SDF Aerospace and Aerodynamics Corner

Engineer · Jan 1, 2012

MiG-29 said:
reead this in contet and you will see how wrong you and latenlazy are

the variable ramp inlet will reduce excess airflow about 13%

gracias feliz 2012

Thanks for providing the extract that shows how wrong you are. I have highlighted the important bit for you for your convience:

A1 is the capture area, as explained here:

So A1 being constant means capture area is constant. Mass flow ratio is not equivilent to capture area, and claiming mass flow ratio change does not make you argument that F-14 and aircraft in Su-27 family can vary the cowl capture area of their inlets any less incorrect.

As far as mass flow ratio is concerned, the change is attributed to the change in oblique shock waves. This is not a unique phenomenon that occurs on variable-geometry inlet. Fixed inlets and DSI have oblique and normal shock waves as well, so these two inlets will also see change of mass flow ratio.

In any case, capture area is constant contrary to your claim.

Engineer · Jan 1, 2012

latenlazy said:
The problem is you assume ramp to mean adjustment of capture area, when a ramp can be used to either adjust both throat and capture area, or just one of the two.

Actually, the ramps cause throat area to change. MiG-29 took this and invented his own definition of capture area to mean throat area, in order to make his argument appear more correct. When shown to be incorrect by diagrams in the book, he now substitute mass flow ratio into the definition of capture area.

He is basically using his typical tactics of quoting something then slam on to it with his own meanings.

latenlazy · Jan 1, 2012

Engineer said:
Actually, the ramps cause throat area to change. MiG-29 took this and invented his own definition of capture area to mean throat area, so his original argument would seem correct. When shown to be incorrect by diagrams in the book, he now substitute mass flow ratio into the definition of capture area.

He is basically using his typical tactics of quoting something then slam on to it with his own meanings.

I was going by the picture, where the mouth of the inlet changed when the ramp changed. What exactly defines the capture area if it's not the mouth of the inlet?

Engineer · Jan 1, 2012

latenlazy said:
I was going by the picture, where the mouth of the inlet changed when the ramp changed. What exactly defines the capture area if it's not the mouth of the inlet?

I'm just going by what the books say. First, from

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From

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, A1 is the capture area, As is cross sectional area of intake at position S.

latenlazy · Jan 1, 2012

Engineer said:
I'm just going by what the books say. First, from
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From
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, A1 is the capture area, As is cross sectional area of intake at position S.

I'm just going to guess that it's defined by the cross section of air that gets sucked in, which could be a bit outside the mouth of the inlet. The whole point I think is that the capture area defines the cross sectional area of air that gets captured by the inlet, and the throat area is a narrowing behind the capture area that generates shocks and compression.

Engineer · Jan 1, 2012

latenlazy said:
I'm just going to guess that it's defined by the cross section of air that gets sucked in, which could be a bit outside the mouth of the inlet. The whole point I think is that the capture area defines the cross sectional area of air that gets captured by the inlet, and the throat area is a narrowing behind the capture area that generates shocks and compression.

Throat is the narrowest part of the intake, so throat area is the cross sectional area of the throat section. The shocks are not generated at the throat. They are created by the ramps in front of the throat. The throat is formed as a conseuqence of having to actuate the ramps.

The reduced mass flow that MiG-29 talked about occurs at the throat, where there is a discontinuity of the upper wall of the intake. In short, there is a gap there where air can escape. See

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.

At no point is there change of capture area though.

latenlazy · Jan 1, 2012

Engineer said:
Throat is the narrowest part of the intake, so throat area is the cross sectional area of the throat section. The shocks are not generated at the throat. They are created by the ramps in front of the throat. The throat is formed as a conseuqence of having to actuate the ramps.

The reduced mass flow that MiG-29 talked about occurs at the throat, where there is a discontinuity of the upper wall of the intake. In short, there is a gap there where air can escape. See
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.

At no point is there change of capture area though.

Which I'll presume is the whole point of bypass doors, to regulate the pressure build up generated at the throat.

Equation · Jan 1, 2012

Dear Engineer,

I have a design question, does the size and shape of the spill door/intake flaps on intakes effects the air flow and performance of the engine (just curious)?

Engineer · Jan 1, 2012

latenlazy said:
Which I'll presume is the whole point of bypass doors, to regulate the pressure build up generated at the throat.

One purpose of bypass doors is indeed regulate pressure within the intake, so as to keep the normal shocks at the intake's opening. Yet, this should not be confused with relieving pressure from the throat. The other purpose is to dump excess air out of the intake. The function of the bypass doors should not be confused with the functions of the intake ramps.

Equation said:
Dear Engineer,

I have a design question, does the size and shape of the spill door/intake flaps on intakes effects the air flow and performance of the engine (just curious)?

Are you talking about the

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? If so, the answer is yes. These books will answer your question more clearly and correctly than I can:

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Anyway, DSI dispense with all these mechanisms, yet achieves better performance than variable-geometry inlets like those found on the F-4D. This already says variable-geometry inlets are not always better.

MiG-29 · Jan 1, 2012

Engineer said:
Thanks for providing the extract that shows how wrong you are. I have highlighted the important bit for you for your convience:

A1 is the capture area, as explained here:

So A1 being constant means capture area is constant. Mass flow ratio is not equivilent to capture area, and claiming mass flow ratio change does not make you argument that F-14 and aircraft in Su-27 family can vary the cowl capture area of their inlets any less incorrect.

As far as mass flow ratio is concerned, the change is attributed to the change in oblique shock waves. This is not a unique phenomenon that occurs on variable-geometry inlet. Fixed inlets and DSI have oblique and normal shock waves as well, so these two inlets will also see change of mass flow ratio.

In any case, capture area is constant contrary to your claim.

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

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

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