In aircraft towing , or F-towing for short , a glider is attached to a sufficiently strong towing aircraft using a 30 to 60 meter long tow rope . In addition to "normal" motorized aircraft , licensed ultralight aircraft or sufficiently powerful motor gliders can also be used as tow planes .
The tow pilot brings the glider into an updraft where possible, where it can continue to climb without the help of the engine.
Not only gliders, but also loads such as an advertising tow banner or a target for target practice can be towed with an airplane.
The first successful aircraft tow documented with photos, films and newspaper or magazine articles took place on October 1 and 2, 1922 on Manhasset Bay, Long Island, NY. Glenn Hammond Curtiss and his team were able to pull a water glider into the air with a Curtiss Seagull. After clicking out, the glider landed on the water again.
The Dutchman Anton Fokker started thinking about getting gliders into the air with a motorized airplane back in 1912. He got the idea under the title towing a glider v. Patenting Lilienthal by means of a powered airplane , but its practical implementation initially failed when the First World War broke out . Unfortunately, no relevant official documents have yet been found. In 1926, Fokker gave the German aviation pioneer Antonius Raab permission to use his American patent in public demonstrations.
The third aircraft tow was prepared by the employees Gerhard Fieseler , flight instructor, and Paul John Hall , chief engineer, of the company Raab-Katzenstein-Flugzeugwerke GmbH at the Kassel-Waldau airfield . Hall built the necessary cable couplings in the spring of 1927 based on ideas from Fieseler. On Saturday, March 12, 1927, Fieseler, as pilot of the towing aircraft RK 6 "Kranich" (aircraft registration D-975) and Gottlob Espenlaub as pilot of his towed "E 8", initially with low towing heights "of over ten meters", proved that Towing is possible.
The next successful aircraft tow documented with a picture is dated April 13, 1927, took place at the Kassel-Waldau airfield and shows an RK 6 “Kranich” as a “tug aircraft” , flown by Kurt Katzenstein and an RK 7 as an “aircraft trailer” "Butterfly" , flown from Raab. A few days later, on the occasion of the “major flight day” on April 18, 1927 in Kassel-Waldau, the aircraft tow was demonstrated in front of an audience by Fieseler and Katzenstein.
There are divergent statements about the people involved: If Beckmann's Sport Lexikon from 1933 still mentions Gottlob Espenlaub and Raab as glider or tow pilot, Gerhard Fieseler is specified as tug in the 1938 Handbuch des Segelfliegens . In his autobiography from 1984, Antonius Raab himself blames the propaganda of the Nazi regime for deliberately replacing people who were “unpleasant” with the regime, such as his Jewish partner Kurt Katzenstein and, for political reasons, himself with “approving” ones . In truth, in his role as head of Raab-Katzenstein-Flugzeugwerke GmbH , he himself decided in February 1927 to put Fokker's idea into practice. Since the company did not own a glider itself, they rented Gottlob Espenlaub's machine. In a first, unsuccessful attempt, Gerhard Fieseler in the crane tried to drag the sailor with Gottlob Espenlaub at the wheel. In the process, "the keel fin and the rudder with a loud noise" were torn out of the glider built by Espenlaub when it was towed, as it was not designed for such loads. In the first successful attempt, however - according to Raab on March 15, 1927 - Antonius Raab steered the specially designed glider butterfly while it was being towed in the crane by Kurt Katzenstein .
Development and establishment in the 1930s
In the following years, the new technology was initially only practiced by Raab-Katzenstein as a special attraction at flight days . In April 1930, the American Frank Hawks had himself towed from San Diego to New York - a distance of over 4000 km. A telephone cable was part of the tow to allow communication between the pilots. At this point in time, aircraft tow had not yet established itself as an alternative starting method for gliders.
Only through further developments by the Rhön Rossitten Society and extensive tests by Günther Groenhoff in collaboration with Peter Riedel as a tow pilot did the method gain in importance. In autumn 1931 the first “towing course” was held in Griesheim , Hesse , and by 1938 at the latest, aircraft towing was finally established.
Since radios suitable for gliders were hardly available at the time, simple waving signals were initially used for communication between tow and glider pilots. So the tug could give a student pilot to understand that he had to pull or push the control stick , or that the tow had flown in an updraft and the glider pilot should now release the tow rope and circle it. The so-called tow telephone was later developed : a telephone cable was braided into the tow rope and the pilot's hoods were equipped with headphones and a microphone. However, with the development of suitable radio devices, this technology became obsolete.
