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Mar 27, 2010, 01:24 PM
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38% Bill Hempel Decathlon

I had been looking for a new, big, scale tug for a while and had considered several options. I was concerned about cost, size, ease of assembly at the field, airfoil, quality, etc, etc. Most everything winds up with compromises and my decision to go with the Hempel 38% Decathlon was no different. I found this airplane by chance after talking with Steve Rojecki at the JR Aerotow in 2009. He mentioned the Hempel 50% Cub and remarked he thought it was a pretty good quality offering at a very reasonable price. I went to Hempel’s website, looked at what was available and decided nothing there would fit my requirements. I wanted a tug with at least a semi-symmetrical airfoil, not too much area, and something that would accept a 150cc engine. Hempel didn’t have anything at the time that fit these criteria but there were rumors….

I kept visiting Hempel’s site and learned that new 38% and 57% Decathlons were in the works and would be available in early 2010. The 57% was off the chain for me but the 38% looked very interesting. I lurked around the giant scale airframes thread over at Flying Giants and learned a lot about Hempel’s planes. They were held in high regard for quality, flight characteristics and overall value. I asked a few questions which were promptly answered and a check for the cost of a 38% Decathlon was on the way to Tucson.

I received the plane a few weeks later. It was carefully packed and doubled boxed. Initial inspection proved most everyone’s observations to be correct. A great covering job, immaculate glass and paint work and a complete hardware package including some really spiffy wheels were evident. I carefully unwrapped all the components and placed them on benches in my shop. I commandeered the dinning room table for the wings. There just wasn’t enough room in the shop to accommodate the 72” long by 24-1/2” wide wings without standing them on end in front of my wife’s car and that just didn’t seem to be the safe bet. She made me promise to move them out as soon as possible, which, I pointed out, would require a lot of time spent in the shop in order to finish the plane within her time frame.

I evaluated the plane for about a week determining the assembly sequence, tow release position and installation, position of fasteners and construction and just generally becoming familiar with it. There are no instructions with the model, just tiny slips of paper with writings in Mandarin and English in each hardware bag denoting the assembly to which they belong. That helps quite a bit. It seems the accepted theory is if you’re ready to fly such a beast, you’ll be able to figure out how to put it together. I also got some good insights by visiting a couple of build threads for Hempel’s Cubs over at Flying Giants. I’m pretty much right-brained so the pictures included in those threads told me what I needed to know.

I like to make as many parts as possible removable on my models. I’m always looking ahead trying to anticipate recover jobs, repairs, general service, replacement of parts and transportation. I try to design assemblies and attachments to allow me to remove them for access, replacement and even initial finishing and covering. The Decathlon has removable stabs (one of my requirements) and of course wing panels. The gear isn’t easily removable but I’m working on that one.

So what are the compromises at this point? The wing struts and empennage flying wires will be a bit tedious at the field, but not overly so. A DA 150 won’t quite fit in the cowl, it needs a hole for each sparkplug cap and wire. The new DA 120 would be perfect and I’m also looking at a BME 150 which should fit nicely with a bit more uummpphh for towing the “fittycenters”. The structure is a tad lighter than I’m used to. Not a bad thing, just designed for the IMAC crowd; hovering and general cattawampusing. I hope to place this in the advantage column in the future.

Pluses include a symmetrical airfoil for reduced drag and increased speed, plenty of room for anything you might want to put in, good looks, pretty much built, high quality and a great overall value for what you get.
Last edited by Asher Carmichael; Mar 31, 2010 at 09:52 PM.
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Mar 27, 2010, 04:31 PM
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Tow Release Setup

The first order of business was to set up the tow release. The Decathlon has a pretty good expanse of glass and no real bulkhead near the trailing edge of the wing. Too, the Hempel version has a single hatch latch precisely at the most logical location for the release. This pin seemed a bit light to me. I’ve had planes with too much positive cabin pressure lose their hatches in flight that used similar single point pins so the decision was made to scrap the latch pin, install a bulkhead in its place with reinforcements and replace the latch with a couple of 10-24 nylon bolts. This places the release much closer to the CG which is a good thing.

As an afterthought I should have doubled the cabin roof with 1/8” ply from the start. One reason I didn’t was because the release (from MK at Little Soaring Fleet) doesn’t have much room between the attachment screws and the top of the release. This requires a fairly thin roof in order for the release to stick above it adequately. I opted to double with ply around the release bulkhead, front and back, and added I-beam strips on the cabin roof “X” braces.

