Build your first flying scale model - Chapter 12 - Will it fly?

I was optimistic that the Swift could be persuaded to fly without too much difficulty, so did not want to go through a long winded and methodical trimming procedure. This tends to be a last resort with me if I have a model that refuses to behave. However, one thing that needs to be checked before the first glides is the balance point. This can only be done accurately with the rubber motor installed, so this is the first thing we need to sort out.



Firstly you need to open up the holes for the rear motor peg. Holding the model up to a bright window can help to locate them. I use a round needle file to gently enlarge the holes until the peg is a tight fit.



The best thing to use for a rear peg is a length of aluminium tubing of suitable diameter. This allows you to to put a wire through the middle to hold the model securely if you are using a winding stooge.

The more traditional approach is to use a piece of hardwood dowel or bamboo. I used a length of bamboo from a kebab skewer for the simple reason that I discovered I had somehow run out of aluminium tubing!



Now to the rubber. For most flying, the currently available Tan sport rubber is just fine. The selection here was bought from Mike Woodhouse, who will supply a mixed batch like this on request. You can see 1/8", 3/16" and 1/4" rubber here. To avoid degredation over time, you should keep rubber in a light-proof container such as an old biscuit tin. This lot will keep me going for ages.



One loop of 3/16" rubber will give a very spritely performance for this model - in fact I am sure it would fly OK with rather smaller rubber than that (maybe 0.150"?) However, as 3/16" is widely available, let's see how we go with that. I am starting with a loop that is 2 times the peg to prop hook distance. We can experiment with longer motors after the thing has been trimmed out.



The knot is just a standard reef knot, but tied twice.



The strip rubber needs to be lubricated before use, usually after the knot has been tied. On the right are a couple of bottles of traditional rubber lubricant, from FAI Model Supplies and SAMS Models. On the left is a jar of silicon oil, as supplied by Mike Woodhouse, who also sent the jar of white powder next to it, which is colloidal silica binding agent. If you dip the ends of your slippery silicone covered rubber into this stuff, you can then tie a knot that does not come undone.

I know some of my US friends use silicone sprays sold in car accessory shops for lubricating their motors, and I believe "Son of a Gun" is one brand that is popular.



To help keep the rubber on the prop hook, I wrap a small rubber band round and round the rubber as shown. Once the rubber is placed on the prop hook you can roll the small loops up so they sit almost at the hook as shown.



So, now you need to persuade your lubricated rubber motor into the fuselage until it gets past the rear peg position. Some sort of stuffing stick is useful for this. On the Swift you are hampered by the small nose aperture which you can barely get the knot through. I find it best to keep the knot at the rear, so it does not come out of the nose again while you are stretch-winding the motor. By peering through the nose you should be able to push the motor peg through the fuselage so it goes through the loop of rubber.



After winding a few turns on to get the rubber more or less evenly distributed along the fuselage, have a look at where the model balances by holding it at the wingtips. You can (hopefully) see that the balance point is well behind the front spar, and would certainly result in the model stalling if we tried to fly it.

So, some nose weight was added (very rarely do you get away without having to do this on a rubber powered scale model). How I did this was a bit hit and miss, but rather than adding unsightly lumps of Blu-Tac to the outside of the nose, I squeezed a bit of Formula 560 canopy glue inside the nose aperture so it dribbled down inside the nose and formed a small puddle at the bottom (at least I assumed it did - I could see nothing!) I then dropped some small lead shot (from a fishing shop) through the nose so they fell into the glue inside. I assumed they got stuck, because I could hear no rattling when I shook the model. The model was left for 24 hours for the glue to set. This moved the balance point forward until it was about 3 mm behind the wing spar, so somewhere where I was happy to start trimming glides.

Click here for movie of test glides (930 KB)

This should play if you click the link, or you can right click and select "save link as" to put it on your hard drive. The glide looked pretty close straight off, though note how it has a sharp tendency to turn right. This is because my fin came out slightly curved, and I did not stick it on quite straight.

I thought it was a shame that I could not demonstrate what a stall looked like, because that is the classic thing that usually happens trying to trim a rubber model like this. Often test glides show a stall, so you add nose weight until it goes away. So, just to show you what a stall looks like, click here (760 KB) to see test glides after I added a lump of Blu-Tac to the tail, to simulate not enough weight in the nose.

With the glide looking hopeful, a few winds can be added to see what happens under power. The next movie here (650 KB) shows a hop with about 250 winds on. It still looks fairly good.

