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reversers and a/c speed

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reversers and a/c speed

Old 4th Jan 2011, 11:48
  #81 (permalink)  
 
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Originally Posted by PBL
With which engine component or components is that momentum-reduction primarily associated?
Answer: The thrust reverser, and it's not a component of the engine but of the nacelle (or exhaust duct). I wonder if you have ever seen the demonstration of a Harrier at a Farnborough Air Show, and thought about how it "vectors" its exhaust thrust and controls the engine that produces the exhaust "momentum"?

regards,
HN39
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Old 4th Jan 2011, 12:01
  #82 (permalink)  
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Originally Posted by PBL
With which engine component or components is that momentum-reduction primarily associated?
Originally Posted by HN39
Answer: The thrust reverser, and it's not a component of the engine
Then it isn't an answer to the question, is it?

But interpreting you more widely, if the aircraft component primarily associated with the momentum reduction is the reverser itself, why would anyone want to call the resultant force "inlet ...", or or "intake...." anything? Why wouldn't they call it "reverser vane load"?

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Old 4th Jan 2011, 12:14
  #83 (permalink)  
 
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PBL;
In my vocabulary I wouldn't call it a momentum 'reduction', because the energy added by the fuel burned in the thermodynamic cycle increases the momentum of the air mass flow that passes through the engine. The reverser merely changes the direction of the exhaust momentum, and doesn't appreciably affect either the intake momentum or the internal cycle of the engine itself.

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Old 4th Jan 2011, 14:20
  #84 (permalink)  
 
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But interpreting you more widely, if the aircraft component primarily associated with the momentum reduction is the reverser itself, why would anyone want to call the resultant force "inlet ...", or or "intake...." anything? Why wouldn't they call it "reverser vane load"?
Because that's not the case.

If blowing exhaust gasses on cascade vanes or reverser buckets were to produce a retarding force, we'd have a whole new law of physics at play (and we'd be revisiting Newton's third). We could transfer that new law to sailing ships, and let the ships use giant fans to blow on ship-mounted sails to propel the ship along.

Using a rearward engine-produced force to act on an engine-mounted or nacelle-mounted reverser mechanism to produce a greater retarding force is nonsensical.

You can certainly attempt to separate components of drag into lip drag, form drag, intake drag, or any number of other components of the ram drag equation (already provided, here); but it's largely irrelevant and meaningless. Particularly to one sitting in the cockpit.

Ram drag is a collective term which accounts for numerous components. One could blame the compressor, the diffuser, the intake, etc; "ram drag" is a useful collective term with a viable equation (again, already given here, with link provided) with accounts for the retarding value that is what slows us down during reverse action on the runway.

So far as exhaust mass gas flow being used to account for reverse thrust, it's the wrong flow. We're considering intake flow (hence, the acceptable use of "intake drag" or "ram drag," used here interchangably), not exhuast mass airflow. If we are to consider airflow through the exhuast on a high-bypass turbofan, we can only consider it as a portion of the mass airflow through the nacelle inlet; it's the mass airflow through the inlet that accounts for the retarding force in reverse operations. Considering the fuel flow in the equation, when it's only added to a small component of airflow through the engine assembly is important to the thermodynamic equation of thrust production, but not necessarily to reverse thrust. The chief value of burning fuel during reverse operations is to increase RPM and mass intake airflow, and thus intake or ram drag.
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Old 4th Jan 2011, 15:37
  #85 (permalink)  
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Originally Posted by PBL
But interpreting you more widely, if the aircraft component primarily associated with the momentum reduction is the reverser itself, why would anyone want to call the resultant force "inlet ...", or or "intake...." anything? Why wouldn't they call it "reverser vane load"?
Originally Posted by Guppy
Because that's not the case.
I'll let you argue that with HN39.

Originally Posted by Guppy
If blowing exhaust gasses on cascade vanes or reverser buckets were to produce a retarding force, we'd have a whole new law of physics at play (and we'd be revisiting Newton's third). We could transfer that new law to sailing ships, and let the ships use giant fans to blow on ship-mounted sails to propel the ship along.
New laws of physics? I think we can do that with the ones we have. I can think of at least two designs where that would work. Not that it would necessarily be efficient, mind.

It would work for airplanes also. Mount the engines in the other direction, blowing onto the wing. Smoothly, of course. And rise into the air vertically. Better idea: move the wing in the air. You'd probably have to make it go round and round to do that though. Wait, hasn't somebody had that idea already?

