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Thread: Gross Weight Increase?

  1. #1

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    Gross Weight Increase?

    Ok this is my first post. I'm "this" close to purchasing my first, and hopefully only SuperCub. I'm planning floats in the summer of course and Ski's in winter. (Canadian Eh?) I've looked at a few of them and am curious as to what is involved in increasing the gross weight of a PA-18 150 horse from the current 1750 lbs to the 2000 lbs. I understand there is an STC for this but am not at all familiar with what is involved with this. Is this a big/expensive undertaking? Should I save myself a lot of aggrevation and go for one that already has it? The missions will be fishing cottage in summer on floats. this is why I would like the the higher Gross Weight. I would like to take full fuel and 2 adult males with some gear. Any and all advise is greatly appreciated.

    thanks
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  2. #2
    Crash's Avatar
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    The 2000 lb gross weight increase STC belongs to Wipline. It is a big hassle to install unless you're re-covering the wings. It requires installing a cap strip on the rear spar at the lift strut attach bracket. It also requires welding or clamping a small diagonal tube to the rear seat front cross over tube at the door opening. On wheels it requires a 5/8" square tube on the upper gear legs.

    It only allows for 2000 lb GW on Wipline floats or when on wheels.

    Most guys just blow it off and fly like the planes have been for 50 years prior to the Wip STC. The Wip STC is more of a paperwork GW increase then actually making the plane safer.

    Crash
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  3. #3
    kase's Avatar
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    Actually is a 1/2X.035 square tube. Also need Pawnee tail springs and 1280HD bungees.

  4. #4
    Crash's Avatar
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    You're right, the STC is for 1/2" square tube, not 5/8", I stand corrected.

    Crash

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    Rob Murray's Avatar
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    Increase gross

    I thought Cubcrafters also was doing an increased gross on their ships. Or are they using Wip's paperwork? Something to keep in mind is that even with the kit installed you will still have the same engine and in my experience a PA18/150 will fly away with just about anything you can stuff in it or tie to it - floats, wheels or skiis - but it doesn't climb as well and gets pretty mushy when you are overgross. And if you have two large people, full fuel and any amount of baggage you probably will be at gross on floats. Of course I would never do such a thing (just in case my insurance company reads this) but if you keep the luggage on the light side and don't always fill the tanks, the plane will perform just fine, particularly at lower altitudes. Something I've been curious about is how many of you have seen or experienced structural failures in the areas that get beefed up. Or is this just something to keep the insurance people happy? If that's possible.

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    BlackFly's Avatar
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    I just purchased my second SC, and was hoping to find one with the GW increase already installed. After many discussions at the Airmens Convention/Valdez with manufacturers and operators, the feeling I got was that having the third seat (heavy baggage) fuse mod, X bracing, and the Hd gear with the square tubing mentioned above do give you the necessary strength for heavy loads. Several of the (very well known) re-builders really did not like the beef-up to the aft spar, commenting that it would possibly be stronger without it, but an FAA engineer may have thought it necessary at the time. Having some experience in the certification process, I could see this happening.
    The SC I ended up with has all the Mods (new airframe, 160hp) I needed for the job, but with my 48gal I know I am heavy++. The good news, is that I do frequent long legs, and when landing I'm comfortably with in limits. The best GW increase kit I have found is having the pilot loose those extra 30#...you'll keep your Medical longer too!
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  7. #7
    SteveE's Avatar
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    BlackFly,
    You are probably right about the GVW, and what everybody said at the BIG SHOW. But in this day and age, it still doesn't make it legal, and consequently insurance companies and many lawyers can sure make a big deal out of not complying with the "FAA" paperwork. Don't get me wrong, I really could give a rats _ss. It costs a lot of money to be "legal", but it could cost a lot more if you are not. What everybody needs to look at is how big of a gambler are you.

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    BlackFly's Avatar
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    SteveE, I couldn't agree more. I'm just trying to look at strength from a physical safety perspective, not a legal/insurance perspective. The reality of the Beast( Super Cub) is that with full fuel, 2 adults and camping gear, few will ever take off below GW. I also suspect there isn't a more proven Over gross airplane on earth than the SC. Rob Murray posed an interesting question above...where have the failures been on the newer(or well maintained) airplanes due to (>200#) Over gross, and eliminate the obvious incidents like steep turns and overspeed, etc.

