Designing the F2G

Discussion in 'Gallery & Designs' started by Willja67, Jan 31, 2006.

  1. Willja67

    Willja67 Member

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    Yeeehaaa!

    I just had a eureka moment and thought I'd share it. The alierons on the corsair are a quite complex in shape and I have been trying to make them as accurate as possible and they have been frustrating me no end.

    Near as I could tell the hinge line had to be almost underneath the surface of the wing for the leading edge not to hit the top skin of the wing when deflected downward and I could not figure out how it was designed, then today while thumbing through the Corsair in action book I came across a factory picture showing corsairs on the assembly line with the wings up and glory of all glories(drum roll) without the little fairings on the underside of the alierons that cover the bottom of the hinges where they protrude below the bottom skin. The actual pivot points are still inside the alieron(barely) but the structure hangs down slightly. It was so nice to know I wasn't imagining things and that there was a logical explantion.

    Most of you probably think this is a silly thing to get worked up over but I'm dancing a jig!
  2. Willja67

    Willja67 Member

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    This perhaps might help you visualize the stuff in the previous post. It's not the pic I mentioned but should suffice.

    [​IMG]

    I've had this image for awhile and it supported my theory that the hinge line had to be pretty low but without being able to see the underside I couldn't know for sure how far down the hinges actually hung.

    This pic shows the little fairings that I mentioned and how much of the alieron is actually in front of the hinge line.

    [​IMG]

    I used to think that those fairings were weights to balance the alieron to avoid flutter. For those of you unfamiliar with aerodynamics flutter is potentially disasterous and one of the key considerations to avoid flutter is making sure that most of the weight is ahead of the hinge line. That's the reason for all the tabs on the control surfaces on many aircraft, also the reason that most control surfaces of American aircraft of the period were fabric covered(reduced weight behind the hinge line).
  3. Willja67

    Willja67 Member

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    Skinning the outer wing panels part1

    In Paul Matt's drawings he provides three airfoil shapes the smallest of which he labels as "theoretical tip" which I understand to mean would be the shape if the wing didn't get rounded and tapered at the tips(see below).

    [​IMG]

    A little tweaking was necessary to get it to fit tghen it was put on the end of the spar and lofted.

    [​IMG]

    [​IMG]

    Next I copied the outline of the outer wing panel and placed it directly over top of the wing structure then extruded the curve and used that to trim off the excess.

    [​IMG]

    [​IMG]

    [​IMG]

    Next I did the same for the lower surface of the wing. Since the wing is tapered on the bottom (note blue line between top and bottom surfaces) I extended the line that was drawn on the spar where the rib would intersect it and copied it twice placing one in front of the wing and one behind (using ortho so they were in line)then I lofted those 2 lines to make a cutting plane and chopped off the bottom wingtip. That was kind of confusing but I hope the pics will be adequate to explain it.

    [​IMG]

    As you can see the bottom trailing edge doesn't line up with the top trailing edge as it goes towards the tip.

    [​IMG]

    This was not unexpected as it had happened on the previous wing design. This time I will try a slightly different approach to overcoming that problem but you all will have to wait for the next installment to see how I attempt that.

    I would really appreciate some feedback here from the Rhino users. Was my method for trimming the wing a good one (extruding the outline to use for trimming purposes) or is there a better way that I'm not aware of?
  4. Gil

    Gil Active Member

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    Wilja,

    Looked at the Paul Matt drawings. I wouldn't worry too much about the absolute accuracy of the airfoil cross section. Just getting it approximately close is good enough for the purposes of modeling in card just so long as it "sockets" into the rest of the structures.

    The F4U wing has a straight leading and trailing edge tapered toward a nicely formed tip bow. I assume that you're using the standard wing assembly which joins the surface at the trailing edge. I usually take the root chord and scale it for the tip chord aligning the leading edge with both chords as well as the trailing edge with the chords "end points". The bow outline is drawn on the CP surface so that it is flat. It is then "oriented" by using the two point method to the leading and trailing edge tip chord "knots". Angle the tip bow slightly up with reference to the parallel leading and railing edges using 2D rotation (this wasn't done in the image example below). The chord sections are then split into upper and lower halves at the leading and trailing edge knots. Rebuild (under the edit command) all chord halves and the tip bow to 10 points, degree 3. Loft all surfaces with the normal straight line loft. All surfaces lofted in this manner will also unroll properly.

    The tip bows can be fudged back into the unfolded wing halves to make aligning the assembly pieces during construction easier by positioning the leading edge to the tip chord leading edge knots. The radius of the bow is something that can be accomplished with a .5-1 mm thick structure under the tip bow surfaces with which the surface can be worked with a burnishing tool to obtain a nicely rounded appearence. Hope that this all makes sense.

