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  1. #1

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    Scratch Building 1/4-Scale Pitts Model-12

    This started as a plan-build for a Pitts S2A and grew horns. Big, nasty pointy-horns.

    Long story ( http://www.rcuniverse.com/forum/m_11048479/tm.htm) short: the plans were mis-scaled and while wrestling with what to do about that I finnaly decided to take the plunge and just custom-design the whole thing.

    Current critical dimentions are:
    wingspan - 62"
    length - 61" (tip of spinner to end of tail, which seems 'cheating' somehow but seems to be the standard) too short coupled? maybe.. this might change
    Wing are - approx 1200 sq-in
    Take-off weight - approx 11lb
    Power-plant - OS33GT

    Goals:
    - Learn how to design an airplane
    - Use somewhat affordable parts
    - Easy-to-assemble interlocking kit, low part-count.
    - Design for unlimited bone-crunching rip-the-wings-off aerobatics

    Been designing it in 3D using TurboCAD, this is the current snapshot, I consider it 95% complete (still engineering the wing design somewhat and the top hatch/canopy section needs some love)


    A litte bit about me: I've been in the hobby for 20-ish years off and on, built several kits and ARFs. I have a passion for aircraft so much so that I went to college as an Aeronautical Engineer.. and the math kicked my butt into Mechanical Engineering which I graduated as. Which ended up being futile.. my "other" hobby (computers) ended up being my career and I am currently a game-programmer. I hold a private-pilot's licence with an instrument rating and have always wanted to design my own plane

    And holy research, Batman! I cannot beleive how much goes into designing one! Every dimension is the result of a collection of dependancies on every *other* dimension. I have been pouring through books and articles and papers on aeronautical design and can't believe the staggering mountain of analysis the human mind has dedicated to aircraft design. I started out thinking it would be pretty easy to throw down a working plan that was at least mediocre but at this point I am just praying for stability, and expecting to have to actually build/fly/iterate a few times before I would have a worthwhile design, if ever. It is not an exaggeration to say that I have re-designed ever part on this plane at least a half-dozen times, and changed critical dimentions based on new considerations/calculation/WAGs at least that many times.

    It's fun, but a load of work and nail-biting. How heavy is it actually going to be? Computing where the CG is supposed to be is actually pretty straightforward, but where will it ACTUALLY be? Can I tell without building one? Will the first Alien-Wall I do rip the wings off or is it so overbuilt it's overweight? I am deathly afraid this plane will be hugely tail-heavy and then what do I do? strap a brick to the cowling? No.. spend another X-hundred $$ laser-cutting a kit with the wings set 1/2" further back.

    Anyways enough rambling. I welcome any tips or feedback. I'll keep this thread posted.

    -Curt

  2. #2

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Hi Curt.... I identified with some of your thoughts. I just completed my first original design and dealt with the concern (fear) of what the result would be in the way of looks, flight, weight, et all. Those concerns will be nagging stressors to keep you (hopefully) between the lines of propriety of what will produce a good result.

    I accepted up front that I couldn't build as light as an ARF and really didn't want to as I hate the gear and firewall support in most ARFs.

    In the end... the rewards were great. The build was fun and the plane looks good and it flies excellent.

    Best of luck with your adventure... I'm subscribed to follow.

  3. #3

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Hi Curt,

    Nice looking design. I have been using Turbo CAD myself for a couple years. I have nothing close to what you have but hope to design an Extra 300 or Yak 54. Keep up the good work.
    I will be following your thread.


