# What is driving the rotor?

#### ferranrosello

##### Member
Right now, and as I had promised in Understanding rotor rpm thread, and because of some disagreements between some gyro pilots and other people posting in that thread I will try to explain the mechanics of autorotation in the simplest way… Of course, this does not mean that I’m going to succeed.

Initially we are not going to enter in the complexities of the real aerodynamics acting on the blade. We are going to replace them by single forces showing the final results we can see on rotor rpm.

Instead of posting everything in a long and complicated text, I will divide the explanation in smaller steps, easier to follow. It is absolutely necessary to understand and agree every step to be able to understand and accept the new ones. Because I have not a lot of time available for posting, it will take me some days to put all the information on the thread. Of course, if you find that something is wrong, or that something is not understandable, please, tell it to me. Learning by the discussions is one of the qualities of this forum…

1st Step. The driving force.

We are going to start by the results. For a specific gyrocopter, (fixed weight and flying in a fixed air density) sometimes the rotor spins faster, and other times spins slower.

Without discussing the reason for this behavior, it is obvious that something is driving the rotor blades. And because of its rotation, the rotor has to overcome a resistance force, a drag. This drag will be bigger when the rotor is spinning faster and smaller when the rotor is spinning slower. If the rotor is stopped there is not drag to overcome.

The drag that the blade has to overcome when it is rotating is a force. And the only way to overcome a force is with another force. Then, just because we can see that a gyro rotor spins, there has to be a driving force acting on the blade, that overcome the drag force. We are going to call it “Driving Force”.

It is obvious that in the prerotation this force is produced by a mechanical device. But once the gyro starts flying this is not the case anymore. This driving force will have an exact magnitude to overcome the drag. If the driving force is bigger, then the rotor blade will accelerate its spinning speed. This will produce an increase in the drag. The rotor will reach a new balance in which driving force = drag at a faster rotor rpm.
If the driving force is reduced the rotor rpm will decay until reaching a new balanced state.

Then, the first agreement we need to reach is that it is driving force acting on the blade what keeps the rotor spinning, and that rotor rpm are totally linked to the value of this force.

Is there any disagreement with that?

Ferran

#### ferranrosello

##### Member
2nd Step. What is the Gliders driving force?

Do you know why a glider or plane without power is flying forward?

It is obvious that the gravity force, the weight, is pulling it vertically downwards. Probably we think that this is a very intuitive and natural thing that does not require more explanation.

But, Is it really clear what is the origin of the force that keeps it moving forward while descending? What is the difference between this gliding plane and a falling stone or a ball?

Is anyone willing to answer these questions?

Ferran

#### Illini85

##### Active Member
This might be a good place to insert the following video link

Leading edge vortex stably attached near the base of a freely flying maple seed Acer pseudo-platanus - YouTube

In my effort to understand the how's and why's I envision the votex created over the leading edge to cause a low pressure area that is the source of lift and a component of that gets translated into rotational energy and hence autorotation, the speed of which is the resulting balance of all of the associated forces. My engineering credentials are listed here___________. So I suppect that I may be completely wrong.

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#### ferranrosello

##### Member
Illini, it is not a question for engineers. Obviously what you say make sense, but you have gone much further to the specific question of the glider...

Do you really believe that is the aerodynamic force acting on the glider what provides the forward movement?

Ferran

#### Illini85

##### Active Member
At the risk of showing my ignorance here goes. A component of the force against the wing is directed forward as the angle of attack is reduced. Literally the craft falls forward.

#### WaspAir

##### Supreme Allied Gyro CFI
A glider wing is going forward while descending slightly. The relative wind is not horizontal, but tilted up slightly because of the descent. If we use the convention that "lift" is perpendicular to the relative wind, then the lift vector is not vertical, but tilted forward, and drag is aligned with the relative wind. In steady state flight, with all forces balanced, adding those components must produce a force equal and opposite to gravity (which is truly vertical). As the glider descends, potential energy that it has from its altitude in the gravitational field is slowly lost, providing the work done in moving against the drag force. Gravity provides the power, drawing energy from altitude, the pitch of the glider directs the lift and drag vectors to maintain balance against gravity at a given speed.

If you wish, you could always resolve the gravity vector into a component perpendicular to the relative wind (balancing lift) and a component parallel to the relative wind (opposing drag), but that's really the same thing.

