A question regarding VRS.

[hideki2]

Deer Belsimtek! First I would like to say that I love your work. I am not a real pilot, so the following question is being asked just out of curiosity. Well, yesterday I tried UH 1 by Aerosoft in FSX and it felt awfully unrealistic. But what got my attention was what they wrote in description of this add on.

 

"...Please note that vortex ring state (VRS) is NOT included because the Huey does not suffer from this dangerous condition."

 

http://en.shop.aerosoft.com/eshop.php?action=article_detail&s_supplier_aid=11333&s_design=DEFAULT&shopfilter_category=Flight%20Simulation

 

 

I would like to hear your comment about that.

 

Best Regards!

[TurboHog]

UH-1H/V Army Model Huey manual with all latest changes (Changes 1-19, last change on 31-12-2002, Americans: 12-31-2002) mentions setting with power as a possible condition in the flight characteristics section.

[hideki2]

Thanks for Reply! I see, that means that they just took a shortcut. :)

[sondo214]

Of course it can suffer from VRS. I beleive every heli can... As far as I know FSX does not support VRS apart from HTR (helicopter total realism) which for it's time was quite good... But Since DCS uh-1 came out, just don't try any other heli sim out there... That's a rule. You will find nothing close to it. Only drawback, the lack of models... But we hope for that in the future

[hideki2]

Only drawback, the lack of models... But we hope for that in the future

 

Thanks for reply. Only other drawback I could think about is that DCS hates every computer on the planet :lol:

[Soulres]

Thanks for reply. Only other drawback I could think about is that DCS hates every computer on the planet :lol:

with the right mods,FSX can put DCS to shame,Visually only of course,Tho the lack of Runway Signs,Lights and Markings is still a bummer for night flying.:cry:

[TurboHog]

with the right mods,FSX can put DCS to shame,Visually only of course,Tho the lack of Runway Signs,Lights and Markings is still a bummer for night flying.:cry:

 

We are only considering helicopter flight dynamics here...

 

Graphically FSX definitely beats DCS and will probably even do so when DCSW 2 comes around, provided that you enhance FSX with the right addons...

[kontiuka]

"...Please note that vortex ring state (VRS) is NOT included because our simulation of the Huey does not suffer from this dangerous condition."

fixed;)

[FishBike]

"...Please note that vortex ring state (VRS) is NOT included because the Huey does not suffer from this dangerous condition."

 

Q: "Did the Huey suffer from vortex ring state?"

 

A: "No, it died very quickly."

[Flagrum]

Q: "Did the Huey suffer from vortex ring state?"

 

A: "No, it died very quickly."

LOL :thumbup:

[scrtagnt69]

Q: "Did the Huey suffer from vortex ring state?"

 

A: "No, it died very quickly."

 

 

:megalol: LOL :megalol:

[ff4life4]

Anyone remember what the vrs was like when the huey first came out? Lol it was an invisible hand pulling the aircrsft out of the sky

[scrtagnt69]

Anyone remember what the vrs was like when the huey first came out? Lol it was an invisible hand pulling the aircrsft out of the sky

 

Oh yes the good ole days lol...remember the joy when you defeated VRS for the 1st. time ?

[b00ce]

Anyone remember what the vrs was like when the huey first came out? Lol it was an invisible hand pulling the aircrsft out of the sky

That was just you leaving ETL :music_whistling:

 

VRS is only really a problem when you have any sort of weight (IE Rockets and guns) otherwise you have enough power to arrest your descent.

[fjacobsen]

I´m one of the developers behind the Aerosoft Huey X.

 

The adon was made in close cooperation with a German military helicopter pilot with many years of experience, having flown a lot in the Huey.

 

He told that he never had experienced VRS in the Huey.

It´s not that it couldn´t go into VRS, but it was very hard. It had to be heavy loaded and then still being brought to it by purpose.

 

That's why we choose not to model it.

 

During development we visited him and flew in one of the German army's Huey simulators, and where able to take of, fly and land again.

The flying experience was offcourse different, mostly due to the different view, periphical vision and controls very different from the normal flightsim joysticks.

Btw I have never had any difficulties flying the DCS Huey either.

 

I´m not saying that the Aerosoft Huey is perfect, cause FSX is not very good at simulating helicopters, but we managed to put it custom scripts to enhance helicopter specific flight modeling.

