buffet?

[SNS3Guppy]

Never heard about a TAS-limitation. Only either IAS (Vmo) or M (Mmo).

I don't believe anybody said anything about a TAS limitation. Limitations are described in CAS in some aircraft, however.

I frankly don´t know how my aircraft behaves beyond Vmo/Mmo, but I´m not too curious to find out about it either. Maybe there´s no buffet or the like, but I don´t care if I overstress the structure with a slight elevator movement

The aircraft isn't going to be overstressed by flight up to Vmo/Mmo. Nor are you going to see adverse flight characteristics at that point. A manufacturer isn't putting a limitation at a point beyond which flight characteristics decay or the aircraft is unsafe.

Structural damage can be done at speeds far below Vmo; structural damage can be done, and has been done, below maneuvering speed. A good example of that was the loss of the Airbus A300 vertical stabilizer on American Airlines Flight 587, out of JFK, in November 2001.

You can break your airplane at low speeds, too.

[CONF iture]

Where Mmo, or the mach operating limit applies at altitude, of course Vmo applies at lower levels. Flutter may be a consideration, but with non-reversible controls, not so likely.
...
You can push up the power if you like, but you do have aircraft limitations to consider before you go too far.

...........................................
To paraphrase CONFiture, if I firewall the throttles at low level (clean config, obviously) in a typical modern airliner, what's the envelope wall I'll hit? Is it indeed flutter?

So ... we don’t really know what’s happening … and what we could encounter ?

But the risk looks serious enough that Airbus put in place the High Speed Protection in order to prevent any uncommanded as well as commanded (!) excessive exceeding ...
Their words still remain a bit vague:
There is an increased potential for aircraft control difficulties and structural concerns, due to high air loads

Should we consider that control difficulties would be reached before structural concerns … ?
If so, where would be that limit in term of exceeding VMO + ?

No pressure to answer that question which is obviously over stretched for a day to day operation.
Just my own curiosity.

[SNS3Guppy]

It's not exactly rocket science. Don't exceed Vmo.

[ssg]

Yeah you got it right..

Chris,

Yes that's what I meant...flying in an aircraft that has enough engine to get you into Coffin corner, means your generaly flying a faster aircraft, that would be nice.

Designers that build a plane and set a low Vmo for the stupidist pilot meant to fly it means that the rest of us have to fly around 20/30/40+ kts slower then the plane can handle...inefficient...

I don't pretend to be an aeronautical design expert, but I do believe with the advent of computers around the mid 80s, helping with airflow simulations, ect...we have better wings now then a slide rule designed Lear 24. While I have no experience in that aircraft, my informal survey of Lear pilots on this subject state that high speed characteristics vary from nothing to buzzing, to full on tucks, that have killed some guys...who knows...all this happening in the .82+ region...not very fast...

I do know that if you look at an older Lear, they usualy have more VGs, slall fences, and other add on to make the wing better.

Anyway..

A good point of this is the new Premier One. Swept wing, flies right up to FL410 and does .80 right off the bat. Landing? Nice easy ref of 110 kts, no TRs on board, plane stopped in 2500 ft on my first try without really getting on the brakes. Good low and high speed characteristics. The only swept wing corporate jet that the FAA will let a single pilot loose on.

Just a note: If you can get a hold of some of the flight testing summaries of aircraft modells you might notice some things...

The SJ30, and Citation X for example...tend to roll left before they exhibit any wing overstress or flying issues. Others I am told, certain Citation models will start to vibrate uncontrollably, some are limited by thier windshield...so Vmo many times will not be because of tuck, buzz, or your elevator getting washed out but other things....

Bringing this full circle...the idea that you can fly a plane where nothing limits or happens to you, untill you run out of power or wing, again would be flying an aircraft right up it's max. I like that...

[ChristiaanJ]

Originally Posted by SNS3Guppy

It's not exactly rocket science. Don't exceed Vmo.

It's done routinely, actually, after a major overhaul, precisely to assure that there still is the margin between Vmo, where the "o" stands for "operating", and the limits established during certification, such as Vne.

My stupid question still stands.

