How to work out the thrust

[simong93]

Hi Guys im new to the forums but for the past year i had a great idea that i want to make happen. My first question is i can not work out thrust say for example i have a box 10x10x10x10cm pushing out air then sucking air in. So how would i work out how much thrust this would generate just buy pushing the air out once then how many RPM would it need to do to make a 100g item leviate.

 

I hope i havent confused to many people .

[studiot]

i have a box 10x10x10x10cm pushing out air then sucking air in. So how would i work out how much thrust this would generate just buy pushing the air out once then how many RPM would it need to do to make a 100g item leviate.

 

 

How many dimensions doe this box have?

 

No wonder it levitates.

 

Please try to expand on your question.

[simong93]

 

How many dimensions doe this box have?

 

No wonder it levitates.

 

Please try to expand on your question.

 

Im not to sure how would i know its a box in real life a square i think it would be 4d.

 

How can i expand this question ?

[studiot]

 

How can i expand this question ?

 

A 4D box? yes I think you definitely need to provide more detail to help us understand what you are trying to say.

[simong93]

 

A 4D box? yes I think you definitely need to provide more detail to help us understand what you are trying to say.

 

ok say say you have a 10cmx10cm 4d box filled with air and a little hole at the bottom say 2cm wide now say i was to push all of that air towards the hole. would this generate enough thurst to lift a 100g item

[imatfaal]

OK - we are getting confused with the number of dimensions. A normal box has three dimensions - ie width, length and height, we would describe a box like this as 10cmx10cmX10cm. Is this the sort of box you mean? A box with 4 dimensions - also called a hypercube or tesseract - is something we believe to be possible but that we cannot create or even properly envisage as our perceived reality only has 3 dimensions.

 

If the air was to come out quickly it might for a moment lift the box - especially if it was actually under pressure within the box; think balloon whizzing around the room or a cola-bottle rocket.

[simong93]

OK - we are getting confused with the number of dimensions. A normal box has three dimensions - ie width, length and height, we would describe a box like this as 10cmx10cmX10cm. Is this the sort of box you mean? A box with 4 dimensions - also called a hypercube or tesseract - is something we believe to be possible but that we cannot create or even properly envisage as our perceived reality only has 3 dimensions.

 

If the air was to come out quickly it might for a moment lift the box - especially if it was actually under pressure within the box; think balloon whizzing around the room or a cola-bottle rocket.

Ow sorry about that i didnt no.

 

Yep thats what i am trying to work out is this possibe is this going to generate enough lift to hover. I mean lets say we have 4 of these all at diffrent stages all pushing air out one at a time would this generate enough lift to make 100g hover or maybe more of less.

[swansont]

You need to know the mass flow rate of the air, and its speed. The product of the two will give you the thrust. F = dp/dt, and p = mv. This will work as long as the mass of the system is constant (i.e. blowing air through the system by taking in outside air). If the mass is changing, it's more complicated.

 

Since g is almost 10 m/s² you would need to shoot e.g. 100g of air per second out at 10 m/s. Or more air at a proportionally lower speed, or less air at a proportionally higher speed.

[simong93]

You need to know the mass flow rate of the air, and its speed. The product of the two will give you the thrust. F = dp/dt, and p = mv. This will work as long as the mass of the system is constant (i.e. blowing air through the system by taking in outside air). If the mass is changing, it's more complicated.

 

Since g is almost 10 m/s² you would need to shoot e.g. 100g of air per second out at 10 m/s. Or more air at a proportionally lower speed, or less air at a proportionally higher speed.

 

Ok now this is where i have a massive blond momment give me anything mechnical and i am fine but this i am lost.

 

So i guess the simple question would be could a 10cm square shoot out 100g of air per second out at 10 m/s or how big would i cube need to be

[studiot]

simong93,

 

You seem to be using different terminology from the rest of us.

 

I'm sure if you explained fully in simple ordinary English what you are trying to achieve someone will be able to help you.

Edited February 5, 2014 by studiot

[simong93]

simong93,

 

You seem to be using different terminology from the rest of us.

 

I'm sure if you explained fully in simple ordinary English what you are trying to achieve someone will be able to help you.

 

I am not to sure how else i can explain this.

