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Slope
Soarer Design |
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Radio
Control Model World - Apr '96
by
Stan Yeo
INTRODUCTION
Being
a manufacturer of slope soarer kits I am probably committing business
hara-kiri by writing this article and encouraging you to design
and build your own slope soarers. Well, there is nothing to hide,
all the information is readily available in easily accessible books.
Besides, there is a lot of satisfaction to be gained from designing
and building your own models. I should know, my creations number
over 50.
Slope
soarers are the simplest of radio control models to design, no thrust
lines to worry about, just a few simple rules to follow and the
model should fly straight off the building board providing a systematic
approach is adopted.
DESIGNING
YOUR MODEL
It
is recommended that you start your design career with something
simple. My philosophy is that it is better to make a good job of
something simple than a mediocre job of something difficult. This
does not mean that the difficult jobs are not tackled, just put
off until sufficient expertise has been gained to ensure success
consequently my recommendation is you start with a basic slope soarer
of 60 to 70 inch span. The reasons for offering this advice are
as follows:
1.
Money and time commitments are kept to a minimum so should the model
not meet expectations then not too much is lost!
2.
Simple models are easier and quicker to build making it easier to
maintain enthusiasm and hence motivation to finish the model.
3.
It is a convenient size for the materials that are available.
4.
Structural inadequacies are likely to be less catastrophic!
THE
DESIGN PROCESS
The
design process is universal. First decide what it is you want to
build, draw up a specification, study how other people have approached
the problem, then start drawing. If you tackle the design process
logically then you will find that the answers from the preceding
problem point to the solution of the next problem. In deciding that
the model is going to be fully aerobatic the type of section normally
used would be fully symmetrical. Therefore it is only necessary
to look at symmetrical sections when choosing the section. Failure
to work in a logical manner will result in design conflicts that
are impossible to resolve without serious compromises.
Drawing
up the specification is more like answering a series of questions
organised in a logical order with the answer from the previous question
providing part of the answer to the next. Below is a simple sketch
of a logical design process that can be used to design your model.
The only difference between the one shown and the one I use is that
I do not look at the oppositions' products (I do not want to let
their ideas influence me and consequently be accused of piracy!).
Obviously it is not quite that simple as there is a fair amount
of head scratching before any model takes to the air so I will now
look at different aspects of the design process in a little more
detail.
MODEL
SIZE
As
stated in the introductory paragraphs a 60 to 70 inch (1.5 to 1.75
metre) span model is the recommended size. A model of this size
is relatively economic to build, has good crash resistance and will
accommodate comfortably standard size radio control equipment. Larger
models will require more thought as regards the type of construction
employed. Smaller models could present problems re the finished
weight and housing the radio equipment. The size or wingspan of
a model could of course be predetermined if the model is going to
be designed to meet a particular specification i.e. the up and coming
60 inch pylon racing slope soaring rules.
MODEL
TYPE
Run
of the mill slope soarers fit into one of five categories, basic
trainer, intermediate trainer, intermediate aerobatic, fully aerobatic
and pylon racer. The main differences between the models is the
control configurations and the type of sections used.Having decided
on the type of model to build you can now make a decision on what
controls to fit, taking into account the equipment you have available.
Equipment restrictions may preclude a certain type of model. There
is not a lot of point in designing a fully aerobatic model if you
only have 2 channel equipment and cannot fit a rudder. It would
be better to design a general purpose intermediate aerobatic model
that can be flown in a wider range of conditions. In full house
aerobatic contests the rudder is required for a large proportion
of the manoeuvres.
Recommended
control configurations are:
Basic
trainers Rudder Elevator
Intermediate
Trainer Ailerons Elevator with optional Rudder
Intermediate
Aerobatic Ailerons Elevator with optional Rudder
Fully
Aerobatic Ailerons Elevator Rudder optional Flaps / Flaperons
Pylon
Racer Ailerons Elevator
After
deciding on the type of model, performance targets / desired flying
characteristics can be thought through. This is very important as
the performance expectations could be in conflict with the desired
flying characteristics. An example of this could be in the selection
of the wing section. It is possible to select a section for it's
low drag qualities only to find in practice that it had vicious
stalling characteristics that made it unsuitable for use on a tight
turning pylon racer.
