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  Derek's Favourite Gliders  
  By Derek Piggott
Issue 11/2001


This month Derek has chosen to write about the Mini Nimbus, a glider he considers is now a "lovely glider in every way" once the early problems were sorted

There were already a number of Mini Nimbus flying in the UK and in other countries when someone realised that it had never been tested and officially approved for the British Gliding Association. With so many already in use the last thing I expected was any problem. After all it looked a very simple, straightforward new glass-fibre machine and it came from a very reputable company.

However, once in the air it had an unfamiliar feel and I did not much like its handling. On my first flight in the Mini Nimbus I could not immediately identify what was wrong. Not wanting to carry on with the testing without identifying the problem if it was serious, I went to Frank Irving for advice and got him to make the next flight. Now Frank is an expert on longitudinal stability and he was able to say straight away that although it behaved reasonably in normal flight, it was unstable in pitching manoeuvres with extra g. The stick forces were negative under g.

Sure enough, once I knew what to look for, it was obvious that when trimmed for a certain speed in straight flight in a steep turn, it required a forward pressure on the stick to prevent it getting tighter and tighter. Similarly in any pull up, a forward pressure was needed to prevent the glider continuing up into a loop. This, I was assured by Frank, was a totally unacceptable characteristic and it was obvious it could be very dangerous at higher speeds when it would be only too easy to pull out with very high g and cause a structural failure. This could easily occur if the pilot lost control in cloud or poor visibility and spiralled at high speed.

We were rather amazed that the type had been tested and accepted by both the factory and the Government Authority before getting an export C of A. I should mention at this point that this original version of the Mini Nimbus had an all-moving stabiliser and the stick forces were in any case very light. Later versions, the B and C models, all had a normal tailplane and elevator which totally eliminated this problem.

So who was to let Klaus Holighaus at Schempp- Hirth know that he had a problem to be solved? As the new boy to the Test Group I would have that honour. It was a difficult letter to write to tell the designer that his aircraft was not airworthy and I don't think I was very popular at the factory. By this time I believe there were about 50 of these sailplanes all over the world and it would be expensive to put right.

A Modification Was Soon Devised

I don't remember having a reply back from the factory, but it was not long before a modification was devised. This was simply to reduce the mass balance weight on the front of the stabiliser which, under extra g, was helping the stabiliser to tip and cause the extra negative stick forces. This certainly eliminated the negative stick force per g, but in return it gave the elevator a nasty snatching feel in turbulent air.

The next modification was to add a sma