Growing a Grumman
- Part 4
was written towards the end of 2004 and a mere 3 years(!!!) have gone
by to bring us to the point where Part 4 of this odyssey can be written!
Apart from some work
on the airframe and other smaller components, the main effort of the past
18 months has been the engine and particularly the home-brewed PSRU. Incidentally
the original plan was for a chain-drive unit using HY-Vo chains - but
on investigation it was found that these in SA were difficult to source
and very costly, in the sizes and lengths needed at least!
Therefore we reverted
to the HTD belt design - easily obtainable, not so pricey and well-known
The first step in
any such project is to get something down on paper - well I am not much
of a draughtsman and my engineering skills are more aligned to the shade-tree
variety than anything like a degree hanging on my office wall! But wait
- what are friends for? - I have a mate who was a big wheel locally in
race car design and another who runs a company which makes all sorts of
fancy armoured vehicles and he is a professional, qualified engineer to
boot! Both these guys are Grumman aficionados, owning and modifying and
even flying the same type of aircraft, albeit behind Lycoming 0-320s -
and both have totally rebuilt their aircraft and made major mods - good
company to have!
After lengthy discussion
with both Trevor and Andrew, I had some clarity in my own mind as to what
should do the trick in engineering terms. Further study of what is done
and how it is done by both Belted Air Power of Las Vegas, Nevada and North-West
Aero of Washington, USA in terms of the way they build their successful
PSRUs was invaluable. The most valuable sources of info here were Contact
Magazine, of which I have a number of back-issues from when I subscribed
and from purchases of back-issues I made at Oshkosh in 1997 or thereabouts.
One of these detailed BAPs Chev conversion carried out on the RV6A. Also
the books 'Alternative Engines' (from Contact Magazine) and Finch's 'Auto
Engines for Experimental Aircraft' have been useful.
Thanks to these publications
and the installation manual available at one time from the NW Aero website,
I could see how it all goes together. These units are quite simple really
and after my researches I came to the conclusion that the key elements
which need addressing in the design are:
1.Rigidity - there
can be no flexing within the unit which can cause misalignment of sprockets
or the belt, leading to the belt stripping teeth or generating excessive
heat build up followed by catastrophic belt failure
loads through and within the unit must be dealt with - there are forces
of some magnitude generated by the rotation of the belt - calculations
show that belt loads can be as high as 1300 NM in the case of my application
(a function of power,rotational speed and sprocket sizes) and that load
acts on the sprockets and bearings etc, which must be able to handle this
3. Propeller reversal
loads - my research shows that these loads are mostly taken care of by
the belt having a "damping' or 'absorbing' effect, unlike a gearbox
where these reversal forces are transmitted directly into gear teeth -
just stand close to any Rotax motor at idle to hear this phenomenon loud
4. Flexing of the
crankshaft under torsional loads felt by the driver sprocket attached
to the crankshaft. By using a small (approx 2 in dia) sealed bearing mounted
on the front plate of the assembly which accepts a "stub" shaft
extending from a flange bolted to the front of the sprocket, such loads
should be minimised and flexing of the driver sprocket minimised. This
is important since flexing of the driver sprocket can cause main bearing
or even crankshaft failure.
During these deliberations
I managed to download from the internet some very handy software (from
Dayco and from Gates) to calculate loads and power ratings of cog-belts.
The software also established other parameters such as center distances
for various sprocket combinations and proved to be very useful. (Download
Drive Engineer from Dayco - CLICK HERE
. Visit gates.com
for Designflex software)
One key item that
changed from my original thinking about the design was triggered by examining
the installation manual from NW Aero - while other units appear to use
bearings at front and rear of the prop shaft, mounted to the front and
rear support plates, the NW Aero unit uses a cantilevered arrangement
by having an extended upper shaft housing in which are fitted front and
rear bearings. This has some advantages it seems to me - the main one
being that one can more easily adjust the belt tension, without fear of
misalignment of the sprocket, as the upper or driven sprocket and the
shaft and shaft housing are essentially one unit which can be moved up
or down in the vertical plane.In addition oil rather than grease can be
used to lubricate the bearings on the upper shaft, since this can now
be enclosed with oil seals. In all likelihood after the initial fill,
only very occasional checking and top up of the oil should be necessary,
unlike greased bearings where I expect I would want to be re-greasing
them all too frequently!
