SDF Aerospace and Aerodynamics Corner

Re: J-20... The New Generation Fighter III

MiG-29 said:

You are wrong i do understand perfectly the bump, what you do not understand how an intake works, they are not only creating shock waves to slow down the flow, but also controling the mass flow, a turbofan needs flow at Mach 0.5 to work, on a DSI you need to slow the flow at such speeds even if outside the inlet the air is flowing at supersonic speeds, you need to control the subcritical and supercritical states where pressure recovery losses can appear, plus flow mass control and bypass is needed for turbofans or any turbine.

These are needed technologies if you use turbines, F-14 and Su-27 for such a reason have bypass doors and SR-71 moves forward and backwards the intake cone.

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Ummm, you didn't understand my criticism. Different geometries=different performance at different Mach numbers. Because the flow state at different Mach numbers are themselves different, one bump shape an perform very well at one speed and very poorly in another. You can thus optmize the bump and inlet geometries for different speeds. The bump and inlet shape and positions must further be optimized for other sources of flow change and compression such as the nose of the airplane and the inlet tunnel. Thus one cannot tell which speeds a diverterless inlet works best at simply by their presence. Not all bump and inlet shapes perform the same. Different planes will have different flow dynamics as air approaches an intake. What you do not understand is that the geometry of the inlet itself can be changed to operate best at higher or lower speeds.

Re: J-20... The New Generation Fighter III

latenlazy said:

Ummm, you didn't understand my criticism. Different geometries=different performance at different Mach numbers. Because the flow state at different Mach numbers are themselves different, one bump shape an perform very well at one speed and very poorly in another. You can thus optmize the bump and inlet geometries for different speeds. The bump and inlet shape and positions must further be optimized for other sources of flow change and compression such as the nose of the airplane and the inlet tunnel. Thus one cannot tell which speeds a diverterless inlet works best at simply by their presence. Not all bump and inlet shapes perform the same. Different planes will have different flow dynamics as air approaches an intake. What you do not understand is that the geometry of the inlet itself can be changed to operate best at higher or lower speeds.

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and how you change the geometry on a fixed DSI intake? how do you control the flow mass to avoid supercritical or subcritical states? how do you control intake capture area?

On SR-71 the cone intake moves forward, on the F-14 the ramps deploy and the by pass doors bleed extra mass flow how do you do it on a DSI?

How do you do it if the intake cowl does divert boundary layer and the bump is fixed?

Re: J-20... The New Generation Fighter III

MiG-29 said:

and how you change the geometry on a fixed DSI intake? how do you control the flow mass to avoid supercritical or subcritical states? how do you control intake capture area?

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I'm no aerodynamics specialist, but I would imagine by changing the shape and position of the bump, inlet cowl, and fore body, using a wind tunnel to test for the specific effects.

On SR-71 the cone intake moves forward, on the F-14 the ramps deploy and the by pass doors bleed extra mass flow how do you do it on a DSI?
How do you do it if the intake cowl does divert boundary layer and the bump is fixed?

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Well, if we're talking a variable solution, inlet tunnel ramps or a movable bump. If we're talking a fixed solution, you optimize the bump shape for the intended speed envelope. I'm not saying the DSI bump is itself variable, but that what shape is used for the fixed solution can be optimized for different speeds. Theoretically you could design a bump that works great at mach 3 but is awful below mach 1, etc. Depending on other factors of compression like the inlet tunnel and the forebody, you could even have a DSI that works great from 0 to mach 3. The point is the DSI bump itself has no speed limit. What speed it operates best at is dependent on its specific geometry.

Wind tunnel tests means building many models and spending weeks or months testing them. Better use computer simulations and test these using wind tunnel models and a few full scale models in test aircraft. Having ever more super computers is really nice for such work.

Mig-29 anyone who reads their wikipedia knows about turbojet/fans limitation, and about the SR-71 inlet cone and shockramps

to further my question and make my point clearer, why can't DSI go above mach 2 if they can design DHI for ramjets, when both needs subsonic air to feed the engine?

delft said:

Wind tunnel tests means building many models and spending weeks or months testing them. Better use computer simulations and test these using wind tunnel models and a few full scale models in test aircraft. Having ever more super computers is really nice for such work.

Click to expand...

computer fluid dynamics is the new cool these days

Re: J-20... The New Generation Fighter III

latenlazy said:

I'm no aerodynamics specialist, but I would imagine by changing the shape and position of the bump, inlet cowl, and fore body, using a wind tunnel to test for the specific effects.

