Increased Max Gross?

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Willy7250

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It's not just the "G" load and fatigue. Remember an increase in MTOW will also increase takeoff and landing distances, reduce rate of climb, service ceiling, cruise speed, etc. Need to look at all of the performance.
 

Dana

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I don't know about European safety factors but it's important to differentiate between limit load factor (3.8 for normal category and 4.4 for utility category) and safety factor (normally 1.5 for metal structures). The limit load factor gets multiplied by the safety factor to determine the ultimate load the plane is designed for. Composites usually use 2.0 instead of 1.5.

It's not uncommon for an aircraft to be certificated at one gross weight for utility category and a higher gross weight for normal category. But you don't want to go below 3.8 at any weight.
 

Victor Bravo

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TJ, my original post above was not meant to frighten or intimidate, the big point was that someone needs to fire up the calculator and calculate the answer to your question. It may well be entirely feasible and 'no problem' to go over MTOW, and it may not. The only useful advice is that it is something you just can't guess at. As I often remind everyone within earshot, I am no engineer at all. Matter of fact, I'm not qualified to have a cup of coffee with a real engineer :)

One thing I will offer a slightly more "educated" opinion on is that 2.5G is definitely not enough of a factor. Unless my old un-calibrated G-meter is way off, I see 2G all the time just driving around straight and level in an old 172, flying through what would be classified as "light chop" in my area of the country (and yours). I never fly at gross, and even an old 172 will likely not get bent anywhere under 6G, so I know I'm nowhere near any limit.
 

narfi

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2 pages and I am more confused than when I started, so I will attempt to confuse it more :)

There are two seperate issues you need to address, 1. Legality and 2. Physics.

1. Legality - From my understanding when you do all of your homebuilt paperwork with the FAA, you tell them the gross weight you want. You are the manufacture and you get to choose.

2. Physics - If the kit/plans designer/manufacturer has published numbers with their weights and load factors, AND you built to plans, then it is a simple matter to run the numbers again with your proposed gross weight and see if you are comfortable with the new numbers or not.
 

Pops

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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.
In being a gov contractor in the past, there is always a gov employee that will try to pressure you to the limit, but if you stand up and say NO when you feel you should, you will be a winner in the long run by getting a reputation of being a good safe pilot. Means a lot for the next contract renewal when the person making the renewal decision might be riding with you.
 

R08Tech

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TJTX, the 1050 lb. and 1150 lb. MTOW numbers make it sound like you're building an older Kitfox or Avid Flyer. If you are, the modifications / improvements each manufacturer took to increase the MTOW are available with a Google search.

For example, the Kitfox series 1 through 4 had changes in spar stiffeners, lift struts and fuselage gussets to go from 850 lbs. to 1200 lbs. There are quite a few Avids and Kitfox planes flying with "upgraded" parts and increased gross weight.
 

galapoola

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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
If you reload, it's propellant. Label usually says something like "Ball Powder Smokeless Propellant". Now here's a relevant aviation question, there used to be huge shot shell type cartriges to turn over radials. You loaded them in a breach and fired. Anyone every used one?
 

Dana

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If you reload, it's propellant. Label usually says something like "Ball Powder Smokeless Propellant".
I believe it's a legal classification, too... for example, black powder for muzzloading firearms is considered a "low explosive" which has special storage requirements in the gun shop (big fireproof safe), whereas modern smokeless "propellant" can be stored in open shelves.

Now here's a relevant aviation question, there used to be huge shot shell type cartriges to turn over radials. You loaded them in a breach and fired. Anyone every used one?
Everybody knows the Coffman starter from Flight of the Phoenix, of course, but I understand they were hard on the engines. I toyed once with the idea of one using a .22 blank to start a paramotor engine, but never tried to build one.
 

Chilton

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The military DHC1 Chipmunk used a cartridge starter on the Gipsy Major engine, in cold weather they would strip the oil pump drive on first start of the day as the cold oil prevented the oil pump spinning up as fast as the starter pushed the crankshaft. At least one maintenance chief disabled all the cartridge starters and left them unserviceable to prevent this, he had enough underlings to hand start the aircraft as needed.
 

TJTX

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TJTX, the 1050 lb. and 1150 lb. MTOW numbers make it sound like you're building an older Kitfox or Avid Flyer. If you are, the modifications / improvements each manufacturer took to increase the MTOW are available with a Google search.

For example, the Kitfox series 1 through 4 had changes in spar stiffeners, lift struts and fuselage gussets to go from 850 lbs. to 1200 lbs. There are quite a few Avids and Kitfox planes flying with "upgraded" parts and increased gross weight.
Its a Fisher Horizon 2. The Dakota Hawk has an extra 100lbs over the Horizon 2, but I believe they have comparable Wings? And I've not taken any offense at all, I greatly appreciate the information provided here. It goes to show nothing will be simple with this project. I need to review the G limit definitions as well. My brain thought the noted G limit of the airframe was how many you can pull, not what its ultimately rated too. I dont want to shrink any safety margins that dont make sense. I will certainly run all applicable performance numbers if I were to increase the weight. As I said, I do enough max performance operations at work that I'd prefer to not deal with that in my fun flying. Excellent info and I'll certainly sit down with my tech advisors and find some definable data to draw from.
 

rv7charlie

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That's what I'd expect. My understanding as a simple man is that composites don't have any 'in between' failure modes like other structures, so where an aluminum or wood acro wing can legally have permanent deformation at 6 Gs (but still function), with ultimate failure at 9 Gs, the composite wing goes from unbroken directly to broken so in the case of an acro design, it must be capable of the full 9 Gs without any failure to meet the 6 G number. Just an example of what can drive composite design margins higher than other materials.
 

