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I've been considering this question. I believe myself to be an average pilot but I'm embarrassed to say that I could not figure out the answer to the following question:

I flare and use power to maintain height, for example coming into the hover. As I slow, I must keep increasing power. I have enough power to hover out of ground effect.

If I attempt to continue maintaining the flare, attempting to come to a stop, I will sink and if I don't do anything about it, I will hit the ground. Why?

I've been trying to think through all of the key aerodynamic effects but keep hitting a brick wall.

P.S. Fixed wing pilots - if you don't have a good understanding of helicopters, please try to resist the temptation to guess. A helicopter in this flight regime is similar to a fixed wing in that it's a powered, heavier than air flying machine. End of list.

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  • $\begingroup$ @mins. I mean continue to maintain the flare attitude, i.e. nose pitch up. I've edited the question. Imagine holding pitch up and attempting to come to zero airspeed. Of course, it won't work. I'm trying to figure out why. $\endgroup$ – Simon Mar 5 '16 at 11:00
  • $\begingroup$ I don't think this is correct Simon. This may be due to your description, which I'm finding difficult to follow. Possibly by your description of maintaining the flare. You should just find you come to a stop in the hover, as mins pointed out. What are you trying to do in order to keep the nose up at that point? $\endgroup$ – Rory Alsop Mar 5 '16 at 11:15
  • $\begingroup$ @Rory. I'm not trying to do anything. I know perfectly well how to come to a hover (otherwise, I wouldn't be a pilot would I?). Are you a rotary pilot? If I try to keep back pressure on the cyclic to maintain nose pitch up, what happens and why? Of course, I am never going to test this out (well, maybe at 1500ft with a CFI beside me). $\endgroup$ – Simon Mar 5 '16 at 11:25
  • $\begingroup$ @mins, that's the problem. I don't know :) At the end of a quick stop, or coming to the hover, we level the pitch and checkup on the collective. I've just never questioned that, it's what we do! But what would happen if I used cyclic to maintain the nose pitch up (say 30 degrees) and collective to maintain height. I know I'll hit the ground. But what happens and why? So between let's say 10 kts after loss of translational lift and coming to 0 kts. That won't stress the blades and I'm not flying into my own downwash, although I guess there is some recirculation at the rear of the disc. $\endgroup$ – Simon Mar 5 '16 at 13:00
  • $\begingroup$ Simon - watch the second video on my answer. A beautiful example of doing it right - while very scary for spectators:-) $\endgroup$ – Rory Alsop Mar 5 '16 at 14:55
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If I attempt to continue maintaining the flare, attempting to come to a stop, I will sink and if I don't do anything about it, I will hit the ground. Why?

if you continue to maintain aft cyclic after the flare, you will hit the ground because some of the lift which is being generated will be used to fly the helicopter backwards, and you won't have enough power in to both maintain altitude and also maintain backwards flight (which is what you are commanding with aft cyclic). This is why we level off after the flare: to use all the lift in the vertical direction.

Its the exact opposite of take-off: if you are in a hover on the runway, and push the nose over to the 60KIAS nose-down attitude, you will descend into the runway.

Although I have never tried it, I imagine that at the peak of the flare, if I add (a lot?) of power without changing the attitude (much:), I will not descend into the runway, but rather I will start to fly backwards.

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    $\begingroup$ Its the exact opposite of take-off - doh! Light bulb moment. Thank you! That last paragraph sounds like fun. Next time I have a CFI next to me.... $\endgroup$ – Simon Mar 18 '16 at 8:23
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As I have military pilots in my family i thought I'd ask one who has flown many helicopters in combat and S&R situations. Here is what he told me:

In normal forward flight, the disc is slanted forward, so some/majority of the thrust is downwards, but some is backwards to counteract drag or accelerate. When you decide to slow, the disc is tilted backwards so now some of the thrust is forwards.

What also happens is that the relative airflow that was impinging on the top of the disc in forward flight is now impinging on the bottom of the disc as you decelerate.

