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Written by Scott Stoops
Flight Training
Column
As seen in the February 2013 issue of Model Aviation.


On a trip with family and friends to Mexico, we were lucky enough to end up in the last row of coach for the four-hour flight. Sitting across the aisle from a family friend, he queried as to why it seemed as though we were severely tilted nose-up, even in cruise flight. In my typical wordy fashion, I proceeded to outline the basics of flight and specifically the angle of attack (AOA).

Seeing his eyes glaze over after a minute or so, I decided that this column would make a better vehicle for that discussion. Let’s explore AOA, some common misunderstandings new pilots have about stalls, and some common recovery techniques. Let’s start from the beginning.

Wings create lift. They do this primarily by manipulating the AOA. AOA is the difference between the chord line and the flight path or relative wind of a wing. Not unlike sticking your hand out the window of a car with it tilted slightly up, a wing creates down force through both its shape, but primarily, the angle it addresses the oncoming air. This is AOA (see Figure 1).




Figure 1


Although the basic shape of the airfoil contributes to the efficiency of the wing and its ability to create lift, the primary factor in lift creation is AOA. Based on the design of the wing and airfoil section, there is a maximum AOA at which the wing section will continue to produce lift. Flight beyond that AOA causes the airflow to become extremely turbulent and detach from the upper surface of the wing. This detachment results in a loss of lift, or a stall. The specific stalling AOA is a constant for that particular wing.

Stalls have absolutely nothing to do with a power failure of the motor or engine. In fact, unpowered aircraft such as sailplanes can also stall. Stall is an aerodynamic term that only relates to exceeding the critical AOA.

During normal flight in most types of airplanes, we avoid flying the aircraft at or close to the critical AOA. It is, however, important to be familiar with the stalling characteristics of your model. Learning to stall your model allows a higher level of awareness of the energy state of the airplane with regard to AOA. Practice is the only way to become familiar with and competent at stall and recovery.

For the airplane to stall, an AOA that exceeds the critical AOA must exist (see Figure 2). In the case of practicing stalls, the best place to start is from level flight with plenty of recovery altitude. You can intentionally stall the aircraft by increasing the elevator input and holding it in an increasing pitch attitude while reducing the power of the motor.




Figure 2


As the aircraft exceeds the critical AOA, airflow over the wing will “detach” from the wing’s upper surface, causing some buffeting and usually a pronounced pitching moment toward a nose-down attitude. Most models have a critical AOA of approximately 17°. Recovery is simple, but not instinctive.

With the nose now pointing slightly down (probably below the horizon), you must reduce the up-elevator input to let the wing recover to a flying AOA. This is not instinctive, because in normal flight we would apply up-elevator when the nose is below the horizon to correct for level flight.

In stalled flight, it is critical to allow the wing to start flying again by lowering the AOA even further. Often, simply releasing any elevator input back to neutral is enough to get the recovery started. This reduction in AOA generally coincides with an increase in thrust and, once the wing is no longer stalled, a gentle correction back to level flight.


Stalls in All Attitudes

Now for the confusing part! The previous example was for level, decelerating flight. Stalls occur when the critical AOA is exceeded, which means they can occur in any pitch attitude. A stall can occur when the aircraft is pointing straight up, straight down, inverted, or at any pitch attitude as long as the critical AOA is exceeded. This is generally tied to a large elevator input, but can also occur with small inputs at higher speeds.

A stall can occur at any airspeed (it is not necessarily a slow speed event, but rather, a high AOA event). This can be confusing to new modelers, because the traditional diagrams of the stalling AOA depict an aircraft in level flight as I have explained.

A model can be stalled going straight up in a loop. If the pilot pulls too hard on the elevator control stick (displacing the elevator up), the critical AOA can be exceeded and the wing will stall while the airplane is pointing straight up. The same is true if the pilot pulls too hard on the elevator during the backside of a loop while pointing straight down.

A good indicator that the model’s AOA is near the critical AOA is the position of the elevator. For the AOA to be high, the elevator has to be significantly displaced. So, wings stall at a specific AOA, not at a specific pitch attitude (see Figure 3).




Figure 3



3-D Flight

The next logical question would be how 3-D airplanes can be flown beyond the critical AOA if lift significantly decreases when the wing stalls. The simple answer is thrust. They use thrust to replace the lift lost from the stalled wing.