    -Gil

    [​IMG]
  5. Willja67

    Willja67 Member

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    Not all of it makes sense but I'm sure it will if you could answer a few questions. I'm assuming CP means construction plane?

    As for editing I'm only vaguly familiar with those commands and have always wondered exactly what the degree option actually did.

    From your comments it seems that you don't think I constructed the wing surfaces properly ie they are not the correct shape but I am at a loss to understand at what point they start being wrong(sorry I'm an idiot but I hope you'll have patience).

    This was one of the reasons I started this thread so that others could look at my struggling attempts and take pitty and say "Uh no, like this". I'll admit I know very little about Rhino's capabilities and am always willing to listen to others advice.
  6. Gil

    Gil Active Member

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    Wilja,

    1.) CP is the Construction Plane.
    2.) Curves are generated mathematically by the use of a polynomial equation. The number of points indicates the number of edit points in the rebuilt line. A degree 1 will yield a line consisting of straight line segments, degree 2 yields circular and elliptical etc. An increased number of points creates a higher number of isocurves on lofts derived from the curve. Keeping the number low is a good strategy as the increased number also increases the compute time required to redraw the line and will slow the redraw times in complicated drawings. Creating curves with the same number of points yields better lofting results with 10 being a pretty good number.
    3.) There is nothing wrong with your method although you will have problems when trying it out in a paper prototype. Card wings can deceive the casual observer into lookiing perfectly normal until an attempt is made to construct them. One of the more difficult thought processes that needs to be conquered is to think in "paper". Imagine the paper to be only flexible in one axis and not in the other planar direction without unsightly folds and wrinkles. You extruded the paper through two chords then trimmed them with a vertical tip box plane intersect. You made an airfoil sectioned tube. Now pinching the top and bottom surfaces together to form the taper at the wing tip will work but the trailing edge will not match up well and wrinkles will form toward the leading edge due to that area deviating most from the correct fit. The nice thing about straight lofts is that they will always build correctly. Your method deviates by lofting a shape which will need to be corrected at assembly time. It might be instructive to build a section as it now exists to see how the design effects ease of assembly. Looking at the F4U wing shows that the tip requires a separate section as the tip bow undersurface is angled up slightly toward the tip. This indicates that a separate loft will be necessary to accomodate this section (loft from the end sectional chord to a tip bow curve). Note that it may be possible to leave the top surface alone and only loft the lower surace to accomodate the original. Keeping the surface simple is always a good idea and this is probably the best tactic to use on this section.

    -Gil
  7. Willja67

    Willja67 Member

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    Thanks I never quite understood that before. As for my wing I think you will be satisfied that I thought of the paper needs when you see the next installment. I consider the method I use (I've tried it but haven't documented it yet) to be quite brilliant.
  8. Willja67

    Willja67 Member

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    Okay documentation has been completed on the method mentioned, but first I'll show my first try that was not very satisfactory.

    [​IMG]

    With a lot of tweaking I had managed to make this work(on my first design) but it wasn't very good, so here is the really pretty way to do it.

    First I decided on a line to chop off the back portion of the underside of the wing and decided to put that line(in yellow) basically where the flap and alieron are split off from the wing.

    [​IMG]

    Then I placed the line overtop the wing extruded it and used it as the cutting plane to chop off the trailing edge of the underside of the wing.

    [​IMG]

    Then I drew in the lines on the trailing edge of the top skin and lofted the cut edge of the bottom.

    [​IMG]

    [​IMG]

    Then to get the aft portion of the wing tip I lofted the curve on the wingtip and the edges of the 2 surfaces I have been fiddling with.

    [​IMG]

    The front piece of wingtip is going to be really fun(said in a very sarcastic way) and I'm not entirely sure yet how I'll do it.

    More to follow.

    -Will
  9. Gil

    Gil Active Member

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    If you split the chord from the leading edge through the trailing edge, split the tip bow such that it starts and ends on these points a loft of the two resulting curves will produce exactly what you want and it will still unfold.

    -Gil
  10. Willja67

    Willja67 Member

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    lower front of wing tip

    Okay I probably made this harder than it needed to be and I did so much fiddling before I got it to where I wanted it that there is no hope of being able to show how I did it. Perhaps nobody would want to see it anyhow.

    Most of my problems were caused by my not quite right drawings.when looking at the wing from the front the wing starts tapering towards the tip looking at the wing from the top the curve or bow as Gil calls it should start at exactly the same place. Mine didn't.

