Critical angle of attack and stall
Good point, Barit. And Love had another one.
One of the things I learned from my instructors ( over 50 years ago, same as you) was the "feel" when "close" to that arena where you could snap roll with a lotta rudder and maybe a reasonable amount of back stick. Never saw one when at a fairly low AoA, but the bent wings I flew wouldn't do it before adverse yaw kicked in and such, plus a lotta shaking/burble.
Love's point about wind tunnel tests of wings are great, and when wings are mounted on an airplane, things change. Just look at the Viper and Hornet AoA stuff ( F-16 and F-18). The vortex "lift" kept the air flowing over the wings longer than planes with no leading-edge extensions/strakes. Prolly also got some lift from the fuselage. However, we're talking about 20 - 30 degrees or more AoA, and the average general aviation dude or airline pilot would never see that part of the "envelope".
My main point is that during training you must fly the plane close to the limits and try to get the "feel" just before the plane goes outta control. Many of the heavies can't allow this during training, and I do not know what can be done about it.
Some folks talk about "zero" AoA to gain energy and recover from a stall. Prolly true, but the certain method is zero gee, which you can feel without any stupid indicator. Don't think the SLF's would like that procedure, but it's damned hard to stall if the plane is not producing lift, ya think?
One of the things I learned from my instructors ( over 50 years ago, same as you) was the "feel" when "close" to that arena where you could snap roll with a lotta rudder and maybe a reasonable amount of back stick. Never saw one when at a fairly low AoA, but the bent wings I flew wouldn't do it before adverse yaw kicked in and such, plus a lotta shaking/burble.
Love's point about wind tunnel tests of wings are great, and when wings are mounted on an airplane, things change. Just look at the Viper and Hornet AoA stuff ( F-16 and F-18). The vortex "lift" kept the air flowing over the wings longer than planes with no leading-edge extensions/strakes. Prolly also got some lift from the fuselage. However, we're talking about 20 - 30 degrees or more AoA, and the average general aviation dude or airline pilot would never see that part of the "envelope".
My main point is that during training you must fly the plane close to the limits and try to get the "feel" just before the plane goes outta control. Many of the heavies can't allow this during training, and I do not know what can be done about it.
Some folks talk about "zero" AoA to gain energy and recover from a stall. Prolly true, but the certain method is zero gee, which you can feel without any stupid indicator. Don't think the SLF's would like that procedure, but it's damned hard to stall if the plane is not producing lift, ya think?
Join Date: Mar 2012
Location: Melbourne
Posts: 28
Likes: 0
Received 0 Likes
on
0 Posts
Been some really interesting replies in this thread, as I'm currently studying aerodynamics it's great to read all the bits about 'how things work in the real world' rather than the perfect examples they seem to use in the textbooks.
Join Date: Jun 2011
Location: france
Posts: 760
Likes: 0
Received 0 Likes
on
0 Posts
derivation
1. Thank you,Henra for that link to old documents (1930')
2. Do you know or does anybody knows where Prandtl's closed curves can be found on the web : lift and drag on the two axis as mesured with dynamometers, for the whole range of angle of attack (-180,+180°) at constant speed ?
I had these precious books for all the studied profiles, but they have been stolen...
On these curves it is easy to understand that stall is not so wild, but aircraft stability and changing speed may do (or not do ) it wild.
3. We often read posts opposing flying speed and flying gee, or Cl max and deplacement of center of lift, as coming from different paradigms. Thanks to Gums who is reminding or teaching us how gravity feeling, hence gee is an accurate mean to pilot speed, stall,aso for well taught pilots with aerobatics :Nz is the derivate of the speed, so it tells us the speed tendance, it previews speed modification, so the best way to pilot speed uses gee !
Similar math reading of cl max does not emphasizes the fact that cl is maximum at that point, but that left of that point lift increases with attitude, and starts to decrease with attitude passing at the right size of that point.
It is mandatory to the airline pilot to understand the tendance (mathematic derivation) to preview the flight. Hiding how FBW ( in normal law) elevator is a math integration of the side stick position or pressure, and Airbus THS is a math integration of elevator are pedagogic holes, with heavy issues for flight safety. The requested level of math is elementary math. Piloting anticipates the plane 's movement, you need to know the tendance.
2. Do you know or does anybody knows where Prandtl's closed curves can be found on the web : lift and drag on the two axis as mesured with dynamometers, for the whole range of angle of attack (-180,+180°) at constant speed ?
