SDF Aerospace and Aerodynamics Corner

MiG-29 · Jan 9, 2012

Engineer said:
For any inlet system whether variable-geometry or fixed, excess air must either be spilled or bypassed. Either way, some performance is lost. For DSI, the excess air is exclusively spilled but this performance lost is less than other inlets.

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low mass flow rates also mean low subsonic speeds, so the article says not better for for supercruise at Mach 1.7 but better near transonic speeds, that is one of the reasons why F-35 its okay flying at a max speed of Mach 1.6

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Engineer · Jan 9, 2012

MiG-29 said:
look you are wrong engineer, for a simple reason, spilled air shows the throat does not equate to capture area.

I have always maintained that throat area does not equate to capture area. There is nothing incorrect or wrong about this. Furthermore, you are unable to point out what is incorrect about my statements, showing nothing is wrong about my statements.

MiG-29 said:
i know this is too abstract for you but i will put you a simple analogy, a sink

If you open the faucet and let the water run on a sink you discover that water can go right away down the drain pipe if the water runs at a small flow rate however if the water runs on a stronger jet of water and the flow rate is higher the sink bowl will fill up with water that takes more time to go and seems to flow slower and so it gets overflowed then you need to close the faucet or a overflow hole to relieve the sink.

Same is a intake, it spills for the same reason, on an fixed intake bypass doors relieve the overflow as the overflow hole on a sink.

Variable geometry means a larger throat for more air to get into the intake at low speeds and a smaller throat to reduce flow

If you apply the same amount of water to a sink with a larger opening and larger drain pipe, it won`t overflow.

This analogy is flawed. Water is an incompressible fluid, whereas air is compressible fluid. Your use of this particular analogy clearly shows that you are unaware of many keys fundamental of aerodynamics.

Compressible means the matter can be squeezed to take up smaller space. This is why air that passes through the inlet mouth will pass through the narrower space at the throat with mass being conserved. The speed of air at the throat will be slightly higher than at the intake mouth to compensate for the restricted space. This is Bernoulli's principle.

What regulates air flow are spill doors. Recall this statement of your's:

MiG-29 said:
If you were right you only would need the by pass doors, not variable geometry throats...

As we have seen on F-22, inlets can work perfectly fine without intake ramps.

Engineer · Jan 9, 2012

MiG-29 said:
supersonic intakes are made relatively smaller than subsonic intakes, so they run the risk to choke the engine at low speeds, thus they need auxiliary intakes, but as they approach their mach number and their intake sizing means bypass doors are needed, on variable geometry intake throats the ramps control mass flow and aid bypass doors or auxiliary intakes reducing drag

your concept is only of a fixed intake

The intake is a fixed geometry,
external compression intake. It comprises a 2-shock
(oblique and normal shock) system, which is
formed by a protruding conical section (see Fig 4).
The cone semi-vertex angle is 23.9o and the intake
throat area is 0.230 m2. To control the position of
the normal shock and prevent the shock system
operating sub-critically a single bypass door was
incorporated on the wing mounted intakes and two
smaller doors on the fuselage intake. The capture
area is 0.673 m2 and this area is complemented with
auxiliary doors with an area of 0.150 m2 for low
speed, low altitude operation.
The intake is primarily designed to operate
at flight speeds of Mach 1.6 with a pressure
recovery of 90 %. The intake has also been
designed with growth in mind, and will still operate
reasonably efficiently up to Mach 1.9 with a
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Variable-geometry inlets have bypass system, and it is bypass system that regulates the amount of air going into the engine, not ramp. All your citation have been saying the same things as I have pointing out to you:

Intake ramp controls position of shock waves.
Flow regulation and maintaining critical condition is achieved using auxiliary doors and bypass.

From the same paper that you use:

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Engineer · Jan 9, 2012

MiG-29 said:
low mass flow rates also mean low subsonic speeds, so the article says not better for for supercruise at Mach 1.7 but better near transonic speeds, that is one of the reasons why F-35 its okay flying at a max speed of Mach 1.6

Nope. The paper clear states sub-critical condition. You cannot have sub-critical condition without shock waves, and the presence of shock waves clearly indicates supersonic speed. Furthermore:

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MiG-29 · Jan 9, 2012

Engineer said:
I have always maintained that throat area does not equate to capture area. There is nothing incorrect or wrong about this. Furthermore, you are unable to point out what is incorrect about my statements, showing nothing is wrong about my statements.

