Lemurs
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Why Is The Vert Stab So Small On 787?

Fri May 06, 2005 7:20 am

On the newest illustrations from Boeing for the 787, the most striking thing I've noticed has been the relatively small size of the vertical stabilizer as compared to other jets in its class.

Now, I realize that the size of that part of the empenage is calculated based on the length of the fuselage, the amount of asymmetric thrust that can be expected on engine failure, and all the other data points around the lever of thrust and CG...so my question is, what's unique about the 787's geometry that I'm not noticing that allows it to be so small?

I know that if your flight computers can react to an asym thrust scenario quickly enough, and you make the tail/rudder/actuators strong enough, it'd be possible to reduce the size since you wouldn't be tied to pilot reaction time. Could this be part of it?

Or is simply my eyes playing tricks on me?
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7E7Fan
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RE: Why Is The Vert Stab So Small On 787?

Sat May 07, 2005 6:11 pm

Hi,

I'd like to follow up on that question; On the latest illustrations of the 787, the awesome shark-fin look of the vertical stab has been replaced by a standard boring looking this-is-what-stabs-been-looking-like-for-a-hundred-years-now type of stabilizer... Does anyone know whether this was actually due to aerodynamics etc or was it just a coward cave-in because there have been some conservative postings around the net from people who don't like the new look? As you can probably tell by now  Wink I loved the shark-fin and I am terribly disappointed that they removed it!

Thanks/Mike
 
pilotpip
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RE: Why Is The Vert Stab So Small On 787?

Sat May 07, 2005 10:07 pm

To the first poster, while the drawings show the small vert. stabilizer, have any dimensions of the aircraft been specified yet? If not remember that it's still all artist's conception.

Second, It could be a combination of both. Boeing may have found a more efficient fairing for use with "traditional" style vert. stabilizer, they may have determined that some sort issue may arise with the sharkfin, or the customers decided that the marginal improvement in efficiency (if there was any) was not worth the radical new design because people don't like change.

These reasonings are all purely speculation on my part and just an attempt to stir the pot a bit.
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Jetlagged
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RE: Why Is The Vert Stab So Small On 787?

Sun May 08, 2005 2:43 am

The latest impressions from Boeing (straight leading edge fin, et al) are described as defining the final external look of the 787. In other words most of the curves have gone forever as most people predicted. The curves might have been causing trouble with compressibility effects, or it might be to reduce manufacturing costs. The three versions of 787 may need different sized fins and this would be easier to arrange with straight edges.

However this does not mean that the fin's size is fixed, and it does look on the small side. The smaller the better for drag, but there are limits.

Quoting Lemurs (Thread starter):
I know that if your flight computers can react to an asym thrust scenario quickly enough, and you make the tail/rudder/actuators strong enough, it'd be possible to reduce the size since you wouldn't be tied to pilot reaction time.

The aircraft must be safe without the automatics, so you can't really reduce the fin/rudder area on that basis alone. Fin area will normally be determined by lateral/directional stability considerations, rather than engine out yaw correction. Things like dutch roll characteristics and spiral stability are set by fin size and moment arm.

I wouldn't like to rely on the avionics working to avoid spiralling in after an engine cut on takeoff. There needs to be enough countering yaw moment from the rudder, regardless of reaction time.
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AsstChiefMark
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RE: Why Is The Vert Stab So Small On 787?

Sun May 08, 2005 4:19 am

THIS is small, especially the Comet


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redflyer
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RE: Why Is The Vert Stab So Small On 787?

Sun May 08, 2005 4:25 am

Quoting 7E7Fan (Reply 1):
Does anyone know whether this was actually due to aerodynamics etc or was it just a coward cave-in because there have been some conservative postings around the net from people who don't like the new look?

I suspect it had more to do with economics. In the final analysis, this plane is intended to deliver savings to airline customers. A fin with the complex curves depicted in Boeing's original illustrations would cost more to manufacture and the benefit of the original fin - designed for aesthetic purposes only - was negligible.

Best regards,

R
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Jetlagged
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RE: Why Is The Vert Stab So Small On 787?

Mon May 09, 2005 12:39 am

Quoting AsstChiefMark (Reply 4):
THIS is small, especially the Comet

The Comet had quite a large tail dihedral angle, adding to the effective fin area.


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