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Helicopters can autorotate back to earth in case of emergency. The FAA recommends a flight profile similar to the following procedure:

enter image description here

Notice that in section (2) there is forward speed: vertical autorotation is not recommended. Why is that?

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    $\begingroup$ Well, the reference frame for lift in a helicopter is the inflow velocity at the blades, not the downward speed of the fuselage. So it is actual lift that is generated. $\endgroup$ – Koyovis Aug 15 '17 at 12:29
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    $\begingroup$ If you have no forward momentum to play with then you have no margin for error. If your descent rate gets too high while you are low to the ground then there is nothing you can do but fall. At least if you have some forward motion you can flare to trade some of that momentum for lift. $\endgroup$ – J... Aug 15 '17 at 14:18
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    $\begingroup$ Even quadcopter pilots are cautioned against this. Descending straight down puts you in your own vortex ring and you could lose lift or lose control. $\endgroup$ – Octopus Aug 15 '17 at 17:09
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    $\begingroup$ Why is vertical gliding in a fixed-wing aircraft not recommended? The answers are not entirely unrelated. $\endgroup$ – reirab Aug 15 '17 at 22:22
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    $\begingroup$ Apart from the physics/aerodynamics of airflow and energy management, what about the practicality of seeing where you are going? If descending vertically, your touchdown point is under your seat. Even with windows in the forward floor, there is still a blind spot directly underneath, isn't there? If you are moving forward you can see where you will touch down, at least until the flare; at that point you would be close enough to the ground that a view directly below isn't necessary, no? $\endgroup$ – Anthony X Aug 17 '17 at 3:13
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When the engine fails in a single engine helicopter it can autorotate back to earth: it gives up potential energy from reducing height to keep the rotor turning, which then continues to provide lift. Autorotation can take place straight downwards in vertical flight, but the general recommendation is to maintain forward speed, like a fixed wing gliding: the rate of descent is much lower if there is forward speed.

enter image description here Image source

We associate helicopters with hovering, but this is a strenuous effort. Even a bit of forward velocity generates translational lift, which makes it much easier for the rotor to generate lift:

  • The forward inflow tilts the lift vector forward, decreasing induced drag and blade torque, and increasing lift. Or stated in a different way: the airstream is flowing in already, and does not need to be accelerated from zero. The rotor now starts to work more like a fixed wing does.
  • The airstream through the rotor experiences less interference from the fuselage.

The effect of forward velocity on auto rotational rate of descent is depicted in a graph in this book:

Minimum descent rate as a function of airspeed

Going back to vertical autorotation: the vertical upward airstream needs to go through the rotor fast enough to reach the windmilling state, so that lift is created by decelerating the air passing through it. If we compute the lift component per area of a vertically autorotating rotor, it is comparable to a $C_D$ value of 1.1 to 1.2 referenced to the rotor area. According to this source:

  • a flat plate has a $C_D$ of 1.28
  • a parachute has a $C_D$ of 1.4.

So in vertical descent, the auto-rotating rotor is almost as good as a parachute of the same area - it’s just that the parachute size is a bit small for the typical payload of a helicopter, resulting in vertical descent speeds of between 3,600 to 6,200 ft/min. By way of comparison:

  • Air France 447 fell to the ground with a vertical descent rate of 10,000 ft/min.
  • Captain Sullenberger's plane took four minutes to reach the Hudson in a proper glide, from 2,060 ft altitude = average of 500 ft/min.

Two factors make autorotation much more survivable though:

  1. The descent rate can be “flared” by pulling the cyclic back (exchanging forward velocity for more rotor kinetic energy) and then collective up just before ground contact, slowing down the rotor and increasing lift. The pilot only has one shot at this.
  2. The descent rate is a lot less if autorotation is done at a forward speed, the best one being the optimum climb speed.

Note that in a vertical descend there is no pullback of the cyclic, only increase of collective - plus the rate of descent is higher. A higher speed and fewer means to brake, that is not recommended!

A side note

The windmilling rotor does not turn the other way! The incoming air from underneath tilts the lift vector forward, like in a glider. Part of the rotor disk is driven by the aeroforces into the same direction as it was when still under power.

enter image description here Image source

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    $\begingroup$ Nice answer, but I can't help disliking that graph. Due to the way it displays its axis, it appears to show the first quadrant but instead shows the second, so appears to show that the vertical velocity is higher when moving. Could be fixed by using positive values for descent, or just labelling the x axis at the top (y=0). Not to take anything away from this answer, it's just unfortunate. $\endgroup$ – Baldrickk Aug 15 '17 at 15:30
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    $\begingroup$ I think the keyword translational lift should be mentioned regarding the shape of that descent rate graph. $\endgroup$ – Jan Hudec Aug 15 '17 at 20:42
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    $\begingroup$ The point in 1. factor is that the kinetic energy due to forward motion is available for generating the lift to arrest the descent in addition to the (very little, unfortunately) energy stored in the rotor itself. $\endgroup$ – Jan Hudec Aug 15 '17 at 20:44
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There are three good reasons to avoid Vertical Autorotation.

