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Physics of Airtrack Operation, specifically, how an airtrack causes the glider float

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adamxrt
#1
Feb14-12, 02:08 PM
P: 28
Hello,

Could anyone help me understand what im doing here. This is NOT a homework question. Im a mechanical engineer in my final year at uni, and my project is to design and build an airtrack for a range of undergraduate mechanical experiments.

Im sure many of you have seen videos of airtracks operating on youtube and the like,.... well my project is to take those airtracks and expand and make them better so that they may be used for a range of experiments through different topics, and eventually actually design and build it.

My understanding of mechanics and dynamics is decent enough, but definitely not my strongest point. Im more of a concept/cad/design minded engineer, but i took this project in order to challenge myself.


Now, Im trying to go through the basics of how airtracks operate, and give near frictionless movement. Obviously i have to create this environment with my airtrack, so i have to know how it works. I know the airtrack creates a layer of air underneath the slider to counteract the downward force of the slider. But i dont know where to start with the actual maths etc behind it.

The airtrack is going to be a 2m long triangular(isosceles with the 90 degree apex pointing up) aluminium tube, and i need to set the air pressure, need to define how many holes to drill through the surface, their spacing, how heavy to make the sliders... etc etc.


So there are alot of things to work out, but in order to do all these i need to do th emaths first.

Can anyone help me start? where do i start? The area in particular i am focusing on at the moment is how the air coming out of the airholes creates a pressure and forces to hold the slider in equilibrium..



heres an airtrack in operation.

http://www.youtube.com/watch?v=l04IlMWNdkU


Many Thanks!!!
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technician
#2
Feb14-12, 03:22 PM
P: 1,506
The apparatus you describe is a fairly standard piece of equipment and usually uses aluminium 90 angle 'sliders'. In my school we made one along the lines you suggest, the air exit holes were very small, about 1mm diameter and spaced evenly on each side of the tube about 50mm apart. It worked well.
We also bought a commercial one which was very similar.
the air source was an old vacuum cleaner set on blow.
I think the main thing to relise that the air arrangement is to supply a thin film of air between the slider and the track rather than to provide a pressure to lift the sliders any great distance. With such small diameter holes the air flow problem is negligible. think it is a practical exercise to get it working rather than a mathematical exercise to design something that will work.
Hope this helps....it is not a difficult thing to make, just take care drilling and de-burring the holes.
PS
You need to go to some trouble to see that the tube is well supported with some means to adjust the level.
Tomorrow I will photograph our set up and if you are interested I will post the photographs.
sophiecentaur
#3
Feb14-12, 04:04 PM
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I guess what you're after is an idea of the actual pressure that's needed. Not an easy question to answer fully but, for your glider to be supported, the pressure underneath it needs to be at least its weight divided by the area of the base. Air will come out of the small holes in the track and, mostly, spray out into the atmosphere. The air from the holes underneath the glider will be 'trapped' or 'delayed' on their path to the atmosphere via the base of the car.
The leakage from under the glider will depend on the clearance and 'flatness' of the contact and the balance. If the car is lopsided then air will escape from one side and lower the pressure enough for the car to 'ground'. Assume that everything is just right, for a start.

From memory, a standard glider will have a mass of about 0.5kg, so a weight of about 5N. The area is around 0.02m2. The minimum pressure to keep it up would be
5/0.02=250Pa.

Atmospheric pressure is about 105Pa so the minimum pressure needed would be only 1/400 Atmospheres. That's not very helpful but is IS a minimum value. Allow a factor of 10 (why not?) and you're up to 1/40 Bar or 2.5kPa. I should think that. if your blower can maintain that pressure when supplying all the holes you have then your glider should 'glide'.

I guess the point is that you need a LOT of air at low pressure, rather than a high pressure source with not much volume.

PS. I feel very smug because I just found a link for an air track system that is requires > 5.8kPa - only twice what I estimated. YES MY SON!. Note the tight tolerances there are for clearance.

adamxrt
#4
Feb15-12, 01:47 PM
P: 28
Physics of Airtrack Operation, specifically, how an airtrack causes the glider float

Sophiecentaur, thats the type of info im after. Surely theres more to it though? I mean what youve described to me is very basic.


