SDF Aerospace and Aerodynamics Corner

Engineer · Jan 1, 2012

MiG-29 said:
tht is incorrect capture area is not definite by A1 alone, it is definited by A1 + As, the text say consider the generalized external compression inlet at fig 10:49 with capture area A1 and area As in fact it says As\A1 is determined by the geometry of the intake, your definition of capture area is like a pitot tube where A1=As, however it is not, As is basicly the lower lip while A1 is the upper lip

Wrong. "Capture area A1" and "area As" are two different areas, otherwise the book would have said "capture area A1+As". A1 is the capture area, and As is the cross sectional area inside the intake at station S. Another diagram from a different book also refers to A1 as the capture area.

The definition of capture area which I am using is what the books are saying, whereas the definition of capture area which you use is an invention by you. The ratio As/A1 being determined by geometry of the intake does not contradict A1 being capture area.

MiG-29 said:
the text goes further into say the variable ramp inlet will reduce excess airflow about 13%

the ramps are moveble not fixed when deployed the can have ramps of 5 degrees or 10 degrees and that gives a difference in flow rate
to put it in other words flow rate is not the same when the ramp are at 5 degrees and when they are at 10 degrees

When you create a gap in the intake, then of course air would escape from the gap. This however has nothing to do with changing capture area. Also, you are confusing the terms airflow with flow rate, the former has to do with volume and the latter has to do with speed.

The graph of 10.53b shows change of mass flow ratio, and this change is resulted from change in the oblique shock waves. Change of mass flow ratio occurs on fixed inlets and DSI as well due to present of oblique shock waves. It is not a phenomenon that only occurs with variable-geometry inlet, thus your argument that variable-geometry inlet is superior because of change in mass flow ratio doesn't work.

The only instance where variable-geometry inlet would be superior is when weight isn't an issue. In this case, you can add as many ramps as you want to boost pressure recovery ratio and inlet efficiency.

MiG-29 · Jan 2, 2012

Engineer said:
Wrong. "Capture area A1" and "area As" are two different areas, otherwise the book would have said "capture area A1+As". A1 is the capture area, and As is the cross sectional area inside the intake at station S. Another diagram from a different book also refers to A1 as the capture area.

The definition of capture area which I am using is what the books are saying, whereas the definition of capture area which you use is an invention by you. The ratio As/A1 being determined by geometry of the intake does not contradict A1 being capture area.

When you create a gap in the intake, then of course air would escape from the gap. This however has nothing to do with changing capture area. Also, you are confusing the terms airflow with flow rate, the former has to do with volume and the latter has to do with speed.

The graph of 10.53b shows change of mass flow ratio, and this change is resulted from change in the oblique shock waves. Change of mass flow ratio occurs on fixed inlets and DSI as well due to present of oblique shock waves. It is not a phenomenon that only occurs with variable-geometry inlet, thus your argument that variable-geometry inlet is superior because of change in mass flow ratio doesn't work.

The only instance where variable-geometry inlet would be superior is when weight isn't an issue. In this case, you can add as many ramps as you want to boost pressure recovery ratio and inlet efficiency.

the statement say this

comparasion of fig. 10,53b with figure 10,52 shows that variable geometry has reduced excess air that must be bypassed at high mach numbers which will result in a corresponding reduction in installation losses

and what is the cause of that ?

the variable ramp inlet will reduce excess airflow about 13%

So is the ramp that has different settings at different speeds
what is the ratio As/A1?

An why is needed in the equation?
without that ratio is impossible to get the mass flow rate, in fact A1 is not by it self alone capable to do it, it needs As in the equation to calculate mass flow rate

in fact i will tell you a fact, fixed intakes are bigger than variable for the simple fact that smaller mass flow rates are needed for variable geometry intakes, on fixed intakes to avoid choking the intake at subsonic speeds you need a larger intakes

Engineer · Jan 2, 2012

MiG-29 said:
the statement say this

comparasion of fig. 10,53b with figure 10,52 shows that variable geometry has reduced excess air that must be bypassed at high mach numbers which will result in a corresponding reduction in installation losses

and what is the cause of that ?

the variable ramp inlet will reduce excess airflow about 13%

The keyword here is "bypassed", and the cause is bypass door. Intake ramp is not the samething as bypass door.

