How safe is safe enough?

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Monty

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OK...so people are constructing indestructible crash proof vehicles (......Steve!....)

And I'm working on a death trap.

I have wet wings....leading edges. The first thing to take damage in a crash, and I'm surrounded by some 1X1X.125 angle and 30 thou aluminum sheet. Not much energy absorption to speak of. At least the landing speed is reasonable, the canopy is big enough for me to wear a brain bucket, and there is a roll over structure.

Basically I see this as a flying motorcycle. I'm OK with the risk.

People who ride Harleys understand they are not in a Mercedes M class...I think.

Thing is physics makes me do the things I do design wise. Otherwise I would have to increase the gross wt A LOT to gain a fairly small margin that would only help in limited scenarios.

Fact is if you hit the ground at a substantial vertical angle...with a speed greater than about 65 mph...you are toast. The Earth is HARD!!

Unless we use some sort of giant external airbag with radar activation and retro rockets...which isn't a bad idea...for Steve's mochine, but I don't have the weight margin.

I'm not trying to pick a fight, just want to know peoples thoughts.

So my question is...how many of you want the M class and how many want the Harley?
 

Topaz

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Color me somewhere in-between. I think the best safety measure you can have is to not crash at all. Unfortunately, I'm not a perfect pilot and whether I crash or not isn't always in my control.

My personal philosophy is to build in as much safety as I can - so long as it doesn't adversely affect performance. 99.999+ percent of the time (and hopefully 100%), I'll be flying the aircraft without incident. The performance matters more than the crashworthiness, 99.999+ percent of the time. However, staring down at the ground rushing up isn't the time to decide to add more safety features.

My goal is to provide some realistic survivability features without killing the performance of the aircraft. Like everything else in aircraft design, it's a balance. For my first aircraft, I'm thinking of including a safety cage. Something steel, welded, and just enough to surround my little pink bod. That and a crushable seat pan and a solid five-point harness. And a seat-pack parachute that will, if it comes down to brass tacks, make all of it moot. But for my aircraft - a small glider and a somewhat larger motorglider, putting in the nth-degree safety feature would kill all the performance. If the aircraft can't perform the mission within the designated budget, it's a failure, regardless of how "safe" it might be.

So like I said, it's a balance. Ignore safety at your peril. Obsess on it at your peril. Pick your personal "balance" point.
 
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Head in the clouds

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Good topic Monty.

On the open country road with minimal traffic I want to enjoy the Harley although it's no fun when a kangaroo decides to turn right in front of you (of course they'd be turning left if they were in US or Europe...). On the highway or the freeway I want the M class - too many maniacs, trucks and incompetents to share the road with.

In the air and flying for fun, the Harley of course.

The thing is that crashing isn't inevitable. Some folks seem to think that since the statistics say that one in every however many flights ends in a crash that it's some kind of a lottery. But you choose how and where and what to fly. If you fly a plane that's airworthy and structurally sound and fly where you always have an outlanding in case of power failure and stay out of bad weather and don't be a hoon then there's no reason why you can't go through life without crashing.

Since you mentioned Steve I read the other day that he's going to have a BRS system as well as all the other wonderful devices to save him (and fiancee) in a crash. It would almost seem a waste to fly around with all that stuff and not have a crash... ;)
 

Monty

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Good topic Monty.

On the open country road with minimal traffic I want to enjoy the Harley although it's no fun when a kangaroo decides to turn right in front of you (of course they'd be turning left if they were in US or Europe...). On the highway or the freeway I want the M class - too many maniacs, trucks and incompetents to share the road with.

In the air and flying for fun, the Harley of course.

The thing is that crashing isn't inevitable. Some folks seem to think that since the statistics say that one in every however many flights ends in a crash that it's some kind of a lottery. But you choose how and where and what to fly. If you fly a plane that's airworthy and structurally sound and fly where you always have an outlanding in case of power failure and stay out of bad weather and don't be a hoon then there's no reason why you can't go through life without crashing.

