## CHAPTER 4

LIFT AND STALL

Factors in Lift and Drag equations

1. Airstream velocity V (knots)

2. Air density ratio (sigma)

3. Airfoil planform area square feet

4. Profile shape of the airfoil

5. Viscosity of the air

6. Compressibility effects

7. Angle of attack (degrees)

## Factors in Lift and Drag equations

1. Airstream velocity V (knots)

2. Air density ratio (sigma)

3. Airfoil planform area square feet

4. Profile shape of the airfoil

5. Viscosity of the air

6. Compressibility effects

7. Angle of attack (degrees)

Items 1 and 2 define dynamic pressure

Items 4,5 and 6 explain drag on an airfoil

## Coefficient of Lift

Is a measurement of lift at a certain angle of attack or said another way, it is the ratio of the lift pressure and dynamic pressure and is a function of the shape of the wing and angle of attack.

Dynamic Pressure and Lift

Recall that the dynamic pressure possessed by a moving fluid is equal to the density ratio (sigma) times the velocity squared in knots divided by 295.

## Dynamic Pressure and Lift

It is also true that when dynamic pressure is exerted over a certain amount of area measured in square feet, it yields a force proportional to the amount of the area. F=pressure x area

## Dynamic Pressure and Lift

The amount of lift obtained from a wing should thus be proportional to the dynamic pressure and the wing area.

It is not exactly equal to the product of these 2 quantities so the difference is made up by the CL in this equation:

## The lift equation

The equation says that lift is equal to the lift coefficient times the dynamic pressure times the wing area.

The CL can be thought of as a measure of how efficiently the wing is transforming dynamic pressure into lift.

## The lift equation

Lets say we have a wing that has an area of 2 square feet, is subjected to a dynamic pressure of 1.5 psf and yields a lift force of 1.2 pounds.

The CL would be determined as:

## The lift equation

This result would be obtained at a certain angle of attack.

At a higher angle of attack the same wing and dynamic pressure would have a higher CL up to the point of stall.

Each angle of attack produces a particular lift coefficient since the angle of attack is the controlling factor in the pressure distribution.

## CL change for flaps up and flaps down

1. A very sharp drop off would indicate a sudden stall whereas a softer curve indicates a more benign stall.

2. You can tell if the airfoil has a camber to it because of the zero lift angle hits base line in the negative numbers.

3. The very top of the curve indicates the CLmax or maximum lifting ability.

## Stall Facts

Remember stall AOA does not change for weight, attitude or altitude.

Stall Speed does change with weight, density altitude and Load factor or G loading

## The V stall speed occurs at CLmax so the equation is:

## Load factor = G’s x weight

2gs x 2150 = 4300

All factors being equal the stall speed varies as the square root of the lift.

The equation is:

Where W2 is current weight and W1 is max gross weight

This formula works for Va, approach speed, gust penetration speed and stall speed

## The Controversy:

A blend of pitch and power should be used to control altitude and airspeed.

To rely on AOA for airspeed control only would result in rough control technique.

AOA indicators are used to determine Vx, Vy and max range.

## Airfoil Savvy

Airfoil design exploded in about the 1920’s making some sort of id system necessary.

The National Advisory Committee for Aeronautics (NACA) the forerunner of NASA, developed a wind tunnel test and numbered the airfoils.

## Airfoil Savvy

The first series are the 4 digit series.

E.g. 2412

The first number is max camber in % of chord (in hundredths)

The second is location of max camber in % of chord (in tenths)

The last 2 are max thickness in % of chord.

## Airfoil Savvy

Example: 2412 with a 60 inch chord would be .02c, .4c and .12c

max camber 1.2 inches

max camber location 24 inches behind the leading edge

max thickness 7.2 inches

## Airfoil Savvy

A symmetrical airfoil would have two zero’s 0010

In the 1930’s the max camber was moved forward for a 10 to 20% greater possible lift.

This dictated a new numbering system– the 5 digit system.

## Airfoil Savvy

NACA 23012 is the Bonanza airfoil.

The first number is the same with the 3 indicating max camber in twentieths.

The 0 indicates a straight aft meanline and a 1 indicates a curved aft meanline.

