Talk:Propelling nozzle

Net forward thrust

 * "For airbreathing engines, if the fully expanded jet has a higher speed than the aircraft's airspeed, then there is a net rearward momentum gain to the air and there will be a forward thrust on the airframe."

This needs clarification. Even if the rearward jet is slower, the overall thrust (with components from the inlet system and combustion chambers) is still forward. As it's worded, it sounds as if exhaust speeds higher than airspeed are a necessary condition for any thrust. The SR-71 is the infamous example of this, gaining some enormous proportion of its thrust at full speed from the inlet system. Andy Dingley (talk) 15:36, 1 July 2010 (UTC)


 * I can see what you're saying but the standard analysis is done in the frame of reference of the aircraft, so what you say isn't the case; the exhaust velocity (which is always taken relative to the aircraft) must be greater than the aircraft speed to give positive thrust; it's not true for rockets though.- Wolfkeeper 18:23, 1 July 2010 (UTC)

Doing edits
Correcting or 'picking fault' is much easier than original writing and my following observations recognize the time and dedication that went in to the original work. Pieter1963 (talk) 17:00, 2 January 2014 (UTC)

acceleration control

 * "Engines that are required to generate thrust quickly from idle use propelling nozzles with variable area. While at idle, the nozzle is set to its open configuration for minimum thrust and high engine rpm, but when thrust is needed, (e.g., while initiating a go-around) constricting the nozzle will quickly generate thrust."

This statement I believe applies to augmented engines during non-aug operation. So not every jet aircraft installation which is intimated in the statement (since all jets are required to generate thrust quickly).

So will require some clarification. Any comments welcomed. Pieter1963 (talk) 16:50, 1 January 2014 (UTC) DonePieter1963 (talk) 16:10, 17 March 2014 (UTC)

Rocket nozzles

 * "rocket motor con-di nozzles have a much greater area ratio (exit/throat) than those fitted to jet engines. The Convair F-106 Delta Dart has used such a nozzle design, as part of its overall design specification as an aerospace interceptor for high-altitude bomber interception, where conventional nozzle design would prove ineffective"

Including the F-106 in this section might lead readers to think it had a rocket-like area ratio. With a con di ejector nozzle it was typical of other US turbojet installations at that time. The YF-106,when unable to reach M2, was fitted with a con-di ejector nozzle (as well as intake improvements) ref John Fitzpatrick, Convair test pilot in "Test Pilot".

I will remove the F-106 mention from Rocket Nozzles. Pieter1963 (talk) 02:12, 2 January 2014 (UTC) DonePieter1963 (talk) 00:45, 6 March 2014 (UTC)


 * I don't really understand that bit either.GliderMaven (talk) 23:34, 5 January 2014 (UTC)

Afterburners

 * "However, the afterburner present on many combat aircraft's exhaust systems increases exhaust flow volume to the point that upstream turbomachinery can aerodynamically rematch; although some engines achieve maximum net thrust by allowing a modest rematch, all others must have a nozzle that can accommodate this upsurge by increasing its throat area. To accomplish this task, the nozzle consists of a series of moving, overlapping petals with a nearly circular nozzle cross-section."

Again, ref acceleration control, this description is based on augmented turbofans controlling the fan running line with AB nozzle area changes both with and W/o AB. So some clarification needed I think.DonePieter1963 (talk) 16:10, 17 March 2014 (UTC)

"although some engines achieve maximum net thrust by allowing a modest rematch" Unless I can find examples of this, and why, I will remove the statement as I think it leaves us hanging..wanting to know more but with no reference. Pieter1963 (talk) 02:48, 2 January 2014 (UTC) I've clarified the above in 'nozzle control during dry/wet operation'Pieter1963 (talk) 22:53, 16 March 2014 (UTC)

Principles of operation

 * "The energy to accelerate the stream comes from the temperature and pressure of the gas- the gas expands adiabatically, when done against a nozzle, this largely reversibly (and hence efficiently) cools, expands, and accelerates the gas, with the heat and pressure of exhaust gas being proportional to its speed"

