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Stall / spin avoidence


Ed Cesnalis

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In 1997 an FD test pilot pulled the cute because he got in a maneuver he couldn't recover from. I think anyone purposefully spinning a CT is not a very bright individual. If a pilot is willing to go this far how far will he go in failing maint., pre-flights, and risk taking with passengers? Done wrong what stresses will he cause on the plane's stab which is our weak point? A CT pilot in Italy, like all of us, was warned not to do aerobatic type maneuvers. So at a show he is low, fast and does 1 too many hard maneuvers. He's dead now because the stab failed at 75'.

 

Like Forest Gump said: Stupid is as stupid does.

Roger, correct me if I am wrong. The Flight Design test pilot was testing a change to the stabilator or pitch trim system when the controls locked up at a speed well above VNE when he needed to pull the chute. The Italian accident was because of high speed passes and pitch ups. The last of which was in excess of 300 kilometers, and well beyond VNE.

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Roger, correct me if I am wrong. The Flight Design test pilot was testing a change to the stabilator or pitch trim system when the controls locked up at a speed well above VNE when he needed to pull the chute. 

 

Interesting.  I have not heard of very many BRS activations on the CTs, so I have wondered how well it performs.  Do you know any details of this accident?  Were there any injuries to the pilot?  It's nice to know that at least in this case a "beyond Vne" activation was still effective.  I know that Cirrus had at least one successful BRS deployment outside the published BRS envelope, but have not heard of one before regarding a CT.  It sounds like BRS is very conservative when engineering their deployment specs.

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Gosh, CT, I know there is a difference in P-factor, torque, perspective and other factors but I'd also like to know the answer for the right base to final turn. I hope this doesn't complicate the expected plethora of responses. :)

 

Jim,

 

I wonder what it takes to get a CTSW to spin to the right? I think its reluctant.  Perhaps thinking this way could result in being safe flying left traffic and less so flying right.

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A macho attitude kills.  Why even discuss pushing a craft not rated Utility or Aerobatic to do spins or other aerobatic maneuvers?   It's like fantasizing riding a bicycle over 100mph...  Even if you could could find a mountain road steep enough to do it, why?

You do realize our aircraft don't use the Standard, Utility, or Aerobatic type ratings don't you.

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Until I simulated a base to final save with a skid and a high sided skid I had no clue how quickly and with how little warning this happens.  It is easier to avoid something that you have done and harder to avoid something you have never seen.

 

What did you use as a recovery procedure?  Nose down and neutralize rudder?

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Vne for a Sonex is 197mph.  He pulled out of the dive at 254mph.  No damage was done to the airplane, which is a testament to the strength and conservative margins of the airframe.  But it could have gone much worse.

I'm no physicist nor metallurgical engineer nor test pilot so my comment is only passing on what I have heard. That is, the airplane structure may survive such an incident and maybe many such, but it does stress the aircraft to the point where it may let go at what is normally considered not an extreme speed.

 

I understand that aluminum has no memory but I don't know about fabric or carbon fiber construction. Maybe someone with knowledge can add to the idea that overstressing may not cause immediate failure but can bring on later failure if done enough.

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Vne for a Sonex is 197mph.  He pulled out of the dive at 254mph.  No damage was done to the airplane, which is a testament to the strength and conservative margins of the airframe.  But it could have gone much worse.  

Vne is somewhat arbitrary. It's purpose is to be set at a value of known guaranteed safety. After that, theres a generous safety margin. After that, it's unknown exactly where the giving point is for each aircraft until someone finds it!

 

 

Im no physicist nor metallurgical engineer nor test pilot so my comment is only passing on what I have heard. That is, the airplane structure may survive such an incident and maybe many such, but it does stress the aircraft to the point where it may let go at what is normally considered not an extreme speed.

 

I understand that aluminum has no memory but I don't know about fabric or carbon fiber construction. Maybe someone with knowledge can add to the idea that overstressing may not cause immediate failure but can bring on later failure if done enough. 

 

To my knowledge, ALL metals have a yield point. In bolt metallurgy as an example, each time you tighten the bolt, the yield point is lowered. That's one of the reasons the military requires new hardware to be used on aircraft, period.

 

The less you travel up the stress graph, the less the yield point is affected. Different metals are also affected differently by stress, but they all become weaker as you put more and more stress on them.

 

Composites are unusual, in that they don't really suffer the same problem (they do, but to a significantly lower degree). However, the failure mode tends to be catastrophic as the yield point and the fracture points are very close on the graph, whereas there is still a lot of room on metals.

