One of the lesser-known and more catastrophic aspects of flying RC airplanes is the dreaded stall. So what causes an airplane stall? Many new pilots don’t realize such a thing exists and it’s very common for even the most experienced pilots to not fully understand how a stall occurs. A stall can happen even if your thrust-to-weight ratio is greater than 1:1 and a stall doesn’t mean that your aircraft has stopped moving forward rather, your wings have stopped producing lift.
“I don’t know what happened. It just stalled and I lost control.” ~ many rc pilots
What is an airplane stall?
A stall is the sudden reduction or loss of lift generated by your aircraft’s wing. Basically when you stall you’ll suddenly lose altitude. However, this doesn’t mean you stop moving forward rather it means that your aircraft is no longer going to keep or gain its current altitude.
What causes an airplane stall?
An airplane stall occurs when you exceed your aircraft’s critical angle of attack.
What is a critical angle of attack?
When your airplane’s wing is traveling forward think of it as attacking the oncoming air (this will be the ‘attack’ in our critical angle of attack phrase). Now in order for our wing to properly attack the air it needs to be within a set range of what I’ll call a “safe” angle of attack. If your wing angle stays within that “safe” range the results are clean laminar airflow over the wing which provides the lift you need to keep the airplane in the sky.
If we continue to increase the angle at which the wing is attacking the air, eventually we will reach a point that is dangerous or “critical”. Once you’ve reached this point any further increase in the angle of the wing greatly decreases and eventually eliminates any clean airflow over the wing. When this happens our clean airflow turns dirty and our wing can no longer generate lift to keep the airplane level and at its current altitude. Here we’ve exceeded our critical angle of attack and will begin to create a stall situation with the aircraft.
What does a stall look like?
The most frequent stall is an asymmetrical stall. This is where the airplane’s nose drops down while both wings remain level. The other common stall is the asymmetrical stall. This is where one wing drops before the other. Though the asymmetrical stall may look more violent, neither are deadly when you put to practice the proper recovery techniques shown below.
Recovering from a stall
Reducing the angle of attack is the only way of recovering from a stall regardless of the amount of power used. – Airplane Flying Handbook; U.S. Department of Transportation – Federal Aviation Administration
As stated above, in order to recover from a stall you simply need to reduce your angle of attack. Reducing the angle of attack can be accomplished by reducing your engine’s power, keeping your wings level with the horizon, and allowing the airplane to go into a shallow dive. Through this dive, you’ll build up the necessary airspeed to begin producing lift on your wing. Then by pulling back with just a touch of elevator, you’ll get the airplane to level out. Only after you’ve regained level flight should you slowly begin to add power back to the aircraft.
A common mistake among beginners is to add power in order to recover from a stall. While adding power is not necessarily wrong, it can cause all sorts of problems. Simplify your situation and reduce your power. By reducing your power (i.e. throttle back to idle) you now have one less element to manage when recovering from a stall.
Practicing an RC airplane stall
The purpose of practicing an intentional stall is to learn how to stall and to learn how to recognize when you are in a stall and take the correct actions to recover.
- Be sure you have plenty of altitude to practice entering into and recovering from a stall. It’s best to be 3 mistakes high.
- Perform your stall on a straight and predictable flight path.
- Try to stall the aircraft right in front of you so you have a good side profile to observe. This way you can see exactly how your airplane behaves as it enters into and exits a stall.
While flying straight and level, slowly reduce your throttle through a countdown of “Three, Two, One… idle”. Once at idle, keep your wings level and slowly pull back on your elevator to maintain as much of your current altitude and increase your wing’s angle of attack. Eventually, your airplane will exceed its critical angle of attack and enter into a stall. At this point you need to employ these techniques:
- Keep your wings level as you enter into the stall.
- Do not apply up elevator as your plane enters a nose-down attitude.
- Let the plane continue nose-down until you gain airspeed.
- With increased airspeed you will create “smooth” airflow over your wings – this is critical.
- Once you have speed, add light up elevator to level off.
- You’ve recovered from your stall. Congratulations!
Understanding and safely practicing stalls give you the necessary tools to sense and avoid a potentially dangerous situation. Stall practice allows you to hone your flying skills safely and will give you a strong base to improve all other aspects of flight.