Tailplane lift
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rrr
That's because you are going one way down the control path, I'm going the other!
Let me quote again from my "Bible" - Dick Shevell's book (p.315 in Iss 2)
"Stick fixed stability means that the pilot maintains the control column in a fixed position so that the control surfaces, specifically the elevators, are not permitted to move as the airplane changes its angle of attack. Stick-free stability describes the value of dCm/dCL with the elevators assuming the position, at each airplane angle of attack, for which the elevator aerodynamic hinge moment is zero. This will be the angle at which the elevators will float with no control force. Stick free stability is important for airplanes with direct cable control of the surface and/or its control tabs mounted on the surface. Aircraft with full irreversible powered controls will have stick fixed stability even when the pilot is flying "hands off" provided the pilot has trimmed the column to zero force at the initial flying speed"
In other words stick free stability depends on the feedback from control aerodynamics, whereas you, I think, are talking about the direct yoke/control surface path? Your B707 would have had stick free stability, but not the B747. And on my definition, stick free stability is meaningless on the A320 even in direct law, because it still uses irreversible powered controls.
On aircraft without any artificial feel (any left? - well maybe your Cherokee and similar) the control forces felt at the column would be proportional to the aerodynamic hinge moment on the control, but otherwise you get what the designer thinks you want (which I agree may not be the same as you actually do want
)
That's odd because, from B707s ({#1} no hydraulic power controls) to 747s, the control yoke has always caused the same deflection in the flight control surface position despite any aerodynamic changes. The effort to move the yoke may have changed due to the artificial "feel" (Q pot on the 707; - Air Data Computer adjustment to spring stiffness on others), but we always knew how much the control surface was displaced by simply looking at the yoke (similar to the power steering on your car).
Let me quote again from my "Bible" - Dick Shevell's book (p.315 in Iss 2)
"Stick fixed stability means that the pilot maintains the control column in a fixed position so that the control surfaces, specifically the elevators, are not permitted to move as the airplane changes its angle of attack. Stick-free stability describes the value of dCm/dCL with the elevators assuming the position, at each airplane angle of attack, for which the elevator aerodynamic hinge moment is zero. This will be the angle at which the elevators will float with no control force. Stick free stability is important for airplanes with direct cable control of the surface and/or its control tabs mounted on the surface. Aircraft with full irreversible powered controls will have stick fixed stability even when the pilot is flying "hands off" provided the pilot has trimmed the column to zero force at the initial flying speed"
In other words stick free stability depends on the feedback from control aerodynamics, whereas you, I think, are talking about the direct yoke/control surface path? Your B707 would have had stick free stability, but not the B747. And on my definition, stick free stability is meaningless on the A320 even in direct law, because it still uses irreversible powered controls.
On aircraft without any artificial feel (any left? - well maybe your Cherokee and similar) the control forces felt at the column would be proportional to the aerodynamic hinge moment on the control, but otherwise you get what the designer thinks you want (which I agree may not be the same as you actually do want
)
Last edited by Owain Glyndwr; 20th May 2012 at 11:16.
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Wait, Now thinking I guess that certification requirements must account for failure of the FCS. Is that so? The aft CG limit of an A320 or a B777 is determined because of the direct law possibility?
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It can be so for certain high performance a/c but not air transport category.
Failure modes have to be taken into account. Unfortunately, things go wrong.
In an ideal world all a/c would be designed with all surfaces lifting (Canard?) but we are not in that world.
Failure modes have to be taken into account. Unfortunately, things go wrong.
In an ideal world all a/c would be designed with all surfaces lifting (Canard?) but we are not in that world.
Don't start me off on canards - they have their disadvantages
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Microburst,
As you probably guessed I am (was) an engineer, so perhaps not the best person to judge whether Shevell's book is apt for non-engineers. Dick (I met him once) was Chief Aerodynamicist at Douglas and then went to Stanford, so his book is full of realistic data. Looking through it it seems to be about 95% good, understandable text and explanations, but some maths is inevitable - only you can judge if it is too much, but I think you could just take the maths for granted and read the text.
Anyway, it is : Fundamentals of Flight, by Richard S Shevell, published by Prentice Hall, ISBN 0-13-339060-8. I don't know whether it is still in print though, and I doubt you will get a .pdf version - it is over 400 pages.
As you probably guessed I am (was) an engineer, so perhaps not the best person to judge whether Shevell's book is apt for non-engineers. Dick (I met him once) was Chief Aerodynamicist at Douglas and then went to Stanford, so his book is full of realistic data. Looking through it it seems to be about 95% good, understandable text and explanations, but some maths is inevitable - only you can judge if it is too much, but I think you could just take the maths for granted and read the text.
Anyway, it is : Fundamentals of Flight, by Richard S Shevell, published by Prentice Hall, ISBN 0-13-339060-8. I don't know whether it is still in print though, and I doubt you will get a .pdf version - it is over 400 pages.
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TURIN....
I have to completely disagree with your stance. Not to be personal but your attitude contributes to the lack of scientific literacy throughout North America and many other places. I can't stand people who justify using ambiguous and/or incorrect terminology or explanations, because in reality they don't understand the topic themselves. Frankly, if you don't understand it, you shouldn't be the one telling everyone how it should be described! You should look into Richard Feynman - he was a physicist who is known for his excellent explanations yet he also shares the same point of view as I do on this matter. If you don't want to listen to me, at least listen to a man who definitely had a good idea of what he was talking about. [/rant]
To add onto what John Tulla posted, for those of you who still don't quite understand/believe that a tail almost always will produce downlift, look at this video at 0:40.