Let's take a 737 or an A320. No passengers, little fuel, sea level, level flight.
I fully open the throttles (without exceeding engine limits).
I ignore any overspeed warnings, and override any system limitations.
You with me?

Now, what limits do I run into? Mach buffet? I doubt it, at sea level. Structural failure? Flutter, as has been suggested here? Or do I simply run out of "puff" anyway? Let me add I do not intend to pull significant 'g' in the process, just do a real high-speed run to the real limit.

Stupid question from an ancient aeronautical engineer, who's done mostly stability and control, but is still interested in the subject.

CJ

[Mark1234]

@SN3:

Never heard about a TAS-limitation. Only either IAS (Vmo) or M (Mmo).

I did! (It was a question) - Part 1:

I shouldn't have said 'TAS', as it's not quite that: I'm given to understand Vne as marked on the ASI is predicated on standard temp & pressure, and should be factored for altitude - i.e. the IAS will underread at alt due to density, but VNE is a constant *airstream* speed. Or to put it really simply, cruising along at 20,000ft you could hit flutter while the ASI was well short of the redline.

Part 2:

I gather big tin generally doesn't have Vne, but Vmo - am I correct in thinking that varies / is varied with altitude. If so, how does that work - presumably it's driven by factors different to Vne/flutter (not just airstream speed).

[411A]

Let's take a 737 or an A320. No passengers, little fuel, sea level, level flight.
I fully open the throttles (without exceeding engine limits).
I ignore any overspeed warnings, and override any system limitations.
You with me?

Should you find a bird at these low levels and high speeds, you might better perhaps rethink your actions.

In addition, if Vmo was exceeded at low levels with a more classic swept-wing jet aeroplane, the B707, you would rapidly notice increased nose-up trim (mach trimmer action) that would soon run out of stabilizer authority...bad, very bad.

[SNS3Guppy]

Now, what limits do I run into? Mach buffet? I doubt it, at sea level. Structural failure? Flutter, as has been suggested here? Or do I simply run out of "puff" anyway? Let me add I do not intend to pull significant 'g' in the process, just do a real high-speed run to the real limit.

Transport category aircraft certificated under Part 25 do not require a Vne speed, as Part 25 doesn't require establishing Vne. Part 25 requires that up there be no adverse characteristics up to Vmo/Mmo, and above that speed up to the maximum design speeds, no adverse characteristics that are not easily handled by the crew in recovering back to Vmo/Mmo. What will happen if you accelerate through Vmo at low level? Nothing. The specific characteristics that will be encounterd depend, as has been repeatedly stated thus far, on the specific aircraft and configuration.

You want to know what's going to happen if you push the power up and fly to the maximum design speed? Nothing.

Will you get buffeting? Possibly. Depends on the design. Depends on the configuration. Depends on the loading and other factors. Will you run out of trim? No, not up to the design speed, but again, this will really depend on the airplane, how it's configured, loaded, etc.

Does the airplane have the power to reach that speed in level flight? Depends on the airplane and it's loading. It might. Some aircraft do, some don't.

Is exceeding Vmo part of a windshear recovery, as previously suggested? Of course not. It's a ridiculous idea.

Do manufacturers and repair facilities routinely push the airplane to it's design limit speeds to see if the old gal can still do it? No.

While I have no experience in that aircraft, my informal survey of Lear pilots on this subject state that high speed characteristics vary from nothing to buzzing, to full on tucks, that have killed some guys...who knows...all this happening in the .82+ region...not very fast...

Sim warriors. Those characteristics just don't exist in the Lear, especially at .82 Mach. Pete Reynolds, a test pilot for Lear, wrote a good paper on the subject, in which he described taking the airplanes to speeds well beyond those numbers with no adverse characterstics. I've heard the stories too, mostly from pilots who saw it happen in the sim and assumed it happens in the airplane...it doesn't. I've had the airplanes to speeds well above .82, and there are no bad habits lurking there. Even with the earlier wings.

I recently spent time in a LR35A that not only was modified with pylons on the wings and hardpoints, but flying it configured with external stores and gear throughout it's flight envelope, including frequent flights in turns to the shaker with all ranges of AoA explored...and still no bad habits...despite plenty of ways to disrupt airflow over the wing.

