SDF Aerospace and Aerodynamics Corner

Engineer

Engineer

Major
MiG-29 said:
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
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Click to expand...

If ramp angle change acts a value to regulate amount of air flow into the engine, you wouldn't need bypass doors.

Intake ramp controls the position of oblique shock waves, bypass doors don't do this. You see both on F-14's intakes because the two are doing different jobs. As a matter of fact, you only would need bypass doors in the case of F-22's inlet system.
 
MiG-29

MiG-29

Banned Idiot
Engineer said:
How do you believe Bernoulli's principle works then? :rolleyes:



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. :rolleyes:



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.
Click to expand...

not correct

The mass flow ratio is defined as the ratio of air mass flow at inlet entry
to air mass flow at free stream conditions.

when the ratio is one no air is spilled, spilled air does not enter the intake, remains free stream flow
FlTO8.png



A0i/A1 is air mass flow ratio or A0/Ac
 
Engineer

Engineer

Major
MiG-29 said:
not correct

The mass flow ratio is defined as the ratio of air mass flow at inlet entry
to air mass flow at free stream conditions.

when the ratio is one no air is spilled, spilled air does not enter the intake, remains free stream flow


A0i/A1 is air mass flow ratio or A0/Ac
Click to expand...

There is nothing incorrect about my statements.
  • Mass flow ratio does not equate to mass flow, and this statement of mine is correct. The former has no unit, the latter has the unit of kg/s.
  • By Bernoulli's principle "air mass passing through the mouth equates to air mass passing through the throat". This is also your statement, so by claiming I am incorrect you are also saying you are incorrect.
  • Spill air occurs even with no variable-geometry involved, because it is due to the position of normal shock wave. There is nothing incorrect about this statement, and in fact it is the subcritical condition which you referred to earlier. By claiming I am incorrect it means you do not believe air can be spilled at sub-critical condition.
  • Intake ramp positions the shock waves. There is nothing incorrect about this either.

Spill air occurs with fixed geometry inlets and DSI as well, but you are assuming this spill is caused by variation in throat area. This is not the case, because it is the position of normal shock wave that determines the spill, thus your claim that variation in throat area regulates flow remains unsubstantiated.
 
MiG-29

MiG-29

Banned Idiot
Engineer said:
There is nothing incorrect about my statements.
  • Mass flow ratio does not equate to mass flow, and this statement of mine is correct. The former has no unit, the latter has the unit of kg/s.
  • By Bernoulli's principle "air mass passing through the mouth equates to air mass passing through the throat". This is also your statement, so by claiming I am incorrect you are also saying you are incorrect.
  • Spill air occurs even with no variable-geometry involved, because it is due to the position of normal shock wave. There is nothing incorrect about this statement, and in fact it is the subcritical condition which you referred to earlier. By claiming I am incorrect it means you do not believe air can be spilled at sub-critical condition.
  • Intake ramp positions the shock waves. There is nothing incorrect about this either.

Spill air occurs with fixed geometry inlets and DSI as well, but you are assuming this spill is caused by variation in throat area. This is not the case, because it is the position of normal shock wave that determines the spill, thus your claim that variation in throat area regulates flow remains unsubstantiated.
Click to expand...

incorrect


mass flow ratio is the ratio of two mass air flows in kg/s, with variable geometry you adjust the critical state according to the speed


how do you do that?

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

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

DSI has no translating cone or lip, thus no mass control, it is fixed, then you need bypass doors on a fixed intake.

However this create troubles, the fixed intake has to be make bigger plus bypass doors generate more drag, the solution is a smaller variable geometry intake.

Fixed intakes are bigger to avoid choking the engine at low subsonic speeds where a larger area is requiered, but as speed goes faster you do not need auxiliary intake doors to ingest more air, the concorde transforms those auxiliary intake doors into by pass doors and reduces thoat area, the same is F-14.

SR-71 does the same, how? well it simply reduces or increases throat area

in fact without some type of variable geometry remains most of its time on subcrital and supercritical states
 
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Engineer

Engineer

Major
MiG-29 said:
incorrect
Click to expand...

There is nothing incorrect about my statements. If you think I have said something incorrectly, then quote it specifically and say why it is incorrect. Using one word to blanket my entire post only shows that you are unable to point out flaw with my statements.

MiG-29 said:
mass flow ratio is the ratio of two mass air flows in kg/s...
Click to expand...

