A return to model building and flying. Part 7

The Wing

September 2004

It always surprises me how my mind can get stuck in a groov and ignore the obvious. After struggling for quite some time to enlarge the wing plan and print it out in A4 size sheets using a graphics program, and never quite getting it right, I had one of those "Duuh!" moments. Suddenly it dawned on me that I have a CAD program sitting in my system that is designed specifically for such a task. After 15mins with Qcad, and that inluded time taking the measurements and double checking them, the job was done and I was sticking A4 sheets together.

wing plan

Then came the inexplicable problem of the size not being right. I'm sure there is some mischievous imp that keeps altering the chord of the wing. Inspite of carfully double checking the dimensions, every time I had the whole thing taped together and measured it just to be sure, it was always a few millimetres short on the chord. And of course, I had not bothered saving the cad file as I thought it was just a temporary job! After redrawing it all again, mercifully a quick simple job in Qcad, It took three altered printouts before everything finally matched up.

ribsAs all the ribs are the same size, I decided to make a cardboard template and cut them all out inividually. Not too arduous a tast. The drawing pins help stop too much movement between the template and the wood while cutting. Every time a make a temporary template like this, one side is alwas different to the other. By rotating every other one, and sandwiching them in a block, the final sanding can eliminates most of the differences.

At this point I was beginning to get a sense of the divergence between old and new technology. This model was designed in a time when tissue and dope was the currency of the day. The wing relies on the tensile strengh of the covering to give it most of it's rigity; uncoverd it is quite easy to twist and bend, resembling a freeflight wing in a lot of ways.

My looming problem is that I will have to cover it in plastic film; not through choice, but practical necessity. It would be very unpleasant, and it goes without saying, unhealthy, to use cellulose dope in the cramped confines of my flat. My experiences, admittedly not extensive, with plastic film have not been good. I actually did some tests on dummy wing structures coverd in lightwight and heavywight tissue, compared to film. I was quite dismayed to find the test pieces deflected about three times as much with a given load as the tissue and dope samples. The film was also considerably heavier. This led to the conclusion that the airframe must be built to in a manner that regains some of the rigity and stiffness that is lost if film is used as a covering material. This in turn leads to an increase in weight in the structure plus the increase in weight of the covering marerial itself. It's a bit of a vicious circle, so I have avoided it in the past.

However, now that my back is against the wall, I do not have much choice, I have to learn to adopt my building methods to suit. One easy stop gap measure was to retro fit webs between the main spars this I know from experience will give a very rigid wing for up and down defections, and to a more limited extent in the twisting deflections in the wing. I hope this will be enough as the alternative is to completely redesign the wing structure, which I could do quite easily, but the main aim is to get a flying model out of this process, not to get bogged down in the details.

rib damage Of course one should watch what one is doing when sanding down the spar webbing, and not let the sandng block take chunks out of the wing ribs, thus having to patch up the holes. The results can be seen opposite.

As I have mentioned in the past, this model was designed in a way that make it difficult to align things relative to each other. Fitting the wing to the fuselage accurately meant removing wood from the fuselage/wing joint to give zero incidence. The only reference I have, is the top join line of the fuselage sides., and this was aligned with the engine bearers by eye; so I hope the whole thing is resonably zero zero.

Because of the hybrid way of wing mounting the undercariage, I have to sheet the top wing centre section, align everything with the fuselage, fit the under carriage, finish sheeting the underside of the center section, all in the at order. This model was not designed for easy construction.

Flying to date has consisted of throwing one of Pete Catlow's old PAW 2.5 powered Warlords (combat wing) about, in a manner that could be best described as, "almost in control." Mufflers rob the engine of power which can make things a bit hairy at times, but great fun none the less.

goodyearTrying to pilot Bob Stanley's West Eurotech T1 36 ex-helicopter engine powered goodyear, was hair raising and head spinning. I was really dizzy after that one, but at least capable of standing up at the end of the flight.

I got nearly as dizzy trying to video one of Pete Moore's sedate flights from the centre of the circle, It's very difficult squinting through a viewfinder, walking round the pilot, and trying to keep some sense of where every thing is. I would like to do more videoing now I can borrow a camera. At present processig the files to a size that would be practical to place on this website is proving a little elusive, but I am working on it.

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