Understanding of The Stall

The stall is one of the least understood aspects of flight, especially with R/C models. What is it, why does it occur, when does it occur, and—above all—how can we stop it from happening?

Stalls can and will happen in a variety of attitudes, not only at low power in straight and level flight, when the weight of the aircraft is too much for the lift produced by the wing (the helicopter beats the air into submission). The weight of a model is among the most crucial considerations. A model's performance in flight and stall speed will both improve with weight. This applies to all models, regardless of size. It's a misconception that the more robust a model is, the more probable it is to survive a collision. In a crash, a model will always shatter at its weakest place, regardless of how much strength is packed into it.

A model is only as strong as the weakest part.

Consider two identically sized and designed models. One weighs 6 pounds, while the other weighs 7 pounds. Because the heavier model must fly at a greater airspeed to produce the same lift needed to keep it in the air, the lighter model will have a lower stalling speed than the heavier one. Because the light model will be able to land at a lower speed due to its lower stall speed, there will be less chance of damage from a harsh landing. Additionally, a slower landing speed will allow the pilot more time to plan how to maintain the model heading down the runway and the wings level.

Assuming that a particular model's stall speed is 25 mph in straight and level flight, this would be the optimal velocity for the model to stall and flare in order to land on the runway. The identical aircraft's stalling speed increases to roughly 30 mph when its wings are at a bank, say, 30 degrees. The stalling speed rises to about 45 mph in a 45º bank. The stalling speed may now be between 50 and 55 mph when the bank angle is increased to 60 degrees. The stalling speed rises significantly as the bank angle increases, and the airspeed begins to decrease due to generated drag if the elevator is also used to start a turn. If the pilot is unaware, the model will quickly lose height and crash if corrective action is not taken. Because there is less airflow over the flying surfaces in an EDF, the scenario above is considerably more pertinent. A propeller-driven model has airflow from the propeller across the control surfaces. Please be aware, though, that the airflow will only be over the rudder and elevator. In the direction of the wing's tip, the ailerons will have little to no authority.

Now, think of a landing circuit. The model is on the pattern's downwind leg; you're losing height and power. At a particular moment, you turn 90 degrees onto a base leg while lowering the power a little bit to lower the height. If the pilot is not aware, the model is most likely to stall and crash at the final turn onto the approach. Rolling the aeroplane into a bank during the turn causes you to apply more elevator and maybe the rudder at the same time. For a spin, they are the same control inputs. Always make the last spin onto the landing approach extremely gently and shallowly.

On the final, the model is perfectly aligned, and as it approaches the landing, everything appears to be going smoothly, but then, all of a sudden, the model drops a wing and crashes. What just happened was that you had a tip stall. I hear you ask what causes a tip delay and how to avoid it. When lift at the wing tip ceases producing lift while lift at the wing's root or center continues to do so, this is known as a tip stall. Stated differently, the tip of the wing will sink because the center of the wing generates more excellent lift than the tip. Thus, the moniker "Tip Stall."

It generally occurs with a tapering wing, and once more, the model's weight is crucial; it should be low. There isn't much you can do once the model is constructed and completed. To raise both ailerons slightly over the trailing edge of the wing, however, somewhere between 1/32" and 1/16", is a simple method that works. This is known as "Wash Out" and has the effect of producing lift at the tip. Anything beyond this must be taken into consideration throughout the planning and construction phases. Note that this only functions while the model is flying upright. When inverted, it produces the opposite effect and is referred to as "Wash In." The model will be acceptable if you only want to fly it inverted or the correct way up and not less than 50% throttle inverted.

Aerobatic aircraft have tip-stalling capabilities built into their design. They need to be able to snap, roll, and spin cleanly and predictably.

The final one I want to discuss is the power stall. In order to take off, your model speeds down the runway, lifts off and begins a pleasant, short rise. At this point, you can adjust the angle and climb rate. While still running at full power, the model starts to slow down and loses airspeed more quickly. Adding more elevators will exacerbate the problem. The model will stall and probably crash if you don't respond right away. Pushing the nose down to boost airspeed is the only way to avoid a collision.

In order for the wing to produce enough lift for the model to fly in a stall situation, you must increase the airspeed. Depending on the model's attitude, you should level the wings first.

Lastly, this helps you avoid crashes and may even help you in the event of an inexplicable accident.