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Author Topic: Comet Nickel scale Monocoupe 90 for Liam  (Read 2410 times)
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Flyenz0
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« Reply #25 on: October 30, 2015, 09:58:05 PM »

Day 14:
Finally bit the bullet and made the propeller, but can't say I'm satisfied with it. I started methodically and made sure my average bottle diameter was equal to my 3 inch propeller diameter, then opted for a 1.2 pitch to diameter ratio and got 24 degrees to the left of vertical (11:00 position) for cutting out the propeller blades. I settled on a simple hourglass shape capped by 3mm diameter half circles (the size of my handy coin) and cut out 6 blades before running out of space on my Sprite bottle cone. I made three 10mm 1/16 balsa squares for the hub, poked axle holes and glued them cross grain, then reinforced the back with a styrene disk and a 2mm long 2mm diameter styrene tube. Then given my 1.2 ratio I got a blade angle of 30 degrees at the 2/3 propeller radius from a nomogram.

Things started to go wrong when I forgot the pitch was measured at the 2/3 radius and cut a 30 degree slots at the hub, then after CA gluing the blades it was all I could do to ensure both blades had the same pitch (little chance of getting the real angle right). As it is I think it's overpitched but at least the blades balance, but there is a heavy side at right angles to the blades. Then as I was gluing the thrust button I dropped it and it rolled into the space between the floor and electric heaters where I couldn't retrieve it. Luckily there was another used pen of the same type that I quickly sawed up so the new thrust button only took half an hour to make - that's why it now looks purple in the new picture.

I think I was confused because I ended up with a propeller that unwinds clockwise viewed from the front,  and I remember I used to wind my kit propellers clockwise so they unwound counterclockwise. At least it works, just have to remember to wind the other way - all I need to do before final assembly is to paint these last parts white.

As for the tissue, I guess white would be lightest but let me describe how I weighed it. That same ruler in the pictures is my crude beam balance, and I place a 2.5g penny or 5g nickel on one side and tissue on the other. It just happened that it balanced with a penny, plus another penny halfway to the fulcrum so by the method of moments I effectively have an additional 1.25g weight. The rest was calculated given the sheet surface area stated on the pack. I don't think it'll catch weight differences between different color sheets unless I do the method of moments systematically, but even then there're errors from the penny and nickel weights of roughly the same values. The irony is my brother actually does have a digital postal scale, but it only works connected to his PC and gave so much trouble he disconnected it.
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jym6aw6
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« Reply #26 on: October 30, 2015, 11:07:44 PM »

Good luck with your new prop. Just FYI, Comet never intended for their Nickel series models to ROG, hand launch only. You might want to try prop diameters starting at 4'' and possibly working up to the 5" range.

Jim (6aw6)
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Flyenz0
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« Reply #27 on: November 01, 2015, 07:05:16 PM »

Day 15 and 16:
Had to wait for my brother's car to leave the garage so I could paint the parts white, and with temperatures just above freezing it took a while to dry between coats. Rushed it near the end, so now I need to sandpaper one blade to balance them again - left everything to dry overnight. As it is the Monocoupe is slightly nose heavy after multiple paint coats, on my crude balance beam it weighs about 6.25g (a nickel and half a penny) and is my lightest build ever.

Next day I finished the wing details and decided it was time for final assembly, so here are the pictures. I don't have any rubber yet, and was thinking of chaining stationery rubber bands 2 at a time (which the old timers did) - no propeller hook yet, I'm considering a lighter one than the safety pin as it'll be even more nose heavy.

Did no-propeller test glides while mounting the stabilizer and rudder, it rolled right as the left wingtip has washout and the other doesn't, possibly due to the number of times they had to be reglued. Shimmed the stabilizer 1/16 at the rear in addition to the upward twist of the rear fuselage from the swayback reshaping, so even after the wing was shimmed 1/16 at the rear the decalage still seemed OK. Glued the rudder front about 1/16 left of center to counteract the roll and it now glides straight indoors.

