buffet?
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buffet?
No not Warren.
Can anyone explain high speed and low speed buffet for airliners, with reference to the 'red army' on the speed tape and aerodynamically.
Many thanks in advance.
Can anyone explain high speed and low speed buffet for airliners, with reference to the 'red army' on the speed tape and aerodynamically.
Many thanks in advance.
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Henry,
High speed and low speed buffet are terms which simply refer to the airplane aerodynamic response to certain flight conditions. A low speed buffet is nothing more than a stall buffet; it's based on angle of attack and in it's simplest terms, just like any airplane in training, get it slow enough and it will stall. Bank it steep enough and it will stall. It's just a stall buffet.
As an airplane climbs higher and higher at a constant indicated airspeed, true airspeed increases...the airplane goes faster and faster. This is due to air density. Again, on the most simple level, where there's less air resistance the atmosphere is thinner), the airplane goes faster over the ground in still air while indicating the same airspeed in the cockpit. The airplane must move faster through thinner air to produce the same lift. The airplane must move faster to produce the same pressure to produce the same indication in the pitot tubes which tell the airspeed indicator how fast the airplane is going.
Conversely, if the airplane goes faster and faster (higher true airspeed) for the same indicated airspeed, it also holds true that the airplane flies at a slower and slower indicated airspeed for the same true airspeed (other airspeed descriptions fit better, but become more complicated..such as equivilent airspeed, etc...it's easier right now to think in terms of indicated, and true airspeeds showing how fast the instrument says you're going, and how fast you really are). If you hold a given true airspeed and keep climbing, you'll be flying at a lower and lower number on the airspeed indicator as you climb.
While that relationship is going on, another is important. The upper limiting speed of the airplane is the maximum operating speed, known as Vmo at lower altitudes, and Mmo (mach limiting airspeed) at higher altitudes. On the airspeed indicator Mmo is shown as a barber pole (above which on a tape is the "red army" to which you're referring, though there are a number of ways to display information, ranging from angle of attack indications to tapes to round dials...I'm going to use round dials to keep it simple). As you climb higher and higher the mach limit of the airplane decreases. The barber pole striped needle on the indicator may start out at say, 400 knots indicated airspeed at sea level, but at cruising altitude may indicate a much lower number, such as 230 knots.
At altitude, we cruise in relation to mach airspeed rather than knots indicated airspeed. Mach is simply a measurement of the speed of sound in a given air density...that speed decreases as we climb. Much like indicated airspeed. For our purposes, we really don't care how fast sound goes, but it's a good reference number for us because it measures certain functions of air density...most important for us, mach effects...or in other words, what the atmosphere does for us and doesn't do for us at high altitudes and high airspeeds in relationship to our reference speed, the speed of sound.
Hopefully I'm not confusing you. We cruise at .84 Mach all the time. In our airplane, it's the number we use pretty much everywhere at altitude. .84 mach, or in other words 84% of the speed of sound at any given air density, occurs at a much higher indicated airspeed down low than it does up at cruising altitude. .84 is well below any high speed aerodynamic limitations, and keeps us fast enough that we don't stall while flying level or when turning (remember that your indicated stall speed goes up in the turn, so we want to cruise with a high enough airspeed that we can make a standard turn at altitude and still be comfortably above stall margins.
As we climb higher, the stall speed increases. Air is less dense, the airplane must fly at a higher true airspeed, but lower indicated airspeed, to make the same lift, and at a higher angle of attack. A low speed stall or buffet is a function of angle of attack; at some point you reach an angle of attack, the angle at which the wind meets the wing, when the wing will stall...every bit as much in an airliner as on a paper airplane. The speed at which that occurs straight and level will depend on several factors, (weight, altitude, temperature, bank angle, configuration and power setting, among other things), but represents the lower buffet limit. Just think of it as don't-get-too-slow-or-you'll-stall. That speed gets higher and higher as we climb. For a given weight, the stall margins, or space between the airspeed/mach number at which we're cruising now, will shrink with an increase in altitude. We might have 50 knots to play with here at this altitude, for example, but if we climb higher there might be a 25 knot or 10 knot difference between our cruising airspeed and the low speed buffet or stall. This plays part of the role in calculating our cruising altitude, and the altitudes of which we are capable at any given time based on some of the things we've discussed.