I had these precious books for all the studied profiles, but they have been stolen...
On these curves it is easy to understand that stall is not so wild, but aircraft stability and changing speed may do (or not do ) it wild.
3. We often read posts opposing flying speed and flying gee, or Cl max and deplacement of center of lift, as coming from different paradigms. Thanks to Gums who is reminding or teaching us how gravity feeling, hence gee is an accurate mean to pilot speed, stall,aso for well taught pilots with aerobatics :Nz is the derivate of the speed, so it tells us the speed tendance, it previews speed modification, so the best way to pilot speed uses gee !
Similar math reading of cl max does not emphasizes the fact that cl is maximum at that point, but that left of that point lift increases with attitude, and starts to decrease with attitude passing at the right size of that point.
It is mandatory to the airline pilot to understand the tendance (mathematic derivation) to preview the flight. Hiding how FBW ( in normal law) elevator is a math integration of the side stick position or pressure, and Airbus THS is a math integration of elevator are pedagogic holes, with heavy issues for flight safety. The requested level of math is elementary math. Piloting anticipates the plane 's movement, you need to know the tendance.
Join Date: Jun 2008
Location: Cambridge UK
Posts: 192
Likes: 0
Received 0 Likes
on
0 Posts
re: derivation
roulishollandais
Do you know or does anybody knows where Prandtl's closed curves can be found on the web?
... I had these precious books for all the studied profiles, but they have been stolen
Sounds interesting, what were the books called?
Do you know or does anybody knows where Prandtl's closed curves can be found on the web?
... I had these precious books for all the studied profiles, but they have been stolen
Sounds interesting, what were the books called?
Join Date: Jun 2011
Location: france
Posts: 760
Likes: 0
Received 0 Likes
on
0 Posts
Prandtl
@Peter H, others
I no more remember the tittle of the books.
They are written in German, the tittle could be something like General aerodynamic works (Generale aerodynamischen Studien ???)
As the author is now well known some German University have probably the original books recovered with dust.Perhaps the name of Prandtl was only inside. The books were green middle sized, , perhaps 100-200 pages for each book.
I already searched on the web some of the curves, but without success. I did not tried to find books again. Mine were in the library from my ornthologist oncle and I was surprised to find them after he died. Nobody in the family was interested,they seemed not modern science ! For me it was a treasure to teach my new pilots.
Very pedagogic works for today pilots.. and for aviation administration, BEA, etc. who seem to have very raw and low documentation about stall (AF447 final report !!)
Decade after decade the knowledge taught in the books was poorer, often false in french litterature, included to teach airline pilots and ENAC APTL preparation... And finaly really ignorant airline pilots diving in the Ocean.
I no more remember the tittle of the books.
They are written in German, the tittle could be something like General aerodynamic works (Generale aerodynamischen Studien ???)
As the author is now well known some German University have probably the original books recovered with dust.Perhaps the name of Prandtl was only inside. The books were green middle sized, , perhaps 100-200 pages for each book.
I already searched on the web some of the curves, but without success. I did not tried to find books again. Mine were in the library from my ornthologist oncle and I was surprised to find them after he died. Nobody in the family was interested,they seemed not modern science ! For me it was a treasure to teach my new pilots.
Very pedagogic works for today pilots.. and for aviation administration, BEA, etc. who seem to have very raw and low documentation about stall (AF447 final report !!)
Decade after decade the knowledge taught in the books was poorer, often false in french litterature, included to teach airline pilots and ENAC APTL preparation... And finaly really ignorant airline pilots diving in the Ocean.
Last edited by roulishollandais; 31st May 2013 at 03:48.
Join Date: Jun 2008
Location: Cambridge UK
Posts: 192
Likes: 0
Received 0 Likes
on
0 Posts
re: Prandtl
roulishollandais, you said books. Are we looking for several books, or several volumes belonging to a single title?
How many books were there?
FYI a world-wide library search for books in German written by L Prandtl can be found at:
Results for 'au:L Prandtl' > 'German' [WorldCat.org]
Do any of these titles trigger your memory?
Peter H
From the limited information so far -- and not having any German -- I'll just observe that Prandtl published a multi-volume work in German called
Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen.
... which does seem to have the green covers you mention.
Text from [all?] four volumes can be found at:
http://tinyurl.com/kgk3c6j
http://tinyurl.com/ljws7un
http://tinyurl.com/kubffpc
http://tinyurl.com/lhl7v4r
... is this of any interest?
