Gross weight between patrol and bearhawk

[zkelley2]

There was an interesting article by the designer of the Vans airplanes about builders specifying max weights above his limits. I think it ran in Sport Aviation but here is a thread about it over at Doug's place:
http://www.vansairforce.com/communit...ad.php?t=73492

The meaty bits:
Who Owns the Margin?

It seems common practice among homebuilders to second-guess the factory engineers, particularly regarding gross weight increases.* Because of all of the “I gotta have” added features, empty weight creep erodes the aircraft’s useful load.** The simple solution for the homebuilder is to “pencil in” a new gross weight limit.* It’s only 100 lbs. (3.7%) more; how much effect can that possibly have?”* Imagine this example: you are on a mid-size airliner with a gross weight of 270,000 lbs.* Just before leaving the gate, the captain comes on the PA system and says: “we’ve overbooked more than usual today, so we’re going to assume that the factory engineers over-designed this airplane and allowed an abundant safety margin. We’re going to take off at 280,000 lbs. instead.** So move over, there are 50 more passengers coming on board.”* Run the numbers; it’s the same over-weight ratio as simply pencilling in an additional 100 lbs to the gross weight of an RV-10.

Along with gross weight increases, some builders take the same liberties with horsepower increases and speed increases, betting their lives on the assumption that the airplane is designed with a huge margin of safety---it is really far stronger than in needs to be.* This is not really true.* Certificated aircraft, and well-designed kit aircraft, are designed to withstand limit loads at specified maximum weights.*** During testing, they are subjected to ultimate loads, which are higher than design limit loads by a specified margin.* Yes, there is a margin between the design and ultimate strengths.** But that margin belongs to the engineer.* He owns the margin.* It is his insurance against the things he doesn’t know or can’t plan for, and the pilot’s insurance against human error, material variations, and the ravages of time.* Wise pilots respect this design safety philosophy and leave this insurance policy in effect by operating strictly within established limits.* They don’t try to steal the margin from the designers.

I agree with that entirely. Which is why I left the margin in my calculations. You'd be insane not to.

As far as the airliner part goes, it's actually quite common to be well overweight, but the official "weights" say you aren't. Standard summer weights are 190lbs and winter 195. That's you, your clothes and everything you carry on. Every checked bag weighs 30lbs regardless of what it weighs, unless it's over 50lbs, then it's 60lbs. So on a 200 seat aircraft(about 250,000lbs) you're looking at being over somewhere in the 5-10,000lb range. At the commuters we'd usually store our overnight bags in the lower cargo. If you were really close getting the last pax or a js on, we'd move the crew bags out of the lower cargo and into the cabin. Viola! They magically now weigh nothing. source - AC120-27E

It's a little better on the freighters since things are actually weighed, but in my experience cookie sheets are weighed inside. If it's raining, you're going to be a lot heavier than what the paper says.

[svyolo]

Yep, but all that is accounted for in the safety margins.

My worst experience ever in an airplane was a takeoff out of Vegas in August of '92 (110 degrees F). Navy DC-9. The Captain briefed the pax (all pilots) that we were a bit heavy, and he was leaving the flaps up until later down the runway. He back taxied to the end of the displaced threshold, and we finally rotated and got airborne in the overrun/displaced threshold on the other end. I think most of the armrests were damaged by the passengers, and the seat cushions all had a brown stripe. Including mine.

[zkelley2]

In the performance margins...sometimes(we were heavy enough the other day that we couldn't take off and meet a climb gradient, well under MGTOW), but part 25 aircraft, from a gross weight and loading perspective can be as low as 2.5G per regulation. IME, most of them are 2.5 to take advantage of additional useful load for the weight required to build to that strength.

[PaulSA]

Can someone point me to any "official" paperwork showing approval by the designer (Bob) to operate the 4-seater to 2700lb T/O weight with 2500lb LW? I know the float version is approved to 2700lb. I understand the arguments over the g limits so if Bob has actually approved 2700lb with a 3.8g limit then I'd like to see the paperwork so I might get to use it.

[schu]

The four place is the same in some ways. I cannot exactly measure the aft CG, but I suspect I have been more than close to the limit. Weight is harder in the 4 place, I put mine at 2,700 T/O max weight and I haven't been able to get that heavy. I would need to start over-eating.

But in other ways, its different to the Patrol as reported. When flying near the weight limits, the takeoff performance degrades remarkably at 2,600lbs compared to 2,100lbs. Every horsepower counts.

I guess that's one reason to not buy a nice composite prop. It lightens the front as well as your wallet.