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Old Nov 30, 2011, 09:05 PM
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Billinaz's Avatar
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Tom Herr J3 Cub build

Yeah, I know. I just finished the Mini Sport and have plenty of work to do repairing her to keep her in the air. So what? I like to build 'em more than fly 'em!

Anyway, I'll try to do a little better with this Build Log than I did with the Mini Sport.

As with the Mini Sport, this kit comes very well packed and every piece was accounted for and undamaged. Care had to be taken as many pieces fell out of their sheets with little or no pressure and stray parts are very susceptible to being broken. One thing I noticed right off is the ribs for this model's wings are solid ribs and not lightened like the ribs for the Mini Sport and Air Boss.
The Instructions Manual is 5-1/4 type written pages, including the cover sheet, stapled together. They are very easily read and intelligible.

This kit is also designed to be a three channel airplane but I am electing to add ailerons. However, I am going to add them per their scale locations and not as "strip" ailerons as I did on the Mini Sport. I'm thinking about operating them differently, though, using a single servo. I have an idea, and if it works, will be pretty neat. I'm sure someone else has done the same thing, but I'm still going to save this idea until it is time to put it together.

As a change of pace, I started with the tail surfaces first. They are simple sheet stock pieces and the elevators are joined with a dowel. The rudder is 2 pieces that have to be joined. Hinging will wait until final assembly.

Bill
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Old Dec 01, 2011, 11:38 AM
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Start of the Fuselage.

A bare beginning of the fuselage.
The first step is to join the side pieces, which immediately indicated a slight problem. Pieces F1A and F1B did not align perfectly. It was necessary to trim the tab of F1B so that the tab would mate with the recess in F1A and would still create the slot for former F4 in perfect alignment. Not a difficult thing to accomplish, but if one was moving right along and not checking these things, the improperly shaped slot intended to accept former F4 would create an issue later on. As it is, no big deal, just a small void at the aft end of the F1A/F1B joint. I suspect the difference in wood density between the two parts and a change in humidity is probably the culprit as the other side had a similar issue but with only about half of the dimensional difference.

Bill
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Old Dec 01, 2011, 05:01 PM
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More Fuselage Trouble...

It appears my estimation of misalignment due to temperature and/or humidity is likely in error. It is more likely an error in the cad and/or laser cutting/cnc files.

I have been progressing strictly following the manual instructions.
Installation of formers F6 and F7 were uneventful. Installation of part F8 was mostly uneventful except the slots in the center of the part to accept the tabs from F6 and F7 were so very slightly misaligned and the very aft ended arrow shaped piece was slightly short for its tab intersection. A little persuasive pressure was all that was necessary to bring the parts together for gluing and pinning.
Installation and gluing of parts F9 and F10 went well as the fitting of these parts was actually pretty good.
Installation of part F11 is a completely different story. The misalignment and incorrect cutting of this part is requiring some interesting adjustments. If the part is installed starting at the rear of the fuselage, things line up ok until former F6 where the mismatch is minimal. At the point where the part is cracked to make the bend the mismatch is so significant it is impossible to mate the parts without trimming approximately 1/16" off the tabs at former F3, the next tab forward and just aft of former F4. When installed, there is a gap of 1/16" between the forward-center notch in part F11 where it mates (or is supposed to mate) with former F4. The legs of the notch just touch the edge of former F4. Part F12 fits the space between former F4 and firewall former F5 perfectly. But it is not supposed to. The main landing gear is mounted on the forward face of former F4 and is captured and secured via sandwich method by parts F4A and F4B with a tab on F4B that is supposed to index with the aft notch on part F12. But because part F12 fits the space perfectly, it will be necessary to remove 1/16" of material from part F12's notch and also make clearance for the landing gear wire to exit the bottom of the fuselage by making the notch wider by the diameter of the gear wire on each side of the notch. Oh, by the way, the landing gear wire was not found in this kit. A minor thing as I have plenty of wire on hand.
Once the discrepancies with the parts for the fuselage have been corrected, construction will be greatly simplified.

As this design is also intended for an inrunner motor, I will be making modifications to the firewall similar to those I did for the Tom Herr Mini Sport, and should be done very soon. I have another of the same motor I used in the Mini Sport and also an E-Flite Park 250. I am leaning toward the E-Flight motor because it will be adequate but not overpowering. It is a Cub after all.