First, Vertical Autorotation is way more likely to lead to a Vortex Ring condition where the air exiting the rotor disk gets recirculated back through the rotor disk. The recirculated air in the Vortex Ring then limits the amount of airflow/energy available for conversion to Rotor Disk rotational velocity which is critical to successfully completing that part of the Auto Rotation maneuver essential to survival (the flare up to touchdown).

Second, the forward velocity increases the amount of energy available for conversion to rotor disk speed.

Third, forward velocity allows landing shock to be shared between different harnesses and seat shock absorbers thus decreasing the load on any particular axis.

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    $\begingroup$ A simple way to understand the problem is this: To get lift, the rotor has to push air down. So there will be descending air just below the rotor. If you descend straight down, you're staying with that descending air, and it will carry you down with it. Flying forward gets you out of the descending air the rotor created so it can't push down on you as much. $\endgroup$ – David Schwartz Aug 15 '17 at 17:11
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    $\begingroup$ Here. I found a source for that: It is not possible to enter the vortex ring state whilst the helicopter is in autorotation. skybrary.aero/index.php/Vortex_Ring $\endgroup$ – TomMcW Aug 15 '17 at 19:30
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    $\begingroup$ @TomMcW: Even though air is moving upward through the rotors during auto-rotation, the helicopter is still going to be disturbing the air underneath it. Efficient lift generation requires entering undisturbed air. $\endgroup$ – supercat Aug 15 '17 at 21:20
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    $\begingroup$ @DavidSchwartz To be in a pure windmilling state, the helicopter has to descend downwards with quite a high velocity. Obviously that is not desired - VRS is the region between hover and windmilling, and in order to reduce the rate of descent as much as possible the pilot will stay as close as he can to the VRS region. There is usually a little induced down flow through portions of the rotor disk, although most of the flow will be upwards. ref $\endgroup$ – Koyovis Aug 16 '17 at 4:26
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    $\begingroup$ @DavidSchwartz If the air flows through the rotor fast enough, the lift is obtained by decelerating the air - outflow is in the same direction as inflow. $\endgroup$ – Koyovis Aug 16 '17 at 5:41
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Forgetting THEORY... a vertical autorotation IS possible and can be accomplished safely. To answer this question put to me by my Co-pilot that day in Vietnam, I took our H-Model Huey up to 5,000' and confirmed hover. I lowered the collective and chopped the throttle to split the needles and monitored rotor RPM to ensure no over-speed while keeping the cyclic centered for true vertical. I used collective to stabilize our sink rate. Once satisfied that the maneuver was safe, I recovered at about 1,000' by pushing the cyclic forward and recovering power.

We experienced no issues with vortex ring theory or any indication that this maneuver was dangerous. Had I needed to actually put it down, I would have initiated a forward cyclic aspect in order to perform a standard flare and set-down once below that 1,000' I chose. In Army flight school in the 1960's, we practiced every possible aspect of auto-rotations ALWAYS to the ground with NO power recovery at 50' nonsense. I personally believe it's irresponsible and insane to not train a helicopter pilot for full ground contact autos. What? Is God going to suddenly show up and help in that last 50' or so?

SIMS are NOT an acceptable substitute for actual practice. We did hovering autos, run-on autos, 180° and 360° autos, full-speed deceleration autos, but my all-time favorite was called "Spot Autos." The IP picked a spot on the runway - usually one of the numbers and I initiated an auto from pattern altitude and speed on the runway heading noting my steep glide angle through the chin bubble.

Keeping collective down and using only cyclic, I flared the chopper to an almost dead stop - closely monitoring rotor rpm and pulling collective when needed to prevent over-speed. Cyclic was pushed forward to start the wonderfully fast descent again - oh wee - we could get at least two and sometimes three flares on the way down - the last one was to a stop right on the number chosen and pull what was in essence a hovering auto to land.