I suppose I ought to describe the technique of stretch-winding here, which is the way you get the most turns on a given length of rubber motor. Basically you hook your winder, if you have one, on the prop hook and stretch it way out of the front of the nose of the aircraft - maybe three times the rubber length. You then put the winds on, and gradually move back to the model, still winding, as you come up to the required total of winds. This technique gets the knots in the rubber spaced as evenly as possible. You learn to feel the tension in the rubber, and when things start to really tighten up, you know you are close to the limit. Best not to go there unless you are making a competition flight where the time aloft is important. A blown motor can make a right mess of your fuselage.

I use the KP Aero Models 10:1 winder for virtually all my models - it seems to cope with up to 4 strands of 3/16" rubber, and is a very handy size. SAMS Models sell it, as do Flitehook and several other Free Flight specialists.

Next we have a flight with 500 winds on - click here (1.6 MB) to see it.

Now this does reveal a problem when the rubber runs out. Because the rubber is driving the prop, the model will react and try to rotate in the other direction, though obviously much more slowly because of all that wing area resisting it. So, under power, the left hand wing is pushed down (model tries to rotate to the left as viewed from the rear), which resists the natural tendency of the model to turn quite sharply right. When the rubber runs down, the torque effect disappears, and the model is free to follow its natural tendency to turn more steeply to the right. The right turn is too steep however, so the model spirals in.



What we need to do is reduce the amount of right turn on the glide, so a small acetate trim tab was glued into a slot in the trailing edge of the fin and bent left as shown. The movie here (1.8 MB) shows what happened on the next flight. The model now goes almost straight on the powered part of the flight, but begins to turn right as the rubber runs out, then glides to the right, though not as sharply as before. Straight powered flight is not a good idea if you want to hang onto a model, so we need to do something to get the power and glide parts of the flight more similar.

The way to do this is to alter the thrust angle of the nose bush. You can balance out the torque effect of the prop by adding right thrust to the nose bush. Glue in a strip of 1/64 ply to one side of the hole in the nose. To give right thrust, glue it to the left hand side of the nose when viewed from the rear. The prop should now be pointing a bit more to the right. You can add additional strips later as needed, but it is best to gradually add thrust adjustments so you can see the cumulative effect as you add each strip. In that way the model should now turn right during the power phase without adjusting the rudder trim tab.

At this point, things took an unexpected turn (literally) when the model went left after launching, and ended up rather high in an oak tree.

Now if you put right thrust in a model that was just about turning right anyway, there is no way it should turn left, so the nagging thought came into my head that perhaps I had put the ply packing piece on the wrong side of the nose, and given it some left thrust instead. No way of telling unless I managed to get the model back though.

We had to leave it where it was, and the plan was to come back with a pole three days later on the following Sunday (it was about 30 feet up). I was not optimistic of ever seeing it in one piece again, as on the Friday we had torrential thunderstorms. However, much to my amazement, when I went back to the field on Sunday evening, there was the model now just 12 feet up, with its prop wedged in a fork in a branch, swinging merrily on the end of its rubber. The prop was poked free using a branch we found on the ground, and down the model came. Amazingly there was no damage at all, except one wing strut missing, which is easily replaced. It must have got very wet on Friday, because one of the decal aileron lines had come adrift and floated over to the wingtip, before drying in a crumpled sort of heap. It just goes to show the waterproofing effect of a thick coat of enamel paint!!

Well, guess what - I discovered the packing WAS on the wrong side of the nose, so I quickly pulled it off and glued it on the correct side before anyone noticed. They say confession is good for the soul - but perhaps this is also a good illustration that even experienced modellers make mistakes sometimes, and is a useful reminder to check all your trimming adjustments twice before launching the model.

I wound it up to see if all the rain had affected the trim at all, and can report that the model set off in a steady right hand circuit as if nothing had happened.

I will put some more video footage on the site when I get to chance to fly it at a bigger field. It is certainly great to have it back!

Many thanks to Laurie for manning the video camera.


Here is a quick update on the little Comper's flying exploits during July 2005:

At the Scale Weekend at Old Warden I managed to break the wing off twice when it decided to do a RH death spiral from launch under maximum winds. I thus chickened out and altered the trim to make it fly left. This involved being fairly brutal with the fin - in fact cracking it down the hinge line and regluing. I then nearly lost it on Sunday when it did a thermally assisted 130 second flight, fortunately dropping out of the lift just at the boundary hedge.



Thanks to Richard Crossley for sending this great shot of mine and Chris Strachan's Swifts just after we had launched them together.

Graham Potter took a video of the Swift flying a couple of weeks later at the SAM 35 Gala at Old Warden - click here to see it (3.3 MB).



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