Building Frank's SR-71

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

aerostadt

Well-Known Member
TRF Supporter
Joined
Oct 26, 2009
Messages
4,471
Reaction score
1,119
Location
Brigham City, UT
Last Fall when I crashed my HobbyLab SR-71 Frank Burke suggested to me that I might want to build his profile SR-71. As many of you know Frank cranks out these profile rocket gliders very fast and has had an article published in Sport Rocketry. So, I have started on this one. A survey of the build can be seen on Frank's RC group posting here:

https://www.rcgroups.com/forums/showthread.php?t=1524183

Here's Frank's first message to me:

Hi, if you are at all rc savy and want to try another sr-71, I have free plans on my rcgroups page for a profile version, it weighs about half of the hobbylab version although a bit bigger, but does require the lighter thrust E-6 aerotech reloads, it's pretty docile on glide as the wing loading is low. It fits onto one sheet of 6mm depron. I can help with any advice if interested.
 
Last edited:
The plans can be downloaded here.

View attachment SR-71V2.pdf

There are 9 pages on 8.5" X 11" sheets with a scale on the first page to check if the pages copied correctly. The pages copied correctly for me and the plans are full scale, although, you need to cut and tape the pages together to make a full scale pattern for tracing onto Depron. Here's some more information from Frank:

It should fit onto a single sheet of 39x27" 6mm depron and that is really the lightest/best material for it. Two servos and use any modern radio for elevon mixing.

It's light, so you don't want to fly it on E-15 or E-18 reloads, the E-6 RC reload is perfect and will take it to about 700 feet. glide and landing are pretty slow/tame.

Let me know if/when you get around to trying it and I can help with more pictures if needed. On these small planes, I just use thick foam safe CA to glue the servo in place once I've set the surface at neutral.
 
Last edited:
I ordered my 27" x 39" 6 mm Depron from RCForm that I found on the internet. The first order was bent on shipping and the manufacturer did not reimburse me. I ordered a new sheet and it arrived in good shape. (As Frank says the entire model can be cut on one sheet.) The Depron was only about $9, but there is about another $9 for shipping. I also ordered one 3.2 mm x 1000 mm carbon rod and one 1 mm x 1000 mm carbon strip at the same time. It took me some time to cut and tape the pattern, but the long time might have been due to the holidays and other commitments. Once, I was determined to start cutting foam, things went pretty fast.

IMG_2789.jpg IMG_2788.jpg IMG_2786.jpg
IMG_2785.jpg IMG_2784.jpg IMG_2782.jpg

I used a black Sharpie that was probably a little bit too wide in the tip, but things ended up working out pretty good. I have since found a finer tip black Sharpie that works a little bit better. The foam is simply cut with an Xacto knife.
 
Last edited:
Along with cutting the fuselage and wing profiles, one needs to cut the engine nacelles. If modeler is really adept, he can cut the nacelle slots in the wings at a slant so that the nacelle fins can be angled toward the fuselage later. One can clearly see on the HobbyLab model that the nacelle fins are slanted toward the fuselage. I did not do this and sanded things later in order to get the fins slanted.

IMG_2790.jpg
 
Looking good so far, found the thread, I'm here if you have any questions.

The angle in of the nacelles isn't critical, just for the look.....Note, you want to cut and hinge the elevons before you attach the nacells, so you can be sure the V cutout on the nacelle at the rear will clear them.....

Here is how I would approach the build:

Cut out templates and trace them on the foam.
Cut out the outlines of the pieces and sand the edges round if you want.
Cut the slots in the wing top view portion using a straight edge.
Use the top view portion as a guide to mark where the tabs and slots will be on the fuselage side view, then cut them out and test fit.
Cut the nacelle slots in the wing, and then mark and cut the tabs/slots on the nacelle pieces. You can cut the nacelle slots at a slight angle so that the nacelle pieces angle inward a bit, it isn't critical for flight, just want to be symmetric left/right.
Cut the elevons out and cut a 45 degree bevel on the leading edge bottom of the control surfaces.
Hinge the control surfaces.
Cut and insert/glue the wing spar and then tape over with hinge tape.
Slide on the fuse side view piece and glue in place once happy with the alignment.
Slide on and glue the nacelles.
Glue the carbon rods onto the neck to reinforce it.
Install the elevon micron horns on the bottom of the surface, glue them in place about 1.25" in from the inboard end.
Make your pushrods, and install them onto the horns and servos.
Glue the servos in place once they are centered with the radio on so that the surface is at zero deflection.
Make the motor mount and glue it in place.
Install your receiver and bec as far forward as possible for CG.
Install velcro to mount your battery as far forward as possible.
Tape down all wiring with the hinge tape.
Install a loaded motor, and check CG, add nose weight if needed, or move the battery back if needed.
Remove receiver and bec, tape around servo, and paint the model flat black using model master flat black spray.
Re-install all components and set your trims to neutral for boost, and about 3/16" up for glide.

