local motors wing design.pdf

7/25/2019 LOCAL MOTORS WING DESIGN.pdf

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Local Motors Unmanned Aircraft Design Guide

iTABLE OF CONTENTS

INTRODUCTION TO UNM NNED IRCR FT 2

P RTS OF N IRCR FT 10

INTRODUCTION TO ERODYN MICS 21

PRINCIPLES OF FLIGHT 35

IRCR FT ST BILITY ND CONTROL 41

CONCEPTS OF DESIGN 57


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INTRODUCTION TO UNMANNED AIRCRAFT

This guide will provide brief descriptions and details of the major

components and subsystems of a UAS.

This Design Guide is intended to familiarize the non-aviation communitywith some of terminology and design philosophy used to create unmanned

aircraft systems.

This guide is only to be used as a reference material, and is not intended to

satisfy any Civil Aviation Authorities requirements for training a UAS pilot.

Remember to follow your local national requirements before you attempt to

fly any unmanned aircraft.


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Aircraft

Unmanned Aircraft (UA), although ranging in size and appearance, ultimately

share common goals and design. Take a hand-launched MicroUAS and

compare it to a larger aircraft, and it will be obvious that they are designed

for the same purposes.

Desirable characteristics of a UA are:

Stability

Maximize payload capability

Endurance

Ruggedness

Maintainability

For the most part, UA are of composite construction. This means that

multiple types of materials are used. For example, the fuselage may becarbon fiber reinforced plywood and the wings may be cut from foam and

overlaid with carbon fiber. Ultimately the design will attempt to use

materials that will provide the strongest properties while remaining light

enough to maximize the payload capability.

Within the larger sized UA, the configuration of the space available to carry

the payload is important. Increasingly, over the last several years, more

diverse collections systems have been incorporated onto UA. The ability to

easily reconfigure the payload space to accommodate these various form

factors is desirable.

Compared to the UAS as a whole, the UA has the greatest concentration of

subsystems, all of which have to be designed to occupy the least amount of

space possible and weigh the least possible.


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Autopilot

The autopilot is essentially the brain of the UA. After reception of the uplink

commands through the antenna and receiver, the control computer in the autopilot

processes and distributes the commands to the respective components aboard the

UA. For redundancy purposes, the autopilot may incorporate several computers thatmay process independent information such as flight control and mission control. It

may also assume control of the aircraft and execute a pre-programmed series of

maneuvers if communication with the ground station is lost.

ElectricalSystem

The electrical system may consist of batteries, some form of engine driven generator,

relays, and switches. The electrical system may be divided into segments known as

busses, which power essential and nonessential equipment. In the event of an engine

failure, resulting in a loss of power generation, the autopilot may incorporate logic to

switch off the non-essential busses, thus reducing the load on the emergency or back-up batteries. An example of non-essential equipment may be the payload. Essential

components would be the flight control servos. In addition, the electrical system may

be designed to provide redundancy to critical components in the event of damage or

failures.

Flight Sensors

The flight sensors consist of the instruments that measure UA attitude, altitude, speed

and heading. Many UA incorporate solid state circuits and accelerometers referenced

to GPS position to measure these values.

Propulsion System

The propulsion system consists of the propeller, engine and all associated

components.


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Payload

The payload may consist of one large item or several smaller items

transported by the UA.

When designing the UA, consideration must be given to:

Location on the aircraft. The aircraft must be able to be safely and

efficiently operated throughout the full range of possible payload weights.

This will mean the design must take into consideration the change in aircraft

weight when the payload is transported and when it is offloaded.

Stability of the payload. Payloads and the containers that hold them

must ensure that the loads do not shift during flight. This can cause loss of

aircraft if the autopilot cannot compensate quickly enough.

Loading and unloading procedures. Payloads must be added to and

removed to aircraft with relative ease and minimal risk of damage (both to

cargo and the aircraft).

For this challenge, the payload focus is on cargo.


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Data Link System

The Data link system of a UAS can generally be described as the collection of

equipment, both ground based and airborne, that provide the path, through the use

of Radio Frequency (RF), to send commands to the UA (uplink) and receive reports and

status from the UA (downlink). The subsystems of the data link can be found in theGround Control Station (GCS), on the UA and as stand-alone equipment, such as the

antenna.

UAS data link is usually discussed in terms of uplink and downlink. The forward (up)

link controls the activities of the platform itself and the payload hardware. This

command and control link requires a sufficient degree of security to ensure that only

authorized agents have access to the control mechanisms of the platform. The return

(down) link transmits critical data from the platform payload to the warfighter or

analyst on the ground or in the air. System health and status information must also be

delivered to the ground control station or UAV operator without compromise.

Data link systems are designed as either line of sight (LOS) or beyond line of sight

(BLOS).

LOS- As implied, LOS data links are limited primarily by the curvature of the

earth, but also by local obstructions. Transmitter output power and antenna

sensitivity also contribute to the limitations of LOS systems. Most UAS that

are


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Spectrum Usage

Radio frequency is a term that refers to alternating current (AC) having

characteristics such that, if the current is input to an antenna, an electromagnetic

(EM) field is generated suitable for wireless broadcasting and/or

communications. These frequencies cover a significant portion of theelectromagnetic radiation spectrum, extending from kilohertz (kHz) the lowest

allocated wireless communications frequency (it's within the range of human

hearing), to thousands of gigahertz(GHz).

With respect to RF, there are two basic forms used in UAS data links: analog and

digital.

Analog- Analog is the process of taking an audio, RF or video signal and

translating it into electronic pulses or waves.

