Vertical stabliser

[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 ?

[ChristiaanJ]

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...Disproportionally, no. Proportionally, yes. ....and does this then lead to a greater risk of over stressing the VS if the dynamics go off-nominal ?No.
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

[lomapaseo]

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.

[ChristiaanJ]

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

[lomapaseo]

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.

[ClippedCub]

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.

[john_tullamarine]

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 ?

[Graybeard]

This is where the twins with tail mounted engines excell.

[ClippedCub]

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

Half the thrust, twice the arm, same moment as losing one on a two, until you retard 4, smaller tail for comparable fuselage length.

Vmca2 for losing two on a side is usually over 200 knots. Not considered for the takeoff case.

[john_tullamarine]

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.

[ClippedCub]

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.

[727gm]

This is where the twins with tail mounted engines excell.

This is where trimotors with tail-mounted engines excel. :)

[john_tullamarine]

This is where trimotors with tail-mounted engines excel.

.. "Check Essential"

[Mr Optimistic]

...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).

[ChristiaanJ]

Mr Optimistic,

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).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

[Mr Optimistic]

I don't have an unlikable attitude or a 'case' (as far as I know). http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/worry.gif 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.

[Mr Optimistic]

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.

[ClippedCub]

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.

[john_tullamarine]

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)

[Graybeard]

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

[411A]

Lets look in the distant past to see some of the other considerations that govern vertical stabilizer size on a swept wing jet transport airplane.

Type, B707 Intercontinental (707-320/436, straight-pipe powered).
There was a definite problem with this airplane regarding the vertical fin size, not driven by the engine out case, but yaw stability.
These aircraft had a parallel type of yaw damper (which needed to be switched OFF for takeoff/landing).
When this airplane was submitted to the British ARB for certification, DP Davies demanded that the rudder power and yaw damper be modified.
The rudder power Q-feel system was re-designed, and the normal 13 inch ventral fin was increased to 39 inches in width, to provide increased yaw stability.
When this series of airplanes were fitted with fan engines later on, Boeing once again redesigned the vertical fin...this time by increasing the height and, a series type full time yaw damper was fitted.
However, in extending the height of the vertical fin, Boeing just barely avoided another problem...IE: too large a vertical fin, and the spiral stability certification standards could not be met.

Aircraft design...a very large compromise, in many areas.

[ClippedCub]

Sounds like those Boeing guys had their hands full. Increasing the height increases the tail aspect ratio giving a steeper lift curve slope and making it more effective. Think Grumman Bearcat. Even with that big prop and short fuselage, it required minimal rudder on takeoff. Tails on the WWI fighters have tiny aspect ratios. They can be deflected to extreme angles without stalling and when they did, it is a gradual break in the lift curve - good things to have in a tail.

[Mr Optimistic]

thanks for the replies (bar one): I am better informed now. Was half expecting someone to suggest it is vital to have a clear surface to paint logo on. Will we never see a 'vee' ?

[ChristiaanJ]

Mr Optimistic,

I presume you're referring to this photo?

http://www.bollywoodsargam.com/jokepics/6331.jpg

Excellent illustration of two totally different VS designs, BTW.

CJ

[Mr Optimistic]

CJ, point taken:)

[hawk37]

When Airbus shortened the 320 series, were there modifications to the VS? Thinking A319 and A318, where now the arm would be smaller.

Or was this all possible due to the magic of FBW?

[Kelly Hopper]

Forget tri-motors, what about the VC 10?

[SincoTC]

However many engines there are in the tail, will we ever see todays huge diameter turbofans mounted that way in a future airliner? Mounted close to the fuselage and back far enough so that their thrust lines could be canted towards the CG, the tailpalne issues would be much reduced; except maybe getting a rough time from reverse thrust and then there's the worries about rubbish thrown up from the main gear and water ingestion. I suppose mid-mounted engines something like a scaled up A10 Warthog, so that the wing (swept though) would provide a shield.

If we are stuck with under wing pylon-mounted engines as in all current designs, I wonder why twin Vertical Stabs fitted to the end of the HS are not more popular? They seem to work very well for the Antonov 225 and I think they look very good. I realise that in this case, the reason behind the design, was to put the VS in clean airflow when carrying external loads on top of the fuselage, but this configuration would seem to have a lot to offer in engine out/exterme conditions that might come close to overloading a single cantilevered fin.

Mounted at about their midpoint to the HS, the rudder forces would be a straight push/pull with little bending moment, and I would have thought that being downstream of the engines (but not too close), they would benefit from the increased local flow velocity to enhance rudder authority in low speed/high thrust situations and they wouldn't be masked by the fuselage at high AOA. I suppose that they may work out heavier, which wouldn't please the "beancounters", but you can tell them that their expensive new hangers wouldn't need to be so high and there's plenty of vertcal billboard area to paint their Logo on!

[Graybeard]

Why not a triple tail, like the Lockheed Constellation, and the "Cardboard Connie", the Bellanca Cruisair/Cruisemaster?

Auxiliary fins were added to the 1937 Bellanca Cruisair because it would tend to go flat in a spin, and not recover by itself within the required six(?) turns. Some 40 years later, they found it could be predicted which planes would spin more readily by the intersection of the fuselage and aft area of the wing root. The more continuous the surface between wing roots, the more spinnable. It was evident on the Grumman Yankee/Traveler/Tiger of the 1970s.

GB