Speed Hold

[AeriaGloria]

Anyone got speed hold to work at any time? 
 

  I know it is intended to engage at the same time as altitude hold, and manuals have it supposedly being effective enough to hold you within +/-10kmh

   However, if I taxi along a runway, keep all my controls the same, but change speed with Wheel brakes and/or collective, I cannot get pitch channel to move even after a 100 kmh speed difference 

 

  I know it uses the pitch channel, and the pitch channel is also influenced by cyclic movement from the compensation transducers, so I performed my tests engaging altitude channel when my cyclic was in the needed position and not moving it. 
 

I can upload a track shortly, but wanted to see if anyone else had this experience or if it is a long known missing feature

 

[AlphaOneSix]

My documentation indicates that the KZSP indicated airspeed controller doesn't start working until 150km/hr IAS.

[AeriaGloria]

56 minutes ago, AlphaOneSix said:

My documentation indicates that the KZSP indicated airspeed controller doesn't start working until 150km/hr IAS.

Interesting, I’ll try it out 

[AeriaGloria]

   Here is a track. I tried to start various dives/climbs, and enabled roll/pitch and altitude channel when only at a stable pitch, trying to minimize as much pitch channel movement as possible from attitude hold/compensation sensors 

   From 150-300 kmh, could not see any noticeable change. Perhaps it is too small to see or the test isn’t correct, but I expected it to be noticeable since Mi-8/17 documentation including ED manual says speed hold should be good for +/-10kmh holding, and for Mi-24 this value is +/-5 kmh. In Mi-24’s case, the speed hold causes the pitch channel to deflect 100% at just a +/-6.6 kmh difference from the speed it’s turned on at.

   Most other channels between Mi-8/24 appear to be identical, roll channel attitude hold holding a 12.5 degree difference from set attitude, compensation sensors using 40% lateral cyclic to create 100% deflection at attitude turned on, and altitude channel causing a 100% deflection when only a 10m difference from initialized altitude, yaw channel heading needing a 7 degree difference for 100% deflection. The largest outlier being pitch attitude hold which is 3x more aggressive in Mi-24, 5 degrees vs 15 degrees. But its compensation sensor is also identical between Mi-8/24, causing 100% pitch channel deflection for 50% stick movement. 
 

  So for speed hold, I expected if not a similar value, to be within a similar ballpark. Here is a track 

kzspspeedhold.trk

[DeMonteur]

On 11/8/2023 at 3:56 AM, AeriaGloria said:

Anyone got speed hold to work at any time? 
 

  I know it is intended to engage at the same time as altitude hold, and manuals have it supposedly being effective enough to hold you within +/-10kmh

   However, if I taxi along a runway, keep all my controls the same, but change speed with Wheel brakes and/or collective, I cannot get pitch channel to move even after a 100 kmh speed difference 

 

  I know it uses the pitch channel, and the pitch channel is also influenced by cyclic movement from the compensation transducers, so I performed my tests engaging altitude channel when my cyclic was in the needed position and not moving it. 
 

I can upload a track shortly, but wanted to see if anyone else had this experience or if it is a long known missing feature

 

As far as I know speed hold is not installed on this version of Mi-8. It is sepparate switch left of the AP indicator. At least on Mi-171Sh from Ulan-ude.  I never heard or read that altitude hold is connected to speed hold also

Edited January 21, 2024 by DeMonteur

[AeriaGloria]

7 hours ago, DeMonteur said:

As far as I know speed hold is not installed on this version of Mi-8. It is sepparate switch left of the AP indicator. At least on Mi-171Sh from Ulan-ude.  I never heard or read that altitude hold is connected to speed hold also

 

From the DCS Mi-8 manual

[zerO_crash]

It definitely works. I tested it in low/medium/heavy winds, and it holds speed practically on point (10-15km/h deviations that keep correcting themselves). In heavier winds, the Mi-8MTV2 will fluctuate somewhat on altitude, trying to correct for uneven wind and turbulence, but it stays within 5-6m +- from the altitude set when you engaged the channel.

 

To add to what AlphaOneSix wrote. It has to be activated above 150km/h, otherwise the AP will hold the desired pitch and not speed.

 

I can post you track if you still have trouble with it.

Edited May 3, 2024 by zerO_crash

[AeriaGloria]

On 5/3/2024 at 6:04 AM, zerO_crash said:

It definitely works. I tested it in low/medium/heavy winds, and it holds speed practically on point (10-15km/h deviations that keep correcting themselves). In heavier winds, the Mi-8MTV2 will fluctuate somewhat on altitude, trying to correct for uneven wind and turbulence, but it stays within 5-6m +- from the altitude set when you engaged the channel.

 

To add to what AlphaOneSix wrote. It has to be activated above 150km/h, otherwise the AP will hold the desired pitch and not speed.

 

I can post you track if you still have trouble with it.

 

You can see the pitch channel giving inputs as speed changes 5-6 kmh when engaged? 

[zerO_crash]

3 hours ago, AeriaGloria said:

You can see the pitch channel giving inputs as speed changes 5-6 kmh when engaged? 

Depending on winds, I can see the AP holding the speed within 10-15km/h of my selected speed (a short spike of 15km/h deviation was in heavy winds and turbulence, otherwise, a maximum was 10km/h, before the AP stabilized back on speed). That's to say, in the worst case, I saw the speed deviate up to 15km/h, before quickly going back. Generally, when you engage it, the speed-hold will take a couple of seconds to adjust. That is also when you will experience those deviations, otherwise in really heavy wind/turbulence.

 

The altitude changed by 5-6m from my preset (it was oscillating), but normalized over time (not that it should be held, it was simply to give a reference of AP stability).

