SDF Aerospace and Aerodynamics Corner

MiG-29 said:

variable throat means it can expand or retract, become smaller or bigger at will.
At Mach 1.8 variable geometry gives you better pressure recovery and Sr-71 was never shot down it could fly at Mach 3.2

speed and variable goemetry go hand by hand
why? as intake mass flow grows, turbofan mass flow needs decrease, thus smaller intakes achieve better intake mass flow matching, that is the reason you have Mach 3 variable geometry intakes The problem is, as the static pressure and mass flow rate are increased, the
pressure recovery reduces while the drag increases. Hence an optimum design needs
to be arrived at.

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And yet, hypersonic test vehicles employ fixed inlet. Your argument that variable-geometry inlet is better because it can go faster is therefore flawed. Furthermore, not all aircraft that employ variable-geometry inlets can reach Mach 3. This means for an aircraft to fly at Mach 3, it involves way more than just adopting variable-geometry inlets. Finally, for those aircraft that can reach Mach 3, all of them maneuver like an airliner and are useless as fighter. Their complexity and weight of the inlet are just too much, so these aircraft can just fly fast and are useless in every other performance criteria.


Performance of an inlet is measured based upon its pressure recovery ratio, not speed. Looking at pressure recovery ratio, we see that for DSI, this value is higher than 0.91 at M1.8. At M2.0, the coefficient decreases to 0.87. As comparison, F-4D's variable-geometry inlet only has a ratio of 0.89 at Mach 1.8. 0.91 is higher than 0.89, which means DSI has better performance. Your claim that variable-geometry inlet is always better is incorrect.

MiG-29 said:

By the way DHI means divertless hypersonic intake and also needs mach number cowling or in few words sizing, they are not universal, they need to be optimised to different mach numbers,

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No inlet design is universal. Above their design operating speed, the pressure recovery ratio for variable-geometry inlets decrease just like any other type of inlet. Because of this, F-14 and F-15 cannot reach Mach 3 despite having variable-geometry inlet. This effect can be seen in the following graph:


For variable-geometry inlets, their performance suffers at low supersonic speed as compared to fixed-inlet:


You are under the assumption that variable-geometry inlet has constant performance throughout all Mach number. This assumption is wrong.

Engineer said:

So what you are saying is that external compression inlet becomes internal compression inlet when shock wave is ingested. In other words, you yourself are admitting the same effects of compressible flow occur to inlets of different designs. What we see here is that the video is perfectly applicable, and all you do is deny, deny, deny and argue just for the sake of arguing.

The video shows how compressible flow behaves inside the duct of an inlet. In internal, external and mixed compression designs, oblique shock waves reduce the speed of supersonic flow, and a final normal shock occurs at the throat to turn the supersonic flow into a subsonic one. They are all based on the same principles, because the design of the inlets does not alter the rules of fluid dynamics.

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let us see with the F-111, and i will prove you how wrong you are

first check the symbols for Mass flow rate





here it says different position of the F-111 intakes were tested




it says clearly a reduction in throat area reduces mass flow rate

MiG-29 said:

let us see with the F-111, and i will prove you how wrong you are

first check the symbols for Mass flow rate

here it says different position of the F-111 intakes were tested

it says clearly a reduction in throat area reduces mass flow rate

Click to expand...

Oh Mig-29. After careful consideration of your evidences, I must admit I was wrong -- about you. I genuinely thought that you would change; that after all this time, you would come up with valid arguments... for once. What you have done instead is a great disappointment. All you have done here is just rely on your old tactics: making strawman arguments, quoting out of context, and put words into people's mouth.

Now, let's deal with the . Not only did your not prove me wrong, but it proved what I have been saying as correct. This diagram shows change in mass flow ratio to variable-geometry. Regarding mass flow ratio, I have specifically said it changes. My first response that I've found regarding mass flow ratio is as follow:

Engineer said:

The reason of change in mass flow ratio is actually quite clear within this diagram:

The reason of the change is due to the change in angle of the oblique shock wave. For this reason, mass flow ratio will also increase at high Mach value on a DSI and F-22's fixed inlet. That graph of mass flow ratio vs. Mach number is applicable to all intake design, and the phenomenon shown is not unique to variable-geometry inlet.

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Then later on, I said:

Engineer said:

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.

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At no time did I mention mass flow ratio does not change, so your strawman argument is invalid.

You keep on claiming I am wrong, and disagreeing with everything I said. Now, your very own says the same thing that I have been saying. Essentially, you showed you are the one who is wrong for disagreeing with what I have said. Ironic, isn't it?

What is wrong is your inability to grasp simple concepts such as ratio, which results in you coming up with wrong interpretation for more sophisticated ideas. An analogy is like a building that have no proper foundation, which eventually collapses. You originally claimed that variable-geometry controls mass flow, which I quote below:

MiG-29 said:

what you do not understand how an intake works, they are not only creating shock waves to slow down the flow, but also controling the mass flow...

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You are unable to backup your claim, so you find the closest thing - "mass flow ratio", because it shares two words with "mass flow". However, sharing two words doesn't make them the same, which is your mistake. I have mentioned this before as well:

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.

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Then not soon after, I have to correct you again:

Engineer said:

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.

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You are arguing just for the sake of arguing and not adding anything new to this discussion... if we can even call it a discussion.

Why would anyone still argue with our dear knowledgeable aerospace engineer/rocket scientist? Please just ignore him.

Engineer said:

Oh Mig-29. After careful consideration of your evidences, I must admit I was wrong -- about you. I genuinely thought that you would change; that after all this time, you would come up with valid arguments... for once. What you have done instead is a great disappointment. All you have done here is just rely on your old tactics: making strawman arguments, quoting out of context, and put words into people's mouth.

