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Old Mar 18, 2014, 03:40 PM
If it flies I like it.
HotdogX's Avatar
Madison, AL
Joined Sep 2004
465 Posts
Build Log
Cherokee RM 1/3 Scale

I am starting a build log for my 1/3 scale Cherokee RM sailplane. The Cherokee RM is a further development of the Cherokee II sailplane. The differences between the Cherokee RM and Cherokee II will be explained below.

Some of you might be familiar with the Cherokee II since Dave Smith designed a 1/4 scale model that was put out in a short kit by TMRC some years back. Having owned a full scale Cherokee II I am a big fan of the design and I built the 1/4 scale from the TMRC kit back in 2008. The version Dave designed flies very well; handling is excellent and it thermals quite well. It is also very well behaved on aerotow, making an excellent first sailplane to learn aerotowing. World Models recently released a 1/4 scale ARF version. There have been a number of build logs on RCG covering the 1/4 scale Cherokee II. Dave also designed a 1/3 scale Cherokee II a few years ago and there are now several of those flying. Dave's 1/3 scale version is now out in a short kit from Marc's Laser Shop http://www.scalesailplanekits.com/#!...97&id=21589563

Back when I owned the full scale Cherokee II I learned of the higher performance Cherokee RM via the Soaring Society of America's Sailplane Directory. The RM was designed by Terry Miller and built by Bill and John Ree; hence the RM designation. The primary difference from the Cherokee II was the wing. The span was extended from 40' to 44', the aspect ratio changed from 12.8 to 16.4, and the airfoil changed from a Gottingen 549 to the NACA 63(3)-618. The new wing resulted in a considerable improvement in max L/D and high speed L/D while also decreasing the sink rate slightly. Max L/D went from 23 to 28; high speed L/D went from 67 mph to 88mph; and sink rate went from 2.7 fps to 2.6 fps.

There are a few other less apparent differences between the Cherokee RM and Cherokee II. The RM has a fully sheeted horizontal stabilizer and fully sheeted fin. The fuselage is a bit more streamlined due to the upper nose area being smoothed out and the canopy being longer. The turtledeck shape on the RM has a non-elliptical cross section, and the RM horizontal tail is mounted slightly lower. I have attached 3 views of the Cherokee RM and Cherokee II so you can see the differences.

When I drew the 1/3 scale RM fuselage using AutoCAD, I started with Dave Smith's 1/3 fuselage (thanks Dave!) and made changes to get to the RM fuselage. This turned out to be more work than expected. The RM root chord is smaller and the LE and TE are in different spots, so the formers between the wings ended up a different size and in a different spot. Same for the canopy formers; the RM canopy being longer and a different shape. The forward fuselage formers are also different due to the RM fuse shape being different on top. And all the turtledeck formers are different.

The RM wings I designed for a 6G limit load (9G ultimate load) and the spars and wing tube are sized accordingly. I should note the full scale RM had dive boards on the bottom instead of spoilers, and they were reported to not be very effective, so I went with spoilers on my RM (which is what I would do were I building the full scale RM). The horizontal tail is similar to Dave's 1/3 tail except for the sheeting and the elevator horn arrangement which is a different setup.

As for the performance differences between Dave's 1/3 Cherokee II and my 1/3 Cherokee RM, here are my thoughts. Dave has extended the span on his wings out to the same as the RM (176"). Dave and I also use the same airfoil family, the Quabeck HQ35xx series. So the main difference is the aspect ratio and area. My RM will have a higher wing loading as well as a higher aspect ratio. I also expect my RM to be a little heavier than Dave's due to the design of the wing and using basswood fuse longerons. So I expect the RM will have a better high speed L/D and perhaps a slightly higher max L/D. I'm expecting the minimum sink rates to be similar, or slightly better for Dave's II. We will see once she is flying.

I am looking forward to getting the RM into the air. I think she will be a nice compliment to the tried and true Cherokee II. Marc's Laser Shop also plans to kit the 1/3 Cherokee RM, and I expect he will release the kits once I get the prototype flying and verify the performance and make the final drawing revisions. I am using laser cut parts from Marc's Laser Shop on the prototype build, and the laser cutting and wood densities are very nice.

I hope to make fairly steady progress on the build, but will there will be times when family commitments take precedence, and I will post updates as time allows. I should be posting on the horizontal tail soon.