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    Christina Young's Avatar
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    I think the insurance thing would be more of a worry than the FAA. Unless you are involved in an accident, how would they know?

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    There is a strength issue with both spars at the wing lift strut fitting at higher gross weights and possibly longer wingspans. The rear spar will be the primary failure point in most loading situations. I am only familiar with the Piper Pawnee mods (2500lbs gross) and the CubCrafter wing mods (spar and rib improvements) but I have yet to see one mod that would weaken the spar or wing. The spar improvements are a critical piece of the improvements necessary to increase the gross weight safely and still maintain the no permanent damage margins in the certification regulations.

    clmaxx Aero
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  11. #11
    Rob Murray's Avatar
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    Increase gross

    I've seen good evidence that beefing up the aft fuselage and a second diagonal tube at the top of the cabin area are worth doing but I haven't any first hand experience with other areas of the airframe that need to be strengthened. I guess my question about the Wipline kit should have been is it based on engineering projections or have any of you seen structural failures in the areas that have to be modified. Sounds like the answer is yes for the rear wing spar. And, is the Cubcrafters mod the same as Wiplines?

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    I helped with the wing load testing for the Super 18 so I'll try to answer some of the questions about why and where the wing failed.

    We pulled that wing in every direction and simulated every situation imaginable. Keep in mind that the front spar is much more sturdy because the leading edge is attached to it so it takes a lot less abuse. It was the torsion test that was the most interesting which simulated the twisting the wing takes when you're at gross weight and close to red-line and make an abrupt deflection of the controls (even accidentally bumping it, though rare) bent the front spar a little less that 3" and the rear spar almost 8 1/2" (from the lift strut to the wing tip) with no permanent damage. The brand new wing didn't fail, the rear spar was beefed up and it was tested to beyond 2300# GW... but by the example it shows how hard it is on the older piper rear spar and how close the wing is to failing if you're over GW on a standard wing.

    I realize that it's been done for years and will continue to be done for years to come. Just food for thought to help keep yourselves safe. And to not over do it any more than you must to get by.
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  13. #13

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    And, is the Cubcrafters mod the same as Wiplines?
    The 2000 lb. GW increase that Cubcrafters sells is the Wipline STC. In fact, the kit is drop-shipped from Wip. Cubcrafters is developing their own 2300 lb. GW increase, but the details were uncertain when I talked to them several months ago. Presumably it would require using CC's wing assembly.

  14. #14
    mvivion's Avatar
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    Folks,

    Wipline actually does have a couple engineers on staff. They actually did test the wing, and they actually did document that the increased gross weight kit DOES in fact, strengthen the wing.

    But, as everyone already knows, Cubs will fly at 7,000 pounds with absolutely no problems at all.

    Just dont' lower the flaps or deflect an aileron in the process.

    The Wip kit is worth doing. It strengthens the wing, it strengthens the landing gear.

    And, last and possibly least, it makes the airplane legal at a pretty high gross weight.

    What in the world could POSSIBLY be wrong with those concepts?

    Sorry, a little rant here.

    MTV
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    No matter what, put that half-inch tube in the gear leg. If you then cover the leg, it will stay more or less wrinkle-free. And lots of reasons for the x-brace overhead.

    The question was asked, not answered, and assumed - have there been any recorded metal spar failures while in flight?

  16. #16
    Rob Murray's Avatar
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    gross increase

    I don't think anyone is questioning Wipline's mod for the GW increase. Which was done to increase the useful load for their amphib floats. I simply posed the question - does anyone know of any problems with standard Cub airframes in the areas that Wipline feels need to be beefed up. And I found the comments about the rear spar pretty informative.

  17. #17
    mvivion's Avatar
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    Rob,

    Really?

    "Most guys just blow it off and fly like the planes have been for 50 years prior to the Wip STC. The Wip STC is more of a paperwork GW increase then actually making the plane safer"

    MTV

  18. #18
    Crash's Avatar
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    I haven't personally installed a Wip GW increase kit on a Cub but I understand that the rear spar "beef up" is an aluminum channel that is slid under the ribs on the top spar cap and pop riveted in place. The channel is located above the rear lift strut attachment bracket.

    There have been questions raised about the wisdom of drilling holes and pop riveting in such a high flexing / stress location on the spar. The concern is that the holes being drilled in the spar cap may lead to cracking of the spar.

