Crashworthiness

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Dana

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That's right, NO mechanical failure modes. The rotary has no mechanical failure modes that result in sudden or total power loss. They are bullet proof...
I still don't buy it. I listed several potential (if unlikely) failure modes. Just because failure hasn't been reported in a particular installation, doesn't mean that a completely different won't have a failure mode... or that you won't get the odd defective part. There is no such thing as 100% mechanical reliability.

-Dana

Inflation is a result of legalized counterfeiting.
 

addaon

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Yep. Throwing a prop blade might not stop the engine immediately, but will you really care if it's running without a prop?
 

Topaz

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...That's right, NO mechanical failure modes. The rotary has no mechanical failure modes that result in sudden or total power loss.
Sorry, I'm not buying that either. I can think of several right off the top of my head, and I'm far, far from being an engine expert. Seals, output shaft fatigue, gear wear between the rotor and output shaft, and so on. The chamber itself has seals, and those'll give out eventually. Bearings have a finite life before the wear out or seize, and a Wankle has 'em too. Simple defective parts in any of those components (which can happen in any manufacturing operation) can drastically shorten the time before you're looking for an unplanned landing field.

They are bullet proof up to 10,000 RMP in automotive racing. I'll be loping along at a leisurely 5,000 RMP....
Not a good comparison. Racing engines run for relatively short periods and are carefully maintained between uses by a mechanic usually more qualified than your average shade-tree. Professional teams tear the engine down completely between races and replace components on a regular basis, whether or not they're approaching their design life.

An airplane engine runs for hours, months, and sometimes years before any significant maintenance. Things are generally only replaced either when the engine reaches its TBO, or if something fails.

These are completely different operating environments, and comparison between them isn't valid.

Dana's point is probably the best: No matter how "bulletproof" the simple mechanics of an engine might possibly be, the vast majority of failures are in the various systems of the engine: carburation/fuel injection, lubrication, cooling, ignition, etc. There's nothing special about a rotary in any of these aspects.

I personally would love to use a rotary in an airplane - they're tiny and light for the installed power. Just don't have any illusion that they're "bullet proof". Nothing is.
 

Dan Thomas

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Is a low wing truly safer? It could absorb energy... but you're more likely to catch a wingtip on a rock or bush, spinning you around. Also tanks in low wings are more likely to rupture in a landing on rough ground than tanks in a high wing.

Tricycle gear... I'm prejudiced in favor of taildraggers, I know, but there's a good reason why all bush planes are taildraggers. Nose gears tend to be a weak spot, and when they don't fail, they take the firewall with them.

Heavy elevator control to provide feedback to pilot. Use of bobweight."
Nope. Not all bushplanes are taildraggers. Not anymore. The Quest Kodiak, the Gippsland Airvan, and the Cessna Caravan are all trikes and operate off rough strips. I'm associated with an organization that used to use 185s and so on in some really rough areas but has shifted to the trikes. Their nosegears aren't the stupid flimsy setups we find on 172s and 182s.

But I still prefer taildraggers, though. Even a stout nosegear is more prone to digging into soft ground and nosing the airplane over
a lot sooner than the taildragger. I know there'll be an argument arise out of that one. Separate thread, maybe.

Low-wing airplanes can be harder to get out of if they end up on their backs. I'm building a Hummel Bird and have a Jodel, both low-wing airplanes with canopies that would be almost impossible to get open once the airplanes are resting on them. Not a pleasant thought, considering fire or whatever. I want to fix that with the HBird. Dunno how, yet. Make it a high-winger?

Rear-engined airplanes also bother me. That heavy engine is going to want to come through my space if I hit something hard enough, so the mount and all the structure ahead of it all have to be stronger, usually meaning heavier, which is a dirty word around airplanes.

There are seats designed to collapse. Fairchild holds the patent to one design used in light aircraft like the Cessna 185 or the Helio. Its side frames are made of steel tubing bent into an "S" shape; rigid enough for normal service but will collapse under the vertical load that damages the spine. Seats like that need some distance to work, but in a lot of homebuilts there isn't that sort of room. In a low-winger you're often sitting on the spar. Even many high-wing homebuilts are too slender to have any room for such stuff.