Digital- Digital is the process of breaking the signal into a binary format

where the audio, RF or video data is represented by a series of "1"s and

"0"s. The nature of digital technology allows it to compress many 1s and

0s together into the same space, thus requiring less bandwidth (space)

for an equal amount of data as an analog signal. Digital signals are more

tolerant to noise (interference), but digital signals can be completely

corrupted in the presence of excess noise. In digital signals, noise could

cause a 1 to be interpreted as a 0 and vice versa, which makes the

received data different than the original data.


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Ground Control Station

All UAS incorporate some form of control station, from which the flight of

the Unmanned Aircraft (UA) is controlled and monitored. The grouping of

ground based, Command and Control (C2) equipment, necessary to do this is

commonly referred to as a Ground Control Station (GCS).

The inception of many GCS can be traced to the engineering efforts of the

individual UAS manufacturers. The GCS was a tool simply used to

manipulate and test the platform from the perspective of the design

engineer. As such, standardization of displays and architecture, across

multiple UAS has not existed until recently.

The GCS of a UAS is the collection of equipment and structure to house the

equipment (aircraft mounted or other) required to control the specific

platform. The individual subsystems of the GCS will be discussed later in this

module.


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GroundSupport Equipment

Larger UAS (>55lbs) typically require specific support equipment to sustain

operations.

This equipment may include:

Generators

Ground vehicles used to transport the UAS

Specialized tools

Petroleum, Oil, Lubricants (POL) containment and dispensing systems

One of the more distinguishable characteristics of a UA is the method by

which it is launched and recovered. Some are designed strictly to be launched

by a catapult mechanism, while others operate by the more traditional

method of rolling takeoffs. Regardless of the method there will be additional

system support equipment commonly described as Launch and Recovery (LR)equipment.

This equipment may include:

Launcher

Compressor

Arresting gear

Specialized GCS and antenna for LR operations

Future challenges may address Service stations for cargo UAS.


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PARTS OF AN AIRCRAFT

Fuselage

Wings

Empennage

Powerplant

Rotors


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Fuselage

The body of the aircraft is called the Fuselage. This shape provides the space

for all of the components the aircraft.

Cargo compartments should be designed to be remain within the fuselage.


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Wings

Wings are the lifting surfaces for the aircraft, and are vital to the performance

of any airplane.

To be practical, an unmanned aircraft must be able to fly at very low speeds,

and also have acceptable cruise performance.

Chord Line: The theoretical line running from the leading edge of the wing to

the trailing edge. The chord line is frequently used in calculating the efficiency

of an airfoil.

The design challenge is to design a wing with a high lift coefficient so that the

wing area is as small as possible, while take-off / landing speeds are as low as

possible. Relatively short wings make the aircraft easier to taxi, especially when

operating in an off-airport environment with obstructions, and requires less

space for hangaring, while being easier to build, and stronger (less weight andwing span to support).


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Empennage

The main purpose of the empennage is to give stability to the aircraft.

Also called a tail assembly, the empennage includes static and dynamic control

surfaces.

Elevators, rudders and vertical stabilizers are dynamic control surfaces (highlighted in

color on the image to the right)


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Rotor Blades

Multirotors use counterrotating rotors for lift and directional control. A

quadcopter uses two clockwise and two counter-clockwise propellers.

Propellers are classified by length and pitch. For example, 94.7 propellers are

9 inch long and has a pitch of 4.7.


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Flight Control Surfaces and their Axis

A properly designed UA is stable and easily controlled during normal

maneuvering.

Control surface inputs cause movement about the three axes of rotation

through the Center of Gravity (C.G.)

The types of stability a UA exhibits also relates to the three axes of rotation.


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Helicopter Flight Control Surfaces

Rotorcraft require a different method of control than airplanes and are much

harder to master for teleoperation.

Flying a helicopter requires constant concentration by the pilot and a near-

continuous flow of minute control corrections.

Multirotors use differential control instead of cyclic control surfaces or anti-

torque petals.


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Flight Control Surfaces

The ailerons, elevator, and rudder constitute the primary control surfaces and

are required to control a UA safely during flight.

Aileron (Roll Control)

Elevator (Pitch Control)

Rudder (Yaw Control)

Hybrid control surfaces reduce the need for additional servos by combing

different primary controls:

Elevons- A control surface(s) that combine the functions of the elevator

and the aileron.

Ruddervators (V-Tail): This control surface combines the functions of

rudders and elevators


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Aileron Flight Control Surfaces (Longitudinal Axis-Roll)

Roll is controlled by the ailerons.

The UAs motion about its longitudinal axis resembles the roll of a ship from side

to side. The motion about the UAs longitudinal axis is roll.

Longitudinal axis is nose to tail

Roll or Bank rotates around the longitudinal axis

Ailerons must be sized correctly to make sure the aircraft is controllable. Ailerons

can be made more effective by increasing their surface area or moving them

farther away from the centerline of the aircraft.


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Elevator Flight Control Surface (Lateral Axis/Pitch)

The elevator controls pitch about the lateral axis. Deflecting the

trailing edge of the elevator surface up (up elevator) pitches

the nose of the UA up.

Deflecting the trailing edge of the elevator surface down (down

elevator) pitches the nose of the UA down.

Lateral axis is wing tip to wing tip

Pitch pivots around lateral axis

Elevators play a crucial function in aircraft control. Their

effectiveness depends on size (surface area), deflection range,

and distance from the lateral axis. This distance is often related

to the length of the fuselage. The deflection of the elevator

contributes to drag. So, an efficient design will have theminimum amount of elevator defection during cruise. When a

aircraft has a wide-range of operating weights, this optimum

setting can be a challenge.