Look at it this way.. how do i go about finding an arrangement and a pattern for drilled holes to provide me with adequate pressure of 250Pa??



Hi technician, i see what you are saying. Unfortunatley i cannot just get one to work in a practical sense...this is my final year project and everything will have to be pre thought out and calculated before anything gets made. so Unfortunatley i will have to mathematically design it!!



And surely there would be some flow rate equations to be applied due to air flowing though the holes???

After all this is my final year project and detail is required where it can be found!!!
adamxrt
#5
Feb15-12, 02:06 PM
P: 28
I have attached a cad picture to this post of what i have for a preliminary design. (very Early) the airtrack will have modifiyable angle of incline via a leadscrew. im also looking at the use of reed swith magnet circuids and bicycle pedometers for the measurement of the slider postion times. We're also looking into a laser. but what you see along the edge of the track are small magnets.
Attached Thumbnails
airtrack 2.JPG  
technician
#6
Feb15-12, 02:54 PM
P: 1,506
I don't know what your brief is but I would suggest that air flow and pressure problems were the last thing you needed to worry about.
The only other practical advice that I can offer is that the holes along the sides of the track should be towards the edges of the riders, I would say about 2/3 of the way from the apex. Thefirst one I made had the holes near the apex and the riders tilted and touched the track. I blocked these holes with plasticene and drilled a new set furthe down the track and that worked.
If I had to guess I would say it had something in common with hovercraft. The first hovercraft was a large fan blowing air down from the centre of the craft. A great improvement was to deflect the air into a skirt, this meant that the air from the fan 'trapped' air underneath the hovercraft and less power was wasted because less air escaped.
I like to think that the holes lower on the rider 'trap' air above them and ensure a more stable hover.
I would also add.... as a guide.... that you need at least 3 holes along the length of a rider.
If your brief requires a tilt mechanism do you realise that you will need a bearing of some sort to cope with the changing angle of the screw as the track is raised?
adamxrt
#7
Feb15-12, 03:24 PM
P: 28
Quote Quote by technician View Post
I don't know what your brief is but I would suggest that air flow and pressure problems were the last thing you needed to worry about.
The only other practical advice that I can offer is that the holes along the sides of the track should be towards the edges of the riders, I would say about 2/3 of the way from the apex. Thefirst one I made had the holes near the apex and the riders tilted and touched the track. I blocked these holes with plasticene and drilled a new set furthe down the track and that worked.
If I had to guess I would say it had something in common with hovercraft. The first hovercraft was a large fan blowing air down from the centre of the craft. A great improvement was to deflect the air into a skirt, this meant that the air from the fan 'trapped' air underneath the hovercraft and less power was wasted because less air escaped.
I like to think that the holes lower on the rider 'trap' air above them and ensure a more stable hover.
I would also add.... as a guide.... that you need at least 3 holes along the length of a rider.
If your brief requires a tilt mechanism do you realise that you will need a bearing of some sort to cope with the changing angle of the screw as the track is raised?

Yeah. Im well behind on all this stuff. Bearings etc, will all come into play in the final design. What you see there is my early final concept. My Tutor wants me to concentrate on defining the variables included in the mechanics of the airtrack, and also the dynamics behind the possible experiments 1st. My brief is very open.

For example, we're looking at having a dc motor on a lang slider and investigating the vibrations on a frictionless surface,

Possibly attaching a CPU fan to a slider to investigate thrust, have clay end stops to investigate collisions. theres a whole range of things to go through. All in due course though.


This is the thing man, theres so much going on that i literally havent a clue where to start!


In my notes for my next meeting, i have to determine load capacity, airflow, hole diameters spacing and air pressure, begin looking into a starter mechanism, and begin looking at sleigh design.
technician
#8
Feb15-12, 03:31 PM
P: 1,506
3 holes per slider near the bottom edge. Start with that
adamxrt
#9
Feb15-12, 03:38 PM
P: 28
Quote Quote by technician View Post
3 holes per slider near the bottom edge. Start with that


Sure, sounds like a decent idea, do you mean that the rider will always be "over" 3 holes at anyone given time in the length of the slider direction????

