Model World - Nov '95
by Stan Yeo
A slope soarer is
probably the easiest of radio control model aeroplanes to design yet surprisingly few
modellers attempt it (fortunately for me and others like me!!). There are probably a
variety of reasons for this ranging from not having the facilities, lack of time to not
having the confidence or necessary knowledge. The purpose of this article is to address
the crisis of confidence by providing a few simple rules that will help you design a
successful model. For me the hardest bit of designing a new model has always been
acheiving a pleasing shape and an attractive colour scheme. Hopefully you will feel the
same way after reading this article.
Stage One Design
The first step in the
design process, whether it is designing a new model aeroplane or a new kitchen is to
identify in your mind what the design criteria are. You need to draw up a simple design
Type of model - Trainer,
Intermediate, Aerobatic etc.
Controls - Rudder,
Elevator, Ailerons Flaps.
Performance - Floater,
Fast, Fully aerobatic etc.
Wing span - 65ins etc.
In choosing the type of
model to be designed you are also indirectly specifying some of the performance criteria
i.e. how many basic trainers are fully aerobatic?. This applies throyghout the design
process and in a way is your guarantee of success if you follow the rules and are
realistic in your design requirements.
For your first design
exercise remember KISS (Keep It Simple Silly) so I
would suggest a model of 60 to 70 inches span (1.5 - 1.75 metres). The main reasons are it
is big enough to have a reasonable performance and yet small enough to not require special
construction techeniques to withstand the aerodynamic loads. Also it is cheaper and can be
built fairly quickly.
Stage 1 Number
In Stage 1 we decided on
the design criteia for the model. In Stage 2 we do the design calculations. These are very
striaghtforward and should present no problems but some explanation of the 'variables' is
1. Aspect Ratio
The Aspect ratio (AR) is
the number produced when the wingspan is divided by the mean wing chord. Power model
generally have a low AR (5 - 6) whuilst thermal soarers have much highers aspect ratios
(12 - 20). General purpose 'kipper' slope soarers have modest aspect ratios of around 8 to
1 (range 6 -9 to 1). As a rule the higher the aspect ratio the more efficient the wing but
large aspect ratio wings pose structural problems due to the increased bending loads at
the wing root.
2. Wing Loading
The wing loading is the
weight the wing has to support in normal level flight measured in ounces per square foot.
The target wing loading of your model wil depend on the type of model you decide to build.
If you want the model to fly in very light winds then it will need a low wing loading
Aerobatic models benifit from a little extra weight as it helps to maintian speed through
the manouvres, providing of course the drag is kept low. A good starting point for
intermediate aerobatic models is 11 ounces /sq ft. With light wind models 7 - 8 ounces is
more appropriate. For pylon racer type models aim for around 11 -12 ounces but make
provision for ballasting up to 24 ounces /sq ft.
The size of the
tailplane is going to have a direct impact on the model's pitch stability along with the
tail moment arm (distance between the mean chords of the wing and tailplane). Within
reason larger the tailplane / moment arm the more stable the model. It is possible however
to have too powerfull a tailplane whereupon in certain dive situations the tailplane takes
over and holds the model in the dive until up elevator is applied. I experienced this on a
number of occasions when I flew single channel gliders in the mid-sixties.
A starting point for
tailplane area is 15% of wing area with a mooment arm of 3 x mean wing chord. The
tailplane on 'Tee' tail models is more efficient than one fitted at the base of the fin so
a slightly smaller tail can be fitted (12 - 15%). 'Vee' tail models have perform the
function of both the fin and the tailplane. As a rough guide the fin area is approximately
half that of the tailplane so the 'Vee' tail angle must be set to attain this ratio when
the tailplane is veiwed from above and the side. If you do your sums this works out at
approximately 110 degrees but for convenience I always use 120 degrees (60 / 30 set
squares). Actual tailplane area needs to be increased by 2 - 3% to make up for the area
'lost ' due to the angle but it is still less than the total area of a conventional fin
and tailplane. I have built a number of models that have been fitted with both a
conventional tailplane and a Vee tail and in my experience the vee tail out-perform the
conventional tail but they are aerodynamically less abusable without biting back! Basic
trainers need good in pitch stability so fit a slightly larger tailplane of 18 - 20% of
As mentioned above the
general rule for calculating fin area is half the tailplane area or 7 - 9% of wing area.
Again the further aft the fin the more effective it will be. Please remember though that
the fin still has to perform like a wing even though it is fully symmetrical and mounted
vertically. It still has to produce 'lift', albeit horizontally. It is not just a paddle
that is stuck out into the airstream.
Choosing the correct
moment arm is a bit of a compromise. The longer the tail moment arm the morte stable the
model will be in pitch and yaw for any given area but the model will require more nose
weight to achieve the correct balanc e point . Long fuselages also increase the wetted
area and the fuseladge volume therby increasing parasitic drag i.e. drag not associated
with lift production. Likewise a short nose moment will increase the weight required in
the nose . Another side issue and quite an important one is that loong fuselages are more
vulnerable to damage on an arrival due to the 'whiplash' effect.
