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Discussion in 'Rocket Boosted Gliders' started by Rktman, Jun 24, 2019.
What is dope (brand, etc.)? Briefly, how do you do the tissue covering?
Since dope is basically nitrocellulose lacquer, I use a far less expense alternative than the butyrate dope from SIG, Brodak, etc.: Miniwax Clear Brushing Lacquer. I dilute it 40% lacquer thinner to 60% lacquer (sometimes 50/50).
When I first started, I used to paint on a coat or two, let dry, then lay the tissue and wet it with thinner. That would briefly liquify the lacquer beneath so that the tissue would stick. Messy, and since the tissue would adhere in seconds, it wasn't uncommon to have wrinkles and small bubbles that I couldn't remove even by spraying the tissue afterward with alcohol/water to shrink it.
I find it much easier to use a glue stick. I use the purple stuff that dries clear, so I can see if I've missed any spots. I also have about 5 minutes before the glue sets (vs about 20 seconds with lacquer thinner) so I can pull out any wrinkles before they become permanent. Once the tissue is in place, I give it about a half an hour to make sure that the glue is completely dry everywhere, then brush on a coat or two of lacquer to seal it.
Beginning the tissuing process today. At least I can work indoors in air conditioned comfort instead of a stifling hot fume-filled garage that had my eyes watering. And I actually sort of enjoy this part.
Most times it be like:
Then the very next one be like:
Then I be like: #@*^%<*&!!
After years of tissuing I thought I’d perfected my technique and rarely if ever have an issue laying it down wrinkle-free… but not in this instance. Got the top side of the right wing adhered smoothly and effortlessly but for some reason the left side is refusing to cooperate. Same sheet of tissue, same side down, but $##@! bubbles and wrinkles everywhere. Wouldn’t bother me if I weren’t so OCD about getting it applied perfectly. (Well, maybe not quite so OCD; if I were I’d sand it all off and start over). Guess now would be a good time to take a coffee break.
Well, I guess that break lasted a little longer than I expected. Got sidetracked with an easy spool build and ordering parts for another retro-clone before deciding to take a drive to test fly a scratch-built. Doing not-frustrating things cleared out the mental cobwebs and made me realize the problem was likely too thin glue coverage or missing some spots, as well as wrangling a much larger sheet of tissue over a much bigger area of balsa than I’ve ever had to. The purple glue stick I use is supposed to show where you’ve already laid down glue, and it works well enough on white paper…but balsa isn’t exactly white. More like straw-colored and textured, making it difficult to see the glue. So moving forward, all parts get a heavy coat and I’ll hold them at an angle to a light source so I can see the reflection from the wet glue to tell which areas didn’t get any.
Made quite a lot of progress. I decided what the heck and tissued everything for color as well as strength, especially as I’ll probably move up to an E11 after its first flight on a D12 (which I’m concerned may not get it to a safe enough height to deploy the canard flap. The bare balsa wings alone were far heavier than I expected). Here it is, rudders attached and waiting to be assembled.
As the de facto “backbone” that most everything is attached to, the fuse bottom is the key part that goes in first.
Now that the wings and fuselage bottom are together, I can exactly reference the CG location that was marked on the balsa: it’s 8 7/16” from the aft end, measured from the slot in the bottom of the wings as shown below.
Doesn’t look like much, but this unassuming part is the bulkhead that will not only support the fuse sides and canard, it will take the brunt of the piston impact that deploys the canard flap.
It’s cemented into the forward end of the ejection vent hole.
Been looking at other motor alternatives: what is your opinion on the Aerotech E18-4W (longer sustained peak thrust and 2.1 sec. burn time) and the Aerotech E15-4W (peak thrust happens almost immediately but at a lower max thrust, though not sustained as long as the E18, and has a shorter 1.6 sec burn duration)? I'm thinking the E18 would get it off the pad at a decent speed and maintain it longer for a straighter trajectory, and since it has a longer burn time than a D12 it might loft it to a safer height?
I agree about the E18-4W . . . You need a good "kick" off the pad, followed by a decent sustainer.
30.357 @ 0.134s, that's a kick in the pants! Sim it on all three of them, ballpark how it does.
Wish I could. Don't have Rocksim, and Openrocket, which I do have, doesn't do glider sims (or I haven't figured out how if it does).
Remember, the E-15 does not exist any more in the previous form of 40ns, the new ones with thrust ring built in and forward ejection reservoir are rebranded E-20's which are something like 35ns. Aerotech changed them but never re-certed because their argument was they were close enough in performance. Thrustcurve still shows the old E-15 40ns performance curve.
I've never had any problem with mine (built stock, bare balsa) on D12-3 and E9-4, even in mild wind. I've seen them flown on E18s and it works, not sure what delay was being used.
The E15 is still listed on Aerotech's 2018 Master Motor matrix so has it actually been dropped? The E20 is a slightly shorter motor so I didn't consider it.
Thanks, I had considered it until I read E9s have been discontinued.
The current E15s are the same as the old E20s. As always, how or why Aerotech sells the same motor under two different designations remains a mystery to me.
