Approach angles

[thecontroller]

normal approach is 10 degrees..

steep is 15 degrees..

shallow is 5 degrees..

right??

when doing a steep approach in an r22, you level at 300ft agl until steep approach angle is reached - ie spot just above console. this looks more like a 30-40 degree angle to me. not 15 degrees.

likewise a normal approach looks greater than 10 degrees

can anyone explain this?

[Flingwing207]

Per the FAA:
Shallow - less than 8 degrees
Normal - 8 to 12 degrees
Steep - more than 12 degrees (with 15 degrees being the recommended maximum)

As far as what it looks like, remember that the "spot on the windscreen" technique only works to identify the place where you start your descent. As you decelerate, the aircraft attitude changes (more nose-high), so the spot will "move down" even though the angle remains the same.

[NickLappos]

Fling is right on, the normal deceleration that we continuously perform during an approach requires about 2 to 3 degrees nose up, which makes the approach look steeper. Each degree nose up from normal causes about 1/2 knot per second deceleration.

The limiter to steepness is forward speed. If you are faster, the steepest approach you can make becomes more shallow, because the helo will only descend at a given amount to stay above autorotation. If you slow to 35 or less, the autorotation boundary is very far below your flight condition, so you can descend in full control at angles up to about 20 degrees. If you want to be steep and safe, be steep and slow.

No, Vortex Ring State is not possible in steep approaches, within normal reason (anything is possible for the truly ungifted pilot.)

[Hughes500]

Spot on the screen works very well from entry into descent until about 150 ft agl and about 250m away from landing zone. Only really need to decelerate when you are getting that close. Bear in mind this works very well for a 300 when your approach speed is 40 kt ground speed, imagine the R22 would be about the same.
Vortex ring is possible if you have a very steep approach in no wind with slow slow fwd speed( less than 20 kts) high rate of descent ( 500ft plus) and then take an armful of power. As Nick said for the truly ungifted - I think stupid would be better than ungifted, Nick is being too kind.

[thecontroller]

Mmm.. i remain to be convinced. The reason why is that when i teach a steep approach i say "a steep approach angle is 15 degrees", then we go fly it and the student says "that looks more like a 40 degree angle to me"

[IHL]

From the cockpit the angles always seem steeper than they actually are.

If want to do the math to determine your approach angle use your GPS to determine distance back (in feet if your altimeter reads in feet) for the start of your "steep approach" and record your altitude.

use the following formula : (ALt/dis) which will give you slope percentage. Then take the INV TAN of the slope and voila decent angle .

i.e. 200 feet alt and 1520 feet (1/4 NM) back from the spot=
200/1520=.13157 (13% slope)
the INV TAN(ARC Tangent) of .13157 =7.49 degrees.

Therefore a continuous approach angle from 1/4 NM and 200 feet = a 7.5 degree approach angle to touch down.

[Hiro Protagonist]

In an attempt to win a argument (with my chief instructor at the time) I developed a spreadsheet where you can input a groundspeed, and a rate of descent and it will tell you the angle you're moving down.

If the math is right a constant 30kts groundspeed with a 500'/min r.o.d. should yield a 9.46 deg. angle, keeping 30 kts., and increasing r.o.d to 800'/min should give a 15 deg. angle. (Interestingly, 1700'/min at 60 kts groundspeed gives a an angle if 15.8, so a typical r22 auto should demonstrate a textbook steep approach angle)

Of course we don't fly approaches at these constant conditions, but you could set up a steady state descent holding these numbers to demonstrate an angle to a student.

The best solution if you really wanted to fly and visualize precision glideslopes would be to build an alignment of elements visual glideslopt as mentioned in 2-1-2 e. in the AIM (http://www.faa.gov/atpubs/aim/Chap2/aim0201.html)
You could make it adjustable to demonstrate different angles! ;)

I think students should be taught to fly apporaches with a lot of variation, i.e. slow and shallow, fast and shallow, slow and steep, etc.. as this develops a great feel for the ship which can then make flying a "normal" approach a no brainer.

