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Author Topic: Miller "Little Gem" build for FAC Goodyear mass launch  (Read 6873 times)
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MKelly
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« Reply #75 on: July 27, 2017, 08:53:16 PM »

John,

The blue tape is just holding down some plastic wrap I put over the standoff sticks to keep the doped tissue from sticking.

I'm going to give the dope several days to cure with the flying surfaces restrained in hopes of precluding major warps.  In the meantime today I played with propellers.  I've got two candidate props for this model - a 5" Tern and a 5.5" North Pacific.  The Tern did a great job pulling my A2D Skyshark around, which has similar weight and wing area to the Little Gem.  The North Pacific is half a gram lighter, which may be important given all the weight this model already has up front.  I made an extra noseblock and spinner, so I can build up both and see which works best.

The Tern prop was white, and the NP prop was red - neither of which would do for this model given that the original had a natural metal prop at the race.  For grins, I tried a product I'd read about for years in plastic modeling circles, Rub-n-Buff.  It is a wax-based silver finish that you rub in until dry, then polish to the desired luster.  Although a bit messy, it worked great!  We'll see how the finish holds up over time.

The s-hook prop shafts are a new product from Peck - nicely made, and perfect for folks like me that don't really care for music wire gymnastics.

Mike
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OZPAF
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« Reply #76 on: July 28, 2017, 03:37:40 AM »

One of the English scale fellows used Rub n Buff on a simulated Alum cowl. It's certainly effective.

Reverse S hook Shafts from Peck - that's interesting.

Waiting patiently for your trim flights.

John
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Pete Fardell
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« Reply #77 on: July 28, 2017, 05:47:40 AM »

What a lovely model this is. "Little Gem" is right!
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MKelly
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« Reply #78 on: July 29, 2017, 05:58:02 PM »

Thanks Pete - here's hoping it flies well.  Not much to show for the last few days - working on the canopy tonight.

Mike
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« Reply #79 on: July 30, 2017, 02:31:17 PM »

Fabbed and fitted the canopy and pilot. 

I wanted the canopy frame to match the tissue best as I could, so I chalked a piece of tissue, glued that to layout paper with spray adhesive and doped that.  Cut out the inner area where the glass goes, then glued the frame onto clear giftwrap plastic using Micro Krystal Clear (my favorite canopy glue).  Once dry I cut the frame and glass to shape, rolled it on a pencil to get the curves, then trial-fitted it to the fuselage.

I made a profile pilot, copying the image of Bob Downey from the picture Mark B. posted here of Little Gem at Reno in 1964.  I made it double-sided by mirroring the picture in PowerPoint and glued it in.

The real canopy is a single piece over the top, and sits a bit proud of the fuselage at the top rear.  I couldn't form that part of the frame with the rest of the canopy as a single piece, so I did it separately.  After the canopy was glued on, I trimmed off the top paper frame.  I cut a canopy top from the doped tissue sandwich, then burnished it over a firm rubber pad to coax as much complex curve as I could get into it.  Glued that on and burnished the edges to make it as smooth as possible.  Not perfect, but it looks pretty good.

Mike
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MKelly
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« Reply #80 on: July 30, 2017, 04:25:20 PM »

Finally got to the point of assembly!  The fin, stab and wings all went on fairly easily.  I used the magnet board to block up the fuselage square, measured the height of the wing at the root, then glued the wings on and blocked the tips up to match the plan's 3/4" dihedral per panel.  Seems like it's been a long haul to get to this point...

I really like the magnet board for this kind of work - makes it much easier to block everything in place and double-check measurements and alignment.

Cheers,

Mike
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MKelly
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« Reply #81 on: August 06, 2017, 12:13:33 AM »

Home stretch.  Built up a tailwheel and fitted it - the model has the rudder stretched a bit, so the tailwheel mounts a little aft of scale in order to line up with the back end of the rudder.

I put off working on the model for about a week thinking about how to mount the spinner.  Decided to fit a backplate to help line the spinner up and hold it in place.  Cut the backplate from scrap .015 styrene using a circle cutter, trimmed a flat on the back of the prop hub, then glued the backplate to the prop.  Next was a long session of fit-trim-fit again to cut slots in the spinner for the prop blades.  Once I was mostly happy with the fit I bent and trimmed the prop shaft, then glued on the spinner and fiddled with it to make it run as close to true as I could get.  It looks pretty good - still need to match some paint to the white tissue color and paint the spinner to get rid of the plastic look.

With the prop and tailwheel fitted and the motor peg installed the cg is about 1/4" forward of where it should be.  The hook is about 3" forward of the cg and the peg is about 5" back, so it should come in pretty close.  I haven't glued the cheeks on yet, so I can fit a motor, check balance and if necessary hollow the cheeks out a bit more to lighten the front end.

Mike
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MKelly
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« Reply #82 on: August 06, 2017, 06:10:14 PM »

Complete!  Ground another tenth of a gram out of the cheeks, drilled holes for the exhausts and glued the cheeks to the model.  Final step was to insert sections of black heat-shrink tubing in the cheeks to represent the exhausts.  20.75 grams without rubber, for an empty wing loading of .40g/sq in.  May still need a bit of tail weight, we'll see once the rubber is in.  Not much calm weather forecast for the next few days, so not sure when I'll get to start trimming.

