Basic Aeronautics and Aerodynamics

Forces of Flight

Four Forces of Flight

• 4 fundamental forces act in an aircraft in flight

• Forces have a magnitude and direction which makes them “vectors”

• Vectors can be added and subtracted to give resultant or net force When forces add to zero, aircraft is in steady flight When a force is unbalanced, aircraft accelerates in direction of net force

All Forces Balance in Steady Flight

Forces in a Turn

LIFT

Daniel Bernoulli (1700 - 1782)

Bernoulli did the pioneering work on fluid dynamics which forms the basis for aerodynamics

Showed that pressure of a fluid decreases as velocity increases

His work described the relationship between pressure and velocity in a fluid

Airfoil Design

How an Airfoil Works

Higher speed flow (relatively low pressure)

Lower speed flow (relatively high pressure)

Relatively low pressure force downward

Relatively high pressure force upward

The shape of an airfoil generates lift when air flows over it

Wingtip Vortices

Vortices From Large Aircraft Are Very Dangerous

THRUST

Propellers

Propellers Convert Engine Revolutions to Thrust

Increasing speed

• A propeller is a spinning airfoil• Spinning propeller produces thrust• First successful aircraft propeller invented by the Wright Brothers

• Notice the pitch decreases further away from the hub

• This keeps the load even

• The faster moving tip doesn’t need as much pitch as near the hub

DRAG

Drag is Air Resistance to Motion

Flat plate Sphere or Cylinder

Airfoil

• Which has the most drag?

• Which has the least drag

Laminar Flow

• Laminar flow is smooth

• Turbulent flow has a lot of swirling and mixing

• Laminar flow is desirable for aircraft as it has lower drag than turbulent flow however there are other considerations we shall see later

Turbulence

• Turbulent flow results when airflow is no longer laminar and becomes “swirly”• Straight streamlines become unsteady and mix • BUT turbulence isn’t all bad……it “sticks” to a surface better than laminar flow

Notice how strings are not in straight lines

Highly Turbulent Becomes Separated Flow

• Separated flow is highly turbulent flow that is no longer “attached” to the surface• Separated flow results in a HUGE increase in DRAG• In the case of an Airfoil it results in a STALL

Stalls

• Stall occurs when flow over wing separates

• Separated flow results in high pressure

• Result is loss of lift (and life if not corrected!)

• Turbulent flow remains “attached” better than laminar flow

• Sometimes it’s better to have a turbulent flow than a laminar flow

• For example, do you know why a golf ball has dimples??

Using Turbulence for Advantage

Aircraft in Motion

Center of Gravity

The center of gravity is the average location of the weight of an object. In flight, both airplanes and rockets rotate about their centers of gravity.

Aircraft Move in 3 Axes

Longitudinal (Roll) Axis

Roll controlled by ailerons on the wings

Vertical (Yaw) Axis

Yaw controlled by the rudder on the vertical stabilizer

Lateral Axis

Pitch controlled by the elevator on the horizontal stabilizer

Flight Maneuvers Use all Three Axes

Aircraft Engines

Reciprocating Engines• Reciprocating engines are used in many aircraft• Same type of engine used in cars and trucks• Based on 4 stroke cycle also known as Otto cycle

– 1 power stroke for every 4 strokes

Nicolaus OttoGermany

1832 -1891

#1 Intake Stroke

StartIntake valve opens and piston

drops down and sucks in a mixture of air and fuel

Valves

Fuel/air mix

#2 Compression

Valves close and piston moves upward compressing the fuel/air mixture

#3 Ignition and Power

Spark plug fires at top of compression stroke

Exploding gas pushes piston downward

#4 Exhaust

Exhaust valve opens and piston pushes burned gases out of the cylinder

Hot exhaust gases

Review

Can you follow the four stroke cycle??

Turbine Engines

Why Turbines??Turbines (jets) are:• Simpler (higher reliability)

Recip TBO 1500 to 2000 hrs Jet TBO 5000+ hrs

• Smoother running• Produce much higher thrust at high speed• Capable of very high speeds (much higher than propeller engine)

Props loose efficiency at high speeds due to tip speed approaching Mach 1• Much higher power per unit weight (as much as almost 4:1)

But• Require special high temperature materials• High fuel consumption• Not efficient for low speed operations• Very loud (environmental issue)

A Quick Comparison

CharacteristicLockheed 1049G

Boeing 747

Weight, W, lb 112 000 700 000

Speed, V, mph 330 530

Altitude, ft 23 000 35 000

Lift-drag ratio, L/D 15 16

Number of engines 4 4

Total cruise power, hp 6 585 59 934

Power per engine, hp 1 646 14 984

Dry engine weight, lb 3 675 8 600

Power to weight ratio 0.45 1.74

Wright 3350 Pratt & Whitney JT-9

Turbojet Engines

Turbofan Engines

Turbofan has some of the good properties of a propeller engine such as better low speed efficiency but retains high speed jet performance

Turboprop Engines

Turboprop is more like a propeller engine with high low speed efficiency but lower top speed compared to pure jet

Ramjets and ScramjetsInstead of compressor turbines, ramjets use the “ram” pressure of inlet air for compression

• Ramjets are very simple with no moving parts• But they can only operate at very high speeds

Structures and Other Systems

Fuselage Structure

3 main types of aircraft structures:

• Truss (used in early fabric covered airplanes)

• Semi-monocoque– Loads carried by frame and skin

• Monocoque– Loads carried by skin

Truss Monocoque

Semi-monocoque

Landing Gear

• Conventional (“Tail dragger”)• Tricycle• Tandem• Fixed and Retractable - why is retractable gear a good thing??

Fuel Systems

• Fuel usually stored in wings• Fuel “Feed”

– Gravity– Force (pumped)

Fuel tanks in wings

Wing lower than engine

Wing higher than engine