  4. #4

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Some more views representing some design decisions.
    My current design for the upper cabane mount is to have two ribs cut from 1/8 lite plysandwiching a 1/2" x 1/16" flat aluminum strut I plan to bend to shape. I plan to mount it with a 6-32 screw + washers + locknut. This is one of those "measure 10 times drill once" things where the wingincidencewill be set and that bothers me. I'd rather have a slot in the metal but am afraid of slippage in flight, would I have to crank it down so hard it would crush the ply? even with generous washers? a lock-washer even? I think I'm safer with non-adjustable but that is not set in stone. I am planning on 0-degrees for top and bottom wing but would like that to be adjustable for triangulation-trimming.
    Next up the firewall. A little unconventional, a 1/4" ply doubler is behind the front fuse former. I started with a huge air baffle behind the engine and have taken great pains to maintain it. I want gobs of cooling and plenty of airflow to carry the smoke away from the plan in hover.
    The rear profile shot shows some design decisions I made early on that may or may not haunt/kill this design:
    - The wings have as large a separation as I could manage. The bottom wing is practically flush to the fuse (the line you see there is the center of the rib). I am hoping this reduces the venturi-effect between the wings, and minimizes interaction. In addition providing slightly large torsional moments for the roll axis.
    - The rear fuse is very thin, getting the taper right on the entire radial axis for this was a royal pain let me tell you, but I think I have an approximate airfoil. The main idea is to push the center of pressure forward for stable knife-edge flight. That's the theory anyway. The reduction in material is minor.
    - Thestabilizeris centered at the halfway point between the wings. Originally I wanted to make sure the air coming off the wing surfaces affected it as little as possible, but the wings are staggered so my gut says it should be a bit lower to really achieve that. But then how does p-factor affect this? Does it matter? Should I be moreconcernedwith the thrust line? These are maddening questions because 3D flight is really not covered in the literature. Tail plane design is all centered around proper authority in imminent stall conditions. Okay fine I want it to harrier well but is that a function of just the tail design or more due to obscene power/weight to just muscle through?
    - Can't really see the rudder but I pretty much figured "more is better" as long as the large surface was as close to the thrust line as possible to avoid the p-factor blowing the model sideways. I have compared ratios to known-good 3D designs and I am pretty sure I'm in the ballpark.
    The more "finishing touches" I put on this design the further from done I seem to be. I really wanted this out for laser-cutting
    quote this week.
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  5. #5

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    I've finished the wing design, which was quite an excercise in wetware stress analysis. Trying to make sure the wings are securely anchored for positive and negative G's. I have read where aerobatic models routinely undergo 12g's and on up to 20 g's. I've seen that figure quoted a few places and it does pass the sniff test, which leads to an interesting calculation.

    Lets assume the weight of the fuselage is 8 pounds at 1G (yes I know I should talk slugs or grams, but easier to think about) then at 20Gs it impart a 160lb force acting about its center of mass. Assuming uniform wing loading (a bad assumption, but probably not terrible) then that force will need to be reacted by 4 cabane connections and a wing saddle in positive Gs (not so bad) but in negative Gs it will be reacted by the same 4 cabanes and... two dowels and two nylon screws. hmm.. so..

    160 lbs / 2 wings = 80lbs per wing == 20lbs per cabane or dowel/nylon screw.

    That's not.. terrible.. but that force is going to then be transmitted through the frame, and while it's cliche it remains true: a chain is only as strong as its weakest link. Want to lose a week? Download this game: http://www.bridgebuilder-game.com/bbg-info.phpIt teaches you how to construct bridges and results in some surprising stress concentrations.

    So anyways I have that on my mind (inverted alien wall, anyone?

    I think I'm finnaly ready to collect all the parts and send them out for a cutting quote. I have tried to reduce partcount but there are still a lot of intricate parts here, like how much is this bulkhead going to run me cut from 1/8" lite ply.. ? $5? $10? $50? I doubt I could cut it myself to the precision required. Guess I'll see.

    I've attached some shots that include the built-up wing, but I think at some point this just becomes noise so I'm not going to post gratuitous pics anymore probably until I actually get some parts in.


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  6. #6

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    If you are just talking about wing separation on the stresses, then keep in mind that in negative G situations, it is not the 5 lb fuse but the lighter wing that is trying to separate. the two wings will support each other and spread those stresses around. I don't think it will be as bad as you describe.

  7. #7

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Have been sanity-checking the design, just could not shake the feeling that the wings were set too far forward and I think I was right.

    I checked the ratio of as many photos as I could find of similair aircraft (Beast, Ultimate) and the ratio of the measurement from the front of the cowl to the front of the top wing to the back of the top wing to the tail was around .23:1 quite consistently, whereas the Dash (working title of this Pitts) had a ratio closer to .2:1

    Meaning what my gut was telling me was right, the wings were set too far forward. So I'm moving them back by the requisite 1" to bring it in line with other 3D biplane ratios. There is sincere flattery and there is NOT flushing $500 down the commode.

