LOSS OF POWER GRID: Survey says... [SkyVenture tunnels only]

[pope 0]

So in the interest of safety, maybe we could just get a feel for who's presenting the most accurate information in the previous Novel-post with a similar title.

I know there are other points of view included in the post, but I've just taken a couple of the main ones. I can't vote in the poll apparently, but I tend to side with the manufacturer on this issue myself.

cheers!
pope

[Paulipod 0]

Erm - Who is 'The Manufacturer'? exactly... Is this a SV spokesperson... salesman or Toshiba technical rep about VFD's?

Bodyflight Bedford
www.bodyflight.co.uk

[Hooknswoop 19]

Quote

It takes about 15 seconds for the wind to reach 30-mph after taking the throttle from 90% to zero."

That is a quote of what I wrote. It is the data from a test I did at the SVCO tunnel to show the difference that the 'braking' sytem makes to slow the wind down faster than what would happen if the power failed or the e-stop was hit. The wind takes about 45 seconds to reach 30-mph from 90% after the e-stop is pressed and power is cut to the motors.

If you go through my posts, I already wrote what the designer of the control system said about what happens if the power was cut, almost exactly the same as if the e-stop was pressed.

This only applies to SVCO, I do not know what happens in other tunnels.

Derek

[ryoder 1,384]

From my point-of-view, Hooknswoop's answer is the most plausible. However, just in case Diablopilot is right, if the event ever occurs I intend to cushion my fall by landing on my tunnel rat.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[Jewels 0]

LOL--excellent answer! Hooknswoop better hope he's right, then, 'cause there's a good chance that he'll be the one spotting me. OUCH!! No mercy.

TPM Sister #102

[mdrejhon 8]

Multiple choice was a good idea.

I think all tunnels have the potential to stop immediately. However, math shows there is enough momentum stored in a very efficient windtunnel to sustain flight for quite a while upon outage, if enough external resistance factors are eliminated. Air weighs 1.29 kilograms per cubic meter, so that is over 100 kilograms rocketing at 120mph - assuming the tunnel is low resistance enough, especially the fan. (The fan needs a complete disconnect from the grid in a power outage - like an e-stop, so it does not go generator and bleed electricity into a dead electric grid as it slows down)

Very minor things can mean the difference between paulipod, diablo, and Hooknswoop. Each and every one happens and has happened before.

Kudos to SVCO and their great efficiency. It obviously bleeds momentum more slowly.

Out of curiousity, how much of a power surge does SVCO need to get started from 0 to 120mph in the chamber? And how long does it take? This can, essentially be pictured is the little surplus of "energy" charged into the windtunnel (in the form of momentum) that will bleed out when power is cut...

Much like a small surge of gas is needed to accelerate a car to a certain speed on level ground, and the car will coast for a bit upon sudden engine failure (of the kind that doesn't suddenly brake the engine - I.e. Engine braking, etc). More efficient cars with good aerodynamics and rolling efficiency, will coast for longer. The like can be applied an efficient windtunnel will have significant momentum for quite some time after power loss. (Where the fans are not put into braking mode)

It just makes total sense.

[Paige 0]

I'm not sure if this is the case or not but I can lend some experience in different tunnels to the discussion. Every tunnel I've flown in has faulted when I was flying and one portable caught on fire lol! I'm still here and if you are a good flyer, you can feel the air starting to die off and prepare for the net if you are high up.

I found SVNH & SVAZ to shut down the slowest (to me it seemed like 1,2,3,4,5, shut off) while Orlando shut off faster (1,2,3 shut off). I could be crazy however and this could all be in my head

shut off=you aren't flying anymore, there is no air

Tunnel Pink Mafia Delegate
www.TunnelPinkMafia.com

[mdrejhon 8]

This also brings up an interesting question: What are the differences between SVNH and SVCO. Motors? Controllers? Firmware on the controllers? Controller configuration during power outage? Etc? (You Skyventure employees might want to research this matter.)

A scientific test would be useful. It appears for some reason, multiple posts in the past (not just yours) indicate SVCO shuts down more slowly than SVNH.

[pope 0]

Quote

I think all tunnels have the potential to stop immediately.