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Rudder Flight Control Surface (Vertical axis/yaw)

The rudder controls movement of the UAs about its vertical axis. This motion

is called yaw.

Like other primary control surfaces, the rudder is a moveable surface hingedto a fixed surface, in this case to the vertical stabilizer, or fin

The vertical axis runs top to bottom

Yaw- pivots around the vertical axis

Yaw control is increased with a larger rudder or increased distance behind the

vertical axis. In some cases, it might be more efficient to use two separate

rudders instead of one large one.


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INTRODUCTION TO AERODYNAMICS

Introduction - Aerodynamics is the study of forces and resulting

motion of objects through the air. In order to understand

aerodynamics, an understanding of the physical properties of the

atmosphere and certain laws of physics are required. This module willdiscuss these principles and laws

Thrust, lift, weight and drag are forces that act upon all Unmanned

Aircraft (UA) in flight. Understanding how these forces work and

knowing how to control them with the use of power and flight

controls is essential to flight. This module discusses the aerodynamics

of flight- how design, weight, load factors, and gravity affect a UA

during flight maneuvers.


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Aerodynamic Forces

The four forces acting upon a UA are:

Thrust

Lift

Weight (Gravity)

Drag

Understanding how these forces work and knowing how to control them

with the use of power and flight controls is essential to flight.


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Thrust

The forward force produced by the Powerplant/propeller or rotor that sets

a UA in motion. It opposes or overcomes the force of drag. As a general

rule, in fixed wing UAS, it acts parallel to the longitudinal axis of the

airframe.

Thrust:

Puts a UA into motion

Overcomes drag

Induced by:

External (catapult or hand-launched)

Internal (onboard propulsion- sustained flight)


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Lift

Lift opposes the downward force of weight, and is produced by the dynamic

effect of the air acting on the airfoil. In fixed-wing UA, acts perpendicular to

the flight path through the center of the airfoil.

Lift is the upward force created by the wings moving through the air that

sustains a UA in flight.

Lift operates to overcome weight. It must be equal to or greater than the

weight of the object in flight and acting in the opposite direction in order to

sustain flight. Lift can be increased by increasing the forward speed of the

UA, or by increasing the wings angle of attack.

Lift produced by air flow across wings is affected by:

Wing size

Wing shape

Lift opposes weight by producing

High pressure under wing

Low pressure above wing

Lift is also affected by the density of the air

Lift is higher at lower altitudes

Lift decreases as altitude increases Causes aircraft to have an operational ceiling


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Weight

Weight is the load factor of the UA itself, the payload, the fuel (including

batteries). Weight pulls the UA downward because of the force of gravity. It

opposes lift, and acts vertically downward through the UAs center of gravity

(CG). The CG may be considered as a point at which all the weight of the UA isconcentrated and is a critical aspect for determining UA performance and

stability.

Total weight of the UA is a measure of the force of gravity

CG is a critical aspect for determining UA performance

Pulls the UA downward

Opposes lift


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Drag

Drag is a rearward, retarding force caused by disruption of airflow by the

wing, rotor, fuselage and other protruding objects. Drag opposes thrust, and

in a fixed wing UA, acts rearward parallel to the relative wind.

There are two basic types: parasite drag and induced drag. The first is called

parasitic because it in no way functions to aid flight, while the second,

induced drag, is a result of an airfoil developing lift.

Drag:

Acts rearward

Opposes thrust

Two Types of drag:

Parasitic Induced


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Density Altitude (Air Density), Temperature and Humidity

An increase in altitude, temperature, or humidity results in a corresponding

decrease in air density. These changes contribute to the resulting decrease in

lift and thrust being created by the power plant and lifting surface.


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Effects of Altitude

As we move upward through the atmosphere, the weight of the air above becomes

less and less. Under standard conditions at sea level, a column of air that has a

footprint of one square inch would weigh 14.7 pounds. That same column of air at

18,000 feet is approximately 7.4 pounds, almost 50 percent less than the sea level

column.


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Temperature

This concept is best described by the Kinetic Theory of Gases. This theory

states that a body of gas is composed of identical molecules which behave

like minute elastic spheres spaced relatively far apart and continuously in

motion.

The degree of this molecular motion depends on the temperature of the

gas. An increase in temperature will result in an increase in molecular

motion with a corresponding increase in collisions between the molecules

and the walls of the container. The increase in collisions results in an

increase in pressure because a greater number of molecules strike against

the walls of the container.

Because of the Kinetic Theory, warm air also rises. Heat causes the air

molecules to spread apart, becoming less dense and lighter than the

surrounding air. As air cools, the molecules pack together more closely,

becoming denser and heavier than warm air and therefore it descends


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Effects of Pressure, Temperature and Humidity on Density

Since air is a gas, it can be compressed or expanded. When air is compressed,

a greater amount of air can occupy a given volume. Conversely, when

pressure on a given volume of air is decreased, the air expands and occupies a

greater space. At a lower pressure, the original volume of air contains a

smaller mass of air. The density is decreased because density is directly

proportional to pressure. If the pressure is doubled, the density is doubled; if

the pressure is lowered, the density is lowered. This statement is true only at

a constant temperature.

Because of airs compressibility, flight conditions will vary depending upon the

altitude. This is due to the air density. More molecules for a given volume of

air will generate greater lift with less thrust. Fewer molecules for a given

volume of air will require greater thrust to generate the same lift. Air density

decreases with an increase in altitude.


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Temperature/Density

Increasing the temperature of a substance decreases its density. Conversely,

decreasing the temperature increases the density. Thus, the density of air

varies inversely with temperature. This statement is true only at a constant

pressure. In the atmosphere, both temperature and pressure decrease withaltitude, and have conflicting effects upon density. However, the fairly rapid

drop in pressure as altitude is increased usually has the dominating effect.