[Arm out the window]

Perhaps to convince unbelievers, you could try getting your students to sit in their normal seating position and see where 45 degrees from the horizontal is.
To do this, sit in the seat, stick your arm out level in front of you, then straight down beside you, and then point to the position that's half way between the two. For me, it's a couple of inches in front of my feet.
Then look at where the top of the instrument panel is with reference to the horizon; it won't be too far down. The angle between your eyes and the horizon is basically horizontal, so you can judge angles between your eyes and aim points on the ground with reference to that.
Then adding in the illusory 'steepening' effect that is brought in by the decelerative attitudes you get into while approaching (can be quite exciting on a dark night), and the whole picture should make more sense.

[TheFlyingSquirrel]

Mr Lappos and Vortex ring again....who do you believe ? My current instructor swears by Nick's beliefs so much, that VR is not even considered in steep approach training. Is it something that is over emphasised to trainees ? Hopefully, i'll never find out.

Confused but aware, TFS.

[Hilico]

As an (extremely) low-timer, I always thought the standard approach angle was like in a Cessna, 3 degrees or so. Hitting the spot was like trying to widdle through a letter box from the other side of the road. I would have preferred to start with something much steeper, as though jumping off a wall - much easier to judge where you'll end up.

[delta3]

I like your attempt to answer thecontroller's question. It is probably a human misperception, even without descelerations.
For me it does not only happen in a heli, but also when being on mountain slopes: it always looks a lot steeper than it is.

d3

[NickLappos]

TFS,

The issue with VRS is that the name is often applied to all those many more accidents where the power is simply not enough to both hover the aircraft where the pilot desires, and stop its descent prior to the hover. That temporary burst of energy needed on a normal steep, fast approach is often 10 or 15% more than that needed to perform a still hover. When a pilot makes a poor approach, drops through, droops the rotor, and whacks the ground, he says "Vortex Ring" and we all nod knowingly. The helicopter got the poor chap. If he spins around at the bottom (that extra 15% torque, remember?) we call it "LTE!"

I do try to keep our folly in the properly labeled boxes in the hope that when it is all said, we all at least know WHAT caused the accident, as a slight first step to actually not having one.

As a little refresher - you cannot experience vortex ring state in any helicopter at less than about 1000 feet per minute vertical descent, and at more than about 8 knots forward speed.

[bladewashout]

The Robinson training regime at the two centres I have used preaches limits of not exceeding 300 fpm when IAS is below 30kts.

Is this just making sure we are massively within the envelope, or to keep us clear of incipient state?

BW

[NickLappos]

bladewashout,
Remember the joke about the little boy who blew a small horn every minute. When asked why, he said "It keeps the elephants away." When told there were no elephants within 100 miles, he said, "See, it works!"

To make an approach and at the bottom be much less than 300 fpm is good pilotage, and likely to produce a very satisfactory approach. I used to tell students that at 100 feet they better be at 300fpm or less, or I would start yelling, even in a three engined monster with 13,000 horsepower. Nothing to do with VRS, it is because it keeps the elephants away.

Regarding descending vertically, if you are in a helo with barely the power to hover IGE, who wants to try and hover OGE? You will fall like a rock, hit the ground and say "VRS!" and we will all nod sagely. Meanwhile, US Army attack helicopters fly profiles where they hover OGE to shoot over a ridgeline, bob back down, straight down, at 300 to 500 fpm, stop quickly in space and move laterally to the next fire point. I guess VRS is against Army regs, or perhaps these helos have 10% power margin while HOGE.

I even saw a poster above who was worried about pulling in power while at low speed in a descent, as if the power pull would CAUSE the VRS! The misunderstanding about the low speed portion of our envelope is aweful, and the old guides and sage advice from practical but (forgive me) slightly misinformed instructors does not help.

There are great reasons not to descend vertically in underpowered helos, but they are not VRS. Follow the advice about 300 fpm, as a limited case where underpowered helos must be babied, just don't take the aerodynamics to the bank. In a helo with enough power, that vertical region is a whole new degree of freedom. Ask photo chase pilots, folks who tend power lines, those who fly NOE for a living.

VRS is:

Caused when you descend vertically fast enough to start catching up with your downwash.

Never experienced at less than 50 to 75% of your downwash speed

Not at all likely to occur in any helo at 300 fpm

Often confused with "over pitching" or "hovering without proper power"

Grossly misunderstood by almost every old line instructor and training guide

Harder to experience at high altitude or high gross weight

Very very seldom the cause of any helo accident (see "over pitching" for the real reason, in most cases.

