reversers and a/c speed

[HazelNuts39]

Originally Posted by PBL

Does the aircraft rise vertically? (...) What do you think?

Yes, initially. In the longer term, you need to take care of drag and moments. It's called powered lift. The FAA has even proposed a set of regulations for large airplanes using that principle. The Breguet 49something (forgot the number) came in that category.

Any chance of returning to subject?

regards,
HN39

[SNS3Guppy]

Suppose we do that, and we do it in such a way as to generate uniform airflow across the wing (by channelling it). And we start the whole contraption up. Does the aircraft rise vertically?

No. The rearward vector of the thrust provided by the fan will provide forward propulsion. If you intend to provide enough thrust to cause adequate airflow over the entire airfoil to develop lift, you're going to be providing enough thrust to drive the entire vehicle forward, too; it's not a vertical operation, but a forward operation while developing lift.

Some here want to say that can't possibly happen.

What do you think?

I think you're still playing spin doctor with ridiculous theories. You might be alluding to coanda effect and attempting to reinvent the airplane and helicopter, but you're mixing your efforts and disregarding basic physics.

You're dangerously close to resurrecting the old treadmill hypothesis, which had no merit from the start. Your attempt at a parallel with a wind tunnel is without merit.

While certain aircraft have used bleed air injected along the top edge of the airfoil to enhance boundary layer aerodynamics, and various devices such as slats and slots are used to enhance boundary layer aerodynamics or delay separation, and some multi engine airplanes do benefit from propeller airflow over the wing to some degree, mounting engines or fans over the span of the wing to produce wing lift vertically isn't happening.

While I suspected early-on that you had some clue whence you speak on these matters, I'm more and more convinced that you're neither pilot nor engineer, and appear to be tossing smoke in the air for lack of something intelligent to offer. Your posts are tantamount to flame-bait.

Loma, consider. A mass of air is accelerated towards the engine inlet duct (by sucking). Some of it goes in the duct, some of it goes around. But it all gets stopped (relative to the aircraft); the stuff inside by plates, the stuff outside by encountering reverse-velocity air (formerly the stuff inside).

You have to put energy in to stop all that air, but stopped it gets.

Now, that air had momenttum. All that momentum goes somewhere (conservation of, and so on). Where does it go? If you have designed things cleverly, it goes into force. In which direction? The same: negative-x (that is what conservation of momentum means). That negative-x direction force is experienced as braking by the airplane+occupants.

The mass of air accelerated toward the engine duct occurs in part by "sucking," and in part by ram effect (hence the term, "ram drag."). The airflow outside the nacelle is irrelevant. Whether it it is accelerated or not by reverse ran gas flow is irrelevant.

The function of reverse gas flow (by blocker doors or cascade vanes) is that of Newton's third law. The opposite reaction to vectoring gas flow forward is a retarding force which contributes to what we experience as "reverse thrust." The reverse flow diverted fan and exhaust gasses do not push against the free airstream outside the nacelle, in order to slow down. While an interaction with the diverted airflow will certainly occur, that interaction doesn't contribute to slowing the airplane down during reverse thrust operations.

The second means of assisting the braking of the aircraft is to increase the engine nacelle drag (frontal area of the engine) by slowing the air down that passes through the engine without producing significant forward thrust or turning the reverser air forward enough to cause reingestion.

Frontal area isn't the issue, nor the equation: this isn't a flat-plate drag issue.

You're probably already familiar with the concept of a windmilling propeller causing substantially more drag in a light twin than a stopped prop: you're hopefully also familiar with the fact that the windmilling propeller produces more drag than a flat plywood disc of the same diameter as the propeller. The issue, then, isn't simply frontal area or flat plate aerodynamics.

While energy is extracted from the slipstream to rotate a propeller and engine, and all it's associated drag, the effects of the turbine engine are substantially more complex with respect to the energy extracted. This extraction occurs at multiple points during entry and processing of the mass airflow, from the inlet through the compressor and diffuser and turbine, to say nothing of the fan itself.

Disregarding airflow through the engine and it's various complexities, consider only the airflow through the fan, bypassing the core. The fan acts as a series of propeller blades or airfoils, each one producing "lift" in a rearward direction: thrust. A function of the production of lift in an airfoil is also induced drag. Airflow through the fan isn't simply imparted energy to thrust without any loss; that loss occurs as drag across the fan. This is but one of many areas in which energy is absorbed in the engine, or lost from the airstream, which account for the difference between gross thrust and net (usable) thrust. That difference is properly termed "ram drag," and is a collective figure comprising various components of drag.

If the reduction of the engine reverser efflux is done at low aircraft speed and low thrust settings, the pilot should perceive no changes in retarding force.

Very little benefit will be perceived (or found) at slower speeds. The disparity between gross and net thrust diminishes as ram drag diminishes, at lower power settings and lower speeds, just as previously shown.

When at a standstill, the only tangible retarding force, or reverse force, will be the redirected fan or gas flow.

It's called powered lift. The FAA has even proposed a set of regulations for large airplanes using that principle.

Powered lift is entirely different: that's the V-22 Opsrey, and other aircraft of the same pattern design.

[TheChitterneFlyer]

I'm now somewhat seriously confused by trying to follow all of the contributions to this thread.

I have now considdered those "hot and cold nozzles" of the Harrier and, how we have managed to balance several tons of airframe upon a vertical stream of thrust; all of that "inlet drag", or whatever each individual might wish to call it, has stopped the aeroplane from plummeting to earth by redirecting the thrust path into a "more useless path" other than a "forward thrust" direction!

I've no wish to become "Devil's advocate", but...

Do forgive my lack of scientific understanding of what has been previously debated, but, "How does that work"?

TCF

[Pugilistic Animus]

----------

PBL as Guppy asks where's the forward component of thrust?



The Encyclopaedia Britannica: a ... - Google Books

a brilliant example of mathematical induction from Bernoulli senior--that's totally wrong

[SNS3Guppy]

PBL as Guppy asks where's the forward component of thrust?

Say again?

[PBL]

Originally Posted by Guppy

While I suspected early-on that you had some clue whence you speak on these matters, I'm more and more convinced that you're neither pilot nor engineer, and appear to be tossing smoke in the air for lack of something intelligent to offer. Your posts are tantamount to flame-bait.

Guppy, who I am is no secret, and neither is elementary physics, although both these things seem to be a mystery for some here.

I admit here I really don't know how to discuss with you. Since we're talking physics and aerodynamics, it would seem that a common understanding of the basics of these sciences is necessary to have a fruitful discussion. That isn't there, because one of us apparently doesn't have those basics.

You think it needs new laws of physics to get a ship to sail forward under own-generated wind (your example), whereas any aero student would fail a first course if heshe couldn't see how to do it. You think a wing can't fly under self-generated wind alone, without "forward thrust", whereas in the standard intro-aero text to which I often refer, thrust isn't even mentioned until half-way through, after four hundred plus pages of how airfoils work. We're worlds apart, and I am not about to alter mine, because my day job and those of my colleagues are way too bound up with it. And I'll suggest that yours is too!

When I find out the answers to my questions about the relative contributions of the various engine parts to the loads under reverse thrust, I'll share them here. Thank you, Guppy and HN and others, for a lively debate!

PBL

[de facto]

Im lost,jeez requesting radar vectors

[SNS3Guppy]

You think a wing can't fly under self-generated wind alone, without "forward thrust", whereas in the standard intro-aero text to which I often refer, thrust isn't even mentioned until half-way through, after four hundred plus pages of how airfoils work.

Thrust isn't necessary for an airfoil to develop lift, obviously.

That a text doesn't address thrust as one of the four forces of flight until lift and drag have been thoroughly treatised is no surprise. Again, however, you mix apples and oranges.

Your mythical airplane which develops lift by blowing air over it's airfoil, independent of any motion of the free airstream, is more akin to a perpetual motion machine than the result of anything more than junk science. These magic fans, spaced the full span of the wing, will produce thrust if attached to the airframe, if producing enough thrust to produce lift. That amount of thrust will produce forward motion, whether or not you desire it, whether or not the airfoil requires thrust to create lift.

