飞机航模基础知识

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First Model

Some people consider a glider as the obvious choice for the first

model.

Although a glider normally flies slower and is supposed to be more

forgiving,

I think that's just a matter of taste.

Being a skilled glider pilot doesn't necessarily mean being also a skilled

powered aircraft pilot and vice-versa.

Assuming that a powered model was chosen, the beginner is advised to

start

with a so-called trainer.

This type is usually a high wing aircraft model with nearly flat bottom

airfoil

that produces high lift, permitting slow landing speeds without stalling.

It also has some dihedral angle to give a good lateral stability.

However, a flat bottom high wing

with

dihedral is more sensitive to

crosswind

gusts, so the first flights

should be

done during calm weather.

A beginner should avoid wings

with

too sharp leading edges, as it

will

worsen the stall

characteristics.

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A well-rounded leading edge is therefore preferable, as it better

conveys the

airflow onto the upper wing surface allowing higher angle of attack at

low speed.

A trainer model should not be too small, as it would be difficult to

assemble and

maintain and would be more sensitive to strong winds.

It should not be too large

either,

as it would be difficult to

transport,

require a larger flying

field and

would be more expensive.

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A reasonable size is about 150cm wingspan (60 in) with a high aspect

ratio,

which means the wingspan being about 5.5 times the wing chord.

A square wing is advisable, as it distributes the weight of the aircraft

evenly

over the entire surface of the wing.

In order to allow a reasonable low landing speed without stalling, the

wing

loading should not be greater than about 60g/ (19-oz/sq. ft).

Wing loading is the aircraft's weight divided by the wing area.

Some degree of wing washout also improves the stall characteristics.

The basic parts of a trainer model:

Engine

- provides the power to rotate the propeller.

Propeller

- (also Prop) is attached to the engine's shaft to convert rotational motion

into thrust

and speed, which depends on the Prop's diameter, pitch and the Engine's power.

Spinner

- streamlined part that covers the end of the Prop shaft.

Fin

- (also Vertical Stabiliser) provides directional stability (stability in yaw).

Rudder

- moveable part fitted to the Fin's trailing edge, is used to change the aircraft's

direction.

Stabiliser

- (also Horizontal Stabiliser or Stab) provides longitudinal stability (stability

in pitch).

Elevator

- moveable part fitted to the Horizontal Stabiliser's trailing edge, is used to

make the

aircraft climb or dive.

Ailerons

- movable parts on both sides of the wing, are used to make the aircraft roll

about its

fore - aft axis. When one aileron moves up the other moves down.

Wing

- provides the aircraft's main lifting force.

One may build a model aircraft based on drawings (plans). This

requires some

building skills and also time and effort to find out and gather the

materials needed

for the construction.

An easier approach (albeit more expensive) is buying a kit of parts.

There are many kits on the market with different levels of prefabrication

depending

on their price.

The cheaper kits have most of parts included, but some pieces come

either pre -

cut or printed on sheets of wood, so the builder is expected to do some

extensive

job, such as to cut out the fuselage formers and wing ribs, glue the parts

together,

apply the covering material, etc.

For those who are not so keen on construction, there are almost ready

to fly (

ARF

)

kits with an extensive prefabrication, requiring one or two evenings to

assemble.

There are also ready to fly (

RTF

), which normally come complete with

the power

plant and some of them even with the radio preinstalled.

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First Flight

It's highly recommended to have an experienced instructor beside you

during

your first flight, however, it is not impossible to get succeed by doing it

alone.

Max wind speed recommended is 5 - 8Km/h (3 - 5mph) including gusts.

Check the CG location with empty fuel tank by supporting the model

with your

fingertips underneath the wings. Find the position where the fuselage

gets level

or its nose is pointed slightly downwards.

Transmission range check should be performed on the ground before

the flight.

This is usually done with the Transmitter aerial collapsed. The control

surfaces

should respond without glitch at a distance of about 80 meters (263ft).

This distance is only an approximately guide line, as the actual range

may vary

depending on the environment.

The effective range may only be half of this value if located at mountain

bowl site

or close to a public radio transmitter, radar station or similar.

The range may suffer adverse effects if the receiver aerial is close to

metal parts

or model components reinforced with carbon fibre.

Some transmitters allow the aerial to be totally collapsed inside a metal

case,

which also may reduce the radiation.