Bill
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Old Dec 02, 2011, 03:05 AM
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You should send this post as an email to Herr engineering. Who knows, maybe future customers will get a correctly revised kit?
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Old Dec 02, 2011, 08:16 AM
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Looks good so far. Nice clean build. Like Odd said, contact them. I believe Herr is part of Sig now, and knowing Sig, I'm sure they would re receptive to any suggestions for improvement.

I've gotta ask.... Where did you get all those pins. I use about 1/2 that amount on a plane twice the size

But I use T-pins, and after about 20 years I'm running low (they are like socks... they just disappear one by one) and would really like to get the ones you have.
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Old Dec 02, 2011, 10:46 AM
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Quote:
would really like to get the ones you have.
They are called "Grip Pins", marketed by "Midwest Products Co., Inc" Stock #587.
http://www.midwestproducts.com/store...Grip_Pins.aspx

I like them because the pin wire is about 1/2 the diameter of the small T-pins.
Part of the reason I use so many pins is I don't push them into the wood very far and use more with less withdraw resistance.
I hope this helps.

I did send an e-mail to Sig regarding the issues I have encountered. We'll see what I hear back. I have built four Sig kits in the last 6 months and this one being my 5th, is the first one that I have had any significant issues with.

Bill
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Old Dec 02, 2011, 01:31 PM
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Bill

I followed your Four star build and I am new to building. What exactly do all the pins do.

I am building a Herr Cloud Ranger and really only use pins to hold things in place on the plans.

Thanks, John
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Old Dec 02, 2011, 02:04 PM
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Quote:
Originally Posted by Billinaz View Post
They are called "Grip Pins", marketed by "Midwest Products Co., Inc" Stock #587.
http://www.midwestproducts.com/store...Grip_Pins.aspx


Bill
THANKS!!! I'll have to pick up a few packs..

Just ribbin' ya bit on the pins. I'll confess; I use tape in addition to the pins.
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Old Dec 02, 2011, 03:57 PM
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Quote:
I followed your Four star build and I am new to building.
Jeb,
Welcome to the great obsession! I hope you will find, like I do, there is a special sense of accomplishment to see your plane, constructed by you, fly. The only thing better would be if it was your own design, too. (I'll wait on that one, for sure!)
I hope the Four Star build log was helpful. You may want to check out my Tom Herr Mini Sport build, too.
Quote:
What exactly do all the pins do.
Pins are kind of a universal tool. They hold parts and pieces to the plans, but also, they hold pieces together for gluing. In my case, I am very sensitive to CA glues, so with very few exceptions, I have been strictly using aliphatic resin and epoxy resin adhesives. If you are using CA glue, you can pin pieces together and then hit all the joints with thin CA and it will glue the joint. With aliphatic resins and epoxy resins, the pins hold the pieces in place while the adhesive cures. The depth pins penetrate a particular joining chore depends on several things but mostly personal preference. Obviously a pin doesn't penetrate plywood as easily as basswood or balsa, for instance. I tend to use a lot of pins with shallow penetrations because balsa can be very prone to splitting. If I can use 2,3 or four pins and avoid splitting the balsa, I will. If I can use a clothespin, tape or rubber bands to hold pieces together without pins, I will often do so, but I have to admit, I kind of have a thing for pins.


Bill
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Old Dec 02, 2011, 07:48 PM
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Those tabs not aligning look a lot like the tabs on the Fuselage of my Pete n Poke. Some due to slight ply warps, other times the slot and tabs just seemed to have crap tolerances due to the limitations of die cutting. Nothing additional epoxy can't cure though. The fuselage probably ended up being the truest part of my build despite the non interlocking parts.
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Old Dec 02, 2011, 09:38 PM
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Quote:
Some due to slight ply warps, other times the slot and tabs just seemed to have crap tolerances due to the limitations of die cutting.
Possible, I am sure, with your Poke. However, all of these misalignment issues in this TH J3 Cub are with laser cut balsa parts. Considering the uniformity of the misalignment across the piece's (port to starboard) edges, it's probably not a material issue.

Bill
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Old Dec 02, 2011, 10:00 PM
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While messing around with part F-12, it occurred to me that using F-12 as the battery tray was going to set it below and behind the bottom edge of Former F-4. This will make battery manipulation rather tedious and may cause some damage to Former F-4. To remedy this situation I added a piece of 1/8" thick balsa the shape necessary to fill the void from the top of the bottom strap of Former F-4 to the rear face of Former F-5. This will make a sturdy battery tray and will at the same time protect and reinforce Former F-4. It may be a little overkill and I should be more careful about weight gain. A gram here and a gram there and before you know it she's a porker.

