SDF Aerospace and Aerodynamics Corner

MiG-29 said:

no i am not trying to mislead anyone because the throat area is reduced thus the capture area is reduced too

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Throat area is not the same as capture area. This is like your use of design-speed of an inlet and argue it is the top-speed of the inlet. Stop misleading.

MiG-29 said:

The intakes are of multi-ramp wedge configuration and offer a straight path for the air entering the engines. Each intake has a pair of adjustable ramps attached to the upper part of the inner intake. Hydraulic actuators in the upper part of the intake adjust the positions of the first and second ramps in the upper surface of the inlet and of the diffuser ramp located further aft, reducing the inlet air to subsonic velocity before admitting it to the engine. A gap between the back edge of the second ramp and the leading edge of the diffuser ramp allows bleed air to escape from the inlet, passing overboard via a bleed-air door in the outer surface of the inlet. The inlet ramps are under the automatic control of a computer, which calculates the optimal position for the ramps based on engine speed, air temperature, air pressure, and angle of attack. At supersonic speeds, the hinged panels narrow down the throat area while diverting the excess airflow out of the ducts through aft-facing spill doors at the top of the intakes. At low speeds (especially during takeoff) when more engine air is needed, this airflow is reversed and extra air is sucked in via the spill doors

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Why open at low speed the throat area? why reduce the throat area at high speed and bypass the extra mass flow?

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Throat area is reduced as a consequent of having to position the ramps to optimally place the oblique shocks. It is explained :

The angle of the variable throat area intake automatically varies with aircraft speed and positions the shockwave to decrease the air velocity at the engine inlet and maintain maximum pressure recovery within the inlet duct.

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As the ramps move downward, it is going to displace space within the inlets. But the reason of moving the ramp is to position the shockwave optimally. Bypass doors behind the ramps are not the ramps themselves, and increase or decrease of flow into the engine is attributed to these bypass doors, not ramps.

MiG-29 said:

11. At higher supersonic speeds, the pitot type of air intake is unsuitable due to the severity of theshockwave that forms and progressively reduces theintake efficiency as speed increases. Amore suitabletype of intake for these higher speeds is known asthe external/internal compression intake (fig. 23-8). 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).The airflow velocities encountered in the higher speedrange of the aircraft are much higher than the enginecan efficiently use; therefore, the air velocity must be decreased between the intake and the engine air inlet.The angle of the variable throat area intake automatically varies with aircraft speed and positions the shockwave to decrease the air velocity at the engine inlet and maintain maximum pressure recovery within the inlet duct. However, continued development enablesthis to be achieved by careful design of the intake andducting. This, coupled with auxiliary air doors to permitextra air to be taken in under certain engine operatingconditions, allows the airflow to be controlled withoutthe use of variable geometry intakes. The fuselageintakes shown in fig. 23-10 are of the variable throat area

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This is the same principle of all supersonic intakes, including DSI. Oblique and normal shocks are created by the bump, which slows the supersonic flow down to subsonic flow. No one is disputing the working principles behind supersonic inlet, so explaining the purpose of shocks does not strengthen your original arguments.

What is clear however is that variable-geometry inlets are complicated, with movable ramps, their associated actuators, spillage channels and bypass doors. This complexity leads to increase in weight, which cancels out benefits of increased pressure recovery ratio. DSI is far simpler, and although it can only generates two shocks, it nevertheless has higher performance that three shocks variable-geometry inlets.

From

From , A1 is the capture area.


To vary capture area, mechanisms like that on F-15 are needed as shown in this . F-14 and Su-35S cannot vary the capture area.

I also come across the while looking up illustrations:

Aircraft engine design said:

The improved performance of variable geometry mixed compression inlets and external compression inlets at high Mach numbers comes with some reduced performance at low supersonic Mach numbers due to the increased frictional losses.

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So unlike what you are trying to portray, variable-geometry inlet is not absolutely superior, nor can this type of inlet operates at all Mach number. Your argument "DSI cannot function at all Mach number therefore is inferior" does not work.

Engineer said:

From

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sorry my friend your drawing shows a fixed intake

Engineer said:

From

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, A1 is the capture area.

To vary capture area, mechanisms like that on F-15 are needed as shown in this

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. F-14 and Su-35S cannot vary the capture area.

I also come across the

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while looking up illustrations.

So unlike what you are trying to portray, variable-geometry inlet is not absolutely superior, nor can this type of inlet operates at all Mach number. Your argument "DSI cannot function at all Mach number therefore is inferior" does not work.