The stories of ailerons causing the flight controls to slam from side to side, the terrible dutch roll charactersitics at high altitude following a yaw damper failure, and so on...all untrue. The mach tuck characteristics are so mild that the solution for it is mach trim...slight nose up trim. The only reason it's a concern at all is that the controls become light enough that they don't quite meet the Part 25 requirements for stick pressure...but it's fully manageable, fully controllable...all myth floating around out there. How many of those pilots that you questioned have actually experienced those effects in the airplane...and how many had this first hand knowledge from their sim experience? You're not going to find pilots who experienced those problems in the airplane, because they're sim-isms.

I'm given to understand Vne as marked on the ASI is predicated on standard temp & pressure, and should be factored for altitude - i.e. the IAS will underread at alt due to density, but VNE is a constant *airstream* speed. Or to put it really simply, cruising along at 20,000ft you could hit flutter while the ASI was well short of the redline.

No, not really. Flutter isn't a function of actual speed, it's a function of air density, because it's a function of the atmosphere acting on the control surface or flying surface. The surface has no idea how fast it's actually going. That at a higher density altitude the TAS may be higher has no bearing on fluter issues.

IAS is an imperfect medium through which to consider flight characteristics. Corrected for position error, CAS is often used as a reference in charting and data, but is also inadequate to consider the aerodynamic effects on the airplane, and should be discussed in terms of Equivilent Air Speed, or EAS. To simply our use in the cockpit, and discounting minor error excursions, we stick with Indicated Air Speed (IAS).

If the mass airflow around the wing and it's relevant pressure distributions indeed account for flutter, we can measure it in flight with indicated airspeed. If indicated airspeed drops, we move farther from flutter, or in other words, create a bigger margin. It's air density that determines the speed necessary to produce the same impact force in the pitot tube, and it's the same density that determins flutter. Airspeed decreases, flutter margins increase.

Again, this is simplification, but simply because density altitude has increased does not imply that flutter is achieved at speeds less than Vne. Moreover, whereas Vne is established with a margin already built in, it's just not going to happen unless control balance has been changed or other changes are made to the wing or control system which decrease susceptability to flutter to a lower speed.

Some elements of flutter do occur at lesser speeds, but are addressed by airframe limitations. In the B747 Classic, for example, a limitation exists regarding reserve fuel between FL290 and FL340, with reserve fuel on board, related to speed characteristics. This speed limitation does not exist at other altitudes or when the aircraft weight is outside the parameters of the limitation, or if the additional reserve tanks are not filled. The issue is addressed by the manufacturer.

One would need to differentiate between control flutters or airframe flutters...and we could diverge off on all kinds of tangents. I don't believe the original questions intended that, however, nor are they particularly relevant to the central question of what we experience when accelerating in level flight to a particular design speed. In the range of operating speeds we're going to see, within prescribed limitations for the aircraft we fly, we're not going to encounter adverse characteristics.

The question was asked about accelerating to Vmo on engine thrust...are we going to run into some kind of wall or limiting flight characteristics? No. We're either going to run out of excess thrust as the drag rise rapidly exceeds available thrust and thus stop accelerating, or we're going to hit airspeed limitations and need to reduce the thrust in order to maintain them. This may be complicated by maneuvering and loading the airplane/increasing angle of attack, but then we're back to buffet margins...and even at the upper speed limits at low altitude we're exploring the lower buffet margins when we either operate at critical AoA's...we've simply raised the buffet margin to a higher number. We're not touching upper buffet margins...the mach limits of the airplane aren't usually attainable because they're far above the IAS limitations at lower levels.

[gr8shandini]

I think what's confusing some of the folks here is the fact that Vmo and Mmo are usually independent of one another. Vmo keeps you out of harm's way structurally (either flutter or just plain air loads) and while Mmo protects from adverse Mach effects.

