A return to model building and flying. Part 15

Rats in small spaces

May 2005


I included this picture of myself because after I took it, I was surprised by how little space I actually manage to work in.

This rat has been quite a challenge, and learning experience. After deciding to make the leadouts internal, I rapidly found out why people use rotary bellcranks. There is no lateral displacement with a round crank, so no hacking great big holes in the wing to allow for the extra movement, so that's a mental note for next time..

As its entirely my own creation, it's just a question of sawing up bits of wood and trying them together until it looks right. This method also has the disadvantage that some problems don't come to light until construction is well under way, the bellcrank for instance. Side mounting the engine seemed like a good idea at the time as I could hang all the hardware off the bottom bearer, disadvantage, it occupies space in the tank compartment. However this did give me somewhere to screw the tank to.

ra tank rat uc

Continuing my exploration of shrink films, I decided to cover the flying surfaces with Pro Film and paint the fuselage, I have problems getting any shrink film to go round compound curves.

This has thrown up two problems. The film has a habit of trapping air when trying to cover a sheet surface that naturally expands when heated causing lifting of the film from the wood. This is a sort of catch 22. need to heat the the film to shrink it, this softens the adhesive, air expands, separates film from wood. Not sure how to deal with this one except with fast application of a rag to press the surfaces together again as the film cools.

The second problem, covering sheet with film shows up every imperfection in the wood surface. Unlike a paint finish, where if you notice the odd dint or lump, it can be filled, sanded down and re-coated. Film can't, it's a one off process that leaves very little room for errors or sloppy workmanship (my building is far from precise, and usually involves copious amounts of filler).

I have just remembered a third problem, film does not go round small sharp radius corners well and requires a lot of tedious manipulation for it to look reasonably neat. So far I have never managed to get it to look good. On a large airframe these imperfections would not be too noticeable, but on a relatively small model like this one they can become an irritation.


When I came to fly the rat I was expecting trouble starting the engine. After years of storage it was seized up almost solid. Placing it in the oven for a few minutes freed it up enough to turn it over and get some lubrication into it. However, on the day, it had gone tight again.

Then came one of those strange moments when I squirted some fuel into the venturi connected the battery and after a few flicks it fired and ran. This did not last. When I tried to start it prior to a flight, the engine would slow and stop when the glow clip was removed. putting in a new plug cured that problem.

The next thing was inconsistent motor runs. After takeoff it would cough and splutter for around 15 laps then die. Rearranging the plumbing to make a exhaust pressure feed seemed to solved that problem, with it then running consistently for around 70 laps

The model is a bit twitchy on the controls but I can do something about that. I was also happy to find the under carriage worked as I hoped, with the model rolling serenely over the bumps in the grass.

I have decided to name my creation, 'Rattus Buutus.' Silly name really, as it's a bit of softy to fly; 'Soft Fluffy Cuddly Rat' hasn't quite got the right ring to it.

After repairing the Danish Challenge combat wing, and having a chance to throw it round the sky for a while, I got Bob (I'll fly it!) Stanley to pilot it so I could get some idea of what it looks like from outside the circle, and take some pictures. The PAW 19 really likes an 8x6 prop, it's suddenly become very sprightly indeed. It must also be run in as it is now starting second or third flick

dnaish callenge flight2

I have a problem in that it turns much tighter in outside loops than inside. The only way I can think of altering this is by adjusting the thrust line?

danish challenge flight

My flying however, needs a lot of work I am breaking two props a week on average, although I did have an excuse for the last time. There was quite a strong wind blowing at the time, that was bringing a few RC models to grief. The difference is, my mistakes cost a few pounds without the the trailing RC zeros: thankfully!

I am beginning to hate modern props that seem to be made of materials that are guaranteed to break at the slightest contact with the ground. Mind you, not much would survive this sort of landing, One of my classic misjudgments of altitude combined with a moment of hesitation.


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