|Aerobatic airplanes built up in several parts are en vogue. In the distant past control line models traditionally were small, simple, one piece designs. The first European Champion used a 40 inch span airplane with a 2,5 ccm diesel engine. Americans had bigger capacity motors already, but international competition and travel were not the norm then. Depending on increased demands on later designs the size of our airplanes began to grow. Soon flyers had some problems to solve transportation problems of family and airplane in one car. For competitors travelling to contests via airliner the real troubles began. Gone are the times when most pretty and friendly stewardesses helped to find a free row of seats to rest our airplanes on. A solution was needed.|
have always been simple ways to assemble models, like rubber bands; a method
taken over from free flight tradition. Even for the most simple models we
should erase such thoughts very quickly out of our brain. With the power
of modern engines, vibration, oil smeared parts, and unreliable rubber bands
this is an accident waiting to happen. Add to this the flight loads of our
modern aerobatic craft and it’s quite obvious that a most serious
method is required.
The first time I ever read about such a system was an article in the British AEROMODELLER magazine in 1972. Jim Mannall published the plan of his Nimrod design which was built up in several parts, with detachable wing, tail, and fin. Shortly later he wrote another article with more detailed information, including precise drawings of his system. I never quite understood those fine intricacies of the way he mounted the tail, but I was convinced that this was the way to go. As it turned out - this is still one of the most common systems we use today.
|Before I forget - let me say that all my building methods are very simple and purely traditional. I will - and I can - address advanced builders only. There’s no mention and no help for experts building two part wings and multi part fuselages. I have neither the building skills nor the technical prerequisites to fabricate those elaborate metal wing joiners or the accuracy to build a three part fuselage. What follows is a guidance for a simple way of building a detachable F2B airplane for the advanced modeler with decent building experience. I’ll describe the variation with a detachable one piece wing and detachable tailplane and fin.|
basic thoughts about a detachable airplane consider several aspects of design,
construction, building, and airplane parts.
1) Occuring loads should be spread most evenly on as much area as possible to reduce loads, to prevent highly loaded stress points, and to improve load distribution. Few connections, with even small dimensions, concentrate loads in small places, thus increase loads, stress the connecting parts, and may easily cause failure.
2) Unlike RC airplanes, where loads occur in flight only and mostly in vertical direction, our airplanes have to stand horizontal loads, too. Apart from the usual flight loads in manoeuvres we also feel centrifugal force in our circular flight. Add to this increased load when flying manoeuvres in high wing. PLUS - last but not least - the horror of every stunt pilot: the pull test !!! In this test the whole pulling load is carried by only a few spots; those parts which connect different airplane parts to each other. Depending on how and where we hold our airplane during the pull test, we must build these places very carefully and solidly ( more about this later ).
3) Bending loads which occur during manoeuvres are not a problem for the fuselage of a one piece airplane. However if the wing is detachable, the vertical height of the fuselage (at the point of highest thickness of the wing) is strongly reduced, thus weakening the fuselage structure. Also, at this point the fuselage is not a “closed box” layout which - as you know - is a preferred design element. So rigidity of the fuselage is badly impaired and we have to think about some strengthening implicitely.
4) The connecting parts and their installation must be carried out completely stress free, in order to avoid constant bending loads.
5) Last but not least we have to consider weight. Additional required parts should be carefully designed considering dimensions, shape, and expected loads.
flyers who accept long distance journeys to contests via airliner the fully
detachable model has become the standard equipment. In fact there’s
no other possibility. The advantages are obvious. However the take apart
airplane has even more benefits.
1) The most important argument is less required space for transporting, especially if we have to transport more than one airplane in not so huge a family car. Single parts can be stored much more economical.
Airline transport is not possible at all without the model in a box. Some
flyers have even had problems with one piece airplanes in a big box (seems
the hole in the airliner where the box has to go in was too small). Even
with a one piece wing my model box gets so slender that I’ve never
had any transport problems (except for other nasty annoyances which have
nothing to do with box size).
3) Right from the beginning we will experience some fine pleasantness during construction of the airplane. Building and handling smaller parts is very practical , especially in limited working space, and will definitely reduce the number of dings and dents so common with big airplanes. A so called “paint stand” can be used already while finishing the wood work, sanding, covering, etc. All the more separate parts will considerably ease complicated paint work with that hated removing of masking tape!
4) Sometimes we may feel the desire - or necessity! - to modify, repair, or replace single parts. Maybe a small modification is required, new parts have been built for test purpose, or a damaged part needs replacement. With a detachable airplane all these problems can be solved much more easily and quickly.
