Propeller - Rearward flow of air

[JME23]

Hi,

Could someone please help me in understanding how the air is thrust rearward by a propeller?
I am specifically interested in how an axial-flow compressor, which uses aerofoil shaped blades to force the air through the engine does so, however I feel a good start is a propeller.

It makes sense to me with the propeller how the forward force is created. Like a wing the air passing over the forward face of the aerofoil lowering the pressure on that face and the higher pressure on the rear of the propeller blade pushing forward creating the forward motion.

How the air is passed rearward however is beyond me. Especially in the case of the Compressor where its very design is to send that air rearwards.

Thanks in advance,

Jamie.

 

[.Scott]

As the propeller moves through the air, it acts exactly as a wing does.
The forward face of the propeller blade develops a lower air pressure than the rearward face. Just as the top of the wing develops a lower pressure than the bottom surface.

 

[JME23]

Thanks for the reply Scott. That is however the only bit i do understand. It's the part where the air is sent rearwards that is the sticking point with me. I assume there is also a downward motion of air on a regular wing but that isn't something discussed when looking into basic aerodynamics so I can't even use that as a comparison. I accept with the wing there is also downwash at the trailing end of the wing but that appears to have a much greater horizontal component that downward and seems to be as a result of the air flowing over the wing rather than by some induced thrust.

 

[.Scott]

Yes. There is a downward flow of air - especially during aircraft rotation (the moment the airplane pulls off the runway).
The specific name for it is "Wake Turbulence", see https://en.wikipedia.org/wiki/Wake_turbulence

I can tell you from personal experience as a sail plane pilot, that the turbulence is well below the aircraft generating it. Although, of course, it start right at the wings and wing tips. One of the standard exercises for sail plane student pilots is to box the turbulence from the tow craft. Fly left-to-right over it, down to get below it, right-to-left passing under it, then back up - all while not actually hitting the turbulence itself.

It is also important when sharing a runway with larger planes to keep a close eye on where and when they rotate. For example, while in a Cessna 172, I waited 2 minutes after the Concorde SST rotated, then went well past that point before rotating myself.

In the planes I flew, the prop blast was immediately deflected by the aircraft fuselage - forming part of the slip stream. But it is still readily distinguishable from the rest of the slip stream. Once you move about 20 inched beyond the fuselage of a Cessna 182, the relative airspeed is very noticeably lower. (yup, done that too).

There are two elements of "downwash". One is in the local downwash from the immediate area of the wing or prop. The other is over a larger area that is the result of the local downwash and the wing tip vortices. That second one represents the net downwash required to support the mass of the plane.

The loading per surface area on the prop is much greater than it is for the wings. Also, the travel of the plane is not in the direction of lift - so the wing's net downwash is spread over a much much wider area that the prop backwash.

You can combine the two. The planes we have been talking about are called "fix wing aircraft". With rotary wing aircraft (helicopters) there is very obvious downwash - as I've sure you've seen in any video of helicopter approaching a landing.

 

[boneh3ad]

I assume there is also a downward motion of air on a regular wing but that isn't something discussed when looking into basic aerodynamics

This is absolutely discussed as part of any basic aerodynamics course. Any technical discussion of lift on a wing is incomplete without discussing the resulting downwash.

I accept with the wing there is also downwash at the trailing end of the wing but that appears to have a much greater horizontal component that downward and seems to be as a result of the air flowing over the wing rather than by some induced thrust.

Sure, but the shape of a wing is optimized for different things than the shape of a propeller or an axial compressor. A typical axial compressor, for example, uses a much more curved airfoil OML than what you would see on a wing, and it changes substantially from root to tip.

 

[JME23]

Thanks also boneh3ad.

So is the airflow which opposes the thrust on a propeller (that I'm struggling to grasp) directly analogous to the downwash on an aircraft wing?....and as you say the shape of the blade is key. If a propeller blade was made of the same cross section of a wing (scaled down) rather than optimally shaped would the flow of air take a path rearward but not along the axis as expected by a conventional propeller?

 

[anorlunda]

Don't forget the analogy to ordinary fan blades. They may not be optimized like a wing, but they use the same physics.

 

[Tom.G]

If you look at propeller you will see that the blades are twisted, the leading edge is forward of the trailing edge. That means they also act like a screw.

 

[BvU]

Less problems understanding why a ships propeller pushes the water backwards ?

