"Forget Bernoulli's Theorem"

[Aerofoam]

The difference between "drive" and "lift" is just that the lift vector on the driving region is both upward and forward, whilst that of the driven region is upward and perhaps slightly aft. Look again at the vectors Jean Claude has kindly provided.
The forward component must at least match any backward component, plus all drag, else the rotor will start slowing down.
When there is excess forward component, the rotor will speed up, of course.

Understood, just looking for more detail, there has to be a gradual transition of these zones, the diagrams are very general
and there also has to be a span wise flow component as well as the vortexes that are formed by the outward spill off from the rotor disk.

 

[Andino]

To me, the absence of "prop suction" warning signs is clear evidence that few people are stupid enough to walk in front of a plane pointed at you with spinning blades, and the warning is unlikely to be necessary. Talk to any Navy veteran who worked on a carrier deck, and you'll find people who had to be concerned about "suction". And don't walk in front of the nacelle of a running jet engine unless you want to end up like one of those frozen chickens they toss in to test robustness against bird strike ingestion.

Sorry, but that seems rather a silly reasoning and comparison, both respectively negating each other. To wit: untrained civilians are not "stupid enough" and routinely avoid prop suction without the necessity of warning signs, yet highly trained and daily working Navy carrier deck crews are "stupid enough" to need jet intake suction warning signs? Please.

Also, jet engines operate on different principles than props; the comparison cannot jive.

There is, of course, net downwash for any heavier than air flying machine,...

Yes, agreed, thank you.

As to propellers, they are not a good comparison to autogyro blades because they are very positive AOA (can not auto rotate) and are being mechanically driven and have airfoils designed to maximize thrust.
A modern windmill is a slightly better analogy, however their airfoils are designed to maximize rotational thrust (extract max. energy from the wind) and minimize the lifting forces that are not contributing to forward movement. they are much lower AOA than lifting foils and would push air the wrong direction if driven.

There are a few jump-start gyroplanes, whose rotor is both driven and provides (brief) vertical lift before sufficient airspeed to maintain autorotation, and their driven rotor does not "push air the wrong direction."

I used the propeller as an example to illustrate a rotating wing's thrust. A rotating prop makes thrust, but a rotating gyroplane wing does not, and produces instead lift? With the Bernoulli lift metaphor, shouldn't a prop produce very strong suction from the top side of its blades, sucking the aircraft ahead of itself? Nobody thinks of a prop in that way, and we all routinely describe a prop's work as Newtonian thrust. Why not also describe a rotating wing's action as "thrust" (which has the concurrent benefit of lift)? To me, Bernoulli's portion of lift has been overstated, often to such a degree that the palpable downwash in ground effect is minimised if not denied outright. I believe it more accurate to describe a wing's benefit as providing a downward thrust of air vs. a lifting suction, and this is demonstrable regardless of the wing: prop, rotor, or fixed wing. All wings shove air perpendicularly to its plane. Although that thrust is nearly universally viewed as lift from the wing's other side, I am unemotionally challenging the notion of that. Thust vs. drag? Sure. But, lift vs. weight? Why not thrust vs. weight? We can witness effective thrust of air from props, helicopter rotors, and gyroplane rotors. But not their Bernoulli suction.

 

[Doug Riley]

Keep in mind that "lift" and "drag" are concepts we have invented to help us analyze what we're observing. Neither lift nor drag exists as a separate phenomenon.

Air impacts the airfoil, and the airfoil experiences a reaction force. There's just ONE reaction force, not two.

The reaction force points generally upward, but at angles that vary along the blade's span. The reaction force "leans forward" near the blade's tip. It "leans backward" near the blade's root. At one point along the blade's span, it leans neither forward nor backward.

We'd like to be more precise than saying "forward" and "backward". We can use vector geometry to divide up our one reaction force into components -- in effect, we pretend that it's two forces, just for ease of discussion.

We can divide up the reaction in this "pretend" way, so that the two imaginary component forces are aligned in any direction we wish to define. A handy pair of directions for our two pretend components is (1) right angles to the rotational axis and (2) parallel to the rotational axis.