Originally Posted by Guppy
Using a rearward engine-produced force to act on an engine-mounted or nacelle-mounted reverser mechanism to produce a greater retarding force is nonsensical.
Really?

Originally Posted by Guppy
You can certainly attempt to separate components of drag into lip drag, form drag, intake drag, or any number of other components of the ram drag equation (already provided, here)
So now you are saying that "intake drag" is "a component of the ram drag equation", whereas, before, you were saying that ram drag and intake drag were two names for the same thing:

Originally Posted by Guppy, note 13
you're left with ram drag, also referenced as inlet drag or intake drag
But wait a minute ...... you weren't only saying it before, you're saying it again now!

Originally Posted by Guppy
We're considering intake flow (hence, the acceptable use of "intake drag" or "ram drag," used here interchangably), not exhuast mass airflow.
So, if you would please, I'd like you to decide. Is "intake drag" a "component" of "ram drag"? Or is "intake drag" identical with "ram drag"? You can have one, but not both. And when you've chosen, I'd like you to stick with the choice.

Originally Posted by Guppy
If we are to consider airflow through the exhuast on a high-bypass turbofan, we can only consider it as a portion of the mass airflow through the nacelle inlet; it's the mass airflow through the inlet that accounts for the retarding force in reverse operations.
Well, no. There is also the mass of accelerated air which passes just outside the cowl, which will be arrested in the x-direction by the reverser exhaust. That loses momentum also, so that is going to enter the momentum-loss equation too.

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Old 4th Jan 2011, 16:14
  #86 (permalink)  
 
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Well, no. There is also the mass of accelerated air which passes just outside the cowl, which will be arrested in the x-direction by the reverser exhaust. That loses momentum also, so that is going to enter the momentum-loss equation too.
No, it's not.

The perception that diverted fan or exhaust gasses cause a "wall of air" which somehow serves to slow the airplane down is largely a myth. Only where such airflow causes aerodynamic losses for the airplane can it possibly serve to slow the airplane, or where some forward component of that gas path exists. It's effect on the free airstream, or even the boundary layer adjacent to the cowl, is largely inconsequential to the process of slowing the airplane on landing.

While re-directed gas path may impinge on the free airstream and divert of slow it, this doesn't cause the airplane to slow. Only the forward component of that exhuast gas path has an effect. It's not a sail, and it's not a solid, attached component to the airplane. It's a fluid, and once vectored by the cascade vanes or buckets, it's done, insofar as any benefit to the airplane.

It would work for airplanes also. Mount the engines in the other direction, blowing onto the wing. Smoothly, of course. And rise into the air vertically. Better idea: move the wing in the air. You'd probably have to make it go round and round to do that though. Wait, hasn't somebody had that idea already?
No. If you're talking about helicopters, find a helicopter that works by mounting turbojet engines on the rotors and using the exhuast gas to flow over the rotor to produce lift. Then again, find an airplane where the same thing happens. Even then, you're talking about something entirely different than considering exhuast gas velocity with respect to reverse; entirely different mechanisms, absolutely apples and oranges.

A rotor passing through the air operates in a very similar manner to a wing passing through the air. This has nothing to do with reverser operation, however.

You seem to have run in circles and are entertaining yourself with nonsensical teasers that do nothing but cloud the issue.

Really?
Yes, really.

So now you are saying that "intake drag" is "a component of the ram drag equation", whereas, before, you were saying that ram drag and intake drag were two names for the same thing:
For the purposes of discussion, as explained here ad nauseum, yes.

When considering whether redirected exhaust or fan gasses serve as the chief retarding force, or whether the chief retarding force in reverse operation is airflow through the engine inlet, yes. We will refer to the collective inlet force interchangeably. You're welcome to break it down ad infinitum into numerous components, even to attempt to decide which compressor stage, diffuser, or even fan blade absorbs the most energy from the ram mass airflow influx, but I have no need to do so. Collectively, the drag produced by the various components that compose the total retarding force may be referred to as ram drag, and yes, I also interchangeably use intake drag, for reasons previously cited. Deal with it.
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Old 4th Jan 2011, 18:29
  #87 (permalink)  
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My, my, we do seem to be getting confused!

