Mini R/C SR-71 rocket gliders

[burkefj]

I've been pondering how to do a mini version of my SR-71 for 18mm D2.3 motors and so here is what I have so far, it may be a bit heavy in the end, we'll see. Shooting for sub 6 oz rtf. This is a 75% downscale of the kit I used to make for 24mm motors. 27.5" long, 16" wingspan.

If it is too heavy I may make iit for D-13 reloads, the 3mm stabs might have flutter issues on the higher thrust so I may need to change those to 6mm if I go that route.

Like my kit I saved weight by splitting a tube lengthwise for the engine nacelles and glued them only on the upper portion of the wing because it looks fine like that from most angles and when the model is flying you just see the bottom view. In this case they are BT-60 11" long.

The tail surfaces are 3mm depron slotted and glued into the nacelles with the slot at a slight angle, the rear of the nacelles is angle cut to clear the elevon which is full span.

For the wing/chine top view I decided to use two sheets of 3mm laminated with 3m-77 spray adhesive with a 2.5mm spar in-between the laminations. This provides a bit more stiffness than a normal single 6mm sheet piece and hides the spar. I cut the top view(had to splice a short piece near the nose since my sheets are a little small) and then layed the spar in place and laminated the other piece to the top view, then I cut out the new piece using the top view as a guide. I sanded and cut/hinged the tail surfaces then proceeded.

Total wing with spar and forward chines weighed an ounce.

The intake cones were rolled from glossy photo paper and glued to the triangular shape in the wing, the nacelles were then glued onto the wing. The intake cones are also just on the top portion.

I'm going to use BT-55 for the body tube, split lengthwise and glue it to the top of the wing only to save weight, and make the nose/cockpit from foam.

I'll mount the servos under each nacelle through the foam wing from the bottom up into the nacelle to get them near flush with the bottom of the model and avoid issues on landing.

Probably all I can do for now till the body tubes get here.

 

[David Schwantz]

Frank, I am going to ask for you to be banned unless you start making all this neat stuff into kits

 

[burkefj]

Frank, I am going to ask for you to be banned unless you start making all this neat stuff into kits

Well, you never know, something might be kittable in all this junk I'm making

 

[David Schwantz]

Should be small enough to go into the new mailing size.

 

[burkefj]

Should be small enough to go into the new mailing size.

The problem is motor supply for the D2.3 as they only run once a year maybe.. really limits interest.

 

[burkefj]

I laminated some pieces and carved a half cone and shaped it and added a cockpit. Used a soldering iron to melt it out to make room for nose weight.

 

[burkefj]

I wasn't happy with the weight so far, just too heavy, so I started over with a single ply 6mm wing, Cut the vertical stab slots in the engine nacelle tubes at 90 degrees to the lengthwise split, then trimmed off 1/4" lengthwise on the inboard portion so that the fins would get a natural cant in and remove some material and save some weight. I decided to forego the keel and leave the bottom clean. I sanded a rounded recess in the top of the wing to nestle the motor mount in so it doesn't stick below the bottom of the wing and the drag of the tail/pods should be offset by the slightly higher thrust line.

Total weight should be 5.6 oz plus paint, the previous version at the same stage was looking like 5.9 oz plus paint and was getting heavy. This gives me some margin to stay below 6 oz. 0.3 oz savings is pretty good at this size...

 

[burkefj]

I hated to have a nice airframe go to waste so I decided to reinforce my first laminated airframe for D-13 motors. I removed the 3mm stabs, re cut the slots in the nacelles for 6mm versions and glued them in place. I will use a complete body tube and nose cone for this version. Weight for the D-13 version should be about 7.5 oz rtf which will keep the boost speed reasonable.

 

[burkefj]

Servos mounted in the light model, wires routed up to the top to go forward through the body tube to the receiver. You can see on this one I have moved the servos all the way forward under the nacelles to move cg forward and used longer pushrods with support tubes to prevent flexing. Servos are nested up into the nacelles.

 

[burkefj]

I went to the lhs and found a pnc-55 cone so I trimmed out the wing and chines on the heavier wing to fit the forward tube and cone, the tube will be slotted about 2/3 length and slid on from the front and glued to the wing top and bottom. The chines will then be glued to the tube and cone and then the chines will be cut to allow the cone to come off for battery access. This is more like my larger kit was put together.

Cockpit was carved and glued to the cone.

 

[burkefj]

Servos installed in the heavier model, nested up inside the engine nacelles and wires will go up front through the body tube into the rx and battery in the nose cone. Servos further back to compensate for the nose cone mass.

