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Bolivia 4-Place Build

Done with Phase 1. The biggest relief is taking that Cherokee prop off. Seemed like the Catto has less than half the vibration of the metal prop. Should be able to get my hands on a dynamic balancer tomorrow for an even smoother ride...happy pilot!20231023_184203.jpeg

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The Catto is WAY, WAY smoother than the 76/60 Sensenich. I put the prop balancer on it and that nice smooth feeling was still .37 ips (not great). Scared to know what the other one was... Got the Catto down to .06 at 2350 rpm.

The Catto turns 2350+ static, is 10-12 mph slower at the top end but still runs 100 mph at 2500 rpm on 26 GYs, climb is at least 30% more and takeoff feels like half.

I got to shoot 7 or 8 landings with a pax yesterday . It is very much a Cub. Everything Kirk said. It stays very nose low on approach and touches down at Cub speeds.

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Out with the family today visiting mission A&P programs in Michigan. Plane performs well in these cool temps. Loaded it up with 3 pax (2000 lbs) for a couple of rides and it still gets off short (150 ft with 6kts headwind) and climbs well. Got humbled by the trademark gusty crosswinds at Andrews University, my home flight school...no respect. I guess I don't quite measure up after 15 years of landing into the wind, post graduation.

View over the nose is great on approach. Last picture is ~1900 lbs and 45-50 indicated (low to mid 50s gps). I expect it to still be good at slower approach speeds when I work into that realm.signal-2023-11-01-11-17-19-178-1.jpegsignal-2023-11-01-18-04-03-276.jpegsignal-2023-11-01-11-17-19-178.jpegIMG_3303.jpeg

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How is the trim working? Did changing the incidence fix the problem or did you work on the tail.
DENNY
 
It helped a lot. At 100 mph it still needs a little forward pressure with aft CG loading. Will the longer gear and bigger tires add drag below the thrustline and help?

My empty CG is similar to many of Kirk's builds. Some are further aft. I need to ask vj88 how his behaves. I wouldn't mind more nose down trim.

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It helped a lot. At 100 mph it still needs a little forward pressure with aft CG loading. Will the longer gear and bigger tires add drag below the thrustline and help?

My empty CG is similar to many of Kirk's builds. Some are further aft. I need to ask vj88 how his behaves. I wouldn't mind more nose down trim.
Are you certain the stabilizer is actually hitting the full nose down stop? Have you left it at the full nose down position in flight, then landed and looked at it?

If it is, then this is a possible solution to install the lower cable and spring as in this drawing. https://www.supercubproject.com/drawings/pdfs/A3310041.pdf

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Flew the Cub down from Michigan to Chattanooga, TN a week ago.

Hopped over to North Carolina last Friday to visit the folks at Acme Aero. Cub is flying well. Now we get to wait for import permits from Bolivian civil aviation authorities.20231114_104327.jpeg20231117_065452.jpeg

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I needed more nose up trim when I went from 31 to 35 in Bushwheels so that should help. Gap seals in the tail help with trim authority at slower speeds in my cub. Not sure if that would help you but it is a simple thing to test.
DENNY
 
Finally headed south of the border! Beside the mission boys that I lined up to help, the local EAA chapter in Lafayette, GA helped patch up some holes in logistics and showed up for the wing pull. Thanks Stacey, Scott, Dave and Don!
 

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Here is the update on the trim issue.

The 4 place fuselage comes with a custom rear stabilizer geometry. The rear attach bracket for the horizontal stabilizer is raised 1/2 inch, shifting the trim range towards nose up. Kirk has had very good success with this mod.
With Kirk’s help we identified several factors that were contributing this 4 place flying differently.
1. Zero thrustline
2. Lowered engine mount (more to this story)
3. Fairly light FWF
4. Trim bungee tension
And possibly
5. Short landing gear and small tires

These factors were creating two issues with a rear CG loading, one at low speed and one at max cruise.

At low speed the plane could be trimmed into a spin that it would not recover from by itself. It required a healthy forward push to break the stall. I felt that this was a bit of a safety concern. This was remedied by loosening the trim helper spring. Kirk runs them fairly loose on the 4 place. With no spring at all, the minimum power off trim speed with aft CG increased to 38 indicated from 27 indicated (deep in the stall buffet). I can reinstall the spring with less tension to fine tune this but for now without a spring the safety concern is handled.

The other issue was at cruise with an aft CG there was not enough nose down trim to cruise at more than 86 mph. At forward loadings there was no issue.

Shimming the thrustline 2 degrees down helped some (about 3 mph). Kirk’s planes have closer to the 4 degree down PA-18 mount configuration so that alone might fix the issue but would require cowl mods.

Longer gear and 35s will likely provide some nose down force in cruise at the high end.

There is room in the tail to get an extra turn of nose down trim (worth 4 mph trim speed in cruise) if I want to cut fabric and pull out the jackscrew to trim the upper bushing

I can install a lower trim helper spring like a Top Cub or Tango Cub if I feel the need and don’t want to remove the jackscrew.

In the end we are blaming my custom zero thrustline, lowered mount for the issues. We had an engine mount shop copy the dynafocal Airframes mount to create a conical zeroed mount but in the process it ended up 2 inches lower as well. Lesson learned, buy the Airframes mount and find a dynafocal case engine to fit. The 4 place is sensitive to thrustline angle according to Kirk’s testing.

Kirk also says the 4 place fuselage really doesn’t like to go more than 100 mph without climbing. The STCd version is listed at 100 mph max structural cruise and 126 Vne. I think I can get it to trim out at 95 mph with a full load without any major mods. That works for me.
 
It’s been a long road but I’m sure smiling now. Big milestone. Nothing was damaged in transport, engine runs and everything is back together. Maybe only 3 more months till we get permission to fly. It’s a BIG Cub. Love the interior space and ease of maintenance.

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