Originally Posted by PBL
There is also the mass of accelerated air which passes just outside the cowl, which will be arrested in the x-direction by the reverser exhaust. That loses momentum also, so that is going to enter the momentum-loss equation too.
Originally Posted by Guppy
No, it's not.
Oh, yes, it is!

(And the response is.....?)

Originally Posted by Guppy
The perception that diverted fan or exhaust gasses cause a "wall of air" which somehow serves to slow the airplane down is largely a myth.
Yes, I seem to remember someone saying something like that earlier in the thread. Who was it now? Maybe...

Originally Posted by PBL
This business of a "disc of air" generating some kind of resistance seems to me to be poppycock. As usual, being a bit familiar with basic physics helps enormously.
Your suggestion about ships was that you needed to change the laws of physics to get a self-blown sail to work. It took me a minute to think of two designs where that would work. If it takes you longer than a minute, we may conclude that I am better at aerodynamics than you are.

Concerning blown wings,
Originally Posted by PBL
It would work for airplanes also......
Originally Posted by Guppy
No. .... find an airplane where [that] happens.
Does this count? Powered Lift: Novel GTRI Design

If not, why not?

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Old 4th Jan 2011, 18:45
  #88 (permalink)  
 
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Does this count? Powered Lift: Novel GTRI Design

If not, why not?
no...it does not

why? boundary later control is what those designers are discussing, blc adds weight and complexity that's why it never is really implemented [as of yet] in a successful commercial design...I could expand this...if you wish

any questions?
we may conclude that I am better at aerodynamics than you are.
But are you better than Lester ?
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Old 4th Jan 2011, 18:52
  #89 (permalink)  
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PA

I think someone should inject Coanda, No??
 
Old 4th Jan 2011, 19:07
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Bear,

Coanda effect is based upon the law of conservation of momentum 'p'

p=mass*velocity

if mass flow is held constant then the only effective change in momentum can come about through a change in the velocity so if the velocity vector is changes this is a momentum change

a momentum change through a certain time interval produces a force [written dmv/dt] the most common example is cambered surface.held to a moving jet of a fluid...such as a spoon being held so the it's curvature faces a running faucet...also practically speaking one is adding momentum to the boundary layer...similar to what the designers mentioned in PBL's post were aiming for...in essence Coanda effect is exploited to modify the boundary layer in such a manner as to produces an aerodynamic force..

a practical example of Coanda effect is Demonstrated in the Boeing/ McDonnell Douglass NoTaR system...I hope I'm being understandable if not let me knowin fluid mechanics the concept is derived from a concept known as 'continuity'... but I try to temper my posts...sometimes
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Old 4th Jan 2011, 19:12
  #91 (permalink)  
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So, what's the answer, PA?

Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?

Do notice I said "physically possible" and not "a practical proposition"!
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Old 4th Jan 2011, 19:14
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If Boeing can do it.....I think the solid need not be cambered, though that is the default shape in a flexible mass due friction. A ceiling fan has greater efficiency and a broader disc when it is close to the ceiling. Sticky has its place. Actually it is all about placement. No mysteries then.......
 
Old 4th Jan 2011, 19:20
  #93 (permalink)  
 
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It gets murlier and murkier the further that we go in this discussion.

I thought I was satisfied several pages back and posted my undrestanding of the issue for any further enlightment of myself. Since then I have seen a lot of personal semantic arguments between a few individuals. So I would propose adding some new paricipants eager to learn communication skills which might satisfy most of us lurkers.


I'm really not sure that I yet have this correct but here's where I'm at right now.

There are two parts of the engine as a propulsion system assisting in the braking of the aircraft.

The first is the deflection of some of the engine's propulsive force (energy) forward against the motion of the aircraft. This is accomplished by turning the air entring the inlet by more than 90 degrees (reverser cascades, buckets, clamshell deflectors etc.) This is only practical at high aircraft speeds due to the inherent risk of reingesting the air and runway debris.

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.

This is accomplished by an idle condition on a running engine where the compressor and fan work are just enough to slow the inlet air down and maintain the engine cycle with very little net thrust produced. Thus the effective drag on the engine nacelles assume a plate blockage (not just the lip) while at the same time without effective rearward thrust the net effect is almost entirely a perceptive increase in drag assisting the brakes.

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.

