Help finding values for ram air pressure

[Gary Warner]

I've seen it before but I'm having no luck finding it today.

It's a chart that shows the values of forces (pounds, Pka, etc.) for ram air in a pitot tube for any given airspeed.

As I type this, I think it was an Ap-note on the Motorola's sensors web site (now Freescale). I'll try there.

If you have any other sites, please share.

Gary
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[Gary Warner]

I found "Half Rho V squared".

Is this what I need?

(1/2 Rho (1.22)) times V (velosity: meters per second) squared

Answer in Pa

Correct?

Gary
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[Sparky Paul]

"V sqvare, rho half" is the way my aero prof would say it.
That is the dynamic pressure, which the plane flies by.

[Gary Warner]

Tan'ks Sparky.

Gary
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[chrisgood]

Dynamic fluid pressure is defined as:
P(dynamic) = 0.5 (r) (v2) ,
where
v = velocity of fluid (air),
r = density of fluid (air)

r(air) @ sea level, incompressible (low Mach number) = 1.229 kg/(m3)

If choosing a pressure transducer, we need a maximum airspeed to design for. Assume a max velocity of 50 m/s (111 mph), you get:
Pmax=.5 (1.229 kg/(m3)) (50 m/s)2 = 1536.25 kg/(m s2)

We need to convert this to PSI.

1 pound(mass) = .4535 kg
1 pound(force) = 32.174 pound(mass) ft/sec2 (multiplied by gravity at sea level)
1 ft = .3048 meter
1 ft2 = 144 in2

After all these numbers are put in the equation, we get:

P(dynamic, air) = 32 pound(force) / ft2 @ 111 mph
= .22 pound(force) / in2 @ 111 mph
= .22 psi @ 111 mph

[Gary Warner]

So .22 PSI = 1.516 kPa or = 1515.8 Pa?

Is this right?

Gary
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[Gary Warner]

Also, if I were to open the end of the pitot tube by twice the area, what would be the effect on the sensor? Twice the pressure for a given airspeed?

Gary
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[Sparky Paul]

You might choke the flow.. but the dynamic pressure wouldn't change.
The inlet on the probe is designed for smooth flow to the sensor.
Futzing with these wouldn't be a sensible thing to do.

[chrisgood]

Yes.

1 atm = 14.696 psi = 101325 pa

.22 psi * (101325 pa/14.696 psi) = 1516.8 pa

Chris

[Gary Warner]

Sparky,

Since this a ram pressure system there won't be any 'air flow' to speak of, right?

My thinking was if I increase the area (cross-section) of the ram air tube, I'd get more static pressure for a given airspeed. I guess I was wrong.

Gary
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[Sparky Paul]

Static pressure is the pressure around the airplane.
It's usually picked up at flush ports along the fuselage side where airflow is constant.
The pitot probe sees total pressure.. ram (due to airspeed) plus static (due to altitude).
These probes can have seperate lines for the ram pressure, and the static pressure, which would be picked up by holes around the surface of the probe.
Static pressure is plumbed to the altimeter.
Ram pressure includes the pressure due to forward motion and static pressure.
Ram pressure (total pressure) is plumbed to the airspeed indicator, usually a curved tube uncoils with more pressure, moving the airspeed needle in the instrument case which is vented to the cabin.. in an unpressurized airplane, this is static pressure.
(That's the Piper Cub, P-38 way)
Modern digital transducers still have to seperate the total pressure from the static pressure.
Changing the sizes of the pressure ports, either total or static won't affect the situation.
http://www.allstar.fiu.edu/aero/PSSI.htm

[Gary Warner]

Thanks Sparky. I've visited that page before. In re-reading the page, I was looking for any comments about cross section changes affecting airspeed readings. Much of what is there refers to total blockage. But one part sort of addresses a reduction in pitot tube cross section:

"CAUTION:
Pitot icing can occur at a relatively slow rate, causing a gradual reduction in pitot pressure. This results in a slow decrease in indicated airspeed rather than a frozen condition."

Doesn’t this suggest that a reduction in cross section will reduce indicated airspeed? If so, wouldn't the reverse be true?

I don't think I've been clear about my intentions. What I want to do is increase a transducers sensitivity (output) for better reading of low speed readings by “flanging” the end of the ram pitot tube to a larger cross section.

