SDF Aerospace and Aerodynamics Corner

MiG-29 · Jan 10, 2012

Engineer said:
Change in throat area is a consequent of having to adjust the intake ramp to position the shock waves. Air spillage is caused by the position of normal shock wave, not by variation in throat area. Air can get spilled regardless of whether the inlet can vary its inlet geometry or not as shown in the following diagram.

Spill air doesn't enter the inlet mouth, but the amount of air that does is going to be the same as the amount of air that flows through the inlet throat. This is Bernoulli's principle shown in the diagram below.

In addition, air is a compressible fluid. Your water-in-the-sink and toilet-plunger examples are flawed because water is incompressible and do not behave like air. What does it mean by compressible? Think of a spring, now squeeze it and fit it into a small area. That's what going to happen to the intake air too when it encounters narrower space.

Thus, variation in throat area is not going to regulate air flow. To regulate air flow, bypass system are needed. In fact, air inlet with bypass doors only can operate without intake ramp, as is the case on F-22. Remember what you have said:

Right now, you are arguing just for the sake of arguing.

on a fixed intake you have only one setting for the ramp simple like that and you have to make the intake bigger for low speeds. on a variable geometry you can make it smaller because the ramp and throat are adjustable tothe mass flow so you can adjust the ramp with a bigger throat at low speeds therefore you do not need to make the intake bigger.

Benefits? well better pressure recovery

remember engineer Air is spilled, capture area mass flow can get spilled and is the engine demands that creates that spilling

Engineer · Jan 10, 2012

MiG-29 said:
on a fixed intake you have only one setting for the ramp simple like that and you have to make the intake bigger for low speeds. on a variable geometry you can make it smaller because the ramp and throat are adjustable tothe mass flow so you can adjust the ramp with a bigger throat at low speeds therefore you do not need to make the intake bigger.

Benefits? well better pressure recovery

remember engineer Air is spilled, capture area mass flow can get spilled and is the engine demands that creates that spilling

Nope. You are assuming variable-geometry adjusts mass flow, but that's not the purpose of variable-geometry. What adjust the mass flow is bypass system, as explained below. There is no mentioning of variation in intake throat to regulate air, contrary to your claim.

Remember Mig-29, by Bernoulli's principle the amount of air going into the inlet mouth is the same as the amount of air going through the throat. Air is compressible unlike water, so the throat cannot act as a valve like on a faucet. As long as you assume air to act like water, you are wrong.

Variable-geometry is for manipulation of shock wave patterns, it is that simple. It is due to this ability that gives variable-geometry better performance over fixed-inlet. However, DSI has better performance than variable-geometry inlets like those found on F-4D. This means DSI's two shock system beats the performance of three shock variable-geometry inlets.

You can add as many ramps as you like to force variable-geometry inlet to get better performance than DSI. However, for each ramp that is added, complexity is increased which drives up weight. This added weight in turn reduces benefits gained from raised pressure recovery. So while your aircraft can function at higher speed, it also loses ability to maneuver and becomes useless in dog fighting. In addition, variable-geometry inlet loses performance at low supersonic speed, and it is not a magical solution that works wonderfully for all flight condition:

MiG-29 · Jan 10, 2012

Engineer said:
Nope. You are assuming variable-geometry adjusts mass flow, but that's not the purpose of variable-geometry. What adjust the mass flow is bypass system, as explained below. There is no mentioning of variation in intake throat to regulate air, contrary to your claim.

Remember Mig-29, by Bernoulli's principle the amount of air going into the inlet mouth is the same as the amount of air going through the throat. Air is compressible unlike water, so the throat cannot act as a valve like on a faucet. As long as you assume air to act like water, you are wrong.

Variable-geometry is for manipulation of shock wave patterns, it is that simple. It is due to this ability that gives variable-geometry better performance over fixed-inlet. However, DSI has better performance than variable-geometry inlets like those found on F-4D. This means DSI's two shock system beats the performance of three shock variable-geometry inlets.

