Oberursel 100 HP Engine

Discussion in 'Gallery & Designs' started by Gil, Mar 2, 2004.

  1. Gil

    Gil Active Member

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    Grebgory,

    Thanks for the informaton. The color in the photograph was not corrected and tended to show color into the green bands giving the effect of a "khaki" glow. At least it got some to respond about cylinder colors and baked on castor oil. The engines took on a kind of brownish black color very similar to the outside rim of a cast iron frying pan even though the cooling fins were turned from aluminium stock (edit: they were made of forged nickel steel). Experiments show painting the base with aluminium paint followed by several coats of medium black wash misxed with a small amount of sienna gives the best effect. I'll try and take shots that are color corrected so the colors are presented fairly accurately.

    Best regards, Gil

    P.S. Further research resolved the materials issue as regards the cyclinders...., they were turned out of steel so the frying pan analogy is pertinent.
  2. Gil

    Gil Active Member

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    Hi All,

    Reduced the scale of the cylinders to 1:33 to derive building dimensions (English) as per the drawing below. Continuing to investigate best media mix effect for least work. Some have been a lot of trouble and were discarded (have learned a good method to center hole punches though). Even tried the "wrapping with thread" method to imitate the cooling fins. Monofilament fishing line was used instead of thread. Fin count came out right but the overall effect was underwhelming. A lesser fin count allows this detail to be better detected by the eye, the fins seemed to disappear in the thread method unless examined under a magnifying lense. Easiest, so far, was the "acrylic modeling paste combing method" but still does not equal the "turned acrylic modeling paste" method. This method is proving to be the best overall but does require that the modeler take the time to practice till they feel competent enough in the technique to make it work. I guess this will have to be turned into a tutorial if there's enough interest to make the work worthwhile.

    Best regards, Gil

    [​IMG]
  3. charliec

    charliec Active Member

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    May I add a small correction - the cylinders of the Gnome rotary engines were turned from solid Nickel Steel billets. The metallurgy of the time wasn't up to Austenitic steel liners with Aluminium cylinder body and fins. A rotary engine doesn't need hard cylinder liners because there isn't any reciprocating motion of the pistons within the engine. The cylinder machining was very fine - the wall thickness of the Gnome rotaries was about 0.15 in (about 4 mm) I guess the German copy of the Gnome engine was the same.

    You can see the shininess of the cylinders from the Nickel Steel in the photos of the Fokker E.III - Aluminium usually goes quite dull after a few hours of engine operation.

    Regards,

    Charlie
  4. Gil

    Gil Active Member

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    Hi Charlie,

    Quite correct, forged Nickel Steel at that. Thanks for the clarification.

    The pistons do reciprocate within the cylinder, just not in the normal sense we're used to. The cylinders of this engine type were prone to warp due to uneven cooling causing the pistons to seize. This was partly overcome by the addition of a very flexible obturator ring (to stop up or close up), made of thin bronze, above the normal piston rings.

    The wall thickness was kept thin for practical reasons of weight and higher heat transfer for cooling (which was an ever present problem). Engines of the time had very low compression ratios (5 or 6) requiring only thin walls for the cylinder. Most of the strength was required just to hold the cylinder on the crakcase due to the 100 g centripetal force acting on it.

    I've noticed from pictures that the inner portion of the cylinder is lighter and metallic in appearence while the upper working section of the piston is dark in color with highlights of metallic steel at the outer periphery of the associated cooling fins. A coat of silver-steel followed by two different wash coats of black should yield a very similar effect.

    Best regards, Gil
  5. Gil

    Gil Active Member

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    Hi All,

    Interesting the things you inadvertently learn in attaining some goal that seemed so easily attainalble when you began.

    The drawings below are the result of finally understanding how to get TurboCAD under control (as with most software). These are not intended to be shop drawings so some license has been taken in their construction. Exhaust valves, pushrods, spark plugs and ignition wires need to be added to conclude the engine drawing(s). In 3D CAD you only have one assembly drawing with all other drawings being parts of, or different views of the same. Popeller hub mounting hardware will be included with the propeller detail.

    Best regards, Gil

    [​IMG]
    [​IMG]
  6. wunwinglow

    wunwinglow Active Member

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    Forces in a Rotary engine....

    60 hp Gnome, wall thickness 1.5mm, max gas pressure contributes stress of 6000lb/sq in to the wall, centrifugal (? centripetal) is only 2100lb/sq in. Total max 8100lb/sq in. Not excessive, even for 'old' nickel chrome steels.