One of the best ways to kick off a good thread on pprune!

Regarding listening to your flight instructor, please remember your Mother told you to bundle up, so you don't catch a cold. She was off base (viruses do not care how cold you are!), but the advice was sound, and the intention was excellent!

[Stringfellow Dork]

Nick Lappos - please, please, please write a book!

It might help put to bed all the myths we get taught as helicopter neophytes! It's very frustrating and confusing to read /be taught/told one thing and then read from yourself that it's just not the case!

Please? :ok:

[2PWRR]

Not sure how steep the angle needs to be for a normal approach in the UK, however in Canada using the flight instructor manual a normal approach is between 6 and 8 degrees anything steeper is a steep approach anything less is a shallow approach.

To work out your angle of approach draw two lines on an A4 (8 by 11) sheet of paper at 6 and 8 degrees from the corner up then when flying hold the sheet level look down the lines and you have your approach angle for a normal approach , you will see from this even if the instructor guide for the Jaa(caa) says ten degrees most of you will have been doing steep approaches as normal approaches.

Vortex ring state: aircraft descending into its own down wash
High rate of descent: greater than 3/500feet per min
Low air speed: below translational lift
Power applied: more than 20/30% power applied

If you don't have all three factors no VRS
Recovery Get airspeed altitude permiting reduce collective setting

Or autorotate

Hope this helps

[WHK4]

I am interested in tc's original question.

If I can restate it:

He and his students experience an illusion on approach. The approach angles appear 2-3 times larger than they actually are. What is the basis of this illusion?

The explanation about nose-up attitude due to decelleration only accounts for a few degrees. In any case tc appears to be trying to rule out that factor by supposing a level attitude prior to picking out the landing point.

So I also experience this illusion, but don't have an explanation. It looks like a human factors question to me. Maybe there is a psychological basis for it.

[NickLappos]

WHK4,
Who says the poor judgements of untrained people are worth debating? What training or experience does the student have in telling 12 degrees from 16 anywhere, let alone in a cockpit? Why the mystery and need for "human factors" explanations for people who have no experience and no basis for it? This does not take away their need for info, nor our wish to help.

Let me restate their question as I see it: I have no earthly idea how to tell the angle of approach, any more than I can guess the weight of a prize bull or the height of a far tree. Can someone give me some tips?

In that case, I see lots of good thoughts on this thread.

[Arm out the window]

A couple of factors that probably contribute to the over-estimation of approach angles would be:

1. The fact that our eyes don't actually end up at the aim point spot on the ground - in the hover with the mast over the spot, our eyes are some distance above and in front of it, which would contribute to an 'overshoot' illusion, especially on short final.

2. A true appreciation of where we're going on an approach takes a while to build up; i.e. we look out the front at the aim point, imagine a direct line between us and the point, and over period of time (a few seconds, say) assess how we are travelling with respect to that line (over- or undershooting, or right on).
Just a quick look out the front, especially with a high nose attitude, will probably give an inaccurate mind-picture of how the approach is going.

[PPRUNE FAN#1]

N. Lappos about VRS:Not at all likely to occur in any helo at 300 fpmThere is something that troubles me about statements like this and in fact all that we "know" about VRS/SWP: That Canadian RAF Sea King at the airshow in upstate NY some years back.

If you can find the whole clip, you'll see the big Sikorsky hovering around up at 100 feet or so (it's hard to tell) in an OGE hover. It is nice and stable, and the cameraman is easily able to "track" the ship without a lot of jerky camera movements. The ship does a sort of sliding-pedal-turn without losing much altitude (again, hard to tell), when all of a sudden the bottom falls out and it starts descending vertically, blades coning like the proverbial ballerina, collective obviously up under two armpits, until it smashes into the ground like a ton of bricks, landing gear fails and it rolls over. (Most of the internet video clips I've seen only show the last few seconds of the flight as it is already in...whatever flight regime is causing the downward cable-snapped elevator ride.)

