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An Aerodynamics Question  
User currently offlineAstuteman From United Kingdom, joined Jan 2005, 10012 posts, RR: 96
Posted (8 years 1 month 2 weeks 2 days ago) and read 2001 times:
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Seeing the release of the A350X with its "extra-large" body prompted some thoughts about "drag".

My background is Naval architecture, and I've studied the Hydrodynamics of a body travelling though a more dense fluid (seawater  Smile ), but I'm not sure how much of the theory carries over in practice.
Example - wavemaking resistance is THE key drag for a fast moving warship (or sub).
So, if you lengthen a ship, making it heavier in the process, you actually reduce the drag, because there's less interference between the bow wave formation and the stern wave formation. This effect way overcomes any increase in frictional drag.

As I understand it, an aircraft experiences drag from 2 main sources, Lift, and "parasitic" drag (non-lift).

Q1 - what is the typical ratio of lift drag to total drag, and "form" drag to total?

I'm guessing "non-lift" drag breaks down into elements too, such as "form" drag (e.g. frontal area, aerodynamic shape etc), and frictional drag.

Q2 - what are the proportions of these.

Q3 - what proportion of form drag is attributable to a) the fuselage, b) tail, c)other appendages (appendage drag is disproportinately large in ships hydrodynamics....)

Q4.- Hence what is the anticipated increase in drag attributable to the A350X, for example, having a fuselage some 300mm bigger than the old A350?

If anyone can answer, or at least point to some user-friendly publication, I'd be grateful.

Regards

10 replies: All unread, jump to last
 
User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 1, posted (8 years 1 month 2 weeks 1 day 23 hours ago) and read 1982 times:

I can't answer all your questions completely but I'll try to get started...

First, something that was not a question but deserves special attention:

Aircraft experience three basic types of drag: induced drag (caused by lift), parasitic drag (caused by friction and form) and wave drag caused by the compressibility effects (i.e. the drag associated to the existence of shockwaves). Ships never have compressibility effects since water is conveniently not compressible  Wink

Q1: Lift/Drag, or L/D for short, is maximized when induced drag equals the sum of non-lift related drag. If your induced drag is more than the parasitic it means you are flying a high lift coeficient - you can gain efficiency by flying faster at a lower angle of attack. If your parasitic drag is higher than the induced, it means you are flying fast, but are burning more fuel to do so - you can gain efficiency by flying slower at a higher angle of attack.

Q2: About the breakdown of non-lif drag, I don't what proportions are typical. Depending on flight regime (i.e. low transonic vs high transonic) and the type of wing (conventional vs supercitical) the contribution of wave drag can be minimal or very significant, for example.

Q3: I really don't know. Appendages are certainly a problem though, specially with wave drag (sudden shifts in local airspeed - due to an irregular shape - are a great causer of shockwaves).

Q4: I think the effect on drag due to the increased diameter will pretty insignificant. Weight is much more of an issue. In fact, all fuselage shapes are longer than the ideal diameter/length relation for minimizing form drag (check a raindrop for the ideal). Increasing width adds drag due to the larger skin friction, but a better aspect ratio means the increase is at least a little mitigated.

mrocktor


User currently offlineAstuteman From United Kingdom, joined Jan 2005, 10012 posts, RR: 96
Reply 2, posted (8 years 1 month 2 weeks 1 day 7 hours ago) and read 1913 times:
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Thanks for replying Mrocktor. I'll take time to assimilate that lot. Just one point, though....

Quoting Mrocktor (Reply 1):
wave drag caused by the compressibility effects (i.e. the drag associated to the existence of shockwaves). Ships never have compressibility effects since water is conveniently not compressible

Water is NOT compressible - that's why we get REAL waves  Smile.

Regards


User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 3, posted (8 years 1 month 2 weeks 1 day ago) and read 1877 times:

Quoting Astuteman (Reply 2):
Water is NOT compressible - that's why we get REAL waves

Yeah, that's why I tried to explain what "wave drag" means in an aeronautical context  Big grin

mrock


User currently offlineWingedMigrator From United States of America, joined Oct 2005, 2212 posts, RR: 56
Reply 4, posted (8 years 1 month 2 weeks 16 hours ago) and read 1833 times:

Quoting Astuteman (Thread starter):
If anyone can answer, or at least point to some user-friendly publication, I'd be grateful.

I found just what you need! Torenbeek, Synthesis of Subsonic Aircraft Design Methods, ISBN 9024727243, chapter 11. There is a very enlightening overview of all the different kinds of drag (form, wake, vortex, induced, friction, wave, parisitic, basic, viscous, interference, there are so many I lost count!) and most importantly, how they all relate to each other... which is rather confusing at first blush. It's an excellent treatment. He cuts right through to the heart of the matter, explaining how we resort to all these weird classifications to describe what really boils down to a pressure distribution over a complex shape.

