> >
>

# Theory on lift

Tech Log The very best in practical technical discussion on the web

# Theory on lift

25th Jul 2012, 22:19

Join Date: Mar 2011
Location: engineer at large
Posts: 1,409
now you understand why winglets look like they do....
25th Jul 2012, 22:46

Join Date: May 2010
Location: PLanet Earth
Posts: 801
The funny thing is that both theories (Newton and Bernoulli) are indirectly linked to each other.
Newton assumes that air behind the wing is accelerated downward. If you follow the air movement further downstream at one point the diretion of flow will be longitudinal in line with free stream (vortices aside).
The vertical distance between the TE and this point will a) relate to the vertical component of the mass volume flow (Newton) and b) relate to the horizontal volume flow and thus the acceleration of the air above the wing (Bernoulli).

Edit: It is worth looking at different wing polars to cross check. E.g. NACA4506 vs. 2Rz12.
The former being a very thin profile with cl > 0,3 at Alpha = 0°
The latter being a rather thick profile with cl < 0,05 at Alpha = 0°.

The accleration of the air above the wing does not only depend on the thickness of the profile alone but rather on the entire 'expansion' of the flow also behind the wing itself. And that is where Bernoulli will meet Newton.

Last edited by henra; 25th Jul 2012 at 22:57.
26th Jul 2012, 01:23

Join Date: Jun 2010
Age: 32
Posts: 382
Lift is very complicated. Nobody currently knows why a wing can create lift. It's similar to the fact that nobody currently knows why there is inertia. We have detailed theories and laws where we can accurately predict how inertia will affect an object and we know inertia is proportional to mass but we don't know why. To get the best understanding of lift I've found that you need to understand as many theories regarding lift as you can. All the theories tell the story of lift that's not complete and all from different angles. When you analyze lift from all these different angles you start to build a pretty decent understanding of lift. I'd recommend getting the basic understanding of all the theories (or as many as you can understand) and then keep going over them, and every time that you do go over them you should be able to go further in depth.

Regarding the rotating cylinder: it's showing that a rotating cylinder has a similar effect as an airfoil does. They both produce lift, they both increase the speed of flow over the top and decrease the flow over the bottom. There is upwash and downwash. This will tie into Kelvin's circulation theorem. Eventually this Magnus effect should seem somewhat intuitive. Magnus effect - Wikipedia, the free encyclopedia

John Tulla posted a nice explanation here too.

No. A prop pulls, a jet pushes. A wing is the surface area which is pushed by the fluid.
That's hard to define. I think it's easy to see that a jet pushes as it accelerates a flow of air behind it at a higher velocity. But a propeller actually pulls 50% and pushes 50%. Consider a prop that accelerates the local flow velocity by 100 m/s. There will be a 50 m/s increase by the time the air reaches the propeller and the last 50 m/s increase will happen somewhere down stream of the propeller. So with either a Cessna 182 or a Piaggio Avanti II, they both pull AND push.

http://www.gidb.itu.edu.tr/staff/emi...R_THEORIES.pdf

Equation 13 and the paragraph below that show this. I like that document as the way lift is created for an airplane is the same way thrust is created for a ship. A different way of looking at it might make it clearer. The document is orientated more towards the physics inclined person.

Last edited by italia458; 26th Jul 2012 at 01:26.
26th Jul 2012, 01:40

Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
Hi Italian....

"That's hard to define. I think it's easy to see that a jet pushes as it accelerates a flow of air behind it at a higher velocity. But a propeller actually pulls 50% and pushes 50%. Consider a prop that accelerates the local flow velocity by 100 m/s. There will be a 50 m/s increase by the time the air reaches the propeller and the last 50 m/s increase will happen somewhere down stream of the propeller. So with either a Cessna 182 or a Piaggio Avanti II, they both pull AND push. "

A turbojet does the same push/pull...

In extreme cases, the intake can produce well over half of the thrust of the exhaust.

In such a case, the exhaust can be redirected to flow against the a/c forwards, and the aircraft will still fly. A jet engine is full of propellors

Put propellers in a tube, and spin them, will there be thrust? Yes.

Power these propellers with an engine that drives them mechanically, still, thrust.

Inject fuel behind the propellers, and ignite it. Thrust.

Last edited by Lyman; 26th Jul 2012 at 01:56.
26th Jul 2012, 02:10

Join Date: Jun 2010
Age: 32
Posts: 382
Lyman...

A jet engine's blades aren't there to propel the airplane forward (assuming we're talking strictly about a turbojet). They're called compressor blades because they compress the air then fuel is added and the air is ignited. The expansion of the fuel/air mixture is directed out the rear of the engine which produces the thrust that propels the airplane forward. A turbojet is really not related to a propeller.

