Aerodynamics for Commercial Maneuvers

Module 2 · Aerodynamics for Commercial Maneuvers · Lesson 1 of 6

Load Factor and Stall Speed

Why a level turn needs more lift than the airplane weighs, and why stall speed goes up with the square root of the extra load.

Load factor is the total lift the wing makes divided by the weight of the airplane. It is measured in G. In straight-and-level flight the wing makes lift equal to weight, so the load factor is 1 G. Anything that makes the wing carry more than the airplane weighs raises the load factor: banking, pulling out of a dive, or a gust that suddenly raises the angle of attack.

In a level turn, the lift vector tilts with the wings. Only the vertical part of the lift holds the airplane up; the horizontal part turns it. To keep the vertical part equal to weight, total lift has to grow as the bank steepens. In a coordinated level turn that works out to:

Why Stall Speed Rises

Lift depends on angle of attack and on dynamic pressure, which goes with the square of airspeed. If the wing must make twice the lift at the same airspeed, it needs a higher angle of attack. It reaches the critical angle of attack, and stalls, at a higher airspeed than it would wings level. Because lift goes with airspeed squared, doubling the required lift raises the stall speed by √2, not by 2. This is the root of every accelerated stall in Lesson 2.5.

Limit Load Factors by Category

The structure is certificated to a limit load factor for its category. Above the limit load, the airframe may be permanently bent. Certification adds a safety factor of 1.5, so the structure should not fail until the ultimate load factor, 1.5 times the limit. That margin is for gusts and errors, not for flying.

Values from PHAK p. 5-35. Many airplanes are certificated in more than one category; the limits that apply depend on how the airplane is loaded. Check the POH limitations section.

Module 2 · Aerodynamics for Commercial Maneuvers · Lesson 2 of 6

Maneuvering Speed and Weight

The placarded V_A is right only at maximum gross weight. Lighter, the safe speed is lower.

Design maneuvering speed, V_A, is the speed at which the wing reaches the limit load factor and the stall at the same moment. Below V_A, a full, abrupt pull on the elevator stalls the wing before it can load the structure past its limit. The stall relieves the load. Above V_A, the same pull can reach the limit load before the wing stalls.

From Lesson 2.1, the stall speed at the limit load factor is V_S × √n_limit. That speed is V_A. For a normal category airplane, V_A ≈ V_S × √3.8, about 1.95 times the 1-G stall speed.

Why V_A Goes Down With Weight

A lighter airplane needs less lift for 1 G, so it stalls at a lower speed. Because V_A is tied to stall speed, it drops too, by the square root of the weight ratio:

A lighter airplane is also hurt more by turbulence. The same vertical gust changes the wing's angle of attack by the same amount, so it adds the same amount of lift. That extra lift is spread over less weight, so the load factor jumps more. Slowing down reduces both the gust load and the speed at which the wing will stall to relieve it.

What V_A Does Not Protect You From

• V_A covers one full, abrupt deflection of one control. It does not cover full deflections in more than one axis at once, or rapid back-and-forth inputs such as rudder reversals. Those can exceed structural limits below V_A.
• V_A is not marked on the airspeed indicator. Find it in the POH limitations section or on a placard, often listed for several weights.
• V_A is not V_NO. V_NO, the top of the green arc, is the maximum structural cruising speed for smooth air. In turbulence, slow to V_A for your weight, or to the POH turbulence penetration speed if it gives one.

The ACS steep turn and chandelle Tasks both ask you to explain maneuvering speed, including the effect of weight. The steep turn is flown at the manufacturer's recommended speed or, if none, a speed not above V_A (CA.V.A.S2). In a light airplane, that means V_A for today's weight.

Module 2 · Aerodynamics for Commercial Maneuvers · Lesson 3 of 6

Rate and Radius of Turn; Overbanking

Bank and airspeed together set how fast you turn and how much room you need. Bank alone sets the load factor.

The horizontal part of lift turns the airplane. A steeper bank gives a bigger horizontal part, so a tighter, faster turn. A higher airspeed means the airplane has more momentum to redirect, so at the same bank the turn is wider and slower. The formulas show how strongly each one matters:

The 11.3 in the radius formula is the same constant as in the pivotal altitude formula you will use for eights on pylons in Module 13. Both come from the same physics of a coordinated turn.

