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RADIO CONTROL
BY BOB YOUNG
Jet engines in model aircraft; Pt.2
While the jet-powered model has been like the
“Holy Grail” to aircraft modellers, there have
been intractable problems to solve in scaling
down the jet engine to make it fit into typical
model aircraft. This month we look at the
fundamental principles governing the design of
jet engines for model applications.
Jet propulsion of a body such as
an aircraft is quite simply explained.
The propulsive force is developed in
reaction to the ejection of a high-speed
jet of gas. In other words, it is action
and reaction. The action is to squirt a
lot of gas out at high velocity and the
reaction is that the aircraft zooms off
into the distance.
The jet-driven turbine or turbojet,
consists of four basic parts: compressor, combustion chambers, turbine
and propelling nozzles. Fuel is burnt
in the combustion chamber, after being mixed with air coming from the
compressor. The combustion process
generates expanding gases which spin
the rotor of the turbine.
The shaft of the turbine is connected
directly to the axis of the compressor
so the turbine drives the compressor.
After passing through the turbine, the
gas is exhausted to the atmosphere at
high speed through a nozzle.
In the propeller-driven turbine or
turboprop, the turbine not only drives
the compressor but also drives a normal propeller.
A ramjet engine relies on its own
forward motion to compress the air
that enters it. The Turboprop and
ramjet have no model equivalents and
thus will not feature in this series.
Fig.1 shows the basic layout of a
typical jet engine.
The development of jet engines for
use in models has proven to be a very
difficult task, largely because of “scale
effect”. Briefly, there are two separate
Fig.1: this shows the basic layout of a typical jet engine. Air enters the compressor at left and is mixed with
fuel which is burnt in the combustion chamber. The expanding waste gases then drive the turbine before
being exhausted. The turbine is directly connected to the compressor.
80 Silicon Chip
Fig.2: these are the three basic forms of jet engine compressor. Because of its lesser sensitivity to scale effect, the radial
compressor is most suited for use in jet engines for models.
problems relating to scale effect. First,
we have the problem of machining tolerances. For example, as the compressor and turbine are reduced in size, the
gap between the rotor and its housing
becomes more significant when expressed in terms of a percentage of air
leaking past the compressor/turbine
relative to the volume flowing through
the compressor/turbine. Compounding
this are the problems of metallurgy and
expansion due to heat.
Second, we have the problem of
the loss of aerodynamic efficiency
as the compressor/turbine blades are
reduced in size. The engine designer
would refer to the latter problem as
“difficulties with Reynolds numbers”.
In plain English, this simply means
that as the size of a wing, propeller
or turbine blade moves closer to the
size of air molecules, the laws of aero
dynamics start to break down.
Now of all of the modern propulsion units, the jet engine is perhaps
the most reliant upon aerodynamic
theory for its successful operation.
We have all heard that in theory the
bumblebee should not be able to
fly. Among the reasons that aerody
namicists would give for this, Reynolds number is high on the list.
Without going too deeply into the
complex mathematics of Reynolds
numbers with their strange units
(slugs), it is sufficient to state for this
series of articles that the Reynolds
number is given by the formula:
R = Density x Velocity x Size/Viscosity.
The higher the Reynolds number,
Built by Chris Patterson of Brisbane, this superb 1/7th scale F18 carries the
colours of 75 squadron of Williamstown, NSW. It is powered by two OS91
motors driving a Ramtec fan unit. The model has a length of 2.49 metres and a
wingspan of 1.82 metres.
the greater the efficiency.
Reynolds numbers for full size
flight vary from about 2,000,000 for
small slow speed aircraft up to about
20,000,000 for large high speed aircraft.
Combine this with the fact that lift
increases with the square of the velocity and the large high speed aircraft
becomes very efficient indeed. This
is largely the reason that a modern jet
fighter can carry much the same load
as a World War II bomber
Thus it is quite clear that as size
decreases and the velocity falls to
model speeds, the Reynolds number
falls away rapidly and the efficiency
of any aerodynamic device tumbles.
