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jws3

Wingsuit Indicated Airspeed

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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.

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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.

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Different wingsuit models have extremely different internal pressures. This is related to the design and has nothing to do with airspeed. Take a look at this pic. The Phantom has like zero pressure in the legwing. But it's going the exact same speed as the highlight pressured Vampire next to it. Hell, modern suits allow you to to vary the pressure with a zipper. You could do this while flying at a continuous speed.

Internal pressure is a measure of suit rigidity and nothing else.
www.WingsuitPhotos.com

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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.

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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.

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jws3

If you're intentionally trying to throw off the system, you totally could.



True. But that's not what I was talking about at all. You don't need to "try" to throw off the "system" because it is already broken. The internal pressure is more related to inlet size, inlet shape, inlet location, overall suit design (which plays into general orientation during flight), and flight mode than it is to airspeed (which does admittedly play in, but it takes a far smaller role compared to all those other things).

jws3

You could also drill a hole in the back of the pitot tube on a plane if you wanted.



Funny you mention pitot tubes, because a pitot tube is a good example of how to correctly measure airspeed. Notice where it is installed? In the free airstream. That is a place where you can measure airspeed. Not inside an inflated wingsuit.

I've flown a few dozen wingsuits in the past decade. All in flocks at about the same speed. Some of those suits have practically zero internal pressure, I can collapse them without even trying. Others have so much pressure that I cannot collapse them in flight no matter how hard I try. Even after deployment, under canopy, some suits retain some internal pressure. These are not "little variations" as you suggest. These are night and day differences.
www.WingsuitPhotos.com

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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.

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jws3

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



Look at the picture I posted. The legwing was not even inflated on one of the suits! A lot of earlier suits were like that, and I could get 3 minute flights in those suits with roughly the same glide I can get in my modern suits. Even today, Phoenix Fly suits trend toward much lower inflation pressures than Squirrel and Tony suits, and you can fly all of those suits side by side, at exactly the same speed, with very different internal pressures. Inlet design is the biggest difference in all of these suits.

jws3

Inside a pitot tube ...



This isn't about how a pitot tube works. It's about what you're attempting to measure, and how/where. If you want to measure the pressure in the free airstream, you need a gauge in that free airstream.

jws3

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.



My first college degree is in Aerospace Engineering, and I earned my living for a decade designing bodies that fly in both air and space, before I moved on to a better career. I would not take your word for it, nor would I trust implications culled from "playing with CFD" when those implications directly contradict actual freefall observations. One of the things about science is that real world data (i.e. those observations) beat simulator data every time. And CFD is one of the most notoriously hard to get right simulations in existence. The math models behind CFD were numerically arrived at only after mountains of real data had been collected from dynamic airflow for decades. And those math models in turn are best applied to those same kinds of dynamic airflow (which the static air inside your wings is anything but).

What you're proposing might work, but only if you calibrated your measurements for a specific suit/human/rig combo (and that calibration would need to be done using real speed data that you measured externally... at which point you already have what you need so your whole "solution" becomes unnecessary). In no way could work as a general purpose solution for any wingsuit.

And as long as we're talking about science...

jws3

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



Yes, air in a sealed volume with one open surface will push out as hard as it is getting pushed in. But that has nothing to do with the "conservation of mass and momentum" as you keep repeating. It's simply Newton's 3rd law. The things is, wingsuit models have a huge variety of inlet designs, which among other dissimilarities, are located at different points in the airstream. Meaning that two wingsuits flying at the same speed but with different inlet locations will have very different pressures wrt to the "air outside trying to push in." And inlet size does make a difference too since for small enough inlets you get interference drag. All of this is why those two suits flying side by side will have massive differences in internal pressure.
www.WingsuitPhotos.com

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The111

What you're proposing might work, but only if you calibrated your measurements for a specific suit/human/rig combo (and that calibration would need to be done using real speed data that you measured externally... at which point you already have what you need so your whole "solution" becomes unnecessary). In no way could work as a general purpose solution for any wingsuit.



