LESSON 12 Chapter 11 Low Speed Flight ANA Chapter 2

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Flight Theory

Chapter 11

Hazards of Slow Speed Flight

Sweepback vs straight wing

4 differences can be seen from the CL curves

  1. The straight wing has a higher CLmax than the swept wing with equal wing area and aircraft weight.

This means that the stall speed will also be lower because of the greater CLmax value.

Sweepback vs straight wing

  1. The sweptback wing must fly at a higher AOA to achieve max lift
  2. There is a sudden reduction in CL for the straight winged aircraft at the stall but not for the swept wing plane

Sweepback vs straight wing

  1. Since the CL is greater the straight winged plane is more sensitive to AOA changes

Stall Patterns, Elliptical

The elliptical wing has constant local CL over the entire wing.

This is the most efficient planform for a wing.

This means that the wing produces lift equally and will therefor stall all at the same time across the wing.

The disadvantage is that the ailerons would stall at the same time and recovery could be difficult.


This wing generates high local CL at the root and low CL at the tips.

This means a stall begins at the root and works outward giving the pilot adequate stall warning and control during the stall.

This wing also exhibits high induced drag from the big pressure differential at the wing tips.

Moderately Tapered

This wing closely resembles the elliptical wing and the lift pattern is much the same however local CL is lower at the tip allowing a slight advantage in control.

Highly Tapered

The highest CL occurs just inboard from the tip thus the stall pattern starts just about where the ailerons are located.

A stall in this airplane would be characterized by aileron buffet followed by the wing dropping.

This tip stall tendency would not give any buffet warning through the elevator and there would be no strong nose down tendency and the ailerons would be useless.

Pointed Wingtip

Extremely high CL is generated at the tips and would be in the stalled condition nearly all the time unless stall allaying devices where in place.


The sweepback wing is similar to a tapered wing when it comes to local CL.

Since the wing is swept there is a strong cross flow of a low energy boundary layer toward the tip because of the increased suction at the tip.


This thickened low energy boundary layer is easily separated thus the stall pattern occurs at the tip and moves inward.

In addition, when the tip stalls, the center of pressure shifts forward causing an inherent nose up tendency.


The aircraft designers must take this into consideration with regards to tail placement and cg range.

When sweepback is large and is combined with a low aspect ratio such as in the concord, max CL may be achieved at very high angles of attack some as much as 45 degrees.


This would lead to absurd landing gear arrangements and the stability may seriously deteriorate.

Manufactures then place airspeed limits based on the CL value and not the stall value.


The same transonic reducing characteristics are found in this wing planform as in the sweepback.

Due to taper, vortex effects, and spanwise flow, sweepback wings are more loaded at the tip than the rest of the wing thus not allowing the wing to operate at its max angle of attack before stalling.


Since spanwise flow is at a minimum the tips will remain unstalled past the point where the inner portion stalls, thus more of the wings lifting potential is utilized.

The sweepback is designed for elliptical loading pattern at cruise speeds because this generates the lowest amount of drag.


At low airspeeds, due to spanwise flow the tip loads up and stalls out.

The sweep forward wing produces the valued elliptical pattern at max lift which in turn provides a better L/D ratio.


The ailerons will remain effective throughout the stall.

  1. More lift than a similar size sweptback wing or a smaller wing for the same amount of lift.


  1. Less induced drag, shorter takeoffs and landings, due to more efficient flaps slots and such
  2. Increased aileron effectiveness at high alpha
  3. The same lower transonic and supersonic drag as sweptback wings


However, the tip twists upward when stressed and increases the angle of attack. The increase in angle of attack further bends the tip and structural failure results.

The rectangular tip bends upward no appreciable increase in alpha


The sweptback wing tip twists downward when a load is put on it thus decreasing the angle of attack and causing it to unload aerodynamically.

Region of Reverse Command

If we look at the typical thrust required curve we know that on both sides of the L/Dmax point, drag increases with either an increase in airspeed or a decrease in airspeed.

The point to the right of the L/Dmax is called the region of normal command.

In the region of normal command, thrust is directly proportional to velocity.

Region of Reverse Command

The point to the left of the L/Dmax is called the region of reverse command or when referring to prop planes, behind the power curve.

In the region of reverse command, thrust is inversely related to the velocity.

The slower the airspeed the greater the thrust required.

Region of Reverse Command

Refer to fig 11.6 pg 162 Dole

If the guy at point A wants to climb he just needs to pull back on the stick.

This will slow the plane but there will be a decrease in thrust required leaving an excess in thrust causing the aircraft to climb.

Region of Reverse Command

If the guy at point B wants to climb and he pulls back, there will be a increase in drag as the airspeed slows.

This causes an increase in thrust required.

The region of reverse command is where we do our approaches, stalls, takeoffs, climbs, slow flight and landings.

Region of Reverse Command

Dole recommends: airspeed with the stick and rate of climb or descent with throttle.

Heavy Rain

Rain effects the aerodynamic of the wing in some unusual ways:

  1. The momentum of the drops impact the plane down and back, Newtons 3rd law action, reaction
  2. An increase in weight because of the water film

Heavy Rain

  1. The water film is roughened by the impact of the drops and the aerodynamic properties of the wing are adversely affected.
  2. The raindrops hit the plane unevenly causing possible rolling and pitching moments.

A 30% increase in Cd and a 30% decrease in lift is possible

Heavy Rain

The AOA for Clmax AOA may be reduced by 2 to 6 degrees.

9 commercial plane accidents have been involved with heavy rain.

Effects of Ice and Frost

pg 169 shows Cl curves with and without ice and frost

note the sharper and lower curve with the frost.

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