normal approach descent rate and VRS

[slowrotor]

My instructor is teaching me to perform 12 degree normal approaches in a hughes 269C.The 12 degree glide slope seems very steep to me and I do not understand why such a steep approach is used, as it is difficult to perform.My concern is that the descent rate is more than 300fpm and could lead to possible vortex ring state as the speed drops to 15kts just before landing.
Shawn Coyles book states "a good rule of thumb is to make sure the rate of descent is less than 300 ft per minute as you pass 100 AGL."
I drew a 12 degree slope with a protractor and calculated as follows:
50 knots = 1066fpm descent
25 knots = 533fpm descent
15 knots = 319fpm descent

I have an old "Basic Helicopter Handbook" that shows 12 degree approach angle as "normal" and 5 degree as "shallow". Shawn's book (1996 edition) doesn't discuss the approach angle or mention what is "normal approach angle"
What is considered a normal approach angle today?

[NickLappos]

The 300 fpm is actually quite wrong, but has rested unchallenged for decades. The actual downward speed of the helo has to be about 50% of its downwash speed, which is usually quite a bit faster than 300 fpm.

Most VRS/settling with power accidents are actually the problem of trying to hover out of ground effect with too little engine power, but the understanding of this is not pervasive in the community, even among accident investigators (especially when they read the books that quote 300 fpm as the VRS boundary!)

That being said, most light helos are unable to hold a 12 degree glideslope at 50 knots speed because you are in autorotation. The typical steep approach is shot by holding about 300 feet agl and slowing down to begin the descent, and the 12 degrees is not achieved until you have slowed to below about 30 knots, where the autorotative boundaries are not an issue. Most steep approaches are actually curved glideslopes that get progressively steeper, and achieve the 12 degrees as the aircraft passes over the barriers, where the steepness is needed.


Here is the web site I set up based on the last time this subject went around!

http://www.s-92heliport.com/vrs.htm

[Bronx]

Rotorheads must be the only place where a student- PPL can ask a training question and in a few hours get an answer from one of the top helo test pilots in the world.

Good question Slowrotor.

Well done Nick.

Over to you Shawn ................... ;)

[Shawn Coyle]

And Nick is right. Future editions of the book will have this erroneous statement expunged (taken out for you fixed wing guys lurking here).

[Heliport]

Oh No! :eek:

How can we have a debate if the experts agree?

Where's Rich Lee when you need him?

[Hughes500]

Slowrotor

As a rule ot thumb I teach mystudents in a 300c to hold about 45 knots 16 inchs. This will give approx a 500 ft a minute descent rate.( depending upon wind , temp and weight ). Use a normal sight picture approach, as you get to 150 ft agl where sight picture does not work start gently flaring the helicopter and you will be able to touch down with a zero zero using no more than about 18 inches to cushion on to the ground. Nodanger of vortex ring.

[slowrotor]

Yes indeed, this forum is truly amazing!
I like to study the details perhaps to the point of being annoying.
Shawn, Thanks for taking comments about your book with a positive mood. I wish you could be my instructor, your book has saved me hundreds of dollars so far with tips not found anywhere else.

Nick, Your response, debunking the long held views, is always a pleasure.
Sounds like you are saying the 12 degree approach is actually a very steep approach.
As a fixed wing pilot, the 12 degree approach seems to require a large amount of carefully timed collective to stop the descent.
I am wondering if a shallower approach is really the "normal" approach to an airport. Should I just shut up and learn to approach at 12 degrees?

Hughes500,
My calculations show a 500fpm descent as you suggest to be about a 6 degree glide slope. More like a fixed wing.

[LMCCOY]

There is nothing wrong with learning to do steep approaches. you will need to do them on many jobs. External loads with a long line to small areas is an example.
I have found that a high rate of closure will make a steep approach feel uncomfortable. A slow rate of closure feels more comfortable and allows a more gradual increase in power.
Planning is very important, knowing you have a power margin for the OGE hover is mandatory for a safe comfortable approach.
I have noticed different characteristics between a 212 and412 doing external load work. (OGE hovering) Do the different rotor blade designs effect VRS?

[NickLappos]

slowrotor,

From my experience, the typical helo approach ends at about 4 to 6 degrees, and a steep one might be 8 to 12 degrees from the last 150 feet or so.

