Vx Airspeed

[Jenni Israelsen]

Inaviation, airspeed is the speed of an aircraft relative to the air it is flying through (which itself is usually moving relative to the ground due to wind). It is difficult to measure the exact airspeed of the aircraft (true airspeed), but other measures of airspeed, such as indicated airspeed and Mach number give useful information about the capabilities and limitations of airplane performance. The common measures of airspeed are:[1]

The measurement and indication of airspeed is ordinarily accomplished on board an aircraft by an airspeed indicator (ASI) connected to a pitot-static system. The pitot-static system comprises one or more pitot probes (or tubes) facing the on-coming air flow to measure pitot pressure (also called stagnation, total or ram pressure) and one or more static ports to measure the static pressure in the air flow. These two pressures are compared by the ASI to give an IAS reading. Airspeed indicators are designed to give true airspeed at sea level pressure and standard temperature. As the aircraft climbs into less dense air, its true airspeed is greater than the airspeed indicated on the ASI.

Airspeed is commonly given in knots (kn). Since 2010, the International Civil Aviation Organization (ICAO) recommends using kilometers per hour (km/h) for airspeed (and meters per second for wind speed on runways), but allows using the de facto standard of knots, and has no set date on when to stop.[2]

Depending on the country of manufacture or which era in aviation history, airspeed indicators on aircraft instrument panels have been configured to read in knots, kilometers per hour, miles per hour.[3] In high altitude flight, the Mach number is sometimes used for reporting airspeed.

Indicated airspeed (IAS) is the airspeed indicator reading (ASIR) uncorrected for instrument, position, and other errors. From current EASA definitions: Indicated airspeed means the speed of an aircraft as shown on its pitot static airspeed indicator calibrated to reflect standard atmosphere adiabatic compressible flow at sea level uncorrected for airspeed system errors.[4]

An airspeed indicator is a differential pressure gauge with the pressure reading expressed in units of speed, rather than pressure. The airspeed is derived from the difference between the ram air pressure from the pitot tube, or stagnation pressure, and the static pressure. The pitot tube is mounted facing forward; the static pressure is frequently detected at static ports on one or both sides of the aircraft. Sometimes both pressure sources are combined in a single probe, a pitot-static tube. The static pressure measurement is subject to error due to inability to place the static ports at positions where the pressure is true static pressure at all airspeeds and attitudes. The correction for this error is the position error correction (PEC) and varies for different aircraft and airspeeds. Further errors of 10% or more are common if the airplane is flown in "uncoordinated" flight.

Indicated airspeed is a better measure of power required and lift available than true airspeed. Therefore, IAS is used for controlling the aircraft during taxiing, takeoff, climb, descent, approach or landing. Target speeds for best rate of climb, best range, and best endurance are given in terms of indicated speed. The airspeed structural limit, beyond which the forces on panels may become too high or wing flutter may occur, is often given in terms of IAS.

This expression is based on the form of Bernoulli's equation applicable to isentropic compressible flow. CAS is the same as true air speed at sea level standard conditions, but becomes smaller relative to true airspeed as we climb into lower pressure and cooler air. Nevertheless, it remains a good measure of the forces acting on the airplane, meaning stall speeds can be called out on the airspeed indicator. The values for p 0 \displaystyle p_0 and a 0 \displaystyle a_0 are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated.

The true airspeed is important information for accurate navigation of an aircraft. To maintain a desired ground track whilst flying in a moving airmass, the pilot of an aircraft must use knowledge of wind speed, wind direction, and true air speed to determine the required heading. See wind triangle.

TAS is the appropriate speed to use when calculating the range of an airplane. It is the speed normally listed on the flight plan, also used in flight planning, before considering the effects of wind.

Some airspeed indicators include a TAS scale, which is set by entering outside air temperature and pressure altitude. Alternatively, TAS can be calculated using an E6B flight calculator or equivalent, given inputs of CAS, outside air temperature (OAT) and pressure altitude.

Equivalent airspeed (EAS) is defined as the airspeed at sea level in the International Standard Atmosphere at which the (incompressible) dynamic pressure is the same as the dynamic pressure at the true airspeed (TAS) and altitude at which the aircraft is flying. That is, it is defined by the equation

EAS is a measure of airspeed that is a function of incompressible dynamic pressure. Structural analysis is often in terms of incompressible dynamic pressure, so equivalent airspeed is a useful speed for structural testing. The significance of equivalent airspeed is that, at Mach numbers below the onset of wave drag, all of the aerodynamic forces and moments on an aircraft are proportional to the square of the equivalent airspeed. Thus, the handling and 'feel' of an aircraft, and the aerodynamic loads upon it, at a given equivalent airspeed, are very nearly constant and equal to those at standard sea level irrespective of the actual flight conditions.

