FallDownGoBoom

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In reading some of the other threads in Kit Building, there are some folks thinking about building UltraSports this winter. After trying mechanical retracts on my first US, I'd almost sworn retracts off. But one of the guys at our field showed up with a H9 Jackal equiped with the new E-flite 60 sized electric retracts, and the robust nature of their construction, along with the simple operation (as well as a great deal from the LHS!) convinced me to try a set in my new US60. I thought I'd share the construction in case people might be interested. My US is built as a tail-dragger; if you're contemplating a tricycle gear set up, you'll have to figure out how to move the tank back in the fuse to accomodate the nose gear, and probably have to add some kind of pump to the fuel system.

Retracts in an US require some trade-offs, as well as a little planning before you start cutting ribs and glueing. Part of the construction is figuring out how long you need the struts to be to generate adequate prop clearance, keeping in mind that the longer you make the struts, the more they're prone to bending on less-than-perfect landings. You may also want to run larger diameter wheels if you're flying off a rough field. But wheel diameter is limited by the size of the wheel-well you can get in front of the spar. The closer to the center line of the wing the more space you'll have for a wheel-well, but you also end up cutting a big hole in the fiberglass re-inforcement in the wing center. It also helps to install the retracts one rib bay further out than shown on the plans (thanks Mike B for the tip) to help ground handling.

Long story made short: I put the wheel-wells just outside of the fuselage line on the wing, with 5.5" struts and 2.75" wheels.

After having watched all sorts of problems with the retract support structures in an unnamed P51, I wanted the retract rails to span a couple of rib bays, with plywood reinforcement whereever I ended up cutting into a rib. So I ended up with the retracts going in between rib 4 and 5. Since two ribs wouldn't give me the support I thought I needed, I ended up cutting some additional half-ribs and placing them between ribs 3-4 and 4-5. I also cut 1/16" ply rib doublers for rib 4 and 5. One tip: if you're cutting half ribs, get their outline off the appropriate rib before you glue the ribs/spars/leading/trailing edge together.

First picture is the top of the wing: rather than open rib bays, I planned to sheet the entire structure in 3/32" balsa. I did the top rear sheeting to add strength to the wing structure before I started the retract install.

2nd pic is the bottom of the wing. You can see the hinge block placement (flaps and ailerons will use hinge points).

3rd and 4th pics are the half-ribs and rib doublers. The 3-1/2 and 5-1/2 ribs are 1/8 lite ply.

FallDownGoBoom

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The next step is to glue in the 3.5 and 5.5 "ribs". The placement of the half-ribs isn't real critical, but half way between each of the real ribs works out pretty well. Try to get them as square as possible to to the real ribs.

FallDownGoBoom

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My next step was a preliminary installation of the retract to better judge the width of the retract rails, and check the overall location of the wheel well. I was leary of using 1/4" retract rails, so I used some 1/2" hardwood ply I had sitting around to fabricate the rails.

Pics 1 and 2: first cuts to rib W4 to clear the retract mechanism, and the inboard ribs to clear the strut.

Pics 3 and 4: a "dummy" installation of the retract mechanism. You can see the bevel required on the rear rail. The real rails will end up being about 5" long, all this was intended to do was check the fit and make modifications as necessary.

Pics 5 and 6: preliminary fitting of the retract mechanism between ribs W4 and W5. I found I needed to increase the bevel angle a little bit, as well as widening the read rail to get the strut to center in the eventual wheel-well location.

More tomorrow...

MinnFlyer

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I see one small mistake. You don't have the 1/16" plywood doublers on the ribs (or am I just not seeing them?)

The plywood retract rails need to be glued to the plywood rib doublers.

FallDownGoBoom

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Mike:

The doublers haven't entered the picture(s) yet... both literally and figuratively

MinnFlyer

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Ok, I was just making sure that you didn't forget them! [X(]

FallDownGoBoom

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Back to the salt mine...

A couple of unpictured steps: (1) I flipped the wing over and drew lines on the outboard side of W4 and inboard side of W5. These lines locate the retract rails on the ribs. (2) Then I cut new front and rear rails that fit between the W3.5 and W4.5 half ribs. The new rear rail was 1/4" wider than the test rail.

