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Slow Flight
Most any one can skate or ride a bike fast. It is at slow speeds that true skill and control can be demonstrated. The same is true about flying. When I first sought to be a flight instructor an old time CFI took me out on his nickel and told me that we would explore the outer-limits of the aircraft's controllability. Under his guidance I learned to fly. Prior to that I only had control.

We would fly without flaps on heading and altitude with power as required to maintain level flight. We would fly slow, slower, and slowest. We flew well below the 10-knot margin given by the stall warner. A student is not expected to do this in any configuration. The PTS standards are to fly at Vs1 without flaps, with flaps, with partial flaps on heading and at altitude plus ten and minus five knots; within 100 feet of altitude and 10 degrees of heading. Then they add turns, climbs and descents with specified angles of bank.

Most any Vs1 slow flight can be performed in a ten degree bank. To the left just relax the rudder. To the right add rudder and opposite aileron. If you go beyond the 10-degrees you look forward to a cross-control stall. By adding some power you can make a 30 degree bank. Now the stall spin possibilities are increased. Time for a distraction to be introduced. Slow flight near the stall is called minimum controllable. The power of the rudder in controlling the stall and yaw is best demonstrated in this exercise. The proper rudder application is proven when the stall break is straight ahead without any wing drop. Any application of aileron will be counter productive by further stalling the wing and causing a more abrupt wing drop.

Aircraft Stall Factors
Wilbur Wright used the word 'stall' in 1904 to describe how in a turn Orville allowed the aircraft to pitch up too much and stall. The potential of an aircraft to stall or spin is in its design. A pilot's ability to detect and react to this potential is a criteria of flying skill. When an airplane is flown at an angle that exceeds the critical angle of attack, the airplane will stall. In deliberate training stalls we decrease airspeed and avoid the abusive control inputs that cause unusual attitude stalls. Low speed is not the cause of the stall; the cause is the angle of attack.

The pilot has control of the elevator. Pressures on the elevator determine if the wing will develop an angle of attack sufficient to stall. When the angular difference between where an airplane is pointed and the way it is going exceeds about 11 degrees to the wing's chord line a stall occurs. This is called the critical angle of attack. Exceeding the critical angle of attack of the wing with elevator inputs will cause the airflow to break from the upper wing surface. This break in air flow reduces the coefficient of lift, increases the coefficient of drag and transmits to the pilot a series of aerodynamic, mechanical and physiological cues.

Stall warners give a ten-knot warning of impending stalls as normally performed. The accidental inadvertent stalls that I have encountered occurred simultaneously with the sound of the horn. The same plane could stall at 40 knots when weighing 1600 pounds an at 30 knots weighing 1300 pounds. Of course, weight is always a factor in that a 20% weight increase will give 10% higher stall speeds. while a corresponding 20% reduction in weight will give a 10% lower stall speed. . The real objective is not so much performance as recognition by sight, sound, and feel.

The critical constant in stall speeds is weight. Book  (POH) figures are based on gross weights. This provides most flight operations with a built in safety margin. This safety margin may be over-ridden by knowing that your actual weight is a certain percentage less than the gross. You can reduce your approach speed by a percentage that is half the percentage of lower weight difference. Some aircraft have critical approach airspeeds that do not follow the rule because of control and ability to go-around considerations.

Opinion
I'm working my way through Gleim's FIRC (Flight Instructor Renewal Clinic) and came upon the statement that:...
"Load factor is the ratio of the total lift generated by the wings to the actual weight of the airplane and its contents [...] In un-accelerated flight, the load factor is 1; i.e., the total lift equals the gross weight of the airplane."

As a broad statement, that's true, but there's a nit.
The lift generated by the wings in level flight is the weight of the airplane PLUS the down-force generated by the tail. If you were being pedantic, would it not be more accurate to say that the load factor in level
flight is slightly greater than 1?

It just so happened that the immediately preceeding question had to do with the decrease in stall speed as the CG moved aft, due precisely to the change in down-force of the tail, which is what got me thinking along these lines.
Roy Smith

For those who may be a little weak in doing the figuring, make a proportion like this:
Difference between actual and gross = Percentage of gross
Gross weight 100
Cross multiply difference x 100 and divide product by gross weight to get percentage of gross.