 

EDIT: GRAPHS!

 

tensile-diagram.jpg

 

 

 

 

Carbon fiber is a brittle material, whereas aluminium is a ductile material.

 

 

 

stress_strain.gif

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Metal memory is probably not the right attribute to be concerned about if overstressing and aircraft. I told you I was no metallurgist! :)

My analogy is bending a piece of aluminum stock back and forth. It seems that with each bending the metal is fatigued/weakened at the bend point, and if done through enough cycles it will break.

 

I've always thought that over-stressing an aluminum spar, let's say, could weaken it going forward - that's what I think of when I hear "memory". That a Bonanza that was repeatedly over-stressed might seem fine, but could bite a subsequent owner going forward.

 

But I'm a long, long way from a metallurgist as well!

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Vne is somewhat arbitrary. It's purpose is to be set at a value of known guaranteed safety. After that, theres a generous safety margin. After that, it's unknown exactly where the giving point is for each aircraft until someone finds it!

 

 

 

To my knowledge, ALL metals have a yield point. In bolt metallurgy as an example, each time you tighten the bolt, the yield point is lowered. That's one of the reasons the military requires new hardware to be used on aircraft, period.

 

The less you travel up the stress graph, the less the yield point is affected. Different metals are also affected differently by stress, but they all become weaker as you put more and more stress on them.

 

Composites are unusual, in that they don't really suffer the same problem (they do, but to a significantly lower degree). However, the failure mode tends to be catastrophic as the yield point and the fracture points are very close on the graph, whereas there is still a lot of room on metals.

 

EDIT: GRAPHS!

 

tensile-diagram.jpg

 

 

 

 

Carbon fiber is a brittle material, whereas aluminium is a ductile material.

 

 

 

stress_strain.gif

 

This is all true.  Vne is sometimes set due to structure, sometimes due to flutter.  The Vne is just a point where it is assured that, if built and maintained properly, the aircraft structure will not be damaged and no flutter will occur.  There is customarily a 10% margin built in, so in theory if you made a boo-boo and went to the Vne+10% speed you *should* still be safe.  Beyond that it's a crapshoot.

 

Jim's point is well taken, but I believe for a metal airplane, if you have not reached the yield point of the metal or cause any joints (rivets/bolts/screws) to deform or break, there should not be permanent structural issues.  Which ties to the discussion of yield strength vs ultimate (breaking) strength.  So if nothing is bent, deformed, or broken, a metal airplane structure is probably unharmed.  This may not be true for very high speed aircraft, where atmospheric frictional heating and cooling cycles can damage the strength of metals.  That's the reason the MiG-25 used a large amount of steel in the structure; at Mach 2.8 speeds aluminum weakened due to heat stresses.  

 

One of the great things IMO about metal airplanes, is that metal has a yield load often significantly different from the failure load.  Many metal (or metal framed fabric) aircraft have landed after overstressing due to aerobatics, encounters with turbulence, or other problems with bent wings and wrinkled skins, but have landed.  Composites don't really have a yield strength, they simply stay rigid until they break at their ultimate failure load.  

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And many more have crashed with no excessive loads at all. Fatigue issues commonly make any argument about yield strength esoteric. 

 

Carbon fiber composite has no significant fatigue issues whereas aluminum is very poor and will not give infinite life regardless of the level of stress it has been subjected to. If you want to call that memory (by virtue of a finite life), aluminum will fail eventually even at low cyclic stress. Steel is better in that at some reduced stress levels, it will go on forever and never fail.

The fatigue limit for steel is typically one half the ultimate tensile strength and it will come as a surprise that a high strength steel with, say, double the ultimate tensile strength (UTS) can have double the stress applied yet still go on forever. This steel will also be more brittle in that the yield point will be closer to the UTS. In other words, under cyclic stress, the steel following the red line above will have a substantially higher fatigue strength than the one following the green line and fatigue is the main concern in aircraft structures and in fact most dynamically loaded structures.

 

So you might get away with a certain number of high stress situations with a metal plane but it will catch up with you quickly. Alternatively, a carbon composite plane, if you are still alive, you have likely done no lasting damage to the structure.

 

There are no aluminium aircraft, even if operated in normal duty, that will last forever unless structural members are replaced periodically.

 

But back to yield (which is unrelated to fatigue) in metals - the yield point is unaffected by stress cycles up the the elastic limit ie the knee on the stress strain curve). The yield will remain unaffected but fatigue cycle limit will dramatically reduce at high stress.

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