This is what I just don't get. If a turbine stopped "immediately," then it would blow itself apart. It HAS to spin down gradually. I'm no physics guru, but the "lights out, hit the net" theory makes absolutely no sense to me. But hey, I guess that's my problem, right?
pope

[sartre 0]

Quote

From my point-of-view, Hooknswoop's answer is the most plausible. However, just in case Diablopilot is right, if the event ever occurs I intend to cushion my fall by landing on my tunnel rat.

I managed to do that last time I was in there. I must say, it was one of the better moments of my day. I doubt poor Brad would say the same!

[Paulipod 0]

Quote

I'm no physics guru, but the "lights out, hit the net" theory makes absolutely no sense to me. But hey, I guess that's my problem, right?

Ok - I'll try one last time to make this clear... It takes many many kW of power to create enough lift... and the blades idly spinning once the power has gone is not even close. This whole air has mass enough to keep going idea works two ways : It takes energy to keep the mass moving and it also has a little momentum. However the thing to note with Mark's maths idea is that the wind speed around the tunnel is not even close to the test section velocity. ie Closer to stationary (around 10mph) than flying (100+mph)

As has been said prior - different tunnels have different ER values (energy ratio) which is a guage of the operating losses around the circuit. However all recirculating tunnels that I am aware of have an ER which is reasonable.

Ours like other facilities I have experienced power loss in lose flying lift quickly. ( Note there are as I have been informed cool facilities that have no such problems so I am not talking about those )

Unlikely my enthusiasm for typing in this thread is going to continue past there being nothin much on TV right now so hopefully I have said nothing too contentious here

Bodyflight Bedford
www.bodyflight.co.uk

[Jewels 0]

Oh, yeah, right. Brad was reeeaaallly suffering. I was there. I saw.

TPM Sister #102

[sartre 0]

tee hee

[mdrejhon 8]

Quote

Quote

I think all tunnels have the potential to stop immediately.

This is what I just don't get. If a turbine stopped "immediately," then it would blow itself apart. It HAS to spin down gradually. I'm no physics guru, but the "lights out, hit the net" theory makes absolutely no sense to me. But hey, I guess that's my problem, right?
pope

Rephrase:

I think all tunnels have the potential to stop too quickly for a human to react.

[mdrejhon 8]

Quote

Mark's maths idea is that the wind speed around the tunnel is not even close to the test section velocity. ie Closer to stationary (around 10mph) than flying (100+mph)

I seem to recall a figure of about 30mph for the skyventure recirculating, at least at full tilt. Can anyone confirm?

If it's only 10mph the ducts would have to have 12 times the cross section of the main tunnel area (correct me if I am wrong). A 12 foot diameter octagonal windtunnel chamber would have about 112 square feet of cross-section (assuming 12 feet measured flat side to flat side) (this is 10.4 square meters) For 120mph wind in the chamber, that'd be a whopping 1344+ square feet of cross section for a duct that had wind going only 10mph. That would mean the ducts would have to be over 36 x 36 feet wide for a square duct (SV recirculating tunnels appear to use square-ish ducts) for a 12-foot octagonal chamber. That would mean probably well over 100 cubic meters of air volume. Anyway, I'm switching bewteen metric and imperial, but SV measures the diameter of the chamber in imperial, but I'm more familiar with meters for many things. Anyway, there's likely far over 100 cubic meters of air volume, which would then anyway compensate for the slow 10mph speed. (at 1 cubic meter = 35.3 cubic feet).

Even so, that does not make sense at least from the sizes of recirc skyventure tunnels - they are way too small looking for such massive ducts. I'm inclined to go with a 30-40mph quote I read in some skyventure literature. This would require a duct cross section only 4 times the cross section of the chamber... This looks about right from the sizes of the tunnels I'm seeing in literature, the ducts look approximately twice the diameter of the chamber, maybe less than that.

