Glider tug design

[Basil]

Just watched a tug and glider from Booker which did not seem to be climbing very well and wondered if anyone has designed a glider tug whose aerodynamic characteristics more closely approximate those of a glider, for instance high aspect ratio wings with a profile matching the glider L/D curve?

[airfoilmod]

Absent cost, the best "tug" would be a Helo. Up and down, many cycles, etc. Altitude is expensive, the faster you capture it, etc.

[Denti]

Yes, touring motor gliders make excellent tugs if they are powered well. However for heavy gliders just very well powered planes probably beat anything. A tow behind an Extra 300 is something to be experienced .)

[Blinkz]

Wilga is a pretty darn good tug. Huge engine, high lift wing....

[MCR01]

They used to tow gliders with Pilatus Porters at Gap-Tallard, France; even with a full load of parachutists climb rate was spectacular - probably like the fuel burn!

[longer ron]

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

[Dan Winterland]

You have to remember that once the tug/glider combination has accelerated to climb speed, the only force the tug has to overcome is the drag of the glider. A glider weighing 400kg with a L/D ratio of 1:40 will only provide 10kg of drag to the combination, plus a little bit added for the towrope. This is why even a fairly low powered tug such as a Chipmunk with a Gypsy Major and the standard Fairey Reed prop will still climb at close to 500fpm with a standard class glider on the back.

Getting the combination up to flying speed is a different matter though.

[longer ron]

Hmmmm...my first solo aerotow (K13) was behind a Condor (Ahhhh Condor !!!),whilst strapping in I was approached by the Tuggie who asked me how good I was at thermalling on tow !!!
Well i said I would give it a go...certainly improved our climb rate!!
Written on the Condor wingtip was 'Half scale Pawnee'
You cannot beat adequate power for any aviation related activity

[john_tullamarine]

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....

[MIKECR]

As a tuggy, flown a few different type's over the years - chipmunk(with converted lycoming 180 horse power), pawnee, super cub and robin DR400. By far the best tool for the job was the pawnee. The nicest to fly however certainly go's to the chippie. Such a fun aeroplane.

I would suspect the turboprop Porter or perhaps the big radial Wilga may be a force to reckon with though.

As a slight aside, the 'beefed up' grob 109 motor glider is no slouch either. We trialled one recently and it would certainly give the chippie a run for its money.

As for thermalling on tow - making use of any extra energy is always a boost...especially if the club charges 'per minute on tow' rather than by 'height'. If the glider pilot is willing to crank the bank...then all the better!!

[Mark 1]

You have to remember that once the tug/glider combination has accelerated to climb speed, the only force the tug has to overcome is the drag of the glider.

Dan, your analysis doesn't sound right to me. There's the weight of the glider to consider too and the work done by increasing its altitude. The force required is M.g.Tan(theta).
When gliding, this is a negative term which when added to the glider drag equals zero for a still air glide. When climbing the Tan(theta) is positive and adds to the tension in the tow-line. 500 ft/min at 60 knots is about 12:1 gradient which would add another 33.3 KgF to the drag experienced by the tug.

I'd rate the ability to climb efficiently at glider speeds and plenty of surplus power as the main desirable qualities in a tug. I've seen the super-falke towing, and that doesn't do bad on 100hp(?).

[22/04]

Depends whether your the pilot or an "accountant". In terms of operating cost, visibilty and ease to fly leaving you free for other things, the Robin 400 is hard to beat.

The Wilga is good but not as good as it could be because of the need to throttle back (ideally) aftr take off to conserve life an the big radial.

[FullWings]

I'd second the Robin 300/400 series as being close to the ideal. You have to factor in the fuel burn as well as the 'ooomph' and in that I think they are very efficient. The DR400 with a VP prop is an excellent performer in this respect and the undercarriage is far more sturdy than it looks.

If you can ignore the initial investment, the best tug I've ever experienced was the Turbo Bravo at Schaenis in Switzerland. It's a mil-spec two-seater with an 'experimental' 360shp Allison turbine in the front instead of the normal 4-cylinder Lycoming... The first launch was in an ASH-25 and the acceleration was so rapid we got airborne in negative flap! I've had 1,500fpm+ behind it in a single seater; the best thing is that it was actually cheaper to launch to height as a) it ran on turbine fuel and b) the cost/min. was high but you needed very few min. on tow. It could also fine off the prop, pull the dive brakes and be ready to go again on the ground from a 7,000' launch in about a minute. Class.

