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.

Using the lift equation we can see that as V decreases, CL has to increase to provide the same lift

Sigma and area remain the same

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

So this means the stall characteristics are much different between a straight wing and a swept wing

The straight wing will give a more definite break

The swept wing will tend to mush into the stall and may suddenly enter a spin

Wing Planforms and Stall Patterns

Wing planforms influence how the stall starts and progresses

Where the stall occurs first is determined by where the highest ratio of local CL to the overall wing CL occurs

Manufactures use geometric wing twist or “washout” to decrease the wing loading at the tip

This builds in a lower α at the tip, causing stall to occur at the root first

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.

The Supermarine Spitfire

This airplane due to its elliptical planform was notorious for its treacherous stall

Several pilots were caught unaware and perished as a result

Rectangular

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.

In addition, washout may be added to reduce the α at the tips delaying the stall.

Stall strips may be added at the wing root to induce airflow separation

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.

Local CL to Total Wing CL Ratio

Sweepback

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.

Sweepback

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. 

Sweepback

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. 

Sweepback

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.

Sweepback

How does it work?

Only a component of the forward velocity will hit the wind cord-wise.

The relative wind strikes the wing at an angle equal to the sweep

Sweepback

Actually this component is equal to the cosine of the sweep angle times the free stream Mach Number

Cos 30 = .866(.86)=.75 Mach

So in this example the wing flies as if it is at .75 Mach

Sweepback

Sweeping the wing also lowers its aspect ratio

Low aspect ratio leads to higher drag at slow airspeeds

Thus highly swept wings leads to a condition known as the region of reverse command

This condition may be especially challenging when making carrier landings

Solved by using a variable sweep wing such as the F-14

Sweep Forward

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. 

Sweep Forward

On the swept forward wing, 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.  (ailerons not stalled)

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

Sweep Forward

On sweptback wings, 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.

Sweep Forward

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.

Sweep Forward

  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

Sweep Forward

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 wing tip bends upward with no appreciable increase in alpha

Soviet SU-47 Berkut

Sweep Forward

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.

Additionally he suggests for a climb

  • control airspeed with yoke
  • if airspeed is constant Tr will be constant
  • climb is then controlled by excess thrust
  • thrust available is controlled by throttle

So his conclusion is that throttle controls rate of climb or descent

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

Turbulence

Generally turbulence hurts performance

Variations in α increase drag and decrease the pilots ability to hold a constant airspeed

Really bad turbulence may cause structural damage

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.

Spins – Straight Wing Aircraft

2 conditions are required for a spin to occur:

1.Stall

  1. Yaw

In a spin to the right, the downward moving wing has a higher alpha than the left

Both wings are above CLmax but the left wing is producing more lift even though it is stalled

This produces the autorotation

Inertia and gyroscopic forces also add into the condition

Use opposite rudder and forward stick to recover

Spins – Sweepback Wing Aircraft

Requirements are different for the swept wing:

1.Slow speed

2.Yaw

Since the CL curve does not have a well defined CLmax point, wing autorotation forces are weak

The CD for each wing is much different causing substantial yaw forces

The low aspect ratio means most of the mass is along the longitudinal axis which forces the flat spin to develop

This results in very high alpha and high sink rates

Along with rudder, forward stick, and ailerons against the spin are used in most cases to recover

azards of Slow Speed Flight

 

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