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Longer Plane, Smaller Tail: Even For The A380-900?  
User currently offlineA380900 From France, joined Dec 2003, 1118 posts, RR: 1
Posted (1 year 6 months 3 weeks 4 days 2 hours ago) and read 4196 times:

Some tubes with wings have had different tail sizes depending on their lengths. I definitely remember that being the case for the A318 having a larger tail than other A320 family members. I think that was also the case for the A330-200/300 or maybe between 500/600? Are there any other case of special tail designed for specific derivative?

So I wondered too about the A380-900, wouldn't the weight gains be spectacular if they could reduce the tail's size? But wouldn't it cost a lot to engineer that compared to keeping a slightly oversized tail? Can the weight gains compensate the engineering costs?

Wouldn't an A380-900 with an A380-800 tail be a much lesser plane than if the tail was adjusted? I mean the bigger the plane, the higher the impact on weight and drag of reducing the size of such a massive component.

21 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17177 posts, RR: 66
Reply 1, posted (1 year 6 months 3 weeks 4 days 2 hours ago) and read 4174 times:

Fin and rudder size is dependent on the requirement to have enough authority during engine out. As you say a longer body means more moment arm, in theory allowing a smaller fin and rudder. However the engines are more powerful, so this advantage might not be realizable. For example the 345 and 346 have the taller 330 fin despite being longer than the 342 and 343.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinerwessel From United States of America, joined Jan 2007, 2412 posts, RR: 2
Reply 2, posted (1 year 6 months 3 weeks 4 days 1 hour ago) and read 4147 times:
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The 747SP had a larger tail than the "normal" models.

While a longer fuselage would enable a smaller tail, other design considerations might reduce that. For example, the A389 will certainly have bigger engines, requiring more rudder authority to handle an engine out. Also, it depends on how the stretch is done - if more of it happens forward of the CG (this might happen for CG purposes, or convenience in where fuselage sections can be stretched), the lever arm for the tail will not improve as much, and the destabilizing influence of the forward fuselage will increase.

As to the cost/benefit, while a smaller tail would certainly help (assuming the tail would otherwise be oversized), the cost to reengineer/change tooling/recertify that obviously has to be justified by the performance gains (as those lead to improved sales).

In any event, the A389 is not expected to be all that much bigger than the A388, so we’re probably not looking at any huge oversizing of the tail feathers, even if they’re not clipped.


User currently offlineferpe From France, joined Nov 2010, 2805 posts, RR: 59
Reply 3, posted (1 year 6 months 3 weeks 3 days 22 hours ago) and read 4084 times:

Quoting rwessel (Reply 2):
As to the cost/benefit, while a smaller tail would certainly help (assuming the tail would otherwise be oversized), the cost to reengineer/change tooling/recertify that obviously has to be justified by the performance gains (as those lead to improved sales).

There are a number of considerations when it comes to wether one could and should reduce the tail sizes or not and it differs for the Vertical and Horizontal tailplanes.

HTP
This is working all the time to balance a naturally stable aircraft. This means it produces drag due to lift (induced drag) and drag independent of lift (mostly skin friction drag) all the time. When the tail arm gets longer the induced part reduces so you have a reduction of HTP drag with a longer tail without reducing the HTP size. If you still want to reduce the size you want to reduce the area but not the span, otherwise there is not much gain from the operation. Hence for the most part nothing is done for the HTP.

VTP
This is normally not generating much of a force hence its drag due to lift (once again induced drag but in the horizontal direction) is low, its parasitic drag dominates. Here one can see that the height of the tail and thereby the VTP surface can be reduced sometimes be clipping the tip if the induced drag when one engine goes inop (OEI) is still Ok for the OEI certification criteria, you would gain reduced wetted area for the cruise phase and for the life of the aircraft.