The high speed buffet is a little different. That's a result of what are called "mach effects," or simply put, the effects of going too fast. At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible. At higher altitudes at higher mach numbers, especially transonic numbers between about .75 and 1.2 mach, the way the airflow around the airplane affects us changes dramatically. It's the reason we have swept wings and flying surfaces, and the reason we measure our speed in relation to mach (percentage of the speed of sound); it's a measurement of our relationship to actual mach, which is a measurement of how the air is bunching up and compressing around us.
Air compresses to form shock waves and bow waves which change the way pressure is distriubted in front of, across, and behind the airplane. It changes the way the airplane displaces air, and displaced air is how we create lift. Lift is nothing more than a change in pressure around different parts of the airplane, and can be thought of as more pressure underneath than on top. As we move into higher mach numbers, pressure also builds in front of the airplane and forms a wave of compressed air. This wave moves aft and across the wing, and the properties of that air and the way the wing affects it, and the way it affects the wing, change depending on where it is.
Among those effects are things you've probably read about, such as mach tuck, and of course, high speed buffet. You can think of high speed buffet as a warning that we're going too fast. Just like a low speed buffet being produced by air burbling around the wing and striking the wing, tail and other parts of the airplane in a turbulent, random fashion, a high speed buffet is the airplane starting to react to airflow in a different manner. The drag rises sharply, lift can decrease, control effectiveness can be reduced, the airplane may experience a loss of downward force on the horizontal stabilizer or a change in the center of lift on the wing as the shock wave moves aft across it. It may want to speed up even more or begin losing elevator authority or reach it's trim limits if one goes too far. It may shake, or buffet, or it may display nothing really significant, depending on the airplane, it's weight, etc.
We're given upper and lower buffet limits as an operating range in which we must stay. It provides margins in which we can safely operate without going so fast we're into the mach effects, and without going so slow we stall. Sometimes the buffet margins are referred to as stall margins; a high speed tall and a low speed stall. You can also think of the upper buffet margin as a high speed stall in that the airflow changes about the wing (in some cases drastically) to where aircraft control may be difficult or impossible in some airplanes (if flown fast enough), lift is reduced, control forces may become too light (or in some cases too heavy), etc...and the drag rise is so high it saps all the efficiency out of the flight. You think of a low speed stall as a buffet because of some of the same reasons; the changes in pressure about the wing and airplane change drastically, lift is lost, control is affected, etc. Both can produce a buffet (or in some aircraft no buffet, but other warning devices are installed to replicate the buffet for the pilots in the cockpit).
Operationally, at cruise altitude, data is available in form of buffet margins. I use an old fashioned round dial airspeed indicator. I set one of the little plastic bugs in the window of the airspeed indicator to show me where the low speed buffet margin is at cruise, based on data available to us in the cockpit. I use the barber pole or mach limit as my upper margin...though actual buffeting may take place a little before or after that point.
The higher I climb, the closer those numbers are together. You may have heard the term "coffin corner," which is a place where those two numbers come together, or very close together. You can't slow down because you'll stall, and you can't speed up because of mach effects. You're trapped in a very narrow operating range or speeds. Some aircraft like the U-2 operate at such high altitudes that they literally spend most of their flight in the coffin corner...with only a few knots either way before they're either into mach effects of going too fast, or stalling for going too slow. Hopefully that helps.
High speed and low speed buffet are terms which simply refer to the airplane aerodynamic response to certain flight conditions. A low speed buffet is nothing more than a stall buffet; it's based on angle of attack and in it's simplest terms, just like any airplane in training, get it slow enough and it will stall. Bank it steep enough and it will stall. It's just a stall buffet.
As an airplane climbs higher and higher at a constant indicated airspeed, true airspeed increases...the airplane goes faster and faster. This is due to air density. Again, on the most simple level, where there's less air resistance the atmosphere is thinner), the airplane goes faster over the ground in still air while indicating the same airspeed in the cockpit. The airplane must move faster through thinner air to produce the same lift. The airplane must move faster to produce the same pressure to produce the same indication in the pitot tubes which tell the airspeed indicator how fast the airplane is going.
Conversely, if the airplane goes faster and faster (higher true airspeed) for the same indicated airspeed, it also holds true that the airplane flies at a slower and slower indicated airspeed for the same true airspeed (other airspeed descriptions fit better, but become more complicated..such as equivilent airspeed, etc...it's easier right now to think in terms of indicated, and true airspeeds showing how fast the instrument says you're going, and how fast you really are). If you hold a given true airspeed and keep climbing, you'll be flying at a lower and lower number on the airspeed indicator as you climb.