Sorry if it sounds like I'm shitting on your idea, but I'm just trying to connect it to my observations. I'd recommend making all of your measurements in the free airstream. If you really want to keep trying with internal measurements, do it with a lot of different suits (e.g. small suits, big suits, different manufacturers, tri-wings, mono-wings), and compare to data from GPS and purely external measurements. I'd wager a large amount of money you'll find the purely external measurements have some strong correlation to GPS data, and your conclusions drawn from internal data will vary widely (and if not, you need to include some Phantom 1's in your test). ;)

The video is cool and the numbers sound reasonable, but I still think it has a lot to do with the inlet design on that suit. Come to think of it though, maybe the true number is really just an upper bound on what would be measured from poorly designed inlets (which can only let in less than the total pressure)... so if all modern suits have good inlet design (letting in close to the full total pressure) then maybe it is actually pretty close to reality in each case, and my thinking is just biased by all the badly designed suits of the past. ;)

I still think pure external would be more reliable, and user yuri_base did something with that years ago, a huge pitot setup coming off of his helmet.
www.WingsuitPhotos.com

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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.

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So for the average jumper this doesn't matter, but for a competitive person sure, and they will want it to be very accurate. So in the interest of accuracy and consistency, why not just put a gimbaled pitot tube on a foot long chest or helmet mount? Then all the internal pressurization speculation is irrelevant.

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For a very modest cost you can get pitot/static systems for RC models and drones that are remarkably accurate. It would be trivial to modify one of these for attachment to a helmet or wing gripper.
...

The only sure way to survive a canopy collision is not to have one.

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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.

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It's called a passive gimbal (as opposed to the active/powered kind).

DIY or some fancier machined stuff out there.

Wood

PVC

Fancy

3D Printed

Just stick some fins on it like a wind direction arrow and voila.

(I've actually seen one of these made from carbon fiber and jumped, there was just no point really other than "how neat is that?")

You could probably find a good swivel to do the job too and be really small.

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jws3

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.



Your video mentions that you're already measuring static pressure outside of the wingsuit. Do you have a picture of how you have this set up?
www.WingsuitPhotos.com

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Quote

I don't feel like putting much effort into selling this idea to people who don't care.



Here's a thought: when your idea instantly garners a lot of informed criticism and essentially zero support the problem might not lie with your potential customers, it just might lie with your idea.
Do you want to have an ideagasm?

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jws3

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.



Yes, and you could expend a trivial amount of effort to come up with a passive way of keeping it pointed into the freestream, far less than is required for your idea to be practical.
...

The only sure way to survive a canopy collision is not to have one.

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Sorry, brother but I gotta weigh in with Matt and Doc, here. You're trying to pull data from a source that doesn't produce the data you want. I'm currently experimenting with an unusually large-surface-area suit with some unusually bizarre properties and an unusually wide range of flight modes, shapes and settings. It is an extreme example of the range of pressures 111 was referring to.
I can depressurize the thing until it is almost limp in flight, to reduce the drag and make it easier to manipulate the fabric areas, or I can zip it up drum-tight and it acquires the handling and feel of a raft dive, where most of its handling properties are dominated by sheer scale and inflation pressure. The internal pressure inside this set of wings has almost no relation to its airspeed. I can fly it fast with high pressure, which just puts a sharp speed limit on the top end due to drag, or I can fly it just as fast with almost no pressure, which simply allows me to fly it faster than that. I can fly it slow with no pressure, resulting in nimble handling properties, or I can fly it slow, while pressurized to the max and so rigid that deploying past it is a -major- technical challenge.

The range of internal pressures available bears almost no relation to airspeed unless I've stalled the thing to an almost complete stop. One setting might get me 5 PSI doing 40 with the zips shut... another setting might get me that same pressure with the zips wide open and the suit being flown so hard in a balls-out 170 mph dive that it's almost coming apart and a hurricane of airflow passing through the suit. An internal pressure measurement system is mostly just going to indicate how closed the winglock zippers are, with airspeed a distant second-place in terms of how much effect it has on the internal pressure.

