SDF Aerospace and Aerodynamics Corner

Engineer said:

Just because two inlets generate the same type of shocks, that does not make them the same type of inlets. Variable-geometry and fixed inlet both generate oblique shocks, but the two inlets are clearly of different types. Likewise:

• F-104's inlets were designed in 2D whereas DSI is designed in 3D;
• F-104's inlets have diverter while DSI has none;
• F-104's inlets feature axis-symmetric cone whereas DSI does not

mean the two inlets are not of the same type. They cannot be the same when they have difference.



Go on and keep on comforting yourself with that. The fact is that fixed inlet has application on hypersonic test vehicles, meaning fixed inlet can function above Mach 3, nice and simple. This means your argument that fast flying aircraft use variable-geometry inlets because only variable-geometry inlets can fly at high speed is a flawed one.



It means you are still trying to divert attention away from the fact that fixed inlet is employed in flight condition where speed is above Mach 3.

Operation limits exist on variable-geometry inlets too, as evident by the following graph. This is why F-14 and F-15 cannot fly at Mach 3. Also, the fact that many aircraft that employ variable-geometry inlets cannot reach Mach 3 is quite telling, because it shows factors other than variable-geometry determine the top-speed these aircraft can travel. Factors such as the number of shocks generated, whether the airframe has low enough drag, or whether the engines are powerful enough.



Variable throat is a consequent of having to vary the ramps' angle to position the oblique shock waves. This means if you want the oblique shocks at certain position, then throat area would become certain size regardless of whether you want more or less air. Bypass doors are what control mass flow, and this is why an inlet that has variable-geometry requires bypass doors, yet an inlet that has bypass doors does not require variable-geometry.



Fixed inlet can be found on hypersonic test vehicles that travel at higher speed than Mach 3. This is a fact. Whether or not the test vehicles can propel themselves to their operating speed without the aid of rocket does not eliminate the above fact.

Furthermore, being able to employ mixed compression is not an ability exclusive to variable-geometry inlet. Fixed inlet can operate in hypersonic regime precisely because mixed compression is employed. Mixed compression allows an inlet to operate at high speed because the compression method generates higher number of oblique shocks than external compression, which means it is the number of oblique shocks that correlates inlet performance. Adopting a variable-geometry inlet would not magically make an aircraft travels at Mach 3, and indeed F-14 and F-15 cannot fly at Mach 3.




Once again, whether or not the test vehicles can propel themselves to their operating speed without the aid of rocket does not eliminate the fact that fixed inlet can operate at above Mach 3. An operational range does not mean the maximum speed of an inlet is capped at certain Mach number. The lower bound and the upper bound of that operational range can shift, meaning maximum speed at which an inlet can operate can shift, too.

Variable-geometry inlet also has operational range, as evident by the following graph. This is why aircraft like F-14 and F-15 cannot reach Mach 3 despite employing variable-geometry inlets.

It is also why as speed decreases, variable-geometry inlet loses performance. From

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:

So, it's not like with variable-geometry inlets, a flight vehicle can go from 0 to Mach 5 either. Theoretically, it could be done, but the inlet would be so heavy as to be practically worthless. Then there's the issue of engine: at hypersonic speed, a ramjet or scramjet is needed.

Click to expand...

Look engineer, all what you write is utterly false, you might think you are right and all the fallacies you say are true, but the reality is this:

DSI and F-104 are cone fixed intakes, the difference is the F-104 uses a diverter which is a 1950s solution, even wikepedia disagree with you.

second the variable thoat controls the air mass flow, the F-111 paper shows it, the books confirms it and many webpages say it.

Third there is not a single jet with fixed intakes that uses turbofans or turbojets and reaches mach 3
every design shown incorporated some sort of variable geometry, be it a translating spike, hinged doors or a variable throat. Variable geometry was needed, of course, because the range of flight speeds for the vehicle (or engine) required operation under a variety of conditions. The variable geometry was used to improve aerodynamic performance, match engine airflow requirements, and to maintain inlet starting



The purpose of the variable throat and bypass system, as with the SR-71, was to position the terminal normal shock slightly downstream of the aerodynamic throat to promote stable operation and to maintain self-starting.











