Calibrated airspeed (HUD) vs Air/Ground speed (HSI)

[lascaris]

I notice in the F-18 that the airspeed in the HUD is significantly different than the mach number would suggest or the air and ground speed shown in the HSI. For example, I was doing a free flight and the HUD airspeed was in the 380ish range, the mach number was around 1.05, and the airspeed and ground speed in the HSI was, as I would expect, in the upper 600's. I'm sure there's an obvious answer to the differences but as I'm a bit of a noob, I don't understand what is going on and would appreciate some info.

 

Thanks! :helpsmilie:

[ricktoberfest]

I notice in the F-18 that the airspeed in the HUD is significantly different than the mach number would suggest or the air and ground speed shown in the HSI. For example, I was doing a free flight and the HUD airspeed was in the 380ish range, the mach number was around 1.05, and the airspeed and ground speed in the HSI was, as I would expect, in the upper 600's. I'm sure there's an obvious answer to the differences but as I'm a bit of a noob, I don't understand what is going on and would appreciate some info.

 

 

 

Thanks! :helpsmilie:

 

 

 

The F18 hud shows indicated air speed, which generally gets lower the higher up you are as there’s less air pressure.

[tweet]

You are seeing

 

as Rictoberfest says the the effect of altitude (air density), temperature, mach number on the several differences in airspeed.

 

 

IAS, Indicated AirSpeed, is what is raw indication from the airspeed indicator system. It is a measurement of dynamic pressure sensed by the pitot tube compared to static pressure from a static source. It is potentially inaccurate and must be corrected.

 

 

CAS, Calibrated AirSpeed, is IAS corrected for installation errors found in the indicating system.

 

 

EAS, Equivalent AirSpeed, is CAS corrected for the compressibility of air. These corrections are found at higher mach numbers. Greater than .80 mach is generally the area where the correction become significant.

 

 

TAS, True AirSpeed, is EAS corrected for density. Air density decreases with increased altitude and decreased temperature.

 

 

CAS is generally a solid number to use for pilots considering maneuvering and handling qualities. TAS is essential to navigation calculations. Modern aircraft remove the messiness of calculating all of this through the use of Air Data Computers (ADC). We get real time calculations rather than having to make time consuming hand calculations.

 

 

A great rule of thumb is : TAS increases by 2% for every 1000' of altitude gain. For example, 300 KIAS at 10,000' is 360 KTAS on a standard day. This rule of thumb is close enough up to about 30,000' and .80 mach. Exceeding either of those numbers increases the inaccuracies to a point that they're not too useful.

 

 

The relationships of IAS - CAS - EAS - TAS looks like a square root symbol. That is: IAS >- CAS <++ EAS ~ TAS most of the time.

 

 

Class dismissed.

[lascaris]

Thank you very much!

 

So if I understand, the Calibrated Air Speed is not the actual velocity of the plane through the sky but it does give a good indication of the energy state of the aircraft, i.e. the ability to maneuver without stalling or becoming easy meat for an adversary. The speed on the HSI is the actual velocity which is used for navigation.

 

Do I have that right?

[Etirion]

To put it simply and omitting a lot of the detail tweet already mentioned: Airspeed tells you how much air is flowing over the wings, so thats whats most important to keep the thing flying.

True Airspeed tells you the speed you are going over the ground combined with wind forces (again really simplifying things here)

Ground Speed ignores the air completly and tells you how fast you are moving over... well the ground. So wind is also taken out.

 

Edit: Please dont shoot me if you're a pilot or an Aerospace engineer, I know theres a lot more to it :D

[tweet]

That is the essence and what you should understand.

 

 

I like to think that CAS is the airspeed that an aircraft "senses" as it flies through the air. That isn't a perfect explanation but works well except in extremes when the effects of air density (pressure and temperature dependent), compressibility, and mach effects do weird things to aerodynamics. Almost everything pilots use to know the performance they can get out of an airplane is IAS or CAS (where displayed) dependent. It is what is readily available. In the higher altitude and speed regions of the flight envelope mach becomes the go to performance indicator.

 

 

Almost everything else that is necessary to do whatever job you're trying to do with an airplane is TAS dependent. Optimum and maximum altitudes, fuel burn, time-distance calculations, and the like all use TAS or mach.

[tweet]

No worries Etirion. We all find a way of thinking about complex subjects in terms we understand. You're spot on - minus the details that do get really messy.

 

 

Almost forty years of flying jets for a living, including three teaching in USAF UPT and another three at the instructor school (PIT), left me with a nearly unforgettable understanding of all of this stuff from a pilot's perspective. Instructing is a really incredible experience. A guy/gal learns way, way more than most pilots need to do their job or they get happy with embarrassing themselves in front of their peers. Apparently, it has a lasting effect.