[vorticey]

why is it that a little forward airspeed is needed for VRS?
is the autorotation boundry to close at 0 air speed? (it doesnt look like it in the picture):confused:

[Jcooper]

So if you have a higher disk loading (higher down wash) it would be harder to get into VRS correct? If so why does the V22 seem to have so many problems with VRS since it has double the downwash (disk loading) of even the heaviest helicopter?

[PPRUNE FAN#1]

Some pilots seem inordinately preoccupied with VRS. But to get into fully-developed VRS, one would have to be making a nearly-vertical approach in calm wind or perhaps even a slight tailwind, in a helicopter with very limited power reserves.

Nick has pointed out many times that what we call "settling with power" is not really classic VRS but in reality settling with insufficient power. Here is what the U.S. Army FM 1-203 says about the subject:"blah blah blah...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."Well, the R-22 certainly qualifies there! But Frank's infamous trainer is by no means the first underpowered helicopter.

I can't repeat this often enough: You CANNOT get into VRS if you are above ETL. Thus, one sure-fire method of avoiding VRS is to remain comfortably above ETL. This means that you need only maintain something higher than about 20 knots on approach until you are ready to settle into your ground cushion. This is not incredibly hard to do.

If you want to avoid VRS, merely ensure that all of your approaches are INTO the wind! In all of my years of flying, I've seen relatively few dead-calm days when there wasn't at least some breeze. Use it to your advantage. It sounds so basic and simple, yet I've seen many helicopter pilots who've become cavalier and careless about where the wind is while they are on approach.

Remember, the VRS chart is based on calm wind, because VRS is a function of AIRspeed. As the wind increases, the shaded areas of the chart virtually disappear (unless you're making your approach with a tailwind, of course, then they're even bigger!). In a 25 knot breeze, you could make a completely vertical descent and the rotor would *think* it was still moving forward at 25 knots. (Please keep in mind that this discussion is about VRS/SWP and not the H-V curve. Thank you.)

So. Make your approaches into the wind, and keep it above ETL for as long as possible. If you do this routinely, you shouldn't have to worry much about VRS, and the specific approach angle you use won't matter.

One final note: If you are doing your steep approaches in such a manner that they require either a large fuselage pitch attitude adjustment or a large power increase at the bottom, you are probably doing them wrong. A steep approach is not necessarily an autorotation.

[Devil 49]

Slowrotor, if you'll indulge me for a second, I'll try to add a little real world to the most excellent replies you've gotten already-

Steep and slow seems hard because you're just learning to do it. And it's strange and dangerous to your airplane sensibilities.

I remember wishing I could just YANK the power and leap into the air- gradual, controlled liftoffs seemed a waste of time getting to 3 feet- then I saw the results of a dynamic roll-over, and talked to an amazed pilot "THe cyclic wouldn't stop it. I hit the stop and it just kept going!" Point taken, now I try to count all four corners off the ground as I lift straight up.
Next big lesson for me- After some time at the "Southeast Asian Unpleasantness," I had to break the bad habit of "diving-glide-flair-crash" landings. Took me a while to appreciate the risks I was incurring by "not wasting time" and "staying outside the deadmans curve": tail rotor strikes; main rotor strikes; hitting stuff in general- the stuff that grew or got added to familar helipads- Antennas, wires, signs, loose stuff, people, the list is endless. Add the fact that an approach flown fast is seldom stable.

Steep and slow is worth learning. It will reward you many times as you fly. This is a helicopter and it hovers.

Helicopters exist to go places off airport. The more this you do this, the more likely you're going to have to contend with obstacles and limited space. When you're landing to an LZ, the mechanical act of flying the helicopter is the very least of your work. No matter how many time you've been into that LZ, this time something will have changed, and you're coming in s-l-o-w to give yourself time to find it. Making it even more difficult, you're going to be looking down into the LZ limiting your view and from a very poor aspect. You want to start your search for obstacles as soon as you can and look as long as you can.

VRS is a factor to consider in these operations. I'm not going to minimise the risk, just put it in the proper place. VRS isn't your greatest hazard. If you're alert to the conditions allowing it and the signs of onset, it's easily corrected. I suggest you become very familiar with them, do some fully developed, and that bogey man will lose a lot of it's terrors. Collisions are the killer here.