You can add as many ramps as you like to force variable-geometry inlet to get better performance than DSI. However, for each ramp that is added, complexity is increased which drives up weight. This added weight in turn reduces benefits gained from raised pressure recovery. So while your aircraft can function at higher speed, it also loses ability to maneuver and becomes useless in dog fighting. In addition, variable-geometry inlet loses performance at low supersonic speed, and it is not a magical solution that works wonderfully for all flight condition:

Mi is the air entering the throat, Mo is the air needed by the engine, however the Mi varies with the As area.

M1-Mi=air spilled around the inlet, so as a sink the intake air is spilled around the intake, spilled air never enters the intake

5949d1326242237-sdf-aerospace-aerodynamics-corner-engine2.jpg

5948d1326242209-sdf-aerospace-aerodynamics-corner-engine.jpg

Engineer · Jan 10, 2012

MiG-29 said:
Mi is the air entering the throat, Mo is the air needed by the engine...

Some what correct, although mi is the mass flow rate of the inlet, not just the throat. The difference between the air flowing in the inlet (mi) and the air that the engine needs (m0) is the excess air. This excess air is removed by bypass system as explained in the same source. As the lack of mentioning of intake ramp shows, the regulation of air flow is not done by the ramp, contrary to your claim.

MiG-29 said:
however the Mi varies with the As area.

Nope. Mass flow rate mi does not vary with throat area, as mass is conserved. From this screen capture, you can see that the source specifically mentioned conservation of mass, and that mi is equal to ms.

MiG-29 said:
M1-Mi=air spilled around the inlet, so as a sink the intake air is spilled around the intake, spilled air never enters the intake

Nope. Air is compressible, water is not. Your analogy of a sink is flawed and does not work. A correct analogy would be a spring, and by squeezing it you can fit it into a smaller space. The same occurs with air because air is compressible. In addition, by Bernoulli's principle, the narrower throat will merely cause air there to flow faster. Thus, the ramp does not regulate air flow.

MiG-29 · Jan 10, 2012

Engineer said:
Some what correct, although mi is the mass flow rate of the inlet, not just the throat. The difference between the air flowing in the inlet (mi) and the air that the engine needs (m0) is the excess air. This excess air is removed by bypass system as explained in the same source. As the lack of mentioning of intake ramp shows, the regulation of air flow is not done by the ramp, contrary to your claim.

Nope. Mass flow rate mi does not vary with throat area, as mass is conserved. From this screen capture, you can see that the source specifically mentioned conservation of mass, and that mi is equal to ms.

Nope. Air is compressible, water is not. Your analogy of a sink is flawed and does not work. A correct analogy would be a spring, and by squeezing it you can fit it into a smaller space. The same occurs with air because air is compressible. In addition, by Bernoulli's principle, the narrower throat will merely cause air there to flow faster. Thus, the ramp does not regulate air flow.

look A1 capture area free stream flow can be spilled. As is throat area, Sr-71 and F-14 vary the throat area for flow control.

The variable intake does vary the throat as such it does not need to be bigger, it spills less air and bypass less air, as such it gets better pressure recovery than fixed intakes

5917d1325380966-sdf-aerospace-aerodynamics-corner-intake.jpg

5950d1326244375-sdf-aerospace-aerodynamics-corner-spilledintake1.jpg

spilled air or water are the same

[video=youtube;3IFxKgViyJc]http://www.youtube.com/watch?v=3IFxKgViyJc[/video]

Engineer · Jan 10, 2012

MiG-29 said:
look A1 capture area free stream flow can be spilled. As is throat area, Sr-71 and F-14 vary the throat area for flow control.

Nope, that is your opinion. Fact says mass flow is conserved, which means the amount of air flowing at the inlet mouth equates to the amount of air flowing through the throat. Hence varying the throat size does not cause change to the flow.

In addition, there is also the compressibility of air itself and Bernoulli's effect that have to be taken into account. The former allows larger volume of air to fit into smaller space. The latter says that narrower space simply causes air to flow faster to compensate.

