Flying Machines 3: Tug Aircraft

The description “Tug Aircraft” is a bit vague as a tug is just a light aircraft with a towing hook at the back. For the purposes of this page I will limit it to aircraft I have actually towed gliders with.

Super Cub G-AWMF at Booker

Super Cub G-AWMF at Booker

My first experience of towing was in the Piper Super Cub. G-AWMF had been at Booker since it was new in 1968, although it was acquired by the gliding club two years later. It is still there, although de-rigged in a hangar after an accident in 2012. In truth a fair amount of that particular airframe has been replaced over the years (the wings have been completely rebuilt at least twice), so it is a bit like the hammer with two new heads and six new handles.

The Super Cub is probably a little too lightly built to be an ideal tug, but its other attributes more than make up for that. It has an incredibly forgiving wing section and can be flown safely very close to the stall without danger. The same wing section is efficient at low speeds and much less efficient at higher speeds, just what you need in a tug.

Engine cooling was always a problem as the cylinder head temperature would get quite high in the climb (high power, low speed) and would cool rapidly in the descent. Various techniques were used to mitigate this, but none have ever been totally successful and cracked cylinders were part of the cost of towing.

By the end of the summer of 1976 the 150 HP engine was running out of life and the decision was made to fit an O-360 engine of 180 hp which was done that winter. The conversion turned a good tug into a great one with a massive improvement in the rate of climb, particularly with a heavy glider on tow. It was so successful that a subsequent Super Cub (G-BFFP) was also converted.

Champion Citabria

Champion Citabria

Alongside the Super Cub in the early 1970s was a Champion Citabria. This had been acquired after the loss of the second Super Cub (G-APZJ) in an accident. On paper the Citabria looked good, the same 150 hp engine as the Super Cub on a stronger airframe, the reality proved less so. The rate of climb was less and it never really came up to expectations. It was more comfortable, though.

The name Citabria is actually airbatic backwards and in theory it was semi-aerobatic, but after a number of in flight failures in the USA, Booker Gliding Club decided to ban aerobatics in it.

One weak point was the “U” bolt that held the rear of the tailwheel spring in place which was inclined to fail the result of which was a ground loop on landing. On the wide open spaces of Booker airfield it was never a problem, but eventually it chose to do it at Enstone on a narrow runway with a fence down the side resulting in quite a lot of damage.

It was finally sold to someone in Northern Ireland and I got the job of ferrying it to Carlisle for the handover, so my last flight in it was probably the longest flight it ever made out of Booker.

After the summer of 1976 it was obvious that another tug was needed, so arrangements were made to lease a PZL Wilga. Another bizarre Polish aeroplane. I really have no idea what the designer was thinking of! It was a large high wing monoplane with a 260 hp radial engine at the front (the engine was a Russian copy of a Pratt & Witney Wasp Junior). The fuel consumption had to be seen to be believed even at its cruising speed of 75 knots. Why anyone thought that there was a need for a 4 seat, 260 hp aeroplane that only did 75 knots I can’t imagine. The visibility was so poor that at Booker it was always flown with a lookout in the right seat.

PXL Wilga

PXL Wilga

The engine was not only supercharged, but also drove a very large constant-speed propeller. The starting system was a compressed air arrangement that was moderately efficient with a good charge of air, but therein lay the problem, the engine driven compressor was often unserviceable so the engine had to be started from an diving air bottle plugged in to an external connection. The nearest place the bottle could be recharged was a diving shop in Old Windsor!

When the compressor was working the offload valve would operate with an explosive noise, usually on take off about 50 ft above the ground. Another trick the engine had when it thought it needed a service was to cut out just after take off with all seven cylinders refusing to fire once and then resuming normal operation. The undercarriage consisted of two long steel tubes with a sort of kneeling suspension arrangement at the lower end As the suspension was fairly soft this meant the applying the brakes caused the undercarriage to go down which was very alarming in a crosswind because as you attempted to correct any swing into wind with the brakes, the downwind undercarriage would get shorter resulting in a roll in the wrong direction.

The Booker Wilga had two accidents, the first resulting in it ending up on its back after landing and the second after it ran out of fuel, the pilot tried to get back to the airfield and crashed into a road. In the second accident the lookout was killed.

The Wilga was a very unusual aircraft and my only experience of radial engines.

The Auster 6 Terrier at Rest

The Auster 6 Terrier at Rest

Lurking in the background at Booker was an Auster 6 Terrier that had been modified to take a 160 hp Lycoming engine by the BA apprentice school (that sort of thing used to happen in those days). It had been taken back to Heathrow for a C of A a few years before and had never returned. After some persuasion it was eventually brought back in good order and put to work.

Perhaps now is a good time to talk about Austers. The original Auster design came from an aircraft called the Taylorcraft. This evolved into the AOP Auster of WW2; they should have stopped then. All the Auster aircraft after that got heavier and less useful, the line ending with the appalling Auster 9 of the British Army that appeared to have been chiselled out of the solid.The Terrier was somewhere in the middle of this downward spiral.

