Getting it right with a large three-bladed prop

Discussion in 'Aircraft & Aviation' started by Leif Oh, Sep 6, 2004.

  1. Leif Oh

    Leif Oh Member

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    This weekend I built a large three-bladed Hamilton standard prop (as encountered on many WW-II fighters, and, in the four-bladed version, on bombers and civilian transport planes).

    I started from materials supplied in a regular (but up-scaled) kit, and didn't really add anything important (only a solid, instead of hollow, center-piece for the spinner).

    I'm reasonably happy with the result, although several mistakes remain quite prominent. Still, I explored a few techniques which might be worth sharing. They have to do with:

    1. Getting the spinner sufficiently well aligned on its axis, so that you could mount it on an electric motor without encountering wobbling.

    2. Hiding the joints between segments in the spinner

    3. Getting the twist and curvature of the blades reasonably right.

    I'll recount them in a couple of postings below, but first an image of the finished result, and the main tool, so we know what we are talking about.
  2. Leif Oh

    Leif Oh Member

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    1. Getting the spinner aligned

    The idea was to use a hobby drill mounted on a stand as a way of getting the axis of the spinner correctly aligned. First every part of the spinner was drilled with its 2 mm hole (which is the diameter of the electric motor's axis).

    I then turned the drill bit upside down in the chuck, and used the smooth part to mount all segments on, pushing them downwards and glueing to bottom parts, one by one. While the glue was still wet, the drill was run while holding the glued parts still, in order to make each segment align vertically.

    The completed two-tiered inner section of the spinner was then pushed off the drill and mounted on a 2 mm bolt, with nuts and washers, and the whole assembly put back into the drill, for sanding to perfect roundness and correct angles for glueing the outer parts on. This is of course the same technique as for making wheels.

    The end result so far was a perfectly aligned basic spinner segment. There was absolutely no wobbling in any dimension, while running it in the drill. (As it turned out, it was difficult to avoid introducing a tiny amount of wobbling when mounting it on the electric motor, because that axis is such a short stub - all the more reason for wanting to have it as near perfect as possible to start with.)
  3. Leif Oh

    Leif Oh Member

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    2. Painting the spinner

    After having glued on all the outer parts I tried to disguise the joint lines by using water colour mixed with white glue, and later matt acrylic varnish.

    Water colours don't cover dark lines very well. If you want a covering effect, you'll have to mix it with opaque white from a tube, and it takes a lot of experimenting to get the right shade, since you have to wait until the test paint dries to compare its shade with the original to be patched up.

    Mixing white glue with paint was very good for filling in joint lines in need of filling.

    The most successful part was mixing water colour with matt acrylic varnish. It is sort of semi-opaque. If you need to cover the joint further, you just give it another layer.

    I am not completely satisfied with the finished result, more due to clumsy glueing in the first place, however, than to touch-up painting. I'll keep experimenting with this, but I really have to learn and practise how to make better joints in the first place.
  4. Leif Oh

    Leif Oh Member

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    3. Getting the blades right

    A propeller blade is a small wing, most commonly using the simple Clark Y airfoil, with a flat underside and a curved upper surface. So I started with glueing in the bottom (back) sides of the propeller, trying to keep them flat (except of course at the center, where they are attached to the hub).

    At the same time a suitable amount of twist was introduced (roughly 45 degrees I guess, from center to tip). The angle of attack of the center parts of the propeller is much higher than that for the outer parts (in order for the angle of attack to be the same along the whole blade, once it's running and the plane also is moving forwards through the air).

    Upper (front) parts were preshaped with the most curvature in the front (leading edge) third of the blade. They were glued on to the twisted (but flat, as far as it was possible to accomplish) bottom parts in the following order: 1) centre, rounded part; 2) tips, getting the general alignment correct; 3) leading edge, working the flat bottom part slightly up towards the curved leading edge of the top part (rolling a coarse crochet needle back and forth along the bottom part of the leading edge); 4) trailing edge.

    While the glue was still semi-dry, adjustments were made to get leading and trailing edges straight (but still twisted, i.e. a straight line at an oblique angle), sighting from tip to root.