Since you mentioned Steve I read the other day that he's going to have a BRS system as well as all the other wonderful devices to save him (and fiancee) in a crash. It would almost seem a waste to fly around with all that stuff and not have a crash... ;)
I think you have hit on an important distinction that seems obvious but is not. My airplane is designed as a fair weather fun flyer. I have no desire to tangle with bad weather, IMC or such in this aircraft. If I were flying hard IMC with my wife, parents, or other pasengers, where I had to contend with mission risk, and scheduling pressure....I'd want Steve's airplane, or a King Air...no question. I've flown a Piper Malibu, in bad weather...aircraft and mission are inseparable.
 

Vigilant1

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Like everyone, I think it makes sense to make tradeoffs. If we want to take to the air, we are taking risks. But it makes sense to make the craft strong enough, and with impact mitigation featrues good enough, to give the occupants a good chance of survival in the most common types of accidents. And, where it can be done without undue compromise to other goals, it makes sense to protect even against unlikely but high-lethality eventualities.

Specifics:
I think a basic structure that can hold it's shape to avoid impingement on occupants in a 60 knot impact at a pitch of 45 degrees onto a paved surface is a reasonable goal for the type of aircraft I'll likely be building. I think we should take all reasonable steps to reduce risk of spinal injury due to vertical accelerations (pancake style), but recognize that, from a practical standpoint, we'll be limited to an effective total seat stroke of about 4-6". A lot of good can be done within that range--far better than many certified aircraft. On a cost-benefit basis, probably nothing exceeds the value of well-designed restraints (hooked into parts of the structure that will stay put). Reducing the risk of post-crash fire through well-built fuel tanks put in places that are not especially prone to damage is prudent. Building in robust rollover protection and engineering in a way to egress quickly from a rollover is also important.

Well, that's a start . . .

We should also recognize that some things done in the name of safety can increase the chances of an accident, if we're not careful. Avoidable, but if someone "Safetyfies" an existing design by adding weight so that it barely climbs, that plane isn't safe. Sometimes the thing that would add greatest to safety is lower wing loading or an extra 15 minutes of fuel aboard. Everything is a tradeoff, and if safety costs weight, then something else suffers.

I'm betting we'll hear from others. . . .
 

SVSUSteve

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Monty said:
Fact is if you hit the ground at a substantial vertical angle...with a speed greater than about 65 mph...you are toast. The Earth is HARD!!
Honestly, most ground has a lot more "give" than people think. It's not uncommon to see hard-packed soil dented a foot or more from a survivable crash landing at the point where the belly made first contact. It really isn't until you get above 65 degrees from the horizontal that you really have to start relying largely on the front of the aircraft to crumple. Most crashes are at angles less than that- probably something like 30-45 is more common because even with "a loss of control", a lot of folks get their nose up some before impact. With a lesser angle, as long as your engine doesn't fold down and dig in and you have the bottom of the firewall canted back (no less than 45 degrees off the vertical), chances are you'll slide more than you plow which extends the deceleration distance and cuts down on your impact loading. Even if you're going

There are a lot of crashes where a more or less vertical impact has been initially survived only to have the occupants killed by the ensuing fire. A flight into a mountainside or hill is actually one of the more common ways to have a "higher" angle because of the difference between the direction of travel and the impacted surface CFIT crashes often involve an aircraft that is more or less level and going at a higher rate of speed than your average low-altitude spin stall. A good number of these (most of them that can maintain a reasonable cockpit or structural volume have initial survivors that are killed by an ensuing fire.

