Duralium??

[Anonymous]

Ah, the Pawnee! Our club has just bought a second Robin DR400 to replace the Pawnee we sold a few years back. Makes financial and operational sense, but I was holding forth yesterday about it being boring to fly. The Pawnee was much more fun.

Full size there can be a problem aerotowing older gliders, in that their maximum aerotow speed can be less than the minimum towing speed for the tug. Some years ago I towed the T21 at our club with the Pawnee. The glider pilot was also our chief tug pilot. There was a lot of discussion about not towing too fast and I must be below 60 knots at all times and so on. After the tow the only comment was that they didn't know the Pawnee flew that slowly. The stall warning was going off most of the way up the tow, but it was a bit pessimistic so not really a problem.

Andrew

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Andrew Johnston on 24/04/2016 10:40:13:The stall warning was going off most of the way up the tow, but it was a bit pessimistic so not really a problem.

Glad your optimism was justified!

Russell.

[Ian S CParticipant @iansc]

Bring back the Tiger Moth.

Ian S C

[christopher olsonParticipant @christopherolson21818]

Hand launching isn't impossible and when flying larger sailplanes from the slope 2 or three helpers are often used but One can also use a bungee to gain airspeed and "ping" the model from the ground but it again needs helpers to assist as the bungees have considerable pull.

But as stated above there are many club level organised events throughout the year and the accepted way of getting up among the thermals is by a powerful towplane/tug using reliable petrol engines of 60cc and over they get the sailplane up safely and quickly.

[Anonymous]

Chris: Thanks for the information, very interesting. I've read about bungee launching (full size) but have never done one. It's an area where my mum is one up on me, as she has had a bungee launch off the South Downs.

Andrew

[Ian S CParticipant @iansc]

Another one is a car tow, don't know if much of this is done.

Ian S C

[NogginParticipant @noggin]

If you are still looking for some material. we stock 1.5mm and 3mm 2014 sheet and can cut to the size you need. Link

[mike TParticipant @miket56243]

Years ago we used to bungee lunch full size gliders off the bank. The bungee was pulled into tension by a huge retired dray horse. The horse had spent all of it's working life pulling a brewers dray around North Yorkshire, so was well used to pulling heavy loads. The horse ambled slowly down the bank with one end of the bungee attached to it's halter. When the tension was high enough the glider was released into the air.

One day, someone noticed that the old horse always shut both eyes when it started pulling on the bungee cord. Well wouldn't you? Even the old horse could imagine the consequences, if the bungee cord suddenly snapped without warning.

[Anonymous]

Posted by Ian S C on 26/04/2016 11:06:24:

Another one is a car tow, don't know if much of this is done.

Seems pretty rare now? I'm sure Aston Down used to launch by car and reverse pulley, but having just looked at their website they now use conventional winches. Of course one problem with car launching is that you need a long runway. Having said that I've done a lot of car launches. I learnt to glide at Farnborough where we had a 6000ft runway. We used an old Austin Cambridge, and a Jaguar, running on some awful mix of used solvents from the 'system'. We used Parafil as the cable; expensive and a pain when it broke and had to be joined. It was also a PITA when it fell off the runway had to be lifted over all the runway lights. The worst was dropping it over the ground radar antenna, usually between the two warning lights on the top.

Once we sold the Tiger Moth at Thurleigh we reverted to car launching using an old Rover 3500 and ordinary rope. Od course we had a 10500ft runway. The Rover V8 engine was bomb proof. The car fell apart, literally as the body shell rusted and broke in half, but the engine kept going.

Car launches are an elegant way to get airborne, cheap and no need to pull hard at the bottom of the launch as the cable isn't getting any shorter.

Andrew

[Martin KyteParticipant @martinkyte99762]

Andrew

You comment "they didn't know the Pawnee flew that slowly"

No idea to the answer to this but does the action of towing from a plane decrease the stall speed in any way. I know nothing (well very little about aircraft) but I'm thinking that the engine(s) must be working that much harder and shifting more air. Some of this would go over the wing (especially in a twin) coupled with maybe an increased angle of attack may provide more lift at slower speed? I don't know perhaps it's irrelevant and as you as they were just being pessimistic on the stall alarm.

As you have actually done it I would find your comments interesting.

[Ian S CParticipant @iansc]

Stall on the Pawnee is 50 to 53kts depending on 150 or 235/260hp aircraft, and the stal warning horn is better it goes off higher than low.

On the auto tow, the reverse tow cuts the length of ground required, but it has the same draw back as a winch tow, as the rope shortens as it nears the pulley.

Ian S C

[Anonymous]

Posted by Martin Kyte on 27/04/2016 09:42:22:

No idea to the answer to this but does the action of towing from a plane decrease the stall speed in any way. I know nothing (well very little about aircraft) but I'm thinking that the engine(s) must be working that much harder and shifting more air. Some of this would go over the wing (especially in a twin) coupled with maybe an increased angle of attack may provide more lift at slower speed? I don't know perhaps it's irrelevant and as you as they were just being pessimistic on the stall alarm.

Martin: On tow the engine is at full throttle, usually at or near the red line rpm as fine pitch props are normally used on tow planes for better ground acceleration. The angle of attack of the wing needs to increase as the speed decreases to maintain the lifting force. This is irrespective of the engine power. Of course the induced drag also increases sharply as the stall is approached, so from the performance viewpoint it is not always the best operating point. I guess the extra airflow over the wing from the prop wash would help, but there is also a a slight upwards component of the thrust vector which effectively reduces the weight and hence the stall speed when at full power. The stall warning in our Pawnee was known to be pessimistic by a couple of knots so I wasn't actually at the stall. Certainly there was no buffet, although the ailerons were a bit sloppy.

A big problem when towing at slow speed is engine cooling. The engine is running at full throttle, but the speed is relatively low, so not much airflow over the cylinders. We normally run very close to the cylinder head temperature red line.

Andrew

[Ian S CParticipant @iansc]

Another problem with engine handling is after the release of the glider, the tow plane has to get down again and with reduced power, increased speed, the engine temperature drops quickly, glider towing and agricultural flying must be fairly similar, with the stress of high cylinder head heat, followed by rapid cooling. On our Cessna 180/185 and 188 Agwagons this was helped with cowling flaps which helped to slow the cooling on let down, where as on take off and climb with them open the airflow is increased. This C-185 has the cowl flaps open.

Ian S C

[Martin KyteParticipant @martinkyte99762]

Thanks for the response Andrew. I always find it interesting to think about systems when they are outside their 'normal' working parameters. It often gives you a real insight into the dynamics of the thing.

regards Martin

[NJHParticipant @njh]

"I'm sure Aston Down used to launch by car and reverse pulley, but having just looked at their website they now use conventional winches."

Quite right Andrew they did – I had such a launch on a visit there many years ago. It was all a little sedate as I recall – but then I was used to winch launches of the " sat there at one moment then, at the next, flat on your back going straight up!" variety.

Norman

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