Rough Air Penetration Airspped - VRA
02-24-2009, 07:50 PM
#41
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Supposedly. Like I say I don't recall Va defined for a transport category jet (I've flown A320, 737, DC9, F28 - YMMV). It can be calculated easily. The assumption for it is that the elevator is limiting at lower speeds. We find that on the AA crash it was the rudder that was limiting (I don't think they were going very fast) - aggravated by repeated "back-and-forth" control inputs. Another issue was just how far the rudder deflects with a given "apparently small" control input. It's much more than we think - and more than the pilots thought that day. This is all "IIRC". There was also a repair done on the tail fin - NTSB said this wasn't a factor.
02-25-2009, 06:44 AM
#42
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Va seems to have been well-defined on this thread. Theoretically, one can apply full control deflections and not break anything. There are some exceptions, well-noted previously.
Vb would be a max speed / mach where you can encounter a max gust (defined by FAR 25 - I don't recall the figure, maybe 50 fps) and not have any structural damage - theoretically. This doesn't necessarily relate to full control deflection, as as such is not directly related to Va. The speed is usually quite a bit higher. Indeed, I don't recall flying an airplane on which both were defined. I suspect that's due to different certification rules. I ain't no expert, but I think I got this right.
And note the liberal use of the word "theoretically". Y'all be careful out there ...
Vb would be a max speed / mach where you can encounter a max gust (defined by FAR 25 - I don't recall the figure, maybe 50 fps) and not have any structural damage - theoretically. This doesn't necessarily relate to full control deflection, as as such is not directly related to Va. The speed is usually quite a bit higher. Indeed, I don't recall flying an airplane on which both were defined. I suspect that's due to different certification rules. I ain't no expert, but I think I got this right.
And note the liberal use of the word "theoretically". Y'all be careful out there ...
Joe
03-11-2009, 01:11 PM
#44
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More Info
Dougdrvr:
I was just about to point that out (BOAC). A Japanese Army unit, with a tracking telephoto camera, happened to be filming the airplane when it went down. Spooky. It has no engines, no emmpenage, and is streaming fuel from the engine pylons. It is falling straight down in a level attitude--90 degrees angle of attack.
rickair:
American had a BAC-111 lose the vertical fin due to a horizontal CAT gust in the mid 1960s; somewhere in the midwest.
Boeing lost the vertical fin from a B-52H while researching CAT and flight-loads on an instrumented aircraft they leased from the Air Force; about 1961. They managed to land it, and it is still serving at Barksdale (it of course, has a new fin). They learned a lot about stresses and strains from that mishap.
The T-38 Va is 420 kts. At that speed, if you plant the stick in your lap, you should stall before you exceed the allowable g-limit.
Vb is 280, and I point that out as earlier posters said the two (Va and Vb) should be "about the same." Depends on the airframe.
So, what determines Vb? It is a compromise between controllability, structural limits, performance, and stall speed.
Generally, the slower you go (and higher the angle of attack), the smaller percentage change in coefficient of lift for any given gust factor. That means the gusts don't "feel" as strong or bumpy.
But, as you go slower, you may hit a negative shear, and lose speed, or a strong enough vertical shear that you momentarily stall. Or, in a strong downdraft, you may not have much ability to climb if you are near stall speed.
On the other side of the spectrum, if you go too fast, those same vertical shears now represent a much bigger % change in AOA, which means without stick input, you could exceed the structural limits. Any stick input on top of that just makes it worse. It could also be so rough that it is just a miserable ride.
Finally, in all the fighters or tactical aircraft I've flown, the rolling-g limit (whether due to rudder, aileron, or outside forces), is generally 30-35% lower than the symmetrical limit. I won't bore you with discussions of Mohr's-circle stress-strain-shear diagrams. It just means that metal that is twisted at the same time it is compressed or pulled breaks earlier.
It isn't published, but the "rolling" Va for the T-38 is about 350 knots.
I don't know if any specific FARs determine Vb or if it is an arbitray selection from the manufacturer.
rotorhead:
A lot of speculation went on that the composite tail failed, but actually, 587's steel attach-lugs broke off from the fuselage. The composite fin, and its repair, were fine.
I was just about to point that out (BOAC). A Japanese Army unit, with a tracking telephoto camera, happened to be filming the airplane when it went down. Spooky. It has no engines, no emmpenage, and is streaming fuel from the engine pylons. It is falling straight down in a level attitude--90 degrees angle of attack.
rickair:
American had a BAC-111 lose the vertical fin due to a horizontal CAT gust in the mid 1960s; somewhere in the midwest.
Boeing lost the vertical fin from a B-52H while researching CAT and flight-loads on an instrumented aircraft they leased from the Air Force; about 1961. They managed to land it, and it is still serving at Barksdale (it of course, has a new fin). They learned a lot about stresses and strains from that mishap.
