a380900
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Tail Size Vs Plane Length

Mon Feb 14, 2011 8:00 pm

It is well known that the longer the airplane, the smaller the tail needs to be as there is a stronger leverage arm for elevator and rudder and thus better control authority with less size.

The 747 SP and the A318 have larger tails than the rest of their respective family because of that. Yet, the 747-8 has the same tail and it does not look like aircraft makers are taking advantage of this most of the time.

A340-500 and 600 have same tails. All the A32- except A318. All the 777 have same tail... I haven't checked

Can anyone shed more light on this subject?
Do you guys think a longer A380 would have a smaller tail than the 800?

[Edited 2011-02-14 12:05:57]
 
KELPkid
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RE: Tail Size Vs Plane Length

Mon Feb 14, 2011 8:25 pm

Quoting A380900 (Thread starter):
Can anyone shed more light on this subject?
Do you guys think a longer A380 would have a smaller tail than the 800?

For the Rudder/vertical stabilizer (technically only one half of the "tail", as the horizontal stabilizer and elevator are also part of the "tail"), it will depend on how much engine-out rudder authority that the engineers (and the regulations!) deem as being necessary. On a large multiengine plane, this is probably the primary factor that drives the size of the rudder and vertical stabilizer. And yes, in the end, it is about how large of a moment the rudder can create  
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
tdscanuck
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RE: Tail Size Vs Plane Length

Mon Feb 14, 2011 8:29 pm

Quoting A380900 (Thread starter):
The 747 SP and the A318 have larger tails than the rest of their respective family because of that. Yet, the 747-8 has the same tail and it does not look like aircraft makers are taking advantage of this most of the time.

That's because "this", in your context, means a lot more than just required tail authority. There are major manufacturing (read: cost) efficiencies to only running a single tail. You have to weight all the design/manufacture/upkeep/maintenance/support costs of multiple tails against the potential weight savings of the smaller tail. Sometimes it wins, sometimes it doesn't.

Tom.
 
SchorschNG
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RE: Tail Size Vs Plane Length

Mon Feb 14, 2011 8:40 pm

Quoting tdscanuck (Reply 2):
That's because "this", in your context, means a lot more than just required tail authority. There are major manufacturing (read: cost) efficiencies to only running a single tail. You have to weight all the design/manufacture/upkeep/maintenance/support costs of multiple tails against the potential weight savings of the smaller tail. Sometimes it wins, sometimes it doesn't.

The A321 has a smaller tail than the A320 or A319. I guess here the production disadvantages were outweighted by the advantages. Or a classical case of German tradition to over-engineer a system for optimum performance irregardless of the associated costs. The downside of the hailed German engineering.
The A320's tail weights less than 500kg, and the saving by making it slightly smaller is probably limited.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
dw747400
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RE: Tail Size Vs Plane Length

Mon Feb 14, 2011 9:53 pm

Quoting KELPkid (Reply 1):
it will depend on how much engine-out rudder authority that the engineers (and the regulations!) deem as being necessary.

Exactly. The higher thrust engines likely require more authority, which is achieved by increasing arm while maintaining the same sized tail. It probably was not feasible to make any substantial tail reduction (or any reduction at all). Thus, with not significant benefit it makes no economic sense to do a redesign.
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Western727
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RE: Tail Size Vs Plane Length

Mon Feb 14, 2011 11:08 pm

Quoting A380900 (Thread starter):
The 747 SP and the A318 have larger tails than the rest of their respective family because of that.

Not only does the fuselage length affect the required size of the vert stab, but so does the distance of the thrust from the plane's fuselage centerline. The more "outboard" the overall thrust is, the more impact an assymetrical-thrust situation would have...requiring a larger vert stab. Good 'ol physics.  
Jack @ AUS
 
474218
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RE: Tail Size Vs Plane Length

Tue Feb 15, 2011 12:18 am

Quoting KELPkid (Reply 1):
For the Rudder/vertical stabilizer (technically only one half of the "tail", as the horizontal stabilizer and elevator are also part of the "tail")...


Technically, the vertical stabilizer, rudder, horizontal stabilizer and elevators make up the "empennage"!
 
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SEPilot
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RE: Tail Size Vs Plane Length

Tue Feb 15, 2011 10:59 pm

I suspect that the bigger reason for making the empennage smaller with longer fuselages is drag, not weight. But I suspect that the difference would be far outweighed by the higher costs involved. The 747SP was not considered in the initial design of the 747, and one of the requirements that they wanted to meet was that its handling would match the regular 747. I believe that that requirement, as much as the engine out control authority requirement, drove the size of the empennage (the horizontal stabilizer/elevator is larger as well).
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
kalvado
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RE: Tail Size Vs Plane Length

Wed Feb 16, 2011 2:02 pm

Extending original question.. Is there similar size dependence for horizontal stabilizer?
My understanding is that horizontal stabilizer has to be big enough to neutralize momentum of CoG shifted with respect to lift generated by the wing. For longer plane momentum generated by same CoG shift s larger due to higher wight of a plane - but I doubt it's linear with length.
If that is correct, there should be some correlation between plane balance limits and stabilizer size - but it's just speculation on my side.
 
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SEPilot
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RE: Tail Size Vs Plane Length

Wed Feb 16, 2011 2:24 pm

Quoting kalvado (Reply 8):
Extending original question.. Is there similar size dependence for horizontal stabilizer?

Yes. Note that the 747 horizontal stabilizer is also larger than the standard one.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
musang
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RE: Tail Size Vs Plane Length

Sun Feb 20, 2011 4:48 pm

Quoting kalvado (Reply 8):
For longer plane momentum generated by same CoG shift

Your argument's basically correct, but its "moment". Momentum is a quantity of kinetic energy or something like that, the product of an object's mass x its velocity. You're not alone, we had an instructor on staff who always said "momentum" and wouldn't be told.....

Anyway, I believe the A330-200 has a larger fin than the -300, and BWIA's TriStars had enlarged fin area by means of a rudder chord extension, so they could handle wing engine failures on take-off from short runways in high WAT conditions.

On the subject of tailplanes (horizontal stabilizers to some), I always wondered whether the MD-11 stab, smaller than that of the DC-10, was because design had improved such that a smaller one could do the same job, or because the longer fuselage didn't require the large one, and weight saving considerations made the lighter one viable.

Regards - musang
 
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SEPilot
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RE: Tail Size Vs Plane Length

Mon Feb 21, 2011 12:58 pm

Quoting musang (Reply 10):
On the subject of tailplanes (horizontal stabilizers to some), I always wondered whether the MD-11 stab, smaller than that of the DC-10, was because design had improved such that a smaller one could do the same job, or because the longer fuselage didn't require the large one, and weight saving considerations made the lighter one viable.

My understanding it was because of the longer fuselage, more rearward CG, and using artificial stability (FBW). The longer fuselage increases the moment arm, the rearward CG reduces amount of downforce required, and the FBW makes it more effective. They had to make the horizontal stabilizer as small as they could to reduce drag to meet performance goals.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
musang
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RE: Tail Size Vs Plane Length

Tue Feb 22, 2011 4:28 pm

Clear, consise, convincing reply. Thanks!

musang

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