Hence, pilots can expect the density to decrease with altitude.


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Humidity/Density

The preceding paragraphs refer to air that is perfectly dry. In reality, it is

never completely dry. The small amount of water vapor suspended in the

atmosphere may be almost negligible under certain conditions, but in other

conditions humidity may become an important factor in the performance of

an aircraft. Water vapor is lighter than air; consequently, moist air is lighter

than dry air. Therefore, as the water content of the air increases, the air

becomes less dense, increasing density altitude and decreasing performance.

It is lightest or least dense when, in a given set of conditions, it contains the

maximum amount of water vapor.

Humidity, also called relative humidity, refers to the amount of water vapor

contained in the atmosphere, and is expressed as a percentage of the

maximum amount of water vapor the air can hold. This amount varies with

temperature. Warm air holds more water vapor, while colder air holds less.

Perfectly dry air that contains no water vapor has a relative humidity of zero

percent, while saturated air, which cannot hold any more water vapor, has a

relative humidity of 100 percent.


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Pressure Altitude (PA)

In the standard atmosphere, sea level pressure is 29.92 inches Hg and is used as the

Standard Datum Plane (SDP). Pressure altitude is the height above or below this SDP.

Example: If an aircraft were at an airfield located at sea level, and standard atmospheric

conditions prevailed, the altimeter was set to 29.92, the altimeter would indicate 0 feet.

As atmospheric pressure changes from standard, the SDP may be below, at, or above

actual sea level.

There are several ways to determine or calculate what the pressure altitude is at any

location.

The first and easiest method is to set the barometric pressure of a sensitive altimeter to

29.92. The altitude indicated on the face of the altimeter is pressure altitude.

The second method is to calculate it as follows: subtract the current barometric pressuresetting from standard setting (29.92) and either add or subtract this number depending on

whether it is a positive or negative number, to/from field elevation.

Example: standard atmosphere 29.92 minus current atmosphere 29.98 equals -.06. This

represents -60 feet. At an airfield with a surveyed altitude of 1200 feet, the pressure

altitude would be 1140 feet (1200-60).

The third method is through the use of a chart typically found with the rest of the

performance charts for your UA/aircraft. An example of this type chart is to the right.


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Density Altitude

Air density is affected by changes in altitude, temperature, and humidity. High

density altitude refers to thin air (the molecules are spread out) while low

density altitude refers to dense air (the molecules are tightly packed).

The conditions that result in a high density altitude are high elevations, lowatmospheric pressures, high temperatures, high humidity, or some combination

of these factors.

Density altitude is determined by first finding pressure altitude, and then

correcting this altitude for nonstandard temperature variations.

Since density varies directly with pressure (the higher the pressure, the more the

molecules are compressed, or. denser), and inversely with temperature (the

higher the temperature, the more the molecules spread apart, or. less dense), a

given pressure altitude may exist for a wide range of temperatures by allowing

the density to vary. However, a known density occurs for any one temperature

and pressure altitude. Regardless of the actual altitude at which the UA/aircraft

is operating, it will perform as though it were operating at an altitude equal to

the existing density altitude. The density of air has significant effects on the

UA/aircrafts performance.

As air becomes less dense, it reduces:

Power because the engine takes in less air.

Thrust because a propeller/rotor is less efficient in thin air.

Lift because the thin air exerts less force on the airfoils


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PRINCIPLES OF FLIGHT

Introduction

The theories defining lift have historically been the adaptation over the past few

centuries of basic physical laws. The fundamental physical laws governing the forces

acting upon an aircraft in flight were adopted from postulated theories developedbefore any human successfully flew an aircraft.

Sir Isaac Newton, formulated the law of universal gravitation and also described the

three basic laws of motion.

In 1852, a German physicist and chemist, Heinrich Gustav Magnus made experimental

studies of the aerodynamic forces on spinning spheres and cylinders. (The effect had

already been mentioned by Newton in 1672) These experiments led to the discovery of

the Magnus Effect, which helps explain the theory of lift.

A half-century after Newton formulated his laws, Daniel Bernoulli, a Swissmathematician, explained how the pressure of a moving fluid (liquid or gas) varies with

its speed of motion. This principle explains what happens to air passing over the

curved top of the airplane wing.


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Bernoullis Principle

As the velocity of a moving fluid (liquid or gas) increases, the pressure within the

fluid decreases.

A practical application of Bernoullis Principle is the venturi tube.

The venturi tube has an air inlet that narrows to a throat (constricted point) and an

outlet section that increases in diameter toward the rear. The diameter of the

outlet is the same as that of the inlet. At the throat, the airflow speeds up and the

pressure decreases; at the outlet, the airflow slows and the pressure increases.

Since air is recognized as a body and it is accepted that it must follow the above

laws, one can begin to see how and why an airplane wing develops lift. As the wing

moves through the air, the flow of air across the curved top surface increases in

velocity creating a low-pressure area. This pressure difference results in an

upwards lift force.

The carburetor used in many reciprocating engines contains a venturi to create a

region of low pressure to draw fuel into the carburetor and mix it thoroughly with

the incoming air. The low pressure in the throat of a venturi can be explained by

Bernoulli's principle; in the narrow throat, the air is moving at its fastest speed and

therefore it is at its lowest pressure.

The principle also makes it possible for sail-powered craft to travel faster than the

wind that propels them (if friction can be sufficiently reduced). If the wind passing

in front of the sail is fast enough to experience a significant reduction in pressure,

the sail is pulled forward, in addition to being pushed from behind.