If you’ll note, most 3-D airplanes have dramatically oversized flight controls and optimized airfoils that allow full control through thrust vectoring and clean transition in and out of stalled flight. As your skills improve, consider learning some of the basics of 3-D flight, because it can only make you more comfortable flying at AOAs around and even beyond the stall!


Take-Away

Although it can be scary to slow your model to the point where you’re uncomfortable with how it is going to perform, learning stalls and stall recovery is critical to becoming a well-rounded RC pilot. Start high, and with a buddy box if necessary. Most importantly, remember that simply releasing the elevator input will often allow the model to recover on its own!

In the columns going forward, I’ll do my best to further explore stalled flight through some 3-D maneuvers as well as snap rolls and spins, so give the basic stall a try.

Fly safely, and remember that learning is fun, and fun is what this great hobby is all about.

-Scott Stoops






9 comments

Hi Scott. I really enjoyed reading your article on how and when a wing can stall. I would like to offer one additional comment. I was flying a Venus 40 and was practicing turns. Actually I was performing aerobatic turns with the wing in a near vertical position, and pulling significant elevator. What I experienced was a snap roll. I believe the snap roll happened because I exceeded the wing's critical AOA and one wing stalled before the other. As a result of this experience I always caution new students to watch out for unexpected stalls at any attitude.

Our club training program typically helps set up a plane for new students. One of the key parameters is that we do not allow enough elevator control to allow the plane to stall. A Kadet LT 40 can usually loop fine, but still does not have enough elevator to execute a stall or a spin. Once the student can solo, we go back to look at increased elevator control.

Thanks again for an excellent article.
Jim Orsborn

I commend you for adding the Sub-Paragraphs about All Attitude and 3D. I helps somewhat to understand that a Stall is not just a wings level, slow event.

I beg you tho, PLEASE remove all references to the horizon in future sketches. More importantly CORRECT NOW the arc that depicts the angle of attack as measured from the horizon line on Figure 1. The angle of attack is measured from the relative wind vector, not the horizon. It will help a lot more to future readers of this article if the sketches were not drawn parallel to the horizon (or their perceived horizon on their computer screen/magazine) either. But removing the horizon reference to begin with, will help.
Thanks
Rafael

Mr. Scott Stoops

Great article- should help aircraft modelers with the tricky "stall" modes of their models.

However, I would propose an expanded definition of "aircraft stall", going beyond the traditional definition to include high-drag aircraft or low airspeed stalls occurring even at proper AOA".

This different definition might be "reduction of wing lift to values below the gravitational attraction, resulting in aircraft loss of altitude regardless of wing AOA."

Some of my scratch-built model aircraft designs have intentional high-drag, low air-speed characteristics that will lose lift and altitude if I reduce the air speed below the lift-gravity profile regardless of wing AOA. This isn't a dumb design flaw of mine, but a natural result of creating a powered aircraft efficiently flying at low air speeds.

After all, this is what happens in any powered aircraft when the throttle is reduced below the critical air speed wing-lift-forces and the wing loses lift aka "stalls" long before any critical wing "pitch AOA" is reached.

Taken to extremes, imagine an efficient wing at zero air speed. Regardless of wing "pitch AOA", the wing will create zero lift. So in this case, the ability of the wing to produce lift is more a function of airspeed than AOA. In fact, with zero airspeed, there isn't any meaning of AOA.

Keep flying- it adds years to our lives!

Best regards,

Frank

You should not include the horizon in your figures. Angle of attack is measured as the angle to the relative wind not the horizon. If you are descending fast you can still be in a stall even if your wings are flat with respect to the horizon. If you are descending fast enough, you may need to nose down (below the horizon) into the relative wind to break the stall and build up some speed.

AOA

Great subject!

In future articles discussing stalls it would be a good idea to mention that the stall speed increases with increasing angle of bank. It might save some low time RC pilot a stall-spin-crash scenario on a turn to final.

One thing confusing to a beginner like me is that the Bernoulli Effect basically says that the curved airfoil means that lift is created with a zero angle of attack, as long as there is thrust to create forward motion. Wouldn't this mean that as long as thrust is increased (increased throttle), lift would increase and the plane would climb, even with zero angle of attack? Thanks

I am not sure about the statement in par. 3 that says a wing produces a down force. Down acting force on an aircraft is gravity. A wing only produces lift following Bernoulli's Principal which states lift is produced when the airflow over the wing is faster than the airflow under the wing.

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