#### hillberg

##### Active Member
Hamsters & Gerbles ! Yes that's it to a T !

#### ferranrosello

##### Member
A glider wing is going forward while descending slightly. The relative wind is not horizontal, but tilted up slightly because of the descent. If we use the convention that "lift" is perpendicular to the relative wind, then the lift vector is not vertical, but tilted forward, and drag is aligned with the relative wind. In steady state flight, with all forces balanced, adding those components must produce a force equal and opposite to gravity (which is truly vertical). As the glider descends, potential energy that it has from its altitude in the gravitational field is slowly lost, providing the work done in moving against the drag force. Gravity provides the power, drawing energy from altitude, the pitch of the glider directs the lift and drag vectors to maintain balance against gravity at a given speed.

If you wish, you could always resolve the gravity vector into a component perpendicular to the relative wind (balancing lift) and a component parallel to the relative wind (opposing drag), but that's really the same thing.
That is exactly the point.

2nd Step
The gravity force is what drives a glider. Is the component of the weight on the glide path what pushes the aircraft forward. This is similar to what happens to a car in a slope, it is able to run because of the weight.

The main difference between a gliding plane and a falling ball is that the plane flies, and the ball falls. The aerodynamic forces acting on the glider wing balance the weight. The result is not a fall, it is a flight. The gravity provides the energy required creating these forces, it is the glider driving force: it is the engine.

The result is a glide path which implies always a rate of descend. It is not possible to glide without a rate of descend. The rate of descend is a measure of the power required to fly. If we introduce some power then the rate of descend will be reduced, showing the difference between the power provided by the engine and the power required for flying.

They can be replaced by a mechanical force coming from an engine, and this is the only way to get a leveled flight. In the case of the falling ball is the parasite drag what balanced the gravity force in the vertical fall.

Then, the second agreement we need to reach is that the gravity is the driving force in gliding plane. It is not an aerodynamic force what drives the glider forward: it is its weight.

Is there any disagreement with that?

Ferran

#### NoWingsAttached

##### Unobtainium Member
I finally grasped autorotation of rotorcraft by making a foam airfoil, or wing, of about 8 inch cord, about 1 foot long.

I'd studied (after getting interested in flying gyrocopters) about lift forces acting only in the perpendicular to any wing surface, which is what produces forward "pull" on any airfoil, as opposed to drag. I only have a rudimentary knowledge and yeoman's understanding of all this.

Wish I'd made a video of the experiment, it might be helpful to demonstrate just how this works.

Holding the wing perfectly flat, I dropped it over and over. Each and every time it dropped straight down until airflow over the surfaces acted upon the wing and eventually PULLED the unit forward. In order to convince myself that the air was not being redirected to the rear, and that this was not some force PUSHING behind the balance point of the wing and/or directing the wing to tilt down at the front and thus direct the unit forward, I added a degree or two of UP to the wing before dropping it. It still shifted and got PULLED forward. I kept adding tilt, leading edge up, until I reached a stall point, whereupon the wing fell fluttering to the floor straight down. At no point can you get the airfoil to fall backwards at all. It either falls controllably and goes forward, or it stalls and loses all semblence of flight. Indeed, the lift acting upon the sufaces forward of the fat part of an airfoil are what makes autorotation work, with speed of the finite differential areas of the disk interacting to overcome drag and opposing lifting forces to move the rotor fast enough in the forward rotational direction to fly the gyrocopter safely.

Adjusting disk AOA which has a fixed blade pitch is what makes the entire aircraft fly in a controllable manner.

Is any of this what you were looking for in your thread?

#### JAL

##### Member
Ferran I will bite.

I think I know where you are heading and I will be first to have a crack and I think it is going to be counter to your logic.

It is airspeed and AoA that allows the glider to fly. The weight is irrelevant, weight provides the energy but that isn't the reason it flies.

A glider will climb if it is in a thermal because the energy of the thermal exceeds its weight. That is the airspeed and AoA increases and produces more lift.

The rotation of the rotor blades is a function of airspeed and AoA not weight. The driving section of the blade that results in autorotation is the section of the blade where the lift vector is tilted forward of the chord as opposed to the lifting area of the blade where it mostly perpendicular to the chord. lift is created by airspeed and AoA.

a rotor will produce x amount thrust at y AoA and z airspeed. No weight variable there.

Weight only dictates the energy required for flight not for flight itself.