Now, let's deal with the

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. Not only did your

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not prove me wrong, but it proved what I have been saying as correct. This diagram shows change in mass flow ratio to variable-geometry. Regarding mass flow ratio, I have specifically said it changes. My first response that I've found regarding mass flow ratio is as follow:


Then later on, I said:


At no time did I mention mass flow ratio does not change, so your strawman argument is invalid.

You keep on claiming I am wrong, and disagreeing with everything I said. Now, your very own

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says the same thing that I have been saying. Essentially, you showed you are the one who is wrong for disagreeing with what I have said. Ironic, isn't it?

What is wrong is your inability to grasp simple concepts such as ratio, which results in you coming up with wrong interpretation for more sophisticated ideas. An analogy is like a building that have no proper foundation, which eventually collapses. You originally claimed that variable-geometry controls mass flow, which I quote below:


You are unable to backup your claim, so you find the closest thing - "mass flow ratio", because it shares two words with "mass flow". However, sharing two words doesn't make them the same, which is your mistake. I have mentioned this before as well:


Then not soon after, I have to correct you again:


You are arguing just for the sake of arguing and not adding anything new to this discussion... if we can even call it a discussion.

Click to expand...

what does it say?

If the throat area (A2) is too small, corresponding to point c in Figure 2.25, a detached shock will stand ahead of the inlet, as shown in Figure 2.25a. By increasing the throat area, the shock can be moved to the inlet lip. When the operating point a of Figure 2.25 is reached, the shock will reach the inlet lip and ...



haha that is called air mass control without bypass doors, of course jets use both, by the way i am right, mass flow ratio means the ratio of air that enters the inlet and the potential air of free stream, since air is spilled outside the intake,, throat area controls the amount of air that is spilled

cn_habs said:

Why would anyone still argue with our dear knowledgeable aerospace engineer/rocket scientist? Please just ignore him.

Click to expand...

probably you shoould have quoted

two—dimensional throats to control the position of the normal shock in each

MiG-29 said:

what does it say?

If the throat area (A2) is too small, corresponding to point c in Figure 2.25, a detached shock will stand ahead of the inlet, as shown in Figure 2.25a. By increasing the throat area, the shock can be moved to the inlet lip. When the operating point a of Figure 2.25 is reached, the shock will reach the inlet lip and ...

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Click to expand...

Using large font size isn't going to make it says what you want it to say, nor does it enhance your argument in anyway. All the text says is that variable-geometry positions the shock waves for optimal operation, which is one of the things that I have said before. Here is one such example:

Engineer said:

Variable-geometry inlets vary their geometry to keep the oblique and normal shockwaves optimally positioned with respect to the intake lip.

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So, not only does it not contradict anything I have said, you still have nothing to back up what you have claimed. This is nothing more than yet another one of your strawman arguments.

MiG-29 said:

haha that is called air mass control without bypass doors, of course jets use both, by the way i am right, mass flow ratio means the ratio of air that enters the inlet and the potential air of free stream, since air is spilled outside the intake,, throat area controls the amount of air that is spilled

Click to expand...

Nope. Aircraft don't need to use both. Variable-geometry inlets need both, whereas fixed-inlets like those found on F-22 only need bypass doors. Furthermore, spillage occurs on fixed inlet and DSI without any need of variable-geometry, which results in change in mass flow ratio. Recall what you have said:

MiG-29 said:

If you were right you only would need the by pass doors, not variable geometry throats.

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So right now, you are continuing the argument just for the sake of arguing.

MiG-29 said:

probably you shoould have quoted

two—dimensional throats to control the position of the normal shock in each

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Click to expand...

This is yet another source which says what I have said. Here is one of the things that I have said earlier:

Engineer said:

The purpose of variable-geometry is to optimally position the shock waves...

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Here is an extract of your source verifying what I have said:


So, this is nothing but yet another one of your . You are misrepresenting my position so as to create an illusion that you are refuting what I have said, when in reality you haven't refuted any of my point.

Engineer said:

Using large font size isn't going to make it says what you want it to say, nor does it enhance your argument in anyway. All the text says is that variable-geometry positions the shock waves for optimal operation, which is one of the things that I have said before. Here is one such example:


So, not only does it not contradict anything I have said, you still have nothing to back up what you have claimed. This is nothing more than yet another one of your strawman arguments.



Nope. Aircraft don't need to use both. Variable-geometry inlets need both, whereas fixed-inlets like those found on F-22 only need bypass doors. Furthermore, spillage occurs on fixed inlet and DSI without any need of variable-geometry, which results in change in mass flow ratio. Recall what you have said:


So right now, you are continuing the argument just for the sake of arguing.

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haha the text shows perfectly how wrong you are, first air can be spilled, you initially claimed no air was spilled but it was bernuolli`s principle, second you claimed the Throat does not control the sock`s position, it does, reducing mass flow ratio, in few words reducing the amount of air mass the intake takes.

the text show an inlet without bypass doors can just by varing the inlet throat do air mass control just like by pass doors.

Engineer said:

This is yet another source which says what I have said. Here is one of the things that I have said earlier:


Here is an extract of your source verifying what I have said:

So, this is nothing but yet another one of your

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. You are misrepresenting my position so as to create an illusion that you are refuting what I have said, when in reality you haven't refuted any of my point.

Click to expand...

your argument was the F-14 does not use the throat size to positon the shock. you claimed air mass control was done uniquely by bypass doors, now that you know the throats do the same thing bypass doors you deny it


in fact i wil leave you a present

The results show that under design point condition,the average total pressure recovery coefficient at exit of bump inlet is nearly 0.87 at design Mach number of 2.0,and no less than 0.91 at mach number of 1.8
What F-16 has the same performance than DSI almost 20 years ago! ah i see DSI is used simply because it is stealthier and cheaper to build and maintain