Scariest Day/Night Flying

[Jim123]

Yeah, but a number somewhere in that region is where they told you to "never exceed".. It may have even been a 57 NATOPS limit/warning. I vaugely remember hearing some number like that a HSL-40, but I have drank most of those braincells clear by now. It's been over 10 years.

FWIW, in the 2003 revision of the 60B (sorry to open old wounds) NATOPS manual, had a pretty badly written aero chapter. There was a bunch of new stuff, some of which was good, and some of which was simply wrong (NATOPS isn't always written in blood, occasionally it's written in "WTF").

The 800fpm/40kts gouge is an approximation that applies to a few popular models made by Bell. It's also a lot like telling someone they'll never lose control of their car as long as they obey the speed limit.

Best short technical explanation of VRS entry conditions- decelerate through translational lift (that detail gets left out a lot) followed by some come control inputs that make your descent rate approach the speed of your rotor's downwash. At a low power setting, you can get into it at a much lower descent rate (and vice-versa). And if you're going faster than translational lift, then you're far enough in front of your downwash that you basically can't settle into it.

 

[phrogdriver]

I've always been under the impression that 40KIAS was because pitot-static instruments aren't, or weren't, accurate below that speed, and so it was set at the limit because once one slowed past it, he didn't know how slow he was actually going.

 

[exhelodrvr]

I've always been under the impression that 40KIAS was because pitot-static instruments aren't, or weren't, accurate below that speed, and so it was set at the limit because once one slowed past it, he didn't know how slow he was actually going.

That's what we were told. (H-3's)

On a somewhat related note, are "freestreams" still done if there is a problem raising the sonar?

 

[JTS11]

I've always been under the impression that 40KIAS was because pitot-static instruments aren't, or weren't, accurate below that speed, and so it was set at the limit because once one slowed past it, he didn't know how slow he was actually going.

There was a good article in Approach on Vortex Ring State (Jun 03) called "The Vortex Ring State Fallacy", but I can't find an archived link.

From an abstract of the article: "NATOPS manuals generally state VRS may occur during rates of descent greater than 800 fpm, at forward velocities less than 40 knots. These guidelines are extremely conservative and do not reflect differences in aircraft characteristics, nor do they account for the actual mechanism leading to the vortex ring state. Anyone who has tried to stand under a CH-53E in a hover can attest that the wind velocity (induced velocity) is far than from a Huey. This induced velocity is of the air being "pumped" downward and is a of disk loading (gross weight/rotor disk area) and air density.
A computational study by the U.S. Army Aviation Laboratory, in May 1971, developed vortex ring-envelope diagrams for the Vietnam-era Huey as a function of the aircraft's descent angle, horizontal speed, and induced velocity. This information subsequently was carried over to the primary training helicopters for naval aviation and may have been the genesis of the 800-fpm/40-knot guideline. At their respective normal operating gross weights and "Pensacola, Fort Rucker" standard sea-level conditions, the TH-57 and UH-1E/L yield VRS diagrams with approximately an 800-fpm upper threshold (Figure 1). The 40-knot guideline most probably was based on the inherent limiting characteristic of antiquated pitot-static, airspeed-sensing systems that are unreliable or erratic below 40 knots. For some reason, the 800-fpm/ 40-knot guideline was adopted by virtually every rotary-wing NATOPS manual without consideration for the vast range of gross weights and rotor-disk areas of modern helicopters."

Also this: "For example, look at the VRS diagram for a CH-53E at 55,000 pounds and standard sea level (Figure 3). The severe vortex ring state can be expected when reaching approximately 1,900 fpm at a horizontal speed of approximately 20 knots."

 

[JTS11]

I've always been under the impression that 40KIAS was because pitot-static instruments aren't, or weren't, accurate below that speed, and so it was set at the limit because once one slowed past it, he didn't know how slow he was actually going.

There was a good article in Approach on Vortex Ring State (Jun 03) called "The Vortex Ring State Fallacy", but I can't find an archived link.


From an abstract of the article: "NATOPS manuals generally state VRS may occur during rates of descent greater than 800 fpm, at forward velocities less than 40 knots. These guidelines are extremely conservative and do not reflect differences in aircraft characteristics, nor do they account for the actual mechanism leading to the vortex ring state. Anyone who has tried to stand under a CH-53E in a hover can attest that the wind velocity (induced velocity) is far than from a Huey. This induced velocity is of the air being "pumped" downward and is a of disk loading (gross weight/rotor disk area) and air density.
A computational study by the U.S. Army Aviation Laboratory, in May 1971, developed vortex ring-envelope diagrams for the Vietnam-era Huey as a function of the aircraft's descent angle, horizontal speed, and induced velocity. This information subsequently was carried over to the primary training helicopters for naval aviation and may have been the genesis of the 800-fpm/40-knot guideline. At their respective normal operating gross weights and "Pensacola, Fort Rucker" standard sea-level conditions, the TH-57 and UH-1E/L yield VRS diagrams with approximately an 800-fpm upper threshold (Figure 1). The 40-knot guideline most probably was based on the inherent limiting characteristic of antiquated pitot-static, airspeed-sensing systems that are unreliable or erratic below 40 knots. For some reason, the 800-fpm/ 40-knot guideline was adopted by virtually every rotary-wing NATOPS manual without consideration for the vast range of gross weights and rotor-disk areas of modern helicopters."