Unusual developments of this time are the towing of gliders with particularly short ropes (short tow , up to 1 m) and with rigid poles (rigid tow) to facilitate blind flight. The flight behavior in rigid tow was significantly more stable. Gliders and unmanned flying trailers were also developed in Germany and the Soviet Union, from which fuel supplies for the towing machine could be taken during the flight.
Since 1976, hang gliders have been towed by ultralight aircraft in Germany . In this method, known as UL tow , weight-controlled ULs ( trikes ) are preferably used, which are relatively similar to hang-gliders in their flight characteristics.
Probably the first glider tow by a motor glider took place in Austria in the early 1980s . The HB-21/2400 used for this purpose was registered there as a tow plane around 1983. Systematic tests on the towing behavior of touring motor gliders were carried out by the German Alpine Sailing School Unterwössen in the mid-1990s . In 1998 , the Samburo used for this was the first motor glider in Germany to be officially approved for aircraft towing. A little later, aerodynamically controlled microlight aircraft began to be used as towing machines for gliders. The additional requirements that an ultralight aircraft must meet in Germany were published in 2001 in the " Nachrichten für Luftfahrer" newspaper.
Pre-flight check: Usually a helper hooks the tow rope with the metal ring provided at the end of the rope into the motor aircraft tow coupling , which is usually located at the end or under the fuselage of the motor aircraft. With some tow planes, the rope can also be retracted and is then pulled out by the helper before take-off. As soon as the glider pilot has indicated that he is ready to take off ("thumbs up"), the helper latches the tow rope onto the glider. It is important to ensure that the tow rope is not knotted anywhere for reasons of stability. Only at the transition from the rope to the two snap-in metal rings is a structure-friendly knot possible with a special technique; another connection option is to interweave the rope with the metal ring ( splice ), which is possible without any significant loss of tensile strength.
Take-off procedure: After having hooked the rope to both machines as described above, an assistant holds the glider's surfaces horizontally. If the pilot of the glider gives the readiness to take off via radio by “tightening the rope!”, The motorized pilot in the tow plane confirms this e.g. B. by "pulling the rope taut". The towing machine rolls forward slowly. Usually, the glider pilot then gives the radio command "Tighten the rope, start!", Whereupon the tow pilot accelerates the tow plane to take off. The glider usually takes off first and signals this to the tow pilot via radio with the command "Free". Occasionally the commands are also signaled by hand signals. The helper signals on the surface with hand signals when the rope is taut. When accelerating on the runway, the helper runs along until the glider can keep the surfaces horizontal even without his help, i.e. has enough wind pressure on the ailerons . It is important for the pilot in a powered aircraft not to pull the slack tow rope taut at full speed, as the elasticity of the tow rope would cause the tow rope to jerk and relax and thus introduce load peaks into the aircraft structure or tear the predetermined breaking point .
When taking off, the glider takes off first, as its aerodynamics make it flyable at significantly lower speeds than the towing machine. As long as the towing machine has not taken off, it is important to keep the glider flat on the ground (approx. 30 to 50 cm) so as not to pull up the tail of the towing aircraft with the tow rope and thus prevent the towing aircraft from taking off or even cause an accident . The tow plane takes off at a speed of approx. 100 km / h. The glider pilot now has to follow the tow plane exactly. In straight flight, the glider pilot flies in such a way that the landing gear of the tow plane is on the horizon line from his point of view or, in the case of low-wing planes, the elevator halves the wing of the tow plane. When turning, the glider pilot steers onto the surface of the towing machine on the outside of the curve, with a slightly lower bank angle.
The so-called box flight is trained in the training. The pupil should learn to control his flight in the tow. The student should first fly sideways to the tow plane, then descend, fly under the propeller jet of the tow plane to the other side and steer back to normal flight position. The flight path should describe a rectangle behind the towing machine.
Aborted take-off: Due to the numerous factors of the tow plane / towed aircraft system, it is difficult to calculate the take-off distance (also because performance data are usually not available). A common procedure is to disengage for safety reasons when the towing team is not yet in the air after 70% of the available runway length.