If the throat area (A2) is too small, corresponding to point c in Figure 2.25, a detached shock will stand ahead of the inlet, as shown in Figure 2.25a. By increasing the throat area, the shock can be moved to the inlet lip. When the operating point a of Figure 2.25 is reached, the shock will reach the inlet lip and ...




why DSI are used?
The DSI concept was introduced into the JAST/JSF program as a trade study item in mid-1994. It was compared with a traditional "caret" style inlet. The trade studies involved additional CFD, testing, and weight and cost analyses. The new inlet earned its way into the JSF design after proving to be thirty percent lighter and showing lower production and maintenance costs over traditional inlets while still meeting all performance requirements.



To evaluate the validity of the intake analysis tool abpi and to obtain the general rules in the specific conditions
applied to the existing in-flight or model intakes, the following intakes are investigated with abpi:
- Concorde intake
- XB-70 intake
- Japanese HYPR scaled intake
- Japanese S-engine intake for hypersonic pre-cooled turbojet engine propulsion
All the intakes above are rectangular, variable geometry intakes. This configuration corresponds to the expected
selection of the intake type to the LAPCAT M4 intake.
Following [8], the supersonic intakes applicable subsonic in-duct configuration, which means excluding
SCRam configuration, are classified into three; Mach2+, Mach 3 and Mach 4-6 intakes



In the case of the simplier external compression inlet, such as those on the Concorde M01⁄4 2 supersonic transport, internal ramps are extended by an actuator, thereby decreasing the throat area for the reduced mass flow requirement in supersonic cruise as shown in figure 9.17......... In the take-off configuration shown in Figure 9.19a, the bleed door is opened and the hinged inlet ramp is deflected upward to permit inflow of the additional air needed for high thrust situations......

Air Force Brat said:

Yeah thanks guys, engineer I am reading the J-20 thread where you pointed out to me, very interesting reading, the math is way over my head but I can try to wrap my mind around the fluid aspect of air and try to picture what is happening. I knew a little about turbulent flow off the close coupled canards, but this is making that more clear. Thanks to you both. The J-20 thread is real boring without the aero part of the discussion although there are some neat pictures and vids that also help.

Click to expand...

this article i think is good

The F-35 transonic acceleration specifications were written based on clean-configuration F-16 Fighting Falcon and F/A-18 Hornet fighter, Burbage said.

But unlike the Hornet or the F-16, the F-35 has the same configuration unloaded as it does loaded with weapons and fuel, Burbage said. When an F/A-18 or F-16 is encumbered with weapons, pylons and fuel tanks, those jets are robbed of much of their performance.

“What is different is that this airplane has accelerational characteristics with a combat load that no other airplane has, because we carry a combat load internally,” Burbage said, the F-22 Raptor notwithstanding


Now just apply it to F-22 versus F-15 or T-50 versus Su-27 or MiG-29 and of course J-20 versus Su-27/J-11

MiG-29 said:

Look engineer, all what you write is utterly false, you might think you are right and all the fallacies you say are true, but the reality is this:

Click to expand...

Nope.

First of all, I am actually right: DSI and F-104's inlets are not of the same type; number of oblique shocks correlate with performance of an inlet and the speed at which the inlet can operate; fixed inlet can operate at above Mach 3.

Second of all, fallacies refer improper reasonings employed in argument, and the only fallacies in this thread are your arguments. Example of your fallacies include:

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  because you use isentropic shock to generalize DSI and F-104's inlets are of the same type;
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  because you post facts that are irrelevant with the point of contention;
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  because you keep on trying to shove words in my mouth.

MiG-29 said:

DSI and F-104 are cone fixed intakes, the difference is the F-104 uses a diverter which is a 1950s solution, even wikepedia disagree with you.

Click to expand...

Wrong. DSI is not cone intake, Wikipedia actually disagrees with you. Referring to DSI:

Wikipedia said:

It consists of a "bump" and a forward-swept inlet cowl...

Click to expand...

A bump is not a cone. Furthermore:

Wikipedia said:

The DSI can be used to replace conventional methods of controlling supersonic and boundary layer airflow, such as the intake ramp and inlet cone, which are more complex, heavy and expensive.

Click to expand...

The keyword is "replace", meaning the ramp and cone disappear and sitting in their place is something else.

Also, although DSI is fixed, it specifically refers to inlets that do not employ diverter. Fixed inlet has diverter. That's why when we refer to the inlets on F-35, J-10 and J-20, we use the term DSI and not fixed inlet.

MiG-29 said:

second the variable thoat controls the air mass flow, the F-111 paper shows it, the books confirms it and many webpages say it.

Click to expand...