    I'm doing this by extending the cowl forward by 1" and shortening the tail by 1" rather than re-do.. well.. everything.

    -Curt


  8. #8

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Hi Curt,
    I always look forward to see a new design of Pitts. Wish you a lot of luck with this project.
    As I can see you took care of some important problems that were found on "Spirit of Pitts" design, like taking weight of nose and material from center box to reduce weight.
    I am one very happy builder and owner of S12 built from plans by Bernard Burton.

    This bird flies great, and if your design proves to be good, maybe my Pitts could get a little brother.
    Subscribed.

    Regards, Sasa

  9. #9

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Thats a really nice looking plane. I sure hope the Dash ends up being viable. I really only planned on building this one for myself but I would be over the moon to see others

    Laid the Dash out for lasercut and it was an interesting feeling looking at all the parts splayed out like a boned fish.. it seems a little less initimidating now, that big 3D mass just seems so impossible to actually build outside of a computer but looking at the individual components now I think it seems a lot more approachable. Just have to find a kit cutter willing to do it.

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  10. #10

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Got a few laser-cut quotes and went with one in the low $200's which I think is a pretty darn good deal. Have ordered the rest of the balsa I know I'll need and am keeping careful track of materials and sources.

    Don't know if I will make this a 1-place or 2-place aircraft. I think the 2-place looks cooler but don't know what kind of canopy I'll be able to make. Pretty sure the ol' Soda-bottle trick will get me a small one, but the top hatch was designed to accept either. Maybe I can source a cooler one.

    Planning on a single 5/8" carbon wing-tube on the bottom wing, pretty sure the top wing will be okay with the wide cabane butressing combined with the outter struts. I will probably add flying/landing wires on the first iteration but I don't think they will be required. Speaking of which I don't know if I'll go with traditional Pitts anti-flutter cables on the tail, I'm pretty sure they'd just be for show, but not sure how I'll know for sure unless I mount a rear-facing camera or the tail is ripped off in flight. I'm hoping to affect plan 'A' there.

    I added the sheeting and stringers to see how it was going to look and I couldn't be happier with the rear profile, that says "Pitts" to me. but the front? well that kin of says "beer can" to me and I'm hoping it doesn't look that way in the final equation. I may change that profile a bit if I don't like how it ends up turning out. Can do wonders with color-blocking

    In addition I'm working up a "plan" view which is basically a piece of cake! I just project the 3D model, delete a few non-visible objects and viola, done! Trick is going to be printing it out on large-format. I was going to try Kinkos or some other McDonalds-of-printing but I think I read where the largest they'll do is like 24" or something. I'll research other options, but if anyone knows where I can print out 24" (ish) by 65" (ish) from PDF's or something, send me a PM please.

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  11. #11

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    I've printed plans at Kinkos before. Not sure exactly how big they were, but they can definitely do 24" x 65".

    Just make sure they don't scale it down by 5%.

    The one I used seem to know what they were doing, but the OfficeMax I tried right before that scared me off.

  12. #12

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    They parts came today! Surreal that just a week ago this was a drawing in a computer, and now I am holding the actual physical piece cut out of wood exactly as I drew it. I know this is old-hat to most of you but I am just in awe of how relatively easy it was to go from screen to real-world! Gotta shout out to http://www.laseredgecutters.com for holding the hand of a newbie, they did an excellent job and the price was more than reasonable, I do not hesitate to reccomend them.

    So time to.. get to work? Wait.. how do I cut these pieces out of the plywood? My hobby knife is too thick as is my keyhole saw. I was about to drill them out but thought I'd ask first if the vets had any tips on the best way to get laser-cut parts out of their 'tree'.
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  13. #13

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    I take an Xacto to first one side - just lightly scoring the wood, and then flip it over and do it again from the other side.

    It usually pops out at that point, but if not you can just do it again.

  14. #14

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Curt,

    You're doing an excellent job there. Kudos!

    Nice "nesting," of all your parts. None of that is as easy as it looks! Especially the making of the line art for the parts.

    Thanks for sharing.

    Charles
    Owner: CFC Graphics. "Model Airplane Graphics from a Model Airplane Builder." cfcgraphics.com

  15. #15

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    So I fit checked the parts and so far so good!