MiG-29 said:
So is the ramp that has different settings at different speeds
what is the ratio As/A1?

An why is needed in the equation?
without that ratio is impossible to get the mass flow rate, in fact A1 is not by it self alone capable to do it, it needs As in the equation to calculate mass flow rate

in fact i will tell you a fact, fixed intakes are bigger than variable for the simple fact that smaller mass flow rates are needed, on fixed intakes to avoid choking the intake at subsonic speeds you need a larger intakes

The fact is, A1 is clearly defined as the capture area. Change in As is not change to capture area.

MiG-29 · Jan 2, 2012

Engineer said:
The keyword here is "bypassed", and the cause is bypass door. Intake ramp is not the samething as bypass door.

The fact is, A1 is clearly defined as the capture area. Change in As is not change to capture area.

here there is no magical words as you claim, to start, the formule 10:18a, shows the different geometries of intakes will have different mass flows.

You are claiming the F-14 intake with ramps deployed will have the same mass flow with ramps collapsed.

To start an intake is considered not the upper intake lip, but both upper and lower intake lip, on the F-14 the intake is raked, the lower intake lip is positioned below and aft the ramps why? well F-14 has external compression, so the ramps are ahead the lower intake lip, so the As does indeed count, first because the opening at the lower intake lip is narrower than at the upper lip when the ramps are deployed.

Second when you add a spike or intake cone you get a reduction of capture area even on a circular base intake lip as MiG-15.

You are claiming Sr-71`s intake with intake cone and without it have the same capture area, you claim the circular lip of A1 diameter will have the same capture area with and without intake cone, of course that is not true and the aft or forward movement of the cone intake changes capture area.

and this proves my point

As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent.

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and the same applies to a Mirage 2000 half cone intake as it applies to a SR-71 or MiG-21

Engineer · Jan 2, 2012

MiG-29 said:
here there is no magical words as you claim, to start, the formule 10:18a, shows the different geometries of intakes will have different mass flows.

I will put your quote up here for your convenience:

You are now claiming that bypass was not mentioned, yet there it is. Bypass wouldn't be necessary if the intake ramp acts as valve as you claimed. The present of bypass shows ramp is not a value like you imagined, and the fact that the book specifically said A1 being constant, proves that your claim of variable capture area is incorrect.

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.

MiG-29 said:
You are claiming the F-14 intake with ramps deployed will have the same mass flow with ramps collapsed.

It is called

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.

MiG-29 said:
To start an intake is considered not the upper intake lip, but both upper and lower intake lip, on the F-14 the intake is raked, the lower intake lip is positioned below and aft the ramps why? well F-14 has external compression, so the ramps are ahead the lower intake lip, so the As does indeed count, first because the opening at the lower intake lip is narrower than at the upper lip when the ramps are deployed.

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.

MiG-29 said:
Second when you add a spike or intake cone you get a reduction of capture area even on a circular base intake lip as MiG-15.

You are claiming Sr-71`s intake with intake cone and without it have the same capture area, you claim the circular lip of A1 diameter will have the same capture area with and without intake cone, of course that is not true and the aft or forward movement of the cone intake changes capture area.

and this proves my point

As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent.

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and the same applies to a Mirage 2000 half cone intake as it applies to a SR-71 or MiG-21

F-14 doesn't use intake cones, nor do aircraft within the Su-27 family.

MiG-29 · Jan 2, 2012

Engineer said:
I will put your quote up here for your convenience:

You are now claiming that bypass was not mentioned, yet there it is. Bypass wouldn't be necessary if the intake ramp acts as valve as you claimed. The present of bypass shows ramp is not a value like you imagined, and the fact that the book specifically said A1 being constant, proves that your claim of variable capture area is incorrect.

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.

F-14 doesn't use intake cones, nor do aircraft within the Su-27 family.

