SDF Aerospace and Aerodynamics Corner

MiG-29 said:

actually external compression works for only critical states, mixed compression swallow the shock and start the intake, you did not know that mixed compression has supercritical states

Click to expand...

More pseudo-aerodynamic theories.

No. Sub-critical, critical, and super-critical conditions refer to the position of normal shock wave with respect to the throat location. All compression method works best at critical condition, and worse at sub-critical condition. Super-critical condition is to be avoided entirely.

The fact the word "compression" is used to describe different type of inlets tells you one thing: air is a compressible fluid and this compressible property is being exploited in an inlet. As long as compression exists, your analogy with water-in-the-sink is flawed.

Engineer said:

Whether it is internal, external, or mixed compression inlet, does not negate the fundamental employed in the design of supersonic inlet, which is that of a compressible flow. The behavior of compressible gas is shown in the video, and we see when air encounters a narrower space, the flow rate is increased to compensate. The air at the narrower space also compresses. The combined result of the two is the terminal shock wave.

You do not know about compressible gas. You assumed air acts like water. You do not know about Bernoulli's effect either. This shows you have no understanding whatsoever of aerodynamics.

Click to expand...

to start the video has fixed throat, second a external compression never get started, and third intakes spill at subcritical conditions

Engineer said:

More pseudo-aerodynamic theories.

No. Sub-critical, critical, and super-critical conditions refer to the position of normal shock wave with respect to the throat location. All compression method works best at critical condition, and worse at sub-critical condition. Super-critical condition is to be avoided entirely.

The fact the word "compression" is used to describe different type of inlets tells you one thing: air is a compressible fluid and this compressible property is being exploited in an inlet. As long as compression exists, your analogy with water-in-the-sink is flawed.

Click to expand...

haha subcritical means the shocks are outside the intake to start, in fact intakes spill, like a a sink and there is where you can not understand, it is not a closed circuit

MiG-29 said:

To start DSI can not swallow the shock unless is a DHI, DSI are external compression simply because the shocks are outside the lip cowl and do not get supercritical states.


F-14 also does not swallow the shocks, a mixed compression at Mach 3 does not spill you unchoke the intake throat how engineer? well by moving the shock outside the intake and impinging on the intake cowl lip, in an subcritical state you will see the shock outside the cowl not even impinging the intake lip and spilling

Click to expand...

No one claimed shock is swallowed in DSI, quite setting up strawman.

The paper is pretty clear. As Mach number increases, sub-critical condition occurs and excess air is spilled. Spill air represents loss of inlet efficiency, but the paper found that for DSI, the loss is less than traditional inlets. This is attributed to stronger oblique wave at higher Mach number.

MiG-29 said:

to start the video has fixed throat.

Click to expand...

Wrong. The video shows both a fixed throat and a variable-geometry throat. Your only referencing of the fixed portion is not going to magically makes the variable-geometry portion disappear.

Compressible fluid behaves the same regardless of whether it is passing through a fixed throat or a variable-geometry throat. This compressibility is seen as choking and shock waves in internal, external, and mixed compression inlets. What the video shown is applicable to all type of inlets.

The video also shows that mass flow rate is conserved. Air flow is slower at location where the cross section area is larger. As the air flows into a narrower space, the flow picks up. This is Bernoulli's principle at work.

MiG-29 said:

second a external compression never get started.

Click to expand...

Irrelevant. This is not the point of contention.

MiG-29 said:

and third intakes spill at subcritical conditions

Click to expand...

Correct. Sub-critical condition occurs because the normal shock wave moves out of the inlet mouth. This occurs with fixed inlet and especially on DSI, without needing any variable-geometry throat. Thus, the fact remains that variable-geometry throat does not regulate air flow, and your claim is incorrect.

MiG-29 said:

haha subcritical means the shocks are outside the intake to start, in fact intakes spill, like a a sink and there is where you can not understand, it is not a closed circuit

Click to expand...

Sub-critical means the normal shock is outside of the inlet mouth. But you are wrong in using your water-in-a-sink analogy. We have already established that air is compressible, which is why shock wave can be formed. We have also seen Bernoulli's principle at work, as evident by the increase in speed at the throat area which results in a normal shock. Finally, we see from the screen capture below that mass is conserved. The mass flow (mi) of the un-spilled portion is equal to the mass flow (ms) at the throat region.


