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HUD AoA indicator


Fox One

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I don’t think the AoA indicator on HUD is working correctly. Precisely, I don’t think the upper mark on the scale should correspond to 24 deg.

 

On the real aircraft, AoA indicator appears only in display modes “Takeoff” and “Landing”. In DCS we don’t have a “Takeoff” mode, but we have a “no-selection” mode.

 

I don’t think the indicator on the real aircraft was intended as an aid to allow performing aerobatics “HUD only”. It appears only in “Takeoff” and “Landing” modes – in takeoff/landing configuration the real aircraft is restricted to 20 deg AoA. Why would they put a mark on the scale at 24 deg, when you shouldn’t exceed 20?

 

In “Takeoff” mode I think the role of the AoA scale is to help the pilot during rotation. The Su-27 has a pitch angle indicator on the HUD of a type that provides poor pitch rate awareness. So the AoA indicator is helpful in this case. I am 99% sure that in “Takeoff” mode the upper mark on the scale corresponds to 10 deg AoA. The manual recommends 10 deg pitch for takeoff – you rotate until the AoA reading index on the HUD is at the upper mark on the scale. And here is also the explanation why the AoA scale extends so much above the upper mark – because the upper mark corresponds to 10 deg only, not 24.

 

In “Landing” mode the AoA scale’s role is more like to display how much AoA “reserve” the aircraft has for approach, flare, touchdown. I believe in “Landing” mode the upper mark on the scale corresponds to 12 deg instead of 10, however I can’t demonstrate that. But it is worth saying that in the flight manual the landing speed diagram clearly states it is for a 12 deg AoA at touchdown.

 

In the image below I analyze what can be seen in this video

 

In the video the aircraft is an Su-27UB, but it doesn’t matter.

 

The “Takeoff” and “Landing” display modes are similar. I think the pilot in the video selected “Takeoff” display mode for landing because he didn’t wanted to make a slower landing, with 12 deg AoA at touchdown. It was not necessary. So he preferred to have the AoA indicator on the HUD show him 10 deg as a reference.

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I have tried to recreate flight condition seen in this video at 2’29’’

 

Speed 520, altitude 2300, pitch angle 2.

 

I have used close to 100% fuel, just to make the AoA higher. See the image below. The cockpit indicator shows about 3 deg AoA.

 

The AoA indicator in the video is at about 25% between zero AoA and max AoA. Assuming the upper mark corresponds to 10 deg would mean the AoA is 2.5 deg – that seems close to my experiment. If we assume the upper mark corresponds to 24 deg like in DCS that would mean the AoA is 6 deg – that is way too much for current flight condition. At 6 deg AoA in those conditions the aircraft would be pulling more than 1G – notice in the video the pitch angle changes so slowly, it can be considered constant. The aircraft in the video was at very close to 1G.

 

I repeated the experiment at very low fuel level – AoA was about 2 deg.

 

In my video capture included in the screenshot the vertical velocity shows -6m/s descent, while my experiment shows -2, that is because FC aircraft don’t have variometer delay. Notice that in the video the descent rate reduces continuously.

 

Minor differences between AoA scale in DCS and real aircraft:

- on the real aircraft the left “zero bank angle/AoA reading index” mark is touching the AoA scale

- the gap between zero AoA mark and min AoA mark is smaller

- the horizon line is crossing the AoA scale

- the AoA scale is shorter

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