Talk:Autorotation

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Most Lift

Is there any reference for the "produces the most lift" part of this claim? It seems reasonable enough, but I wasn't able to find any mention of it in any of the references.

The driven region, also called the propeller region, is the region at the end of the blades. Normally, it consists of about 30 percent of the radius. It is the driven region that produces the most lift, but also the most drag. The overall result is a deceleration in the rotation of the blade.

--Klee Dienes 01:48, 13 July 2007 (UTC)

Good catch. It should probably read more like that the driven region is most responsible for the slowing effect on the rate of descent of the helicopter rather than the most lift. It is the angle of the TAF up and to the rear that slows the descent as the aircraft moves down and forward. The percentage of the driving region would mean that it is producing the most lift and the angle forward drives the rotor system allowing the driven region to provide the slowing of the rate of descent. --Born2flie 09:54, 15 July 2007 (UTC)

Rotorcraft

Considering that the autogyro is also a rotorcraft which utilizes autorotation, I think this article should be expanded and balanced to discuss the autogyro's use of autorotation, and that the article should then be moved to Autorotation (rotorcraft). --Born2flie (talk) 18:54, 23 September 2009 (UTC)

• When a helicopter is helicoptering, it is not autorotating! When a helicopter stops powering, that is, stops helicoptering, then in fall the device becomes a gyroglider under the effects of autorotation. The article has a misleading name. I vote for new article Autorotation (aerodynamic) so that gyrokites, gyrogliders, engine-off helicopters in glide mode, autorotating blades, windmills, and the like can be rightly treated. Please change to a new name and delete the extant name. 68.123.235.29 (talk) 22:47, 5 February 2014 (UTC)

Requirement of certification

The words "all helicopters must demonstrate [autorotation] capability in order to be certified" are not sourced. I understand that helicopters with counterrotating blades are not capable of autorotation, yet they are still certified and flown. If anyone knows about this, please respond. 97.125.51.63 (talk) 00:29, 12 March 2010 (UTC)

I agree that the statement regarding autorotation and certification must be sourced. I believe §27.71 of the US Federal Aviation Regulations is a suitable source and I will cite this in the article.
Helicopters with counter-rotating blades are equally capable of autorotation. The phenomenon of autorotation is related to the angle to which each blade can be moved so that air flowing upwards through the blade disk generates aerodynamic forces that drive the blades in the desired direction rather than slowing them and driving them in the undesired direction. Autorotation is not related to the number of blade disks or, if there is an even number of disks, whether the disks are counter-rotating.
For the purposes of certification, safe autorotation must be demonstrated by single-engine helicopters and some mulit-engine helicopters (known as Category B helicopters). For the most capable multi-engine helicopters (known as Category A helicopters) autorotation is not necessary because it is assumed failure of both engines will never occur in flight. When one engine is inoperative, the other will still be operating. The helicopter may not be able to maintain height, and it may drift down towards a forced landing, but power will be available to the rotor and the resulting flight is not autorotation. Dolphin51 (talk) 01:49, 12 March 2010 (UTC)

Good job on such a quick edit (I was surprised). And I personally appreciate the detailed explanation here! 97.125.51.63 (talk) 02:15, 13 March 2010 (UTC)

Physical mechanism, differences between helicopter and autogyro

The article lacks a clear explanation of the physical principle. In particular, *no* article about autorotation and autogyros I found so far (neither in Wikipedia nor anywhere else) could answer the question whether the angle of attack is inverse to that of a helicopter (and thus must be inverted and not just reduced during the transition from powered rotation to autorotation) or not, and if not, why not. Consider, for example, a Christmas pyramid. Similar to an autogyro the rotor is powered by upwards flowing air (causing a clockwise rotation of the pyramid in the first image). However, a powered rotation in the same direction with similarly angled wings would force the air to flow upwards, resulting in a downwards force (i.e. the exact opposite of an uplift). A rotor spun to create uplift would, once the engine stops and the rotor rotates free, first stop to rotate and then rotate in the opposite direction.

