Jump to content

Unrealistic VRS?


sondo214
 Share

Recommended Posts

Hello everyone,

 

I think I have found a total unrealistic behaviour of the huey. I will try to explain it as good as I can.

 

When you are going to land and lets say you are travelling with 60 knots, if you try to make an approach at a specific point to land and your aproach is with 500ft/m descent, nose up 5 dec to bleed speed, then when the speed is close to zero and you are over the LZ and it's the point to push the joystic fwd to level the heli, apply torque because you want to stop the descent and come to hover, the heli will continue descending till it crashes. I beleive it enters VRS, but it's not rigth with 500ft/m descent. It's too low to enter VRS.

 

If you have leveled off before 40 kts and the last 40kts you bleed the heli is level, then when you stop apply fwd stick and collective the heli will come to hover.

 

Please tell me what you think.

Link to comment
Share on other sites

It's not about what one thinks. It's about what one knows:

 

a 500ft/m descent rate is more than enough in many circumstances to get a UH-1H into a VSR.

...and it can happen at much more less decent rate when winds a blowing from your back.

Link to comment
Share on other sites

My fault

 

Petep you are right. I spoke with a fellow pilot that flies the hueyN (real life) version and verified that 700ft/m is the limit to enter VSR. I don't fly Huey (I fly another heli) in real life, and because our approach descent rate is 500 (even can reach 700-800) ft/m thought the same would be for huey but I was wrong! So I should have read the F... manual and the various diagrams before posting!

 

Thanks


Edited by sondo214
Link to comment
Share on other sites

[quote name=

...and it can happen at much more less decent rate when winds a blowing from your back.[/quote]

 

 

If you try landing with the wind from your back, then... what can I say. We will live to remember you... :megalol:

Link to comment
Share on other sites

If you try landing with the wind from your back, then... what can I say. We will live to remember you... megalol.gif

 

;) ... just saying...

...because you can read sometimes very absurd things when people claim to found a 'Bug' about the FM - and when you than finally convinced him to upload a track with the 'issue' happening - you quickly understand where the problem is really buried... :)


Edited by PeterP

Link to comment
Share on other sites

Minimum rate of descent for VRS is 300ft/min.

 

Don't forget that the VSI lags. You could be well beyond 500ft/min rate of descent even if the VSI says 500ft/min.

 

 

You mean maximum rate is 300ft/m. Beyond that VRS kicks in

Link to comment
Share on other sites

Minimum rate of descent to enter VRS is 300ft/min.

 

Maximum rate of descent to avoid VRS is 300ft/min.

 

That doesn't make any sense to me so if I am descending at 300ft/min whats happening am I entering VRS or am I avoiding VRS theres no middle road.

 

From your figures it appears the whole VRS is a total toss of a coin gamble here we go we might get in VRS we might not.

Eagles may soar high but weasel's don't get sucked into jet engines.

 

 

System Spec.

Monitors: Samsung 570DX & Rift CV1

Mobo: MSI Godlike gaming X-99A

CPU: Intel i7 5930K @ 3.50Ghz

RAM: 32gb

GPU: EVGA Nvidia GTX 980Ti VR Ready

Cooling: Predator 360

Power Supply: OCZ ZX Series 80 Plus Gold

Drives: Samsung SSD's 1tb, 500g plus others with OS Win10 64 bit

 

Link to comment
Share on other sites

That doesn't make any sense to me so if I am descending at 300ft/min whats happening am I entering VRS or am I avoiding VRS theres no middle road.

 

From your figures it appears the whole VRS is a total toss of a coin gamble here we go we might get in VRS we might not.

 

No it is not - it gives you the absolute save decent rate - If you leave this area you will have to check back the conditions, if they allow it and if you are save. - a coin gamble is something very different .

Link to comment
Share on other sites

That doesn't make any sense to me so if I am descending at 300ft/min whats happening am I entering VRS or am I avoiding VRS theres no middle road.