Now to this observer, it does not *look* like the ship was in a big vertical descent before The Event. In fact, just the opposite, it looks like the crew was being pretty cautious and slow. They didn't seem to be all that heavy, so available power shouldn't have been a problem. But "something" happened, and I'd sure like to know what. Engine failure?

[NickLappos]

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/092104-sea_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

[helmet fire]

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.

[Wunper]

Nick

So the Seaking clip was the result of the Elephants eating the Beans, phew!!!

Great thread Lu can't be far away

W :ok:

[PPRUNE FAN#1]

Nick sez: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. 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.

[delta3]

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

[NickLappos]

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/dschrage/AE3310/RD%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

[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. Getting frisky in an OGE hover is certainly courting disaster.

[PPRUNE FAN#1]

Nick: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!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: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.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: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.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: 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.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.

[NickLappos]

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/~kevinogdump/vid/Accident_sideways_helicopter_crash.mpeg

[JimL]

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?

[Simon853]

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

[NickLappos]

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.

[Flingwing207]

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!...so ya wanna be a test pilot...

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?

[wiisp]

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!

[NickLappos]

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.

http://webpages.charter.net/nlappos/downwash.jpg

[delta3]

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

[WHK4]

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.

[NickLappos]

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:

http://webpages.charter.net/nlappos/VRS.jpg

Note they use 66% as the typical onset.

[Flingwing207]

Thanks Nick, Delta,

I'll take that as a "yes"... :8

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.

[NickLappos]

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.

[thecontroller]

>"It seems that a helicopter would be somewhat less prone to VRS the more heavily it was loaded"

err.... is this right??? if a helicoter is heavier, then it would be operating at a higher pitch angle, and therefore more tip vortices, and therefore more danger of VRS, not less???

[[email protected]]

controller - heavier helo = more MR thrust required = higher AoA = increased downwash velocity, therefore more RoD required to catch the vortices and get VRS.

[NickLappos]

the controller,
Do not think of pitch angles, they are the result, not the cause, and they are quite confusing, and actually, I would bet not one ppruner in 100 can tell me the real angle of attack (pitch) angle the blade assumes in ANY flight condition.

It is more intuitive, and more accurate, to think of the rotor making a stream of air run downward from it. Think of that stream as what supports the helo, as if it were swimming upward in a sea of air, and by pushing air downward, the helo raises itself. If the helo is heavier (for a given size rotor) it must toss more air downward, so the stream is pushed faster.

When VRS is encountered, it is because the rotor is lowering itself downward faster and faster until it catches up with some of its downwash. When the rotor is shooting downward at more than half its downwash velocity, it begind to catch up and the ring starts to form. By 75% of the downwash speed (maybe 800 fpm in a light helo) the VRS is truly encountered.

If the helo is heavier, it takes more descent speed to catch up with the faster downwash, so VRS is HARDER to find in a heavier helo.

If you are at a higher altitude, the air is thinner, and lighter, and so it must be pushed faster by the rotor, so VRS is HARDER to find at higher altitude.

Why do instructors teach otherwise? because they mix up VRS with running out of power to hover, where the aircraft falls through as it tries to hover. Falling through is EASIER at high weight, because it takes more power than normal. Falling through is EASIER at high altitude, because the engine makes less power, and the rotor needs more.

Don't mix up VRS with "Falling Through" or "Over Pitching". They look similar enough, but they are caused by two different things.

[puntosaurus]

A couple of pages back from Nick ...

Regarding listening to your flight instructor, please remember your Mother told you to bundle up, so you don't catch a cold. She was off base (viruses do not care how cold you are!), but the advice was sound, and the intention was excellent! Couldn't resist possibly my only ever opportunity to prove Nick wrong - see here (http://news.bbc.co.uk/1/hi/wales/4433496.stm)

:D

[delta3]

Quote : "Do not think of pitch angles, they are the result, not the cause, and they are quite confusing, and actually, I would bet not one ppruner in 100 can tell me the real angle of attack (pitch) angle the blade assumes in ANY flight condition."

My model/simulator does exactly do this, inch per inch, degree per degree or 0.001 sec at a a time if you which, and it took me ages to understand/verify/believe the results. So allow me to think that I do understand most of it, at least for the R44-I.

