Turbulent penetration...
Guest
Posts: n/a
Turbulent penetration...
It got to that time of day again..around beer o'clock... and the subject of turbulent penetration speed came up again.....and again I have several interesting theories 
so what i suggested was lets farm it out to the guys on PPrune to help solve this riddle...
Theories did vary as the night progressed, the most popular was that Vturb was dependant on weight and could be calulated out with various methods, we did find this thoery and a proof calculation for a PA28 on the web but the power died before we could save it, so we ajourned back to the bar to continue.
1)My theory was that usually the one published in manual is calculated for the max all up weight of the aircraft, and reduces with weight to give you almost a Vturb max and Vturb min for want of a better description for max and min weight, and that speeds outside of this range either higher or lower, would be similarly detrimental to the structure of the aircraft... as in flying 20 knots above would do similar degree of stress and damage as flying at 20 knots below
2) The second theory was that any speed below the one published in the manual was acceptable to decrease the stress's on the airframe while doing the bumpy stuff.... some how this dont seem right to me but I cant prove either way if its right..
way off the mark...??? well I hope not too much... pppplease put me out of my misery as next week I'm sure we going to get onto that no-no subject of how does a fly manage that approach, flare and landing onto the ceiling..... 
HELP !!!
so what i suggested was lets farm it out to the guys on PPrune to help solve this riddle...Theories did vary as the night progressed, the most popular was that Vturb was dependant on weight and could be calulated out with various methods, we did find this thoery and a proof calculation for a PA28 on the web but the power died before we could save it, so we ajourned back to the bar to continue.
1)My theory was that usually the one published in manual is calculated for the max all up weight of the aircraft, and reduces with weight to give you almost a Vturb max and Vturb min for want of a better description for max and min weight, and that speeds outside of this range either higher or lower, would be similarly detrimental to the structure of the aircraft... as in flying 20 knots above would do similar degree of stress and damage as flying at 20 knots below
2) The second theory was that any speed below the one published in the manual was acceptable to decrease the stress's on the airframe while doing the bumpy stuff.... some how this dont seem right to me but I cant prove either way if its right..
way off the mark...??? well I hope not too much... pppplease put me out of my misery as next week I'm sure we going to get onto that no-no subject of how does a fly manage that approach, flare and landing onto the ceiling..... HELP !!!
Join Date: Oct 2000
Location: N. Europe
Posts: 436
Likes: 0
Received 0 Likes
on
0 Posts
At or below. What you want is an aircraft that will need more than critical AoA to reach it's structural limits (i e stall first). That way, hitting a sudden up- or downdraught will never rip lose expensive or necessary parts such as engines, batteries of the coffe machine. 
Now, hand over that beer. I need it more than you. Maintenance type course in progress.
Cheers,
/ft
Now, hand over that beer. I need it more than you. Maintenance type course in progress.
Cheers,
/ft
Join Date: Jun 2001
Location: Australia
Posts: 889
Likes: 0
Received 0 Likes
on
0 Posts
Turbulence penetration speed is related to max manoeuvring speed Va. However, instead of being the fastest you can go where the wing stalls prior to exceeding max load limit as a result of control mishandling, Vturb (Vb?) allows for sudden wind gusts momentarily increasing the TAS or angle of attack. How strong a gust you ask? I'm not sure, but FAR25 probably has the answer.
So if you fly faster than Vb, a sudden updraft or side-draft or whatever, may cause the aircraft to exceed max load limit / Vno / Mmo before the pilot can react. If you fly much slower though, a sudden patch of turbulence may cause you to reach low speed buffet.
Moral of the story: if you fly at Vb, you will be protected from Vs on the one hand and max load limit on the other, from all but the most statistically improbable wind gusts.
All the above dates back to when I did ATPL's about three years ago, so all care and no responsibility eh?
cheers,
O8
[ 30 October 2001: Message edited by: Oktas8 ]
So if you fly faster than Vb, a sudden updraft or side-draft or whatever, may cause the aircraft to exceed max load limit / Vno / Mmo before the pilot can react. If you fly much slower though, a sudden patch of turbulence may cause you to reach low speed buffet.