The first thing we
needed to do was to get the plates cut which would form the "box"
or "frame" in which everything would run and to which almost
every component is mounted. These plates are of 12 mm (approx 0.5in) aluminium
and the grade available here in SA was 8082-T6 - the strength of which
is adequate, but if I could have obtained 2024 T3 I would have preferred
that. The front plate has to be wide enough to give space for the largest
sprocket (driven) and belt and deep enough to allow for positioning of
the driven sprocket according to the length of belt used - in my case
we used the shortest belt available of 966mm pitch length, as this would
preserve most closely the original thrust line and cowl design.
The plates were water-jet
cut by a local supplier, and then began the task of mounting first the
back plate on to the existing bell-housing mounting bolt-holes. Obviously
for a different engine the procedure would be essentially the same, but
the bell-housing bolt-holes being differently placed would require a differently
designed or shaped back-plate.
The front plate needed
the driver-sprocket bearing housing to be positioned accurately then fabricated
and installed - this can only be done once the sprocket is in place, so
we proceeded to order that item. The sprocket, with 30 x 14mm pitch teeth
and made from grey iron (ductile cast iron) came as a solid cylinder of
metal weighting 9 KG (approx 20 lbs)! After judicious machining this happily
was reduced to 3.5 Kgs and perhaps could be made even lighter - but let's
not take chances!
Of course the Chev
crankshaft uses imperial threaded bolts - the originals were no use as
the engine had a flex-plate so they were too short and no similarly threaded
high tensile bolts of correct length could be had for love or money in
South Africa! We therefore had to make the bolts by buying oversized HT
metric bolts of the correct length and machining and threading these to
Once we had the bolts
the driver sprocket could be mounted to the crankshaft, with an alloy
spacer between it and the crank flange to position the sprocket correctly
between the front and rear plates of the "box".The support bearing
housing (a square block of alloy) was positioned and attached to the front
plate with 4 x 6mm bolts.
Next on the menu came
the driven sprocket and belt - enquiries revealed that belts of the width
required - 90mm - were available off the shelf only from the Gates distributor
in Johannesburg - and they had only the latest Polychain design, which
are extravagantly expensive - I was quoted no less than R8000 for the
belt I wanted (that's about US$1150)! Since all earlier belt-drives on
aircraft used the original HTD belts since the 1970s when such belts became
available, I figured that if such a belt could handle the power and torque
in the 1970s it should be able to do the same in the 2000's - that then
would be my choice, as the straight HTD belt is vastly cheaper. Beware
however - today there are some very cheap and very nasty belts coming
out of India and China which are of inferior quality and WILL FAIL. Good
enough for a bottling machine firmly bolted to the factory floor perhaps
- not for your aerial conveyance!
Incidentally the tooth
design of HTD and Polychain belts is not the same and once your sprockets
are cut for one or the other, you are stuck with that belt - they are
Using the downloaded
software mentioned earlier showed that a standard HTD belt of 90mm, with
my sprocket combination and rotational speeds, can handle 375 hp! Good
So far so good - but
just about here the wheels came off - on ordering the belt and sprocket
from a local supplier, who would import the belt and manufacture the sprocket,
I was quoted 2 weeks. Five months later the items were
delivered! The delay took us up to October 2007 and with these items to
hand we could finally get back to the project!
However that damned
work stuff intruded - so it was November before activity resumed on the
Once again the sprocket,
this time made of alloy and the most expensive item in the entire contraption
at around R3200 (US$450) needed machining for added lightness. It was
my intention to connect the propeller shaft to this sprocket using splines
- after a lot of time wasted running around to find some company who could
provide a splined connection, I gave up when I realised that nobody was
interested for one unit - they can supply if you have 20 or 50 or whatever
but one - no way man!
