Thursday, 19 March 2015

World Models Groovy

I have recently acquired (second hand) a World Models Groovy 50. This is a first for me on two counts:

1. I am flying someone else's model
2. Its an ARTF
3. Its a (fairly) modern aerobat

(OK - 3 counts)

It looks a bit like this:

A pretty looking aeroplane and like most new models I thought that it would be the making of my aerobatic career.

It needed a bit of tidying up. The original undercarriage legs had been replaced with an alloy alternative - and the spats had punctured the lower wing sheeting when the original undercarriage had collapsed (along with the lower fuselage - the joy of landing on a grass strip). The previous owner had also kindly bolted the new U/C assembly on  using M5 Nylon bolts - to save the fuselage in the event of another disaster.

I also acquired a S/H OS70FS for this from eBay at a very reasonable price, so all assembled and with new digital aileron servos - off for a maiden flight.

I had already taken the wise precaution of setting 2 stage rates on Aileron and Elevator.

The first flight almost ended in disaster - there was far too much aileron throw and insufficient expo. A quick flick to very low rates and I had a manageable model which I carefully flew for a few minutes - it was clear there was still far too much aileron travel.

A quick landing and readjust the rates and expo and I was away again. The engine was a bit disappointing, not pulling anywhere near as hard as I expected but at this time I was more concerned with the model handling.

After a few basic manoeuvres, I brought it in for a landing and although quite gentle, the soft ground was too much for the undercarriage and the Nylon bolts sheared - I now have 2 spat shaped holes in the lower wing. <sigh>

Back in the workshop, the wing was repaired again, the loose trim was reapplied and an attempt was made to reduce aileron throw.

Now - why cant servo manufacturers make a low travel servo - something like +/- 30 degrees instead of the usual +/- 75 degrees or so? The torque arm was placed on the innermost servo arm hole and two new extended horns added to replace the marginally shorter horns on the ailerons.

Take 2 on the flying field.  This was an evening flight - just before dusk. The model is behaving better but roll rate is still far too high but now more manageable. The first flight almost ended with a pancake as the engine cut on a 3 turn spin. I just managed to recover it and get in a very fast downwind landing. The next hour was trying to work out what had gone wrong as the engine would start but not pick up, so exhaust off, cowl off and I discovered the throttle linkage arm had come loose on the carb - my fault, I should have checked it.

A quick second flight - but now starting to get a bit concerned about lack of power.

Take 3 on the flying field - the day following 16 hours of heavy rain and the strip was sodden again. My problem now was getting the model in the air - it was flat calm. Just visualise a 40mph taxi run and you will get the idea.

Eventually got it away and now really trying to sort out the trim, the model appears to be crabbing in flight and definitely leaning left on an upline. Spent 10 minutes rolling, looping eights etc and just building up confidence in the model. A very nice approach - although rather fast - saw the undercarriage disappear and associated holes in the wings reappear as the Nylon bolts sheared again. A quick repair with tape for 3 further flights and I was left with a list of things to do.

1. Reduce aileron travel even more. I know that I can use rates, but I really prefer to be able to use full travel - if for no other reason to protest the feedback pot in the servos.
2. Add a bit of right thrust
3. Add some expo on rudder, its far too sensitive around the mid position
4. Reduce the height of the tailwheel - get the nose pointing upwards a bit more
5. Service the motor
6. Bolt the undercarriage on using Steel bolts!

To reduce the aileron travel, I drilled another hole nearer the servo output spindle - only about 3mm further in but that is the nearest to the output shaft that  I can get. If it's still not enough then I will need to put even bigger aileron horns on.

The side thrust was achieved with a shim of 1/32 ply just inside the left engine (radial) mount bolts.
When I eventually managed to get the head off the engine -  a bit of a worrying sign as one of the cover bolts hex head was knackered, I found that the tappets had been set using an imperial feeler gauge rather than metric - i.e. 04" rather than .04mm. I was amazed the engine ran - hopefully that should now be sorted ready for the next flight.

We will see.