The Pitot Tube and a Math Lesson

In response to "AE 84"
"The error lies with the fact that the units of grams are mass, not force." So far so good. "Europeans commonly (and incorrectly) refer to their weight in Kilo's whereas Americans (correctly) refer to their weight in pounds." Pounds and kilograms are both measures of mass, simply differing in their quantitative structure, i.e in 10 (metric) or in some old units such as an ounce, a tonne, or the how much water a half-eaten fat ladled big mac displaces... Also NEITHER are truly weight but need to be multiplied with the appropriate acceleration to produce the force - or in simple terms for you: Mass x Gravity = Weight.
What a crock. Kilo's is short for kiloGRAMS and refers to '1000', as in kilohertz, kilowatts, etc. As you just said before, grams are the measure of mass. So again what a stupid, ignorant thing to say. "Pressure is force per unit area, or Newtons/square meter (NEVER Grams per square meter). A Newton is a Kg-M/s^2 and that is where the time comes from." Yeah, but where does it come from in the incorrect psi? Mass per area. Which was alluded to in the article: grams per sq. cent.; pounds per square inch. Get it? SI is correct. Imperial/American is plain wrong. Go read a bok.

An alternative technology could be utilized to develop new aircraft speed sensors. For instance, a new idea is the application of the Doppler principle, which would work as follows: the aircraft could release a sound wave signal with known frequency. A sensor mounted at the tip of the aircraft would then sense the frequency of the returning sound wave. The speed of the aircraft is related to the ratio of the returning sound wave frequency to the original sound wave frequency. With this aircraft speed sensing approach, the speed is measured accounting for the wind speed.

Having written code for avionics used in helicopters, I have experience with the need for sensitivity to icing conditions. In rotary winged aircraft, icing build up can be fatal. The air temp and humidity are monitored to determine if atmospheric conditions could contribute to the buildup of ice. The avionics would alert the flight crew to the need for anti-ice heaters to be activated. These heaters were placed in the rotor, the engine inlets, _and_ the Pitot tubes. Depending on the option level of avionics package, (i.e.: added cost extra), the heaters could be activated automatically, (with the mandatory flight crew alert). It seems fairly straight forward that some combination of scenarios took place: 1) the flight crew ignored icing warnings, and did not turn on the pitot tube heat; 2) the avionics did not recognize the potential for icing; 3) there was a maintenance issue that was not addressed; 4) all of the above

VonKarman solution for a redudndant velocity measurement it's so clear that I wonder if airliners are not using it as a comparison measurement.

Even though the discussion about pitot tubes is a good one as it relates to math, physics, and aeronautics; as some have touched upon and previously brought up, the problem is one of programming and systems. As Mannstein mentioned you do not need airspeed to land the plane using other inputs it can be safely flown, however when using fly-by-wire those choices if not programmed correctly are taken away. Perhaps a fourth system after the failure of the first three which would be manual override with remaining instruments.

On June 21, Ockie D. almost had the answer with the statement, "On airliners, the on board computer decides how much the rudder can move for a given speed." My question is, why have a pilot if the computer automatically overrides him, even if he knows the computer is malfunctioning or has bad information?

A possible design strategy to address this problem would be the use of a redundant measurement of velocity, using an alternative technology. The aircraft speed could be independently estimated by the on-board computer by continuously tracking its position with a Global Positioning System, and deriving the speed by keeping track of time. The redundant and independend airplane velocity could be used in case of a Pitot sensor malfunction.

FWIW, what not place a umbrella type shape in front of the opening. The inlet is unlikely to get plugged in this manner and a heater will cause formed ice to shed easily off the curved surface. Now of course, speed is based on vacuum and not pressure.

I've only flown single engine aircraft so I can't speak to commercial aircraft, but on the small planes there is a static port for the air speed indicator (static-pitot system) on the side of the plane that points perpendicular to the (usual) direction of travel. This static port is checked for blockage during the compulsory pre-flight check. It is also a good idea to remove the pitot tube cover (usually visible from the pilot's seat). Addressing Jaguar's comment, I have had a light aircraft trimmed up in a strong headwind to move at negative ground speed (10 deg flaps at min air speed in a 20knot headwind and the plane creeps backward relative to the ground). GPS could show you a completely blocked pitot tube during takeoff but would be very inaccurate once airborne.

Jaguar commented about using GPS, this would give you ground speed and not air speed, which is the thing you are interested in for maintaining lift. GPS is great for calculating when you will arrive but no good for keeping plane in the air!