The 2006 172 we bought last fall has gone the other way. Its seats are built skookum like a bridge, two or three times heavier than the old ones. Not built to collapse at all. And their weight cuts into the useful load big time. I'm not sure what they were thinking.

A bobweight is a weight on a lever attached to the elevator's control mechanism. As G forces build, the weight pulls downward and pulls the elevator downward to reduce its nose-up authority. It makes the control forces higher when pulling back hard on the stick or yoke to try to avoid overstressing the airplane or getting an accelerated stall.

The Cessna Citations have them. See this:
Service Difficulty Alert - National Aircraft Certification

Dan
 

GESchwarz

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My design is in the same general class as the RV-8A. So another thing I did was increase the span and the aspect ratio while keeping the wing area about the same. With the increased wing thickness there is no weight penalty for the increased span. With these changes I get a better climb rate and glide slope than the RV-8A. I know that my empty weight will be a little greater than the RV-8A but the added safety and other features are worth it to me.

Good point about egress while upside down in a low wing. Will have to think about that one. I plan to have a tool on board for breaking the canopy, and a stout roll bar. Perhaps the canopy skirt can be designed for easy push out following distruction of the canopy. This may allow room for egress.
 

GESchwarz

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Saturday Night at the Forum! I guess we now know what old airplane guys do on Saturday night. I laughed to myself when on my way to Oshkosh last summer (for my first time!), I arrived at the destination airline terminal and I spotted these other single, over 50 guys in plaid shirts and I knew where they were going.

Back to the subject...Someone said that it's best to just not crash. I which it were that easy, but we all know that even great pilots can and do find themselves in forced landings. With Murphy as our constant passenger it's just risky to not prepare as best we can. If it costs a little bit in performance, it's a very small price to pay for being able to walk away.

The issue of the nose wheel digging into soft soil is a good one to raise. I have a couple of thoughts. A trailing link configuration may perform a little better than the standard. And these newer gears that have the strut extending at a rather forward angle may perform well in a soft soil landing versus the more vertical legs.

Another way to help to survive a soft soil landing with a trike is to design the nose gear to fold back under that sort of load without causing damage to the firewall/cockpit cage.

Did you notice how nicely the engines came off that US Airways flight 1549 Airbus that ditched in the Hudson River on Jan 16, 2009. Does anyone know about that?...Did Airbus design the nacelles to snap off in a water landing? It worked great..all 155 occupants got out alive!!! That's just amazing. Talk about a crashworthy design.
 
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Topaz

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...Back to the subject...Someone said that it's best to just not crash. I which it were that easy, but we all know that even great pilots can and do find themselves in forced landings. With Murphy as our constant passenger it's just risky to not prepare as best we can. If it costs a little bit in performance, it's a very small price to pay for being able to walk away.
Let's talk about that for a moment. A forced landing does not automatically equate to bent airplane and potential injury. We're drifting back into territory recently covered, but with adequate training and some practice at it, an off-airport landing shouldn't usually any more than: an off-airport landing. With a reasonably slow stall speed, reasonably sized tires, and a decent glide ratio, you should be able to find someplace flat enough and smooth enough (and close enough), and without too many obstructions, to allow a safe off-airport landing. Soft soil is less of an issue if you don't use tiny hard-pavement-only tires. Granted, I live in the West, where we don't have forests everywhere.

Airplanes do crash, and it's a good idea to build in adequate safety technology to make crashes more survivable. Still, it's far better to design to make crashes as avoidable as possible - the best crash is one that doesn't happen. Designing so that you can glide to a reasonable off-airport landing site, and having a slow stall speed and larger, softer tires go a long way in that direction. Instead of relying upon a crash cage, you can worry about how to trailer the airplane out of there.

