astuteman
Posts: 6483
Joined: Mon Jan 24, 2005 7:50 pm

### An Aerodynamics Question

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   ), 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

mrocktor
Posts: 1391
Joined: Mon Jan 24, 2005 12:57 am

### RE: An Aerodynamics Question

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

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

astuteman
Posts: 6483
Joined: Mon Jan 24, 2005 7:50 pm

### RE: An Aerodynamics Question

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  .

Regards

mrocktor
Posts: 1391
Joined: Mon Jan 24, 2005 12:57 am

### RE: An Aerodynamics Question

 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

mrock

WingedMigrator
Posts: 1770
Joined: Wed Oct 26, 2005 9:45 am

### RE: An Aerodynamics Question

 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.

cptspeaking
Posts: 567
Joined: Mon Apr 24, 2006 2:49 pm

### RE: An Aerodynamics Question

 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.

BTW...what a great question(s); well thought out, clearly stated and with your reasoning behind statements. Awesome!!
...and don't call me Shirley!!

bhill
Posts: 1410
Joined: Thu Sep 13, 2001 8:28 am

### RE: An Aerodynamics Question

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

mrocktor
Posts: 1391
Joined: Mon Jan 24, 2005 12:57 am

### RE: An Aerodynamics Question

 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"

mrocktor

3201
Posts: 813
Joined: Sun Oct 10, 2004 4:16 pm

### RE: An Aerodynamics Question

 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.
7 hours aint long-haul

WingedMigrator
Posts: 1770
Joined: Wed Oct 26, 2005 9:45 am

### RE: An Aerodynamics Question

 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)

astuteman
Posts: 6483
Joined: Mon Jan 24, 2005 7:50 pm

### RE: An Aerodynamics Question

 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

 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
Thanks

Regards

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