While that relationship is going on, another is important. The upper limiting speed of the airplane is the maximum operating speed, known as Vmo at lower altitudes, and Mmo (mach limiting airspeed) at higher altitudes. On the airspeed indicator Mmo is shown as a barber pole (above which on a tape is the "red army" to which you're referring, though there are a number of ways to display information, ranging from angle of attack indications to tapes to round dials...I'm going to use round dials to keep it simple). As you climb higher and higher the mach limit of the airplane decreases. The barber pole striped needle on the indicator may start out at say, 400 knots indicated airspeed at sea level, but at cruising altitude may indicate a much lower number, such as 230 knots.
At altitude, we cruise in relation to mach airspeed rather than knots indicated airspeed. Mach is simply a measurement of the speed of sound in a given air density...that speed decreases as we climb. Much like indicated airspeed. For our purposes, we really don't care how fast sound goes, but it's a good reference number for us because it measures certain functions of air density...most important for us, mach effects...or in other words, what the atmosphere does for us and doesn't do for us at high altitudes and high airspeeds in relationship to our reference speed, the speed of sound.
Hopefully I'm not confusing you. We cruise at .84 Mach all the time. In our airplane, it's the number we use pretty much everywhere at altitude. .84 mach, or in other words 84% of the speed of sound at any given air density, occurs at a much higher indicated airspeed down low than it does up at cruising altitude. .84 is well below any high speed aerodynamic limitations, and keeps us fast enough that we don't stall while flying level or when turning (remember that your indicated stall speed goes up in the turn, so we want to cruise with a high enough airspeed that we can make a standard turn at altitude and still be comfortably above stall margins.
As we climb higher, the stall speed increases. Air is less dense, the airplane must fly at a higher true airspeed, but lower indicated airspeed, to make the same lift, and at a higher angle of attack. A low speed stall or buffet is a function of angle of attack; at some point you reach an angle of attack, the angle at which the wind meets the wing, when the wing will stall...every bit as much in an airliner as on a paper airplane. The speed at which that occurs straight and level will depend on several factors, (weight, altitude, temperature, bank angle, configuration and power setting, among other things), but represents the lower buffet limit. Just think of it as don't-get-too-slow-or-you'll-stall. That speed gets higher and higher as we climb. For a given weight, the stall margins, or space between the airspeed/mach number at which we're cruising now, will shrink with an increase in altitude. We might have 50 knots to play with here at this altitude, for example, but if we climb higher there might be a 25 knot or 10 knot difference between our cruising airspeed and the low speed buffet or stall. This plays part of the role in calculating our cruising altitude, and the altitudes of which we are capable at any given time based on some of the things we've discussed.
The high speed buffet is a little different. That's a result of what are called "mach effects," or simply put, the effects of going too fast. At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible. At higher altitudes at higher mach numbers, especially transonic numbers between about .75 and 1.2 mach, the way the airflow around the airplane affects us changes dramatically. It's the reason we have swept wings and flying surfaces, and the reason we measure our speed in relation to mach (percentage of the speed of sound); it's a measurement of our relationship to actual mach, which is a measurement of how the air is bunching up and compressing around us.
Air compresses to form shock waves and bow waves which change the way pressure is distriubted in front of, across, and behind the airplane. It changes the way the airplane displaces air, and displaced air is how we create lift. Lift is nothing more than a change in pressure around different parts of the airplane, and can be thought of as more pressure underneath than on top. As we move into higher mach numbers, pressure also builds in front of the airplane and forms a wave of compressed air. This wave moves aft and across the wing, and the properties of that air and the way the wing affects it, and the way it affects the wing, change depending on where it is.
Among those effects are things you've probably read about, such as mach tuck, and of course, high speed buffet. You can think of high speed buffet as a warning that we're going too fast. Just like a low speed buffet being produced by air burbling around the wing and striking the wing, tail and other parts of the airplane in a turbulent, random fashion, a high speed buffet is the airplane starting to react to airflow in a different manner. The drag rises sharply, lift can decrease, control effectiveness can be reduced, the airplane may experience a loss of downward force on the horizontal stabilizer or a change in the center of lift on the wing as the shock wave moves aft across it. It may want to speed up even more or begin losing elevator authority or reach it's trim limits if one goes too far. It may shake, or buffet, or it may display nothing really significant, depending on the airplane, it's weight, etc.
