tendency to flatten out in a C152 but not in a C150. (In all fairness to you and to Cessna, I should say this is just my firmly held belief and not the result of any rigorous testing.) First, the facts.

A C152 has an extension of it vertical stabilizer that extends forward from what I would call the ‘real’ vertical stabilizer. You can see it in the diagram on the first page. It isn’t very tall, but it doesn’t have to be. The C150 does not have this extension.

Recall that in a flat spin, the airplane is in a near horizontal attitude, descending vertically and spinning around its center of gravity. (The CG in a C150 and a C152 is between the pilot and his passenger.) One wing is moving forward, the other backward, the wind is blowing over the nacelle in one direction and over the fuselage in the other.
Here is the difference: The C152 has the little extension of the vertical stabilizer that acts like a spoiler on the top of the fuselage and the C150 doesn’t. The C150’s nacelle generates lift but the fuselage, being even farther from the CG, moves faster and generates even more. In contrast, the C152’s nacelle is 4 inches (10cm) longer, produces more lift and its fuselage has a spoiler on it, producing even less. This would certainly pull the nose up on a C152 more than on a C150.

Please remember that the purpose of the discussion of the spin was to explain boundary layer separation and its consequences and not a scientific analysis of flat spins in a Cessna 152 – take that part as it was intended: food for thought.

* If terms like angle of attack, boundary layer separation, vertical stabilizer, or attached flow are new to you, they will be explained in the upcoming posts.

© Douglas Daniel 2011-2015 all rights reserved

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