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
The next modification was to add a sma