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Yesterday I shaped the underside of the patterns up to section BB. The shape is slightly oversized, and approximate at this stage. In the first photo you can see where I checked the blade shape and angle against the “extra” profile. It is very flat. Most RAF propellers have a curved underside until Section CC or DD. The T.6296 is unusual from the point of view that it seems to transition from a symmetrical section at Section AA to a flat underside approximately 2” outboard of Section AA. Although it does not seem possible, I was able to achieve this without any sharp bends or awkward transitions. It will be interesting to check the complete T.6296 to make sure that this is correct. I am very confident that my patterns will be very close. Unfortunately I have not been able to locate any photographs taken at the time. I suspect they are on an iPhone 4, which crashed. I have not been able to retrieve any data from this phone, but I will try to at least salvage the photos I took of the T.6296 blade.
As a result of the lost data I do not have a record of the shapes of the top surface of the blade at Section BB. Starting from Section CC to the tip I have quite good measurements. The blade at Section BB was difficult to measure, so I was relying on photographic evidence. Still, I am very confident that I will be able to determine the shape of the blade out to Section CC. I will of course check these shapes and dimensions against the complete T.6296 at Narromine when I am able to get up there.
The second photo shows the underside of the blade out to Section BB. The third photo shows how close I was able to get to pencil line on top of lamination 8, when I smoothed the laminations using information I had used to work upwards from laminations 1 through to 8. I am happy with the proximity to the pencil line. There is only a 3mm or so gap, which is about the amount I expect the original boss would have shrunk over the last century, hence the discrepancy. Closer to the boss the margin is much larger. I suspect I need to do a lot more sanding and shaping to obtain the correct shape. The fourth photo shows a quick check of the incidence at Section BB. The underside is slightly more shallow angle, which is ok because I will be able to rectify this with further sanding. Also, particularly towards the boss the leading edge radius tends to be larger than the trailing edge radius. As a result, the underside of the blade, even if it is flat tends to be at a shallower angle than the angle of incidence. The centreline is determined by the centres of the leading and trailing edge radii, not necessarily by the angle indicated by the underside of the propeller. When the leading edge radius is the same as the trailing edge radius, the flat underside is equivalent to the angle of incidence. It can be misleading if the principals used when designing these propellers are not known.
As a result of the lost data I do not have a record of the shapes of the top surface of the blade at Section BB. Starting from Section CC to the tip I have quite good measurements. The blade at Section BB was difficult to measure, so I was relying on photographic evidence. Still, I am very confident that I will be able to determine the shape of the blade out to Section CC. I will of course check these shapes and dimensions against the complete T.6296 at Narromine when I am able to get up there.
The second photo shows the underside of the blade out to Section BB. The third photo shows how close I was able to get to pencil line on top of lamination 8, when I smoothed the laminations using information I had used to work upwards from laminations 1 through to 8. I am happy with the proximity to the pencil line. There is only a 3mm or so gap, which is about the amount I expect the original boss would have shrunk over the last century, hence the discrepancy. Closer to the boss the margin is much larger. I suspect I need to do a lot more sanding and shaping to obtain the correct shape. The fourth photo shows a quick check of the incidence at Section BB. The underside is slightly more shallow angle, which is ok because I will be able to rectify this with further sanding. Also, particularly towards the boss the leading edge radius tends to be larger than the trailing edge radius. As a result, the underside of the blade, even if it is flat tends to be at a shallower angle than the angle of incidence. The centreline is determined by the centres of the leading and trailing edge radii, not necessarily by the angle indicated by the underside of the propeller. When the leading edge radius is the same as the trailing edge radius, the flat underside is equivalent to the angle of incidence. It can be misleading if the principals used when designing these propellers are not known.
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