Now comes the moment of truth: up to this point, other than the simple notch in W4, nothing in the basic wing structure has been modified, and you can still opt for fixed gear. Well, deep breath and here goes...

I used a razor saw, hobby knife, and sanding block to cut flat areas on the bottom of the W4 and W5 ribs. The new retract rails were used to test the depth of these cuts, so now the wing looks like this picture.

FallDownGoBoom

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Test fitting the retract rails to the modified wing ribs

FallDownGoBoom

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Now to deal with the rib doublers Mike mentioned above. I used three doublers, one for each side of W4, and one for the inboard side of W5. (There were two reasons I didn't make a doubler for the outboard side of W5: (1) I was going to reinforce the rib/rail joint with a 1/2" piece of balsa, and (2) I was running out of scrap pieces of 1/16 ply to make the doublers

Pic 1 shows two of the doublers temporarily clamped in place, with pencil lines drawn to show the cut locations.

Pics 2 and 3 show the doublers before and after cutting.

FallDownGoBoom

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This is how the doublers will fit on the ribs, and then you can use the rear and front rails to finalize the fit.

FallDownGoBoom

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Now it all begins to come together: Pic 1 is one of the retracts on the rails, and Pic 2 shows the installations in both wings. Note that nothing has been glued in.

FallDownGoBoom

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Rather than gluing in all the rails at this point, it seemed natural to build the wheel wells. I've mentioned that I planned to use 2.75" wheels, so I wanted the maximum diameter wheel well I could fit into the wing, without compromising the strength of the wing by having to sacrifice some of the fiberglassed area in the center of the wing, or cutting away significant areas of the main spar or leading edge. This turned out to be an area about 3-3/8" wide located from the inboard side of W3, and extending about 1/2" past the W2 rib. At which point it hit me: how do you cut a 3.375" hole? It's not exactly a standard "size", and I was reluctant to just start cutting away at the ring structure. I did end up sanding about 1/8" into the main spar, but there's so much reinforcement in this area of the wing it shouldn't matter.

I figured I could wrap some 80 grit sand paper around a circular shape and sand out the areas I needed. All I needed was something the right diameter. Well, the Foster's lager can was too small, the 1-liter wine bottles were too small, the JD bottle was square, and the Woodbridge 1.75 liter wine bottles were too big. (Sampling the content of these bottles did not seem to affect their diameter in any fashion [&:]) But sitting on the top shelf of the fridge was an old ranch dip bottle that was... just right!

Pic 1 is testing the diameter

Pic 2 shows the areas that need to be cut out

Pic 3 & 4: the highly technical wheel well cutting tool

FallDownGoBoom

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I drew vertical lines where it was necessary to cut the W2 rib, and then very carefully began to sand the the stubs of W2 and into the spar to create the round shape for the well.

FallDownGoBoom

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Pictures from the above post...

FallDownGoBoom

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To support the wing sheeting, as well as keep the W2 stubs in place, I needed to fill in part of the space between the W1 and W2 ribs. (Mike used expanding urethane foam for this when he built his last US.) I was a bit leary about the expansion factor, so I used a block of 2" think styrofoam and cut and sanded it to shape. I expoxied it to the W2 stubs, spar, and leading edge, then sanded it so it to match the wheel well diameter.

FallDownGoBoom

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To form the inside surface of the well, I used 1/16" balsa. I edge-glued three pieces of 4" wide balsa together, then soaked them in a 50/50 mix of alcohol and water, then wrapped them around the alcohol bottle to dry into a circular shape. Once the balsa was dry, I test fit it in the area of the well until it was round and contacted the LE, spar, styrofoam, and inboard side of W3.

More tomorrow, gotta go watch "Revenge" in case I ever get rich and famous...

