SDF Aerospace and Aerodynamics Corner

MiG-29 · Jan 9, 2012

Engineer said:
Your citation says the following:

The intake ramp controls pattern of external shock wave;

Auxiliary doors (bypass) is used to dump air.

These are exactly what I have pointed out to you. The fact remains that intake ramp is used to control position of oblique shock waves, consequently causing change in throat area. However, the throat does not act as a valve to regulate flow, which is why bypass system must be installed.

This citation made the following points

Excess airflow flows out of the duct through spill doors;

Extra air flows in via spill doors;

Spill vales are needed to control changing volumes of air;

Angle of throat varies with aircraft speed to position shock waves that decrease air velocity flowing into the engine.

Once again, your citation makes the exact same points as I have. Bypass system is needed when there is a requirement to adjust the amount of air flowing into the engine. The variable-geometry does not serve this role. What the variable-geometry contributes is the adjustment of shock waves to maintain pressure recovery ratio.

The intake ramps shown in your pictures are for positioning of oblique shock waves. By Bernoulli's principle, the amount of air flowing into the inlet is the same as the amount of air flowing through the throat. Hence, adjustment of the amount of air going into the engine needs bypass system.

This is indeed the case, showing that I am right. Inlet only needs bypass doors, not variable-geometry throat. The inlet on F-18E does not have variable-geometry float. The inlet on F-22 does not have variable-geometry throat. On the other hand, variable-geometry inlet must incorporate bypass system, because the intake ramp does not regulate the amount of airflow as you claimed.

no you are not right you just claim to be right, variable geometry reduces the air flow to be bypassed, air can be spilled so As is not A1 all the time capture area flow is not compressed and kept 1, in subcritical states air is spilled , so A1-Ao=spill,

You are claiming As=As1 which is wrong because Sr-71 reduces throat area and reduces mass flow
As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent.
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5917d1325380966-sdf-aerospace-aerodynamics-corner-intake.jpg

MiG-29 · Jan 9, 2012

Engineer said:
Nope. The caption of the picture says "Mach number distribution on a horizontal plane (parallel to x-y plane) near design mass flow (left) and low mass flow ratio (right) at M∞ = 1.7." The paper clearly is talking about low mass flow ratio, and the flight condition is not subsonic speed unlike what you have claimed. This is evident in the following screen capture:

In the discussion of the right picture, the paper points out that pressure recovery ratio at sub-critical condition is higher than traditional inlet. This is because the oblique shock wave becomes stronger at high Mach number.

The pressure recovery ratio of DSI at sub-critical condition is higher than traditional inlet. This is the finding of the paper:

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why you did not talk about pressure recovery, it drops to the same level of Mach 1.2, and the reason is the shock system does not empige into the cowl lip, so at Mach 1.7 at low level mass flow rates the pressure recovery drops much lower than the one needed at the max design mach number

Engineer · Jan 9, 2012

MiG-29 said:
Bernoulli's principle also work for water and sinks overflow hahaha

Your water in the sink example is flawed because water is incompressible. Air is a compressible fluid, this means that air can be fitted into smaller space. Therefore, the amount of air flowing through the inlet mouth equates to the amount of air flowing through the throat, and mass is conserved.

To regulate air flow, such as at high supersonic speed where excess air needs to be removed, bypass system must be used. All variable-geometry inlets incorporate bypass system for the purpose of air flow regulation. On the other hand, variable throat is not needed in an inlet, as we have seen in the case of F-22. This shows your claim of intake ramp regulating air flow as incorrect.

MiG-29 · Jan 9, 2012

Engineer said:
Your water in the sink example is flawed because water is incompressible. Air is a compressible fluid, this means that air can be fitted into smaller space. Therefore, the amount of air flowing through the inlet mouth equates to the amount of air flowing through the throat, and mass is conserved.

To regulate air flow, such as at high supersonic speed where excess air needs to be removed, bypass system must be used. All variable-geometry inlets incorporate bypass system for the purpose of air flow regulation. On the other hand, variable throat is not needed in an inlet, as we have seen in the case of F-22. This shows your claim of intake ramp regulating air flow as incorrect.

however the air spills and does not enter the intake

Engineer · Jan 9, 2012

MiG-29 said:
no you are not right you just claim to be right...

This is purely your opinion. You keep claiming I am not right, but you are unable to point out any statement of mine which is incorrect. Why is this? This is because my statements are actually correct. Right now, you are arguing just for the sake of arguing.

MiG-29 said:
variable geometry reduces the air flow to be bypassed, air can be spilled so As is not A1 all the time capture area flow is not compressed and kept 1, in subcritical states air is spilled , so A1-Ao=spill,

Variable-geometry reduces the air flow going into the inlet by employing shock waves. The excess air must be dump using bypass system.
By Bernoulli's principle, the amount of flow going into the inlet equates to the amount of airflow going through the throat.
Spillage occurs because normal shock wave is outside of inlet's mouth, this is called sub-critical condition. This phenomenon occurs on all inlets, such as DSI. It is not caused by variation of throat area.

Furthermore, the inlet can be maintained at critical condition by merely employment of bypass doors. This is the approach taken on aircraft such as F-18E and F-22, with no variable-geometry involved.

MiG-29 said:
You are claiming As=As1 which is wrong

I have made no such claim.

MiG-29 said:
because Sr-71 reduces throat area and reduces mass flow
As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent.
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Your own citation says the same thing as what I am telling you: variation in geometry is to keep cone-shape and normal shock waves optimally positioned. The paragraph which follows your citation says the following:

How Things Work said:
It is a constant balancing act to keep the normal shock wave in the right position. The inlet has an internal pressure sensor, and when it detects that the pressure has grown too great, it triggers the forward bypass doors to open, expelling excess air. The inlet also has a set of aft bypass doors, controlled by the pilot. The forward and aft bypass doors work in opposition to each other: Opening the aft doors causes the forward doors to close, and when the pilot closes the aft doors, the forward doors open in turn.