To start, the formula 10:18a claims

Ao1/A1=mass flow, Ao1 is the free stream area divided by the inlet highlight area A1.

Now Ao1/A1=A01/Aa As/A1.

It means the free stream flow divided by the intake highlight equals the free stream area divided by the As Area multiplied by the ratio of division of As area by intake highlight area A1.

the text says:consider the generalized compression external compression inlet of fig 10,49 with capture area A1 and As and later goes to say As and A1 ratio is determined by the inlet geometry

and now let us resolve the equation showing how you have made a fallacy and mathematics will prove you are wrong

when As=A1 it means the ratio is 1, so Ao1/A1= Ao1/As times As/A1 can also be written A01/A1 =Ao1/As multiplied by 1 hahaha so capture area is indeed As+A1 hahaha math proves your fallacy and proves As at the lower lip of F-14`s intake correct, why? because when As is not equal to A1, it means the flow rate has changed

see

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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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.

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look at the F-14 intake at max supersonic it is

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5919d1325426423-sdf-aerospace-aerodynamics-corner-f14-detail-airintake-07l.jpg

at subsonic speed

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5920d1325426456-sdf-aerospace-aerodynamics-corner-f14-detail-airintake-02l.jpg

Engineer · Jan 2, 2012

MiG-29 said:
To start, the formula 10:18a claims

Ao1/A1=mass flow, Ao1 is the free stream area divided by the inlet highlight area A1.

Now Ao1/A1=A01/Aa As/A1.

It means the free stream flow divided by the intake highlight equals the free stream area divided by the As Area multiplied by the ratio of division of As area by intake highlight area A1.

the text says:consider the generalized compression external compression inlet of fig 10,49 with capture area A1 and As and later goes to say As and A1 ratio is determined by the inlet geometry

and now let us resolve the equation showing how you have made a fallacy and mathematics will prove you are wrong

when As=A1 it means the ratio is 1, so Ao1/A1= Ao1/As times As/A1 can also be written A01/A1 =Ao1/As multiplied by 1 hahaha so capture area is indeed As+A1 hahaha math proves your fallacy and proves As at the lower lip of F-14`s intake correct, why? because when As is not equal to A1, it means the flow rate has changed

see

Wrong. The fallacy is your argument that As=A1 by assuming As=A1. It is known as

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. Furthermore, your example does not show capture area equates to A1+As, as the left-hand side of the equation is mass flow ratio and not capture area. To continue from the last step of your example:
A0i/A1 = A0i/As * 1

Since you assumed As=A1, we can substitute A1 into As which gives:
A0i/A1 = A0i/A1 * 1 = A0i/A1

So the ratio is independent of As. Since you refer to As as the throat area, we know that A1 is not As as A1 is the capture area.

Let's look at this another way. By equation 10.18a, the mass flow ratio is:
A0i/A1 = A0i/As As/A1

We can also substitute numbers into the above variables and show that you are wrong via your own method. First, we let A1=1.0, A0i=0.9, and As=0.8. From these value, the ratio left-hand side of the equation gives:
A0i/A1 = 0.9/1.0 = 0.9

Now, the right-hand side of the equation is:
A0i/As As/A1 = 0.9/0.8 * 0.8/1.0 = 1.125 * 0.8

The final result for 1.125 * 0.8 is 0.9, which is identical to A0i/A1. We can substitute any value to these three variables and we will get a value not related to As. The general case which is applicable to any value used in the three variables is as follow:

By equation 10.18a:
A0i/A1 = A0i/As As/A1

The two As on the right-hand side cancel out each other, and you are left with the ratio:
A0i/A1 = A0i/A1

which does not include As.

So you see, mathematics show you are wrong. This is expected, since your claim about capture being A1+As is wrong in the first place.

Whether As is the throat area is irrelevant to the capture area, since capture area is the projected area A1 bounded by the upper and lower lip of the intake. We have two sources for this that explicitly states A1 as the capture area. Let us look at a third

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:

Here the paper uses a different symbol Zs, but it essentially says the same thing as the other two sources did.

