[RESOLVED] Wrong HUD energy chevrons?

[Kercheiz]

Today, after a cruise at M2.2, I dived. At about M2.0 / 650kts during the dive, My airspeed (kts, HUD) was clearly increasing, while the chevrons were well under the speed vector.

 

 

Is this an airspeed/ground speed effect (wind), or a bug?

 

 

Thanks!

[Eldur]

Airspeed is indicated and will increase when diving. Check the Mach number which should stay the same.

[Kercheiz]

I know airspeed increases when diving, that's not the issue, please read the title...

 

The problem is energy chevrons below the speed vector while the aircraft is accelerating. They should be above the speed vector.

 

This is either

A) A bug

B) An effect of energy chevrons being based on ground speed, and despite having my airspeed increasing, my ground speed is decreasing because I'm leaving strong high altitude tail winds. But I do not know if the energy chevrons are ground or air speed based.

[myHelljumper]

I know airspeed increases when diving, that's not the issue, please read the title...

 

The problem is energy chevrons below the speed vector while the aircraft is accelerating. They should be above the speed vector.

 

This is either

A) A bug

B) An effect of energy chevrons being based on ground speed, and despite having my airspeed increasing, my ground speed is decreasing because I'm leaving strong high altitude tail winds. But I do not know if the energy chevrons are ground or air speed based.

 

I think the FM guy at Razbam said it was correct, as the energy chevrons are generated by the INS, so they would use your true speed.

[jojo]

Please post a video or q track, we are talking without a clue here.

 

But you could try to make the maths (total energy = kinetic energy + potential energy) at various sample points during your descent to assess if it’s right or wrong.

 

Chevron aren’t just speed indicators but total energy indicators.

[Frederf]

The chevrons (if it's the same math as the infos linked below) have a very interesting math. The positioning of the chevrons relative to the horizon represent an energy angle. It's a sort of theoretical flight path that if you were to fly it at constant kinetic energy the potential energy change rate would equal the actual total energy rate of change.

 

For example if you add energy (either kinetic or potential or mixed) and the chevrons jump up to +5° above the horizon that means you're adding an equivalent of a constant speed 5° climb. The chevrons aren't positioned relative to the VV but relative to the horizon. The result is that if you climb at constant kinetic energy then the chevrons will jump up (or down) an equivalent climb (or dive) path but since you're actually putting all energy change into potential energy the VV and chevrons will match up.

 

The angle Gamma_T is your pure-climb-equivalent-path angle and Gamma is your actual path angle. The VV is Gamma above or below the horizon. The chevrons are Gamma_T above or below the horizon. If you are flying in the kinetic+ regime then Gamma_T>Gamma and kinetic- regime Gamma_T<Gamma. So in a certain way you can compare the relative positions of VV and chevron to see if you are aimed above or below the potential path energy equivalent. It is inescapable that if VV is above chevron that kinetic energy is decreasing, below chevron that kinetic energy is increasing, and coincident that kinetic energy is constant.

 

Chevron representing total energy and only speed are both true depending on what symbol the chevron is placed relative to. When considering relative position of chevron and horizon, chevron shows total energy. When considering relative position of chevron and VV, chevron shows speed change.

 

The calibration of the chevrons can tested by flying level at constant speed and pulling the power and seeing what angle chevron goes to relative to horizon. Then do the exact same test but put VV at this path angle after pulling throttle. If calibration is good then speed will be constant in this descent.

 

https://www.fun-mooc.fr/c4x/isaesupaero/25001S02/asset/SymbologieTeteHaute.pdf

hudF.pdf (sorry, don't have link at the moment)

[Kercheiz]

I think I found the explanation, the HUD displays CAS, not TAS. So even with no wind (Where TAS = ground speed), CAS is much lower (by a factor of 0.6 or so) than TAS at high altitudes. That factor goes to 1.0 at sea level.

Upon diving the CAS may increase while the TAS decreases.

 

So the chevrons are correct, I was just not expecting CAS to increase that much while I was not accelerating, because I forgot how much CAS is lower than TAS at high altitudes.

Edited September 17, 2020 by galinette

[Kercheiz]

You can do the following test:

Climb at 45000ft, M2.0

Dive -25° while trying to keep a speed of M2.0

Between 25000ft and 15000ft this is very visible. Energy chevrons under, HUD airspeed (CAS) increases a lot. True airspeed and ground speed decrease a lot (as shown on the INS).

chevrons.trk

[RAZBAM_ELMO]

You can do the following test:

Climb at 45000ft, M2.0

Dive -25° while trying to keep a speed of M2.0

Between 25000ft and 15000ft this is very visible. Energy chevrons under, HUD airspeed (CAS) increases a lot. True airspeed and ground speed decrease a lot (as shown on the INS).

 

Hello Galinette, can you confirm this is still occurring in the latest OB patch 23/09/2020? If so please attach a new trk file

[RAZBAM_ELMO]

Marking as RESOLVED and moving to RESOLVED.