A design study
To illustrate how I approach design, I’ll describe a particular model I produced recently.
How the Bambino came about.
For some time, I had been thinking about making a small, simple, 3-channel model for flying in light conditions. It wasn’t necessarily going to be a trainer, though it would be nice if the design could later be enlarged and used as such. Additionally, I had for some time been thinking about building a scale model of a particularly interesting aircraft I’d stumbled across during an internet search. That aircraft was the Aermacchi AM-3CM Bosbok. With all this in mind, what better than to design a model based on the Bosbok? My rationale was simply that it looked attractive to me, and the old truism “if it looks right, it will fly right!” has so far held me in good stead.
For this project, I decided only to model the basic profile of the fuselage. There was no attempt to be made (yet) to produce a fully accurate scale model. I was going to give it a simple tapered wing with a flat bottom for good lifting capability. Also, since there would be no ailerons, it would need reasonable dihedral and a sufficiently large rudder.
I wanted to power this model with an electric motor and an outrunner design was preferred. With the small size of the model, I decided not to build in a hatch for changing batteries. Simply attaching the wing with rubber bands would give easy access to the battery compartment.
So, there are my basic thoughts and design concepts. Let’s get to work and design this little baby….
But first, a word from our sponsor…..
The Aermacchi AM-3 was designed to fulfil the requirement for a new observation and reconnaissance aircraft to replace the Cessna L-19 Bird Dog aircraft serving at that time with the Italian Army. Designed as a joint venture by Aerfer Industrie Aerospaciali Meridionali SpA of Turin and Aermacchi of Varese, the first prototype flew on 12 May 1967 with the second prototype following on 22 August 1968. The AM.3 used the wing design of the Aermacchi AL-60 utility aircraft, strengthened to incorporate two hardpoints. The fuselage was a new design.
Although the AM-3 lost the Italian Army contract to the SIAI-Marchetti SM.1019, South Africa ordered 40 AM-3C aircraft, the first aircraft delivered in May 1972. It was known as the Bosbok (Bush Buck) in SAAF service. Three aircraft were also ordered by Rwanda.
Pilot and observer sit in a tandem arrangement, and the aircraft features dual controls. In its utility role, the Bosbok has space for two stretchers or seat space for additional passengers.
Armament configurations are diverse, as well. Two standard underwing pylons were able to carry 170 kg (375 lb) of stores each. Typical armament includes machine guns, rockets and bombs.
The Bosbok saw extensive action during the South African military operations in Angola, at which time it wore an olive-drab/dark earth camouflage instead of the light-grey scheme in which it was delivered.
Roles performed by the Bosbok in SAAF service include:
· Forward air control
· Radio relay (referred to as “Telstar duty” by the SAAF)
· Target marking (using smoke-rockets)
Start with a 3-view
All aircraft, model or full size, need to have lifting surfaces of sufficient size to carry the weight of the craft, but not so large that they cause unnecessary drag or weight. The easiest way to develop a feel for the various sizes is to find or draw a 3-view. In my case, I had found many photos of the Bosbok that showed a reasonable profile of the fuselage, and a few of the plan view too. The actual aircraft has an essentially constant chord wing with tapered outer panels.
Although this would not be difficult to reproduce, I decided that a constantly tapering wing would be aesthetically pleasing and quite simple to produce.
The only 3-view I found for this aircraft was actually quite a small, low resolution image, so I decided to work with photographs instead. There were a number of high quality photos available, so I simply downloaded one that suited my purpose and imported it into CorelDraw.
My first task was to rotate the photograph so that the longitudinal axis was horizontal. Then, came the somewhat simple task of tracing the outline of the fuselage and major components. As you can see from the accompanying image, I initially traced around the edges as closely as I could. From there, I smoothed out some of the bumps and bulges of the scale outline, and simplified the outline.
