Debunked: The Role of Aeroelastic Flutter in the Events of 9/11


I have already explained why you are wrong using the FAR you sourced while giving you calculations and instruction proving you are wrong. You just didn't like the answer and continue to censor. You will continue your pointless censorship since the answers are not to your liking and repeatedly prove you wrong. Your censorship is pointless due to the fact people are reading my posts no matter where they are.... in your "Rambles" section, or at the P4T forum.

Another great example which proves you wrong is in the alleged text from TWC's "Test Pilot" regarding the A380. I have referenced it many times for you, weedwhacker has even posted the video above, and now the "Test Pilot" has also made reference.

.96 Mach is Vd/Md for the A380. Not "Vd+20%". You can watch the Pilots visibly shaking in their seats due to the onset of flutter as they approach the aircraft Vd/Md. In fact, the airplane broke at .93 Mach, the flutter test had to be aborted, the airplane was then modified, and then the test resumed to certify the airplane to Vd/Md.

Again, scroll to the bottom of this page and learn....
http://theflyingengineer.com/tag/vdmd/

... and actually watch the video...
Playing the Vd definition by a journalist card, which is not based on the real definition is not good for much. Using a journalistic piece as an expert is not going to hack it.

1.2Vd is the design 767 was under. Thus the fake Vg diagram fails, since the curves are not for a 767. And there is no structural failure speed for a 767. Never will produce a source from Boeing for the structural failure zone you made up in the fake Vg diagrams for a 767.

Can you explain the Gish Gallop, "in equivalent airspeed at both constant Mach number and constant altitude", and why this only means a lack of aeronautical engineering knowledge? A detailed explanation is in order.

Who can supply proof a 767 was not designed to meet the 1.2 Vd specification as required? It looks like the OP is right, and has supporting evidence, with real requirements.
 
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No. The "airplane" didn't break.
A piece of the aircraft did come loose. However, that was the test flight AND it was redesigned to address that problem. The production model, after test flight certification, would not do the same thing.

Note also that 0.96M +15% would be above 1.0 M so that would not apply.

However, we have also seen a 727 hit 0.96M WITH one slat extended and still manage to land despite this being well over Vd for a 727 let alone for one so oddly configured.
 
A piece of the aircraft did come loose.

Yes, I know. A fairing on the fuselage. Not a structural component (as "SpaceCowboy" would seem to have inferred). And as you said, it was addressed so that in production, the problem would not re-cur (not that the full production fleet were ever expected to achieve those Mach speeds anyway, in normal use).

While on the subject of "structural damage" (the term so often touted) this seems to have been a moving goalposts sort of concept, for the PfT. For airplanes, "structure" is specifically designated when determining whether any extant damage to an airplane (aircraft) is an "Incident" or an "Accident", by definition.

For instance, ding the wingtip? "Incident". An inspection panel that is improperly secured, and comes off in flight? "Incident". These items are part of the over-all 'structure', yes...but NOT critical STRUCTURE.

For reference, there are (U.S.) NTSB definitions (49 CFR Part 830)

A narrative explanation (emphasis added):

The NTSB defines an “aircraft accident” to mean “an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight and all such persons have disembarked, and in which any person suffers death or serious injury, or in which the aircraft receives substantial damage.” The first portion of this definition is especially important to note for maintenance and ramp personnel. The initial threshold for classification as an aircraft accident is that the aircraft is being operated “with the intention of flight.”
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And, specifics:
The NTSB defines “substantial damage” as “damage or failure which adversely affects the structural strength, performance, or flight characteristics of the aircraft, and which would normally require major repair or replacement of the affected component.”
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There are various other criteria that immediately trigger the term "Accident", even if there is no aircraft damage related...such as the death or severe injury to a person.

But, those other criteria are not the question, here.
 
