Underwater Lift

[Mtns2Skies]

Is it doable? Water is in-compressible but also acts as a fluid as air does. Perhaps Bernoulli's principle will not work? Would AoA achieve the same result?


Just an odd theoretical equation spurred by hydrofoils.

 

[Non Compos Mentis]

Every water skier depends on lift to get up on top of the water when they yell "Hit it!"

Since water is so dense, even a lousy "airfoil" like a ski will lift quite the un-aquadynamic load.

 

[N801BH]

Every water skier depends on lift to get up on top of the water when they yell "Hit it!"

Since water is so dense, even a lousy "airfoil" like a ski will lift quite the un-aquadynamic load.

That really is not "lift".. it is more like deflection of one surface to another...

 

[Mtns2Skies]

That really is not "lift".. it is more like deflection of one surface to another...

Ben... so is a wing. The curverature of the wing just aids the lift but without it such as flying inverted it is created more inefficiently by the deflection of air.

 

[N801BH]

Ben... so is a wing. The curverature of the wing just aids the lift but without it such as flying inverted it is created more inefficiently by the deflection of air.

Agreed.... to a degree.... A Slight degree..

 

[PilotAlan]

Dunno.... I know there's underwater craft that have positive buoyancy and use inverted 'wings' to control depth. But I don't know if they are airfoil shaped or simply are a flat plate creating downforce.

 

[ClimbnSink]

Plenty of underwater foils that lift boats. Google foiling moths or the vespa sailrocket for a start.

 

[Capt. Geoffrey Thorpe]

Airfoils work fine underwater.

Newton and Bernoulli do not evaporate in a liquid.

 

[Mtns2Skies]

Airfoils work fine underwater.

Newton and Bernoulli do not evaporate in a liquid.

Well... if water is not compressible does that mean that the water ontop of the foil will not be able to move faster and be less dense?

 

[N801BH]

Well... if water is not compressible does that mean that the air ontop of the foil will not be able to move faster and be less dense?

I think you meant WATER instead of AIR.... But..

it is a great question.....

I ain't smart enough to answer it either..

 

[Seanaldinho]

Hydrodynamics and aerodynamics are very similar.

I would think that "lift" is certainly achievable.

 

[N801BH]

On second thought... There has to be "lift" in water... Propellers on boats act like an airfoil and on subs,, those things are TOP secret..

 

[PilotAlan]

On second thought... There has to be "lift" in water... Propellers on boats act like an airfoil and on subs,, those things are TOP secret..

There's no question of lift in water.
The question is does an 'airfoil' shape generate lift, or does water's incompressibility preclude Bernoulli effects and limit you to Newtonian effects only?

 

[dmspilot]

What part of Bernoulli's theory depends on air being compressible? In fact, it is just the opposite, it works because air is not compressible at low speeds.

 

[PilotAlan]

What part of Bernoulli's theory depends on air being compressible? In fact, it is just the opposite, it works because air is not compressible at low speeds.

Beats me. It's way over my limited physics knowledge.
I'm good a clarifying and specifying the question, not so much as answering at this level of physics.

Air is incompressible as low speeds? Didn't know that.

 

[Capt. Geoffrey Thorpe]

Beats me. It's way over my limited physics knowledge.
I'm good a clarifying and specifying the question, not so much as answering at this level of physics.

Air is incompressible as low speeds? Didn't know that.

Bernoulli's equations (whicn can be derived from Newtons Laws and a couple assumptions) assume incompressible flow. Air at well under sonic speeds behaves like it is effectively incompressible.

It's just pretty much conservation of of energy - the equations trade off kinetic energy for pressure.

 

[BillTIZ]

Sails on boats work as airfoils work.
Shaped keels on boats work as airfoils work.

Look at some of the studies done on keel design with America's Cup racing sailboats.

 

[PilotAlan]

Bernoulli's equations (whicn can be derived from Newtons Laws and a couple assumptions) assume incompressible flow. Air at well under sonic speeds behaves like it is effectively incompressible.

It's just pretty much conservation of of energy - the equations trade off kinetic energy for pressure.

Thanks, that actually made sense to me! (and that's not easy)
One who really knows a topic can explain it in simple language (which is good for me).

 

[Sac Arrow]

Is it doable? Water is in-compressible but also acts as a fluid as air does. Perhaps Bernoulli's principle will not work? Would AoA achieve the same result?


Just an odd theoretical equation spurred by hydrofoils.

Bernoullis actually does not apply to compressible fluids, but the approximation is close enough for subsonic speeds.