## Airfoil Savvy

Airfoils capable of speeds in the 300 to 400 mph range needed a new classification

So they went to the 6 series airfoil such as 65 – 415

6 = series

5 = min pressure at 5/10ths chord

2 = range of low drag above and below the design CL

Airfoil Savvy

4 = design CL

15 = max thickness at 15% chord

The series 6 airfoil was first used on the P-51 mustang because of the low drag qualities.

In order to achieve a far aft min pressure point, the max thickness in also far aft.

Mooney’s also use this series airfoil.

## Boundary layer

velocity profiles for laminar and turbulent flow (pg 50)

Reynolds Number

“Ozzy” Osborn Reynolds found that whether the boundary layer was laminar or turbulent depended upon 3 things:

the fluid velocity

the distance downstream

the kinematic viscosity.

The formula is as follows:

## Reynolds Number

RN varies directly with velocity and distance back from the leading edge and inversely with viscosity.

The curve indicates lower CDrag with increasing RN’s because the velocity gradient decreases as the boundary layer thickens.

## Reynolds Number

High RN’s are obtained from large chord surfaces, high velocity and low altitudes.

Low RN’s are obtained from small chord surfaces, lower velocity and higher altitudes.

Remember high altitudes produce higher values of kinematic viscosity (the air is less thick).

## Reynolds Number

Probably the main use for the RN is in analysis of skin friction.

Since laminar flow indicates small values of skin friction then a lower RN is desirable.

The higher the RN the more chance of developing a turbulent flow and a correspondingly higher value of skin friction.

## Reynolds Number

The higher RN indicates the turbulent flow closer to the leading edge.

RN’s less than 500,000 indicate mostly laminar flow

RN’s from 1 and 5 mil are partly laminar and partly turbulent

RN’s above 10 mil give mostly turbulent boundary flow

## Adverse Pressure Gradient

Basically the lower pressure gradient on top of the airfoil causes the airflow to want to separate from the airfoil.

As the air flows back on the airfoil pressure increases until at the trailing edge, pressure should more or less equal.

The more the pressure is reduced the slower the velocity.

## Adverse Pressure Gradient

In fact near stall conditions, the airflow can actually reverse and flow backwards up the wing.

Where these two meet is where airflow separation will take place.

The boundary layer is acted on by 2 things:

1. friction

2. adverse pressure gradient

## Adverse Pressure Gradient

Laminar flow will lend to earlier airflow separation because of lower energy velocity

Turbulent flow will resist early airflow separation because of higher energy forces.

## Stall

Stall is airflow separation of the boundary layer from a lifting surface. Stall starts at the trailing edge and advances forward.

The accelerated stall is where airflow separation occurs at the leading edge first because the air can’t make the corner.

## High CL Devices

Camber changers:

Flaps – Plain, Split, Slotted, Fowler

Leading edge slats and flaps

## Helio Courier

## The Advantage of Leading Edge Slats

It is a 6-place bush/utility/sport-utility aircraft that features the 350 horsepower turbo charged six-cylinder Lycoming TIO-540 engine.

It is designed to have a minimum controllable airspeed of 26 knots and a cruise speed of 147 knots.

The Helio Courier is anticipated to have a useful load of 1,700 pounds and a range of 912 nautical miles with standard fuel capacity.

It can safely operate out of a 500 foot unimproved clearing at full gross weight and take-off and land in areas otherwise only accessible to aircraft such as 2-place Piper Super Cubs.

The Helio Courier is capable of safely taking off and landing in less than 250 feet.

On floats, it can land on and take off from many smaller lakes (not accessible with the de Haviland Beaver and Otter or the Cessna 180/185 and 206).

## The Vx Takeoff Looks a Little Different eh?

## High CL Devices

Energy Adders

Vortex generators stir up the boundary layer mixing higher velocity air higher up in the boundary layer with lower velocity air lower in the boundary layer.

Reduces takeoff and landing dis. and speeds

## Vortex Generators

These are mounted on a J3 Cub, lowering stall speed 5 – 6 kts! Like mini-wingtip vortices. Note how the vortex brings the high energy air closer to the wing surface, lowering drag.

## High CL Devices

Leading edge slot may allow airflow through to energize airflow pattern over the wing.

## High CL Devices

Boundary Layer Control is when a vacuum pump is used to suck faster moving air closer to the wing to artificially raise boundary layer velocity

Sometimes using pressurized air usually from the turbine is directed over the wing energizing the airflow