Update: I have made the 1/3 scale Cherokee RM plans available for free: http://www.rcgroups.com/forums/showthread.php?t=2246733 Also, the wood list is available here: http://www.rcgroups.com/forums/showthread.php?t=2246779

Regards,
Al Clark
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Old Mar 18, 2014, 04:21 PM
Registered User
Birmingham, AL
Joined Sep 2003
422 Posts
I've been hoping this build would happen soon. Look forward to the progress as you go along. I'm sure it will fly great when you finish. Cherokees rule!
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Old Mar 18, 2014, 05:15 PM
"Fly"Fisherman
Asher Carmichael's Avatar
Guntersville, AL
Joined Jun 2004
731 Posts
Subscribed. Yippee! An instruction manual for my future RM. Al, thanks for sharing.
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Old Mar 18, 2014, 06:28 PM
Vintage wood is the best!
SZD16's Avatar
In a house
Joined Sep 2002
2,835 Posts
I was talking with Marc the other day and he mentioned this project......looking forward to following your progress!
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Old Mar 18, 2014, 08:04 PM
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United States, KY, Taylorsville
Joined Mar 2010
1,626 Posts
Nice! Looking forward to the build!
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Old Mar 18, 2014, 08:18 PM
If it flies I like it.
HotdogX's Avatar
Madison, AL
Joined Sep 2004
465 Posts
Horizontal Stabilizer

I started construction on the horizontal tail a few days ago and began with the stab first. The laser cutter has a 24" length limit so the stab TE (or spar web if you prefer) had to be made in two pieces with a scarf joint. The laser curf leaves the stab TE pieces slightly undersize so the total length (span) will be too short if the parts are just glued together as is. The easy fix was to add a strip of 1/16 hard balsa to the scarf joint. I used CA glue. See pix 1-3 below.

Next glue on the 1/8 square basswood spar caps. Use a straight edge on these to make sure they are straight before hitting them with CA. No need to cut the caps at the center - just bend them across the center so they remain in one piece. See pic 5.

Glue all the ribs on next. Make center marks or draw center lines on the ribs so you can check them for squareness with the building board. Make sure the stab TE is pinned down well. A tiny bit of fitting is required for the spar cap notches on each rib, and I like to lightly sand the ends of the ribs square (laser cut leaves a very slight angle) for a good glue joint. All ribs are glued using CA (or CA+ if you have any gaps). Just make sure you check each one as you install it using some kind of square.

After you get all the ribs glued, sight down the ends and adjust any that look out of line. Then glue on the 1/8 balsa false LE pieces and carefully sand them down to the ribs with the edge angle as shown on the plan horizontal tail cross section. See pic 9.

Before sheeting, the center blocks need to be cut to size, fitted, and glued into place with CA. Carefully sand them down to the rib contours. I like to cover adjacent ribs with blue painters tape while sanding, then remove the tape and touch up with the fine sanding block. This keeps me from sanding into the adjacent rib contours. See pic 10.

Sheet the stab frame with 1/16 balsa. I use medium-hard on the bottom and medium on the top. Each side is done in two pieces. You can use 4" wide balsa by adding the trimmed off piece to the front of the sheet to get the extra width at the center. Leave a little extra width at the LE and tips so you can trim it off later for a nice fit. The first side (two pieces) is glued on with CA. The second side is glued with a combination of Titebond, CA+, and CA. We will do the left piece first: 1)put Titebond on the center blocks and all ribs except the tip rib and center rib (left side only) 2)run a bead of CA+ down the spar cap 3)carefully position the sheet along the stab TE and press down onto the CA+ for a few seconds until it sets 4)starting at a rib in the middle of the sheeted area, pull the sheet down along the rib and tack it to the false LE at the front of the rib with CA 5)do this for each of the other ribs, alternating from side to side 6)glue each end rib to the sheeting using CA 7)glue the remainder of the false LE to the sheeting using CA 8)run CA glue along the aft edge of the sheeting along the TE to make sure it is completely glued all along the TE 9)now repeat these steps for the second piece of sheeting on the right side. See pix 11 & 12.