 

[Mtns2Skies]

Thanks for clarifying Alan. Thanks for the info Capt. Geoffrey and Engineer Sac Just an odd curiosity that I wasn't sure of.

 

[wanttaja]

On second thought... There has to be "lift" in water... Propellers on boats act like an airfoil and on subs,, those things are TOP secret..

And don't forget, a little-known naval manufacturer called "Boeing" built several vessels that depended on underwater lift.

Ron Wanttaja

 

[gismo]

Beats me. It's way over my limited physics knowledge.
I'm good a clarifying and specifying the question, not so much as answering at this level of physics.

Air is incompressible as low speeds? Didn't know that.

Ever try to squeeze air in a balloon? When a (subsonic) airplane wing pushes against the air, the air moves out of the way.

 

[tonycondon]

fluid is fluid.

 

[TMetzinger]

fluid is fluid.

Not to a crime scene tech!

Sent from my SCH-I200 using Tapatalk 2

 

[flyingcheesehead]

Is it doable? Water is in-compressible but also acts as a fluid as air does. Perhaps Bernoulli's principle will not work? Would AoA achieve the same result?

Just an odd theoretical equation spurred by hydrofoils.

The mere existence of working hydrofoils proves that underwater lift is possible - That's how they work!

That new Lisa amphib LSA uses a pair of hydrofoils sticking into the water rather than a boat hull shaped fuselage - Very cool!

Also, I've seen an airplane "fly" in water (on video). The one I've flown that went to the bottom of Lake Michigan nearly 8 years ago. To get it to shore so the NTSB/FAA could take a look, they floated it up using dive bags in the cabin, which brought just the top few inches of the plane above water. When they began towing it toward shore, the plane began to lift out of the water. It raised up a couple of feet, so the wings were still underwater but over half the fuselage was above water when they were moving. With the water being so much thicker than air, the low speed didn't matter.

 

[guardrail]

fluid is fluid.

Was going to say the same thing. Anything in a gaseous or liquid state is readily considered a fluid. Bernoulli's Principle applies to any fluid. Even some solids aren't actually solid in the sense that they never change, think old lead glass windows...after a couple hundred years they begin to sag towards the bottom.

 

[jab3racer]

Hydrodynamic lift is the primary method that submarines maintain depth control. The faster the sub is going the less overall positive or negative buoyancy matters. At slow speeds the control surfaces have less effect.

 

[MAKG1]

Was going to say the same thing. Anything in a gaseous or liquid state is readily considered a fluid. Bernoulli's Principle applies to any fluid. Even some solids aren't actually solid in the sense that they never change, think old lead glass windows...after a couple hundred years they begin to sag towards the bottom.

No. It applies to any INVISCID fluid. No fluid is truly inviscid, but air is a decent approximation at scales above a millimeter or so. Air has much smaller viscosity than water.

There is a similar effect, but it is not an identical effect.

You're assuming spherical cows.

 

[PilotAlan]

Ever try to squeeze air in a balloon? When a (subsonic) airplane wing pushes against the air, the air moves out of the way.

That's a expanding structure. Put air in a syringe with the end capped, and you'll see that you can compress it.
I guess all those cans of compressed air at the computer store are falsely advertising.

But what I am hearing is that at the speeds and pressure differentials with a subsonic airfoil, air behaves the same as an incompressible fluid.

 

[Henning]

Is it doable? Water is in-compressible but also acts as a fluid as air does. Perhaps Bernoulli's principle will not work? Would AoA achieve the same result?


Just an odd theoretical equation spurred by hydrofoils.

Yes, every propellor on a boat functions under the same NEWTONIAN rules of motion, as Bernouli's 'law' is merely an observation of the results of Newton. (This is the most fun thing you can say at a party full of aerodynamic engineers, even guys with PhDs behind their name will get into fist fights over it after a couple of Jack and Cokes...) I had an old Aqua Zep that I mounted a canard on for ease of handling, and there is a very cool positive buoyancy dynamic sub that looks like an F-18 that requires forward motion to dive. When the battery dies or motor fails, it automatically rises to the surface. It also has a lift bag incase you get in real trouble. It's a pretty cool rig if you got a mil and a half to spare and a boat to haul and launch it with. They also offer training in it without purchase for a few grand.

 

[N801BH]

That's a expanding structure. Put air in a syringe with the end capped, and you'll see that you can compress it.
I guess all those cans of compressed air at the computer store are falsely advertising.

But what I am hearing is that at the speeds and pressure differentials with a subsonic airfoil, air behaves the same as an incompressible fluid.