Well, if we're talking a variable solution, inlet tunnel ramps or a movable bump. If we're talking a fixed solution, you optimize the bump shape for the intended speed envelope. I'm not saying the DSI bump is itself variable, but that what shape is used for the fixed solution can be optimized for different speeds. Theoretically you could design a bump that works great at mach 3 but is awful below mach 1, etc. Depending on other factors of compression like the inlet tunnel and the forebody, you could even have a DSI that works great from 0 to mach 3. The point is the DSI bump itself has no speed limit. What speed it operates best at is dependent on its specific geometry.

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Sorry but you do not understand how DSI works niether how intakes work, and the reason is you do not understand and know why do you need to control the air mass flow at different speeds.

And i will explain it to you


F-111 and F-14 for example do control the air mass flow, on F-111, the intake semi cone expands or collapses depending on the speed and air mass flow, why? simple engines do not only need to slow dow the flow from supersonic speeds or transonic, but also regulate the volume of air that gets into the intake.

Capture area refers to the volume of air the intake takes, on F-111, and basicly SR-71 or Mirage III/2000 by moving the intake longitudinaly or collapsing or expanding the cone/semicone control the air volume, thus the air mass flow is control, they do this to prevent subcritical or supercritical states, subcritical and supercritical states mean the oblique and normal shocks change their location lowering pressure recovery, the critical state is the highest pressure recovery at an ideal mass flow.

On F-14 the air mass flow is controlled and bled by a bypass slot and bypass doors, the air mass flow is bled so only the amount of air the jet needs enters.

As speed goes higher so air mass flow increases, variable geomety and bypass doors are a most as you go faster and faster on a Sr-71.

On the DSI you have several limitations in shockwave generation and fixed geometry

paintgun said:

Mig-29 anyone who reads their wikipedia knows about turbojet/fans limitation, and about the SR-71 inlet cone and shockramps

to further my question and make my point clearer, why can't DSI go above mach 2 if they can design DHI for ramjets, when both needs subsonic air to feed the engine?




computer fluid dynamics is the new cool these days

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because pressure recovery goes down thus affecting the operation of the engine before it can reach mach 3.5 where is the speed the RAMJET becomes operatonal.

On the SR-71 they have a variable geometry intake with mixed compression.


however i will be honest, the webpage you present has several claims that if they are true and i understood well you are right,

The DHI has been computationally proven for at least Mach 3 to Mach 10 applications, which is well above the nominal operating envelope of traditional boundary layer diverting systems. The DHI is highly effective in boundary layer reductionabove Mach 3, which is important for ramjet and scramjet applications. The present invention facilitates successful engine start-up and enhanced operability and performance at much lower Mach numbers than those demonstrated in the prior art. A lowerMach number start speed for a ramjet system closes the gap between the maximum speed of a gas turbine accelerator and the minimum speed at which a ramjet can take over. This design enables a dual mode, air breathing vehicle to propel itself tohypersonic speeds from a standing start on the ground. The DHI utilizes an elegant design method that converges on an optimized solution with few iterations, minimizing computer time, and requiring relatively few man hours. It provides a uniformflowfield at the cowl plane and can be modified to include low-observable features.

if this is correct, the new DHI will allow take off and Mach 3.5 operations, any way thanks for the information, but this is not a conventional DSI

However i do not understand why they say The turbojet, ramjet, and scramjet engine applications forthe DHI of the present invention include flight speeds of approximately Mach 2.5 to 10.

why they say mach 2.5 to 10 for turbojet engines? they should say 0-mach 10, of course they say the word include, but for me is unclear if it is only able to opeate from Mach 2.5 to Mach 10

Re: J-20... The New Generation Fighter III

MiG-29 said:

Sorry but you do not understand how DSI works niether how intakes work, and the reason is you do not understand and know why do you need to control the air mass flow at different speeds.

And i will explain it to you


F-111 and F-14 for example do control the air mass flow, on F-111, the intake semi cone expands or collapses depending on the speed and air mass flow, why? simple engines do not only need to slow dow the flow from supersonic speeds or transonic, but also regulate the volume of air that gets into the intake.

Capture area refers to the volume of air the intake takes, on F-111, and basicly SR-71 or Mirage III/2000 by moving the intake longitudinaly or collapsing or expanding the cone/semicone control the air volume, thus the air mass flow is control, they do this to prevent subcritical or supercritical states, subcritical and supercritical states mean the oblique and normal shocks change their location lowering pressure recovery, the critical state is the highest pressure recovery at an ideal mass flow.