No. Mass flow is in kg/s, mass flow ratio has no unit. For demonstration, let's divide 1kg/s by 1kg/s to calculate mass flow ratio:

(1 kg/s)/(1 kg/s)
= 1 kg/s * s/kg
= 1

So as you can see, a ratio of two entities in the same dimension has no unit. This is simple mathematics.
 
Engineer

Engineer

Major
MiG-29 said:
with variable geometry you adjust the critical state according to the speed


how do you do that?

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

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

DSI has no translating cone or lip, thus no mass control, it is fixed, then you need bypass doors on a fixed intake.

However this create troubles, the fixed intake has to be make bigger plus bypass doors generate more drag, the solution is a smaller variable geometry intake.

Fixed intakes are bigger to avoid choking the engine at low subsonic speeds where a larger area is requiered, but as speed goes faster you do not need auxiliary intake doors to ingest more air, the concorde transforms those auxiliary intake doors into by pass doors and reduces thoat area, the same is F-14.

SR-71 does the same, how? well it simply reduces or increases throat area

in fact without some type of variable geometry remains most of its time on subcrital and supercritical states
Click to expand...

In other words, variable-geometry inlet adjusts the shock wave position, while bypass door dumps excess air and regulates the amount of air actually going into the engines. This is exactly what I have pointed out.

If intake ramp regulates air flow, then there wouldn't need bypass doors. The variable-geometry inlets all have bypass system because mass flow cannot be regulated without such system. On the other hand, inlets can function perfectly with only bypass door and no intake ramp, as we see on F-22.

This existence of an inlet system with bypass doors but no variable-geometry reaffirms that bypass door is what regulates flow ("mass control" in your words), not variation in throat area.

In terms of drag, spill air and bypass air both result in drag. You can maintain critical state by bypassing air, but performance is still lost. For DSI, the air is exclusively spilled but the performance lost is smaller as compared to other inlets. The fact that pressure recovery ratio of J-10B's DSI is 0.91 at Mach 1.8 while the ratio for F-4D's inlet is at 0.89 demonstrates this.
 
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Engineer

Engineer

Major
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.

ZxfTc.png

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Engineer

Engineer

Major
MiG-29 said:
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
Click to expand...

The next paragraph has this to state about bypass system:
How Things Work said:
It is a constant balancing act to keep the normal shock wave in the right position. The inlet has an internal pressure sensor, and when it detects that the pressure has grown too great, it triggers the forward bypass doors to open, expelling excess air. The inlet also has a set of aft bypass doors, controlled by the pilot. The forward and aft bypass doors work in opposition to each other: Opening the aft doors causes the forward doors to close, and when the pilot closes the aft doors, the forward doors open in turn.
Click to expand...
 
kwaigonegin

kwaigonegin

Colonel
Engineer said:
There is nothing incorrect about my statements. If you think I have said something incorrectly, then quote it specifically and say why it is incorrect. Using one word to blanket my entire post only shows that you are unable to point out flaw with my statements.



No. Mass flow is in kg/s, mass flow ratio has no unit. For demonstration, let's divide 1kg/s by 1kg/s to calculate mass flow ratio:

(1 kg/s)/(1 kg/s)
= 1 kg/s * s/kg
= 1

So as you can see, a ratio of two entities in the same dimension has no unit. This is simple mathematics.
Click to expand...


AIIIYAYA you guys are saying the same thing! I don;t think he meant to say there is a unit of measurement either. it's ratio so in your example it's a 1:1 ratio
Ratio by definition has no unit of measure because you are comparing two entities or quantitative variables of similar unit .
 
MiG-29

MiG-29

Banned Idiot
Engineer said:
There is nothing incorrect about my statements.
  • Mass flow ratio does not equate to mass flow, and this statement of mine is correct. The former has no unit, the latter has the unit of kg/s.
  • By Bernoulli's principle "air mass passing through the mouth equates to air mass passing through the throat". This is also your statement, so by claiming I am incorrect you are also saying you are incorrect.
  • Spill air occurs even with no variable-geometry involved, because it is due to the position of normal shock wave. There is nothing incorrect about this statement, and in fact it is the subcritical condition which you referred to earlier. By claiming I am incorrect it means you do not believe air can be spilled at sub-critical condition.
  • Intake ramp positions the shock waves. There is nothing incorrect about this either.

Spill air occurs with fixed geometry inlets and DSI as well, but you are assuming this spill is caused by variation in throat area. This is not the case, because it is the position of normal shock wave that determines the spill, thus your claim that variation in throat area regulates flow remains unsubstantiated.
Click to expand...

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

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.

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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