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Overspeeding

Reviewing my notes for comments I posted on the props and engines thread brought to mind high speed issues in general. Pilots are wisely afraid of exceeding Vne, which is of course very wise. That said, it's not like the whole plane tears itself to pieces at Vne, but your margins begin to narrow rapidly. All the safety reserve built into the plane is reduced or eliminated.

All of the critical aircraft elements are tested with reserves. For speed, at least 10% faster than Vne, and structurally, at least 50% more G. There are also engine and propeller speed reserves.

When you overspeed, you are temping the serious consequences of airframe damage from G loading due to rough air, or poor aircraft handling. You are getting nearer to the flight control flutter margins, which can be variable based upon the condition of the flight control hinges, and linkages. Other less critical bits of the plane can get to buffeting (I once had a camera door try to shake itself off a 172 I was flight testing at the time).

When a modified aircraft seeks a reduced Vne due to an external modification, that is not an automatic "yes". Consideration has to be given to upset margins. If the pilot bimbling along upsets the plane, will he have the speed margin for a reasonable recovery. Some planes are very slippery. I once very foolishly did some quasi aerobatics in a 182RG, and found out how quickly it will accelerate when pointed down. Nothing exceeded, but a wiser Pilot DAR. During my spin testing of a modified Cessna Caravan, I similarly found out how quickly they accelerate when pointed down (though this time I was ready!). The recoveries involved 2.5 to 2.8G at Vne, though otherwise went very well. This considers however, that the PT6 is tolerant of having power rapidly removed, and the propeller actually sits with a tiny amount of braking pitch it idle.

So if you find yourself approaching Vne, try to do the opposite of what JFK Jr. did, and be very gentle, as you get it recovered, and slowed down...

Home runway, in central Ontario, Canada, Canada

Interesting...

I think most IFR planes can reach Vne within 10 or 20 seconds if the bank angle is allowed to remain at say 45 degrees and one is suitably distracted.

If one assumes lack of flutter, what will come off first beyond Vne and what is the cause? I mean if the Gs are very low.

I recall reading a report on the PA46, which the FAA grounded (IIRC) at one time following a number of in-flight breakups, and the investigation gave the airframe a clean bill of health, with an assertion that the wings are good for 1000kt (not sure how they tested that). Nevertheless, Piper did reinforce the airframe during subsequent years. A friend who bought a Jetprop researched this topic in great detail.

Administrator
Shoreham EGKA, United Kingdom

Anyone who flies a CJ2 or CJ3 and says they haven't had the overspeed warning going off is probably being economical with the truth! These aircraft will go past the red line in the cruise at most levels unless it's very hot.

Darley Moor, Gamston (UK)

While in IMC the A.P commanded the Comanche into a spiral dive, things did happened very quickly, while fighting for controll I calculated 2min. to the end.

I had a similar thing happen during a maintenance check flight of a Bellanca Viking a few years back - though happily VMC. The autopilot was happy to command a roll through 120 degrees bank angle (I would not let it go any further, though it would, had I let it). I did not get to Vne, but left unattended, it would have in short order!

Vne is easy to reach quickly in a slippery plane. Establishing Vne includes flight testing to satisfy the test pilot that the "average" pilot can upset the aircraft, and get it back, without exceeding Vne. Thus an unreasonably slow Vne cannot be certified.

Home runway, in central Ontario, Canada, Canada

In case you have not seen it and are interested, here is a glider having flutter tests:



By the way, most if not all things that vibrate will have more than one mode of doing so. I think that for a glider wing, the first mode is simple bending from the roots, then the next is a harmonic with a single null point partway in from the tip, etc.

I have heard that on at least some gliders, the limiting flutter for VNE is the tailplane. I have also known of a glider with probably prior but unknown damage having the whole empennage go into a twisting mode about the fuselage axis, at less than VNE.

Dunno about power aircraft, but I suspect that similarly, there may be several different modes of imminent failure, depending on the model and perhaps its history. There was the well documented (and video’d) aircraft in the USA – a modified Mustang? - which lost part of the tail first, in a mixture of speed and G.

