Measuring canopy airspeed

[mxk 1]

I got my hands on a vane anemometer (General DAF4207SD) that I'd like to use under the canopy to measure airspeed in various configurations. The fastest sampling rate is 1 second, which might be a bit low for recording the dynamic events, but perfectly sufficient for steady flight. If anyone here has some first-hand experience with doing this type of data collection, I'd welcome any tips, especially on the best place and method for mounting it.

I'm primarily interested in figuring out the true airspeed for full glide, different brake and rear riser positions, and possibly front riser input. The anemometer gives me indicated airspeed and temperature, which can be converted to true airspeed along the glide path as long as I have the local pressure setting from the AWOS. I also have an AltiTrack that records my rate of descent, so I could even account for that to get the true horizontal speed. Unfortunately, I don't have the docking station for it, but I'll ask around my DZ to see if someone else may have one. From what I understand, the JumpTrack software only supports XP. Does anyone know of any alternatives?

Thoughts? Suggestions?

[yuri_base 1]

This sensor might be of interest to you:

Airspeed MicroSensor



(they also have data recorder that this sensor can plug in to, as well as wireless dashboard)

I've used Eagle Tree products in the past and although it was a struggle to adapt them to my purpose, they might work for you.



If you're interested in using your smartphone with a wireless pressure sensor, check out SensorTag from Texas Instruments.

Android+Wear/iOS/Windows apps:
L/D Vario, Smart Altimeter, Rockdrop Pro, Wingsuit FAP
iOS only: L/D Magic
Windows only: WS Studio

[pchapman 261]

Doesn't an anemometer style airspeed indicator give a True airspeed (changing with altitude) rather than an Indicated airspeed (constant with altitude)? Because they deal with the linear amount of air flowing through instead of just the pressure. Can't be sure offhand but I thought that was the thing to remember with them, if using for flight test purposes.

Then the Altitrack would be used in TAS mode not SAS. (adjusted to 3000' with I guess an ISA assumed temperature).

Then both sets of numbers would need to be converted to ISA conditions so you can compare tests down low vs. up high (and on different days) on an equal basis.

Ideally one would have the anemometer on a trailing bomb (as they say in the flight test world) but I always just held one way out to the side and up a little, as far from my body and torso as possible, to minimize errors due to increased velocity around a blockage (one's body).

[councilman24 36]

Just for giggles I used a much simpler anemometer about the size of a stick of old style gum. Press the button and read the airspeed. Wasn't into the aeronautical science. Just wanted to see how canopies compared. Seemed to work for what I needed. Decide on your needed error tolerance and don't over engineer. Plus or minus 1mph seems fine to me. Do the math to see what corrections and kind of data are actually needed.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[Hellis 0]

JumpTrack works on Win7.
You need to update it and it should work.

[mxk 1]

yuri_base

This sensor might be of interest to you:

Airspeed MicroSensor

(they also have data recorder that this sensor can plug in to, as well as wireless dashboard)

Cool! I guess my next project should be to build a full face helmet with an array of sensors and a projected HUD inside the visor. I guess Google Glass would work as well

pchapman

Doesn't an anemometer style airspeed indicator give a True airspeed (changing with altitude) rather than an Indicated airspeed (constant with altitude)? Because they deal with the linear amount of air flowing through instead of just the pressure. Can't be sure offhand but I thought that was the thing to remember with them, if using for flight test purposes.

No, I expect to get the same raw readings at any altitude (the only caveat is that the meter may be doing some correction for the temperature, I'll have to look into that). It's dynamic pressure that is causing the propeller to spin, which is the same thing measured by the pitot-static system on an airplane. At high altitude, the air molecules will be moving faster through the probe (TAS), but there will be fewer of them because of lower density, so the propeller will spin at the same RPM, giving the same airspeed reading.

Hellis

JumpTrack works on Win7.
You need to update it and it should work.

Good to know, thanks!

[hackish 8]

I built a pito one using a MEMS pressure sensor but I was never able to calibrate it. I think it had a sample rate of 10hz. In the time since with all these drone projects you can now buy a unit for an RC model airplane and they're pretty cheap.

-Michael

[pchapman 261]

mxk

No, I expect to get the same raw readings at any altitude [...]It's dynamic pressure that is causing the propeller to spin, which is the same thing measured by the pitot-static system on an airplane.

Negative, Ghost Rider!

But your questioning did get me to try to get a definitive answer. Check out NACA (predecessor to NASA) Report 420 from 1933 on airspeed measurement. It's out on the web. Relevant excerpts are in the attachment.

It notes that an anemometer will read true airspeed and not indicated airspeed, which is why it is not particularly suitable for aircraft speed measurement.

(This is for normal style anemometers which attempt to have a bearing of negligible friction. If it were a wind turbine with torque resistance, then it would act more like a set of airfoils and vary measured speed with air density -- showing IAS.)

This is a subtle thing about anemometer airspeed measurement which isn't obvious at first glance!

