It’s 100hourly is now due, not because of hours but the 12 months is
up, however we were able to do 80 hours in that 12 month period, it has now
done around 400 launches. The
only defect during that period was a leaking head gasket which appeared ok and
was not blown, but tests showed there were traces of CO in the coolant.
Head gaskets and head bolts were replaced. I also replaced the re-drive belt,
not because of any visual defect but because the manufacturers claim they have
a shelf life of 10 years and this one was 12 years old. The engine itself has not missed a
beat and has not dropped any power (RPM,s) at all since it’s first start up in
1992.
Performance and economy is better than
expected, especially fuel costs which shows an unbelievable (conservative) saving of $2000.00 per 100 hours.
This is
the breakdown:
According to reports from several clubs that operate PA25,s with 0-540
engines (which they all have) their fuel consumption varies from 58 to 60
litres per hour towing. The
reason for this high consumption is that they all operate at full throttle
with full rich mixture to assist in cooling. Upon glider release (usually at 2000
ft) the throttle is left fully open on the initial letdown or until the speed
builds up from towing speed of around 65 knots to 100 - 110 knots IAS with the
RPM getting close to the redline, this is done to reduce the incidence of
shock cooling, then the power is trickled back to maintain that speed slowly
reducing RPM,s (fixed pitch props) to around 2000rpm until final
approach. Thus a lot of fuel is
consumed on the decent (still at full rich). Any deviation from this practice has
shown to cause cracked pots.
On the other hand the auto engine
(spinning the same standard PA25 prop at the same rpm) does not have to run at
excessively rich mixture because of the ability of water internally cooling
the valve stems and seats. Upon
glider release the throttle is immediately fully closed and only a trickle of
opening is required if decent is too severe, thus little or no fuel is used on
decent (when the coolant drops by approx 15deg c the thermostat closes thus
shutting off the coolant circulation).
Records over a long period show fuel consumption at 32 litres per hour
towing.
Therefore:
At 58 litres per
hour at 90c a litre for 100 hours (58 * 100 * .90) = $5220
At 32 litres per hour at 90c a litre for 100
hours (32 * 100 * 90) =
2880
$2340 saving
These figures are assuming that both fuel prices are
the same.
Until recently mogas was 10c a litre cheaper than avgas
which
would improve the situation further should that
variation return.
Further to this the auto engine (because
of faster letdowns) is capable of at least one extra tow per hour. We often do ten tows in the hour and
frequently do 30 tows without refueling with a good reserve left, the tank
holds 151 litres.
I sent a circular to all clubs with performance/economy
details requesting expressions of interest in financially supporting us
obtaining limited certification for dedicated glider tugs, the response has
been disappointing. It’s strange but every time a club
finds itself due for an engine overhaul I get a call, “when can we get one of
your engines”
Regards
Dave.
----- Original Message -----
Sent: Thursday, March 15, 2001 10:04
AM
Subject: [aus-soaring] Autotug
With the Aussie dollar continuing it's downward slide, why is
it that we are still powering our tugs with imported engines. The costs
associated with engine overhauls and replacement are escalating as the
dollar declines. Can anyone answer these questions.
1. Where is our
Autotug project?
2. Why was it abandoned?
3. Is an Aussie made, auto
engine viable as a tug engine.
4. Where is all that money the GFA
membership sunk into the project all those years ago?
I have launched
behind a Pawnee with an auto engine many years ago when it visited Gawler.
(I think it came from Port Lincoln). Is there now a economic incentive to
get this thing off the ground?
Am I wrong or am I not
right?
Comments appreciated?
PS In anticipation of this one,
some might say that the answer is winch or auto-wire launching.
Unfortunately in many cases wire launching is not practical, ie in the
presence of nearby main roads, powerlines, built-up areas or at airfields
used by lots of GA. Aerotow is a very popular launch mechanism.
Andrew Wright (VH GAM)
--
* You are subscribed to the aus-soaring mailing list.
* To Unsubscribe: send email to [EMAIL PROTECTED]
* with "unsubscribe aus-soaring" in the body of the message
* or with "help" in the body of the message for more information.