How many books were there?
FYI a world-wide library search for books in German written by L Prandtl can be found at:
Results for 'au:L Prandtl' > 'German' [WorldCat.org]
Do any of these titles trigger your memory?
Peter H
From the limited information so far -- and not having any German -- I'll just observe that Prandtl published a multi-volume work in German called
Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen.
... which does seem to have the green covers you mention.
Text from [all?] four volumes can be found at:
http://tinyurl.com/kgk3c6j
http://tinyurl.com/ljws7un
http://tinyurl.com/kubffpc
http://tinyurl.com/lhl7v4r
... is this of any interest?
Last edited by Peter H; 31st May 2013 at 12:54. Reason: still getting refs to pdfs correct
Join Date: Jun 2011
Location: france
Posts: 760
Likes: 0
Received 0 Likes
on
0 Posts
@Peter H
Thank you very much !
My books effectively are
"Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen"
Different "Lieferung" !
Congratulations for all these references I will read again 30 years after I discovered them (I was only begining to feel me allowed to have a critical analysis of official aerodynamics theory)
Without surprise I already found something wrong
we discussed in the Thread about Lift !
We have to sort what is worth from these old studies who were forgiven with dispersion of german scientists : someone had to fly away nazism, others have been made prisoners and worked on other subjects.
Thank you very much !
My books effectively are
"Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen"
Different "Lieferung" !
Congratulations for all these references I will read again 30 years after I discovered them (I was only begining to feel me allowed to have a critical analysis of official aerodynamics theory)
Without surprise I already found something wrong
we discussed in the Thread about Lift !We have to sort what is worth from these old studies who were forgiven with dispersion of german scientists : someone had to fly away nazism, others have been made prisoners and worked on other subjects.
Last edited by Jetdriver; 2nd Jun 2013 at 06:29.
Join Date: Jun 2008
Location: Cambridge UK
Posts: 192
Likes: 0
Received 0 Likes
on
0 Posts
Ergebnisse der Aerodynamischen Versuchsanstalt zu Göttingen
roulishollandais,
A pleasure to re-unite you. I'll also mention a couple of web sites
that you might find of interest.
The open-acess archives of the university of Goettingen:
Open Access
The vintage gliders bookcases of Scale Soaring UK:
Scale Soaring UK - Book Case 2
A pleasure to re-unite you. I'll also mention a couple of web sites
that you might find of interest.
The open-acess archives of the university of Goettingen:
Open Access
The vintage gliders bookcases of Scale Soaring UK:
Scale Soaring UK - Book Case 2
Last edited by Jetdriver; 2nd Jun 2013 at 06:28.
Join Date: Sep 2002
Location: Hampshire
Posts: 113
Likes: 0
Received 0 Likes
on
0 Posts
A few thoughts on CL and stall
Sometimes it helps to use the equations:
Lift = nW = 0.5 * density * wing area * speed^2 * CL
where n = load factor (number of g) and W = weight = mass x gravitational acceleration
Put simply, to sustain flight at a given load factor CLmax determines the minimum speed you must have. In the equation above you can use either TAS with actual air density or EAS with sea-level density. The relationship between speed and g is thus:
n proportional to V^2.
This is plotted on a V-n diagram; google for one if you need reminding!
CLmax is a factor of AOA and wing configuration, nothing else. It is the maximum CL the wing can produce and it occurs at the critical AOA specific to each configuration (i.e. flap and slat settings). However, what happens if you exceed the critical AOA varies massively depending on the aircraft: most GA aircraft will just pitch down a bit or "porpoise", perhaps with a bit of wing drop. Deltas and highly swept wing aircraft tend to increase sink rate and not much else. Many twin propeller aircraft and some swept wing jets will flick-roll on to their back and try to kill you, hence the need for a stick-pusher. But the common factor is that for every aircraft there is an AOA above which no more CL is available, so to sustain non-stalled flight you must reduce AOA.
CL vs AOA graphs can vary hugely with wing shape, even for an identical wing section, due to complexities such as spanwise flow. However, in simple terms the critical AOA is the point at which there is a reversal of the gradient of CL with AOA. On some aircraft the change is gentle, on some it is severe. The worst I know of is the F104 Starfighter, where the reduction of CL was extremely rapid above the critical AOA, earning the aircraft the nickname of "widowmaker" because a stall during the finals turn was often fatal.