    After drying out, excess material (of the bottom, "flat", part) was sanded; joints were covered with white glue mixed with black acrylic (to get a slightly rounded joint at leading and trailing edges); and all black areas except the text & logo touched up with diluted (to blend in with the original print) black acrylic. The finished blades got two coats of matt acrylic varnish.

    This is the part were I learned most about how propellers really are made up, and what I must get right in modeling them.

    Finally the blades were joined to the spinner using Tonino's propeller making tool (I cheated, and didn't use the small parts for getting the angle of attack of each blade correct; in retrospect I regret that and will not lapse again!). The finished unit got another coating of matt varnish.

    I learned much this weekend, also about what I must get better at. Propellers take time to make, and it pays to figure out how you can divide the work up in many small stages, so that you have time to get each part really right before continuing to the next stage. Previous disasters were mainly caused by trying to make a whole blade in "one go". I no longer think that is possible.

    Leif Oh.
  5. Gil

    Gil Active Member

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    Leif,

    A great tutorial! The cursed segments are the bain of card modeling. Another fine addition to the Card Modeling Builders Library.

    I notice that you're using acrylic varnish which I recommend highly. A paste can be made using the varnish as a binder out of talc, plaster, fine saw dust, micro balloons or flour. It can be used like spackle to fill cracks and add curvature to segmented joints. You don't have to be neat just get it approximately right as you can sand out any imperfections after the mixture is dry. You can purchase acrylic hard modeling paste, of course, which is ready made for this purpose and has many other uses beside.

    Another point is that it's a good idea to varnish, shellac or lacquer the paper before applying the filler paste. It keeps the paper from "sogging out" and allows it to be sanded.

    Personally I'm glad you've undertaken research in this area as the B-17G has four of these to get right. Your blade forming is to be commended.

    Best regards, Gil
  6. jrts

    jrts Active Member

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    Leif

    Great work now I know how to stop mine from looking as if the have already crashed :lol:

    Info assimilated and will be put to very good use.

    Thanks

    Rob
  7. Tonino

    Tonino Member

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    YOU DIDN'T USE THE SMALL PARTS?????

    AAAAAAAGHHHH! :evil:

    This is a very bad stab in the back! I'm really scandalized...

    Joke! Leif, to see my tool was really and profitably used is the best of the pleasures. I really wasn't sure about posting it or not. I was figuring all the experienced modellers like you laughing a lot...

    Thanks Leif, I want to see a complete manual on paper modelling written by you. I will be the first to reserve it.
  8. barry

    barry Active Member

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    Leif and Tonino

    Thanks propellors have always baffled me and put me off airplanes.

    I love cheap tools out of cardboard.

    barry
  9. RyanShort1

    RyanShort1 Member

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    Lief,
    Great Job!
    'Nuff said.

    Ryan
  10. Willja67

    Willja67 Member

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    I saw this thread back when it was new and thought it was cool but didn't take time to take it to heart:cry: , now I'm designing the prop for my Super Corsair and was wondering if I could get some of the pics posted again? Please please please? I'm on hands and knees begging.
  11. Willja67

    Willja67 Member

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    I saw this thread back when it was new and thought it was cool but didn't take time to take it to heart:cry: , now I'm designing the prop for my Super Corsair and was wondering if I could get some of the pics posted again? Please please please? I'm on hands and knees begging.
  12. jasco

    jasco Member

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    All of your links take me to the homepage. What gives? (crying)
  13. jasco

    jasco Member

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    All of your links take me to the homepage. What gives? (crying)
  14. Texman

    Texman Guest

    Jasco,

    If you look at the date of the posts containing the links, you will see that
    they are quite old. Since those posts, there have been server crashes, DOS attacks (if I remember), and a change of hosts and board program. The reason you keep going back to the home page is those links may not have
    been reconnected, or even brought over. Peter will be the subject matter expert on this.

    Ray
  15. Texman

    Texman Guest

    Jasco,

    If you look at the date of the posts containing the links, you will see that
    they are quite old. Since those posts, there have been server crashes, DOS attacks (if I remember), and a change of hosts and board program. The reason you keep going back to the home page is those links may not have
    been reconnected, or even brought over. Peter will be the subject matter expert on this.

    Ray