SDF Aerospace and Aerodynamics Corner

Engineer said:

I appreciate the gesture. I will not argue against your statements with regards to SR-71 and MiG-21's intakes, as I realize they do not use intake ramps and obviously work differently, so for now I will assume you are correct about them.

I also want to say that a lot of ideas you have are fine by themselves, but whenever you put them together to convey a more complex idea things always seem to fall into pieces. Future discussion would be more fruitful if you can work on your reasoning skills and avoid some common

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Well engineer, mistakes are human, it is impossible not to have them plus sometimes we get too overexcited, the formula did indeed show to you and me i did not think well my argument, just by fractions i was wrong.

But okay, after a more exaustive reading i have see, the mas flow ratio is considered as :
Mass Flow Ratio
The mass flow ratio is defined as the ratio of air mass flow at inlet entry
to air mass flow at free stream conditions.


An the formula we were discussing deals with it, you are right A1 is capture area as As is throat area, however you are not right in the aspect the mass flow ratio at the throat actually is the air mass flow at inlet entry, while A1 is air mass flow at free stream conditions.

Aoi is also the air mass flow at inlet entry, why? well mi=ms and this is the same as p0VoA0i=psVsAs where P = air density
A = cross-sectional area
v = air velocity

So if i am not wrong, and increase of S such as Si means the the air mass flow at inlet entry has increased.

If i am correct it means the variable throat indeed regulates mass flow and F-14 uses it as concord to regulate mass flow


Efficiency of the intake in all flight conditions is taken care
of by a variable-angle ramp in the upper surface of the intake
throat and by an auxiliary door in the floor of each intake.
The primary function of the ramp is to control the external
shock-wave pattern in front of the intake. The ramp is also
used to assist the lower surface auxiliary door when it is
necessary to spill air from the intake—for instance, when slow
engine failure occurs of there is a need to reduce excess thrust

Mig 29 & Engineer agreeing? Whoa, is it raining cats and dogs outside? Just kidding, good debates there gentlemen. I enjoyed reading the knowledge about aerodynamics displayed from the both of you. Thanks.

Equation said:

Mig 29 & Engineer agreeing? Whoa, is it raining cats and dogs outside? Just kidding, good debates there gentlemen. I enjoyed reading the knowledge about aerodynamics displayed from the both of you. Thanks.

Click to expand...

well communication is a exchange of ideas, and i have learnt more just by talking to engineer because i have to read more and more, plus not always i am right, communication is a double way you hear and learn from others, now i am reading how variable geometry intakes reduce mass flow. and it seems that after all F-14 does move the ramps to reduce mass flow by deploying the variable throat and slot.

When he is right, you read many articles where the same terminology is used, so i had to admit i was wrong, i am interested in learning, so i have to admit mistakes

However a variable geometry is indeed smaller than a fixed one see graph

Engineer said:

I appreciate the gesture. I will not argue against your statements with regards to SR-71 and MiG-21's intakes, as I realize they do not use intake ramps and obviously work differently, so for now I will assume you are correct about them.

I also want to say that a lot of ideas you have are fine by themselves, but whenever you put them together to convey a more complex idea things always seem to fall into pieces. Future discussion would be more fruitful if you can work on your reasoning skills and avoid some common

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Intake types
First generation of supersonic intakes:
– sharp-lipped pitot intake
– long subsonic duct (high internal friction)
– large total pressure loss due to normal shock wave
Second generation: addition of conical spike (Mig 21)
– Houses radar dish
– Improves supersonic pressure recovery (oblique
shock)
Horizontal ramp inlet
– Fuselage boundary layer diverter required
– Long ramp lengths due to inlet aspect ratio (thicker
boundary layer)
Variable geometry capability in the ramp angle
changes for mass flow regulation This is for F-14

Possible inclusion of variable cowl devices to enhance
inlet engine matching F-15 case




Half cone inlet
– Variable geometry for mass flow regulation via translating
cone


this is for Mirage III/2000/kfir



Vertical ramp inlet
– Variable geometry capability in ramp angle changes
for mass flow regulation
Stable mass flow ratio at Mach 2.0: approx 10 to
30% with and inlet design Mach number of 2.2
this is for MiG-23 and F-4

You were right in the sense only the F-15 has variable capture area, you are right F-14 does not change capture area just throat area, by increasing the throat area it achives a higher inlet flow since

Aoi/A1 equals mass flow rate, thus when As increase its area Aoi grows.