Engine Pressure Variation - EPR

In extreme cases, the intake can produce well over half of the thrust of the exhaust.
Do you have a reference for that on a turbojet engine? - specifically where it says that it produces "well over half the thrust".

I'm assuming you're talking about ram effect. There can be a noticeable increase in thrust due to the compression of the air prior to entering the engine (ram effect) if the airplane is traveling at high speeds. But I'm unsure what you're trying to point out.
26th Jul 2012, 02:35

Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
I am proposing negative ram. The tailpipe is dramatic, but the gas comes from somewhere, in the case of the over square intake, cool air comprises more mass than the exhaust. If you could paint the exhaust red, and the intake blue, the huge blue cone in front of the engine is lower in pressure than the small red cone in the back. The aircraft is falling forward into its own 'blue hole', and the front of the intake disc is massively more involved in propulsion than the doughnuts on a rope in back.

Pratt/Whitney. J58.
26th Jul 2012, 03:26

Join Date: Jun 2010
Age: 32
Posts: 382
Regarding the J58:

At Mach 3.2, 80% of the engine's thrust came from the ramjet section, with the turbojet section providing 20%.
A ramjet is not the same as "ram effect" on a turbojet engine. You will not get even near half the thrust from ram effect on a turbojet.

Last edited by italia458; 26th Jul 2012 at 03:33.
26th Jul 2012, 03:45
Moderator

Join Date: Apr 2001
Location: various places .....
Posts: 6,528
specifically where it says that it produces "well over half the thrust".

If one looks throughout an engine, we get a bunch of pressure variation. Integrating over the various surfaces lets us express that in terms of normal forces. The observation is that, in a well designed installation, a large proportion of the nett forward thrust can be achieved from surface pressure contributions.

As to proportions, this will vary between installations.

The Concorde thread had some discussion on the subject so far as it relates to the Olympus installation and may be worth a read.

This page suggests that the Concorde had a prodigious proportion of non-engine thrust contribution in supersonic flight .. something in excess of 90%.

... standard engine installation design consideration, regardless of which OEM and model.
26th Jul 2012, 03:51

Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
Hi Italian

"The turbojet components of the engine thus provide far less thrust, and the Blackbird flies with 80% of its thrust generated by the air that bypassed the majority of the turbomachinery undergoing combustion in the afterburner portion and generating thrust as it expands out through the nozzle and from the compression of the air acting on the rear surfaces of the spikes."

To be precise, the J58 is at no time a "ramjet". The 80% thrust referenced above is not combusted air, but intake air, isolated from the turbo machinery and the after burner. It has more in common with "high bypass" turbofans in this mode than with a pure ramjet, a device that has no internal compressor section, only ignition. It is in this regard that the division of pressure at the intake cone resembles the propellor, though the zones are defined more by geography within the ducting than the metal stators, due the intense differentials.

IMO. What say you?
26th Jul 2012, 05:36

Join Date: Jan 2008
Location: Denver
Posts: 976
As to whether a given engine "pushes" or "pulls" - simply put a stressmeter on the connection between engine and airframe and determine whether the stress is in tension ("pulls") or compression ("pushes.")

You can fly with a barn door for a wing - with enough power. Water skis provide lift with little or no Bernoulli input. Just downwash and AoA.

However, Bernoulli effect makes the process much more efficient, and makes the difference between being able to fly, and being able to fly functionally (with a useful amount of payload capacity and range).

With barn doors for wings, Orville and Wilbur would have needed closer to 1,200 hp to get off the ground, rather than 12.
26th Jul 2012, 05:47

Join Date: Jun 2010
Age: 32
Posts: 382
John...

I should have been more clear. I was talking about ram effect on a basic turbojet (subsonic) engine which would provide a slight rise in compression at the entrance to the engine which would produce more thrust as it went through the burner stage.

The Concorde engine is quite a bit different than a basic turbojet. I don't fully understand the engine but it's quite interesting! I'll have to read through some of those Concorde threads... there are some very knowledgable people on here.

Lyman...

To be precise, the J58 is at no time a "ramjet".
Reference this: Ramjet - Wikipedia, the free encyclopedia

You will see this quote: "The SR-71's Pratt & Whitney J58 engines act as turbojet-assisted ramjets at high-speeds (Mach 3.2)."

Another reference: Factsheets : J58 Turbojet Engine

It's a turbo-ramjet. A ramjet won't function when there is no airflow. The turbojet is what propels the airplane up to speeds that will have the ramjet part of the engine function. The J58 is essentially a turbojet inside a ramjet.