Plan B then had to
be adopted - this meant that we would be using a simple bolted flange
at the backend of the propshaft - not wanting any weldments in this shaft
whatsoever, I found a rear axle shaft from some ancient truck at a breakers
yard. The diameter of this solid shaft was 41 mm, and unlike many modern
automotive drive shafts it had no taper along it's length - and the flange
onto which the brake and wheel would fit on a truck was flat and a perfect
85mm diameter - again most auto units are dished in the centre of the
flange - don't ask me why! This shaft was also 6 feet long, so I knew
it could be made to fit!
Now we had in our
grubby paws - a shaft - a sprocket - a drive belt - and after purchasing
the bearings and oil seals, plus a log of aluminium of 155mm length and
185mm diameter from which would be machined the shaft housing we were
in business. Something like a month later, and many drawings printed out
by my computer, out of the machine shop came the kit of parts for the
top end of my PSRU!
Having assembled the
unit with these (almost final) parts, we now have a "burning and
turning" PSRU - no prop yet though - which looks like it will do
the job and be plenty sturdy enough to cause me no worries when aloft.
Weight wise we came out at just over 60 lbs - but I reckon that later
I can shave at least 10 lbs by adding lightness, in the form of lightening
holes or cutouts to the plates which make up the "box" - right
now it's all solid.
Just a word on the
drawings - at the start of this project to build a PSRU, I spoke to a
fellow who does engineering drawings as a sideline to his day job as a
draughtsman. He indicated some rough pricing that would make your eyes
water, and I wondered how to do such drawings - obviously, buy the necessary
tools and get to it! Actually no - I discovered that the programme installed
on my desktop computer, which I never use, MS Word (and I have an ancient
version MSWord 97!) can do drawings using lines and shapes - and they
can be scaled! A little experimentation later it was clear that this software
would do the job perfectly well - and it did too!
Working with an experienced
machinist also means that he will be able to adjust fits and clearances
in line with "best practice" as he goes along, so the necessity
for super-accurate drawings falls away to an extent anyway. I suspect
that if one was doing such machining on a modern NC machine then whatever
is on the drawing will be whatever is produced - right or wrong - old-style
craftsmanship still has a value, I am pleased to say!
Next instalment will
be when the prop is bolted up and we start to blow things around!
Author's note: I received
the propeller shaft and shaft housing yesterday (2007-12-16) from the
machine shop after 4 weeks of waiting. Amazing how these guys work - the
parts sat on his shelf for 3 weeks and 6 days - then he knocked them out
in a day at the very last minute before toddling off on holiday for three
So we will have to
wait until January 2008 before we can get on with the last piece which
will be the propeller hub and flange. Meanwhile I can check how this assembly
runs and whether belt alignment is correct., so that once the prop hub
is ready we can just bolt the prop on and go fly - NOT!!
Here are some pictures!
Click the image for larger (1024x768) view - I have provided the large
size images as I know you guys want to see DETAILS!
view of PSRU
this unit weighs in at 28.15 Kg (62 lbs) - which is a bit
more than hoped for but then we can still take a bit of weight
(maybe 5 Kgs) off by cutting lightening holes in the alloy
and steel plates. That would give a weight of 23 Kg or 50lbs.
green coloured side channels are simply zinc-chromate painted
view of PSRU
There are three bearings in this assembly - a small (lubed/sealed
for life) bearing at the bottom in the square housing and
a front and rear bearing inside the conical upper shaft housing.