When looking at the whole aircraft, remember that if it is longitudinally stable in pitch without a FBW computer helping you, then there is a down-force on the horizontal stabiliser. Thus for >1g flight the down-force will increase, needing greater CL from the wing to achieve a given load factor than if there was no tailplane. To avoid the pilot having to think about this, stall is determined by AOA. After all, even simple GA aircraft stall warners are activated by AOA exceeding a set angle, thus moving the vane on the leading edge.
Hope this helps, or at leasts adds to the discussion!
Lift = nW = 0.5 * density * wing area * speed^2 * CL
where n = load factor (number of g) and W = weight = mass x gravitational acceleration
Put simply, to sustain flight at a given load factor CLmax determines the minimum speed you must have. In the equation above you can use either TAS with actual air density or EAS with sea-level density. The relationship between speed and g is thus:
n proportional to V^2.
This is plotted on a V-n diagram; google for one if you need reminding!
CLmax is a factor of AOA and wing configuration, nothing else. It is the maximum CL the wing can produce and it occurs at the critical AOA specific to each configuration (i.e. flap and slat settings). However, what happens if you exceed the critical AOA varies massively depending on the aircraft: most GA aircraft will just pitch down a bit or "porpoise", perhaps with a bit of wing drop. Deltas and highly swept wing aircraft tend to increase sink rate and not much else. Many twin propeller aircraft and some swept wing jets will flick-roll on to their back and try to kill you, hence the need for a stick-pusher. But the common factor is that for every aircraft there is an AOA above which no more CL is available, so to sustain non-stalled flight you must reduce AOA.
CL vs AOA graphs can vary hugely with wing shape, even for an identical wing section, due to complexities such as spanwise flow. However, in simple terms the critical AOA is the point at which there is a reversal of the gradient of CL with AOA. On some aircraft the change is gentle, on some it is severe. The worst I know of is the F104 Starfighter, where the reduction of CL was extremely rapid above the critical AOA, earning the aircraft the nickname of "widowmaker" because a stall during the finals turn was often fatal.
When looking at the whole aircraft, remember that if it is longitudinally stable in pitch without a FBW computer helping you, then there is a down-force on the horizontal stabiliser. Thus for >1g flight the down-force will increase, needing greater CL from the wing to achieve a given load factor than if there was no tailplane. To avoid the pilot having to think about this, stall is determined by AOA. After all, even simple GA aircraft stall warners are activated by AOA exceeding a set angle, thus moving the vane on the leading edge.
Hope this helps, or at leasts adds to the discussion!
Join Date: Jun 2011
Location: france
Posts: 760
Likes: 0
Received 0 Likes
on
0 Posts
free / open / shared information
@Peter H
Thank you for these two very inteesting new links.
The open-acess archives of the university of Goettingen: Open Access
The vintage gliders book cases of Scale Soaring UK: Scale Soaring UK - Book Case2
They are both in that open access/sharing/published information movement ICAO choosed to increase Flying skill and freedom of trade.
Since some decades someone would like to do secrets with these informations. Sometimes patent Offices accept to privatize already public knowledge and traditional culture or names...
I hope that in the future ICAO after WWII spirit will be stronger again, helped by the web and communities like PPRuNe against today's tendancy to hide information : CVR after crash, algorithms of flight laws, name of report writer ; the latter shows that such secrets don't protect writer's/searcher's/inventor's rights. The real State and Defence Secrets are better protected when comunication rules are not the result of obscurity and tiranny.
Civil aviation development is a Precious Gift of the Military world.
Thanks so much for sharing here and in the shadow,
Thank you for these two very inteesting new links.
The open-acess archives of the university of Goettingen: Open Access
The vintage gliders book cases of Scale Soaring UK: Scale Soaring UK - Book Case2
They are both in that open access/sharing/published information movement ICAO choosed to increase Flying skill and freedom of trade.
Since some decades someone would like to do secrets with these informations. Sometimes patent Offices accept to privatize already public knowledge and traditional culture or names...
I hope that in the future ICAO after WWII spirit will be stronger again, helped by the web and communities like PPRuNe against today's tendancy to hide information : CVR after crash, algorithms of flight laws, name of report writer ; the latter shows that such secrets don't protect writer's/searcher's/inventor's rights. The real State and Defence Secrets are better protected when comunication rules are not the result of obscurity and tiranny.
Civil aviation development is a Precious Gift of the Military world.
Thanks so much for sharing here and in the shadow,
