What is a stall?


The dreaded stall... To hear people talk about it, you'd think it was some sort of mythical beast that flies around, pulling aircraft out of the sky with one swipe of its mighty paw. As you might have already surmised from my slightly sarcastic tone, this simply isn't the case. :-)

In reality, a stall is nothing more than the ultralight trying to do "the right thing". (Not some politician's ethical views about what is right; your ultralight follows the laws of physics.) ;-) A stall is nothing to be afraid of; it is a perfectly natural occurance, and there is nothing "wrong" with an ultralight when it stalls.

First of all, a stall has nothing whatsoever to do with the engine. When an ultralight is said to have "stalled", it is the wing which has stalled, not the engine! To understand why an ultralight stalls, we first have to understand how it flies. Without getting into a whole bunch of gobbelty-gook about the theory and physics of flight, the basic reason why an ultralight flies is because of the shape of its wing. (Which a whole bunch of people just love to argue about. As strange as it may seem, it doesn't appear as if we really fully know for sure why an airfoil flies, even after all of these years of flight.) Air molecules rushing over the curved, smooth surface on the top of the wing create an area of low pressure above the wing. (As compared to the relative high pressure under the wing.) This is because air which speeds up will create an area of low pressure. The shape of the top of the wing causes the molecules to speed up. (A similar thing happens in a carburetor; the air that is being forced into it is sped up, which creates a low pressure area, which sucks fuel into the air stream.) Because of this, the aircraft is more or less "sucked" upwards. There are those who say that the air is actually being pushed downward (by the bottom of the wing), or a combination of a multitude of factors, but whatever the actual reason the wing flies, the smooth flow of air over the top surface of the wing is an essential part of flight. This is also the reason why frost or icing will cause an ultralight to stop flying. The air can't flow smoothly over the top of the wing (the ice or frost is rough), so the low pressure system doesn't develop to the extent that it needs to, and the wing can't generate enough lift to overcome the weight of the aircraft. The reason the ultralight flies of course, is because it happens to be attached to the rising wing. (Note: If you think you've got a better or more accurate explanation of why a wing flies, I'd be interested in hearing it.)

Ok, now that we know we need a smooth airflow over the wing, what is a stall? Basically, it is an interruption of that smooth airflow. As you increase the angle of attack*, the air has to travel over a larger and larger "hill" before running down the top of the wing, and the high pressure air under the wing can more easily move around the back of the wing toward the low pressure air on top of the wing, thus weakening that low pressure area. The air on top of the wing begins to roll and burble. Eventually, the low pressure system is completely destroyed, and there is no longer anything holding the wing up at that exaggerated angle. The wing drops, and as it does, it picks up speed, and the relative wind flows smoothly over the top of the wing again. As it does, the high pressure area increases, and the ultralight is flying once more.

Below is a depiction of a dynamic stall (a stall caused by a rapid maneuver). However, the basic principles should remain the same for most any stall, I should think. In panel 1, the wing is in level flight, and the airflow is smooth, and attached to the wing. As a result of the viscosity of the air, the individual fluid particles spin. The red color indicates particles that are spinning clockwise; the blue color indicates particles that are spinning counter-clockwise. As the angle of attack increases in panels 2 through 5, the airflow at the trailing edge begins to seperate from the wing. Lift is increasing. Panel 6 shows the clockwise spinning particles interacting with the counter-clockwise spinning particles at the trailing edge. Panel 7 shows a small blue bubble appearing on the upper surface of the wing (near the leading edge). In panel 8, the interaction of the counter-spinning particles has formed a vortex, which grows rapidly in panels 9 through 12, until it is driven away from the upper surface by the blue counter-rotating particles, and a stall occurs.

