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Old 09-21-2014, 05:47 AM   #1
grosbeak
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Hobbistar tail repair


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One day this summer I took my Hobbistar to the field to burn some holes in the sky. I had a hard landing but everything looked good, so I took to the air again.

I should have checked the control surface travel. I needed full elevator just to get the plane off the ground and as soon as I let a bit off, in she went. Cartwheeling, the prop broke...



Followed by the tail wheel steering extension on the rudder...



And, worst of all, the tail broke off. Completely severed, held together only by the elevator and rudder push rods.




Well, it looked bad... but it's just wood. I've been there before with this plane: Hobbistar wing repair
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Old 09-21-2014, 06:21 AM   #2
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Re: Hobbistar tail repair

To start, I disconnected the push rods from the elevator and rudder and removed the clevis from each; this allowed the empennage to be completely removed. I disconnected each push rod from its servo, pulled them out, reassembled the parts and labeled them for later. They're not badly bent and will be fine for re-use.




Next, I removed the tail wheel assembly from the empennage. Going forward I plan to use a direct servo connection for the tail wheel so I'll have modify this one or go with a different assembly




With the two sections apart I took a closer look at the break. Fuselage end...



Empennage end.

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Old 09-21-2014, 06:33 AM   #3
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Re: Hobbistar tail repair

It's not obvious in the photos but when I tried fitting the two sections back together I realized there were a lot of mating surfaces where a glue bond could be made - at least enough to support the weight of the tail. I propped the plane one its nose and used my trusty cabinetmaker's glue to put it together again.



There is a very distinctive smell to certain woods from China. I lived in Beijing (then Peking) for two years as a young child when Dad took a secondment to External Affairs to work at the Canadian Embassy. We brought some furniture back and the scent is firmly tied to my sense memory. I've caught it in my 29% Slipstream SBach (especially the new replacement canopy) and I smelled it again in the broken sections of this airplane. Ah, memories. But I digress.

The glue-up turned out very well and supported the weight of the tail as hoped.





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Old 09-21-2014, 06:41 AM   #4
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Re: Hobbistar tail repair

Static and dynamic forces are two different things. Although the repair supports the weight of the tail sitting in the stand in the workshop I wouldn't trust it in flight. My solution is to make four external reinforcing plates out of thin ply; I started by recording the measurements in rough drawings.




I took them to the computer and worked them up into scale drawings on Visio.




Here I'm testing one of the side plates.




I have every confidence that these four plates will provide more than enough strength. Next step - cutting them out. Stay tuned.
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Old 09-21-2014, 08:50 PM   #5
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Re: Hobbistar tail repair

Cutting out the plates from the paper templates. Straight cuts first, then a Forstner bit in the drill press for the round part.




Testing fitting the top and starboard plates.




All plates cut out and chamfered. Note the hole in the top plate for the rudder push rod exit.

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Old 09-21-2014, 08:55 PM   #6
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Re: Hobbistar tail repair

After a test clamping to make sure pressure was applied with the right force and in the right places, here's the top plate glued and clamped.




Top plate cured.




Starboard plate glued and clamped, from above the horizontal stabilizer...




And below.




That's it for today. I'll continue with the port plate next time.
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Old 09-22-2014, 03:29 AM   #7
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Re: Hobbistar tail repair

Couldn't sleep... may as well spend some time in the shop.

Starboard plate cured.




Port plate, glue on (after the usual test clamping).




Glued and clamped - top.




Bottom.




Next episode - bottom plate.
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Old 09-23-2014, 06:54 AM   #8
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Re: Hobbistar tail repair

Here's the bottom plate installed. I've rounded the edges of all the plates.




The actual tail repair is complete, structurally at least. I've changed tail wheel assemblies; note the ply reinforcing plate for the new one.




Here's the new assembly. It will connect to the rudder servo via push rod and will employ a control over-ride servo saver to keep rough jolts from the wheel from shortening the life of the servo. The elevator push rod exit is visible below my finger; there's another one on the starboard side that's perfectly positioned for the tail wheel control horn.




Before I recover the tail I'll install and connect the tail wheel assembly... stay tuned.
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Old 09-23-2014, 09:23 PM   #9
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Re: Hobbistar tail repair

Here's the tail wheel push rod tube through the exit on the starboard side of the tail.




Here's the control over-ride servo saver on the servo horn.




And here's a view of the push rod support plate, glued and curing.




The push rod is in perfect position to connect to the tail wheel control horn.

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Old 09-24-2014, 06:59 AM   #10
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Re: Hobbistar tail repair

This morning I cut the push rod tube shorter at the rear exit and applied some medium CA to the at both ends to help hold it in place. I was planning to use Gorilla glue but it's not safe for some plastics and I don't know the tube is made of. In any case the fit in the forward tube is very tight and I don't expect it will shift.






I added a third hole to the tail wheel bracket and mounted it.




Hardening the threads with thin CA.

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