 

You have a 10cm wide by 10cm high 3d box filled with nomal air we breathe.

 

At the bottom of the box there is a little whole at 2cm wide.

 

Now a plunger pushes all the air from the top to the bottom and out the little whole.

 

Then the plunger comes back up and sucks air back in.

 

This is then repeated over and over again my question is would this system have the ability to make a 100gram item hover?

 

If it could how many times per minute would this plunger need to go up and down to sustain a stable hover?

 

If the load is to heavy what would be the weight it could carry?

 

Or would the cube need to be bigger to get the necessary lift?

[swansont]

A 10 cm cube will have a volume of 1000 cm³, or 1 liter. A mole of an ideal gas at STP takes up 22.4 L with a mass of ~30g, so your box has just over 1 gram of air in it. The time it takes to move the plunger gives you your mass flow rate (e.g. if it takes 1 sec, it's 1g/s. If it's 10 ms, then the rate is 100g/s)

 

The speed would probably be determined with Bernoulli's equation + continuity equation and/or some other fluid dynamics. It will vary depending on the hole size and probably wall thickness (at some point, anyway. A thick wall will look like a tube).

 

From a practical standpoint you'd want the exit holes to be small, and a check valve on a larger opening that would let you draw air in quickly.

[studiot]

OK this is bit clearer.

 

I suggest that you use a cylindrical chamber rather than flat sided as the corners introduces unwanted effects.

 

The original hovercraft principle was proved using a domestic vacuum cleaner as a blower.

You could emulate this experiment.

 

Here are some pics and other useful links to the principle.

 

https://www.google.co.uk/#q=hovercraft+principle

 

This link offers a calculator and analysis of the thrust equations.

 

http://www.hovercraft.org.uk/showthread.php?18868-Fan-Thrust-Calculator

 

100g sounds rather light, don't forget you have to allow for whatever motor drives your piston although an axial fan may be better.

Edited February 5, 2014 by studiot

[simong93]

A 10 cm cube will have a volume of 1000 cm³, or 1 liter. A mole of an ideal gas at STP takes up 22.4 L with a mass of ~30g, so your box has just over 1 gram of air in it. The time it takes to move the plunger gives you your mass flow rate (e.g. if it takes 1 sec, it's 1g/s. If it's 10 ms, then the rate is 100g/s)

 

The speed would probably be determined with Bernoulli's equation + continuity equation and/or some other fluid dynamics. It will vary depending on the hole size and probably wall thickness (at some point, anyway. A thick wall will look like a tube).

 

From a practical standpoint you'd want the exit holes to be small, and a check valve on a larger opening that would let you draw air in quickly.

 

ok if i have worked this out right i can get 1 reveloution per 18.18ms so that would be something like 55g/s so a little under what i need but if i had four of these little blighters but the overal weight is still 100g that would mean this should hover with plenty left.

 

If i also shrink the hole down to 1cm this should make it alot easyer.

 

Is my maths correct or am i way off in the distance.

OK this is bit clearer.

 

I suggest that you use a cylindrical chamber rather than flat sided as the corners introduces unwanted effects.

 

The original hovercraft principle was proved using a domestic vacuum cleaner as a blower.

You could emulate this experiment.

 

Here are some pics and other useful links to the principle.

 

https://www.google.co.uk/#q=hovercraft+principle

 

This link offers a calcualtor and analysis of the thrust equations.

 

http://www.hovercraft.org.uk/showthread.php?18868-Fan-Thrust-Calculator

 

100g sounds rather light, don't forget you have to allow for whatever moter drives your piston although an axial fan may be better.

 

Many thanks for the links i will look at them tonight.

 

My current weight would be 100g this allows for 4 dc motors that weigh 20g each and allows for 20g for perspects.

 

I never thought of using a cylindrical chamber i will look into this.

[John Cuthber]

The speed of a fan in RPM tells you very little about how much air it moves.

You could imagine a fan with blades that are very nearly flat and perpendicular to the axis. It would hardly move any air at all no matter how fast you spun it.

It's complicated

http://en.wikipedia.org/wiki/Propeller#Propeller_thrust

I think most people would look at the manufacturer's data and see if there was a propeller that would do what they wanted.