What books and various websites say is that variable-geometry controls position of shock, with bypass to control mass flow. What the F-111 paper show is the change in mass flow ratio, but change in mass flow ratio occurs on inlets with no variable-geometry and is not an exclusive phenomenon on variable-geometry inlet.

Mass flow ratio changes on fixed inlet can be seen in the graph below, showing the change is independent to throat geometry:

MiG-29 said:

Third there is not a single jet with fixed intakes that uses turbofans or turbojets and reaches mach 3

every design shown incorporated some sort of variable geometry, be it a translating spike, hinged doors or a variable throat. Variable geometry was needed, of course, because the range of flight speeds for the vehicle (or engine) required operation under a variety of conditions. The variable geometry was used to improve aerodynamic performance, match engine airflow requirements, and to maintain inlet starting

The purpose of the variable throat and bypass system, as with the SR-71, was to position the terminal normal shock slightly downstream of the aerodynamic throat to promote stable operation and to maintain self-starting.

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If the throat area (A2) is too small, corresponding to point c in Figure 2.25, a detached shock will stand ahead of the inlet, as shown in Figure 2.25a. By increasing the throat area, the shock can be moved to the inlet lip. When the operating point a of Figure 2.25 is reached, the shock will reach the inlet lip and ...

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Click to expand...

Yet, fixed inlet can operate above Mach 3, proven by their existence on hypersonic test vehicles. This is contrary to your claim about top-speed limit of inlets.

You wish fixed inlet cannot operate above Mach 3, because one of your premises is that speed represents inlet performance and only variable-geometry inlet can operate in high speed. Now that you are shown fixed inlet has operated at higher speed that variable-geometry inlets, you try to divert attention away by saying that hypersonic test vehicles cannot power themselves from 0 to hypersonic speed, which is and is a fallacy. It does nothing to eliminate the fact that fixed inlet can function perfectly above Mach 3.

MiG-29 said:

why DSI are used?
The DSI concept was introduced into the JAST/JSF program as a trade study item in mid-1994. It was compared with a traditional "caret" style inlet. The trade studies involved additional CFD, testing, and weight and cost analyses. The new inlet earned its way into the JSF design after proving to be thirty percent lighter and showing lower production and maintenance costs over traditional inlets while still meeting all performance requirements. [/SIZE]

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Click to expand...

A lighter structure with performance improvement over variable-geometry inlets is a net performance increase, and this is why DSI is used on J-10B.

In contrast, to get performance improvement on variable-geometry inlet, then the complexity and weight must also increase. The extra complexity and weight are penalties.

MiG-29 said:

To evaluate the validity of the intake analysis tool abpi and to obtain the general rules in the specific conditions
applied to the existing in-flight or model intakes, the following intakes are investigated with abpi:
- Concorde intake
- XB-70 intake
- Japanese HYPR scaled intake
- Japanese S-engine intake for hypersonic pre-cooled turbojet engine propulsion
All the intakes above are rectangular, variable geometry intakes. This configuration corresponds to the expected
selection of the intake type to the LAPCAT M4 intake.
Following [8], the supersonic intakes applicable subsonic in-duct configuration, which means excluding
SCRam configuration, are classified into three; Mach2+, Mach 3 and Mach 4-6 intakes

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Click to expand...

This doesn't dismiss the fact that fixed inlet can operate above Mach 3.

From the same source, what enables the higher speed is the switch from external to internal/mixed compression. Indeed, mixed compression is the method employed by fixed inlet on a hypersonic test vehicle.

MiG-29 said:

In the case of the simplier external compression inlet, such as those on the Concorde M01⁄4 2 supersonic transport, internal ramps are extended by an actuator, thereby decreasing the throat area for the reduced mass flow requirement in supersonic cruise as shown in figure 9.17......... In the take-off configuration shown in Figure 9.19a, the bleed door is opened and the hinged inlet ramp is deflected upward to permit inflow of the additional air needed for high thrust situations......

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Click to expand...

This is a fallacy called , because you removed materials in such a way to misrepresent the passage's original meaning. Also, I have see that you have once again increased your font size, from the previous 6 to 7. No matter how large a font you use, screaming isn't going to add any more strength to your argument.

But I digress, so allow me to highlight those parts that you have conveniently left out:


Throat area is reduced and indeed there is a reduction in mass flow, but this reduction is attributed to the gap which is part of the bypass system. In addition, at supersonic speed the ramp is undeflected and bypass door is opened to regulate flow:


Furthermore, performance of an inlet correlates with the number of shocks, not whether the inlet has variable-geometry. Thus, by employing mixed compression, the fixed inlet on a hypersonic test vehicle has higher number of shocks, resulting in higher performance than a variable-geometry inlet that utilizes external compression.