At standard sea level pressure, CAS and EAS are equal. Up to about 200 knots CAS and 10,000 ft (3,000 m) the difference is negligible, but at higher speeds and altitudes CAS diverges from EAS due to compressibility.

For aircraft that fly close to, but below the speed of sound (i.e. most civil jets) the compressibility speed limit is given in terms of Mach number. Beyond this speed, Mach buffet or stall or tuck may occur.

The airspeed sensor is a JMT F19129/30, and the pixhawk tested are version 2 (cube) and Pixhawk 4. The fact that it works well on a cube show that the airspeed sensor works. I use the latest stable version of PX4, QGC is up to date. Circuit breaker for airspeed is 0.

Thank you so much !!! The airframe was the problem. I have to admit that I feel a bit stupid, it was a custom airframe which my company configured, and nobody thought to check that. Going back to a default airframe enabled airspeed.

Hi there, I have a similar problem in which I am trying to use an airspeed sensor for a Holybro s500 quadcopter. It seems that the airspeed sensor tab is not displayed for this configuration. Is there any way to still use the airspeed sensor with a quadcopter using QGC or do I have to use an Arduino to read in the airspeed values instead. I am using the MEAS MS4525DO pitot tube airspeed sensor plugged into the I2C port on the Pixhawk 4.

Hey @Sindusagar_Kamurthi, I solved this issue by using Mission Planner with ArduPilot, I was unable to configure the airspeed sensor using QGC, and I switched to mission planner and was able to configure it relatively easily.

Link to docs: here

Sindusagar_Kamurthi

Yes, It depends on the airframe you are using.

if you still want to use airspeed sensor, then may be you can create your own airframe and you can enable airspeed sensor there.

Or you can use Generic Quad VTOL, where you use it only for quadcopter mode but you can still use airspeed sensor.

Recently, my team tested a large quadplane model, and ran a satisfactory flight in AUTOTUNE mode. However, I noticed from the log analysis that there are some noise issues with our airspeed sensor; see the screenshot from our log below:

It looks like a cabling issue. The key thing to look at is the ARSP.Temp reading, which is the airspeed temperature:

image.png801423 39.1 KB

The good news is it shows the issue even when you are on the ground. I suggest you put a good logic analyser (eg. Saleae Pro) on the I2C pins and see if you can spot what is happening. Also try without the differential extender.

Cheers, Tridge

It will probably fly OK, but it is risky. I suggest you fly with ARSP_USE=0, which means it will log the airspeed sensor but not use it. It will use a synthetic airspeed estimate instead. Make sure you set TRIM_THROTTLE to a value a bit above what you expect to be needed in cruise flight (ie. at least 50 on your airframe).

btw, your ARSPD_FBW_MIN looks a bit low for that airframe. Are you sure it stalls at below 13m/s ?

Also, your vibration levels are far too high:

image799427 57 KB

you need to add some good isolation foam below the cube (the internal isolation of the cube is not suited to the frequencies in your airframe). This is causing significant pitch estimation errors in flight.

Cheers, Tridge

Thanks @tridge ! Airspeed sensor temperature was not something I had thought to look at previously, and my understanding of I2C is pretty limited. How does the airspeed temperature affect the performance of the sensor?

So, for all of you far more knowledgeable than me, what would be some causes of the Airspeed indicator strip on the G5 being about 10knots slower than the indicated airspeed on the analog gauge? I will preface this with, I do understand that the strip on the G5 is not primary. I also am leaning towards trusting my analog gauge because if I follow the analog gauge the airplane stalls at the correct indications on the gauge where it is a lot lower on the G5, when I landed according to the airspeed on the analog the airplane landed fine, if I land with the G5 at the correct landing speed, the analog is way high and I float half way down the runway. So I am leaning towards trusting my analog airspeed indicator but I am curious as to what could be the cause of the discrepancy. My avionics shop is on notice but they are extremely busy and trying to fit me back in but I thought I would see what the collective has to say and if anything could point me in the right direction. Thanks.

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