“A design for the delivery of humanitarian food aid from an airborne platform”
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Aerodynamics and internal layout
There is no escaping the fact that the DBX does not have an aerodynamic shape. The internal contents will be packed in the lower 2/3rds of the DBX which will ensure the center of gravity is below the mid point. The flaps at the base of the DBX are designed to fold up on the outside of the two sides and lid. This ensures the air flows over the sides and base and does not try to pull the DBX to pieces.
The 1m square flat end may cause instability but it may be possible to secure some sort of curved profile to improve airflow. Should this be necessary, the profile could be manufactured from maize or similar and coated with a layer of some material to prevent disintegration in flight. I guess the more “bomb like” this can be the more stable the descent.
The dead space inside the top 1/3rd of the DBX needs to be filled with packing material and I know that an edible type similar to polystyrene chips does exist.
Toggles
The key to the DBX is the use of a toggle system that keeps the DBX together in flight and allows the contents to be ejected.
Fig shows the toggle assembly. There are three components: nylon washer, nylon pin and domed star fastener. The domed star fastener is similar to those used as retainers on plastic & steel shafts and can be found on modern printers, children's bikes etc. Once fitted, a star fastener is almost impossible to remove.
The domed star fastener is placed over the end of the nylon pin to form a peg. The peg has a 4mm hole drilled at the other end which will accept 3mm steel cable. A washer will be glued to the peg and the sub assembly pushed through and fixed to the appropriate side of the Triwall. The opposite side of the fixing point will have a washer secured to the Triwall. Once the peg is located, steel cable is threaded through the hole to secure the assembly and two layers of Triwall.
This construction method ensures that when the DBX cables are removed there is not a shower of loose components raining down on the ground.
The operation of the toggles and steel cable is crucial to the way the DBX functions. When fully assembled the DBX will be a strong structure but remove all the cables from the pegs and the box will collapse. The motive force for this will come from the main parachute deployment and various groups of toggles will be released in turn. When all cables are clear the parachute bridal will apply considerable tension to the web straps located on the rear of the DBX. At this point the DBX's velocity will be severely retarded and, as the DBX is not held together with anything, the contents should be ejected downwards.
Although there will be many toggles to release the actual force required will be quite small. Steel cable running in nylon with silicone grease will reduce friction to a minimum. A staggered release will also help matters but I am confident that the force available will be substantially more than required.GPS & Telemetry
Successful deployment of this system is Dependant on releasing the toggles at the correct height and the subsequent dispersal of food packs. It may seem to be just a question of using a GPS to achieve this but a GPS will only give height information based on the distance between the receiver and MSL (mean sea level). It will not give height above the actual terrain.
As the earth is not a true sphere and is more like an oval, the GPS needs to use a system to determine exactly where sea level should be. Various standard models of the globe have been produced but the most common is called WGS-84. I would say that the majority of receivers base their calculations on this model.
It is vital that we obtain accurate terrain height to allow the system controller to release the toggles at 1000'. It is very important to ensure the DBX GPS and ground control GPS are using the same datum (WGS-84) else there will be inaccuracies and the system will not operate as planned.
The GPS module is hot of the production line and I have purchased direct from Taiwan. Although there are many different types available I needed one that would continue to function at any orientation. When the DB is ejected from the aircraft it will most likely tumble before stabilizing so any loss of tracking is not desirable.
The new design may help to solve another problem, namely lack of GPS signals inside the aircraft fuselage. For normal aircraft operation this is not an issue as there will be an externally mounted antenna with a clear view of the sky. The DBX will have the GPS mounted at the top of the unit and several DBX's will be together. The DBX's will be horizontal until deployed so providing the worst conditions.
During my original design stage for the Drift Marker I identified a system that will “repeat” the GPS signals inside the aircraft. This GPS will quite happily use these signals to give position information based on the location of the repeater aerial. These repeaters were extremely expensive at the time but, thanks to the explosion in car satnav systems, they are down to a few hundred pounds each. Various models are available and the most suitable will broadcast over a distance of about 30'. The repeater GPS antenna can be positioned anywhere in the aircraft so long as the collecting antenna and repeater antenna cannot “see” each other. The system does not have to be permanently mounted to the aircraft.
The control system will provide a series of indicators to show correct operation of the GPS and also a set of dial switches that will be set at the deployment height. The controller will determine the correct altitude for release based on this figure. Should the DBX system be successful, these switches will be replaced with a non contact method of setting the initial height.
Telemetry
During the testing phase it will be vital to have various parameters transmitted to a ground based receiver to monitor the decent and deployment. This will be achieved using a radio modem. The link will be bi-directional so there will be a facility to send release commands in the unlikely event of the internal control system malfunctioning.
The radio modem operates at 2.4GHz. Other frequencies are available but that one is licensed for use in the UK. Although licensing is not of particular importance when deploying in a real situation I feel it is better to keep the DTI happy at this stage