Engineer said:

Nope.

:

But I digress, so allow me to highlight those parts that you have conveniently left out:

Throat area is reduced and indeed there is a reduction in mass flow, but this reduction is attributed to the gap which is part of the bypass system. In addition, at supersonic speed the ramp is undeflected and bypass door is opened to regulate flow:

Furthermore, performance of an inlet correlates with the number of shocks, not whether the inlet has variable-geometry. Thus, by employing mixed compression, the fixed inlet on a hypersonic test vehicle has higher number of shocks, resulting in higher performance than a variable-geometry inlet that utilizes external compression.

Click to expand...

let us see if it is the gap


If the throat area (A2) is too small, corresponding to point c in Figure 2.25, a detached shock will stand ahead of the inlet, as shown in Figure 2.25a. By increasing the throat area, the shock can be moved to the inlet lip. When the operating point a of Figure 2.25 is reached, the shock will reach the inlet lip and


where does the article mentions the bypass doors?

In fact the drawing has no bypass doors it is a venturi tube.


there is no bypass door in that example, that shows first is not the shock since the shock is moved by the throat size, and in that case the throat goes bigger or smaller, and by controlling the shock position the throat controls mass a small detail engineer you did not think.


hey why in the F-111 the mass flow also decreases when the throat varies and it reduces? would not be more logic to just close the by pass doors? why expand or collapse the spike?

So what F-104 has 2D intakesengineer, hey i am waiting for you to tell me a jet that goes 0-mach 3 on fixed intake

MiG-29 said:

let us see if it is the gap


If the throat area (A2) is too small, corresponding to point c in Figure 2.25, a detached shock will stand ahead of the inlet, as shown in Figure 2.25a. By increasing the throat area, the shock can be moved to the inlet lip. When the operating point a of Figure 2.25 is reached, the shock will reach the inlet lip and

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where does the article mentions the bypass doors?

Click to expand...

Bypass is mentioned here:


The extract specifically points to the gap in between the intake ramp and the divergent ramp where excess air is bleed. Thus control of mass flow is attributed to the bypass system. Then the bypass is mentioned again in the next page:


This time, the extract says the bypass door is opened to reduce excess flow.

So once again, your own source shows what I have been saying is correct. Now you are resorting to fallacy again with . How predictable.

MiG-29 said:

In fact the drawing has no bypass doors it is a venturi tube.

there is no bypass door in that example...

Click to expand...

That example demonstrates how an inlet is started, and whether its authors decided to include bypass or not has no relevance on bypass system that exists on real inlets. It also does not alter the fact that you purposely misquoted in your post in an attempt to mislead.

MiG-29 said:

...that shows first is not the shock since the shock is moved by the throat size, and in that case the throat goes bigger or smaller, and by controlling the shock position the throat controls mass a small detail engineer you did not think.

Click to expand...

First of all, repeating your original claim ad infinitum isn't going to add anything new to your arguments.

Secondly, when the throat area is insufficient for the flow condition, the normal shock shifts location until the area of the shock is large enough to accommodate the flow. By controlling the position of normal shock, the throat is responding to flow condition. More simply, the amount of flow causes throat area to change, not the other way around.

Thirdly, since you want to bring up small details, I will entertain you. A detail which you have missed is that signal of a restriction (the throat) cannot warn the incoming air to move away. That's because the flow backs up at the rate of speed-of-sound, which cannot travel faster than the supersonic flow ahead of the normal shock. In kiddies' terms, the air inside the inlet cannot tell the air outside the inlet to move away, so the air outside will go into the inlet until it is too late. This means variation in throat area cannot control the flow going into the inlet mouth.




In the above diagram, m1_dot refers to the mass flow in front of the inlet, and mi_dot refers to the mass flow behind the normal shock. Note that once the normal shock is inside the intake, the two mass flows remain the same regardless of position of normal shock. Thus, variation in throat area to change the location of normal shock does nothing to control the mass flow. Without bypass system, the only way of altering mass flow is to build up pressure inside the duct of the inlet to push the normal shock out of the inlet's mouth, resulting in spillage. This is what's done in DSI, and has no relevance to variation in throat geometry.