We're given upper and lower buffet limits as an operating range in which we must stay. It provides margins in which we can safely operate without going so fast we're into the mach effects, and without going so slow we stall. Sometimes the buffet margins are referred to as stall margins; a high speed tall and a low speed stall. You can also think of the upper buffet margin as a high speed stall in that the airflow changes about the wing (in some cases drastically) to where aircraft control may be difficult or impossible in some airplanes (if flown fast enough), lift is reduced, control forces may become too light (or in some cases too heavy), etc...and the drag rise is so high it saps all the efficiency out of the flight. You think of a low speed stall as a buffet because of some of the same reasons; the changes in pressure about the wing and airplane change drastically, lift is lost, control is affected, etc. Both can produce a buffet (or in some aircraft no buffet, but other warning devices are installed to replicate the buffet for the pilots in the cockpit).
Operationally, at cruise altitude, data is available in form of buffet margins. I use an old fashioned round dial airspeed indicator. I set one of the little plastic bugs in the window of the airspeed indicator to show me where the low speed buffet margin is at cruise, based on data available to us in the cockpit. I use the barber pole or mach limit as my upper margin...though actual buffeting may take place a little before or after that point.
The higher I climb, the closer those numbers are together. You may have heard the term "coffin corner," which is a place where those two numbers come together, or very close together. You can't slow down because you'll stall, and you can't speed up because of mach effects. You're trapped in a very narrow operating range or speeds. Some aircraft like the U-2 operate at such high altitudes that they literally spend most of their flight in the coffin corner...with only a few knots either way before they're either into mach effects of going too fast, or stalling for going too slow. Hopefully that helps.
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SNS3Guppy,
It may be more than he asked for, but I am impressed by your excellent write-up on the subject.
To the extent that I've saved it all!
I hope you won't mind my plagiarizing some of your text sometime (if the same kind of question crops up) in appreciation of the time you put into writing it.
It may be more than he asked for, but I am impressed by your excellent write-up on the subject.
To the extent that I've saved it all!
I hope you won't mind my plagiarizing some of your text sometime (if the same kind of question crops up) in appreciation of the time you put into writing it.
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It may be of interest to some that the early models of the B707 and DC8, those with turbojet engines (rather than turbofans) it was indeed possible to climb to such a high altitude that the wing would no longer be efficient, and the mach buffet could be encountered with some regularity.
This was well above the altitude mandated according to the aircraft weight versus cruise mach buffet onset, and led to early jet upsets...until the effect was clearly understood by line crews who had transistioned from four engine piston types.
This was well above the altitude mandated according to the aircraft weight versus cruise mach buffet onset, and led to early jet upsets...until the effect was clearly understood by line crews who had transistioned from four engine piston types.
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Thank you Guppy for this great reading.
Does it mean we could, on an airliner, at low altitude, firewall the thrust levers, without major inconvenience or loss of control, just ignoring the overspeed warnings ?
What kind of speed would be reachable ?
The high speed buffet ...
At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible.
At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible.
What kind of speed would be reachable ?
Last edited by CONF iture; 19th Apr 2008 at 19:10.
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CONFiture and typhoid,
You've just found my 'coffin corner' of ignorance.
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?
Christian
You've just found my 'coffin corner' of ignorance.
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?
Christian
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Does it mean we could, on an airliner, at low altitude, firewall the thrust levers, without major inconvenience or loss of control, just ignoring the overspeed warnings ?
What kind of speed would be reachable ?
What kind of speed would be reachable ?
In order to reach a region where the mach effects of compressibility become a factor at low altitudes you'd have to increase your airspeed beyond Vmo...hence the reason at low altitudes we're IAS limited, and at higher altitudes, mach limited. You can push up the power if you like, but you do have aircraft limitations to consider before you go too far.
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Nice job Guppy...
Well written...
A note: Not all wings do the same at high speeds...some go right through the Mach region without so much of a bump ( Many Falcons) other are notorious for thier issues right after Vmo (Lears).
I like to joke it would be nice to fly a plane with more engine then wing...
A note: Not all wings do the same at high speeds...some go right through the Mach region without so much of a bump ( Many Falcons) other are notorious for thier issues right after Vmo (Lears).
I like to joke it would be nice to fly a plane with more engine then wing...
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I've met a lot of pilots who obtained their type in the LR 20 or 30 series, who believe that somewhere right around Mmo the airplane will begin to experience a sensational aileron buzz and all sorts of mach effects...because they experienced this in the simulator.
