Increased Max Gross?

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TJTX

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I had a friend, who I trust and whose built two very beautiful birds, tell me today that you can increase your kits limiting max gross takeoff weight above what the kit design has published. He said all you have to do is show it'll carry the extra weight. I'd kill for just 50 to 100 extra lbs on my max gross for my Horizon 2 build. He used an example of an Oshkosh award winning Velocity that runs almost 800lbs over the "normal" max gross limit. Is this true? As I said, I trust him immensely but jm just trying to make sure I dont so something stupid.
 

TFF

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The FAA will allow you to set the gross you want. If the plane has excess strength, there should not be issues as long as you might have to restrict the G loading. The ability to take gusts goes down two. The FAA does not allow you to go backwards though. If the plane is LSA and you reset to above LSA weight, it will forever be not LSA. It’s all
About how strong the plane is on how much of a chance you are taking.
 

TJTX

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My build has a listed max gross of 1050 I believe, I wouldn't think taking that up to 1150 would be that detrimental?
 

TFF

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It depends on the G rating of the airplane. Most are plenty strong. It’s not flying around in calm air that will get you. It will be cumulative. Bad landings, turbulence, abrupt control response. I’m not saying don’t, I’m saying understand. I’m not the one who has to trust it.
 

TJTX

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I fully agree with that. I guess a better way for me to state my goal is, "I want that extra 100lbs but I want to know I haven't excessively dug into my safety margins either". I will work with my tech counselors and other guys who are knowledgeable to see what I can do. My plans came with a structural analysis packet that hopefully can help us find my answers.
 

rv7charlie

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RV builders do it all the time. Lots of 'buts' though. 100 lbs in normal category numbers for a plane rated at 1650 in utility category is, relatively speaking, nothing, compared to 100 lbs on something that's starting out at say, 1000 lbs gross, normal category. That would be, well, not nothing.

Whether the FAA goes along on changing the oplims on an existing a/c probably depends largely on which FSDO you have to deal with. On a new build, I'd bet you would have more trouble from a DAR than a FSDO rep. None of the legal questions alter the safety questions, though.
 

Victor Bravo

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You have a giant revolver with 50 chambers, and you know there is already one bullet in one of the chambers. Now you are talking about putting one more bullet in another one of those chambers.

If a real trained firearms engineer studies the bullets, he can probably verify if the bullets do or don't have any gunpowder in them. If the bullets don't have gunpowder, then it doesn't matter if there are one or two bullets.

But if the bullets are actually real and have gunpowder in them, then you just took your odds of having a problem from 1 in 50 to 1 in 25.

Do you ask for an engineer, or do you ask for a gypsy woman with the tarot cards?
 
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Topaz

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My build has a listed max gross of 1050 I believe, I wouldn't think taking that up to 1150 would be that detrimental?
Not unless the wings come off, no.

... "I want that extra 100lbs but I want to know I haven't excessively dug into my safety margins either". ...
How many G's or pounds are there in one "excessively"? Every extra pound you add over the POH "Maximum Take-Off Weight" is shaving a little off that margin. How much? Without doing the engineering analysis, you just don't know. It might be fine. It might not be fine. You won't know until the wings come off.

The other problem here is a mental game called "normalization of deviancy." You fly at 100lbs over MTOW for a few months. Nothing bad happens. Then you have a situation where you "need" to fly at 105 lbs over MTOW. Nothing bad happens. So you start routinely flying at 105 over, until one day you "need" to fly at 125...

Don't think you'll do that? NASA lost two space shuttles and 14 astronauts to this very problem. If they can fall prey to it, so can you.

Stick to the published numbers unless you have a full and professional engineering analysis saying that it's safe to increase the MTOW. The best place to get "permission" to increase the MTOW is from the original designer. If they say, "no", don't do it, regardless what your friend tells you.
 

Toobuilder

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Interesting that the LIMFAC on most of the STC'd conversions is climb rate, not airframe strength. Put a giant Continental and floats on a Cessna and you magically get 100 pounds of added gross. Any spar mods done? Not usually.

My Rocket is a 6g airplane at gross and has fighter jet climb performance - I would have zero concerns about adding 100 pounds above gross on an "airliner type" cross country mission. One simply needs to know that it's no longer a 6g airplane when heavy.
 

wsimpso1

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I am with the guys above. "Here be dragons"

IFF you can show that structure is not limiting max gross weight from both raw strength and fatigue aspects, then going up in max gross might be acceptable. This far from easy. Then you only have to worry about takeoff distance, landing distance, climb rate (feet per minute), and climb gradient (feet per mile). If you have lots of margin on every last bit of structure and plenty of climb, you might be fine.

