TOPIC: Catapult flight testing

To avoid wasting motors, duff flights and damaging your models, all model aircraft require ‘trimming’. Trimming is careful launches to check for a gentle glide and confirm that all the surfaces are true and not creating unwanted turns and also that the C.G. is in the right place. This is particularly relevant to all Jetex, Rapier and TSP powered models as they fly faster and the results of uncorrected behaviour can be final for the model!

Test launching a ‘jet’ model with the help of a 4” rubber band is very helpful here as (a) the launches are more consistent than hand a launches and (b) the higher speeds under power can be gently explored before ever lighting up a motor.
Proceed as follows: stick a pin in under the nose block nose and crop off the head, fix the pin in place with a drop of cyano then find a rubber band about 4” long and you are ready. No breeze is most helpful but a slight breeze is no problem.

Launch the model with the aid of the band at about half stretch, straight into any breeze with the wings level and pointing level and see what happens. What we are looking for is a nice even path down to earth going straight ahead. If it zooms up, there is not enough nose weight or you have too much difference between wing and tailplane (3 degrees or a bit less is good). If it dives steeply it is too nose heavy. Any turns left or right should be countered by gentle tweaks upward to the wing tip outside of the turn or to the rudder in opposition to the turn.

As confidence is gained, work up gradually to full stretch launches with the model banked to left or right. A slight difference in the flight path between left and right is good, but a big difference needs to be eliminated with some more careful tweaking. What we are looking for is the model to complete perhaps half a banked circuit and then gradually returning to wings level straight flight before the touching the ground. If the model climbs in the turn it needs a little more nose weight and if it doesn’t get far round the turn the nose is too heavy.

Good luck with your model trimming, I have always found it fun to do and usually do a quick test before flying any of my models.

Sage advice Howard. I may well adopt this with Fiat once it’s ready.

Good advice from the sage Howard, who knows his onions and that of which he speaks. I trim my proper (built up) models with a catapult, but tend to aim for a 'straight & level' glide without turn.

Here's Howard showing how to catapult launch his tricky Concorde with the help od Daniel. Note Daniel has survived with his right ear undamaged.

Here is another way of using a catapult for our models, I called it a Catstick. It is especially useful for tricky to handle models like the Harrier or Concorde where you need eight hands or there is no place to hold it for launching without getting your fingers burnt!

For what it's worth, below is Albert Hatfull's advice how to launch jet models. I've yet to see one of these at Old Warden and other flying fields, but, I think it looks worth a try.



Heavier models, like Roj Reese's Thunderflash, might go a lot better with this techique.





The estimable AH was of course the designer of Keil Kraft's Jetex models (apart from the MiG 15).

Here is his advice about trimming his Jetex 100 powered Cutlass:

That is a great idea!

Roger adds (sorry!)

Might work well with micro EDF jets as they need a bit of a oomph to get going.