Building the Sig DO 217 Kit – Adding Retract Gear (2)

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“A scale WWII twin-engined fighter is supposed to have wheels…landing gear!” That was pretty much my first response when FLY RC asked me to consider doing a kit review of Sig’s DO 217. As presented, the kit doesn’t include any at all, although the instructions do mention an optional fixed landing gear.  “No way…it deserves better,” I insisted, and after a phone call to explain what I had in mind, the folks at Sig agreed. What follows here is an explanation of my conversion of the Sig Dornier DO 217 kit to E-Flite electric main gear retracts.

I have been working with RLG (retractable landing gear) systems for RC airplanes since the days of the first servo-operated mechanical designs, which were a sure recipe for frustration and broken airplanes. Over the years better mechanics and especially pneumatic actuation have brought vast improvement in actually making these things work where it counts…out at the flying field. However, I was ready to try my first set of electric RLG’s, and when some research determined that the new E-Flite 10-25  main gear system would easily fit a modified Sig DO 217, I was hooked. Here’s how I made it work.

The optional fixed main gear struts described in the instructions mount directly to nacelle formers N-2 at the wing leading edge. Fixed struts would work that way but the E-Flite units need a much wider mounting base and in fact would not fit in the vertical orientation that mounting on N-2 would require. I had to design and integrate a horizontal RLG mounting surface. This turned out to be an entirely new structural element, a 1/8″ aircraft plywood plate that accepts the RLG  mounting screws AND provides a structural base that transfers all the new loads into the existing structure. As it turned out, that meant that I would have to design substantial modifications to N-1, N-2 and N-3 as they are supplied in the kit. Rather than create a whole new set of drawings for those of you who want to do this on your own, I’ve provided photos of each of these parts with all the critical dimensions for the modifications drawn directly onto them…and…I did the same for the entirely new RLG mounting plate. The photos will take you all the way through the extra building steps. Just install everything in the same order you would if you were doing a fixed gear version.

The E-Flite main gear units I used are designed to be mounted with the central axis of each unit running spanwise, so that the strut will retract straight IN toward the fuselage (unless of course you’re building a Spitfire). The DO 217 mains retract straight BACK , so I turned the retract units 90 degrees and built modified struts to align the wheels properly. In the process I added a simulated double-strut assembly…check out those photos. I’ve included all those details in the captions. The bottom of each engine nacelle on the full scale DO 217 is turned into a pair of equal-sized doors that split (and open and close) at the centerline and hinge along each side. To replicate these doors and their hinge mechanisms accurately would require some heavy duty engineering. However, since this airplane is not exact scale to begin with I was comfortable approximating the appearance of the doors as they follow the strut-wheel assemblies up into the nacelle openings. Again, check out the photos. I did not go into detail on the door hinging and actuating mechanisms. Getting to this level of complexity on a model this small demands some expertise. If you’re good enough to make use of any instructions I might provide, you are probably going to want to work out those fiddly bits, as our British friends call them, on your own anyway. If you do take your own Sig DO 217 to this level, please be sure to send photos to FLY RC so the rest of us can enjoy seeing the results of your work.

Once I decided that the E-Flite 10-25 TLG units would fit this airplane, I had to engineer away to make them work. Here you can see each of the N-1, N-2 and N-3 parts as they came out of the laser cut parts sheet.On the plans below them you can see my pencilled-in structural modifications that are going to require changes in each of the N- parts.

Once I decided that the E-Flite 10-25 RLG units would fit this airplane, I had to engineer a way to make them work. Here you can see each of the N-1, N-2 and N-3 parts as they came out of the laser cut parts sheet. On them and on the plans below them you can see my pencilled-in structural modifications that are going to require changes in each of the N- parts.

Here you see oneset of the nacelle formers as-is and the other set all marked up for modification.

Here you see one set of the nacelle formers as-is and the other set all marked up for modification.

This is N-1 with all the delineations and dimensions you'll need.

This is N-1 with all the delineations and dimensions you’ll need.

Same for N-2.

Same for N-2.

AND...for N-3, too.

AND…for N-3, too.