    Some very minor issues not worth mentioning, did not need to do very much sanding at all to get it to fit perfectly. The pictures you see here are held together with friction only

    Just makign notes and figuring out an assembly order when Iwhip out the epoxy and CA, Iexpect this plane to go together very quickly. The wings are another matter but their design is fairly conventional so no real biggie.

    So far the full-up weight of the fuse plus wing ribs is just over 1.5 pounds. figure weight of glue plus the balsa Ineed to finish it (sheeting/stringers/tail) should have me well under my 6lb structural weight budget.




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  16. #16

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Got the odds and ends I needed to start building. Most importantly a pair of 4-foot 1/2" steel tubes to act as the build-jig ($7 apiece from online metals, sure wish I could build airplanes out of steel). The idea was to design-in a pair of removeable hole-carriers running the long -axis of the fuse so it could be easily built guaranteed-straight without needing a plan or pins or anything. Worked perfecly! build this up in all of 3 hours

    Next up will be the stringers, some sheeting, and then the wings and tail-feathers.

    I re-worked the landing gear a bit and am quite happy with the new swept-forward design bolting directly to the cabane-ribs, it's also lighter and simpler as it no longer requires it's own 1/4" ply mounting plate.
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  17. #17

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    On a recent design and build, I missed the wheel position slightly forward where the tail was reluctant to come up on takeoff and caused porpoising on landing. Will your strut mounting have any ability to be adjusted for optimum wheel position?

  18. #18

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Tell you the truth I hadn't thought about it. I researched conventional gear design (not more than X degrees from CG, not less than Y, track at least Z.. etc etc.. )and then computed those values for a volume representing where the CGlocation might ever be (all the way forward and a lot aft)and then put the gear right in the middle (which turned out to be an area about half an inch wide)

    Ifigured this location will "work" for any CG but will obviously not be optimal for every one. The gear is split, and Ihad envisioned mounting them with 3 #8 threaded rods (to act as supports between them) . They will be accessible from the underside, but not the outside through the sheeting. I guess Icould slot the top screw a bit and allow some (maybe 1" either way?)adjustability, but hadn't planned on it.

    Frankly with the powerplant I'm planning to use I was not expecting this plane would be "reluctant" to take off and as for landing, I just want it to be ground-loop resistant.

    If it flies well but the ground-handling is poor I'll work up a bracket to move the wheels around (will be ugly but just a testbed)and fine-tune the location, then redesign the landing-gear struts to reflect my findings.

  19. #19

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    I think the location is more critical if if the plane has generous wing area as if it is ready to fly before the tail comes up... it will fly before elevator authority is optimal. You are right however that with adequate power, that is not much issue unless it is simply disconcerting to someone used to otherwise. Even though my design lifted off without the tail coming up... I never sensed any elevator stall condition.

    On the other end was where the greater issue existed... as when the mains touched down, the tail went down quickly and if there was a little too much speed on, it porpoised. I'd given thought to this before finishing the build and had asked about it in the aeronautics forum and a fair discussion took place. It was offered that there is a rule that applies and I hope I get it right here... that an ideal placement exist when there is a seventeen degree angle between a CG to wheel contact line and back perpendicular to the datum line. If the angle goes above 21 or so degrees... porpoising might be expected and again I think that is somewhat dependent on if there are generous inches.

    I adjusted with shims and found the sweet spot to be quite narrow where the tail comes up and the tail stays up on fast landings long enough to bleed off speed without porpoising.... and there is yet good ground handling to prevent nose tipping.

    Keep in mind that my plane has generous wing area so might not be typical.

  20. #20

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    So I have been busy, added a lot of filler to the fuse, and most of the stringers.

    The lower wing is structurally complete, waiting on some balsa to finish the aileron buildups, then some sheeting and covering and it'll be done.

    Worked on the upper wing most of today, ran into some issues in the center joint I had not anticipated. Mainly.. I did not include one at all. As designed the wing would just fold in half. oops. After some noodle-time I have a solution I really like and have amended the plans to reflect it. I'm making zillions of little improvements to the plans as I go, the Dash-B will be so much easier to build, and a superior design in a lot of ways, including lighter. I have identified at least 4oz of weight I can shave off this design with no compromise in strength, doesn't sound like much I know but it adds up, I am a miser.

    purchased the aluminum I'll need to make the cabanes today, installation will be straightforward but I need to complete the upper wing first.