Jordan

#### bryancobb

##### Junior Member
A glider doesn't FLY...It's "Falling With Style!"

#### ferranrosello

##### Member
Gerg, in your experiment you have the blade falling in a very deep attitude. When the speed builds up it starts flying, and the aerodynamic force acts vertically on the blade making it to change its nearly vertical path to a forward one.

But it is not the aerodynamic force (which acts vertically) what drives the airfoil forward. It is the weight.
It is airspeed and AoA that allows the glider to fly. The weight is irrelevant, weight provides the energy but that isn't the reason it flies.
Airspeed and AOA lets the glider to create aerodynamic force, but the weight is not irrelevant. Without weight you will have not forward movement, that is to say that no airspeed will be built and no flight.
A glider will climb if it is in a thermal because the energy of the thermal exceeds its weight. That is the airspeed and AoA increases and produces more lift.Sorry, I agree with your first statement, but not with the second. The airspeed and AOA in not accelerated flight are dependent only on the weight. And the lift produced will be the same than the one produced when flying outside a thermal.

The rotation of the rotor blades is a function of airspeed and AoA not weight.It is function of the weight too. You cannot put any AOA with any airspeed, only a combination the produces a lift force equalizing the weight. Everything in flying is related to the weight. The driving section of the blade that results in autorotation is the section of the blade where the lift vector is tilted forward of the chord as opposed to the lifting area of the blade where it mostly perpendicular to the chord. lift is created by airspeed and AoA.I´m not still speaking about this complicated aerodynamic system, and it has no sense to enter in this kind of discussion if we are not able to fully understand and agree the much simpler mechanics of a fixed wing glider.

a rotor will produce x amount thrust at y AoA and z airspeed. No weight variable there.I have stated at the beginning of this thread that we are talking about steady flight conditions. Once more: You cannot put any AOA with any airspeed, only a combination the produces a lift force equalizing the weight. Everything in flying is related to the weight.[/

Weight only dictates the energy required for flight not for flight itself. Jordan
Please, Jordan it is very important to clarify that without weight there is no flight. If necessary we can stop here until this question is fully accepted.

Ferran

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#### WaspAir

##### Supreme Allied Gyro CFI
A glider will climb if it is in a thermal because the energy of the thermal exceeds its weight. That is the airspeed and AoA increases and produces more lift.
Airspeed does not increase in a glider in a thermal, nor does the AoA. The lift produced by the wing is constant, exactly matching your weight. Instead, the airmass through which you are moving rises with respect to the ground and carries you with it. If you keep the same control inputs, you continue to descend through the air around you at the same airspeed, lift, drag, and AoA whether you fly through still air, rising air, or descending air.

Imagine a public place with an up escalator, a staircase, and a down escalator all parallel to each other, as you might see in many public buildings.

A glider is always walking down the stairs, as if you are taking steps at a steady rate and with a steady size of stride. A fixed staircase is like still air. If you shift sideways from the stairs to an escalator that is rising quickly (like a thermal), you will be walking down an "UP" escalator, still stepping downwards at the same rate with respect to the steps, but going upward with respect to the bottom floor because the steps themselves are rising. If you shift sideways onto an escalator that is going down (like sinking air) you will still be stepping at the same rate but going down quickly as if walking down a "DOWN" escalator.

In the image, you have a thermal on one side, sink on the other, and still air inbetween. The glider would "walk" down each of these at the same stepping rate.

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#### ferranrosello

##### Member
3rd Step. What is the true meaning of descend?

That is a perfect and clear explanation. Waspair. I want to address a less academic explanation on this issue. It is a crucial point for the next step, that will be no other to understand than a gyro rotor blade it is always gliding, that is to say, descending driven by the gravity force.

However, by now I´m going to return to the glider explanation, in order to fully understand that what is driving the glider forward movement is always its weight.

But If the gravity is driving a glider forwards, what is driving a glider when it is climbing in a thermal? Then the vertical glider track is tilted upwards, not downwards, and there is no gravity driving component on this flying path.

The answer to this dilemma is that it is not a dilemma, is a question of mixing two different reference systems. We are used to think in the Earth surface as our main reference system, and we are relating nearly everything of our flight technique to the horizon, that’s to say to the earth surface. And this is correct sometimes, but not always.

The correct reference system for any aircraft is the air, the small atmosphere bubble that surrounds it. So the gliders track related to earth does not matter. What it is really significant is the glider path related to air that is surrounding it.

When we say that a glider that is climbing in a thermal, we are using the bad reference system to analyze the glider flight. The good reference system is the air mass inside which it is flying.

