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Advanced Aerodynamics

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AF 202 – Chris Dimoulis

Advanced Aerodynamics

A little bit of Theory

A little bit of Physics

A little bit of Math

A whole lot of fun!

Leading edge

Trailing edge

Chord Line

Relative Wind

Angle of Attack

What is a force?

Any influence to an object that causes a change in speed, direction, or shape.

What is acceleration?

A change in velocity (speed and direction)

An unbalanced force is required for an acceleration.

Force, acceleration, velocity are ‘vectors’

The most known force formula:

Force = Mass x Acceleration

You can rewrite this to say that:

Acceleration = Force/Mass

The 4 Airplane Forces are:

Weight

Lift

Thrust

Drag

Remember what an acceleration is

Change of speed and/or direction

Requires and unbalanced force

Opposing forces must be equal

Thrust = Drag

Lift = Weight

If weight and lift are not balanced…

Weight > Lift = Airplane decends

Lift > Weight = Airplane pitches up (NOT CLIMB!!!!)

If thrust and drag are not balanced…

Thrust > Drag = Aircraft speeds up

Drag > Thrust = Aircraft slows down

It is simply a matter of adding the vectors together…

The resulting force is…

This will produce an acceleration in the direction of that force.

-90

100

10

Weight is a force

The acceleration in F=ma is the pull of gravity (9.8 m/s2)

The total weight of your plane is the downward force applied at the center of gravity (CG).

We feel weight

It is equally opposed by the ground.

Therefore there is no acceleration

Opposes weight

Acts perpendicular to the flight path

Occurs at the center of lift/pressure

Two principles are used to explain lift:

Bernoulli’s Principle

Newton’s 3rd law of motion

When the velocity increases the pressure decreases

High pressure always seeks a low pressure

High pressure below the wing applies a force upward

For every action there is an equal and opposite reaction

What ways can lift be changed?

Pilot Controlled:

Change Speed

Change Angle of Attack

Non-Pilot Controlled

Change Wing Surface Area

Change Air Density

L=1/2 (CL V² Ρ A)

CL= Coefficient of Lift

V = Velocity (ft per sec)

1 knot = 6076 ft/hr = 1.68527 ft/sec

P = Air Density

A = Wing Surface Area (sq.ft.)

The coefficient of Lift increase as Angle of Attack increases

Velocity also has a great impact.

Velocity increases – Lift Increases

Air density and the design of the wing have an affect, but cannot be changed by the pilot

Altitude Density Speed of Sound

(Feet) (d) (Knots)

0 .002377 661.7

1,000 .002308 659.5

2,000 .002241 657.2

3,000 .002175 654.9

4,000 .002111 652.6

5,000 .002048 650.3

Angle of Attack = 5 degrees

Coefficient of lift = .4

Airspeed = 100 knots = 607600 ft/hr = 168.7 ft per sec

Air density = .002175 (3000 feet)

Wing surface area of 172 = 174 sq.ft.

L = .5(.4 x 168.72 x .002175 x 174)

Lift = 2154 lbs

Increase the speed to say… 150 knots and let’s see what happens

150 knots = 253.167 ft/sec

L = .5(.4 x 253.1672 x .002175 x 174)

L = 4853 lbs

THAT’S 2G’S AT FULL WEIGHT

With just a little rearranging we can learn other wonderful things about lift

Rearrange for velocity and we can see what angle of attack will do to it (assuming you remain level)

Rearrange for Coefficient of Lift to see what Angle of Attack you need for a given Air speed

Keep L = 2154 lbs. for level flight

V = SqRt(2L/ (CL Ρ A))

Make the Angle of Attack 10 degrees

Coefficient of lift = .8

V = SqRt(4308/(.8 x .002175 x 174))

V = 119.2 ft/sec

V = 70 knots

Once again keep lift at 2154 lbs.

CL = 2L/(V² Ρ A)

Slow airspeed from 100 to 80 knots

80 knots = 135 ft/sec

CL= 4308/(1352 x .002175 x 174)

CL = .62

Angle = 8 degrees

ASSUMING LEVEL FLIGHT IS MAINTAINED

VelocityAngle of Attack

1005 degrees

906 degrees

808 degrees

7010 degrees

6013 degrees

5016 degrees

As Angle of Attack increases we decrease speed

As speed decreases we need to increase angle of attack to maintain lift

As speed increases we must decrease angle of attack.

Airplane stalls at Critical Angle of Attack

An airplane can stall at any airspeed

Why then do we have Vso and Vs?

The Airplane will stall at the critical angle of attack regardless of

Speed

Pitch

Angle of Bank

Below Vso and Vs you the required angle of attack to produce lift is beyond the critical angle of attack

Types of stalls

Landing

Takeoff

Trim

Cross-Control

Secondary

Accelerated

Drag is the rearward acting force opposing thrust

Two types

Parasite

Induced

Three Types of Parasite Drag

Form Drag

Skin Friction

Interference

Increases when speedincreases (exponential)

Inherent whenever a wing produces lift

Increases when speed decreases

WHY??????

As speed decreases we need to increase angle of attack

Induced drag increases as AOA increases

Induced Drag is also caused by wingtip vortices

High pressure below the wing is pulled toward the low pressure above the wing.

D =1/2 (Cd V² Ρ A)

Cd = Coefficient of Drag

V = Velocity (ft per sec)

P = Air Density

A = Wing Area (Sq.Ft.)

Speed = 100 knots = 168 ft/sec

Air Density = .002175

Wing Area = 174 sq.ft.

Angle of Attack = 2 degrees

Coefficient of Drag= .03

D = .5(.03 x 1682 x 174 x .002175)

D = 160 lbs of Drag

After a certain speed drag increases and so the required thrust increases.

Region of reverse command refers to the need for MORE power to fly SLOWER speeds

This is reversed from normal (hopefully that is obvious to you)

As one enters ground effect, much of the downwash, upwash, and wingtip vortices are reduced effectively increasing your Coefficient of Lift

Forward force pulling/pushing plane through the air.

Thrust is most easily described as lift in the horizontal direction

The propeller aerodynamically functions similar to a wing

By spinning it creates its own relative wind.

Why does the propeller have a twist in it?

Just like with lift, thrust on the propeller increases with angle of attack and with speed.

The outside of the propeller spins faster thus requiring a smaller pitch

No Propeller is 100% Efficient

Effecitve Pitch

Geometric Pitch

Slippage

Asymmetrical Thrust (P-Factor)

Gyroscopic Precession

Spiraling Slipstream

Torque from the engine

When pitching up, the angle on the DOWNWARD moving blade is greater than that on the upward moving blade. Causes a left yaw.

A gyroscope is basically something that spins

A force applied to a spinning object is felt 90 degrees in the direction of rotation.

Pitch up – right yaw

Pitch down – left yaw

The slipstream strikes the left side of the rudder yawing tail right and the nose left

The engine is rotating Clockwise from a pilot’s view. The opposing reaction makes the plane want to bank left