Profile photo of Bosbok, showing how I rotated it to the “flying” attitude and then proceeded to trace around the basic outline. For a truly scale model, this needs to be done very accurately, but for my purposes, an approximation was sufficient. I only needed the aircraft to have the “feel” of the real Bosbok.
Preliminary profile drawing. Note that I’ve slightly reduced the area of the vertical stabilizer, and elongated the fuselage to compensate for the otherwise reduced stability. I did this purely because it looked more appealing a little bit “skinny”. I also straightened out the nose as it looked a little like it had been broken. A cursory comparison of the two figures shows that we now have a model that is markedly different in outline from the real aircraft. Since this is not a scale model, the changes are all aimed at making the model more attractive and flyable.
Having decided that I would not be using the AM-3’s wing plan, I traced the plan view of the fuselage and then drew up a constantly tapering wing that had about the same surface area. At this point, I used my best judgement in determining the span and taper. Due to the intended size of the model, I felt it best to make the wing a little larger in area rather than risk it being too small. Anyway, the end result was a plan and profile view of the intended model. I set them up with the plan view directly below the profile view. In this way, any parts drawn on either view can then be directly aligned on the other view using guidelines. In more complex models, I also draw an elevation view (head on), aligned with the profile view, and this aids in developing the former shape and other details. I then had to decide the actual dimensions (I chose a 700mm wingspan) and scale the views to suit.
Once you have the scaled the basic drawing, you can then start planning for the wood thicknesses and rib and former spacing. A design can succeed or fail at this point, simply with the design of the internal structure. It is important that the structure be strong, yet light, being neither too heavy, nor too weak. Any unnecessary structure simply adds weight, but too little imparts weakness that may later lead to failure.
For the Bambino, I planned for 1.6mm balsa sides as well as top and bottom sheeting.
Having decided how thick the outer skin needs to be, you then produce an inner contour of that thickness.
Once the outline and scale has been established, draw a parallel line inside the outer line to begin planning the skin thickness and other details such as tailplane and firewall positions. In this case, note that I’ve planned for an open motor bay, so the top and bottom skins stop at the firewall.
Firewall and Formers
With the contour in place, now is the time to define where the firewall and other formers will go. In a model of this size, it is important not to add too much weight, so it definitely suits us to have a minimum number of formers and since we’re planning flat fuselage sides, two will do. We’ll place them at either side of the wing position. The firewall will be positioned at the most convenient place, allowing for the length of the proposed electric motor. As I was getting the parts laser cut, I went with 3mm ply firewall and formers as I could easily lighten them with holes. Otherwise, 3mm balsa would have been perhaps a little lighter and just as strong.
Here are the profile and plan views, aligned one above the other to aid placement of formers and other details. Note that I’ve also added guidelines in strategic places, and started adding the firewall and formers on both views. If you make a mistake and need to move or add formers it isn’t difficult, but it is important to keep in mind the eventual size and weight of the model. You want sufficient strength, but minimum weight.
Now, since we are looking to build an essentially rectangular fuselage, it is easy to determine both the height and width of each former. Taking into account that we want to be able to install radio gear and a battery into the fuselage, we of course need to shape the formers appropriately, but leaving enough wood to impart the required strength.
Former shapes and dimensions can be planned using both the side and plan views already developed. If the shape of the fuselage was to be circular or oval, or some other combination, it gets a little more difficult, but can be assisted by drawing the end elevation view – normally from head on.
Former 3 in the Bambino is easy – just leave a reasonable amount of wood around the edges, but former 2 is to be used for mounting the undercarriage, so the lower part of it needs to be a little deeper. Similarly, the firewall can have the holes planned for the engine mount, and centrelines drawn on them. Because the Bambino will not be large, we don’t need to leave any bracing in the middle, and can therefore have excellent access to all the space for battery and radio gear.
