Thanks to all those who have declared an interest in the Trigear bungee
replacement springs. At this moment 13 builders have said they are
interested.
Here is some background so you know where we have got to.
Two springs have been used to replace the nose wheel bungee and this has
been done on at least five aircraft (on the Dutch register). The spring
specification was done by Nico Groot and it took three attempts to get a
suitable spec. Andy Draper was aware that the work was being done and had
some reservation as to how successful it might be. Those who have them are
very happy with the result.
I have produced a drawing of the spring (complete with English translation
of the specification) and calculated forces "on the stop" and at the extreme
of the spring travel (hitting the wire stop).
Andy's reservation was that the spring rate (Young's modulus) of the spring
is much higher than that of the bungee. The modulus of a steel spring is
linear whereas that of the bungee is highly non linear. This is because a)
The Young's modulus of rubber is non linear, b) the bungee is multi strand,
c) the sheath probably plays a significant role, d) the bungee may have a
safety thread, e) the installation may have anything from 12 to 20 working
strands and f) these may be of two lengths. So the Young's modulus of your
bungee might anything. Based on some very crude measurements I think mine
may be about 25N/mm. The spring rate of the two springs is 65N/mm. While
this arithmetic is interesting it is somewhat academic as only about half
the load on the nose wheel is known (the weight of the aircraft). The
remaining loads are dynamic loads caused by bumps so they are almost
impossible to quantify.
I have talked this through with Andy and reached the following conclusion:-
a) The initial springing is done by the nose leg, just as with the bungee.
b) Once the nose leg is bounced off the stop the spring will give a "harder"
ride but this may prevent the wire stop being reached. In fact the higher
spring rate may actually reduce the chance of a bent leg or prop strike.
c) Once the nose leg hits the stop, and if sufficient further force is
applied, the nose leg is going to bend.
Following this logic, as long as the spring tension at full extension to the
stop (wire loop) is less than the strain at which the nose leg will bend
then there is no disadvantage to using the springs. Andy has offered to do
the calculations on the strain needed to bend the nose leg. Andy does not
wish to see the protection, against bending the nose wheel leg lost.
The disadvantage of using the spring is a weight increase of about 1KG. The
advantage is "fit and forget" and less blood on the floor.
Assuming Andy is satisfied that the springs do not significantly increase
the danger of bending the nose leg then I will get a modification
application off to the PFA as soon as possible (the paperwork is already
prepared). If Andy's calculations suggest these springs could be a problem
then we may have to look at the spring design again.
Everyone should be aware that the details on my drawing are more a
requirement than a complete specification. They do not specify the material,
heat treatment etc so my remarks apply to only those springs made by the
Dutch manufacturer. It will be perfectly possible for others to manufacture
suitable springs but it will require some development work and may not be
cost effective.
As Tim Weert has clearly explained (his posting dated 31/10/2005) the
manufacturing cost of the springs has a high fixed overhead. To offset this
it will be of great benefit to order in bulk. My preference would be for
Europa(2004) to handle this but I am prepared, if necessary, to act as
co-ordinator.
I will keep you all informed as to progress.
Ian Rickard #505 G-IANI XS Trigear
Europa Club Mods Rep (Trigear)
e-mail mods@europaclub.org.uk
or direct g-iani@ntlworld.com
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