    [​IMG]

    The yellow line is where my curve originally started and it yielded this result:

    [​IMG]

    If done right the bow should come right to the very tip if the airfoil without jogs or abrupt changes in angle in any way. I had to modify the front of the "theoretical" airfoil to get it right and do a lot of other stuff that I probably couldn't duplicate if asked to do it again. So here is the finished product:

    [​IMG]

    I could have done it without the surface that I have highlighted and it would have meant fewer seams but I don't like the idea of that edge just being sharp. The real thing was rounded and by putting that surface in I hope I improve the look of the model but only test building will prove if I'm right or not.
  11. Gil

    Gil Active Member

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    Wilja,

    I alluded to this in an earlier post but the easiest way to get the tip edges to look real is to insert a sanded form underneath the tip surfaces. The edges are then "wetted" with PVA and burnished down or "onto" the form. The photo below shows this technique applied to a WWI wing tip. I use inserts of sheet foam which are easily cut and sanded to shape. It is an insanely easy way to do wing tips.

    -Gil

    [​IMG]
  12. Willja67

    Willja67 Member

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    Thanks Gil I was a little in the dark about what you meant so thanks for clarifying. I should probably do my models a favor and try and learn some advanced building techniques so I can incorporate them into the design.

    Are there any special considerations needed when designing something like that? Should I add maybe 1/16" to the edge of the bow so there is enough material to burnish it?
  13. John Griffin

    John Griffin Member

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    It's going great

    Willja, I can't believe how fast you seem to have picked up the Rhino stuff. Your F2G looks really good, and all the extra stuff like flaps and oil cooler throats are great.

    I'm jealous! Looking at the images you posted has spurred me to get off my rear and finish my Tempest Mk II radial (I've been 95% done for about 3 weeks!).

    Cheers, John.
  14. Gil

    Gil Active Member

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    Wilja,

    I usually make a template for the foam which is approximately 1 mm smaller than the tip bow. Note that the trailing edge section of the insert needs to be tapered to match the section profile. You can also use thick cardstock but it is a lot more trouble to form than is the foam. The foam can be derived from commonly availiable vacuum molded food packaging such as meat trays or Mc Donald's hamburger containers.

    -Gil
  15. Willja67

    Willja67 Member

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    @John I'm glad this is encouraging. I've found that doing the documentation actually makes the project go faster because I have to set goals and stay focused on them, normally I jump all over the place and can never seem to finish anything. Good luck on your tempest. Why not show some screen shots I'm sure there are quite a few guys who will start drooling when they see your next incarnation of your very well received Tempest.

    @Gil, Maybe a dumb question but I guess the mold remains inside the wing? Also I just realized that the glue you were referring to isn't super glue so what exactly is PVA? I'm defineatly going to have to try this method.
  16. Bowdenja

    Bowdenja Active Member

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    PVA is plain ole white glue. Elmer's Aileen's etc.
  17. Willja67

    Willja67 Member

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    Thanks I probably should have known that:oops: But oh well. I know the thick stuff is generally preferred cause it won't soak the paper and wrinkle it, so for this application what is best thin or thick?
  18. Gil

    Gil Active Member

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    Wilja,

    PVA is Poly Vinyl Acetate or plain old white glue. I dilute it with a small amount of water to facilitate application with paint brush and hypodermic syringe squeeze bottle. Avoid the wet paper problem by sealing it with matte acrylic or lacquer spray. The form is left inside the wing as you guessed. I first glue it to the "flat" areas of both sides before beginning the burnishing process. PVA is applied to the area of both top and bottom overhanging wing surfaces and brushed out to even the application out. I usually let it "tack up" before beginning the burnishing. The slightly dried PVA acts like a contact cement that still has some workability. Working delicately but deliberately will yield filleted curves worthy of a Rhino two rail, two section loft and render...,

    -Gil

    P.S. The Krylon matte sealer will whiten out areas of the paper which can really bring on a good case of the angsts..., not to worry though, a coat or two of Krylon crystal clear will make this mal effect disappear. To tone down the clear coats gloss give it misting of the matte spray. It won't whiten as it did on the untreated paper before.
  19. Willja67

    Willja67 Member

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    Thanks for the tips Gil, now I need more help.

    The following are pics of the underside of the wingtip as it looks on the model and as Rhino unrolled it.

    [​IMG]

    [​IMG]

    As you can see on the unrolled surface the corner doesn't come to a nice point like it does before unrolling. Why? I have yet to figure out if there is anything that can be done or if I just have to deal with it.
  20. Gil

    Gil Active Member

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    Wilja,

    I guess you've already read the help topic regarding pitfalls of surface development. This may be one of them. Try reversing the order that the curves are chosen for the loft. Yes, it does make a difference sometimes. Rebuilding the two lofting curves using degree 2 instead of 3 might yield better results although the leading edge may come a little pointed requiring the curve representing it in the loft to be somewhat more detailed in the radius area of the curve. In fact try using a circular arc for the very leading edge if all else fails. It will always develop nicely when limiting the curves to degree 2...,

    -Gil