The fear of stalls has led many pilots to grief. As they pad their airspeed to avoid stalling in the pattern, for example, they set themselves up for a poor approach and a less-than-optimal landing. Hounded over the years to avoid a low-altitude stall that can lead to a fatal spin, some pilots fear stalls like the plague.
There is no doubt that pilots should have a healthy respect for stalls, but they need not live in terror. Understanding stall factors can mean the difference between extracting the maximum capabilities from the airplanes flight envelope and just getting average performance.
Every pilot has learned, from the first days of pilot training, that an airplane in a bank has a higher stall speed, due to the increase in g load on the wing. This is a definite law of physics.
The increase in wing loading is a function of g load, and g load is a function of bank angle. Gravity always acts straight down, toward the center of the earth.
As the airplane banks, the lift vector is no longer directly opposite the weight of the airplane. The lift vector now has a component acting vertically (the cosine of the bank angle times total lift) and a component acting horizontally (the sine of the bank angle times total lift).
It is the vertical component of lift that continues to support the weight of the airplane. The horizontal component of the lift vector is what causes the airplane to turn.
From a physics standpoint, the horizontal component of lift is called centripetal force and is what causes objects to move in a curved path.
The end result is that the total lift of the wing has to be higher than when the aircraft is in wings-level flight.
The reason for this is that the vertical component of the lift still needs to be equal to the weight if the aircraft is to maintain altitude. Thats why so much back pressure is needed in steep turns, for example.
In a constant altitude turn at a 30-degree bank, the g loading will be 1.15. That is, you need 15 percent more lift in a 30-degree bank turn than in wings-level flight in order to maintain altitude. Because the wing has to work harder (maintain a higher angle of attack) in a turn, a constant altitude 30-degree bank will result in an increase in stall speed of 7 percent.
Add in the third dimension, however, and the situation gets more complicated. Some pilots profess that the wing will unload in a descending turn, and this will lower the stall speed. This is true – but only to an extent.
When an aircraft is in a descent or climb, a component of the aircraft weight acts parallel to the flight path (the sine of the flight path angle times the airplanes weight), and a component of weight acts perpendicular to the flight path (the cosine of the flight path angle times the weight).
The weight is still acting straight down toward the center of the earth but the airplanes flight path is more aligned with gravity when the airplane is climbing or descending. As the airplanes flight path becomes steeper, either in a climb or descent, more weight becomes aligned with the flight path. Absent a power change, the airplane will accelerate in a descent or decelerate in a climb.
In addition, less weight acts perpendicular to the flight path as the flight path becomes steeper. Since the wing supports the component of weight acting perpendicular to the flight path, the stall speed drops as the component of weight acting perpendicular to the flight path is reduced.
However, in a 25-degree descent or climb, the wing still has to support 90 percent of the weight of the airplane – and a 25 degree descent is quite steep (unless you fly the space shuttle).
Apples Plus Oranges
If the wing has to work harder to support the airplane in a bank and it has to work less in a climb or descent, what happens when you combine the two?In a 10-degree descent with a 30-degree bank, the g load would be 1.14, or about 1 percent less than the g load in a constant altitude 30-degree bank turn.
But lets make the situation even more radical. In a constant altitude 60-degree bank turn the g load is 2. If your wings level stall speed is 60 KIAS, your stall speed in a constant altitude 60-degree bank would be 85 KIAS.
In a 10-degree descent, a 60-degree bank would produce a g load of 1.97 – a g load change of 1.5 percent – and the stall speed would drop by a single knot.
The steeper the descent angle, the greater the change.
A 30-degree descent and a 60-degree bank would provide a g load of 1.73, about a 14 percent change, and the stall speed would now be 79 KIAS.
A 30-degree descent coupled with a 60-degree bank is definitely outside the realm of normal flying for anyone not actively participating in aerobatics or dog fighting.
In order to truly unload the wing, the aircraft has to be pushed over to a less than 1 g, and in fact must approach 0 g. By putting the wing to 0 g, induced drag falls to zero, which means an instant increase in acceleration or decrease in deceleration.
Induced drag is a function of the square of the lift coefficient. When the aircraft is pushed over to 0 g, the lift goes to zero, which means induced drag vanishes as well. The stall speed also drops to zero because the load factor is zero.
However, there is no free ride. When pilots perform this maneuver, the aircraft begins arcing downward, losing altitude at an ever-increasing rate. But the airplane cannot stall or spin.
This procedure is most useful in an inadvertent nose-high pitch attitude, in which the airspeed is bleeding off rapidly. The maneuver will keep you from stalling, and increase your acceleration or decrease the deceleration rate.
