Hydrodynamics of the Nautilus - some thoughts and observations

[tk-7642]

The front of an airplane

The front of an airplane wing curves down, yet airplanes fly up - The front of an F1 cars rear airfoil curves up, yet it pushes down - just like the bow of the Nautilus points up - and it dives down AT A POSITIVE AOA. In other words orientation to flow is all that matters (AOA). There is nothing counter-intuitive here - nothing. My explanation supports many in this thread - Bob's position as well as Jeff's. I read them all and can't recall seeing any faulty reasoning off hand. The Wright brothers airplane wing angles down at the front and points down - yet their plane flew up.

[captain nemo]

TK,

It has to do with

TK,

It has to do with the differences between aerodynamics and hydrodynamics. When it comes to submarine hull shapes, control surfaces, and the like, the primary factor is increased "impact" pressure, not reduced pressure "lift" generated in accord with Daniel Bernouli's theorum. Why? Because water is about 800 times denser than air is, and is virtually incompressible.

Pat

[tk-7642]

Bernoulli's theory also applies to

Bernoulli's theory also applies to incompressible fluids such as water. This is why curved undercambered propeller blades are used on boat propellers. Viscosity is taken into account by Reynolds number in the equation. Here is some interesting info you may enjoy (cut and paste into the address line if it doesn't link)]www.uvi.edu/Physics/SCI3xxWeb/Plumbing/ ... amics.html[/url]

[JWLaRue]

I understand how airfoils work.

[color=#000000]I understand how airfoils work. The shape of the upper surface (vs. the lower) causes an area of lower pressure above the upper wing surface, causing lift.

The Goff Nautilus hull cross section is not a 'diving plane'.

The side rakers/chines do not have an airfoil cross section, rightside up or down......so there is no airfoil effect.

Which brings us to an important difference when comparing fluid dynamics in air vs. water]

[captain nemo]

Thanks for the Link, TK.

Thanks for the Link, TK. Actually, I'm pretty well-read on Bernouli]http://www.subcommittee.com/forum/icon_biggrin.gif[/img]

Pat

[captain nemo]

Jeff,

Of course, you're right.

TK's

Jeff,

Of course, you're right.

TK's fixated on a misconception. He'll learn.



Pat

[wbnemo1]

I have just one thing

[color=#000000]I have just one thing to add........Sorry there Bob didnt meant to insult you..hope all is well

William aka Nemo z(]

[captain nemo]

Go ahead and insult him!

Go ahead and insult him! He called me "the psycho sub builder!!!"

[tk-7642]

I am a pilot as

I am a pilot as well and have been designing airplanes with low Reynolds numbers (of around 100,000, and less) with variable camber airfoils, and variable planform geometry for many years. Reynolds number includes flow speed. Not everyone must agree with me or with Bernoulli's equation - it just helps to understand how things work, until something better comes along. I choose to believe it based on my experience not because someone taught me to believe it.
The Goff Nautilus is an airfoil shape in cross section of the lateral keel chines - the same cross section airfoil the Wright flyer had, Otto Lilenthal, Bats, Pterosaurs, Flying Squirrels, Frisbees, boat propellers and hang gliders have - this shape is called an undercambered airfoil.
Hang a spoon (undercambered airfoil) loosely by your fingers and put it under a running kitchen faucet, convex part facing the running water and watch what happens. Will it be pushed away from the fast moving water or will it be sucked into the water stream as Bernoulli suggests? See and feel what happens to the spoon. According to some on this board it will be pushed out of the water stream by the water. I disagree. See it for yourself. Bernoulli's equation applies to fluids such as water. Try it at different speeds. This is how I believe the Nautilus gets sucked down in the water until someone explains why it's wrong - no one has yet. Bernoulli's equation takes into account speed, and viscosity via the Reynolds number. It applies to fluids such as air, and incompressible water.

[captain nemo]

Sorry TK, but your spoon

Sorry TK, but your spoon analogy is invalid.

If you put only the positive camber of the spoon against the stream, it is drawn into it]http://www.subcommittee.com/forum/icon_wink.gif[/img]

Place the entire spoon into the center of the stream and angle it to a neutral angle of attack]http://www.subcommittee.com/forum/icon_wink.gif[/img]

You guys who have NAUTILUS models that are diving uncontrollably from level trim need to look elsewhere for the gremlin. It aint the upswept bow.

Pat

[captain nemo]

And just for kicks, consider

[color=#000000]And just for kicks, consider this]

[PaulC]

Guess I'll just have to

[color=#000000]Guess I'll just have to build one of these boats and see for myself!

]

[captain nemo]

Anyway, nuff about what the

[color=#000000]Anyway, nuff about what the problem aint...about what it might be]

[captain nemo]

PS]

[color=#000000]PS]

[bob the builder]

Wow! I take a couple

Wow! I take a couple of days away from the board and miss out on my whole Hydrodynamics course. I'll flunk the final for sure!

Pat, I've carefully designed the interior to allow as much trapped air out as fast as possible. The skiff is actually a solid piece, with the hard angle of the deck aft of the skiff slot filled in. All of the deck grating is practical, composed of 1/16" stainless steel mesh. The only place that air seems to get trapped slightly is in the wheelhouse, and even then it's only because my vents there are only (2) 1/16" diameter holes disguised just above the 'alligator eyes' and below their shrouds and they vent a little slower than everything else. On a side note, it makes for a really cool double stream of bubbles for a few seconds when she gets just under the surface. I don't think that trapped air is my problem.

And I use the term problem loosely here. I want to clarify that I find my model infinitely easy to drive, very responsive, and almost self-navigating. The only reason for this, however, is the addition of my third set of planes aft of my propeller.

Before this modification, I could still experience the pleasure of diving my model, but only at lower speeds. I didn't use the dive planes at all in those early days. A little forward throttle, and she'd slowly drift under the surface in a beautifully controlled dive. Kicking the rudder in helped pull the aft end down to keep her upward oriented if I needed to.

Now that I've finished the mod, I can use full throttle and the APC will keep her dead-level, with only a minute porpoising effect that occurs at full speed only.

My intent in posting this effect in this string was simply to let other Goff Nautilus builders know what I've experienced. Apparently this is a common problem, as at least three other people have experienced the same phenomenon with their models. Coincidence? I think not.

I'd consider closing in my ballast grates, except that I like having eight monstrous vents to let my trapped water out of my hull when I pull her from the water. You can imagine that a (nearly) six foot long model with two inches of free space around the entire length of the WTC will hold a lot of water. She weighs a ton when she's first pulled out!

I guess my motto is "if it ain't broke, don't fix it!" I can't imagine how my model could work better (other than a bit more speed... you can never really have enough, right?), so I'm very happy leaving it as it is.

She's currently drydocked while I do some tweaking to my WTC mounts, my battery holder, and I install a new high-output LED lighting system. After I get her underway again, I'll taker her out to Langford Lake and shoot some more video so everyone can see what I'm talking about.

This has been a great thread, (most of the technical jargon partially over my head, but educational nonetheless).