The T-38 Va is 420 kts. At that speed, if you plant the stick in your lap, you should stall before you exceed the allowable g-limit.
Vb is 280, and I point that out as earlier posters said the two (Va and Vb) should be "about the same." Depends on the airframe.
So, what determines Vb? It is a compromise between controllability, structural limits, performance, and stall speed.
Generally, the slower you go (and higher the angle of attack), the smaller percentage change in coefficient of lift for any given gust factor. That means the gusts don't "feel" as strong or bumpy.
But, as you go slower, you may hit a negative shear, and lose speed, or a strong enough vertical shear that you momentarily stall. Or, in a strong downdraft, you may not have much ability to climb if you are near stall speed.
On the other side of the spectrum, if you go too fast, those same vertical shears now represent a much bigger % change in AOA, which means without stick input, you could exceed the structural limits. Any stick input on top of that just makes it worse. It could also be so rough that it is just a miserable ride.
Finally, in all the fighters or tactical aircraft I've flown, the rolling-g limit (whether due to rudder, aileron, or outside forces), is generally 30-35% lower than the symmetrical limit. I won't bore you with discussions of Mohr's-circle stress-strain-shear diagrams. It just means that metal that is twisted at the same time it is compressed or pulled breaks earlier.
It isn't published, but the "rolling" Va for the T-38 is about 350 knots.
I don't know if any specific FARs determine Vb or if it is an arbitray selection from the manufacturer.
rotorhead:
A lot of speculation went on that the composite tail failed, but actually, 587's steel attach-lugs broke off from the fuselage. The composite fin, and its repair, were fine.
Last edited by UAL T38 Phlyer; 03-11-2009 at 08:07 PM.
03-13-2009, 12:11 PM
#45
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[quote=UAL T38 Phlyer;576730]
Are you thinking of the Braniff BAC-11 in Nebraska? IIRC, they think that one actually flew into a tornado.
I remember that from a long time ago as the chase pilot (F-100) lived in my neighborhood when I was a kid. They flew around for quite a while with the aft gear extended for stability and eventually landed at Little Rock AFB for crosswind reasons I would guess.
American had a BAC-111 lose the vertical fin due to a horizontal CAT gust in the mid 1960s; somewhere in the midwest.
Boeing lost the vertical fin from a B-52H while researching CAT and flight-loads on an instrumented aircraft they leased from the Air Force; about 1961. They managed to land it, and it is still serving at Barksdale (it of course, has a new fin). They learned a lot about stresses and strains from that mishap.
03-24-2009, 09:59 AM
#47
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Okay so question:
In the CRJ Turbulence Penetration Speed is 280/.75 whichever is lower.
Last night we were penetrating a line of thunderstorms, and before we arrived at the line, I reduced the speed to .74. The captain said "What's penetration speed?" and I said .75, so he said "Well then fly .75!" He's kind of anal about the speeds (like cruise speed was .78 yesterday, and the airplane crept up to .79 and he got onto me about it), but oh well.
The reason I wanted to fly .74, is because when you are in turbulence and around convective activity the speed fluctuates, sometimes considerably. I didn't want to be right on .75 and risk being in moderate or greater turbulence and having the speed go up to .77 or .78. At least .74 gives me some margin of error. We were at F370, and he got nervous when the speed got down to .742, or .738. It can be difficult to maintain exactly .75 during turbulence.
So my logic, was I'd rather be slower than .75 for that margin of error right? And he said because we were at a high altitude, it was very important to keep the speed up. But if we have climb performance numbers for 250/.70 up to FL410, wouldn't it be okay if the airplane slowed to .72 in cruise? I'm so confused. What do you all think?
In the CRJ Turbulence Penetration Speed is 280/.75 whichever is lower.
Last night we were penetrating a line of thunderstorms, and before we arrived at the line, I reduced the speed to .74. The captain said "What's penetration speed?" and I said .75, so he said "Well then fly .75!" He's kind of anal about the speeds (like cruise speed was .78 yesterday, and the airplane crept up to .79 and he got onto me about it), but oh well.
The reason I wanted to fly .74, is because when you are in turbulence and around convective activity the speed fluctuates, sometimes considerably. I didn't want to be right on .75 and risk being in moderate or greater turbulence and having the speed go up to .77 or .78. At least .74 gives me some margin of error. We were at F370, and he got nervous when the speed got down to .742, or .738. It can be difficult to maintain exactly .75 during turbulence.
So my logic, was I'd rather be slower than .75 for that margin of error right? And he said because we were at a high altitude, it was very important to keep the speed up. But if we have climb performance numbers for 250/.70 up to FL410, wouldn't it be okay if the airplane slowed to .72 in cruise? I'm so confused. What do you all think?
