Aerodynamicists Question
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I'm still struggling to comprehend how smart people get something so simple, so wrong!
You are talking about when the wing stalls. The others are talking about the relationship between the stall and the aircraft's weight/airspeed - and the importance of understanding those relationships.
Another relationship regarding the stall is that associated with angle of bank/load factor.
Every textbook on aerodynamics emphasises those relationships and I think you will find that every syllabus of instruction related to flight principles also insists on some coverage of those relationships.
Important for every pilot to know these relationships? What do you think?
Moderator
I normally don't come back into a thread which goes wildly off the rails but this one is a tad frightening (and I mean that quite sincerely) in respect of standards and (a dreadful level of) general knowledge ..
Some observations -
(a) You can stall at high speed, you can stall at low speed, you stall anytime you exceed the critical angle of attack.
Certification (ie Flight Manual) stall data is for a slow speed (max 1 kt/sec) speed reduction to stall with specific trim conditions, CG, etc., and, as such, is a not all that relevant to real world operational risks. However, to a reasonably practical (and conservative) accuracy, a "normal" low speed stall will/should approximate the certification figures.
High(er) speed stalls fall into at least two categories -
(i) an accelerated stall where the load factor provides a higher speed observed at departure - higher load factor works much the same as a higher weight
(ii) for very high pitch rates (recollection is 70-80 deg/sec or more - there was an interesting RAeS paper on the topic several years ago), quite a different situation where a leading edge vortex forms and significantly higher angles than "normal" are achievable - generally not a problem on fixed wing but may be a concern with some of those devices which thrash the air into submission (humble apologies to JE, NL, et al)
(b) An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed
I am a tad confused by this .. if we are at stall angle (and, as you have observed, stall angle is a pretty important consideration in stalling) how do we then fly at a higher angle just because we have a higher weight (other than by generating a very high pitch rate as described in (a) (ii), above .. and that is probably quite irrelevant) ? .. or have I missed some underlying and important physical principle somewheres along the way ... ?
(c) Cl varies with Reynolds number ie with rho, V and chord (a big compnent of S) and that variation is different for different airfoils.
Now if the reduction in stall speed is achieved with wing extension then we have a change in aspect ratio; whole new sets of things to play with
.. all of which is interesting and factual ... but quite irrelevant to the present discussion on weight and speed variations at constant altitude with a constant configuration
(d) You're an INSTRUCTOR ? .. and following impertinent comments ..
I might have been a tad more polite than our contributor ... but the underlying sentiment so expressed was not too far short of the mark, I suggest
(e) FAR 25.333 gives a nice picture of the low speed stall lines. For a certification style of stall you just can't operate outside the envelope in the curvy sections as they represent the stall condition. For interest only, I guess ?
Some observations -
(a) You can stall at high speed, you can stall at low speed, you stall anytime you exceed the critical angle of attack.
Certification (ie Flight Manual) stall data is for a slow speed (max 1 kt/sec) speed reduction to stall with specific trim conditions, CG, etc., and, as such, is a not all that relevant to real world operational risks. However, to a reasonably practical (and conservative) accuracy, a "normal" low speed stall will/should approximate the certification figures.
High(er) speed stalls fall into at least two categories -
(i) an accelerated stall where the load factor provides a higher speed observed at departure - higher load factor works much the same as a higher weight
(ii) for very high pitch rates (recollection is 70-80 deg/sec or more - there was an interesting RAeS paper on the topic several years ago), quite a different situation where a leading edge vortex forms and significantly higher angles than "normal" are achievable - generally not a problem on fixed wing but may be a concern with some of those devices which thrash the air into submission (humble apologies to JE, NL, et al)
(b) An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed
I am a tad confused by this .. if we are at stall angle (and, as you have observed, stall angle is a pretty important consideration in stalling) how do we then fly at a higher angle just because we have a higher weight (other than by generating a very high pitch rate as described in (a) (ii), above .. and that is probably quite irrelevant) ? .. or have I missed some underlying and important physical principle somewheres along the way ... ?
(c) Cl varies with Reynolds number ie with rho, V and chord (a big compnent of S) and that variation is different for different airfoils.
Now if the reduction in stall speed is achieved with wing extension then we have a change in aspect ratio; whole new sets of things to play with
.. all of which is interesting and factual ... but quite irrelevant to the present discussion on weight and speed variations at constant altitude with a constant configuration
(d) You're an INSTRUCTOR ? .. and following impertinent comments ..
I might have been a tad more polite than our contributor ... but the underlying sentiment so expressed was not too far short of the mark, I suggest
(e) FAR 25.333 gives a nice picture of the low speed stall lines. For a certification style of stall you just can't operate outside the envelope in the curvy sections as they represent the stall condition. For interest only, I guess ?
I love it. Green Goblin & rmcdonal (who?) correcting Mr Tullamarine on a point of P of F. Can't work out whether it's funny or tragic. But it's certainly priceless, either way.
