AIRCRAFT BULBOUS BOW 3

[rickyaaaa]

Would a bulbous bow on the front of an airplane, similar to a bulbous bow on a ship, reduce drag on the airplane.

 

[KENAT]

OK, I'll start with a simple question of what flight regime you're talking about?

Certainly if going fast enough that compressibility is a concern the simple answer is no.

However, depending on exactly what you mean then aerospikes are occasionally used in very high velocity applications.

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[rb1957]

i think the main reason for the bulbous bow is it improves the water/air boundary, and so reduces drag. as such, doesn't sound particularly applicable to aircraft.

there have been some examples of what you're thinking, compare a standard DHC8 with a CT142 (a radar trainer for the Canadian Forces).

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[rickyaaaa]

reference rb1957 post.
I don't understand your comment "bulbous bow is it improves the water/air boundary" In a ship the bulbous bow is completely below the water line therefore is doesn't seem like the air/water boundary would come into play?

 

[rb1957]

look carefully at how the bow wave sits on top of it. i think what it's doing is giving the water a small vertical velocity and that affects/reduces the bow wave proper.

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[IRstuff]

The simple answer is no. I suggest you read the Wikipedia article and

http://www.dieselduck.info/library/01 articles/bulbous_bows.htm

The bulbous bow solves issues related to moving in an incompressible fluid, while air is not.

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[debodine]

May I recommend this site for some good photos of an aircraft "bulbous bow"? I was not part of the design team, I was maintenance for 4950th Test Wing so I know nothing of the design characteristics or limitations. I can tell you the bulbous bow was not to reduce drag but to house an antenna. However, you may find someone on the site (former pilot maybe?) who could discuss drag characteristics.

http://www.flyaria.com/index.html

Look under the link that says "The Aircraft".

 

[btrueblood]

What IRstuff said, but more so: a bulbous bow is used for vessels travelling on the surface of a fluid, not through it. The bulb creates a bow wave that (theoretically) exactly cancels the bow wave from the actual bow of the ship, reducing the wave resistance of the hull. There are no examples of vessels designed for speed underwater having bulbous bows, as the entire hull is submerged and the surface wake doesn't exist (ok, a surface disturbance is created, but it's a far-field effect). The theory of bulbous bows is discussed in a bit of detail at Wikipedia -

http://en.wikipedia.org/wiki/Bulbous_bow

and

http://en.wikipedia.org/wiki/Hull_speed,

and links from there will take you as deep into the theory of naval architecture as you care to travel.

If you want to discuss wave drag in compressible fluids (air for example), that's a different subject. Read up on the area rule (see F104), aerospikes like Kenat mentioned, and Bussman biplanes.

 

[IRstuff]

While bulbous bows do not exist for undersea vessels, other approaches, like supercavitation, have been demonstrated to drastically reduce drag for torpedoes

http://en.wikipedia.org/wiki/VA-111_Shkval.

The cited torpedo supposedly has a max speed greater than 200 kt, which is faster than almost all surface vessels; the fastest boat speed record is only about 75 kt faster than the cited torpedo. Of course, that speed record was accomplished on a river and not in open ocean.

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[GregLocock]

Actually some work was done on the Collins class subs to investigate wavemaking drag, since they spend a fair bit of time near enough to the surface to create a wake, which can be seen using side scan radar from orbit. I would guess from the actual hull shape that was used that no visually obvious solution was found.

If you ever want to discuss wave making drag of ships, let me know!

Incidentally one interpretation of the bulbous bow is that it is an impedance matching device between the sea surface and the ship.






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[tbuelna]

While it's not exactly a "bulbous bow", the "coke bottle" fuselage shape has long been used to improve the aero performance of high-speed aircraft.

 

[btrueblood]

"impedance matching device between the sea surface and the ship."

Nice analogy, I think. Like tuning an exhaust muffler, or a mass-spring damper, one would tune the bulb for a given hull speed?

"If you ever want to discuss wave making drag of ships, let me know!"

Go for it, Greg. I'm no expert, but willing to be educated.

 

[rb1957]

"If you ever want to discuss wave making drag of ships, let me know!"

an article for the writer's guild ?

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[KENAT]

Both ships and aircraft have had bulbous bows/noses installed to house sensors (typically sonar on ships, radar on A/C) however in these cases effort typically had to be taken to minimize impact on drag - not get a benefit from it.

I have a vague recollection of the odd exception where the larger dia nose cone did improve drag but I can't remember details.

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[GregLocock]

b-yes the bulb is only a benefit at the speeds where wavemaking drag is high. At low speeds the extra surface area causes an increase in drag.

The fundamental work on this was done in the 1800s when people had too much time on their hands. Havelock approximated the hull of a moving ship by a source at the bow and a sink at the stern, each with a strength v*A. From this he worked out the interference pattern of the surface waves, which results in a v shape from the bow and a trailing pattern of curved transverse waves following the stern. When the ships length is the same as the wavelength of the surface wave the pattern is at its strongest, as its drag.

All subsequent analysis builds on that simple model, it is a brilliant piece of work. That model is good enough for typical catamaran type hulls, with semicircular cross sections of constant cross section.

http://www.cyberiad.net/library/pdf/tsl00.pdf

is a nice intro, Leo L is a very cluey and helpful guy.


Cheers

Greg Locock


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[rb1957]

"solved in 1898 the problem of a thin ship moving steadily forward in a calm sea, and was able, in spite of the total absence of
computing equipment, to evaluate with 2-figure accuracy, the triple integral for the resulting inviscid drag force or wave resistance."

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