De Agostini RB7 Manual de usuario

Red Bull Racing RB7: Step by Step

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370

Stages 77-80

Pack 20

RB7

RADIO CONTROLLED • BUILD IT YOURSELF • NITRO ENGINE

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Red Bull Racing RB7: Step by Step

™

371

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RB7

Editorial and design by Continuo Creative, 39-41 North Road, London N7 9DP

RED BULL RACING RB7 complies with CE regulations.

NOT SUITABLE FOR CHILDREN UNDER THE AGE OF 14. THIS PRODUCT IS NOT A TOY AND IS

NOT DESIGNED OR INTENDED FOR USE IN PLAY. ITEMS MAY VARY FROM THOSE SHOWN.

Photo credits: All photographs copyright

Visit our website www.model-space.com

Intro

The GX21 silencer system

The GX21 air lter

Cleaning the air lter

The RC power switch

Stage 77

Fitting the silencer

Stage 78

Fitting the air lter

Stage 79

Fitting the RC power switch

Stage 80

The battery box and antenna tube

Page 372

Page 374

Page 376

Page 378

Page 381

Page 389

Page 395

Page 397

Contents

Red Bull Racing RB7: Step by Step

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372

THE GX21 SILENCER

SYSTEM

UNLIKE THE SILENCER SYSTEM OF A FOUR-STROKE ENGINE, THAT OF A TWO-

STROKE NOT ONLY EXPELS THE EXHAUST GASES AND REDUCES THE NOISE, BUT

ALSO HELPS COMPRESS THE FUEL-AIR MIXTURE IN THE COMBUSTION CHAMBER.

On a four-stroke engine, expelling the exhaust gases and

reducing noise are the main tasks of the silencer system,

but the silencer of a two-stroke – such as your GX21 – is

used primarily to optimise its performance.

With a four-stroke, the inlet and exhaust ports of the

combustion chamber are opened and closed by valves.

When the piston moves downwards on the rst (induction)

stroke, the inlet valve opens and the fuel-air mixture ows

into the combustion chamber. After the compression and

combustion strokes comes the exhaust stroke, when the

exhaust valve opens and the piston drives the exhaust

gases out of the chamber.

The situation is dierent with the two-stroke engine,

where it is the movement of the piston that opens or

closes the inlet and exhaust ports (see the intro section in

Pack 16). When the piston is at the top of the combustion

chamber, it is closing o both the inlet and the exhaust

ports. As it moves downwards on the combustion stroke, it

opens the exhaust port and the exhaust gases are ejected

by the pressure generated by the explosion. As the piston

moves further downwards, it also opens the inlet port,

while suction is created in the combustion chamber by the

escaping exhaust gases. Since both ports are open and the

piston is at the bottom of the cylinder, the fuel mixture is

drawn not only into the combustion chamber but also into

the silencer system. This is a really undesirable eect, since

it means that a portion of the fuel-air mixture can escape

unused and is wasted.

Dome-headed screws

O-ring

Silencer cap

Silencer (expansion chamber) assembly

Manifold

Tailpipe

The GX21 silencer system

components

Dome-headed screw

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373

In the 1950s, East German motorcycle engineer Walter

Kaaden converted this disadvantage into an advantage.

He designed a system that prevented the unburnt fuel-air

mixture from owing out with the exhaust and at the same

time led to a considerable increase in engine performance:

the tuned pipe.

THE RESONANCE EFFECT

Basically, this system consists of nothing more than an

expansion chamber in the silencer pipe. It is shaped so

that the exhaust gases do not escape freely but are slowed

down, and some of the exhaust gas is trapped. The length

of the pipe and the volume of the expansion chamber are

selected so that the exhaust gases resonate in time with the

piston as it goes up and down – they expand as the piston

descends and are reected back towards the combustion

chamber as it rises again. In this way, the silencer system

causes a stronger compression of the mixture in the

combustion chamber.