[Panel 1]  The wing is in level flight. [Panel 2]  Angle of attack increases. [Panel 3]  Angle of attack increases. [Panel 4]  Angle of attack increases. [Panel 5]  Angle of attack increases. [Panel 6]  Clockwise spinning particles interact with those spinning counter-clockwise at the trailing edge. [Panel 7]  A small blue bubble appears on the top of the wing, near the leading edge. [Panel 8]  Counter-rotating particles have formed a vortex. [Panel 9]  Vortex grows. [Panel 10]  Vortex grows. [Panel 11]  Vortex grows. [Panel 12]  Vortex is driven away from the upper surface of the wing and a stall occurs.

[Stall Animation]

As the ultralight approaches a stall, it will usually give you some warning signs. Everything may get very quiet, and you may hear a sound similar to mice running through your wings as the air burbles over the surface, and your wing will probably be at a high angle in relation to the horizon. (Usually... Keep in mind that it is the angle of attack which is important, not the angle of the wing with the horizon, but in straight and level flight, the horizon and the relative wind are generally parallel. You can actually stall your wing in any attitude, at any speed.)

The proper recovery for a stall is to let the stick forward and add full power. If you do not let the stick forward, you may stall the ultralight all the way to the ground. (If you can keep it from spinning*.) If you are approaching a stall near the ground, it can be difficult to let that stick forward, but it is nescessary to prevent the stall. If you have enough altitude, stalls are prefectly harmless, and even a lot of fun! Practicing stalls at altitude will alert you to the warning signs as your ultralight tells you it is about to stall, and it also gives you an opportunity to perfect your recovery techniques. You'll want to practice trying to lose as little altitude as possible, something that becomes increasingly important for stalls that occur near the ground. If you don't have any stall experience, it would be a very good idea to spend some money and a little time with an AFI, BFI, or CFI and have them "show you the ropes" before going off and trying it on your own. (If you aren't careful, a stall can develop into a spin*.)

* - Explanation of terms:
Angle of attack:
Is the angle created by the chord* of the wing and the relative wind*.
Is the immaginary line that would be created if you drew a line from the leading edge of the wing to the trailing edge.
Relative wind:
Is the wind created by the onrushing air. For example, if you were riding a bicycle at 10mph on a calm day, the relative wind would be coming straight at you at 10mph. Ride directly against a 10mph wind at 10mph, and the relative wind would be coming straight at you at 20mph. Ride with a 10mph wind at 10mph, and there is no relative wind.
A spin occurs when you are fully stalled, but one wing is more stalled than the other, so that wing drops faster than the "less stalled " one. The nose points almost straight down, and the ground rapidly rotates around you in a most disconcerting manner as your ultralight rushes toward it, similar to a spiral dive. (To determine wether you are in a spin or a spiral dive, look at your airspeed indicator. In a spin, you will be stalled, and hence, not gaining speed. In a spiral dive, you will be rapidly gaining speed.)
Some people do spins for pure enjoyment, but don't mess with them unless you know the proper recovery technique and your aircraft is rated for spins. Spins are generally not recommended in ultralights. A well-behaved aircraft will bring itself out of a spin if you just release the controls, but there are some that will not. You cannot spin an aircraft without stalling it first. If you want to do spins, find a CFI to work with you. Do not try to spin unless you have a full understanding of what is going on, and you know the proper recovery technique!! Your first few spins should be done under the direction and guidance of a CFI.

The history of this article:

I found some great pictures of a stall which were created by researchers at the University of Cincinnati, which I thought would look neat as an animated .gif. I scanned 'em in, and struggled with them for a long time before I was moderately happy with the results. (They could be better, but at least they're not horrible.) ;-) I decided to write an informative article about stalls around 'em, and this is the result. I'm no expert, and this is my first attempt at such an article, so if you find anything missing, misleading, or just pain wrong, please let me know and I'll correct it ASAP. --Jon

Many thanks to Walter Grooms (CFI), for correcting my spin explanation. Many thanks to Mark E. Carruth (CFI), for correcting my stall recovery technique.
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Jon N. Steiger / stei0302@cs.fredonia.edu / SUNY College at Fredonia