Bob has done things in a different order in a few places because I wasn't smart enough to volunteer to tell him first.....so it makes things just a bit harder to fit like the hinges....but not impossible.
 
Last edited:
Looking good so far, found the thread, I'm here if you have any questions.

The angle in of the nacelles isn't critical, just for the look.....Note, you want to cut and hinge the elevons before you attach the nacells, so you can be sure the V cutout on the nacelle at the rear will clear them.....

Frank

Frank,
I have already attached the nacelles, but from looking at the model I don't think that this will be a problem.

It is probably a good idea to cut the elevons out of the wing before gluing on the fuselage. This is probably not absolutely necessary, but is most likely easier. Also there is a notch to cut out of the elevons. Inspection of Frank's photos on the RCGroup post shows all these parts. Also, it is probably good to put in the wing spar or carbon 3 mm strip before gluing the fuselage to the wing. The location of the spar is found by looking at Frank's drawings. I did the spar out of order, but it worked out with some good tips from Frank. You want to use safe foam (no CA) glue. I bought some from my local Hobby Shop. The bottle says 12 hours curing time, but the stuff gets tacky within a few minutes, which I find to be good, because parts tend to stay in place without much trouble after a few minutes.

IMG_2801.jpg

Following Frank's instructions I slit the fuselage profile from the first slot down the center to the aft end, but left the nose to the first slot connected. Then I could slide the fuselage starting at the front of the wing until the fuselage was entirely on. I had a piece of coarse sand paper that worked good for adjusting the tabs and slots. When the dry fit was satisfactory, I just ran glue along the joint on all sides.

IMG_2794.jpg IMG_2796.jpg

Having forgotten to put in the wing spar (3 mm carbon strip) I followed Frank's suggestion to put a light cut on the wing according to the drawing location and a small slit on the vertical fuselage profile at the wing joint. I then slid in the carbon strip starting at one wing edge through the profile all the way to other slide. I may have moved the spar about 1/8" back in order to miss a heavy glue spot on the profile joint. I did a few dry fits and re-cut things lightly in order to get a good fit without cutting too deep into the wing. I then glued the spar into place. The spar goes on the underside. Frank says to use hinge tape on top of the spar, also, but I am waiting for my hinge tape to arrive in the mail.

IMG_2797.jpg IMG_2798.jpg
 
Last edited:
I just found my drawings for the 1/16th scale SR-71 I wanted to make a few years ago (balsa static model). My plans were to making it much like the old Guillows' kits but add in a trio of carbon fiber tubes that would allow me to assemble and disassemble it much like you do with fiberglass tent poles. I still want to make it, but now I'm thinking that this might be a little more fun.
 
I have glued the nacelles to the wings. BTW I thought of buying gray depron, but Frank says that gray might be weaker. I split the nacelles in half in the same manner as the main fuselage starting at the rear of the forward slot and proceeding to the aft end, so that the forward end of the nacelle is still connected. I sanded the slots in a manner such that the nacelles will tilt towards the center fuselage. I then slid the nacelles on to the wing starting at the front. Once I was satisfied with the dry fit I just ran a thin bead of glue along the joint. Actually, after a few minutes I realized that I made a mistake on the first nacelle and had glued it in upside down. I was surprised that I could pull the nacelle off with some force, but, nevertheless, the nacelle broke in one spot. However, I proceeded to put the nacelle back on with some additional glue including gluing the broken nacelle at the break and it seemed to work pretty good. I was surprised that the construction was that forgiving.

IMG_2799.jpg
 
Last edited:
Looks good so far, you know you can buy the accellerator spray at the hobby shop, that will help to set the CA.

I can't tell from the picture, is the fuse really straight from front to back, it looks like it has a little jog in the middle....

Frank
 
Yes, there is a jog (I would say slight) on the aft end or lack of straightness of the fuselage profile towards the aft end. Also, in the same vicinity the fuselage is not exactly perpendicular to the wing. At some point in the construction I noticed it after the glue had dried. It is a testimony to my lacking to pay attention to details. Probably when I was sliding the fuselage onto the wing, was paying more attention to the front of the model. It is less than perfect, but I am hoping that it will not affect flight too much
 
Last edited:
When the RCFoam sent me the 2 Depron sheets the first time the 1 meter carbon rod in the same box was broken into pieces. One piece was about 22.5" long and the other about 16.5" long. I asked Frank if the carbon rod was necessary and this was his reply.