WING
SELECTION CRITERIA
For
the purposes of this article wing sections are divided into three
categories, flat bottomed, semi-symmetrical and fully symmetrical.
Also, as a general rule, it can assumed that the thicker and the
more cambered (curved) a section is the more lift and drag it will
produce and that the section will have more forgiving handling characteristics.
This is not always the case but it is a good point from which to
start when selecting a section.
Basic
trainers require a good lifting section that will allow the model
to be recovered from near disaster situations quickly without inducing
a high speed stall to make the situation worse. The extra drag that
usually accompanies these sections is also an advantage as it slows
down the model's acceleration in a dive giving the pilot more time
to recover in an out of control situation. The negative side of
course is that model cannot cope with the very strong winds without
ballast. Sections recommended for basic trainers are Clark Y and
the NACA 6412 with the slight undercamber removed. If a bit more
performance is required try the Eppler 205. This is by no means
the only suitable sections but again it is a point from which to
start.
For
intermediate models the Eppler 374 takes some beating. It has been
around for nearly 30 years now but whereas there has been alot of
development on fast thermal soaring sections, some of which are
suitable for intermediate slope soarers, there seems to have been
little on general purpose aerobatic sections. I look forward to
all your letters proving me wrong because I would be delighted to
find a semi-symmetrical section that outperforms the ubiquitous
Eppler 374. Two sections that are popular with flat field fanatics
that are good intermediate slope sections, particularly on intermediate
aileron trainers, are the Eppler 205 mentioned previously and the
Selig S3021. Both soar well, as you would expect, and have some
inverted performance.
Fully
aerobatic models require fully symmetrical sections. Anything less
will compromise the models inverted performance. Trailing edge flaps
/ flaperons can be used to restore the inverted performance but
it will be at the expense of extra drag. Flaps may not be an option
but if it is then my inclination would be to use a fully symmetrical
section and drop the flaps to gain height for manoeuvres. The fully
symmetrical section I use is the Eppler 374 (the top and bottom
co-ordinates are added together then halved to provide the plotting
co-ordinates) but the NACA 641 A012 will do equally well.
Pylon
racers need fast efficient sections to be competitive therefore
the section must have low drag characteristics but still able to
produce the lift necessary for tight pylon race turns. The section
in favour at the time of writing this article is the RG15. It is
very efficient but it does require strict adherence to the profile
if the potential performance is to be realised. Also, because it
is a specialised section, the handling characteristics of the model
could be suspect if the design is not quite right. On my latest
pylon racer I have opted for the more predictable Selig S3021, simply
because it is more suitable for kitting.
Once
you have decided on the type of model to build choosing the section
is usually fairly straightforward as the number of sections within
a category with full published data is limited. There is quite alot
of choice in the intermediate model category, mainly due the developmental
influence of F3B, but outside this area not so much.
WING
DESIGN
The
major decision in designing the wing is the planform, is it to be
constant chord, tapered, straight, swept back or swept forward.
The decision you make will depend on the design 'theme' you are
striving to achieve i.e. sleek looking, fighter appearance etc.
A semi-scale or sleek theme will dictate a higher aspect ratio wing
design than a mock fighter appearance where a short stubby wing
is in keeping. For run of the mill models an aspect ratio (wing
span to wing chord ratio) of 7 or 8 : 1 is the norm.
With
the Wing Span and the Aspect Ratio known the Mean Chord (Span /
Aspect Ratio) and Wing Area (Span x Mean Chord) can be calculated.