I stated:
Note also that 0.96M +15% would be above 1.0 M so that would not apply.
The regs as applies to the Airbus: ( bolding mine)
For normal conditions without failures, malfunctions, or adverse conditions, all combinations of altitudes and speeds encompassed by the VD/MD versus altitude envelope enlarged at all points by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude. In addition, a proper margin of stability must exist at all speeds up to VD/MD and, there must be no large and rapid reduction in stability as VD/MD is approached. The enlarged envelope may be limited to Mach 1.0 when MD is less than 1.0 at all design altitudes, and
(2) For the conditions described in Sec. 25.629(d) below, for all approved altitudes, any airspeed up to the greater airspeed defined by:
(i) The VD/MD envelope determined by Sec. 25.335(b); or
(ii) An altitude-airspeed envelope defined by a 15 percent increase in equivalent airspeed above VC at constant altitude, from sea level to the altitude of the intersection of 1.15 VC with the extension of the constant cruise Mach number line, MC, then a linear variation in equivalent airspeed to MC + .05 at the altitude of the lowest VC/MCintersection; then, at higher altitudes, up to the maximum flight altitude, the boundary defined by a .05 Mach increase in MC at constant altitude.
 
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Wasn't an F-4 Phantom a subsonic fighter but quite capable of going above Mach 1 in a dive? Doesn't apply to thread but seems to me I read that when I was a kid( which at my present age, means some time before I hit 30)
 
the whole pft concept is silly anyway. theres no point in building a specially strong plane to crash into a building when it's easier to just teach the kamikaze pilot how to fly the plane properly.
Further to that, PfT makes the claim it was done so in order to get the plane to NYC before the interceptors. However flight 175 slowed down after being hijacked. They could have avoided having to modify an aircraft by simply pushing to max cruise speed and staying there until lined up with WTC 2.

Just more ridiculousness from the pilots for truth
 
It was a while ago, let me see if I can find it again. I'm at home with a bad cold so I have time on my hands and daytime TV sucks.
 
It was a while ago, let me see if I can find it again. I'm at home with a bad cold so I have time on my hands and daytime TV sucks.
OK, in the video "9/11 Intercepted" at time approx 20:20 the narrator asks "Was this aircraft modified to beat Otis fighters to its target..."
 
Just added the following to the OP:
Does Vd vary with altitude?

Yes, however the Vd limit of 420 Knots is the Calibrated Airspeed (essentially the same as the indicated airspeed) , which is the same as the true airspeed at sea level. The FAA certification document says:


VD = 420 KCAS to 17,854 ft/.91M above 23,000 ft, linear variation between these points. VFC = 390 KCAS to 17,600 ft/382 KCAS at 23,000 ft/.87M above 26,000 ft, linear variation between these points.
VMO = 360 KCAS/.86M
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Note that Vd is a certification speed, not an actual operating limit, so it does not show up in the 767 flight manual. However the lower speed Vmo does:
http://www.scribd.com/doc/210896551/BOEING-767-Airplane-Flight-Manual


closer in for clarity:



Notice the Vmo is a constant indicated airspeed below 26,000 feet. This means that the Vmo of a 767 is 360 knots at sea level. The same applies to Vd, meaning Vd of a 767 is 420 knots at sea level.
 
Notice the Vmo is a constant indicated airspeed below 26,000 feet.

Yes, for large commercial airliners (in my experience) there is always a pressure altitude where some parameters switch from Vxx​ to Mxx​ (the 'xx' as a placeholder for any appropriate designation).

Although the chart (great find, BTW!! Man, I've hunted to no avail to find online sources!) is labelled for the -300 variant, there should be no differences at all between the -200 and it, in terms of that graphed chart. (Except of course, for any MZFW or MTOW references**).

(** 'MZFW' = "Max. Zero Fuel Weight" 'MTOW' = "Max. TakeOff Weight")

(Keep in mind that we see a LOT of such charts when related to large commercial airplanes {{and of course, they are biased towards pounds, etc...which I am used to. Other operators who purchase airplanes receive data already in metric units, for their convenience}}. As compared to relatively less complicated smaller airplanes that a student pilot might encounter early in his/her training. However, the principles in displaying data are the same, even if the variables become more complicated.)
 
