Control Model World - Apr '96
by Stan Yeo
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.
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
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.
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
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.
Basic trainers Rudder
Ailerons Elevator with optional Rudder
Ailerons Elevator with optional Rudder
Fully Aerobatic Ailerons
Elevator Rudder optional Flaps / Flaperons
Pylon Racer Ailerons
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.
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
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.
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.
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.
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.
Wingspan 60 inches
Aspect Ratio 8 : 1
Mean Chord 60 / 8 = 7.5
Wing Area 60 x 7.5 = 450
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
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
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 Area = (TP Aspect
Ratio x TP Chord) x TP Chord
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 =
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
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
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
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.
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.
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
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
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
Aerodynamics By Martin Simons ISBN 0 852429 15 0
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
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)
Area 15 - 20% of Wing
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
Movement for all moving
tailplane + or - 10 degrees
Section Flat plate
approximately 1/4in (6mm) thick
Incidence 2 -4 deg.
relative to Fuselage
Area 18 - 20% of Wing
Angle 110 - 120 degrees
(120 deg. easiest to work with)
Area 6 - 8% of Wing area
Rudder 40 - 60% of total
Nose Length 1.25 x Wing
Tail Moment Arm 3 x Wing
Mean Chord (distance between **Aerodynamic Centres of Wing and Tailplane)
Width To suit radio
Basic Trainer Clark Y,
NACA 6412 with undercamber removed
Eppler 205, Selig S3021
Fully Aerobatic NACA
641A012, Eppler 374 (equalise co-ordinates to plot)
Pylon Racer Selig S3021,
** The Aerodynamic
centre is assumed to be 25% back from the leading edge for the purposes of this article.