jws3

Does anyone know why there would be a problem with using rubber bands to stow excess chest or leg strap webbing? I heard someone at the dropzone the other day say that it degrades the nylon, but I can't find anything online about why rubber would degrade nylon any more than any other material.

The only thing I can think of that makes sense would be that as you pull the stowed webbing out repeatedly over time, the higher friction could cause wear because the rubber band holds it tighter than standard stows (the reason for using it in the first place). Apart from that, it seems like it could easily be one of those things that someone just made up and started passing around.

Yeah, you could put a pitot tube on your hand or helmet and just make sure to always point your hand or head in the right direction. You could also have a long pole sticking out of your chest with a pitot tube on the end then figure out how to keep it oriented the right way. If that sounds easier, then sure. I don't feel like putting much effort into selling this idea to people who don't care. There's nothing in it for me.

Thanks for the constructive criticism but I can't tell much from that picture. The inlets on the other side of the Phantom could be collapsed, which was a problem in early suits like you said. The suit could be stalling. The guy with the Vampire has his legs bent quite a bit, so maybe they're all flying on the verge of stalling.

Quote

But that has nothing to do with the "conservation of mass and momentum" as you keep repeating. It's simply Newton's 3rd law.

Newton's laws are conservation of momentum for point masses and rigid bodies.
N1L: In the absence of a force, an object's momentum stays the same.
N2L: If an object's momentum changes, that change comes from a force.
N3L: If an object gives another object momentum through a force, the second object gives back an equal and opposite amount of momentum.

Navier-Stokes is conservation of momentum for a fluid.

Which suit have you flown that had basically zero pressure no matter how fast you flew it?

Inside a pitot tube, there is zero airspeed just like there's zero airspeed inside a ram air cell. The viscous boundary layer at typical wingsuit speeds is on the order of a few millimeters thick, so the majority of the inlet sees about the same airspeed as if it were sticking out 10 ft from your body.

I'm not asking you to take my word for it. One of the things I love about science is that you never have to take anyone's word for anything. You can always try it yourself. The hardware I used to do this was pretty cheap and the software I used to do the computational fluid dynamics was free. It's called OpenFoam and I'm happy to send you the case files if you want to run them. You can try different geometries, different mesh sizes, different initial conditions, different finite-volume schemes, etc and I'll bet you'll get approximately the same results.

Not when the pressure outside is ~63 kPa. This is what I used to make the measurements. It's an Adafruit Feather with a BMP280 and a battery. It was zip tied to the mesh inside the leg wing.

Edit:
When you measure the pressure of a tire or a paddleboard or whatever, you're usually measuring the difference between the pressure and the surrounding pressure. When you have a flat tire on your bike and the pressure gauge reads zero, it doesn't mean it's a vacuum inside your tire.

If you're intentionally trying to throw off the system, you totally could. You could also drill a hole in the back of the pitot tube on a plane if you wanted. If you're flying the suit on the verge of stalling or the inlets collapse or you're backflying without backfly inlets, that could all ruin the readings too.

Yes, it'll vary a little from one suit to another but conservation of mass and momentum say that if the inlet is wide open and exposed to the flow and the rest of the cell is reasonably air tight, the air inside has to push back about as hard as the air outside is trying to push in. This is for people who care about knowing their airspeed, which might be no one. I don't know.

That's why it's indicated airspeed, not true airspeed. It's like the pitot-static system on a plane. The airspeed instrument gives you the IAS. If you want TAS, you just need density. The indicated airspeed is basically an indication of the dynamic pressure, which is what matters. Your lift and drag depend on dynamic pressure.

With respect to IPR, yes, that's why I'm posting this now while I'm still an unemployed bum.

I've been working on a project to try to measure indicated airspeed in a wingsuit based on the pressure in the leg wing and I put together a demo video:

https://youtu.be/jQ7ZT_hvThs

The basic idea is that the pressure in the leg wing is going to be pretty close to the total dynamic pressure of the flow, so you can compare it to static pressure and get a pretty good estimate of the IAS, which differs from true airspeed depending on density. I think getting within about 5% of the "real" IAS is pretty realistic. The inaccuracies will come from the angle of attack and the porosity of the fabric. I think the porosity of the fabric is going to make a really small difference and the alignment with the flow will be an issue with any total pressure measurement system.

It makes sense based on conservation of mass and momentum. Apart from the small amount of air that leaks through the fabric, the mass coming into the inlet has to equal the amount going out. The freestream keeps wanting to ram air into the inlet and the only way the inlet has enforce conservation of mass is by pushing back with a pressure gradient equal to the dynamic pressure.

I'm imagining that rather than being a stand-alone system, this could be part of one big Kalman filter with a bunch of other sensors. The basic idea is that you make a prediction, make a measurement, meet somewhere in the middle, then repeat. Where the "middle" is depends on how sure you are about your predictions and your measurements.

On the sensor side, we could attach some uncertainty to the IAS measurement, then combine it with GPS, inertial data, magnetic data, etc (all with their own uncertainties). On the model side, we could start with the most general model possible: a rigid body that can rotate and translate in all three dimensions. Then we can refine it by putting restrictions on it that match the physical system: constant gravitational force, lift only generated perpendicular to the flow and normal to the body, lift and drag are proportional to airspeed, etc.

The CFD software is OpenFOAM, which is free, and the hardware I've been using is a few Adafuit Feathers with BMP280s. I'm about to start a new job where they basically own everything I come up with, so it might become hard to publish this kind of stuff, even for free, so if anyone is interested in playing with or adopting this project, let me know.

