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T.6296 hub and replica propeller

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  • #16
    Calculating the pitch and lamination 2

    I spent a few days thinking about how I could calculate the pitch, and deciding how to determine the shape of lamination two, which I have cut today. I am primarily working on the first specified profile, which is about 8”, or 8 1/8” from the centre of the boss. I will be able to determine this measurement more precisely as I progress along the entire blade. Mike spent the last ten days at the museum, and took some key measurements for me. With a bit of luck, he was able to trace the edge of the boss and at least two blade trailing edges. I have been having trouble working out the exact location of the trailing edge. The information available from different parts of different propellers contradict each other. The complete T.6296 in Narromine is currently the only complete T.6296 I am aware of and have access to. Consequently, information from it will carry a lot more weight than information I have been able to source from other incomplete propellers.

    From analysing the pitch of the T.6296 it is evident that it has been designed in millimeters. The pitch, 3040mm is very close to 10’. I am using 19mm Radiata Pine. Interestingly, other Royal Aircraft Factory drawings I have specify 3/4” timber boards. The difference between 3/4” and 19mm is negligible, but there is a difference. Although 3/4” timber is specified, I am very confident that the Royal Aircraft Factory engineers used 3/4” interchangeably with 19mm. To complete a full revolution, the propeller needs to travel forward exactly the equivalent of 160 laminations, which are 19mm thick. This also gives a very nice figure to work with, 2.25 degrees of rotation per lamination. I cut some pieces of timber which hinge at the centre of the boss and extend to the first profile, 8” from the centre. Because I am not yet sure of the exact measurement, I have cut these pieces of timber a bit longer, at 8 1/4”. Later, I will cut and / or sand them to a much more precise measurement. I am very happy with the pitch at the first profile. I can check it mathematically before I determine the size of the blade profile at that location.
    Attached Files
    Last edited by Garuda; 01-15-2021, 09:50 AM.

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    • #17
      Estimated shape of section 1

      I have drawn the pitch at the correct angle and drawn the estimated shape of the propeller blades at section 1, approximately 8 1/8” from the centre. I have a photo of a T.6296 which has been cut off at this location, or very close to it. It is a good match. I will check my patterns against the original T.6296 when I am able to travel and study it. This area of the propeller is where I have the least information, so determining the blade sections as I get closer to the tip will be much easier.
      Attached Files

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      • #18
        Here's another on ebay.
        'R.A.F. Wooden propeller hub from a WW1 British Reconnaissance/Light Bomber 1916'
        That's the heading I can't seem to find the item number.

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        • #19
          RAF propeller hub on eBay

          Thank you very much for letting me know. I searched eBay, using the phrase you posted but I was not able to find it. If you would not mind copying the link, pressing the Manage Attachments button and pasting it into the Upload File from a URL box, that would be greatly appreciated, thank you. I see plenty of them on eBay, but I can't always afford them!

          Thanks,

          David.

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          • #20
            Every time I try and post the link it says it's spam and won't let me.
            The phrase is copied and pasted from the ebay listing and works when I copied and pasted it back.
            Maybe try ebayuk.

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            • #21
              T6296 boss on eBay UK

              Thank you very much for the tip. As suggested, I was able to find it by searching an eBay UK page. It is a very nice example of a T.6296 hub, but the £395 price tag is a problem! I will post some photos, and I will post a link to the listing if anyone is interested. The diameter and pitch figures stamped on this boss are different to those stamped on other T.6296 bosses I have seen. The diameter, at 2971mm is only 1mm more than the 2970 I observed stamped on another boss, but the 3170mm pitch is 130mm more than the 3040 I observed on the other boss. Does anyone know what the standard pitch and diameter are for a T.6296? Although it does not make a lot of difference to the shapes of each profile and the propeller overall, these are differences I would like to investigate. Presently I am constructing these T.6296 patterns with a view to manufacturing static reproduction propellers, but in the future I plan to build T.6296 and other propeller types for flying reproduction aircraft, if there is a market for them. Such a difference in the pitch in particular will become an issue for a propeller constructed for flight.
              Attached Files
              Last edited by Garuda; 01-22-2021, 06:29 AM. Reason: different pitch and diameter specifications

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              • #22
                Laminations 1 - 7, up to section AA

                I have shaped the undersides of laminations 1,2,3,4,5,6 and 7 from the edge of the boss up to a point about 2" outboard of Section AA. It is oversized for two reasons. Firstly, I don’t know the exact shapes and dimensions in this area, and secondly I don’t know the exact angle and position of the blade as it emerges from the boss. With every relic and drawing I study for clues I am continually revising the shapes and dimensions. Although I don’t know the exact dimensions yet I’m getting continually closer to determining them.

