other modelling categories Precision Aerobatics offers a wide variety of
shapes to the aesthetic eye. From fanciful to functional styles and from
elegant lines to martial scale renditions all styles have been tried and
used successfully. However we don’t see so many radial cowls. There
are a few remarkable exceptions; namely those outstanding creations by Al
Rabe ( his Bearcat and Seafury ) have proven the suitability of this concept.
Other examples include Jack Sheeks’ Corsair and Focke Wulf, Trostles
and McClellans Focke Wulf, and Beringers YAK and GeeBee.
Most modellers seem to shy away from what they believe is way too much frontal area. While there may be some increase, the additional area does not necessarily increase drag proportionally. This depends much more on the body shape of the subject. Since we have left the typical 35 size stunter behind us long ago, those modern powerful engines with their ability to swing big propellers no longer limit our choice to extra slim fuselages.
|I got my first impression from a picture in an Aeromodeller issue in the early sixties. Dave Day from England had built a design with a radial cowl. This inspired me to produce such an airplane shape. It took me several decades to realize this idea, but over the years I’ve built quite a number of airplanes with this feature. In my opinion these cowls are not much more work than the more common version. The only problem might be the question of how to shape the fuselage section right behind the cowl. Usually I have added two thick balsa fuselage doublers and carved a nice round shape. These doublers|
|reach to the high point of the wing airfoil. If made from very light balsa the added weight is neglectable, and you wouldn’t see an interruption in that smooth line of the fuselage ( top view ).|
|Construction depends on personal preference and on outward shape of the fuselage nose ( lengthwise cross section ). Basically there are three types: cylindrical, conical, and curved outline. Let me try to describe these variations.|
|One method is to use thick balsa blocks. This method is suitable for all kinds of cowl shape, but for convex or cancave shapes it’s supposition. The blocks are small trapezoids and glued to multi edged formers; in my case it’s 12 blocks glued to a 12 edged former. We can use two or more formers to build the front end. There’s also one provisional rear former which helps to align the blocks and glue them together. This rear former is removed later. All formers are glued to a 10 Millimeter beech dowel as seen in the pictures. The trapezoid blocks have to be cut precisely in order to provide a snug fit. I prefer to use some old carpenters|
( a powder mixed with water ). It takes some time to dry thoroughly, but
it sands as easily as balsa and you won’t see glue seams on the finished
product. The whole unit is fixed in an electric drill. At low revolutions
the shape can be formed with various grades of sandpaper.
|While still in the chuck we can prepare the front of the cowl. I do this with a sturdy balsa knife with a very sharp blade, held very rigid. It helps a lot to rest our hand on a suitable support. The knife is held in such a position that the wood of the turning cowl is “running away” from the blade and not against it ( look at the picture to see what I mean ). Don’t forget to wear safety glasses when doing this kind of work. We can already shape the front of the cowl and carve a groove into the rear former. Don’t cut “through”! This groove will help to remove the formers later. After taking out of the chuck the formers are removed and the shape is finished. Usually some ply doublers will have to be added on the inside to help mounting the cowl. At last all holes for engine, muffler, and needle are cut out.|
|Admittedly for me making these trapezoid blocks is a nightmare. However for fancy outlines we need enough thickness for sanding the desired curves. Another method would be to use circular formers and add several layers of balsa sheet around the formers, glued to each other. However the sheer thought of possibly having to sand through several glue layers would give me fits.|
|When our design calls for a cylindrical or conical cowl shape another method can be applied. We simply wrap a “jacket” ( is it jacket?) around round formers. These can be sawn out roughly, then glued to the dowel and shaped carefully on the electric drill. Measuring repeatedly should produce the exact diameter. Now we have to prepare the jacket. On a cylindrical shape it’s quite easy; we can calculate the dimensions or take them from the formers diameter. For the conical version we have to find the exact shape first. This can be done in two different ways. One simple method is to wrap a|
|sheet of stiff paper around the formers, keeping it with pins or rubber bands. A marker is fixed and held steady so that it just touches the jacket. Slowly turning the cowl unit by hand will draw a line at front and rear. When cut out we have the desired shape which can be transferred to plywood ( I made mine from 0,8 mm ). Length of the jacket is exactly the dimension of the girth, there’s no overlap which would be difficult to sand. A small ply strip inside will hold both ends together later.|
solution to find the jacket shape is by drawing. We make a scaled down side
view drawing of both formers and the exact distance between them, sitting
on a centre line. A tangent line touching both formers will somewhere cross
the centre line. This will give us the precise front and rear radii for
drawing the jacket. Okay, now that we have the plywood jacket it can be
glued to the formers. The rest of the procedure is the same as mentioned
above. The pictures will
show the process.
|For whatever solution we have decided on, it’s a good idea to glue a narrow rim of thin plywood on the rear inside to stiffen the cowl. Especially with big cutouts it will not be very rigid. Also it has to be mounted somehow and some ply supports are necessary anyway. In any case I cover all my cowls with glass cloth on the outside. This helps to prevent the inevitably scratches and dents.|
|Of course there are other methods of cowl production. When deciding for a fibre glass or carbon product there’s no design limit. However building raw models and moulds for a one unit production line in several years doesn’t appear very attractive to me. One of my friends had a clever idea. He used a Cognac glass as a mould. According to his advice this method works best AFTER the Cognac is enjoyed.|