"Yes, you do need some sort of rod along the fuselage to help protect the skinny neck from damage.....I didn't do it on my original one and didn't have any problem, but if you pull a hard manouver under thrust you may have an issue, I saw one guy who built one as a pusher electric with a heavy battery up front and did a hard loop and broke the neck. Alternatively I guess you could do a doubler for the fuselage top and bottom portions to add strength...you only need it to be long enough to go near the nose back past the wing/neck joint a few inches, not sure where the break in your carbon rod was.......you may have a piece long enough."
 
When the RCFoam sent me the 2 Depron sheets the first time the 1 meter carbon rod in the same box was broken into pieces. One piece was about 22.5" long and the other about 16.5" long. I asked Frank if the carbon rod was necessary and this was his reply.

"Yes, you do need some sort of rod along the fuselage to help protect the skinny neck from damage.....I didn't do it on my original one and didn't have any problem, but if you pull a hard manouver under thrust you may have an issue, I saw one guy who built one as a pusher electric with a heavy battery up front and did a hard loop and broke the neck. Alternatively I guess you could do a doubler for the fuselage top and bottom portions to add strength...you only need it to be long enough to go near the nose back past the wing/neck joint a few inches, not sure where the break in your carbon rod was.......you may have a piece long enough."

"I don't think you need the rod to go all the way to the tip of the nose, it's really to protect the neck up to the battery. If you can put the carbon rod from the spar forward enough to where the battery will go that's good enough. You could glue the long piece on the bottom of one side and the short rod on the other side just to double up on the neck area, it doesn't add much weight."

I followed Frank's advice and glued the long carbon rod to about 1.75 inches from the nose. The other piece is glued from wing spar going forward as shown in the photo.

IMG_2809.jpg
 
Last edited:
Putting in the motor mount tube is simple, but I may have made it a little bit more difficult than necessary. I found that two different E6 motors are available. The Apogee Medalist, which is single-use, is fairly expensive, but I have one in my inventory and I want to use it. The other choice is the Aerotech RMS 24/40 (re-usable) case. Frank pointed out that E6 re-loads are available at Red Arrow and the price is pretty good. When you go to that website use the pull-down menu that gives rocket glider motors. If you use the pull-down menu for Aerotech, you will miss the E6 reloads. After looking at the two different motors I realized that they have different lengths. The overall length is different and the main 24 mm barrel length is different.

IMG_2820.jpg

So I made the motor overall motor mount tube to be 3.75" long and I cut off one end of the metal engine clip. I took a standard Apogee 24 mm centering ring and put a piece of a wooden dowel in the center and glued that into the motor tube with yellow glue to make a solid bulkhead. I then bent back the forward end of the engine clip so that it would not penetrate the motor tube and I used a small piece of drywall tape to epoxy the clip to the side of the motor tube (I had to use masking tape to hold down the edges of the drywall tape until the epoxy cure. I then used the Foam Safe glue to glue the motor tube into the mainframe. Frank says that the Foam Safe glue is sufficient to hold the motor tube in place. I glued a small piece of green centering ring to help secure the forward engine clip, but this may not be necessary.

IMG_2811.jpg IMG_2819.jpg
 
Last edited:
I just use the dubro hinge tape to hold the motor hook in place. Since there is no ejection charge you don't really need a hook, you can just use a small piece of tape to keep the motor from falling out and making the model too nose heavy post boost. I'd make the tube so that the motor is as far forward as possible using the longest motor, and then just use a spacer for the shorter 24/40 case, in reality the 24/40 case has a lip and it won't go past the motor tube end anyway.

Frank
 
Here is how I do the hinge.

I take the control surface, turn it upside down and cut a 45 degree bevel on the leading edge bottom.

I then flip it over so it is on top of the wing, and apply the first layer of hinge tape.

I then move the control surface down so it is bottomed out on the 45 degree bevel and apply the tape to the top side.

The advantage of this is the tape seals the gap, there is absolutely no binding, and done this way the tape cannot come loose and allow the control surface to move away from the wing. If you try to do a 45 degree bevel on each side and then try to tape both sides with the tape going into the bevel it can loosen and pull way over time changing your trim. I don't trust flex type hinges where you have to slot the foam and insert them to work well on the foam and they are never as free moving as a tape hinge. You can also use monokote trim to do this, however I've found once the trim monokote is nicked it will tear, the dubro hinge tape(blenderm 3M) will never tear.