*Okay, going to throw yet another spanner in the works, even the stalling angle will vary with air density & speed so even the statement of an aerofoil stalls when its angle of attack exceeds a fixed angle isn't quite correct. For practical purposes it is though.
GG
By failing to adequately read the posters actual query you have started a completely pointless technical spat!!
The poster referred to a situation where the aerodynamic qualities of an airfoil had been altered to change the critical AoA. He simply wanted to know how to derive the new weight which would result in the same stalling parameters as the unmodified aerofoil!!
By failing to adequately read the posters actual query you have started a completely pointless technical spat!!
The poster referred to a situation where the aerodynamic qualities of an airfoil had been altered to change the critical AoA. He simply wanted to know how to derive the new weight which would result in the same stalling parameters as the unmodified aerofoil!!
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Ohhhhh! My head hurts!
Dr
Dr
Dr. FYI there's another thread currently running with information where you can get some medical relief from this condition, albiet not the legal kind from a chemist.
Quote:
Originally Posted by The Green Goblin
An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed.
So initially, you said weight had zilch to do with it, but now you say it does.
Originally Posted by The Green Goblin
An increased weight will result in an increased Vs simply because we require a larger angle of attack to generate the additional lift for a given airspeed.
So initially, you said weight had zilch to do with it, but now you say it does.
JT - you normally don't get catty.
On occasion, technical incompetence of such exquisite proportion requires respectful acknowledgement.
On occasion, technical incompetence of such exquisite proportion requires respectful acknowledgement.
An Aerofoil stalls at a particular:
A) Airspeed
B) Angle of Attack
C) Combination of airspeed and weight depending on the conditions.
GG
By failing to adequately read the posters actual query you have started a completely pointless technical spat!!
The poster referred to a situation where the aerodynamic qualities of an airfoil had been altered to change the critical AoA. He simply wanted to know how to derive the new weight which would result in the same stalling parameters as the unmodified aerofoil!!
By failing to adequately read the posters actual query you have started a completely pointless technical spat!!
The poster referred to a situation where the aerodynamic qualities of an airfoil had been altered to change the critical AoA. He simply wanted to know how to derive the new weight which would result in the same stalling parameters as the unmodified aerofoil!!
From the PMs I'm getting many people agree
No he wanted to know what speed he would stall at for a given weight. I was merely pointing out that airspeed did not matter, he would stall at a particular angle of attack regardless of airspeed or weight.
Airspeed does matter in this case, because, despite the fact we are all aware that the aerofoil will stall at a particular angle regardless of weight, the original poster wants to know what speed this will occur at in straight and level flight.
There is no need to expressly state that we are talking about straight and level flight because that is implied whenever the term 'stalling speed' is used.
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No he wanted to know what speed he would stall at for a given weight.
His question was perfectly valid and was well answered by other posters.
You're a clown GG.
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glekichi, thanks for your injection of reality and sanity.
Have to agree with the others who imply that GG is about as helpful as a hip pocket in a singlet. Have look at how the people who certified most of the aircraft which we fly have to say on the subject.
V S means the stalling speed or the minimum steady flight speed at which the airplane is controllable.
V S0 means the stalling speed or the minimum steady flight speed in the landing configuration.
V S1 means the stalling speed or the minimum steady flight speed obtained in a specific configuration.
V SR means reference stall speed.
V SRO means reference stall speed in the landing configuration.
V SR1 means reference stall speed in a specific configuration.
V SW means speed at which onset of natural or artificial stall warning occurs.
Of course, the FAA never had GG as an instructor, so they may well be remiss in their definitions. Don't see any mention of critical angle, and how do you measure that in your aircraft? Only ones I have experience on that had AoA all flew off carriers, as in navy floating type.
How many? We need to know how many prats can dance on the point of a pin.
bluesky, yours maybe V SR1. Defer to JT.
Have to agree with the others who imply that GG is about as helpful as a hip pocket in a singlet. Have look at how the people who certified most of the aircraft which we fly have to say on the subject.
V S means the stalling speed or the minimum steady flight speed at which the airplane is controllable.
V S0 means the stalling speed or the minimum steady flight speed in the landing configuration.
V S1 means the stalling speed or the minimum steady flight speed obtained in a specific configuration.
V SR means reference stall speed.
V SRO means reference stall speed in the landing configuration.
V SR1 means reference stall speed in a specific configuration.
V SW means speed at which onset of natural or artificial stall warning occurs.
Of course, the FAA never had GG as an instructor, so they may well be remiss in their definitions. Don't see any mention of critical angle, and how do you measure that in your aircraft? Only ones I have experience on that had AoA all flew off carriers, as in navy floating type.
From the PMs I'm getting many people agree
bluesky, yours maybe V SR1.
Defer to JT.
You have lost me here John. I’m not sure what I have said that is incorrect. Aircraft stall at the critical angle, Vs is just a measurement of that angle based on a set of parameters (S+L, WT, ISA etc).
