Hot Air Blimp

Spherical hot-air balloons experience the same stability problems as helium balloons. If they are free flying, they are simply uncontrollable, and in tethered mode they start turning erratically even at quite low wind velocities. In addition, the internal pressure inside the hot-air balloon needs to be

FIGURE 8-14 Urban, industrial scene, Kansas City, Kansas, United States. Missouri River on right side. Blimp was launched from a small park next to the river. Taken from Aber and Aber (2009, fig. 58).

relatively high in order to prevent deformation of the envelope in light wind. Therefore, hot-air balloons are not well suited as camera platforms. For these reasons, it is no surprise that this kind of camera platform is almost forgotten nowadays, although it was the first platform used for aerial photography by Gaspard Tournachon (see Chapter 1). In Jahnke (1993) valuable hints on the construction and operation of model hot-air balloons can be found.

Tethered hot-air systems as camera platforms have emerged since the late 1970s particularly in archaeology. In this field of research, a high need exists for quickly producible, detailed images for documenting excavations in remote areas (Fig. 8-15; Heckes, 1987). The hot-air blimp

FIGURE 8-15 In research cooperation of the German Archaeological Institute (DAI) with the German Mining Museum Bochum (DBM) and Frankfurt University, the hot-air blimp is used as a medium-format camera platform for documenting archaeological excavations at the Sabaean city of Sirwah, Yemen. Photo by U. Kapp.

FIGURE 8-15 In research cooperation of the German Archaeological Institute (DAI) with the German Mining Museum Bochum (DBM) and Frankfurt University, the hot-air blimp is used as a medium-format camera platform for documenting archaeological excavations at the Sabaean city of Sirwah, Yemen. Photo by U. Kapp.

Tethered Mode Balloons

FIGURE 8-16 Small 100 m3 hot-air blimp built by GEFA-Flug. (A) Carrying capacity and wind susceptibility of this model reach their limits in the alpine conditions of the Spanish Pyrenees. (B) Small packing size makes it easy to transport to remote regions such as high mountain ranges; no single part is heavier than 15 kg. Photos by IM, 1996.

FIGURE 8-16 Small 100 m3 hot-air blimp built by GEFA-Flug. (A) Carrying capacity and wind susceptibility of this model reach their limits in the alpine conditions of the Spanish Pyrenees. (B) Small packing size makes it easy to transport to remote regions such as high mountain ranges; no single part is heavier than 15 kg. Photos by IM, 1996.

introduced by Busemeyer (1987, 1994), which was also used by Wanzke (1984) for the documentation of excavations in Mohenjo-Daro in Pakistan, is the prototype on which all such blimps constructed by GEFA-Flug (Aachen, Germany) are based. These hot-air blimps combine the principle of an open hot-air system with an elongated egg-shaped blimp form equipped with tail empennages.

In comparison to the spherical shape of balloons, the blimp is considerably more stable in the air. Owing to the streamlined shape and tail empennages, the blimp is much more aerodynamic and aligns itself with the wind. Thus, the air vehicle becomes much easier to control from the ground even with a light breeze. In case of increasing wind, the envelope may get pushed in at the nose, but a safe landing is still possible as this affects the uplift properties only slightly. The danger for an ignition of the envelope owing to a wind gust blowing the fabric into the burner flame is rather low for the blimp in contrast to a balloon. The concept and

FIGURE 8-17 The hot-air ''Goethe monitoring blimp'' carries the logo of Johann Wolfgang Goethe University, Frankfurt am Main, Germany. Blimp and frame constructed by GEFA-Flug and by the technical workshop staff of the Faculty of Geoscience and Geography. Adapted from Ries and Marzolff (2003, fig. 2).

FIGURE 8-17 The hot-air ''Goethe monitoring blimp'' carries the logo of Johann Wolfgang Goethe University, Frankfurt am Main, Germany. Blimp and frame constructed by GEFA-Flug and by the technical workshop staff of the Faculty of Geoscience and Geography. Adapted from Ries and Marzolff (2003, fig. 2).

functionality of this airship, its construction, and its use for aerial photographic surveys are presented and discussed in detail by Marzolff (1999).

Two of the authors, IM and JBR, have experimented during the last 15 years with two different sizes of hot-air blimps, a smaller one with about 100 m and a load capacity of 5 kg and a larger one with 220 m3 providing a carrying capacity of approximately 25-40 kg. The smaller version ranges at the lower border for model airships, but has the crucial advantage that the individual parts can be transported by one person each (Fig. 8-16). Only four people are necessary to carry the equipment even to remote study areas. Unfortunately, the smaller model turns out to be comparatively susceptible to wind influence because the net-lifting capacity of only 5 kg does not leave a wide scope for "heating against the wind.''

