DG Flugzeugbau GmbH / Passion, Power + Performance

Aerotow with the turbo engine running

The Aero Club in Lesce/Slovenia, which has been my main gliding holiday destination for the last 10 years, have two Piper Pawnee and one Super Cub for aerotowing. However, this impressive fleet is less than useful when one of the Pawnees has been waiting for a new engine for several months, and the other one is grounded because of a minor technical problem.

The Super Cub only has 150 bhp, the DG-1001T with two rather heavy pilots (my friend and me) has a takeoff weight of approx. 700 kg.

Faced with these challenges, Werner Jansen, the ever tireless tug pilot, looked slightly worried despite the length of the runway (1,000m): the grass was wet, the ground soft and there was only a light wind. Well, we calculated that everything was still within the limits, and so we were soon ready for take-off.
Sure enough, after 500m we had reached the required 15m height, but we still only went over the houses and trees at approx. 50m at the end of the airfield - that wasn't very nice!

For months now we have been trying to convince the German LBA that aerotowing while the turbo engine on the glider is running is safe and works well, especially behind a weak tug. As this method is not explicitly illegal, it does not have to be specifically approved. However, we would like to go one step further and describe it in the approved part of the DG-1001T manual.

This method of aerotowing has been possible for years with another manufacturer's open class glider after all. The LBA's response to our claim "same rights for everyone" unfortunately was that the approval process had changed since then and that they needed a bit more proof. Well, we are all for providing the required proof if it's in the interest of safety!

The emphasis here lies in ensuring that usable emergency procedures for engine failures on the tug or glider are in place. A "multi-engine" operation such as this is a rather complex matter after all.

We decided to do a few test aerotows - we had to do them for the LBA anyway:
The next day we decided to start the aerotow (behind the Super Cub again) with the turbo running. I had the opportunity to gain valuable impressions of what any pilot should bear in mind who wants to use this method - as yet on their own responsibility though!

As a general rule, using the turbo engine for aerotows behind a powerful tug is pointless. Ordinary tugs normally tow at speeds exceeding 65kts (120kph). At this speed the DG-1001 engine must be throttled down quite a lot to avoid overcranking, which means that it just about compensates for the drag caused by the propeller. The turbo shortens the ground run in this case but won't have any other beneficial effect!

If a weak and slow tug (like an ultralight or a TMG) is used, however, it's a different matter:
In this case both aircraft take off together and have a dramatically better climb rate at a speed of approx. 50 to 55kts (90 - 100kph) than without the turbo. Towing a heavy two-seater might only be possible with the turbo in this scenario.

The Super Cub is probably between the two types of tug:
Werner towed us very gently at approx. 60kts (110kph), the average climb rate increased from 2kts (1m/s) to 5kts (2.5m/s), and we felt very safe at a comfortable height when we reached the airfield boundary. The whole ground run, take off and aerotow was perfectly ordinary, with the only exception that the noise of the engine meant we had to use the headset for communicating.

The complete aerotow took less than half the time it had taken the day before, and it cost us 2 litres of fuel.

A few days later, a different tug pilot towed us at a speed of 65 to 70kts (120-130kph). I had to throttle down all the time, and the climb rate after take off was significantly worse.
To sum up: The conclusion therefore is that the aerotow with a running turbo engine works really well if the tug can fly slowly - if it can't, don't bother.

The tow rope was 50m long. This should really be the minimum length. With a shorter rope, a light tug with a heavy glider might not be able to counteract excessive and incorrect control inputs by the glider pilot. Needless to say that you must only use the nose hook for aerotowing!

The other issue is that if the glider is in the low tow position, the stall warning of the DG-1001 is activated. Therefore the normal tow position just above the prop wash is recommended.

Emergency procedure

What now does one do in an emergency, i.e. if one of the two engines fails? Please make sure you've memorized the following options before the launch:

Initial tow up to approx. 40kts (75kph)

40kts (75kph) up to reaching a safe height

Problems during the remainder of the aerotow

The important thing is that if the turbo fails, the additional drag from the turbo's prop should not cause any additional load for a weak tug.
The DG-1001T has two retraction methods for the engine:
The automatic system will retract the engine completely. If this doesn't work - e.g. because the engine has seized - you need to actuate the emergency switch until the prop lies flat on the engine flaps. Aerodynamically this will not make a huge difference compared to a completely retracted engine.
You can see the prop and the open engine flaps on this side view. This frontal view - i.e. in the direction of the airflow - shows only the a cross-section of the prop:

 

Conclusion:

Based on our experience this launch method provides a significant safety factor and should always be used if a slightly underpowered tug (e.g. a UL or a motor glider) is used.

You can already use this method legally as it's not actually banned.

However, the official approval by the LBA did not come before Dec 2007. 

Aerotow with turbo engine running "officially certified"!

The German "Luftfahrtbundesamt" was demanding some very special things, to be proven by a videoclip.

So during my soaring vacation in Bartholomä/Germany in Julyi 2007 I have been a test pilot. This was quite new for me but was absolute unspectacular.
We made a short movie with some explanations in German, but I suppose that you will understand what can be seen there:


Video: Turbo Aero Tow Testing (11,6 MB)

Many thanks to the "Fluggruppe Heubach", who came with their Dimona Towing plane.

At Dec. 5th the "Officially Certification" was given by the EASA.


Now it is "officially certified" and allowed
to use the turbo engine during an aero tow!

 

- friedel weber -

Translation: Claudia Büngen

 

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