Here is something to fly indoors that is a little bit larger than most of the duration classes and I even think that the R/C flyers amongst you might also enjoy having a go at?  There are no plans available that I am aware of for indoor Co2 Duration, but designing a model is not rocket science, as I hope to show as we read on.  What follows is an account of how I go about designing and making one of these classes of models, most of which have been very competitive in RAFMAA comps.  So how did I start?

Having been using a Telco Co2 motor in a scale Sopwith Triplane and learning the techniques of operating these motors back in 1988, Co2 Duration interested me as a comp at the RAFMAA Champs, then held at RAF Upavon.  I made a sort of 3ft version of an EZB model and covered it in 4 colours of Jap tissue.  This model was a bit bendy and wallowed about in a tight circle and either climbed through the roof or just floated around a couple of feet off the floor; my Telco motor was a bit tricky to adjust to say the least, but a 3.5 minute flight was fairly consistent.  What I needed was a better model; just in time I came across an old Aero Modeller drawing c1978 reproduced below, which I followed fairly closely and covered it in 5 micron silver Mylar.

his model worked very well, but the Telco was still very awkward to adjust, the power adjustments needed to be very fine. Too high and these models will climb steeply and hit the roof, too low, they will slowly descend from launch.  I re-motored the model with a Gasparin 63BB, which was a close alternative to the Telco for displacement and this motor gave a reliable and consistent run, but this motor also gave problems with fine power adjustments?  Perhaps a quick explanation of the different methods of throttling may be in order.  The Telco uses an eccentric bush in the front crankcase extension that the crankshaft runs in, by rotating this bush; the top dead centre of the piston can be varied allowing more or less time for the gas to discharge.  The Gasparin uses the same idea that the American Brown Co2 motors use, this time the whole of the cylinder screws into or out of the crankcase, thereby lifting or lowering the ball valve seat from or to the piston pin.  The Brown cylinder has a split threaded skirt that gives a damped friction fit to the crankcase; the Gasparing has a locking ring to secure the cylinder from unwanted rotation.  This lock ring I think was the cause of not being able to fine tune the motor; when the ring was locked, it tended to move the cylinder.  My solution was to follow the Brown idea and I removed the lock ring and put a turn of PTFE tape around the threads and this now allows very fine adjustments to be made.

At the same time as I produced this model, one of my main challengers to glory was Ian pallister; whose model uses a single bladed folding propeller and a Brown 100 Co2 motor.  His motor/propeller combination gave a very consistent and long run from a charge and as happens; his model looking similar to mine, was based on the same drawing?  Ian’s model is still alive and well and still very hard to beat.

Back to the plot!  The rules for this event are dead simple and you can view these by opening or downloading the indoor rule book from the BMFA web site ( a link will be found on the RAFMAA web pages) these PDF files are FREE and you need only print off the pages you need, if you want.  As far as the model is concerned; the rules state: Motive power to be a Co2 powered motor using a tank whose capacity with associated pipework shall not exceed 3.25cc.  The rest explains timing and flying rules; so anything goes really?

Over the many years that I have been flying Co2 Duration, I have been toying with ways to prevent the motor from stopping after collisions with walls, pendent lights etc and I have made a few bizarre models; not all of which were successful.  What follows are some of my ideas for airframes; not all of them built but most were tried.  I try all my ideas on paper like this to get some idea of the end result.  The idea was to hide the motor to prevent the propeller from contacting obstructions and allowing the model to deflect itself away from danger?

This design had a cowled motor with the wing mounted on top of the cowling and the fuselage fixed underneath.  The motor was fixed on a cruciform inside the cowling with the option of a fine carbon web cage across the front of the cowl.  The final model flew OK, but was very heavy and did not have enough wing area.

A new spec was dreamed up to use a large high aspect ratio wing with an under slung motor and a shoe type skid for landing on and protecting the propeller.

This model had a wingspan of 56 inches and a chord of only 4 inches; the twin finned tail plane was attached to the 2mm carbon tailboom  by a rolled paper tube (see the EZB article in the Oct 06 newsletter for details of how to make paper tubes).  This allowed the tail tilt to be easily adjusted to control the turning circle and also for ease of transport as the whole model could be disassembled and stowed in its transport box.  This model recorded a 6 minute 12 second flight at its first RAFMAA event.

The problem with such a large high aspect model like this was a lack of torsional rigidity in the wing; any over speeding caused the tips to wash-in and the model would tuck under and bunt towards the floor.  In flight; the model was a show stopper; lots of wing bend and Gossamer Albatross appearance as it passed overhead.  This led on to a similar design with a conventional rolled balsa fuselage, but a vulnerable propeller.

After re-kitting these models after many years of service and getting thrashed by other members like Tony Buchingham, Mike Goldby and later BMFA guest Steve Glass; all flying small 30inch type models and now exceeding 7 minutes flight durations consistently; a serious re-think was needed.  I decided to follow suit and try my luck with a much smaller model.  The search was on for something new!

In the end I decided on something more conventional and fell in line with what was obviously working.  This time, weight would be the challenge; the larger models all scaled up at around 26g or a little more, very nice considering the size of the airframe, but drag was requiring a high power output from the motor and so duration was falling?  Steve Glass came to the 2005 Duration meet with a new 13g AUW model and after a few hours trimming put in two timed flights of just under 9 minutes each; the highest Co2 duration flights recorded at any RAFMAA event.

Back to sketch pad, and something along those lines was drawn up on the drawing board.

Bench running a Gasparin 28BB motor at around 1000rpm was giving what seemed a usable duration of 12 to 13 minutes; what was needed was an airframe that would fly on cruise at around 800rpm.  The 1200rpm+ output from a freshly charged tank  would climb the model to the rafters in about 1minute 20 seconds and then the cruise rpm would drop in; and what could be a RAFMAA record flight is in the making? (QED)  The model was made and is now undergoing development and is showing some promise to break the 9 minute duration; fingers crossed!

Bench running a Gasparin 28BB motor at around 1000rpm was giving what seemed a usable duration of 12 to 13 minutes; what was needed was an airframe that would fly on cruise at around 800rpm.  The 1200rpm+ output from a freshly charged tank  would climb the model to the rafters in about 1minute 20 seconds and then the cruise rpm would drop in; and what could be a RAFMAA record flight is in the making? (QED)  The model was made and is now undergoing development and is showing some promise to break the 9 minute duration; fingers crossed!

This is the latest design off the Morgan building board, I give thanks to all who have inspired my thinking; now come and try to beat me.  The AUW of this latest model is 15.5g and I am very pleased with its performance.

Phil Morgan

Indoor Comp Sec