"Most people find mathematics a very useful common language in these circumstances. It introduces a discipline that avoids the sort of confusion etc etc. "
I have to disagree. Most people use the spoken word to explain anything. Mathematics confuses almost everybody at one level or another. See above.
I have to disagree. Most people use the spoken word to explain anything. Mathematics confuses almost everybody at one level or another. See above.
To add onto what John Tulla posted, for those of you who still don't quite understand/believe that a tail almost always will produce downlift, look at this video at 0:40.
Oh dear! Italia 458, As I'm not from North America I can't say either way if your rant is accurate our not.
However, one thing is certain. You need to settle down. My quoted post was meant to be light hearted. Life is too short. If you really can't stand me because I don't use high level maths to understand/explain a simple opening post such as the one above that is your problem. I'll stick to normal, conversational English if you don't mind. I seem to hang on to more friends that way. Be well.
However, one thing is certain. You need to settle down. My quoted post was meant to be light hearted. Life is too short. If you really can't stand me because I don't use high level maths to understand/explain a simple opening post such as the one above that is your problem. I'll stick to normal, conversational English if you don't mind. I seem to hang on to more friends that way. Be well.
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TURIN...
What exactly do you mean by 'light hearted'? Is that supposed to mean that you actually didn't mean a word of what you said?
I never asked you to use "high level maths". But what I would ask is that you use "accurate" descriptions and examples to explain something. Math is a good way to do that - I suggest you learn a little bit sometime!
Hmm... that attitude seems to go hand-in-hand with the attitude that you should explain everything as simply as possible, even if it's not correct. So if your friends are wrong about something you just disregard it? My friends are generally happy to have an engaging discussion and so I don't have a problem with pointing out a potential error in our discussion.
My quoted post was meant to be light hearted.
If you really can't stand me because I don't use high level maths...
I'll stick to normal, conversational English if you don't mind. I seem to hang on to more friends that way.
Like I said. Life's too short to get into pointless discussion such as this. I enter into pprune to learn and hopefully pass on some of my experience that others may also learn. If you want to get into a fight I suggest you join a gymnasium. I'm not interested.
Good night.
Good night.
Microburst2002, Owain Glyndwr;
Fascinating thread - thank you for this interesting exchange between the two of you. Microburst2002, I can highly recommend Owain's contributions on the AF447 threads. You can use mm43's PPRuNe search tool to search/find anything in PPRuNe.
I searched for a pdf copy of Shevell's work and while doing so I found the following related pdf on Aircraft Design - Synthesis and Analysis. One of the authors is Ilan Kroo, Professor of Aerodynamics and Astronautics at Stanford but the introduction states, interestingly:
Fascinating thread - thank you for this interesting exchange between the two of you. Microburst2002, I can highly recommend Owain's contributions on the AF447 threads. You can use mm43's PPRuNe search tool to search/find anything in PPRuNe.
I searched for a pdf copy of Shevell's work and while doing so I found the following related pdf on Aircraft Design - Synthesis and Analysis. One of the authors is Ilan Kroo, Professor of Aerodynamics and Astronautics at Stanford but the introduction states, interestingly:
Richard Shevell was the original author of several of these chapters. He worked in aerodynamics and design at Douglas Aircraft Company or 30 years, was head of advanced design during the development of the DC-9 and DC-10, and taught at Stanford University after that for 20 years. To a large extent, this is his course.
Last edited by PJ2; 26th May 2012 at 15:53.
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Microburst,
If "you guys" are PJ2 and myself, I was referring to the alternative source of material similar to that contained in Shevell's book (which you were interested in obtaining) and which can be found at the URL PJ2 gave.
If "you guys" are PJ2 and myself, I was referring to the alternative source of material similar to that contained in Shevell's book (which you were interested in obtaining) and which can be found at the URL PJ2 gave.
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Owain
Thankyou very much for your message, i received it well.
I just wanted to humoristically thankyou two for it without "incriminating" myself for breaching some rule
But it didnt go well...
Thankyou very much for your message, i received it well.
I just wanted to humoristically thankyou two for it without "incriminating" myself for breaching some rule
But it didnt go well...
I understand the tailplane lift downwards for stability purposes, but for manoeuvring, or for any other reason, would tailplane lift ever be upwards in flight on your typical FAR 23 or 25 aircraft - Airbus/Boeing computerised relaxed stability aside. I say in flight, because I know about lifting the tail of a tail dragger on take off.
Thanks Gals/Guys.
Thanks Gals/Guys.
megan,
Is short, no. It's a design requirement that full elevator in available up till Vmo without overstressing the airframe, and that would not be sufficient to generate positive lift on the tail.
BTW, it isn't the negative lift that is needed for stability- just that the lift component of the tail changes sufficiently with changes of AofA to ensure a corrective moment, while not stalling.
It is the last bit that means that most tails create negative lift, but it is not an actual requirement.
Is short, no. It's a design requirement that full elevator in available up till Vmo without overstressing the airframe, and that would not be sufficient to generate positive lift on the tail.
BTW, it isn't the negative lift that is needed for stability- just that the lift component of the tail changes sufficiently with changes of AofA to ensure a corrective moment, while not stalling.
It is the last bit that means that most tails create negative lift, but it is not an actual requirement.
I realise it's not a requirement, but as wondering if any aircraft actually produced positive (upward) lift on the tailplane in order to reach its negative "g" limit as per its V-n diagram.