Non French in France
User currently offlineViscount724 From Switzerland, joined Oct 2006, 26005 posts, RR: 22
Reply 4, posted (1 year 6 months 3 weeks 3 days 6 hours ago) and read 3813 times:

Quoting Starlionblue (Reply 1):
As you say a longer body means more moment arm, in theory allowing a smaller fin and rudder. However the engines are more powerful, so this advantage might not be realizable. For example the 345 and 346 have the taller 330 fin despite being longer than the 342 and 343.

When the Convair 340/440 was converted to a 580, a major part of the modification involved increasing the area of both the vertical and horizontal tail to compensate for the much more powerful Allison 501 turboprops with about 50% more power than the original P&W R-2800s (roughly equivalent to adding a 3rd engine to the 340/440). The vertical stabilizer/rudder is about a foot taller on the 580 and the span of the horizontal stablizers was increased about 3 feet.

A few photos here of North Central aircraft undergoing that very successful modification.
http://northcentralmemories.blogspot...h/2009/07/convair-conversions.html


User currently offlineBEG2IAH From United States of America, joined Apr 2004, 980 posts, RR: 18
Reply 5, posted (1 year 6 months 3 weeks 3 days 4 hours ago) and read 3780 times:
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Quoting ferpe (Reply 3):
HTP
This is working all the time to balance a naturally stable aircraft. This means it produces drag due to lift (induced drag) and drag independent of lift (mostly skin friction drag) all the time. When the tail arm gets longer the induced part reduces so you have a reduction of HTP drag with a longer tail without reducing the HTP size. If you still want to reduce the size you want to reduce the area but not the span, otherwise there is not much gain from the operation. Hence for the most part nothing is done for the HTP.

VTP
This is normally not generating much of a force hence its drag due to lift (once again induced drag but in the horizontal direction) is low, its parasitic drag dominates. Here one can see that the height of the tail and thereby the VTP surface can be reduced sometimes be clipping the tip if the induced drag when one engine goes inop (OEI) is still Ok for the OEI certification criteria, you would gain reduced wetted area for the cruise phase and for the life of the aircraft.

ferpe, is there any reference (not really aerodynamics for dummies, but something close) where those of us who like aviation but are not engineers can relatively quickly learn some of the basics? I tried searching for good elementary books on Amazon and alike, but I end up biting more than I can chew. Any recommendation would be much appreciated.



FAA killed the purpose of my old signature: Use of approved electronic devices is now permitted.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17177 posts, RR: 66
Reply 6, posted (1 year 6 months 3 weeks 3 days 1 hour ago) and read 3733 times:

Quoting BEG2IAH (Reply 5):
is there any reference (not really aerodynamics for dummies, but something close) where those of us who like aviation but are not engineers can relatively quickly learn some of the basics? I tried searching for good elementary books on Amazon and alike, but I end up biting more than I can chew. Any recommendation would be much appreciated.

"Stick and Rudder" by Wolfgang Langewiesche might be a good start. It was written in the 1930s and as such the language is somewhat dated (elevators are called "flippers") but it is still an excellent text that explains flying well to the layman.

Once you've figured that one out, D.P. Davies' "Handling the Big Jets" is superlative. Written in the 1960s by the Chief Test Pilot of the UK aviation authority, it explains the aerodynamics and other aspects of modern aircraft (incl. spiral divergence, dutch roll, etc...) from a pilot's point of view. The language of this one is also a bit dated but it is an excellent text even today.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineferpe From France, joined Nov 2010, 2805 posts, RR: 59
Reply 7, posted (1 year 6 months 3 weeks 3 days 1 hour ago) and read 3728 times:

Quoting Starlionblue (Reply 6):
"Stick and Rudder" by Wolfgang Langewiesche

Thanks Starlionblue, those are reference that I have never read, sounds interesting. Another good start is NASA, they have web courses where this one is an excellent start and very pedagogical and you can gradually dig deeper and deeper:

http://www.grc.nasa.gov/WWW/k-12/airplane/bga.html



Non French in France
User currently offlineairmagnac From Germany, joined Apr 2012, 321 posts, RR: 52
Reply 8, posted (1 year 6 months 3 weeks 2 days 21 hours ago) and read 3680 times:

