Low Speed Directional Static Stability appears incorrectly modelled

[Samythevilsnail]

Came back to the Hind after not flying it since initial early access FM release. Did the following test on DCS Open Beta on 2 Dec 2023:

 

Conditions:

SAS OFF in all 3 channels

IGE Hover, heading perpendicular to the runway

 

Test method:

Accelerate laterally to the right along the runway, gradually increasing lateral translation speed towards 100kph while maintaining ground track.

Repeat with translation to the left.

 

Result:

During translation to the left, the Hind yaws to the left, indicating positive directional stability (weathercock stability).

During translation to the right the Hind yaws to the left indicating negative directional stability.

 

Discussion:

I've tried to find information on the Hind's behaviour in the low speed environment (such as Trimmed Flight Control Position data), but was unable to, and so cannot verify if the above behaviour is correct. However I find it quite unusual to have such a large difference in directional stability whilst translating to the left vs to the right. 

Am happy to discuss above behaviour using any real world test data the team is referencing, or help with further data gathering in DCS.

Kind regards.

[AeriaGloria]

3 hours ago, Samythevilsnail said:

Came back to the Hind after not flying it since initial early access FM release. Did the following test on DCS Open Beta on 2 Dec 2023:

 

Conditions:

SAS OFF in all 3 channels

IGE Hover, heading perpendicular to the runway

 

Test method:

Accelerate laterally to the right along the runway, gradually increasing lateral translation speed towards 100kph while maintaining ground track.

Repeat with translation to the left.

 

Result:

During translation to the left, the Hind yaws to the left, indicating positive directional stability (weathercock stability).

During translation to the right the Hind yaws to the left indicating negative directional stability.

 

Discussion:

I've tried to find information on the Hind's behaviour in the low speed environment (such as Trimmed Flight Control Position data), but was unable to, and so cannot verify if the above behaviour is correct. However I find it quite unusual to have such a large difference in directional stability whilst translating to the left vs to the right. 

Am happy to discuss above behaviour using any real world test data the team is referencing, or help with further data gathering in DCS.

Kind regards.

There isn’t real world data for right translational hover that I’m aware of. There is real world tests on side slip, but they begin at 120 kmh.
 

People noticed that on release its right translational hover has little weathervaning effect, and the consensus was that VRS of the tail rotor was modeled. I personally feel that is what is personally happening 

Edited December 1, 2023 by AeriaGloria

[AeriaGloria]

I believe this vsterminus video has a section on it. Most of the other issues he talks about have since been fixed 

 

[Samythevilsnail]

8 hours ago, AeriaGloria said:

People noticed that on release its right translational hover has little weathervaning effect, and the consensus was that VRS of the tail rotor was modeled. I personally feel that is what is personally happening

It's definitely possible, however that would usually manifest when a certain lateral speed is achieved, so you should still get positive (or neutral) weathervane behaviour at lower lateral speeds. That said it's also possible that the Hind has neutral or negative directional stability when translating right. Without test data to refer to however there's no way to know.

[AeriaGloria]

12 hours ago, Samythevilsnail said:

It's definitely possible, however that would usually manifest when a certain lateral speed is achieved, so you should still get positive (or neutral) weathervane behaviour at lower lateral speeds. That said it's also possible that the Hind has neutral or negative directional stability when translating right. Without test data to refer to however there's no way to know.

If VRS usually occurs at 5 m/s, that is only 18 kmh. By the time the hover speed indicator shows max right translation, you are probably VRSing the tail rotor. 
 

Would be great If it was published or mentioned in the aerodynamic manual, but for reference the wind speed limit for wind coming from the right is 10 m/s, 36 kmh. 