 

From your figures it appears the whole VRS is a total toss of a coin gamble here we go we might get in VRS we might not.

 

 

Well I have not much experience with the game (RL only) but I think that 300ft/m is not going to enter VRS, the problem starts at about 500ft/m so flying 300ft/m you are ok. Point is that because of the way VSI works there is a delay to show you the exact descent so when you establish 300ft/m descent be jentle with the collective so that you don't accidentaly fall below 500ft/m, and by the time VSI shows it you will be deep in VRS, some feet above the ground where there is no way out of it.

 

Someone with more in game experiance can tell us If I am wrong or not

 

Thanks

Link to comment
Share on other sites

What about that real life Huey driver, who said something about 700ft/m to be a known threshold beyond which the VRS will occur?

 

I personally have found that i can maintain a 1000ft/m constant descend rate but with a lower angle of attack on the blades, because i only had 100% fuel and no weapons which makes it much lighter, so this matters and that's a reason why i could descend at a higher rate..., yet i don't even know what to believe anymore, because the ED testers are saying that a 300ft/m should be the barrier beyond which the vortex ring state would occur (of course this should happen for a given total helicopter weight and height (density counts))! I've done this on a 0 wind component.

 

Another thing which i find quite bad is that even if dropping almost perfectly vertical at 1000ft/m constantly..., you'll end up "hitting" the ground effect like a leaf falling on the grass...! The ground effect seems to have such tremendous power, that even at the inertia and momentum that the helicopter had accumulated in a 1000ft/m (5.08 meters per second) constant vertical descend, it stops the heli very abruptly at about 0.3...0.5m from the ground giving it a vertical deceleration of around 0.5..0.6g's as it enters the ground effect, (so the blades would carry 1.6g's in that stopping distance), leaving you at ~0,3m above the ground like everything was ok!:music_whistling:

 

Here's a video of it..., because i can't use that track/recording feature that our sim is provided with..., it's totally useless, because i've tried 3 particular times to do another track then review it and..., a different story came up every time!:thumbup:

 

https://www.youtube.com/watch?v=2ZP5euMNXDQ

 

As a short conclusion...: Exaggerated/very sharp transition from normal flight behaviour (unstalled blades) to an abrupt lift loss from the blades when entering the vortex ring..., it launches like a rocket towards the ground! Also that air cushion which comes linked with the ground effect is quite exaggerated, it almost stops you in place over the ground, reducing like more than 50% of the horizontal force component. At higher speeds and close to the ground however, the ground effect is close to null though!


Edited by MaverickF22

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



Link to comment
Share on other sites

I don't think any tester has said that beyond 300ft/min rate of descent that VRS would occur. Only that is could occur.

 

In order to enter VRS, the following conditions must ALL exist simultaneously.

 

1. At least 300ft/min rate of descent

2. At least 20% of available power applied

3. Airspeed at or near zero (certainly below ETL).

 

If ALL of the three above conditions exist simultaneously, the helicopter MAY enter VRS. The faster the rate of descent, the higher the probability of entering VRS. The more power applied, the higher the probability of entering VRS.

 

At some point, VRS is a certainty, but as has already been mentioned, several factors all combine to affect the probability (e.g. air density and aircraft weight)

  • Like 1
Link to comment
Share on other sites

If ALL of the three above conditions exist simultaneously, the helicopter MAY enter VRS. The faster the rate of descent, the higher the probability of entering VRS. The more power applied, the higher the probability of entering VRS.

 

At some point, VRS is a certainty, but as has already been mentioned, several factors all combine to affect the probability (e.g. air density and aircraft weight)

 

Yes, it's my bad, sorry..., nobody stated that it should certainly occur at 300ft/m and i also know all of the factors which need to be taken into account, part of which you've already mentioned. I'm certain too that there could be a big difference in how much should these factors vary in order to enter a VRS situation, which can definitely start from what you've said..., around 300ft/m (probably when very heavily armed or in overweight at lower air density, requiring more AoA to provide 1G lift) up to more than 1200ft/m (when the heli has it's fuel tank dry and is flying in a very dense air, needing less AoA on the blades).