[mxk 1]

pchapman

***
No, I expect to get the same raw readings at any altitude [...]It's dynamic pressure that is causing the propeller to spin, which is the same thing measured by the pitot-static system on an airplane.

Negative, Ghost Rider!

But your questioning did get me to try to get a definitive answer. Check out NACA (predecessor to NASA) Report 420 from 1933 on airspeed measurement. It's out on the web. Relevant excerpts are in the attachment.

It notes that an anemometer will read true airspeed and not indicated airspeed, which is why it is not particularly suitable for aircraft speed measurement.

(This is for normal style anemometers which attempt to have a bearing of negligible friction. If it were a wind turbine with torque resistance, then it would act more like a set of airfoils and vary measured speed with air density -- showing IAS.)

This is a subtle thing about anemometer airspeed measurement which isn't obvious at first glance!

Interesting... I'm not one to argue with NACA, but this paper seems to disagree:

http://www.mdpi.com/1996-1073/5/3/683/pdf

Quote

Ambient conditions do have an effect on the anemometers' behavior, as their response (changes in the rotational velocity, w) depends on the aerodynamic and frictional torques (QA and Qf respectively)... The aerodynamic torque, QA, is a function of the air density, p, among other parameters, and the frictional torque, Qf, is a function of the ambient temperature, T, and the rotational velocity, w [6].
[...]
The major conclusions resulting from this work are: 1. Both calibration constants, A and B, of the anemometer's transfer function are affected by changes in air density...

Intuitively, what would you expect to happen if you placed a vane anemometer into a full or partial vacuum and moved it in a circle at 10 mph? In a full vacuum, it should read zero. There is nothing to move the propeller. In a partial vacuum, it should read something above zero, but not the full 10 mph because the pressure acting on the propeller is less than it would be in the standard atmosphere.

I guess we'll find out when I do the measurements. I might try it this weekend if it doesn't get too cold or windy for a high-pull.
[edit]

Also, NACA report from 1915:

http://books.google.com/books?id=xEQjAAAAMAAJ&pg=PA95#v=onepage&q&f=false

Quote

D. Rotary anemometers.--Regarding rotary anemometers, Jones and Booth say:

"The principal advantage possessed by instruments of this type is that they read the actual travel through the air independently of variations in density."

It seems likely, however, that this independence is only approximate and not complete. The rotation of cup or vane speed to wind speed depends on the value of the least wind speed which will just keep the anemometer turning against friction. And since each vane or cup when moving very slowly acts as a pressure plate, it seems that the wind speed required in order to furnish the torque for very low speeds of rotation must depend on the air density. Hence it seems probable that at higher speeds the action of instruments of the Robinson or of the screw type is somewhat influenced by air density. Exact information on this is lacking.

[pchapman 261]

Perhaps we could agree that anemometers largely read TAS within reasonable limits of air density. For our purposes, we can ignore what issues there might be in using an anemometer on a re-entering Space Chuttle.

(I get the impression that the idealized anemometer with zero mass or torque would be free of density effects but I'm not sure if that would eliminate all error or only some of it.)

That was a nice find of a scientific article, with updated information at a level of measurement detail not found in the 1933 paper. On the one hand the new paper shows that air density has an effect:

Quote

The major conclusions resulting from this work are:
1. Both calibration constants, A and B, of the anemometer’s transfer function are affected by changes in air density.

However the effects are small.

They did their calibrations based on the calculated speed being equal to a constant times the anemometer rotation rate, plus a fixed offset constant. V=Af+B

That linear constant "A", its rate of change with density changes could be either positive or negative depending on the model tested. And the data was pretty messy with a lot of scatter in the graphs, with different results from different studies.

I took a sample graph, the one at the top left of p14 of the pdf, and used the linear formula they came up with. If that can be extrapolated to much higher density changes than they tested, the factor "A" would change from about .0512 at sea level to .0474 at half atmospheric density (or about 18,000') Ok, that's about an 8% change, or error from TAS. That's not negligible. But meanwhile IAS will have changed by 30% (root 2 = .707).

But as I said, other anemometers may have very different changes in their constants (and even whether the number rises or falls with increasing altitude), so there's no consistent way of determining density altitude effects without, say, calibrating a particular anemometer in variable density conditions.

But thanks for pointing out some of the lower level effects of air density on anemometer output.

(The paper does mention the effects exist for both vane and cup type anemometers, but focuses on cup type ones. The implication is that vane types ones are similarly only lightly affected by density, but that's open to further investigation).

So as to whether an anemometer basically measures IAS or TAS, it is still very much closer to TAS -- even if they can't quite approach the ideal.

What you will be able to do is to see how consistent your own data is. If you do a high hop and pop and record a given airspeed with brakes off at 10,000', do you get nearly the same readings down at 2,000'? Does the speed decrease as you go down, roughly in line with the density increase, or does it stay nearly constant like an IAS?