That same page also says:

Quote

Even if you do not have an AAD
in your container, a CYPRES loop will markedly
improve your safety. The use of CYPRES accessories
(loops, discs, setups) with other brand
AADs is prohibited as we have not performed any
compatibility tests.

They justify the benefits of using Cypres loop on page 42 saying that

Quote

Previous reserve container closing loops were made
from old parachute suspension lines or similar material
consisting of Kevlar, Dacron, Spectra etc. They
were often thick, rough and became stiff while under
tension in a packed container for a long period of
time. As a result these loops could delay the reserve
container opening or even avoid it after the ripcord
was pulled because they became trapped between
the grommets ... Although the loop is really narrow, its breaking
strength is in excess of 408 lbs (185 Kp).

Have closing loop materials like Spectra really caused total malfunctions historically? It seems pretty unlikely unless someone just pencil packs the same pack-job for like 10 years. The 408-lb rated strength for Cypres loops also seems kind of unlikely.

Edit:
Good point though. I've been mostly packing Vigils and rigs without AADs lately, so I haven't looked closely at the Cypres manual in a while.

My loops aren't particularly tight. I usually close everything up to the pilot chute with just my hands, then start using the leverage tool after the PC.

When I'm packing a rig without an AAD, I like to use 725-lb Spectra mostly because I feel like it's a lot tougher than Cypres loop. I guess in that case, it's probably okay to use for several pack-jobs. I have broken a Cypres loop closing a reserve before, but I can't imagine breaking 725-lb Spectra.

I wish they just made beefier AAD cutters that could handle loop materials that aren't so delicate.

When I learned to pack reserves, I was taught to make a new loop for every pack job but I've noticed that none of the manuals actually tell you to. What's the general consensus on whether or not this is necessary? Maybe necessary if it's a used rig that you've never seen before and you don't know how old the loop is? Maybe not if you did the last packjob and you know you made a new loop last time?

Cool. I sent UPT an email asking about it and I'll post the answer I get. I feel like the most likely response is that it's a major repair or alteration, requiring a master rigger. I'm a senior rigger and if I had a sewing machine that could do harness work, I'd be tempted to just do it myself. I don't have one though.

Edit:
Just got a response from them. They say anything on the harness is supposed to be done by a master rigger.

It's just really long and it's annoying having so much excess webbing to stow away every time, in response to the question on why.

Is shortening a chest strap (by 3 or 4") a major repair? (i.e. Do you need a master rigger to do it?)

Yeah, but I'm asking why. What makes the slider act differently coming down the lines one way vs the other?

I've been told that sliders with brass grommets go with the smooth end to the risers and ones with steel grommets go the other way. Does anyone know why this is?

I ask because I'm having issues with my slider coming down about 2/3 of the way, then getting stuck until I pump the brakes to get it down. Here's a video of what I'm talking about:

https://www.youtube.com/watch?v=6b-FCLFRNN4

It's a Troll 265 (BASE canopy) with a ZP nose but no vents freepacked into an accuracy rig. When I double checked my rigging, I noticed that the slider is on upside down (brass grommets with the smoother end towards the canopy). I'm wondering if flipping it over is likely to make any difference with the openings or if the slider direction conventions have more to do with long term wear or something.

I'm not extremely worried about it because I can get the slider the rest of the way down and the canopy still feels controllable before it comes all the way down, but I'd like to fix it if I can. If it's not the direction of the slider, does anyone have any ideas of what else might cause this? The canopy is also brand new so I'm wondering if brand new dacron might somehow produce more friction with the grommets than broken-in dacron. Thanks!

Here's a picture of what I'm talking about.

I have a Rigging Innovations rig (Classic Pro) and I saw that the reserve bridle has a little F111 pocket built in. I don't see anything in the manual about it. Does anyone know what the purpose of this is? Thanks

When you use a thicker material like Spectra or Type IIA as a closing loop, what do you use as a washer?

That's kind of what I was thinking. I ask because I'm a newer rigger and I learned to just always use the Cypres loop material regardless of whether or not the rig has an AAD. It seems like most of the newer riggers I've talked to learned the same thing.

It seems like a common practice but I noticed that the manuals don't necessarily say you have to. Since the right answer to everything is "follow the manufacturer's instructions," I started wondering if regular 725-lb Spectra would be better if the rig doesn't have an AAD.

Are there any advantages to the Cypres loop material other than just fitting through the narrow cutter opening more easily? Do you still use the silicon gel when you use something other than Cypres loop material?

The Cypres2 manual says that you have to use the Cypres disc and loop material, but it seems like the manuals for most containers don't specify. For example, I don't see anything in the Wings manual saying that you have to use Cypres loop material. Some manuals seem to recommend it, but not strictly require it.

Does anyone use something other than Cypres loop material (like 725lb Spectra) when packing a reserve with no AAD? Just curious.

I'll measure the see exactly what the PC and bridle differences are. Why does the canopy size matter? It seems like as the canopy is coming out of the D-bag, the pilot chute collapses and is done doing its job.

Cool. I didn't see any reason why it would be a problem, but I thought I'd ask in case. When you don't know something, you usually don't know that you don't know it.

I got a new (used, but it's new for me) container and I'd like to use the bridle and pilot chute from my old container with it. The main reason is that it's newer, less worn, and I like the handle. My original PC is slightly larger than the one that came with the container I bought (by an inch or two).

My question is: how much does pilot chute size matter? I'm going to use the same main canopy, but the new container and D-bag are a bit smaller. Is there anything wrong with extracting the D-bag a little faster?

Greenies? What are those?