Recognize and Prevent Airplane Stalls
Be aware of and safely recover from airplane stalls
An RC model airplane’s wings produce lift to keep the aircraft safely airborne. Under certain conditions during fixed wing flight, an aerodynamic stall can occur whereby the wing suddenly experiences a rapid reduction in lift. An airplane’s wing stall can result in loss of control and a crash if it occurs too close to the ground. Note that an aerodynamic stall has nothing to do with engine failure or aircraft airspeed. The effect of an aerodynamic stall - a swift loss of lift - is the same for any fixed wing aircraft, to include an unpowered glider.
Airflow around a wing during stall
The good news is that knowledge of stalls is well understood by model airplane designers and RC pilots. Stalls can be practiced on a flight simulator. Once you see the regime of flight where a stall is most likely to occur, the procedure is to stay out of this area of the flight envelope. If you find yourself accidentally in the performance region where a stall may occur, there are simple and effective procedures to safely fly through this danger area. A good example of where an inadvertent stall could occur is during an engine out recovery when you are low on altitude and airspeed during the final gliding approach.
Stall definition
An aerodynamic stall is defined as the point where the angle of attack increases such that the wing experiences a sudden decrease in lift. Angle of attack is the angle between the wing’s chord line and the vector of the relative motion between the aircraft and the atmosphere. It is important for the pilot to understand that the angle of attack must always be controlled to maintain safe flight.
Airplane stalls typically occur when the RC pilot flies too slowly. For example, if you are returning to the field with a failed engine, the airspeed of this gliding dead stick model is controlled by nose position. If you see that your model’s airspeed is too low, the only option is to lower the nose to fly faster. If you keep the airplane’s nose up or even level, the airspeed will decrease further. You can easily get too slow in this situation, enter a stall and lose control of your model.
Clark Y airfoil
Model plane design and stalls
Model aircraft design plays a large factor in a model airplane’s stall characteristics. Different airfoils exhibit varying stall behaviors. Large sections such as the Clark Y airfoil produce benign stall manners, which is why this wing section is used on such a wide variety of RC models. A constant chord wing offers predictable stall indications, while a tapered wing might be less easily controlled if you permit the angle of attack to get too high.
Design can influence stall behavior
Aircraft weight has a large impact on how and when an airplane stalls. A greater wing loading (ounces per square foot of wing area) will produce a higher stall speed. The same aircraft, if loaded with additional weight, will stall at a higher airspeed than when at a lighter weight. See further examples of high performance and homebuilt aircraft stall behavior at the EAA website.
When you are flying an RC model airplane, it is important to be aware that an aerodynamic stall can occur at any time that the wing’s angle of attack gets too high. Keep this in mind if you attempt a really abrupt pull out from a steep, high power dive. As few model planes have an angle of attack indicator, the best visual indicator of angle of attack resides with airspeed.
Practice stalls
A stall is a normal part of aviation. There is no way to prohibit a stall from ever occurring. The best way to avoid a stall is to keep your model’s airspeed in a safe range. If your model plane is getting too slow, add power. If flying a glider or in an engine out situation, lower the nose to increase the airspeed.
High wing trainers like the Yard Ace have gentle stall characteristics
There is nothing to fear from a stall. Practice getting into airplane stalls and recovering with your model. Sport aircraft such as the Pietenpol Air Camper show docile stall characteristics. Be sure to do this at a safe altitude. Reduce the power to idle and slowly raise the nose. As the airspeed decreases, the angle of attack increases. Once the angle of attack reaches the critical value for your model, the wing will stall. Lift will swiftly reduce, the nose will tend to go down and one wing may drop. The stall recovery procedure is always the same. Increase power, lower the nose and as airspeed increases return to level flight.
Demoiselle needs power during all phases of flight
Practicing stalls is important with RC flight due to the large variety of models you will fly. Lightweight micro indoor RC planes almost bypass the classic stall behaviors. Even if you manage to get one of these tiny aircraft into a stall, they fly slow enough that there is no damage if they drop to the ground.
Stalls and the Demoiselle
Other aircraft, such as the Sig Demoiselle, have such a large wing area combined with a high drag configuration that they fly on the edge of a stall during most of its flight. Once you understand this unique behavior, you understand the need to always carry some amount of power during the landing phase.
If you attempt a power off glide with the Demoiselle in the same way as you might with other aircraft, the antique plane will simply drop to the ground. There is essentially no time to recover from this landing situation. An understanding of airplane stalls, as applied to the Demoiselle, dictates the need to always fly with some amount of power.