As soon as the pilot has released the tow rope, the glider turns to the right and the powered aircraft to the left to avoid collisions. After the tow pilot has made sure that the glider is actually no longer hanging on the rope (e.g. by a radio message from the sail pilot or by checking the rearview mirror), he flies the aircraft back to the airfield in a steep descent, the flight time short and so to keep the towing costs low. The temperature of air-cooled aircraft engines must be closely monitored. Since a descent phase in glider tow immediately follows a full throttle phase with low air speed (unusual for a "normal" flight), this can result in shock cooling with costly engine damage.
Unless the glider club / glider school concerned has agreed to another procedure, the tow rope, which only hangs on the motorized aircraft after being released, is released shortly before the threshold. If the runway is not long enough, the motor pilot turns another traffic lap without a rope and then lands conventionally. If the tractor has good STOL properties (short take off and landing), it is also possible to land immediately after the rope has been released. Other tow planes are equipped with a rope retraction device with which the tow rope is pulled into the fuselage after it has been released, so that there is no need to drop the rope. If the release or rope retrieval function on the tow plane is defective, it must land with the rope still in place. The tow pilot must ensure that he does not damage any aircraft or vehicles standing on the ground or injure people with the tow rope.
Aircraft tow from the glider pilot's point of view: The elasticity of the tow rope mentioned in the above paragraph can - especially with inexperienced sail pilots - lead to the glider hanging like on a bungee rope and thus periodically braking the aircraft flying ahead by tightening the rope, after which the rope is back in the air sags, etc. To avoid this, box towing is practiced for the glider pilot during the instruction phase.
Flight errors in aircraft tow can be particularly dangerous for the powered aircraft. If the glider exceeds the towing machine, it pulls the tail of the motorized airplane up and forces it to descend. The downforce of the powered airplane horizontal stabilizer cannot overcome the lift of the much larger glider wings. At a low altitude, only an immediate release of the tow rope can prevent an accident. If the glider sinks under the towing machine, it can get caught in the highly turbulent propeller wind of the powered aircraft in front. This should also be avoided, since the control movements of the glider - especially with very short tow ropes - cannot immediately compensate for these forces and the twist of the air flow requires increased counter-holding around the longitudinal axis. Some pilots, on the other hand, prefer deep towing, in which the glider deliberately flies very low behind the towing machine in order to remain below the propeller vortices. According to LuftPersV , this type of tow is also part of F-tow training.
The towing motorized pilot has to monitor in particular the speed, the speed change and the roll rate / yaw rate of his motorized aircraft when towing. Too low a speed can bring it close to the stable , too high a speed can lead to dangerous fluttering on the glider, a rapid decrease in speed of the motorized aircraft can necessitate an emergency release of the glider that is flying behind, as it "runs up" due to its comparatively excellent aerodynamics and not has sufficiently effective braking devices; Due to the very fast rolling / yawing of the motorized aircraft, the glider pilot may no longer be able to follow the curves of the tug in front, especially beginners in gliding are overwhelmed here.
Tug pilots rarely follow set F-tow routes (also known as volts or F-tow traffic circuits), but fly a slightly different route with each tow in the direction of the best thermals in order to avoid the noise for the airfield residents on the ground - especially on thermally good days, where a lot is flown - to be distributed as best as possible.
With double towing, which is rarely used , two gliders are towed on tow ropes of different lengths behind a tow plane. They are staggered laterally and in height during the tow. The front glider takes off first and climbs slightly sideways to the tow plane against the wind direction (i.e. with the cross wind). The rear aircraft flies laterally offset in the windward direction (in the crosswind). A double tow represents a high workload in the cockpit for glider pilots. That is why in Germany experience of at least 50 aircraft tow starts is a prerequisite for a double tow.
Double tow, troika tow and multiple tow
There are also the variants with several aircraft, which were used for cargo gliders in World War II . Three Messerschmitt Bf 110 pulled a Messerschmitt Me 321 . This towing method was called troika towing. Since it was very accident-prone, it was soon abandoned.
Two or three sailors can also be pulled behind a tow plane, for example three SZD-30 pirates behind a Wilga or even five pirates behind an Antonow An-2 . In 2006, nine L-13 Blaniks were towed behind a Z-137 T in Slovakia . Also in the USSR in the 1930s up to nine gliders were towed with a twin-engine TB-1 bomber.