More indoor glides revealed the landing gear wire wire was rigid enough for vertical shock but not sideways, and bent backward after hard landings on a smooth wooden floor. Had to put in some toe-in as in the pictures to somewhat counteract this - will try something else next time. Outdoor glides over grass still showed a tendency for right turns but it glides better than I expected, even going 20ft after catching some wind.

I'll be heading back to the Philippines on 12 November and unfortunately won't get a chance to visit Liam in Virginia again, so I'll just Photoshop the Monocoupe with a picture of me and Liam before I conclude this build.
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Balsa Ace
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« Reply #28 on: November 01, 2015, 07:49:04 PM »

Well Done,looks great.

Scott
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jym6aw6
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« Reply #29 on: November 02, 2015, 07:38:07 AM »

+1 what Balsa Ace said, very nice model  Smiley .

Whenever a plan doesn't indicate the balance point, I use the old rule of "1/3 of the chord distance back from the L.E." Seems to work well as a starting point for flight trimming a scale model.

Jim (6aw6)
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Flyenz0
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« Reply #30 on: November 05, 2015, 06:49:24 PM »

Thanks guys, your comments are appreciated.

Day 17:
Liam's Monocoupe has made several powered flights and I'm calling it done!

After 30 or so test glides I finally made an S-hook for the propeller (not reverse as my propeller unwinds clockwise seen from the front) from the paper clip wire I rejected for landing gear, and a helpful PDF download made it straightforward. Previously I soaked a bunch of 1/16 wide office rubber bands from my brother in dish soap for 48 hours, then took two loops and chained them to another two loops with something like a square knot. I'm glad I tested this before installing in the plane because the first time I went 100 turns one propeller blade went flying off when I let go. I should have seen this coming as the hub slots are so shallow that both blades are practically just edge glued and even CA glue has limits. So after gluing it back on then reinforcing the blade joints with a coat of PVA glue, I rebent the S-hook so the right angle bend that locks the propeller has a longer tip that I now use as my winding crank - I'm really proud of that idea, and will use it for future builds with soda bottle propellers.

Flights using this rubber motor with 50-100 turns were hopelessly overpowered, as it would zoom up, stall, then glide down to a hard landing - the first time it smashed into the only rock border in the garden and broke the left wingtip again, so more superglue. With 100 turns it would take off from the driveway, but just barely rise 1/2 foot before landing. My first S-hook broke after 10 flights (from all the false starts bending) so I had to stop and make a new one.

Next I chained together just two loops and did 10 test flights with 100, 150 and 200 turns and found only the first 100 turns matter. The Monocoupe can only manage a powered glide, and will only do a weak right turn climb (chandelle) before landing so the extra turns just unwind on the ground.
I'm not wholly disappointed, it is a nickel model after all - and it does fly!

I'm using a pin to hold the rubber in the back for now, so will hunt around for a bamboo skewer substitute before my brother delivers it when he visits Liam in Virginia next weekend. It'll be a surprise housewarming present.
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Flyenz0
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« Reply #31 on: November 06, 2015, 01:31:38 PM »

OK, this will be the final post about Liam's Monocoupe 90, except for replies and such.

As promised here's a composite picture of Liam, me and the Monocoupe at their old house when he was 8 months old. The other picture is the reference I used for the Nationalist markings, you can judge whether I pulled it off. Total model related purchases comes to, wait for it, $2 for the chuck glider and tissue (with plenty left). If I throw in CA gel glue, PVA glue pen and no-name hairspray (since I almost used up my brother's CA glue) that's another $3 that's good for more models. Kits? What kits?