Frank

WP_20150128_001.jpg

WP_20150128_002.jpg

WP_20150128_003.jpg

WP_20150128_004.jpg

WP_20150128_005.jpg

WP_20150128_006.jpg

WP_20150128_007.jpg

WP_20150128_008.jpg
 
Last edited:
Frank,
Thanks for the tips. They look good. I had already sanded my elevon joint edge into roughly a V-shape as described on the drawings, so I decided to attach as they were to the aft end of the wing. To some extent I pushed the joint tape into the joint as much as I could with my fingernail. I noticed that I needed to keep the inside edge away from the wing-motor-mount edge in order for the elevon to move smoothly. I applied joint tape at 4 locations per elevon; two on top on either side of the fin and likewise on the bottom. The elevons do move freely. I have 15 yards of joint tape that I bought from Tower Hobbies. I am thinking that if I made a mistake that I can recover from, I will have plenty of tape for corrections.

IMG_2833.jpg IMG_2834.jpg IMG_2829.jpg IMG_2835.jpg

IMG_2830.jpg

I need to mark the location of the torque rod, the control horns, and the servo's. It would have been easier, if I had done this back when the wing was flat and not assembled. I have some length of torque rod, but it needs to be cut to length and, Frank, you talk about z-bends. Do I need any special tools for cutting and bending?
 
Last edited:
Bob, you aren't making torque rods, just pushrods, I make the pushrod about 5" long and just use a needle nose pliers to make a z bend on each end, one facing one way and one facing the other. I'll show a picture.

WP_20150129_003.jpg
 
I used the wing pattern designs to draw the location of the push rods with a black sharpie as shown below. With this information one can find where the micro control horns and the servo's will go.

IMG_2840.jpg

I bought the micro control horns from Tower Hobbies. Each control horn has two prongs that go into the elevon once the horn is positioned over the hinge center line. I placed the horns over the sharpie black line and it made marks that I used to poke holes in the depron with a small nail. I then used 5 minute epoxy to put the horns in place. Frank has used CA and CA accelerator for the horn placement.

IMG_2842.jpg IMG_2844.jpg

IMG_2853.jpg IMG_2849.jpg IMG_2848.jpg
 
Last edited:
Following Frank's parts list on his drawing I bought two HS-55 feather servo's from Tower Hobbies and I cut off 3 servo arms with a wire cutter tool.

IMG_2847.jpg IMG_2856.jpg

Before I glued on the servo's I checked to see if the 0.032" diameter push rods would fit through the holes on the servo arm and the control horn. They fit the servo arm hole fine, but did not fit in the control horn holes (4), so I drilled them out slightly with the second smallest drill bit in my Dremel set. I am taking the push rods from a push rod system kit that I bought from Tower Hobbies. I will not be using all the parts in the kit. I then attached the servo's to the Depron with 5-minute epoxy using Frank's photos from the first post as a reference.

IMG_2858.jpg IMG_2859.jpg IMG_2860.jpg IMG_2861.jpg
 
Bob, typically I use the long arm in the servo package with just two arms for my servo connection, and cut off one arm. This gets me a bit more throw but you can probably increase throw with your radio to get what you need if the throw is not sufficient. Did you glue the servos in place after you connected everything to the pushrod? that way you get a zero control surface without using any trim, if not you can use the trims to zero them.
 
Frank, I did not put the push rods in place, yet. I did notice that I can unscrew the servo arms, but it takes a tiny screw driver. It would have been easier to unscrew the servo arms before gluing them in place, but perhaps I can unscrew them later. I am thinking that I might put the push rods in later after the receiver and battery are in place to see how much throw there is. If I do wait to put in the other electronics, I think I am about ready to paint. I still need to put on the launch lug. I do have a Krylon Chalkboard black paint spray can that I used for HobbyLab SR-71. I am wondering if it would be safe to use on this model. What would you recommend?
 
The only paint I trust completely on bare depron is Model Master(testors) small cans. For flat black, I think #1949, it dries fast and covers well, assume anything else will melt the foam. You need to try a large test piece if you want to try anything else. Hobby shops should carry it. Regular testors small cans may work as well #1249, it's the enamel paint.

Frank
 
Last edited:
I tried the Krylon Chalkboard spray paint on a test sample of depron and it seemed to work alright, but then the nozzle clogged. So, I ordered the Testors flat black #1949.
 
Back
Top