A good starting point is
to set the tail moment arm at 3 x Mean Wing Chord. The tail moment is the distance between
the aerodynamic centres of the wing and tailplane. The aerodynamic centre of a section is
assumed to be 25% back from the leading edge. Nose length can be provisionally set at 1.25
x Wing Root Chord.
Stage 3 Choices
This is the stage where
the wing section is chosen and the construction method is outlined along with the size of
the control surfaces. In line with choosing the construction moethod basic design are also
Choosing the Wing
A lot is written about
wing sections in the modelling press and it is very refreshing to read about the amount of
research work going into designing model specific sections. Whilst it is not necessary to
have a deep understanding of airfoil sections it is still worthwhile to do some background
reading on sections and how lift is produced as a little background knowledge will help
you choose the section best suited to your needs. The 'Prepare for Liftoff' article in
April '95 RCMW is a good starting point.
The basic rules are the
thicker the section and the more camber (curvature) it has the more lift it will produce
and the more stable it will be. The down side of course is it will also produce more drag.
The type of model you are going to build will determine the type of section you use. Below
is a list of basic model types with suggestions.
A basic trainer requires
a stable section of modest thickness, capable of producing high lift coefficients with
some built in drag to stop the model accelerating too quickly when out of control (to
increase thinking time!!). Suitable sections are the NACA 6412 (with the undercamber
removed) Clark Y i.e. moderately cambered flat bottom sections of around 12% thickness.
Here a slightly sleeker
section can be used to increase the speed range of the model as loosing control should not
now be the problem it was. The Eppler 205 and the Selig 3021 are ideal sections for the
more advanced rudder elevator models and primary aileron trainers. These sections have
been used extensively on all types of thermal soarers with notable success.
For the intermediate
aerobatic model we not only need good upright performance but some inverted capability as
well. One is always at the expense of the other and to my mind there is a limmited choice
in this area. I always come back to the ubiquitous Eppler 374. I have tried other sections
but not achieved the same all round performance. If you know a better section please write
and tell me.
With the fully aerobatic
model the inverted performance should be as good as the upright performance. This almost
dictates the use of a fully symmetrical section. When choosing this type of section be
careful that the maximum thickness point is not too far back or too far forward. About 35%
is my optimum. Aft maximum thickness points generally mean lower camber and consequently
lower lift coefficients with a decrease in aerobatic performance. Of the sections I have
tried the Eppler 374 with the co-ordunates equalised has given the best results. A point
worth mentioning concerning the use of fully symmetrical sections is that the model, to
perform to its full potential, does require better lift conditions. Too often modellers
are disappointed with this type of sectioned model because they expect it to perform like
an intermediate model in less than ideal conditions. My advice is to always to compliment
a fully symmetrical section model with a semi-symmmetrical section model.
A pylon racer not only
has to be quick but it must also be able to turn tightly at the end of each leg. This
means the section must be capable of producing generous lift coefficients. Low camber
sections may be quicker but they also produce less lift. Take this into account when
choosing your section. Suggested sections include Selig 3021, RG14 and RG15 although at
the time of writng this article new alternative sections are beginning to emerge.
The starting point here
is the fuselage datum line. A slope soarer is a glider with a natural glide angle when
flying straight and level hands off. If fuselage drag is to be kept to a minimum then the
datum line of the model should be parallel to the glide angle with the mean chord line of
the wings at a positive angle of attack (up to 5 deg.) to produce the required lift. The
net effect of this is the model flies with a slight nose down attitude. The tailplane
chord line is then set at the same angle of attack as the wing (aerobatic model) or
slightly less if additional pitch stability is required. You know when you have got it
right because with the balance point in the correct position the model flies with nuetral
elevator. One reason for zero longitudinal dehedral (difference in angle between the wing
chord line and tailplane chord line) in aerobatic models is to reduce drag when inverted
and to make rolls more axial.
It is important that the
tailplane is not producing lift when the model is in stable flight because tailplane lift
is high drag lift due to the poor section profile and the low aspect ratio of the
tailpane. On an all flying tailplane the situation is a little easier because once the
balance point has been correctly located and the model trimmed the tailplaane will be at
the correct angle and producing minimum drag.
The balance point is
probably the most critical parameter on the model, get it wrong and the model is either
very difficult to fly or very sluggish during manouvres. A good starting point for most
models is 35% (30% on basic trainers) back from the leading edge. It is then a case of suck
Trim the model for
straight and level flight. Note the elevator trim setting. Put the model in a shallow dive
and let the stick go. If the model slowly recovers from the dive it is OK, if not move the
balance point back or forward as if it were an elevator trim control and try again. With a
fully symmetrical section areobatic model the amount of down elevator required to sustain
inverted flight is another good indicator. If it is impossible to trim the model
satisfactorily then it is likely the wing incidence is wrong.