[Schiller]

One of the factors that determines the efficiency or otherwise of a glider tug is the rate at which it can get down again after dropping the tow - the quicker down, the more launches it can carry out during the day. Unfortunately, closing the throttle and hurtling earthwards causes rapid cooling of an air-cooled engine, and if carried out as a matter of routine will give rise to a high rate of cracked cylinders. We found that out at Dunstable; the solution was to keep plenty of power on during the descent, although this delayed the return for the next launch.

A turboprop or water-cooled engine doesn't have the same problem. The Wilga has, I believe, an arrangement of shutters in front of the engine that can reduce the air flow through the engine during the descent.

[x933]

Wilga needed a good tug pilot - it was easy to climb faster than the space shuttle but it gave a speed of 45-50kts down the back end. Which isn't fun in a heavy open class machine - somewhat akin to a fat bloke walking a tightrope.

Pawnee is hard to beat - a good mix of speed and climb rate, and nigh on indestructable.

Worst tow ever was an ASH behind an original chipmunk. Walking would have been quicker.

[chrisN]

A very good compromise was the one-off Swedish conversion of a Pawnee with a Volvo car engine (6 cylinder in line), 3:1 reduction gear, big prop, very quiet, very economical, but not repeatable without huge certification issues.


Chris N.

[noooby]

During the 1995 World Gliding Champs in NZ we had a number of different towplanes. Wilga, 260hp Pawnee, 235hp Pawnee, Cessna Agwagon, Cessna 180 and 182, Cessna Bird Dog and a Gippsland GA200 Fatman.
The best was the Bird Dog, by far, even though it had the lowest power (210hp?). Nest best was the GA200. Looks like a Pawnee, but performs much better. You can also get dual controls, great for towpilot training. Engine never got above 212F CHT due to great cowling design.
Next was the 260 Pawnee, 235 Pawnee, then the Wilga (I thought it would be better), and then a race for last between the Agwagon and the Cessna 180. I can't remember how well the 182 did, but I do remember helping to lift it out of a large rabbit hole it got stuck in!!
The Bird Dog however was a sight to behold. Good speed for the ASW22's/Nimbus 4's, and fantastic climb rate. With a power on spiral dive it also got down pretty quick. I hear the electric flaps can give trouble, but they really do tow well.
Most of my early towing was behind a 180hp Super Cub. That also towed really well, but Cubs are getting too expensive to buy nowadays and you also have the extra hassle of fabric.
For a new tug, I'd go with the GA200. For a second hand, I'd go with a second hand GA200 :) or a Bird Dog.
Turbines are great for the no shock cooling thing, but a lot of the components (turbine discs for instance) are lifed on cycles (landings). The more landings you do, the quicker you use up the life of the component. Not good if you do lots of short flights, like say, with glider towing!!
I thought Pik were working on a liquid cooled diesel powered single seat tug but I can't find a link to it at the moment. Now that would be a good way to go.

[noooby]

Pik 27. Rotax 914 power. Not sure if it will be powerful enough for the open class guys.

Windcraft Oy PIK-27 (http://www.windcraft.fi/pik27/welcome.htm)

[RatherBeFlying]

Been towed by Bird Dog, SuperCub, Wilga, Citabria 200Hp. and Pawnee (235 and 260). Behind a Pawnee is like being in a slingshot and yes, it goes up and down quick:ok:

We seem to have a considerable number of people who fail to lock spoilers before takeoff and it once took me a while to realise the spoilers had popped after a bounce on takeoff:ouch:

No problem behind a Pawnee:E

[Jim59]

Just watched a tug and glider from Booker which did not seem to be climbing very well and wondered if anyone has designed a glider tug whose aerodynamic characteristics more closely approximate those of a glider, for instance high aspect ratio wings with a profile matching the glider L/D curve?

Grob 109B turbo with a constant speed prop. meets the above, and having water cooled cylinder heads and air-brakes gets down very quickly, but rate of climb when towing is less than Robin or Pawnee.

