What's the difference between indicated air speeds?

[imacken]

Why are the 2 air speeds shown on the HSI (true and ground) so different to the indicated air speed on the HUD?

[danny875]

Rather than try and explain it myself here is a link with a simple explanation:

 

https://www.boldmethod.com/blog/lists/2019/11/the-four-types-of-airspeed-and-how-each-works/

[imacken]

Thanks a lot. Very helpful!

[Drag80]

Indicated airspeed is indicated by instruments. As the air gets thin it shows less speed. Ground speed is the actual / true speed given by GPS

[imacken]

Ground speed is the actual / true speed given by GPS

Ground speed is different to true speed as shown in the descriptions in the link above and on the Hornet HSI.

[drallabco]

Indicated: what the indicator shows

Ground: how fast you are moving over the ground

True: how fast you are moving past the air molecules

 

True airspeed differs from indicated because as the air gets thinner, the ram air pressure on the pitot tube goes down, thus indicating lower, but we are still moving past those air molecules very quickly. They're just not producing the same pressure as they would at low altitude where the air is thicker. So, true airspeed is corrected for air temperature and pressure at a given altitude.

[Svend_Dellepude]

Indicated is the speed you would relate to when maneuvering. Up high you need to be going a lot faster to have the same G's available to you as when you are low.

 

The U2 for instance had like a 10 Knots window in which it would either stall because of low IAS or overspeed due to high Mach, when cruising at max altitude.

[sk000tch]

That link is useful but it doesn't explain "Why" different values are important. I don't have a link to provide but I am sure a few minutes on google will yield plenty of reading material. fwiw, the Hornet displays the various values where they are the most relevant.

 

IAS is the most important number as it tells you what's going on with your airfoils. The Hornet actually displays CAS in the HUD, or calibrated airspeed, but for our purposes consider it the same thing (its IAS corrected for instrument error). Its measured by a pitot tube, which is just a tube with a hole down the center and perpendicular holds around the outsides, that measures the difference between air hitting the front of the tube vs the air around the outside. It functions on the same principal that makes your wing work, and thus tells you what's going on with your wings. If you're very slow, flying the pattern, etc., it tells you your proximity to stall speed, etc.

 

TAS is of less utility, and will always be greater than IAS. Its mostly useful for planning when we used to have to do that by hand (as an intermediate step to GS). GS is the net of TAS and wind, which is useful mostly for navigation. As such, they are displayed on the HSI. TAS to the left, GS to the right. Useful for a quick check on winds aloft. Note TOT depends on GS for obvious reasons.

 

For the same reasons, the radar, rather than GS, displays ownship mach and, I believe CAS? I'm still using rift so I can't read it anyway. The more relevant number is target mach, displayed to left of track, and Vc - closure velocity, on the right hand side next to range carrot.

 

I'm not sure what the SA screens display, as that is an interesting combination of navigation and radar/targeting sensors. My hunch is it displays mach but we'll have to see in a couple days.

[sk000tch]

The U2 for instance had like a 10 Knots window in which it would either stall because of low IAS or overspeed due to high Mach, when cruising at max altitude.

 

Interesting tidbit, I have never heard of "coffin corner" before. Amazing to think in the 1950s we designed a plane that was so close to the envelope that it could reach an altitude where Vne and stall speed approach 0. If you look at its stall/mach boundaries it makes sense, as pressure decreases its mach decreases, I just didn't realize it got so close. Amazing to think they were flying around at 85 indicated at 70,000 ft, RWR going apeshit... makes sense why we built the SR-71.

 

Coffin Corner from wikipedia: https://upload.wikimedia.org/wikipedia/en/7/75/CoffinCornerU2.png