We can call #1 "drive". Note that #2 is NOT "lift." Call it "thrust."

Our #1 component force points toward the leading edge of the blade at stations near the root of the blade. It points the other way (toward the trailing edge) at stations near the blade's tip. The sum of the forward-pointing and backward-pointing components on a blade in equilibrium is zero.

Our #2 force is the trickier one to think about. Of course, the rotor's rotational axis leans aft, perhaps 12 degrees. By definition #2 is parallel to this axis -- so it. too, is inclined aft 12 degrees.

We can further break down #2 into two compnents -- a component that points straight up, opposite gravity, and a component that points straight aft, parallel to the flight path. The straight-up component is lift. The straight-aft component is drag.

Again neither of these vector components actually exists as a separate thing. "Lift" and "drag" are just make-believe analytical concepts to help our thoughts along. The rotor exerts one force -- thrust -- on the gyro. Thrust, in turn, is the total net force on the blades after the driving and driven forces (#1) cancel each other out.

 

[Aerofoam]

[QUOTE="Andino, post:
There are a few jump-start gyroplanes, whose rotor is both driven and provides (brief) vertical lift before sufficient airspeed to maintain autorotation, and their driven rotor does not "push air the wrong direction."

These are de-pitched to over normal RPMs, then the pitch is dumped back in when the blades are at approx. 50% higher rpm than normal cruise.
The momentum and sudden increase of AOA creates thrust as you are saying. It also creates a low pressure area above the rotor, so it too is sucking.
The AOA of the rotor blade as a gyrocopter is low enough to stay in autorotation
I would make a classifying statement to the terms and state that:
Autorotation is a condition where the blades are being driven by the airflow while creating sufficient lift to maintain controlled flight.
That requires a narrow AOA range, too high, they spin backwards, too little, they don't create sufficient lift.
With an AOA that is too negative, I would term it as "Windmilling" and it will produce drag but not sufficient lift because the lift
vectors are rotated forward (Neg. AOA).
This is a simplification, but there is a distinction between a propeller, a gyro copter blade and a windmill blade.


"I used the propeller as an example to illustrate a rotating wing's thrust. A rotating prop makes thrust, but a rotating gyroplane wing does not, and produces instead lift?"

The rotor also produces thrust, but is operating in a low AOA that enables the induced apparent wind over the top of the foil to drive the blade through the air.
Re-read my prior propeller/windmill statement in the earlier post a couple of times, I try to point out the differences, maybe not adequately...

" With the Bernoulli lift metaphor, shouldn't a prop produce very strong suction from the top side of its blades, sucking the aircraft ahead of itself?"

It does, but if you get close enough to notice it, you won't be telling anyone about it and the airplane will need to be hosed off.....

" Nobody thinks of a prop in that way, and we all routinely describe a prop's work as Newtonian thrust. Why not also describe a rotating wing's action as "thrust" (which has the concurrent benefit of lift)? To me, Bernoulli's portion of lift has been overstated, often to such a degree that the palpable downwash of ground effect is minimised if not denied outright. "

I agree with this concept completely, but think the"Thrust" component is also made up from accelerated airflow from over the foil which also creates a low pressure area creating additional lift vectors . As stated before, the high pressure side(Bottom) is providing more lift at low Reynolds numbers (Below 300,000), in the higher Reynolds numbers the upper surfaces multiple lift vectors become increasingly important.
There is a sliding scale, has anyone quantified it?
A lot of people agree with you and I have read multiple articles describing airplane wings as deflecting airflow downwards...
Maybe they are uncomfortable with the word "Thrust"?

"I believe it more accurate to describe a wing's benefit as providing a downward thrust of air vs. a lifting suction, and this is demonstrable regardless of the wing: " prop, rotor, or fixed wing. All wings shove air perpendicularly to its plane. Although that thrust is nearly universally viewed as lift from the wing's other side,"

I haven't seen that as universally viewed in the last 20 years, I have had many discussions with others in the aerospace industry and would say that most would agree with you to some extent.
The Bernoulli theory is persistent because all the older writings adhere to it and they are still very prominent.
Old theories die hard, especially when they are partially true. Look at the Prandtl D lift distribution curve. It has been rediscovered after 70 years and the only designers who exploited, or understood it in the 1930's were the Horten brothers. It is only now starting to be used after being rediscovered about 18 years ago.
Conceptually I agree with this premise ( the 600+ small airplanes I have built operating under RE. 150000. would agree too) and fall on the side of pressure being more important than vacuum, but again, it is a dance between the two, Bernoulli was at least 50% right, although I do not know if he claimed ONLY the low pressure side was lifting, or if it was just a component, but he was not wrong, it just wasn't the whole story...