To AE84 comment:
Dear AE84, in your comment you’ve made several mistakes.
First of all – you were very generous referring to Europe. If this is what you had in mind, Europe is the whole world except Liberia, Myanmar (Burma) and USA. Only those countries did not adapt SI system.
Second – you see, we (all-Europeans) are not so liberal with writing our units, therefore this is what we mean:
1G = one gram of force, respectively 1kG
1g = one gram of mass, respectively 1kg
- with relationship 1G = 1g * 9.81 m/s^2
- kilo is just the unit multiplier equal to 1000; ex: kV, ks, km, kA; popularly an abbreviation “kilo” is used at the store to buy an AMOUNT of something, a MASS of something – who cares how much it weighs; even k$ are used
- Kilo is popularly used as binary unit multiplier equal to 1024; ex: KB (Kilobyte) vs. Kb (Kilobit of memory = 1024 bit) vs. kb/s (kilobit/s = 1000 bit/s for transmission speed)
Third – based on above, 1N = 1 kg*m/s^2
and not Kg – not used but could be decoded as 1024g
and not M – which refers to mol, a unit of mass, not length
Forth - as weight we consider a specific form of a force of the mass is being exposed to at particular point – acceleration “a” depends on location and circumstances. Sometimes it may be even zero. Therefore we usually use a term “force” instead as more general.
And more, not personal to AE84 – often made mistakes in American publications:
ma – is not milliamper, it means atomic mass
MA – is not milliamper, it is megaamper
sec – is not second, it is secant = 1/cosinus
S – is not second, it is siemens = 1/ohm
Dear AE84, please, do not take it personally; I do not have any intention to insult you nor to make you feel sorry. Simply, in my mind, we engineers, we shall use precise language for our mutual benefit, for our own sake, to make our life easier. Referring to your post is just a pretext to correct some common misunderstandings.
Sincerely,
Krzysztof

On airliners, the on board computer decides how much the rudder can move for a given speed. The faster you go, the less movement is allowed. The computer gets the same information about speed from the pitot. Wrong pito info, wrong speed to computer, and perhaps too much deflection on the ruder. Was the rudder not found miles away from the mains wreck?? Hmmm....

SI units are not "more fundamental" in any objective physical way than any other consistent set of units for length, time, mass, and force. It has been widely argued that it is "better" for all people in the world to use a common set of units to make commerce and understanding easier and perhaps reduce chances of error.

Why is it that hundreds of millions of dollars have been spent to place satellites in orbit to support a highly accurate GPS system that can even tell rental car companies if I was speeding and that same system can't be utilized in an aircraft navigation system to determine airspeed, at least as a backup or in a cross check capacity?

The comments are very informative showing how much there is to what is perceived as a simple measurement. The question asked at the end of the article was what to do about the problem. It seems to me that adding a 3-way solenoid valve to the impulse line which has the third port connected to a calibrated air supply with pressure greater than the maximum expected in flight would allow a periodic check on the Pitot tube function. If the overall flight worthiness of the aircraft is dependent on this instrument (and it is redundant)the additional complexity seems a small price to pay. The small puff of air might even take care of the insect problem.

Units are indeed important. Let's not forget Pathfinder.

As a pilot I turn on the pitot heat to prevent icing whenever operating IMC, that is, in clouds or precipitation. If an insect should happen to block the pitot inlet port no amount of heat will give a correct reading. This has happened to me once in 2000 hours of flying. Losing the airspeed because of a blockage does not necessarily mean disaster. The vertical speed, arificial horizon, and engine instruments are sufficient to safely land the aircraft. All pilots receive training on how to land in the event of an airspeed insturment failure.

The wind blowing from the opposite direction will exert extra pressure on the tube, hence velocity calculations will not be accurate right ?

Does my heart good to see a reference to my college physics text! I think I still have the '63 edition in the basement.

Hello! Listen to
Mike Wryly (6-17-09) when all fails go power up 100%, not down.

Actually, the time element is contained in any measurement unit of pressure in essentially the same way it is for Pascals. Pressure is force per unit area. Force, in turn, equals mass times acceleration, which may be expressed in metric units as grams times meters per second squared, or simply as Newtons. Pressure, then, may be expressed as either Newtons per square meter or as gram-meters per second squared per square meter. The English-unit equivalent of Newtons would be pounds. The time element (e. g., "per second squared") is contained within the unit of force, but explicitly expressed when force is written out as mass times distance per time unit squared. In physics, mass is one of the three basic units, and force is derived; while in engineering, force is one of the three basic units, and mass is derived. There is nothing "new" versus "old" about it.

AS A PILOT THE BEST WORD OF ADVICE I WAS GIVEN WAS ANY VISUAL SIGNE OF MOISTURE TURN ON THE PITOT TUBE HEAT. TURN OF AFTER YOU LAND TO KEEP FROM BURING THE LINE BOY DOING FUELING.