By the time we arrive at turbine blades
of a size suitable for model engines,
efficiency is very low indeed.
As a result, the design of successful turbines for models has centred
around components which are the
least sensitive to scale effect. This has
lead to the almost universal adoption
of the centrifugal compressor for model aircraft jet engines.
Fig.2 shows the three basic forms
of jet engine compressor in order
of common full size usage. Fig.2(a)
shows the axial compressor, Fig.2(b)
February 1998 81
Fig.3: gap losses increase as the gap between a compressor and its housing are
increased. These effects are magnified in jet engines for model use.
shows the centrifugal (or radial)
compressor and Fig.2(c) shows the
diagonal compressor.
Early full size jet engines tended to
favour the centrifugal compressor for
a variety of reasons but the resulting
engine is shorter and greater in dia
meter than the axial flow type and
thus not the ideal shape to fit into a
slender fuselage or engine nacelle.
However, for model size engines
the centrifugal (radial) compressor is
the ideal choice. Once again we must
consider scale effect in the choice of
the compressor. Referring back to Fig
2, note the gap between the tips of the
compressor blades and the housing in
the axial and diagonal compressors.
No matter how tight the machining
tolerances, there will always be some
leakage between the blade tip and the
housing.
Fig.3 shows gap losses at various
gap widths.
Now look at the situation for the
centrifugal compressor. By the very
nature of the design all of the air is
thrown off the tip into the collector
(diffuser) ring. True, there will be
some leakage past the compressor face
but that is more than made up for by
the much larger size (higher Reynolds
number) of the centrifugal compressor
blades. Also it is possible to curve the
blades as in Fig.5 or even fit a cover
plate, reducing leakage losses even
further. By virtue of these facts the
model engine designer has almost
been forced into using the centrifugal
compressor.
However, this choice is not as
one-sided as it would first appear.
There are other good reasons why a
radial compressor is a wise choice for
a model jet engine. As we have already
noted, the Reynolds numbers are
higher and the tip losses are less. In
addition, they are easier to construct,
are much more robust and therefore
more reliable in operation.
Constructing a model size axial
compressor with its rows of tiny compressor and diffuser blades would be
a very difficult and tedious task. Then
there is the problem of anchoring the
blades solidly enough to withstand
speeds in excess of 100,000 rpm and
possible ingestion of foreign matter.
What must be borne in mind at all
times is the very high rotation speeds
encountered in these engines. Shaft
speeds in excess of 100,000 rpm
are routine in model size turbines.
When combined with very high temperatures there is a real danger of
compressor or turbine failure and this
must be guarded against at all times.
Fig.4 shows the typical operating conditions for a model jet engine.
There is also a more subtle consideration to the radial compressor and
we will deal with this shortly. The
downside of the radial compressor
Fig.4: typical operating
conditions for a model
jet engine. Note also
that the engine may be
rotating at up 100,000
rpm!
82 Silicon Chip
Fig.5: leakage
effects in a
radial (centrifugal)
compressor can
be minimised by
various curvatures
of the blades or
by fitting a cover
plate.
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is the more rotund appearance of the
completed motor. It is nowhere near
as slender as the axial flow engine.
Notwithstanding this, the final size
of a successful centrifugal compressor
type of engine is well within the limits
available in a reasonable size modern
jet fighter model.
Automotive turbo chargers
All that aside, the most important
factor in the choice of centrifugal
compressors in model engines is the
fact that turbo superchargers for cars
use radial compressors which are an
ideal size for model work.
Now automotive turbo superchargers are very highly developed devices.
Just what drove the turbo designers
to radial compressors is not known
but the preceding considerations
probably played a large part in the
development of these devices.
Whatever the reasons, the automotive turbocharger provided a
perfect jumping off point for early
experimenters and radial automotive
turbo-compressors found their way
into many an experimental model jet
engine. As supplied, turbocharger
compressors are accurately dynamically balanced, a very important
point. They achieve efficiencies of
between 70 and 80%, depending upon
their size; the larger the compressor,
the higher the efficiency.