03-24-2009, 02:02 PM
#48
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Joined APC: Aug 2008
Position: 777 Left
Posts: 347
Okay so question:
In the CRJ Turbulence Penetration Speed is 280/.75 whichever is lower.
Last night we were penetrating a line of thunderstorms, and before we arrived at the line, I reduced the speed to .74. The captain said "What's penetration speed?" and I said .75, so he said "Well then fly .75!" He's kind of anal about the speeds (like cruise speed was .78 yesterday, and the airplane crept up to .79 and he got onto me about it), but oh well.
The reason I wanted to fly .74, is because when you are in turbulence and around convective activity the speed fluctuates, sometimes considerably. I didn't want to be right on .75 and risk being in moderate or greater turbulence and having the speed go up to .77 or .78. At least .74 gives me some margin of error. We were at F370, and he got nervous when the speed got down to .742, or .738. It can be difficult to maintain exactly .75 during turbulence.
So my logic, was I'd rather be slower than .75 for that margin of error right? And he said because we were at a high altitude, it was very important to keep the speed up. But if we have climb performance numbers for 250/.70 up to FL410, wouldn't it be okay if the airplane slowed to .72 in cruise? I'm so confused. What do you all think?
In the CRJ Turbulence Penetration Speed is 280/.75 whichever is lower.
Last night we were penetrating a line of thunderstorms, and before we arrived at the line, I reduced the speed to .74. The captain said "What's penetration speed?" and I said .75, so he said "Well then fly .75!" He's kind of anal about the speeds (like cruise speed was .78 yesterday, and the airplane crept up to .79 and he got onto me about it), but oh well.
The reason I wanted to fly .74, is because when you are in turbulence and around convective activity the speed fluctuates, sometimes considerably. I didn't want to be right on .75 and risk being in moderate or greater turbulence and having the speed go up to .77 or .78. At least .74 gives me some margin of error. We were at F370, and he got nervous when the speed got down to .742, or .738. It can be difficult to maintain exactly .75 during turbulence.
So my logic, was I'd rather be slower than .75 for that margin of error right? And he said because we were at a high altitude, it was very important to keep the speed up. But if we have climb performance numbers for 250/.70 up to FL410, wouldn't it be okay if the airplane slowed to .72 in cruise? I'm so confused. What do you all think?
03-24-2009, 02:33 PM
#49
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Second that Motion
I agree too. While you can get to a point where too slow is too bad, M0.01 isn't going to be it.
What Mach or Indicated (preferably both) do you climb to FL370 with? You should be able to slow to that speed in level flight at level-off, plus or minus whatever you are used to seeing +/- in the climb.
They don't really teach this, but it is IAS that will get you if you are too slow at high altitude, not Mach number. Too slow IAS-wise means you get on the backside of the power curve, and there isn't much excess thrust at FL370 to correct with. (Full power is probably only 40-50% of sea-level, depending upon whether that power is limited by RPM, ITT, or EPR).
What Mach or Indicated (preferably both) do you climb to FL370 with? You should be able to slow to that speed in level flight at level-off, plus or minus whatever you are used to seeing +/- in the climb.
They don't really teach this, but it is IAS that will get you if you are too slow at high altitude, not Mach number. Too slow IAS-wise means you get on the backside of the power curve, and there isn't much excess thrust at FL370 to correct with. (Full power is probably only 40-50% of sea-level, depending upon whether that power is limited by RPM, ITT, or EPR).
03-24-2009, 07:24 PM
#50
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I agree too. While you can get to a point where too slow is too bad, M0.01 isn't going to be it.
What Mach or Indicated (preferably both) do you climb to FL370 with? You should be able to slow to that speed in level flight at level-off, plus or minus whatever you are used to seeing +/- in the climb.
They don't really teach this, but it is IAS that will get you if you are too slow at high altitude, not Mach number. Too slow IAS-wise means you get on the backside of the power curve, and there isn't much excess thrust at FL370 to correct with. (Full power is probably only 40-50% of sea-level, depending upon whether that power is limited by RPM, ITT, or EPR).
What Mach or Indicated (preferably both) do you climb to FL370 with? You should be able to slow to that speed in level flight at level-off, plus or minus whatever you are used to seeing +/- in the climb.
They don't really teach this, but it is IAS that will get you if you are too slow at high altitude, not Mach number. Too slow IAS-wise means you get on the backside of the power curve, and there isn't much excess thrust at FL370 to correct with. (Full power is probably only 40-50% of sea-level, depending upon whether that power is limited by RPM, ITT, or EPR).
In the Bus we would see throttle up and perhaps a pitch change and on the Boeing I would guess you might get some sort of stick shaker or push, but that would be in a VERY extreme state. Can't really see much more happening than having to pitch down a bit to recover and that would be very little. Seems like the window is pretty wide here.
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