Edited May 11, 2024 by zerO_crash

[zerO_crash]

Attaching a short track of using the speed-hold AP in Caucasus. To stress-test it, I created an empty mission with ridiculous winds/storm (at the limit of what you are allowed to take off with in just about any helicopter, except Kamovs.). One would never use any hold modes, other than heading, in this weather:

 

Wind:

8000m - 45m/s

2000m - 40m/s

500m   - 32m/s

10m      - 15m/s

 

Turbulence:

4.5m (45*0.1m)

 

Over a course of multiple runs for exhaustive testing, the highest deviation I had was 20km/h, but that only for an isntant, quickly falling back to within 10km/h. This isn't visible in the track, btw.

 

As for the track, I engaged the speed-hold at approximately 197km/h, and you can see how well it holds it with this weather. It's truly a fantastic AP-suite. 

Mi-8MTV2 Speed-hold AP.trk TEST1.miz

Edited May 11, 2024 by zerO_crash

[AeriaGloria]

On 5/11/2024 at 2:24 AM, zerO_crash said:

Attaching a short track of using the speed-hold AP in Caucasus. To stress-test it, I created an empty mission with ridiculous winds/storm (at the limit of what you are allowed to take off with in just about any helicopter, except Kamovs.). One would never use any hold modes, other than heading, in this weather:

 

Wind:

8000m - 45m/s

2000m - 40m/s

500m   - 32m/s

10m      - 15m/s

 

Turbulence:

4.5m (45*0.1m)

 

Over a course of multiple runs for exhaustive testing, the highest deviation I had was 20km/h, but that only for an isntant, quickly falling back to within 10km/h. This isn't visible in the track, btw.

 

As for the track, I engaged the speed-hold at approximately 197km/h, and you can see how well it holds it with this weather. It's truly a fantastic AP-suite. 

Mi-8MTV2 Speed-hold AP.trk 271.91 kB · 2 downloads TEST1.miz 9.81 kB · 0 downloads

 

I will try this mission with and without the alt/speed. I not only want to see a difference between the mode on and off, but also want to see pitch channel deflection: it should be very obvious and move pitch channel 100% for 10 kmh deviation 

[zerO_crash]

On 6/9/2024 at 12:48 PM, AeriaGloria said:

I will try this mission with and without the alt/speed. I not only want to see a difference between the mode on and off, but also want to see pitch channel deflection: it should be very obvious and move pitch channel 100% for 10 kmh deviation 

There is a difference between what it "should" and what it "will". As I have already spoken with PilotMi8 on RU-side, there are numerous functions modelled in the Mi-8 (and Mi-24) which mimic real aircraft imperfections. As such, manuals (IRL) have always been guides, more than anything else. They are not consecutive with respect to individual aircraft. As such, for the speed hold, I've seen the AP fluctuate in performance, but a 15-20km/h deviation results in practically instant correction. Therefore, it works, and is actually simulated as per realistic individual properties, rather than a theoretical scheme described by the manual.

 

This discussion goes very much into what we've discussed before about the main rotor RPM threshold. One thing is the manual, something else are practical considerations. In the same way, I noticed recently that during heavier loads on engines (winter - much electrical equipment needed plus worse thermic characteristics), upon pulling in more collective, the power synchronizers will not be able to keep up. That's to say - one engine (left) will be be at higher RPM, than the right one. IRL, this is the case (engines are almost never even), thus individual differences between both engines are modelled.*

 

* 

 

 

Point being, go ahead and check yourself, but nothing is ever precisely as described by the manual. As an instructor myself, that is both from experience as well as military pilots claim.

Edited June 11, 2024 by zerO_crash

[AeriaGloria]

On 6/9/2024 at 5:41 AM, zerO_crash said:

There is a difference between what it "should" and what it "will". As I have already spoken with PilotMi8 on RU-side, there are numerous functions modelled in the Mi-8 (and Mi-24) which mimic real aircraft imperfections. As such, manuals (IRL) have always been guides, more than anything else. They are not consecutive with respect to individual aircraft. As such, for the speed hold, I've seen the AP fluctuate in performance, but a 15-20km/h deviation results in practically instant correction. Therefore, it works, and is actually simulated as per realistic individual properties, rather than a theoretical scheme described by the manual.

 

This discussion goes very much into what we've discussed before about the main rotor RPM threshold. One thing is the manual, something else are practical considerations. In the same way, I noticed recently that during heavier loads on engines (winter - much electrical equipment needed plus worse thermic characteristics), upon pulling in more collective, the power synchronizers will not be able to keep up. That's to say - one engine (left) will be be at higher RPM, than the right one. IRL, this is the case (engines are almost never even), thus individual differences between both engines are modelled.*

 

* 

 

 

Point being, go ahead and check yourself, but nothing is ever precisely as described by the manual. As an instructor myself, that is both from experience as well as military pilots claim.

 

This is actually the work of the power synchronizer, the droop compensator working one one engine and synchronizer on the other, so during changes of engine rpm you will almost always see a small difference in RPM, and they will over time gradually equalize out. 
 

My point is, for speed hold, whatever effect it “should” have or not, I cannot see it working on the pitch channel as the autopilot panel shows. Checks for speed hold in the aircraft include checking for differences with the pitch channel IN-1 gauge. You do not see it here, and I even if I never move the cyclic during speed changes the pitch IN-1 only ever shows pitch AP, and no difference with speed hold engaged. If you fly Mi-24, however difference it’s characteristics, and you will both see the speed hold effecting the pitch channel IN-1, but feel it quite extensively as well. 
 

I’ll run a better test with instant pause, and maybe you will see if speed hold is indeed working, it is not appearing to do so through pitch channel as it should 

Edited July 1, 2024 by AeriaGloria