This is designed to show you how to recognize the stall and to help you get to know your airplane. Someday you may have to fly the plane close to the stall, and knowing how the plane flies in that region may save your neck.
Such flight (sometimes called “slow flight”) is usu- ally considered to be level, sustained flight at a speed close enough to the stall (about 5 K above) so that if power was chopped, or if the angle of attack or load factor is increased, you would get an immediate stall.
You will be expected to demonstrate minimum-speed flying to the FAA examiner during the private practical (flight) test. You will be asked to make minimum-speed climbing, gliding, and level turns in various configura- tions and banks. The flight test tolerances in straight and level slow flight will be to maintain within 10° of heading and 100 feet of altitude.
12-10 Part Two / Presolo Don’t get the idea that the speed for slow flight (or
minimum controllable airspeed) is always the same for your airplane. The angle of attack at the stall is always the same (for a given flap setting), but the airspeed at the stall varies with the weight of the airplane. The less a particular airplane weighs, the less its airspeed when that fixed maximum angle of attack is reached. The stall airspeed is also lowered by the lowering of flaps.
Take a look back at Figure 9-5. The stall speeds given there are at the maximum certificated weight of 1,600 pounds and are power-off. Note that the stall speeds decrease with an increase in flaps for any condition of bank. For instance, looking at the effects of flaps at 0°
bank (wings level), the stall speed decreases from 55 mph to 49 mph at 20° of flaps, and then down to 48 mph at the 40° flap setting. Note that the second 20°
only lowered the speed by another 1 mph. Added drag is a greater factor than added coefficient of lift in this area (20°– 40°) of flap setting.
Okay, take an airplane that stalls at 55 mph (no flaps) at 1,600 pounds with the power at idle. If you wanted to fly slow flight in this condition (idle), you would probably set up a speed about 5 mph faster, or 60 mph calibrated airspeed (CAS).
Now use 20° of flaps. At this same airplane weight you would add 5 mph to the stall speed (49 mph) and set up a descent at about 54 mph. For 40°, 53 mph (CAS) would be a good number.
At a lighter weight of 1,300 pounds you might use 55, 49, or 48 mph respectively in slow flight, to have a 5 mph margin.
A little earlier in the chapter it was noted that the use of power decreased the stall speed (Figure 12-8) so that the airspeed, as an example, could be lowered another 3 mph for each condition. Now the new speeds would be 52, 46, and 45 mph CAS, respectively, for slow flight in the three flap settings, at a lighter weight, and using power.
While numbers have been cited here to make the point that flaps, lower airplane weights, and more power will all lower the airspeed given for stall (and hence the required slow-flight speed if the same margin is kept), the idea is to fly the airplane by your feel and senses.
Figure 9-5 and the discussion about it in this chapter use miles per hour as examples. The principle remains the same whether expressed in miles per hour or knots.
Procedure
1. Throttle back to a power setting much less than required to maintain level slow flight. After the first time, you’ll have an approximate idea of what’s needed. Maintain altitude as the plane slows. This
means that the nose must be slowly raised. As the required speed is approached, start adding power as necessary to keep the altitude constant. You can see in Figure 12-13 that the power required to fly the airplane increases again after the airspeed is decreased from cruise down past a certain point.
2. Maintain your heading. You have power on and a low airspeed, and torque must be taken care of.
3. Notice that elevators and throttle are coordinated in maintaining airspeed and altitude. If you are losing altitude, add power and adjust the nose position to maintain the proper airspeed. If you are too slow but are maintaining altitude, ease the nose slightly lower and adjust your power to assure staying on that altitude. Keep checking the altimeter and air- speed. You’ll also be watching for other planes.
4. Make a shallow turn in each direction. Maintain altitude in the turn. This will mean increased power and a slight raising of the nose.
5. Level the wings and gradually throttle back to idle as you lower the nose to maintain a glide at the min- imum control speed of 5 K above the stall. Make 20°– 30° banked turns in each direction.
6. Return to level, slow flight by applying power and easing the nose up. Try to keep the airspeed from varying during this transition. You’ll note in slow flight, as in other phases of your flying, that an increase in power tends to make the nose rise, which will make it seem as if all you have to do as power is applied is to think about the nose easing up.
