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Growing a
Grumman - Part 13
2010-10-17
Just to clarify
- when one speaks of an 'experimental' aircraft, many people think
of something built from plans, or even better, from a kit of parts.
I should be so lucky as to actually have a proven set of plans,
with all the kinks worked out, or a kit to work with!
No - with a
true 'experimental', one where we really are experimenting with
just about everything, and particularly of course firewall-forward,
then the process we are suffering at present is the actual experimentation
- thus we find that many things need a rethink/redo.
So - one step
forward and two steps back! That's life I guess - but I have been
busy with rectifications for the past few weeks, putting right my
own blunder as a result of bad planning and lack of foresight!
Once the wings
were attached to the aircraft and some fuel in the tanks, it was
discovered that the prop-clearance was too little at about 6 inches.
This appears to be a result of the CG having moved forward once
the wings were in place, thereby putting more weight onto and compressing
the nose leg.With only 6 inches prop-clearance the airplane will
not pass muster with CAA and rightly so - it is bound to happen
that I will ding that prop on any less-than-smooth runway, and certainly
the blades will pick up stones etc on run up, damaging the tips.
After much pondering
(there's lot's of that involved in doing this conversion!) it seemed
the simple way to remedy this was to buy a slightly longer HTD belt
for the PSRU. This would allow the assembly of the driven sprocket
to which the prop is attached to move upwards by a few inches -
et voila, problem solved.
Whoa!, not so
fast there pardner! The next longer size in belts available (maybe
available here in SA, maybe not!) would increase the centre distance
between sprockets by more than 4 inches and that much extra height
I do not want or need - darn it.No way could it be so easy - silly
me! If I went to the longer belt therefore and wanted to add less
than 4 ins to prop clearance, I would also need larger sprockets
to take up belt length whilst maintaining drive ratio - snowball
effect again!
Another solution
proposed by an aviator friend was to make a new nose-gear fork with
the necessary angles built in to get the clearance required - looking
at this I did not fancy the idea, as there seem to be lots of possible
pitfalls in this approach, not to mention suspected large costs!
Therefore I
simply had to embark on a process of mounting the engine some 2
- 3 inches higher, which meant disconnecting the motor, lifting
it off it's mount-points and welding in new and longer mounting
brackets.
First though
I had to buy an arc welder to get this job going and I had also
to learn to use it - 'learn' describes an ongoing process by the
way - one which is incomplete at this time, as can be seen from
my welds made with this device! Remember, as mentioned earlier,
this engine mount will not fly as it is a prototype in it's third
of fourth iteration!
This 'simple'
job took two days of solid and bloody hard sweat - cutting, grinding,
shaping, bending and welding 5mm steel is hard and dirty work, let
me tell you, when everything has to be done by hand!
But now at least
I can now measure a good 9 inches clearance at the prop tips from
the ground, which should help protect my prop and gives me peace
of mind.
Of course, as
they say about mods to airplanes, 'no change will go unpunished'
- and my top cowling and nosebowl now need to be remade to suit.
On the upside,
the thrust line is now closer to the original, which cannot be a
bad thing. In the looks department the airie's nose will look better,
closer to the original.
Right now I
have made cutouts to the top cowl so that I can install it, and
the carbs and the distributor can poke through.I can either remount
the carbs on longer inlet tubes to get them tucked away in a lower
position where there is space behind the engine, instead of on top,
or I can simply form a bigger 'speed-bulge', about 2 inches higher
than before, on the top of the cowling.This would also have to be
remade to cover the distributor which used to be fully under the
standard cowling level but which is now also raised. The first 'longer
inlets' option seems best, although again a lot of work - but then
the risk of carb-ice is increased and we are adding complexity and
weight - I always recall Colin Chapman's motto (he who started Lotus
Cars and won many Grand Prix championships) - 'Simplificate and
add lightness!"
After a little
more 'ponder-time' I will have an answer to these questions as to
what to do next!
2010-12-15
Lots of pondering
later, I see that a higher cowl bulge to accommodate the carbs is
not the way to go - so I started to remount the carbs on extension
tubes, repositioning them from the very top of the motor to the
sides, sitting just above the rocker-boxes.