MinnFlyer

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Suggestion: (you may already plan to do this, but I thought I'd mention it in case you didn't)

Mount the gear to the rails and keep them in place when you glue the rails in place. This will keep them lined up with the gear mechanism so it doesn't torque the casing later when you bolt them in for good.

hrrcflyer

Fred,

A simple reminder if you don't mind. Don't forget to drill holes through the inboard ribs to get the retract lead wires back to the receiver. Please don't ask me why I'm mentioning this..........[:'(]

David

FallDownGoBoom

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Mike: a great suggestion. As we'll see, I didn't glue in the rails that way, but ended up getting lucky. My retracts ended up fitting, for all practical purposes, square to both rails. Where bolting in the retracts would have really helped would have been in maintaining the 1/16" clearance on the sides. I ended up with things a bit "tight", and eventually had to do some cutting and sanding to make the retracts work.

David: Not to worry, that's one thing I didn't forget about (although I didn't drill holes in the ribs, I ground a semi-cirular channel in the top of the styro and ribs W2 and W3.5). It's really nice to be able to work without all the wing sheeting installed.

FallDownGoBoom

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Now that all the preliminary work is done, it's time to break out the epoxy. (For those watching with eagle eyes, you have to suspend time right now, as some of these pictures were taken after the rails were glued into one of the wings. We'll cover that in the next post.)

Pic 1: the wheel well lining glued into the wing, with contact points on the spar, LE, and the styrofoam. I used 5-minute epoxy because it won't disolve the styrofam, and it dries quick. I also lost style points because it's not a perfect fit: there's a 1/4" gap where the strut will end up, instead of the ends mating perfectly.

Pic 2: The bottom side of the wheel well, trimmed to match the rib contours. I had to make sure there was enough sanded off so the sheeting would fit under the lip on the LE.

Pic 3: Both sides of the wheel well sanded down enough allow the sheeting to be applied.

FallDownGoBoom

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Now to glue in the rails. As Mike suggested, it's a great idea to screw the retract body between the rails so it fits flat on both rails. I didn't do it that way because I wasn't totally sure where the retract body would end up, and didn't want extra holes in the retract rails. For the most part, I think the retracts will end up located between ribs W4 and W5, so drill the holes and screw in the retracts.

Anyway, I brewed up a load of 30 minute epoxy and glued in all the doublers and both rails at the same time. This insured that there was good epoxy contact with all the parts. You think clamps!

FallDownGoBoom

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Once the epoxy has cured, 3/8" balsa triangle stock was glued to the inside surfaces of the rails for further reinforcement. I glued these pieces across the rails on the outboard side of W4, and the inboard and outboard side of W5.

FallDownGoBoom

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Once the glue set, I used a razor saw to cut the triangle stock on W4 and the inboard side of W5 so the retract body will fit.

FallDownGoBoom

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We now take a pause while several other things get completed on the wing. Minor stuff, like installing the aileron push rod tubes, finishing up the wing sheeting (or the center, LE, and TE sheeting, and rib caps if you're opting for open rib bays,), joining the wing, and putting down the fiberglass cloth reinforcement in the wing center.

Seriously, I haven't figured out the best order of doing all of the above. I went to great pains to make sure the W1 ribs were glued in at 90* angles to the spars, and the dihedral guage was used to check the W1's angle to the building surface. Then I finished sheeting the wing and started to join it. And when I got done, after it was carefully blocked up by 1-1/8" inch at the tip of the wing, there was still a 1/16" gap between the bottom of the W1's! Working with a 5-foot long wing is a PIA, but the best method of putting the wing together might be to build each half separately, then block up each tip and glue both halves of the wing and the W1's together at the same time. Then sheet the appropriate areas. But I digress...

As David pointed out (and I cleverly forgot to take a picture of it) make sure you put some kind of channel/hole in the foam support between ribs W1 and W2 to run the servo lead for the retract. You also need to put some type of hole in the wheel well lining at W3 to get the servo lead into the retract.

FallDownGoBoom

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So by now you have this beautifully sheeted wing, and have to figure out where the wheel wells are. Make sure you look on the bottom side of the wing! I used a 1-1/2" hole saw to make a minor opening, then sanded it out so it it to match the wheel well lining. (I loose style points here, the wells aren't perfectly circular!) In Pic 3 and 4 you can see the channels for the servo leads.