As you can see, I made the servo and release bulkhead removable so it is easy to service. Just take out 6 screws and it slips out. I started using this system with my Drag Queen tug because there was no easy way to install it otherwise and I have used it since. It works fine and is plenty strong. I’ve been in enough awkward situations towing some pretty big planes to convince me it’ll do the job.
Last edited by Asher Carmichael; Mar 29, 2010 at 03:50 PM.
Mar 27, 2010, 04:52 PM
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I anticipate a recovering job in the future if the Decathlon proves to be the plane I hope it is. While the factory covering job is more than adequate, I think covering it in Stits Lite will be in order. We all know what happens to film coverings and details over time.
For this reason, I wanted to be able to remove the windows. I decided to use the basswood strips on the cabin windows and balsa fillers on the doors. There is a tiny rabbet on the basswood strips to match the thickness of the windows and the strips are glued to the windows using thick Zap. No adhesive is used on the balsa fillers used on the door windows except for some thin Zap in the screw holes to harden them up. This allows me to screw all the windows in place using #2 x ½” screws.
The windscreen is also held in place using these same screws but requires adding hardpoints as shown. The 1/16” music wire is pushed through the screw hole in the fuselage so the end of the music wire is at about the centerline and easier to access. You slip the hardpoint, which has thickened epoxy on the backside, onto the wire and push it along the wire until it seats against the fuselage. Remove the wire and the hardpoint is pretty much where it needs to be.
Last edited by Asher Carmichael; Apr 25, 2010 at 09:54 PM.
Mar 27, 2010, 05:03 PM
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The ailerons on this plane use a knuckle type hinge with a carbon tube for the pin. This is a really nice feature which allows you to remove the ailerons for service or recovering. I wanted the same feature for the stab, rudder and doors so I decided to replace the kit supplied hinge points with rivetless Robart hinge points and use 1/16” music wire pins. I captured the pins as shown in the photos and used brass cotter pins on the doors. This makes for easy removal if required. Thanks to Al Clark for the idea for using the screw to lock the pins in place.
Here are a couple of thoughts concerning hinge points. I cut my hinges to length so that the minimum amount sticks beyond the attach block or spar. I also wash my hinges with soap and water, allow to dry and then do a quick rinse with denatured alcohol to remove any possible molding residue. A brushing with a wire wheel in the Dremel will scarify the surface nicely and give the epoxy plenty of tooth. If you hold the Dremel as shown in the photo, you can actually let the tool orbit around the hinge point which makes quick work of it.
Last edited by Asher Carmichael; Mar 31, 2010 at 09:34 AM.
Mar 27, 2010, 05:13 PM
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Hidden "Agenda"