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Magnus Effect

Flow of air against a non-rotating cylinder - If air flows against a cylinder

that is not rotating, the flow of air above and below the cylinder is

identical and the forces are the same.

A rotating cylinder in a motionless fluid - If a cylinder is rotated and

observed from the side while immersed in a fluid, the rotation of the

cylinder affects the fluid surrounding the cylinder. The flow around the

rotating cylinder differs from the flow around a stationary cylinder due to

resistance caused by two factors: viscosity and friction.

Viscosity - This is the property of a fluid or semi-fluid that causes

it to resist flowing. High-viscosity fluids resist flow; low-viscosity

fluids flow easily.

Friction - This is the resistance one surface or object encounters

when moving over another and exists between a fluid and the

surface over which it flows.A rotating cylinder in a moving fluid - If a cylinder rotates in a fluid that is

also moving, the result is a higher circulatory flow in the direction of the

rotating cylinder.


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The Magnus Effect is applied to an airfoil in the following ways:

The highest differences of velocity are 90 from the relative motion between

the cylinder and the airflow. Additionally, a stagnation point exists where

the air stream impacts on the front of the airfoils surface and splits; some

air goes over and some under. When viewed from the side, an upwash iscreated ahead of the airfoil and downwash at the rear.

The highest velocity of air flow is at the top of the airfoil with the lowest

velocity at the bottom. This concept can be readily applied to a wing or

other lifting surface, because there is a difference of velocity above and

below the wing. The result is a higher pressure at the bottom of the wing

and a lower pressure on the top of the wing. This low-pressure area

produces an upward force known as the Magnus Effect.

To summarize the Magnus effect, an airfoil with a positive Angle of Attack (AOA)

develops low pressure air circulation above the upper surface of the wing/rotor.


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Angle of Attack/Stall (AOA)

AOA is the acute angle between the chord line of the airfoil and the direction of

the relative wind. A UA stall results from a rapid decrease in lift caused by the

separation of airflow from the wings surface brought on by exceeding the critical

AOA.

A stall can occur at any pitch attitude or airspeed. As long as the airfoils lift (L in

the figure to the right) is less than the aircraft weight (W in the figure to the

right) the aircraft will stall.

Stalls are one of the most misunderstood areas of aerodynamics because drone

pilots often believe an airfoil stops producing lift when it stalls. In a stall, the wing

does not totally stop producing lift. Rather, it cannot generate adequate lift to

sustain level flight.

Critical AOA

Angle at which AOA produces maximum lift

Varies with wing design

When exceeded, lift decreases

Stall

Decrease in Lift

Critical AOA exceeded

Lift not sufficient to counter weight (L


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Dissymmetry of Lift

Unique to rotary wing aircraft is a phenomenon known as dissymmetry of lift.

This is the difference of lift created by the advancing and retreating sections

of the rotor disk.

The differential that results from the relative wind (i.e., the airflow over therotor disk) caused early helicopters to be uncontrollable. In traditional

rotocraft, technological solutions, such as articulated rotor blades and hinged

flapping masts, are used to counteract this condition.

Multirotor aircraft, configured with counter-rotating blades, are inherently

resistant of the effects of dissymmetry of lift.


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AIRCRAFT STABILITY AND CONTROL

Application

An airfoil is constructed in such a way that its shape takes advantage of

the airs response to these physical laws.

As the air stream strikes the relatively flat lower surface of a wing or

rotor blade when inclined at a small angle to its direction of motion, the

air is forced to rebound downward, causing an upward reaction in

positive lift. An airfoil is shaped to cause an action on the air, and forces

air downward, which provides an equal reaction from the air, forcing

the airfoil upward. At the same time, the air stream striking the upper

curved section of the leading edge is deflected upward. If an airfoil is

constructed in such form that it causes a lift force greater than the

weight of the UA/aircraft, the aircraft will fly.

If all the lift required were obtained merely from the deflection of air by

the lower surface of the airfoil, an UA/aircraft would only need a flat

wing like a kite. However, the balance of the lift needed to support the

UA/aircraft comes from the flow of air above the airfoil. Herein lies the

key to flight. Applying Bernoullis Principle of Pressure, the increase in

the speed of the air across the top of an airfoil produces a drop in

pressure. This lowered pressure is a component of total lift. The

pressure difference between the upper and lower surface of an airfoil

alone does not account for the total lift force produced.


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Center of Gravity

The center of gravity (CG), or center of mass, is the point at which the entire

weight of the UAS is concentrated. If supported at this point, the UAS would

remain in equilibrium (balanced) in any position. The CG is the intersection of

the lateral, longitudinal, and vertical axes.

The CG can be determined by calculation, graphically, physically hanging the

vehicle by a hook from a simple jig, or supporting it from the underside on a pair

of posts.

The point where the wings and fuselage are level (as measured by a laser/bubble

level) and parallel to the floor is the CG. This method will yield the lateral and

longitudinal CG locations. The vertical CG will normally have to be calculated or

determined graphically.


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Stability

Stability is the inherent quality of an UA/aircraft to correct for conditions that

may disturb its equilibrium, and to return to or to continue on the original

flight path. It is primarily a UA/aircraft design characteristic. If a UA/aircraft

is to fly straight and steady along any arbitrary flight path, the forces acting

on it must be in static equilibrium (When disturbed, it has a tendency to

return to its previous position or attitude). The reaction of any body when its

equilibrium is disturbed is referred to as stability. The two types of stability

are static and dynamic. Stability in an UA/aircraft affects two areas

significantly:

Maneuverability This is the quality of a UA/aircraft that permits it to

be maneuvered easily and to withstand the stresses imposed by

maneuvers. It is governed by the aircrafts weight, inertia, size,

location of flight controls, structural strength, and powerplant. It is a

UA/aircraft design characteristic. Maneuverability requires a certain

lack of stability to be inherently designed into the aircraft. Complete

stability would result in an aircraft that could not be maneuvered at

all.