Given that this is a forum that sets out to address scientific and technical claims , and this thread concerns the claimed by Pilots for 9/11 Truth wrt flight 175 and aircraft exceeding their Vd;
Given that no PfT member is willing to detail , on this forum, their claim or adequately address issues raised here;

What is being said in the PfT forum that could be considered response to these issues?

From a discussion on that forum;
I notice you posted this in the thread, which I need some clarification on.

"in equivalent airspeed at both constant Mach number and constant altitude"

So does this mean that the 15 percent increase in all combinations of altitudes and speeds encompassed by the VD/MD only applies when the altitude and speed are constant?

Rob Balsamo replies:
The regulation is clear. What do you think it means? MikeC understands it.... and in fact I replied to his post in agreement... but Mick deleted it.

Therefore this means that this regulation does not apply when aircraft are in a dive, decreasing in altitude, and increasing in speed, because the speed and altitude are not constants.

Am I correct?


Hmmm... seems you know more than the self-proclaimed "Private Pilot" known as "Mick West"
Another poster also has an opinion concerning Vd and Balsamo again responds :


My take is that those values are for certification purposes only, for engineering purposes, but I am no engineer.

Operational considerations are shaped by the regs of course, but how the aircraft actually flies, and what might happen in any given situation, doesn't have much to do with the regs.


Very good point, and one that the 'duhbunkers' continually fail to understand.

The regs (in this case) were meant for a brand new airplane being certified. This does not compare in any way to a nearly 20 year old airplane with tens of thousands of cycles, and precedent proves as such

Had SpaceC replied to the issue as above, agreeing with the first poster that the shallow dive and increasing speed of AA175 negate the flight certification standard, (which i personally indicated in this forum, appeared to be the stance of pft) and stating that while a new aircraft may need to meet such criteria, it cannot be said to still apply to a 20 year old in service aircraft, that I expect would have not been diverted to the Rambles thread.

So, debate may now move to these claims.
 
My take on flutter as a resonance phenomena:
We have seen discussion that refers to damping and excitation but how to explain these concepts?

Let's take a simpler set up:
Imagine a long rope hanging plumb and anchored at the ground. It is under no tension or load, other than its own weight generating a tension in the rope. A slight wind is applied and the rope will respond with a slight deflection. Keep that wind steady and the deflection will remain constant. Slowly and steadily increase that wind and a greater lateral force will cause greater deflection and no fluttering will be likely to develop until a very high wind is being applied.

However, now do this again but at regular intervals "pluck" the rope. This is an excitation. Similarly you could have a wind gust apply the excitation. This excitation itself would have various parameters, amplitude, rapidity of onset(akin to frequency) and location of excitation along the rope.
There will be combinations of excitation states and lateral load that will introduce a resonant fluttering of the rope.

Now alter the set up by putting a tension load on the rope and repeat all of the above.
This will reduce the combinations that result in flutter. The loading has introduced another damping effect.

On an aircraft there are numerous and various parts on the air frame that will respond independently to the air flow , excitations and have differing types of damping. In level flight at constant Mach number with all expected excitations, the aircraft must not experience flutter at least until Vd plus 15% ( 20% was in effect at the time of flight certification of the 767).
So, what of increasing speed and decreasing altitude?
Descent removes loading and decreases damping.
Increasing speed however increases loading thus increasing damping

What of manoeuvering?
Thus increases loading and damping but also alters air flow over varying surfaces.
So any particular surface sees a constantly changing set of parameters, loading, and air flow speed and direction. Any particular surface may experience conditions that would introduce destructive resonances but with changing conditions may rapidly move out of those conditions.
 
Now, what about the effect of age on an airframe?