The Overbanking Tendency

In any turn, the outside wing flies a bigger circle than the inside wing in the same time, so it moves faster through the air. Lift grows with the square of airspeed, so the outside wing makes more lift. That difference tries to roll the airplane further into the turn.

Coordination in Steep Banks

Holding opposite aileron to stop the overbanking creates adverse yaw away from the turn. That is why a steep turn often needs more rudder into the turn than you expect, with the ball centered. A skid (too much rudder) raises the inside wing's angle of attack and sets up the cross-control stall in Lesson 2.5. A slip (too little rudder) costs altitude and makes the turn sloppy.

Climbing and descending turns add one more effect. In a climbing turn, the outside wing travels a longer, shallower path, so it meets the air at a slightly higher angle of attack, which adds to the overbanking. In a descending turn, the inside wing has the higher angle of attack. That matters in the steep spiral and in a gliding turn to final: if the airplane stalls in a descending turn, the inside wing tends to stall first.

Module 2 · Aerodynamics for Commercial Maneuvers · Lesson 4 of 6

Energy Management

Think of the airplane as an energy account. The throttle changes the balance; the elevator moves energy between airspeed and altitude.

An airplane in flight holds two kinds of mechanical energy. Altitude is potential energy. Airspeed is kinetic energy. You can add them into one number, the energy height: the altitude you would reach if you traded all your airspeed for altitude with no drag.

Kinetic energy goes with the square of airspeed, so knots are worth more at high speed. Slowing from 120 to 90 knots, with power unchanged, buys roughly 280 ft. Slowing from 90 to 60 knots buys only about 200 ft. This is why a chandelle gains most of its altitude early, while the airspeed is still high.

Two Speed Regimes

Above the speed for minimum power required, the airplane behaves the way you expect: slowing down needs less power. Below it, in the region of reversed command (the back side of the power curve), slowing down needs more power, because induced drag is rising fast. There, raising the nose without adding power makes the airplane sink faster, not climb. Close to the ground, low and slow, the elevator has no energy left to give you. Only power, or altitude you no longer have, can fix it.

Energy in the Commercial Maneuvers

Module 2 · Aerodynamics for Commercial Maneuvers · Lesson 5 of 6

Angle of Attack and Accelerated Stalls

The wing stalls at its critical angle of attack. Airspeed, attitude and power only change when you get there.

A wing stalls when its angle of attack, the angle between the chord line and the relative wind, exceeds the critical angle of attack. For a given wing and flap setting, that angle does not change. The airplane can stall at any airspeed, in any attitude and at any power setting. The published stall speed is only the speed at which a 1-G, wings-level airplane at a stated weight reaches the critical angle.

What Moves the Stall Speed

The Accelerated Stall

An accelerated stall is a stall at a load factor above 1 G, and so at an airspeed above the published stall speed. It usually comes from pulling too hard in a turn or a pull-up. It tends to be more sudden than a 1-G stall, and if the airplane is not coordinated one wing usually stalls first and drops. In the ACS Task, you roll into a coordinated 45° bank at no more than V_A, then increase back pressure smoothly and firmly until the stall is impending, and recover no lower than 3,000 ft AGL (CA.VII.D.S2–S5).

Recovery is the same as every stall: first reduce the angle of attack with forward elevator pressure. Once the wing is flying again, level the wings with coordinated aileron and rudder, add power as needed, and return to the desired flight path without a secondary stall. Rolling the wings level first, while still stalled, can deepen the stall on the down-going wing.

The Cross-Control Stall

The classic case is the turn from base to final. The pilot overshoots the centerline, often with a tailwind on base, and tries to tighten the turn with inside rudder without steepening the bank. The rudder makes the airplane overbank, so the pilot holds opposite aileron, and pulls back to keep the nose up. Now the airplane is skidding, cross-controlled, at a load factor above 1 G, and low. The down aileron on the inside wing raises that wing's angle of attack further. If it stalls, the inside wing drops sharply and the airplane can roll inverted and spin with little warning, too low to recover. Prevent it: fly a normal pattern, keep the bank to a reasonable angle, stay coordinated, and go around rather than tighten the turn.