Hence the reason some aircraft have angle of attack gauges. However as most GA aircraft are not fitted with this kit they use Vs as a guide as to when the aircraft is about to stall.
Yes I know that is not what the topic question was about, however I don’t enjoy being told I’m incorrect when every manual on the topic I hold shows otherwise.
It is possible to stall at any speed, it is the Angle of Attack that determines when it happens.
Hence the reason some aircraft have angle of attack gauges. However as most GA aircraft are not fitted with this kit they use Vs as a guide as to when the aircraft is about to stall.
Yes I know that is not what the topic question was about, however I don’t enjoy being told I’m incorrect when every manual on the topic I hold shows otherwise.
It is possible to stall at any speed, it is the Angle of Attack that determines when it happens.
Let's say you had a current MTOW of 1,000 kg's with a stall speed of 50 knots. Easy round figures to start with.
You somehow lower the stall speed of the aircraft, eg. wing extension, wing-tips, vortex generators, flap modification etc. As a result you have lowered the stall speed by 5 knots, to 45 knots.
I then need to work out what the weight needs to be to stall the aircraft back at 50 knots.
You somehow lower the stall speed of the aircraft, eg. wing extension, wing-tips, vortex generators, flap modification etc. As a result you have lowered the stall speed by 5 knots, to 45 knots.
I then need to work out what the weight needs to be to stall the aircraft back at 50 knots.
Quote:
No he wanted to know what speed he would stall at for a given weight.
So what did you do? You posted an "answer" that had nothing to do with the question.
His question was perfectly valid and was well answered by other posters.
No he wanted to know what speed he would stall at for a given weight.
So what did you do? You posted an "answer" that had nothing to do with the question.
His question was perfectly valid and was well answered by other posters.
You're a clown GG.
Have to agree with the others who imply that GG is about as helpful as a hip pocket in a singlet. Have look at how the people who certified most of the aircraft which we fly have to say on the subject.
Of course, the FAA never had GG as an instructor, so they may well be remiss in their definitions. Don't see any mention of critical angle, and how do you measure that in your aircraft? Only ones I have experience on that had AoA all flew off carriers, as in navy floating type.
Of course, the FAA never had GG as an instructor, so they may well be remiss in their definitions. Don't see any mention of critical angle, and how do you measure that in your aircraft? Only ones I have experience on that had AoA all flew off carriers, as in navy floating type.
So am I wrong Mr Abraham?
Regardless of if an aircraft has or has not got an instrument to measure the AoA, it is still the determining factor in a stalled aerofoil. If you understand the underlying principles then when you approach your minimum steady in flight speed for your particular configuration you are armed with the knowledge of what you can and cannot do in the event of a mishap.
Anyway this topic is starting to become laborious.
Moderator
You have lost me here John
I've reread the earlier posts and acknowledge that I have lumped you (somewhat unfairly) in with The Green Goblin's unhelpful chamaeleon posting pattern.
My humble apologies for being a little heavy handed in my commentary. Your observations are, indeed, typical of pilot level training in the topic. I was a little concerned with, and probably misinterpreted, your apparent emphasis on a simplistic explanation of the thread subject and overreacted by including you in my earlier post.
Every now and again even the most relaxed of us can get just a tad irritated and frustrated with difficult and gratuitously argumentative folk such as The Green Goblin. This was particularly so as I am aware of the original poster's circumstances from a side email discussion with him and the stalling angle distraction was not overly pertinent or helpful to what he was after ...
As a side comment, the usual emphasis on stalling angle is a bit simplistic as the inferred value is associated with a particular set of constraints which may not be applicable in a set of real world circumstances. Likewise, Vs involves a few more considerations than simply being an indication of a specific stalling angle.
Pass, friend.
I've reread the earlier posts and acknowledge that I have lumped you (somewhat unfairly) in with The Green Goblin's unhelpful chamaeleon posting pattern.
My humble apologies for being a little heavy handed in my commentary. Your observations are, indeed, typical of pilot level training in the topic. I was a little concerned with, and probably misinterpreted, your apparent emphasis on a simplistic explanation of the thread subject and overreacted by including you in my earlier post.
Every now and again even the most relaxed of us can get just a tad irritated and frustrated with difficult and gratuitously argumentative folk such as The Green Goblin. This was particularly so as I am aware of the original poster's circumstances from a side email discussion with him and the stalling angle distraction was not overly pertinent or helpful to what he was after ...
As a side comment, the usual emphasis on stalling angle is a bit simplistic as the inferred value is associated with a particular set of constraints which may not be applicable in a set of real world circumstances. Likewise, Vs involves a few more considerations than simply being an indication of a specific stalling angle.
Pass, friend.
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The Green Goblin, about your initial post to this thread you said:
What exactly were you correcting? Do you believe posters were somehow making suggestions contrary to what you initially posted?
My answer was a factual correction.