In forward flight this flow is a thrust penalty because it means that the change in downward flow through the disc by the rotor blades is more limited. ie the air is already flowing down and the rotor blades find it harder and harder to shove it any faster. They would need to rotate faster and faster to get any lift and then hit supersonic compressibility problems. (This is why modern blades tend to be funny shaped like Lynx/Merlin etc)

Same problem propellers have on fixed wing. There is a speed at which they just can't propel the aircraft any faster.

Conversely, when the disc is tilted back to flare, the airflow is all bonus. It is known the rotary aviation as the green arrow and what allows autorotation.

I have zoom climbed hundreds of feet upwards whilst assaulting targets in full autorotation with the collective lever full down and essentially nor requiring engines at all.

The upshot of this is that as you transition from normal forward flight by tilting the disc back you need to lower the collective to maintain level flight or you will zoom climb.

However, you don't get something for nothing. You are trading the forward energy of the whole helicopter for rotational energy in the head.

Eventually you slow to a stop, and at this point you require engines to sustain the rotation of your rotor system, and if you have maintained a lot of nose up it is a very significant amount of power.

The power required curve for helicopters is a u shape with most power being required at hover and max speed and min at about 70kt in most helicopters. The upshot of this is as your speed tapers off you need to steadily lower the nose or can get bitten hard by the power requirement when the forward momentum ceases especially if you are flying an underpowered toy which almost all hobby helicopters are.

This thrust deficit is exacerbated if the nose is held high because not all the thrust is supporting lift, some is accelerating you backwards.

https://m.youtube.com/watch?v=qMDIN_R0e68

This is an example. The pilot is coming in too hot and holds the nose very high until a very late stage. He then let's a rate of descent build and doesn't have time to arrest before hitting the ground. Hovering out of ground effect, even military aircraft don't have a lot of spare power, so it is important to correctly judge yeeha flying manuevers and throw them away before the hover if screwing up. These guys should have thrown it away at about the moment the video starts. I have a feeling that they are approaching downwind as well which makes it all twice as bad.

https://m.youtube.com/watch?v=jGMCiDXeCF8

This is it done properly. He simultaneously pitches the nose up and enters auto. You can see the relative aspect of the disc to the airflow switch from slightly downwards to very much up through the disc. As the speed bleeds off he is progressively raising the lever to maintain height as he runs out of forward energy, then as he approaches the hover he pitches back nose down.

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  • $\begingroup$ Thanks Rory, but sadly, this doesn't answer my question. I know all of the above, it's just basic rotary flying and that second video is just a plain old quick stop. It's my fault, I clearly can't explain what I'm trying to understand. Let me try it this way. Imagine he didn't level off at the end of the quick stop, he would descend until he hit the ground, not matter how much collective he pulled. I'm trying to understand what happens. Is in an overpitch? Is it rotor droop? Do the blades stall? The title of the question is the nub of it. $\endgroup$ – Simon Mar 5 '16 at 16:15
  • $\begingroup$ Is that not covered off by that power paragraph and the one on thrust deficit? $\endgroup$ – Rory Alsop Mar 5 '16 at 16:38
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    $\begingroup$ Because if you keep your nose pitched up, you must surely start to accelerate backwards - but that means you require much more thrust, as a good portion of it is pointed forwards. If you have enough power this is not necessarily a problem. $\endgroup$ – Rory Alsop Mar 5 '16 at 18:25
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    $\begingroup$ "You are trading the forward energy of the whole helicopter for rotational energy in the head. Eventually you slow to a stop, and at this point you require engines to sustain the rotation of your rotor system, and if you have maintained a lot of nose up it is a very significant amount of power." < This answers the question entirely. $\endgroup$ – Sanchises Aug 20 '17 at 21:11
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If you continue to hold flare attitude, and start applying power in same attitude, you will settle with power (vortex ring state). You will lose control of your helicopter and even if you yank full collective lever... you will hit ground tail down.

Worse, in case of actual power plant failure and auto recovery, if you continue to hold flare attitude.... rotor RPM will decay rapidly and you will lose control power of disc. Hence you will hit ground tail down.

Tail down attitude impact may not be safe for all helicopters, as this leads to tail rotor loss..... hence vicious yaw in powered recovery!

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