The radial compressor can be built
in many configurations, all with widely differing characteristics. First
ly,
there is the matter of cover plate or no
cover plate, the former being known
as an “enclosed wheel” compressor.
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February 1998 83
Fig.6: under certain conditions the airflow from the compressor can collapse,
leaving a lower pressure at the compressor than inside the engine. A reverse
flow of air begins which continues until the internal pressure falls below that of
the compressor. This cycling effect can destroy the engine.
Secondly, there is the matter of
blade curvature. Fig.5 shows radial
compressors with various configurations. Fig.5(a) shows radial tipped
blades, Fig.5(b) shows slightly retro-curved blades and Fig.5(c) shows
an enclosed wheel with highly retro-curved blades.
Throttle response
Experiments have shown that the
compressor with retro curved blades
is more efficient overall than the
straight blade compressor. However,
more subtle effects of blade curvature
are to be found in the very important
feature of throttle response. In aircraft
work, it is imperative that throttle
response be as close to instantaneous as possible. The Me262 was very
vulnerable during landing and takeoff due to poor throttle response and
allied airmen exploited this weakness
to the full. Kurt Schreckling’s FD 3
84 Silicon Chip
model engine uses a retro angle of 45
degrees and responds to the throttle
almost as quickly as a well-adjusted
piston engine.
I could write an entire chapter on
throttle response, throughput and
blade curvature as it really is at the
very heart of the jet engine and it is
here that we encounter the dreaded
surge line. The “surge limit” of a
compressor refers to the tendency
to supply the working medium cyclically instead of constantly. This
may sound a little innocuous but to
the full size aviator it is viewed with
considerable alarm, since the usual
result is damage to the engine which
may progress to the very serious.
In model size engines the effects
are not as dramatic but the compressor can still be damaged if the surge
limit is exceed
ed. To simplify an
exceedingly difficult subject, compressor surge is often the result of
mismatched components at the design
stage, particularly too small a turbine
which restricts the airflow through
the engine.
Under certain conditions the airflow from the compressor can collapse, leaving a lower pressure at the
compressor than inside the engine.
A reverse flow of air begins which
continues until the internal pressure
falls below that of the compressor and
the compressor begins to deliver air
again – see Fig.6.
In a model jet engine, the cycles follow on so quickly that all you hear is a
loud unmistakable growling sound. If
this occurs, then you need to close the
throttle immediately for the condition
will not clear itself and the end result
is overheating and engine damage.
Once the air leaves the compressor
it passes through a diffuser which
straightens the flow and slows the
air in order to raise the pressure in
accordance with Bernoulli’s Theorem.
In the streamline flow of an ideal
fluid – ie, one which is not viscous
– the sum of the Energy of Position
(Potential Energy) plus the Energy
of Motion (Kinetic Energy) plus the
Pressure Energy will remain constant.
In other words, the residual speed
energy of the air is converted into
pressure energy inside the diffuser.
In this case the energy of the gas
is proportional to the square of its
speed. Therefore if we can halve the
gas speed we have already converted
three-quarters of its energy.
It is here that the radial tipped
compressor blades vary from the retro curved blades. The radial tipped
blades use the diffuser to raise the
pressure whereas the retro-curved
compressor begins the process inside
the compressor itself. Thus the losses
are higher in the radial compressor.
Once the air passes through the
diffuser it enters the combustion
chamber and then the hard part begins. Burning the fuel/air mix evenly
and efficiently, avoiding overly long
flames which result in localised hot
spots on the turbine, and preventing
raw fuel pooling in the engine or running out onto the tarmac are all very
difficult tricks to master.
A model jet belching a metre-long
flame may look spectacular but it ain’t
gonna last long!
Next month, we’ll talk about taming the combustion chamber and
SC
turbine.
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