 

The bad thing is though..., that violent transition to stall, hence the ring vortex state being followed and also the ground effect and air cushion, are just some exaggerated behaviours..., it isn't that they aren't simulated, but the values aren't quite realistic or need a little tweak in order to look as they should. Of course, the way they seem to be (for a more or less trained eye) and the way the real values must be, might have a gab in between, yet the way it looks now..., definitely don't need a trained eye!

 

 

Good day!


Edited by MaverickF22

Mistakes, obviously, show us what needs improving. Without mistakes, how would we know what we had to work on!











Making DCS a better place for realism.

Let it be, ED!



Link to comment
Share on other sites

This seems to be such a huge problem that I thought I would look through my old Huey TM-55-1520-210-10 Operators Manual and try to have everyone get some perspective on the subject. Near the end of this topic are suggestions for avoiding Settling with Power (also know as Vortex Ring State-VRS) and procedure for getting out of this condition. All helicopter pilots should realize this is a real world problem not just a DCS "bug".

 

" Recovery can be accomplished by lowering collective pitch and increasing forward speed. Both of these methods of recovery require altitude to be successful."

 

Settling with Power

 

Settling with Power is a condition of powered flight where the helicopter settles into its own downwash. The condition may also be referred to as the vortex ring state.

 

Conditions conducive to settling with power are a vertical or nearly vertical descent of at least 300 feet per minute and low forward airspeed. The rotor system must also be using some of the available engine power (from 20 to 100 percent) with insufficient power available to retard the sink rate. These conditions occur during approaches with a tailwind or during formation approaches when some aircraft are flying in turbulence from other aircraft.

 

Under the conditions described above, the helicopter may descend at a high rate which exceeds the normal downward induced flow rate of the inner blade sections. As a result, the airflow of the inner blade sections is upward relative to the disk. This produces a secondary vortex ring in addition to the normal tip vortex system. The secondary vortex ring is generated about the point on the blade where airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the disk which causes loss of rotor efficiency even though power is still supplied from the engine.

This figure shows the induced flow along the blade span during normal hovering flight:

Fig_2-79.gif

Downward velocity is highest at the blade tip where blade airspeed is highest. As blade airspeed decreases nearer the disk center, downward velocity is less. This figure shows the induced airflow velocity pattern along the blade span during a descent conducive to settling with power:

Fig_2-80.gif

The descent is so rapid that induced flow at the inner portion of the blades is upward rather than downward. The upflow caused by the descent has overcome the downflow produced by blade rotation. If the helicopter descends under these conditions, with insufficient power to slow or stop the descent, it will enter the vortex ring state:

Fig_2-81.gif

During the vortex ring state, roughness and loss of control is experienced because of the turbulent rotational flow on the blades and the unsteady shifting of the flow along the blade span.

This figure shows the relationship of horizontal speed versus vertical speed for a typical helicopter in a descent. Straight lines emanating from the upper left corner are lines of constant descent angle. Superimposed on this grid are flow state regions for the typical helicopter. From this illustration, several conclusions regarding the vortex ring state can be drawn:

 

Fig_2-82.gif

  • The vortex ring state can be completely avoided by descending on flightpaths shallower than about 30 degrees (at any speed).
  • For steeper approaches, vortex ring state can be avoided by using a speed either faster or slower than the area of severe turbulence and thrust variation.
  • At very shallow angles of descent, the vortex ring wake is shed behind the helicopter.
  • At steep angles, the vortex ring wake is below the helicopter at slow rates of descent and above the helicopter at high rates of descent.