" I am unemotionally challenging the notion of that. Thust vs. drag? Sure. But, lift vs. weight? Why not thrust vs. weight? We can witness effective thrust of air from props, helicopter rotors, and gyroplane rotors. But not their Bernoulli suction."

You can witness suction, it has been measured, but you are also right and a lot of people have challenged the suction/lift theory for some time now.
You are not alone!
[/QUOTE]

 

[Tyger]

Understood, just looking for more detail, there has to be a gradual transition of these zones, the diagrams are very general
and there also has to be a span wise flow component as well as the vortexes that are formed by the outward spill off from the rotor disk.

Of course it's gradual. At the line between driving and driven the vector will be straight up, with neither fore nor aft component.

 

[Smack]

As to propellers, they are not a good comparison to autogyro blades because they are very positive AOA (can not auto rotate) and are being mechanically driven and have airfoils designed to maximize thrust.

I've been behind any number of propellers where the engine was not running, yet the propeller was "auto-rotating" (rotation of airfoil due to air going through the blade system). In fact, one generally has to reduce either airspeed or the blade pitch (to zero) to STOP a "windmilling" propeller.

 

[Aerofoam]

I've been behind any number of propellers where the engine was not running, yet the propeller was "auto-rotating" (rotation of airfoil due to air going through the blade system). In fact, one generally has to reduce either airspeed or the blade pitch (to zero) to STOP a "windmilling" propeller.

A gyrocopter falling upside down vertically would windmill too if the the blades didn't flap off first....

 

[WaspAir]

These are de-pitched to over normal RPMs, then the pitch is dumped back in when the blades are at approx. 50% higher rpm than normal cruise . . . Bernoulli was at least 50% right, although I do not know if he claimed ONLY the low pressure side was lifting, or if it was just a component, but he was not wrong,

The pitch "dumped back in" in my 18A was about 8 collective degrees to make the jump, far beyond the autorotative range, automatically reduced to about 4 degrees by pitch-cone coupling as the rotor slowed to flight rpm (very different behavior in those two regimes).

Bernoulli was 100% correct in his work, completely consistent with conservation of energy. He died in 1782, never having seen an aircraft wing or even a carburetor venturi. It is the application of his principle by modern folks that some question.

 

[Aerofoam]

I did not know that much pitch was involved!

 

[Mayfield]

Bernoulli was 100% correct in his work, completely consistent with conservation of energy. He died in 1782, never having seen an aircraft wing or even a carburetor venturi. It is the application of his principle by modern folks that some question.

Very nicely stated Jon. We sometimes casually use the phrase "Bernouli's principle" when we are explaining lift to our students. Many of us, including me, gloss over the equations. The operative principle is a certainty that pressure of a fluid decreases as velocity increases. For this part of the discussion, we don't have to talk "inclined plane" or "impact pressure" to realize that the principle is very real.

Example: driving on the freeway. A large truck passes you while you are driving your small car or motorcycle. What happens? Your vehicle pulls toward the passing truck. Why? Because the velocity of the air between the vehicles is accelerated which reduces the pressure of that air. Your vehicle is "pulled toward the decreased pressure area.

A possibly better example: Stand near the edge of a highway or a train track. What happens when a truck or bus, or at the railway, a train passes you? You feel you are being pulled toward the passing object. Why? Because the pressure between you and the passing vehicle decreases as the velocity of the air increases.

No impact pressure on your back, just lower pressure on the side toward the passing vehicle.

I fully realize, that in the two tortured paragraphs above, I could have said, "The low pressure causes the high pressure at my back to push me toward the vehicle passing. For me, that complicates the phenomenon; unnecessarily.