Did you notice how nicely the engines came off that US Airways flight 1549 Airbus that ditched in the Hudson River on Jan 16, 2009. Does anyone know about that?...Did Airbus design the nacelles to snap off in a water landing? It worked great..all 155 occupants got out alive!!! That's just amazing.
Eh. It is and it isn't. Not to take away from Sully, but he did what he was supposed to do, had some nice smooth water (no big waves) to land in, and everything went about right. Where he gets his kudos is in choosing to land where he did, and the fact that he landed gently enough that one of the engines remained on the aircraft, even after being dragged through the water. All nacelle-mounted jet engines are held on with bolts designed to shear away in a belly landing. In fact, there was a DC-10 some years ago that crashed because the rear bolt (of three) fatigued and failed, and the engine rotated up over the wing and departed the aircraft, taking various systems with it. A good example of the double-edged-sword nature of safety innovations.
 

wsimpso1

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OK, Powertrain Engineer for 18 years, safety engineer for 3, confidant of Everitt Hatch on gear and bearing sizing for the Powersport engine. Failure modes are ways that it can fail. Just because someone's perception of the frequency is zero does not mean the mode does not exist. After that, we do get into frequencies...

Mazda rotary failure modes that result in zero power:

Failure of any of the pump or seals or gaskets or components that contain glycol or oil for the engine or reduction gear;

Failure leading to seizure of any of the bearing sets on the main shaft, distributor shaft, rotors to main shaft, reduction drive;

Failure of the rotor timing gear, distributor shaft gearing, reduction drive;

Failure of the distributor, cap, rotor;

Failure of the fuel injection pumps;

Failure of the throttle linkage resulting in closed throttle;

Blockage of the inlets or ducting for induction air and coolers. This includes stray charts and paper, icing, duct failure and collapse;

Blockage of fluid flow including fuel flow, glycol and oil flow. Many means including contaminants, channel failure, etc.

And none of this includes parts not made to specs, which happens...

Together we already have many dozen modes with their own sub-modes (how many independant ways can a fuel pump or bearing fail?) resulting in thousands of individual failure paths. And we have not gotten into power reduction failures.

Now there are things that can be and/or are done. The Mazda rotaries are characterized by sturdy design. Add to that redundant fuel pumps, simple robust air ducts and fluid lines, spring loaded throttles, redundant full electronic ignition, careful pre-flight inspection, large reservoirs for fluids. In the end, you still have many ways for things that can cause an engine to stop making usable power.

I knew Ev Hatch. He went through a bunch of designs to get a reduction drive that works for the Powersport engine. Torsional resonance is still in there trying to tear up the path between engine and prop. He knew how to build durable race engines (His engines were noted for being raceworthy at a winning level for a whole season) and went to a lot of trouble to make the reduction gear system sturdy. Alan Tolle still did a lot of dead stick landings with those engines because torsional resonance destroyed the gear drives. Are they bomb proof? Nope. If the lash is set too large or the lash adjustment slips or primary rotor gear contamination is large and trashes the teeth or the oil leaks out and the poor thing bleeds to death, then the power goes to zero...

You can eliminate some failure modes, and the Mazda rotaries do that pretty darned well. And they have made a bunch of modes seem non-existent. And some other failure modes have been reduced in severity. They just might be a more reliable power system. But do not fool yourself or try to convince anyone else that they don't have failure modes... They do.

Billski
 

GESchwarz

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Good points Billski. Many of the modes you described are common to all types of powerplants, and therefore are valuable inputs for all of us to consider. My arguement was regarding what is unique to the rotary. The seal failures result in only partial power loss and I believe are neither sudden nor catastrophic. The reduction unit is a concern and I haven't settled on which one I'm going with; gotta look at the reliability data. Because it is true that Murphy is there all the time, by designing in several layers of safety measures, if one lets you down there are others there to save your skin: the added wing span, the roll cage, the seat suspension, 5-point harness, the interior that's free of hard, blunt or pointy things, etc.
 

MalcolmW

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Hi,

all very interesting and creative. Thank goodness home builders don't have to take 'bailout money,' otherwise they could face a future as grim as the automotive industry...