This one was a bit of a chore at times, but it looks pretty neat all put together.

Mike
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Pete Fardell
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« Reply #83 on: August 06, 2017, 06:23:00 PM »

Really nice job! I love these little Goodyears but don't think I've ever seen this one modelled till now. You've really done it justice. Bet it will fly a treat too.
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MKelly
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« Reply #84 on: August 06, 2017, 06:53:17 PM »

Thanks Pete!  This airplane is a great subject as it was raced from 1949 into the 1970s, with multiple rebuilds/configurations and a number of great color schemes.  Tony Proux did a peanut plan for the early 1950s configuration, and Walt Mooney's plan is a great starting point for the configurations from 1957 on.  The Society of Air Racing Historians Facebook page that Mark Braunlich pointed me to earlier in this thread has a bunch of great pictures of the aircraft across the years (see the 1954 pic below).  I'm tempted to do another from the Proux plan in that blue/grey color scheme after I've recovered from this build and finished the next few projects.

Cheers,

Mike
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« Reply #85 on: August 10, 2017, 05:50:39 AM »

The wheel spats amaze me a bit Mike. I guess it's because I'm a lousy carver and your spats almost look like they came out of a mould.

Really fine effort.

John
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MKelly
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« Reply #86 on: August 10, 2017, 10:26:35 PM »

Thanks John. After the Waco and this one I think my favorite thing about the Tigercat I'm building now is that it doesn't have landing gear...

Cheers,

Mike
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Mark Braunlich
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« Reply #87 on: August 10, 2017, 10:49:20 PM »

Early versions drawing.
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« Reply #88 on: August 10, 2017, 11:07:45 PM »

Thanks Mark - great drawing!  I had a drawing that showed the side view of the 1953 bubble-canopy version, but this drawing is much better.

Mike
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Mark Braunlich
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« Reply #89 on: August 17, 2017, 10:30:15 PM »

Mike,
This is a fairly well known photo of the 1949 midgets from the roof of a hangar in Cleveland.   Notice #14 in lower right corner, Little Gem in it's earliest racing configuration with it's first wing.  Can't find any more views of it in '49, have you seen any?
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« Reply #90 on: August 19, 2017, 08:28:28 PM »

Mark,

That's the only one I've seen with the '40s wing. 

Mike
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MKelly
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« Reply #91 on: August 19, 2017, 09:15:48 PM »

Finally got a free day with some calm skies to put this one in the air.  It has promise, but needs a little more sorting out. 

Started out with a dummy nose plug (no prop or motor) ballasted to put the cg right on the spar where the McCombs numbers said it should be (this is about 1/16" ahead of the plan cg).  A few glides got the stab shimmed for a decent flat glide. 

Today's flights used a 12" loop of 1/8" rubber (1.5x hook-peg).  Put in the motor and prop, checked balance and repeated the glides.  Made a few tosses with hand winds, didn't see anything dramatic, so I put the Gem on the stooge and started working up in winds, selected flights here:  https://youtu.be/qvXd7KMct2g

Pitch trim wasn't consistent through the flights - everything on the model looks straight, and the stab shims aren't moving around, but twice, after a couple decent flights, the model climbed normally, started downwind cruise, then gradually pitched over into a power dive, breaking off the left wheel pant on impact.  After the first incident I played a bit with upthrust - a little bit may be helpful (second dive was after I took all the upthrust back out), but not much, as even a tiny shim had a big effect (see the 4th and 5th flights in the video).

The model also is spiral diving right when the power runs out - will add some left wingtip weight to see if I can tame that, and a maybe little left rudder if also needed.

I double-checked the balance this evening after repairing the wheel pants and it's still right at the spar.

Not a perfect first outing, but I think this will fly like a gem (ha!) once sorted - hope to get another calm day soon to work on it a little more.  Any observations or suggestions would be most welcome.

Cheers,

Mike
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« Reply #92 on: August 20, 2017, 04:24:26 AM »

I dont like to sentence on youtube films because it is to easy to loose the proportions and see things which never happened. And also because I like to fly against torque and not with it, right that is.

But maybe one impression looking at the film, what about tightening the flight path climbing in a somewhat smaller circle. From the headline I see the model is intended to fly in a competition and with such a wide flight path it will immediately getting out of sight on a light wind day. And a tighter flight circle will help a lot to stabilise the airplane.

My 1-pence worth.

Urs
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« Reply #93 on: August 20, 2017, 06:22:49 AM »

Mike I looked at the flights before reading your comments.  Saw the right spiral as the torque came off.  in fact you can see it wanting to straighten even while turning left.  whatever you have on to counteract the torque is causing the issue.  I think my first line of enquiry would be some right thrust and take off the right rudder or warp.
Regards Mike
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« Reply #94 on: August 20, 2017, 10:31:08 AM »

Ok, so first thing to do I think is to check that you have adequate tail volume and the correct cg. You can use of one the various TVO/cg calculators to do this. When you have confirmed that your stab is large enough (TVo in the 0.65 to 0.75)  take the prop off, remove rubber, balance using a bit of clay and work on the glide. You probably know what you are looking for: a floating glide to the left.

