Aces High Bulletin Board
General Forums => The O' Club => Topic started by: Widewing on August 27, 2012, 05:07:17 PM
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As the subject says...
530 mph at the time of the failure.... Way fast!
http://www.msnbc.msn.com/id/48801527/ns/us_news-life/ (http://www.msnbc.msn.com/id/48801527/ns/us_news-life/)
We discussed this on the BBS at the time:
http://bbs.hitechcreations.com/smf/index.php/topic,320374.30.html (http://bbs.hitechcreations.com/smf/index.php/topic,320374.30.html)
http://bbs.hitechcreations.com/smf/index.php/topic,320672.0.html (http://bbs.hitechcreations.com/smf/index.php/topic,320672.0.html)
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A highly modified vintage aircraft was pushed beyond its structural limits before it crashed into spectators......
AND then IT SAYS:
Loose screws on a crucial tail assembly are also blamed in the September 2011 crash
If the trim tab came off because of loose screws, I can't see how the first statement is applicable. (OBviously, having trim tab come off at 200 is heaps better than 530, but that doesn't mean the plane was pushed beyond its limits?)
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AND then IT SAYS:
If the trim tab came off because of loose screws, I can't see how the first statement is applicable. (OBviously, having trim tab come off at 200 is heaps better than 530, but that doesn't mean the plane was pushed beyond its limits?)
The same exact thought came through my head as well...
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there's a structural limit to everything. with lose screws the structural limit was lower than normal. when the plane went above a certain speed the lower structural limits were exceeded and the tail assembly came off.
or the tail assembly would not have come off at that speed if the screws would have been properly tightened.
semp
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I fail to understand how a Trim Tab could lead to total control loss,
unless something else critical was damaged. :headscratch:
:cheers: Oz
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When I was racing cars it was important after every race to check bolts. Vibrations and stress are maxed when racing at the edge of performance. A trim tab deflecting air improperly at 500 MPH where there is little room for error proved to be fatal.
We must learn from mistakes or we are doomed to repeat them. :frown:
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I fail to understand how a Trim Tab could lead to total control loss,
unless something else critical was damaged. :headscratch:
:cheers: Oz
oz, the trim tab was taking a significant amount (hundreds of pounds) of pressure off the control stick. When that tab broke, the aircraft immediately pitched downwards putting forces over 17Gs on the pilot. He would have immediately blacked out and the plane rendered uncontrollable. At that airspeed, even if Mr. Leeward could anticipate the tab breaking, he would not have been able to control the aircraft.
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the aircraft immediately pitched downwards putting forces over 17Gs on the pilot.
No it didn't. The airplane pitched up at according to the report it was a 9g pitchup.
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No it didn't. The airplane pitched up at according to the report it was a 9g pitchup.
This.
I fail to understand how a Trim Tab could lead to total control loss,
unless something else critical was damaged. :headscratch:
:cheers: Oz
Sudden pitch of the nose up causing a moment of greater than 6 (some estimates being thrown around a year ago were ~10) times the force of gravity on the pilot, rendering him unconscious. Such a sudden failure and incdient at lesser Gs can cause signifigant disorientation if not unconsciousness to a pilot, so taking into account a prepared and trained pilot can barely sustain greater than 6 and then compare that to the 9 stated in the report. It's like he got knocked right ontop of the head by a sledge hammer he never saw comming when the trim tab failed.
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I fail to understand how a Trim Tab could lead to total control loss,
unless something else critical was damaged. :headscratch:
Oz,
As a 51 flies faster it requires more force to keep the nose down owing to the nose up force caused by the angle of the horizontal stabilizer. To counteract this, the trim tab provides a convenient way for the pilot to relieve stick pressure and enable him to keep the nose down as opposed to the forces pushing up. When the trim tab departed from the aircraft, that nose down force assistance was instantly lost resulting in the rapid pitch up that caused the pilot to lose consciousness owing to the almost instant onset of 9 G's. Without the trim tab as can be seen from the video, the aircraft immediate headed skyward and entered into a loop type maneuver owing to the non-countered nose up forces.
Obviously we all know how this ended.
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The faster you fly the more lift is produced by the wing. If the wing produces more lift than the weight of the aircraft it introduces a vertical speed component causing the aircraft to climb. The tail surfaces then makes the aircraft weather-vane into the climb thus further increasing the rate of climb. To counteract this, small fixed or adjustable trim tabs are mounted on the control surfaces to move the control surfaces in the desired direction. This movement of the control surfaces reduce the wing's angle of attack and thus lift, balancing lift and weight and allowing the plane to fly straight without control input from the pilot. The force the trim tabs exert on the control surfaces can be extreme at very high speeds, and if they fail can cause almost instant over-stressing of the airframe as the control surfaces snap back into neutral position, increasing the angle of attack and thus causing the wings to produce too much lift.
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The exact same thing happened to Bob Hannah in 1998 (yes, the motorcycle ace).
He woke up on the floor of the cockpit, put forward pressure on the stick, and brought the airplane in safely.
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No it didn't. The airplane pitched up at according to the report it was a 9g pitchup.
The pitched downwards was a typo.
Here's an excerpt from the NTSB:
On September 16, 2011, as the experimental single-seat P-51D airplane “The Galloping Ghost,” traveling about 445 knots, or 512 mph, in the third lap of the six-lap race, passed pylon 8, it experienced a left-roll upset and high-G pitch up. During the upset sequence, the airplane’s vertical acceleration peaked at 17.3 G, causing incapacitation of the pilot. Seconds later, a section of the left elevator trim tab separated in flight. The airplane descended and impacted the ramp in the spectator box seating area, killing the pilot and 10 spectators and injuring more than 60 other
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I would suggest for those who go on an intro or scenic flight from time to time, to ask the pilot to show you what an aircraft feels like when it isn't properly trimmed. Even a 172 flying at 100 kts feels heavy on the controls.
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I read the plane had been 'lightened up' too much.
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http://dms.ntsb.gov/pubdms/search/document.cfm?docID=374510&docketID=51746&mkey=81814
In terms of the language about flying beyond it's structural limits, take a look at the image study report. Figure 17, 18, and 19 really show the forces being applied to the airframe at those speeds. The structural limits language was probably taking into account that the plane was the most highly modified mustang in history, although I could be wrong. All I know is Strega and Voodoo were going faster and we've never seen these kind structural issues on those airframes.
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I'm assuming that by structural limit they mean the limit beyond which the plane starts failing, not the point at which it fails. My understanding is that the most direct cause of the accident was flutter breaking the trim tab control rod. 20 year old bolts and reused nylon lock nuts make flutter more likely, deforming the airframe by flying beyond its structural limits can too.
I don't see how you could make this activity safer without spending more money on engineering and testing, or putting limits on what's supposed to be unlimited.
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I didn't think about him being knocked out.
Hopefully it'll lead to more/better maintenance and checks.
It's sad to realize that if he had "woken up", he might have been able to save himself and plane,
providing he had enough alt.
:cheers: Oz