Finding all the various attach points for the hardware took a bit of feeling around and poking with a round end dowel. You can see the general positions of most all of them in the photos.
Mar 27, 2010, 09:32 PM
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The servo mount for the rudder is obvious on the mid tray. I added a stiffener under the right side top tray as shown. I would also add stiffeners under the X bracing if you plan to put something on top like a pilot bust, etc. I made the Rx battery tray removable as shown and the ignition battery nestles right into the oblong cutout near the windscreen. A piece of ¼” LitePly and some basswood crossbraces make a neat tray for use with Velcro.
I’m using two 72MgHz receivers, with half the plane on each. The opposite aileron and elevator are in the same Rx. Ditto with the other side. Using dual Rx’s is a debatable point but in the end you do what you have confidence in so that’s where my head is. I’ve never used power boxes, regulators, soft switches, etc so can’t really comment on them. I plan to use either 6 cell NiMH or A123 batteries. One Receiver is located on the upper shelf and the other is located behind the battery tray on the lower level.
I made a big switch, at least for me, on this plane. I’m pretty much stuck on JR but I’ve been reading about the new programmable “7 series” servos from HiTec. I decided to go with HS-7985MG’s on the flying surfaces except for the Rudder which uses a HS-7950TH. I also used this same servo on the release. There’s 486 in ozs of torque in that little box, more than enough to do what I need.
There are two servos on each aileron and I wanted to experiment with matching those servos with the new HPP-21 programmer. This system is designed to eliminate Matchboxes though the Matchboxes or programmer isn’t really required on a nine channel plane with my 10X transmitter. Still, it is neat to have the option of matching the servos outside of the TX. Another nice feature with these servos is the OLP or over load protection. This allows you to program the current going to a “stalled” servo after a few seconds in order to prevent burning up the servo motor. Hope I don’t ever need to prove if that function really works.
I made up my own extensions using servo wire from Servo City and plug kits from MPI. I always use a tiny bit of solder on the plugs after crimping them. Just a bit of insurance. All extensions and leads into the receiver are tied with fishing line. BTW, the 22g twisted wire from Servo City is the most supple I’ve found. Neat stuff,
Last edited by Asher Carmichael; Mar 31, 2010 at 09:36 AM.
Mar 27, 2010, 09:52 PM
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The wing and jury struts were pretty much installed as supplied except for the addition of brass sleeves and epoxy in the jury strut ends and the use of 6-32 all-thread in the cross members. The jury strut cross members had pre-bored hardwood dowels installed in the ends and it looked like the idea was to attach them to the “T” block and straps with the 6-32 cap screws supplied. The problem here is that the jury strut “T” blocks that attach the jury to the main struts are forced into alignment with the main struts by the straps that wrap around them. This means a straight shot through the “T” blocks into the cross member is almost impossible. I actually had to open the hole in the “T” blocks to 7/32” diameter in order to get the all-thread rod to pass through. I have seen some build threads for Hempel Cubs where the modeler has simply bolted the cross member on the outside of the jury strut assembly but that didn’t suite my taste and besides, the dowel in the end and the length of the cross member seemed to point towards the installation I chose. I replaced the supplied cross member tubing with heavy-wall T6 aluminum of similar size rather than boring out the kit pieces. This system seems to work OK.
I also had to modify the ends of the jury struts in order for them to fit in the channel blocks attached to the wing. I used the 6-32 cap screws provided for attaching the channel blocks to the wing and they don’t leave enough room for the end of the jury struts and the cross-hole for attaching them. I should have replaced the cap screws with round or pan head for attaching the channels but chose to grind the end of the struts instead. This didn’t leave much room between the end of the strut and its attachment hole so I decided to add the brass sleeve and epoxy. An advantage in doing this is the “anti-crush” action on the strut.
I use US Composite epoxy, sometimes West Systems, and cabosil and 404 high density filler where appropriate. I make a “cakefunnel” applicator as shown using freezer paper and masking tape. Be sure to put the plastic on the freezer paper to the inside. This little device allows you to precisely apply epoxy where needed or you can open the end as required and pile it on. I use this anytime I need to make a fillet or apply epoxy in tight areas.
Last edited by Asher Carmichael; Mar 31, 2010 at 09:47 AM.
Mar 28, 2010, 07:18 AM
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More please.
Mar 28, 2010, 09:07 AM
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Thanks for the invite to your blog. I received my Decath a about 5 weeks ago and its been begging me to build it. I have a couple projects to get accomplished first. Maybe a month away from getting it going.
I learned some things in life, One is to ask before you steal. So I'm asking you if I can steal all your idea's. These modifications you are making seem very well thought out. I don't completely follow the strut issue, but I'm sure it will be clear when I start working on it. I agree about the future re-cover. The current covering is very nice and quite well done. But after 750 tow's on the Pilatus, the plastic iron on covering is getting a little worn.
I had it out of the box Just yesterday with my friend and tow pilot Rob, we were trying to figure why the servo's for the elevator were so low. At least thats what it appears from the first look. Also, the tabs to attach the stab seem like an afterthought. Again, these are quick observation's of my kit without any close inspection or dry assembly, so take it with a grain of salt at this point. I love the fiberglass work. I plan to use the 150. Having the caps stick out is not a big deal for me. If someone makes a comment about them sticking out, they dont get a tow. lol

Good luck and please keep the information coming. Its wonderful and you are doing an outstanding job. Hope to see you at JR this year.


Ps, Check your pm for a personal note.
Mar 28, 2010, 11:15 AM
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You're the primary reason for posting this so use at will. There are any number of ways to "skin a cat" so use what you can and improve as well. I think I'm the first builder at this point so I'm just trying share some initial experiences and observations.
I agree about the stab elevator servo locations but several of Hempel's planes seem to share this. It actually gives a pretty good mechanical advantage to make use of the max servo travel by using a relatively short servo arm and longer stab horn to make the servo travel further in its arc to achieve enough surface throw.
I, too, questioned the stab attach plates as an afterthought but reconsidered when I thought about the relatively thin, soft balsa plate with corresponding short stab tube sockets as the only hold downs at the root. I plan to use the screws.
More to come.
Last edited by Asher Carmichael; Mar 28, 2010 at 03:59 PM.
Mar 28, 2010, 11:18 AM
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Asher -

Thanks for doing this blog, it's helpful as well as inspirational.

I'm going to order some epoxy soon, and I'm curious about US Composites. How does their resin compare to West Systems?