    I don't see how adding this small channel to the TOP flange can add much strength to the rear spar.

    I feel Dakota Cub and Smith did it right. Dakota Cub installs a doubler inside the spar web (both front and back) at the lift strut attachment bracket. They only do this on their extended wings.

    Smith has the spar blanks made .011 thicker in the web.

    The fact is....These planes have been flown "over gross" for 50+ years and I can't remember a single in flight failure of a rear spar.

    Take care.

    Crash
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  19. #19
    Steve Pierce's Avatar
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    The Husky has the strap on top of the spar cap at the lift strut fitting. The short wing Piper spars have the doublers like Crash describes on the Dakota extended wings spars. The only failures I have seen is when one wing hits the ground and bends the rear spar. Does the FAA require cycle fatigue testing? I know Air Tractor has a wing spar fixture set up in downtown Olney that runs continuously flexing the spars to simulate flight to show them when and where they will fail.
    Steve Pierce

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  20. #20
    Rob Murray's Avatar
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    GW increase

    OK, I wasn't criticizing Wip's mod. Maybe others were. I don't know enough about the particulars. I do know there were some weight issues with EDO's 2250's. Wonder what PK is doing?

  21. #21
    mvivion's Avatar
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    Steve,

    Actually, only the model A-1A Husky has the spar cap. This was essentially a means to increase the gross weight of the model A-1 Husky.

    It is interesting to me that two completely separate engineers, working with two different certification offices came up with almost precisely the same alterations to increase the legal gross weight of two airplane models which are very similar in wing design.

    The Model A-1B Husky (and now also the model A-1C Husky) have a rear spar made from a higher strength material, as opposed to using the spar cap. Those aircraft are able to go to a higher gross weight than the -A, in fact 2200 pounds for the -C.

    I don't think there's any question that strengthening the spar itself is a better approach than adding a spar cap, but you'll find spar caps on other aircraft as well, for the same reasons, more than likely.

    Nevertheless, adding the Wipline kit to an existing wing can be done, it isn't that big a deal, and it does strengthen the wing, according to some people who know a lot more about stress loading than I do.

    MTV

  22. #22
    Crash's Avatar
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    Engineering is one thing, pleasing the FAA is another. I feel (and this is just my opinion Mike) that the FAA has to see some change in order to issue an STC for increasing the GW on any aircraft (in this day and age).

    The photos I've seen of the testing that Wip did showed a Cub wing on saw horses with a pile of lead shot bags placed on top of the wing. To my simple thinking, this in no way similates real world flying conditions.

    Most likely they saw more flex in the rear spar and to satify the FAA came up with the spar cap and maybe even noted that the Husky also had one (copy cat approval) for their higher GW.

    Anyway my point is...."It's not always sound engineering that makes up an approval, it's what the FAA will buy in order to get the approval".

    A case in point is the PA12 / 14 GW increase. I can't see how making the turttle deck out of tubing with no cross bracing adds much if any strength. Also the PA-12 / 14 GW increase doesn't require adding a rear spar cap strip or welding in a 1/2" square tube on the gear leg to replace to fabric former.

    I think the guys that work one these planes day in and day out, most likely know a lot more about them (what breaks and what doesn't) then some collage engineering grad. who's never lifted a wrench to one and is only piling lead shot bags on a wing suspended between two saw horses to prove a point.

    Crash

  23. #23
    mvivion's Avatar
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    Crash,

    I don't totally disagree with you, and I don't mean to start a fight, but piling sand bags or shot bags on a wing is how everybody tests wings, as far as I know, and has been done for a long time. It is a time tested process, but as you say, may not in fact be a perfect replication of in flight forces.

    One other thing to remember is that these things are tested to well OVER their design operating weights, so one would HOPE that they'd never actually fail in flight. That is the point of the overdesign, of course. They make the assumption that we'll do something goofy every once in a while.

    As to college trained engineers, I give them a little more credit, I guess. I've seen some that were pretty accomplished pilots as well, and who think pretty practically.

    I've been told by a couple folks that gear strengthening was a major factor in a couple of recent aircraft certifications and recertifications. I was told that the "heavy duty" Cub gear failed the drop test at around 2200 or 2300 pounds, which may not be an accurate number. The point is, it failed, and had to be strengthened.