Bill
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Old Dec 02, 2011, 11:30 PM
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Wing construction...

Wing construction for the Tom Herr J3 Cub is pretty straight forward. The only tricky part is the wing tips, but they are not that bad.
It's not clear why, but the wing tips are made up of 3 parts. One part is a complete 1/2 circle of the shape of the tip and two more parts with tabbed ends are laminated to the single part to double the tip's thickness to 1/8". It works, but why 3 parts instead of 2?
Also, the wing tips are not flat with the plan/bottom of the wing. They have an angle both from front to back and from side to side. This is easily accomplished with a 1/4" block to prop up the end like the plans show and by pinning the ends to the centerline of the leading edge and even with the trailing edge.
I'll say this right off, the wing construction is where I first deviate from the instructions manual. The manual says to leave the excess material of the spars to extend beyond the out board end of the wing. With the round wing tips, the necessity to crack the spars to bring them into contact with the wing tips, etc., I have found it easier and less trouble to locate the crack locations, trim the spar ends close, but not perfectly, to the wing tips and allow the excess to extend inboard past rib W-1. This is especially true for the trailing edge and the leading edge, because the wing tip parts are supposed to meet perfectly at the trailing edge and centered on the leading edge but flush with end rib W-3, it is necessary that the TE and LE be flush with end rib W-3 before fitting the tip. Expecting to remove the wing from the plan and trim the ends before fitting the tip is asking for broken parts, IMHO. Also, nowhere in the instructions nor on the plan does it tell you where the end of the bottom spar occurs. It does not appear necessary to support the tip as that support is provided by the upper main spar and 2 secondary spars, therefore I cut the bottom spar flush with the outboard face of end rib W-3. Besides, when covered, I think seeing that single wide, flat, spar through the covering would look out of place.
Another slight deviation I did from the instructions was dealing with Rib W-1. This rib is angled to accommodate the dihedral when the wings are joined. This angle is set by spar web part A. The Instructions say to add spar web part A after the bottom spar and Rib W-2 are in place then add Rib W-1 and then the rest of Ribs W-3, then the top spars. I did this for the first wing half but had a minor issue with the alignment of spar web part A with the upper main spar. To alleviate this alignment issue on the other wing half, I set all the ribs except W-1 after the bottom main spar and trailing were placed and then placed the top main spar. Once the top main spar was in place it was a simple thing to install spar web part A snug to Rib W-2 and tight to both bottom and top main spars. Placing Rib W-1 into position only required setting it in place at an angle toward the opposite wing and rotating it into place. Slick.

Bill
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Old Dec 03, 2011, 03:21 PM
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You're doing an excellent job documenting this, for sure.
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Old Dec 04, 2011, 10:46 PM
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This Cub kit is not designed for ailerons but she's going to get them anyway.

First step was cutting out the aileron from the wing. I elected to forgo accurate scale for the ailerons and settle for a width of three rib bays and a fore-aft dimension equal to the "scale" ailerons indicated on the plans. Once cut out, it was necessary to install rib ends in the end of the ailerons so a photo copy of a rib punch-out was made four times. The photo copies were then pasted onto a 3/32" piece of balsa scrap and cut out slightly over sized with an exacto knife and sanded to final dimension. The two cut ribs were shortened slightly to compensate for the new aileron spar. The leading edge of the aileron ribs/rib ends were chamfered to provide a down deflection angle allowing about 40 degrees and a balsa leading edge was applied to the face of the rib ends. The ailerons will be continuously hinged at the top using Ultracote. Balsa gussets were cut from 3/32" balsa scrap and fitted to both sides of the ribs at the ends of the aileron spar to reinforce the aileron bay and to support the rib ends and wing tip.

Initially, I intended to use a "Goldenrod" cable and a single servo, but I felt the required bends were just too tight because of such a small plane. I happened to have a couple micro servos on hand, so I decided to use them instead. The wing on this small Cub is not deep enough to allow mounting the servos vertically with the actuating arm exposed below the bottom wing surface so they are mounted flat and the actuating arm will extend below the bottom of the wing. The servos are mounted on basswood blocks, one glued to the wing main spar and the other to a wing rib.

Some very welcome work has come to me suddenly and unexpectedly, so progress on this project will slow considerably, I am sure.

Bill
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