    Source, The Rotary Aero Engine by Andrew Nahum, published by Science Museum, HMSO books, ISBN 0 11 290452 1.

    Quite a dry read, but some excellent explanations. Debunks the Castor Oil myth, and describes why the rotary engine went from front line combat to history in less than 5 years. Lots of pics, drawings end stuff. Worth watching eBay for, as it is long out of print.

    Also, the petrol they had in those days was truly AWFUL stuff.....

    Tim
  7. Gil

    Gil Active Member

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    Tim,

    Looked on Amazons Used Books...., average price when found is around $60.00 U.S. I put in a bid to see what shows up. The castor oil caused some pilots to become inadvertent POWs when the necessity of nature required them to land immediately. The invention of the cowl was to more or less contain the continous fire that these engines put out. They did have one saving grace, they were releatively vibration free due to the cyclinders reciprocating in a circular motion.

    Best regards, Gil
  8. Gil

    Gil Active Member

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    Oberursel-U1-100 HP Rotary Engine Drawing Development

    Hello All,

    This is the final draft of the engine. Seems that the softwares' hidden line, hidden surface algorithm doesn't recognize a single line as an object (Spark Plug wire is still visible in left drawing). Adding a small radius will probably work.

    A Propeller and hub detail drawing will follow after engine design in paper (which is next). A faster computer would help reduce the rendering time for these shots (a good reason for a new box).

    Best regards, Gil

    [​IMG]
  9. Gil

    Gil Active Member

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    Experimental Cylinders Development

    [​IMG]


    Hi All,

    The photo above shows the development process history in my quest for realistic looking cylinders for the Oberursel engine and in effect for all WWI type rotary engines with good effect toward WWII air cooled types.

    The cylinder on the right uses and interesting technique...., card stock was first coated with acrylic modeling paste after which the threads of a machine screw were used to form contiinous ridges and allowed to dry. The cylinder was then cutout (with the final edge carefully taper cut with a very sharp chisel blade) rolled and the edge affixed with glue and painted.

    The middle three were made using the original cylinders as supplied as a "base". Each cylinder was cutout, rolled and and a top added, then coated with acrylic modeling paste and allowed to dry overnight. Each was then mounted on a tapered mandrel in a Dremel tool and rough shaped with a sanding file before using a metal thread gauge to "lathe out" the cooling fins. Various tools were tried to deepen and increase the fin definition with varying degrees of success. The second from the left almost became the standard for making the rest before I let the whole experiment rest a day or two to think about progress to date when a better method occured to me.

    One of the issues is the way in which many designers make their cylinders by overwrapping layers to build up the contour. The problem is that all use a "step" to the cooling fins giving the cylinders a "fat" cylinder top connected to a "skinny" cylinder bottom. By using a rectangle followed by a tapered quadrilateral roll the profile of the cylinder can be more closely approximated using 24# paper. Application of a line down the center of the inside of the cylinder also helps in keeping the roll, and hence, the cylinder profile accurate. The rolled result was topped with a leather/paper punch punchout and allowed to dry. It was then mounted on a tapered bamboo mandrel in a Dremel tool. The Dremel was powered on slow speed and the cylinder was then dipped into the acrylic modeling paste. A thread gauge was used to directly "lathe" the ridges on the cylinder. The "paste" will begin to harden while being worked on the mandrel which suits the work pace beautifully. The top was finished with applying the thread gauge at a right angle to the spin axis. The assembly was then allowed to dry for several hours before paint was applied.

    This completes the search for a fairly easy, straight forward and repeatable method for building cylinders at the 1:32 scale level.

    Crankcase development now begins.

    Best regards, Gil
  10. Gil

    Gil Active Member

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    Paper Only Cylinder Head

    [​IMG]

    Hi All,

    As I was preparing the final drawings for the Oberursel U1 cylinders I decided to try one last "all paper" cylinder design (24# inkjet print paper used). The result was rather suprising and makes me wonder whether the "acrylic paste lathing method" is worth the effort in light of the overall effect that this construct has. I didn't use the same mathematically derived cutout for any of the acrylic trials and probably need to do that before making any real decisions. A paper only design is a "purist" approach and may have its place amongst card model designs. The cylinder is 0.340 inches tall by 0.202 inches wide and is mounted on a 3 mm diameter mandrel for scale reference (yes, it is that precise(see former drawing in this thread)).

    What do you think? Let me know.