Of which airplane in the world are you aware that develops lift and rises vertically by employing fans to blow over it's airfoil, without need nor means of forward propulsion? This miracle method of yours, of which every first-year student is intimately aware: why is it not standard in the industry (or more precisely, why is it not employed at all)?

When I find out the answers to my questions about the relative contributions of the various engine parts to the loads under reverse thrust, I'll share them here.

Do what will float your boat. As I stated at the outset, I'm not particularly interested in the specific values to which each component may contribute: it's enough to address the salient point that ram drag, and not reverse gas flow, accounts for the retarding force in reverse thrust landing operations. More appropriate to the point of the thread is the relationship of rollout speed to the effectiveness of reverse thrust; this is owing to the effects of ram drag (and such components as may interest you, of which the collective ram drag is composed).

[HazelNuts39]

Wind? (edited)

Originally Posted by SNS3Guppy

These magic fans, spaced the full span of the wing, will produce thrust if attached to the airframe, if producing enough thrust to produce lift.

Guppy, I agree with most of what you write, but here there might just be a slight misunderstanding. You are quite right if the "wind" over the airfoil is generated by propellers or fans mounted in front of the wing. But that's not how I interpreted PBL's hypothetical proposition:

Originally Posted by PBL

Now, does it matter where this "relative wind" comes from? No. Just that it is present.

What if the source of the "wind" is a large reservoir of compressed air?

it's enough to address the salient point that ram drag, and not reverse gas flow, accounts for the retarding force in reverse thrust

Why not both?

regards,
HN39

[PBL]

Originally Posted by HN39

What if the source of the "wind" is a large reservoir of compressed air?

Or, as I mentioned above in Note 98,

Originally Posted by PBL

..... solid-rocket motors

And to the question "what airplane flying would do this"?, I gave the answer in Note 91:

Originally Posted by PBL

Do notice I said "physically possible" and not "a practical proposition"!

In both these cases, it would obviously be more practical to turn the compressed air, respectively rocket motors, towards the ground, rather than towards a wing which was to lift one off the ground. This is a matter of understanding physical principle, not of practice.

As to the suggestion that

Originally Posted by HN39

Guppy, I agree with most of what you write

I am having trouble finding any agreement between attributing most of the braking force to "intake drag"/"inlet drag"/"ram drag" (which we are told all mean the same thing) and attributing most of the momentum-reduction to

Originally Posted by HN39

[t]he thrust reverser, and it's not a component of the engine but of the nacelle (or exhaust duct)

And lomapaseo had this difficulty also, in Note 93.

PBL

[HazelNuts39]

Originally Posted by me

Why not both?

Let's consider a straight (non-bypass) jet engine with thrust reverser. Let's assume that the reverser buckets can be set in four positions and deflect the exhaust gas stream without loss of momentum (hypothetically). In the first position the buckets are stowed around the jet pipe: forward thrust mode (zero deflection). In the second position the buckets intercept the exhaust gas stream and divide it in two 'plumes', deflected 60 degrees from the x-direction, still producing forward thrust, but less. In the third position the buckets deflect the gas stream 90 degrees from the x-direction for zero forward thrust. Finally, 120 degrees deflection from the x-direction results in negative thrust, i.e. drag, or thrust for power-back from stand-still. Then we could have for example the following thrust or drag forces:

Deflection (degrees) ........... 0 .... 60 .... 90 ... 120
Gross thrust ................. 100 ... 100 ... 100 ... 100
X-component of gross thrust .. 100 .... 50 ..... 0 ... -50
Ram drag ...................... 40 .... 40 .... 40 .... 40
Net thrust .................... 60 .... 10 ... -40 ... -90

regards,
HN39

P.S. Just in case you don't have your slide rule handy: cos 60 = 0.5 and cos 120 = -0.5
P.S.2 As said earlier, the ratio between gross thrust and ram drag changes with airspeed and power setting.