In other words: To keep the rotor running by vertical updraft, the angle of attack has to be inverted. Since air flow is converted into rotational energy, this would surely slow down the descent, but not cause an uplift. However, this simple model ignores the effect of the horizontal component of flow. Due to the angle of the wole rotor the net force will have a vertical component sufficient to lift the whole autogyro (or to slow down the descent of a poweless helicopter to allow a safe landing). This is my interpretation as a non-expert on aerodynamics. However, without citable sources this would be OR. Could anyone confirm or falsify this explanation and give reliable sources?--SiriusB (talk) 21:55, 2 April 2012 (UTC)

It can be a complex subject and the concept of the autogyro can look like black magic. In helicopters and autogyros the aerodynamic force on the rotor has an upward component that supports the weight of the machine; therefore the angle of attack on the blades is similar and not inverted as you have suggested. (The aerodynamic force is mostly lift, but also partly drag.) If the aerodynamic force on each rotor blade is parallel to the rotor shaft there is no resulting torque and the rotor will continue to turn at constant speed without any input from an engine. However, in a helicopter in level or climbing flight the aerodynamic force on the rotor has a component that points "backwards" relative to the rotor shaft. This produces a torque that will slow the rotor unless an equal and opposite torque is applied to the rotor shaft by an engine. In an autogyro (or in a helicopter in unpowered descent) the aerodynamic force on the rotor has a component that points "forwards" relative to the rotor shaft. This produces a torque that will accelerate the rotor until the drag increases to such an extent that the aerodynamic force is parallel to the rotor shaft and the rotor turns at constant speed.
To comprehend autorotation in a helicopter, and level or climbing flight in an autogyro, it is necessary to see diagrams of the vectors and the axis of rotation applicable to the rotor. Any technical book on autogyros or helicopters will have suitable diagrams and explanations. I am presently looking at Aerodynamics of the Helicopter by Alfred Gessow and Garry C. Myers Jr (1952), Frederick Ungar Publishing Co., New York. Chapter 6, "Autorotation in vertical descent" is particularly relevant. Have a look in your local library for this book, or one similar. Dolphin (t) 22:57, 2 April 2012 (UTC)
I think the issue is about making an understandable explanation of autogyro lift without using the confusingly misleading words of "air flowing up through the blades" on page 16-1 in FAA-RFH. All non-bouyant aircraft must accelerate air downwards to maintain altitude according to Newton's laws of motion, until someone invents anti-gravity. Some say that the full explanation is not easy to understand, but we should try. TGCP (talk) 14:57, 3 April 2012 (UTC)

Thanks so far. The next step should now be to find a description that is understandable to the average reader of the article. A first step might be the analog with a gliding airplane: Its wings do also have a positive angle of attack and would feel a backwards force (besides a huge drag) if falling down vertically at a fixed angle (unphysical for real planes, but possible to demonstrate with a static model in a wind tunnel). Another analog may be a sailboat sailing against the wind (I don't know the English term; in German it is called "kreuzen" (linear translation would be "to cross"; it may be the maneuver described in section Beating or "working"). The boat is heading at an angle towards the wind changing its direction alternatingly to keep a net travel in the desired direction. But I am not a sailor. Additional note: On page 30/31 (or "3-9" and "3-10") in the FAA-PDF you linked to are diagrams which suggest that indeed air is flowing upwards though the rotor (relative to the rotor, of course), but its flow is slown down by the rotor (an external observer would indeed notice the initially static air accelerated downwards). This change of momentum causes the uplift.--SiriusB (talk) 08:07, 4 April 2012 (UTC)

I think there is a significant difference between a helicopter autorotating down and an autogyro moving straight-and-level forwards, and if so, the article should make that clear. Chapter 3 is about helicopter, chapter 16 is about autogyro. I think you are right about comparing with a fixed wing, but the sail comparison might confuse unnescessarily. I think we should leave the explanation to knowledgable experts (or at least gain understanding first), as it seems to be a complex subject. TGCP (talk) 08:35, 4 April 2012 (UTC)

V-22 Osprey claim

The artlicle starts by definining autorotation as something found in helicopters or other similar vehicles such as the V-22. However, the V-22 cannot autorotate as confirmed by its own article. I appreciate the example has been given here more to illustrate that it doesn't only have to apply to helicopters, but could we think of something else to put here? — Preceding unsigned comment added by 79.13.98.136 (talk) 06:53, 28 April 2013 (UTC)