Apogee SR-72 Darkwing

boost glider

The SR-72 is an ejecting power-pod boost glider, much like one of Astron Mike's Marauders or the Estes SR-X, with an overall length near twelve inches and wingspan of about 7-1/2 inches. It is designed to be used with 13mm mini-motors. When the power-pod is ejected, it releases the elevons for glide.

Tim (van Milligan) packed it in with an order I had placed the week before for body tubes, centering rings and nose cones, and included a letter saying it was the first production kit out the door. It had been on back-order waiting for 18mm nose cones from Tim's supplier. Since the nose cones were still not in sight, Tim has produced the kit with a vac-formed nose cone and a coupon good for a replacement injection-molded (he's still waiting) or balsa (in stock) nose cone. The vac-formed nose cone and base look very good, and the only question about it (according to Tim) is how well it'll hold up to the inevitable crashes any boost glider is subject to.

The kit includes two sheets of nicely die-cut balsa, various body tubes (18mm and 13mm) and centering rings, some clay nose weight, several small (dental) rubber bands, some template sheets and a good construction manual. The photo of the elevon hook placement didn't reproduce very well (too dark to see much detail), but the rest of the instructions read well and have good illustrations. I was mildly disappointed that the balsa was die-cut rather than laser-cut like his micro-V2, but the cuts *are* clean and the pieces almost fall out of the carrier. The elevon adjustment looks like it may be a bit of work to get the initial setting, but it should hold well once set.

Construction

22 April, 1998	glide trimming
	I brought the SR-72 to show off at the April meeting of WOOSH. While waiting for everyone else to show up, I trimmed the glide of the model. Tim provided two squares of clay for balance, but I had only installed one to start with. The first few glides were pretty nose heavy (good thing there was some soft grass to land in) and I wound up removing almost half of the clay I had initially loaded in before I got a decent glide. Whe I got home after the meeting, I glued in the nose cone and re-attached the sub-rudder that broke off when it landed on the street on one of the test glides. It looks like all that's left is to get out and fly this bird!
16 April, 1998	installed launch lugs
	I lined up the launch lugs against the wing/body joint - I just prefer to have them snugged up there rather than hanging entirely out in the breeze. The streamer has been taped to the power-pod. I'm also adding some chrome tape to the front of the power-pod to help assure a safe CG/CP relationship for boost. I did this because I'm NOT using all the clay supplied for the nose cone (lighter gliders (tend to) glide longer <G>). All that remains before painting is test glides, gluing the nose cone in place and the first launch.
15 April, 1998	Finished taxes, installed other elevon stop.
	The elevon stops are tiny pieces of balsa. With my fat fingers <G>, it was a challenge to get them positioned properly. I think next time I'll have to get some tweezers to make handling these small pieces easier. After the fillets for the elevon stops dried, I installed the orthodontic rubber bands to see how close I was to the proper initial elevon setting. At this point it looks pretty good. We'll see how it goes on test glides.
11 April, 1998	added elevons.
	To help ensure I didn't glue the elevons to the wing, I cut a narrow (about 1/16" wide) piece of waxed paper and placed it between the wing and the elevon. I used a piece of masking tape on the back of the hinge line to hold the pieces while I attached the hinge material to the elevon with thin CA. It might be easier to do the hinge attachment before attaching the wings to the body tube - I may have to try this if it ever comes time for a rebuild. I also added the elevon stops to one of the wings. I was pleasantly surprised to find the elevon angle came out very close to the recommended initial setting. I was expecting to have to use gap-filling CA to build up the stop and sand down for adjustments, but it looks like I'll only need to use thin CA to strengthen the contacting surfaces of the stops.
10 April, 1998	added elevon hinges and fillets.
	I attached the elevon hinges to the wing using thin CA. I did this by positioning the hinge material on the wing, adding a drop of CA to one of the corners and pressing with a small piece of waxed paper until the CA set. Then I added sufficient CA to adhere the remainder of the front half of the hinge to the wing, again pressing it in place with waxed paper until the CA set. I then added glue fillets to the wingtips and rudders.
7 April, 1998	added rudders
	Again using the kit-supplied template I attached the rudders (double-glued and with glue rivets) to the engine pods.
5 April, 1998	Added wingtips
	I cut out the wingtip dihedral template and laminated it to the cardboard backing for a pad of paper. After trimming the laminated template, I reinforced the edges with thin CA per the kit instructions. The template made it very easy to add the wingtips at the proper dihedral angle.
3 April, 1998	Added left 'engine pod'.
	Put second fillet on bottom wing/body joints and attached left 'engine pod'. Also attached lower rudder extensions to bottoms of the engine pods.
2 April, 1998	Added right 'engine pod'.
	Added second fillet to top wing/body joints and attached right 'engine pod'.
29 March, 1998	Rounded edges of wings and wingtips.
	Attached the wings to the main body tube and applied the first glue fillet to the body/wing joints. I expect to apply two or three thin fillets to provide strength and prevent the sinkholes common to carpenter's glue fillets. I also attached the engine inlet cones to the outboard tubes. Tim recommends cyanoacrylate (CA) for this step, and I've got to agree with him. I tried tube plastic cement (Testor's), but it turned out to be too aggresive and deformed the bases of the inlet cones. It might have worked better if I had laid a bead of cement in the body tube, allowed it to dry and then used thin liquid cement to join the inlet cone to the cement bead. I was able to get them installed passably by using a sharpened dowel to center and support the tips on the outboard tubes while the glue dried, but CA is likely to work MUCH better (and faster). Once the inlet cones have fully set, I'll be punching holes in the outboard tubes so they can be attached using glue 'rivets' for a stronger bond with the wings, wingtips and rudders.
28 March, 1998	Finally, a chance to start working on the Darkwing!
	Today, I marked the body tubes, glued together the pieces for the wings and wingtips, loaded clay into the nosecone and glued it together and assembled the power pod (except for attaching the streamer). I only used one of the two pieces of clay provided in the kit. If it glides acceptably with only the one piece of clay, I'll adjust the boost CG by wrapping some chrome tape around the nose of the power pod. I'm using Elmer's ProBond Carpenter's glue for most assembly, with some specialty glues as required depending on materials. For the nose cone, I used some Tenex-7 liquid glue (in moderation!) to attach the vacu-formed base and tip. I only used a drop, counting on capillary forces to pull it into the joint, since I wasn't sure how strongly it would attack the styrene. It turned out very well.
10 March, 1998	My SR-72 kit arrived today!

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Last Updated:	11 March, 1998
Author:	Alan Rognlie
Email:	arrognli@execpc.com