    Now, I personally have never dropped an airplane in from 10 feet up, but...... .

    Anyway, as I noted, if I were to buy a Cub or rebuild one, it would get the Wip kit. If nothing else, it'll increase the value of the plane by the cost of the mod, I'd bet.

    MTV
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  24. #24

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    Examples of actual failures was the request that I thought had been ignored. We have had actual failures of lift strut forks, and maybe the struts themselves (I am talking about in flight failures, not ground or hangar door impact failures). So far, nobody has mentioned a metal spar failure while in flight in a Cub of any type, at any weight.

    I had a failure of one of those vee-shaped gear top things, and that's why I went to the half-inch tube. It failed when a buddy slammed the brakes on right after I had lubed the bolts. But it was non-structural, as far as I was concerned.

    The Citabrias have been having wing spar failures in-flight, but almost all of them have been traced to previous impacts, and un-detected compression cracks that followed the ground impact.

  25. #25
    Steve Pierce's Avatar
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    Mark Ericson did a talk at the Cub Club meeting at Sun and Fun. He had pictures of the Dakota Cub wing in a wiffle tree that does a lot better job of simulating actual flight loads. I was very impressed with the work that went into building the thing and the results of how the wing stood up to the forces. The wiffle tree had arms all over the place pulling on the individual parts of the wing. I was very impressed with the time, effort and money Mark is putting into his product.

    Kit manufacturers typically only static test with sand bags. This limits the accuracy and spectrum of the test. Certified aircraft require dynamic testing with a hydraulic "wiffle tree". This allows rapid multiple load deflection tests and cycle testing. The system also allows neutral loading so torsional and attachment problems can be revealed.
    Mike, The Husky wreck I have is a Christian model that was wrecked by the border patrol years ago. It has the doublers. Not sure if that is something that was added later or what. Lots of Univair PA18 stamps on the parts in those wings also like compression struts, hinge fittings etc.
    Steve Pierce

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  26. #26
    Steve Pierce's Avatar
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    Found this picture of a Rans wing in a wiffle tree.

    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

  27. #27
    mvivion's Avatar
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    Steve,

    The A-1A was retrofitable to model A-1 Huskys by adding the spar cap, and a couple other things. No Model A-1 Husky came equipped with a spar cap. The retrofit raised the gross weight by 90 pounds.

    I too heard Mark discuss the Dakota wing, and I too agree that the Wiffle tree is probably a more accurate means of replicating in flight wing loading.

    Nevertheless, even on wings that were tested with weights, you just don't hear of many in flight failures, in positive loading. I believe some if not many of the go fast airplanes like Bonanzas which have had wing failures the failures occurred in negative after a tail failure put a heavy negative loading on the wing.

    In any case, it appears to be quite rare to actually have an in flight wing failure in any kind of aircraft.

    That, I would think, is a good thing.

    MTV

  28. #28

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    I have talked with the guys at Wip that did the testing on the 2000# stc. It was actually the fuselage in the tail area that limited them to 2000 lbs.

    The rear spar would fail by kinking forward or aft at the top spar cap, that's the reason for the doubler on the cap and the clips on the ribs.

    Keep in mind that they were trying to keep the mods required to a minimum, so they could be done without a rebuild. They install this stc on a regular basis by just opening up the wing where the mods are installed.

    Frank
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    mvivion, I just pm'ed you. thanks Joe Dory

  30. #30
    Crash's Avatar
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    Quote Originally Posted by mvivion
    Crash,

    I don't totally disagree with you, and I don't mean to start a fight, but piling sand bags or shot bags on a wing is how everybody tests wings, as far as I know, and has been done for a long time. It is a time tested process, but as you say, may not in fact be a perfect replication of in flight forces.

    Anyway, as I noted, if I were to buy a Cub or rebuild one, it would get the Wip kit. If nothing else, it'll increase the value of the plane by the cost of the mod, I'd bet.

    MTV
    I also agree with a lot of what you're saying Mike, just two different perspectives. It's good to know you Government boys never operated above gross when flying the Cubs.

    That said.... today if I were building up a set of wings for a certified Cub I would ALWAYS install Wip's 2000 lb gross weight kit. I didn't install it on my current Cub at rebuild and now wish I had. Still not convinced it makes the plane any stronger, just legal hauling what we've always hauled.