Remember that a well restrained and reasonably fit person in a well designed structure can survive up to 214 G for a fraction of a second (racing has demonstrated this). The primary reason for the difference in survival between an Indy Car and a GA airplane (despite the lower impact velocity of a GA aircraft) is not some massively heavy structure (two people can pick up the survival cell of an Indy car; it weighs less than the engine of most mid-range GA aircraft) or impossible engineering (the Formula 1 regulations literally spell out how to make most of the parts for the **** cell down to what materials, their sources and the ply orientations!) but the fact that the accepted three concepts that folks in GA want to believe are unachievable or impractical:
1. Survivable volume
2. A strong restraint and seating system with energy attenuation
3. A crash resistant fuel system

The first two are achievable in a number of ways and has been demonstrated in a number of aircraft from STOL aircraft (Helio Courier) to military helicopters (with regards to "stroking" seats, they have had free falling Blackhawks after RPG/MANPOD strikes with a vertical impact velocity in excess of 800 fpm with survivors) demonstrate various ways of achieving this.

The second is pretty simple as well. Avoiding a wet wing is a good start. The answer to the question of why someone would put fuel in front of the spar ("because I need the volume") should not be the end of the discussion but rather the start. How can you do it safely? What are the other options for fuel location? Even a breakaway tip tank might be a safer option honest if you are willing to do the engineering work necessary. There are tradeoffs in everything but while Occam's engineering razor is often a good approach there are times where it is not.

Monty said:
Unless we use some sort of giant external airbag with radar activation and retro rockets...which isn't a bad idea...for Steve's mochine, but I don't have the weight margin.
Actually a giant external airbag tends to be bad since it would increase the tendency to bounce the aircraft or cause a loss of control in the second or two before impact. It would be kind of helpful to extend flotation times during a ditching though..... ;)

Monty said:
Thing is physics makes me do the things I do design wise. Otherwise I would have to increase the gross wt A LOT to gain a fairly small margin that would only help in limited scenarios.
As far as weight margins....from the preliminary engineering calculations: The percentage of empty weight on the Praetorian due to structure is roughly 5% more than what you would see with a similar composite aircraft design that isn't designed to be crashworthy and it probably has more to do with it being a pressurized design and the flight loads requirements (basically it has the G targets to give it comparable tolerance to higher end aerobatic aircraft). Most of the "extra weight" compared to another 4 seat aircraft is due to acoustic insulation and other creature comforts and fuel for a longer range.

In some ways, a smaller one- or two-seat aircraft are easier to protect because you have a smaller cockpit volume to protect. A decent 4-seat aircraft (where you're not snuggled up to your neighbor in a way that might constitute carnal knowledge if either of you are above the 75th percentile in terms of size) is more of a challenge. If you want my help or advice, I am happy to offer it. When I say that it's not because I want to change your goals but rather that I want to help you achieve them so I can earn a ride in your plane.

Monty said:
how many of you want the M class and how many want the Harley?
A luxury car is a pretty poor comparison. They are bloated in terms of weight due to their engines and other things that have nothing to do with safety. A better question might be "Who wants an Indy car and who wants a Harley?". Both have the "get up and go" to satisfy speed junkies like us. The difference is that if you crash or flip one, you're probably dead. The other you get out and wave to the crowd and go change your underwear.

As Topaz pointed out, there has to be a balance between the two. Mine is that I want the best chance of surviving a middle of the road crash (not the "firewalled into a vertical granite cliff followed by a thousand foot fall down the face") so I figured out what it would take to do that and then added a factor of stupidity (important since I am not "that kind of engineer" nor a composite manufacturing specialist) of 2 to 2.5 depending upon how critical the structure is. That is sufficient for my purposes and if broadly applied would provide a survivable deceleration for probably 50% of the currently fatal crashes but still light enough to allow for a performance that would put me and three friends in south Florida in about 4 or 5 hours. However, as much as I would like to be the homebuilt safety equivalent of Tony Bingelis, Burt Rutan or Bill Husa, I have to admit that my goals are simpler than that: I want to protect my own ass and those of the people I care about if the **** hits the fan. I want to be as safe and as comfortable I can while being able to travel.

