Su-57 intakes, supercruise performance and 2nd stage engine

icyplanetnhc (Steve)

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Correction, as I had visualized the angle incorrectly.

Based on the conic shock tables and visual estimates of radome and shock angles in the image, seems to be between Mach 2.67 and Mach 3. This is a rather absurd number, but again, I'll caveat by saying that the nose is not an axisymmetric conic object, so I wouldn’t use this as any kind of serious prediction; there’s a lot with the figure that we don’t have information about. Note the sensitivity of the shock angle to Mn for a given cone angle; this goes to show why these visual "estimates" have such a wide margin of error that they're generally useless, as any distortion on the image, or even angle of attack, will throw the number off considerably.

Furthermore, the complex interactions that occurs with compressible flow and shock generations mean that even a 3D cone can’t accurately predict the shocks of a complex body, which may explain the disparity between the visual “estimate” and the published operating maximum Mach of ~Mn = 2 or so. Likewise, the Mach limit may be a structural or some other factor rather than drag, similar to the case with the F-35.
 
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sferrin

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The tips of the blades on modern engine fans are already supersonic at Mil power,
Wow, I did not know that, doesn't it create the kind of shocks and flow unevenness that the subsonic diffuser precisely tries to avoid? It looks that is a limit the design has already reached, unless some improvement in aero design allows to minimize the negative effects of such speeds, the same way supercritical airfoils do for airliners

Wouldn't increasing the number of fan blades have an effect on the airflow?
The first engine that I am aware of with a transonic compressor was the P&W J91, the front two stages of which became the Fan of the initial TF33 turbofan and was scaled down 25% to become the JT11 (J58).

I believe that current blade tip speeds are limited to around 1.4M. Yes, shock waves coming from the blade is a major consideration in the design of the fan/compressor. Much work has been done over the years with the airfoil profiles to improve efficiency, stall margin, and flow capacity. It is more that just individual blades, since they are in a cascade with each blade affecting the flow of the adjacent blades as well as its own flow. Many hours of CFD analytics are spent trying to perfect compressor design.

Increasing the number of blades affects a design parameter known as “solidity”. Increased solidity would probably reduce airflow, since the blades are occupying area that could be used for airflow. The trend of lowering the aspect ratio of the blades has been accompanied by lower blade counts, not more.
Do you have any details about the J91? I've heard it referred to before but nothing really in the way of details.
 

sferrin

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By the way, the flight time of the MiG-31 at supersonic speed is no more than 15 - 30 minutes, while the speed is no more than 2500 km / h. Thus, we get a flight range of no more than 625 km - 1250 km in this mode
There is an account, by an F-15 pilot, of a Mig-31 making a 620 mile flight at Mach 2.6. (Armed presumably.)
 

paralay

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The supersonic practical range is:
with 4 missiles, range 1249 km, duration 43 min (average speed 1743 km/h),
with 4 rockets and a 1/2-way launch, 1300 km, duration 44.4 min (average speed 1625 km/h)

Subsonic practical range and duration without suspended tanks and infrared system off is:
a) without missiles: range 2480 km; duration 2 h. 44 min.
b) with 4 missiles and launching them in the middle of the way: a range of 2400 km; duration 2h. 35 min.
c) with 4 missiles: range 2240 km; duration 2h. 26 min.

this is official data
 

F119Doctor

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The tips of the blades on modern engine fans are already supersonic at Mil power,
Wow, I did not know that, doesn't it create the kind of shocks and flow unevenness that the subsonic diffuser precisely tries to avoid? It looks that is a limit the design has already reached, unless some improvement in aero design allows to minimize the negative effects of such speeds, the same way supercritical airfoils do for airliners

Wouldn't increasing the number of fan blades have an effect on the airflow?
The first engine that I am aware of with a transonic compressor was the P&W J91, the front two stages of which became the Fan of the initial TF33 turbofan and was scaled down 25% to become the JT11 (J58).

I believe that current blade tip speeds are limited to around 1.4M. Yes, shock waves coming from the blade is a major consideration in the design of the fan/compressor. Much work has been done over the years with the airfoil profiles to improve efficiency, stall margin, and flow capacity. It is more that just individual blades, since they are in a cascade with each blade affecting the flow of the adjacent blades as well as its own flow. Many hours of CFD analytics are spent trying to perfect compressor design.

Increasing the number of blades affects a design parameter known as “solidity”. Increased solidity would probably reduce airflow, since the blades are occupying area that could be used for airflow. The trend of lowering the aspect ratio of the blades has been accompanied by lower blade counts, not more.
Do you have any details about the J91? I've heard it referred to before but nothing really in the way of details.
Only that it was a large 400 lb/sec airflow single shaft turbojet. https://en.wikipedia.org/wiki/Pratt_&_Whitney_J58
 

LMFS

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Based on the conic shock tables and visual estimates of radome and shock angles in the image, seems to be between Mach 2.67 and Mach 3. This is a rather absurd number,
Well, it seems you have found further evidence that the max speed of the plane is not 2 M, as the presence of intakes explicitly intended for speeds between 2 and 3 M was not so subtly suggesting already. We don't know if previous statements about max speed were true, disinformation or simply wrongly interpreted (ie. related to max cruising speed), but this adds weight to the opinion that the plane is indeed intended for high max speeds and supports the claim that it fulfils the roles of strike plane, fighter and interceptor in one.