OK, I'm quite sure that I haven't got this all correct so anybody is welcome to tweak it or entirely discard my understanding. Either way I should learn something more from the discussion.
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Old 4th Jan 2011, 19:43
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I think the solid need not be cambered
You are correct it can be a flat plate but C'mon it's Boeing


Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?

Do notice I said "physically possible" and not "a practical proposition"!
No it's not possible....a similar proposition to a conveyor belt'

That article discusses boundary layer control i.e an air jet blown though slots add energty to the boundary layer, delaying turbulent flow separation beacause of the increased kinetic energy...but that energy is very expensive and heavy...and exhaustive of fuel

I
t gets murlier and murkier the further that we go in this discussion.
I think Guppy's explanations should suffice and perhaps a review of Davies

otherwise we are all just beating Tin Lizzie

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Old 4th Jan 2011, 19:52
  #95 (permalink)  
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Originally Posted by PBL
Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?

Do notice I said "physically possible" and not "a practical proposition"!
Originally Posted by PA
No it's not possible
I see. How do you then explain the function of wind tunnels?

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Old 4th Jan 2011, 20:00
  #96 (permalink)  
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Wind Tunnels test aerodynamics by supplying free energy. Free in the sense that the test model isn't producing power, that's later, at Edwards.

Did you not say "Its own engines"?? Is this a trick question, Professor??
 
Old 4th Jan 2011, 20:04
  #97 (permalink)  
 
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or,

you can not fly an aerodynamic aircraft, as opposed to an aerostatic aircraft...without a relative wind...where is the Wind PBL?
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Old 4th Jan 2011, 20:24
  #98 (permalink)  
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No, it's not a trick question, bearfoil. It's a despairing question.

Let me explain. You can generate lift in an airfoil by blowing air across it.

I hope we can agree on this.

Suppose you attach an airfoil to a fuselage with pilot. Can you lift it vertically? Yes, by blowing air across the airfoil.

How much air? Enough.

Can you generate "enough" air from devices mounted in front of the wing? Theoretically, yes. Try the kinds of solid-rocket motors used to power the Space Shuttle.

Can these be attached to the aircraft, or must they be physically separated? Doesn't matter.

Let's go back to ships.

Mount a rotating cylinder vertically on a ship. Mount a keel under the cylinder to keep the ship from moving perpendicular to its bow. Mount a fan on the ship, let's say on the left side of the ship, which blows air across the cylinder perpendicular to the bow.

Will the ship move?

In which direction will the ship move?

My problem at the moment is that I seem to be discussing aerodynamics with people more interested in repartee than they are interested in physics or aerodynamics. Further, the repartee is not very entertaining. What a waste of time!

Except for lomapaseo's intervention, which I missed first time around (sorry!)

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.

Now, exactly how which bit of air gets momentum-reduced, and by how much, is a question no one here has yet answered. Some people don't care; they can stick with the story above. I care, and I'd like to find an answer. I will find it, but I don't think any more through this discussion.

PBL

Last edited by PBL; 4th Jan 2011 at 20:40.
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Old 4th Jan 2011, 20:53
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Originally Posted by PBL
Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?
The schemes you propose in your last post could work in principle but are blatantly inefficient.

I'm intrigued by another question of yours, but am not sure that I understand it:
How do you then explain the function of wind tunnels?
As I see it, the function of wind tunnels is to provide wind (moving air) in a laboratory environment. You can place (models of) various objects (buildings, cars, bridges, cyclists, aeroplanes, pitot tubes, etc.) in the tunnel to study the flow around them by various visualization techniques, or to measure the forces acting on them, or the pressures on their surfaces. What more needs to be explained?

regards,
HN39
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Old 4th Jan 2011, 21:24
  #100 (permalink)  
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Originally Posted by HN39
The schemes you propose in your last post could work in principle but are blatantly inefficient.
Yes and yes.

The point of the wind-tunnel comment is that you generate lift on an airfoil by blowing air across it. If you blow sufficient air across it sufficiently fast, the airfoil generates more lift than the weight attached to it, and we know from commercial aviation that this can be more than enough to lift numbers of hundreds of human beings. You just have to blow sufficient air, etc, etc.

Now, does it matter where this "relative wind" comes from? No. Just that it is present. Is there enough energy present in sources known to mankind to generate a relative wind of the required energy? Obviously, yes. Could we attach such an energy source structurally to the wing, in front of the wing? In theory, yes.

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?

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

What do you think?

PBL
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