Just finishing Langewiesche myself, after going through DP Davies. Great books indeed.
Here are a couple other links (but a bit more advanced) :

http://adg.stanford.edu/aa241/AircraftDesign.html

http://www.av8n.com/how/#contents



One "oh shit" can erase a thousand "attaboys".
User currently offlineTrnsWrld From United States of America, joined May 1999, 959 posts, RR: 0
Reply 9, posted (1 year 6 months 3 weeks 2 days 14 hours ago) and read 3517 times:

Or like the DC8 which is long and skinny and has a very small vertical stabilizer. Infact so small that I always thought the real long DC8 models look kind of funny with that tiny tail.

User currently offlineferpe From France, joined Nov 2010, 2805 posts, RR: 59
Reply 10, posted (1 year 6 months 3 weeks 2 days 13 hours ago) and read 3505 times:

Quoting airmagnac (Reply 8):
Here are a couple other links (but a bit more advanced) :

The stanford link is really good, I based my model on it. It contains a lot of wisdom from R Shevell, the former Douglas head of advanced Aero design. Many practical numbers like the skin friction coefficients etc, climb fuel adders and so on. I verified his formulas and daa with some other well known books like Thorenbeek and Roskam, but the most I got from Shevell.



Non French in France
User currently offlineViscount724 From Switzerland, joined Oct 2006, 26005 posts, RR: 22
Reply 11, posted (1 year 6 months 3 weeks 2 days 10 hours ago) and read 3442 times:

Quoting TrnsWrld (Reply 9):
Or like the DC8 which is long and skinny and has a very small vertical stabilizer.

Distance from the ground to the top of the DC-8 vertical stabilizer is almost identical to the same dimension on the 707, in some cases a few inches greater depending on the model. DC-8 fuselage is only one inch narrower than the 707.


User currently offlinethegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 12, posted (1 year 6 months 3 weeks 2 days 6 hours ago) and read 3379 times:

Quoting ferpe (Reply 3):
HTP

I'm not aware of any aircraft which has varied the HTP size without a complete overhaul, besides the MD-11. And we know how that one went.

Quoting rwessel (Reply 2):

While a longer fuselage would enable a smaller tail, other design considerations might reduce that. For example, the A389 will certainly have bigger engines,

Unlikely to be much bigger engines than the 575t A388 though. I can't imagine any A389 breaching the 625t MTOW limit from the landing gear, for example.

Quoting rwessel (Reply 2):
As to the cost/benefit, while a smaller tail would certainly help (assuming the tail would otherwise be oversized), the cost to reengineer/change tooling/recertify that obviously has to be justified by the performance gains (as those lead to improved sales).

I expect this is the consideration which will dominate. It just won't be worth it to shrink the tail, especially with only a few expected copies to be sold.


User currently offlineferpe From France, joined Nov 2010, 2805 posts, RR: 59
Reply 13, posted (1 year 6 months 3 weeks 2 days ago) and read 3328 times:

There is an absolutely lovely book that I forgot to mention, CM tipped my to have a look in it;

The Anatomy of the Airplane by Darrol Stinton.

He's CV is below, he writing is really with a practical eye first and foremost (and yes it is all slugs and feet etc as it has some years since made but that does not matter because it is his simplistic view on things that count). Though I might have read 10+ books on aerodynamics he got me to understand new things  but virtue of his masterfull explanation of things from a very footed standpoint, the signs of someone who have really understood it    :


Darrol Stinton MBE, PhD, CEng, FRAeS, FRINA, MIMechE, RAF(Retd) was born in New Zealand and grew up in England. He is a qualified test pilot and aeronautical engineer who worked in the design offices of the Blackburn and De Havilland aircraft companies before joining the RAF. His test flying spanned 35 years and more than 340 types of aircraft, first as an experimental test pilot at Farnborough; then 20 years as airworthiness certification test pilot for the UK Civil Aviation Authority on light airplanes and seaplanes, before turning freelance.