MiG-29 said:

hey why in the F-111 the mass flow also decreases when the throat varies and it reduces? would not be more logic to just close the by pass doors? why expand or collapse the spike?

Click to expand...

. An example is that I took a dump this morning and the sun rises, and my action of taking a dump does not cause the sun to rise. Likewise, just because throat area change occurs simultaneously with change in mass flow rate, that does not mean variation in mass flow rate is caused by variation in throat geometry.

Change in mass flow ratio has to be done outside of the inlet with shock geometry. Mass flow ratio is A0i/A1, which is independent of throat area As:

MiG-29 said:

So what F-104 has 2D intakesengineer, hey i am waiting for you to tell me a jet that goes 0-mach 3 on fixed intake

Click to expand...

These are logical fallacies called , to which I have no obligation to respond.

What is clear are these:

• DSI and inlets on F-104 are not of the same type, primarily because the former is diverterless while the latter requires diverter;
• Fixed inlet can operate above Mach 3, as evident by its existence on hypersonic test vehicles.

Engineer said:

Bypass is mentioned here:

The extract specifically points to the gap in between the intake ramp and the divergent ramp where excess air is bleed. Thus control of mass flow is attributed to the bypass system. Then the bypass is mentioned again in the next page:

This time, the extract says the bypass door is opened to reduce excess flow.

So once again, your own source shows what I have been saying is correct. Now you are resorting to fallacy again with

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. How predictable.




That example demonstrates how an inlet is started, and whether its authors decided to include bypass or not has no relevance on bypass system that exists on real inlets. It also does not alter the fact that you purposely misquoted in your post in an attempt to mislead.



First of all, repeating your original claim ad infinitum isn't going to add anything new to your arguments.

Secondly, when the throat area is insufficient for the flow condition, the normal shock shifts location until the area of the shock is large enough to accommodate the flow. By controlling the position of normal shock, the throat is responding to flow condition. More simply, the amount of flow causes throat area to change, not the other way around.

Thirdly, since you want to bring up small details, I will entertain you. A detail which you have missed is that signal of a restriction (the throat) cannot warn the incoming air to move away. That's because the flow backs up at the rate of speed-of-sound, which cannot travel faster than the supersonic flow ahead of the normal shock. In kiddies' terms, the air inside the inlet cannot tell the air outside the inlet to move away, so the air outside will go into the inlet until it is too late. This means variation in throat area cannot control the flow going into the inlet mouth.


In the above diagram, m1_dot refers to the mass flow in front of the inlet, and mi_dot refers to the mass flow behind the normal shock. Note that once the normal shock is inside the intake, the two mass flows remain the same regardless of position of normal shock. Thus, variation in throat area to change the location of normal shock does nothing to control the mass flow. Without bypass system, the only way of altering mass flow is to build up pressure inside the duct of the inlet to push the normal shock out of the inlet's mouth, resulting in spillage. This is what's done in DSI, and has no relevance to variation in throat geometry.

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. An example is that I took a dump this morning and the sun rises, and my action of taking a dump does not cause the sun to rise. Likewise, just because throat area change occurs simultaneously with change in mass flow rate, that does not mean variation in mass flow rate is caused by variation in throat geometry.

Change in mass flow ratio has to be done outside of the inlet with shock geometry. Mass flow ratio is A0i/A1, which is independent of throat area As:

[/list]

Click to expand...

All what you write are a bunch of fallacies, why? simple why add variable throat to intakes?
all you fallacies come to a simple illogic way, if throat area does not change mass flow and only by pass doors control flow then you do not need variable throats in the first place since they do not change mass flow, then you just need by pass doors.

As simple as that the rest is a bunch of fallacies that you are trying to say to give the impression that you know what you are talking about, which is basicly is pure posture and all what you say are pure fallacies.


To start by translating and expanding the the spike in the F-111 you change the throat size, by doing that you change the mass flow.