This is an N-2 cut out so the RLG mounting plate will slip fit into it.

This is an N-2 cut out so the RLG mounting plate will slip fit into it.

This is one of the RLG plates marked out onto a sheet of 1/8" aircraft plywood.

This is one of the RLG plates marked out onto a sheet of 1/8″ aircraft plywood.

I have cut out one RLG plate and set it next to the pattern for its mate.

I have cut out one RLG plate and set it next to the pattern for its mate.

I'm beginning the assembly of one nacelle by ZAP'ing N-2 at right angles to the RLG plate.

I’m beginning the assembly of one nacelle by ZAP’ing N-2 at right angles to the RLG plate.

 

Now I have N-1 and N-2 in place using the RLG plate as an assembly crutch, with both N-4 nacelle sides in place as well.

Now I have N-1 and N-2 in place using the RLG plate as an assembly crutch, with both N-4 nacelle sides in place as well.

 

This is the same nacelle with the N-4's attached at N-3 and drawn together at the rear.

This is the same nacelle with the N-4’s attached at N-3 and drawn together at the rear.

 

The E-Flite RLG unit slips throuigh the slots I made in N-2. You are looking up from the bottom of the nacelle.

The E-Flite RLG unit slips through the slots I made in N-2. You are looking up from the bottom of the nacelle.

When the portion of the nacelle ahead of N-2 is sheeted, ther ewill be no access to the forward two mounting screw holes in the TLG unit, so I designed a mounting pocket for it to slip into. Here the front portion...the "stop plate"...is in place and the cover plate rests off to one side.

When the portion of the nacelle ahead of N-2 is sheeted, there will be no access to the forward two mounting screw holes in the RLG unit, so I designed a mounting pocket for it to slip into. Here the front portion…the “stop plate”…is in place and the cover plate rests off to one side.

Now the cover plate is in place as well. This arrangement allows mme to slide the RLG unit into place from behind and below, then hold it in place with mounting screws in the two rear holes only.

Now the cover plate is in place as well. This arrangement allows me to slide the RLG unit into place from behind and below, then hold it in place with mounting screws in the two rear holes only.

 

This is the same nacelle from the bottom, with the N-5 bottom sheeting in place.

This is the same nacelle from the bottom, with the N-5 bottom sheeting in place.

I closed up each nacelle with the remaining N- parts exactly per the instructions,mounted eaqch nacelle to the wing, again exactly by the book, and sanded the entire outer surface to finished contour. Now I'm starting the cut line that will free the lower portion of the sheet balsa nacelle to become the landing gear doors.

I closed up each nacelle with the remaining N- parts exactly per the instructions, mounted each nacelle to the wing, again exactly by the book, and sanded the entire outer surface to finished contour. Now I’m starting the cut line that will free the lower portion of the sheet balsa nacelle to become the landing gear doors.

A nice new sharp razor blade works best for me to cut the door free along the line. Never mess up a job like this by trying to economize using worn, dull blades.

A nice new sharp razor blade works best for me to cut the door free along the line. Never mess up a job like this by trying to economize using worn, dull blades.

Thisis the entire lower mnacellem skin cut free...I'll split it into two equal sized halves to make the working doors.

This is the entire lower nacelle skin cut free. I’ll split it into two equal sized halves to make the working doors.

The E-Flite RLG unit looks like this out of the box. I'm going to modify the stock steel wire gear leg to fit the DO 217.

The E-Flite RLG unit looks like this out of the box. I’m going to modify the stock steel wire gear leg to fit the DO 217.

The wire gear leg must be rotated 90 degrees so the pre-bent coil spring shock absorber will work as part of a rearward-retracting strut. I covered part of the strut with masking tape to protect it from the vise that's about to grab it.

The wire gear leg must be rotated 90 degrees so the pre-bent coil spring shock absorber will work as part of a rearward-retracting strut. I covered part of the strut with masking tape to protect it from the vise that’s about to grab it.

I'm using a high speed abrasive disc in my rotary tool to grind a new "flat" for the retaining setscrew in its new position rotated 90 degrees. I left the original flat as ground...this airplane is small enough that I'm not worried about loss of strength from multiple flats.