On F-14 the air mass flow is controlled and bled by a bypass slot and bypass doors, the air mass flow is bled so only the amount of air the jet needs enters.

As speed goes higher so air mass flow increases, variable geomety and bypass doors are a most as you go faster and faster on a Sr-71.

On the DSI you have several limitations in shockwave generation and fixed geometry

Click to expand...

I know perfectly well how inlets works. It's not a challenging concept. I'm pointing out that you're making an assumption about a limitation that isn't theoretically there, and so far you have pointed at nothing that disproves it. In fact, you seem not to have read anything I've said.

I'm saying that different bumps can be optimized for different speeds, because a different geometry can be best optimized for the air flow at that speed. For example, let's take a variable inlet. The variable inlet's shape at mach 1.5 is best optimized for mach 1.5, we'll call this inlet 1. The variable inlet's shape at mach 2.5 is best optimized for mach 2.5, we'll call this inlet 2. A fixed solution that has the same geometry as inlet 1 will perform best at mach 1.5. A fixed solution that had the same geometry as inlet 2 will perform best at mach 2.5. These two fixed solutions are optimized for different speeds. What does this mean? You can design a fixed inlet that can perform well at higher than mach 2. We know this because there is nothing special about a variable inlet's geometry except that it can be adjusted for different speeds. If you designed a fixed inlet that had the same shape as a variable inlet at higher mach operations, it would be optimized for that same speed.

Now, if we talk about older inlet designs, the reason a variable solution was employed is because the mass air flow's interaction with complex geometries is hard to predict, and thus they employed simpler geometries that, if designed for higher mach numbers, would mean reduced mass air flow at lower mach numbers, thereby choking the engine. The DSI is significant because it is the first inlet to use a complex 3D geometry. The shock interaction of a bump is much harder to predict and far more complex than a 2D geometry. That means you can't just say "oh look it only has one shock generator it must have limited pressure recovery at high mach". It's not whether the plane employs a bump, but the shape and position of the bump and inlet that determines its performance.

That said, as mentioned many times before bump isn't the only thing that can generate a shockwave for the air flow. The nose of the air plane and the inlet tunnel can also generate shockwaves to compress the air flow to the engine at higher mach speeds.

MiG-29 said:

because pressure recovery goes down thus affecting the operation of the engine before it can reach mach 3.5 where is the speed the RAMJET becomes operatonal.

On the SR-71 they have a variable geometry intake with mixed compression.


however i will be honest, the webpage you present has several claims that if they are true and i understood well you are right,

The DHI has been computationally proven for at least Mach 3 to Mach 10 applications, which is well above the nominal operating envelope of traditional boundary layer diverting systems. The DHI is highly effective in boundary layer reductionabove Mach 3, which is important for ramjet and scramjet applications. The present invention facilitates successful engine start-up and enhanced operability and performance at much lower Mach numbers than those demonstrated in the prior art. A lowerMach number start speed for a ramjet system closes the gap between the maximum speed of a gas turbine accelerator and the minimum speed at which a ramjet can take over. This design enables a dual mode, air breathing vehicle to propel itself tohypersonic speeds from a standing start on the ground. The DHI utilizes an elegant design method that converges on an optimized solution with few iterations, minimizing computer time, and requiring relatively few man hours. It provides a uniformflowfield at the cowl plane and can be modified to include low-observable features.

if this is correct, the new DHI will allow take off and Mach 3.5 operations, any way thanks for the information, but this is not a conventional DSI

However i do not understand why they say The turbojet, ramjet, and scramjet engine applications forthe DHI of the present invention include flight speeds of approximately Mach 2.5 to 10.

why they say mach 2.5 to 10 for turbojet engines? they should say 0-mach 10, of course they say the word include, but for me is unclear if it is only able to opeate from Mach 2.5 to Mach 10

Click to expand...

Because it's talking about a specific kind of geometry.

Re: J-20... The New Generation Fighter III

latenlazy said:

I know perfectly well how inlets works. It's not a challenging concept. I'm pointing out that you're making an assumption about a limitation that isn't theoretically there, and so far you have pointed at nothing that disproves it. In fact, you seem not to have read anything I've said.