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Another fixed intake, and a mistake Su-30 and F-14 do vary the capture are like F-15 simply because they have moveable ramps, same concorde or Tu-22M, in fact any jet that has moveable ramps.

the low subsonic area is around Mach 0.3 which usually modern jets can not fly well, but transonic, where F-15 is even superior to MiG-29 or Su-27, makes the Eagle a good machine, and your graph only shows F-15 having low performance but F-14 has better performance, that is the reason you cropped the fig illustration from top

Engineer said:

Throat area is not the same as capture area. This is like your use of design-speed of an inlet and argue it is the top-speed of the inlet. Stop misleading.



Throat area is reduced as a consequent of having to position the ramps to optimally place the oblique shocks. It is explained

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:


As the ramps move downward, it is going to displace space within the inlets. But the reason of moving the ramp is to position the shockwave optimally. Bypass doors behind the ramps are not the ramps themselves, and increase or decrease of flow into the engine is attributed to these bypass doors, not ramps.



This is the same principle of all supersonic intakes, including DSI. Oblique and normal shocks are created by the bump, which slows the supersonic flow down to subsonic flow. No one is disputing the working principles behind supersonic inlet, so explaining the purpose of shocks does not strengthen your original arguments.

What is clear however is that variable-geometry inlets are complicated, with movable ramps, their associated actuators, spillage channels and bypass doors. This complexity leads to increase in weight, which cancels out benefits of increased pressure recovery ratio. DSI is far simpler, and although it can only generates two shocks, it nevertheless has higher performance that three shocks variable-geometry inlets.

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i will tell you why your whole argument is wrong first the intake ramp is like a door, at low subsonic the door is open at high supersonic speeds the door is semi closed.


same is the spike, it is like a plug, on SR-71, the moving of the cone closes or opens the tube like a valve, simple like that they regulate the amount of air by decreasing or increasing the throat area

MiG-29 said:

sorry my friend your drawing shows a fixed intake

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It is a generic drawing of an intake, which includes fixed and variable-geometry inlets. The intake of SR-71 looks similar. The diagram illustrates clearly the difference between capture area and throat area. Your attempt to equate the two says you are inventing definition for terminologies in order to mislead others.

Engineer said:

It is a generic drawing of an intake, which includes fixed and variable-geometry inlets. The intake of SR-71 looks similar. The diagram shows the capture area as the area between the intake lip and the tip of the intake cone, which is different from the throat area.

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no my friend it is a fixed intake, the ramp collapses on F-14, the ramps becomes lower angled

hahaha the intake has moved aft, the increase in area has happened, in fact look at the F-14 intake at max supersonic it is

at subsonic speed

MiG-29 said:

Another fixed intake,

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You are shown that your claims are wrong and now you go into denial. The diagram is a generic representation of an intake system, where both fixed and variable-geometry inlet are included. The terminologies used on one type of inlets do not vary when used on another type of inlets.

Note that capture area refers to the physical area within the intake mouth, not the area beneath the ramps. The ability to vary the inlet area can be found on F-15, but not F-14 and Su-35S as you wrongly claimed.

MiG-29 said:

...and a mistake Su-30 and F-14 do vary the capture are like F-15 simply because they have moveable ramps, same concorde or Tu-22M, in fact any jet that has moveable ramps.

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You are wrong in confusing movable cowl on F-15 with ramps. The movable cowl of F-15 is situated on the outside of the inlet and it is what varies the capture area of the inlet. Ramps are positioned inside the inlet, and their purpose is to control the angle of the shock waves.

F-14, aircraft in the Su-27 family, Tu-22 do not have adjustable cowling like F-15.

MiG-29 said:

the low subsonic area is around Mach 0.3 which usually modern jets can not fly well, but transonic, where F-15 is even superior to MiG-29 or Su-27, makes the Eagle a good machine, and your graph only shows F-15 having low performance but F-14 has better performance, that is the reason you cropped the fig illustration from top

Click to expand...

The figure is cropped due to the limited vertical resolution of my screen. The full figure has been posted many times already, but if you like, here it is:




The paragraph says the following:

Aircraft engine design said:

The improved performance of variable geometry mixed compression inlets and external compression inlets at high Mach numbers comes with some reduced performance at low supersonic Mach numbers due to the increased frictional losses.

Click to expand...

So there you have it: variable-geometry inlet is also designed to operate within an optimal range of speed. From the graphs, we can see this is true, as the pressure recovery curve of variable-geometry inlets has an apex. As Mach number moves away from the optimal speed, pressure recovery ratio decreases.

You claimed that the above phenomenon only occurs on DSI, but this is not the case. Therefore, your argument that DSI is inferior based on drop in pressure recovery ratio doesn't work.