You are now forcibly argue your position by re-invention of definition of terminologies. This is not going to work, since your definition is incorrect in the first place. Basing your argument on this false definition is essentially basing your argument on a false premise, resulting in a wrong argument.

Engineer · Jan 2, 2012

MiG-29 said:
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

Exactly! Spill doors are what responsible of diverting excess airflow out of the ducts. The intake ramps do not play such a role.

MiG-29 said:
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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.

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This says two things:

Intake ramps slow air down by controlling position of oblique shock waves.
Spill valves are used to control the changing volume of air.

The paragraph which you quoted proves my points.

Why are inlets with intake ramps called external compression inlets? That's because the air gets compressed as it passes through the shock waves. Compression means making something takes up less space, so the same mass of air gets fitted into smaller space to fit through the narrower throat region. In addition, mass is conserved by

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.

MiG-29 said:
look at the F-14 intake at max supersonic it is

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at subsonic speed

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So? The purpose of movement of the ramps is to position the oblique shock waves. Your claim that inlets on F-14 and aircraft in Su-27 family can vary capture area remains unsubstantiated. Your other claim that throat area equates to capture area is also unsubstantiated, and is contradicted by three different sources all stating capture area is the projected area between upper and lower lip of the intake.

MiG-29 · Jan 3, 2012

Engineer said:
Wrong. The fallacy is your argument that As=A1 by assuming As=A1. It is known as
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. Furthermore, your example does not show capture area equates to A1+As, as the left-hand side of the equation is mass flow ratio and not capture area. To continue from the last step of your example:
A0i/A1 = A0i/As * 1

Since you assumed As=A1, we can substitute A1 into As which gives:
A0i/A1 = A0i/A1 * 1 = A0i/A1

So the ratio is independent of As. Since you refer to As as the throat area, we know that A1 is not As as A1 is the capture area.

Let's look at this another way. By equation 10.18a, the mass flow ratio is:
A0i/A1 = A0i/As As/A1

We can also substitute numbers into the above variables and show that you are wrong via your own method. First, we let A1=1.0, A0i=0.9, and As=0.8. From these value, the ratio left-hand side of the equation gives:
A0i/A1 = 0.9/1.0 = 0.9

Now, the right-hand side of the equation is:
A0i/As As/A1 = 0.9/0.8 * 0.8/1.0 = 1.125 * 0.8

The final result for 1.125 * 0.8 is 0.9, which is identical to A0i/A1. We can substitute any value to these three variables and we will get a value not related to As. The general case which is applicable to any value used in the three variables is as follow:

By equation 10.18a:
A0i/A1 = A0i/As As/A1

The two As on the right-hand side cancel out each other, and you are left with the ratio:
A0i/A1 = A0i/A1

which does not include As.

So you see, mathematics show you are wrong. This is expected, since your claim about capture being A1+As is wrong in the first place.

Whether As is the throat area is irrelevant to the capture area, since capture area is the projected area A1 bounded by the upper and lower lip of the intake. We have two sources for this that explicitly states A1 as the capture area. Let us look at a third
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:

Here the paper uses a different symbol Zs, but it essentially says the same thing as the other two sources did.

You are now forcibly argue your position by re-invention of definition of terminologies. This is not going to work, since your definition is incorrect in the first place. Basing your argument on this false definition is essentially basing your argument on a false premise, resulting in a wrong argument.

i have to admit i was wrong, yes indeed capture area, is the A1 number As is considered throat area, in the case of the F-14 i do not exactly how does the throat area is used to reduce flow, so i will be very honest with you, while i admit i was wrong about the throat area in the F-14, i know the SR-71 or MiG-21 do change capture area, in that i am right, as i am right in that variable geometry ramps do actually reduce excess flow.

However now i am reading articles to make me an idea how concord uses the variable ramps to reduce excess flow.
If i find you were right i will admit it, and i agree capture area A1, but in some points i know i am right
So i will return later i we can continue arguing this topic.

Engineer · Jan 3, 2012

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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