 

 

FinnJ

[AlphaOneSix]

The UH-1 has pretty heavy disc loading, which is what make it fairly resistant to vortex ring state.

 

The FAA, and most online sources, will state that any decent rate above 300 ft/min is inviting vortex ring state (in addition to the requirements of having some power applied and near zero airspeed). However, this is a tremendously conservative number.

 

The rate of descent required in order to enter vortex ring state is actually pretty easy to calculate, and I will show you all how to do it so you don't have to look it up.

 

It all starts with the velocity of the rotor downwash. In order to enter vortex ring state, you have to descend at a rate that is somewhat close to the velocity of your rotor downwash. To be on the safe side , you can expect that below roughly .5 times your rotor downwash velocity, you've got practically zero chance of entering vortex ring state. Once you start descending vertically faster than half of your rotor downwash velocity, the danger of getting into VRS increases. Once the two are equal (i.e. the downward velocity of your helicopter is equal to the downward velocity of the air that the main rotor is pushing), you are in big trouble for sure.

 

Okay, so how to figure out your rotor's downwash velocity? Here is the fomula:

 

downwash velocity (ft/sec) = √((Acft weight in lb.)/(2*air density in slugs per cubic feet*rotor disc area in square feet))

 

Okay, so let's plug in the numbers:

 

First, for air density, we'll use the ISA value for air density at sea level, but note that lower air density results in faster downwash, which makes VRS even harder to get into. The ISA value a sea level for air density is .002378 slugs/cubic foot.

 

Next, let's figure rotor disc area. The UH-1H has a rotor diameter of 48 ft. That will give you a rotor disc area of 1810 square feet.

 

Next comes aircraft weight, I saved it for last because it's the tricky one. Note that the heavier the helicopter, the faster you must descend to get into VRS. Seems counter-intuitive at first, but it's true. If you're heavier, you need faster downwash to counteract the force of gravity, so you need to descend faster to reach that same velocity. For our purposes, let's assume a slick UH-1H with a crew of four and full of fuel, but no cargo or passengers. I'm going to call that 7600 lb. but anyone is free to play with that number.

 

Okay so plugging these numbers into the formula:

 

downwash velocity (ft/sec) = √((7600)/(2*.002378*1810))

downwash velocity (ft/sec) = 29.7

 

Convert that to ft/min and you get 1783 feet per minute.

 

So to ABSOLUTELY enter VRS, the UH-1H in my example would need to be descending vertically with near zero airspeed with at least 20% power applied at 1783 feet per minute. Divide that in half to get our "safe zone" and you can be pretty certain that VRS will not be a problem as long as you descend at less than basically 900 feet per minute. A far cry from the 300 ft/min used in most examples. To get anywhere near VRS at 300 ft/min, you need to be in an aircraft with low disc loading, like an R22, for example. UH-1H disc loading is around 5.25 lb/sq ft, while the R22 is 2.6 lb/sq ft.

 

I'll let you guys fiddle around with plugging in different numbers as you see fit, but you get my drift. It is obviously ABSOLUTELY possible to get into VRS in a Huey, but you have to be descending probably beyond 1000 ft/min. to do so.

 

EDIT: Got high vs. low disc-loading backwards, now fixed.

Edited October 21, 2014 by AlphaOneSix

[hideki2]

I´m one of the developers behind the Aerosoft Huey X.

 

The adon was made in close cooperation with a German military helicopter pilot with many years of experience, having flown a lot in the Huey.

 

He told that he never had experienced VRS in the Huey.

It´s not that it couldn´t go into VRS, but it was very hard. It had to be heavy loaded and then still being brought to it by purpose.

 

That's why we choose not to model it.

 

During development we visited him and flew in one of the German army's Huey simulators, and where able to take of, fly and land again.

The flying experience was offcourse different, mostly due to the different view, periphical vision and controls very different from the normal flightsim joysticks.

Btw I have never had any difficulties flying the DCS Huey either.

 

I´m not saying that the Aerosoft Huey is perfect, cause FSX is not very good at simulating helicopters, but we managed to put it custom scripts to enhance helicopter specific flight modeling.

 

 

FinnJ

 

So, knowing that and having flown actual Huey, would you consider DCS Huey realistic? Because this is whats most important for most of us.

