Hi Ian
If I'm not already on the list, please add me to it, also if Europa
choose not to stock the springs, then let me know & I will help if
required.
Cheers
Alan
-----Original Message-----
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of G-IANI
Subject: RE: Europa-List: Nose Gear Springs
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
|