Chris N

PS:

http://en.wikipedia.org/wiki/2011RenoAirRacescrash

“The NTSB thoroughly investigated the extensive modifications made to the airplane. The modifications had made the aircraft lighter and reduced drag, but decreased stability. Leeward took the plane to 530 mph during the race, about 40 mph faster than he had ever gone before.[24] There was evidence of extreme stress on the airframe demonstrated by buckling of the fuselage aft of the wing and gaps appearing between the fuselage and the canopy during flight (clearly visible in high resolution photographs taken by spectators). However, the investigation (released in August 2012) found that probable cause of the crash was old reused locknuts in the left elevator trim tab system that loosened. This led to a fatigue crack in an attachment screw and allowed the trim tab to flutter. This flutter caused the trim tab link assembly to fail which led to loss of control of the aircraft. Untested and undocumented modifications to the airplane contributed to the accident. Particularly, the right trim tab had been fixed in place. Had both trim tabs been operational, the loss of the left trim tab may not have alone caused loss of control. When the trim tab failed, Leeward was exposed to 17Gs which quickly incapacitated him and likely rendered him unconscious.”

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Ridgewell, Essex

This flutter stuff is very interesting.

Socata TB owners will know about the "elevator trim hinges issue" whereby the hinges wear at a rate which bear no relation to the fact that they almost never move. I reckon they wear about 100x faster than they should.

There is much debate about this but I have found (by getting very involved in the Annuals since I went N-reg in 2005) that about half the play comes from 3 or 4 bushes in the trim linkage (and on older planes possibly from wear in the jackscrew) and the other half comes from wear in the hinge pins (PTFE coated on the GT aircraft). The cost of replacing the hinges is about £1000 and it's a big job because Socata used screws on the half of the hinge which doesn't wear and used rivets on the half which does wear! A good halfway solution is to fix the linkage bushes (which are quite cheap) and replace just the two outermost hinge pins which are about £100 each.

Anyway, I've noticed quite a lot of dark residue coming out of the hinges and this is the aluminium of the hinge material. Yet the hinge almost never moves...

And exactly the same on the TB20/21 gear doors. Their hinges, and their linkages, get trashed at an alarming rate, again despite the fact that they hardly ever move intentionally.

So I think there is flutter going on.

In fact one can hear the gear doors rattle in the propwash during taxiing.

The Q is what one can do about it. Legally, nothing, because even if you rebuilt the trim tab assembly using brand new factory parts, you will still get almost the same amount of play - as some people who have done that have reported.

Administrator
Shoreham EGKA, United Kingdom

Flutter and vibration are not the same thing.

Vibration can simply be excitation by turbulent airflow over an element. I would think that is the u/c door issue, and very likely the trim tab mechanism wear issue too.

Flutter occurs when a system moves from a stable state (vibrations would damp out in time) to unstable/positive feedback situation. Consider a slightly flexible wing with an aileron at the end. If it is unstable, slight disturbance of the wingtip upwards tends to leave the aileron behind, creating more lift and pushing the wingtip even further up from its resting position. This positive feedback continues until either the aileron flexes back into a neutral position, or the resistance of the wing to further bending is so great, or both, that the tip stops flexing upwards and starts instead to move down. Now, the aileron gets left behind, and in flexing upwards increases the downward forces on the tip, resulting in an eventual downward deflection even greater than the initial upward deflection. Etcetera.

Mass balancing the aileron helps avoid this. The stiffer everything is, the more resistant to flutter.

HTH.

Chris

Ridgewell, Essex

In that case how would you go about improving that, Chris?

Tightening up the tolerances?

One needs to be careful with thermal expansion, though I notice that steel and phosphor-bronze (the best simple bearing materials) expand at virtually the same rate.

Then lubing becomes really important, too, and that is routinely neglected in GA maintenance.

Administrator
Shoreham EGKA, United Kingdom

Do you mean how improve the trim hinge wear issue?

Assuming it is vibration that is the cause, triggered by prop wash and other things (there is always vibration throughout the airframe, I suspect), tighter tolerances would probably help. Different materials might help too, maybe including a liner which could perhaps absorb vibration. Redesign using ball or roller bearings might help, though there is a risk there – if it never moves, vibration can result in the balls in a bearing indenting the ball races. I believe it used to happen in car wheel bearings on cross channel hovercraft which had considerable vibration. (There are plenty of mechanical things working in vibration-saturated environments – think cars among others – which don’t repeatedly fail in the way you describe.)

All design is a compromise of many factors, including weight, development costs and ultimate product cost. I guess the airframe manufacturer does not want to do a redesign.

Regards –

Chris

Ridgewell, Essex
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