Radio Control 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 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 throughout 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 criteria 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 soarer’s 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 maintain speed through the manoeuvres, 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 moment 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.
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 air stream.
Choosing the correct moment arm is a bit of a compromise. The longer the tail moment arm the more 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 balance point . Long fuselages also increase the wetted area and the fuselage volume thereby 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 long 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 method 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 Lift-off’ 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 under camber 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-ordinates 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-symmetrical 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 writing 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 neutral elevator. One reason for zero longitudinal dihedral (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 tailplane. 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 tailplane 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 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 aerobatic 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.