The top bearings are Deep_Groove Ball Bearings of 62mm (front)
and 83mm (rear)
view of PSRU - oil fill plug visible and front oil seal, behind
which is a 62mm Deep Groove Ball Bearing.At the back of the
housing is a 83mm Deep Groove Ball Bearing and a seal.
run in EP90 gear oil, which fills the shaft housing to between
1/3 and 1/2 the depth of the bearing. There is a similar plug
at bottom to drain the oil.
side channels - 120mm steel sections - would have preferred
alloy but unobtainable in South Africa - even these steel
sections were very difficult to obtain! The steel is heavy
at 2.8Kg per side but some lightening can be done later to
probably halve that weight. The rectangular block on the front
of the front plate is part of the adjustment mechanism, not
yet completed - a simple jackscrew arrangement to raise
or lower the shaft housing assembly.
sprocket. This is cast iron and after machining the centre
out of it, weighs 3.5Kg. The shiny bit between the flywheel
and the sprocket is an alloy spacer to place the sprocket
in the right position fore and aft between the front and rear
plates. The funny shaped green thingy on the flywheel is the
Chev balance weight, without which this engine wants to shake
itself off the mount!
that I am using a full-weight flywheel - the Chev motor came
with a flex-plate for an auto-box so to match the starter
teeth I am using a Nissan flywheel, machined to fit. it weighs
in at close to 20 lb and according to Fred Geschwender a full-weight
flywheel is essential to dampen combustion pulses.
of engine from front.Tubular mount is rough version for test
purposes made of mild steel tube 22mm od.I bought one length
of tube (6 metres) for this mount.
Note "bobbin" enclosure for rubber isolator at front
- this is a rear leaf-spring shackle rubber mount from a Nissan
3 litre LDV - works just fine!
pipes (headers) 3 into 1 and (almost) tuned length and mild
steel, just like on a car! Without silencers they make a terrible
racket - by making an insert of a diameter to slide neatly
inside the exhaust outlet pipe, packed with wire wool like
a normal straight through automotive silencer, the engine
noise is changed to a nice deep and smooth tone - and not
is Nissan 3 litre LDV (1.4 Kw) on fabricated welded mounting
which is attached to the three threaded holes in the Chev
block where I presume, the original starter lived. Pardon
my rust - it is 8mm thick mild steel, not painted or plated
and we have been having rain lately, so the humidity gets
to the steel!
as a result of my poor welding skills, this mount has cracked
twice already in test running - so I will eventually have
something machined out of alloy.
is what in a car would be the front of the motor but in the
aircraft is the rear. Alternator and water pump (from Ford
Essex 3.0 V6 motor because it was light, looked like it should
pump enough volume and was cheap, reliable and turned the
correct way for my setup!) hung from fabricated brackets attached
to convenient existing pick up points. The stock Chev water
pump cannot be used on my aircraft, as I have so little space
between the firewall and the motor. If I used that water pump
the motor would have to move forward two inches which is no
good for my CG
stand on which the motor has lived for past two years - instead
of making up a new instrument panel I just took the one out
of the airplane - seemed the logical solution as a temporary
is custom built unit made from a new BMW 740 rad, cut horizontally
and welded back-to-back - don't know yet whether it will do
the job, as have never had any airflow through it while running
the motor for short periods on test but it is of the recommended
size and capacity.
airflow the motor can run at idle or low revs for 10 - 15
minutes before getting to 100 deg C on the eater temp gauge.
are twin 13/4 inch SU type off Rover V8 motor - these were
used on all UK produced Rover V8s (3.5 ltr) and they are installed
on the original Chev TBI manifold after major mods to that
item! SU carbs are ancient design but actually a good carburettor
as they have altitude compensating characteristics due the
the sliding needle design of the built-in mixture control.
The motor runs well with these carbs - but over 4500 rpm there
is a misfire which may be due to mixture or ignition.
Plate with shaft. Note the steel collar which acts as main
thrust washer to transmit prop thrust loads into rear bearing
thence to front plate and shaft housing
Plate with shaft
Plate with shaft and driven sprocket (48 teeth)
Plate with shaft and driven
sprocket (48 teeth)
- Nissan 3.0 modified to fit Chev crankshaft - with driver
pulley made of cast (grey) iron secured with 6 high tensile
cap-screws. This pulley came as a solid cylinder - you can
see that we have machined out the centre leaving 20mm metal
at the rear fixing point.
(Nissan 3.0 modified to fit Chev crankshaft) and back-plate
with steel side channels
Plate with shaft and driven sprocket (48 teeth) - shaft housing
machined from solid billet of alloy contains front and rear