Planing speed

Surely JSL you are not suggesting that because a hovercraft displaces some water, that it is held up by buoyant forces ------------are you?

 

Interested to hear the answer to this.

Barry said earlier "buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object due to the volume of water ( or alternative fluid) that it displaces"

 

When a hovercraft is sitting still it has a pressure under the hull that supports the weight of the craft. That pressure pushes down on the water, and it does absolutely displace a volume of water equal to the weight of the craft. And if you cut a cross section you would see the water level under the hull is lower than the surrounding water level. That level would be the same as if there was a hull supporting the weight of the craft, and the volume of water displaced will exactly equal the weight of the craft, so yes, a hovercraft acts exactly like a large flat bottomed displacement hull when it is sitting still.

Just because the area is large and the depth of water displaced is small doesn't mean it doesn't displace water. Something has to keep the craft from sinking when it isn't moving, and that is the displacement of water.

 

That looks like a pretty sound analysis to me. Barry seems obsessed with disproving the existence of non-dynamic lift at speed, it appears he has the job ahead of him.

 

The only obsession that I have is for someone to quantify the buoyant forces at planing speeds.

Yellowjacket says "and it does absolutely displace a volume of water equal to the weight of the craft."
"Something has to keep the craft from sinking when it isn't moving, and that is the displacement of water."

I assume that you mean that when the hovercraft is powered up but not moving forward, ie the fan is creating enough pressure under the hull to support its weight and there is a opening beneath the skirt to water boundary.

Something has to keep the craft from sinking and that is the air pressure that the fan can create under the hull. It is not dependent on the amount of water it displaces. The impression of the water under the hull is just dependent on whatever the pressure that the fan creates.

If your guess is correct, then as the hovercraft accelerates the depression will remain the same for whatever speed it is moving at. A hover craft does not fly, nor does it benefit from ground effects, ie it does not have any method of creating lift and lowering its depression or wake, if you will, but for a fact, a hovercraft as it increases speed reduces its imprint
Can you explain this?

 

This is not a "guess", it is a fact. The increase in pressure below a static hovercraft creates a "depression" in the water whose displacement is equal to the mass of the hovercraft. Think about a pot turned upside down with air pumped into it. The amount of lift is equal to the difference in the water level inside the pot versus outside. Same thing for a hovercraft. The leakage around the sides is not material. This only holds true for the static case.

When in motion, a hovercraft planes just like a boat. The dynamic forces under the hull are not the same as a boat, but the net effect is the same. It has hump effects and has a reduction in wake at speed just like a boat hull. The hovercraft can be thought of as a "non-rigid" hull. Very similar to an inflatable boat. The reason that a hovercraft has lower drag is that there is no skin friction that a normal boat traveling on the water would have. Here is a link to a video of a hovercraft planing off, just like a boat does.

http://www.youtube.com/watch?v=CO4PX55uNnk

And here is a link to an excellent discussion of hovercraft planing effects, how it works and what happens. If you go down near the bottom of the page you will find typical drag curves for a hovercraft that show planing effects that are identical to a similar boat hull.

http://hoverclub.invisionzone.com/forum/index.php?showtopic=1091&page=11

Barry, please just quit your trolling and go read up and understand what you are talking about before you go questioning or theorizing. The information is out there, and it has become rather irritating trying to educate you when you go off on a tangent like this. All you would have to have done is google planing and hovercraft and all of your questions would have been answered.

I am done, unsubscribed.

 

Yellowjacket,

Most of this seems true but there are a couple of incomplete or missed points.

First, the hovercraft is also accelerating air (mass) downward and this must provide some dynamic lift in addition to the buoyancy from the water depression. Don't know how much but it must be there or other fan aircraft could not fly. Air that escapes the skirt has been accelerated downward by the fans or jets and must have provided some dynamic lift.

Second, the total lift on a boat at any speed (not including aero effects) must always equal the weight of the boat. The waves from a fast planing powerboat are not really less than from a slower boat, they are just spread out over time and each wave porton is therefore of lower magnitude. The total "work" to support the boat, per unit of time, must always equal the weight of the boat. At rest, there is no work done in supporting the boat since the time element is zero. Either that or Newton was wrong. I think there are some other factors involved in waves but as for lifting forces, I think this is essentially correct.