MY tutor will probably ask me how i came to such an number but ill just have to tell him i made assumptions for this par.

Did you play around with hole spacings in the breadth of the slider direction??? or did you have them square 50*50 mm
technician
#10
Feb15-12, 03:41 PM
P: 1,506
I came to the conclusion 3 holes per rider because I do a lot of sawing (metal and wood) and it soon becomes obvious, from a practical point of view, that you need at least 3 teeth in contact with the material.
Thin metal needs fine teeth
sophiecentaur
#11
Feb15-12, 05:26 PM
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Quote Quote by adamxrt View Post
Possibly attaching a CPU fan to a slider to investigate thrust, have clay end stops to investigate collisions. theres a whole range of things to go through. All in due course though.
I have used several blowers for a single run of demo airtrack, used in School. We tried various makes of cylinder vacuum cleaner (on blow) and none of them was anything like as good as a purpose built blower. I think a CPU blower would probably be inadequate - because that isn't what it was designed for - and you may be wasting your invested time if you spend too much time on trying it.

If you are doing a design study then I don't think there can be any objection to seeing what existing designs are like ('the shoulders of giants' ,as Sir Isaak said and "genius steals" which I believe Picasso said).
I like the idea of having the holes near the edges of the sliders and the other practical points made by technician.

The sort of calculation that I showed in my other post are pretty certainly what your tutor would like to see - as long as you can justify your assumptions about masses, areas etc. You could also look at data on the flow rates of air through various sizes of small hole for various pressures. Using a value for required pressure and multiplying by the number of holes should give an indication of the volume of air needed per second and help you to come up with a realistic spec for a suitable blower.
Any more calculations about the rate of air escaping from under the slider for a given ride height would be a bit difficult - you'd need to do actual measurements.
Do you have access to a seriously beefy blower or is your suggested CPU blower all you have? You will need to be pretty ingenious with a short length of track design as leaks are a real issue with home made gear of this sort. Consider a vacuum cleaner instead.
Integral
#12
Feb15-12, 05:50 PM
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From looking at your picture my thought is that you do not need that much angle adjustment. If these are very slightly out of level you get a very good acceleration. As steep as you pic shows it is not clear to me that the glider would ride correctly.
adamxrt
#13
Feb15-12, 05:53 PM
P: 28
Quote Quote by sophiecentaur View Post
I have used several blowers for a single run of demo airtrack, used in School. We tried various makes of cylinder vacuum cleaner (on blow) and none of them was anything like as good as a purpose built blower. I think a CPU blower would probably be inadequate - because that isn't what it was designed for - and you may be wasting your invested time if you spend too much time on trying it.

If you are doing a design study then I don't think there can be any objection to seeing what existing designs are like ('the shoulders of giants' ,as Sir Isaak said and "genius steals" which I believe Picasso said).
I like the idea of having the holes near the edges of the sliders and the other practical points made by technician.

The sort of calculation that I showed in my other post are pretty certainly what your tutor would like to see - as long as you can justify your assumptions about masses, areas etc. You could also look at data on the flow rates of air through various sizes of small hole for various pressures. Using a value for required pressure and multiplying by the number of holes should give an indication of the volume of air needed per second and help you to come up with a realistic spec for a suitable blower.
Any more calculations about the rate of air escaping from under the slider for a given ride height would be a bit difficult - you'd need to do actual measurements.
Do you have access to a seriously beefy blower or is your suggested CPU blower all you have? You will need to be pretty ingenious with a short length of track design as leaks are a real issue with home made gear of this sort. Consider a vacuum cleaner instead.


I think you maybe misunderstand what i mean about computer fans. We havent specced a blower yet. I meant that a fan can be attached to a battery, and placed on a SLIDER, to investigate thrust of a fan under frictionless conditions!


Thanks for the suggestions on looking up flow rates! ill do that now.


Can you clarify some things about your last calculation please??


You mention including a factor of 10....that brought you up to up to 2.5kPa...why? i dont fully understand.


Also when you were speaking about area of the slider..did you mean just like a flat area? Look at the attached picture to see what i mean.



