RC pilots who have been flying for a while have heard the term “stall” and most know that it is a bad thing. Most RC pilots understand that a stall causes the airplane to “stop flying” and is usually the result of some mistake on the part of the pilot or builder.
However, I believe much of what I hear at the RC field regarding stalls is based in tribal knowledge (knowledge passed among a community by word of mouth…in this case the “tribe” of radio control fixed wing aircraft pilots). This lack of depth to the knowledge causes problems for an RC pilot as he or she advances in the hobby.
With this in mind, RCGS is starting a new series on the stall. This will, again, be a multi-part series with weekly or bi-weekly updates until we feel the topic is sufficiently covered to move on to other topics.
With this first post we are going to define the stall:
A stall occurs when the smooth airflow over the
airplane’s wing is disrupted, and the lift degenerates rapidly. This is caused when the wing exceeds its critical angle of attack. This can occur at any airspeed, in any attitude, and with any power setting. -FAA Airplane Flying Handbook
The FAA definition of stall is suitable for our discussion despite the oversimplification of a complex aerodynamic (and, more generally, a fluid dynamic) principle. One key point to emphasize for our ongoing discussion: “This can occur at any airspeed, in any attitude, and with any power setting”. Put a pin in that and we’ll get back to it in a later post.
Another term we need to define, angle of attack, is the angle between a line from the center of the leading edge to the center of the trailing edge (referred to as the ‘chord line’ of the wing) and a line parallel to the wing’s path through the air, or in other words, it is the angle between the chord line and the direction of the surrounding undisturbed air. This is a relatively hard thing to explain with words and a picture is always better (from http://smoknjoe.tripod.com/aviation.htm):
Finally, the first part of the definition: “A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly”
There are two things we need to discuss in this statement:
1. Smooth airflow disruption is the key to a stall. A wing operating efficiently has smooth, laminar airflow over the top and bottom of the airfoil. As angle of attack increases it becomes more difficult for the air to stay attached to the top of the wing, and it begins to separate. Since lift is created by the difference in pressure between the top and bottom of the wing, and the difference in pressure is created by the airflow over the top of the wing smoothly moving faster than the airflow on the bottom of the wing; when the airflow begins to separate lift will decrease in the local area of the wing where that separation is occurring. Once enough of the wing experiences this condition and lift is no longer great enough to overcome gravity, the wing essentially “stops flying” and we have a stall.
2. Lift degenerates progressively as angle of attack increases, but it begins to degenerate rapidly at or near the critical angle of attack. Essentially, at high angles of attack (i.e. slow flight speeds while maintaining altitude), some of the wing is experiencing separation, but the occurrence is gradual. Once the critical angle of attack is exceeded, the separation is rapid and even sudden…accompanied by a nose drop and often a wing drop as well. The point is that stall is progressive and it accelerates approaching and passing the critical angle of attack. This will become more important as our discussion moves to wing design, stall recovery, and the spin.
We have covered a lot of ground in a very short time, but the RC pilot doesn’t need a degree in aerodynamics to have a suitable depth of knowledge. If these concepts have piqued your interested, the FAA is a great source of free information on basic aerodynamics. In addition, Aerodynamics for Naval Aviators is an excellent resource for those who really want to dig in. There are also free courses in fluid dynamics available from M.I.T. and other renowned institutions.