I do Agree completely that understanding VRS via angles, is the hard way, impuls theory is a lot more intuitive/simple for explaining this behaviour. In the simulator, both models are of course connected via the Math, but zillions of calculations happen to do this quantitatively, so don't assume that via one simple angle of attack drawing, the case gets explained in general.


d3

[SHortshaft]

NickLappos quipped in a parting comment in this thread on 10 Nov:

“Regarding listening to your flight instructor (when he preaches limits of not exceeding 300 fpm when IAS is below 30kts), please remember your Mother told you to bundle up, so you don't catch a cold. She was off base (viruses do not care how cold you are!), but the advice was sound, and the intention was excellent!”

So imagine how shocked I was this morning to learn from the BBC News that “Mothers 'were right' over colds” and that “Scientists say they have the first proof that there really is a link between getting cold and catching one”. (http://news.bbc.co.uk/2/hi/uk_news/wales/4433496.stm).

I guess you pay your money and you take your choice!

[delta3]

Interesting topic again.

Nick stated : "viruses do not care how cold you are!"
The article states (imho): "Cold may reduce our defensive system effectiveness, when infected"

I would infer : "if you are not infected, cold (for not too long time probably), will not infect you"

Back to the topic : "in clean air VRS will not occur, if staying out of the area's shown in Nick's graph", so exceeding 30kts, 300 fpm should pose no problem. I also believe this.
But similar as in the case of a cold, you may be affected, meaning that real speeds are not what they seem, since at slow speed moving air masses may significantly change these speeds, even if this happens only momentarily, too short to be picked up by slow instruments.
Furthermore air may not be clean, for instance main rotor takes in some disturbing turbulence from tail rotor, a case that is not covered by the VRS model (as it assumes clean air)

So using "mothers rule" to keep for instance within 30/300 is probably not only well intended, but also correct if one has to assume some extra disturbances will occur, disturbances that may not be anticipated by the pilot (similar as not knowing you were already infected)

Doing a steep approach over a nice big runway, is different from doing a steep approach into an area with obsticles around creating non laminar air flows.

My 2 cents...


Added : I also do believe that (unanticipated) tail winds are one of those disturbances, see my earlier remarks/questions. It would be interesting to know what Nick's graph becomes when flying backwards for instances. My feeling is that margins may get slimmer because of the possibilty of tail and tail rotor influences.

d3

[2beers]

Whooooa there!

Just catching up on this very good thread and had to comment on something quite important: 1m/s equals 60m/min equals approx 200fpm in my head. The exact figure according to Online conversions (http://www.onlineconversion.com) is 1 meter/second = 196.8503937 foot/minute. (Nice place for conversions by the way).

Oops, just read D3's post again and maybe he already took half away from it to get the critical Vi, in that case forget this post except the part about online conversions...

Cheers!
/2beers

[delta3]

Right on 2beers

d3

[offshoreigor]

Hi Nick,

Great comments. I know this thread is old but I am just catching up.

Firstly. When I was flying CH-124A's I believe the Maximum Normal Landing ROD was 8ft/sec and the absolute demonstrated Max was 10ft/sec (Emergency: 12ft/sec). Maybe I'm getting old and can't remember but I know on the CF's DDH's, we only registered a hard landing at the LSO/FFS station, above 8ft/sec.

Secondly, one factor about Mr. G's accident in the SeaKing was a lack of vertical reference training, ie, had he been trained in VRef, the accident may not have happened. I have spoken to both Pilot's as I know them personally and Mr. G. and Mr. B. (the Pilots involved) had no VRef training. Fatigue was also a cause factor in the accident investigation among many other factor's, all non-crew related.

Great Posts!

Cheers,

:eek: OffshoreIgor :eek:

[NickLappos]

Igor,
Thanks for the details, I know the normal civil 61 is 8 ft/sec (derived from US Navy requirements as the HSS-2). The other numbers 10 and 12 are with some damage, at known points, thus the inspection after you write up the hard landing. I flew off the first 124's with de-ice blades at St. Hubert back in 1974, it was a fun summer, working with a great Canadian pilot, Ross Lennox, who flew for P+W Canada back then. Had a few drinks with the Snowbirds at St. Hubert Officer's Mess, and a big headache the next day!

If you have an accident report or any details on that CH-124 can you share them?
Nick