Since the flow in the nozzle is compressible the relationship between the pressure, temperature and velocity is not easily expressed in words and is best left to the appropriate equations. I don't believe terms like 'adiabatically, reversibly, efficiently', although correct, are appropriate in this article. A statement such as 'the velocity increases through the nozzle as the pressure and temperature decrease' would be better. Any comments? If not I will modify. Pieter1963 (talk) 16:52, 2 January 2014 (UTC)


 * Nozzles are an excellent approximation to adiabatic expansion, so 'the velocity increases through the nozzle as the pressure and temperature decrease', unless you stick the word 'adiabatic' in there or somewhere else in the article, in my opinion that's not nearly as good.GliderMaven (talk) 23:28, 5 January 2014 (UTC)
 * Thanks for your comment. I agree it's not as good and can certainly add it. My thinking was it was one step too far for the technical level at which this article is written. I intend to rewrite this article with what I hope is a better lead-in to terms like adiabatic and all the other ones relevant to understanding aircraft nozzles, and at the same time giving a fuller chronological account of how the nozzles developed with examples to make it interesting. Hope it works OK.Pieter1963 (talk) 03:52, 6 January 2014 (UTC)

"producing a jet of exhaust gas, which, when fully expanded, has a speed that exceeds the aircraft's airspeed."

This statement is misleading. The exhaust doesn't need to be fully expanded to exceed the aircraft speed. All nozzles are underexpanded at high supersonic speeds because the exit area required for full expansion would be bigger than the nacelle/fuselage diameter. eg see p203 "Design for Air Combat" by Ray Whitford. It is risky for an author to express things in their own words unless they have a good grasp of the subject. It is much better to quote verbatim from a reliable source. This is the case with a lot of statements in this article. I hope my rewrite will be betterPieter1963 (talk) 19:26, 12 January 2014 (UTC)

Types of nozzles

 * "Disadvantages are average performance (compared to the other nozzle type) and relatively high drag due to the secondary airflow"


 * "For higher performance, it is necessary to use an iris nozzle"

Since the right nozzle for the job depends on it's particular application the above statements are misleading. For example the Concorde and SR-71 nozzles were designed to excel at their M2 and M3.2 cruise as well as at their transonic acceleration conditions. and using the secondary airflow which was an essential requirement for good intake performance, acceptable compressor distortion,external engine cooling, etc as part of the nozzle arrangement was an elegant solution. So no disadvantages there. The F-111 installation can be dismissed as being at the bottom of the learning curve for nozzle/airframe integration rather than an example of the disadvantages of the ejector nozzle. I think the above statements should be removed.


 * "Although more complex than the ejector nozzle, it has significantly higher performance and smoother airflow. As such, it is employed primarily on high-performance fighters such as the F-14, F-15, F-16"

Again, words like 'higher performance, smoother airflow' are misleading.The Concorde and SR-71 had the "highest performance",whatever that means,as demonstrated by their cruise performance. The fighter nozzles, on the other hand, had no requirement to pump secondary air for afterburner duct cooling (as with the earlier turbojet) and other things. For example, the ducts and nozzles are cooled on the inside with fan air.
 * "For higher performance, it is necessary to use an iris nozzle"

Here is an illustration of why this sweeping statement should not be used - The F-16 has an iris nozzle, because it's a bypass engine installation with low nacelle ventilation requirements (cool engine skin). Replace the engine with a J79 turbojet and the best nozzle is an ejector. Cooling air is required for the hot engine case and exhaust nozzle and is provided by the new inlet and pumped by the nozzle (see 'Improved supersonic performance for the F-16 inlet modified for the J-79 engine' by Hunter and Cawthon).

