Build your first flying scale Jet - Chapter 11 - Getting the thing to fly

Now comes the rather nerve-wracking process of persuading our newly finished jet to fly. I should explain that the videos included here were taken some time after my first attempt at trimming, which was at the 2010 Flying Aces Nationals. This was hampered by a shortage of Rapier motors of the appropriate thrust - all I had were gutless standard L2 motors or very powerful L2 HP motors. After a couple of extended glides with the weaker motors, I foolishly tried an L2HP and ripped a wing off. The model was put back in the box and taken home for repairs. Fast forward to July 2011 and a windy Old Warden. Not the ideal conditions to try and trim a model, but I thought I'd have another go.



Before we start, here are two photos of a modification I made to the model since I last flew it. As you can see, I added a downthrust tab in the trough, to deflect the thrust from the motor and hence push the nose of the model down. As the thrust builds up during the flight, the tab acts more strongly, compensating for the tendency of the model to pitch nose up (as the motor is below the centreline of the fuselage). The tab is made from laminations of the self adhesive foil I used to line the trough. One useful by-product of the tab is that it keeps the hot exhaust gasses off the lower parachute housing. Initial flights without the tab resulted in blackening and blistering of the orange paint. You can see I added an extra little piece of foil at the front of the housing to help protect it.



To get an acceptable glide I found a little clay needed to be added to the nose -this can be conveniently hidden inside the intake. You don't want too floaty a glide for these jets and you also need to get it as straight as you can. This model seemed to have a natural tendency to turn slightly right so I added a small trim tab to the right hand wing trailing edge at the tip, angled slightly down. This seemed to straighten the glide reasonably well - you can see the test glides below.



A powered flight using a Rapier L2 rated at 120 mN is shown below.



On a calm day, the model might have got away with this much turn, but it didn't get high enough before it headed downwind, so hit the deck early. At least there was no damage.



Rather than bend the tab further downwards, I decided to add a second tab next to the first one, again bent down to further lift the right wing. The photo below shows the angle of the lifting tabs. They are very sensitive, so it is best to make small adjustments and keep test gliding until you are happy that the model is flying straight.



The video below show the glide after the new tab was added.



This really did look good and straight now, so time to try another motor. Again this was an L2 rated at 120 mN.



Success at last. I was surprised how well the model climbed out with the 120 mN motor. It looks like the Thunderstreak should go OK on any L2 motor rated between 110 and 140 mN. The higher the rating of the motor, the more important it is to start off with a dead straight glide.

I am acutely aware that I have managed to time the publishing of the final chapter of this epic so that it coincides with Rapier motors disappearing from the dealers shelves. This is most unfortunate. You can still get Rapiers if you know who to ask, but it is a bit of a lottery which thrust ratings and sizes are available.

A possible light at the end of the tunnel is the development of small, lightweight electric ducted fan units that could power models of this size - I know Derek Knight and Stefan Gasparin are working on this. I'll miss the smoke trails, but this could offer a viable alternative to help us keep these small scale free flight jets flying.



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