This pushover is an aggressive anti-stall technique and pre-stall recovery. It will allow you to maintain lateral control even if the airspeed drops down below the 1 g stall speed.
By pushing over to 0 g, the wing assumes a very low angle of attack. The air flow remains predominantly attached.
Ailerons work by not only changing the pressure distribution over the aileron, but by also changing the pressure over the fixed surface chord ahead of it – as long as the airflow is attached. Once the airflow separates, the aileron can no longer influence the pressure on the fixed wing surface ahead of it.
If the flow separates, as in a stall, the aileron will create a lot more drag than lift. Deflecting ailerons in separated flow can be very risky, and can start an airplane spinning very easily.
When an airplane stalls, of course, you must lower the nose and decrease the angle of attack. The ailerons will only work properly as long as the airflow is attached.
If you drop a wing during a stall, pick it up with rudder. Dont ever try to pick up the wing with aileron until after you have lowered the nose and reattached the airflow. Even then, use a lot of rudder to assist, just in case the airflow has not fully reattached yet and you are a little premature with the aileron.
If the airspeed is rapidly approaching stall speed, even with full power, you need to act aggressively in getting the nose down, and this may require unloading the wing to 0 g.
You can prevent a stall and maintain some measure of control. If a crash is inevitable, it is better to crash wings level with some control than to crash out of control and with the airplane in a bank.
The disadvantages of this maneuver are that going to 0 g can be quite uncomfortable to people who have never experienced it and that fuel flow to the engine may be momentarily interrupted, causing the engine to sputter briefly.
The engine sputter is more likely in a high-wing, airplane with gravity-fed fuel, but can occur in airplanes with boost pumps. Usually the engine will come back to life as soon as positive-g flight resumes.
The altitude loss that results from such an aggressive pushover is not much greater than in a normal stall recovery, but by unloading the wing you will prevent both a stall and a possible spin. You will be able to continue flying the airplane until the recovery is complete, and not experience a momentary loss of control as in a full stall or spin entry.
If the airplane actually stalls, you have no choice but to complete a normal stall recovery. Unloading the wing is an aggressive pre-stall recovery and, just like in a normal stall recovery, you must avoid a secondary stall.
The objective of such a maneuver is to go to 0 g but not push over enough to go to negative g. Aircraft have lower negative-g limits than they do positive, and a negative-g maneuver that is within the airplanes structural capacity can still cause some problems.
You and your passengers can literally hit the roof, even with seat belts tight. Headsets, pens, charts, etc. go flying. In some airplanes, the oil cap can dislodge. The negative-g load will also cause you to be pulled away from the controls, and the tendency to grab onto the yoke or stick can make control problems worse. In the absence of a g-meter, push just enough to be a little light in your seat.
When the Balls Not at Home
Stalls that happen during skidding or slipping turns are more complicated. The lateral pressure gradients that develop on a wing in a slip or skid, along with crossed-controls, will cause an asymmetrical stall. One wing will stall before the other and the stage is set for a spin entry.
A skidding turn will cause the low wing to stall first, with a resulting roll toward an inverted position – not a good attitude to be in on a turn from base to final.
A slipping turn will cause the high wing to stall first, with a resulting roll back through level and then into the spin.
Dont be fooled when a stall in a slipping turn takes you back through level first. The rate of departure can be very rapid and the spin entry can happen before you know it.
Just how rapid that entry is depends on the particular airplanes characteristics. Some airplanes require gross slips and skids to start them spinning, others require very little.
In addition, most airplanes will have a change in the static system error during a slip or skid, with a resulting inaccurate airspeed indication.
Pushing over to unload the wing in a turn stops the turn because the zero angle of attack eliminates the horizontal component of lift. However, the ailerons will still produce a rolling moment because of the local changes in the camber of that section of the airfoil.
You will be able to roll out of the turn, thereby leveling the wings and preventing the slip or skid from inducing a spin.
Although unloading the wing does make the stall speed in a climbing or descending turn lower than in a constant-altitude turn, the change is not great enough in normal pitch angles to be of much practical use. However, when the airspeed is bleeding off rapidly, aggressively pushing the airplane over to a 0 g state will keep you from stalling and spinning. It retains your lateral control and gives the airplane higher acceleration or slower deceleration until you can get the airplane flying normally again.
Wing loading may seem like the province of aeronautical engineers, but knowing the dynamics of reducing the load may give you the ammunition you need to kill a low-altitude stall/spin before it happens.
-by Michael Friese
Michael Friese is an ATP, CFII and airframe mechanic. He is a former assistant chief pilot at Embry-Riddle and currently flies for the U.S. government.