About the Project
The Little Jet Company make bespoke large-scale UAV aircraft for those that are inspired by a love of aviation. Each aircraft is individually crafted to meet the clients’ requirements and all the parts are designed and assembled in the UK.
The US NAVY YF2Y-1 Sea Dart: designed by Convair in the early 1950’s was the world’s first and last supersonic water-based interceptor. It was the only seaplane to fly beyond the speed of sound. It was made to take-off and land on water to overcome the problems with jet planes taking off and landing on aircraft carriers. Only 5 of these prototypes were built and only 4 remaining, after one disintegrated in a tragic accident.
The Challenge
The Little Jet Company was approached by a client to commission the design and build of the Convair Sea Dart on a smaller scale that would have the ability to be controlled remotely. This lead to the first step in the process which began with the manufacture of a replica static model produced by Fighteraces, whom produced the model from a pack of 1950’s drawings of the original design. The dimensions from the drawings were scaled down by a quarter, which became the ‘Master’ in which the entire external panel work tooling were produced from.
3D Scanning
PES Scanning came into the process using a high fidelity 3D scanner to capture the surface geometry of the Master Model. The model geometry was captured in the form of point cloud data which is polygonised to provide a facetted 3D stl file. The stl file format is flexible as it allows you to directly 3D print from, but it is currently not suitable for reverse engineering, as stl files are not high-quality CAD surfaces that can be manipulated in most modern CAD tools.
Imagine a series of tiny flat faces that when viewed on a larger scale appear as a single smooth surface. The resolution of the captured data will change the size and amount of these facets. For example, a CAD surface would not be facetted in this way and the surface finish of tooling manufactured direct from CAD can have an automotive bodywork quality finish to it, this would not be possible with a facetted model. Also, post manipulation and optimisation of the surface profile and other such CAD operations is also made possible with CAD surfaces. The GOM scanner is impressively accurate for example, a 600mm measurement volume can produce a 250-micron (0.250mm) point spacing/resolution.
PES Performance
The engineers at PES Performance began the process of taking the facetted data and surfacing this so that the body can be shelled to the desired thickness in order to match the laminate layup of the scale aircraft’s outer panel work.
In the meantime, the Master model was then used to emboss the tooling with an impression of the Sea Dart geometry which was then used to replicate parts. In this case we are only producing one aircraft, but this process can be used to make a much higher quantity until the surface quality of the parts degrades; at which point a new set of tools would be produced again using the master model. K.S Composites produced all the composited panel work and tooling.
Use of 1951 drawings
The engineers at PES Performance used original photos and drawings from 1951 to ensure the airframe geometry, surface data and structural detail were correctly designed as per the original aircraft. The engineers had to ensure that the internal framework was structurally sound and operated in accordance with the original design brief from the client.
The time taken was made longer since the Master Model was not completely symmetrical, due to the hand-crafted nature of the model. So, instead of mirroring one side of the aircraft, which is typical of modern airframe design these days, the structure had to be individually tailored for both sides of the plane. Incidentally mirroring the left side of the model to the right side, the difference was only within a couple of millimetres; testimony to the model makers painstaking attention to detail and craftsmanship.
Aerodynamic Design
The skis were a difficult part of the design as they formed both an aerodynamic and hydrodynamic surface i.e. they had to be a smooth surface on the aircraft and also act as skis when deployed. The fact that proved this aspect of the design was difficult in the original aircraft was made obvious when the drawings and the scan of the aircraft differed slightly from each other, indicating that subtle tweaks were applied to the aircraft to ensure the mechanism worked correctly in reality.
The deployment was linked front and back, however deployed and retracted with a very troublesome compound angle. When the skis are deployed they mimic an almost water skier style position. When retracted they raise back into the body to perfectly close and seal the ski well opening. If you can only imagine the scale of challenge that this would have given the engineers at the time, especially without the use of modern CAD tools.
The engineers had to also consider the choice of materials used, as the salinity in the seawater can negatively affect different types of materials. Saltwater corrodes metal five times faster than fresh water does and the salty, humid ocean air causes metal to corrode 10 times faster than air with normal humidity.
CAD Modelling
Once the entire CAD model was complete, detailed drawings of all the machined and fabricated components were produced. This enabled the manufacturing partners to produce all the necessary components that were required to complete the entire build of the Sea Dart. Components have been manufactured from aerospace grade carbon fibre, aluminium, stainless steels and titanium. All composite components are made using pre-pregnated carbon or glass fibres.
Testing
Currently, the Little Jet Company are in the process of testing the aircraft and the Sea Dart is due for its maiden flight in 2022. See footage from the latest water trials here.