Vmo and Vne are essentially the same thing. One's used on jets, the other on props and helos. Why the difference? I don't know, ask the FAA. It's in IAS because flutter depends (among other things) on dynamic pressure. However, it's not where the bad stuff happens, that's called Vd (for V "dive") and the aircraft has to be demonstrated to be structurally sound out to that speed. Then a series of upset maneuvers are flown to ensure that a pilot flying at Vmo will not exceed Vd due to inattention, disorientation, wind gusts, etc. And despite what was stated earlier, Vmo cannot be legally exceeded for a maintenance test flight (unless it's specifically stated in the flight manual, which I doubt any manufacturer would do).

Mmo is much the same, but it's based on Mach, of course. The aircraft must be demonstrated to be safe out to Md and Mmo is determined with the same upset manuevers. The difference is that handling qualities can also come into play where Mach effects are concerned, so Md may or may not be a structural limit.

And finally, I'm not sure where the concept of an "airstream" speed came from, but that would refer to TAS, and doesn't really come into play unless you're using it to calculate Mach.

I hope that clears up some of the confusion.

[gr8shandini]

Guppy, I guess you beat me to the punch while I was typing. I agree with what you're saying except for one small thing:

"You want to know what's going to happen if you push the power up and fly to the maximum design speed? Nothing."

Probably true, but keep in mind that Vd is demonstrated on a relatively new aircraft with control surfaces rigged to production tolerances (the extremes, to be sure, but in tolerance nonetheless). If one were to try the same on a bird that's been in the fleet for 20 years, there's no guarantee that you wouldn't encounter flutter at a lower speed due to mis-rigging, excessive freeplay, and reduced airframe rigidity due to fatigue.

Not to say that anyone here is foolish enough to try it, but I'd still hate to have the impression that exceeding Vmo is no big deal.

[SNS3Guppy]

Not to say that anyone here is foolish enough to try it, but I'd still hate to have the impression that exceeding Vmo is no big deal.

That's why I previously specifically stated that one shouldn't exceed that speed. I also specifically pointed out that an airplane can be damaged at speeds well below other design speeds, such as maneuvering speed. Whereas pilots are often instructed that the airframe will experience a stall before design limits are exceeded when full control applications are made below that speed (which in turn is well below Vmo, for example)...it's not true. You can break the airplane at speeds well below Vmo or even Va.

So far as control surfaces go, very specific direction is provided for maintenance purposes that spells out tolerances for play in control runs, positioning of controls, control alignment, and control balance.

If the aircraft is properly maintained, flutter should not be an issue.

If the aircraft is improperly maintained, then all other discussions are immaterial. Excess freeplay in control runs or hinges, etc are items which make the aircraft unairworthy. Any discourse on what might be expected of an unairworthy airplane is pointless. It should be expected not to fly until returned to an airworthy condition in fully compliance with applicable approved publications.

[ssg]

Well if this is all just about Vmo

Guppy,

You bring up a good point...seems that many pilots seem to want to dramatize stories that they heard...just based on what I know about how planes are tested, seems very illogical that 5 kts over Vmo a Lear would have problems..

In fact...when testing the Premier one, F5 in tow...the number they tried to hit to certifiy for Vmo was 50 kts over...then do some kindvof of a pullup maneuver, with a specified number of Gs. So your right that Vmo testing has to do with recovery back to Vmo, most people don't know that.

What I have wondered in old aircraft, that once certified 50 kts over Vmo, ten thousand hours later, would come apart or do something funny at a lower speed. Again, sage advice, don't exceed Vmo.

I have always felt that the numbers aside, the pilots should know the numbers that aren't in the book, that the plane could fly to should emergency situation dicate. I wouldn't recommend it but if you were flying along and someone decided to start dogfighting you, going 50kts past Vmo to get down might not seem such a stretch at that point...

Which brings up a thought..

Does anyboy remember that movie where a plane crashed in the jungle, canibals all around, and the pilot said he couldn't take everyone, because of the reduced thrust(broken plane), and runway requirements. So he came up with the max passenger load...(those were pilots back them) It was a little bit like flight of the Pheonix, except they were in the jungle...it was like a single engine takeoff on a twin or something...so they left the bad guy to get eaten...lol

[slip and turn]

SNS3Guppy, just wanted to add my praise too - that was beautifully clear imparting of knowledge. Thank you. You are a born communicator. If you write books they'll be bestsellers!