Mr. Balsamo seems to forget that aircraft must undergo constant checks to ensure no adverse structural effects. This year Boeing 727s were required to increase frequency of inspections of their wing components to ensure that micro fractures have not developed.
These are not private passenger automobiles, "beaters" ready to drop parts or break down due to age. There are still DC-3s, and Norseman aircraft flying that are as old or older than I am and still in very good shape( wish I could say the same for me). Those that aren't have been removed from commercial operations.

So, we must accept that if an aircraft is in passenger carriage, commercial operation, it also meets continuing flight certifications and is at least close to the new aircraft pristine condition it was in when it rolled out of the Boeing plant.

Is it still good to Vd +20%? Quite possibly not.
Vd of the 767, 420 knots was it?
20% = 84 knots
10% = 42 knots
 
Seems to me that some PfT members had things to say about twa841 as well. Perhaps I'll look for that as well since all SpaceC could do was flatly deny what I said about the NTSB report and the data on the fdr, was true.

Incidentally this is by far the most time I have spent viewing PfT forums for many years. Robby gets one of his wishes, my visits are increasing his traffic stats incrementally.:(
 
"in equivalent airspeed at both constant Mach number and constant altitude"

Posting this, and they are unable to explain. Then post the airbus test in a dive, and self-debunk. Making up silly claims based on nonsense.

To say an aircraft built to specifications can't live up to the specifications later is nonsense.
To say the specifications don't apply to a real aircraft flying is extra credit nonsense.
 
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It's interesting that that while the regs for Flutter and divergence prevention say "at both constant Mach number and constant altitude", the flutter testing is actually done in a shallow dive, and with the speed gradually increasing.

Of course you can't actually just magically fly a plane at Vd, you need to accelerate up to that speed. Nor can you instantly fly a plane at Vd+20%. So a plane being safe up to Vd+20% must logically encompass the process of actually getting to that speed. The only way of doing this would be to accelerate, possibly in a shallow dive.

These are additional details though. The basic claim of Vd being a hard limit with no safety margin has been debunked. I really don't think we want to waste too much time debunking "claims" that we have to infer from passing comments on other boards. Especially on page 3, which very few people will read.
 
It's interesting that that while the regs for Flutter and divergence prevention say "at both constant Mach number and constant altitude", the flutter testing is actually done in a shallow dive, and with the speed gradually increasing.

Of course you can't actually just magically fly a plane at Vd, you need to accelerate up to that speed. Nor can you instantly fly a plane at Vd+20%. So a plane being safe up to Vd+20% must logically encompass the process of actually getting to that speed. The only way of doing this would be to accelerate, possibly in a shallow dive.

These are additional details though. The basic claim of Vd being a hard limit with no safety margin has been debunked. I really don't think we want to waste too much time debunking "claims" that we have to infer from passing comments on other boards. Especially on page 3, which very few people will read.
The constant stuff is what you see in aeronautical engineering. This will not be explained by pilots for truth, they avoid reality based sources.

When you find technical work on flying, the charts are done for constant altitude and constant mach. They make little sense to pilots for truth, and doubt they read technical aeronautical engineering work. This anti-intellectual approach is reflected in the fake 767 Vg diagram.
No need to study pilots for truth forum for knowledge on flying. What can you learn visiting pftf, someone was projecting the term ghost town.
 
I see that a possible confusion was set up in my first line of post 98.
It should have read:
My take on flutter is as a resonance phenomena:
. The following three paragraphs being simple demonstrations of resonance, which is a concept far from exclusive to aircraft, and I thought might be better envisioned in a simpler scenario.

The next paragraphs was my attempt to bring it back to airframe application and my intent was to illustrate that there are many factors involved in the effects on an airframe at such high speeds.
In fact of course, this even applies to slower speeds as demonstrated by one aircraft suffering a catastrophic loss of vertical stabilizer on take off due to pilot commands that generated huge forces on that component.