The real airplane doesn't do that. In fact, the airplane can exceed Mmo by a considerable margin before any such effects occur, and they're not nearly as pronounced as the sim. The effects experienced in the sim are exaggerated to make a point, but they don't replicate the actions of the airplane at those speeds. Even the mach tuck experienced by the airplane is very minor, and amounts to little more than a lessening of stick force below certification minimums...which is the reason for the puller and mach trim above .74 M.
You're quite correct that the high speed habits of an airplane very considerably with the design.
The real airplane doesn't do that. In fact, the airplane can exceed Mmo by a considerable margin before any such effects occur, and they're not nearly as pronounced as the sim. The effects experienced in the sim are exaggerated to make a point, but they don't replicate the actions of the airplane at those speeds. Even the mach tuck experienced by the airplane is very minor, and amounts to little more than a lessening of stick force below certification minimums...which is the reason for the puller and mach trim above .74 M.
You're quite correct that the high speed habits of an airplane very considerably with the design.
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There is one hopefully rare time when firewalling the throttles IS required at least on some aircraft. I was roped into taking cockpit video of sim trials of windshear encounters on the CRJ-200 (CL65). The finding there was that, as well as following the Windshear guidance (if set), the best method was to firewall everything immediately as the extra thrust just might make the difference.
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In a windshear encounter, assuming a microburst type situation, the goal is to avoid ground contact. A windshear recovery maneuver involves emergency thrust and pitching as necessary to avoid the ground, right to the stick shaker if necessary. That would be the low speed regime, or the lower buffet margins.
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I agree ... but in the same time that tail microburst type situation could as suddenly die or even evolve in an head one, and in the confusion of the moment, the speed red upper band limit could be easily well exceeded ... At least it's a simulator déjà vu scenario.
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No, in a microburst encounter, if you're experiencing a tailwind, you're in the outflow, and decreasing performance. As you fly out of the shear effect and begin to recover, you're doing so from a very slow condition. If the airspeed is increasing from that point and you're approaching Vmo, then you're certainly not flying the airlplane (it's flying you), and you're far out of danger. Time to either convert that extra thrust into a climb rate or get the power pulled back. Exceeding upper speed limitations during a windshear encounter, unless you were right on the edge of Vmo/Mmo at the outset (not particularly likely...and wouldn't require additional thrust if experiencing an excursion above your speed limits)...you're not going to go there when you're pitching for the shaker.
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Confiture
This doesn't mean the air ISN'T compressible - it is. It's the same air, just a little denser. Lighties just don't fly fast enough to enter into compressibility effects. We refer to Vne (Velocity Not to be Exceeded) beyond which lies flutter (which is an interplay of airframe rigidity and aerodynamics), rather than Mmo / Vmo. VNE is a fixed number, and TAS related rather than IAS. (or so I have been taught...)
It's still possible to experience 'Mach effects'/compressibility at low altitude, but less likely / common for reasons phsical (*generally* the speed of sound reduces with height due to the reducing temperature), and more practical (there are speed limits in place below 10,000ft, though they can be cancelled).
Beyond that, I'm into guessing: but I suspect the most speed limiting factor down low is economics! The fuel penalty in operating fast 'n' low in all that draggy air is probably the main reason why civil a/c don't.
What I would like to understand is why airliners are able to operate to a Vmo which (I believe) is variable with altitude, and can typically fly higher TAS at altitude - which suggests that flutter isn't limiting at altitude for them? Wheras I understand flutter to be an airstream speed issue, NOT a dynamic pressure issue. Or are they simply not flutter limited at any speed / alt? In which case, what does limit them low down, other than economics?
Quite frankly, the rationale behind which some limitations apply to IAS, and some TAS escapes me - I know which I have to watch for various, but not WHY!
At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible.
It's still possible to experience 'Mach effects'/compressibility at low altitude, but less likely / common for reasons phsical (*generally* the speed of sound reduces with height due to the reducing temperature), and more practical (there are speed limits in place below 10,000ft, though they can be cancelled).
Beyond that, I'm into guessing: but I suspect the most speed limiting factor down low is economics! The fuel penalty in operating fast 'n' low in all that draggy air is probably the main reason why civil a/c don't.
What I would like to understand is why airliners are able to operate to a Vmo which (I believe) is variable with altitude, and can typically fly higher TAS at altitude - which suggests that flutter isn't limiting at altitude for them? Wheras I understand flutter to be an airstream speed issue, NOT a dynamic pressure issue. Or are they simply not flutter limited at any speed / alt? In which case, what does limit them low down, other than economics?
Quite frankly, the rationale behind which some limitations apply to IAS, and some TAS escapes me - I know which I have to watch for various, but not WHY!