If F-14 does not change mass flow by enlarging the throat or reducing it, then why do it? you just need to leave the ramps fixed and open the bypass doors. what the F-14 in reality does is enlarge the throat to increase mass flow and close the bypass doors since they are not needed to bypass as much air as at supersonic speeds

However these jets they have variable throats to change the mass flow, in fact why enlarge the throat? there is no point in doing that if i follow your fallacies, you claim the by pass doors are closed when the ramps collapsed, however if enlarging the throat does not increase mas flow , then you do not need collapse them just need close the by pass doors.
Of course in reality F-14 enlarges the throat and the bypass doors narrow their gap


the basic fallacy of yours is claim variable geometry throat do not change mass flow, F-111, SR-71 do change mass flow and the bypass doors are in the intake cowling, if you are right then there is no need to translate the spike, just open or close the bypass doors, however F-111 does use bypass doors and translate the spike, you fallacy is to think because the F-111 and F-14 use both bypass doors and variable throats, variable throats do not change mass flow, however what can i expect frrom you? admit it, no you won`t you will invent time after time fallacies

To provide for the large airflow rates at transonic condition the inlet throat area must be larger than at the cruise condition, this increased area is provided by the variable inlet geometry hahaha

MiG-29 said:

All what you write are a bunch of fallacies, why?

Click to expand...

Nope. Unlike you, there is no fallacy in my statements. Covering your ears and claim I am making fallacies does not make it so, especially when we consider the fact that you are unable to point out what types of fallacy I am making. This only illustrates you do not know what a fallacy is and you should refrain from using a term that you do not understand.

Fallacy refers to improper reasonings. When you point out irrelevant facts in your arguments to try to divert attention, that's and is a fallacy. When you try to put words in my mouth to misrepresent my position, that's which is a fallacy. When you take materials out of a citation to skew the authors' viewpoint, that's called , and is a fallacy.

So you see, the difference between you and me is that you are actually employing fallacies and I can pinpoint exactly which one you are making.

MiG-29 said:

simple why add variable throat to intakes?
all you fallacies come to a simple illogic way, if throat area does not change mass flow and only by pass doors control flow then you do not need variable throats in the first place since they do not change mass flow, then you just need by pass doors.

Click to expand...

Correct, you do not need variable throats. Inlets on F-22 only have bypass doors. DSI does away with bypass doors altogether. Recall what you have said earlier:

MiG-29 said:

If you were right you only would need the by pass doors, not variable geometry throats.

Click to expand...

So you know I am right, but you are arguing for the sake of arguing.

MiG-29 said:

As simple as that the rest is a bunch of fallacies that you are trying to say to give the impression that you know what you are talking about, which is basicly is pure posture and all what you say are pure fallacies.

Click to expand...

You are . You are ascribing your own attributes, thoughts and emotions on to me, but in fact you are the one who doesn't know what he's talking about. And you have been trying to cover up that fact by employing fallacies and being illogical.

MiG-29 said:

To start by translating and expanding the the spike in the F-111 you change the throat size, by doing that you change the mass flow.

Click to expand...

Nope. This is a . Mass flow ratio is dependent on shock geometry ahead of the inlet, and throat area has no relevant whatsoever in the ratio. This fact has been illustrated in numerous books you have cited already, such as the diagram I have included below.


By insisting on your point of view, you are essentially claiming the authors are wrong.

MiG-29 said:

If F-14 does not change mass flow by enlarging the throat or reducing it, then why do it? you just need to leave the ramps fixed and open the bypass doors. what the F-14 in reality does is enlarge the throat to increase mass flow and close the bypass doors since they are not needed to bypass as much air as at supersonic speeds

Click to expand...

Wrong.

First is your confusion between throat area and speed. Throat is enlarge and bypass doors are closed at take off, but not to increase mass flow. It is done because the throat is not needed. At supersonic speed the reverse happens; throat size is reduced and bypass doors are opened. However, variation in the throat size merely causes the normal shock to shift and does not alter the mass flow.

Second, you claim bypass doors are not needed but the fact that bypass doors exist despite a variable-geometry says otherwise. Your claim that F-14 close its bypass at supersonic speed is purely your own opinion, unsubstantiated, and one of a desperate gesture as a result of having been proven wrong.

The intake ramps position the oblique shocks optimally at different flight condition, so that the highest performance can be obtained. That's the purpose of variable-geometry. It is the movement of these ramps that cause the throat size to change, but the variation in throat size plays no role in altering mass flow ratio. Observe that the ratio is A0i/A1, which does not include throat area (As).

