Vertical stabliser
Thread Starter
Vertical stabliser
To those who might know: given the move towards twin engines, does the need to cope with an engine out situation mean that the VS is now disproportionally larger than it used to be and does this then lead to a greater risk of over stressing the VS if the dynamics go off-nominal ?
Join Date: Jan 2005
Location: France
Posts: 2,315
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Mr Optimistic
To those who might know: given the move towards twin engines, does the need to cope with an engine out situation mean that the VS is now disproportionally larger than it used to be...
....and does this then lead to a greater risk of over stressing the VS if the dynamics go off-nominal ?
It's all a matter of design criteria, SOPs, and a few other factors.
You can pull the wings off an airliner, if you want to try...
And you can overstress the VS on an eight-engined aircraft.
There is a famous photo of a B-52 minus nearly all the VS, and they actually brought it back.
CJ
I'll try a short answer 
Its a moment arm relationship that sets the size.
Consider the thrust loss for a outboard wing engine vs the distance ouboard from the fusealage. Now size the fin-rudder combination to handle this at the slowest manuevering speed.
Since to control the moment of an engine out you need aerodynamic forces against the fin and rudder, then you size the rudder stiffness and attachment strength to keep the stresses within acceptable range.
As engines increased in thrust so did fins increase in size. As fins increased in size and thus aerodybnamic forces their stiffness and stresses remain about the same due to clever structural design.
Its a moment arm relationship that sets the size.
Consider the thrust loss for a outboard wing engine vs the distance ouboard from the fusealage. Now size the fin-rudder combination to handle this at the slowest manuevering speed.
Since to control the moment of an engine out you need aerodynamic forces against the fin and rudder, then you size the rudder stiffness and attachment strength to keep the stresses within acceptable range.
As engines increased in thrust so did fins increase in size. As fins increased in size and thus aerodybnamic forces their stiffness and stresses remain about the same due to clever structural design.
Join Date: Jan 2005
Location: France
Posts: 2,315
Likes: 0
Received 0 Likes
on
0 Posts
lomapaseo,
I mostly agree...
But the usual design case for the size of the fin is the one-engine-out case at V1. Nearly the same as two-engines-out on a four-engined aircraft: you lose all thrust on one side, and you need a bigger fin....
I still remember wondering about the proportionally bigger fins on the A300 and A310... until I got my mind round that.
CJ
I mostly agree...
But the usual design case for the size of the fin is the one-engine-out case at V1. Nearly the same as two-engines-out on a four-engined aircraft: you lose all thrust on one side, and you need a bigger fin....
I still remember wondering about the proportionally bigger fins on the A300 and A310... until I got my mind round that.
CJ
Christiaanj
Yes I believe you're right. I was trifle sloppy and mixing it up with some other stuff in my brain.
Well we'll just have to see if we did answer the original posters question in only two replies.
Yes I believe you're right. I was trifle sloppy and mixing it up with some other stuff in my brain.
Well we'll just have to see if we did answer the original posters question in only two replies.
Join Date: Jun 2009
Location: ATL
Age: 67
Posts: 131
Likes: 0
Received 0 Likes
on
0 Posts
As opposed to four engines. Losing number 1 out of four, offhand, would require a larger VS, than losing 1 out of two, because the moment arm is greater on the four engine airplane. But this is alleviated on some aircraft by reducing power, 50%, on number four. You still have more than half the total thrust left. On the twin, you cant reduce power on number 2, because you've already lost half. Hence the larger tail.
During statistical aircraft sizing, the engine out yawing moment is balanced by a certain historical value of vertical tail volume coefficient,
(Vv = lv Sv / b Sw)
Where Vv is the tail volume coefficient and will be in a statistical range.
lv is the distance from the cg to the a.c. of the tail
Sv is the area of the VS
b is wingspan
and Sw is wing area.
As the design progresses, and more information on engine thrust becomes available from engine development, the VS tail size becomes locked in to balancing that engine out moment, allowing for future growth.
During statistical aircraft sizing, the engine out yawing moment is balanced by a certain historical value of vertical tail volume coefficient,
(Vv = lv Sv / b Sw)
Where Vv is the tail volume coefficient and will be in a statistical range.
lv is the distance from the cg to the a.c. of the tail
Sv is the area of the VS
b is wingspan
and Sw is wing area.
As the design progresses, and more information on engine thrust becomes available from engine development, the VS tail size becomes locked in to balancing that engine out moment, allowing for future growth.
Moderator
Nearly the same as two-engines-out on a four-engined aircraft: you lose all thrust on one side, and you need a bigger fin....
.. or you accept the usually rather higher Vmca2 ?
because the moment arm is greater on the four engine airplane.
.. but the moment involves the arm and the thrust .. the four motor bird doesn't usually have the engine thrust of the two motor bird
But this is alleviated on some aircraft by reducing power
Certification requirements usually don't allow any thrust reductions ?
You still have more than half the total thrust left.
.. except the concern is T-D, not just thrust ?
.. or you accept the usually rather higher Vmca2 ?
because the moment arm is greater on the four engine airplane.
.. but the moment involves the arm and the thrust .. the four motor bird doesn't usually have the engine thrust of the two motor bird
But this is alleviated on some aircraft by reducing power
Certification requirements usually don't allow any thrust reductions ?
You still have more than half the total thrust left.
.. except the concern is T-D, not just thrust ?
Join Date: Jun 2009
Location: ATL
Age: 67
Posts: 131
Likes: 0
Received 0 Likes
on
0 Posts
but the moment involves the arm and the thrust .. the four motor bird doesn't usually have the engine thrust of the two motor bird
Vmca2 for losing two on a side is usually over 200 knots. Not considered for the takeoff case.