I'm also especially curious about minor differences bewteen SVCO / SVNH / Airkix. For example, the turn vanes are important here for efficiency - spacing between turn vanes; specific spacing may reduces the resistance of looping air. Even the type of paint or surfacing on the inside walls of a recirc tunnel has a non-zero effect effect on friction: It's already proven in windtunnel testing of automotive finishes and paints that there can have a small effect. (Is there a difference in the interior finish of SVCO and SVNH / Airkix, for example?) Difference in bumps in walls (like "X" crossbraces in aluminum siding, versus perfectly flat walls) - that would have an effect. Blocking off the corners (i.e. a turn vane touching corner) versus dead air in corners that, etc.

Redoing the math, there's still enough momentum for enough time to do safe recovery, but does require the tunnel to be even more efficient to give a human time to fly.

Anyone from Skyventure who can confirm the number of cubic meters of air that a typical Skyventure recirc tunnel holds?

I was using conservative figures earlier, but let's plug more exact numbers. Let's first calculate the amount of cubic meters in a typical Skyventure chamber. We know that SVCO uses a 12 feet diameter tunnel.

Chamber Diameter = 12 ft (3.6 m)
Chamber Height = 20 ft (6.1 m)
Area of cross section = 112 sq ft (10.4 m^2)
Volume of chamber = 2240 cu ft (63.4 m^3)

Guesstimates:
Height of building = 62 ft (Source: SVNH)
Width of building = 80 ft (Source: Photos show tunnels in 4:3 aspect ratio)

Quoted 30-40mph airspeed in ducts, suggests cross section of 4 times the chamber, approximately 450 square feet, or approximately 20 feet wide square (20x20 = cross section of 400 square feet) for the slower 30mph figure. This seems a little bit fat even for the photos of recirculating SV tunnels, assuming scale of 62 feet height quoted for Skyventure Newhampshire. Photos clearly show ducts appear to be closer to 15 feet by 15 feet for most cross sections, although the bottom part appear to be 20 feet deep. This is probably where the 30-40mph Skyventure quotation comes from for the "slowest air", the thinner sections of the ducting probably hits faster airspeeds during 120mph bellyfly. Let's go with a conservative average of 15x15 feet for all ducting involved, for simplicity of math.

In all photos of recirculating skyventure tunnels, there's two long horizontal ducts (top and bottom) and three vertical ducts (left, center, and middle). Let's go with average 15x15 ft cross section for all ducting (I know it chokes to less than that through the fans and the chamber, but it appears to wider at the bottom of the SVCO building). We've already calculated the chamber to be 2240 cu feet (63.4 m^3).

Let's calculate only the top, bottom, left, and right ducts, and not bother with the ducting above/below the chamber, and not bother with the thickness of cladding for now, for simplicity of math:

Air volume of bottom part (including corners)
15ft x 15ft x 80ft = 18,000 cu ft

Multiply by 2 to get air volume of both top and bottom part
18,000 cu ft x 2 = 36,000 cu ft

Height of vertical sections minus top/bottom
62ft - 15ft - 15ft = 32ft
(This represents the connecting ducts between top/bottom at left/right, assuming the known 62 feet height of SVNH is equal to SVCO, and also represents approx duct height)

Air volume one vertical connecting duct
15ft x 15ft x 32ft = 7,200 cu ft

Multiply by 2 to get air volume of both left and right connecting ducts
7,200 cu ft x 2 = 14,400 cu ft

Total of horizontal and vertical sections
36,000 cu ft + 14,400 cu ft = 50,400 cu ft

Add central chamber:
50,400 cu ft + 2,240 cu ft = 52,640 cu ft

Let's even be generous and divide this by 2 to account for turn vanes, smaller cross sections, fan sizes, inaccurate estimates, thickness of walls, blocked-off corners, etc...

52,640 cu ft / 2 = 26,320 cu ft

We know that one cubic meter is about 35.3 cubic feet, so:
26,320 / 35.3 ~= 745.6 cubic meters

There you go. SVCO contains approximately 750 cubic meters of air, give or take probably 25%.