It may not impress the Blue Thunder and Airwolf crowd, but I admire the approach flown such that the helo's attitude doesn't change after loss of ETL, and the bird creeps onto the pad. That's a patient pilot who correctly evaluated conditions, set his approach up properly and kept it stabilized all the way, making minute corrections.

[Shawn Coyle]

All great discussion.
Vortex Ring State requires you to be descending through your own downwash at very close to the vertical speed of the air being pumped out by the rotor. The downwash velocity from the rotor will depend on the power being developed at the time.
Classical requirement aside from the vertical speed is to be descending into that downwash - normally by descending straight down in a no wind situation, but also by being in a no wind situation by confusing groundspeed with airspeed (and actually being downwind and having zero airspeed, but some groundspeed).
Different aircraft will have different downwash velocities - light disk loading (i.e. large rotor diameter and light weight) will have a much lower downwash velocity than a small rotor diameter and high weight, and the low disk loading can actually get into trouble at a lower rate of descent than a high disk loading machine.
What is not common knowledge on the V-22 crash is that the rate of descent was extremely high- so high in fact that the lead aircraft, which did not get into vortex ring state, hit the ground so hard it was written off.
I've only ever been able to demonstrate VRS by doing with a tailwind, at altitude of course. Trying the same thing into wind from the same starting conditions results in something completely different and not VRS.

[NickLappos]

Jcooper,

The VRS issue for tilt rotors is complex, but the answer is simple.

In a helicopter, VRS leads to an unacceptable rate of descent which can most of the time be powered up through, and in the worst case leads to a hard landing. Yes, most helicopters can be flown up by raising the collective and climbing up out of VRS. The older helicopters back on day one had too little power margin, and so could not do this, but most turbine helos can.

With a tilt rotor, the issue is not that the descent that builds up, it is that the rotors enter VRS at different times, so that the aircraft departs controlled flight, so that powered recovery is impossible after VRS experienced, since the aircraft is upside down by then. Upside down on approach is a real problem.

[slowrotor]

Great discussion!
To review:
(1)VRS can be eliminated as a concern for average landing situations into the wind and the 300fpm rule is incorrect.
(2)What a lot of pilots think is VRS is actually settling with "insufficient" power.

With that established, what is the ideal approach for a normal airport landing?
Obviously, My instructor should be teaching the proper approach,but I am not sure he is.
I hate to second guess what he says but it is pretty clear that even instructors do not always have the proper information.

This is what he drew on the chalk board for a normal approach:
A 12 degree glide slope with 50kts at 50 feet then level the fuselage and be at 20 feet with 20kts and 10feet and 10kts.Easy to remember! He said "just keep a bug on the windshield fixed to maintain the same glide angle all the way to touchdown and bringing in the power will stop the forward travel without any need for backward cyclic."
I showed the instructor a drawing I had prepared of a 12 degree glide slope on the next lesson. A 12 degree slope is 4.75 to 1.
So at 50 feet agl we would have 237 horizontal feet (50 x 4.75 = 237) to bring the ship to a stop and I thought that would require more of a quick stop maneuver.
He then agreed that, as a student, I could start the slowing process at tree top level, about 100ft instead.(looking at the trees along the runway as we fly down the runway aiming for the windsock area about mid field)

As Devel 49 suggested above I would much prefer to come in slower, perhaps 30kt.That's what helicopters are all about, right! My assumption is,the instructor is trying to teach a method of approach that improves the chances of a successful landing in the event of power loss by coming in faster than I would prefer.
Since VRS can be eliminated as a concern, only the HV curve need be factored into the approach method, I think.
Looking at the HV curve and my glide slope drawing, 12 degrees is into the danger zone (25kts at 25 feet).
At 6 degree glide slope the danger zone is avoided.
If you look at the HV chart and try to draw a glide slope on the chart itself,that doesn't work,the units do not match with an actual glide slope.The data from the HV chart has to be transfered to a different glide slope chart layed out in feet on both the side and bottom of the chart.
So I think maybe the instructor got the 50kt-50feet 20kt-20feet rule from the HV chart.
I understand sometimes flight in the HV danger zone is necessary and look forward to practicing steep and vertical approaches. But still, I need to define a normal approach.

[Devil 49]

My recommendations are general in nature. You're talking specifics and I've never flown a Robbie, so can't put myself in that seat, h/v or emergency.