To achieve the resonance eect, the cross-section of

the silencer pipe downstream of the exhaust port must

increase. In the case of your RB7 racer this occurs at the

transition from the manifold to the main chamber of the

silencer. This creates suction, which helps to ush the burnt

gases from above the piston and at the same time speeds

These two diagrams illustrate operation of the expansion chamber. Fresh

mixture ows into the manifold, but is forced back into the combustion

chamber by the reected exhaust gases.

Fresh mixture ows into the

combustion chamber

The mixture in

the combustion

chamber is

compressed more

The exhaust

gases

ow into

the expansion

chamber

Piston descends Piston rises

Reected

exhaust gases

ow back from

the resonance point

Resonance

point

The exhaust gases

are reected o

the walls

A portion of the

mixture ows into

the manifold

up the ow of fresh mixture into the combustion chamber

(see diagram above left). When the exhaust gases have

expanded, they collide with the walls of the expansion

chamber, and because the cross-section of the silencer

system narrows again towards the tailpipe, only a small

proportion of the gases can escape. Most of the gases are

reected back from the walls of the expansion chamber

and are concentrated at the so-called resonance point (see

diagram above right).

The resulting excess pressure pushes some of the exhaust

gases towards the manifold, forcing the fresh mixture that

has owed into it back into the combustion chamber. Only

when the piston rises again and closes the exhaust port will

the reected exhaust gases move through the tailpipe to

the outside atmosphere. This releases the pressure build-up

and the exhaust gases of the next combustion cycle can

ow into the expansion chamber.

The challenge in producing the resonance eect is to

match the oscillation frequency of the exhaust gases in the

silencer to the speed of the engine. This is dicult, because

the resonant frequency of the silencer system is xed at

a certain value dened by its shape and size, while the

speed of the engine varies from 3,000rpm when idling to

over 30,000rpm at full throttle. The silencer system of your

GX21 is designed so that the resonance eect starts when

the extra power can be most eective – when accelerating

out of a corner. This corresponds to an engine speed of

24,000rpm.

Piston descends Piston rises

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374

THE GX21 AIR FILTER

ANY DIRT ENTERING THE GX21 ENGINE THROUGH THE AIR INTAKE COULD DAMAGE

ITS CARBURETTOR, PISTON AND CYLINDER. A FINE AIR FILTER ENSURES THAT DIRT

PARTICLES CANNOT PENETRATE INTO THE INSIDE OF THE POWER UNIT.

The core of the GX21 air lter unit is a ne-pored foam

element 23mm in diameter and 20mm long. The centre of

the cylindrical element has a 15mm bore, which is pushed

onto a star-shaped projection on the lter base. The

cleaned air is sucked into the carburettor through a hole in

the base, also about 15mm in diameter.

FILTERING THE FINEST PARTICLES

The element is held in place by a plain plastic disc with

a diameter of 21mm (slightly smaller than that of the

element), xed onto the upper end of the star-shaped

projection by a single screw. Because the element is

slightly compressed from end to end, the thickness of the

cylindrical wall through which the air is sucked is about

4mm. Dust grains and other ne particles get trapped in

the ne pores of the foam.

The components of the air lter, when assembled, will t onto the

carburettor, which can be seen here just in front of the red cylinder head

of the GX21 engine.

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375

The air lter unit is tted inside the right and left parts

of the driver’s helmet in the running bodywork. This

concealment is necessary because there is no room

beneath the bodywork of the model to house the engine

assembly. Because of this space restriction, the air lter

has to protrude from the body somewhere in the area

of the cockpit, and the obvious solution to the problem

of siting it was to integrate it with the driver’s helmet.

In this way, this indispensable but somewhat inelegant

technical component is hidden inside one of the car’s most

important external details, decorated with authentic logos.

A slot below the helmet visor, plus six semi-circular

openings at the base of the air lter in the ‘neck’ of the

helmet, allow a good supply of clean air to be safely sucked

into the engine. From the lter, the cleaned air passes

through a curved rubber pipe, 50mm long, which feeds it

into the carburettor.