Jim

 

[WaspAir]

Sorry, but that seems rather a silly reasoning and comparison, both respectively negating each other. To wit: untrained civilians are not "stupid enough" and routinely avoid prop suction without the necessity of warning signs, yet highly trained and daily working Navy carrier deck crews are "stupid enough" to need jet intake suction warning signs? Please.

I made no such implication and showed nothing but respect for Navy personnel. People who are forced to work routinely in hazardous areas (otherwise assiduously avoided) such as carrier deck crews deserve additional warning efforts to prevent accidents when there is a risk of becoming inured to the danger and making mistakes from the comfort level that acceptance of risk and repeated exposure can produce.

Meanwhile, you have entirely missed the purpose of the post, which was to point out the silliness of arguing (as it appeared to me you did) that the absence of suction warning signs is evidence that the Bernoulli contribution to lift is not dominant. I consider that absurd, and brought up my example to illuminate my opinion. Do you still cling to that argument?

By the way, both turbojets and propellers suck air from one direction and blow it in another. You can't have one without the other.

 

[Smack]

A gyrocopter falling upside down vertically would windmill too if the the blades didn't flap off first....

Translation: I said something stupid and someone pointed it out, so I followed it up with something stupid AND irrelevant...

 

[Aerofoam]

Translation: I said something stupid and someone pointed it out, so I followed it up with something stupid AND irrelevant...

It is totally relevant because the airplane would have to be falling tail first to be in the same blade orientation of an autogyro, but the prop would be spinning backwards
because of the high angle of attack.
An autogyro rotor would also TEND to start backwards from a complete stop if it were falling vertically, but if it has enough forward speed, it will rotate the right direction.
Conversely, the airplane propeller will NEVER autorotate in the correct direction with the wind coming from the THRUST side. But it will WINDMILL in the right direction when moving forward through the air. It will not be an efficient windmill because the airfoil is facing the wrong direction for efficiently being driven.
So yes you did say something in error, or at least in ignorance of the actual physics involved.

 

[Andino]

I made no such implication and showed nothing but respect for Navy personnel. People who are forced to work routinely in hazardous areas (otherwise assiduously avoided) such as carrier deck crews deserve additional warning efforts to prevent accidents when there is a risk of becoming inured to the danger and making mistakes from the comfort level that acceptance of risk and repeated exposure can produce.

Meanwhile, you have entirely missed the purpose of the post, which was to point out the silliness of arguing (as it appeared to me you did) that the absence of suction warning signs is evidence that the Bernoulli contribution to lift is not dominant. I consider that absurd, and brought up my example to illuminate my opinion. Do you still cling to that argument?

Yes, of course you respect Navy personnel; steady now. However, I've toured your USS Intrepid carrier berthed off Manhattan, and don't recall warning signs about prop suction from their WWII Hellcats and Corsairs. To bolster my waggish example: It would be an interesting experiment to place equidistant fore and aft (and safely) of a prop identical height flags or ribbons, and observe their respective behaviour from thrust vs. suction. Anecdotally, I can say that prop blast can easily be felt 50'+ away, but I've never felt a similar force from the low-pressure front side of a prop at 50'+ (or even 20'). All of us have walked in front of running pusher prop gyroplanes, and don't feel as much low-pressure there as we do the prop blast when behind it. To be sure, the low-pressure can be felt, but it is not alarming.

I do not dispute Mayfied's highway lorry example, however, I do not believe that the Bernoulli contribution to lift is dominant. If it were dominant, then we'd feel the powerful force of prop suction as much or more so than the powerful force of prop blast. I do not believe that a spinning prop primarily pulls the aircraft ahead from low-pressure. I believe that the aircraft is primarily propelled forward by thrust, hence the word "propeller." Thank you all for an engaging discussion.