Like an airplane, a good approach starts long before lining up and power reduction. You've asked about a "normal" approach, so I'll tell you how I want it to go, from the terminating hover back up to where the start, assume the old 10 degree normal approach.
1. I want to "coast" from no lower than 15-20 feet AGL to a stationary hover at 3 feet. The fewer and smaller control changes (in a perfect situation), the better I've controlled speed and power to this point.
The aim point will move markedly on the bubble.

2. THe segment from 300 ft AGL to the above has larger, but still small adjustments. Entered from 300 and 60 or 70 knots with a power reduction and decel pitch up. The initiation of descent is the most marked change. The decel pitch up will increase gradually until I hit the bottom of the "power bucket." It's been a long time since I used the correct term for starting up the region of reversed command, power-wise. Until that pont primary angle control is pitch, after that it's power. Remember you're aiming at 20 feet stabilised and coasting to the hover...
Initially, the aim point is startionary but moves down the bubble slowly towards the end.
This segment should also be stable- but you have to be able to anticipate the initial power setting and adapt early for existing winds. My calculator says initial descent at 70 knots is 1160 fpm for angle, at 60 it's 1000 fpm, slowing as you decel, maintaining angle.
To be honest, I think I shallow initially and settle onto angle as I slow, and rarely hit that. Hard to say because I usually do a turning approach wit a 90 deg turn onto final, if not a constant arc all the way down from cruise.

3. Hardest way to get to 300 AGL and 60/70 knots, straight in from cruise. Very hard to adjust and anticipate the wind spot on, but to do the approach you have to be exact. The transition from cruise or high recon is adapted as necesary to get to this point.

As I said, I prefer a constant turn in the approach. Transitioning from a 90 kt high recon or 120 cruise is easier that way- you load up by turning harder to slow down more quickly and shallow out as you get speed under control. Usually a straight line in is not favorable to forced landings so I'm adapting for survival. It's harder to teach but easier to fly.

[Red Wine]

Whilst we have all the "experts" attention....

Why, in the OEI configuration does the ROC at VTOSS exceed that of Vy.....?

Its bloody frustrating to explain why setting Vtoss has this "extra" ROC than is achievable at Vy....

I have heard the explanation before that it is a transient phenomenon, but when you are only in this phase for a minute or less to reach "Safe Flight"....it is hard to provide a satisfactory explanation when the IVSI is right there.

OK...over to you??? [But practical, not theory please.....theory doesn't hurt us]

[NickLappos]

Red,

The answer is that the climb rate and angle at the early oei segements is set at a higher power, so the poorer climb efficiency of Vtoss is sometimes offset by the greater power.

VTOSS is used in the initial climb segment, so is predicated on 2 minute power (or 2.5 minute power, depending on the engine rating scheme). The ROC at Vy is predicated on a lower power, usually 30 min max contingency power (or 30 min oei power, or max continuous power, again depending on the rating scheme that was selected by the engine manufacturer).

[vorticey]

does anyone know why full vortex ring is when a little airspeed is on, is it somthing to do with the tail rotor? looks like going strait down in nill wind will only be buffering and twitching.:confused:
is it because autorotation will preceed vortex ring when going strait down?:confused:

[Barannfin]

i was always taught that u needed little or NO speed. + power applied to rotorsystem(not auto), and ROD greater than 300ft/min

[Red Wine]

Just taking that one step further......

Assuming you simulate the 2.5 minute power setting with a lesser figure for training....once a failure is introduced and the pilot sets Vtoss with this training power setting, [a constant] the aircraft climbs at a certain ROC.
Without touching any power setting, but just "beep' the aircraft to Vy, the aircraft's ROC will decrease.
Then without adjusting any power setting "Beep" the IAS back to Vtoss and watch the ROC increase.

I thought it may be a transient 'benefit", so I allowed the aircraft to stablise at Vy for a minute, but yet Vtoss still gave a better ROC????

It happens in practice, but its difficult to get a good explaination that can be passed on.

[NickLappos]

Red Wine,
I think part of the thing you are seeing is that you must readjust the collective during speed changes, especially at low speeds, since the inflow velocity is an important part of the flow equation. When you leave the controls untouched, don't confuse that with leaving the power constant. If you do not raise the collective, the torque will fall somewhat, and the climb will deminish.