Take half of the percentage of gross as the percentage to reduce your approach speed.
Example: 1600# C-150. Take out 200# instructor and figure reduction of approach speed.

200# = % 200 x 100 = 12.5% Taking half leaves 6%
1600# 100 1600

6% of a 60 knot approach speed is about three knots. 60 - 3 = 57-knots
This example can be used as a basis for interpolation only for the C-150.

When stalling speeds are determined for aircraft they are set at the most critical CG condition. Thus the speeds are set in the manual for "indicated" speeds with a forward CG position. This gives the highest stalling speed. Since training aircraft are seldom flown at the most forward CG the usual stall speed will be lower. This accounts for the book differences you should have noted. The way an aircraft behaves entering, during, and recovering from a stall is used to determine its stall characteristic. These characteristics are determined at the aft CG when stall speed is at its slowest.

Desirable Stalls
A "stall" occurs as a result of one of two events:
1. The wings can not support the load of the weight being carried.
2. The horizontal tail can not provide the pitching authority needed to support the wing loading (tail stall)
3. 1 and 2 have to do with an aircraft that has exceeded its critical angle of attack.

The normal stall is when the wing stalls. When the tail stalls it is called a tail-stall. The tail stalls are very abrupt and the nose pitches down near the vertical. This stall increases the effective AOA of the tail. The stall can tuck the aircraft inverted with negative G-forces. The most desirable stall occurs when the wing root stalls first and moves outward to the wing tip. This desirable stall can be built into the wing by twisting the wing, adding slots to the wing tip, putting stall/spoiler strips to the leading edge of the root.  The noise you first hear is the vibration of erratic air hitting the tail surfaces. 

Every aircraft type and even aircraft of the same type will have stalling characteristics affected by weight distribution, wing loading, its critical angle of attack, control movement, configuration, and power. Higher powered aircraft can often be flown out of the stall by the addition of power. The purpose of such a stall recovery is to minimize any loss of altitude. This is a more aggressive stall recovery than the usual lower the nose technique.

Stall characteristics are often 'discovered' after the aircraft has gone into production. The manufacturer-government license agreement requires that all production aircraft adhere to original construction so some modifications are incorporated. The most expensive fix is construction of a leading edge slot. A 'cuff' or drooped leading edge may be used, a series of protrusions on the upper wing surface may be used to direct air flow even to the extent of being full chord 'fences' to prevent span-wise flow. The addition of a small triangular strip on the leading edge of the wing can cause the airflow over the surface to break and burble sooner than otherwise. This, rather common, method, is the least expensive fix of all. The design should be such that the stall occurs progressively from root to tip. The tips have a lower angle of attack than the root. Recovery of a stall begins at the tips and proceeds to the root. This design allow ailerons to remain effective for longer periods.  This is a defense against the rudder-shy pilsot who reacts with aileron for a wing drop rather than rudder..

Government stall tests are not made with slips or skids. While the old saw of slips being good and skids being bad may be true, it is only partially true. A stall that occurs in a slip or skid may occur at a higher speed than expected. Any deflection of the ailerons will increase the stall onset. Any aggravation of the stall by increasing the back pressure may result in sudden attitude changes due to turning and unequal wing speed. The attitudes resulting may be a combining of yaw, roll, and spin entry.

As the stall approaches the ailerons become ineffective first. Elevators follow when the airflow from the wing becomes turbulent. This turbulence is your natural stall warning. As the stall approaches, students tend to under react with the required rudder pressure to keep the wing speeds balanced. A more aggressive application of rudder in the beginning is more desirable.

When the stall occurs that will kill you it won't be at 2500 feet AGL….It won't be done intentionally and you won't expect it. It'll happen on short final, right after takeoff or on the go around from a short strip  You'll be distracted (which is why you've allowed this to develop) and will need to make an immediate and proper corrective action. The only way to develop that reflex is with practice but not a low altitudes.

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