OPTIMISING FILTER PERFORMANCE

For the air lter to do its job of ltering ne particles

eectively, the foam rubber insert must be impregnated

with a specially formulated air lter oil (see photograph

above), which is available from model shops and online.

This fairly viscous lubricant narrows the pores of the foam,

making the lter even ner, and it also helps the foam to

create an airtight seal at the joins between the element and

the base and the element and the lter cap. This airtight

seal ensures that no stray dirt particles can sneak under or

over the ends of the element and into the engine.

After ve tanks of fuel have been used, the air lter

should be removed and cleaned. To do this, you have to

split open the driver’s helmet to remove the lter assembly,

then take the lter apart so that you can clean and re-

lubricate the foam element.

It’s advisable to saturate the lter element with air lter oil before tting it

onto the base. Knead the element to work the oil into it and distribute it

evenly across the element’s exterior.

Right helmet air lter cover

Air lter cap Air lter element

Air lter base

Air lter pipe

The air lter unit is concealed within the driver’s

helmet, and a rubber pipe connects it to the

GX21 engine.

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376

Some nitro fuel

Phillips screwdriver

(size 1)

Absorbent cloth

Small container

Air lter

oil

Protective

gloves

Craft knife

To clean the air lter, you will need

these tools and materials.

CLEANING THE

AIR FILTER

TO PROTECT THE ENGINE FROM CONTAMINATION AND TO INCREASE ITS LIFESPAN,

YOU SHOULD GIVE THE AIR FILTER A THOROUGH CLEAN AT REGULAR INTERVALS.

HERE’S HOW YOU DO IT.

The air lter of your RB7 model contains a cylindrical

foam insert. This traps grains of dust and dirt that would

otherwise be carried along with the air sucked into the

carburettor and then into the cylinder, where they could

cause damage to the cylinder liner. When your RC car is

running, the air lter intake is only a few centimetres above

the ground. So as well as having to lter out the dust that

exists in the air, it has to deal with dirt from the track and

the residue of worn tyres. Consequently, the lter can

become clogged after quite a short time. The engine no

longer receives enough air to burn the fuel in the cylinder

completely, and the result is a reduction in engine power.

CLEANING THE FILTER

Depending on the condition of the track, it is advisable to

clean the air lter at intervals of ve to ten tanks of fuel.

Cleaning it doesn’t take long, and the process requires little

eort. The tools and materials needed are illustrated in the

photograph on the right, and they include items that you

may already have in your RC workshop case. The cleaning

agent is the same nitro fuel that you use to run your car.

Please note that this uid can cause skin irritation, so

make sure to protect yourself by wearing gloves when you

use it to clean the lter. Just follow the steps on the next

page, and your GX21 engine will reward you by delivering

consistent performance.

Before any prolonged period in which the engine will not

be used, you can supplement this regular basic cleaning

of the lter element by washing it in a bowl of warm water

containing a little dishwashing liquid, which will remove

all traces of oil residue. Then place it in clean water to rinse

away all the detergent.

Red Bull Racing RB7: Step by Step

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377

The air lter of the GX21 engine is concealed inside the driver’s helmet. To open the two half-shells

of the helmet so that the air lter can be reached, you have to cut through the two decorative

stickers on the front section of the helmet because they cross the join. It is best to do this with a

sharp craft knife or cutter. The pieces concerned are the sticker on the helmet visor and the one

that extends across the chinguard of the helmet. The Red Bull logo on the visor is already made of

two halves so it does not need to be divided. Position the tip of the blade at the top edge of the

visor and slide it carefully downwards.

After cutting the two stickers on the front of the helmet, the half-shells of the helmet can be pulled

apart enough to allow the air lter to be removed from its holder. To do this, loosen the top cover of

the lter by undoing the screw in the centre with a size 1 Phillips screwdriver. Now you can pull the

dirty foam lter element from the base of the lter. Wear gloves when doing this – the dirt particles

could lead to skin irritation.