MiG-29 said:

However these jets they have variable throats to change the mass flow, in fact why enlarge the throat? there is no point in doing that if i follow your fallacies, you claim the by pass doors are closed when the ramps collapsed, however if enlarging the throat does not increase mas flow , then you do not need collapse them just need close the by pass doors.
Of course in reality F-14 enlarges the throat and the bypass doors narrow their gap


the basic fallacy of yours is claim variable geometry throat do not change mass flow, F-111, SR-71 do change mass flow and the bypass doors are in the intake cowling, if you are right then there is no need to translate the spike, just open or close the bypass doors, however F-111 does use bypass doors and translate the spike, you fallacy is to think because the F-111 and F-14 use both bypass doors and variable throats, variable throats do not change mass flow, however what can i expect frrom you? admit it, no you won`t you will invent time after time fallacies

To provide for the large airflow rates at transonic condition the inlet throat area must be larger than at the cruise condition, this increased area is provided by the variable inlet geometry hahaha

Click to expand...

You cannot grasp what's going on because you are hindered by your own fallacies, such as the fallacy of which you have demonstrated in your statement above. It is that simple. of your own attributes, thoughts, and emotions on to me doesn't actually mean I employ fallacies as you do.

The collapsed position of the ramp is the default position. Once the aircraft goes supersonic, the ramp moves downward to create oblique shocks, and the resulting throat creates the normal shock. This is called creating a throat when it is needed, and putting it away when it is not, which is not controlling mass flow rate as you have claimed.

The fact that moving of the normal shock wave as a result of change in throat area shows mass flow rate is constant. When the throat area is too small and the cross sectional area of the resulting normal shock is insufficient for flow, the normal shock moves until its area is large enough for the flow condition. Thus variation in throat area only shifts the position of the normal shock, and does nothing to alter the mass flow.

Another fact is that signal of a restriction at the throat cannot travel out of the inlet. This is because air backs up at a rate of speed-of-sound, which is slower than the supersonic flow that travels into the inlet's mouth. In kiddies' terms, the air inside the inlet cannot tell the air outside that there is a restriction, thus the outside air goes into the inlet until it is too late. This is another reason why variation in throat area does not alter the mass flow.

Finally, inlets on the F-22 have bypass doors but no variable-geometry. Opening or closing the bypass doors is sufficient for controlling the mass flow.

Time and time again I have brought the above facts, and time after time you invent fallacies to preach your opinions. That's the difference between you and me. But what can I expect from someone who argues with emotion instead of logic, and thinks that using large font size increases the strength of his arguments?

Engineer said:

Nope. Unlike you, there is no fallacy in my statements. Covering your ears and claim I am making fallacies does not make it so, especially when we consider the fact that you are unable to point out what types of fallacy I am making. This only illustrates you do not know what a fallacy is and you should refrain from using a term that you do not understand.

Fallacy refers to improper reasonings. When you point out irrelevant facts in your arguments to try to divert attention, that's

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and is a fallacy. When you try to put words in my mouth to misrepresent my position, that's

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which is a fallacy. When you take materials out of a citation to skew the authors' viewpoint, that's called

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, and is a fallacy.

So you see, the difference between you and me is that you are actually employing fallacies and I can pinpoint exactly which one you are making.



Correct, you do not need variable throats. Inlets on F-22 only have bypass doors. DSI does away with bypass doors altogether. Recall what you have said earlier:


So you know I am right, but you are arguing for the sake of arguing.



You are

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. You are ascribing your own attributes, thoughts and emotions on to me, but in fact you are the one who doesn't know what he's talking about. And you have been trying to cover up that fact by employing fallacies and being illogical.



Nope. This is a

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. Mass flow ratio is dependent on shock geometry ahead of the inlet, and throat area has no relevant whatsoever in the ratio. This fact has been illustrated in numerous books you have cited already, such as the diagram I have included below.

By insisting on your point of view, you are essentially claiming the authors are wrong.




Wrong.

First is your confusion between throat area and speed. Throat is enlarge and bypass doors are closed at take off, but not to increase mass flow. It is done because the throat is not needed. At supersonic speed the reverse happens; throat size is reduced and bypass doors are opened. However, variation in the throat size merely causes the normal shock to shift and does not alter the mass flow.

Second, you claim bypass doors are not needed but the fact that bypass doors exist despite a variable-geometry says otherwise. Your claim that F-14 close its bypass at supersonic speed is purely your own opinion, unsubstantiated, and one of a desperate gesture as a result of having been proven wrong.

The intake ramps position the oblique shocks optimally at different flight condition, so that the highest performance can be obtained. That's the purpose of variable-geometry. It is the movement of these ramps that cause the throat size to change, but the variation in throat size plays no role in altering mass flow ratio. Observe that the ratio is A0i/A1, which does not include throat area (As).