MiG-29 said:
The variable intake does vary the throat as such it does not need to be bigger, it spills less air and bypass less air, as such it gets better pressure recovery

than fixed intakes

Variable-geometry inlets have better performance than traditional fixed-inlet because the former can position the oblique shock waves optimally. It has nothing to do with throat area. At the same time however, variable-geometry inlet loses some performance at low supersonic speed, as described here:

Control of air spillage is achieved by controlling the position of normal shock wave. This can be done with bypass doors without the need of intake ramp at all. Such setup is seen on F-18E and F-22. Thus, the intake ramp does not regulate air flow, contrary to your claim.

DSI has better performance than 2D three shocks variable-geometry inlet. DSI spills excess air exclusively, but DSI loses less performance from this spill as compared to traditional inlets. This is explained in the following extracts from

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.

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:

Engineer · Jan 10, 2012

MiG-29 said:
spilled air or water are the same

[video=youtube;3IFxKgViyJc]http://www.youtube.com/watch?v=3IFxKgViyJc[/video]

Wrong. No matter how large a font size you use, air is not the same as water. Water is incompressible. Air is compressible, meaning larger volume of it can be fitted into a smaller area, and it is this reason that shock waves can occur. Your analogy is flawed for this very reason.

[video=youtube;JhlEkEk7igs]http://www.youtube.com/watch?v=JhlEkEk7igs[/video]

MiG-29 · Jan 10, 2012

Engineer said:
Wrong. No matter how large a font size you use, air is not the same as water. Water is incompressible. Air is compressible, meaning larger volume of it can be fitted into a smaller area, and it is this reason that shock waves can occur. Your analogy is flawed for this very reason.

[video=youtube;JhlEkEk7igs]http://www.youtube.com/watch?v=JhlEkEk7igs[/video]

The intake has a fixed throat area, and they are testing different air speeds, at minute 6:00 they say the area of the throat is fixed here they are talking the aeodynamics of the intake duct once the air is inside the intake, however air is spilled out side the intake

Engineer · Jan 10, 2012

MiG-29 said:
The intake has a fixed throat area, and they are testing different air speeds, at minute 6:00 they say the area of the throat is fixed here they are talking the aeodynamics of the intake duct once the air is inside the intake, however air is spilled out side the intake

Nope. In the video there are two throats, the one upstream is fixed, the one downstream is variable. It is specifically mentioned in the video that mass flow rate is unchanged. This is due to Bernoulli's principle, and the fact that air is a compressible fluid.

Your analogy with water is flawed, and your claim that varying of the throat regulates the mass flow simply does not occur in reality.

MiG-29 · Jan 10, 2012

Engineer said:
Nope, that is your opinion. Fact says mass flow is conserved, which means the amount of air flowing at the inlet mouth equates to the amount of air flowing through the throat. Hence varying the throat size does not cause change to the flow.

In addition, there is also the compressibility of air itself and Bernoulli's effect that have to be taken into account. The former allows larger volume of air to fit into smaller space. The latter says that narrower space simply causes air to flow faster to compensate.

Variable-geometry inlets have better performance than traditional fixed-inlet because the former can position the oblique shock waves optimally. It has nothing to do with throat area. At the same time however, variable-geometry inlet loses some performance at low supersonic speed, as described here:

Control of air spillage is achieved by controlling the position of normal shock wave. This can be done with bypass doors without the need of intake ramp at all. Such setup is seen on F-18E and F-22. Thus, the intake ramp does not regulate air flow, contrary to your claim.

DSI has better performance than 2D three shocks variable-geometry inlet. DSI spills excess air exclusively, but DSI loses less performance from this spill as compared to traditional inlets. This is explained in the following extracts from
Please, Log in or Register to view URLs content!
.

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:

pressure recovery of that DSI is not better than the one of F-111, F-15, F-14, XB-70 or Sr-71.

It is only better than the F-104 intake which is fixed, F-16 that is pitot tube and F-4 that is vertical ramp and has a max mach number of Mach 2.2

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