So, lessons learned for future builds, in no particular order:
1) Try something new at least once, but keep it simple. If I insisted on the Sturman I-152 to I-153 kitbash, I doubt if that would get finished before I had to leave. Not good.
2) For lightness 1/32 balsa laminations for curved shapes are the way to go - pre-forming is a must (rolling/crushing, steaming or soaking) for neat and clean parts, less frustration and sticky fingers.
3) I'm sold on using CA gel glue for rigid curved laminations that resist tissue shrinkage, but wingtip bows must be reinforced with triangulated stringers as they'll hit the ground often, and hard.
4) Use stiffer landing gear wire - just because it can resist end-on forces doesn't mean it can resist sideways forces as well. Or just omit landing gear entirely.
5) PVA glue pens work well for gluing tissue to frames, but not as cleanly for tissue to tissue, or card. A glue stick (or spray mount, which I found too late) might work better.
6) Balsa wheels are light and easy to make, go for it.
7) Scratchbuilding thrust bearings isn't that hard if you have scrap plastic.
Cool For light rubber motors, paper clip wire makes easy S-hooks (and you can add my winding crank too).
9) Covering top wing and stabilizer surfaces only works fine and reduces weight - if building them to fly, not as pretty static models.
10) The cracked wing rib technique works well with a Kline Fogelman style airfoil, and looks lighter and stronger than the traditional airfoil wing ribs (plus it works if the airfoil rib printwood patterns are missing).
11) Hairspray works amazingly well, combining wet shrinking and doping. Just use light coats, spray both sides at the same time, and dry in positions with good air circulation. Five coats gave a smooth flexible surface strong enough to draw markings on.
12) Propellers - they can be scratchbuilt, but stick to what works and do one that winds clockwise. My next soda bottle propeller will be at least 4 inches diameter, made outrigger style with round bamboo dowels fitting in the hub for precise setting of angles, and will probably use a minimum induced loss shape for the blades that slot into the outriggers for a stronger bond. To minimize painting I'll make it from a clear uncolored plastic bottle, so any departure from scale isn't obvious.
13) It can't hurt to scrounge around the house for stuff to use, my building board was the box the bathroom scale came in - if you've been reading the whole thread it's just been one long scrounge until the Monocoupe was finished.

Thanks for sticking around this long, and I hope you've enjoyed reading along with my build. I'm looking forward to starting my next project after I get home to the Philippines, and am planning a kitbash of a 1/35 IMAM Ro.37bis Italian biplane bomber. I've printed the Scientific and Hi-Flier plans for the Waco Military Model D, and it's amazing how similar they are to the Ro.37bis but scaled down a little. I'll have to change the cowl, top fuselage, landing gear, rudder and stabilizer but it should be a fun challenge. I think I'll use 1/16 bamboo skewers for wing leading edges, landing gear and wing struts for strength, but I also like Arno Diemer's idea of using thin aluminum sheet (from disposable pans?) for wing struts that are scale thickness, way stronger than balsa and self-aligning. I have an Italian interwar scheme in mind that's beige with a red striped top wing and a tricolor rudder.

Then in the tradition of the Sterling peanut double kits I'll follow that with a 1/35 Grumman F4F Martlet in RAF desert pink, and already have a styrene Domino's pizza sauce tub that'll work as the nose cowl. After that, who knows? I'm tempted to try Mike Stuart's Saab J29B Tunnan for twin motor infrared RC and finish it in Tiger Meet colors (a use for zebra stripe wrapping tissue!).
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Balsa Ace
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« Reply #32 on: November 07, 2015, 08:23:41 AM »

That's a nice pic of you with future balsa modeller,Liam.
The Italian biplane bomber sounds like an interesting pro-
ject.

Scott
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jym6aw6
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« Reply #33 on: November 10, 2015, 01:25:16 AM »



Next I chained together just two loops and did 10 test flights with 100, 150 and 200 turns and found only the first 100 turns matter. The Monocoupe can only manage a powered glide, and will only do a weak right turn climb (chandelle) before landing so the extra turns just unwind on the ground.


You should be able to easily get 2-3 times the 200 turns you are now getting if you stretch-wind and if the rubber bands have been stored properly. Try that and see if performance improves  Wink .

Nice pic of you and Liam with Monocoupe inset. Liam should be about old enough to be your mechanic on your next flights  Smiley .

Looking forward to your future builds. This one was a lot of fun, thanks for sharing!


Jim (6aw6)
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