Pops

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Its a Fisher Horizon 2. The Dakota Hawk has an extra 100lbs over the Horizon 2, but I believe they have comparable Wings? And I've not taken any offense at all, I greatly appreciate the information provided here. It goes to show nothing will be simple with this project. I need to review the G limit definitions as well. My brain thought the noted G limit of the airframe was how many you can pull, not what its ultimately rated too. I dont want to shrink any safety margins that dont make sense. I will certainly run all applicable performance numbers if I were to increase the weight. As I said, I do enough max performance operations at work that I'd prefer to not deal with that in my fun flying. Excellent info and I'll certainly sit down with my tech advisors and find some definable data to draw from.
The Dakota Hawk has a different wing from the Horizon 1 and 2. Different airfoil and different spar construction. The airfoil of the 202, Super Koala and Dakota uses the same airfoil with just different wing cords. Good airfoil for the designs.
This is a friends Dakota Hawk that was on the front cover of Sport Aviation one time with my SSSC.
 

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Dan Thomas

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Usually it's better to choose a design that carries what you want it to in the first place. Higher gross hurts performance, so more power is needed, which adds more weight, and more fuel is burned so the tanks get bigger.....
 

Lendo

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TJTX and Markproa, The things that stands out to me is the lack of understanding the processes of Aircraft Design, as everything affects everything else, when crunching the numbers, In all engineering the standard practice for KNOWN and PROVEN Materials is a Safety Factor of 1.5 - for Composites it's 2 or 3 for reasons of Material Type/Quality and Workmanship. Factor in the MTOW, and anticipated G-load factor and the number is getting high. Factors as Climb, Speed (including Landing Speed) is affected by Power and Lift, which is affected by the Airfoil and Drag.

Basically what I'm trying to say is you really need to crunch the numbers yourself, so that you know exactly what your wanting to do and how safe that might be.
If buying a kit quiz the manufacturer to see what SF and G-loads are tested to.
Hope that helps.
George
 

Daleandee

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When we built my Corvair powered Sonex (second one) the gross weight was increased from 1150 to 1250 lbs. There was a quite a bit of research that went into the decision to do this. It had been done many times by other builders and I found no concerns that were encountered by them. Still, I had to find out for myself if I was going to be comfortable with this change. While I'm not an engineer I did obtain information from some about my particular airframe and configuration.

The Sonex is an extremely strong aerobatic airframe. The DAR & I had a conversation about this. He seemed to have no concerns about this increase but we did go through all the numbers for weight & balance, CG, and expected performance. Since this had been done by others and the results had been documented I had a good frame of reference to begin with.

The results have been very good and I wouldn't hesitate to do the same thing again. But as others have cautioned, you have to understand that you are making a major difference in the structure as to strength and in the performance of the airplane. Increasing gross is done by many but please do the research so that you have a safe and enjoyable plane to fly.
 

Mad MAC

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This thread is lacking a clear explanation of causes.

The primary load cases for GA aircraft are flight loads and gust loads

Flight loads are normally (limit) Pos 3.8 & Neg 1.5, larger loads for utility and aerobatic.

Gust loads are defined by wing loading and airspeed (the critical cases are for minimum wing loading)*

Typically for most small aircraft with low wing loadings they are Gust load critical, however this does not apply to how the load gets to the wings nor the landing gear, or the effect on the load cases on the tail. It should also be pointed out that crack growth is at~4th power, gross weight increase of 10% could result in crack growth increasing by 46%, small increase in loads can really eat airframe life.


*yes the wing makes the same force at both min and max wing load however since the G load is force/ mass, the lowest wing loading generates the highest G force, and since departure of items of mass (say the engine) impedes father flight it is considered a critical case.
 

12notes

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If a plane is capable of safe flight at 5G with a 1000 lb gross weight, would the flight loads be different at 4G with a gross weight of 1250 lbs? Assuming an appropriate increase in power to compensate for climb rate, plane is in correct cg range, etc, doesn't the physics work out to be the same?

Is there some part of the structure that would see significantly more loading in these two situations?
 

Map

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All good arguments, but you maybe forgetting something. The FAA is not going to check up on you if you fly an experimental over gross, but if you have an accident, you bet the insurance company will look at it, and has a good reason not to pay if you were outside of the envelope.
 

Marc Zeitlin

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All good arguments, but you maybe forgetting something. The FAA is not going to check up on you if you fly an experimental over gross, but if you have an accident, you bet the insurance company will look at it, and has a good reason not to pay if you were outside of the envelope.
All true.

But what's the definition of "outside of the envelope"? Since an E-AB builder is the manufacturer of the aircraft, and as has been noted before, the MFG gets to set the MGW of the aircraft, and since the OL's determine what you need to do in order to make a major modification to the E-AB aircraft (which a MGW increase most certainly is), one would only have to follow the instructions in the OL's for a major change in order to re-establish compliance with 91.319 and be completely legal.

The insurance company could NOT claim that one was operating outside of an "approved" envelope if one followed this procedure.

In my case, for my COZY MKIV, I've changed the MGW of the aircraft from 2050 lb. (per the POH) to 2155 lb. My OL's require that to re-establish compliance with 91.319, I re-test the plane for 5 hours and make a logbook entry noting the results of the tests (with some particular language). I've done that, and the insurance company can't say squat, since I'm in 100% compliance with the FAA's requirements and with their policy's requirement that I have a current AC and registration.

Now, if one did NOT follow the OL's instructions on how to re-establish compliance with 91.319 after a major change, then yes - all bets are off wrt insurance.
 
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