I can see we are all queueing up to ask questions here, but ever since I learned a bit about coffin corner I've wondered how big a deal it is in say a 737NG if you drop out one side or the other inadvertently? Maybe you encounter Clear Air Turbulence unexpectedly while you are up there high riding a jetstream with a range of 30 or 40 knots to play with ...

Is the 737NG forgiving, much like most training aircraft don't bite at the onset of stall?

I remember chatting to some old military guys who flew the heavy rear engined stuff and when they talked of 'coffin corner' they knew they had to go there sometimes but they treated it like real bandit country, because if they dropped out tail heavy, they might never escape from a deep stall (?) which I understood to be a taildown plummet due to the mass of the engines.

How much of a deal is dropping out one side or the other thesedays on modern transport aircraft types?

[ChristiaanJ]

slip and turn,
Can't answer all yours, but in brief.

A "deep stall" had nothing to do with the engines, but it happened on T-tailed aircraft, when an excessive pitch-up led to the stalled turbulent flow from the wings totally "blanketing" the elevators, so that the aircraft refused to pitch down even with full down elevator, and just went in nose-up with little forward speed but a very high descent rate, and totally uncontrollable.

The 'hint' about the engines is not totally irrelevant, because most of the T-tails involved in such accidents had rear-mounted engines.

CJ

[gr8shandini]

Although it's more common on T-tails, you don't necessarily need one to have a deep stall, but you do need an aft CG (e.g. F-16). Same description, though, airplaine sits relatively level or nose high and just drops like a rock. It's not an automatic death sentence, though. In the F-16 you can pitch-buck out (if you're good) and on other types, you may have enough lateral or directional control to enter a spin and recover from there.

One more interesting thing about the "coffin corner"; I've heard that the U-2 can actually get so far into it that in a turn you can actually get stall buffet on the inboard wing and Mach buffet on the outboard. Don't know if it's true or not, but it sure sounds like a scary place to be.

[SNS3Guppy]

I can't really address the flight characteristics of a 737NG, but many aircraft lack performance capability to put themselves high enough to be in coffin corner...truly in coffin corner (where any increase in speed will cause a high speed buffet, and any decrease will cause a stall). Margins do decrease as one climbs higher, but no airliner or business aircraft is operating on a razor edge at altitude.

If an encounter with turbulence occurs which causes airspeed excursions such that buffet margins are encountered, then one maintains attitide, accepts altitude and airspeed excursions, and responds to any departures from controlled flight. This doesn't mean one lets control get away, but if one reaches a low speed condition to the point of a stall, then one reacts to the stall with a stall recovery. If one experiences a high speed upset to the point that adverse mach effects are encountered, then one performs a high speed upset recovery. These are standard recoveries that are part of any normal training regime.

Minor excursions require minor corrections in most cases; perhaps just pulling the power back slightly in the event of an overspeed condition, for example. Large excursions, particularly attitude and pronounced airspeed losses or increases, require bigger corrections.

So far as older aircraft...as an airplane ages, it's not granted greater tolerances or more latitude in what's allowed to slide by. When the aircraft rolls out the factory door, it's handed over with maintenance publications spelling out exactly what cable tensions, control rod end play, system limits, clearances, etc, are acceptable. At no time, even 10, 20 or 30 years later, or 80,000 hours later, can the airplane be operated outside those tolerances...so regardless of the hard life and age of the airplane, it's still got to be maintained within the same tolerances that it had the day it was born.

Low speed buffet is tempered by early warning using horns, airspeed indications, angle of attack indicators, stick shakers, and finally pushers...these occur before the critical angle of attack is reached. Some respond to trends in airspeed and angle of attack, others respond only to set A0A in degrees. Each system resets based on aircraft configuration (the stall speed and A0A changes with the deployment of flaps and slats or other leading edge devices, etc...and the aircraft warning systems change to reflect this based on configuration. What all of this has in common is that it warns of what's coming before adverse flight characteristics are encountered.