We always completed all our autos to the ground with all of these maneuvers and they were truly FUN to do! Our IP's defined expertise and competence and did an excellent job of instilling those same characteristics into all of us, knowing that where we were headed we would need every training advantage we could handle. My two cents (1,955 combat helicopter flight hours in Vietnam slicks and guns).

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  • $\begingroup$ Thanks for sharing this, experience from the real world. A highly trained helicopter pilot like yourself would be able to land the ship in almost any situation, but would the FAA recommend anything else than the safest option? $\endgroup$ – Koyovis Jun 5 at 8:13
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You have to consider the power required by the helicopter in flight, represented by the red curve in the graph.

enter image description here

The blue line is the power provided by the rotor in autorotation. As you can see at zero airspeed the power requiered is much bigger than at 50, 60 or 70 kts. Therefore, you power deficit will be also much bigger at zero airspeed, resulting in an increased rate of descent.

Another advantage given by speed is the ability to flare (steps 3-4 in the flight profile recommended by FAA). During the flare you use the kinetic energy provided by forward speed to reduce your descent. With zero airspeed you completely lose this posibility, the only chance of reducing rate of descent remaining will be increasing the collective (and most likely it will not be enough to prevent a very hard landing).

A third reason would be that for the pilot it's easier to judge height above the ground if there is some forward speed, and also the helicopter is more stable directionally.

Bottom line, the preferred airspeed range for autorotation is the one shaded green in the graph, between minimum-power-speed and best-range-speed. There are situations where the yellow range is used (confined area, some procedures for night autorotation), but this area doesn't extend below 30 kts, anyway.

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  • $\begingroup$ Wonderfully illustrative graph. $\endgroup$ – Wayne Conrad Aug 20 '17 at 16:04
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During autorotation, forward speed is used to control rotor head speed. In auto, the engine is disconnected (in the case of tail rotor failure) or just plain dead, so it can't be used to control rotor speed.

In a vertical descent, the rotor head will eventually overspeed from the air rushing past the blades, and the rotor blades will depart from the aircraft. Not good.

The air hitting the blades from forward flight is used to induce drag on the rotor blades, slowing them down. The faster the helicopter moves forward, the greater the drag.

So the pilot, in auto, uses forward airspeed to control rotor speed. If the rotor head speed builds up too much, nose down a bit to gain airspeed and slow the rotor down. If the rotor head starts slowing down, then back off the forward velocity to let the rotor head pick up speed.

Forward flight also aids in maneuvering the helicopter when choosing a touchdown point, but mainly, it is used to keep the rotor speed in the green.

Every helicopter has an ideal autorotation forward flight velocity, that should keep rotor head speed in the green. In a Bell JetRanger, it's around 60 kts I believe, though this is a general guideline, not an absolute.

Theoretically, one could increase blade pitch to an overspending head to slow it down, but forward flight is a more precise way of doing this, and avoids the risk of inadvertently slowing the head down too much.

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  • $\begingroup$ In your last paragraph you say that in a vertical autorotation the rotor can be prevented from overspeeding - pull the collective up far enough and the blades will stall. Again something that it is much harder to do without any horizontal speed. And the descend speed remains frighteningly high even with the rotor at 100%. $\endgroup$ – Koyovis Aug 17 '17 at 3:36
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There are many wrong answers in this thread but I want to address one that any rated helicopter pilot should be able to answer (which judging by the answers in this thread may be not as true as I’m hoping) and that is:

YOU CANNOT GET VORTEX RING STATE IN AN AUTOROTATION

The three things you need for VRS or “Settling with Power” (FAA definition) are:

  • Rate of descent greater than 300fpm
  • Speed less than ETL
  • POWER APPLIED

Obviously if the engine has quit you cannot have power applied.

That being said, you do still have energy stored in the helicopter in the form of potential energy (in altitude above ground level) and kinetic energy (in the helicopters airspeed). Kinetic energy increases exponentially with airspeed i.e. if you double your airspeed (ex. 30 to 60 kts) you triple the amount of kinetic energy you can use to maintain your rotor RPM in the flare (that you will need a lot of to arrest the rate of descent and forward airspeed prior to touching down)

If you have no airspeed you have a higher rate of descent in the auto and less energy to arrest that rate of descent with prior to landing.

Answers to these questions and more can be found in the Helicopter Flying Handbook which is free to download from the FAA website!

-FAA RW Commercial Pilot and CFI-I

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    $\begingroup$ Kinetic energy increases quadratically with speed, not exponentially. If you double the speed, you have quadrupled the kinetic energy. Specifically, kinetic energy = 1/2 m * v^2. $\endgroup$ – reirab May 19 at 3:24

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