7. Increase the power and ease the nose up to a climb at minimum speed. Make shallow turns in each direction. (Don’t climb too long, since the engine will overheat at low airspeed/high power.)
Making the transition from level flight to glide to climb, etc., without varying the airspeed more than 5 knots takes some heavy concentration, but when you can do it, you’ll have an excellent feel for the aircraft.
The instructor will probably not do any more before solo than have you fly the plane at the minimum speed to get the feel of slow flight. You will not be required to maintain as close tolerances of altitude as later in your training. However, if you seem to get the hang of slow flight, you may go on with the transition to climbs and glides. You will probably review slow flight before going on the cross-country solo.
Remember that you should also be able to do slow flight by referring to the instruments. In this condition, you don’t rely on your senses but on the instruments.
The instructor will give you a chance to practice this, both hooded and with visual references during periods of dual.
Chapter 12 / Stalls and Slow Flight 12-11
Figure 12-13. The power required to maintain a constant altitude (sea level) for a fictitious, very efficient light trainer at a particular weight (attitudes exaggerated).
You should get slow-flight practice in as many combinations of flaps and power as possible. If your trainer has retractable gear (it’s doubtful), you should fly at minimum controllable speeds in cruise and land- ing configurations. You might repeat steps 1 through 7 in various flap configurations. (Don’t try to climb very long at the low speed with the flaps extended.)
Your instructor may cover the airspeed indicator during the visual slow-flight practice so that you can fly the airplane by pitch attitude and power and may also cover the tachometer so that you will use power as needed to maintain altitude rather than staring at the rpm numbers.
Common Errors
1. Losing or gaining altitude in the transition from cruise to slow flight.
2. Poor speed control in the transitions.
3. Stalling the airplane.
4. Poor altitude control during the level portions of slow flight. (Slow flight is also called “maneuver- ing at critically slow airspeeds.”)
5. Failure to hold heading due to high torque and reduced rudder effectiveness at low airspeeds.
12-12 Part Two / Presolo point where parasite drag and induced drag are equal.
Incidentally, this is the point of maximum cruise effi- ciency, or maximum range, of the airplane. You’ll get more miles to the gallon while maintaining altitude at that speed, which is about 150 percent of, or 1.5 times, the stall speed (for trainers). This would be the airspeed at which to fly if you got in a bind for fuel and had to stretch it to make an airport some distance away. This 1.5 ratio is calibrated airspeed.
As you move back down the curve, point 3 is the airspeed at which the minimum power is required to maintain altitude. This would be the airspeed you’d use if you were looking for maximum endurance, or lon- gest time airborne, and is roughly about 120 percent of the stall speed (or 1.2 times the stall speed, however you like to think of it).
Decreasing the airspeed below point 3 results in the power required to fly the airplane (at a constant altitude) starting to increase sharply; induced drag is beginning to make itself known. The part of the curve from point 3 back to the stall is called the “backside of the power curve,” which implies that something unnat- ural is occurring. It’s only unnatural if you don’t have the full picture of what happens to drag at various air- speeds, and the term is a rather poor one (like “eleva- tors”). Note that this airplane requires as much power (60 horsepower) for slow flight, 5 K above the stall (point 4), as it does to cruise at an airspeed of 94 K.
Finally, as the stall is approached at point 5, all of the power is needed to maintain altitude; if the angle of attack is increased further, the stall occurs even though full power is being used. (The attitudes of the airplane at the extreme ends of its speed range are exaggerated in Figure 12-13.)
Practice-flying at minimum controllable airspeeds is extremely important; it shows you what to expect when you are flying at speeds fairly close to the stall (as might be done on a power approach to a short field). If you find yourself too low and slow and close to the stall on the power approach, you would not pull the nose up.
This would demand more horsepower and make you sink faster. You would instead add power and ease the nose lower. In less critical situations you wouldn’t even have to add power but would just ease the nose down to pick up a slightly higher airspeed (where less power is required).
Notice also how steeply the curve goes up at the maximum level-flight speed, point 1. If this same air- plane had another 30 horsepower, it wouldn’t have a much greater top speed. In fact, an added 30 horsepower (an added 30 percent) would only mean an increase in top speed of about 10 percent. As a rule of thumb for high cruising speeds or top speeds, the increase in