This completes the basic construction of the horizontal stab. The elevator is next.
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Last edited by HotdogX; Mar 21, 2014 at 09:30 AM. Reason: added notes about laser curf
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Old Mar 18, 2014, 09:21 PM
If it flies I like it.
HotdogX's Avatar
Madison, AL
Joined Sep 2004
465 Posts
horizontal elevator

First the elevator LE must be made. Not sure why I didn't get this part laser cut like I did the stab TE, but somehow I left it out. I will correct that for the short kits. I cut it from medium 1/4 sheet balsa. Use the laser cut elevator rib heights to check the correct dimensions on the LE. You can double check the size of the elevator LE against the completed stab by just adding a couple pieces of 1/32 ply to the width and see how it matches the sheeted stab. See pic 1 for the finished elevator LE.

Glue all the ribs onto the elevator LE just as was done for the stab. I didn't take any pix of this.

Before we go any further we need to make the elevator control horn assembly. First make the brass horn from a piece of 1/16 X 1/2 brass stock. Next bend one leg of the horseshoe shape into 1/8 dia. music wire, slide on the brass horn, and then bend the second leg of the horseshoe. I used a K&S wire bender and left the legs a bit long, then trimmed them to final length. If you experiment with the K&S bender a bit, using Sharpie marks on the wire, you will see where the wire needs to be positioned to get the correct bend locations. Music wire is cheap so if it takes several tries don't despair. The brass horn must be silver soldered to the wire and this means a jig needs to be made to hold it all in place. I made the jig from a scrap of 1 X 2 with holes drilled for the music wire, and some small pieces of 1/4 ply CA'd to the 1 X 2 to hold the horn in position. I used Kester silver solder. Make sure the wire and brass is sanded clean with 220 paper where it will be soldered, and that you have enough heat. It takes a fairly hefty iron to get it all hot enough. I used a little Bernzomatic micro torch. The wire and the brass both need to be hot enough to melt the solder which will then flow around the joint. Apply solder to both sides. See pix 2-4.

The elevator center block must be cut to fit and then notches made for the control horn assembly. The ends are notched 1/8 X 1/8 and a 1/16 wide slot must be made to clear the brass horn. I used a razor saw and small files. The elevator LE piece must also by notched to allow the control horn assembly to fit flush with the aft surface of the LE. I used a razor blade, X-Acto, and small file to do this. It might have been easier to make this notch before i glued on all the elevator ribs! Check the fit of the control horn assembly and block and glue into place with 15 minute epoxy. See pix 5-9.

After the elevator control horn assembly glue has cured, sight along the ends of the ribs and adjust any if required. Cut the four 1/32 ply TE pieces (leave them a tad long) and glue the bottom pieces to the ribs. Using some light weight 3/16 thick balsa, glue pieces to the 1/32 ply TE with CA+ between all the ribs; make sure the TE is held flat to the building surface with pins or whatever when you glue in the webs. Carefully sand the balsa webs down to the rib surfaces; I used blue painters tape on the ribs again to prevent sanding into the rib contours. Glue on the top 1/32 ply TE strips using 15 minute epoxy. I used a thin layer of epoxy on the webs and a little thicker strip of epoxy along the aft edge (last 3/32 inch) of the ply pieces. Pin the TE down to the building surface well with lots of pins ( I use the fantastic little Rocket City RC Pin Clamps) when epoxying the top TE strips and make sure they don't slide out of position. You will have to do one side at a time as you can't pin down both sides at the same time. See pix 10 - 13. When the glue has cured you can sand the excess epoxy off the aft edges.

Cut the 1/32 ply strips that will be glued onto the elevator LE on the top and bottom. These are really just wide 1/32 ply spar caps. The bottom one must be notched a bit to clear the elevator control horn. Glue the bottom one on first using CA, then add the 1/16 balsa webs at the aft edge using CA. The 1/16 webs should be medium or medium-hard density balsa. Sand the webs all flush with the top of the ribs and glue on the top 1/32 ply piece using CA+ on the center block and ribs, and CA on the webs. Add the 3/16 wide, 1/32 ply cap strips to the ribs with CA. Sand the ends of all the ply pieces flush at the elevator tips. See pix 13 - 15.

Next will be hinging the elevator, adding the LE, and adding the tips.
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Last edited by HotdogX; Mar 19, 2014 at 02:11 PM. Reason: made changes relating to laser curf
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Old Mar 19, 2014, 11:16 AM
Quebec Aerotow Association
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Canada, QC, st-jean-sur-richelieu
Joined May 2010
582 Posts
Great thread !! I'll be following this closely as I'll be building a Hall Cherokee II in the near future. Lots of great info for someone like me who is a novice in kit building.

