Physics of Flight: The Stall
One of the more deadly physics of flying for the radio control aircraft pilot is the stall. While some pilots don’t even realize such a thing exists, others do not fully understand how or when a stall occurs. Contrary to what you may think, a stall does not necessarily mean that the engine(s) of your aircraft have stopped working or that the aircraft has stopped moving. Just because you have awesome power and the ability to fly through the sky at lightning speeds doesn’t mean that you can’t or won’t stall. Did you know you can be flying at full throttle and still stall your aircraft? Surprising, isn’t it.
Like we’ve said before, understanding the physics of flight is just as important as understanding and learning how to drive a car. Only in the case of stalling your car, odds are in your favor that it won’t end in a crash.
What is a stall?
In aerodynamic speak, a stall is the sudden reduction or loss of lift generated by your aircraft’s wing (see ‘Physics of Flight: Lift’). Basically when you stall you suddenly loose altitude. This doesn’t mean you stop moving forward, however. It just means that your aircraft is no longer going to gain or keep its current altitude.
How do you stall?
By exceeding your critical angle of attack.
“Critical angle of, what…?”
Now that we’ve gotten the simplest explanation out of the way as to how you stall, let’s break that sentence down and take a closer look.
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 we’ll call ‘good’ angles of attack.
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’. It is at this critical point that any further increase in the angle of the wing greatly decreases and eventually eliminates any clean airflow over the wing.
Our clean airflow turns dirty and our wing can no longer generate lift to keep the airplane level and at its current altitude. At this point we’ve exceeded our critical angle of attack and have now gone into a stall.
What does a stall look like?
The most frequent stall is a symmetrical stall. This is where the airplane’s nose drops down while the 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 that we show you 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 Admin.
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 a stall (for RC)
The purpose of practicing an intentional stall is not to learn how to stall, but rather to learn how to recognize and take the correct actions to recover from a stall. First, be sure you have plenty of altitude to do your practicing. We recommend a minimum height of 75 feet until you have a solid grasp of how to enter and exit from a stall. Second, be sure to line up your airplane’s flight path so it will fly in front of you when entering into the stall. This gives you a good side profile to observe exactly how your airplane behaves when it enters in to and eventually reaches the stalling point.
While flying straight and level, slowly reduce your throttle during a countdown of “Three, Two, One… idle”. Once at idle, keep your wings level and slowly pull back on your elevator to maintain your 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 all you need to do is employ the techniques discussed above for a smooth recovery.
By understanding and safely practicing stalls you not only gain the necessary tools to recover from a potentially dangerous situation, you can also begin to hone your slow flying skills for the likes of, dare we say, a sweet landing.