Gary
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[Sparky Paul]

Ice etc plugging either the total or static ports alters the system volume, and makes the system read wrong.
The 737 that hit the 14th St bridge in DC had that problem.
Flanging the opening on the pitot for a bigger gulp of air... That would introduce venturi effects, speeding up the air and giving a false higher reading.
The pitots I'm familiar with are finely designed and manufactured to be consistent in their readings.. and we would still calbrate them at definite intervals to keep them honest.
The pressure end in particular is very sensitive to alterations in shape.
.
Models operate in very low dynamic pressure ranges.
As Chris mentions, 111 mph is only .22 psi. dynamic pressure.
50 mph would be 1/4 of that.
A more sensitive transducer would be better (more expensive) than making uncalibrated alterations to a working system.
But GPS would be a better choice.. it gives true airspeed, not the calbrated airspeed of a pneumatic system, which changes with altitude.
100 knots at 70,000 feet is 475 knots true, for instance.

[Gary Warner]

I think what I'm getting out of this is that flanging would increase the sensitivity. I know what you are saying about tinkering with a calibrated system, but this will just be a relative reading. I won't even care about converting the data to real world numbers. I just want a more sensitive way of reading low airspeed flight. Freescale has a 10 kPa sensor and (at one time) sold an 8 kPa sensor. At these scales, getting noise free reading at 10 to 30 MPH would be difficult for me.

Explain one more thing for me. The pitot tube measures air 'compression'. There wouldn’t be any air 'flow', so why would there be a venturi effect?

Thanks for your time Sparky.

Gary
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[deh6]

Gary,

There isn't much that can be done to increase the pitot-static pressure.

As for increasing the signal/noise of pressure sensors, Motorola has an app note where they pulse the sensor with 40v to get a 4x increase in signal but the noise remains the same. A lot of electronics to put it all together, though.

About 20 yrs ago Technical Soaring (or Soaring?) had an article on building a variometer (before the days of mems ICs). It was a sandwich of metallized mylar that made a differential capacitor. It would certainly be a bit of an undertaking, however.

With such small pitot-static pressure at 10 mph, it seems like other schemes might be better.

I've toyed with the idea of using ultrasonics to measure the airspeed, and a very small free-spinning propellor may have merit and not cause much drag at the 10-30 mph speeds. The hot-wire (thermistor) schemes are another possibility.

GPS is good for navigation, i.e. groundspeed, but not much use for stabilization/control where indicated airspeed is desired.

The airspeed indicators from helicopters were used in sailplanes (at least when I was flying them in the '80's) because of better sensitivity at low speeds. As I remember the scale dropped off in the 20-30 mph range. It gets tough to measure speed via pitot-static pressure at low speeds.

[Gary Warner]

Yep, I starting to see that. Plan "B" was to use an enclosed impeller that was driven by an outside ram air tube. No spinning 'thing' on the outside of my sailplanes. The rotating impeller would be tach'ed for speed. I dislike making sensors, but for this one it looks like the best option.

Gary
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[Salto]

Gary,

It might be worth looking at what the hang glider and paraglider airspeed indicators do. I suspect they use a more sensitive pressure measurement device, to give accurate readings at low airspeeds.

In the 80's I built an ultralight, and made my own static/dynamic pitot probe that fed a commercial airspeed meter. It was a special high sensitivity (pneumatic/mechanical) meter and read reliably down to about 10-15 mph. My stall speed was about 24 mph. The pitot probe was just 1/4" steel tubes. The dynamic tube had a plain open front, and the static had a streamlined bullet on it's nose and some holes about 1/2" back on the parallel portion. Worked fine - I'm still alive!!

To prove the theory to myself I tried some different tube diamaters for the dynamic probe, and hung them out the car window for testing - sure enough, the airspeed reading was constant, independent of tube diameter.

Am I right in thinking that the current solid state pressure sensors would be more sensitive than the old mechanical meters? If so, then you should have no problem measuring down to 10mph.

Graham.

[Gary Warner]

Grham,

Ok, so increasing the pitot cross-section won't change the reading (still scratching my head on this one).

I think it's more likely that a mechanical meter would be more sensitive, since the pressures can be amplified mechanical with little increase in noise. The sensor I found (best sensitivity) was 10kPa full scale (about 1.45 pounds per square inch).

I'll use a mechanical tach for now.

[brnjones]

But GPS would be a better choice.. it gives true airspeed, not the calbrated airspeed of a pneumatic system, which changes with altitude.
100 knots at 70,000 feet is 475 knots true, for instance.

GPS gives ground speed, not true airspeed.

[Gary Warner]

GPS gives ground speed, not true airspeed.

Yep. Sparky Paul (the ascended master) is not always right. Good call.

Gary
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[Sparky Paul]

One strike in 7286 swings isn't bad. :)

[Gary Warner]

One strike in 7286 swings isn't bad. :)

LOL. That's cool, you're still th'a man :D

Gary
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