Thus, air does not act like water and variable-throat does not regulate air flow.

Engineer said:

Wrong. The video shows both a fixed throat and a variable-geometry throat. Your only referencing of the fixed portion is not going to magically makes the variable-geometry portion disappear.

Compressible fluid behaves the same regardless of whether it is passing through a fixed throat or a variable-geometry throat. This compressibility is seen as choking and shock waves in internal, external, and mixed compression inlets. What the video shown is applicable to all type of inlets.

The video also shows that mass flow rate is conserved. Air flow is slower at location where the cross section area is larger. As the air flows into a narrower space, the flow picks up. This is Bernoulli's principle at work.



Irrelevant. This is not the point of contention.



Correct. Sub-critical condition occurs because the normal shock wave moves out of the inlet mouth. This occurs with fixed inlet and especially on DSI, without needing any variable-geometry throat. Thus, the fact remains that variable-geometry throat does not regulate air flow, and your claim is incorrect.

Click to expand...

It is obvious you do not know what is starting and what the video refers.

Mixed and internal compression intakes start the engines. on the XB-70 the start of the engine happens after Mach 2, on the Sr-71 after Mach 1.6.

Boeing XB-70
The XB-70 was designed and built to be a long-range, supersonic cruise, bomber with a gross weight of about 500,000 pounds and a cruising speed of Mach 325. The schematic in Figure 1.8 shows the design features. The design highlighted a 65.6o swept back leading edge delta wing with folding wing tips (creating a quasiwaverider design). The vehicle propulsion system resided underneath the aircraft and consisted of two, two-dimensional inlets mounted side-by-side. Each inlet fed three YJ93-GE-3 afterburning turbojet engines that generated 30,000 pounds of thrust. Each engine consisted of an 11-stage, axial flow compressor, an annular combustor, a two-stage turbine and a variable area, converging-diverging nozzle.



The inlet26 was designed as a mixed-compression system as shown in . The inlet started at Mach 2.0 (i.e. the terminal shock laid across the inlet cowl face). Each inlet was composed of variable ramps and bypass doors that could be positioned to maximize performance through the flight profile.

http://www.sinodefenceforum.com/att...erospace-aerodynamics-corner-xb-70-intake.jpg
An intensive review of the inlet operation is given in the SR-71 Flight Manual13. The external system utilized a translating conical spike that retracted 1 ⅝ inches per 0.1 Mach number starting near Mach 1.6 and ending at Mach 3.2 with shock-on-lip condition.
The purpose of the translation, along with a complex sequence of bleeds15-17 and bypasses, is to control the amount of flow entering the engine and to hold the terminal shock downstream of the throat (when M∞ > 1.6) to avoid unstart




The system you have on the video is an internal compression intake and the they show for the XB-70 is mixed why?

Starting the intake means work with the termnial shock inside the intake.


DSI spills but at Mach 3 XB-70 does not

Engineer said:

Sub-critical means the normal shock is outside of the inlet mouth. But you are wrong in using your water-in-a-sink analogy. We have already established that air is compressible, which is why shock wave can be formed. We have also seen Bernoulli's principle at work, as evident by the increase in speed at the throat area which results in a normal shock. Finally, we see from the screen capture below that mass is conserved. The mass flow (mi) of the un-spilled portion is equal to the mass flow (ms) at the throat region.

Thus, air does not act like water and variable-throat does not regulate air flow.

Click to expand...

On the video they have an internal compression system see


Internal compression intakes
The entire system of shocks in this case is located inside the intake duct. A
simple example of such a system is a convergent-divergent nozzle with reversed flow
(refer Fig. 1.2). Compression in this case is isentropic up to the throat after which a
normal shock, whose location is dictated by the back pressure, decelerates the flow.
This is followed by diffusion in the divergent portion of the intake.This configuration is however unstable with regards to fixing the position
(anchoring) of the normal shock. A system of oblique shocks too can be used to the
same effect.
The main advantage offered by internal compression intakes is the lower cowl drag,
due to the fact that flow turning is contained within an almost parallel sided duct.
Some of the major problems associated with internal compression intakes include
boundary layer ingestion (shock boundary-layer interaction) and the problem of
‘starting’. The later is discussed in detail in the next chapter. These problems
complicate the design of these intakes.