In the following sections, the most important components and their functioning are explained using the larger hot-air blimp—the "Goethe monitoring blimp'' belonging to the Department of Physical Geography of Frankfurt's Johann Wolfgang Goethe University (Fig. 8-17). The monitoring blimp consists of four main components—the envelope, burner frame with camera mounting, remote-control device, and tether ropes. Additional equipment for the inflation phase includes a tarpaulin, a large inflation fan and miscellaneous tools.

Blimp envelope: The envelope consists of tearproof, polyurethane-coated, 52 g/m2 heavy, rip-stop nylon and has a length of 12 m, a height of 6 m, and a width of 5 m when inflated. The airship has a volume of 220 m3 and does not need any additional inner stabilization. For the three tail empennages a specific airflow is necessary. On top of the blimp the hot air is drawn off into a tube that is externally sewn onto the envelope. Through this tube, the air reaches the distributing chamber which is separated from the main body in the rear end. From there the hot air is distributed into the three empennages. These fins are characterized by an unfavorable ratio of large cooling surface to low volume; therefore, they can be kept in shape only with a constant influx of hot air.

At the bottom of the combustible envelope, the so-called scoop is attached, which is made of fire-resistant Nomex material and connects the blimp body to the burner frame. It is open to the front in order to allow an inflow of outside air to compensate for the permanent air loss through the envelope seams. At the scoop, two steel cables encompassing the envelope end in metal chains to which the burner frame is fixed with carabiners.

Burner frame with camera mount: The burner frame consists of a steel-tube frame with a collapsible burner panel, an exchangeable gas bottle, and a plug-in camera unit (Fig. 8-18). Four 100,000 kcal/h Carat liquid phase burners produce flames of about 90 cm height. They are capable of heating up the air inside the envelope to a maximum of 140 °C. The burners are screwed on the

FIGURE 8-18 JBR testing the blimp burner before the survey. All burner functions such as ignition, valves, and pilot flame, as well as leak-tightness of the gas tubes need to be thoroughly checked. G, main gas bottle; P, pilot gas bottle; C, control box with receiver; B, battery packs. The empty space next to the gas bottle is for the camera unit (see Fig. 7-15). Photo by A. Kalisch.

FIGURE 8-18 JBR testing the blimp burner before the survey. All burner functions such as ignition, valves, and pilot flame, as well as leak-tightness of the gas tubes need to be thoroughly checked. G, main gas bottle; P, pilot gas bottle; C, control box with receiver; B, battery packs. The empty space next to the gas bottle is for the camera unit (see Fig. 7-15). Photo by A. Kalisch.

burner panel beneath which the electronic control with two magnetic valves is situated. The gas is kept in an aluminum bottle with a net weight of 5.6 kg and a filling capacity of 11 kg of gas. The gas bottle is strapped underneath the burner but easy to retrieve. The gas is extracted in the liquid phase with an immersion pipe and led to the valves through a steel-reinforced tube equipped with a gas filter. Additionally, the burners are supplied with a mixture of propane and butane from a commercially available 0.7 L multigas cylinder. This gas is used to produce an approximately 20-cm-high pilot flame, which is permanently ablaze in all burners and can be ignited by an electronically controlled piezo igniter.

As for conventional manned balloon-flight (Federal Aviation Administration, 2007), uplift is achieved by intermittent, not permanent heating. When the magnetic valve of the burner is opened by remote control, liquid gas

FIGURE 8-19 (A) Goethe blimp during a survey in the Hoya de Baza, Province of Granada, Spain. (B) Blimp is navigated with two tether lines usually kept parallel. In difficult terrain and during landing, the two pilots (here JBR and M. Seeger) may take turns in holding the airship. In order to keep the lines taut, constant reeling and unreeling is required while the blimp changes altitude. (C) IM with the remote-control device operating ignition, heating, and camera functions. Photos by JBR, IM, and A. Kalisch.

flows through the burner-coils, streams out the orifice, and is ignited by the pilot flame. The burner valves and the piezo igniter are powered by 12-V rechargeable batteries, whereas the remote-control receiver and the pilot-flame valve are supplied by 6-V rechargeable batteries. The burner frame is equipped with a manual control that enables opening of the piezo igniter and the main burner valve during the inflation process. The remote control is not used until the blimp floats and is ready for departure. The gas supply is sufficient for approximately 45 minutes of flight.

The net carrying capacity of the hot-air blimp ranges between 25 and 40 kg depending on ambient temperature and the filling level inside the gas bottle. The system is particularly suitable for different flight altitudes, especially regarding the fact that it becomes increasingly lighter during the flight owing to gas consumption. Accordingly, in the beginning of the flight the lower, and later on the higher flying heights should be scheduled. The camera mount is inserted into the burner frame as a separate plug-in unit (see Chapter 7.3.4); it sits underneath the burner and next to the gas bottle. Power supply for the mount is established automatically with a plug connection. The cameras are protected from damage in rough landings by a robust wicker basket or polystyrene-walled aluminum frame, open at the bottom to ensure free sight for the cameras (see Figs. 7-13 and 7-14).