For example, ponder the aeronautical equivalent to this auto:

(Coming soon to the Congressional Dealer near you)

MalcolmW (slightly tongue-in-cheek)
 
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Inverted Vantage

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Funny...not sure if you could get anymore right-wing with it lol, but funny :D
 
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Grimace

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Good points Billski. Many of the modes you described are common to all types of powerplants, and therefore are valuable inputs for all of us to consider. My arguement was regarding what is unique to the rotary. The seal failures result in only partial power loss and I believe are neither sudden nor catastrophic.
You're referring to the apex seals, right? Which basically serve the same function as the piston rings in a normal piston engine. You may note that piston rings don't commonly fail "catastrophically" either... pretty much every piston engine out there with more than a few hundred hours has a cylinder or two that's a little low on compression... it's not catastrophic for them either. Or, is there a different type of seal that you are referring to?

As for reduction drives on engines, that's another failure mode that doesn't even exist in normal piston powerplants...

I like the rotary engines a lot. They have a lot of potential. But if it weren't a matter of cost, there's no question in my mind that a well-made Lyc/Con engine will offer greater reliability in an aircraft application...
 

Dan Thomas

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You may note that piston rings don't commonly fail "catastrophically" either... pretty much every piston engine out there with more than a few hundred hours has a cylinder or two that's a little low on compression... it's not catastrophic for them either.
Depends how they're flown and looked after. Our seven Lycomings still have compressions in the mid-70s when we removed them at timeout (2000/2400 hours. Zero leakage is 80). They're flown frequently and hard (flight training) and are kept in a heated hangar and fed 100LL and Aeroshell 15W50.

When compressions do fall off it's usually the exhaust valves, not the rings. Aircooled engines are hard on valves and seats. If rings are leaking it's often due either to poor air filtration letting dust into the intake ( a teaspoonful is enough to lunch the engine) or corrosion from disuse or short flights with their water-accumulating problems in the crankcase.

Dan
 

wsimpso1

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Well, Mazda rotaries do sometimes have apex seal failures. When an apex seal goes, it tears up the rotor and housings and that rotor's power goes to zero, but the other rotor is unaffected, and you have a partial power situation. I have seen a rotor from a supercharged RX7 door slammer drag racer that trashed all three apex seals, messed up the rotor, and presumably caused the installation of a new rotor housing set too, but the guy drove the car home. Don't count on climb, but you can definitely reduce to best glide speed, maintain altitude and land where you want to.

My comments about seals were relative to main seals for keeping oil and glycol in their places.

Billski
 

GESchwarz

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Here's another safety feature...

Isolate and maximize separation of all electrical conductors from the fuel system.

A master circuit breaker can be used to accomplish the same purpose if separation isn't possible.
 

Birdmanzak

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... In fact, there was a DC-10 some years ago that crashed because the rear bolt (of three) fatigued and failed, and the engine rotated up over the wing and departed the aircraft, taking various systems with it. A good example of the double-edged-sword nature of safety innovations.
To be fair to McDonnell Douglas, that was American Airlines' fault: instead of following manufacturer's instructions to remove a pylon they used a forklift to lift the whole engine and pylon in one go. It saved several hours, but led to cracking in the structure around the shear pins.
 

Topaz

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Yep. Another good reason to "do it by the book", no matter how much the manager on-site wants to save a few pennies.

It used to boggle my mind at my last employer. We'd get a project and I'd work up an estimate for the money and man-hours involved for my department to complete the work. Sometimes, under pressure from Sales, my manager would overrule my estimate and direct me to do things one or the other 'shortcut' way - usually by trying to do too much concurrently. Every time we did that (and it happened much more than once), we'd end up spending more money and time by the time it was all done than if we'd just done things the "right" way in the first place.

Ask the 787 team if that doesn't sound familiar right about now.

One of my favorite sayings: "There's never enough time to do it right the first time, but there always seems enough time to do it over again."

This is one of the reasons I'm now self-employed. :gig:
 
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