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Hempel 38% Decathlon

Discussion in 'Giant / Scale ARF Build Threads' started by Len, Feb 3, 2015.

  1. Hi guys,
    I'm new to this site and thought I'd share with you some modification we've done to the Hempel 38% Decathlon over the last few years.
    We use the Decathlon as a front line tug for aerotowing, (www.RcAerotowing.com/forum) and most of the modifications were done to make the plane ultra reliable and easy to maintain. At an aerotowing event, its very common for the tow plane to take-off and land 200-300 times over the weekend. When a plane gets that much use, and is depended on to launch scale gliders all day long, it needs to be reliable.

    The Hempel Decathlon is a fantastic plane. The size is impressive and the flight characteristics superb.

    Off we go.

    The order was placed with Bill, http://billhempel.com/decathlon/index.php
    and a week later the plane is sitting in my shop. (Expect a big box)

    First out of the box is the giant fuselage. Most of the modifications are done to the fuse, so we'll just get right into it.

    I'm mounting a DA 150 on the nose and therefore it only required shimming the motor off the firewall to get spinner clearance. Because the plane is really set up for a 120cc motor, the canister tunnel is too short.

    Opening the tunnel is simply a matter of cutting and removing the rear cover from inside the door, extend the
    sides of the tunnel and installing a new rear cover. I made the side removable so I can access that end of the setup.

    DO2C6225.jpg DSCN0732.jpg DSCN0733.jpg DSCN2000.jpg DSCN2004.jpg DSCN2005.jpg DSCN2009.jpg DSCN2013.jpg DSCN2015.jpg
    thurmma likes this.
  2. Next up now that the tunnel is extended is to get the motor mounted so we can get and exact fit for the canisters. The DA 150 is a perfect choice for this plane, given its mission.
    Once bolted on, we figure out exactly where to mount the cans and supports. I've used both front dump and rear dump canisters on different ones, so I'll speak to both.
    The front dump cans are good, but leave the possibility of getting gunk on the gear. The rear dump are a good option to help eliminate that issue. In either case, figure out where the holes need to be and cut them
    The modifications for landing gear is next.
    The Decathlon by design has spindly gear, add to that China metal and a 4o some odd pound plane, constant take-off and landings on rough grass... What do you get? Bending gear.
    To fix this I made a quick call to TNT and we designed a different gear for the Decathlon. The new gear is as wide as the seat area, then tapers down to the bend where the axle bolts to. While not 100% scale, I think it actually looks better and definitely performs better. To make this all work, you need to make a spacer for the gear to sit on since the strut mounting tab is also bolted in this location too. A 1/8 or so plywood piece was cut to fit the gear seat area and then a notch made for the strut bracket. Carbon fiber was added for good measure ( if you're going to do it, over do it, that's my motto)

    Drill the new gear to match and you're all done with that

    IMG_0994.JPG IMG_0997.JPG IMG_0770.jpg
    thurmma likes this.
  3. The Decathlon in knife edge is actually our plane taken on its maiden flight. Bill like the picture he asked if he could use it on his site. This was Decathlon #1 in our series. lol
    Last edited: Feb 3, 2015
    bigblueswope likes this.
  4. SleepyC

    SleepyC 150cc

    Looks great! Keep it coming!
  5. Cool thread @Len . Welcome to the site and thanks for the info:way_to_go:.

  6. Thanks guys, glad to be here.

    The bottom of the plane has a defined soft spot in front of the landing gear. Because of the way I have to lift the plane to put it onto my trailer rack, my hand will eventually go through the sheeting on the bottom. Not to mention, Bill has these planes designed and built to be so light they'll hover. Which always surprises me because I've never seen a real one hover.....

    A modification that was done on one of these Decathlons was to pull the covering back on the belly, forward of the gear and pull off the sheeting. One the sheeting is off, you can apply two layers of 1/64 ply with glue between them. This becomes a micro-lam beam is is nearly indestructible.
    You don't pick more than an ounce or two, will worth the effort.
    The belly section behind the gear can get balsa doublers on the inside between the structure. Again, apply a thin layer of glue to the sheets and lay them in. Hardly and weight gain, HUGE STRENGTH,

    Here is Wayne Hovering his Hempel Decathlon with a DA 120 I believe. Awesome pilot.

    Decathlon 015.jpg
    View attachment 11221 IMG_0792.jpg IMG_0793.jpg IMG_2419.jpg IMG_2425.jpg
  7. A couple quick additions.

    On top of the plane there is a removable hatch, I installed a tray, painted it black and thats where the batteries for the airborne system mount, along with the on / off switches. Its much better than laying on the ground trying to work through a side door.

    A tow release was added inside the fuse also, this mounts right at the back edge of the removable hatch and is servo activated. A must when towing sailplanes.

    DSCN2022.jpg DSCN2029.jpg DSCN2035.jpg
  8. stangflyer

    stangflyer I like 'em "BIG"!

    Wow....very cool!!! Ever since I seen the big Decathlon's at Wenatchee, I've wanted one. May have to seriously consider one soon.
  9. The Decathlon has always been a favorite of mine, you wouldn't be disappointed.

    As you can see in the photos, I paint this interior black. I think this one was satin, however I've learned it better to go with flat.
    Making the interior black takes a little effort but really looks nice when its all done when the smoke colored windows are installed.

    On the bottom, you need to have exit air for the canisters and motor cooling, so I cut some good size holes in at the rear of the canister bay and glued in some screen that I painted white. The screen is " drywall repair" screen I purchased at the local hardware store and cleaned the sticky surface with thinner, then just painted with white spray bomb.
    Its a small detail, but looks pleasing when the plane flys over.

    Motor installation is straightforward. DA 150 bolted to the front, headers and cans. I run a three blade 28.5x12 Carbon fiber prop. We're all about keeping the noise as low as possible.

    DSCN2021.jpg IMG_0804.jpg IMG_2394.jpg DSCN2029.jpg DSCN2021.jpg DSCN2018.jpg
    stangflyer likes this.
  10. stangflyer

    stangflyer I like 'em "BIG"!

    Completely cool... What do they come in for an auw?

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