This is complicated even more by the fact that that internal pressure is also variable at any given airspeed by the angle of attack I'm currently using and how much the air inlets are scooping air as a result. I could choke off the instrument just by changing shape and altering the presentation of the inlets to the airflow with no change in airspeed. Or by changing shape so that although the inlets are still presented the same way, my arms are occupying a different part of the armwing sleeve and allowing free-er airflow through the zips and out the sleeves. Simply moving around inside the wing will create more noise in the data and of a wider range than the data you're trying to measure.
You could set up something that was verifyably measuring accurately -when flown a certain particular way-, but that measurement could be made wildly inaccurate just by flying the suit differently. I think you're going to find that relative to any modelling you do, the best you're ever going to see is a few frames here, a few frames there, brief transients where the conditions happen to pass through the range where the device's specs, the suit pressure and the airspeed actually align and result in a measurement that happens to be accurate- for half a second. It'd be like having a speedometer in the car that is affected by everything from the speeds of other nearby cars, to the steering wheel and how fast you turn it, to whether or not you've got the windows open and how far, to the texture of the road you're driving on. Watching such a speedometer would result in a reading that was very visibly nonsense and would simply look like the speedo was broken. Too many uncontrollable and unquantifiable variables to calibrate around.
Besides, why reinvent the wheel? We already have a very good, lightweight and compact way to measure our airspeed. GPS.
-B
Live and learn... or die, and teach by example.

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lurch


Besides, why reinvent the wheel? We already have a very good, lightweight and compact way to measure our airspeed. GPS.
-B



I agree with the technical assessment but just wanted to comment on the last little bit.

Reinventing the wheel as it were is necessary for any technological advancement. The wheel has in fact been reinvented many times, the wheel and the tire that is. From stone to wood and steel to aluminum alloys, non-pneumatic to pneumatic, to non-pneumatic again, to tracks etc. Subtle changes that alter how it can be used, effectiveness, and efficiency. Innovation is important, and sometimes requires challenging the status quo. In that respect, I applaud the OP's effort, flawed as it is.

The question he is trying to answer is also a somewhat important one for the more nuanced aspects of wingsuit flight. GPS speed is irrelevant when you are talking about aerodynamics because the GPS has no concept of air speed over a wing, which is the determinant factor of aerodynamic performance and handling. Everything from gyro-copters and sail planes to commercial airliners and super sonic jets, rely on "indicated airspeed" to define their performance envelopes. IAS tells you flight characteristics, GPS just tells you speed related to an arbitrary fixed position. You can land some small airplanes backwards in a strong enough head wind, that is a function of IAS, not speed over the ground. GPS doesn't even give you true air speed (which is a speed-over-ground equivalent).

If you maintain a constant GPS speed throughout a jump you are in effect changing your air speed continuously to accommodate changes in pressure at differing altitude (even with zero wind). i.e. If you register 100kph GPS speed at 10,000ft and 100kph GPS speed at 3,000ft; then you would actually be experiencing a higher "indicated airspeed" at 3,000ft, and therefore different flight characteristics. (the air at 3,000ft is denser, so there are more molecules traveling around the wing, and therefore more lift and more drag).

But yes, GPS is "good enough" for the vast majority of wingsuiters and for competitions when everyone is jumping under the same conditions.

** To give a real world example, I've exited at 13k ft into a 70mph headwind in a small wingsuit and my ground speed was virtually zero, a GPS would assume I wasn't moving (IAS or TAS would tell me how fast the air around me was moving), turning 180 degrees, my ground speed (aka GPS speed) would have been my airspeed + the 70mph wind, making it appear as though I was traveling through the column of air much faster than I actually was while IAS and TAS would remain the same (at the same altitude).**

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Anachronist

***
Besides, why reinvent the wheel? We already have a very good, lightweight and compact way to measure our airspeed. GPS.
-B



I agree with the technical assessment but just wanted to comment on the last little bit.