We know that 1 cubic meter of air at normal atmospheric pressure is 1.29 kilograms per cubic meter, so:
745.6 cubic meters * 1.29 kg/cubic meter ~= 961.8 kilograms

(please feel free to refine my estimates -- I'm sure I am off by several hundred kilograms to either side, the math shows we've now pretty much ascertained it's in the "multiple-hundred kilograms" range)

See -- That's about 1000 kilograms of air inside a recirculating Skyventure tunnel :-)

That's over 2000 pounds. Even at 10mph, that's still a fair bit of momentum. But the math shows that the air in the ducts aren't that slow in a Skyventure tunnel - One Skyventure staff owner quoted 30mph for the slowest and widest section. The average airspeed is probably closer to 50mph for all of the air, but let's be generous and go with 30mph. 1000 kilograms at 30mph -- imagine a car crash at 30mph. That's a lot of momentum. Just figure out how to keep the tunnel efficient so the air keeps looping like a spinning top full of momentum (minimum resistance everywhere, including fans), and at least a fair bit of energy is going to be preserved for the energy to dissipate itself via pushing the human body aloft for several seconds after a power outage. So it appears an efficient-enough recirculating tunnel is definitely going to keep a bellyflyer flying for a few seconds, assuming efficiency is high enough including minimum resistance through fans).

Convinced yet?

[Paulipod 0]

Quote

Convinced yet?

erm.. Not really lol

If I were convinced based on the size of the air mass - youd think that our tunnel would have more mass huh? Seeing as we are moving in excess of 4million cubic meters/per hour?

Also - just to be picky you are stating the mass of air based at 0 degrees which in the recircs is quite a bit out

Bodyflight Bedford
www.bodyflight.co.uk

[mdrejhon 8]

Quote

If I were convinced based on the size of the air mass - youd think that our tunnel would have more mass huh? Seeing as we are moving in excess of 4million cubic meters/per hour?

I have a critical question How much of the air is self-contained? The amount of air contained inside the wind tunnel. This is very important since this is the amount of momentum "charged" into the windtunnel. The momentum of air pushing air (because it's a loop) can make the airspeed take a little longer to slowdown, because air is not colliding with stationary air around a non-recirc windtunnel, so there's less resistance on air to air itself, and its momentum will dissipate itself to other sources such as friction with surfaces, air being forced into turns, pushing through fan blades of dead fans, resistance of tunnel flyer body, etc. It's not frictionfree obviously (air is not superfluid, so it is not like a superconductor -- otherwise the human body in the chamber will now become the only source of air resistance). (People familiar with superconductors, know that a loop of superconducting wire, with electricity 'injected' into it can conduct electricity in a circle an infinite circle for very long periods with nearly no power loss - after disconnection of power source. This is called a SMES)

I believe that this is why a well-designed recirculating wins over non-recirculating here -- air behind air helps keeps pushing the air forward --rather than being dissipated in the atmosphere. From what you claim, your tunnel has plenty of brute force power, but not "efficient enough" in a power outage to sustain flight for as long afterwards. As a metaphorical comparision, just because a Hummer or M1 Abrams has more power, it won't coast for as long on a highway after engine stoppage, as say, a Honda Civic. Efficiency isn't the same thing as sheer power. You have one of the most powerful windtunnels in the world, but how does wattage per square feet compare? A well-designed recirculating tunnel uses less electricity (per square feet of chamber cross-section), which means air likely flows more "easily" through the tunnel, helping towards a more gradual slowdown.

The math I've done appear to suggest that momentum stored in a motor is FAR less than the momentum of the total air inside a recirculating tunnel...

People who play with smaller fans and try to spin them while the power is cut off, will often notice resistance differences -- some will tend to self-brake, while some tend to keep spinning for a long time. (Some efficient desk fans seem to keep spinning for a minute when you unplug the fan -- w

Quote

Also - just to be picky you are stating the mass of air based at 0 degrees which in the recircs is quite a bit out

That's an acceptable nitpick, but we're still talking about several hundred kilograms of air nontheless. My rough estimates will have more error margin than this variance anyway...

[Paulipod 0]

Hiya,

The ER for our facility is very efficient... and would not be dissimilar to the SV facilities (with 1 unit for sure) and as a totally recirculating tunnel with no external pressure loss (no vents etc) our results would not be a million miles apart.

The only frictional loss comes from our cooling coils which offer a sub-40 pascal resistance.... and are at the slowest part of the tunnel (under 10mph)

Bodyflight Bedford
www.bodyflight.co.uk