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I am in the middle of my PPL(A) theory and I am experiencing a mental breakdown studying propellers. Until yesterday I was happily living in my ideal world where I was sure I really understood all the relevant aspects and details about how propellers are designed and actually work.

Then today I stumbled upon a webpage that destroyed all my self-confidence with just 2 out of 3 sentences:

Firstly, the shape of the propeller is such that it cork screws through the air. The cork screw motion pulls the propeller and the plane through the air

Ok, I thought this was just a huge misconception. I thought propeller blades, as aerofoils, generated Lift and Induced Drag, which we consider as Torque and Thrust in relation to the rotational plane for a number of reasons. All this having nothing to do with "cork screwing into the air" for the same reason why a wing doesn't go up because it cork screw itself into the airflow. It simply generates Lift. Is there something I am missing?

Secondly, the propeller spins at high speeds pushing air back and thus pushing the plane forward. The more air that is pushed back the more the plane is pushed forward

Ok.. no. I really thought this was how a jet engine works, not a propeller. Again, I must be missing a LOT.

Thirdly, [...] the blades are actually wings that spin around. The curved surface of the blades face the front of the plane and this creates a low pressure region in front of the plane sucking it forward

Oh finally!

Now, since I believed only the 3rd point made sense, are the first 2 described effects real?

user815129
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  • Welcome to avaition.stackexchange.com! – DeltaLima Aug 20 '21 at 09:41
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    If you are suffering from a mental breakdown, you should see a qualified mental health practitioner. – Gregory Currie Aug 21 '21 at 08:56
  • Cork is just a very dense form of air. Rotating corkscrew creates lift and pushes itself forwards ;-) – szulat Aug 21 '21 at 09:16
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    I really thought this was how a jet engine works, not a propeller Actually, both work the same way in principle. If you think of them as a black box, both accelerate air. Jets accelerate little air by much while propellers accelerate much air by a little. – Peter Kämpf Aug 21 '21 at 10:30
  • If you change your perspective from the blades to the air, which, if you put tracer smoke around, you would see the corkscrew effect of the air it may help – Michael Durrant Aug 21 '21 at 14:40
  • Is it more intuitive to you to visualise how a helicopter flies? In that case, each blade of the rotor is effectively a rotating wing. Now see the Bell Boeing V-22 Osprey (wikipedia). It has two helicopter rotors which also vector by 90 degrees to become propellers. – Paul_Pedant Aug 21 '21 at 20:16
  • Helicopter blades, yes. Because they utilize AoA much more than they use airfoiling, just like everything else does that points its nose into the sky to climb. – Mazura Aug 22 '21 at 21:57

4 Answers4

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All three sentences make sense to me.

Firstly, the shape of the propeller is such that it cork screws through the air. The cork screw motion pulls the propeller and the plane through the air

The cork screw reference here is to say that with every revolution, the propeller advances through the air, pushing more air behind it.

A nice visualisation of that occurs when the propeller tip vortices are causing condensation. See the cork screw? a Lockheed C130 surrounded by cork screwing vortices from its propellers

Photo: Ron Kellenaers / Airlines.net Source

Secondly, the propeller spins at high speeds pushing air back and thus pushing the plane forward. The more air that is pushed back the more the plane is pushed forward

That is Newtons third law in action (or reaction?). By accelerating air backwards, the propeller (and the plane attached to it) must be forced forwards.

Thirdly, [...] the blades are actually wings that spin around. The curved surface of the blades face the front of the plane and this creates a low pressure region in front of the plane sucking it forward

Yup, that is how a propeller accelerates the air backwards, just like a wing accelerates air downwards.