    Best regards, Gil

    P.S. Note that both seams were purposely photographed to show.
  11. Pierre

    Pierre New Member

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    Looks pretty realistic to me. How did you do it exactly?

    Pierre
  12. Gil

    Gil Active Member

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    Hi Pierre,

    I'll post construction details later on today.

    Best regards, Gil

    Here's the post:

    [​IMG]
  13. Sticky Fingers

    Sticky Fingers Member

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    Gil, was in the local hobby shop yesterday and he had a Williams Brothers 1/8th scale Rhone rotary along with a Wright Radial. I'm tempted just for the heck off it!
  14. Gil

    Gil Active Member

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    Mark,

    I have the Le Rhone kit. Take a look at Williams Bros. site. Their engine renditions are very good. Sooner or later I'll build the kit after which I can acquire the J5, the engine that jump started the Golden Age of aviation.

    Best regards, Gil

    P.S. The technique represented above can be adapted easily for other radial engine types includeing the J5....,
  15. NOBI

    NOBI Active Member

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    That's really helpful formula for designer especially me :) , thank you very much Gil
  16. Gil

    Gil Active Member

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    Hi Nobi,

    Your thanks is my reward!

    I feel this technique can, with a little imagination, be applied to a wide area of shapes in card modeling. I'm beginning to think that designers need to segregate or at least stipulate the thickness of the paper or card stock to be used as this formula shows that the length varies inversely to the thickness of the paper.

    Best regards, Gil

    P.S. I edited the original to correct scale errors in the dimensioning...,
  17. Peter H

    Peter H Member

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    Outstanding concept and documentation!!!

    I store few items offline for reference purposes but your construction page is a "must have".

    *Many thanks* for putting the time into writing it all up.

    I looked at your "Beta" cylinder and it's texture reminded me of a sand cast, cast iron cylinder, I'm not sure if the texture/look qualifies as your expression of the cylinder for radial engine. How does it go, "beauty is in the eye of the beholder".

    My thoughts are this technique would be magic for cylinders on motor bikes upto the late 1920's. I've had a secret desire to build a Henderson and it's cylinders were very close to your's in shape and form.
  18. wunwinglow

    wunwinglow Active Member

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    Thanks Gil!

    You could use this technique for any round solid object; I'm thinking tyres, life rafts, etc.

    Tim
  19. Gil

    Gil Active Member

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    Thanks All!

    I've one last go at an alternate technique that just has to be tried. It will be pictures if successfull, words only if it's not.

    The print colors of the test cylinder were medium grey background and dark purple lines for simulating the cooling fins. The actual cooling fin count is scale and should be printed on the smoothest lightweight stock you can find as the paper grain begins to break up the printing if too coarse. I'll post the whole engine in the parts bin when I'm finished so everyone can experiment to their hearts delight...., additional work on coloring needs to be done. I've noticed that the color of the radials went from shiny bright in the center to dull burnt umber/asphaltum at the ends of the cylinders which is what I'll try for the Fokker EIIIs Oberursel. Hopefully the effort will be discernable to the eye. We're getting near the limits of printer technology..., which has just given me an idea. The long axis of the cylinder roll was printed vertically on the printer. Rotating this 90 degreees to horizontal will avail better resolution on the inkjet. Ruling the entire cylinder with lines and then modifying each for luminace and color might be key to increasing the realism even further. Need to try this....,

    Peter, these cylinders should look great on old mototcycle engines as is. The grey should match the motorcycles frame members and go well in "matching in" to that theme.

    Tim, the mandrel shape can be changed to fit the situation. I like the life raft application. You also bring out another good point, the insded edge doesn't necessarilly have to be the widest edge. The "thickness" can vary over the roll length to achieve the cross section desired, like that of a life raft on the side of a destroyer. Would really add to the ovrall effect.

    One other thought befor I go, These cylinders are easily made and can be easily repeated by novices of the art. An important design criteria for card models.

    Best regards, Gil
  20. Gil

    Gil Active Member

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    All Paper Oberursel Cylinder

    Hi All,

    Did a little more work on the cylinder in Adobe Illustrator and the following is the result. It looks as if its been machined to the unaided eye. Color is a little off due to fluorescent lighting. Seam has been included as usual.

    One last experiment to try and then on to the crankcase. Some think this would make a good standalone model but at a larger scale..., the effects may not scale properly. An engine stand to display it would of course be needed. See how distracting this stuff can be!

    Best regards, Gil