When you have the glide sorted you can start working on the power pattern. I fly scale models left like the Czechs (it is generally more stable in my experience) so I use differential warps, less washout on the inboard (left) wing and more on the outboard wing. I typically run the thrust line through the vertical CG, which on a model like this means little or no down thrust. The wing should be ~2 degrees positive relative to the thrust line. I would use a bit of right thrust to counter torque at max turns and prevent the left spiral. I use a small gurney flap on the rudder to initiate the right turn in glide.

This is how I would approach it. You start with the calculation because you can't assume that the person who drew the plan actually ran the numbers... a lot of trimming issues for rubber models are caused by undersized stabs and incorrect CG locations. When you know the Two and CG are good you start with the glide to ensure that the model is in a good aerodynamic trim so that you do not end up solving flight surface problems with thrust line adjustments.

This is how we trim F1B and F1G, old timers, and scale rubber models.

hope this helps.
BG
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« Reply #95 on: August 20, 2017, 02:03:36 PM »

Urs, Mike, BG - thanks for the inputs.  I got in some more flying this morning - a combination of less Gurney flap on the right wing, a good-sized Gurney flap on the left side of the rudder, a little more stab nose-up trim and some right and just a little up-thrust has generated some improvement.  With the rudder adjustment the left turn is tighter and the spiral at the end of the motor run is gone.  With the right thrust the initial climb is OK up to about 800 turns, above that it still torque rolls left into the ground, albeit more gently.  Model climbs well but still hangs on the prop a bit under power.

The torque roll above 800 turns tells me I should try a smaller cross-section motor - will try a 12" loop of 3/32.  If the torque roll persists I'll add a little more right thrust.

Transition to glide is still finicky.  Had a few flights with a decent glide, a few others where the model stalled at the end of the motor run, pitched down and didn't recover.  Glide from hand launch is really nice with and without prop, will recover from a fast launch with just a slight stall.  Transition on the problem powered flights looked like the model stalled and the stab never took effect - maybe blanked by stalled airflow over the wing or cheeks?

BG,  tail volume on this model is way small - about .37.  The wing is so big (~50 sq in - see pic below) you'd have to make the stab about 10" span (at the same chord as the plans) to get tail volume up to .65, or add an inch on either side and half an inch extra chord.  CG is spot on the McCombs calculated position (about 1/16" ahead of the plan position). I suspect the small tail volume has a lot to do with the erratic pitch performance.  The Gurney flap under the right wing should accomplish about the same effect as your differential washout.  Regarding your comments on down thrust, the noseblock had about 2 deg downthrust built in, the upthrust shims I've added are reducing that built-in downthrust.  Tell me more about your "Gurney flap on the rudder to initiate the right turn in the glide" - my (limited) experience has been that for low/mid-wing models I've had to add left rudder to tame the right spiral dive after power runs out.

Thanks again for all the observations and suggestions!

Cheers,

Mike
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« Reply #96 on: August 20, 2017, 02:15:21 PM »

This has to be one of my favourite builds of the moment  Smiley
Enjoyed watching the video through FB earlier.
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« Reply #97 on: August 20, 2017, 02:21:42 PM »

I know Mike will have it going even better very soon, He really is building and flying very nicely. Thanks Mike for the effort .
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« Reply #98 on: August 20, 2017, 03:03:29 PM »

Mike,
I must start by saying the Little Gem is and the flying looks promising too.
I must start by saying I am with Urs, I fly rubber models right under power and a correct sized power circle always seem to improve longitudinal stability as well but I will keep off that track.
She is obviously unsettled longitudinally which almost certainly will mean moving the cg forward. However I notice on most small scale models the tailplane have thin 'flat plate' tailplane sections which are liable to flow breakaway as soon as lift is demanded from them. I suggest trying a Gurney strip the full length of the TP trailing edge which ar least will give some semblance of a section. (just changing the incidence may cause breakaway even sooner.
Don't waste time putting weights on wing tips they have little effect compared to warps and tabs.
I don't understand the right spiral when the power runs out, is the freewheel working?
You are moving so fast with testing I had better stop here and wait for the latest report.
John
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« Reply #99 on: August 20, 2017, 09:30:57 PM »

Thanks guys.

John,  I've struggled getting low-wing models to fly right - forcing the right turn under power seems to exacerbate the spiral dive after power runs out.  I'm probably not doing it correctly...  I assume you're suggesting the Gurney flap on the upper side of the stab, to generate a nose-up moment with less negative stab incidence?

Here are the best two flights from this morning's session:  https://youtu.be/MU8AeirGj6s

Both are a little stally under power, but the turn is better and the model climbs fairly well.  Note the glide at the end of the power run on the first flight, and the stall-dive-tuck under after power runs out on the second flight.  Although the CG is right where the tail volume says it should be, I'm thinking I need just a little bit of nose weight.

Thoughts?

Cheers,

Mike
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