Perhaps the tug you watched had forgotten to raise flaps after t/o or had left carb. heat on... It may also have been 2-up if a tug pilot training flight - or any combination of the above!

[Big Bad Dave]

I used to tow using a 235hp Pawnee at Lleweni Parc (North Wales) in the early 90's in some very Interesting (Turbulent!) conditions and can honestly say that although the Pawnee is not the most comfortable aircraft to fly (being male and this is assumption only - like sitting in a birthing chair?!?) on more than one occasion the brute power and controllability ensured that I remained a member of the human race!! Give me a big throbbing powerplant and a short field take off ability anyday!! - a chippy with 235hp Mmmmmmmmmmm!!!

Expat Dave, Perth WA

[Tankengine]

Cessna 150 with 180HP mod and uprated landing gear has almost the same performance as Pawnee 235 but more economical, especially for ferrying. Initial ground roll is slightly slower but can still handle ASH25s etc.:D
The Pik27 looks good but perhaps an o360 instead of the Rotax may be better.

[Fantome]

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 (http://www.gfa.org.au/index.php?option=com_content&task=view&id=382&Itemid=387)

[john_tullamarine]

Let it not be said that I didn't love driving any of the Pawnees .. all great fun. However, the little one didn't leave much to spare on a gusty, bumpy day with a two-seater on tow. At one field, where I was wont to tow many decades ago, I recall that only a couple of us were interested in towing in the little Pawnee .. the others preferred the Super Cubs... mind you that all changed when the little Pawnee was exchanged for a bigger brother.

I'm surprised that no-one has mentioned the Callair .. similar to the Pawnee and, stripped down, a really good tug .. especially when operating from a military aerodrome where one was not quite so constrained in respect of circuit procedures during the recovery. However, I always found the near total absence of aileron control a tad disconcerting during manoeuvring onto short final.

Shock cooling has always been a worry with tugs (and on parachute operations). While I have no recent knowledge of what is done, going back to the 60s/70s it was very apparent that those clubs which used fewer pilots and exercised reasonable control had fewer problems .. largely due to the better training and operational controls which could be exercised.

84 aerotow launches

Good Heavens .. I had a few excellent days but I don't recall ever getting over the 60-odd mark. Ian must have had one unbelievable boomer of a day with a majority of quite low releases ? He would have been one very tired little teddy bear that night ...

[longer ron]

Forgot to add before...one of the good things about being towed by a powerful tug is that you dont have to worry so much about that tiresome 'bowing' (drooping) of the tow rope !!Being towed by (say) an Auster or other low power/highish drag tugs one always had the feeling you would overtake the towplane,especially if you encountered sink !!

[an3_bolt]

My vote is still for the REAL mans Pawnee. But you are right - the sissy 160 is useful and does not need to be bagged..... (just winding you up - can not resist it!:O)

The Supercub is a different story - so sweet, so lovely - love at first flight!!!:ok:

[bspatz]

Been towed by many types of tug but without doubt most memorable was Morane Saulnier 503 Criquet (Fiesler Storch) which was always airborne before the glider and stayed well above the horizon until release - it was more like being underslung than towed!

[noooby]

Best tow I've ever had was in a Ka6 behind a 260 Pawnee :E
Worst I ever had was 2 up in an ASH25, behind a 172 with the old O-300 Continental (was it an O-300 or GO-300??). Man that was a long takeoff and tow!! :eek:

[Fantome]

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.


Macca says -

Yes it was and it was a Friday in May '87 I think. After a wet week and was a Friday catch up. The previous week was 78. Those were the old days of 6 students a week.

[MarkerInbound]

The Cessna Bird Dog is great to fly but they're getting expensive. All the War Bug crowd who cann't afford a P-51 or Spitfire have driven the price up. And we started having to do a lot of work where the landing gear meets the frame in the belly.

[OpenCirrus619]

See: Glider Towing with a Turbulent (http://www.tigerclub.co.uk/Default.aspx?Guid=&MenuId=.index.htm&Action=Public&ActionOption=&ToolAction=Open&Path=%2fTigerTales%2fGlider+Towing+with+a+Turbulent&Script=True&MenuId=.index.htm&File=index.htm) :eek: