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how exactly do autos work?


Jon

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well, how do autos work? how does it actually keep the helo from falling out of the sky? does it just use the inertia from the blades since they where already spinning? but if it started to descend wouldent the air comming up though them slow them down to fast? ive read that it uses the air comming up from under them to keep them spinning, but wouldent that spin them the wrong way? still learning alot about helo's and this would help, thanks!

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Jon,

 

Of course this is difficult question to answer simply and without going into all the aerodynamics.

 

However, here is an attempt to give an answer appropriate to the level of question with the help of some well known diagrams and a little imagination. It will take a novice a couple of reads through to understand.

 

Initially, the inertia is needed to keep the blades spinning. This is because after, say an 'engine failure' there is a moment when the pitch is still high. Here the blades are slowing down. Once the pitch is flat, the drag is reduced. As well as that 'autorotation' takes over to keep the blades spinning. Here's how.

 

To demonstrate the concepts, I want you to cut a 1-foot section of a helicopter blade.

 

Get a broom handle and stick it into the ground. Next, using a hammer, nail the blade section horizontally onto the end of the broom handle (to form a sort of T shape). Angle it a few degrees upwards just for good measure. This is the 'test-wing'. Next get a hair dryer. This will provide the airflow.

 

The first demonstration is to stand in front of the blade and blow the air onto the leading edge of the test-wing. This is simulating the diagram below. There is lift going straight upwards (lift always acts 90 degrees from the airflow), because there is angle of attack. There is also resistance to the airflow. If the blade were moving forwards, this resistance would slow it down. It would be as if someone were pulling the blade backwards. This is 'drag'. So drag can be thought of as working backwards.

 

The resultant is the combination of lift and drag. This is also known as the 'total aerodynamic force'. You can see this is also backwards.

15.gif

 

The next diagram shows what is happening along the length of the whole blade.

 

fcevectvert.gif

 

The next test (Top Diagram - Driven Region) is to blow the air from infront, but slightly (let's say 2 degrees) below the test-wing. Look again at the top diagram above. This would be how it would be if the helicopter is in autorotation, with the airflowing from slightly below (but still from infront). Now we have introduced some more angle of attack, which produces more lift.

 

Remember that lift acts 90 degrees from the direction of the airflow. As you are blowing from 2 degrees below, the lift (as well as working upwards) is also working 2 degrees forward! This would be like someone pulling the blade forward into the airflow. This is called 'Thrust'. With such a small angle of attack though, the thrust doesn't overcome the drag, and if the wing was moving, it would still slow down. In other words, the 'Total Aerodynamic Force' (the result of the thrust and and the drag) is still backwards.

 

The next test (Middle Diagram - Point of Equilibrium) is to lower the hairdryer even more to create more angle of attack. With experimentation, you should be able to find the angle where the forward component of thrust is the same as the backward component of drag. The total aerodynamic force is neither forward or backwards. There is equilibruim.

 

The last test (Bottom Diagram - Driving Region) is to increase the angle even more to about 6 degrees. This of course means that the lift is even more forward before. In fact, now the foward component overcomes the backward component. The 'Total Aerodynamic Force' is forward. If the test-wing were allowed to freely move, it would actually move forward. This is the key to autorotations.

 

The point is that along the same blade on a helicopter that's autorotating each of these 3 states exist. This is because the different velocities of the different parts of the blades and some twisting along the blade results in the different 'angles of attack' like the diagrams. Mostly though the 'Driving Region' is largest and this is enough to keep the blades spinning. Hence, autorotation.

 

vertreg.gif

 

Well, that took long enough. Of course, these diagrams are found in the 'Rotorcraft Flying Handbook' and there is text to go with it. The above is simply my alternative text which might help you. There may be inaccruacys, but it is close enough.

 

Joker

 

P.S. The negative pitch aspect is relatively a minor concept (applicable mainly to model helicopters), and this is not really how autorotation works in most helicopters. Try to understand the relationship between the total aerodynamic force and the angle of attack.

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I don't know of any real helicopters that permit negative pitch, other than a few Navy models, which were given that to help hold them on the deck of pitching ships while they were being tied down. Model helicopters can have negative pitch, and even hover upside down. Models and the real thing have little in common, though.

 

Think of autorotation being like windmills turning, or pinwheels. The air blowing through the rotors turns the blades just like wind turns a windmill. In flight, the airflow goes through the rotor disk from above, downward, because of the engine power driving the rotors. When autorotation is entered, the airflow changes, going upward through the rotors, and driving them like a windmill. That's certainly a simplification, but good enough for an initial understanding.

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man, thanks. but im pretty much lost after the first diagram. i understand everything from there and up. i just dont understand how the heli falling downwards gets enough lift to stop it. is there a less technical way you could explain it? lol sorry, still learning, only 15

 

ahh and yea, gomer, i understand that, but if the airflow is reversed, the air going through them from the bottom up, wouldent it reverse the direction of the blades, also pushing the helo down instead of lifting it up?

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man, thanks. but im pretty much lost after the first diagram. i understand everything from there and up. i just dont understand how the heli falling downwards gets enough lift to stop it.

 

 

There isn't enough lift to stop it from falling, thats why when you enter an auto you fall pretty quickly. The whole idea is go keep the rotor rpm's in the "green arc" so that when you get close to the ground, you can actually pull the pitch up and arrest the fall.

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OK, Sorry to lose you!

 

"During autorotation, the upward airflow hits most of the 'rotor disc' at just the right angle that clever aerodynamics cause it to speed up its RPM in the same forward direction."

 

I can't really simplify it any further.

 

The problem with the 'child's windmill' analogy is that childrens' windmills work mainly by 'action-reaction' and not by clever aerodynamics! This confuses the issue, because logic would have the blades change direction. But this doesn't happen!

 

Its the clever aerodynamics which are vital for autorotation.

 

Joker

 

Edited for clarity!

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and by speed up the rpm, do you mean like the blades actually arent going backwards? i was assuming that since its falling down, this windmill effect would cause the blades to start spinning backwards, unless you flicked on beastly amounts of negative pitch, which they dont have as stated before

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If you're just learning, I would start with relative airflow, and how that effects angle of attack before you start understanding the drive associated with different sections of an airfoil in an auto rotative state, or a windmill brake state. Jokers explanation is dead on, and about as simple as you can make it, but if you don't appreciate that the airfoil is not static, but moving, and that rotation, in turn, creates a local relative airflow and subsequent AoA, and resultant thrust, you won't be able to get your head around the concept of a perpetuating lift without an engine simply by an up flow of air.

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Jon remember = and opersite reaction, so the blades are being driven by the airflow UP through the blades you remove\adjust the pitch to keep them rotating within the safe RPM, just when you think its all over you pull pitch this trades blade rotatioanal energy for lift, if you get it correct you put her down real neat (some helios have a LOT more energy stored in the blades than others )

This is real simple explanation It is not how it is done in real life.

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A layman explanation of autorotation: Air flows upwards through the rotor system keeping the rotor turning and slowing the rate of descent (through the miracles of aerodynamics), so that at the bottom of the autorotation when the pilot turns the autorotational energy into lift (pulling up on the collective) the rotor has enough energy to cushion the landing at the bottom.

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No, the blades never reverse direction of rotation. If they did, the helicopter would fall like a brick when they stopped. It's just not possible to reverse the direction of rotation. The blades keep spinning due to inertia, and when the flow reverses, it drives the blades in the same direction they were already going. The airflow reverses, not the blade rotation.

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Try thinking about the "flying pencil" toy. The one that has a "rotor" on top of a pencil. You spin it between your hands and load it up with inertia and it produces lift and takes off. At the top of it's flight, it's no longer producing enough lift and starts to fall. You'll notice that it speeds back up a bit as it falls (though not as fast as when it's producing lift.) It's auto-rotating. It falls at a slower rate because it turns all that energy from the air rushing past it into rotational energy.

 

Now imagine if you could vary the pitch of those blades. At the top of the flight, you'd lower the pitch so that you were producing less drag. You'd sacrafice the energy to slow your decent for energy to rotate those blades faster. Now right before you touch down, you'd raise the pitch back up and trade that rotational engery back into lift. In a perfect world, you'd have enough energy in that rotor to bring your rate of decent to zero right as you touch the ground. In the real world, you need more energy than just falling straight down will give you.

 

This is the Fisher Price explination, it's certainly more complex than this. Just trying to add a bit a balance for the accurate, and sometimes overwhelming, information already provided. And please, feel free to correct me if this is possibly wrong or misleading.

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Do you remember that helicopter toy that you shot into the air with a rubber band and as it reached apogee the thin blades would flap out out and it would start spinning and land softly on the ground ready for another flight? How's that for a run-on sentance?

 

I'd think it worked basically the same way. Airflow through the blades keeps it spinning and causing a bit-o-drag to slow its decent.

 

I gradiated the fourth grade so I know these things.

 

Later

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You also don't "fall like a rock" - in fact, in a steady auto, lift=weight. Otherwise you would keep accelerating. So, in a steady climb, lift=weight. In a steady cruise, lift=weight. And in a steady auto, lift=weight.

 

The only times lift is not equal to weight are the initial stages of a climb,( where the rate of climb increases from 0 (in the cruise) to 500 or maybe 1000 feet per minute) or acceleration to the cruise, or entering a descent. Once the ROC/ ROD stabilises, lift equals weight.

 

Remember also that in an auto, the blades are extracting energy from the airflow as it goes from bottom to top. Previously they were adding energy to the airflow to make it go top to bottom.

 

Puzzling, ain't it. But that's science for you.

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I don't know of any real helicopters that permit negative pitch, other than a few Navy models, which were given that to help hold them on the deck of pitching ships while they were being tied down.

Huh? Must have been WAY before my time.

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in fact, in a steady auto, lift=weight. Otherwise you would keep accelerating. So, in a steady climb, lift=weight. In a steady cruise, lift=weight. And in a steady auto, lift=weight.

 

The only times lift is not equal to weight are the initial stages of a climb,( where the rate of climb increases from 0 (in the cruise) to 500 or maybe 1000 feet per minute) or acceleration to the cruise, or entering a descent. Once the ROC/ ROD stabilises, lift equals weight.

 

Puzzling, ain't it. But that's science for you.

 

 

puzzling indeed ! altho not sure if i believe it.

 

If you just look at the physics vectors & add numbers to them? when weight exceeds lift=ya go down,,,,,,,in the retrospective? ya go up

 

kinda like archemedes principle of buoyancy NO?

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Jon,

 

Of course this is difficult question to answer simply and without going into all the aerodynamics.

 

However, here is an attempt to give an answer appropriate to the level of question with the help of some well known diagrams and a little imagination. It will take a novice a couple of reads through to understand.

 

Initially, the inertia is needed to keep the blades spinning. This is because after, say an 'engine failure' there is a moment when the pitch is still high. Here the blades are slowing down. Once the pitch is flat, the drag is reduced. As well as that 'autorotation' takes over to keep the blades spinning. Here's how.

 

To demonstrate the concepts, I want you to cut a 1-foot section of a helicopter blade.

 

Get a broom handle and stick it into the ground. Next, using a hammer, nail the blade section horizontally onto the end of the broom handle (to form a sort of T shape). Angle it a few degrees upwards just for good measure. This is the 'test-wing'. Next get a hair dryer. This will provide the airflow.

 

The first demonstration is to stand in front of the blade and blow the air onto the leading edge of the test-wing. This is simulating the diagram below. There is lift going straight upwards (lift always acts 90 degrees from the airflow), because there is angle of attack. There is also resistance to the airflow. If the blade were moving forwards, this resistance would slow it down. It would be as if someone were pulling the blade backwards. This is 'drag'. So drag can be thought of as working backwards.

 

The resultant is the combination of lift and drag. This is also known as the 'total aerodynamic force'. You can see this is also backwards.

15.gif

 

The next diagram shows what is happening along the length of the whole blade.

 

fcevectvert.gif

 

The next test (Top Diagram - Driven Region) is to blow the air from infront, but slightly (let's say 2 degrees) below the test-wing. Look again at the top diagram above. This would be how it would be if the helicopter is in autorotation, with the airflowing from slightly below (but still from infront). Now we have introduced some more angle of attack, which produces more lift.

 

Remember that lift acts 90 degrees from the direction of the airflow. As you are blowing from 2 degrees below, the lift (as well as working upwards) is also working 2 degrees forward! This would be like someone pulling the blade forward into the airflow. This is called 'Thrust'. With such a small angle of attack though, the thrust doesn't overcome the drag, and if the wing was moving, it would still slow down. In other words, the 'Total Aerodynamic Force' (the result of the thrust and and the drag) is still backwards.

 

The next test (Middle Diagram - Point of Equilibrium) is to lower the hairdryer even more to create more angle of attack. With experimentation, you should be able to find the angle where the forward component of thrust is the same as the backward component of drag. The total aerodynamic force is neither forward or backwards. There is equilibruim.