Moral of the story: if you fly at Vb, you will be protected from Vs on the one hand and max load limit on the other, from all but the most statistically improbable wind gusts.
All the above dates back to when I did ATPL's about three years ago, so all care and no responsibility eh?
cheers,
O8
[ 30 October 2001: Message edited by: Oktas8 ]
Join Date: Feb 2001
Location: London, England
Posts: 44
Likes: 0
Received 0 Likes
on
0 Posts
Oktas8 referred to gusting wind, just read in a 'I Learned About Flying From That' -type anthology and one incident saw the airspeed indicator on a commuter prop in extreme turbulence move from 140 kts to hitting the stop next to zero and back TWICE in a couple of seconds. Which would get your attention.
Join Date: May 2000
Location: Seattle
Posts: 3,196
Likes: 0
Received 0 Likes
on
0 Posts
Turbulent penetration speed is a compromise between stall margin and protection from overstress. You want to be as fast as possible to avoid stalling the airplane in low-speed excursions, but don't want to exceed the speed where you will break the airplane in high-speed excursions.
If you don't have a stall margin indication in the cockpit (as in the 747-400) or maneuvering (Vg/Vn) diagram for the gross weight range you fly in, your best bet is to use a speed in the middle of the range given in your FHB. If you have a cockpit indication, fly in the middle of the maneuvering margin, as long as that speed is within the FHB limitations.
There will always be some uncertainty. For example, the 747-400 turbulence penetration speed is 290-310 KIAS or .82-.85 IMN. Cruising at FL310+, the KIAS range and IMN ranges do not coincide. However, .84-.85 IMN will keep you in the middle of the maneuvering margin displayed on the PFD.
If you don't have a stall margin indication in the cockpit (as in the 747-400) or maneuvering (Vg/Vn) diagram for the gross weight range you fly in, your best bet is to use a speed in the middle of the range given in your FHB. If you have a cockpit indication, fly in the middle of the maneuvering margin, as long as that speed is within the FHB limitations.
There will always be some uncertainty. For example, the 747-400 turbulence penetration speed is 290-310 KIAS or .82-.85 IMN. Cruising at FL310+, the KIAS range and IMN ranges do not coincide. However, .84-.85 IMN will keep you in the middle of the maneuvering margin displayed on the PFD.
Moderator
Some observations ..
Vb is described as a factor on the relevant stall speed and considers a range of things - including
(a) wing loading, geometry and lift curve
(b) density (ie altitude)
(c) design cruise speed
(d) design gust velocities (which vary with height)
(e) weight (to which stall speed relates)
At higher cruise levels where Mach becomes limiting, the balance changes to consider high and low speed buffet.
As in all these certification matters, nothing is a guarantee, only a set of reference information. Caution is always an admirable attribute as the bumps get nastier.
Epsom Hold 2
Consider that extreme fluctuations in IAS generally indicate fluctuations in the static pressure sensed rather than actual speed variations. The ASI is only a pressure diff gauge, not a speedo, and is very sensitive to static errors. In general, if the IAS is varying at any significant rate, then the reading should be viewed with a grain of salt until quasi steady conditions return.
[ 03 November 2001: Message edited by: john_tullamarine ]
Vb is described as a factor on the relevant stall speed and considers a range of things - including
(a) wing loading, geometry and lift curve
(b) density (ie altitude)
(c) design cruise speed
(d) design gust velocities (which vary with height)
(e) weight (to which stall speed relates)
At higher cruise levels where Mach becomes limiting, the balance changes to consider high and low speed buffet.
As in all these certification matters, nothing is a guarantee, only a set of reference information. Caution is always an admirable attribute as the bumps get nastier.
Epsom Hold 2
Consider that extreme fluctuations in IAS generally indicate fluctuations in the static pressure sensed rather than actual speed variations. The ASI is only a pressure diff gauge, not a speedo, and is very sensitive to static errors. In general, if the IAS is varying at any significant rate, then the reading should be viewed with a grain of salt until quasi steady conditions return.
[ 03 November 2001: Message edited by: john_tullamarine ]