Deep stall characteristics aren't particular only to T-tailed aircraft. Control authority may be degraded with other horizontal stab configurations, too. Additionally, the use of power to aid in recovery or to slow the decay through increasing angle of attack may have little effect against the drag rise or degraded control authority. This is all really another subject entirely, as it's a condition that's departed well beyond buffet margins. However, allowing most large aircraft, in fact many turbojet airplanes, to degrade well into a stalled condition can put the airplane in a state where recovery may be difficult, very prolonged, or even impossible. The whole idea is to never let the aircraft go that far. The Lear has been used as an example several times now; a protracted stall maneuver in a LR35A may take 10,000' or more for recovery, where as a stall to the shaker or pusher under normal circumstances may result in little or no altitude loss if dealt with properly.

One doesn't simply "drop out" of flight when approaching buffet margins. These are points where adequate warning is given well before adverse conditions occur. An aircraft which is allowed to fly beyond these points by substantial degrees can be brought to a state where adverse flight characteristics can take place.

[slip and turn]

Thanks SNS3Guppy, as I don't plan on getting on any Tu154s or VC10s anytime soon, I can loosen my safety belt a couple of extra notches now in the low-cost cruise, ... or can I ? deep stall not just T tails you say ...

I note what you say about adequate warnings of buffet. But surely the main point about 'coffin corner', if as you say, your aircraft has the performance capability to take you there, and you let it, is that you are potentially boxed in a bit of a corner with potentially no speed range for manoeuvre (as with the U2 example in the extreme) ... frying pan or fire?

I guess what you are saying is that the graph plots of the onset of buffet define the corner, but that after onset there is a further unplotted area of 'forgiveness' in which recovery can be effected without severe upset?

Let me read it again ...

[SNS3Guppy]

I guess what you are saying is that the graph plots of the onset of buffet define the corner, but that after onset there is a further unplotted area of 'forgiveness' in which recovery can be effected without severe upset?

I'm not sure where you're getting that, but no. You seem to be implying that once buffet starts, progressing farther into that region the effects go away. Not the case. Generally as the low or high speed effects begin, any further decay or increase in airspeed will make the effects more pronounced. The exception will be aircraft utilizing artificial means to create the effects, such as a shaker; the aircraft itself may not actually be experiencing any effects at this stage, but rather the shaker may be serving as a warning several knots (or a degree of AoA) in advance of the forthcoming stall.

One may not experience any actually buffeting or stall at the airspeed operating limits, or one may. Where mach or stall effects are encountered, however, you're not going to see them as a brief occurrence that goes away as one progresses beyond the buffet boundaries.

[ChristiaanJ]

SNS3Guppy,
I would think slip and turn is not implying that it goes away, but simply that there is a zone beyond the onset of buffet, where it is mild enough to allow standard recovery procedures, i.e., the onset of buffet does not mean instant loss of control.

[slip and turn]

First, SNS3Guppy, continued thanks for indulging in answering my questions.

What I was getting at (actually still well confused about - be glad I don't fly the things! ) was your earlier emphasis / correction on my comment about 'dropping out' of the corner.

Actually let me quote you:

One doesn't simply "drop out" of flight when approaching buffet margins. These are points where adequate warning is given well before adverse conditions occur. An aircraft which is allowed to fly beyond these points by substantial degrees can be brought to a state where adverse flight characteristics can take place.

I took that (wrongly?) to imply that in many modern aircraft, coffin corner was not the 'bandit country' that it once was or still is on more sensitive types.

But perhaps what you are really saying is to pilots of all types capable of getting up there, is that a healthy degree of respect for these numbers is imperative, because they are the numbers, and can be ignored only at peril of untold severe recovery problems (adverse flight characteristics)?

And yes, thanks ChristiaanJ, or maybe no, not quite. I now read that if you are indeed unfortunate or reckless enough to get into the adverse stuff, which includes the buffet itself, then far from 'going away' you'd better expect things to turn violently and shake you by the throat until you win, or die trying?

Or is the onset of buffet a bit like a narrow second chance zone like the onset of stall in a light trainer - not a place you should ever plan to be, but demonstrably recoverable in standard ways within the slightly broadened speed range that includes low speed and high speed buffet?

Which is it SNS3Guppy? Or have I completely lost myself?