The 80% thrust referenced above is not combusted air, but intake air, isolated from the turbo machinery and the after burner.
Not what I've read. That hot, compressed intake air is fed to the afterburner section.
26th Jul 2012, 06:49
Moderator

Join Date: Apr 2001
Location: various places .....
Posts: 6,528
I was talking about ram effect

Understood .. but that is a secondary consideration.

The end result of the pressure distribution acting on ancilliary structure can produce a significant force on that structure. Include some clever design .. and one gets a lunch heavily subsidised by Mother Nature.

CliveL, djpil (and, no doubt, others whose specific backgrounds I am not familiar with) being experienced aerodynamicists .. can speak on such things for ever and a day, I'm sure.
26th Jul 2012, 06:56

Join Date: Jun 2011
Location: West of Offa's dyke
Age: 84
Posts: 474
For an authentic explanation of how lift is developed check this one out:
The Origins of Lift

It is written around sails, but the explanation is equally valid for wings. Arvel Gentry is a practical professional aerodynamicist so can be believed. After 40 years of accepting the "Bernouilli" explanation he certainly opened my eyes!

PS: the Olympus 593 (Concorde engine) is a pure conventional turbojet and the "thrust" coming from the intake at Mach 2.0 was about 75% of the total.
26th Jul 2012, 07:18

Join Date: Jun 2010
Age: 32
Posts: 382
Owain...

Excellent article! There is a lot more to lift than Bernoulli.

Back to thrust produced by the intake...

Is there a good article that explains how this all works?

My understanding is that the air is compressed by the intake as the intake slows the air to subsonic speeds. The air is heated as a result of the compression. The resulting air is directed around the 'core' of the engine and put into the afterburner section where fuel is added to the hot air and ignited, producing significant thrust.

I had read that the air gets heated to a very high temperature and was led to believe that if fuel was introduced to the air (in the afterburner section), it would auto-ignite. Is that correct?

Since it's the intake that compresses the air that gets burned and produces 63% of the thrust on the Concorde, they say "the intake produces 63% of the thrust". Is that correct?

Last edited by italia458; 26th Jul 2012 at 07:20.
26th Jul 2012, 07:25

Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,342
Newton is the man.

All you have to find is what does the air do when a wing passes through it. It accelerates the downwards and forward, so the air gives the wing an equal force upwards and backwards.

The more air there is, the more lift (either by increased density, wing surface or angle of attack)
the faster the air is, the more air is accelerated,
the faster the air is, the more accelerated it is

hence, lift is proportional to density, wing surface and angle of attack, and to squared speed. then you use the Lift Coefficient as the constant of proportionality, only it depends on the angle of attack, and that is the formula
26th Jul 2012, 07:58
Moderator

Join Date: Apr 2001
Location: various places .....
Posts: 6,528
Is there a good article that explains how this all works?

Really only necessary if you need to be able to run some calculations for design work.

To get a general overview idea, think ...

(a) structure exists .. and we are looking more at the region around and outside the engine itself but still subject to airflow due to the engine.
(b) air is flowing over the structure due to the engine's good work and the aircraft's flight speed
(c) air has pressure which varies with local speed
(d) hence the air over different bits of structure will have slightly different pressure
(e) for each little bit of structure, the force associated with the air pressure acts normal (perpendicular) to the little bit of structure
(f) resolve the little bits of force into whatever directions are of interest (ie typically forward and aftwards are what we are after)
(g) integrate (add up) all the little bits of contributing force
(h) end result gives a nett thrust or drag
(i) the thrust or drag varies through the engine so, in some regions, we are getting useful thrust (nacelle and nozzle) while, in others, it's all drag.

One needs to keep in mind that this is the province of the aeroplane designers rather than the engine folk. The engine has to hang inside some structure and have suitable airflow conditions presented at the front and somewhere for the hot bits to get out at the back.

A similar argument can be structured for what happens inside the engine proper.
26th Jul 2012, 07:59

Join Date: Jun 2011
Location: West of Offa's dyke
Age: 84
Posts: 474
@Microburst

I'm not going to disagree with your overall position that Newton's laws can be used to calculate lift, but that is not quite the same as saying that they explain how lift is generated. If I may take an extract from Gentry's article:

Note that in this entire discussion I have not once mentioned anything about (1) the air having farther to go on the the top side of an airfoil, or (2) Newton’s laws of motion, or (3) about getting lift by “deflecting the air downward”. In the first case, there is nothing in aerodynamics requiring the top and bottom flows having to reach the trailing edge at the same time. This idea is a completely erroneous explanation for lift. The flow on top gets to the trailing edge long before the flow on the bottom because of the circulation flow field.
As for Newton, his laws are included within the aerodynamic theories discussed.
And on the "deflecting the air downward" idea, that is a three-dimensional effect. In our 2-D case, the circulation flow field causes the air out in front of the airfoil to be directed upward around the airfoil and then back down to about the same level as it started out in front. Yet due to viscous effects and resulting circulation, lift is generated. Yes, we can't fly with a two-dimensional wing and, therefore, are influenced by three dimensional effects caused by a complex trailing vortex system. We can reduce these 3-D effects by using very long wings such as on gliders or the around the world aircraft design by Bert Ruttan. On an infinitely long wing, the 3-D effects are gone and we are essentially back to looking at two-dimensional airfoil aerodynamics. If we can reduce the 3-D effects, then "deflecting the air downward" is not essential to the origins of lift.