So, I think it's better to explain how things work rather than trying to specify the relative merits with blanket statements. I intend to remove these statements. Any comments? Pieter1963 (talk) 18:54, 2 January 2014 (UTC) DonePieter1963 (talk) 16:10, 17 March 2014 (UTC)

Introduction

 * "they can have variable geometry, to give different throat and exit diameters so as to deal with differences in ambient pressure, flow and engine pressure; thus permitting improvement of thrust and efficiency."

Inline with replacing incorrect or misleading statements with information from authoritative sources I will replace with '...and exit diameters to match the increasing nozzle pressure ratios which occur with flight altitude (nozzle exit pressure) and flight Mach Number (nozzle inlet pressure), eg see 'Nozzle selection and Design Criteria' AIAA-2004-3923. Pieter1963 (talk) 23:01, 3 January 2014 (UTC) DonePieter1963 (talk) 20:33, 5 March 2014 (UTC)

Convergent nozzles

 * "and in general, narrower convergent nozzles give lower thrust and higher exhaust speed, but wider convergent nozzles give lower exhaust speed and higher thrust"

The reasoning for this statement is not clear as there is no explanation or citation. The terms 'narrower' and'wider' suggest cone angle but I think it means smaller and bigger exit area. Is this referring to the effects during the initial development when the area is being established to set the compressor running line? Anybody know? I think the statement is opposite to what happens during an AB control malfunction, eg flame out causes loss in thrust if nozzle stays open. Pieter1963 (talk) 22:21, 5 January 2014 (UTC) DonePieter1963 (talk) 16:10, 17 March 2014 (UTC)

Convergent-divergent nozzles

 * "variation in the nozzle pressure ratio that engine throttling creates"

Engine throttling, or differences in thrust setting, does not produce nozzle pressure ratios that warrant a divergent section. Although it is flight speed that does that, even at high subsonic speeds the nozzle pr still only requires a convergent nozzle.Neither does the addition of afterburning since it's the nozzle inlet temperature that goes up, not the pressure. So even a high subsonic afterburning engine still only requires a convergent nozzle, eg see AIAA-2004-3923

Replace with '...nozzle pressure ratio that occurs with a large range of flight Mn and altitude'. Pieter1963 (talk) 23:30, 3 January 2014 (UTC) DonePieter1963 (talk) 16:10, 17 March 2014 (UTC)

Low ratio nozzles

 * " At the other extreme, some high bypass ratio civil turbofans control the fan working line by using a convergent-divergent nozzle with an extremely low (less than 1.01) area ratio on the bypass (or mixed exhaust) stream. At low airspeeds, such a setup causes the nozzle to act as if it had variable geometry by preventing it from choking and allowing it to accelerate and decelerate exhaust gas approaching the throat and divergent section, respectively. Consequently, the nozzle exit area controls the fan match, which, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake chokes the throat and causes the nozzle's area to dictate the fan match; the nozzle, being smaller than the exit, causes the throat to push the fan working line slightly toward surge. This is not a problem, however, for a fan's surge margin is much greater at high flight speeds."

This section appears to be verbatim from a textbook, perhaps, but since it has no citation and introduces terms like 'fan working line, fan match' I will add a reference that will help readers to look further if they are interested. OTOH perhaps it is from Ref1 (C, R and S). I don't have the 5th edition. Can anyone confirm this? Pieter1963 (talk) 04:09, 6 January 2014 (UTC)

Review requested
I have added sections and many references. The original content is still there but reworded to clear both ambiguities and statements requiring more clarification.See above for details.

Please give feedback.Pieter1963 (talk) 16:50, 17 March 2014 (UTC)

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Terima Kasih — Preceding unsigned comment added by 36.84.231.107 (talk) 07:27, 15 February 2016 (UTC)

Nozzle operation
In section 'Principles of operation':

"A nozzle operates by using its narrowest part, or 'throat', to increase pressure within the engine by constricting airflow"

Emphasis on "increase pressure by constricting airflow"; the Venturi effect would normally cause the pressure to decrease, and the velocity to increase. An error, or some effect I'm not taking into account? Hayazin (talk) 08:29, 11 May 2016 (UTC)