One of the charts shows a breakdown of drag components by major structural groups... fuselage was about 30% ish percent. I'm sure there's enough in there to extract a rule of thumb.

An invaluable reference for those of us with a tendency to crunch numbers.

Quoting Astuteman (Reply 2):
Water is NOT compressible

Weeeeeellll.... 0.00005 per atmosphere. It's a teeny tiny bit compressible.  Big grin


User currently offlineCptSpeaking From United States of America, joined Apr 2006, 639 posts, RR: 1
Reply 5, posted (8 years 1 month 2 weeks 15 hours ago) and read 1824 times:

Quoting Astuteman (Thread starter):

Q1 - what is the typical ratio of lift drag to total drag, and "form" drag to total?

This depends on the airspeed you're at for your specific wing design. At lower airspeeds, induced drag is high and parasite low, but the higher the airspeed gets, induced decreses and parasite drag takes a bigger role.

Induced (lift) drag is the square of your airspeed inverse, and parasite drag is the opposite, being square of the airspeed. I'm not saying the numbers are squares, thats just how the relationship looks on the chart with airspeed on the horizontal axis.

I've got the chart in a couple books, but I can't find the power cord for my scanner, otherwise you'd see it right now.  Sad

Your CptSpeaking  wave 


BTW...what a great question(s); well thought out, clearly stated and with your reasoning behind statements. Awesome!!



...and don't call me Shirley!!
User currently offlineBhill From United States of America, joined Sep 2001, 966 posts, RR: 0
Reply 6, posted (8 years 1 month 2 weeks 15 hours ago) and read 1821 times:

Hmmm..forgive the question..but is there a "boundry compression" barrier in fluidics that exists in gases, such as the "sonic" boom?.If so, does the laws of Mach apply? Given, fluids can't be compressed..but what about those tiny spaces between the atoms?

Thanks



Carpe Pices
User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 7, posted (8 years 1 month 2 weeks 5 hours ago) and read 1775 times:

Quoting Bhill (Reply 6):
Given, fluids can't be compressed..but what about those tiny spaces between the atoms?

As gas compresses, the atoms are forced together and the distance between them gets smaller and smaller, at one point the atoms are forced so close together that they bond more strongly - the gas has become a liquid. You can still put more pressure on the liquid, but it's volume will not be reduced further.

Water happens to be a liquid at room temperature, since it has a bunch of gasses mixed in with it (thats how fish can breathe) it is a little compressible (you are actually compressing the gas it's mixed with). Pure water is not compressible as far as I know.

Oh, and be careful with using "fluid". Both gases and liquids are "fluids"  Wink

mrocktor


User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 8, posted (8 years 1 month 1 week 5 days 21 hours ago) and read 1710 times:

Quoting WingedMigrator (Reply 4):
I found just what you need! Torenbeek, Synthesis of Subsonic Aircraft Design Methods, ISBN 9024727243, chapter 11. There is a very enlightening overview of all the different kinds of drag (form, wake, vortex, induced, friction, wave, parisitic, basic, viscous, interference, there are so many I lost count!) and most importantly, how they all relate to each other... which is rather confusing at first blush. I

Alternatively, try this page, and on the right side go to chapter 4, "Drag."

Quoting Astuteman (Thread starter):
Q3 - what proportion of form drag is attributable to a) the fuselage, b) tail, c)other appendages (appendage drag is disproportinately large in ships hydrodynamics....)

I have some numbers at work, hopefully can post them Monday, but Torenbeek definitely has them. My numbers and Torenbeek's are for 20-30 year-old aircraft, not sure how much they've changed.


User currently offlineWingedMigrator From United States of America, joined Oct 2005, 2212 posts, RR: 56
Reply 9, posted (8 years 1 month 1 week 5 days 15 hours ago) and read 1654 times:

Quoting 3201 (Reply 8):
Alternatively, try this page, and on the right side go to chapter 4, "Drag."

Wow, by Prof. Kroo at Stanford... I took a basic aero class that he taught (before I branched off into spacecraft)  Smile


User currently offlineAstuteman From United Kingdom, joined Jan 2005, 10012 posts, RR: 96
Reply 10, posted (8 years 1 month 1 week 4 days 13 hours ago) and read 1591 times:
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Quoting WingedMigrator (Reply 4):
I found just what you need! Torenbeek, Synthesis of Subsonic Aircraft Design Methods, ISBN 9024727243, chapter 11



Quoting 3201 (Reply 8):
Alternatively, try this page, and on the right side go to chapter 4, "Drag."

Thanks, WM, and 3201, for the references and the links. Thanks to everyone else too..

That's my spare time taken up for the next few weeks  Smile

Quoting CptSpeaking (Reply 5):
BTW...what a great question(s); well thought out, clearly stated and with your reasoning behind statements

That was about the 8th time I'd written it out  biggrin 
Thanks

Regards


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