In this case the lower section of the aerial should be extended during

the test.

The check should be repeated with the power system running,

alternating the

throttle setting between idle and full-throttle.

The range will be much higher when the model is in the air, normally

about 1Km

or as far as one can see the model.

Take- off:

If you hand launch your model, throw it against the wind horizontally

and

straight ahead, not up.

If you take-off from the ground, taxi the model towards the wind and let

the

model gain ground speed before applying elevator.

Once in the air try to climb at a very small angle, not abruptly upward,

which

would cause loss of airspeed and stall.

The model is more sensitive to the motor torque effect during the

relatively

low take-off speed and may begin to turn left (or right). Use the rudder

or

ailerons to prevent the model from turning during the climb stage,

otherwise

the model may initiate a spiral dive.

Don't try any turns until the model has gained speed and reached a

altitude

in the

air before you try to land.

To prevent losing altitude when turning the model, just give little up

elevator

at same time you make a turn.

Try to keep the model in sight and do not fly too high or too far away.

You may reduce the throttle while high in the air so you may get an idea

how

the model behaves at low speed.

To prevent getting confused about which way to turn when the model

flies

towards you, turn your back to the model slightly while keeping

watching it,

so you can imagine

Some trimming may be needed in order to reduce or eliminate roll, bank

and/or

pitch tendencies.

A flat bottom wing often tends to

climbing

when full throttle is applied. This may be reduced during the flight by

adjusting

the elevator trim or by reducing the throttle.

In worst cases it may be needed to increase the motor's down-thrust

angle

and/or decrease the main wings incidence angle.

Landing:

Reduce throttle to about half so you have to slightly pull up the elevator

to keep

the altitude.

Turn the model towards the wind and let the model sink gradually

towards the

landing area by easing the elevator.

During the last fifteen to twenty meters (45 to 60 feet) of descent,

(which

depends on the model's characteristics) you should idle the throttle.

The model will start sinking at a higher rate now. Try to keep the model

in a

shallow dive and don't use the elevator to gain altitude or to prolong the

flight at this stage, otherwise stall is likely to occur.

Just keep a slightly downward attitude throughout the final approach in

order

to maintain the airspeed.

The higher the wing loading, the steeper the approaching angle may be

however, it is not recommended approaching angles greater than 45

degrees.

If you notice that the model is sinking too fast or is too low to reach the

landing

field - just increase the throttle first before applying elevator to maintain

or gain

altitude to prolong the flight or to repeat the landing approach.

Pull up the elevator slightly about 30-60cm (1-2 ft) before the

touch-down so

that the propeller or nose gear don't hit the ground.

Be prepared to repeat unsuccessful landings several times, since it's

often a

matter of trial and error before one gets used with how the model

behaves.

Don't try to land in a specific spot, avoid turns when the model is flying

low

or at low speed. Just let your model glide into the ground

straight-ahead.

Avoid the proximity of buildings, roads and electric power lines.

Don't fly close to or towards people and animals.

The bigger the field for your first flight, the greater will be your chances

for success.

- Good luck.

It's also advisable to join the nearest model aircraft club there you may

meet

experienced flyers who can provide lots of useful tips and hints.

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Aerodynamics

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Introduction

Aerodynamics is the study of forces and motion of objects

through the air.

Basic knowledge of the

aerodynamic

principles

is highly recommended

before getting

involved

in building and/or

flying

model aircraft.

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A model aircraft that is hanging still in air during strong winds may

be subject

to the same aerodynamic forces as a model aircraft that is flying

fast during

calm weather.

The aerodynamic forces depend much on the air density.

For

example,

if

a

glider

glides

25

met

from a given altitude during low air

density

it may glide 40 meters during high

density.

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The air density depends on the atmospheric pressure and on the

air temperature.

The air density increases with decreasing of the air temperature

and/or with

increasing of the atmospheric pressure.

The air density decreases with increasing of the air temperature

and/or with

decreasing of the atmospheric pressure.

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A flying aircraft is subject to a pressure depending on the airspeed

and the

air density.

This pressure increases exponentially with increasing of the

airspeed.

The aircraft's resistance to the airflow (drag) depends on the

shape of the

fuselage and flying surfaces.

An aircraft that is intended to fly fast has a thinner and different

wing profile

than one that is intended to fly slower.