The regs seek to simplify conditions. While there is no guarantee that an aircraft will survive Vd+20% under complex conditions of flight, it is to survive under the specific conditions set out in the regulations and thus to offer some level of certainty of airworthiness at the edges of the normal flight envelope.

As Mick states, Vd is not published in the pilot's manuals from Boeing. Pilots are not supposed to fly in that realm. That is not to say they cannot, only that they do not have the blessing of Boeing nor the legal permissions of their license, to do so.

Two scenarios can cause an aircraft to enter this realm though; first and most common is by accident such as was the case for TWA841 (although the accident was very possibly caused by flight crew actions)
; the second is a pilot who has decided to ignore the stipulations of flight rules, as in the case of EA990 or all four Sept. 11/01 aircraft.

The certification regs offer some level of certainty, but no guarantee, that an aircraft at Vd or Vd+20% will survive, but it is also erroneous to state that an aircraft definitely will not survive that single condition within the context of the complex conditions of real life situations.

In my simple illustration of resonance effects I did not intend to imply that manoeuvering or a shallow dive would or should impart better airframe survivability. It would be great folly to rely on such a principle. However a combination of relatively slowly changing flight conditions could ( not "would", not "should") in fact aid in keeping it intact over a short period of time.
The 9/11 terrorist pilots had no concern with the stipulations of commercial pilots license, nor did they have much concern in the continued airworthiness of the aircraft. What concern they did have wrt the later, would decrease the closer to the target they got.
They had no intention of trying a go around, hitting well off center....good enough,,, having the plane tear apart in the last ten seconds while trying to manouver on target at full power,,,, good enough,,, deliberately pounding into the Earth in order to not allow passengers into the cockpit,,,, good enough.
 
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Now, what about the effect of age on an airframe?

Mr. Balsamo seems to forget that aircraft must undergo constant checks to ensure no adverse structural effects. This year Boeing 727s were required to increase frequency of inspections of their wing components to ensure that micro fractures have not developed.
These are not private passenger automobiles, "beaters" ready to drop parts or break down due to age. There are still DC-3s, and Norseman aircraft flying that are as old or older than I am and still in very good shape( wish I could say the same for me). Those that aren't have been removed from commercial operations.

So, we must accept that if an aircraft is in passenger carriage, commercial operation, it also meets continuing flight certifications and is at least close to the new aircraft pristine condition it was in when it rolled out of the Boeing plant.

Is it still good to Vd +20%? Quite possibly not.
Vd of the 767, 420 knots was it?
20% = 84 knots
10% = 42 knots

My take on it is that it the margins must apply till the end of the designated "fatigue life" of the airframe. Typically this life is defined as a maximum number of hours and flight cycles.

This would infer some significant over-engineering by the manufacturer in order to achieve this; something that appears to be the case in the examples we have looked at.

It is interesting to note the margin between VMO and Vd for the A330. VMO is 330 and Vd is 365 KEAS. Only 35 knots compared to 60 for the slightly smaller 767.

The question I would pose to P4T is that; does this reduced margin on the Airbus mean that Airbus build much weaker airframes than Boeing, or does it represent the result of changed requirements for the regulatory calculation of Vd due to the advent of Fly-by-wire control systems?
 
There is STILL an open invitation for member "BlindIdiots" to return and post, here, and respond.

Much information has been presented here, freely and accurately.
 
It's interesting that that while the regs for Flutter and divergence prevention say "at both constant Mach number and constant altitude", the flutter testing is actually done in a shallow dive, and with the speed gradually increasing.
The FAA 25.629 Figure 1A for SpaceC/Balsamo

[...] the design dive speed (VD) and design dive Mach number (MD) versus altitude envelope enlarged at all points by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude. Figure I A represents a typical design envelope expanded to the required aeroelastic stability envelope.
 