 

[JME23]

Yes BvU. I've always assumed this was less of an aerofoil shape and more of a reaction type blade. This is slightly more intuitive to me as I can imagine how the acceleration is transferred from the blade to the fluid in this manner. It's when it becomes an aerofoil that I get confused. That's reminded me of another question I had on centrifugal pumps but I'll save that for another thread.

 

[anorlunda]

I think it is easier to visualize with pictures than with words, that the airfoil of a wing pushes the air downward.

 

[JME23]

That definitely helps, anorlunda. So is it safe to say then that the rearward flow is created in the same way as downwash?

 

[CWatters]

I assume there is also a downward motion of air on a regular wing but that isn't something discussed when looking into basic aerodynamics so I can't even use that as a comparison. I accept with the wing there is also downwash at the trailing end of the wing but that appears to have a much greater horizontal component that downward and seems to be as a result of the air flowing over the wing rather than by some induced thrust.

Instead of thinking about air flowing over a wing, think about a wing moving through still air.

The apparent horizontal air flow you refer to is just a result of your frame of reference.

 

[CWatters]

It's interesting to consider a glider. Let's say its moving left to right past you at constant velocity in still air (no thermals). Does it push any air backwards (eg to the left) or drag some along with it (eg to the right)?

I mean overall, ignoring vorticies and local effects.

 

[JME23]

hmm. That's a tricky one. I wouldn't expect any air to move to the left (I suspect by you mentioning to ignore vortices that could have carried some air back, but even them I'm not sure). Any king of body moving through air must carry some air with it (more intuitively in front rather than behind) but even then If its a streamlined wing I'd have to say again that I'm not too sure.

 

[boneh3ad]

Keep in mind that a propeller also does not send air straight back. It move rearward and out in a spiral or vortical sort of motion.

To answer the later question, and object (glider or otherwise) will drag air along with it. The wake behind it as it passes a stationary observer will move in the same direction as the observer. This is a natural result of the finite drag on the object.

 

[rcgldr]

with the wing there is also downwash at the trailing end of the wing but that appears to have a much greater horizontal component that downward

To be consistent, you need to use the air as a frame of reference, since that is the frame of reference being used for the propeller in your question. From the air's frame of reference, as a wing passes through a volume of air, the resulting downwash component (lift) is much greater than the forwards component (drag).

To answer the later question, and object (glider or otherwise) will drag air along with it. The wake behind it as it passes a stationary observer will move in the same direction as the observer. This is a natural result of the finite drag on the object.

Is there a typo here? From an observer's frame of reference, the observer and anything stationary with respect to the observer has zero velocity and no direction.

 

[boneh3ad]

Is there a typo here? From an observer's frame of reference, the observer and anything stationary with respect to the observer has zero velocity and no direction.

Yes, it should have read that the wake would follow in the direction of the object.

 

[JME23]

Thanks everyone for your inputs. I do feel guilty that I am still scratching my head. Having just quickly recapped on what causes downwash on a wing -the vortices at the tips of the wings where the air spills over from the bottom surface to the top and spreading over the span of the wing. Is that what is also happening with the propeller, the air spilling over the tips?
I've been struggling grasping this problem with the propeller for a while but it has actually been recently I've been trying to tackle it from the perspective of an axial-flow fan compressor. I didn't bother too much about the propeller because I at least can accept how the propeller pulls the plane forward (despite not knowing how it accelerates air rearward). Obviously though with the compressor, its sole purpose is to move the air through the engine. If I'm now thinking of this airflow being a result of the air spilling over the tips of the blades then I think that's creating a whole other level of confusion.

 

[CWatters]

Thanks everyone for your inputs. I do feel guilty that I am still scratching my head. Having just quickly recapped on what causes downwash on a wing -the vortices at the tips of the wings where the air spills over from the bottom surface to the top and spreading over the span of the wing. Is that what is also happening with the propeller, the air spilling over the tips?

Downwash happens over the whole span. You would still get it even if there were no wing tips. The tips cause a rotating vortex which modern designs try to minimise. Your idea that the tips cause downwash is incorrect.

A really simple (if incomplete) way to understand how wings and props work is to look at a flat plate mounted at an angle to the airflow. The molecules of air are deflected downwards by the underside of the plate. That's one big source of downwash right there.

A velocity has direction and magnitude. Change either and you have an acceleration. So deflecting air downwards is an acceleration. Newton says force is proportional to acceleration so the resukt is an upward force on the wing (eg lift or thrust in the case of a prop).

