B757/767 wings
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B757/767 wings
G'day
I understand that both the B757 and B767 have supercritical wings, the B757 with a mach speed trim to arrest the mach tuck tendency and the B767 with a trim compensator in order to arrest any longitudinal instability, but not mach tuck, which it doesn't have a tendency towards.
The reason for the differences are that the supercritical on the B767 was moved forward on the fuselage, which brought forward the aft loaded CP and reduced the tuck tendency, whereas the B757 did not have a design compromise in wing positioning and suffers mach tuck tendencies.
Can anyone validate these interpretations?
I am also trying to confirm that the B757 and B767 both have a limiting mach number (Cd) of 0.91. For those not yet cognizant of what the limiting mach number is, I am led to believe that it is the speed at which compressibility effects begin to affect the control of the aircraft - which might be considered as the point at which you will achieve a shockstall.
One other question that comes to mind chaps:
What is actually achieved when wheel plugs melt, from a design standpoint? I realise that the tyre deflates, but what are the designers trying to achieve in deflating the tyres at this point?
Thanks in advance.
I understand that both the B757 and B767 have supercritical wings, the B757 with a mach speed trim to arrest the mach tuck tendency and the B767 with a trim compensator in order to arrest any longitudinal instability, but not mach tuck, which it doesn't have a tendency towards.
The reason for the differences are that the supercritical on the B767 was moved forward on the fuselage, which brought forward the aft loaded CP and reduced the tuck tendency, whereas the B757 did not have a design compromise in wing positioning and suffers mach tuck tendencies.
Can anyone validate these interpretations?
I am also trying to confirm that the B757 and B767 both have a limiting mach number (Cd) of 0.91. For those not yet cognizant of what the limiting mach number is, I am led to believe that it is the speed at which compressibility effects begin to affect the control of the aircraft - which might be considered as the point at which you will achieve a shockstall.
One other question that comes to mind chaps:
What is actually achieved when wheel plugs melt, from a design standpoint? I realise that the tyre deflates, but what are the designers trying to achieve in deflating the tyres at this point?
Thanks in advance.
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Are you sure they have supercritical wings?
I cannot see why the 767 would not have a tendency to Mach tuck? I highly doubt that any of them incorporate such wing designs...
What you refer to as limiting speed is the so-called critical Mach number; the speed where local flow (usually over the wing(s)) reach mach 1.0. When the air decelerates to below M1.0 it does so through a shock wave. The air passing through this shock wave could suddenly separate (detach from the wing) as a result from the great increase in pressure & decrease in speed as it passes through the SW.
When the plugs melt, the Nitrogen inside the tyres is released, and this cools the brakes = no fire! (I think)
I cannot see why the 767 would not have a tendency to Mach tuck? I highly doubt that any of them incorporate such wing designs...
What you refer to as limiting speed is the so-called critical Mach number; the speed where local flow (usually over the wing(s)) reach mach 1.0. When the air decelerates to below M1.0 it does so through a shock wave. The air passing through this shock wave could suddenly separate (detach from the wing) as a result from the great increase in pressure & decrease in speed as it passes through the SW.
When the plugs melt, the Nitrogen inside the tyres is released, and this cools the brakes = no fire! (I think)
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Don' t know about Boeing wings, but the fuse plugs are installed so that the tyres don't burst. The rush of gas exiting the tire through an opened plug isn't damaging, but having that energy released suddenly when a tyre explodes can be very destructive. They also serve this function when the wheels are stowed-tyres have burst after retraction due to built-up heat etc.
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On the subject of fuse plugs...it is my understanding that these are the designed frangible link in a tire/wheel overheat scenario. As heat energy is transferred from brakes to tires/wheels, the designed frangible links are the fuse plugs. Designed so there is not a catasrophic event (read: exposion) of the tire/wheel assembly.
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Spoke to the Ginger Beer on my last departure. Wheel plugs melt at a set temp in order to deflate the tyres and avoid them exploding due to the expanding, hot air inside. Clever. Stops damage and death after a high energy stop. Thanks for the responses to the dudes who did.
Regarding the design compromise, the 76 wing was moved forward as a compromise to the aft loaded CP and thus partly negate the effect of the pitching moment. The 75, I am not sure, but that is the case with the 76.
Ye of little faith, a web search will help you here, but I'd rather see Boeing on the title of the webpage, hence my call for a validation.
Unfortunately, Boeing say next to nothing about their aircraft.
Oh yeah (needing the edit here), careful, as the limiting mach number is not the critical mach number.
Mcrit is as you have described, but the limiting mach is that speed at which compressibility effects begin to affect the controllability of the aircraft, ie, high speed buffet and so on.
