Glider tug design

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john_tullamarine
Moderator

A tow in a lightweight glider behind a big engined Pawnee was almost as good as a winch launch

Indeed .. however, a tow behind a small Pawnee is not much fun at all .. for either pilot. Many fond memories of the former and, likewise, the odd grey hair from the latter.

an3_bolt

My vote goes to the Pawnee (the 0-540 one). Great visibility, good performance and nice handling all at glider speeds. I still have nightmares watching a Pawnee 160 towing open class gliders with water....

Soaring Club of Tasmania's 160 hp Pawnee hauls the Blanik with two 100 kgs in it on 25 degree day and density altitude 1000 feet with ROC 300-400 FPM
Same conditions, nil wind, the combination needs about 600 metres to clear 50 feet. Capital cost, maintenance costs, operating costs way down on the bigger PA-25s. Though not suited to lower latitudes and higher elevations, the PA25-160 doesn't deserve severe bagging. (Only rebagging after many years TLC.)


From the Gliding Federation of Australia's website -

Pawnee

This aircraft meets the requirements of the specifications list if the Lycoming engine is replaced with a high-powered automotive engine. In all other respects it fulfils the requirements.

The Pawnee is an ag-strip cropduster. It was designed to carry up to a tonne of fertiliser into the air off rough farm strips. It is enormously strong. It is a very easy aircraft to fly. There are currently over 70 of them on the register in Australia with an unknown number in sheds and hangars as a spare parts source. The type is still in production via the holder of the Type Certificate in South America.

One major advantage is that Pawnees are cheap. This is because they are being supplanted in agriculture by larger, turbine-driven aircraft and so have ceased to be generally viable as cropdusters. Our experience suggests that they are all for sale, and at reasonable prices.

The Pawnee is also, however, the tug of choice of many of the gliding clubs in Australia, New Zealand and UK. (In NZ and UK, despite exposure to "lighties" as tugs, the clubs we have contacted are reverting to Pawnees).

The Pawnee's capabilities in achieving high numbers of tows per hour is noteworthy. On one occasion at Lake Keepit, a Pawnee flown by Ian McPhee performed 84 aerotow launches in a single day.

Its paddock-retrieve capability is outstanding, because it was designed with rough, short landing areas in mind and it was built to withstand the stresses that these operations generate.

In its existing form, though, the Pawnee does have shortcomings. All of them are engine-related.

The Lycoming aircraft engine is expensive to buy, fuel, maintain, repair and fly. It's also noisy (European clubs don't use this sort of engine in tugs because of increasingly onerous noise legislation).

In glider towing, the Lycoming has to operate close to its performance limits, constantly. The result is that almost no Lycoming engine reaches its 2000-hour overhaul level without first needing new cylinders or a top overhaul. Shock cooling on descent is a permanent problem that increases turnaround time and, if not handled well by the pilot, results in cylinder and cylinder head cracking. All of these problems and processes are costly. A top overhaul can run to $18,000. The 2000-hour overhaul costs between $45,000 and $55,000. Spare parts are expensive, as is avgas and the result of all of these expenses is that an aerotow launch is more costly than it should be.

The solution to all these problems is to dispense with the Lycoming, and install a proven, inexpensive, powerful, modern, extensively tested, readily available, easily maintained automotive engine that is also cheap to run to drive the Pawnee. This conversion process must be approved by the authorities, but there is a specific program to be followed which will provide certification of the modifications.

The perfect candidate for this job is the General Motors LS1 Chevrolet V8 alloy engine (better known in Australia as the Generation 3). GM have produced hundreds of thousands of these engines over the past 5 years. The company has spent billions of dollars developing, testing, refining and manufacturing it. The engine capacity is 5.7 litres. It develops 340hp at 5700rpm. It is fitted to a range of high-powered GM vehicles including the SS Monaro. Outside of motor cars, it powers a remarkable selection of utilities, trucks, dragsters, airboats and aircraft. Complete with prop-drive unit (PSRU) it weighs in within a few kilograms of the 235 Lycoming engine.

It was this combination of airframe and engine the eTug syndicate decided to proceed with. In taking this course, we recognised at once that eTug would become the direct lineal descendant of Autotug.

It is also of the utmost importance to note here that the project to convert numbers of Pawnees in this way can only work if we, hopefully with GFA support and participation, can gain relief from the CASA regulations by achieving a new class of certification for the aircraft, in the area of operation of glider towing.



More on this subject - The Gliding Federation of Australia Inc. - Tug engine developments