    Take care.

    Crash

  31. #31
    mvivion's Avatar
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    Crash,

    When I first started flying for the feds, we carried restricted category A/W certificates in the planes, if any. We operated purely as public use aircraft. In that category, we were permitted by internal policy to operate 10% in excess of the manufacturer's stated gross weight. We did NOT use the Ag gross weight to base that 10% on, by the way. So we were supposed to be operating at no more than 1925 pounds.

    I have no doubt that I launched a bit over that a time or two in those days.

    But, doing the kind of work I did, I quickly found that a heavy Cub flies like a, well---doesn't fly very well. So early on, I started taking things out of airplanes, and trying to keep them light as I could. I really don't like the big Atlee tanks, for this reason--too much temptation to fill those suckers when all you really need is 30 gallons, and hard to measure how much is in there.

    Anyway, to the best of my knowledge, we never had any problems with any of our Cubs operating at those weights, but we also rebuilt and recovered our airplanes much more frequently than average, cause most of them were getting rode hard and we knew it. Besides, we had all YOUR tax money .

    One of the reasons I went to the Husky at work, was because the legal useful load ON FLOATS was much higher than the Cubs (prior to wips kit). Speed and range was also a factor, but...

    As soon as Wip came out with their GW kit, we put that kit on every Cub we had, to the best of my knowledge. Every Cub in that fleet now has the 2000 pound kit, I believe. Adding the kit to a covered airplane really isn't that big a deal, we did it with several.

    MTV
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  32. #32

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    Quote Originally Posted by mvivion
    Adding the kit to a covered airplane really isn't that big a deal, we did it with several.
    It might be a bigger deal for someone like me whose employer isn't going to pay for it .

    Anyone give an estimate how many hours labor to install the Wip 2000lb STC on a covered Cub?

  33. #33
    mvivion's Avatar
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    Garage Guy,

    Give Wip a call. I'd bet they can give you honest labor estimates. It takes some cutting of fabric and such, but again, it's an amazingly simple modification, considering.

    MTV
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  34. #34

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    I don't always understand the technical details but I have the 2000 lbs stc, mostly because both the cub and I both have belly tanks, I can only comment of the spar increased structure this fall my cub was damaged in a wind accident and the wing spar bent just out side of the additional reinforcement. From looking at it I am convinced it provides significant support the spar.

  35. #35

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    There have been previous discussions about Kenmore trying to get a gross weight increase for PA-18 on EDO floats. Anybody have any news?
    Webfoot
    Owned same cub for 26 years!!!

  36. #36
    mvivion's Avatar
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    Webfoot,

    I think they dropped it. In any case, it was to be on their 2130 floats, so wouldn't apply on 2000's.

    MTV
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  38. #38
    jk's Avatar
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    I noticed in CC's upgrade list for their 2300 gw increase, and they say that "fuselages built by piper and Airframes don't qualify". What is the difference in the "new Univair"frame that's not already incorporated into Airframes fuse?
    Anybody know??

    jk

  39. #39
    PA-10's Avatar
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    It looks like the ol' "replace everything but the dataplate" deal.

    The 2300 Lbs gross weight increase is best completed during a complete aircraft rebuild as this STC requires
    a new (or some Univair fuselages can be modified) 4130 fuselage and new or modified (i.e. reinforced) wings.

    The STC also requires new wing struts and CubCrafters heavy duty 3” extended landing gear. The STC is
    compatible with extended ailerons, drooped tips, and Dakota Cub fuel tanks. Some Univair fuselages built
    prior to 2006 will need to be modified to be eligible for amphibious floats. New fuselages ordered through
    CubCrafters will not need modification for use on wheels or floats. Check with us to determine if your Univair
    fuselage must be modified. All Univair fuselages may be modified for 2300 Lbs on wheels. Fuselages built by
    Piper or by Airframes Inc. are not eligible at this time. It is best to check with us prior to beginning work as to
    the question of eligibility and compatibility with other STC’s.

  40. #40
    jk's Avatar
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    Piper or by Airframes Inc. are not eligible at this time.
    Like I said above... What's CC doing that Airframes didn't already do to their frames. Or is it just FM???

    I always thought that pound for pound Lee B. was producing the best frames available for the 18.

    Anybody got the real facts??

    jk

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