DeltaLima
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  • Yes, of course the forward motion of a propeller resembles a corkscrew, but I think this is simply an effect caused by rotation + forward motion, it is definitely not the main reason why a propeller pulls an aircraft (I don't think it even contributes at all). As for the 3rd law, yes I think there must be an effect, but negligible, or maybe just a secondary contributing factor. I think the main principle pulling an aircraft is the last one. – user815129 Aug 20 '21 at 09:50
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    The three sentence are all correct, they are just different ways for looking at the same thing. The 3rd law effect in not negligible at all, it accounts for 100% of the thrust. And so does the lift force on the wing. The pressure distribution over the wing causes the air to accelerate backwards. They are two sides of the same coin. – DeltaLima Aug 20 '21 at 09:58
  • Ok, so please help me: if I understand correctly, the sentence "The cork screw motion pulls the propeller and the plane through the air" actually means "The cork screw motion provides airflow to the blade element, which, thank to the lift generated by its angle of attack pulls the propeller and the plane through the air"? or maybe I still dont get it – user815129 Aug 20 '21 at 10:07
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    Yes, that is a good way of looking at. Suppose we had a propulsion system that consists of a scoop that would throw air backwards. That would work, but we would have to reposition the scoop in the forward position before we could thrust the next parcel of air backwards. The propeller doesn't have that problem; it corkscrews its way through the air, making sure there is always a fresh supply of static air to kick backwards. – DeltaLima Aug 20 '21 at 10:10
  • My bad, I thought the article was really talking about 3 distinct effects, now it makes sense thanks – user815129 Aug 20 '21 at 10:22
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    You're welcome, glad I could help! Can you include a link to the article in your question? – DeltaLima Aug 20 '21 at 10:23
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    of course: http://www.dynamicscience.com.au/tester/solutions1/flight/demonstration%20newtons%20law%20propeller.htm – user815129 Aug 20 '21 at 10:25
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    "Firstly, the shape of the propeller is such that it cork screws through the air. The cork screw motion pulls the propeller and the plane through the air" This sentence is misleading: The propeller pushes air downwards relatively to its chord, as any airfoil does. The fact that there is a rotation makes it appear like an helix. Exactly like the wingtips of an aircraft spinning around its longitudinal axis would also draw an helix. The helix has no particular effect or signification for lift. "The cork screw motion pulls the propeller" is irrelevant, and for me it's a misunderstanding. – mins Aug 20 '21 at 10:59
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    A corkscrew is just an inclined ramp going around in a circle. A propeller is just a wing that wants to fly straight ahead but is forced to go in a circle due to it's root connection to an axle. They're both going up a ramp so to speak. so to the extent that the analogy is useful, it works for the purpose of visualization. – John K Aug 20 '21 at 12:23
  • When I first saw that photo, I thought the corkscrews were photoshopped in. – Acccumulation Aug 20 '21 at 23:08
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    @JohnK Nitpick: a corkscrew is an inclined plane going in a helix. Unless you're in an M.C Escher painting, making a corkscrew out of an inclined plane going in a circle is rather difficult. – Acccumulation Aug 20 '21 at 23:09
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    @mins I agree, IMHO in this form it may be a little misleading – user815129 Aug 21 '21 at 17:34
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    "The curved surface of the blades face the front of the plane and this creates a low pressure region in front of the plane sucking it forward" I think that quote from the question is misleading and needs to be addressed. The front face or upper face of an aerofoil does not have to be curved. Curved aerofoils are more efficient but it is not a necessary condition for thrust or lift. – Smartybartfast Aug 22 '21 at 03:22
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    @user815129: I've elaborated on how this is misleading in a true answer, it was also a good opportunity to clarify other points. – mins Aug 22 '21 at 11:29
  • The "corkscrew" motion is important for understanding how propeller blade pitch, RPM, airspeed, and thrust are related. Consider a fixed-pitch prop for simplicity. In one revolution, the "corkscrew" moves a fixed distance forwards. If the aircraft moves the same distance forwards in the same time, the propeller thrust will be zero. If the prop RPM is higher, the thrust will be positive, and if it is lower it will be negative and decelerates the plane. The same applies to a fixed speed prop, where changing the blade pitch matches the "size" of the corkscrew to the air speed and thrust . – alephzero Aug 22 '21 at 20:50
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Firstly, any new readings or points of view should be carefully reviewed and compared with current information to avoid these lapses in confidence.

As Delta Lima pointed out, "corkscrewing" more aptly describes the combined motion path of the the propeller and the aircraft from a third frame of reference. A good analogy would be "the earth corkscrews around the sun as both revolve around the galaxy". This should not shatter our view that we are safely orbiting our sun.

The propeller indeed is twisted a bit due to the differences in angular rotation speed (of the propeller) relative to the forward motion of the plane (which helps optimize the AOA of more of the prop to the relative wind). But from the aircraft reference the prop always rotates around its engine.

The second and third points (about moving air and creating suction in front, really pull and push) are consistent with lifting theory of wings turned sideways, that is: the bottom of the wing pushes air down, causing the wing to go up. The cambered top of the wing creates low pressure, pulling the wing up.