I'm saying that different bumps can be optimized for different speeds, because a different geometry can be best optimized for the air flow at that speed. For example, let's take a variable inlet. The variable inlet's shape at mach 1.5 is best optimized for mach 1.5, we'll call this inlet 1. The variable inlet's shape at mach 2.5 is best optimized for mach 2.5, we'll call this inlet 2. A fixed solution that has the same geometry as inlet 1 will perform best at mach 1.5. A fixed solution that had the same geometry as inlet 2 will perform best at mach 2.5. These two fixed solutions are optimized for different speeds. What does this mean? You can design a fixed inlet that can perform well at higher than mach 2. We know this because there is nothing special about a variable inlet's geometry except that it can be adjusted for different speeds. If you designed a fixed inlet that had the same shape as a variable inlet at higher mach operations, it would be optimized for that same speed.

Now, if we talk about older inlet designs, the reason a variable solution was employed is because the mass air flow's interaction with complex geometries is hard to predict, and thus they employed simpler geometries that, if designed for higher mach numbers, would mean reduced mass air flow at lower mach numbers, thereby choking the engine. The DSI is significant because it is the first inlet to use a complex 3D geometry. The shock interaction of a bump is much harder to predict and far more complex than a 2D geometry. That means you can't just say "oh look it only has one shock generator it must have limited pressure recovery at high mach". It's not whether the plane employs a bump, but the shape and position of the bump and inlet that determines its performance.

That said, as mentioned many times before bump isn't the only thing that can generate a shockwave for the air flow. The nose of the air plane and the inlet tunnel can also generate shockwaves to compress the air flow to the engine at higher mach speeds.


Because it's talking about a specific kind of geometry.

Click to expand...

i see right away you do not know how an inlet woks because you lack understanding of what is supercritical states, is not the geometry of the inlet, because intake cones are 3D, but the intake size is to capture an specific amount of air, a bump is a body that generates multishocks of very small strength, however on a DSI as the one of F-35, JF-17and J-10B i am right, their fixed geometry means the higher the speed lower the pressure recovery as a result of a fixed area capture and unability to create the 4 shocks a 2d intake with ramps and by pass slot.

Having subcritical states means spillage and lower pressure recovery, in few words subcritical states mean the oblique and normal shocks move forward and do not enpinge any more on the intake cowl lips, supercritical means the normal shock goes further inside the intake duct and decrease pressure recovery due to a stronger normal shock

Is not the DSI intake by it self but the turbine it self, that with low pressure recovery fails, increases burn fuel.

However the DHI seems to increase the ability of the engine to achieve higher pressure recovery or at least increase the ability to start the RAMJET at speeds of Mach 2.5-3, this does not mean the pressure recovery is the ideal for a fighter, but simply they have created a better RAM intake that allows starting the engine before the turbine fails or fuel runs out

MiG-29 said:

because pressure recovery goes down thus affecting the operation of the engine before it can reach mach 3.5 where is the speed the RAMJET becomes operatonal.

On the SR-71 they have a variable geometry intake with mixed compression.


however i will be honest, the webpage you present has several claims that if they are true and i understood well you are right,

The DHI has been computationally proven for at least Mach 3 to Mach 10 applications, which is well above the nominal operating envelope of traditional boundary layer diverting systems. The DHI is highly effective in boundary layer reductionabove Mach 3, which is important for ramjet and scramjet applications. The present invention facilitates successful engine start-up and enhanced operability and performance at much lower Mach numbers than those demonstrated in the prior art. A lowerMach number start speed for a ramjet system closes the gap between the maximum speed of a gas turbine accelerator and the minimum speed at which a ramjet can take over. This design enables a dual mode, air breathing vehicle to propel itself tohypersonic speeds from a standing start on the ground. The DHI utilizes an elegant design method that converges on an optimized solution with few iterations, minimizing computer time, and requiring relatively few man hours. It provides a uniformflowfield at the cowl plane and can be modified to include low-observable features.

if this is correct, the new DHI will allow take off and Mach 3.5 operations, any way thanks for the information, but this is not a conventional DSI

However i do not understand why they say The turbojet, ramjet, and scramjet engine applications forthe DHI of the present invention include flight speeds of approximately Mach 2.5 to 10.

why they say mach 2.5 to 10 for turbojet engines? they should say 0-mach 10, of course they say the word include, but for me is unclear if it is only able to opeate from Mach 2.5 to Mach 10

Click to expand...

I thought if a plane goes higher in altitude the air became thinner and temperature drops (colder), therefore less intake of air flow? (Just curious)