Approach angles
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That guy ran out of power, and pulled the rpm down. That caused his sliding pedal turn. When he hits the ground, he is falling at about one fuselage height every 2 seconds, thus about 500 fpm. If you overpitch in an OGE hover, that is precisely what happens. For vortex ring to occur in an S-61, one must be at least 1000 fpm descent (to get the initial torque kicks, etc.)
Here is a clip of the crash, I cant find the longer one, can you?
http://www.rapp.org/wp-content/09210...king_crash.jpg
In order to get any significant VRS, you must be going about 75% of your downwash speed vertically downward, and even at that, you get 5 to 10% torque jumps.
Overpitching is the reason why most helos have a VRS accident, but the investigators and instructors get the labels mixed up.
The VRS police never got to label this one:
http://www.hughes300.com/H-3_Crash_7-5-2002.WMV
Here is a clip of the crash, I cant find the longer one, can you?
http://www.rapp.org/wp-content/09210...king_crash.jpg
In order to get any significant VRS, you must be going about 75% of your downwash speed vertically downward, and even at that, you get 5 to 10% torque jumps.
Overpitching is the reason why most helos have a VRS accident, but the investigators and instructors get the labels mixed up.
The VRS police never got to label this one:
http://www.hughes300.com/H-3_Crash_7-5-2002.WMV
Wow,
didn't we debate this very thing S-61 thing with you last time Pfan?? Try running a search, and that might reduce Nick's requirements to cut and paste. Again.
There is VRS.
There is terminating with insufficient power.
They are not the same thing, no matter how the unfortunately confusing terms settling with power and power settling make you think you can mix the two. It is always amusing how inaccurate labels can so easily lead to the formulation of amazing aerodynamic theories. See also the LTE in a AS350 thread.
I'm sure Lu is trying to log on about now.
didn't we debate this very thing S-61 thing with you last time Pfan?? Try running a search, and that might reduce Nick's requirements to cut and paste. Again.
There is VRS.
There is terminating with insufficient power.
They are not the same thing, no matter how the unfortunately confusing terms settling with power and power settling make you think you can mix the two. It is always amusing how inaccurate labels can so easily lead to the formulation of amazing aerodynamic theories. See also the LTE in a AS350 thread.
I'm sure Lu is trying to log on about now.
Nick sez:
Huh! So that explains it. I knew it had to be something prosaic and not VRS. Thanks for clearing that up.
Although...I would have thought that a lightly-loaded SH-3 at near seal level would have been capable of an OGE hover and some maneuvering without plummetting out of the sky like a sack of potatoes (and notice I did say "po-tay-toes" and not "po-tah-toes"). I also would've thought that a helicopter designed for shipboard ops would be capable of touching down at a mere 500 fpm RoD, but evidently not. I guess they don't have much reserve power or inherent strength, but I've never flown anything that big and mighty.
That guy ran out of power, and pulled the rpm down. That caused his sliding pedal turn. When he hits the ground, he is falling at about one fuselage height every 2 seconds, thus about 500 fpm. If you overpitch in an OGE hover, that is precisely what happens.
Although...I would have thought that a lightly-loaded SH-3 at near seal level would have been capable of an OGE hover and some maneuvering without plummetting out of the sky like a sack of potatoes (and notice I did say "po-tay-toes" and not "po-tah-toes"). I also would've thought that a helicopter designed for shipboard ops would be capable of touching down at a mere 500 fpm RoD, but evidently not. I guess they don't have much reserve power or inherent strength, but I've never flown anything that big and mighty.
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Glide slopes angles
Are Heli glide slopes significantly different from air-plane glide slopes ?
From most Approach charts one infers an average slope of 5%.
If it is steeper then most charts will make a special note.
A Heli VFR approach will probably be steeper, but how much ?
A 90 Knts / 500 fpm is also about 5%.
Doing standard ILS approaches seems to be a bit boaring in a Heli, but making this much steeper may speed up things too much (times to stabilize etc...)
An ideal Heli IFR angle, how much would that be ??
One of the reasons I am asking this is
- I am building a VITANS (=Intertial digital platform), based 3-dimensional approach assistant (looks like flying 3-D tubes on a simulator), and I am wondering what reasonable slopes are (this is a design parameter when designing the approach tunnels)
- my personal experience is that Plane like slopes give you sufficient time to get things under control, (much) steeper and shorter seems to make things more difficult (ILS/IFR)
- I have a personal feeling that two A109E crashes (nothing against the machine, perhaps just a coincidence) I know of (in SPIFR) are probaly due to VRS (sorry Nick), provoqued by slight tails winds combined with a too slow/steep final approach (could be a wild guess no formal proof of that)
d3
From most Approach charts one infers an average slope of 5%.