All you have to find is what does the air do when a wing passes through it. It accelerates the downwards and forward, so the air gives the wing an equal force upwards and backwards.
Not sure about that acceleration forward bit
The more air there is, the more lift (either by increased density, wing surface or angle of attack) OK
the faster the air is, the more air is accelerated, OK
the faster the air is, the more accelerated it is
Why would that be?

Last edited by Owain Glyndwr; 26th Jul 2012 at 08:04.
26th Jul 2012, 08:24

Join Date: Jun 2011
Location: West of Offa's dyke
Age: 84
Posts: 474
@Italia

My understanding is that the air is compressed by the intake as the intake slows the air to subsonic speeds. The air is heated as a result of the compression. The resulting air is directed around the 'core' of the engine and put into the afterburner section where fuel is added to the hot air and ignited, producing significant thrust.
This is a turbojet we are talking about - there is only the "core", no fan. Fuel is not put into the "afterburner" - at least not in cruise. It goes into combustion chambers aft of the compressor and before the turbine. The 'engine' contribution to thrust is carried on the turbine blades which, contrary to one of your earlier posts, behave very much like a multi-stage propeller.

I had read that the air gets heated to a very high temperature and was led to believe that if fuel was introduced to the air (in the afterburner section), it would auto-ignite. Is that correct?
I dunno, but since the afterburner sits behind the turbine (which has a very high exit temperature anyway) the question has no real meaning

Since it's the intake that compresses the air that gets burned and produces 63% of the thrust on the Concorde, they say "the intake produces 63% of the thrust". Is that correct
Picking up on JT's comments, the intake on a supersonic aircraft is essentially a convergent/divergent passage. In the first bit (convergent) the air is slowed down from freestream speeds to Mach 1.0. In the divergent bit it is slowed from Mach 1.0 to about 0.6M or whatever the engine can accept. Over the front bit the compression generates pressure forces on forward facing surfaces and gives drag. In the subsonic divergence the pressure increases steadily going towards the engine. This part of the intake has aft facing surfaces and the increased pressures generate thrust. Whether you get a net drag or thrust depends on the Mach number and the intake design - hence Brian Abraham's comments.

The Concorde thread is a good source of information on this.
26th Jul 2012, 08:48

Join Date: Jun 2010
Age: 32
Posts: 382
Owain...

This is a turbojet we are talking about - there is only the "core", no fan. Fuel is not put into the "afterburner" - at least not in cruise. It goes into combustion chambers aft of the compressor and before the turbine. The 'engine' contribution to thrust is carried on the turbine blades which, contrary to one of your earlier posts, behave very much like a multi-stage propeller.
I wasn't talking about a turbojet, I was asking about the intake system that creates 63% of the thrust at cruise speed.

Regarding the multi-stage propeller bit: I understand that the blades act like a multi-stage propeller but I was saying their purpose is to compress the air and not to directly provide thrust like a propeller.

This image shows the Concorde's engines while in supersonic flight.

http://www.concordesst.com/graphics/engineairflow2.jpg

What is the "inlet air"? Where does that "inlet air" go? In the picture it appears that some air goes along the top, following the first arrow and bypassing the engine intake, through a tiny passage and then gets dumped at the rear of the engine right before the divergent exit. There is also another passage on the bottom of the engine where air bypasses the engine intake and meets at the rear of the engine right before the divergent exit. Does the air that exits out of those two passages contribute to the "inlet thrust"?

Edit: 8% of the thrust is from the engine in cruise - does that mean that the fuel burned is only related to that 8% that is from the engine? NONE of the thrust from the inlet is made by combustion?

Last edited by italia458; 26th Jul 2012 at 08:53.
26th Jul 2012, 09:18

Join Date: Apr 2009
Location: Wherever it is this month
Posts: 1,212
The 'engine' contribution to thrust is carried on the turbine blades which, contrary to one of your earlier posts, behave very much like a multi-stage propeller.
The turbine removes energy from the exhaust gases in order to power the compressor. In no way does the turbine provide thrust, so the propeller analogy is wrong!