Charles
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Old Mar 19, 2014, 02:12 PM
If it flies I like it.
HotdogX's Avatar
Madison, AL
Joined Sep 2004
465 Posts
Thanks everyone for the interest and nice replies!

Al
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Old Mar 20, 2014, 07:40 AM
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Asher Carmichael's Avatar
Guntersville, AL
Joined Jun 2004
731 Posts
Al,
Does the offset in the brass elevator horn serve a purpose?
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Old Mar 20, 2014, 07:56 AM
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Seadog's Avatar
Canada, ON, Toronto
Joined Jan 2004
808 Posts
Asher,

It puts the clevis hole directly under the hinge line so there is no built-in offset. It's minor, but it's there if you don't align it.

Dave
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Old Mar 20, 2014, 08:21 AM
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Birmingham, AL
Joined Sep 2003
422 Posts
Al - What building surface do you use on your table?
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Old Mar 20, 2014, 10:28 AM
If it flies I like it.
HotdogX's Avatar
Madison, AL
Joined Sep 2004
465 Posts
Elevator Horn Offset

Asher,

What Dave said! Putting the hole directly under the hinge line means there is no differential introduced on the up and down motion. With the modern radios the throws can of course be adjusted to compensate, but I just like to start out with things where they belong mechanically.

Al
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Old Mar 20, 2014, 10:43 AM
If it flies I like it.
HotdogX's Avatar
Madison, AL
Joined Sep 2004
465 Posts
sleep4,

I use Armstrong plain white ceiling tile (their item # 280C) taped to the building board. I used to be able to find the 280C tile at Lowes and/or Home Depot, but more recently they don't stock it. I guess there isn't much call for plain white tile any more. What I have done is to have them order a whole pack of the plain white 280C tiles for me. I forget how many tiles come in a pack, but one pack will usually last me quite a few years. The 280C tile is a good surface for putting T pins into.

As for building boards I have two sizes. My 12 X 48 board is made from an MDF wood shelf with white plastic laminate surfaces. My larger building boards are plain wood hollow-core doors. The last wood door I found was one half of a set of bi-fold doors that had a small hole in it. It is 24 x 80. In the past I have been able to negotiate a good price on doors that have some damage. It's harder to find plain wood doors these days but some door shops and surplus building materials places still have them. On the Cherokee RM the wing panels are 82 long so I will have to add a small extension onto my big building board!

Al
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Old Mar 20, 2014, 02:33 PM
If it flies I like it.
HotdogX's Avatar
Madison, AL
Joined Sep 2004
465 Posts
Finishing the Horizontal Tail

I draw a centerline on the TE of the stab and LE of the elevator using a fine Sharpie, then mark the locations of the hinge slots (Du-Bro 116 standard hinges), and cut the slots using an old hinge slotting tool I've had for years. The tool is a forked looking thing that just gets pushed through the wood. It matches the thickness of the hinges perfectly. After the slots are cut bevel the LE of the elevator per the plan cross section. I also cut away a bit of wood at the front of the elevator hinge slots (makes a sort of V shaped depression) to let the hinges go into the elevator a bit further, thus reducing the hinge gap when the elevator is hinged. I set it up for around 1/32 gap. See pix 1 & 2.

I do a trial fit of all the hinges and make sure the elevator aligns properly with the stab and there are no binds in the hinges. If needed adjust the hinge slots. Next the 1/4 thick leading edges are glued on and sanded to shape. Glue on the 3/8 balsa tip blocks and sand to finished shape. I like to tape the elevator in neutral position on the stab and shape the tips for the stab and elevator at the same time. See pix 3 - 6.

The only thing left is to drill the mounting holes. Use the 10-32 tap drill size for these holes (#21). When you drill the holes it's easiest to use a drill press. Make sure you shim the stab with scraps of balsa so it is level all around, or the holes won't be perpendicular. These two holes will be used later as drill guides when drilling the stab mounts on the fuselage, after which they will be re-drilled to final size for the 10-32 screws. See pic 7.

That's it for the horizontal tail! I think I will build the rudder next while I still have my small building board on the bench.

Al
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Last edited by HotdogX; Mar 21, 2014 at 09:33 AM. Reason: added note about LE
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