The details are more complicated but start by making sure you understand this.

 

[CWatters]

That definitely helps, anorlunda. So is it safe to say then that the rearward flow is created in the same way as downwash?

Rearward flow created by a prop _is_ essentialy identical to downwash.

If I drew a cross section of an airofoil showing the air flow you would not be able to tell if I was drawing a section through a wing or a prop blade.

 

[rcgldr]

A really simple (if incomplete) way to understand how wings and props work is to look at a flat plate mounted at an angle to the airflow. The molecules of air are deflected downwards by the underside of the plate. That's one big source of downwash right there.

The molecules of air are also accelerated downwards from above the flat plate (or wing) to fill in what would otherwise be a void as the angled plate sweeps out a volume of air (this assumes reasonable angle of attack and speed, if the angle of attack is too great for the speed, then vortices form to fill in what what otherwise be a void, a stall condition). Note that small hand thrown balsa gliders use flat plates. By tapering the rear edge, this will make a reasonable wing for a 30 centimeter wingspan balsa glider. Link to archive of an old article:

https://web.archive.org/web/20160227232107/http://www.4p8.com/eric.brasseur/glider2.html

 

[JME23]

Thanks again CWatters and everyone else.

The tips cause a rotating vortex which modern designs try to minimise. Your idea that the tips cause downwash is incorrect.

I think I'm not so much saying that the tips cause downwash so much so as that is where I believed it to occur as that is where the air is able to escape to the upper surface creating the vortices. The downwash being at its greatest at that location and reducing in size over the span but essentially the downwash being a cause of this rather than the air being deflected downward across the span.

What you say does make more sense however rather than me trying to accept that a compresser designer would be exploiting the vortices at the blade tips for rearward flow.

I've also read that the force exerted on the air is equal and opposite to the forward thrust (propeller) or lift (wing). This scrambles my brain when thinking of downwash as isn't this more of a bi-product of lift rather that something equivalent in magnitude to it and the cause of lift?

 

[russ_watters]

I've also read that the force exerted on the air is equal and opposite to the forward thrust (propeller) or lift (wing). This scrambles my brain when thinking of downwash as isn't this more of a bi-product of lift rather that something equivalent in magnitude to it and the cause of lift?

Newton's 3rd law requires that all forces come in equal and opposite pairs.

I think there are two main reasons people tend to not realize wings and propellers are basically the same thing:

1. The rotational motion of the propeller means that unlike a wing it isn't traveling at one linear speed.

2. For lift on a wing it is easier to ignore the downwash becasue it happens behind the wing and isn't part of what you necessarily care about if you don't think about it. For a propeller, it's hard to ignore when it is literally hitting you in the face.

 

[CWatters]

I've also read that the force exerted on the air is equal and opposite to the forward thrust (propeller) or lift (wing). This scrambles my brain when thinking of downwash as isn't this more of a bi-product of lift rather that something equivalent in magnitude to it and the cause of lift?

In any discussion on how aircraft fly or how wings work you will find two groups of people arguing that their point of view is the correct one. One group argues that lift is caused by creating a pressure difference above and below the wing. The other group argue that lift is caused by accelerating air downwards causing an upward reaction force. This is sometimes referred to as the Bernoulli vs Newton debate.

This is a bit of a false dichotomy. Both points of view are correct in their own limited way.

I don't really want to see the debate repeated here. Anyone wanting to comment on this should probably add it to the existing threads.

I think I'm not so much saying that the tips cause downwash so much so as that is where I believed it to occur as that is where the air is able to escape to the upper surface creating the vortices. The downwash being at its greatest at that location and reducing in size over the span but essentially the downwash being a cause of this rather than the air being deflected downward across the span.

Downwash is greatest at the wing root and reduces towards the wing tips. This is because the increased pressure under the wing can escape sideways as well as rearwards and downwards at the tips.

 

[cjl]

The downwash is actually smallest at the tips, not greatest. Compressors also have effectively no tip vortices, as they run the tips very close to the duct around them.

As for whether downwash is a byproduct or a cause? You can kind of look at it either way. You'll never make lift without downwash, and you can't create downwash without making lift, so they basically go hand in hand.

 

[Ryoko]

Lift is proportional to the change in momentum imparted to air. This change in momentum (and hence air velocity) is observed as downwash.