If I learn something that can save others in the process and prevent me from burying more of my friends and colleagues, so be it. My first "flight instructor" (at least the guy who offered me my first logged flight) is dead, gone and far removed from most people's memory but whenever I pull his file out of my filing cabinet it is something that makes me tear up. I may be tilting at windmills but then again I might do something that, had it been done before that tragic day, I might still be able to sit across a table from Scott Crossfield at an airshow and talk about the day he took pity on a nerdy kid who had ridden his bicycle to the local airport to watch the planes work the pattern. Not many people would have given me a second thought....he talked me through my first landing. I remember the terror when he told me that he was going to let me land. He laughed and said "If Chuck Yeager can land a plane, anyone can". I recall not seeing the humor in that until after we were on the ground.

I've worked myself to the point of tears so I probably should wrap this up. Sorry to ramble.
 

lurker

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safe enough for me is: highly unlikely to fall from the sky, backed up with a brs.
flying motorcycle? more like this:
 

SVSUSteve

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highly unlikely to fall from the sky, backed up with a brs.
The BRS is a great tool (probably one of the top ten innovations of the past 25 years honestly) but one with some very narrow applicability because of how few aircraft have issues at high enough altitude for the BRS to deploy. In most cases, it's just another big ignition source for spilled fuel. One of my slightly paranoid friends who works for the NTSB jokes that the "real reason" Cirrus used a solid fuel rocket rather than a compressed air system is to help cover up how poor their design work is ("kind of hard to prove much when everything is burned to cinders" were his exact words).

That said, I am including a ballistic parachute on my own design although I would much prefer a compressed air deployed system. However, there are few of them that handle anything beyond the ultralight or LSA category.

I think a basic structure that can hold it's shape to avoid impingement on occupants in a 60 knot impact at a pitch of 45 degrees onto a paved surface is a reasonable goal for the type of aircraft I'll likely be building. I think we should take all reasonable steps to reduce risk of spinal injury due to vertical accelerations (pancake style), but recognize that, from a practical standpoint, we'll be limited to an effective total seat stroke of about 4-6"
Two pieces of advice:
1. Design with a factor of safety/stupidity in mind. 1.5 is an absolute minimum. 2.0 is probably better. You wouldn't design a spar without a factor of safety so the cabin, seat attachments, restraints and their attachments should be no different.
2. Remember to use that distance in the most effective way possible. That might mean using "subfloor" space to increase the total crushable distance or using energy attenuating devices like cellular foam "beams", crushable CF or metal tubing, wire benders, etc. Even the choice of foam is important (pretty much every major study or organization tends to strongly recommend Confor-Foam 42).


As always, I am happy to offer whatever help I can. Even if we've butted heads before, if input is possible, it is available even if after careful evaluation it is rebuffed.
 

Monty

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@Steve...don't cry man!:depressed. My teacher is dead too. Unfortunately he did not die in an airplane! Sounds sick, but I mean it. Instead he was murdered at an ATM.

I admire what you are doing, and I agree that much can be done.

I debate whether to move the fuel to the outer wing panels-break away. But that decreases spin resistance. W&B & CG requirements put the fuel where it is. The alternative is in my lap, with a heavier spar.

Aluminum is not CF or Kevlar in price or performance.

I can't afford F1
 

SVSUSteve

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Aluminum is not CF or Kevlar in price or performance.
But it's also not a death sentence if well designed. For a lot of things, metal is still a darn fine choice. If it weren't for my desire for a pressurized aircraft (I hate wearing oxygen which might sound odd coming from someone who previously had a career as a respiratory therapist) and the fatigue issues with metal, I'd be building a metal bird. As Peter Garrison says, a perfect shape in a composite design is artificial. To pull it off in metal, that is a sign of a true artisan.

@Steve...don't cry man!
.
Eh.....no worries about it other than I tend to get muddled in my writing when it gets to that point. Not that it's particularly clear to begin with....

I always worry about people who aren't comfortable enough with themselves to cry. It's incredibly cathartic. One of my combat veteran friends (a Silver Star and Distinguished Service Cross recipient as a medic during WWII so the phrase 'tough badass' doesn't begin to cover it) likes to say that anyone who says they haven't cried has definitely never seen war firsthand.

