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Andrew
The Autotug project is still going and by no means abandoned. A full report was
published in (I think) May's AG, followed by another auto engine report in
September. In November I sent a circular to all aerotowing clubs with a
report and asked for expressions of interest, responses have been minimal.
Rather than go through it all again the following is a copy of the latest
(brief) update which another party requested and where the project is
now.
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.
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- [aus-soaring] Autotug ANDREW WRIGHT
- Re: [aus-soaring] Autotug Dave Sharples
- Re: [aus-soaring] Autotug ANDREW WRIGHT
- Re: [aus-soaring] Autotug Peter Stephenson