Thus, bypass system is what regulates the amount of air flow.

Engineer · Jan 9, 2012

MiG-29 said:
however the air spills and does not enter the intake

No one claimed spill air enters the intake, and obviously spilled air is not part of the air that flows through the inlet mouth. However, this does not negate Bernoulli's principle where mass is conserved at two different point in an inlet.

Spillage can occur with inlets with no variable-geometry, such as DSI. You can read about spillage of DSI

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. Thus, spill air is not caused by variation in throat area.

Engineer · Jan 9, 2012

MiG-29 said:
why you did not talk about pressure recovery, it drops to the same level of Mach 1.2, and the reason is the shock system does not empige into the cowl lip, so at Mach 1.7 at low level mass flow rates the pressure recovery drops much lower than the one needed at the max design mach number

Why would I need to? It doesn't negate what the paper says, which is that DSI operates with higher-than-conventional pressure recovery at low mass flow ratio.

With pressure recovery ratio falling, DSI still has better pressure recovery ratio than some variable-geometry inlets such as the ones on F-4D. This means DSI is not inferior, contrary to your claim.

Also, as speed increases with variable-geometry inlet, pressure recovery ratio drops much lower than the one at designed Mach number. It is not a phenomenon exclusive to DSI or fixed-inlet. This is indicated by the graph we have seen so many times and loved:

MiG-29 · Jan 9, 2012

Engineer said:
This is purely your opinion. You keep claiming I am not right, but you are unable to point out any statement of mine which is incorrect. Why is this? This is because my statements are actually correct. Right now, you are arguing just for the sake of arguing.

Variable-geometry reduces the air flow going into the inlet by employing shock waves. The excess air must be dump using bypass system.

By Bernoulli's principle, the amount of flow going into the inlet equates to the amount of airflow going through the throat.

Spillage occurs because normal shock wave is outside of inlet's mouth, this is called sub-critical condition. This phenomenon occurs on all inlets, such as DSI. It is not caused by variation of throat area.

Furthermore, the inlet can be maintained at critical condition by merely employment of bypass doors. This is the approach taken on aircraft such as F-18E and F-22, with no variable-geometry involved.

I have made no such claim.

Your own citation says the same thing as what I am telling you: variation in geometry is to keep cone-shape and normal shock waves optimally positioned. The paragraph which follows your citation says the following:

Thus, bypass system is what regulates the amount of air flow.

5946d1326164811-sdf-aerospace-aerodynamics-corner-criticalsubcritical.jpg

see the drawing spill air outside of the intake cowl it does not enter the intake so A1-A0=spill mass air flow
look SR-71 shrinks throat area like F-14, when it does it reduces the the mass flow.

however you do not only understand what generates the subcritical states and why SR-71 reduces flow rate, the shock system can be critical or subcritical and on the SR-71 the throat is variable, F-14 is the same, by reducing or increasing the throat it controls mass flow

Engineer · Jan 9, 2012

MiG-29 said:
see the drawing spill air outside of the intake cowl it does not enter the intake so A1-A0=spill mass air flow
look SR-71 shrinks throat area like F-14, when it does it reduces the the mass flow.

however you do not only understand what generates the subcritical states and why SR-71 reduces flow rate, the shock system can be critical or subcritical and on the SR-71 the throat is variable, F-14 is the same, by reducing or increasing the throat it controls mass flow

No. Supercritical condition means the normal shock is well inside the inlet. Subcritical condition means that normal shock is outside of the inlet, and this results in spill. Spilling can occurs with fixed-inlet and DSI, with no variable-geometry involved.

No one claimed that spilled air enters the inlet, so stop setting up strawman arguments. Air that is not spilled enters the inlet mouth, and the amount of air that passes through this inlet mouth equates to the amount of air flowing through the throat. This is Bernoulli's principle.

When you have excess air to dump, you need bypass system, not intake ramp. The inlets of F-22 only have bypass doors with no variable-geometry, and can still regulates air flow and maintain critical condition of the inlet. Recall what you have said:

MiG-29 said:
If you were right you only would need the by pass doors, not variable geometry throats.

Right now, you are claiming I am incorrect just for the sake of arguing.

MiG-29 · Jan 9, 2012

Engineer said:
No. Supercritical condition means the normal shock is well inside the inlet. Subcritical condition means that normal shock is outside of the inlet, and this results in spill. Spilling can occurs with fixed-inlet and DSI, with no variable-geometry involved.

No one claimed that spilled air enters the inlet, so stop setting up strawman arguments. Air that is not spilled enters the inlet mouth, and the amount of air that passes through this inlet mouth equates to the amount of air flowing through the throat. This is Bernoulli's principle.

When you have excess air to dump, you need bypass system, not intake ramp. The inlets of F-22 only have bypass doors with no variable-geometry, and can still regulates air flow and maintain critical condition of the inlet. Recall what you have said:

Right now, you are claiming I am incorrect just for the sake of arguing.

As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent

This type of intake produces a series of mild shockwaves without excessively reducing the intake efficiency.12. As aircraft speed increases still further, so also does the intake compression ratio and, at high Mach numbers, it is necessary to have an air intake that has a variable throat area and spill valves to accommodate and control the changing volumes of air (fig. 23-9).

Variable geometry capability in the ramp angle
changes for mass flow regulation

tell me why F-14 and SR-71 have variable geometry throats and the SR-71 reduces the throat area to reduce mass flow?

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