|Slope Soarer Design
Radio 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.
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.
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.
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.
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.
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 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
TP Area = Wing Area x percentage TP area required = 450 x 0.15 = 67.5 sq. ins ounded up to 68 sq. 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.
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.
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.
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.
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
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)
Area 6 – 8% of Wing area
Rudder 40 – 60% of total fin area
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.
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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