[SNS3Guppy]

Why not both?

I did address both from the outset, specific to the point that redirected gas flow is the minor contributor, and that the effects of ram drag variable with airspeed (and RPM) are very germane to the question of why we apply reverse at higher speeds, earlier in the landing, and remove it later on.

In both these cases, it would obviously be more practical to turn the compressed air, respectively rocket motors, towards the ground, rather than towards a wing which was to lift one off the ground. This is a matter of understanding physical principle, not of practice.

Do you realize that you just responded to your own quote? You're quoting yourself.

You're doing so incorrectly, and answering yourself incorrectly, but you're actually talking to yourself. I find that rather telling.

You're now attempting to draw a parallel between a wing which produces it's own lift with no forward motion, to rocket motors, or airflow directed not at the wing but downward? You're now introducing thrust again (after going to the trouble of telling us that it's irrelevant and isn't found until the four hundredth page of your favorite text. You've gone from blowing airflow over a wing and making it rise vertically, to directing thrust downward. Why not get rid of the wing entirely, in that case?

All of which is entirely irrelevant to the matter of reverse thrust. While redirected gas flow, often at a shallow angle, does little to contribute to the slowing action of an airplane during reverser operation on landing, certainly ram drag and the inherent loss of efficiencies between the gross thrust and net thrust produced by the engine do account for the lions share of the change in velocity when in reverse range.

What if the source of the "wind" is a large reservoir of compressed air?

What if it is?

[bearfoil]

I have always seen RT as a modification of the vector aft of thrust to include a narrow feather sliced off in a few locales, and pointing other than aft, Mostly at best to the side, and lacking in actual merit by lacking a redirection forward. Coanda and the Blown whatever notwithstanding, Newton rules, as ever.

As ever Hazelnuts39 has everything nailed down, RATS!!

Wait, two nozzles at the center of the wings, blowing in opposite direction spanwise over the airfoils....nah, that won't work...wait

Pah!!

[HazelNuts39]

Bearfoil;

one nozzle, two buckets!

regards,
HN39

[blind pew]

Sir Richard
during my conversion on to the VC 10 it was demonstrated what happens when you use differential braking at low speed with a large tiller angle - the warning horn went off to say that the nose wheel had disconnected from the tiller.
Fortunately it hadn't gone too far and the skipper managed to get it reconnected without calling for a tractor in the neck at Man.
The engineer and I had a good laugh as the regular copilot and the skipper had "history".
I never had the misfortune of flying with them again.

[Pugilistic Animus]

as Guppy asks where's the forward component of thrust?

No. The rearward vector of the thrust provided by the fan will provide forward propulsion. If you intend to provide enough thrust to cause adequate airflow over the entire airfoil to develop lift, you're going to be providing enough thrust to drive the entire vehicle forward, too; it's not a vertical operation, but a forward operation while developing lift.

Sorry the freebody diagram is burnt into my mind and the thrust arrow points forward...I think we mean the same thing...sorry If I've misquoted you...

I think that the problem that we see with PBL's proposition is there can be no downwash w/o forward movement...how otherwise, does the air get pushed down to push the plane up?...I'm still not seeing any possible contrivance that could do such a thing how does sufficient air get deflected downward to lift the airplane without forward movement..the airplane can not simply rise vertically w/o forward movement

[Turbine D]

Pugilistic Animus

Seeing is believing....

YouTube - F-35 Take Off & Landing

Turbine D

[SNS3Guppy]

Seeing is definitely not believing, as that does nothing to support the case of the aircraft blowing air over it's own wing to produce vertical lift and rise.

The F35 in that image is rising based on raw thrust and nothing more.

[Turbine D]

SNS3Guppy

OOps!

Try this version, a little closer but probably not quite there yet....

YouTube - Ryan XV-5 Vertifan

Turbine D

[SNS3Guppy]

No change.

Again, raw thrust for vertical flight; the aircraft does not rise by producing airflow and blowing it over it's own wings to produce lift.

Still does nothing to illustrate PBL's argument.