5) Very important: since the era of careful, precise, and extensive trimming our airplanes we need to have full access to the control system. The detachable airplane allows full freedom of layout, execution, and range of trim possibilities. After having had some less satisfying experiences in the past with unwilling airplanes which were not detachable - with some necessary ugly repair work - I’ve come to the conclusion: NO MORE one- piece airplanes!
well - nothing is for free. Of course there are some disadvantages, too.
Since the fuselage is not solidly glued to the wing it will be much less
rigid. We have to compensate for this by various methods of strengthening
the fuselage sides and top. In doing so we have to consider several specific
details of our particular airplane design ( I’ll explain this in a
4) All of this takes time - if you feel this a disadvantage ( some people LIKE to build their own creation and see this rather a challenge than a penalty !). So you’ll have to decide for yourself if there are any disadvantages at all.
said “ consider specific details of our particular design”.
That means: if we’ve decided on a special type of airplane, special
design, special shape and contour, special way of handling our airplane,
then we have to consider these aspects already when drawing the design.
Let me explain.
Several designs which were drawn with MY pencil show a fuselage contour which reaches quite a bit below the wing. This requires (let me call it) a deep “fuselage bottom section”; i.e. there’s a big part of the fuselage below the wing. The same is true for high positioned wings, like in those “zero-zero-zero” configurations. On order to not make construction more complicated than necessary, the connecting elements are installed in this fuselage bottom section. The easiest solution is to mount them at the bottom end. This means that they are located at some distance from the wing center line.
|Depending on how you are holding your airplane during the pull test, this is exactly the point when we have to think about construction of this “fuselage bottom section”. Before we continue, let me tell you that I never hold my airplane at the inner wing tip. I just don’t understand how flyers can do this. Holding my airplane at the wing tip during pull test would simply CRUSH my delicate tip construction into pieces. So I hold my airplane with both hands and spread fingers above and below the wing, taking the most part of the load at the bottom section; thus the fuselage doesn’t feel much of the load.|
|During the pull test the load first hits the bellcrank. Then it’s transferred to the bellcrank mount, then to the wing center, then (in my case) to the fuselage bottom section. This is where I hold the airplane, and this means that this connection is of high importance and that this part has to be connected to the wing very solidly. The formers in this fuselage bottom section should have decent thickness to provide sufficient gluing surface to the respective side sheets and the wing center section. Connections to the fuselage have to take flight loads and centrifugal forces mainly. Of course their duty is also to stiffen the fuselage.|
|Another example: I don’t want to have the fin very far behind the tailplane. This can easily move the Center of Gravity to the rear, which might require a longer fuselage nose for compensation. As I see it, the distance “wing to tailplane” is one of the deciding factors for good flying characteristics. The fuselage just helps to keep things together. Any unnecessary length should be avoided. Since I like to have the fuselage nose as short as possible (for aerodynamic reasons), my fuselage sides end exactly where the elevator horn is. That way I can remove the detachable fin and have easy access to the pushrod rear end. I can easily change the trim : on "FULL UP" the pushrod extends beyond the fuselage end and with an adjustable elevator horn I can|
|choose the desired elevator deflection. HOWEVER this requires another feature. With the elevator horn swinging backwards, it needs some room to move into. That’s why I cannot simply build the rudder out of some thin balsa sheet. Instead I have to somehow “elongate” the fuselage and allow some room for the moving elevator horn. Basically the bottom part of my “fin unit” is an elongated fuselage end. I hope the enclosed photo will show what I’ve tried to describe.|
these two examples I’ve tried to explain how some special ( personal
) preferences can dictate the technical layout of a design. For other configurations
other solutions may be required. I don’t claim my solutions to be
superior to any others. It’s just my way of doing things. My aim is
to point out that all construction details have to be considered right before
doing the first pencil line !
