Vertical Descents


I suspect the confusion might lie in the terminology. If the students meant forward airspeed as being air that flows through the rotor from the forward motion of the disk, pushed or pulled, through the air (forward motion relative to the airflow this would include vertical descents) then I think they are right. Considering that the comments are in context of autorotation and helicopters I suspect they were referring to airspeed as the air flow over the blades, which in a helicopter can happen in two ways 1) the driven rotor pulls the air through or 2) the rotor moves through the air forwards relative to the airflow (autorotation). A helicopter can fly with no foward airspeed (hover) and the driven rotor provides the airspeed over the airfoil to create lift, without a driven rotor a helicopter can only fly if the rotor is moved forward (relative) through the air. So I think in that context you do need forward airspeed (relative to the airflow) to autorotate otherwise there is no air passing through the rotors. Also I think the term forward airspeed is just a way of referencing the direction of the relative to airflow so if there were no visual clues and the pilot was in an open cockpit he would have the wind in his face and it would feel to him (or her) that the aircraft is moving foward (even in a vertical descent the pilot would feel the wind coming from the front). You can be flying backwards relative to the ground but you are still flying forwards relative to the airflow.
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Senior Member
JAL, if you're saying what I believe you are saying, then you got it wrong. You CAN have autorotation without the gyrocopter moving forward with respect to the surrounding air. Just read the posts preceding yours in this thread.

-- Chris.


I dont think autorotation is possible unless there is air passing through the rotor disk and since the rotor is not powered the only way to get air moving through the rotor disk is to move the rotor disk through the air. In a vertical decent the rotor is being pushed (or maybe more correctly pulled by gravity) through the air, the air passes through the rotor blade which then spins the blade (I think it is the air passing through the rotor that drives the autorotation). So relative to the air the rotor disk is always moving forward. I cant see how else it can work, unless autorotation is completely self sustaining which clearly is not as you cant hover.

My point was that there may be a distinction between forward airspeed and airspeed when dealing with helicopters, because helicopters do not need forward airspeed but do need airspeed (that is the speed of the air passing over the rotor blade). There are two ways for a helicopter to spin its blades 1) engine power and 2) aerodynamic forces (autorotation) whereas the gyro has only one (autorotation). So when we refer to airspeed it would be forward airspeed in helicopters , an engineer might consider airspeed on helicopter as a how fast the rotor has to turn to fly and this rotational speed can be maintained by either engine thrust and/or forward airspeed (autorotation).
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Supreme Allied Gyro CFI
Perhaps you are using "forward" where you really mean "rotational" or "tangential".

In perfectly still air, no wind of any kind, a helicopter can autorotate vertically with no "forward" airspeed or groundspeed at all. It will have rotational speed on each blade, but that's not truly forward, since the direction of motion of a blade tip is constantly changing as it flies around the hub (sometimes forward, sometimes backward, sometimes sideways with respect to the airframe).


Platinum Member
Autorotation needs a force to power it. gravity will sufice
Mass moving through a fuid will find the less resistant path.


21st Century Crankhandler
In a vertical descent, the blade tips are doing 300mph+ horizontally, 15mph vertically
downward. Get your engineer friends to calculate the angle of attack seen by the blade
tips. If they cant do that, they ain't engineer material.
If the AOA is over 15degrees, they're right, and gyros cant do vertical descents.
Well, not the good kind, anyway. :)

Not everything is directly intuitive. I had difficulty wrapping my head around the fact that my machine has a very similar descent rate at 50mph, power off, and 0mph. But when you think it out, it makes sense.
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The problem is that engineers are taught about autorotation by viewing and subsequently pondering static diagrams that show a 2 dimensional airfoil with arrows emanating from it indicating L (for lift) and D (for drag) along with an accompanying description about how the resultant vector (usually described as "thrust" T) drags the airfoil around in circles - thereby resulting in autorotation.

It's not that easy to mentally segue from that static 2D textbook scenario to the very dynamic and entirely 3 dimensional situation that actually describes true autorotation. Also, if you think about autorotation in this purely bookwork context, it somewhat makes sense that the airfoil would not autorotate without forward velocity. And in actuality it won't - that is without some initial forward velocity in order to get the autorotation started first (i.e. prerotation). Remember that autorotation is a conditional phenomenon. Your gyro blades don't start spinning on their own (even with wind blowing through them) without some preliminary action to get them started in motion first. And once they're set into motion, action must be immediately taken to keep them in autorotation, which necessitates either a headwind or constant forward motion (while still on the ground). At this point, the students are correct. And in fact, up to certain non-flying rotor rpms, they remain correct.

