When to flare?

[Have a Chat]

I would like to here comments on a subject for which I have heard differing options on in the field. My question is for a day flight and concerns the method used to trigger the moment at which a flare at the bottom of an auto should commence? Do you flare when the ground speed appears excessive or at around 100 ft pending what type of aircraft you are flying?

[paco]

Hopefully, you've been looking out and adjusting the groundspeed as you go down. My own choice is to wait till about 70 feet. If it helps, here is an edited version of the autos section from The Helicopter Pilot's Handbook:

"For a clear area, when about 70 feet from the ground, use rearward cyclic to slow down vertically and horizontally. The amount is proportional to your speed and serves to increase the total lift reaction (which stops the sink) and shifts it to the rear (which stops forward movement). It also increases the rotor RPM.

Continue the flare progressively to a nose-high attitude (in a 407, the instrument panel should just about hide the horizon), applying collective as flare effect decreases to allow the machine to flop forward in a self-levelling process, watching for drift. The “check”, which is a more positive application of collective, is only used with some helicopters to level the attitude better. Otherwise, there should only be small pause in a continuous movement, and you should find the rear skids touching the ground gently well before you run out. Get used to the visual clues required for the correct approach and flare attitude – there's no time to look at the ASI, and the one on the 407 is dampened anyway, so is fairly useless under these circumstances. Get up on a nice day and practice autos to a cloud, getting used to the horizon's position through the screen during descent, flare and turns on your machine.

In every flare there is a point called the apex, which is where the trading off of airspeed for lift is essentially all over and you just have to get yourself on the ground. Put another way, it is the point where there is no further benefit from the flare manoeuvre, so you may as well pull the pitch (a little later in a 206). As the flare ends, and the kinetic energy of the rotors is used when the collective is raised, the airflow through the rotors is reversed, assisting the level, ready to cushion the landing with collective. This is where correct use of airspeed during the descent will have had the most beneficial effects—as the kinetic energy stored in the blades is what slows you down, it follows that any you have used already to slow an unnecessarily fast descent is not available for the final stages of touching down."

Hope that helps

Phil

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Have a Chat, presumably you are into wind, wings level and at the recommended auto speed (probably 50-70 kts depending on type), hopefully you have been in that configuration since at least 300 'agl to allow the RoD to stabilise at its minimum.
Your flare height will depend on aircraft type (R22 about 40', Sea King/Lynx about 150', most other things like Gazelle/ 206 etc about 100') and is primarily a function of RoD - the faster you are going towards the ground the sooner you need to do something to stop you hitting it hard.
Learn to try and judge the flare height using external references(heights of trees, houses, sheep etc) rather than slavishly using the radar altimeter if you have one. The concept of waiting until the groundspeed seems excessive is an interesting one but how do you assess excessive? When you are hurtling towards the ground it all seems to get far too quick, far too quickly and you run the very real risk of leaving it too late.
The more EOLs you do, the better you will learn how to not only judge the required flare height but also how to vary the flare depending on wind strength and AUM. A heavy aircraft in light winds will need more flare started higher than a light aircraft in a strong wind.
There are many variables to consider when aiming to carry out a
textbook EOL but the technique detailed in the previous post should stand you in good stead. The British military teach 'Flare, Check, Level, Cushion' as the 4 main parts of the EOL with the Check element only being left out for low inertia rotor systems (R22 ) or big heavy aircraft where you need all the Nr to cushion the touchdown.
Good Luck

[paulgibson]

I was taught to use groundspeed and it does seem logical and more importantly, user friendly to me.

The purpose of the flare is to reduce the landing speed for touchdown, not to control height, and only an appreciation of the groundspeed can assist you in that decision of 'when to'.

Using groundspeed as your cue, takes away all the guesswork with regard to wind direction and speed, AUW and ROD. Of course, the knack of sensing when the groundspeed is excessive is something that comes with experience thru your initial and subsequent training, but that skill is no different, or more difficult, than many other facets of helo ops.

[paco]

I think the flare controls height and speed very successfully. It's only as the machine starts to sink the collective needs to be applied.

phil

[Hughes500]

I teach my students in a 300

Oh **** the ground is coming up quick

Oh f--- the ground is coming up really quick

FLARE, CHECK, LEVEL:)