The most simple solution to mount the wing ( as is done in RC areas ) is to use a hefty dowel in front and a bolt at the rear; this way you’ll save some screwing work. This method has never achieved acceptance in our circles. Load transmissions are not very satisfying, especially during pull tests. Generally we have preferred the method with ply-“tongues”, since this method avoids high punctual loads. Especially for the fuselage this system provides some kind of “closed box” design when the wing is mounted with bolts before and behind the wing, respectively through the fuselage bottom section.
|Sketch 1 shows a typical arrangement. This sketch is from Henk DeJong, Netherlands, and has had quite some influence on my own designs ( thank you, Henk, for permission to use your drawings ). Please note that the front mounting block has to be secured VERY rigidly to the former, in order to take considerable loads during manoeuvres. An early ( bad ) experience required that it must be done with a tenon system to hold the mounting block in the former. It can be installed in front of or behind the former, depending on your desired fuselage nose length. Since I favour a short nose, I install the mounting block in front of the former ( this may even complicate tank installation !).|
|The ply mounting block can be screwed to the former with wood srews or glued in with a small gudgeon (sketch 2 ).|
front mounting tongue must be held savely in the fuselage bottom section
and can be secured by formers ( that’s what I’ve tried to explain
above when using a deep bottom section) . This mounting block will hold
the blind nuts for keeping the wing and can take another nut for holding
the engine cowl. For the rear anchorage we use two tongues as shown in the
When using a detachable wing it’s quite practical to control the elevator by a second pushrod which is driven by the flap horn. I’ve described various methods on another page. Again Henk DeJong has found a brilliant solution. Henk connects the rear pushrod into a little nylon block. This is part of the flap horn and can be “srewed” up or down in order to adjust control surface deflections. He doesn’t need any fixing. A slot in the next former ( see sketch 1 )will guide the pushrod and keep it from disconnecting from the horn. When the wing is removed a little from the fuselage, it can be moved sideways and the pushrod will disengage from the flap horn. Ingenious solution !
|In order to stiffen the fuselage and to reinforce the wing saddle, it’s recommended to elongate the fuselage side ply doublers and run them along the wing cutout till the wing tongue rear end. A narrow strip will do ( see sketch 13 ). The rear fuselage tongue is easily supported by the fuselage former which is sitting at this location anyway.|
|I take great care when making the connecting tongues ( sketch 3 ), shaping them to fit, and when installing them. With the fuselage upside down and the wing precisely adjusted, glue is added to the front tongue and it is bolted to the mounting block! At the same time the two rear tongues are applied - but they are BOLTED TOGETHER before they are glued in! That way I can avoid any misalignment of these two parts, and when tightening the mounting bolts there can be no undesired stress tension when mounting the wing to the fuselage. During this process care must be taken to not glue the tongues together !|
|Several years ago Stanislav Czech from Czechia used a clever idea to mount the wing. Instead of tongues he used slanted formers in the fuselage and bottom section. Mounting bolts ran just through these formers into blind nuts ( sketch 4 ). To strengthen the wing-fuselage connection even more, we can run a long bolt from the bottom right through the wing into the fuselage where a suitable (horizontal) ply former with a blind nut provides additional support. Also some builders glue a flat block on top of the wing center with exactly the inner width of the fuselage at that point. This block will prevent the fuselage to bend sideways under load.|
|The desire to reduce the number of parts and to have one part master several tasks leads to another version. In my first detachable design I used the bellcrank mount to serve as the front mounting tongue as well. The mounting plate is made from 3 mm ply and extends beyond the wing leading edge, where it can be bolted to the mounting block sitting behind the former. The definite advantage of this system is that the load is transferred from the bellcrank directly to the fuselage via the bellcrank mounting plate - a solid part we have to install anyway. The only|
|disadvantage is the somewhat more complicated work to apply the wing sheeting at the wing center. The photo shows this arrangement. Of course with these kinds of systems the wing can be mounted from the bottom or from the top as well. The decision is probably influenced mostly from personal preferences or maybe dictated by a special fuselage shape - I suppose aesthetic reasons play a role here, too. But since the “zero-zero-zero” configuration is not so popular, the wing will go the bottom in most cases. This favours the low wing position and thus installation from the bottom.|
|With the tailplane it’s just the other way round. In most designs the tailplane is sitting high, so it’s quite natural to mount it from the top. My first take apart airplane was a strictly functional design, I was in a hurry, and I didn’t want go through all the trouble with a detachable fin ( at that time I had no idea how to construct this part ). So I decided to mount the tailplane from the bottom ( sketch 5 ). This layout saved me some|