Reinventing the wheel as it were is necessary for any technological advancement. The wheel has in fact been reinvented many times, the wheel and the tire that is. From stone to wood and steel to aluminum alloys, non-pneumatic to pneumatic, to non-pneumatic again, to tracks etc. Subtle changes that alter how it can be used, effectiveness, and efficiency. Innovation is important, and sometimes requires challenging the status quo. In that respect, I applaud the OP's effort, flawed as it is.

The question he is trying to answer is also a somewhat important one for the more nuanced aspects of wingsuit flight. GPS speed is irrelevant when you are talking about aerodynamics because the GPS has no concept of air speed over a wing, which is the determinant factor of aerodynamic performance and handling. Everything from gyro-copters and sail planes to commercial airliners and super sonic jets, rely on "indicated airspeed" to define their performance envelopes. IAS tells you flight characteristics, GPS just tells you speed related to an arbitrary fixed position. You can land some small airplanes backwards in a strong enough head wind, that is a function of IAS, not speed over the ground. GPS doesn't even give you true air speed (which is a speed-over-ground equivalent).

If you maintain a constant GPS speed throughout a jump you are in effect changing your air speed continuously to accommodate changes in pressure at differing altitude (even with zero wind). i.e. If you register 100kph GPS speed at 10,000ft and 100kph GPS speed at 3,000ft; then you would actually be experiencing a higher "indicated airspeed" at 3,000ft, and therefore different flight characteristics. (the air at 3,000ft is denser, so there are more molecules traveling around the wing, and therefore more lift and more drag).

But yes, GPS is "good enough" for the vast majority of wingsuiters and for competitions when everyone is jumping under the same conditions.

** To give a real world example, I've exited at 13k ft into a 70mph headwind in a small wingsuit and my ground speed was virtually zero, a GPS would assume I wasn't moving (IAS or TAS would tell me how fast the air around me was moving), turning 180 degrees, my ground speed (aka GPS speed) would have been my airspeed + the 70mph wind, making it appear as though I was traveling through the column of air much faster than I actually was while IAS and TAS would remain the same (at the same altitude).**

The GPS is great if you only pay attention to the vertical speed. Vertical speed does not tell all. For example you can fly slow like a falling leaf or fly hard fast, with speed generated lift and have the same vertical as flying slow. But removing the element of upper wind (horz speed and glide ratio) you can get a good indication of your performance from vertical speed.

I have a practice of flying hard and vertically slow after breakoff and before deployment if it just a small group. During some hot summer days I noticed my vertical speed lower than what I can normally do. Thermal seem to be adding an offset like a tail wind would. So much fun to play with the numbers.
Instructor quote, “What's weird is that you're older than my dad!”

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dthames

During some hot summer days I noticed my vertical speed lower than what I can normally do. Thermal seem to be adding an offset like a tail wind would. So much fun to play with the numbers.



That's weird... In hot days your vertical speed should be higher as the air will have lower density. Thermal activity wouldn't account for it at 4000ft+ unless you are flying along a major highway or somesuch, since at that height the column would have arranged itself into a rather narrow shaft and there would be the corresponding downdrafts surrounding it which would nullify any effects.

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GoneCodFishing

*** During some hot summer days I noticed my vertical speed lower than what I can normally do. Thermal seem to be adding an offset like a tail wind would. So much fun to play with the numbers.



That's weird... In hot days your vertical speed should be higher as the air will have lower density. Thermal activity wouldn't account for it at 4000ft+ unless you are flying along a major highway or somesuch, since at that height the column would have arranged itself into a rather narrow shaft and there would be the corresponding downdrafts surrounding it which would nullify any effects.

Well, I can't be sure. But twice that happened this summer. In my Rbird, when I am pegged out, I can fly 37 MPH vertical pretty consistent. I was hearing 34 and thought, no way, and tried to hold it. Both times it happened there were noticeable thermals under canopy, so I was guessing that might be the cause.
Instructor quote, “What's weird is that you're older than my dad!”

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