The catch tow is also relatively common, in which the object to be towed ( tow banner or, earlier, cargo glider ) is picked up by a flying aircraft. For this purpose, the tow rope lying on the ground is laid out in a loop and hung loosely between two masts, while the tow plane only has a short rope with a hook to take up the tow rope lying on the ground. This results in horizontal accelerations of over 2 g, and thus higher than with a rubber rope start . In order to dampen the start, either plastically deformable elements are attached to the tow rope, or the rope is unwound from a braked rope drum. Similar to an inelastic impact , part of the kinetic energy has to be converted into internal energy , either as plastic deformation by the rope or as frictional heat by a braked drum. An elastic tow rope would not be suitable because the contraction after take-off could throw the glider in front of the towing machine.
One method tried and tested by the Air Force for the recovery of DFS 230 cargo ships was the so-called catch box. Intermediate floors of different thicknesses made of plywood were installed in a wooden box , between which the tow rope was laid in an S-shape. When the rope was tensioned, these intermediate floors were destroyed one after the other, reducing the energy.
Catch towing was one of the few ways to tow gliders out of short fields and was used for gliders until shortly after the Second World War. At the end of the development phase, the tow was developed to the point that it was even used for training flights at times.
For most aircraft tows, the glider can fly at a slower speed than the tow machine and has a higher quality wing. A towing is possible in which the glider is located above the towing machine and takes over part of its weight through the rope. Both aircraft are connected at the center of gravity and the glider flies like a kite 10–20 m above the towing machine, slightly offset backwards. With this technique, the towing machine can take off earlier because some of the lift is generated by the towed glider, and rise faster because the glider has a better glide ratio , so that it can generate the lift it has taken over with less drag. The lifting tow is particularly favorable when the towing machine flies at the loading limit and would therefore fly with an unfavorably high angle of attack during normal towing.
The theoretical basis of this tractor was published in 1941 by Nils Hiorch and further developed by Akaflieg Munich ; the first attempts at towing were carried out in 1944 with a Klemm Kl 25 and a Mü 17 . Further developments arose when towing the DFS 230 cargo glider with a Junkers Ju 87 B-1. On many flights the glider was so stable that it was sufficient to control either the towing machine or the glider. Compared to the traditional towing of the DFS 230 with the Ju 87, the rate of climb in the lifting tow was about twice as high (5–7 m / s versus 2–3 m / s). In addition, the top height, range and top speed improved with the lifting tow.
The carrier tow is, so to speak, the reverse of the lifting tow. The aircraft to be towed has two tow couplings at the side of the center of gravity on the upper side of the wing. The tow plane has the same couplings on the underside. The tow plane takes off first and begins a flat climb while the towed plane is pulled in a position below the tow plane. From a certain rope angle, the tow plane then carries the towed device upwards. This method was used in the combination of He 111 / Ba 349 "Snake" , whereby the He 111 carried the "Snake" which was still flightless at this flight speed upwards. A controlled landing is also possible with this method. Once the towed device has landed safely, it is disengaged and the tow plane makes a normal landing.
Ropeless towing method
When towing a drawbar, a drawbar of the tow plane is hooked onto the aircraft to be towed, similar to a truck trailer on its truck. This also enabled F-tows to be carried out blindly or at night (combination Ju 52 / DFS 230 )
With piggyback or mistletoe towing, the aircraft to be towed is attached to the towing aircraft with a mechanical device. The first piggyback ride took place in England on May 17, 1917. It was from a Porte Baby I a Bristol Scout C towed into the air and successfully separated. In the Second World War, after successful attempts at DFS, the mistletoe teams were created . These included flight tests in which the towing machine was attached to the glider or the aircraft to be towed, e.g. B. a Kl 35 or a Fw 56 "Stößer" on the cargo glider DFS 230 . These constructions were airworthy, but not self-launching, so they were launched by an additional towing machine on the rope. Later a Bf 109 E was put on, which had enough power to start the combination on its own. Up to five fighters were attached to a TB-3 in the 1930s ( Project Sweno ). The NASA used this method to their space shuttle with a modified Boeing 747 to tow. The USSR also used this procedure for its Buran ferry.