It should be
possible to mount these carbs this way at a low enough level to
provide clearance from the underside of the top cowl - but then
the fuel may have to be sucked uphill, and SU's are not at all designed
to act as updraft carbs.
I also will
make a new nosebowl, as the previous one suited the height of the
previous engine mounting and the extra length occasioned by the
nosepiece housing of the PSRU - there is no way it can be easily
modified for the new height of the drive assembly. Luckily I still
have a mould for the original Grumman AA1 nosebowl.
2011-01-17
Have spent the
last few weeks making a new nosebowl as per the original Grumman
design - and having fitted that to the aircraft I found that the
top cowling would no longer fit properly, since it is slightly longer
front-to-back as it had a downward slope before! So had to make
a new one of those as well! Anyhow, now we have a cowling exactly
as Grumman intended - problem is that it reaches about 10 inches
short of the spinner! So that is the next task, to make it extend
to the back of the spinner.
The new nosebowl
has no holes for incoming cooling air as I have not yet cut those
- and I am thinking I will try it without those holes since my radiator
is under the belly- Maybe we will need a small hole just to let
some sweep air into the cowling for general cooling draught - again
some experimentation will be needed.
Meanwhile I
stripped the PSRU for examination as it has run 12 hours or more
- lo and behold I discovered that both bearings in the nosepiece
housing were rough and noisy - they had failed. Puzzling to say
the least, since these are both large size deep-groove bearings
which should last for thousands of hours.
I took both
to a bearing specialist who took one look at them and noticed the
manufacturer's name engraved on them - "BTC". What is
this rubbish he says - "Better Than Chinese?"!
I think we have
some gash bearings here.At the time of purchase these were all that
was readily available - I should have held out for a decent brand
I reckon, so NOW we will have to find a good brand like Timken,
FAG, SFK etc. Then try again.
Re. the carburettors
- after yet more 'pondering' I have been moving these around the
engine compartment trying to find the right position - not too close
to heat sources like exhaust manifolds and aligned facing each other
so as to keep a simple linkage. Also have to consider carb-heat
to these carbs. I found the place which will achieve this, but will
need some fancy curvy pipes. They will be placed right behind the
rocker boxes of the V6 and slightly lower. In this position they
do not want to stick out of the cowling anywhere at all! Without
pipe-bending equipment I will have to fabricate a mockup of the
whole catastrophe, possibly using PVC pipe (easier to bend with
a heat gun and sand). With the mockup in hand my tame aluminium
welding specialist can copy these exactly in ally tubing.
That's the plan
then - now to see if it will work!
2011-03-17
Several months
down the line we have finally finished the engine raising project
by completing the new cowlings necessitated by that change.
Re. the cowling
I reckon the real answer would be to get hold of cowls from a Grumman
Tiger - these will have the additional length that I need and are
aerodynamically good, being designed by Roy loPresti - however with
only about 4 or 5 of those Tigers in SA chances do not seem good!
I found an auto-exhaust
shop which does mandrel bends in steel pipe - so that is what we
are going to use - no chance of cracking through vibration, and
all around much simpler to make up! This place can also make up
profile-cut mounting adaptor plate and carb flanges.
So the ally
tubing I previously had in mind will not be seen on my engine -
weight wise there will be little difference anyway, since there
is no thin-wall ally tubing to be had in SA!
Of course having
ordered the adaptor-plates and carb-flanges last week and sent drawings,
there was some kind of problem with the fabricator who promptly
lost the drawings but failed to inform me of this fact! Only when
I called to arrange to collect the parts, did I discover that work
had not even started! Hopefully we may see these parts soon - who
knows?
Had to hacksaw
the carb-plinth off the top of the manifold of course - managed
this time to do this without the hospital bills - nice one! Then
I took the manifold for refacing, since it had previously been welded
and needed a clean up, so that I can get a seal between the steel
adaptor, attached with studs and the aluminium surface of the manifold.
Managed to get
decent quality bearings for the PSRU - the brand is RHP from Britain
- boy, these are expensive too at R380 for the big bearing! On the
other hand the "better than Chinese" rubbish was also
very costly even at half that price, given it is now only good for
scrap metal!
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