After a further rinse with nitro fuel, place the lter element on an absorbent, lint-free piece of cloth

or on kitchen paper. Fold the absorbent material over the element, and squeeze the foam-rubber

cylinder thoroughly between the two layers of cloth or paper. Now rub the material gently over the

lter surface, so that it picks up any dust particles remaining in the foam after the rinsing process.

Hold the element up to the light to check the progress of the cleaning. If necessary, repeat the

rinsing and drying procedure until all the oil and dirt has been removed.

To clean the lter element, put it in a small container, such as the lid of a spray can. Drip nitro fuel

onto the element until it is saturated. (Again, wear gloves when doing this.) The fuel will dissolve

the old air lter oil and loosen any dirt residues that have built up in the pores of the element.

Vigorously squeeze the fuel-soaked element several times with your ngers (wearing gloves!) to

distribute the fuel throughout the foam. This process will remove the coarse dirt particles trapped

in the element, and they will settle on the bottom of the container.

To ensure the optimum performance of the ne-pored foam lter, you should soak it with air lter

oil after cleaning. This air lter oil is available in model shops and online, and its thick consistency

ensures that even the nest dust particles are retained. Put a few drops on the lter insert and

knead it well to work the oil into the pores. Now you can reassemble the air lter and t it back in

place between the half-shells of the driver’s helmet. For more information about the air lter and

detailed instructions for tting it, see Stage 78.

Red Bull Racing RB7: Step by Step

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378

The RC power switch and its cables, seen here in the centre of the picture,

connect the RC receiver, at the top of the picture, to the battery box, on the

right.

THE RC POWER SWITCH

YOUR RB7 MODEL’S RC RECEIVER AND SERVOS ARE SUPPLIED WITH ELECTRICAL

POWER FROM ITS BATTERIES. THERE IS NO NEED TO MAINTAIN THIS POWER

SUPPLY WHEN THE CAR IS NOT IN USE, SO THE RC SYSTEM IS FITTED WITH A

SWITCH TO BREAK THE CIRCUIT.

With this pack, you will receive the RC power switch, the

rst electrical component of your model. The switch marks

the beginning of a new phase of the build, in which you will

t the components of the onboard control system – the RC

receiver and the throttle and brake servos – to the chassis

and the RC box.

BETWEEN BATTERY AND RECEIVER

The switch connects the battery box and the RC receiver

of your RB7 model. This arrangement means that is tted

at the most eective point in the circuit, because the two

servos of your model obtain their electrical power via

the receiver. When you use the switch to disconnect the

receiver from the power source, you are also disconnecting

the servos, so there’s no need for them to have individual

switches.

Red Bull Racing RB7: Step by Step

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379

The switch itself is a slide switch. Connected to it are a

pair of two-core cables (one core with red insulation, the

other black). These cables are soldered to the switch, which

is housed in a rectangular plastic casing. The on and o

switching process is carried out by a slider on the top of the

switch, which remains in whichever position is selected.

The ON and OFF markings are on a cover plate surrounding

the slider. You have to remove this plate when you mount

the switch on the right chassis plate of your model, so

always make sure that the plate is the right way round

when you replace it.

The plugs of the two cables attached to the switch are

dierent colours, and the cable with the red plug will be

connected to the battery box and the one with the black

plug to the receiver. When the slider is nearer to the cable

with the red plug, the switch is sitting in its OFF position

and the RC receiver and the two servos are disconnected

from the batteries.

When mounting the switch on the right chassis plate, you rst need to

remove its cover plate by removing the two retaining screws. This plate also

carries a visual indication of whether the switch is in the ON or OFF position,

so make sure that it is the right way round when you replace it.

The cable with the black plug connects the switch to the receiver.

The red and black wires are securely clamped to the metal contacts

in the plastic body of the plug.

The switch is connected upstream of the receiver, from which the servos

receive their power.

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