Wasn't an F-4 Phantom a subsonic fighter but quite capable of going above Mach 1 in a dive? Doesn't apply to thread but seems to me I read that when I was a kid( which at my present age, means some time before I hit 30)
I know this is an old comment but I just saw it. An F4 Phantom was capable of Mach 2.2.
But there was probably at least one plane that WAS exactly as you described. a spitfire would get reasonably close,(one got to 0.92 mach once although the propeller did break off in the process)

you were probably thinking of something like the F86 Sabre. That would seem to fit the bill quite well.
 
I know this is an old comment but I just saw it. An F4 Phantom was capable of Mach 2.2.
But there was probably at least one plane that WAS exactly as you described. a spitfire would get reasonably close,(one got to 0.92 mach once although the propeller did break off in the process)

you were probably thinking of something like the F86 Sabre. That would seem to fit the bill quite well.
Yeah possibly the F86 I was conflating with the F4 for some odd reason.
 
Mick,
I'll give you a hint....

the 1.2VD requirement is based on "an increase of 20 percent in equivalent airspeed at both constant Mach number and constant altitude" - FAR 25.629

Were the aircraft on 9/11 remaining at a constant airspeed and altitude while lining up with their targets?

Anytime you wish to actually learn from real and verified pilots on the matter, instead of interpreting the FAR's based on your bias... feel free to make a post at the P4T forum and become educated.

Hmmm... will you approve this post?
Yes, you admit that VD is not a structural limit "at constant mach and altitude" and that the FAA certification includes a check of zero flutter at 1.2VD in this case ?

We are talking about VD, the FAR 25.629 certification of VD and aeroelastic stability, not the 9/11 planes.
The debate first concerns your lies about : VD is a structural limit.
You are trying to confuse us about the 767 VD certification in 1982 and the 9/11 events !

So, after recognizing that VD is not a structural limit "at constant mach and altitude", can you explain us the figure 1A of the FAR 65.269 posted above (#107), the sea level line and C' value ?
 
Yes, you admit that VD is not a structural limit "at constant mach and altitude" and that the FAA certification includes a check of zero flutter at 1.2VD in this case ?

We are talking about VD, the FAR 25.629 certification of VD and aeroelastic stability, not the 9/11 planes.
The debate first concerns your lies about : VD is a structural limit.
You are trying to confuse us about the 767 VD certification in 1982 and the 9/11 events !

So, after recognizing that VD is not a structural limit "at constant mach and altitude", can you explain us the figure 1A of the FAR 65.269 posted above (#107), the sea level line and C' value ?
While he may very well read posts on this forum, SpaceCowboy no longer posts here after having his derriere handed to him. (though he would probably characterize it as an inability to convince his detractors that they are all stupid)

His avatar shows his status as "banned"
 
Hi Mick

My posts here are still read by Balsamo !
http://pilotsfor911truth.org/forum/...0813200&mode=threaded&start=100#entry10813200

And it's link metabunk to his forum and shows he always answer the same thing to any question : constant mach and altitude .. and the A380 flutter video.

i'll create a login on his forum.
 
and the A380 flutter video.

He still posts that? I used to also....except when he posts it he claims it shows "structural damage". It doesn't. The piece that breaks in the first part of that vid is a fairing, not a structural member. It merely needed to be re-designed to provide a more smooth airflow, so that in subsequent flutter tests it doesn't....flutter.

But of course, P4T mis-represents (as usual).
 
A piece of the aircraft did come loose. However, that was the test flight AND it was redesigned to address that problem. The production model, after test flight certification, would not do the same thing.

Note also that 0.96M +15% would be above 1.0 M so that would not apply.

However, we have also seen a 727 hit 0.96M WITH one slat extended and still manage to land despite this being well over Vd for a 727 let alone for one so oddly configured.
I referred to the roll through 360 degrees, of this a/c, as a barrel roll. I see lately that this is not specifically accurate. It more resembles an aileron roll though in both cases that would imply starting and finishing at the same altitude.
 