Projected flying weight can then be used to calculate the wing loading
(flying weight / wing area) A good wing loading for general purpose
slope soarers is 10 to 12 ozs/sq. ft. this gives a finished model
weight of approximately 2 1/4 lbs. (1Kg).
The
purpose of dihedral is to improve lateral stability (the model's
wing levelling ability) and increase the effectiveness of the rudder
on rudder elevator models. On aileron models dihedral reduces the
effectiveness of the ailerons and is not required but to avoid the
'droop wing look' a small amount (10mm) of dihedral is usually built
in. Rudder elevated models require 25 to 30mm per 200mm of wing
span, sometimes more if a 'modern' laminar flow section is used
or the side area aft of the Balance Point is marginal. If wing dihedral
and side area are not in harmony the model will have a tendency
to 'dutch roll'.
The
purpose of ailerons is to induce a rolling action along the axis
of the fuselage. As with all twisting forces the further they are
applied away from the axis of rotation the more effective they are.
This means that the further outboard the ailerons are fitted the
more effective they become which is why full size aircraft have
outboard mounted ailerons. Unfortunately though, unless the model
has a built up wing or mini servos can be buried in the outboard
wing panel, this is not the most practical solution for our basic
slope soarer. The most practical solution is to mount the aileron
servo in the centre of the wing and fit strip ailerons that are
operated via torque rods. If you choose to go this route then the
ailerons need to be between 15 and 20% of the mean chord wide.
TAILPLANE
DESIGN
The
overall style and size of the tailplane is determined by the wing.
The design of the tailplane must be in keeping with the overall
design theme. All too often this is not the case and the model ends
up looking like a 'bitsa'. The shape of the tailplane is unlikely
to have any effect the performance of the model but it will have
a big impact on it's overall appearance.
The
purpose of the tailplane is to stabilise the model in pitch. If
it is too small the model will be longitudinally unstable. If it
is too large then there is a drag (performance) penalty to pay.
Tailplane area and hence pitch stability is a function of the tailplane
moment arm and wing area. A rule of thumb guide is for the moment
arm to be 3 x Mean Wing Chord measured from the aerodynamic centre
of the wing to the aerodynamic centre of the tailplane. Tailplane
area should be 15 to 20% of the wing area. The aerodynamic centre
of a section can be assumed to be at 25% of mean chord. Tailplane
effectiveness is dependant on how high it is mounted relative to
the wing. A high mounted, ('T' tail) tailplane is more effective
than one mounted at the base of the fin. This means a smaller tailplane
can be fitted to 'T' tail models.
Butterfly
or 'Vee' tails look attractive and they do create less drag but
at the expense of handling characteristics. A testament to their
increased efficiency is the following they attract on the contest
circuit. The best angle to get the right balance between the projected
horizontal and vertical tail areas is 110 degrees, for ease of construction
I use an angle of 120 degrees and a 60/30 Set Square. The overall
area of the tailplane must be increased slightly to make up for
the area lost due to the angle. A total area of approximately 20%
of wing area should be adequate.
Once
the tail area has been calculated ( Wing Area x Percentage chosen)
the tailplane can be designed. The aspect ratio of the tailplane
need only be 50 to 60% of that of the wing. Below is a sample set
of calculations for the wing and tailplane.
WING
CALCULATIONS
Wingspan
60 inches
Aspect
Ratio 8 : 1
Mean
Chord 60 / 8 = 7.5 ins.
Wing
Area 60 x 7.5 = 450 sq. ins.
Projected
Weight 11 x 450/144 = 35 ozs. i.e. 11 ozs/sq ft wing loading)
Root
Chord 8.5 ins
Tip
Chord 6.5 ins
TAILPLANE
CALCULATIONS
TP
Area = Wing Area x percentage TP area required = 450 x 0.15 = 67.5
sq. ins ounded up to 68 sq. ins.
