Friday, February 3rd, 2012 09:16 am GMT -6 Friday, February 3rd, 2012 09:16 am GMT -6Friday, February 3rd, 2012 09:16 am GMT -6
 

A bad day? When you stall your model airplane close to the ground. Let me explain what a stall really is.

 

 

 

Definition

The most common technical definition of a stall is when an increase in angle of attack does not result in an increase in the amount of lift produced. For the typical airfoil, this is about 15 degrees. Very thin airfoils stall closer to 10 degrees. Thick airfoils on full-size airplanes can reach 20 degrees before they stall.

Note that the definition says nothing about loss of lift, as most people assume. A stall is when you pull back on the stick and the model says “no más”. It’s not uncommon for an airfoil to have a plateau at maximum lift where it just kind of sits there without dropping the airplane.

It also says nothing about airspeed. You stall when you fly slowly not because the airspeed is too low, but because you tried to increase the angle of attack too much. It may sound like I’m splitting hairs, but that’s a key difference whose significance will become clearer in a moment.

The definition also says nothing about the direction of lift having to be up into the air. You could be banked at a 45 degree angle and stall. Angle of attack is the angle that the wing makes with the relative wind or direction of motion. The direction that gravity is pulling you has nothing to do with it.

Alternate Definition

Another much more technical definition of a stall is when reverse airflow on the top surface of the wing prevents the creation of additional lift. Let me explain.

Lift

About two-thirds of the lift on a wing is produced by the top surface. As the air flows over the wing, it’s forced around a curve and speeds up. This causes a lower pressure due to the Bernoulli effect. The partial vacuum is what we call lift.

Flow Separation

As an airfoil pitches up (in relation to the oncoming wind) flow separation occurs. It is very hard to predict exactly when this will happen. Flow separation is a circular air current (or vortex) that sits right on top of the wing surface. The shape, size, and location of these circular air currents vary but when small they are typically about six times longer than their height. In other words, when they start out they are pretty flat and right next to the wing surface.

Right at the wing surface these eddies, or circular currents, are flowing forward. When small these separation bubbles (as they are also called) don’t affect the performance of the wing very much. But when they get bigger they start causing real problems.

Instead of being on top of the wing and pulling it up, these larger eddies are mostly behind the wing and create a partial vacuum there. Just like a partial vacuum on top of the wing pulls it up, a partial vacuum behind the wing pulls it back. In other words, large scale flow separation kills the lift and increases the drag on the wing.

When the flow separation is so bad that increasing the angle of attack of the wing just leads to more separation, that’s what we call a stall.

Laminar Versus Turbulent Stalls

The air flowing over a wing always starts out flowing smoothly. This is what is called laminar flow. The air then loses kinetic energy because of drag against the wing surface or because we are asking the air to flow around a curved surface. At some point, it loses enough kinetic energy that it stops flowing smoothly and becomes turbulent.

It’s very hard to predict when this transition to turbulence will occur. A lot of factors affect it. Generally speaking, you can assume that an indoor model airplane will have laminar flow. Assume that a giant scale model will have turbulent flow. In between, you probably have some of both types.

If a flow is laminar, separation bubbles grow from the wing leading edge. Since lift on a wing is produced near the leading edge, it leads to abrupt stalls. In turbulent flows, separation bubbles grow starting from the trailing edge of a wing. As they grow they cause a much more gradual loss of lift.

Accelerated Stalls

You can stall at any airspeed and flight attitude. It all has to do with trying to get the wing to generate more lift than it’s capable of producing. Because they usually come as a big surprise, these types of stalls are often deadly to your model.

For example, let’s say you are checking out a new model and doing slow turns to see how well it handles. It’s common for wind to change speed, direction and character once you get 100 feet up. A 45 degree bank angle on the wings requires about 50% more lift to maintain altitude than straight and level flight. The nose of the airplane could be pointed straight at the horizon, but because of the bank angle and wind, you could stall and enter a deadly spin. I have seen this happen too many times!

Just Scratching the Surface

Airplane stalls are a hugely complicated subject. Intensive research into understanding them and controlling them continues to this day. This long article could just hint at the issues involved. I plan to return and expand on this subject in the future.

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