If it is steeper then most charts will make a special note.
A Heli VFR approach will probably be steeper, but how much ?
A 90 Knts / 500 fpm is also about 5%.
Doing standard ILS approaches seems to be a bit boaring in a Heli, but making this much steeper may speed up things too much (times to stabilize etc...)
An ideal Heli IFR angle, how much would that be ??
One of the reasons I am asking this is
- I am building a VITANS (=Intertial digital platform), based 3-dimensional approach assistant (looks like flying 3-D tubes on a simulator), and I am wondering what reasonable slopes are (this is a design parameter when designing the approach tunnels)
- my personal experience is that Plane like slopes give you sufficient time to get things under control, (much) steeper and shorter seems to make things more difficult (ILS/IFR)
- I have a personal feeling that two A109E crashes (nothing against the machine, perhaps just a coincidence) I know of (in SPIFR) are probaly due to VRS (sorry Nick), provoqued by slight tails winds combined with a too slow/steep final approach (could be a wild guess no formal proof of that)
d3
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PpruneFan#1,
1) If you have a copy of that accident report, please share it with me. I have no idea what the weight is, frankly. But I can say with some certainty the hover weight was more than the available power would support!
2) The maximum design rate of descent for an H-3/S-61 main gear design is 8 feet per second, which varies a very little with gross weight. Combined with the yaw at touchdown (symptomatic of excess torque, saturating the tail rotor) and the bank, the gear failure is predictable at somewhere near 500 or 600 fpm. The fuselage stayed intact, showing that the rate was not too much greater.
3) The downwash velocity of an H-3 is about 80 feet per second (4800 fpm), so we need at least 50% of that to get close to VRS, and 75% before we see any real torque spikes and the like. Thus, the machine had to be dropping at more that 2400 fpm to be anywhere near VRS.
See:
http://www.ae.gatech.edu/people/dsch...201%20Aero.ppt slide 33 for a nice discussion of the downwash velocity and how to calculate it. Remember, the Vortex that forms the ring is running down away from the rotor at that velocity, and you have to drop fast enough to catch it if you want to form a VRS.
See:
http://www.enae.umd.edu/AGRC/Aero/vring.html for a great VRS resource site. Leishman did great work helping define where VRS actually happens (because of the notorious V22 accident) and modeled for the first time the actual vortex reingestion. He shows that no significant power rise occurs until 50% of the downwash speed, based on experiment and modeling.
Here is a movie of his very accurate modeling of the vortex ring. The film shows a rotor as it is accelerated downward, showing the vortex forming as the descent rate increases. Unfortunately, he didn't run a device to show the rate of descent as the vortex forms, but his paper shows that the first vortex touches the rotor at aboyt 60% of the downwash speed ((3000 fpm for that S-61!)
http://www.enae.umd.edu/AGRC/Aero/images/wake1.gif
1) If you have a copy of that accident report, please share it with me. I have no idea what the weight is, frankly. But I can say with some certainty the hover weight was more than the available power would support!
2) The maximum design rate of descent for an H-3/S-61 main gear design is 8 feet per second, which varies a very little with gross weight. Combined with the yaw at touchdown (symptomatic of excess torque, saturating the tail rotor) and the bank, the gear failure is predictable at somewhere near 500 or 600 fpm. The fuselage stayed intact, showing that the rate was not too much greater.
3) The downwash velocity of an H-3 is about 80 feet per second (4800 fpm), so we need at least 50% of that to get close to VRS, and 75% before we see any real torque spikes and the like. Thus, the machine had to be dropping at more that 2400 fpm to be anywhere near VRS.
See:
http://www.ae.gatech.edu/people/dsch...201%20Aero.ppt slide 33 for a nice discussion of the downwash velocity and how to calculate it. Remember, the Vortex that forms the ring is running down away from the rotor at that velocity, and you have to drop fast enough to catch it if you want to form a VRS.
See:
http://www.enae.umd.edu/AGRC/Aero/vring.html for a great VRS resource site. Leishman did great work helping define where VRS actually happens (because of the notorious V22 accident) and modeled for the first time the actual vortex reingestion. He shows that no significant power rise occurs until 50% of the downwash speed, based on experiment and modeling.