Also this: "For example, look at the VRS diagram for a CH-53E at 55,000 pounds and standard sea level (Figure 3). The severe vortex ring state can be expected when reaching approximately 1,900 fpm at a horizontal speed of approximately 20 knots."

 

[MasterBates]

Okay, now I see what you are getting at.

But back to LSO, it's still not a "see light, bottom collective, it'll be cool" maneuver.

 

[picklesuit]

I've always been under the impression that 40KIAS was because pitot-static instruments aren't, or weren't, accurate below that speed, and so it was set at the limit because once one slowed past it, he didn't know how slow he was actually going.

Is that why my airspeed gauge starts at 50 KIAS!? ....Hmmm....where is that digest?

 

[bert]

Just a couple random things to add:

- the 40 kts/800 fpm is just a gouge number, and some aircraft can find VRS easier than others (it is nearly impossible to induce in a 60). Having said that, a low airspeed/high rate of descent situation can easily cause you to run out of power the old fashioned way at the bottom, so it is still a lousy place to be.

- I know of three mishaps where a pilot attempted to "back down" visually at night. It is terribly disorienting. Night/IFR (or VFR in the old unaided days on a dark-as-the-inside-of-a-dog's-ass night) can create optimal conditions for vertigo and trying to hover in it will magnify that. My scariest moment flying was taking off at night unaided from an LSD and after about 5 seconds I had the leans so bad I was bent across the center console. I was flying with another HAC and told him "I'm screwed up overhere; take it when you are ready". He put his hands up over his head and said something along the lines of "I can't I don't know what's happening.". Not good times, and I wasn't able to fly my way out of it until I was able to get some airspeed on.

 

[BACONATOR]

What's with this " blanket 800/40" thing? The vast majority of Navy helos are 60's, and those aren't the numbers of importance for VRS that I've seen in NATOPS anywhere.

 

[bert]

What's with this " blanket 800/40" thing? The vast majority of Navy helos are 60's, and those aren't the numbers of importance for VRS that I've seen in NATOPS anywhere.

It's been around forever in a bunch of different NFMs and MDGs. I think the fear is that even if they have the DT to support the real numbers (and the manufacturers do even if we don't), they still don't want somebody to think that being at 20 kts/1200 fpm on short final is ok.

 

[BACONATOR]

It's been around forever in a bunch of different NFMs and MDGs. I think the fear is that even if they have the DT to support the real numbers (and the manufacturers do even if we don't), they still don't want somebody to think that being at 20 kts/1200 fpm on short final is ok.

I gotcha. I was just making the point that if those are the "two big numbers", I've never seen them. our big number is 700/0-20 and 1500/5-10 (don't have my NATOPS in front of me: think that's right). And even still, not likely to see it. I find it hard to believe that VRS is likely anywhere near 40kts. At least not in the 60 with it's disc area and downwash physics.

 

[Gatordev]

I gotcha. I was just making the point that if those are the "two big numbers", I've never seen them. our big number is 700/0-20 and 1500/5-10 (don't have my NATOPS in front of me: think that's right). And even still, not likely to see it. I find it hard to believe that VRS is likely anywhere near 40kts. At least not in the 60 with it's disc area and downwash physics.

The was the point of my original post, the numbers are bogus and from the Huey (as JTS11 posted). Those numbers you're quoting are fairly new and in the R/S NATOPS. I can't remember if they made it into the Bravo Superhawk.

 

[BarrettRC8]

Forgive my ignorance, but I'm trying to understand this Vortex Ring State, with a beer in hand.

Essentially, what is happening, is that the downwash from the rotors at a high power setting are circling back around to the top of the helo and pushing it down at a faster rate because its lack of forward motion keeps it in said turbulence? Is that right?

 

[HueyCobra8151]

This is a decent video of what's happening. No sound though.

~30 seconds in it gets to VRS

 

[BarrettRC8]

This is a decent video of what's happening. No sound though.


~30 seconds in it gets to VRS

Well, now I'm even more confused. Thanks for the video though. Now to return to my beer.