Moderator
Vmca2 for losing two on a side is usually over 200 knots. Not considered for the takeoff case.
precisely my point .. so the double failure is not a principal driver for fin sizing.
precisely my point .. so the double failure is not a principal driver for fin sizing.
Join Date: Jun 2009
Location: ATL
Age: 67
Posts: 131
Likes: 0
Received 0 Likes
on
0 Posts
With you John. Will have to check ferry takeoff for 3 eng T.O. out of four. Now we only consider at light weight, but can't remember if we retard number 4 say, or just take the chance 2 will keep running if number 1's dead.
Thread Starter
thanks for the replies, I think
...but is that a 'no' ? Just thought that with a whopping big single engine on one side which is capable of keeping things flying, the increased VS area with 1/2pv^2 and centre of pressure higher above the root might cause the bending moment to increase such that 'clever' design might be necessary at the root (with the suspicion that 'clever' might = 'too clever' and limit design margin in extremis).
Just thought that with a whopping big single engine on one side which is capable of keeping things flying, the increased VS area with 1/2pv^2 and centre of pressure higher above the root might cause the bending moment to increase such that 'clever' design might be necessary at the root (with the suspicion that 'clever' might = 'too clever' and limit design margin in extremis).
Join Date: Jan 2005
Location: France
Posts: 2,315
Likes: 0
Received 0 Likes
on
0 Posts
Mr Optimistic,
Yes, that is a "no".
What ARE you talking about?
A bigger VS, of course you redesign the root to deal with the forces in question.
Of course, it might also become slightly easier to wrench off a bigger VS when flying the aircraft beyond the design limits.
I don't like your attitude. Stop trying to be "clever" and state your case.
CJ
Yes, that is a "no".
... 'clever' design might be necessary at the root (with the suspicion that 'clever' might = 'too clever' and limit design margin in extremis).
A bigger VS, of course you redesign the root to deal with the forces in question.
Of course, it might also become slightly easier to wrench off a bigger VS when flying the aircraft beyond the design limits.
I don't like your attitude. Stop trying to be "clever" and state your case.
CJ
Thread Starter
case stated
I don't have an unlikable attitude or a 'case' (as far as I know). 
Sorry if I caused offence. Seemed a reasonable question as to design implications to me: if everything is as it always was and their have been no design challenges (eg need for special materials to allow the design not to be limited, then fine, please feel free to say so). I am not involved in the industry other than as a paying customer, nor am I Boeing v AB, if that's your suspicion.
Sorry if I caused offence. Seemed a reasonable question as to design implications to me: if everything is as it always was and their have been no design challenges (eg need for special materials to allow the design not to be limited, then fine, please feel free to say so). I am not involved in the industry other than as a paying customer, nor am I Boeing v AB, if that's your suspicion.
Thread Starter
a bit more
CJ, thought some more about your message. Let me explain. The first aircraft I ever got on was a Bae 1-11 in ~1964. Simple looking thing, had to look to see the engines. Now an aircraft from the front is dominated by huge gaping engines and the wings are covered in slats, flaps, winglets, boundary layer augmenting bits etc. The cockpit is full of electronics and computers intermediate between the man and the machine. The VS is still the same old single sticky-up thing. Why no apparent innovation there ? The answers given above I think are to the effect that no innovation was necessary.
Join Date: Jun 2009
Location: ATL
Age: 67
Posts: 131
Likes: 0
Received 0 Likes
on
0 Posts
To answer your original question - negative. Design loads would have been established and stress would have designed the structure to with stand them with the required margins. It's not unusual for manufacturers to increase those margins slightly when introducing new technologies, like composites say. Though I didn't or read the accidents reports for the New York Airbus fin failure. I would guess they found that the rudder was commanded through side-to-side oscillations setting up a force hysteresis loop which increased the loads. Don't know if they allowed for that in design.
Moderator
ClippedCub has the basic story -
(a) an aircraft should be designed according to well understood principles while, at the same time, being able to comply with the Design Standards (for certification) or being suitable for negotiating concessions against the Standards
(b) weight and cost are critical
(c) no OEM is going to overdesign structure for an extreme event outside the certification process unless there is a very good reason to do so
(d) there will be rare events outside the Design Standards ambit where a structure is overwhelmed. On such occasions, and with the benefit of hindsight, one was well placed staying in bed and missing the flight.
Overall, the main risk, still, is in getting to and from the airport .... (airline food is another matter, perhaps)
(a) an aircraft should be designed according to well understood principles while, at the same time, being able to comply with the Design Standards (for certification) or being suitable for negotiating concessions against the Standards
(b) weight and cost are critical
(c) no OEM is going to overdesign structure for an extreme event outside the certification process unless there is a very good reason to do so
(d) there will be rare events outside the Design Standards ambit where a structure is overwhelmed. On such occasions, and with the benefit of hindsight, one was well placed staying in bed and missing the flight.
Overall, the main risk, still, is in getting to and from the airport .... (airline food is another matter, perhaps)
Join Date: Nov 2006
Location: SoCalif
Posts: 896
Likes: 0
Received 0 Likes
on
0 Posts
A rudder with enough authority to compensate for a failed wing engine has too much authority when it goes wrong, as in the 737, or in AA-587.
Unfortunately, they are not making Trimotors anymore. I never have understood the economics of two huge engines and attendant extra system redundancies and huge rudder, vs. 3 engines 2/3 the size.
GB
Unfortunately, they are not making Trimotors anymore. I never have understood the economics of two huge engines and attendant extra system redundancies and huge rudder, vs. 3 engines 2/3 the size.
GB