Mar 28, 2010, 11:47 AM
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Your Drag Queen fuselage is laid up with US Composites epoxy. Pretty much the same as West but a good deal cheaper. May not wet glass out quite as easy as some of the others but worth the price. Get the medium hardner as the pot life is pretty much what you need for everything. There is a good thread on the composites forum on RC groups comparing West, MSG, US Comp and some others. It's authored by Wyoming Windworks.
Last edited by Asher Carmichael; Apr 25, 2010 at 09:59 PM.
Mar 28, 2010, 01:33 PM
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I replaced the clevises on the empennage flying wire with Dubro HD lock clevises. One of those supplied with the kit broke while installing it on the rod so out they all went. They seemed a bit hard and brittle after the fact anyway. I removed the pin lock and pin from the clevises, bored a clearance hole for a 4-40 cap screw on one side and threaded the other side to accept the screw. I also zapped some #4 nylon washers onto the inside of the clevises to take up the space between the thin metal attachment plates. I left just enough slack on the attachment plates on the elevator halves so I could rotate them into aligment with the flying wires for transport.
I was initially a bit miffed about the lack of phenolic tubes for the wing and stab joiners. I live in Alabama and it gets pretty humid in the summers so I was concerned about swelling of the sockets and sticking of the tubes. As I began to install the wings and stabs I noticed that things were already a bit tight. I surmised the epoxy used to install the tubes between the spar caps might have shrunk and deformed the tubes as it sometimes does. A quick sanding using appropriate sized dowels wrapped with 120p sticky-back sandpaper made a beautiful sliding fit. I doubt it would have been that easy with phenolic tubes. Maybe paper tubes aren’t all that bad after all.

I exchanged the provided tailwheel with one from J&J Tailwheels. This is as nice a tailwheel as I have ever seen. The one in the kit seemed a bit light for my taste so I opted to go ahead with something a bit beefier from the start. The axles supplied in the kit were also replaced with 1/4" titanium from PSP. After the clevis broke on the empennage wires I didn’t want to take any chances on other critical hardware. Saved a tiny bit of weight, too.
You can see my general setup for the rudder pull/pull, elevator pushrods, tailwheel cable/ springs and rudder horn in the photos. The elevator pushrods are from Central Hobbies with titanium ends and aluminum tubing Zapped over the carbon fiber rods to prevent splitting.

I used Hangar 9 HD 10-32 (HAN3165) Swivel clevises on all surfaces. I do things a bit differently on the 10-32 bolts. Instead of running them completely through the surfaces and using the flathead screw, I use the studs and tap for them to just before breaking through the upper surface. I then use epoxy and plenty of wax and release agent on the studs so I can remove them at a later time. The flange nut spreads the force out on the hardpoint
Last edited by Asher Carmichael; Mar 31, 2010 at 09:46 AM.
Apr 04, 2010, 08:11 PM
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First Assembly

I finally have some shots of the assembled Decathlon sans the power train. I was able to balance using sand bags as shown. It took 180ozs (11.25lbs) to balance at 30% (7.5" from the LE). I don't know if BH planned it that way (OK, I'll say he did) but the first wing bolt hole is exactly at 7.5" from the LE. A convenient spot I'd say. A simple rope sling made things easy. Now I have some figures to work with to help determine which engine, exhaust system, prop and spinner to use.
I was also able to do some work on the wheels, pants, top hatch and few other items. I'll follow up with some thoughts and pics about those a bit later.
Apr 04, 2010, 10:00 PM
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Bits & Pieces

I masked off the top hatch perimeter and painted it with American Accents "Oil Rubbed Bronze" aerosol after sanding with 320p paper. Not quite black and not too glossy. I also decided to temporarily install the supplied main axles while I decide what to do about the wheel pants.
Those wheel pants. I've never been a big fan of them and I know there are some Decathlons without them so I may fly that way for a while. Still, I've decided to go ahead and fit them anyway. I placed them over the wheel and marked along the edges of LG using a Sharpie. I then sanded the recess to fit over the axle with a 1/2" sanding drum in the trusty Dremel. A 1-1/2" block at the back helped me gauge how deep to sand the recess and once that was done I marked the pants for the screws through the pre-bored LG. I opened up the screw holes for the "T"-nuts. I'll glass those in and I'll have a quick-change set of wheel pants. There's a shot of the Fuz with the pants in place.
I filed a flat on the axles for the set screws in the wheel collars but also added a cotter pin as shown. A little bit of safety.
There's also a shot of the 64oz B&B tank in place. Its pretty much on the CG which is where it should be. Notice the stiffeners under the mid-tray for the velcro staps.
Some of the t-nuts for the cowl were not aligned to my liking. I used an X-acto saw to help cut them away and actually it wound up being pretty easy to pop the strips off. A quick cleanup up with a sanding drum in the Dremel and some thickened epoxy put everything back where it needed to be.
Last edited by Asher Carmichael; Apr 04, 2010 at 10:29 PM.

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