Here is a movie of his very accurate modeling of the vortex ring. The film shows a rotor as it is accelerated downward, showing the vortex forming as the descent rate increases. Unfortunately, he didn't run a device to show the rate of descent as the vortex forms, but his paper shows that the first vortex touches the rotor at aboyt 60% of the downwash speed ((3000 fpm for that S-61!)
http://www.enae.umd.edu/AGRC/Aero/images/wake1.gif
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PPF#1, it seems to me that if the blades are coning heavily, then VRS isn't what is happening, it's loss of RPM. Blades don't cone in VRS, AFAIK. They cone when the RPM decreases below what it should be, or the weight is too great, or both. I haven't seen video of that particular accident, but from your description of it, it sounds as if Nick is exactly correct in his evaluation. Getting frisky in an OGE hover is certainly courting disaster.
Nick:
Oh sorry, I thought I made it clear that I was describing the video footage from memory of the original airing when it happened. I was kind of hoping that *you* had a link to the full version. The reason I said that it was "lightly loaded" was because that is the way it was described on the news that evening and anyway it was not full of people (only four guys onboard). Maybe the thing was heavily loaded with full fuel and a bunch of stuff that the guys brought down from Canada to Albany with them for the airshow. And so if you say that the weight was more than the available power could support, I would certainly trust your superior knowledge of the S-61's performance. But like I said, I don't have the accident report handy so I'll refrain from making any declarative statements, only observations and questions.
Nick again:
Izzat all? Jeez, some of my L-model landings on the PAB must have been at least that! Frankly, it astounds me that the N-model/SH-3 gear is so...is there any other word for it?...flimsy. I wish I'd known! Come to think of it, we did later have a little gear problem on the roof. But that was after I left and I had nothing to do with it, I swear.
Nick finally:
I wonder sometimes if an updraft can "fool" the rotor into momentarily thinking that it's in a rate of descent? I know, it would have to be one hell of an updraft. But jeez, it just seems like the classroom theory and the real-world practicality sometimes diverge when it comes to the chaotic nature of...well...nature.
Gomer Pylot:
Well I am certainly *not* an expert in VRS, Gomer, and and I personally cannot vouch for whether the blades would cone in a VRS condition or not, so I will gladly defer to your opinion as to the cause of an accident that I have described but you have not seen. I do seem to recall seeing recently a video of an NH-90 doing some aerobaticals at an airshow, and all of us watching were surprised at how much those big blades were coning during some of the maneuvers. But it might have been an optical illusion.
If you have a copy of that accident report, please share it with me. I have no idea what the weight is, frankly. But I can say with some certainty the hover weight was more than the available power would support!
Nick again:
The maximum design rate of descent for an H-3/S-61 main gear design is 8 feet per second, which varies a very little with gross weight. Combined with the yaw at touchdown (symptomatic of excess torque, saturating the tail rotor) and the bank, the gear failure is predictable at somewhere near 500 or 600 fpm.
Nick finally:
The downwash velocity of an H-3 is about 80 feet per second (4800 fpm), so we need at least 50% of that to get close to VRS, and 75% before we see any real torque spikes and the like.
Gomer Pylot:
PPF#1, it seems to me that if the blades are coning heavily, then VRS isn't what is happening, it's loss of RPM. Blades don't cone in VRS, AFAIK. They cone when the RPM decreases below what it should be, or the weight is too great, or both. I haven't seen video of that particular accident, but from your description of it, it sounds as if Nick is exactly correct in his evaluation.
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pprunefan#1, The coning in airshow maneuvers should reflect the load factor being produced, when I flew in airshows, it was common to develop 2 g's, so that the rotor had to produce double thrust, and double coning.
Regarding how hard we hit, by the time you hit with 4 or 5 feet per second, the cockpit g's are perhaps 2 or so, it feels really hard. You would be surprized how gently we all land, typically much less than 1 foot per second.
Here is a better link, that shows the bottom of the crash. I just noticed, his landing gear was UP on impact, which cuts his energy absorbtion to a fraction of what he would have had with the gear down:
http://home.earthlink.net/~kevinogdu...ter_crash.mpeg
Regarding how hard we hit, by the time you hit with 4 or 5 feet per second, the cockpit g's are perhaps 2 or so, it feels really hard. You would be surprized how gently we all land, typically much less than 1 foot per second.
Here is a better link, that shows the bottom of the crash. I just noticed, his landing gear was UP on impact, which cuts his energy absorbtion to a fraction of what he would have had with the gear down:
http://home.earthlink.net/~kevinogdu...ter_crash.mpeg
Last edited by NickLappos; 12th Nov 2005 at 01:30.