The performance as a tug was not great and the visibility poor. Its one saving grace was its serviceability; the engine installation had been well engineered and gave little trouble. Unlike the other tugs it had a single throttle lever in the centre of the instrument panel which gave trouble to at least one pilot I checked out on it. He started the take off run with his left hand on the stick and right hand on the throttle, but as the tail came up and with a cry of “*** this!” he changed hands and flew the rest of the tow with his hands crossed.

The photo shows it parked and tied down in the weeds, possibly the best place for it.

Robin DR400

Robin DR400

With more modern gliders needing higher speeds on tow the need emerged for a tug that worked better at those speeds. The Robin DR400 was (is) a lovely little aeroplane. The design started life as a homebuilt design called the Jodel. The cranked wing with only the outer parts having dihedral was to make it easier to construct on a flat bench. The construction was all wood and it had a 180 hp Lycoming O – 360, or at least the tugs did. With a coarse pitch propeller fitted it would easily cruise at 130 knots and with a fine pitch prop it could easily launch the heaviest gliders.

The huge bubble canopy gave excellent visibility, the controls were light and effective and it flew very well. I did have a major engine problem in one just after take off at Booker when it blew a cylinder off with a lot of noise, vibration, smoke and loss of power, but it somehow staggered back to land downwind never having got above 200 ft. It could have happened to any Lycoming powered aeroplane.

Piper Pawnee

Piper Pawnee

The Pa25 Piper Pawnee is really a crop spraying aircraft and there have never been many of them in the UK, in particular there were very few of the 150 hp version, most of them being the larger version powered by the O-540 six cylinder engine of 235 hp. I have only flown the 150 hp version and I only flew that when we needed to borrow a tug from Lasham.

The Pawnee is very closely related to the Super Cub in using the same wing planform and aerofoil section, but the wing is mounted low and the struts are on top. The tail is also very similar to the Super Cub. The fuselage is, of course, very different with a large hopper for the chemicals in the centre section and the single seat mounted behind and high up. Because the fuselage width is designed around the hopper the rudder pedals are a long way apart leading to a strange sitting position best described, by Chris Rollings, as “probably a good position to give birth in”.

Visibility is very good and the performance roughly the same as the Super Cub. The cockpit is very basic with almost no concessions to comfort.

The Beagle Airedale

The Beagle Airedale

The Beagle Airdale is possibly the worst tug I have had the misfortune to fly, although, to be fair, I only did one launch. A quick look will show an aeroplane that is closely related to to the Auster Terrier, but with a nose wheel undercarriage. In some way the designer had managed to make the already rather stodgy handling of the Terrier and make it much worse. With a Ka13 behind it the rate of climb was only just positive.

I did fly it again a few hours later with four people on board and no glider. It was still awful, consumed massive amount of fuel and cruised slowly.

The only reason I got the opportunity (?) to fly it at all was because Booker Gliding Club bought it for the 180 hp engine to fit in a Super Cub. The price paid for the complete aircraft was £500 less than the value of the engine alone which indicated that the airframe was worth minus £500. That was a judgement I could not argue with.

De Havilland Chipmunk

De Havilland Chipmunk

The DH Chipmunk has already featured in an earlier page, but I include it again as I flew it on a number of times towing gliders. The version I did not manage to fly was the “Supermunk” with the 180 hp Lycoming conversion.

To be honest the “Chippie” was not much of a tug, there was very little excess thrust from the Gypsey Major to pull a glider. However it was an excellent airframe and after your 10 minute climb to 2000 ft the fun could begin! As you approached 2000 ft a good lookout was kept of everything below so that as the glider released you could half roll to the right and pull through. There was no need to throttle back and you were quickly the right way up, heading towards the airfield at over a hundred knots.

My first experience of the Chipmunk as a tug was about two months after I started work at Booker. I was sent to Dunstable to borrow a tug which proved to be one of their Chipmunks. I was expecting some sort of brief, but I was just asked “You have flown a Chippie before, haven’t you?”. Hearing in my mind Dave Watt’s voice saying “It’s a light aircraft” and since I got the impression I wouldn’t be allowed to have it at all if I said no, I answered “Of course”, not entirely a lie, I had had a couple of AEF flights as an ATC cadet 10 years before. I was very soon forced to ask for help to start it since it was not something Air Cadets ever did. Fortunately this one had had the mod to electric start and I was able to claim that I had only experienced Chippies with a Coffman starter before. Once the engine was going I found Dave Watt was correct, it was just a light aircraft.

AOP Auster

AOP Auster

I only flew the AOP Auster for a day’s towing. Another gliding club had run out of tug pilots and asked for help and I had expected a day of hard work in a close cousin of the Auster Terrier with its heavy unresponsive controls and rather weird flaps.

I was pleasantly surprised and whilst it was no Super Cub, it was a nice little aeroplane. Its towing performance was, of course, severely limited by the Gypsy Major engine, but otherwise it was a very pleasant little aeroplane; a bit basic, but OK.

Advice to aircraft designers, if it ain’t broke, don’t fix it.