Structurally, the hard part is not in knowing that the main spar is OK for FOS, it is in knowing that EVERYTHING is ok for FOS in fatigue. It only takes one critical part or joint to get from OK to broken to bring down the bird, and there are a lot of parts and joints. Know that fatigue is intended by design, but only really proven by either very long expensive tests (rare in airframes) or demonstrated in the field by the fleet - by ratcheting up weight, you are going outside the knowledge basis that exists. The design might have been just barely enough at stated max gross, and you can not know...

Once a pilot gets to flying it, the pilot can get runway and climb performance data by slowly raising takeoff weight and document takeoff, climb, and landing performance, plot them vs weight and see where it gets to be too much. With climb data at several weights, any student of the topic can then project the no-climb weight, but the "that climb scared the crap out of me" climb gradient is not so easy to anticipate. Know that airplanes crossing oceans with overgross takeoffs do so from really long runways and with lots of flat terrain nearby so they can tolerate lowered limb gradients. Do this work from long runways and perhaps do the ballasting with a big water bag or lead shot and a really big dump gate. Same for aft CG checks.

Billsk
 

TJTX

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All wonderful information gentlemen. I want to clarify I'm not hell bent on doing this or anything else crazy with my build. Safety is, and will be my number 1 goal through the build. My intent was to first, understand if it was legal. Then solving that mystery, to find out if its actually something that is realistic to try and pursue. As stated above by someone else, I've flown many aircraft (much larger) that have had substantial increases to MTOW with different modifications that never touched the basic airframe. That is where my very very basic level of thought came about on this subject. I'm currently just a run of the mill pilot who is trying to understand all the ins and outs of this airplane construction business. I would really hate to lose usable carrying capacity just because the designer wanted to be extra safe, but I would also really hate to exceed a tolerance out of ignorance and hurt myself or my family/friends. I'll look into the structural analysis and see what it says. I believe it currently shows 4gs as the limit. Would you think there would be an adequate safety margin to limit it to 2.5g and gain that extra weight? And I will not be trying to operate out of super tight strips where climb gradients are critical. I do enough of that at work.

Once again, thank you for helping to guide me in the right direction!
 

TJTX

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I would not want to fly a sport aircraft with less than a standard category load limit of 3.8 g.


BJC
Completely understandable. Seems like the consensus of the guys much smarter than me are tread very lightly and don't push it. As of know I'll keep it as is.
 

Markproa

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The designer of my aircraft states MTOW as 560kg. It has carbon spars made professionally in an autoclave which have been designed to the European safety factor of 4 instead of the usual 2. My fuel tanks have been moved to the wings instead of in the fuselage so I'm feeling OK with upping the MTOW to 600kg.
 

Pilot-34

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Most of me is in IL but my hearts in Alaska
I wonder if it makes a difference where you live? I’ve been told many times in Alaska that you can get a 10% gross weight increase because the air is cool and the skies are clear.
of course we’re talking about Alaska here were pilots explain to you there such a thing as a moose emergency reading meaning all the moose you can stuff in one because it’s emergency to get it out of the woods.....

So going What the guys above have said and some of the rumors that I’ve heard in Alaska I would say you can probably get away with a 10% upgrade On cool calm days with well centered loading.

From personal experience on hot humid days operating from an airstrip below the general ground level in a creek bottom surrounded by a cornfield and a wonderful piper Cherokee 140 with motor gas STC I downgraded my gross weight to one fat pilot and one well cleaned airplane and at the end of short relocation flights to the local public airport may have occasionally needed assistance removing the pilot seat from the pilots rectum......
 

wsimpso1

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Your gun expert will tell you it's "propellant" not gun powder, Ha!
LOL.

When I was an R&D guy in that business (early 1980's), it was "propellant" in our offices and with Hercules, Valleyfield, and Olin, but it was still "gun powder" in the plant guy's offices and out on the plant floor.

Some folks at the range look at me funny when you use "propellant", other folks go, "He used to work for Remington..." and nod their heads.

Billski
 

blane.c

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14 CFR § 91.323 - Increased maximum certificated weights for certain airplanes operated in Alaska. | CFR | US Law | LII / Legal Information Institute (cornell.edu)

Generally if you are 121 or 135 or it is a gvt. operation most any plane can be operated legally at 115% of its certificated weight in Alaska. There are other provisions to get a weight increase in Alaska listed in the reg.

1050 x 1.15 = 1,207.5

So if Alaska fish and game had your airplane (with normal type certificate) they would operate @1,207.5lb all day long.

Old addage; It'll fly over gross but it won't fly out of fuel.
 
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rv7charlie

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The designer of my aircraft states MTOW as 560kg. It has carbon spars made professionally in an autoclave which have been designed to the European safety factor of 4 instead of the usual 2. My fuel tanks have been moved to the wings instead of in the fuselage so I'm feeling OK with upping the MTOW to 600kg.
What explanation was given for using the safety factor of 4 instead of 2? Was it by any chance due to using composites instead of aluminum or steel?
 
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