Once in the air and the blades are well into autorotation, at that point the oncoming airflow through the rotor no longer has to come from a forward direction, it can also come from directly below (as occurs during a vertical sink). The confusion from the student's part is that the dominate forward velocity component over much of the rotor is now coming from the spinning airfoil element itself. When the student misses this critical observation, the student is missing the very essence of AUTO rotation. The blades at this point rotate by their own spinning action. They're absorbing the energy associated with the oncoming airflow from the vertical sink and converting that energy into LIFT, DRAG and rotational energy. The pitch of the blades is set at a sufficiently low (fixed) angle that the resultant airflow vector causes most of the airfoils to pull themselves around in a circle (unlike a helicopter with collective pitch which can quickly fall out of autorotative status without a rapid reduction in pitch). The rotor has transitioned from the static 2D static image on their textbook page stuck in their heads to the very real 3D (actually 4D) dynamic and self sustaining rotational status associated with true AUTO-rotation.

I would suggest telling the students that autorotation is just that; SELF sustaining rotation. And that autorotation is highly conditional. It will not happen if the rotor is stationary. It will not happen if the aircraft is flying too fast (i.e. where retreating blade stall and advancing blade compressibility halt normal autorotation). But as long as the rotor has in fact transitioned into autorotation and is flying within these conditional parameters, it WILL fly and sustain controlled flight, even in a pure vertical sink.
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I am just trying to say there is in fact two airspeeds to consider:

The first is the airspeed of the air traveling over the airfoil (wing) to create lift that holds the weight of the aircraft. This I imagine is the term used by all aeronautical engineers and is applicable to all aircraft that requires wings to provide lift. For rotorcraft this speed is the speed of the blade rotation and is fast (as as quoted above +300 mph)

The second airspeed which is applicable to rotorcraft (but not fixed wing) is the speed in which the air has to pass through the rotor disk to generate autorotation and is typically a lot less than the rotational speed that it generates (like say 50mph). (In a fixed wing the term I used as forward airspeed and airspeed over the wing are the same.)

In a gyrocopter I think these two speed are directly proportional to each other, the faster the rotor disk moves through the air the faster it spins and the term airspeed would really cover both as one can not occur without the other. In a helicopter this is not the case, because the rotor has two mechanisms in which to drive it 1) engine power and/or 2) aerodynamic power (autorotation). I am sure that the autoroational forces are still acting on the helicopter rotor disk in hover (reducing the amount of power required to spin the blades), just that for autorotation to be sustained air must be passing through the rotor disk. When using power the air is moved through the rotor disk, when there is no power the disk has to be moved through the air at a speed that is fast enough for autorotation. It is this moving the rotor disk through the air that I am calling forward airspeed, this is probably the wrong term and has some other name? I am just using the term forward airspeed (not knowing what else to call it) to mean the direction the air passes through the rotor disk. In a zero "airspeed" descent the relative air is still flowing through the rotor from front to back and relative to this airflow the rotor is moving forward.

The use of forward airspeed that seems to be the proper terminology (which I looks I have been miss using) referred here on the forum is in relation to the pitot tube and not the "rotor disk". A zero airspeed descent does not mean there is zero airspeed, just means that there is zero airspeed in relation to the pitot, the rotor disk still see the airflow and is in fact moving forward through the air at speed that allows autorotation which is what I call the rotor disk's forward airspeed (which is not the same as the pitot's foward airspeed which might be zero).
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C. Beaty

Gold Supporter
Slice off a ¼” section of a 1” wood dowel and attach to a stick with a nail as a propeller or windmill.

Hold it out a car window and it autorotates in whichever direction it is given a starting flip. If you know why, you understand vertical autorotation.


Lee Scatt

Crash-test pilot
Chuck, I thought I had autorotation figured out, and your little sketch's have always helped me understand your point.
But as Vance says, I am confused on a much higher level now.
In my vision, that thing would not autorotate. I'm off to the hardware store for a piece of dowel.

C. Beaty

Gold Supporter
Slice off a piece ~8”-10” long and make sure it turns freely, using a couple of washers between stick and rotor.