I made an assumption of, for the sakes of getting some calculations down,
Slider length=200mm
2 rows of holes
Base of slider pitch =100mm
mass=150g so weight=1.47N

i tried to a bit more indepth than your calculation, worked out a load at 45 degrees to the face of the track for either side, to find the normal force it would take come straight out perpendicular to each face, to hold the glider in equilibrium. Maybe ive tried to be too clever and done it stupidly....



anyway i came out with a force of
0.625N to come out of each face.....and then the pressure supplied would be the area of that face divided into that force...


My area for the face turned out to be 1/150m^2 so pressure is 94Pa, if i included a factor of 10, 940Pa


Obviously ive probably calculated for too light a weight. But its a start anyway.


What do you make of it? But if you could clarify the point above?
Attached Thumbnails
areas of underside of slider.jpg  
adamxrt
#14
Feb15-12, 06:02 PM
P: 28
Quote Quote by Integral View Post
From looking at your picture my thought is that you do not need that much angle adjustment. If these are very slightly out of level you get a very good acceleration. As steep as you pic shows it is not clear to me that the glider would ride correctly.

Yeah that became apparent once i thought about it afterwards. I maybe got over zealous with the CADing up of it. theres no way it would need to be that high i should think. Plus i need to leave room for adding in a starting mechanism.

Thinking a compression spring..., maybe a blast of compressed air (very hard to measure how much force this would give though), or a pendulum impulse hammer, which u see a very very very vague one half modeled in the picture.
adamxrt
#15
Feb20-12, 05:10 PM
P: 28
Ok back onto this tonight! i had a meeting with my project overseer on thursday. He was expecting a lot more indepth work for the pressure and airspeed bwtween the glider and the track! he gave me a lot of stuff on bernoullis theorem and told me to assume incompressible flow (Q=VA) for this area and go work on it further. i wish i had gone straight back onto it after he spoke to me because ive forgotten the most of the things he said and i cant find where i wrote it down! Just have the equations.



anyone have any idea how to apply this?
sophiecentaur
#16
Feb20-12, 05:32 PM
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2adamxrt

You asked me why I chose a factor of 10 up on my estimate. That's a pragmatic / practical approach so as to err on the safe side. Fluid engineering is not my field - in Electronics, you'd be working to within +/- a few % and with antenna design you would be happy within a factor of 2, perhaps.

To be more precise about the requirement, you would need to decide what clearance you would need for the glider over the track (both for the right friction and to clear any bumps). Then you would have to calculate the pressure needed (weight / area) and the rate of flow of air out, under the glider (that's where one of your formulae would come in. Then you'd have to work out how much excess pressure you would need inside the track, to keep this pressure under the glider from the hole size and number of holes. Then you would need to know the capacity of the blower if it is to maintain this internal pressure with ALL the track holes leaking all the time.
Several steps in there but I think that's the logical way to tackle it. The clearance would depend on how precisely you can engineer it but you could google airtrack design perhaps for some details like that. The friction is something else to work out, of course, for the dynamics bit of the experiment. There are formulae for working out friction for air bearings.
rcgldr
#17
Feb20-12, 08:04 PM
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The math for this is somewhat complicated. You need a required rate of flow at some amount of pressure going into the track. There's a pressure loss as air flows through the holes, and a second pressure loss as air flows from the holes outwards towards the edges of the slider. Compounding the matter is that the air flow expands radially outwards from the holes in the track to the edges of the slider which contributes to the pressure loss from the holes to the edges of the sliders.

The sliders themselves are somewhat self-stabilized, since if one side "pinches" the air flow, the pressure increases and corrects the slider. The hole spacing will need to be fairly uniform with the holes lined up at any cross section of the track to maintain stability.
adamxrt
#18
Feb20-12, 08:25 PM
P: 28
thanks for the replies. that seems like the best logical structure to approach from, SC.

rcgldr. If i understand you correctly, pressure loss occurs when the air particle is still in the cutout of the hole. I was aware of that. There is then another second pressure loss as it passes out of the hole just above the surface of the track, again which i was aware of. But you are saying theres a sub loss after this? I dont fully understand how that comes to be? is it something to do with roughness of the glider surface?


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