A return to model building and flying. Part 18

Engine disappointment

August 2005

A bit of an up and down month. I have known for a long time that I have a gap in my engine collection, I only have one 2.5cc engine, it's an old PAW and destined for the Lucky Lady. For those that have jumped in here, an old stunt design that has caused me a lot of problems in the building, but more of that later.

Those following this little saga will know that I have a severely restricted budget, so any decisions are heavily influenced by this fact.

Looking around I came across the MVVS 2.5 diesel, available as glow or diesel, twin ballrace, schneurle ported, side or rear exhaust, 24000rpm (doubt I will get that). This engine also has the advantage of a range of mufflers and pipes that can be fitted, an important, if not essential, precondition. Call me old fashioned, but I wanted a decent modern diesel. There are other East European engines that are probably better, Fora engines seem to go well, but suffer from the lack of readily available silencers. My club has strict noise limits. and I do not want to be responsible for losing the site.

The thing that finally had me reaching for the plastic, was the price, a little over half the price of the equivalent PAW by the time the extras are added on.

Ordered from Just Engines in the UK, it promptly arrived two days later.


mounting it in a test stand and putting a slop of fuel down the choke I, it felt slightly dry, I was rewarded by a quick burst from the engine and an oily circle of exhaust ejecta on the building room wall. So far, very promising.

The following Sunday I ran it properly at the club site. After a few short busts it did not feel right, and I noticed black oil bubbling around the compression screw as the engine was turned over compression. My suspicions of a loose contra piston where confirmed when the comp screw was removed and contra piston could be seen following the piston up and down. Not a pretty sight.

The next day, Monday,the engine was returned to Just Engines. I was quite dejected at this point as I really was impressed by the design on the engine and it's potential. To my surprise Wednesday morning it was back with a new piston liner assembly. I can't fault that sort of service.

After twiddling my thumbs for the rest of the week, it was back to the field to run the engine. This time I had the opposite problem, the contra piston was so tight, I had to use a spanner to get some leverage in the compression screw. However it did run, and keep running. It did sound sweet, with the mini pipe reducing the noise down to quite acceptable levels.

The next day I was still troubled in my mind by the stiff contra piston. Whilst the engine runs OK, I would still like to be able to adjust the compression with my bare hands in a conventional manner.

Fortunately, when the engine was returned it was complete with the old piston liner set. After dismantling the engine I was able to put the old loose contra piston in the new liner and vice versa, both seem to be good fits with no looseness.

A week later my hypothesis was proved correct. The compression can now be adjusted normally. The engine is beginning to sing as I open slowly lean out the settings. Still far from run in but showing a lot of promise.

Now all I have to do is finish the 'Lil Quicky' Goodyear I am building for it.


Back tracking a little, I finally finished the Lucky Lady; something I thought I would never accomplish at times due to it's unnecessarily difficult construction.


The centre of gravity is on the leading edge as I feared. I can't move the engine back as there is no room, so all I could do was add lead to the tail.

The weather was hot and the Solarfilm slackened and wrinkled, Something I half expected, which in spite of it's speeding up of the finishing process does not endear me to it. I wish I had painted the fuselage, but I just wanted it finished after all this time. This is the price I paid.

It flew and showed some promise, surprising after the agony of building it. The flaps are non functional, two weeks of struggling to make a linkage that would fit in the small space was enough. I adapted them so they can be used as a trimming aid so the work is not lost completely.

In my usual cavalier way, on the second flight I started the lower half of a vertical eight much too low in a shifting breeze, resulting in the inevitable SMACK!

The result was a cracked fuselage in line with the mid section of the wing at the weakest point. why I didn't notice this obvious flaw when i was building it I have no idea, probably just fed up with struggling with it. Repairable but annoying none the less.

Another thing that gives cause for concern is the apparent lack of power from the old PAW 2.5, it starts OK but seems a bit gutless, maybe it's just old and tired like me.

A week later and the engine was tried with a 7x6 prop. lots of revs but no improvement. Resorting to an old KeilKraft 8x6 clear nylon bendy prop there was a sudden jump in the performance to nearly right (not there yet).

Now I have seen this happen on three different engines and models belonging to me, and it flies in the face of current thinking on props, which basically says, "Props that flex are no good, use rigid ones." This may be true for modern large capacity engines, but in my experience the opposite seems to be true on engines below 3.5cc. They also have the added bonus of being a lot easier on the pocket when they shake hands with Terra Firma.

As KK nylon props are no longer made, the closest match seems to be German 'Kavan' props but the supply of useful sizes seems to be sporadic.

And the wrinkling Solarfilm is really beginning to (fill in your own expletive!) me off. It's another material I am beginning to develop a hatred of.

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