Remote control: Piezo igniter, pilot flame, and main burner valve as well as camera servos and camera trigger are all operated with a commercial remote-control device (e.g. Graupner mc 16/20). The remote control is equipped with a pulse code modulation (PCM) fail-safe control that is programmed to close all valves automatically in case of problems with radio reception, external signals, or a failure of the transmitter. This is quite important in order to make sure that uncontrolled burning can be avoided—permanent burner blasts would lead to overheating and possible inflaming of the envelope. The radio signals are transmitted on the frequency band locally used for model aircraft (e.g. 40 MHz in Spain).

Tether lines: Being a captive airship, the blimp position can be controlled from the ground only by means of two captive tethers (Fig. 8-19). For this purpose extremely light ropes made from PE-coated Dyneema are used, an extremely strong polyethylene fiber with low elongation and twist and high breaking strength. The ropes are fixed to the envelope via a trapezoid of lines attached to the bow and front flanks of the blimp body. A rubber expander in the

FIGURE 8-20 Launching the blimp. (A) Envelope is filled with cold air. At least four people are needed to hold the nose, tail, and back to the ground. A light breeze is enough to make the blimp writhe like a stranded whale. (B) Once enough space has filled inside the body, the burner is positioned to face the blimp spine and the air is heated with intermittent blasts. (C) As the envelope is released and rises, the most critical phase of the launch begins. The uplift force is already strong, but the blimp still needs to be held tightly and heating discontinued until the burner frame is securely fixed to the scoop. (D) Fully inflated airship in launch position. After testing all valves again, the camera unit is attached and tested. The blimp is quickly heated then and lifts up to a safe height of >30 m. Photos by JBR and IM.

FIGURE 8-20 Launching the blimp. (A) Envelope is filled with cold air. At least four people are needed to hold the nose, tail, and back to the ground. A light breeze is enough to make the blimp writhe like a stranded whale. (B) Once enough space has filled inside the body, the burner is positioned to face the blimp spine and the air is heated with intermittent blasts. (C) As the envelope is released and rises, the most critical phase of the launch begins. The uplift force is already strong, but the blimp still needs to be held tightly and heating discontinued until the burner frame is securely fixed to the scoop. (D) Fully inflated airship in launch position. After testing all valves again, the camera unit is attached and tested. The blimp is quickly heated then and lifts up to a safe height of >30 m. Photos by JBR and IM.

FIGURE 8-21 The blimp is turned upside down after the survey for deflation, and the delicate envelope is swiftly recovered and packed to avoid further exposure to the sun and wind. Photo by V. Butzen.

trapezoid minimizes the jerky pulls that are passed onto the airship during fast maneuvers by the ground crew; thus, constant nicking of the blimp nose can be prevented. The rope of 500 m length is wound on a wooden reel and permits a flying altitude of approximately 350 m. When in flight, the blimp's bow always points toward the wind; this enables stable maneuvering and free influx of fresh air into the blimp. Additionally a third rope, the so-called plumb line, hangs down vertically from the envelope. It is marked every 5 m in order to allow an estimation of the flying height and indicates the current position of the airship (approximate image center) to the crew on the ground.

Launching and landing: For cold inflation, the blimp envelope is spread out on two fabric-reinforced, 50 m2 plastic tarpaulins and inflated with fresh air by means of a 5.5 PS fan of the kind frequently used for manned balloons (Fig. 8-20A). Afterwards the air is heated up with the burner, and as soon as the blimp floats upright in the air the camera mount unit is inserted into the burner frame (Fig. 8-20B-D). A minimum of five people is advisable for helping with these tasks—one each holding the envelope at bow, top, and rear for ensuring that no "free-floating" fabric is blown into the burner flames, and two at the mouth of the blimp for holding it open and operating the fan and later the burner system. Particularly in the hot inflation phase the absence of wind is crucial in order to avoid damage by fire.

After the survey, the blimp is landed on the tarpaulin, first removing the camera mount, then the burner frame. Consequently, the blimp is relieved of its carrying burden and needs to be secured well by the tether lines again, especially in light wind. The hot air keeps the weight of the envelope aloft long enough for a change of gas bottle if the survey is to be continued. If not, the blimp is quickly turned upside down to let the hot air escape (Fig. 8-21). A velcro-fastened opening in the rear allows releasing the air from the empennages, and the air in the main body is carefully squeezed out through the mouth before the envelope is packed into its canvas bag.

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100 Photography Tips

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