Hovercraft Thrust - Velocity verses Static Pressure

[kach22i]

I am trying to get my mind around concept of Velocity verses Static Pressure in simple terms.

Using a hovercraft as our topic model, I understand that high static pressure is desired for the lift system, and high velocity for the thrust system. Therefore it's nice two have two systems one for lift, one for thrust.

Here is where I get a little fuzzy, the case of a strong headwind. In a strong headwind more air enters the propeller or thrust fan at a faster rate, right? I'm not sure that is correct, just asking, and I don't know if it’s all that critical to my final question.

The question is; In a strong head wind is it more desirable to have velocity or static pressure coming out of the tail of your thrust unit?

The reason I am asking is that I think that not only does a strong head wind reduce you "ground speed", it creates a head of resistance which velocity is ill suited to contend with and overcome, and that static pressure is well suited to deal with it and push back.

What head wind creates is more than just resistance and drag, it changes the character of the fluid medium we call air.

Boats never run into "thick" or "dense" water do they? I would assume water is more predictable than air to design for.

Please help straighten my head out on this one, I have run into some real life experience on the topic, trying to paste experience and theory together this time.

NOTE: I am not an engineer, I am an architect. Please try to keep it simple for me, examples and pictures will help, charts and diagrams will not help as much.

 

[kach22i]

I set-up a similar question for waterscrews in a boat forum because the main difference (I think) is that air compresses - rest of the theory basis should remain constant.

I think the main physical property is density of the fluid medium (air).

Water does not compress and the most variance you will get is between salt water and fresh water, or perhaps froth from rapids, this might be the easier question to resolve.

Link:
http://boatdesign.net/forums/showthread.php?t=11121

 

[FredGarvin]

Unless you're talking about a hurricane headwind, I can't imagine a headwind causing the need to worry about compressability. Your concern in a headwind will be form drag. It is really pretty easy to estimate a Reynolds number for your situation and that will dictate where you are in the realm of incompressible vs. compressible.

You are correct that water is more predictible in that it is incompressible. However, with boats, there are other aspects that have to be considered, like wave interaction and drag.

In an answer to your question, let me pose this: To generate thrust, what must be said about the momentum of the air coming out of the thrust producing fan? That should point you in the right direction.

 

[kach22i]

In an answer to your question, let me pose this: To generate thrust, what must be said about the momentum of the air coming out of the thrust producing fan? That should point you in the right direction.

Thrust must be positive for me to go forward, right?

Let's say a big wind comes up from behind me in my hovercraft. I wish to go foreward at 20 mph, but the wind is 60 mph. Do I need a negative thrust of 40 mph? No I would not, because the hovercaft is not a sail catching all the forces put on it. But there would be some pushing, you can feel it. Also the negative pressure pocket of air forming on the feed (front) end of the duct will starve the propeller of feed air won't it?

Example:
This is one of the hardest concepts to get across. I run into this every summer with my wife. She puts the fan an inch from the wall and complains that we need a new fan. I pull the fan a foot or more from the wall and the breeze increases (and you can hear the difference). Therefore the rate the supply air enters the drive fan makes a difference.

Once we had a fan in the upstairs window on and a gust of air pushed the fan blades faster (was blowing towards the inside) which transferred the gust to the interior of the house - papers went flying. The sound was quite remarkable as the electric motor was being pushed and not pulling the fan blades.

Positive and negative pockets of feed air directly effect propeller/fan thrust and performance. What is this phenomena called?

 

[FredGarvin]

What I was getting at is that the thrust provided is actually the change in momentum of the airstream. The greater the difference created, the greater the thrust.

 

[kach22i]

What I was getting at is that the thrust provided is actually the change in momentum of the airstream. The greater the difference created, the greater the thrust.

I have read some waterscrew and air propeller theory so I have an idea what you are saying.

What I picked up so far is that a fan can draw more air in than perhaps a propeller, but it depends on the angle of blade/foil attack.

Also if the motor and blade/foil spend energy drawing air into the the fan/propeller there will be less thrust energy leaving the system.

Also you get more thrust energy if you have more energy entering the system (ramjet style) with Induced Velocity.

Interesting link:
http://www.flightlab.com/~he/flabqna/tblocnts/inflow.html

 

[russ_watters]

Since a hovercraft has virtually no friction drag due to the ground, it is probably simplest to consider the hovercraft's performance on similar terms as an airplane's: speed is airspeed, so if your hovercraft has a top airspeed of 60mph (typical) and you have a 60mph headwind, you'll have to lean on the throttle just to stay stationary.

 

[kach22i]

Since a hovercraft has virtually no friction drag due to the ground, it is probably simplest to consider the hovercraft's performance on similar terms as an airplane's: speed is airspeed, so if your hovercraft has a top airspeed of 60mph (typical) and you have a 60mph headwind, you'll have to lean on the throttle just to stay stationary.

This would be true only on smooth ice. In the real world, hovercraft encounter waves (hate those choppy ones) and lose bottom cushion seal over tall wavy grass.

 

[arthuritus]

How do you know how much static pressure you have in a hovercraft? I can work out velocity pressure alright, but how the heck do you do static pressure? I've asked heaps of people, but nobody seems to know. Can anyone help me here?

We can make it really simple. Say a circular 1m^2 hovercraft. Engine output of 800m^3/h. What else do I need to know?

Cheers

 

[kach22i]

UPDATE:

This comment has me thinking:
http://hovercraft.org.uk/FUDforum2/index.php?t=msg&th=602&start=0&S=9749d76b55ca36b924ecc58941c23071

There is a fundamental difference between good lift and good thrust. For lift you need a large volume of air moving slowly, whereas for thrust, a smaller amount of air moving fast!

In order to achieve a large volume of air flow, a slow fan with lots of blades is best, whereas for a smaller volume moving fast, you need a few blades spinning quickly.

Therefore when setting up a fan to do both jobs, it has to be compromised between the two set-ups.

Just a note on static pressure, the static pressure behind the fan is actually atmospheric (this can be shown by theory), the fan only generates dynamic pressure which is recovered to static pressure as the speed of the air flow drops throughout the lift air chambers.

What is this thing called dynamic pressure anyway?:yuck: