Propellers and rotors

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Propellers and rotors

Postby Thadlerian » Mon Jul 26, 2010 8:21 am UTC

I'm wondering what the principal difference between a propeller and a rotor is. Do they utilize the same aerodynamic principles, or are they completely different?

The reason I'm wondering is because I'm used to seeing Leonardo da Vinci cited as the inventor of the helicopter. But as far as I can see, his design uses a screw, analogous to a propeller (?), whereas working helicopters use rotors, which seem more like wings. Is this a significant difference?
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Re: Propellers and rotors

Postby Hawknc » Mon Jul 26, 2010 8:29 am UTC

Propellers were once referred to as airscrews, so there isn't a huge difference. Fundamentally they work on the same principle - moving an object through the air by rotating with a certain pitch.
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Re: Propellers and rotors

Postby Thadlerian » Mon Jul 26, 2010 2:04 pm UTC

But how do they move the air? I've always had the notion that the wing of the rotor creates lift by creating an under-pressure above itself, whereas propellers push the air more directly. Or does the propeller function similarly to a wing?
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Re: Propellers and rotors

Postby Velifer » Mon Jul 26, 2010 6:02 pm UTC

Thadlerian wrote:But how do they move the air? I've always had the notion that the wing of the rotor creates lift by creating an under-pressure above itself, whereas propellers push the air more directly. Or does the propeller function similarly to a wing?

What's happening to the air in each scenario? Where is it going?
What does lower pressure mean, and how does the lower pressure happen?
See how each of your scenarios are the same thing?

There is a common misconception about how airplane wings work. The lift from the longer travel path above vs. below is a very very small amount, while the air pushed down from the angle of attack is generating some serious lift (Newton's third law. Push air down, push plane up.)
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Re: Propellers and rotors

Postby gmalivuk » Mon Jul 26, 2010 6:07 pm UTC

Propellers, rotors, and wings all move air *primarily* by pushing it the same way as a household fan does. Push the air down, and you can move your rotor or wings up. Push it backward, and your prop moves forward.
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Re: Propellers and rotors

Postby Thadlerian » Mon Jul 26, 2010 8:59 pm UTC

Velifer wrote:the air pushed down from the angle of attack is generating some serious lift (Newton's third law. Push air down, push plane up.)

What?! But that's precisely what I used to think when I was a kid! And then they came along and taught me that (at the time) utterly incomprehensible thing about travel paths!

I will spread the word at opportunities.
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Re: Propellers and rotors

Postby EdgarJPublius » Mon Jul 26, 2010 9:29 pm UTC

I'm having trouble figuring out what exactly the question is, a propeller is a type of rotor, so you're question is basically 'what is the difference between a rotor and a rotor?'
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Re: Propellers and rotors

Postby Thadlerian » Mon Jul 26, 2010 9:45 pm UTC

EdgarJPublius wrote:I'm having trouble figuring out what exactly the question is, a propeller is a type of rotor, so you're question is basically 'what is the difference between a rotor and a rotor?'

The idea behind my question was simply: What's the difference of the thing on the top of a helicopter and the thing in front of a plane. They look different, and no-one's ever explained a propeller to me using the travel path idea, so it seemed to me they were functioning by different aerodynamic principles. My usage of the term "rotor" stems from a direct translation from common Norwegian, where rotor explicitly means the former. It is possible that in English it is a name that encompasses both, but this possibility did not strike me at the time.

The matter has been cleared up for me now. My thanks to all involved.
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Re: Propellers and rotors

Postby Sockmonkey » Tue Jul 27, 2010 12:12 am UTC

Thadlerian wrote:
Velifer wrote:the air pushed down from the angle of attack is generating some serious lift (Newton's third law. Push air down, push plane up.)

What?! But that's precisely what I used to think when I was a kid! And then they came along and taught me that (at the time) utterly incomprehensible thing about travel paths!

I will spread the word at opportunities.

It's sort of both. The travel path area/lift thing is a good rule of thumb for fast n dirty calculations for the performance of a standard subsonic wing so generally only aircraft designers ever bother to get into the details. Really the curve of a wing both creates pressure lift and directs the air down.
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Re: Propellers and rotors

Postby Korrente » Tue Jul 27, 2010 3:31 am UTC

Propellers (on planes) do use the travel path idea (essentially Bernoulli's Principle) but they also use normal Newtonian deflection like a fan would, simply because they're turning much faster and moving a lot more air than would be moving over a fixed wing of the same size. Complex prop systems allow the blades' pitch do be changed, which lowers RPM and is more efficient, but the variable pitch wouldn't be effective unless a large percent of the thrust generated was from deflection.

Helicopter rotors primarily use Bernoulli's Principle, as the principle for a helo is to move the wings, not the aircraft.
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Re: Propellers and rotors

Postby EdgarJPublius » Tue Jul 27, 2010 4:30 am UTC

Korrente wrote:Propellers (on planes) do use the travel path idea (essentially Bernoulli's Principle) but they also use normal Newtonian deflection like a fan would, simply because they're turning much faster and moving a lot more air than would be moving over a fixed wing of the same size. Complex prop systems allow the blades' pitch do be changed, which lowers RPM and is more efficient, but the variable pitch wouldn't be effective unless a large percent of the thrust generated was from deflection.

Helicopter rotors primarily use Bernoulli's Principle, as the principle for a helo is to move the wings, not the aircraft.


Not true. both rely on deflection to a great extent. Helicopter rotors usually operate at a constant RPM, and the only way to adjust thrust and lift is by adjusting the swash-plate, which consists of two parts, a collective plate and a rotating plate. The collective plate adjusts the pitch of the entire rotor assembly, allowing the helicopter to vary it's speed through the air without pitching the entire aircraft, while the rotating plate adjusts the pitch of individual blades, to vary the amount of thrust produced.

There is no difference in how a vertical rotor assembly (such as on a plane) and a horizontal rotor (such as on a helicopter) actually function. The only real difference is in how they are optimized (and even that is a fairly minor difference), a Helicopter rotor assembly is optimized to push a relatively large amount of air relatively slowly, moving more air in this manner generates more lift, and is more efficient at lower speeds.
An airplane rotor however, is optimized to move a relatively small amount of air at very great speeds. Since the airplane doesn't use it's rotor to generate thrust directly, it does not need to move as much air, only enough to provide forward thrust (very few airplanes have enough thrust to accelerate straight up, however, every helicopter does). Also, moving less air is more efficient ant higher speeds, and allows the airplane to reach much greater speeds than a helicopter.

The best way to illustrate this is with the V-22 Osprey, when the Osprey switches from vertical to horizontal flight, or vice-versa, the rotors do not change shape or pitch, just orientation.
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Re: Propellers and rotors

Postby jmorgan3 » Tue Jul 27, 2010 6:55 am UTC

A lot of people are trying to draw a distinction between lift due to a pressure differential and lift due to pushing air downward. This is a false dichotomy. Think of it this way: there are two ways that the air and rotor blade interact: viscosity and pressure.

Viscosity and Why I'm Ignoring It
Spoiler:
Viscosity in a fluid causes shear stress in the presence of velocity gradients (there's a more precise mathematical way to say it, but bear with me). Importantly in this discussion, viscous forces between the air and a rotor blade act in a direction parallel to the blade's surface. An airfoil is mostly flat, so the viscous force acts mostly in the direction parallel to the flow (i.e. the drag direction, not the lift direction). Therefore, it would make sense if viscosity could be neglected in discussions of lift. In fact, empirical measurements of lift show very good agreement with inviscid (ignoring viscosity) calculations. So, despite the many interesting and important phenomena of viscous flow, I'm going to ignore viscosity in the rest of this post.


Pressure, in contrast, acts perpendicularly to the surface of the rotor blade. It is the primary means of interaction between blade and air, and the primary force acting between different morsels of air.

Effect of Pressure Gradients
Spoiler:
In our inviscid model, any acceleration of the air is the result of a pressure gradient (an increase in pressure in a certain direction.) To envision the effect of these gradients, imagine a discrete morsel of air, small enough that pressure, velocity, and other properties are uniform throughout the morsel, but large enough that you don't need to think of it as individual particles. If pressure increases in the direction of this morsel's velocity, then there will be more force pushing on the front than the back, and it will slow down. Conversely, decreasing pressure in the direction of velocity will speed up the air. This is the conceptual basis behind Bernoulli's principle.

Gradients perpendicular to velocity will not change the speed of the morsel, but will cause curvature in the direction of decreasing pressure. The pressure differential between sides of the morsel provides the centripetal force that curves the air's path.

Gradients with both parallel and perpendicular components will change both speed and direction.


Pressure and Deflection
Spoiler:
Now that that's out of the way, let's get back to the rotor blade. Imagine the blade staying stationary, with air flowing past. Because air cannot pass through the blade, it will split into two sections, flowing above and below the blade. The two paths will rejoin at the trailing edge of the blade. If the trailing edge is pointed downward (the front is pointed upwards), then the faster air along the upper surface will have a lower pressure than the slower air along the lower surface, and the blade will experience lift from this pressure differential.

So far, this explanation sounds just like the equal transit time fallacy. The important difference is the mechanism behind this pressure differential. Rather than some anthropomorphic "need" for the airstreams to meet back up exactly as they left, the speed and pressure changes are explained by the effects of pressure differentials described above. (In fact, the faster air along the longer upper surface of the blade will beat the slower air to the edge. If anything, the equal transit time fallacy will understate the lift.)

When the two airstreams meet again at the trailing edge, they will keep going in the direction of the trailing edge. If the trailing edge is pointed downwards, then that means that both upper and lower streams must be deflected downward. This is caused by pressure gradients in both streams, with pressure increasing in the upward direction.

Look at the top stream first. Far away from the airfoil in all directions, the pressure will be uniform. This is called the freestream pressure. For an upward pressure gradient to exist on top of the blade, then, the pressure on top of the blade must be lower than freestream pressure.

On the lower side, conversely, an upward pressure gradient requires that the pressure below the airfoil be higher than freestream pressure. With higher-than-freestream pressure on the bottom, and lower-than-freestream pressure on top, the blade experiences a net upward lift.


Note that the pressure distribution around the wing is intimately linked to the downward deflection of air. It is wrong to try to separate the two as if each contributes to lift individually. It's the equivalent of saying "My car is starting to move partially because my foot is pushing the accelerator and partially because the engine is turning the wheels."

EDIT:It's me, Melchoir.
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Re: Propellers and rotors

Postby Shotglass » Wed Jul 28, 2010 1:19 am UTC

EdgarJPublius wrote:Also, moving less air is more efficient ant higher speeds, and allows the airplane to reach much greater speeds than a helicopter.


the limiting factor of a helicopter top speed is the plane its rotor is operating in and its rpm not the design of the rotor as such

what happens when a helicopter reaches its top speed is the speed at which the air travels past the heli approaches the speed of the baldes as they move towards the rear of the heli (assuming forward flight of course)
thus the blades begin to stall as they rotate in direction of the tail of the heli up until the point where they fail to generate lift and the heli begins to roll
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Re: Propellers and rotors

Postby Velifer » Wed Jul 28, 2010 11:49 am UTC

Shotglass wrote:...up until the point where they fail to generate lift and the heli begins to roll

How the hell does anyone ever get these things to stay in the air?
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Re: Propellers and rotors

Postby Sockmonkey » Wed Jul 28, 2010 2:45 pm UTC

By limiting their top speed or adding auxilliary wings.
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Re: Propellers and rotors

Postby EdgarJPublius » Wed Jul 28, 2010 7:12 pm UTC

Shotglass wrote:
EdgarJPublius wrote:Also, moving less air is more efficient ant higher speeds, and allows the airplane to reach much greater speeds than a helicopter.


the limiting factor of a helicopter top speed is the plane its rotor is operating in and its rpm not the design of the rotor as such

what happens when a helicopter reaches its top speed is the speed at which the air travels past the heli approaches the speed of the baldes as they move towards the rear of the heli (assuming forward flight of course)
thus the blades begin to stall as they rotate in direction of the tail of the heli up until the point where they fail to generate lift and the heli begins to roll


This is only a relatively recent 'limit' with the advent of high-powered turbo-props that can generate much more thrust than earlier helicopter power-plants.
However, even if retreating blade stall is avoided (such as by using auxiliary lift surfaces on the retreating blade side) the performance of a helicopter is still fundamentally limited by the fact that most of the power to the blades is being utilized for lift, and it is much more efficient to generate lots of lift at lower blade speeds.
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Re: Propellers and rotors

Postby Kang » Thu Jul 29, 2010 9:17 am UTC

I'd like to state my agreement with jmorgan3 that the 'different' lift effects are actually linked and would thank him for the pressure-based explanation of the Bernoulli effect. i'd also like to add that if you looked at a world without the Bernoulli effect an airplane stalling would be hard to explain (to see what a stall looks like see last weekend's news :( ), which happens when the airflow over the wing (or rotor in case of a helicopter) is interrupted and becomes turbulence-dominated, either by lack of flow (no speed) or too much obstruction (high AoA or use of spoilers). Anecdotal evidence can be found on a large variety of military plane types, like the Bf-109 or the A-4, which sported so-called 'slats', a mechanism that would let the leading edge of the wing roll out a few inches, which by deflection forces air that would otherwise just go by the wing to flow over it creating lift. That allowed for very high AoA manoeuvres for a short time. Also modern airliners use 'spoilers', flaps on the top sides of the wings to interrupt air flow during (or rather right after) landing, which a solely deflection-based lift would be unaffected by.
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Re: Propellers and rotors

Postby Sockmonkey » Thu Jul 29, 2010 11:36 am UTC

Kang wrote:I'd like to state my agreement with jmorgan3 that the 'different' lift effects are actually linked and would thank him for the pressure-based explanation of the Bernoulli effect.

Yep, he explained it better than I could have.
(which irks me because I'm usually pretty good at that sort of thing)
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Re: Propellers and rotors

Postby Shotglass » Sun Aug 01, 2010 4:56 pm UTC

EdgarJPublius wrote:This is only a relatively recent 'limit' with the advent of high-powered turbo-props that can generate much more thrust than earlier helicopter power-plants.


you dont need a lot of poer to achieve retreating blade stall because the power required to keep the helicopter afloat drop rather noticeably once youre up to speed thanks to translational lift

However, even if retreating blade stall is avoided (such as by using auxiliary lift surfaces on the retreating blade side) the performance of a helicopter is still fundamentally limited by the fact that most of the power to the blades is being utilized for lift, and it is much more efficient to generate lots of lift at lower blade speeds.


youre either not making much sense here or youre not expressing your point even remotely clearly
if you were to add additional lift sruface you wouldnt need the rotor to generate lift anymore so your argument appears to be circular and self contradicting
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Re: Propellers and rotors

Postby EdgarJPublius » Sun Aug 01, 2010 5:30 pm UTC

Shotglass wrote:
EdgarJPublius wrote:This is only a relatively recent 'limit' with the advent of high-powered turbo-props that can generate much more thrust than earlier helicopter power-plants.


you dont need a lot of poer to achieve retreating blade stall because the power required to keep the helicopter afloat drop rather noticeably once youre up to speed thanks to translational lift

However, even if retreating blade stall is avoided (such as by using auxiliary lift surfaces on the retreating blade side) the performance of a helicopter is still fundamentally limited by the fact that most of the power to the blades is being utilized for lift, and it is much more efficient to generate lots of lift at lower blade speeds.


youre either not making much sense here or youre not expressing your point even remotely clearly
if you were to add additional lift sruface you wouldnt need the rotor to generate lift anymore so your argument appears to be circular and self contradicting



If you don't need the rotor to generate lift, then it isn't a helicopter anymore...

EDIT:
Auxilliary lift surfaces don't need to be non-rotors, a countra-rotating blade arrangement can provide lift to the retreating blade side with an advancing blade this record-setting helicopter, though you need to have blades that are rigid enough so as not to collide when opposite sides of the collectives are in RBS.

Contra-rotating blades also have some advantages at lower speeds, but you lose most of that in high-speed flight due to RBS (one advantage, that of not needing a tail-blade or NOTAR to counter the blades' torque is retained) , still, it illustrates my point nicely, even a highly advanced helicopter with RBS-countering design elements is slower than an airplane driven by a comparable turbo-prop.
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Re: Propellers and rotors

Postby Sockmonkey » Sun Aug 01, 2010 7:07 pm UTC

There is one other type where you stop the rotor and the blades work like a fixed wing but mechanically and design wise it's a real cluster f*** to pull off.
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Re: Propellers and rotors

Postby Shotglass » Mon Aug 02, 2010 2:30 am UTC

EdgarJPublius wrote:it illustrates my point nicely, even a highly advanced helicopter with RBS-countering design elements is slower than an airplane driven by a comparable turbo-prop.


no it doesnt
a coax helicopter still experiences retreating blade stall reducing its lift significantly and thus severly limiting its ability to reach high speeds
on top of that if you find ways to counter retreating blade stalling youll soon run into problems with the advancing blade entering transsonic or supersonic flow conditions
all significant problems that fundamentally result from running the rotor in a plane parallel to the direction of flight
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Re: Propellers and rotors

Postby EdgarJPublius » Mon Aug 02, 2010 6:13 am UTC

Only if both props are necessary to maintain lift. in reality contra-rotating props do not generate significantly more lift than a single prop of the same diameter, the main advantages are better efficiency, countering angular momentum (eliminating the need for a tail-prop or NOTAR, which further improves efficiency) and low noise.

Although both props will enter RBS, they will do so on opposite sides of the helicopter, so over-all lift is not affected excessively.

Blades entering transonic or supersonic flow is not a significant problem, and can be alleviated greatly by blade design, many helicopters have actually been designed to intentionally enter this condition.

late night edit:
We're really just arguing in circles, there are a number of interleaved reasons that helicopters and other horizontal-rotor-craft fundamentally can't reach vertical-rotor-craft speeds, and picking just one over the others is silly.
What I should have said is that, while there are other problems that prevent helicopters reaching high speeds which can be corrected to varying degrees, fundamentally, the orientation and lift requirements prohibit it.
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