Although not mandatory, we can now plan for radio and equipment locations. Don’t be concerned that these are definite positions, but more like an opportunity to begin “brainstorming” the options. More importantly, you now have an opportunity to spot the problems – is structure getting in the way of your pushrods? You can also get a better idea of where you can put the equipment to best suit the weight and balance limitations .
This essentially finishes the fuselage. If you are planning to distribute the plan, you will need to label and identify all parts and list the material thicknesses etc. Also, if you’ve hand drawn the plans, you will need to ink them after you’ve flown the prototype and confirmed that the design is sound. However, we are getting a little ahead of ourselves – first we need a wing!
As previously noted, I planned from the start to give the Bambino a constantly tapering wing, Additionally, I felt it would be best with a lifting section, and in this case, a flat bottommed aerofoil is perfect. Previously, I had already selected the desired shape and size that I will be using and a plan view is shown in Figure 9. I will for now work only on one wing, and later, when I’m satisfied that the design is fnished, I will copy and flip it to match. This ensures that both wings are identical in every way.
This is my initial plan view of the Bambino. You can see that the wing is of constant taper, and should be of sufficient area to support the model with ease. From here, we work only with one wing, and later copy and flip the image to provide a complete wing with identical halves.
Here it can be seen that I’ve added leading and trailing edges, and a single main spar, as well as located the positions of the ribs. Note that the inboard ribs are of 3mm thickness, whereas the remainder are of 1.6mm balsa.
All wings need a leading and trailing edge of course, and here I’ve planned for a 6mm square balsa leading edge, and a 12mm x 3mm trailing edge. There is also a full depth, 3mm main spar for strength, and scrap balsa webs in the corners to assist the wing to remain square and strong. With a span of only 700mm, there is really no need for leading and trailing edge sheeting, so I’ve omitted this process. Now all that remains is to decide the aerofoil to use.
A flat bottom section will provide good lifting ability, benign stalling characteristics and ease of building so that’s what I decided to use. Using the plan view, measure the wing chord at the root and tip, and this will allow us to scale the rest of the ribs.
The remaining ribs may now be scaled by using the plan view, as shown above. Additionally, the position of the main spar can now be determined and cutouts planned.
With the desired rib spacing now planned, we can draw in the notches for the ribs, in this case from the top of the spar, allowing us to make corresponding cutouts on the bottom of each rib. We need also to plan for the leading edge and trailing edge, so we cut off the nose and notch the rear of each rib as necessary. As this wing will be joined by two carbon fibre rods, we now need to plan their position and match the height of their holes using the front view of the spar. This spar design has been used in many models – and full size aircraft, and has proven very reliable, strong and light.
About this time it is appropriate to think of the dihedral and determine just how much we need for this model. Dihedral will provide not only stability, but also assist with turning. However, too much dihedral will make the aircraft too sensitive to rudder inputs, even if it is more stable. Therefore, I planned this model to have 22mm of dihedral at each wingtip. With this number in mind, we can now start planning the mainspar and wing joiners. I have found the easiest method is to draw the mainspar full size, using the tip and root ribs to gauge spar thickness.
All that now remains, is to plan the horizontal stabilizer, a simple enough task.
The tailplane and elevators of the Bambino are cut from 3mm balsa sheet. If this were a larger model, we would cut the tips off and use balsa strips with the grain running fore and aft to provide extra strength and rigidity. However, in this application, no additional strengthening is required.
Similarly, we can fine tune the shape and size of the fin and rudder, with the fin fillet being cut with its grain oriented in a different direction to that of the fin.Its importance is not so much for stability – though that is indeed one of its uses – but for additional strength of the fin.The rudder needs to be large enough to provide good control.
We have seen the process from initial concept to completion. At this stage, it simply remains for us to cut wood and assemble the parts, finish the model and go flying. The Bambino flies very well and, due to its light weight and great stability, is ideal for relatively low time pilots. Certainly, it is very simple to build and set up. Lastly, it is a great introduction to building, something of a lost art in today’s modelling fraternity.