Nick,
That’s an interesting point on the rate of descent - when we modelled the PC2e procedures (for helideck landings) we set the arrival limit at 5ft/sec both for take-off and landing; in spite of taking an engine failure from 30ft above the deck on take-off and 40ft on landing we never exceeded that limit - except when we modelled considerably above the maximum masses for take-off and landing.
What would be the affect of a sideways movement on the undercarriage and how much do you consider it could tolerate?
That’s an interesting point on the rate of descent - when we modelled the PC2e procedures (for helideck landings) we set the arrival limit at 5ft/sec both for take-off and landing; in spite of taking an engine failure from 30ft above the deck on take-off and 40ft on landing we never exceeded that limit - except when we modelled considerably above the maximum masses for take-off and landing.
What would be the affect of a sideways movement on the undercarriage and how much do you consider it could tolerate?
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An easy way to prove the approach angle is less than someone believes would be to get them to estimate the distance across ground to the planned touchdown point and compare it with their height AGL. (i.e. if they're the same then it'll be 45 degrees.)
For example, when I'm doing my practice circuits I have to aim to touchdown about halfway along parallel to a runway about 4300' long, and start my descent from about a quarter of the runways length away ahead of the threshold, at about 700' AGL. Which gives me a slope ratio of about 12 degrees.
I know this because I always thought it felt really steep, so I went off and worked it out!
Si
For example, when I'm doing my practice circuits I have to aim to touchdown about halfway along parallel to a runway about 4300' long, and start my descent from about a quarter of the runways length away ahead of the threshold, at about 700' AGL. Which gives me a slope ratio of about 12 degrees.
I know this because I always thought it felt really steep, so I went off and worked it out!
Si
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Jiml,
I agree that 5fps is a good number for the max procedure. At the factory, we instrument the gear structure and work the procedure to get more aggressive as we watch the structural loads climb, more to the specific point. The touchdowns get to be 5 to 6fps at the peak, so your assumptions agree quite well with our test experiences. I have personally flown procedures where we hit 100% of the gear loads and the collective pitch stops, while dropping the rotor to the min limit, all at the same time. Not too much left in the aircraft at that point!
As to side loads, the combination of velocity and bank is a killer, the regs describe the design values as assumed side loads, with 80% of the vertical load applied to one gear inward and 60% outward to the other simultaneously. These are met with virtually no margin (not including the 1.5 safety factor, of course), done analytically.
I agree that 5fps is a good number for the max procedure. At the factory, we instrument the gear structure and work the procedure to get more aggressive as we watch the structural loads climb, more to the specific point. The touchdowns get to be 5 to 6fps at the peak, so your assumptions agree quite well with our test experiences. I have personally flown procedures where we hit 100% of the gear loads and the collective pitch stops, while dropping the rotor to the min limit, all at the same time. Not too much left in the aircraft at that point!
As to side loads, the combination of velocity and bank is a killer, the regs describe the design values as assumed side loads, with 80% of the vertical load applied to one gear inward and 60% outward to the other simultaneously. These are met with virtually no margin (not including the 1.5 safety factor, of course), done analytically.
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I have personally flown procedures where we hit 100% of the gear loads and the collective pitch stops, while dropping the rotor to the min limit, all at the same time. Not too much left in the aircraft at that point!
I think my students do this on a fairly regular basis.
But onto a more serious question. Is it fair to say that the higher the disk loading, the higher the ROD required to get vortex ring? Is there a reliable corrolation between disk loading numbers and the ROD which will lead to VRS? For instance, will an R44, with a disk loading of around 2.9 lb/ft2 get into VRS at a lower ROD than a Schweizer 333, with a disk loading of around 4.3 lb/ft2? Would any helicopter with a disk loading of around 4.3 lb/ft2 get into VRS at the same ROD as the 333?
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Hello,
Forget the fancy math's when you are talking approach angels and how they feel much steeper. Humans have not been programmed to fly, so we have an "error" in our brains. Vertical distances look about 10 times more than horizontal ones. That's why 10 m on the ground is short but 10 m in a tree is very high..
Fly Safe!
Forget the fancy math's when you are talking approach angels and how they feel much steeper. Humans have not been programmed to fly, so we have an "error" in our brains. Vertical distances look about 10 times more than horizontal ones. That's why 10 m on the ground is short but 10 m in a tree is very high..