Controllability This is the capability of a UA/aircraft to respond to

the pilots control.


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StaticStability

Static stability refers to the initial tendency, or direction of

movement, back to equilibrium. In aviation, it refers to the

UA/aircrafts initial response when disturbed from a given pitch

attitude, roll attitude, or yaw attitude. Types of static stability:

Positive Static Stability This is the initial tendency of the

UA/aircraft to return to the original state of equilibrium after being

disturbed

Neutral Static Stability This is the initial tendency of the

UA/aircraft to remain in a new condition after its equilibrium has

been disturbed

Negative Static Stability This is the initial tendency of the

UA/aircraft to continue away from the original state of equilibrium

after being disturbed

Essentially, static stability can be defined as the initial tendency toreturn to or not to return to equilibrium that a UA/aircraft displays

after being disturbed from its trimmed condition.


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Dynamic Stability

Dynamic stability refers to the UA/aircraft response over time when

disturbed from a given pitch attitude, roll attitude, or yaw attitude.

This type of stability also has three subtypes:

Positive Dynamic Stability - Over time, the motion of the

displaced object decreases in amplitude (amplitude is the amount a

value changes from its at- rest, or normal condition to its maximum

condition) and, because it is positive, the object displaced returns

toward the equilibrium state.

Neutral Dynamic Stability- Once displaced, the displaced object

neither decreases nor increases in amplitude. A worn automobile

shock absorber exhibits this tendency.

Negative Dynamic Stability- Over time, the motion of the

displaced object increases and becomes more divergent.


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Longitudinal Stability (Pitching)

Longitudinal stability is the quality that makes a UA/aircraft stable about its

lateral axis. It involves the pitching motion as the UA/aircrafts nose moves up

and down in flight. A longitudinally unstable UA/aircraft has a tendency to

dive or climb progressively into a very steep dive or climb, or even a stall.

Thus, a UA/aircraft with longitudinal instability becomes difficult and

sometimes dangerous to fly.

The following is a simple demonstration of longitudinal stability. If a slight

push to nose the UA/aircraft down were given to the control of an UA/aircraft

and after a brief period, the nose rose to the original position and then

stopped, the UA/aircraft is statically stable. Ordinarily, the nose passes the

original position (that of level flight) and a series of slow pitching oscillations

follows. If the oscillations gradually cease, the UA/aircraft has positive

stability; if they continue unevenly, the UA/aircraft has neutral stability; if they

increase, the UA/aircraft is unstable.

Lift Effects

The primary surface to control longitudinal stability is the elevator. In addition, longitudinal stability is greatly affected by the relationship

between the center of gravity (CG) and the center of lift. The combined (wing and horizontal tail) center of lift must be aft of the CG.


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The closer the two centers are, the more maneuverable, but less stable the aircraft

becomes. This range of locations is known as the Static Margin. The Center of Lift

(aerodynamic center) on a low speed, symmetric aircraft wing is approximately 25% of

the mean (average) chord length measured aft of the leading edge of the wing.

Now that the locations of both the CG and the Center of Lift are known, the distancebetween the two points, divided by the chord length, is the Static Margin.

Typical values range from 5% to 40%. As previously mentioned, the smaller the SM, the

less stable and more maneuverable the vehicle will be. However, in order to maintain

stability, the horizontal tail will play a much larger role. This may mean it must be larger

and may create more drag.

Thrust Effects

Longitudinal stability is also affected by the engine. In some configurations, the increase

in power will create additional airflow over the wings creating more lift. The location of

the CG in the vertical axis may also contribute to stability. As depicted in the diagram,

the relationship between the thrust line and the CG may cause pitching motions. Unless

properly accounted for, these influences may cause the aircraft to become unstable.


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Lateral Stability (Rolling)

Stability about the UA/aircrafts longitudinal axis, which extends from the nose

of the UA/aircraft to its tail, is called lateral stability. This helps to stabilize the

lateral or rolling effect when one wing gets lower than the wing on the

opposite side of the aircraft.Properly sized ailerons aid in controlling all aspects

of lateral stability.

There are five main design factors that make an UA/aircraft laterally stable:

Dihedral- The most commonprocedure for producing lateral stability is

to build the wings with an angle of one to three degrees above

perpendicular to the longitudinal axis. The wings on either side of the

aircraft join the fuselage to form a slight V angle called dihedral.

Sweepback This is an addition to the dihedral that increases the lift

created when a wing drops from the level position. A sweptback wing

is one in which the leading edge slopes backward. When a disturbancecauses a UA/aircraft with sweepback to slip or drop a wing, the low

wing presents its leading edge at an angle that is perpendicular to the

relative airflow. As a result, the low wing acquires more lift, rises and

the UA/Aircraft is restored to its original attitude.

Keel Effect and Weight Distribution A UA/aircraft always has the

tendency to turn the longitudinal axis of the aircraft into the relative

wind. This weather vane tendency is similar to the keel of a ship and

exerts a steadying influence on the UA/aircraft laterally about the

longitudinal axis. When the UA/aircraft is disturbed and one wing dips,

the fuselage weight acts like a pendulum returning the airplane to its

original attitude.

With propeller driven aircrafts, the torque of the engine may cause roll

in the opposite direction of the propeller motion. This is especially

noticeable during changes in power settings.