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At lower altitudes, where Vmo is limiting with a jet aeroplane, to then exceed Vmo by some margin exposes the aeroplane to increased dynamic pressures, with resultant difficulties, including the possibility of increased control surface flutter.
Even with turbopropeller and large piston designs, this was possible.
For example.
The DC-6B experienced elevator flutter during flight test when the maximum authorized indicated airspeed was reached or just slightly exceeded.
A fix involved additional hinges installed at the outer ends of each elevator.
In my considered opinion, if a pilot should decide, on their own and for no good reason, to exceed Vmo/Mmo deliberately, would/should be grounds for license suspension....if not ready for the rubber room.
Even with turbopropeller and large piston designs, this was possible.
For example.
The DC-6B experienced elevator flutter during flight test when the maximum authorized indicated airspeed was reached or just slightly exceeded.
A fix involved additional hinges installed at the outer ends of each elevator.
In my considered opinion, if a pilot should decide, on their own and for no good reason, to exceed Vmo/Mmo deliberately, would/should be grounds for license suspension....if not ready for the rubber room.
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Never heard about a TAS-limitation. Only either IAS (Vmo) or M (Mmo).
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
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 downside of surplus thrust
Quote from ssg:
I like to joke it would be nice to fly a plane with more engine then wing...
[Unquote]
Hope I’m not misunderstanding your remark, but that is precisely what enables you to go into “coffin corner”. As 411A has reminded us, the turbojets of the 1950s and 60s were at their most efficient at high altitude (unlike turbofans); and capable of powering the wings of DC-8s, B707/720s, CV880/990s, and Comets well up into potential trouble.
When later variants of many of these aeroplanes were fitted with by-pass engines and even turbofans − but retained roughly similar wings − the situation changed. Instead of maximum cruise altitude being dictated by the wing, it was thrust that limited it; i.e., you’re at climb thrust and no longer climbing…
In my experience, the two opposing situations were illustrated by the by-pass-powered VC10 and the turbofan-powered B707-320B/C. On the VC10, when looking for a step climb, we merely looked at the low-speed buffet graphs. There was always enough power to get us up to an altitude where 1·35G would cause low-speed buffet at the current weight and cruise Mach; and even higher, where 1·25G would be enough. Up there, the safe range of speed between low-speed buffet and Mmo was interestingly narrow. 1·25G is equivalent to about 35deg bank in smooth air. This leaves little margin for turbulence in turns. Most crews would avoid it unless the ATC alternative was a fuel-costly descent, and reports indicated it was smooth at the higher level.
On the turbofan 707-320B/C, the situation was reversed. Although the graph included an indication of the buffet margins, these were usually 1·4G or more at attainable altitudes. Thrust was invariably what stopped us climbing, particularly when temperatures were above ISA. So, on that type of 707, there was little risk of reaching the coffin corner of the flight envelope.
I like to joke it would be nice to fly a plane with more engine then wing...
[Unquote]
Hope I’m not misunderstanding your remark, but that is precisely what enables you to go into “coffin corner”. As 411A has reminded us, the turbojets of the 1950s and 60s were at their most efficient at high altitude (unlike turbofans); and capable of powering the wings of DC-8s, B707/720s, CV880/990s, and Comets well up into potential trouble.
When later variants of many of these aeroplanes were fitted with by-pass engines and even turbofans − but retained roughly similar wings − the situation changed. Instead of maximum cruise altitude being dictated by the wing, it was thrust that limited it; i.e., you’re at climb thrust and no longer climbing…
In my experience, the two opposing situations were illustrated by the by-pass-powered VC10 and the turbofan-powered B707-320B/C. On the VC10, when looking for a step climb, we merely looked at the low-speed buffet graphs. There was always enough power to get us up to an altitude where 1·35G would cause low-speed buffet at the current weight and cruise Mach; and even higher, where 1·25G would be enough. Up there, the safe range of speed between low-speed buffet and Mmo was interestingly narrow. 1·25G is equivalent to about 35deg bank in smooth air. This leaves little margin for turbulence in turns. Most crews would avoid it unless the ATC alternative was a fuel-costly descent, and reports indicated it was smooth at the higher level.
On the turbofan 707-320B/C, the situation was reversed. Although the graph included an indication of the buffet margins, these were usually 1·4G or more at attainable altitudes. Thrust was invariably what stopped us climbing, particularly when temperatures were above ISA. So, on that type of 707, there was little risk of reaching the coffin corner of the flight envelope.