I would still like to see actual pics of the shape of the hydrofoil surface that is doing the lifting.. Is it really an air /hyrdofoil shape or just a flat piece of metal that changes angle of attack to lift the ship ??..

The pic of the cruiser that Ron posted is interesting.. BUT.. what is the actual shape of the hydrofoil's lifting surface.

Anyone got pics ???

 

[N801BH]

Yes, every propellor on a boat functions under the same NEWTONIAN rules of motion, as Bernouli's 'law' is merely an observation of the results of Newton. (This is the most fun thing you can say at a party full of aerodynamic engineers, even guys with PhDs behind their name will get into fist fights over it after a couple of Jack and Cokes...) I had an old Aqua Zep that I mounted a canard on for ease of handling, and there is a very cool positive buoyancy dynamic sub that looks like an F-18 that requires forward motion to dive. When the battery dies or motor fails, it automatically rises to the surface. It also has a lift bag incase you get in real trouble. It's a pretty cool rig if you got a mil and a half to spare and a boat to haul and launch it with. They also offer training in it without purchase for a few grand.

I know propeller design for subs is TOP secret... But,,, it is more aimed to running silent through the lack of cavitation, not actual propeller efficiency for propulsion.. Correct?

 

[gismo]

That's a expanding structure. Put air in a syringe with the end capped, and you'll see that you can compress it.
I guess all those cans of compressed air at the computer store are falsely advertising.

But what I am hearing is that at the speeds and pressure differentials with a subsonic airfoil, air behaves the same as an incompressible fluid.

That's what I was trying to analog with the balloon. Air is indeed compressible but it pretty much behaves like an incompressible fluid at subsonic speeds.

BTW, "incompressible" doesn't mean you can't raise the pressure by applying force. IOW you can produce "pressurized" water with a "water compressor" also known as a pump. Of course if you packed "compressed water" into a can the pressure would drop rather rapidly when you let some out.

 

[Henning]

I would still like to see actual pics of the shape of the hydrofoil surface that is doing the lifting.. Is it really an air /hyrdofoil shape or just a flat piece of metal that changes angle of attack to lift the ship ??..

The pic of the cruiser that Ron posted is interesting.. BUT.. what is the actual shape of the hydrofoil's lifing surface.

Anyone got pics ???

I know propeller design for subs is TOP secret... But,,, it is more aimed to running silent through the lack of cavitation, not actual propeller efficiency for propulsion.. Correct?

Correct, it's about noise on the props. If you look at the foil on a hydrofoil it looks a lot like a supersonic wing.

 

[Clark1961]

That's what I was trying to analog with the balloon. Air is indeed compressible but it pretty much behaves like an incompressible fluid at subsonic speeds.

Unconstrained is nowhere near incompressible...

 

[wanttaja]

The pic of the cruiser that Ron posted is interesting.. BUT.. what is the actual shape of the hydrofoil's lifting surface.

Anyone got pics ???

From the bottom view, it looks a lot like a wing:

Not having too much luck getting good edge-on pix of a PHM's foils. Here's a shot of a hydrofoil for a model:

When you think about it, given the drag of the water, a wing-type cross section is probably the best pick, both lift-wise and structurally. Most online sources describe them as having curvature similar to an airfoil. One source said the PHM wings were flat-bottomed.

Ron Wanttaja

 

[Henning]

From the bottom view, it looks a lot like a wing:

Not having too much luck getting good edge-on pix of a PHM's foils. Here's a shot of a hydrofoil for a model:

When you think about it, given the drag of the water, a wing-type cross section is probably the best pick, both lift-wise and structurally. Most online sources describe them as having curvature similar to an airfoil. One source said the PHM wings were flat-bottomed.

Ron Wanttaja

One thing you will note on any hydraulic foil be it for pushing or lifting, it will have a square (or double convex 'w' if they are getting really tricky) trailing edge to produce a small set of vortices that reduce drag and cavitation.

 

[ebykowsky]

Both Newton's laws of motion and Bernouli's principles apply in these situations, but Newton's applies to a larger extent in water because it is a more viscous liquid than air, and is therefore able to exert a much higher "equal and opposite" force on the object traveling through the solution--especially at the low speeds realized in water. The pressure differential in water will be less significant than that in air due to the viscous nature of the water, but will still play a role. I wonder what would happen if we got an underwater craft with the same density ratio to water that an aircraft has to air...?

 

[ebykowsky]

fluid is fluid.

Really?

How about this?
^^Zero viscosity=screw gravity! Does that count as lift?
(the second link gets good at 1:00 in--unless you want the background)