That's why many aircraft change their wings' profiles on landing

approach

by lowering the flaps located at the wings' trailing edge and the

slats at the

leading edge in order to keep enough lifting force during the much

lower

landing speed.

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The wings' profile of an aircraft is usually asymmetric, which

makes the

pressure on the wings' upper side lower than the underside,

causing the air on

the wings upper side to accelerate downwards, thereby a lift force

is created.

The air always flows away from areas of higher pressure toward

areas of lower

pressure, thus the air over the wing top accelerates as it enters the

lower

pressure region (where the air curves toward the wing), whereas

the air under

the wing slows down as it enters the higher pressure region.

So, one may also say that the wings create lift by reacting against

the air flow,

driving it downwards, producing downwash.

The top of the wing is often the major lift contributor, usually

producing twice as

much lift as the bottom of the wing.

The lift force of a symmetric profile is based on the airspeed and

on a positive

angle of attack to the airflow, which makes the air react as it was

asymmetric.

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The following picture shows the airflow through two wing profiles.

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The uppermost profile has a lower angle of attack than the lowest

one.

When the air flows evenly through the surface is called a laminar

flow.

A too high angle of attack causes turbulence on the upper surface,

which

dramatically increases the air resistance (drag), this may cause the

flow

to separate from the upper surface resulting in an abrut reduction

in lift,

known as stall.

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Summarising:

The aircraft generates lift by moving through the air.

The wings have airfoil shaped profiles that create a pressure

difference

between upper and lower wing surfaces, with a high pressure

region

underneath and a low pressure region on top.

The lift produced will be proportional to the size of the wings, the

square

of airspeed, the density of the surrounding air and the wing's

angle of

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attack to on- coming flow before reaching the stall angle.

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How does a glider generate the velocity needed for flight?

The simple answer is that a glider trades altitude for velocity.

It trades the potential energy difference from a higher altitude to a

lower

altitude to produce kinetic energy, which means velocity.

Gliders are always descending relative to the air in which they are

flying.

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How do gliders stay aloft for hours if they constantly descend?

The gliders are designed to descend very slowly.

If the pilot can locate a pocket of air that is rising faster than the

glider is descending, the glider can actually gain altitude,

increasing

its potential energy.

Pockets of rising air are called updrafts.

Updrafts are found when the wind blowing at a hill or mountain

rises to

climb over it. (However, there may be a downdraft on the other

side!)

Updrafts can also be found over dark land masses that absorb

more

heat from the sun than light land masses.

The heat from the ground heats the surrounding air, which causes

the

air to rise. The rising pockets of hot air are called thermals.

Large gliding birds, such as owls and hawks, are often seen

circling

inside a thermal to gain altitude without flapping their wings.

Gliders can do exactly the same thing.

Aerodynamics

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Wing Geometry Definitions

A vertical cut through the wing parallel to flight's direction (plan view)

will show

the cross-section of the wing.

This side view (profile) is called Airfoil, and it has some geometry

definitions

of its own as shown on the picture below.

The longest straight line that can be drawn from the Airfoil's leading

edge to

trailing edge is called the Chord Line.

The Chord Line cuts the airfoil into an upper surface and a lower

surface.

If we plot the points that lie halfway between the upper and lower

surfaces,

we obtain a curve called the Mean Camber Line.

For a symmetric airfoil (upper surface the same shape as the lower

surface)

the Mean Camber Line will fall on top of the Chord Line.

But for an asymmetric airfoil, these are two separate lines. The

maximum

distance between these two lines is called the Camber, which is a

measure

of the curvature of the airfoil (high camber means high curvature).

Asymmetric airfoils are also known as cambered airfoils.

The maximum distance between the upper and lower surfaces is

called the

Thickness.

Both Thickness and Camber are expressed as a percentage of

Chord.

Airfoils can come with all kinds of combinations of camber and

thickness

distributions. They are designed for the condictions under which

the plane is

likely to be flown most of the time.

NACA (the precursor of NASA) established a method of

designating classes

of airfoils and then wind tunnel tested the airfoils in order to

provide

lift coefficients and drag coefficients for designers.

Aspect Ratio is a measure of how long and slender a wing is from

tip to tip.

The Aspect Ratio of a wing is defined to be the square of the span

divided

by the wing area and is given the symbol AR.

The formula is simplified for a rectangular wing, as being the ratio

of the span

to the chord length as shown on the figure below.