As the name implies, mixed compression inlets offer a blending of external and internal compression and seek a more practical balance between performance and complexity than that offered by fully internal compression designs in the Mach range from approximately 2.5 to 3.5. The internal portion of the shock train of a mixed compression inlet is less sensitive to flow disturbances than a fully internal design, and has lower cowling angle and drag than a fully external compression inlet designed to the same speed. But mixed compression nevertheless requires a complex control system for starting the internal shock train and for stability management to avoid inlet unstart. Two notable applications of mixed compression include the inlets on the XB-70 Valkyrie and SR-71 Blackbird aircraft.

[0008]External compression inlets are most appropriate for applications below about Mach 2.5. In this speed range, external compression offers a design simplicity that typically outweighs its generally inferior pressure recovery. Because the shock train is completely external, cowling angles, and therefore installed drag characteristics, tend to be higher when compared against internal and mixed compression designs at similar speed. However, because the shock train on an external compression inlet remains completely outside of the internal flow path, it is not subject to the sudden unstart expulsion produced by upstream or downstream flow disturbances. External compression shock stability is therefore superior to mixed or internal compression designs, requiring a significantly less complicated inlet control system. Notable examples of inlets employing external compression include those used on the Concorde, the F-14 Tomcat, and the F-15 Eagle.

Engineer said:

Furthermore, in equation 10.18a the ratio is A0i/A1, which doesn't involve As. Furthermore, A1 is independent from As, while A0i is dependent on shock geometry and not As. Your problem is that you are trying to claim As as A1 to forcibly argue capture area changes, and you go on inventing your own definition when it suits you. Now the book contradicts your claim.

It is called

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In other words, the area of the inlet is the projected area bounded by upper and lower lip. This is A1, which is the capture area.

No one claims As doesn't count, but the change in As is not change in capture area.



:

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no Bernoulli's principle there, the air can be spilled A1 means free stream air see, while As the air that the intake takes, that is the reason it is a ratio, the ratio can less than 1, 1 or more than 1

MiG-29 said:

no Bernoulli's principle there, the air can be spilled A1 means free stream air see, while As the air that the intake takes, that is the reason it is a ratio, the ratio can less than 1, 1 or more than 1

Click to expand...

Your diagrams do not contradict anything I have said.

Bernoulli's principle always occurs. The diagrams you have show what you called a sub-critical condition, where there is too much air in the inlet and pushes the normal shock out of the mouth of the intake. Air spillage occurs on variable-geometry, fixed, as well as diverterless supersonic intakes, except that DSI has better performance compared to others when air is spilled. The shape of the lip also influences how efficient air can be spilled, and a lot of attention is put into designing the lip on J-10B.

MiG-29 said:

Well engineer, mistakes are human, it is impossible not to have them plus sometimes we get too overexcited, the formula did indeed show to you and me i did not think well my argument, just by fractions i was wrong.

But okay, after a more exaustive reading i have see, the mas flow ratio is considered as :
Mass Flow Ratio
The mass flow ratio is defined as the ratio of air mass flow at inlet entry
to air mass flow at free stream conditions.


An the formula we were discussing deals with it, you are right A1 is capture area as As is throat area, however you are not right in the aspect the mass flow ratio at the throat actually is the air mass flow at inlet entry, while A1 is air mass flow at free stream conditions.

Click to expand...

No, I never claimed mass flow ratio at throat equates to mass flow at intake mouth. Mass flow ratio is unitless whereas mass flow is not, and I would not make a mistake like that.

What I pointed out is that mass is conserved as its pass through the mouth and throat, due to Bernoulli's principle. I never claimed ratio is conserved.

MiG-29 said:

Aoi is also the air mass flow at inlet entry, why?

Click to expand...

A0i is area, not mass flow.

MiG-29 said:

well mi=ms and this is the same as p0VoA0i=psVsAs where P = air density
A = cross-sectional area
v = air velocity

So if i am not wrong, and increase of S such as Si means the the air mass flow at inlet entry has increased.

Click to expand...

This is conservation of mass which I have been telling you about. I mentioned Bernoulli's principle so many times already.

As long as air remains compressible, whatever goes into the inlet mouth must fit through the throat. If you want less air, then you must dump the air overboard after the throat. This is the role of bypass doors, not intake ramps.