    Also got the cowling in from Fiberglass Specialties: http://www.fiberglassspecialtiesinc.com/catalog.htm Ordered an off-the shelf model 8 1/2" x 8 1/4" and couldn't be more pleased with it! Looking forward to acquiring the engine so I can make some cutouts and install it.

    Speaking of which I am starting to consider the DLE-35 for this. the OS GT33 looks like a great choice but its a bit spendy, I think I can get more bang-for-the-buck with the 35




  21. #21

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Seems my attachments didn't make it in my last post.. odd.
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    RE: Scratch Building 1/4-Scale Pitts Model-12

    To the person who PMed me offering to plot the plans: thank you! But in the future please turn on your PMs so people can respond to you

    I created some PDFs of the views I needed to build up the wings & tail and brought them to Kinkos, was a very pleasant experience! The one near me (don't know if its standard equipment) had a 36"-wide laser printer that printed onto continuous spool, so 36" by "whatever" is easy to do, they charged $0.15 per square foot, I spent like $8.

    I was careful to put on an X and Y scaling box and they were both spot on.

    Started laying down color schemes for the Dash as I finish off the sheeting. Sorry no pics for this update.

  23. #23
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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Looks great!
    For a kit you are, and to a kit you shall return.

  24. #24

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    After much thought and considerationI've decided to go with the DLE-30.

    I know Iknow I was bouncing around from 33GTto DLE-35 but common sense won out, namely:there are many folks at the field running them so lots of help/experience, it's relatively cheap, and 17-ish pounds is plenty of static thrust.

    Got it in today along with the prop and wow, this is the biggest engine I've ever installed in anything. Being a biplane the Dash doesn't quite seem as big a scale as it is. A 62" wingspan is not that large, and while the body is stout, it's not that long. I just got a sense of the size and power of the model I'm building with the king-sized 18x8 egg-beater It will be swinging.

    Measured 3-times, drilled once and had it mounted in the plane as a fit check, so far so good. Need some hardware to complete the installation but there are still plenty of steps between here and firing it up for the first time. Also completed the surfaces buildup. Iam leaving the holes in the ailerons since I am planning to cover it with transparant blue, but Imay sheet it with 1/32 if Ichange my mind, they are relatively large and I'm not sure I'm fully confident in their robustness, even though those ribs are 3/8 stock, that trailing edge is just not attached with much surface area.

    lower wing is complete except for rib caps, hinges, servos and covering. liekwise the top wing. Still saving up money for servos (the next big buy)but Ihave ordered a sample of this little guy: http://www.servocity.com/html/hs-225bb_mighty_mini.html and for only $18 it seems like it might be okay for the ailerons. Bear in mind there will be 4 of them and this is a biplane so the surfaces are not as large as a traditional 1/4 scale (although they are oversized)

    I am not trying to pinch pennies on the important parts, but the next servos up for consideration are in the $35-$50 range and having to buy 4 of them, well that's worth the savings if it will comfortably do the job. Will report on findings, unless someone has a strong 'no and here is why' answer for me.
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  25. #25

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    RE: Scratch Building 1/4-Scale Pitts Model-12

    Important step today- the first full-sized mock-up. I guess not everyone does this but once Ihave enough pieces Ilike to just do a gratuitous layout to say "hey.. THISis what you are building.." it makes all the minute details-on-bizzare-looking-parts seem worth it.

    Took me a solid two hours to get those cabanes right. had to use a makeshift bending brake and lay them out on the plans to check for correctness.. lot of iterations.

    As Isaid in an earlier post the cowling came in, fits perfectly, and I couldn't be happier with it, has a very nice NON-beercan shape. next step is to finish sheeting. had to install the cabanes and ... oh sanskrit.. Iforgot the gear. how am I gonna bend THAT slab of aluminum. Okay guess I'll tackle that tomarrow so I can finish sheetnig.

    Iam also uncomfortable with how well butressed the firewall is, Iplan to run those 1/4-inch plywood rails all the way forward (wigh some lightening cutouts in the next lasercut) and I think that will lock the model together solid as a tank.
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