Wing Dihedral refers to the angle of wing panels as seen in the

aircraft's

front view.

Dihedral is added to the wings for roll stability; a wing with some

Dihedral

will naturally return to its original position if it is subject to a briefly

slight

roll displacement.

Most large airliner wings are designed with Dihedral.

On the contrary the highly maneuverable fighter planes have no

Dihedral.

In fact, some fighter aircraft have the wing tips lower than the

roots, giving

the aircraft a high roll rate.

A negative Dihedral angle is called Anhedral.

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Stability Concepts

The aircraft's response to momentary disturbance is associated

with its

inherent degree of stability built in by the designer, in each of the

three axes,

and occurring without any reaction from the pilot.

There is another condition affecting flight, which is the aircraft's

state of trim

or equilibrium (where the net sum of all forces equals zero).

Some aircraft can be trimmed by the pilot to fly 'hands off' for

straight and

level flight, for climb or for descent.

Free flight models generally have to rely on the state of trim built in

by the

designer and adjusted by the rigger, while the remote controlled

models have

some form of trim devices which are adjustable during the flight.

An aircraft's stability is expressed in relation to each axis:

lateral stability (stability in roll), directional stability (stability in

yaw)

and longitudinal stability (stability in pitch).

Lateral and directional stabilities are inter-dependent.

Stability may be defined as follows:

- Positive stability: tends to return to original condition after a

disturbance.

- Negative stability: tends to increase the disturbance.

- Neutral stability: remains at the new condition.

- Static stability: refers to the aircraft's initial response to a

disturbance.

A statically unstable aircraft will uniformly depart from a condition

of equilibrium.

- Dynamic stability: refers to the aircraft's ability to damp out

oscillations, which

depends on how fast or how slow it responds to a disturbance.

A dynamically unstable aircraft will (after a disturbance) start

oscillating with

increasing amplitude.

A dynamically neutrally stable aircraft will continue oscillating

after a disturbance

but the amplitude of the oscillations will not change.

So, a statically stable aircraft may be dynamically unstable.

Dynamic instability may be prevented by an even distribution of

weight inside the

fuselage, avoiding too much weight concentration at the

extremities or at the CG.

Also, control surfaces' max throws may affect the flight stability,

since a too much

control throw may cause instability, e.g. Pilot Induced Oscillations

(PIO).

Static stability is proportional to the stabiliser area and the tail

moment.

You get double static stability if you double the tail area or double

the tail moment.

Dynamic stability is also proportional to the stabiliser area but

increases with the

square of the tail moment, which means that you get four times the

dynamic stability

if you double the tail arm length.

However, making the tail arm longer or encreasing the stabiliser

area will move

the mass of the aircraft towards the rear, which may also mean the

need to make

the nose longer in order to minimize the weight required to

balance the aircraft...

A totally stable aircraft will return, more or less immediately, to its

trimmed state

without pilot intervention.

However, such an aircraft is rare and not much desirable. We

usually want an

aircraft just to be reasonably stable so it is easy to fly.

If it is too stable, it tends to be sluggish in manoeuvring, exhibiting

too slow

response on the controls.

Too much instability is also an undesirable characteristic, except

where an

extremely manoeuvrable aircraft is needed and the instability can

be continually

corrected by on-board 'fly-by-wire' computers rather than the pilot,

such as a

supersonic air superiority fighter.

Lateral stability is achieved through dihedral, sweepback, keel

effect and

proper distribution of weight.

The dihedral angle is the angle that each wing makes with the

horizontal (see

Wing Geometry).

If a disturbance causes one wing to drop, the lower wing will

receive more lift

and the aircraft will roll back into the horizontal level.

A sweptback wing is one in which the leading edge slopes

backward.

When a disturbance causes an aircraft with sweepback to slip or

drop a wing,

the low wing presents its leading edge at an angle more

perpendicular to the

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

rises, restoring

the aircraft to its original flight attitude.

The keel effect occurs with high wing aircraft. These are laterally

stable simply

because the wings are attached in a high position on the fuselage,

making the

fuselage behave like a keel.

When the aircraft is disturbed and one wing dips, the fuselage

weight acts like

a pendulum returning the aircraft to the horizontal level.

The tail fin determines the directional stability.

If a gust of wind strikes the aircraft from the right it will be in a slip

and the fin

will get an angle of attack causing the aircraft to yaw until the slip

is eliminated.