                The portion of blade I have shaped up to, about 2" outboard of Section AA is what I call the extra profile. It was not originally specified by the Royal Aircraft Factory when the T.6296 was drawn, but it has proven to be a very useful extra piece of information, since I am reverse engineering the design of this propeller without the convenience of access to a drawing of it. I measured the angle of the underside of the blade at two other locations. In addition I have an original T.6296 propeller blade tip. The more reference material I am able to obtain, the easier this task becomes.
                Attached Files
                Last edited by Garuda; 01-31-2021, 09:01 PM. Reason: correction to information relating to sectional shapes

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                • #23
                  Using the Augmented reality app to sketch lamination 8

                  I was very pleasantly surprised by the accuracy I was able to attain using the augmented app to sketch the shape of lamination 8. I have a rough idea of the shape and position of lamination 8 from shaping the other laminations. Royal Aircraft Factory propeller laminations are numbered from the rear of the propeller. I know that the T.6296 has 8 laminations, from studying the broken T.6296 blade. Bosses and blades I have studied are typically 5 1/2” tall, but I suspect that the original timber used was 8 laminations of 3/4” to give a thickness of 6”. The shrinkage noted in the length of the blade is far less than the shrinkage I have noted in the thickness of the blades and existing bosses. This is a known phenomenon.

                  The augmented reality app proved very difficult to use, but I am very happy with the results. It requires quite a knack to use effectively but I believe the results are very accurate. I can zoom in to a much greater degree than is possible with the iPhone alone. I can also vary the opacity. This is vital, since the only reference points I have to work from are the centre screw and the two screws placed where the bolt holes will eventually be drilled. The extended centreline of these holes viewed in front and rear elevation become the centreline of the propeller. For this reason it is vital that the holes are drilled accurately. When aligning the laminations I had issues with the alignment between the laminations and the jig I made to measure the progression of each lamination. I was easily able to measure these angles on the broken blade which I measured in 2011. Fortunately for me the half laps are a weak point of the design and this, of course is exactly where the blade broke off at the boss. I will explain the error, and the method I used to overcome it in the following few posts.
                  Attached Files
                  Last edited by Garuda; 02-01-2021, 01:05 AM.

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                  • #24
                    Alignment of bolt holes and laminations

                    The first photo shows the screws as I was fixing the laminations together. I have screwed these laminations together and unscrewed them many times. I will have to repeat this many more times. Whenever I am looking at the correct alignment of the laminations I check that the screws are straight. If they are screwed in at an angle, which can happen very easily if I am not paying sufficient attention, it affects the accuracy of the alignment of the laminations.

                    The second photo shows the progression of each lamination. The angle between lamination one and eight is much larger than I would have expected it to be. It is, however correct, at least for Darracq built T.6296 propellers, since the blade I measured was manufactured by Darracq. It will be absolutely fantastic if I am able to find a T.6296 drawing. Other RAF drawings make reference to a separate drawing of the boss. I think I will be lucky enough to be able to find a T.6296 drawing, much less the prescribed drawing for the T.6296 boss. Still, even without the boss detail drawing I will be able to work the shapes and measurements out to a very reasonable degree of accuracy.

                    The third and fourth photos show the discrepancy. Incidentally, a few weeks ago I decided to move the holes slightly anticlockwise, which will resolve these issues. It is yet another example of the constant revisions necessary to tweak the shapes and dimensions, in an effort to improve the accuracy of the reproduction propeller. Once the patterns are made, I will check them against the original T.6296 in Narromine, and make further adjustments as necessary. I will, of course have to allow for shrinkage. I have noted shrinkage of original relics of up to 12%.
                    Attached Files
                    Last edited by Garuda; 02-01-2021, 12:46 PM. Reason: spelling mistake

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                    • #25
                      Misalignment of laminations one and two

                      The following three photos show the misalignment of laminations one and two. Once the holes are relocated, slightly anti clockwise, as viewed from the rear, laminations one and two will creep backwards, in a clockwise direction.

                      Note the pencil line, visible in the third photo which indicates the leading edge of the propeller blade up to Section AA. I will allow about a 5mm margin, for shrinkage of the original boss, which will bring the leading edge to its exact location, within about 1mm.
                      Attached Files

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                      • #26
                        This is certainly a labour of love.

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                        • #27
                          labour of love

                          Yes, it certainly is a labour of love. I helped to restore a DH9. That was a labour of love too. A friend of mine is restoring a P.40, and we agree that we would be lucky to make $0.20 per hour from our projects, and even then this will only be possible if someone purchases them!

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                          • #28
                            Originally posted by JR44 View Post
                            Here's another on ebay.
                            'R.A.F. Wooden propeller hub from a WW1 British Reconnaissance/Light Bomber 1916'
                            That's the heading I can't seem to find the item number.
                            Hmmm, how much are they selling it for or betting right now? Would be curios to know about this specific model. I will try to find more info when I get back to my property in Spain here later on today after work.
                            Last edited by Niduko; 02-12-2021, 11:00 AM.
                            Hi

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                            • #29
                              The seller seems to have doubled the buy it now price to £795 Plus £30 shipping.
                              Dreamer!

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                              • #30
                                £795!

                                Even when it was listed for £350 I was thinking to myself, how could I possibly get that much money together? Like most of us I’m sure, there are times when I have that much money to spend on something as non essential as a relic propeller boss, but this is not the case very often at all. At £795 + £30 postage / shipping it is now even more unaffordable.

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