Another problem associated with internal and mixed compression intakes is
that of ‘starting’. In the ‘not-started’ condition, the mass flow and total pressure
recovery of the intake are very low leaving no room for sub-critical performance. In
such a situation, catastrophic failures due to combustor over-heating and compressor
surge have a high probability of occurrence. To avoid such a situation, the ‘start’ and
‘unstart’ limits of an intake need to be defined clearly at the design stage.

Engineer said:

No one claimed shock is swallowed in DSI, quite setting up strawman.

The paper is pretty clear. As Mach number increases, sub-critical condition occurs and excess air is spilled. Spill air represents loss of inlet efficiency, but the paper found that for DSI, the loss is less than traditional inlets. This is attributed to stronger oblique wave at higher Mach number.

Click to expand...

External compression intakes
In these, the entire system of shock waves is located outside the intake duct. A
simple example of such a system is a divergent duct having a normal shock at entry
do these intakes have starting?


These intakes do no suffer from problems like ‘starting’. Also, this
configuration allows shortening the length of the intake as a whole. There
where is located the shock system on the video?


Internal compression intakes
The entire system of shocks in this case is located inside the intake duct

MiG-29 said:

It is obvious you do not know what is starting and what the video refers.

Mixed and internal compression intakes start the engines. on the XB-70 the start of the engine happens after Mach 2, on the Sr-71 after Mach 1.6.

Boeing XB-70
The XB-70 was designed and built to be a long-range, supersonic cruise, bomber with a gross weight of about 500,000 pounds and a cruising speed of Mach 325. The schematic in Figure 1.8 shows the design features. The design highlighted a 65.6o swept back leading edge delta wing with folding wing tips (creating a quasiwaverider design). The vehicle propulsion system resided underneath the aircraft and consisted of two, two-dimensional inlets mounted side-by-side. Each inlet fed three YJ93-GE-3 afterburning turbojet engines that generated 30,000 pounds of thrust. Each engine consisted of an 11-stage, axial flow compressor, an annular combustor, a two-stage turbine and a variable area, converging-diverging nozzle.



The inlet26 was designed as a mixed-compression system as shown in . The inlet started at Mach 2.0 (i.e. the terminal shock laid across the inlet cowl face). Each inlet was composed of variable ramps and bypass doors that could be positioned to maximize performance through the flight profile.

http://www.sinodefenceforum.com/att...erospace-aerodynamics-corner-xb-70-intake.jpg
An intensive review of the inlet operation is given in the SR-71 Flight Manual13. The external system utilized a translating conical spike that retracted 1 ⅝ inches per 0.1 Mach number starting near Mach 1.6 and ending at Mach 3.2 with shock-on-lip condition.
The purpose of the translation, along with a complex sequence of bleeds15-17 and bypasses, is to control the amount of flow entering the engine and to hold the terminal shock downstream of the throat (when M∞ > 1.6) to avoid unstart

Please, Log in or Register to view URLs content!

The system you have on the video is an internal compression intake and the they show for the XB-70 is mixed why?

Starting the intake means work with the termnial shock inside the intake.


DSI spills but at Mach 3 XB-70 does not

Click to expand...

Nope. The video shows what happen inside the inlet as compressible flow goes through a throat. As the air flows from a wider point to narrower point, the velocity picks up due to Bernoulli's principle. At the same time, compression occurs, as evident by the normal shock. Mass flow rate is conserved before and at the throat region, showing your claim that varying the throat area causes change to flow rate as false. This is also explained in the book as shown in the screen capture below:


The symbol mi is the mass flow the inlet mouth, and ms is the mass flow at the throat. The two values are equal, thus narrowing of the throat region does not regulate air flow, meaning your claim is incorrect.

The terms internal, external or mixed compression refer to the location of oblique shock waves. For an internal compression inlet, the oblique shocks are inside the inlet. For an external compression inlet, the oblique shocks are outside the inlet. For mixed compression, the oblique shocks are inside and outside of the inlet.


The location of the oblique shock waves does not alter how compressible fluid behaves within an inlet. Here is a video showing how compressible flow behaves:
[video=youtube;JhlEkEk7igs]http://www.youtube.com/watch?v=JhlEkEk7igs[/video]