Edited October 21, 2014 by hideki2

[fjacobsen]

So, knowing that and having flown actual Huey, would you consider DCS Huey realistic?

 

First and foremost - I have not flown a real Huey - only simulators !

 

To tell whether its realistic or not is very hard.

The entire environment feels different. The military simulator was very good, because it used a real Huey cockpit and You has almost all-around vision, but it was still a simulator.

 

What is really lacking on PC simulators is periphical vision and pants in the seat feeling. Nothing can substitute these inputs.

 

The DCS Huey definitely feels more fluid that our Aerosoft Huey, I also guess that the flightmodel comes closer, apart from that it is too easy to enter VRS (same probably is true for the Mi-8).

 

 

FinnJ

[Flagrum]

Very interesting, AlphaOneSix!

 

But when I use the formula to calculate and compare the results of the Huey, the Hip and the Blackshark, the latter two should be even harder to get into VRS?

 

It is true, I can't really remember when (if?) I got the Huey into VRS lately - that is since the last one or two FM patches back then. I just tried to force her into VRS and the 2-3 attempts I made, I was unable to do so. If there is enough room under the skids, I could always catch her again just by applying more collective.

 

But the Mi-8 is in fact easy(..ier) to get into VRS. This does not match the results of the mathematical comparisation. So, is the conclusion that the Mi-8 FM has there some "potential for improvements"?

Edited October 21, 2014 by Flagrum

[fjacobsen]

Maybe the problem is rather that the devs know that their flight model will be flamed if the helis wouldn´t go into VRS - or where hard to get to.

 

Just to emphasize... VRS is a danger and should be modeled, but I doubt that it kicks in as easy as we see it on the Huey and Mi-8.

 

FinnJ

[hideki2]

First and foremost - I have not flown a real Huey - only simulators !

 

I'm sorry that I misunderstood you.

 

 

 

What is really lacking on PC simulators is periphical vision and pants in the seat feeling. Nothing can substitute these inputs.

 

I totally agree with you on that! I spend hundreds of hours playing driving sims and this same applies there. FFB Wheel is supposed to substitute "butt sense". The problem with chopper is that you have to feel forces in all directions where in car sim left and right is enough.

Edited October 21, 2014 by hideki2

[AlphaOneSix]

So, is the conclusion that the Mi-8 FM has there some "potential for improvements"?

 

I think that's a fair statement. I don't know if it's an oversight or just a simplification. Also, I am not an engineer. The formula is correct, but the "half" that I use for the "safe zone" is not scientific, as far as I know, it's just something I've seen over and over again from people much smarter than I am.

[fjacobsen]

I think that's a fair statement. I don't know if it's an oversight or just a simplification. Also, I am not an engineer. The formula is correct, but the "half" that I use for the "safe zone" is not scientific, as far as I know, it's just something I've seen over and over again from people much smarter than I am.

 

But as a calculation for triggering VRS in a sim it would make sense, maybe adding a tad of randomisation i.e You will not know exactly when it kicks in - let´s say the calculated vertical speed ±50 ft/min.

For a simulation You need to make some kind of compromises, but as long as the outcome feels believable and can be explained, I´m fully happy.

 

 

FinnJ

Edited October 21, 2014 by fjacobsen
edit

[FishBike]

To get 300 ft/min, you need to be in an aircraft with high disc loading, like a V22, for example. UH-1H disc loading is around 5.25 lb/sq ft, while the V22 is 26.7 lb/sq ft.

 

Wouldn't it be the other way around, e.g. you would need a helicopter with LOW disc loading to have a downwash velocity low enough to get VRS at 300ft/min descent?

 

There is some good stuff written here about VRS, written by a Sikorsky test pilot. It matches up well with what you posted re: disc loading and VRS and confirms it would be damn near impossible to get into in a V-22.

 

http://yarchive.net/air/vortex_ring.html

[fjacobsen]

Wouldn't it be the other way around, e.g. you would need a helicopter with LOW disc loading to have a downwash velocity low enough to get VRS at 300ft/min descent?

 

There is some good stuff written here about VRS, written by a Sikorsky test pilot. It matches up well with what you posted re: disc loading and VRS and confirms it would be damn near impossible to get into in a V-22.

 

http://yarchive.net/air/vortex_ring.html

 

Nice "article" indeed and in a way emphasising that VRS isn´t that common.

 

FinnJ