MiG-29 said:

If i am correct it means the variable throat indeed regulates mass flow and F-14 uses it as concord to regulate mass flow


Efficiency of the intake in all flight conditions is taken care
of by a variable-angle ramp in the upper surface of the intake
throat and by an auxiliary door in the floor of each intake.
The primary function of the ramp is to control the external
shock-wave pattern in front of the intake. The ramp is also
used to assist the lower surface auxiliary door when it is
necessary to spill air from the intake—for instance, when slow
engine failure occurs of there is a need to reduce excess thrust

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Read the next paragraph. What controls the amount of air flow is the auxiliary door in the floor of the intake. The ramp is for controlling the angle of the shock waves.

Engineer said:

Your diagrams do not contradict anything I have said.

Bernoulli's principle always occurs. The diagrams you have show what you called a sub-critical condition, where there is too much air in the inlet and pushes the normal shock out of the mouth of the intake. Air spillage occurs on variable-geometry, fixed, as well as diverterless supersonic intakes, except that DSI has better performance compared to others when air is spilled. The shape of the lip also influences how efficient air can be spilled, and a lot of attention is put into designing the lip on J-10B.

Click to expand...

Bernoulli's principle does occur but not like you claim, the air by being spilled means the true air flow is not the potential A1 but the real As, when the ratio is 1, is when the potential air capture equals the real air mass taken by the throat.

when the ratio is 1>Ao/Ac or in few words lesser than one means the capture area is spilling the air, so in few words you need bypass doors and variable geometry too on F-14 an F-14 to control air mass.

On DSI by being fixed, you can not control as well air mass and flow ratio.

F-14 and F-15 have variable geometry throats which regulate mass flow, variable geometry means variable intake sizing not fixed

Engineer said:

No, I never claimed mass flow ratio at throat equates to mass flow at intake mouth. Mass flow ratio is unitless whereas mass flow is not, and I would not make a mistake like that.

What I pointed out is that mass is conserved as its pass through the mouth and throat, due to Bernoulli's principle. I never claimed ratio is conserved.



A0i is area, not mass flow.



This is conservation of mass which I have been telling you about. I mentioned Bernoulli's principle so many times already.

As long as air remains compressible, whatever goes into the inlet mouth must fit through the throat. If you want less air, then you must dump the air overboard after the throat. This is the role of bypass doors, not intake ramps.



Read the next paragraph. What controls the amount of air flow is the auxiliary door in the floor of the intake. The ramp is for controlling the angle of the shock waves.

Click to expand...

Horizontal ramp inlet
– Fuselage boundary layer diverter required
– Long ramp lengths due to inlet aspect ratio (thicker
boundary layer)
Variable geometry capability in the ramp angle
changes for mass flow regulation This is for F-14

Possible inclusion of variable cowl devices to enhance
inlet engine matching F-15 case

sr-71 and Mirage 2000 do the same
Half cone inlet
– Variable geometry for mass flow regulation via translating
cone


this is for Mirage III/2000/kfir



If you were right you only would need the by pass doors, not variable geometry throats, however the throat by changing its area also changes the amount of air ingested

MiG-29 said:

Bernoulli's principle does occur but not like you claim...

Click to expand...

How do you believe Bernoulli's principle works then?

MiG-29 said:

the air by being spilled means the true air flow is not the potential Ai but the real As, when the ratio is 1, is when the potential air capture equals the real air mass taken by the throat.

Click to expand...

The ratio only involves A1 the capture area and A0i the free stream cross sectional area. Note that this is a ratio, which is unitless and does not equate to air mass flow, and has nothing to do with throat area.

As for your statement that air mass taken at mouth is equate to air mass taken at the throat, it is in fact Bernoulli's principle exactly like I've told you. You are now saying I am wrong yet saying the exact same thing as I have told you regarding Bernoulli's principle.

MiG-29 said:

when the ratio is 1>Ao/Ac or in few words lesser than one means the capture area is spilling the air, so in few words you need bypass doors and variable geometry too on F-14 an F-14 to control air mass.

On DSI by being fixed, you can not control as well air mass and flow ratio.

F-14 and F-15 have variable geometry throats which regulate mass flow, variable geometry means variable intake sizing not fixed

Click to expand...

You are now contradicting yourself by claiming "air mass taken at mouth is equate to air mass taken at the throat" and "throat regulates mass flow". Your first statement is correct, and this statement is what makes your second statement incorrect.

The throat is a result of the intake ramp positioning itself to optimally place the shock waves. Intake ramp does not regulate air flow, and this is the reason there are bypass doors.

Spilling occurs due to change in shape of shock waves, and this occurs on DSI. Spilling air decreases performance, yet DSI loses less performance compared to other intakes.