Regarding the design compromise, the 76 wing was moved forward as a compromise to the aft loaded CP and thus partly negate the effect of the pitching moment. The 75, I am not sure, but that is the case with the 76.
Ye of little faith, a web search will help you here, but I'd rather see Boeing on the title of the webpage, hence my call for a validation.
Unfortunately, Boeing say next to nothing about their aircraft.
Oh yeah (needing the edit here), careful, as the limiting mach number is not the critical mach number.
Mcrit is as you have described, but the limiting mach is that speed at which compressibility effects begin to affect the controllability of the aircraft, ie, high speed buffet and so on.
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Life as a journey, you are correct in that the 767 does not have a mach tuck problem where as the 757 does require the trimmer. The aerodynamics of it all are lost on me now as it has been some time since I flew both. I will dig out some old notes and see if I can find out further for you but bear with me as the roster is heavy this month.
Just a thought, there is a website called "bluecoat" where you could post such questions of a technical nature and it would be answered by Boeing/Airbus engineers. Initially the site was set up by NASA and then expanded there on to encompass civilian aviation. Whether it still operates I do not know but was of use when I had questions that the manuals did not cover during the rating and subsequent flying. If you managed to find it then please send me a message with the link.
Cheers.
Just a thought, there is a website called "bluecoat" where you could post such questions of a technical nature and it would be answered by Boeing/Airbus engineers. Initially the site was set up by NASA and then expanded there on to encompass civilian aviation. Whether it still operates I do not know but was of use when I had questions that the manuals did not cover during the rating and subsequent flying. If you managed to find it then please send me a message with the link.
Cheers.
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Dudes, & Fire Wall
Here is a bit more of an explanation. It was kindly provided by aerospace.org and I recommend them to you all.
Quote:
The 757 uses a supercritical airfoil which is the Boeing TR-51 series. More information on these airfoils can be found at the
following locations:
http://www.aerospaceweb.org/question...ls/q0073.shtml
http://www.aerospaceweb.org/question...s/q0054b.shtml
As you probably know, the reason an aircraft would need a mach speed trim device is because the wing center of pressure (Cp) moves aft at transonic speeds, usually starting at about Mach 0.6 or 0.7. As the Cp moves aft, the moment arm between it and the elevator decreases making the elevator less effective in providing pitch control. The difference in location between the Cp and the center of gravity (located in front of the Cp) causes the aircraft nose to pitch down, so more elevator trim is required to keep the aircraft level.
Now that we know what mach speed trim is, the question becomes why would one aircraft need it and the other not.
What this question suggests is that there is no movement of the Cp on the 767, or at least much less than there is on the 757.
Cp travel is usually associated with the wing airfoil section, but I believe that both aircraft use the same airfoils, or ones similar enough that their Cp movement should be similar. Another method of reducing Cp movement is to use wing sweepback.
This is indeed one major difference between the wings of the two aircraft. The 757 wing is swept 25° while the 767 wing is swept 31.5°. That 6.5° difference seems pretty small, but I think it likely plays a major role.
The 767 also has a thicker wing, greater wing chord, and larger horizontal tail than the 757. A combination of some or all of these variables may have the effect of making the aircraft
less sensitive to the Cp movement.
Enquote.
Here is a bit more of an explanation. It was kindly provided by aerospace.org and I recommend them to you all.
Quote:
The 757 uses a supercritical airfoil which is the Boeing TR-51 series. More information on these airfoils can be found at the
following locations:
http://www.aerospaceweb.org/question...ls/q0073.shtml
http://www.aerospaceweb.org/question...s/q0054b.shtml
As you probably know, the reason an aircraft would need a mach speed trim device is because the wing center of pressure (Cp) moves aft at transonic speeds, usually starting at about Mach 0.6 or 0.7. As the Cp moves aft, the moment arm between it and the elevator decreases making the elevator less effective in providing pitch control. The difference in location between the Cp and the center of gravity (located in front of the Cp) causes the aircraft nose to pitch down, so more elevator trim is required to keep the aircraft level.
Now that we know what mach speed trim is, the question becomes why would one aircraft need it and the other not.
What this question suggests is that there is no movement of the Cp on the 767, or at least much less than there is on the 757.
Cp travel is usually associated with the wing airfoil section, but I believe that both aircraft use the same airfoils, or ones similar enough that their Cp movement should be similar. Another method of reducing Cp movement is to use wing sweepback.
This is indeed one major difference between the wings of the two aircraft. The 757 wing is swept 25° while the 767 wing is swept 31.5°. That 6.5° difference seems pretty small, but I think it likely plays a major role.
The 767 also has a thicker wing, greater wing chord, and larger horizontal tail than the 757. A combination of some or all of these variables may have the effect of making the aircraft
less sensitive to the Cp movement.
Enquote.