TP
Area = TP Span x TP Chord
TP
Aspect Ratio = Wing Aspect Ratio x 0.5 (Span / Mean Chord)
=
8 x 0.5 = 4
TP
Span = TP Aspect Ratio x TP Mean Chord
Substituting
TP Span for TP Chord
TP
Area = (TP Aspect Ratio x TP Chord) x TP Chord
or
TP
Chord = sq. root of TP Area / TP Aspect Ratio
TP
Chord = 68 / 4 = sq. root of 17 = 4.125 ins.
TP
Span = 4.125 x 4 = 6.5 ins
After
doing the calculations all that remains is to design the tailplane
around the span and mean chord. Elevator area is normally 20 to
30% of tailplane area, less if it is a basic trainer. If an All
Flying Tailplane is to be fitted then limit the angular tailplane
movement to + or - 10 degrees. Any more and it is likely the tailplane
can be stalled with potentially disastrous results.
Fin
area is normally 6 to 8% of wing area. Again the design theme adopted
should be adhered to if the model is going to look 'right'. Rudder
area can be up to 60% of total fin area.
WING
AND TAILPLANE INCIDENCE
It
is imperative that the model is rigged correctly. If the model is
rigged correctly it will fly like it is on rails but if it is not
the model will fly like the proverbial sack of potatoes. There are
two sets of incidences to be set, one is the Wing to Tailplane incidence
known as Longitudinal Dihedral the other is the Wing to Fuselage
incidence.
The
wing to tailplane incidence has an effect on pitch stability and
the position of the Balance Point in Neutral trim. For basic trainers
the wing is normally set at 3 - 4 deg. positive (leading edge up)
relative to the tailplane. The angle is measured along the Chord
Line of the section and NOT the bottom of the section. The Chord
Line is the Datum line used for plotting the section. It connects
the start and finishing points of the section on the Leading and
Trailing edges. On intermediate and fully aerobatic models this
angle is reduced to zero to make the model neutrally stable in pitch.
To
reduce fuselage drag to a minimum the normal flying attitude of
the fuselage should correspond to the glide angle of the model.
This is why full size gliders fly in a nose down attitude. To achieve
this the tailplane is set at 2 - 4 deg. positive incidence relative
to the fuselage. The 'draggier' or less efficient the model the
higher this angle needs to be to compensate for the steeper glide
angle. With the tailplane incidence known the wing incidence can
be calculated.
BALANCE
POINT
If
the model is rigged correctly the optimum position for the Balance
Point should coincide with neutral elevator trim. This is normally
30 - 35 % back from the wing leading edge at the Mean Chord position.
The position of the balance point also has an effect on the pitch
stability of the model. The further forward it is the more stable
the model will be which is why on basic trainers the balance point
is normally fairly well forward. Likewise, for initial flights with
a new model it is recommended that the balance point is moved forward.
Some indicators used in finding the correct balance point are how
easily the model enters and recovers from a spin, the sensitivity
of the elevator control, dive recovery and how much down elevator
is required to fly inverted.
To
locate the balance point find the mean chord position on each wing
panel. Decide where the balance point should be relative to the
wing leading edge. Mark this point on each wing panel. Connect the
two points and where the line crosses the centre of the fuselage
is the Balance Point for the model. For constant chord or straight
tapered wings the mean chord is the mid-point of each wing panel.
FUSELAGE
DESIGN
Sufficient
space for the radio equipment coupled with a long enough moment
arm to provide adequate pitch stability (a function of TP moment
arm and TP area) are the main requirements of the fuselage. A secondary
requirement is being able to position the Balance Point correctly
without having to carry an excessive amount of lead in the nose
compartment. This of course is dependant on how far forward the
R/C equipment can be positioned. A good starting point for the nose
length is 1.25 x Wing Root Chord.
Structurally,
the rear fuselage must be strong enough to absorb shock loads from
the tailplane in the event of a crash. This is particularly important
when the tailplane is mounted on the fin. Do not attempt to reduce
the size of the fuselage to a minimum unless it is a pylon racer
as clearances you thought you had do not always materialise in practice.