And in accordance to the air reference system the glider is descending, not climbing.

Imagine that you launch a paper glider which descends at 50 fpm inside a building. In this case both reference systems are the same and you can see directly the real plane’s glide path.

Now we are going to do the same experiment launching the paper plane inside a big transparent elevator that is climbing at a speed of 300fpm. Because we are in the elevator we are seeing the same glide path (the 50fpm one), that is the real one. But for anyone that is watching this experiment from outside the elevator (we can suppose that is provided with glass walls) the paper plane will be climbing at 250 fpm rate...

What I want to remark that the relevant reference system for any flying thing is the air, not the ground. So any aircraft and any airfoil are descending when they are descending in relation to the air.

And any gliding aircraft it is always descending, and the driving force is always gravity. It does not matter the flight path in relation to the ground. It is only the air related glide path the relevant one.

Ferran

#### bryancobb

##### Junior Member
Ferran,

What's Driving the Rotor? was your title.

Looking closer...Where's the origin of the energy?
* On a helicopter in a hover -- The fuel contains the energy
* On a helicopter in a level flight -- The fuel contains the energy
* On a helicopter in a autorotation -- The altitude+mass has potential energy
* On a gyro in level flight -- The fuel contains the energy
* On a gyro in no power autorotation -- The altitude+mass has potential energy

Looking closer...how is the energy transferred to the rotor?
* On a helicopter in a hover -- The engine twists the mast
* On a helicopter in a level flight -- The engine twists the mast
* On a helicopter in a autorotation -- The fall makes air flow around the blades which creates aerodynamic forces that drive the rotor
* On a gyro in level flight -- The engine pushes or pulls the gyro through space and makes air flow around the blades which creates aerodynamic forces that drive the rotor
* On a gyro in no power autorotation -- The fall makes air flow around the blades which creates aerodynamic forces that drive the rotor

Weight doesn't have any inherent or "pent-up" energy like fuel or explosives do. Weight (or more accurately, mass) only develops energy when it is forced into motion or dropped from a height.

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#### GyroDoug

##### Active Member
I have not chimed in here and tried to share my understanding of the topics being discussed because I understand that I am a student and not a master in this area. However I want to chime in now and say "Thanks" to those that are sharing their understanding with us, because I feel I am learning and understanding this subject much better due to the explanations being given. So, Thanks again! and Keep it up!

#### Vance

##### Gyroplane CFI
Thank you Ferran!

Thank you Ferran!

I feel you have an interesting approach to this potentially controversial phenomenon by defining your terms and not letting the less astute redefine terms or pretend you have defined something in a way you have not.

I was asked this question many times at the Cable Air Show and felt I was not up to the task.

The concept seems simple enough and yet I often failed to communicate because I could not find terms simple and clear enough to overcome preconceptions and a misunderstanding of simple terms.

If you succeed I may simply refer those confused about semantics or imagining they have the answers to this thread.

I do not have your patience.

Mass at altitude seems like a lovely place to store energy to me. It took energy to elevate the mass so why not store energy there?

I feel you are doing well so far; I hope you are able to prevail.

Good luck, Vance

#### Rotor Rooter

##### Dave Jackson
Please, Jordan it is very important to clarify that without weight there is no flight. If necessary we can stop here until this question is fully accepted.

Ferran
If 'flight' is defined as remaining airborne then weight is not required.
In fact, a weightless object, should such an object exist, could stay airborne forever.

Here is an example of 'lighter-than-air-flight'.

Perhaps the definition of' flight' should be first item to be defined.

Dave

#### WaspAir

##### Supreme Allied Gyro CFI
Fair enough.

I inferred that the initial intention here is to discuss 1-g wing-borne heavier than air flight, to distinguish it from (1) buoyant lift lighter-than-air "flight", (2) rocket-driven (wingless) "flight", and (3) "weightless" maneuvers, when held at zero-g in ballistic trajectory, in which the wings do not carry a load. Other g-load conditions could perhaps be addressed later.

Note that "wing-borne" could encompass both fixed and rotary, unless otherwise specified.

#### WaspAir

##### Supreme Allied Gyro CFI
I may be putting words in his mouth, but I thought Ferran's effort was to describe the source and nature of the force that drives the rotor. Any force, when moved through a distance, will do work (transfer energy), but I don't think that's what he's getting at, or disputing.

P.S. This was a response to a post that apparently has been deleted, so pardon if it doesn't makes sense here now.

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