Physics of Deceleration in Landing

[AngeloUMD]

Ok guys, if you're coming in for a landing at 15 kts, at what point does your forward momentum commit you to land? 1 second out, 2 seconds, 1 rotor diameter, more? Let's say you're flying a Bell 206 the like, so not too heavy.

Basically, how much time/distance would you need to stop without physically touching down if you're landing fast. Does it depend on the pilot?

[Shawn Coyle]

A lot will depend on:
Weight
Rate of descent
Groundspeed - you're maneuvering with respect to the ground, not the air...
Atmospheric conditions (OAT, PA) so you can determine power available and power required to hover
In-ground-effect or Out-of-Ground effect?

[AngeloUMD]

Let's say you're flying in Bell 206 from 100ft AGL at 20 kts groundspeed and descending at something normal like 10 ft/sec. I'd assume to stop quickly the pilot would need to flair the helicopter to an "extreme" angle, may lose visibility with the ground? Something like 200 ft to stop and land sound right?

[Hawkeye0001]

As Shawn already said the approach / flare depend on a lot of variables, one of them being wind speed and -direction. E.g. with a 20kts tailwind you would maintain a 20kts ground speed by having 0kts airspeed. But you could also attain the same groundspeed with a 30kts headwind and 50kts airspeed - you'd be much safer in what you're doing.

I'm not exactly sure if I'm on the same page with you but with the figure of 10ft /sec (or 600ft / min) and 20kts speed you'd set yourself up for trouble - unless you got a pretty stiff headwind blowing.

I think overall you never want to get into a position where you have so much momentum that it becomes the determining factor whether or not you commit to a landing: in my book that sounds like a loss of positive aircraft control. Normally you gradually bleed of airspeed while you decrease altitude and what started at 60kts (airspeed!) / 300ft altitude on your final approach should ideally terminate at 0kts / 0ft altitude on the spot that you planned to land on. Throughout the approach you can vary any variable at any time.

If you flare to bleed of momentum (e.g. to terminate an autorotation or Quick Stops) you'd have a much higher airspeed to begin with (where your descent rate does not matter as much) and you could either flare gradually for as long as you want over a long distance or commence a very harsh flare to shorten the distance required. All depending on a multitude of factors.

[topendtorque]

you might take a cue from our feathered friends, they seem to stop flapping, start flaring then when it all runs out of puff, hit the deck with only a few steps to slow down or float along a bit if it's a duck on water. after studying them for a while you should be the same, take to it just like a duck to water.
all the best. tet.

[sycamore]

Angel, I know this may sound/seem flippant,but really it is all down to the pilot`s brain `interfaced` with the hands and feet co-ordination to `compute` as a continuation,whether you will `make-it`,whatever `that` is,ie HOGE/IGE,touchdown,run-on,or gentle hover.The pilot should know/feel/assess/judge what he wants to achieve,and what he `feels` the aircraft can produce(with his inputs). Helos are not aircraft that are lined -up with a load of tarmac,flown at a particular speed,and at a certain height(usually reminded by a woman`s voice) close the throttle,and pull back the stick and `arrive`;It is called judgement,and if you `pigs-it` you go-around and try again,having learned from the first attempt;otherwise it will be cast as your `Judgement day`..Reality; it is eyeball and seat of the pants....(OK,performance parameters should have been checked,in case you thought I`d forgotten)....
Said from the armchair ,4000hrs helo,8000 F/W,and just finished a good bottle of red........

ed.you should have the `feeling` it`ll be OK/NOT by about 50ft/5secs before`it`..

[Matari]

Doesn't answer the OP's question, but interesting physics nonetheless:

According NASA Langley researchers:

Helicopter height was 35 feet. The flight path angle was about 33 degrees and the combined forward and vertical speeds were about 48 feet per second or 33 miles per hour.

5B2liYoHbNA

[switch_on_lofty]

Angelo,

I don't fly the b206 but like sycamore says, a lot depends on pilot skill etc. certainly in the lynx if you have 15kts airspeed on a still wind day you can pretty much stop within a rotor span. The limit to how quickly you can stop (in the lynx at least) depends on how much nose up (aft cyclic) you can achieve without entering autorotation (because as you flare you need less collective pitch to maintain height). This however is not going to happen at 15kts because the aircraft has less energy than at say 60kts.
Also consider how much nose-up you can have without striking the tail. Forward visibility is less of an issue if you can judge where you are out the side window.
the key point is to have an overshoot so that if you misjudge the approach for any reason you are not committed to landing.
What is the context of your question?

[topendtorque]

AngeloUMD (http://www.pprune.org/members/405966-angeloumd)

I think you need to address the sequence in reverse. First challenge yourself to use as little power as possible then see how much distance that takes if you wish, or where your sight picture of your approaching target would have been.

Helicopters by their very nature usually don't have surplus power for anything, so the the least power you use for everything the more employable you will be later to carry the most amount of load, either internal or external.


That means, learn how to bring the aircraft to a landing with the most amount of power margin unused and reserved for useful load.

Decelerate slowly, arrive with your rotor as high at half your rotors diameter (most efficient IGE hover height) as you gently pick up ground effect. Because you are still applying aft cyclic it will not accelerate forward but instead come to a hover where you then centralize cyclic (just prior this point measure your power) and then land.

Of course the take off technique is more important as you need to pick up the biggest load with minimum power and gently translate away to possibly a hot high destination where you may have to terminate your approach speed directly as you or your external touch down, your power margins having been eroded by the higher DA.

As far as physics is concerned, no need to get too deep, just learn how to operate the aircraft as well as if not better, smoother and more efficiently than others and you will be in good stead for future employment.

I often remark about one of our pilots once whom I swear never moved that damn collective a fraction as he lifted an external load at MAUW on a fifty foot strop and translated away, neither was there any discernible aircraft attitude change as he translated. He now flies big bells doing lots of lift work.

see - do it smooth and keep the f'n cyclic still, dead still.

cheers tet

[AngeloUMD]

Thanks guys,

I ran some calculation for the Bell 206 based off a report given experimental AH-1G airspeeds for stopping distance. Seems to check out with what you all had said. Appreciate the assistance, here's the link to the chart: https://docs.google.com/file/d/0B3d5_qZn7OYFUHN1M1RPMW50eEU/edit?usp=sharing

Cheers,

-Angelo