This could lead to difficulties in installing the controls / R/C
equipment. If you are designing a basic trainer be generous with
the dimensions as the extra drag created adds to the model's suitability
as a trainer.
CONSTRUCTION
METHODS AND MATERIALS
The
best advice here is stick to construction methods and materials
with which you are familiar. For this type of model I have standardised
on a foam veneer wing, ply fuselage sides, balsa top and bottom
and all sheet balsa tailplane. If cutting foam wings presents a
problem you can either contact one of the foam wing manufacturers
who advertise in the back of the modelling magazines or design a
built up wing.
A
little time spent studying plans and back issues of modelling magazines
is well worth the time and effort as it will yield valuable information
on different construction techniques. A golden rule in designing
any structure is keep it simple and avoid any sudden changes in
section.
Sudden
changes in section = High Stress Points = Damage in Crashes
Design
these weak points out by tapering the ends of doublers, staggering
the ends of spars and avoiding sharp corners.
SUMMARY
A
short article like this cannot hope to be a comprehensive thesis
on model aircraft design. Neither can it hope to encapsulate 30
years of modelling experience. It does however provide a starting
point from which to go forward. If the design process is worked
through logically and the basic rules are followed then there is
no reason why you should not be able to design and build a model
to be proud of. So get the pencil, paper and calculator out and
start designing.
RECOMMENDED
READING
Radio
Control Slope Soaring By Dave Hughes ISBN 0 903676 13 3
R/C
Model Airplane Design By A G 'Andy' Lennon ISBN 0 903676 14 1
Model
Aircraft Aerodynamics By Martin Simons ISBN 0 852429 15 0
DESIGN
SUMMARY
Wings
Wing
Span 50 to 70 ins (1.25 - 1.75 metres)
Aspect
Ratio 6 - 9 to 1 (Wingspan / Mean Chord)
Section
see Table
Section
Thickness 9 - 12% of Chord
Mean
Chord Span / Aspect Ratio
Layout
Constant Chord (parallel) or Tapered
Straight,
Swept Forward or Back (max 25 degrees)
Dihedral
Rudder only wing - 1in in 8in (25 - 30mm in 200mm)
Aileron
wing - 3/8in (10mm) under each wing tip
Ailerons
Strip type 15 - 20% of Mean Chord wide
Incidence
0-4 deg. relative to Tailplane (depends on model type)
Tailplane
(Conventional layout)
Area
15 - 20% of Wing area
Aspect
Ratio 50 - 60% of Wing aspect ratio
Mean
Chord = Sq. Root of (TP Area / TP Aspect Ratio)
Span
= TP Mean Chord x TP Aspect Ratio
Layout
Same as Wing for the model to look right.
Elevator
20 - 30% of Tailplane Area
Movement
for all moving tailplane + or - 10 degrees
Section
Flat plate approximately 1/4in (6mm) thick
Incidence
2 -4 deg. relative to Fuselage
Tailplane
('V' Tail layout)
Area
18 - 20% of Wing area
Angle
110 - 120 degrees (120 deg. easiest to work with)
Fin
Area
6 - 8% of Wing area
Rudder
40 - 60% of total fin area
Fuselage
Nose
Length 1.25 x Wing Root Chord
Tail
Moment Arm 3 x Wing Mean Chord (distance between **Aerodynamic Centres
of Wing and Tailplane)
Width
To suit radio equipment.
Sections
Basic
Trainer Clark Y, NACA 6412 with undercamber removed
Intermediate
Trainer Eppler 205, Selig S3021
Intermediate
Aerobatic Eppler 374
Fully
Aerobatic NACA 641A012, Eppler 374 (equalise co-ordinates to plot)
Pylon
Racer Selig S3021, RG15
**
The Aerodynamic centre is assumed to be 25% back from the leading
edge for the purposes of this article.
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