Lateral CG Location If the CG is offset from the centerline of the

vehicle, this imbalance will tend to introduce rolling motions.


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Vertical Stability (Yawing)

Stability about the UA/aircrafts vertical axis (the sideways moment) is called yawing

or directional stability. Yawing or directional stability is the most easily achieved

stability in UA/aircraft design. The area of the vertical fin and the sides of the

fuselage aft of the CG are the prime contributors which make the UA/aircraft act

like the well known weather vane or arrow, pointing its nose into the relative wind.

Rudder size is also a key factor.

Positive directional stability is ensured by having more fuselage side surface aft of

the CG than ahead of it. To provide additional positive stability to that provided by

the fuselage, a vertical fin is added. The fin acts similar to the feather on an arrow

in maintaining straight flight. Like the weather vane and the arrow, the farther aft

this fin is placed and the larger its size, the greater the UA/aircrafts directional

stability.

If a UA/aircraft is flying in a straight line, and a sideward gust of air gives theUA/aircraft a slight rotation about its vertical axis (i.e., the right), the motion is

retarded and stopped by the f in because while the UA/aircraft is rotating to the

right, the air is striking the left side of the fin at an angle. This causes pressure on

the left side of the f in, which resists the turning motion and slows down the

UA/aircrafts yaw.

Power changes and aileron deflections will also induce additional yaw and must be

properly managed.


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Airspeed change and straight-and-level as required

All of the principal components of flight performance involve steady-state flight

conditions and equilibrium of the UA. For the UA to remain in steady, level flight,

equilibrium must be obtained by lift equal to the UA weight and power plant thrust

equal to the UA drag.

Thus, the UA drag and weight defines the Thrust required to maintain steady, level flight.

For all practical purposes, the airspeed will remain constant in straight-and-level flight

with a constant power setting. Significant changes in airspeed will, of course, require

considerable changes in pitch attitude and pitch trim to maintain altitude.

These required changes are especially evident in low-speed or hovering flight when

performed by rotorcraft.


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Load Factors While Turning

Newtons First Law of Motion, the Law of Inertia, states that an object at rest

or moving in a straight line remains at rest or continues to move in a straight

line until acted on by some other force. A UA/aircraft, like any moving object,

requires a sideward force to make it turn. In a normal turn, this force is

supplied by banking the UA/aircraft so that lift is exerted inward, as well as

upward. The force of lift during a turn is separated into two components at

right angles to each other. One component, which acts vertically and opposite

to the weight (gravity), is called the vercal component of li. The other,

which acts horizontally toward the center of the turn, is called the horizontal

component of lift, or centripetal force. The horizontal component of lift is the

force that pulls the UA/aircraft from a straight flight path to make it turn.

There are three factors associated with turning that a pilot should be familiar

with.

Additional lift is required while turning to maintain constant altitude

Radius and Rate of Turn depend on speed and angle of bank

Stall speed increasesas bank angle increases


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Establishing and Maintaining Altitude

No discussion of climbs (increase in altitude) and descents (decrease in

altitude) would be complete without touching on the question of what

controls altitude and what controls airspeed. The UA pilot must understand

the effects of both power and pitch (elevator control), working together

during different conditions of flight. At any pitch attitude, the amount of

power used will determine whether the UA will climb, descend, or remain

level at that attitude.

Through a range of altitudes from very slightly nose-low to about 30onose-

up, a typical light UA can be made to climb, descend, or maintain altitude

depending on the power used. In about the lower third of this range, the UA

will descend at idle power without stalling. As pitch altitude is increased,

however, increased engine power will be required to prevent a stall. Even

more power will be required to maintain altitude, and even more for a

climb. Typically, at a pitch altitude approaching 30onose-up, all available

power will provide only enough thrust to maintain altitude. A slight increase

in the steepness of climb or a slight decrease in power will produce a descent.


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Basic Aerodynamic Climb

The wings lift in a steady state normal climb is the same as it is in a steady

level flight at the same airspeed. During the transition from straight-and-level

flight to a climb, a change in lift occurs when back elevator pressure is first

applied. Raising the UAs nose increases the AOA and momentarily increases

the lift. Lift at this moment is now greater than weight and starts the UA

climbing.

Since the lift during the bank is divided into vertical and horizontal

components, the amount of lift opposing gravity and supporting the

UA/aircrafts weight is reduced. Consequently, the UA/aircraft loses altitude

unless additional lift is created. This is done by increasing the Angle of Attack

(AOA) until the vertical component of lift is again equal to the weight.

Since the vertical component of lift decreases as the bank angle increases,

the AOA must be progressively increased to produce sufficient vertical lift tosupport the UA/aircrafts weight. An important fact for pilots to remember

when making constant altitude turns is that the vertical component of lift

must be equal to the weight to maintain altitude.

Since the drag of the airfoil is directly proportional to its AOA, induced drag

increases as the lift is increased. This, in turn, causes a loss of airspeed in

proportion to the angle of bank. A small angle of bank results in a small

reduction in airspeed while a large angle of bank results in a large reduction

in airspeed. Additional thrust (power) must be applied to prevent a

reduction in airspeed in level turns. The required amount of additional thrustis proportional to the angle of bank

Rotorcraft encounter this in sideward f light.

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Basic Aerodynamic Cruise

Cruise is the level portion of UA travel where flight is the most fuel

efficient

Forces are balanced

o Thrust=drago Lift=Weight

Neither gains nor loses altitude

Airspeed may vary

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Basic Aerodynamic Bank

Enables UA to turn (change heading)

Less lift is opposing weight

Some lift is acting in the direction of the bank

If the UA were in a bank it would be apparent that lift did not act directly opposite to the

weight, rather it now acts in the direction of the bank.