The two work together. As a final note, distinguishing props from simple action/reaction mass movers is correct. It is their wing-like lifting method of propulsion that gives them an efficiency advantage.

Robert DiGiovanni
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  • The Frame of Reference point is excellent. Using the pilot's seat as a fixed datum, the props are pushing the entire world backward ! – Criggie Aug 22 '21 at 11:22
  • Plus a propeller still generates thrust when the aircraft not moving (i.e. wheel brakes on), so the idea that the thrust is due to the corkscrew motion cannot be correct. – David Waterworth Aug 23 '21 at 03:42
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    @DavidWaterworth Even when the aircraft is motionless with respect to the ground, the propeller is still "corkscrewing" with respect to the air that it's pushing. – Shufflepants Aug 23 '21 at 15:33
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Well, the way a propeller works is a little bit like a corkscrew. There are several similarities:

  • Both machines penetrate a surrounding medium—a propeller penetrates air, and a corkscrew penetrates a cork.
  • Both machines move in a helical path, as seen by an outside observer.
  • Both machines produce a force on the surrounding medium which is approximately perpendicular to their direction of motion.

However, there are some important differences, too.

Cork is a solid, so when a corkscrew penetrates a cork, the angle of attack of the corkscrew is almost exactly zero. Air is a fluid, so it slips out of the way of the propeller, to a certain extent. As a result, the angle of attack of a propeller through air can be substantially different from zero.

Since cork is a solid, a corkscrew doesn't have to have any special shape in order to lift the cork out of the bottle. Since air is a fluid, propeller blades have to be specially designed to produce a lot of lift.

And since cork is a solid, the user of a corkscrew can screw it in slowly, rest for a while, and then pull the cork out. Since air is a fluid, propellers can't be used that way. Propellers have to spin fast in order to produce thrust.

I would say that overall, the notion that a propeller "cork screws through the air" is not very accurate. Your prior understanding of how a propeller works is probably more accurate.

That said, the next paragraph that you quote—

Secondly, the propeller spins at high speeds pushing air back and thus pushing the plane forward. The more air that is pushed back the more the plane is pushed forward.

—is 100% accurate. All aircraft engines (and all spacecraft engines!) produce equal amounts of thrust and backwash. In order for an aircraft (or spacecraft) to push itself forwards, it must produce an equal push backwards on the surrounding air (or, in the case of a rocket engine, on its own exhaust).

Tanner Swett
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    "Corkscrew" the verb has a different meaning than "corkscrew" the noun. – nick012000 Aug 21 '21 at 05:35
  • @nick012000 That's true, but I get the impression that the paragraph about "cork screw[ing] through the air" was intended to make a comparison to an actual corkscrew. If we take "corkscrew" as meaning merely "to move in a helical path," then the paragraph is merely stating that the propeller moves in a helical path and generates a force, but fails to say anything about how or why any force is generated. – Tanner Swett Aug 21 '21 at 13:16
  • @TerranSwett "Move in a helical path" is what "corkscrew" means when used as a verb, yes. It has nothing to do with corkscrews-the-noun aside from the shape. – nick012000 Aug 21 '21 at 15:18
  • @nick012000 My interpretation makes more sense to me. – Tanner Swett Aug 21 '21 at 22:06
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The corkscrew motion of the propeller is what causes the lift that causes thrust.

A propeller is, effectively, a tiny wing that pushes air backwards through the use of aerodynamic forces - that is, it generates lift. Like any wing, it generates this lift through its motion through the air, with the angle between the wing and the air body being referred to as the angle of attack, and the angle of attack plays a key role in the generation of lift.

In the case of the propeller, this angle of attack is created by the corkscrew motion of the propeller through the air: the propeller moves in a circular pattern around its base by its engine, and the plane as a whole moves forward through the air. If you were to break an instance of this motion into its components from the point of view of the propeller blade, you'd see that the rotation produces a motion "back" along the propeller blade, while the lateral motion of the plane produces a motion "down", and you'd add these together to calculate the relative wind velocity vector, which in turn would be used to calculate the angle of attack of the propeller blade along with the lift and drag forces it produces.

Like any wing, a propeller produces lift both by generating a low pressure region on top of the wing, and by redirecting air downwards through the laminar flow of air over its surface.

nick012000
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