Angle of Attack Indicators

An AOA indicator shows how much of the critical angle of attack you are using, whatever the weight, load factor, bank or flap setting. That makes it a direct stall warning in exactly the cases where the airspeed indicator is misleading: a steep turn, a pull-up, a heavy airplane. Stall warning horns also sense angle of attack, so they work in accelerated stalls, but they give only a single warning point. Buffet varies by airplane and may come late. Know the range and limits of the warnings in your airplane (CA.VII.D.R2).

Module 2 · Aerodynamics for Commercial Maneuvers · Lesson 6 of 6

Spins and Upset Prevention

How a stall becomes a spin, how to tell a spin from a spiral dive, and how weight and CG decide whether you can recover.

A spin is an aggravated stall that results in autorotation. Both wings are stalled, but one more deeply than the other. The less-stalled wing makes more lift and less drag, so the airplane rolls and yaws toward the more-stalled wing and descends in a steep corkscrew. A spin needs two things: a stalled wing and yaw. Remove either and there is no spin. That is why coordination at high angles of attack matters so much.

Loss of control in flight is the leading cause of fatal general aviation accidents, and many of those accidents are the result of stalls, spins or accelerated maneuvers (RMH pp. 3-3, G-2). Most happen in the traffic pattern or during maneuvering at low altitude, where there is no height to recover.

The Four Phases of a Spin

1. Entry
   The pilot provides, or fails to stop, a stall with yaw: uncoordinated flight at a high angle of attack.
   Phase 1
2. Incipient
   From the stall until the spin is fully developed. In a light airplane, about 4–6 seconds and roughly the first two turns. Recovery is quickest here.
   Phase 2
3. Developed
   Rotation rate, airspeed and vertical speed settle and stay steady from turn to turn. Airspeed stays low, near the stall.
   Phase 3
4. Recovery
   The angle of attack comes below critical and rotation stops. It may take a quarter turn to several turns, depending on the airplane.
   Phase 4

Spin Recovery

Use the procedure in your POH first. Where the POH gives none, the AFH general recovery procedure is:

1. Reduce power to idle. Power flattens the spin and increases the rate of rotation.
2. Put the ailerons in neutral. Aileron in a spin can make it worse.
3. Apply full rudder opposite the direction of rotation. Use the turn needle or turn coordinator to tell which way you are turning; the ball is not reliable in a spin.
4. Apply a positive, brisk forward movement of the elevator to break the stall, just after the rudder reaches its stop.
5. When rotation stops, neutralize the rudder. Holding it can start a spin the other way.
6. Return to level flight with smooth back pressure, without a secondary stall and without exceeding V_NE or the load limit.

Weight, CG and Spins

• Aft CG makes the spin flatter and the nose higher. The elevator has less authority to lower the nose, so recovery can take longer or become impossible.
• Forward CG makes the airplane harder to stall and spin, and generally easier to recover.
• Higher weight means more inertia, so rotation is slower to start and slower to stop.
• Uneven loading, such as fuel imbalance between the tanks, can change spin behavior and make one direction worse.
• Many airplanes are approved for intentional spins only in the utility category, which limits the weight and keeps the CG forward, often meaning no rear passengers or baggage. A normal category placard reading "intentional spins prohibited" means the airplane was not tested for them, not that it cannot spin.

Spin or Spiral Dive?

A spiral dive is not a stall. The wing is flying, the airplane is in a steep, descending, banked turn, and airspeed and load factor are climbing fast. The two need opposite recoveries. Spin recovery uses opposite rudder and forward elevator. Spiral recovery is: reduce power, level the wings with coordinated aileron and rudder, then raise the nose gently to level flight without exceeding the load limit. Pulling back in a spiral only tightens it and raises the G load.

Upsets and the Startle Response

The AFH defines an airplane upset as an unintended pitch attitude more than 25° nose up or 10° nose down, a bank of more than 45°, or flight within those limits at an airspeed that is wrong for the conditions. A sudden, unexpected upset can trigger a startle response. For several seconds, the pilot may freeze or make a strong, instinctive input, usually pulling back, which is the wrong move in a stall or a spiral. Training to a short, practiced sequence is what shortens that delay.

Module 2 · Aerodynamics for Commercial Maneuvers · Knowledge Check

Module 2 Knowledge Check

Fifteen questions in the style of the CAX test and the oral exam, each tagged with its ACS element. Pass mark: 80% (12 of 15).