I’m using a high speed abrasive disc in my rotary tool to grind a new “flat” for the retaining setscrew in its new position rotated 90 degrees. I left the original flat as ground…this airplane is small enough that I’m not worried about loss of strength from multiple flats.

To create the impression of the double strut Dornier main gear while retaining the usse of theoriginal E-Flite strut, I bent the lower portion...below the coil... OUT about 5 degrees, letting the bend occur within the coil itself.

To create the impression of the double strut Dornier main gear while retaining the use of the original E-Flite strut, I bent the lower portion…below the coil… OUT about 5 degrees, letting the bend occur within the coil itself.

Next I bent the strut BACK at an identical angle so it would lie in the original plane, but offset to one side aboiut 1/2".

Next I bent the strut BACK at an identical angle so it would lie in the original plane, but offset to one side about 1/2″.

I made a pair of cross bars from 1/8" micarta to simulate the cross members of thefull scale gear. Here I'm drilling a pair of 1/8" holes where the working strut and its dummy partner will fit through.

I made a pair of cross bars from 1/8″ micarta to simulate the cross members of the full scale gear. Here I’m drilling a pair of 1/8″ holes where the working strut and its dummy partner will fit through.

Now you should be able to see where all this is going. I have the offset-bent working steel struts assembled into each RLG unit, withthe micarta cross bar slipped into final position and the stock E-Flite axle assembly attached at the lower end.

Now you should be able to see where all this is going. I have the offset-bent working steel struts assembled into each RLG unit, with the micarta cross bars slipped into final position and the stock E-Flite axle assemblies attached at the lower ends.

I made a non-loaded double strut for each assembly from a piece of 1/8" carbon fiber rod epoxied to a drilled-out wheel collar that also serves to hold the wheel in place at the lower end, and stuck the upper end of the rod through the extra hole in the micarta plate. Clothespin clamps work well ghere to keep the entire assembly quared up while the 30-minute epoxy sets hard (overnight).

I made a non-loaded double strut for each assembly from a piece of 1/8″ carbon fiber rod epoxied to a drilled-out wheel collar that also serves to hold the wheel in place at the lower end, and stuck the upper end of the rod through the extra hole in the micarta plate. Clothespin clamps work well here to keep the entire assembly squared up while the 30-minute epoxy sets hard (overnight).

here's the whole deal put together and working. This is the right nacelle. You can see the working strut(the one with the bend) extending from the stock E-Flite coil spring.At the lower (wheel) end of that strut is the stock E-Flite axle assembly which supports wheel and handles all the working loads. The rectangulkar cross bar is the one I cut from micarta...it holds the dummy carbon fiber strut in position. At the top end (inside the wheel well) the dummy strut terminates in a free end; at the lower end it fits into a drilled out wheel collar that rides on the outer end of the axle. This part of the assembly is intended to support itself during operation, but does not carry any working load.

Here’s the whole deal put together and working. This is the left nacelle. You can see the working strut (the one with the bend) extending from the stock E-Flite coil spring. At the lower (wheel) end of that strut is the stock E-Flite axle assembly which supports the wheel and handles all the working loads. The rectangular cross bar is the one I cut from micarta…it holds the dummy carbon fiber strut in position. At the top end (inside the wheel well) the dummy strut terminates in a free end; at the lower end it fits into a drilled- out wheel collar that rides on the outer end of the axle. This part of the assembly is intended to support itself during operation, but does not carry any working load.

The doors are supported by tubing-within-tubingshafts and tensioned "open" with little coil springs. The loose cord across the bay is plastic coated braided beading wire measured "just right" for the double strut to catch it during retraction and pull the doors closed. When the LG extends the coil springs in the side rails pop the doors open and hold them there...just as you see here. It works!

The doors are supported by tubing-within-tubing shafts and tensioned “open” with little coil springs. The loose cord across the bay is plastic coated braided beading wire measured “just right” for the double strut to catch it during retraction and pull the doors closed. When the LG extends the coil springs in the side rails pop the doors open and hold them there…just as you see here. It works!

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