daleandee wrote:But before we get started I have a few questions for the group.
This is not meant as an argument either way for the Corvair or any other modifications, but I think those questions aren't really relevant if we're discussing design modifications.
Let me counter-ask these questions:
- How many times did the Space Shuttle fly before the Challenger blew up?
- Knowing what we know after the fact, were those flights safe or not?
It's the nature of statistics that the presence of a Sonex that has flown with twice the FWF weight of the factory limit tells us little. In contrast, the presence of a Sonex with twice the FWF weight of the factory limit
that had its engine fall off would tell us a whole lot.
Just so you know where I'm coming from with this: I work at SpaceX. Since I started there, we've had two catastrophic failures of orbital launch vehicles. I can't go into details, but they've both been traced to two failures of Helium pressure vessels. They were completely different failure modes, but what they had in common is that we had successfully flown the vehicle numerous times before these anomalies. Everything
appeared to work fine, but in reality, due to larger part tolerances than we were aware of, we unknowingly had much less margin than the design aimed for. Eventually, we hit a situation where the variations stacked up against us and many hundreds of millions of $$$ went up in smoke.
The Sonex design has a certain safety margin designed into it. This is meant to account for tolerances in parts, builder technique, turbulence, pilot technique, design flaws, etc. We don't know what that margin is. John Monnett does, but he's not going to tell us. What we're doing when we exceed the design limits is eat into that margin. That may work. It may work fine forever. It may also only work fine until you hit severe turbulence, or do a hard landing, or pull a few extra Gs. You have no way of knowing unless you actually test it.
I think Dick van Grunsven of Vans Aircraft put it well (from
https://www.facebook.com/notes/vans-aircraft-inc/what-price-a-masterpiece-by-dick-vangrunsven/237594966250883):
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.