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Radio
Control Model World - Nov '95
by
Stan Yeo
INTRODUCTION
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 Decisions
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 specification
i.e
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 Crunching
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 necessary.
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.
2.
Tailplane
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 wing area.
Fin
Area
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.
Moment
Arm
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 and Options
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 sketched.
Choosing
the Wing Section
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.
BASIC
TRAINER
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.
INTERMEDIATE
TRAINER
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.
INTERMEDIATE
AEROBATIC
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.
FULLY
AEROBATIC
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.
PYLON
RACER
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.
RIGGING
ANGLES
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.
BALANCE
POINT
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 and see.
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.
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