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Paul,

The flare does 2 things - it reduces forward speed but also, by increasing rotor thrust, reduces rate of descent; obviously both of these are very important because hitting the ground at 60-70 kts@1500 feet/min will hurt.
Opinions differ as to whether or not during the flare you should raise the lever slightly to contain the rising Nr; the danger is you will raise it too much and decay the valuable Nr - the advantage is that you will further reduce speed and Rod as the rotors total reaction is tilted backwards and you are again increasing rotor thrust.
As Paco rightly says, flare effect doesn't last for ever - it is the change in inflow angle that produces the increased angle of attack and therefore rotor thrust; if you want more flare effect you must keep tightening the flare. The problems with this are a. as speed reduces, the change of inflow angle for a given amount of nose up reduces so the slower you are the less effective the flare and b. you do not want excessive nose up attitude just before you try and select a level attitude for landing. As you push the cyclic forwards to select the landing attitude, you are rotating the aircraft fuselage about the centre of gravity - the C of G is still going downwards so you are effectively accelerating the aircraft towards the ground. The bigger the attitude change you have to make, the worse it is and you are undoing some of the good work you have done to reduce the RoD in the flare.
The check is a positive but small raising of the lever as the benefits of flare effect are lost, to further reduce rate of descent and forward speed (the total reaction of the rotor is still tilted backwards) and is not used to level the aircraft (in fact it tends to make the nose pitch up slightly). If all goes well in the flare, the check should come between 10 and 30 feet depending on type and conditions - however it is not recommended for low inertia rotor systems as it robs you of valuable Nr which you will need to cushion the touchdown.
The selection of the level attitude should be made gently for the reason given earlier and because it is too easy to over rotate and end up a little nose down.
Finally the cushion - this can require the full application of lever but again should be progressive; unless you are about to smack the ground hard, a snatched cushion will often level you off with one or 2 feet to go to the ground and the Nr decaying rapidly.
Although the EOL does seem like walking a tightrope at first, in that even the slightest error will mean disaster - most aircraft are quite forgiving and providing you land in a level attitude with no skid or drift, will at least let you walk away from it.
I spent several happy years deliberately doing EOLs wrongly to give student QHIs practice at intervention and recovery and it surprising what you can get away with on an EOL.
Hope this helps Have a Chat.

[heedm]

When to flare?

Fortunately, this question has been answered for you by the people that wrote the book on your helicopter. Each helicopter has it's own uniqueness that means the when to flare and the how to flare are determined by flight test. Learn to fly your machine that way and only that way.

Go with an instructor and practice the auto with the numbers, until you develop your own visual cues. Then you can fly the auto with your head more outside the cockpit and the instructor confirming that your cues are accurate.

Once in the field, things may change. Do what training you can in the environment you work and discuss with the senior pilots how things change for them.

Now that you're in the field and not in a training environment you may find that the rule above, "[fly] that way and only that way", doesn't apply completely. In any case trust the experience of those who wrote the book, those who instruct you, and those who have flown in your environment and machine.

[Skycop9]

When to flare?

When I teach autorotations in a 300C I have the students start the flare between 50 - 60 feet. I have them slowly apply aft cyclic and have them level the aircraft between 5 - 10 feet no mater what the ground speed is. After several auto's they develope the idea of how much to flare and how long to hold it. It will vary depending on wind conditions, etc.

"disclaimer"
DO NOT TRY THIS WITHOUT A FLIGHT INSTRUCTOR ON BOARD!

When you enter the autorotation and have established a steady state auto increase the collective slightly to bring the rotor RPM to the lower part of the green. 300C 410-420RPM. You will see the rate of descent is slower than when the rotor is in the middle to upper part of the green.

When you flare the RPM will raise to the upper part of the green in the rotor rpm gauge, and you are less likely to overspeed the rotor in the flare. Always try to be in a level attitude by 5 - 10 feet.

DO NOT TRY THIS WITHOUT AN INSTRUCTOR. LOW ROTOR RPM WILL KILL YOU.

I have done this with commercial students (175 hrs - 200 hrs) learning touchdown auto's and found that it made the decent and flare less dramatic at the bottom. The instructor has to stay on his toes to insure rotor RPM does not decay below limits.


skycop9

"never let your education interfere with your learning"

[t'aint natural]

Re when to flare:
My old FIC instructor advocated beginning the flare "when the grass resolves itself from a uniform green appearance to where you can see individual blades..."
I never got the hang of that one myself. Perhaps we had different focal lengths. Looking horizontally, it gives me 80 to 100 feet.
That same instructor, now sadly departed, told of practising engine-offs to a hard runway in a Dragonfly (or a Sycamore, I forget). After a session he went back and told his superior what he'd been doing.
'Very good,' came the reply. 'But try raising the lever in the last six feet, it works a treat.'

[Droopy]

The best advice I can give is:- practice, practice, practice - EOLs, like instrument flight, are a perishable skill.

[MaxNg]

Hava a chat

When to Flare ?

Just as the Sphincta valve goes into overdrive!!!!


Tant natural

Sounds like the late Bill Barrel to me.

Respect

[t'aint natural]

MaxNg:
For it was he.
A truly effective instructor and a sad loss.

[Steve76]

During a type conversion to the R44. The instructor displayed a no flare auto to the ground. He called it a constant attitude auto and it involved a well timed collective pull at the bottom. Seemed to work ok, but is fairly nerve wracking. :D

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Steve 76,

The constant attitude EOL is a splendid technique when you have to get the speed back to make the intended (or only available) landing site. Aim for between 30 and 40 kts, maintaining positive groundspeed and ignoring airpspeed fluctuations in the latter stages due to windshear or turbulence.
This really is a question of waiting for the green to turn to grass as you want to apply the first bit of lever (a check just like the variable flare EOL) at about 30'. This will tend to pitch the nose up so forward cyclic is required to maintain the attitude but the RoD will start to reduce. The end game is to get to about 5' with enough Nr to cushion the touchdown ensuring you are striaght without skid or drift and the skids are level. Some prefer a gradual pull from about 30' as opposed to a separate check and cushion but it works pretty much the same (IMHO it is more difficult to judge the rate of application of lever this way but others like it).