You cannot grasp what's going on because you are hindered by your own fallacies, such as the fallacy of

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which you have demonstrated in your statement above. It is that simple.

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of your own attributes, thoughts, and emotions on to me doesn't actually mean I employ fallacies as you do.

The collapsed position of the ramp is the default position. Once the aircraft goes supersonic, the ramp moves downward to create oblique shocks, and the resulting throat creates the normal shock. This is called creating a throat when it is needed, and putting it away when it is not, which is not controlling mass flow rate as you have claimed.

The fact that moving of the normal shock wave as a result of change in throat area shows mass flow rate is constant. When the throat area is too small and the cross sectional area of the resulting normal shock is insufficient for flow, the normal shock moves until its area is large enough for the flow condition. Thus variation in throat area only shifts the position of the normal shock, and does nothing to alter the mass flow.

Another fact is that signal of a restriction at the throat cannot travel out of the inlet. This is because air backs up at a rate of speed-of-sound, which is slower than the supersonic flow that travels into the inlet's mouth. In kiddies' terms, the air inside the inlet cannot tell the air outside that there is a restriction, thus the outside air goes into the inlet until it is too late. This is another reason why variation in throat area does not alter the mass flow.

Finally, inlets on the F-22 have bypass doors but no variable-geometry. Opening or closing the bypass doors is sufficient for controlling the mass flow.

Time and time again I have brought the above facts, and time after time you invent fallacies to preach your opinions. That's the difference between you and me. But what can I expect from someone who argues with emotion instead of logic, and thinks that using large font size increases the strength of his arguments?

Click to expand...

spill man airflow in subcritical states is spilling air mass, that it never enters the intake hahahaha



here is the proof throat area increase mass flow.

MiG-29 said:

spill man airflow in subcritical states is spilling air mass, that it never enters the intake hahahaha

Click to expand...

This is a fallacy called , because I never claimed spilled air enters the intake. You still don't have proof that variation of inlet throat area controls mass flow.

The spill is a result of pressure built up in the inlet, pushing the normal shock out of the inlet mouth, and allowing the subsonic air spills from behind the inlet. Spill occurs on fixed inlets and DSI as well, and is not a phenomenon unique to variable-geometry inlet. Thus, it does not support your claim that variation in throat area controls mass flow.

MiG-29 said:

Click to expand...

This diagram says mass flow ratio depends on A0i and A1, with no involvement of As, which actually contradicts your claim.

MiG-29 said:

Click to expand...

This diagram says spill flow is depended on free-stream cross sectional area A0 and capture area Ac, with the portion of spill defined to be (Ac - A0)/Ac. The division does not involves throat area At, and this means variation in throat area has nothing to do with spillage.

MiG-29 said:

here is the proof throat area increase mass flow.

Click to expand...

Neither of your citation says throat area controls mass flow. , and just because two events occur simultaneously, that doesn't mean one is the result of another. Here is the proof that variation in throat area doesn't change mass flow:


The fact of the matter is that to control the mass flow, bypass system is needed. Hence, all variable-geometry inlets have some sort of bypass.

Engineer said:

This is a fallacy called

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, because I never claimed spilled air enters the intake. You still don't have proof that variation of inlet throat area controls mass flow.

The spill is a result of pressure built up in the inlet, pushing the normal shock out of the inlet mouth, and allowing the subsonic air spills from behind the inlet. Spill occurs on fixed inlets and DSI as well, and is not a phenomenon unique to variable-geometry inlet. Thus, it does not support your claim that variation in throat area controls mass flow.



This diagram says mass flow ratio depends on A0i and A1, with no involvement of As, which actually contradicts your claim.



This diagram says spill flow is depended on free-stream cross sectional area A0 and capture area Ac, with the portion of spill defined to be (Ac - A0)/Ac. The division does not involves throat area At, and this means variation in throat area has nothing to do with spillage.



Neither of your citation says throat area controls mass flow.

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, and just because two events occur simultaneously, that doesn't mean one is the result of another. Here is the proof that variation in throat area doesn't change mass flow:

The fact of the matter is that to control the mass flow, bypass system is needed. Hence, all variable-geometry inlets have some sort of bypass.

Click to expand...

look you are wrong simple like that, this proves throat area changes and flow rates increase
the rest what you write is a bunch of fallacies you write, basicly you won`t admit it your pride does not let you


cruise airflow < transonic air flow


cruise air flow is 70% of transonic flow

Transonic flow is 100%.


throat area at cruise conditions<transonic throat area