Jon - You said...“Wait a minute, pressure in grams per square centimeter, for example, has no unit of time, so where does the velocity come from?"
The error lies with the fact that the units of grams are mass, not force. Europeans commonly (and incorrectly) referto their weight in Kilo's whereas Americans (correctly) refer to their weight in pounds. Pressure is force per unit area, or Newtons/square meter (NEVER Grams per square meter). A Newton is a Kg-M/s^2 and that is where the time comes from.

Fellas,
there are three pitot systems on the AirBus,
this was not the cause of the crash,
what is needed is a lesson on design and why flimsy composite structures should not be used in aircraft tails,,

Concerning Pressure is force by OldFlyer, if we go back to units, we discover the error in this. Pressure - Lbs/sq.in. or psi, whereas Force is Lbs (force) thus Lbs/sq.in does not equal lbs. Multiple lbs/sq.in. times area Sq.in. and the utis cancel and one nets Lbs. as in Force.

As an engineer that has installed hundreds of pitot tubes with and without static pressure taps, I can assure you that these are very reliable instruments for measuring differential pressure which can then be converted to airplane velocity.
What bothers me on Flight 447 is that the pitot tube instrument had been reported to be malfunctioning 5 days BEFORE that eventful flight. How can that be if it's an icing problem -- unless the deicer heater was broken and the pitot tube iced up earlier but no one bothered to fix it.
In the pitot tubes I worked with, there was often an electronic sensor that measured the pressure difference between static and total pressures. These pressure differences are very small: At, say, 150 mph, the delta P is only .3 psi; at 500 mph (likely speed of Flt.447), the delta P is about 3 psi. If the sensor had been malfunctioning prior to the flight (and it wasn't a heater failure), then I would suspect the differential pressure sensor.
Here's my question: exactly why is it that the pitot sensors were being replaced? They have been reliable instruments for measuring airplane speed for a hundred years. Why were the Airbus sensors so unreliable that a replacement program was already in place?

Wouldn't these devices be heated to avoid icing?

Wouldn't these devices be heated to avoid icing?

In English units confusion arises due to the difference in pound mass versus pound force.
Mass M is in pounds, force (weight) in pounds is Mg. Pressure is a force.

Thank you for the lessons. Keep your first statement in mind (Pitot tubes, and similar devices sense pressure changes, not airspeed) when you talk about other devices. Thermocouples do not sense temperature -- they generate voltage in relation to temperature differences. Also, pito tubes are almost useless for air speed. Sensible people use pitot-static (or Prandtl) tubes. This device, along with the manometer, allows the user to read the delta P, and hence the math to determine airspeed. Since you were wanting to explain the ins-and-outs, you should have gone the extra step and explain that the total pressure is the same as the stagnation pressure referred to earlier in your diagram. Your problem with units is not unique, but it should be pointed out that the "old" measurements have worked well for aviation. Note that icing over one hole or the other will both give opposite, but equally disasterous results when indicating airspeed!

For pressure use Lb/ft^2, for velocity use ft/s, and for density use slugs/ft^3. When you treat the units as any other variable the proper units will result. FYI, remmember when you do your units algebra that 1 Lb = 1 slug*ft/s^2

For physical calculations with units, I have found nothing that comes close to Frink, url: futureboy.us/frinkdocs/
It not only know basic units, but also combinations of units. So, given 1.2 kg/m^3, it knows that is a mass density...and what can be done with it.
The above calculation might look like this:
vel = 550 mph #velocity in miles/hr
30734/125 (exactly 245.872) m s^-1 (velocity)
denAir = 1.27 kg/m^3 #density of air
1.27 m^-3 kg (mass_density)
#result in psi
1 atm + 1/2 * denAir * vel^2 -> psi
20.26361113280574347
#result in kPascals
1 atm + 1/2 * denAir * vel^2 -> kPa
139.71268064384
If you get the units wrong, it even helps you get them right:
1 atm + 1/2 * denAir * vel^2 -> lbf
Conformance error
Left side is: 139712.68064384 m^-1 s^-2 kg (pressure)
Right side is: 8896443230521/2000000000000 (exactly 4.4482216152605) m s^-2 kg (force)
Suggestion: multiply left side by area
or multiply left side by length^2
For help, type: units[area]
or
units[length]
to list known units with these dimensions.

Gravitational constant (32.2ft/sec^2) wil lgive the time units needed. Always keep track of units regardless of the measurement system and you will never go wrong.

Do a better job of never allowing the pitot tubes to ice over (or stop flying through similar cloud formations)...

Nice explanation.

Boy, this sort of unit arrangement has caused me many headaches because texts and reference books don't give the measurement units. Good idea to go to basic meter-kilogram-second units to get to the most basic info. Nicely done Jon.