When the UA banks, lift acts inward toward the center of the turn, as well as upward.

Because some of the lift is used to turn the UA, lift must be increased to increase the

component that offsets weight in order to maintain level flight. This increase in lift

causes an increase in induced drag. Thrust must be increased to maintain airspeed.

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Basic Aerodynamic Descent

A UA may descend either by reduction in thrust or by a reduction in the angle of attack.

When reducing thrust, lift is decreased. With weight now being greater than lift, the UA begins to descend.

When reducing the pitch and therefore the AOA the resulting decrease in lift will also cause a descent.

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CONCEPTS OF DESIGN

Overview

1. Mission of the UA what do I want it to do?

2. Design constraints what limitations are imposed on my design

Size

Weight

Powerplant/type

Takeoff/landing requirements

3. Once the design constraints are known, then you can work through to

determine the critical factors

4. Now an iterative process begins to balance the four forces while achieving

the mission

5. Iterative process is always influenced by inherently dependent variables

Stability

Controllability Maneuverability

Maintainability

Reliability

Serviceability

Operability

6. Once design begins to take shape, it will need to be tested at various

phases, always keeping in mind the ultimate mission. Tradeoffs will need to

be made between various limitations of the design. For instance, a bigger

engine will make it fly faster, but it weighs more. The more weight will needa larger wing. The larger wing produces more drag so it might end up flying

slower. There is an optimal balance between mission and physics. It is your

job to find that harmony.

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Determining Wing Area Configuration and Design

Aspect Ratio: The ratio of the wings length to its chord line.

Aspect Ratio = (wing span)/chord

Aspect Ratio = (wing area)/chord2

A wing with a high aspect ratio will perform well at slowspeeds and produce large quantities of lift, but at the

expense of maneuverability and airspeed.

A wing with a low aspect ratio on the other hand will have

a sleek appearance and allow an aircraft to fly faster, or be

more maneuverable.

Typical aspect ratios are 4 to 5 for fast, maneuverable

aircraft (like a fighter jet); 12-15 for general aviation and

transport aircraft; and 25 or greater for gliders.

Camber: The name given to the curvature of the upper or lowersurfaces of the wing. A higher camber, or more curved surface,

results in an aircraft that can fly at slower speeds while still

generating sufficient lift for flight.

Chord Line: The theoretical line running from the leading edge of

the wing to the trailing edge.

Leading Edge: The front edge of an aircrafts wing.

Trailing Edge: The rear edge of an aircrafts wing.

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Airfoil Designs

The following pages show typical cross-sections for various airfoil shapes. Keep

in mind that thicker wings produce more lift at lower airspeeds but create more

drag as airspeed increases. A more rounded leading edge is less susceptible to

stalls but have more drag over all airspeeds.

The National Advisory Committee for Aeronautics (NACA) airfoils were

designed during the period from 1929 through 1947 under the direction of

Eastman Jacobs at the NACAs Langley Field Laboratory

The numbering system for these airfoils is defined by: NACA MPXX

M is the maximum value of the mean line in hundredths of chord

P is the chordwise position of the maximum camber in tenths of the

chord.

XX is the maximum thickness in percent chord

The graphs are percentages.

A NACA 4418 airfoil that has a chord of 20 inches also has a camber of 4

inches at 6 inches along the chord line

A Clark Y airfoil that has a chord of 20 inches also has a camber of 2.2

inches at 6 inches along the chord line

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Determining Wing Area

Tapered Wing Area:

Average Chord = (Root Chord + Tip Chord ) 2

Wing Area = Wing Span x Average Chord

Rectangular Wing Area: Wing Span x Wing Chord

Ellipse Wing Wing Area = ( 3.14 x Span x Chord ) 4

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Wing Loading

While gross takeoff weight of the aircraft is important, wing loading is even more

so. This is a reflection of the distributed weight across the lifting surface area.

The lighter the wing loading, the slower the aircraft can take-off, fly and land. It will

have a better climb.

Wing Loading: Weight Wing Area

For multi-wing aircraft, divide the overall weight of the aircraft by the total wing

area for all wings.

For a 55lb aircraft, the wing area should be between 3600 square inches (232 dm2)

and 1300 square inches (84 dm2) for best wing loading (10 and 30 kg/m2)

Rotor Disc Loading

Rotor Disc Loading: Disc Load = Helicopter Weight (kg) / (* Blade Length m2)

For a 55lb aircraft, the combined rotor disc should be between 32 and 44

diameter for best rotor loading (24 and 48 kg/m2)

For larger multirotors that carry payloads, large propellers and low-kv motors tend

to work better. These have higher relative rotational momentum, and will more

easily maintain the aircrafts stability.

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Determining Thrust

Short Takeoff and Landing Required Thrust Per Motor

Choose a power system that can provide a pitch speed of about 2 to 3 t imes the stall speed and a 2/3 thrust to weight ratio. This means that a

55lb aircraft needs 36lbs of thrust for a single pusher, or 18lbs for twin pushers.

VTOL lift Required Thrust per motor = (Weight x 2) / # of Rotors

This means that a 55lb aircraft needs 27.5lbs thrust for each motor for quad rotors, or 18.33lbs for hexacopters.

Power Requirements

This operating profile needs 75-100 watts/pound, which equates to 4125 to 5500 watts.

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Calculations (Imperial)

Wing Area (Square Feet) ( )

Wing Loading (Ounces per square feet) ()

( )

Pitch Speed (Propellers) ( )

Stall speed 3.7

Power in (Watts)

Shaft Power Out (Watts)

Shaft Horsepower ()

local motors wing design.pdf

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