 

[burkefj]

Added the rail buttons with styrene disk supports on the light version, once painted the bottom will look pretty clean

 

[burkefj]

I got my BT-55 tube from bms snd I cut it to 23" length snd I split the tube down the center and glued it in place on the top of the on wing light D2.3 model. I had to remove the rx case to fit inside the tube, but otherwise worked as planned. I cut a hatch for the battery and proceeded to add lead to balance, it took a bit more than I hoped so I am at 6.1 oz rtf w/o paint, we'll see how it flies tomorrow.

 

[burkefj]

Here is the heavy D-13 version, I was able to use the the remaing tube pieces and finish her off with 3/16" of tube to spare. Took about 16 grams nose weight, battery and rx in nose cone. 6.9 oz rtf as shown. The rear 17" of the tube is cut to fit over the wing, then slid on from the front and glued with the rail buttons already installed on the bottom, then the chines are glued to the sides of the forward tube and nose cone. The chines are then cut to free the nose cone to access the battery and receiver inside and to add nose weight.

 

[burkefj]

Had just enough paint in one small can to do both, ready to fly the light one for D2.3 motors is 6.3 oz, the heavy version for D-13 motors is 7.1 oz

 

[burkefj]

The light one flew great today!

 

[burkefj]

Put some vinyl trim on the light version, will get some tail art from stickershock next week to finish it off

 

[burkefj]

And here is the heavier D-13 version

 

[burkefj]

I decided to do one more lighter version for D2.3 by hand rolling 3mm depron for the nacelles, same basic construction as the first one, but because it saves .3 oz in the tail and .2 in the nose it came out at 1/2 ounce lighter 5.75 oz rtf which is pretty significant. I routed the servo wires a bit better so I could get the rx a bit further forward as well as the battery and nose weight...I had to splice the lengthwise split BT-55 to make it long enough using part of the cutoff as a half coupler...I already had the foam nose made for my D-13 model when I decided to use an actual plastic nose cone instead, so that saved time.

 

[ReynoldsSlumber]

Nice!

BTW for inspiration along these lines, have you seen this fellow who adapts papercraft scale airplane plans to sheet Depron in order to craft R/C models out of them? He's on Paper Modelers and YouTube.

 

[burkefj]

Nice!

BTW for inspiration along these lines, have you seen this fellow who adapts papercraft scale airplane plans to sheet Depron in order to craft R/C models out of them? He's on Paper Modelers and YouTube.

There are some real artists out there.

 

[David Schwantz]

For even lighter pushrods, have you tried pull pull cables out of thin kevlar string? You stuff always looks great.

 

[burkefj]

For even lighter pushrods, have you tried pull pull cables out of thin kevlar string? You stuff always looks great.

Thank you, I have done that in specific applications but that requires top and bottom connections, a bit fiddly for this model, the pushrods weighed 3 grams total in this case so not a huge %

 

[burkefj]

Finished up paint and decals this morning, right at 5.8oz rtf so a worthwhile project saving a half an ounce, about 10%

 

[burkefj]

Created templates for a kit version of the D-13 powered model and took assembly photos for the instructions.

 

[burkefj]

On the kit I recommend cutting pockets in each side of the body tube and laying the servos on their side and having the servo arms point down for simplicity, instead of mounting them into the nacelles from the bottom and routing/taping the wires onto the wing before adding the body tube like I did on my experimental version.

On this one I experimented with insetting the servos inline into the body tube flush with the top of the body tube so the rail will clear the arms as it passes right over them. It required different length pushrods and adding a foam strip to the bottom of the servos to set how far in they sit and give a way to glue them in, but it made a nice clean low drag install, but is more finicky/critical in installing them and would be more prone to mistakes.

 

[burkefj]

Prototype built from kit templates done..A bit lighter than my scratch built first one at sub 7oz rtf with D-13

 

[tab28682]

Awesome work, Frank!

 

[burkefj]

Test flights on D-13's went great on both kit prototypes, just a few trim adjustments for boost and glide, then nice straight boosts, good glide and flare for landing, about 45-50 second glide times. I also flew my lighter D2.3 model and it also flew outstanding.

I got the tail art from stickershock and finished them off.

 

[burkefj]

This design is so neat and fun to build I decided to do a Fedex version...I covered the body tube in white vinyl then cut the wing slot and trimmed out the vinyl where the chines glue to the tube. Worked great and no paint needed. Tails covered in purple vinyl. Rest of the markings are hand cut vinyl with an exacto and my mischevious little hands. Stickershock will cut the fedex text for me.