[erdnarob 1]

You can compare with the iPhone App named Wind Speed.

The iPhone microphones are used as pressure (speed) sensors.

Learn from others mistakes, you will never live long enough to make them all.

[JohnSherman 1]

http://www.jumpshack.com/default.asp?CategoryID=TECH&PageID=Glide&SortBy=DATE_D

This study might help. We used a Pasco system for both altitude vs. Time (ROD) and an anemometer input. The Pasco company has an excellent data logger system for all phases of parachute testing. We have been using this system for over 10 years.
www.pasco.com

I am attaching a URL for a brochure for the system, modified and accessorized for parachute testing.http://www.jumpshack.com/product_images/PDAS%20Brocure.pdf

We mounted the anemometer to a stand off at about 12 inches in front of the chest of the wearer. Directly in front of the logger in the mounting vest.

Using this system we have a common time line. This is much easier when it comes to selecting data points for speed and ROD.

The Data Studio software (also from Pasco) is excellent for analysing and viewing data. We used it for extracting the data point for constructing the Polar Curve. See attachment.

[mxk 1]

Thanks, John! I actually watched your PIA presentation on this very topic. As an aside, I would say that you can approximate air as an incompressible fluid at the airspeeds that we're talking about, which is why I've never had to calculate the equivalent airspeed in a Cessna. However, it is not correct to say that air is an incompressible fluid, which implies that this holds true for all airspeeds and conditions.

The main thing that was of interest to me is your assertion that pressure inside of a canopy is ambient. What I wanted to ask you was whether this claim was ever measured with pressure sensors located inside the canopy?

Before we get to canopies, what is the pressure inside of a pitot tube on an airplane? The pitot tube measures static + dynamic pressure, or stagnation pressure as it is commonly called:

https://en.wikipedia.org/wiki/Stagnation_point
https://en.wikipedia.org/wiki/Pitot_tube

There is no airflow through the pitot tube (not counting the drain hole, which isn't required), yet the pressure inside is certainly not ambient. You need the static port to subtract the ambient pressure in order for the airspeed indicator to show dynamic pressure.

How is this situation different from a canopy? You've got a stagnation point at the nose and it is that pressure that I would expect to find inside all of the cells, just like in a pitot tube. The lower pressure outside of the cells due to Bernoulli plays a role in keeping the canopy inflated, but I'm not convinced that it's the whole story. Do you have any data that shows otherwise?

[yuri_base 1]

JohnSherman

We mounted the anemometer to a stand off at about 12 inches in front of the chest of the wearer. Directly in front of the logger in the mounting vest.

I've found that 2 feet is the absolute minimum to get accurate airspeed and glide measurements. The more, the better.

Good (L/D Magic):



Good (Analog L/D Meter):

[inline AnalogLDMeter.jpg]

Bad - too short (~12"), recorded airspeed was only ~70% of the actual, and exaggerated L/D due to relative wind being deflected by body and appearing to be closer to horizontal than it actually is (Z-Device):

Android+Wear/iOS/Windows apps:
L/D Vario, Smart Altimeter, Rockdrop Pro, Wingsuit FAP
iOS only: L/D Magic
Windows only: WS Studio

[yuri_base 1]

mxk

ever measured with pressure sensors located inside the canopy?

You can find this out yourself with a couple of SensorTags for a cost of two jumps.

Android+Wear/iOS/Windows apps:
L/D Vario, Smart Altimeter, Rockdrop Pro, Wingsuit FAP
iOS only: L/D Magic
Windows only: WS Studio

[mxk 1]

yuri_base

***ever measured with pressure sensors located inside the canopy?

You can find this out yourself with a couple of SensorTags for a cost of two jumps.

I'm not sure how you would go about packing a parachute with those on the inside, or how to attach them without damaging the fabric. Maybe we could get some CRW guys to throw these sensors into an inflated canopy

[yuri_base 1]

The SensorTag printed circuit board is the size of two quarters side by side, and weighs nothing. If I were to do it, I'd make a tiny pouch from thick fabric or leather and attach it with a few hand stitches to an inner rib (if you look at crossport cuts, you'll see some fraying - manufacturers dod't even bother embroidering the edges, because the stress there is small enough; making a few stitches there to hold a 5-gram PCB won't weaken your parachute). Of course, you'll need to be careful when packing as not to snap the PCB. For the second sensor, I'd make some kind of miniature sock or cotton ball that will equalize the pressure so it won't measure the stagnation pressure, but rather ambient, and mount it between frontriser links after opening. I believe TI's app is able to pair with several sensors at once (but you'll need to switch between screens to read each). I have two SensorTags (which I bought with the purpose of making a Bluetooth Pitot tube for L/D Magic), so I'll gladly answer any questions about them.

Android+Wear/iOS/Windows apps:
L/D Vario, Smart Altimeter, Rockdrop Pro, Wingsuit FAP
iOS only: L/D Magic
Windows only: WS Studio