|some building time. Right from the beginning it was quite clear that I had to draw the shape of the fuselage rear end in such a way that it was strong enough to take the bending loads created by the tailplane. Normally fuselage height at this point doesn’t appear sufficient to fulfill this task. So the fuselage end was shaped accordingly, = means increasing the fuselage height above the tailplane. A ply plate will hold the tailplane. A narrow ply stiffener ( see sketch 5 ) was added to the fuselage sides because this cornered cutout is exactly the point where the loads from the tailplane will strain the fuselage and stress cracks will appear.|
|Again there is a “fuselage bottom section”. It is not glued to the tailplane, but built and mounted separately. It would be very circumstantial to handle the tailplane with this bottom section hanging on to it. Holes in the bottom sheet give access to the mounting bolts. I’d recommend little mounting lugs at the bottom front and rear end of this part ( not shown in the sketch ) to mount it savely and to add some additional rigidity to the fuselage end.|
Probably the most popular way of mounting the tailplane is shown in sketch 6 which again is from Henk DeJong’s drawing board. My own ideas were strongly influenced by this layout, and I think most designs show this or a very similar feature. A plywood plate between the fuselage sides holds the tailplane. The fin - including fuselage rear bottom end - is then fixed; in Henk’s case with a dowel into the fuselage and one bolt at the bottom. The separation line at the fuselage end should run vertical to allow easy access to the elevator horn. Of course that bottom end has to be constructed hollow and carefully designed - it’s very easy to build it too heavy! Some specifics of Henk's construction are shown at right:
Plywood 0,5 mm
2) Blind nuts M3
3) Tongue aluminum 1,5 mm
4) Tail skid 2 mm
5) Plywood 3 mm
6) Aluminum tube or beech dowel
first own attempt on this way is shown in sketch 7. Too bad the separation
line ran slanted, this proved to be a mistake. At this design I wanted to
have an adjustable rudder, so this special solution was chosen. The typical
feature of this layout is that two bolts hold the tailplane and the fin
at the same time. These bolts are inserted through two holes in the bottom
sheet. You’ll need socket head bolts ( it doesn’t work with
other ones ) and a long Allen wrench. A tongue with tongue box was used
for the upper connection, a pin-into-tube system will work as well. One
bolt at the bottom end is used; mainly for achieving additional rigidity
and also for having a tightly closed gap between parts( looks nice! ). I
like the elegant idea of mounting everything together with only two bolts
- at a place where it can be mounted very solidly, at that. However I have
to admit that the mounting process can be tricky at times! A third hand
would be helpful.
For the structure shown in sketch 7 several parts are required. They are shown again sketch 8.
Mounting plate, 3 mm plywood, with two blind nuts. Built into the balsa
block of the fin
B) Covering plate on fuselage sides, 0,5 mm ply.
C) Tailplane mounting plate 3 mm ply. Goes between fuselage sides. Together with B forms a solid mounting board
D) Ply doubler at fuselage bottom 1,5 mm ply. Takes tail wheel strut sewn to it
E) Tonge 2 mm ply
F) Tonge box center
G) Tongue box cover
little arrows in sketch 7 show the fuselage end = separation line. Also
the linkage for an adjustable rudder is shown. To secure vertical alignment
I’ve installed a small tongue which fits into a suitable tongue box
in the upper fuselage end. One bolt at the bottom rear end also helps alignment
and shouldn’t be discarded. At this point the whole fin unit is not
very rigid and may easily benefit from some bracing.
avoid crushing the center section of the tailplane it’s recommended
to “bush” the fixing holes for the fastening bolts. This can
be done by inserting short pieces of aluminum tubes. I prefer short beech
dowels with 3 mm bore holes, length adequate to tailplane thickness.
My present method of building the rear end is shown in drawing 9, individual parts drawing 10.
B) central layer of “fin unit”, Balsa 3 mm
A) fastening plate 3 mm ply with blind nuts
inserted into B
D) fuselage end cover ply 0,5 mm ply,
glued ON fuselage sides
C) mounting plate 3 mm ply,
glued BETWEEN fuselage sides
whole effort of building a detachable airplane may appear very high at first.
I have to admit - when you do it the first time the building process tends
to run pretty tenaciously. There are some more parts to fabricate, they
have to be installed precisely, and the sequence of installation has to
be thought through very carefully right from the beginning. If however you
have overcome it once, the second attempt will run much better, and all
future realizations will be a trifle because the process has become a habit.
In any case the benefits of this method will easily outweigh the disadvantages.
At last - there’s no alternative for transporting a model airplane
in not so bulky a model box.
Dimensions of such a kind of box merely depend on the span and chord of the wing of the airplane. Height simply arises from our ability to arrange and “stack” the various components efficiently. In any case there should be some “air” left safely between individual parts!
|Clicking this little ( detached ) button will show you some construction photos. These - as well as the photos and graphics on this page - show construction work of different designs and designers.|