Power settling is an unstable condition. If allowed to continue, the sink rate will reach sufficient proportions for the flow to be entirely up through the rotor system. If continued, the rate of descent will reach extremely high rates. Recovery may be initiated during the early stages of power settling by putting on a large amount of excess power. During the early stages of power settling, the large amount of excess power may be sufficient to overcome the upflow near the center of the rotor. If the sink rate reaches a higher rate, power will not be available to break this upflow, and thus alter the vortex ring state of flow.

Normal tendency is for pilots to recover from a descent by application of collective pitch and power. If insufficient power is available for recovery, this action may aggravate power settling resulting in more turbulence and a higher rate of descent.

 

Recovery can be accomplished by lowering collective pitch and increasing forward speed. Both of these methods of recovery require altitude to be successful.

 

References Copter.com and Army FM 55-1520-210-10


Edited by flyer49
  • Like 1

[sIGPIC][/sIGPIC]

229th battalion, 1st Cavalry

Link to comment
Share on other sites

Thank you flyer49 for the excellent post. I don't know about the others but I can definitely see from this material how I must be falling into VRS.

 

Do you know what units the horizontal axis is on that final graph? I have a bit of trouble visualizing my flightpath angles while flying so I try to keep certain speeds and rates of decent to achieve the desired flightpath

Link to comment
Share on other sites

Hi thrawn...I'm more of a visual pilot like you. The way I would explain the graph in visual terms is that when doing an approach not only should you keep your forward speed at around 20 to 30 kts and vertical descent around 300 ft/min but also pick a spot on your windscreen about five or six inches above your instrument panel. Pick a spot that's easily tracked like the hash marks at the end of the runway or an intersection, Hold that spot during your approach by increasing and decreasing your collective as necessary.

 

That would be about a 20 degree approach angle. The closer your selected target landing spot get to your instrument panel the steeper your getting and entering the danger area where settling with power exists. As you can see by the graph this danger area is from an angle of 40 (which would be about where the top of your instrument panel is) to 80 degrees (which is about where you would be looking through your chin bubble ...maybe even through your pedals). You should never come to a hover before touchdown. A proper approach to the ground is just that...an approach to the ground. You should "fly" your helicopter to the ground by always moving forward. Don't try to "find" the ground...just fly the aircraft until you touch down. My instructor use to tell me...forward and down...forward and down...forward and down. I still say this to myself today and it helps me remember how to make a proper approach.

 

As I said in an earlier post, your approach may seem exaggerated and exceedingly slow but that is the way a helicopter makes an approach (unless your doing a combat approach which is a much higher speed but still slow in comparison to a fixed wing approach). I bring up fixed wing because fixed wing driver are use to higher approach speeds and steeper approach angles so your approach sight picture is different and this causes fixed wing guys some anguish...I hope this help explain the above graph.


Edited by flyer49

[sIGPIC][/sIGPIC]

229th battalion, 1st Cavalry

Link to comment
Share on other sites

Flyer

 

 

Any chance of making a video of a combat approach please?

 

I have tried it on numerous occasions, but still cannot get it nailed down, for some reason my tail dips hard, even when I am concentrating on it not too.

 

Thanks

“Any pilot should be flying the spitfire, at least once.” – John S. Blyth

Link to comment
Share on other sites

Hi marker...I will be happy to make a video when I figure our how to make the video. If you can instruct me on how to make the video, I will do so.

 

Nice flyer I will look forward to that and your post above is about how I do it give or take a little speed.

Eagles may soar high but weasel's don't get sucked into jet engines.

 

 

System Spec.

Monitors: Samsung 570DX & Rift CV1

Mobo: MSI Godlike gaming X-99A

CPU: Intel i7 5930K @ 3.50Ghz

RAM: 32gb

GPU: EVGA Nvidia GTX 980Ti VR Ready

Cooling: Predator 360

Power Supply: OCZ ZX Series 80 Plus Gold

Drives: Samsung SSD's 1tb, 500g plus others with OS Win10 64 bit

 

Link to comment
Share on other sites

 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...