I anxiously await your vector analysis that explains why it autorotates equally well in either direction.


Platinum Member
Equal arms?
Autorotation, it seems to me, is only a designation, the movement is caused by some kind of power applied to the blades. (wind)
If you stick Chucks rotor out the window and don't give it a starting flip it will not rotate at all.

JAL, The wind going through the rotor is not the force causing auto rotation. Here is how it works. Lift is perpendicular to the relative wind. If we look at the relative wind we see that the rotor has an angle of attack that increases as we move from the tip to the center. That means that the lift vector will also rotate forward as we move toward the center. This forward vector of lift is the power that causes the rotor to turn. That is why we call this portion of the rotor the driving region.

When we start Chucks rotor turning with a sufficient flip we create a relative wind and an autorotative lifting force.
They were saying that RRPM could not be maintained at zero airspeed and that the RRPM would decay. They said the aircraft should drop.
If this statement is actually what they said, then I would say they understand it.

It appears that they fully understand that the aircraft needs to drop to maintain autorotation.

Just sounds like confusion to me, the discussion was probably at two different levels of knowledge. Same facts different terminology or different way of explaining it.
Karl, you could be right. Often in these type discussions the two sides are saying basicly the same thing, but due to terminology differences, are talking past each other.
But two points I take exception with (if they are related here correctly) are;
They were arguing with me that the physics of autorotation required forward airspeed.
This is definately wromg.

They were saying that RRPM could not be maintained at zero airspeed and that the RRPM would decay. They said the aircraft should drop.
In view of the previous quote, one must assume they are refering to horozontal (translational) airspeed. Translational airspeed in NOT required for autorotation.

The term "drop" inferes to me "free fall"! it don't! If they had said "sink", I could buy that.:eek:hwell:


Michael I completely understand what you are saying and I totally agree. But all I am saying is to sustain autorotation the disk must be moving through the air or air must be moving through the disk, if not then is stops. Now I know I cannot be wrong about this is because I observe this everytime I fly my gyro and if my gyro is not moving through the air the rotor stops spinning. The magic of autorotation is that it takes an airspeed of 50mph and converts it to +300mph, using the physics described by you. I am just stepping back to the more basic principle the rotor spins much faster than the velocity of the air the rotor is moving through. The fact it happens is impressive enough in my mind, but what you are saying without aknowledging what drives autorotation (I think of it more like the fuel of autorotation) is as close to perpetual motion as you can get, which it is clearly not the case because if you stop the disk moving through the air you stop the driving force behind autorotation and the blades stop spinning, it doesnt get simpler than that.

This why I keep banging on about two velocities , to me there are quite clearly two velocities involved in autorotation, the velocity of the air moving through the disk and the velocity of the rotation that it produces. I cant believe that I am wrong about this because it is not from some sort of theoritical understanding but a direct observation of what occurs every time I fly. I would have a guess and say when someone designs an airfoil for a wind turbine I that they would try for the fastest rotational velocity from the slowest wind velocity as they can. In a gyro my guess is that a popular design would be one that spins fast enough to produce maximum lift at the slowest forward velocity.

I also realise that the speed of rotation is a function of airfoil shape and angle of attack that is why I assume no one uses an airspeed indcator that uses the RRPM to determine airspeed (each blade would have a co-efficient which would describe its efficiency in converting wind velocity through the disk to rotational velocity but it might be different for each angle of attack). If one could be invented then you would never have to endure a "zero airspeed" descent again.

But to me the fundamental principle of autorotation is that wind velocity through the rotor spins it much faster than the wind itself and I am not sure why I am the only one that seems to think so. I also dont actually know what the proper definition of autorotation is, but this is just what I think it is. I accept that it may actually have some other name (windmilling maybe) and that technically autorotation may have a much narrower meaning that being the precise description of the force created by the lift vector of the airfoil being directed foward within the driving area of the blade that provides the extra rotational speed.
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C. Beaty

Gold Supporter
All gyros without translational velocity sink at a rate of ~25 x square root of disc loading. If the disc loading is 1.5 lb/ft², sinking speed is about 30.6 fps or 1900 fpm.

A purely vertical descent without translational airspeed is difficult to maintain and slight translation dramatically reduces sink rate. Most people won’t see more than 1500 fpm.

Rotor efficiency doesn’t play a significant role is vertical sinking rate. Draggy rotors don’t come down vertically much faster than low drag rotors.