In the 1920s and 1930s piggyback towing was also attempted with airships . The first tests were carried out in 1917 with the airship L 35 (LZ 80). This towing was then practiced in Germany, England and the USA for a little over a decade with both gliders and powered aircraft. After the accident involving the LZ 129 “Hindenburg” , there were no further tows on airships.
The advantage of piggyback tow is either to extend the range of the aircraft being towed or to operate aircraft that are unable to take off.
- Beckmann's Sportlexikon, A-Z . Otto Beckmann publishing house, Leipzig / Vienna 1933.
- Georg Brütting: Start types . In: Wolf Hirth (Hrsg.): Handbuch des Segelfliegens . Franckh'sche Verlagsbuchhandlung, Stuttgart 1938.
- Gerhard Fieseler : My path in the sky . (Autobiography). Bertelsmann Verlag, Munich 1979, ISBN 3-453-01539-8 (unabridged paperback edition HEYNE-BUCH No. 6037).
- Easter Monday at the Kassel airfield . In: Kassel Latest News . April 20, 1927, p. 2. Supplement .
- Rolf Nagel, Thorsten Bauer: Kassel and the aviation industry since 1923. History (s), people, technology . A. Bernecker Verlag GmbH, Melsungen 2015, ISBN 978-3-87064-147-4 .
- Ernst Peter: The aircraft tow from the beginning until today. Development - Methods - Practice - Projects . Motorbuch, Stuttgart 1981, ISBN 978-3-87943-781-8 .
- Antonius Raab : Raab flies - memories of an aviation pioneer . (Autobiography). Konkret Literatur Verlag, Hamburg 1984, ISBN 3-922144-32-2 .
- European manufacturers exhibit at Aero 83 . In: Flight International . tape 123 , no. 3849 . IPC Transport Press, London February 12, 1983, p. 451 ( flightglobal.com [PDF; accessed January 23, 2012]).
- Motor glider towing research project - background information on towing with touring motor gliders on the DG-Flugzeugbau website
- List of the microlight aircraft approved for banner and glider towing in Germany on the DAeC website
- ↑ Raab flies , pp. 91-101
- ↑ a b Kassel and the aviation industry since 1923 . P. 39 ff.
- ↑ Kassel Latest News . April 20, 1927, 2nd supplement.
- ^ Beckmanns Sport Lexikon, A – Z
- ↑ a b c Start types , pp. 124–129
- ↑ Raab flies . P. 94.
- ^ A b Hermann Ruthard, Theo Erb: glider instruments . In: Wolf Hirth (Hrsg.): Handbuch des Segelfliegens . Franckh'sche Verlagsbuchhandlung, Stuttgart 1938, p. 113-123 .
- ↑ Peter Riedel: Training in aircraft tow . In: Wolf Hirth (Hrsg.): Handbuch des Segelfliegens . Franckh'sche Verlagsbuchhandlung, Stuttgart 1938, p. 147-152 .
- ↑ Towing types for gliders
- ↑ a b c d e Ernst Peter: The aircraft tow from the beginning until today. Motorbuch Verlag Stuttgart, 1981.
- ↑ German Hang Glider Association : dhv.de UL tow with hang gliders
- ↑ Chronicle. (No longer available online.) In: hb-flugtechnik.at. HB-Flugtechnik , archived from the original on February 8, 2012 ; accessed on August 18, 2019 .
- ↑ European manufacturers exhibit at Aero 83 . In: Flight International . tape 123 , no. 3849 . IPC Transport Press, London February 12, 1983, p. 451 ( flightglobal.com [PDF; accessed January 23, 2012]).
- ↑ Akaflieg Munich: Samburo XP - not a high-performance device ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , accessed February 17, 2011.
- ↑ How it all started ... History of the Samburo on the manufacturer's website, accessed February 17, 2011.
- ↑ NfL II-81/01.
- ↑ 2. DVLuftPersV: Appendix 5B Curriculum for practical training for the acquisition of a license for glider pilots (to Section 8) , accessed on November 27, 2015.
- ^ German Aero Club e. V. - Glider Flying Commission (ed.): Glider Flying Sports Regulations . January 2001 ( daec.de [PDF; accessed on February 18, 2011] Changes as of September 2009).
- Jump up ↑ Five SZD-30 pirates being towed by an Antonov An-2
- ↑ Video of the 9-way tow 2006
- ↑ Catching tow of a glider