Edited December 2, 2023 by AeriaGloria

[Samythevilsnail]

11 hours ago, AeriaGloria said:

If VRS usually occurs at 5 m/s, that is only 18 kmh

Not sure where you got these numbers from, but I did a bit more digging and found this Mi-24D flight manual from the very awesome Cold War Air Museum (CWAM), link here: https://mudspikefiles.s3.dualstack.us-east-1.amazonaws.com/original/3X/7/a/7ab064a7f951b9f0325354f257f5e58c571936d6.pdf. According to this manual, the airspeed limit for sidewards and backwards flight is 10KIAS, which is very close to your numbers. I've also seen some manuals translated from Russian that state 10m/s, but I'll leave that discrepancy aside for this discussion.

So to check out the tail rotor VRS theory I went back into DCS and repeated the test with no weapons and at 50% fuel (so that pedal authority would be less of an issue). I repeated the test but accelerated from hover into lateral flight to the right much more gradually. Referencing the hover speed indicator, at speeds below ~18KPH the Hind weathervanes to the right, requiring left pedal (or rather a reduction in right pedal) to maintain heading, indicating positive directional stability. Once ~18KPH is exceeded, the yaw tendency rapidly reverses, requiring an accordingly rapid right pedal input. Maintaining this flight condition resulted in a yaw oscillation, requiring alternating left and right pedal inputs to maintain heading control.

This behaviour is consistent with tail rotor alternately entering and exiting VRS as the heading oscillated, with the according pedal workload. I'm really impressed that this aerodynamic phenomenon is being modelled in DCS!

Case closed I think.

[AeriaGloria]

3 minutes ago, Samythevilsnail said:

Not sure where you got these numbers from, but I did a bit more digging and found this Mi-24D flight manual from the very awesome Cold War Air Museum (CWAM), link here: https://mudspikefiles.s3.dualstack.us-east-1.amazonaws.com/original/3X/7/a/7ab064a7f951b9f0325354f257f5e58c571936d6.pdf. According to this manual, the airspeed limit for sidewards and backwards flight is 10KIAS, which is very close to your numbers. I've also seen some manuals translated from Russian that state 10m/s, but I'll leave that discrepancy aside for this discussion.

So to check out the tail rotor VRS theory I went back into DCS and repeated the test with no weapons and at 50% fuel (so that pedal authority would be less of an issue). I repeated the test but accelerated from hover into lateral flight to the right much more gradually. Referencing the hover speed indicator, at speeds below ~18KPH the Hind weathervanes to the right, requiring left pedal (or rather a reduction in right pedal) to maintain heading, indicating positive directional stability. Once ~18KPH is exceeded, the yaw tendency rapidly reverses, requiring an accordingly rapid right pedal input. Maintaining this flight condition resulted in a yaw oscillation, requiring alternating left and right pedal inputs to maintain heading control.

This behaviour is consistent with tail rotor alternately entering and exiting VRS as the heading oscillated, with the according pedal workload. I'm really impressed that this aerodynamic phenomenon is being modelled in DCS!

Case closed I think.

Good job on the test! Very interesting what it showed, I’m glad they modeled it that way. 
 

I got these numbers from the official manual documentation, as you noticed both English and Russian manuals agree. These have wind limits and talks about VRS occurring above 5 m/s. For VRS, the tip speed of the tail rotor and main rotor is very similar, so I would assume similar limits. Under emergency procedures for vortex ring an English translation would be “The helicopter entering the vortex ring is possible during power-on descent and forward speed under 50 km/h and a vertical rate greater than 5 m/s.” (right speed of 5 m/s and 50 kmh descent/climb in tail rotor case)

The manual you reference is originally made for TSMO (Threat Systems Management Office)by OTSA (Operation Test and Evaluation Threat Systems Management Office).

It is basically created based on American intelligence, and used for any American/Western evaluation/use of said airframes, with numbers roughly rounded to convert to knots and other imperial measurements. For example. When English speaking/weatern countries operate Mi-8/24, TSMO manuals are considered the Bible they go by. There is only one small blurb on page 6 that mentions it. They probably have other sections for combat and other flight operations, but I don’t think they are so willing to release under FOIA. 
 

Cool to see they modeled it so well! There was much discussion on release if this was a modeling error or LTE or VRS. And ED said they had not modeled LTE so we were scratching our heads. This completely confirms it’s VRS!