My teacher is dead too. Unfortunately he did not die in an airplane! Sounds sick, but I mean it. Instead he was murdered at an ATM.
That sucks. My condolences man....beyond that, I don't have the words for it.

I admire what you are doing
It's nice to know that it's appreciated. I often wonder if people listen or just get annoyed because of my "broken record".

I debate whether to move the fuel to the outer wing panels-break away. But that decreases spin resistance. W&B & CG requirements put the fuel where it is. The alternative is in my lap, with a heavier spar.
Totally understand. Depending on your CG issues and how much effort you're willing to put into the tank, it might actually be an option to put it aft of the pilot seat. However, that is one time where you want to be sure to overdesign the tank (like a safety factor of 3.0 or 3.5 above the worse case scenario) because if you spill fuel and it doesn't ignite (not a terribly uncommon occurrence), fuel induced chemical burns are one of the levels of Dante's hell.

The other way you can reduce your fire risk is to take steps to reduce ignition sources. The US Army crash survival design guide's volume on post-crash survival (volume V if memory serves; it should come up easily with a Google search or I have it on computer if you can't find it) gets into this with a great degree of detail. Most of what they discuss could be achieved with very little weight penalty (electrical isolation in the first fraction of a second of a crash sequence, etc) and could be put together by anyone with a reasonable degree of mechanical ability with parts available commercially.
 

harrisonaero

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Been in this game a long time and do it for a living and here's my opinion on the "safest" airplane:

Low, over trees, local flights, backcountry: steel tube fuselage, low stall speed, and never ever compromise on two things- weather and fuel

Fast to go XC: don't ever fly low- get up and fly as high as needed so you nearly always can glide to an airport. Have basic IFR instruments and know how to use them. Have a turbo to get above conditions like icing when needed.

Best compromise between the two: Vans RV series with basic IFR panel.
 

SVSUSteve

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Headintheclouds said:
There isn't a plane that's been thought of that could do that in most of inland Oz
Correct me if I am wrong, but I was under the impression that the central part of Australia is rather similar to the central US plains. That is: most of it is relatively flat and makes for a workable landing strip in a pinch.
 

SVSUSteve

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harrisonaero said:
Best compromise between the two: Vans RV series with basic IFR panel.
A Vans performance in a crash is pretty sad especially if it inverts during the crash. It's a great plane aerodynamically and performance-wise not to mention a generally attractive aircraft aesthetic but it's not a very good compromise between the the two "extremes" that you mention. Other than the steel roll cage and low stall speed you mention, most of those things have little to do with safety as Monty was talking about. It's the classic pilot approach of "avoid the crash" rather than making the crash more tolerable. That's important but it's only a single "layer" of the "onion" of safety.
 

Head in the clouds

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Correct me if I am wrong, but I was under the impression that the central part of Australia is rather similar to the central US plains. That is: most of it is relatively flat and makes for a workable landing strip in a pinch.
Google Earth is your friend. Soft desert sands, scrub (small scattered trees), rocks, stumps from cleared land, anthills, shale, the Great Divide, channel country, vast erosion areas but yes you can land, much better to be in what HA described for trees etc than XC. Main point was HA said glide to an AIRPORT..
 

SVSUSteve

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Main point was HA said glide to an AIRPORT..
Yeah, it works at airliner altitude.....not so much at 18,000-25,000 (high GA altitude) unless you're in the eastern quarter of the US or the western quarter.
 

Head in the clouds

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Yeah, it works at airliner altitude.....not so much at 18,000-25,000 (high GA altitude) unless you're in the eastern quarter of the US or the western quarter.
No it doesn't work here, nothing like it. Airliner alt might be say FL400. A GA up there might have 8:1 average gliding down, so 60 miles max incl manoeuvering. With 400 miles between airports in some remoter parts here... our population is on the coast, the majority of the inland is either untouched scrubby grazing or desert
 

akwrencher

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Google Earth Alaska. Best bet here in Southeast is a beach, if the tide is out. Otherwise, hope you can find a gap in the trees wide enough for the fusulage to slide through......:gig:
 
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