 

The last test (Bottom Diagram - Driving Region) is to increase the angle even more to about 6 degrees. This of course means that the lift is even more forward before. In fact, now the foward component overcomes the backward component. The 'Total Aerodynamic Force' is forward. If the test-wing were allowed to freely move, it would actually move forward. This is the key to autorotations.

 

The point is that along the same blade on a helicopter that's autorotating each of these 3 states exist. This is because the different velocities of the different parts of the blades and some twisting along the blade results in the different 'angles of attack' like the diagrams. Mostly though the 'Driving Region' is largest and this is enough to keep the blades spinning. Hence, autorotation.

 

vertreg.gif

 

Well, that took long enough. Of course, these diagrams are found in the 'Rotorcraft Flying Handbook' and there is text to go with it. The above is simply my alternative text which might help you. There may be inaccruacys, but it is close enough.

 

Joker

 

P.S. The negative pitch aspect is relatively a minor concept (applicable mainly to model helicopters), and this is not really how autorotation works in most helicopters. Try to understand the relationship between the total aerodynamic force and the angle of attack.

 

 

 

 

 

I just wanna say thank you for explaining this. I am going over this stuff right now in ground school and some of it was still over my head. I am starting to get a grasp on it now. This is a great thread

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Not so puzzling, Pokey.

 

Newton says F=ma

Force = mass x acceleration

 

If you have an unbalanced force, you will get your mass accelerating until either the force stops, or another force like friction, increases enough to balance the force. Then you get steady speed. In your car, push on the gas pedal, engine force increases, speed increases until friction (tyres, internal, bearings etc) and wind drag are equal to the force. You are going faster, but at a steady speed.

 

On the ground, the chopper's weight (mass x gravity) is a force acting down. The ground pushes back equal and opposite. No movement.

 

Start engine, spin up revs, pull a little pitch. Blades create an upward force (lift) and it is equal to the weight. Still no movement.

 

More pitch, lift is greater than weight. Aircraft lifts off the ground and accelerates upward, ROC goes from zero to some larger number. But with this upward acceleration comes increased inflow air, which reduces the angle of attack and the lift force. When this force reduces to be the same as the weight, then lift equals weight and the machine stops accelerating and is in a steady climb.

 

Same in a descent. Reduce pitch, less lift. Unbalanced force. Machine accelerates downward, IVSI shows ROD. But the airflow from below reduces the inflow from the top, angle of attack goes up, and lift increases until it equals weight in a steady, unaccelerated descent.

 

It doesn't matter if you believe it or not, but this is the way it is. :blink:

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Another way to look at is your Total Energy.

 

This is the sum of your Potential energy (height x mass x g) and Kinetic Energy (half of mass times speed squared) plus others like chemical energy (fuel) and electrical energy (in your battery).

 

In level steady flight, KE+PE is constant, but you burn fuel to stay up there and overcome the forces of evil (gravity, drag, friction.) And of course, Lift = weight.

 

To climb, and increase the PE, the energy has to come from somewhere. You can trade off some speed to zoom climb, or add power to climb at a constant speed. In the steady climb, as above , lift=weight, but you use more power because you are adding to your Potential Energy.

 

This is where the confusion comes in: "How can Lift equal Weight if I am pulling so much power??"

Answer: That power is being traded for altitude.

 

To descend, lift again equals weight, but you use less power because gravity is providing the power, and you are trading off your potential energy. To level off, or to stop losing PE, you must use more power to resume level flight and fight gravity.

 

In an auto, there is no power from the engine. You have your Height (PE) and speed (KE) and that is all. Gravity's force is being slowed down by the rotor extracting energy from the air (lift) and storing it as rotating kinetic energy.

 

At the bottom of the auto, you have to wash off the descent and the forward speed carefully. Some of this energy appears as higher RRPM, which is useful. Once the flare is completed, you have no more KE and very little PE but plenty (we hope!) of RRPM.

 

You then trade these RRPM for a slower descent from that last 3 feet to cushion on.

 

Piece of cake. B)

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I don't know of any real helicopters that permit negative pitch, other than a few Navy models, which were given that to help hold them on the deck of pitching ships while they were being tied down.

 

Here's a neat little tidbit:

 

I was looking at the Robbie POH and on page 1-4, it says that the main rotor has a blade twist of -8 degrees. I don't know if that's Farenheit or Celcius. But anyway, if the collective is down and airflow is coming from under the disk, wouldn't the rotor then act as a windmill? Then as one added collective, the blades will change pitch to reverse the airflow from the top of the disk.

 

Call me Mr. Wizard.

 

Later

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