Fly Safe!
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Flingwing,
Here is the page I referred to in a post above, from Georgia Tech, where the relationship betweem disk loading and downwash velocity is explained. Basically, the disk keeps the helo up by throwing air downward. The heavier the aircraft, and the smaller the disk the more speed the package of air must have to develop the thrust and keep the helo up. The disk loading is a good approximation of the velocity. Two helos with the same disk loading will have the same downwash velocity, and the same vertical speed for getting into VRS. for your helo with 2.9 lbs/sq ft, the downwash speed is 33 miles per hour, or 49 feet per second.
the "p" type symbol in the formula is air density, "rho" and is .002378, if you want to crunch the numbers.
Here is the page I referred to in a post above, from Georgia Tech, where the relationship betweem disk loading and downwash velocity is explained. Basically, the disk keeps the helo up by throwing air downward. The heavier the aircraft, and the smaller the disk the more speed the package of air must have to develop the thrust and keep the helo up. The disk loading is a good approximation of the velocity. Two helos with the same disk loading will have the same downwash velocity, and the same vertical speed for getting into VRS. for your helo with 2.9 lbs/sq ft, the downwash speed is 33 miles per hour, or 49 feet per second.
the "p" type symbol in the formula is air density, "rho" and is .002378, if you want to crunch the numbers.
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Vi, Vi Infinity
From a number cruncher...
A formula I use is (same notation as Nick)
Vi = sqrt(( Trust / ( 2 * ro * Surface) )
Vi inf = 2* Vi
For some heli's (at MTOW)
R22 : 7,36 m/s
R44-I : 7,4 m/s
B206 : 8,47 m/s
MD 500E : 10,28 m/s
EC350B2 : 10,01 m/s
S 76C : 12,26 m/s
S 92 : 14,32 m/s
Take about 2* for Knts
In (strict) vertical descent Vortex starts at half vi. Converting units that critical descent rate in f/min turns out to be 100 times the above values in m/s
So in f/min
R22 : 736 f/min, etc
Vi (cfr Glauert) is quite different in forward speed and from any significant speed on it tends to zero as 1 / ( forward speed * Trust), but in these regimes rotor stall is also different (not at half vi).
d3
A formula I use is (same notation as Nick)
Vi = sqrt(( Trust / ( 2 * ro * Surface) )
Vi inf = 2* Vi
For some heli's (at MTOW)
R22 : 7,36 m/s
R44-I : 7,4 m/s
B206 : 8,47 m/s
MD 500E : 10,28 m/s
EC350B2 : 10,01 m/s
S 76C : 12,26 m/s
S 92 : 14,32 m/s
Take about 2* for Knts
In (strict) vertical descent Vortex starts at half vi. Converting units that critical descent rate in f/min turns out to be 100 times the above values in m/s
So in f/min
R22 : 736 f/min, etc
Vi (cfr Glauert) is quite different in forward speed and from any significant speed on it tends to zero as 1 / ( forward speed * Trust), but in these regimes rotor stall is also different (not at half vi).
d3
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Wiisp,
What you say rings true for me. There may be an innate tendency to forshorten distances in the horizontal plane.
And as Delta3 observes (in his first post), slopes seem steeper than they actually are even when viewed from the ground.
What you say rings true for me. There may be an innate tendency to forshorten distances in the horizontal plane.
And as Delta3 observes (in his first post), slopes seem steeper than they actually are even when viewed from the ground.
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DElta 3,
Right on, and that 50% of downwash speed for first VRS indications is very conservative.
Here is a US Army Safety Center diagram developed to help their accident investigators:
Note they use 66% as the typical onset.
Right on, and that 50% of downwash speed for first VRS indications is very conservative.
Here is a US Army Safety Center diagram developed to help their accident investigators:
Note they use 66% as the typical onset.
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Thanks Nick, Delta,
I'll take that as a "yes"...
It seems that a helicopter would be somewhat less prone to VRS the more heavily it was loaded. However, it would be more prone to settling due to insufficient power, so this difference would be academic at best.
I'll take that as a "yes"...
It seems that a helicopter would be somewhat less prone to VRS the more heavily it was loaded. However, it would be more prone to settling due to insufficient power, so this difference would be academic at best.
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Fling,
You are right, in fact, the difference is worse for the extra weight case, since the real cause of most hover "falling through" accidents is insufficient power, not VRS.
You are right, in fact, the difference is worse for the extra weight case, since the real cause of most hover "falling through" accidents is insufficient power, not VRS.