[Edited 2013-06-02 00:06:33]


Non French in France
User currently offlineBEG2IAH From United States of America, joined Apr 2004, 980 posts, RR: 18
Reply 14, posted (1 year 6 months 3 weeks 1 day 11 hours ago) and read 3148 times:
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Starlionblue, ferpe, airmagnac, thanks for great suggestions and links.


FAA killed the purpose of my old signature: Use of approved electronic devices is now permitted.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 15, posted (1 year 6 months 3 weeks 1 day 8 hours ago) and read 3106 times:

Quoting thegeek (Reply 12):
I'm not aware of any aircraft which has varied the HTP size without a complete overhaul, besides the MD-11. And we know how that one went.

The 747SP has a larger horizontal tailplane compared to the normal length 747.

To put it another way, the MD-11 is one of the very few instances, possibly the only one, where a stretched aircraft has been given smaller tail surfaces. As you hinted, it didn't go so well.

The fin of a stretched aircraft might not have to be quite so large to meet engine out criteria, but being bigger gives the aircraft better margins, lower Vmca and Vmcg. The drag penalty of the slightly over sized fin and tailplane is tiny.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlinethegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 16, posted (1 year 6 months 3 weeks 1 day 8 hours ago) and read 3103 times:

Quoting Jetlagged (Reply 15):
The 747SP has a larger horizontal tailplane compared to the normal length 747.

Thanks, didn't know that one.


User currently offlineViscount724 From Switzerland, joined Oct 2006, 26005 posts, RR: 22
Reply 17, posted (1 year 6 months 3 weeks 1 day 7 hours ago) and read 3095 times:

Quoting thegeek (Reply 16):
Quoting Jetlagged (Reply 15):
The 747SP has a larger horizontal tailplane compared to the normal length 747.

Thanks, didn't know that one.

10 ft. difference. Horizontal stabilizer span 82 ft. 9 in. on the SP and 72 ft. 9 in. on the 100 through 400 (72 ft. 2 in. on the 747-8). The vertical stabilizer is 5 feet taller on the SP.


User currently offlinethegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 18, posted (1 year 6 months 3 weeks 1 day 7 hours ago) and read 3087 times:

Hmm, the HTP was shrunk by 7in. on the 747-8. Perhaps a smaller tail is plausible for the A389.

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 19, posted (1 year 6 months 3 weeks 1 day 7 hours ago) and read 3081 times:

Quoting thegeek (Reply 18):
Hmm, the HTP was shrunk by 7in. on the 747-8. Perhaps a smaller tail is plausible for the A389.

Interesting. But then the 747-8 has a new wing too, so maybe Boeing thought they'd optimise the tail while they were at it.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 20, posted (1 year 6 months 3 weeks 3 hours ago) and read 2842 times:

Quoting Viscount724 (Reply 11):
Distance from the ground to the top of the DC-8 vertical stabilizer is almost identical to the same dimension on the 707, in some cases a few inches greater depending on the model. DC-8 fuselage is only one inch narrower than the 707.

But the 707 was much closer to the ground and the DC-8 had a nose-down stance, which tells us that its fin was smaller.


User currently offlineferpe From France, joined Nov 2010, 2805 posts, RR: 59
Reply 21, posted (1 year 6 months 2 weeks 6 days 16 hours ago) and read 2770 times:

Quoting thegeek (Reply 18):
Hmm, the HTP was shrunk by 7in. on the 747-8. Perhaps a smaller tail is plausible for the A389.

When you change the wing profile but perhaps more important the high lift devices (slats and foremost flaps) you affect the requirement for the max authority of the tail + it's tail arm, i.e. the max moment it has deliver at the most inconvenient (read low) speed (most probably during start or landing).

So this is a cause for also touching the HTP, for the 748 both the wing profile and the flaps were changed. For the 389 the one thing we know is that the wing will stay the same (at least this is what A has said), they might tweak flap angles etc and that can affect=increase the max moment they need. But with the longer tail arm that should not be a problem. So, doubt that one would touch the HTP for the 389.



Non French in France
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