 

[Aerofoam]

Yes, of course you respect Navy personnel; steady now. However, I've toured your USS Intrepid carrier berthed off Manhattan, and don't recall warning signs about prop suction from their WWII Hellcats and Corsairs. To bolster my waggish example: It would be an interesting experiment to place equidistant fore and aft (and safely) of a prop identical height flags or ribbons, and observe their respective behaviour from thrust vs. suction. Anecdotally, I can say that prop blast can easily be felt 50'+ away, but I've never felt a similar force from the low-pressure front side of a prop at 50'+ (or even 20'). All of us have walked in front of running pusher prop gyroplanes, and don't feel as much low-pressure there as we do the prop blast when behind it. To be sure, the low-pressure can be felt, but it is not alarming.

I do not dispute Mayfied's highway lorry example, however, I do not believe that the Bernoulli contribution to lift is dominant. If it were dominant, then we'd feel the powerful force of prop suction as much or more so than the powerful force of prop blast. I do not believe that a spinning prop primarily pulls the aircraft ahead from low-pressure. I believe that the aircraft is primarily propelled forward by thrust, hence the word "propeller." Thank you all for an engaging discussion.

The suction area is not concentrated, the prop is drawing effectively in air from about 160 deg radiating from the hub.
The exit is highly focused in a thrust line that decays and spreads farther away. In many respects it is acting sort of like a
lens and focusing the column of air.
If you put a tube from the front of the prop for about 5', you would have a focused intake and could measure and feel the vacuum in that area.
I think with a very long flexible tube, it is referred to as a vacuum cleaner....

 

[Andino]

The suction area is not concentrated, the prop is drawing effectively in air from about 160 deg radiating from the hub.
The exit is highly focused in a thrust line that decays and spreads farther away. In many respects it is acting sort of like a
lens and focusing the column of air.
If you put a tube from the front of the prop for about 5', you would have a focused intake and could measure and feel the vacuum in that area.
I think with a very long flexible tube, it is referred to as a vacuum cleaner....

Yes, I'd also considered the more diffused area of intake vs. the more concentrated cone of thrust. However, doesn't that very diffusion of low-pressure argue against it being the primary anti-drag force? Isn't the "lens focused column of air" logically doing most of the propelling?

 

[Jean Claude]

I can say that prop blast can easily be felt 50'+ away, but I've never felt a similar force from the low-pressure front side of a prop at 50'+ (or even 20'). All of us have walked in front of running pusher prop gyroplanes, and don't feel as much low-pressure there as we do the prop blast when behind it. To be sure, the low-pressure can be felt, but it is not alarming.

First of all, do not confuse pressure with speed.
When passing near a propeller you do not feel pressure, but only the speed of the air.

And yes, there is less speed in front and more behind, because it is by means of the acceleration of the air speed that the propeller occurs the thrust (Newton law)
The acceleration progressively increases the airspeed from far upstream to far downstream, i.e during more one diameter



This does not allow to conclude anything about the distribution of the ventral and dorsal pressures of the blades.

 

[Aerofoam]

I'm not disagreeing with you on the "Thrust side" doing a lot of work. I think it is a sliding scale from extremely low Reynolds numbers operating
a flat plat "Foil" with mostly "Deflection" (More on that can of worms later) progressing up to very high Reynolds numbers where the top surface is not only creating some "Suction", but it also adding to "Thrust" because the laminar air is leaving the trailing edge at a downwards angle and contributing to the "Thrust" It would be interesting to isolate these components with some empirical testing.

 

[Aerofoam]

PS, all the "Thrust" is composed of air that had to be "Sucked" from somewhere, so diffused intake is really the only option.

 

[Doug Riley]

As a licensed boat captain, I can attest to the reality of the Bernoulli effect on vessels in the water.

Not only will two moving vessels experience acceleration toward each other if they pass close aboard, but we also encounter "bank suction" and "bottom suction" when our moving hull approaches the side or bottom of a waterway. I imagine that the Everclear (or whatever they call themselves) container-ship company can give us a pep talk on that subject, as they keep grounding out in shallow water.

I do like Langeweische's emphasis on the "planing" effect on airfoils, though. It jibes with everyday experience, whether we stick our hand out the car window and give it an angle of attack, or toss a flat-winged balsa glider, or cool ourselves under a ceiling fan with old-fashioned, flat-board blades.

One must draw the line at Langeweische's re-naming elevators "flippers," though. We are not at Sea World.