This analogy is flawed. Water is an incompressible fluid, whereas air is compressible fluid. Your use of this particular analogy clearly shows that you are unaware of many keys fundamental of aerodynamics.

Compressible means the matter can be squeezed to take up smaller space. This is why air that passes through the inlet mouth will pass through the narrower space at the throat with mass being conserved. The speed of air at the throat will be slightly higher than at the intake mouth to compensate for the restricted space. This is Bernoulli's principle.

What regulates air flow are spill doors. Recall this statement of your's:

As we have seen on F-22, inlets can work perfectly fine without intake ramps.

The analogy is correct because air is spilled, in fact i will say to you the following

when the engine needs of mass flow are lower than the intake mass flow the air intake has you get the subcritical states, the engine is matched when the mass flow of the intake equals the engine needs of mass flow, in few words when the engine demands more air than the intake can suply you get supercritical states, if the engine demands are lower than the mass flow the intake suplies then the air is spilled outside the cowl thus the shocks move forward and outside the intake

The air is spilled outside the cowl in the same way a sink is overflowed

MiG-29 · Jan 9, 2012

Engineer said:
Nope. The paper clear states sub-critical condition. You cannot have sub-critical condition without shock waves, and the presence of shock waves clearly indicates supersonic speed. Furthermore:

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and airplane can fly subsonically and some parts have supersonic flow, low mach numbers such as Mach 1-Mach 1.2 are low mass flow rates while Mach 1.8 are high numbers, DSI has a mach design number of Mach 1.7.

So is not mach 1.8

MiG-29 · Jan 9, 2012

Engineer said:
Variable-geometry inlets have bypass system, and it is bypass system that regulates the amount of air going into the engine, not ramp. All your citation have been saying the same things as I have pointing out to you:

Intake ramp controls position of shock waves.

Flow regulation and maintaining critical condition is achieved using auxiliary doors and bypass.

From the same paper that you use:

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that intake is fixed

Engineer · Jan 9, 2012

MiG-29 said:
The analogy is correct because air is spilled, in fact i will say to you the following

The analogy is incorrect for the simple reason that air is compressible where water is not.

MiG-29 said:
when the engine needs of mass flow are lower than the intake mass flow the air intake has you get the subcritical states, the engine is matched when the mass flow of the intake equal the engine needs of mass flow, in few words when the engine demands more air than the intake can suply you get supercritical states, if the engine demands are lower than the mass flow the intake suplies then the air is spilled outside the cowl thus the shocks move forward and outside the intake

The air is spilled outside the cowl in the same way a sink is overflowed

Air spillage occurs when normal shock wave is pushed out of the inlet, or in your own words "sub critical condition". This is not even remotely close to your water in a sink example, as there is absolutely no shock wave exists in your example.

Here is the fact: the throat does not regulate flow. Otherwise, all air intakes would require intake ramp so they can have variable throat. The fact that F-22 inlets only have bypass and no intake ramp shows bypass system is what regulates flow in an inlet.

Engineer · Jan 9, 2012

MiG-29 said:
and airplane can fly subsonically and some parts have supersonic flow, low mach numbers such as Mach 1-Mach 1.2 are low mass flow rates while Mach 1.8 are high numbers, DSI has a mach design number of Mach 1.7.

So is not mach 1.8

Nope. Sub-critical condition occurs because air is not flowing fast enough in an inlet. This phenomenon occurs at high supersonic speed. In fact, the paper gives the flight condition as Mach 1.7:

So you are incorrect in your assumption that this is at subsonic speed.

MiG-29 · Jan 9, 2012

Engineer said:
The analogy is incorrect for the simple reason that air is compressible where water is not.

Air spillage occurs when normal shock wave is pushed out of the inlet, or in your own words "sub critical condition". This is not even remotely close to your water in a sink example, as there is absolutely no shock wave exists in your example.

Here is the fact: the throat does not regulate flow. Otherwise, all air intakes would require intake ramp so they can have variable throat. The fact that F-22 inlets only have bypass and no intake ramp shows bypass system is what regulates flow in an inlet.

you are incorrect air is spilled outside the intake in fact low mass flow rates mean there is a lot of spilling outside the intake, the shocks simply move because the intake does not need that mass flow, there is no spillage if the intake needs the entire mass flow, in few words mass flow rate means how much air does not enter the intake, so its basicly a rest A0-A1 and later is divided by A1 to get the ratio, but basicly it is A0/A1

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