 

The amount of acceleration of air across the fan is small and therefore the direct lift created by the fan is small. You can measure it, but the thrust is essentially the fan area x the pressure rise. So there is a fan component, but compared to the area under the hovercraft, the thrust of the fan is second order. There is a small component of lift created by the acceleration of air through the fan, but if you look at the amount of power that goes into the air from the fan it isn't very large.

And Tom you are correct, and Newton has never been wrong... Force still equals Mass times Acceleration, or at least it did the last time I looked....

If you look at the link I posted you'll see an explanation of what happens under a hovercraft at speed over water. Essentially it acts like a boat, but the surface pressure distribution is not like that of a hard hull, it can't vary, so the water rises in places under the vehicle. Again, this is what would happen if you took an inflatable boat, since the pressure inside an inflatable is essentially constant.

We are digressing here into hovercraft, which are a very different and special case since the pressure at the surface of the water is constant, but is only slightly higher than the pressure outside of the vehicle.

 

I'm sure you are correct that the lift from fans in a hovercraft is small but they are a special case that I don't know much about. Other lift fans like in personal flying vehicles offer greater thrust but still are not all that effective and the "flying saucer" built by Avro could not lift itself more than a couple feet. The F35, Osprey and other similar VTOL craft are a different animal.

The shape of the displaced depression under a hovercraft at high speed is a mystery to me.

 

There are two components to fan lift. One is mass flow rate and other is velocity, the lift produced being the the mass flow rate times the velocity. And power required is essentially the same thing...

What happens with air cushion vehicles is that the amount of mass flow is relatively low. That allows them to pump into a cavity and produce lift very efficiently. The fans don't flow much, and only what leaks out the bottom is lost. Even though there isn't much flow, you have a good bit of pressure, and that is what gives you the lift you need.

When you are out of ground effect and don't have a skirt to capture the lift, you have to move a lot of air with a low velocity change to have efficient lift. As you go to a ground effect machine, you seal the bottom and your only lift losses are the leakage from around the skirt. Lift fans are totally different than fans for air cushions. Lift fans don't move as much air volume as a rotor that produces the same amount of lit, but they accelerate the flow to higher velocity, and that takes a lot of power.

 

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Barry, please just quit your trolling and go read up and understand what you are talking about before you go questioning or theorizing.

Yellowjacket, I almost always read up on a topic if I feel that I want a further understanding.


The information is out there, and it has become rather irritating trying to educate you when you go off on a tangent like this.

Yellowjacket, Almost all the information that people come to the forum to ask about is available in technical papers on the net, but the forum provides more than technical description, it provides, in most cases, some practical experience and solutions from a diverse cross section of people. Sorry if my questioning of some of your comments irritates you, this is not the purpose, but if you are incorrect, or if I want clarification, I will ask a simple question. You do not have to respond, but if you do, please be civil
Re the tangent, I did not bring up the hovercraft and buoyancy issue, JSL did. Years ago, just before Scat went under, we purchased 8 Scat recreational hovercraft in the 10 and 12 foot sizes for resale and when Scat tanked, we changed over to Hovertechnics craft and sold and flew several of those. I have flown these many hours and feel confident talking about them.
Additionally in 1970 I worked for 4 months between engineering semesters, for a company that was trying to develop a hoverdeck that was a platform with skirts, inflated by axial high volume low pressure fans, that carried part of the payload. The balance was carried by hydraulically driven low pressure high foot print tires similar to the tractor pulling it which was called a Delta 3.
( which had tundra tires, wide, low pressure)The goal was to carry heavy loads across the arctic permafrost without causing damage to the road bed.


All you would have to have done is google planing and hovercraft and all of your questions would have been
answered.



/QUOTE

So the next item is you suggest that a hovercraft flies in ground effect
While some of the untechnical discussions on the net discuss ground effect, the ground effect that I am most familiar with does not include a hovercraft within them
Ground effect terminology is applied to a winged entity (without skirts) where the vortices development are impacted by the proximity of the ground, which creates more lift and more drag.
With the exception of the Hoverwing, a combination, hovercraft and short fixed wing unit designed to take advantage of the ground effect, I have not found any technical paper that says that a cushion of air operating under a hovercraft inhibits vortices as there are no wings. But I would be happy to read another explanation of this

 

typo error, "which creates more lift and more drag" should read less drag

 

Another significant lift component to take into consideration, particularly at very high speeds, is the one of the propeller itself.