GoNavy

My Feedback: (15)

Is anyone aware of rules of thumb or guidelines for air tank (bottle) volume, i.e., how many cubic inches of air tank(s) for a given set of retracts? For example, can we determine the volume needed based upon the size of the pneumatic cylinders?

I don't see recommendations on the manufacturers' websites.
Another thread here recommended experimenting with various capacities.

I presume higher pressures can overcome lower volumes, but then the retracts operate faster than desired.
I haven't had much luck with regulating valves, but I've only had one installation.

I like to put the tanks in the wings, if possible, to eliminate air connections at each flying session. I'd rather just connect the lead for the switch servo.

Putting the tanks into the wings normally means smaller tanks, which means more tanks.

Jimmbbo

The Robart 191 tank is a good place to start... Since the sizes of the cylinders vary from one model gear to another, What I do to figure how big a tank to use is to calculate the volume of each cylinder, add the volumes and multiply by the number of retract/extend cycles you want:

e.g.; Let's say you are using three 3/8 bore dia x 1 inch stroke Robart 165 cylinders.

The volume for each would be: pi X R^2 X stroke, or 3.14 * (.187^2) * 1.0 = 0.110 cubic inches * 3 cylinders = 0.330 cubic inches per operation X 2 or 0.660 cubic inches for one retract/extend cycle.

If you want 10 full cycles plus 5 as a safety margin between refills (15 total cycles), that would require 15 X .660 or 9.90 cubic inches minimum tank volume...

If you want 20 cycles plus 5 as a safety margin (25 total cycles) that would require 25 X .660 = 16.50 cubic inches minimum tank volume...

So... http://www2.towerhobbies.com/cgi-bin...?&I=LXET37&P=7 is the Robart 191 tank, 30 cu in capacity... oughta work

Check out www.robart.com for other cylinder bore diameters/strokes and tank capacities

Cheers!

Jim

GoNavy

My Feedback: (15)

Jimmbbo: Thanks! Let's see if I've got it:

2 retracts, 3/4 inch diameter, 1.5 inch throw; One cycle of gear up and gear down consumes 2.65 ci. Ten cycles consume 26.5 ci.

Okay?

I took my measurements from the Robart Midwest AT 6 gear. The diameter is the outside diameter of the cylinder. If Robart identifies the
cylinder size and throw on their site, I can't find it.

I suppose the piston diameter is actually smaller, perhaps 5/8 inch? If so, I calculate 1.84 ci per cycle. That's a significant difference.

Assuming I fill the system to 100 psi, and assuming 30 ci of tank capacity, what pressure remains after 10 cycles at 2.65 ci per cycle? (I don't believe I want to go below 60 psi.) How do we calculate that?

Let's see: Sea level air pressure 14.7 psi......... aw, who am I kidding.

Jimmbbo

Gonavy,

Tried to back out the cylinder dimensions from the Robart website, but couldn't get the cylinder number from their description of the PN 620-108A strut, so no piston dimensions were readily available..

The cubic inch capacity (and the number of cycles) of the tank depends only on the initial pressure (minus minor leaks in the system-hence the safety factor), and in theory will cycle the gear till the tank pressure equals ambient pressure (not that you'd want to do that ). Hence my choice of a number of cycles as a safety margin, since we can't calculate the pressure loss of each cycle. If you feel more comfortable landing with 60 psi, you run tests to figure out how many cycles it takes to drop to that value.

Using your measurements, (which are conservative), your numbers are correct... You win a prize for math!! Since the volume of a cylinder varies as the square of the radius of the piston, decreasing the radius by 1/16th of an inch really decreases the volume per cycle.

Let's say the real cycle volume is conservatively 2.0 ci... Worst case is you get 10 applications with at least five as a safety margin... I'd be happy with that.

Cheers!

Jim

jeffk464

You can always toss an air line restrictor in your system to slow down the retract action.

GoNavy

My Feedback: (15)

Okay, I have a deep thought.

We'll assume that each up and down cycle consumes 3 ci. We round atmospheric pressure to 15 lbs/sq.inch.

If I pump the tank to 6 atmospheres, I have 90psi. I assume that the increase in air pressure is linear, i.e., twice as much air in the same volume doubles the pressure.

If my tank is only 3ci, and I start at 90psi, one cycle removes 1 volume, and 5 remain. Pressure is down to (5 x 15) 75 psi. A second cycle I am down to 60 psi, and it's time to quit.

If my tank is 30 ci, ten times a cycle, and I start at 90 psi, I should get (10 x 2) 20 cycles before I drop to 60 psi.

How say ye men of the jury?

Jimmbbo

Doubling the pressure will halve the volume (Boyle's law: P1*V1 = P2*V2)...

Your answers seem pretty close to the numbers we got earlier.. Gotta love it when a plan comes together....

Will be interested to see how close the "real deal" comes out...

GoNavy

My Feedback: (15)

Captain: The results are in. I set the retracts up for a test. First I used a single 14.5 ci bottle, starting at 90 psi. I then cycled the gear, meaning an up and a down, and read the pressure again. I did this until the gear would not lock in place.
The second test result (which I hope to attach) was a single bottle but starting at 100 psi. The third used two bottles, 29.0 ci, starting at 90 psi.

I conclude: 1. This gear operates very nicely, even down to less than 20 psi.
2. Higher starting pressure doesn't gain much in cycles, but more capacity sure does.

Robart AT-6 (Midwest) pneumatic gear

(One 14.5 ci bottle)

Cycle Psi (rounded)

0 90
1 78
2 66
3 58
4 50
5 42
6 36
7 30
8 25
9 21
10 17
11 lock failure


0 103
1 89
2 77
3 67
4 58
5 50
6 42
7 36
8 31
9 26
10 21
11 17
12 lock failure

(Two bottles, 29.0 ci)

0 90
1 84
2 77
3 72
4 67
5 62
6 57
7 53
8 49
9 45
10 42
11 38
12 35
13 32
14 30
15 28
16 25
17 22
18 20
19 18
20 17
21 15
22 failed to lock

Jimmbbo

Cool! Real world data!! [sm=thumbs_up.gif][sm=thumbs_up.gif]

I put together a spreadsheet with your data, and it proved most interesting... Your analysis of capacity being more useful than pressure is right on the nose!

What was interesting but no real surprise, is that the psi loss per retract cycle is non-linear, and is really noticeable with the larger tank... the last nine cycles running 3 or fewer psi per, while the smaller tank gives up pressure at a much higher rate - with the last two cycles at 4 psi per... Looks like "bigger is better" for sure...

Tried to PM the sheet, but RCU won't upload .xls files... PM me with your email and will forward to you..

Cheers!

Jim

Jimmbbo

Printed and scanned the graph - not a clean as an electronic version, but the results are clear enough... the slope of the curves for the 14.5 CI tank is pretty steep compared to the larger tank...

Cheers!

Jim