A "barrel roll" is (typically, when done correctly and intentionally) is a constant positive (+) 1-G maneuver.

An "aileron roll" can have negative (-) G forces at the inverted stage of the maneuver.
 
The 727 did more of an uncontrolled, diving, stuck slat roll. Probably not something anyone would care to repeat.
 
The 727 did more of an uncontrolled, diving, stuck slat roll. Probably not something anyone would care to repeat.

Yes, indeed. You are correct, here. HOWEVER, the other claims that you have made remain: That airplane greatly EXCEEDED any design "specs" (and of course the "aerobatics" were unintentional....I can explain the circumstances that led up to these "gyrations", but in another thread, perhaps).

POINT is....multiple G-force excursions, well beyond typical FAA (or ICAO) certification "standards". Yet?? Airframe stayed intact.

This was a B727...there is NO reason to presume that a B757 or B767 would be constructed to a "lesser" standard.

It is NOW up to the so-called "P4T" to suggest, and then "prove" otherwise.
 
I can't post on the P4T forum ? My new account is not valided by Balsamo ?

In his first response to my posts here, Balsamo says :
The 1.15 factor above Vd (used to be 1.2) is for flutter and is based on theoretical calculations of equivalent airspeed at both a constant mach and constant altitude as pointed out in FAR Part 25.629. In other words, not maneuvering, diving... etc. In further words, no changes in static and dynamic pressure which would induce flutter. It is not a required flight test.
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In other words, flutter prevention is only "at both constant ..." and not in conditions which would induce flutter ?

Fultter, one of the most dangerous events that can occur in flight, does not require flight test ?

Don't worry about FAR 25.629, it's only theorical calculation, speed over VD can't by free of flutter ?

If VD is a structural limit, why the FAR 25.629 require a verification above VD ? (at constant ... :))
[EDIT: * and even a litlle above 1.15VD by control of damping ratio (and 1.2VD for the 767 in 1982)]

An overview of Flight flutter testing/in flight modal analysis from Boeing :
Every new airplane design or configuration undergoes “flight flutter testing” very soon after its first flight. The new airplane will operate with a severely restricted flight envelope (speed limits) until it has been cleared to higher speeds during the flutter tests. This is sometimes called “envelope expansion.” These tests are in part a demonstration of adequacy, in that the airplane must be adequately stable (damped) throughout its design operating range. This range includes flight envelope (altitude and airspeed), payload, fuel loading and possibly control system configuration.
A new airplane, undergoing flight testing, is instrumented with on-board sensors, signal conditioning and data recording equipment. For a flutter test, data are telemetered to a ground station where engineers monitor the signals, and the data from each flight condition are analyzed. The airspeed is increased in increments, up to its maximum flight speed envelope, as depicted in Figure 1.

Analysts perform modal analysis on the airplane at each test condition. The airplane is excited in some means and the
modal properties of the airplane are identified using the input and response signals. The aero-servo-elastic “modes”
which are identified, of course, include the airplane structure, unsteady aerodynamics and perhaps flight control system.
The purpose of the modal properties are to:
1) Clear to the next speed, 2) Certify adequacy of design, and 3) Validate, and possibly improve a model (flutter analysis).
The airplane flutter speed must be at least 15% higher than the operating flight envelope.
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[EDIT: * FAA AC25-629]
No significant mode, such as curves (2) or (4), should cross the g=O line below VD or the g=0.03 line below 1. 15 VD.
 
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The FAA 25.629 Figure 1A for SpaceC/Balsamo

[...] the design dive speed (VD) and design dive Mach number (MD) versus altitude envelope enlarged at all points by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude. Figure I A represents a typical design envelope expanded to the required aeroelastic stability envelope.
"at both constant Mach number and constant altitude" - Balsamo had no idea what this meant. Will he understand why he was wrong, will he understand the graph.
 
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