What do you mean about "shorter" though? The E15s I have used fit perfectly into standard 2.75" 24mm motor mounts... was the old one actually too big for that?
On the NAR Certified Motors page of their website (https://www.nar.org/SandT/pdf/Aerotech/E20.pdf) they list the E20 as 24mm × 65mm instead of 70mm. Possibly a typo?
Obviously if it flies OK on an E9 it will fly OK on an E12 as well, although I haven't flown mine on one.
I'm pretty sure it will fly all right on an E20, especially given your strengthening. You'll probably want to remove some of the BP as Aerotech motors tend to have larger ejection charges than Estes.
The E12 has a nice thrust curve and some good reviews. Unfortunately my engine block is already in place to fit 70mm motors so the longer Estes E12 won't fit (and the E20 is a tad too short). If it weren't for that they both look like they would've been good candidates.
You can add a spacer for short. Long can be a bit harder to work around if you have a motor hook installed.
The E20 has a thrust ring and should work fine, it doesn't need a block at all. It's unfortunate that you put a block in that precludes the use of 95mm BP motors.
E20s are the normal length, thrustcurve is a typo. I said the e15 doesn't exist in the previous 49ns form but do in the e20 35ns format with e15 name, Gary said they were dropping the three packs though.
Forward fuselage sides are in.
I have to admit I goofed with the bulkhead: It stuck up about 1/8” higher than the forward fuse sides so I sanded it flush because I thought it was a production error. Nope, my mistake; it’s supposed to fit in a matching slot on the underside of the canard wing. Totally my bad for missing that in the instructions, but luckily not a critical mistake.
Thanks for all the feedback on motor suggestions you guys. Now I have quite a few alternatives if the D12 doesn't work out (decided to maiden it with the D12 after all). And thanks to y'all I'm more familiar with the value of thrust curves in selecting appropriate motors for what you're flying and weather conditions.
Found a really cool use for this old tool. Normally I’d use a pin to laboriously poke small holes into critical balsa edges to be joined to let the glue soak in deep and grab tight. Using this perforator tool it took me only seconds instead of minutes. I haven’t thought about this tool in decades. It was given to me by a retiring graphic designer and was used to make perforations for things like tear off coupons or mail-in cards when doing mock-ups of print pieces. By the time I inherited it everything had gone digital, though as a former graphic designer I did use it a few times when doing mock-ups to show management. I’m happy that a neat vintage antique has found a new use.
Okay, guess I got a bit off-topic there. Moving on…
I decided to enlarge the vent hole to be safe. The stock opening is only 1/8” in diameter (the size a paper hole punch makes) and I feel it’s too small to adequately vent the ejection charge pressure rapidly enough. It’s probably the prime reason they tend to spit their motors. One review even mentioned that the fuselage shattered or ruptured just aft of the canard when the ejection charge fired.
My RGs too often fry their pods on their first flights and on a number of occasions have ruptured the pod as well with a vent hole that size.
Since the Thunder has a far more generous opening in the fuselage bottom than its 13mm sibling, I opened the vent hole up to the full ¼” width and extended it longitudinally as far back as I could without uncovering the balsa piston. It’s now almost 3X the original area and should help noticeably with the backpressure.
I was pleasantly surprised to discover how thick, sturdy and high quality the tube was; much more so than “standard” 24mm tubes. I’m guessing that it’s BMS's Heavy Wall tube, since that’s where the kits were fabricated. Nice!
Did some final adjustment sanding on the balsa actuation piston to ensure it slides freely but is still snug enough to hold the canard in place once the ejection charge forces it forward. Although formed of hard balsa, I coated its motor-facing end with a thick protective coat of epoxy to keep it from getting fried
To deal with any motor-spitting tendencies, I decided to install a motor retainer in the MMTs aft end. I didn’t want to mess with trying to install an engine hook (which would have to go on the outside since there’s no room to run it internally under the fuselage sides). I felt the retainer would not only be an easier option, it would also look better and I wouldn’t have to deal with trying to somehow reinforce a wire hook where it penetrated the balsa sides into the MMT.
One possibility for broken fuselages on these is having a vent hole on only one side. On my models I put a symmetrical hole on the opposite side as well. I didn't get any busted fuselages, but my number of flights isn't enough to be statistically significant.
Here's a flight on an E9-4 just to see what typical flights look like.
Seeing the actual boost should be helpful to Eric, I'm sure.
The CG must have been too far forward. That was a "ballistic glide", aka a "controlled crash".
A few more various Ecee Thunder flight video's.
I initially considered putting another smaller hole on top, but since I was able to enlarge the vent hole quite a bit I thought I'd leave well enough alone and not possibly mess up the sliding of the piston. In this case I suspect any broken fuselage may more likely be the result of the piston slamming so hard into the bulkhead and/or the canard flap being slapped really hard against the fuselage as well.
I do agree that having multiple vent holes (with at least two opposite each other) is good insurance against motor pod damage. I have an RG equipped with 3 generous square vent holes 90 degrees apart and the paint around it isn't even scorched.
I agree with what that kid said: "EPIC FLIGHT!"
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