The accuracy of the estimation is not so relevant for the time being IMHO, because the true, accurate technical specs of the plane and therefore its official concept of operations will not be available for a very long time, so this is the best that we have by now.

I believe that current blade tip speeds are limited to around 1.4M. Yes, shock waves coming from the blade is a major consideration in the design of the fan/compressor. Much work has been done over the years with the airfoil profiles to improve efficiency, stall margin, and flow capacity. It is more that just individual blades, since they are in a cascade with each blade affecting the flow of the adjacent blades as well as its own flow. Many hours of CFD analytics are spent trying to perfect compressor design.
That is basically amazing, thanks for sharing it
 

icyplanetnhc (Steve)

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Well, it seems you have found further evidence that the max speed of the plane is not 2 M, as the presence of intakes explicitly intended for speeds between 2 and 3 M was not so subtly suggesting already. We don't know if previous statements about max speed were true, disinformation or simply wrongly interpreted (ie. related to max cruising speed), but this adds weight to the opinion that the plane is indeed intended for high max speeds and supports the claim that it fulfils the roles of strike plane, fighter and interceptor in one.
I wouldn’t interpret this as reliable indicator of maximum speed; even for a ballpark estimate it’s wholly unreliable. It’s literally a blurry image without any accompanying scale or legend. No indications of angle of attack, or even if the image has any was distortion or was stretched, etc.

Regarding the Mach 2 limit, I’m not sure why this is a point of contention, as even aircraft that can exceed Mach 2 will rarely do so operationally. It may be the case that the Su-57 is not limited by drag at that speed, but limited for structural or other reasons.
 
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F119Doctor

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The tips of the blades on modern engine fans are already supersonic at Mil power,
Wow, I did not know that, doesn't it create the kind of shocks and flow unevenness that the subsonic diffuser precisely tries to avoid? It looks that is a limit the design has already reached, unless some improvement in aero design allows to minimize the negative effects of such speeds, the same way supercritical airfoils do for airliners

Wouldn't increasing the number of fan blades have an effect on the airflow?
The first engine that I am aware of with a transonic compressor was the P&W J91, the front two stages of which became the Fan of the initial TF33 turbofan and was scaled down 25% to become the JT11 (J58).

I believe that current blade tip speeds are limited to around 1.4M. Yes, shock waves coming from the blade is a major consideration in the design of the fan/compressor. Much work has been done over the years with the airfoil profiles to improve efficiency, stall margin, and flow capacity. It is more that just individual blades, since they are in a cascade with each blade affecting the flow of the adjacent blades as well as its own flow. Many hours of CFD analytics are spent trying to perfect compressor design.

Increasing the number of blades affects a design parameter known as “solidity”. Increased solidity would probably reduce airflow, since the blades are occupying area that could be used for airflow. The trend of lowering the aspect ratio of the blades has been accompanied by lower blade counts, not more.
Do you have any details about the J91? I've heard it referred to before but nothing really in the way of details.
Only that it was a large 400 lb/sec airflow single shaft turbojet. https://en.wikipedia.org/wiki/Pratt_&_Whitney_J58
 

LMFS

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I wouldn’t interpret this as reliable indicator of maximum speed; even for a ballpark estimate it’s wholly unreliable. It’s literally a blurry image without any accompanying scale or legend. No indications of angle of attack, or even if the image has any was distortion or was stretched, etc.
You have two images above and both have shock angles that look compatible with speeds higher than 2 M. Angle of attack you could discern by the asymmetry in the shocks. In any case an estimate with a relatively big uncertainty is already an improvement, when you know so little as we know about the Su-57

Regarding the Mach 2 limit, I’m not sure why this is a point of contention, as even aircraft that can exceed Mach 2 will rarely do so operationally. It may be the case that the Su-57 is not limited by drag at that speed, but limited for structural or other reasons.
That example does not apply to MiG-31 for instance, but I get what you mean. Regardless, if the designers accept the design compromises required to attain high supersonic speeds, that ability needs to be deemed very important, regardless of being a frequent flight regime or not. Mind you, in the Su-57 the flight at supersonic speed is considered the standard operational condition (I assume in wartime, of course), so the average flight speed vs. 4th gen is probably way higher, contrary to this usual narrative that the value of supersonic flight is overestimated. The actual trend of high end platforms seems to be quite the opposite (F-22/Su-57/NGAD)

As to the structural limit, I agree, but again, why to use the variable intakes in a plane limited to 2 M?
 

icyplanetnhc (Steve)

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You have two images above and both have shock angles that look compatible with speeds higher than 2 M. Angle of attack you could discern by the asymmetry in the shocks. In any case an estimate with a relatively big uncertainty is already an improvement, when you know so little as we know about the Su-57

why to use the variable intakes in a plane limited to 2 M?

The original maximum speed requirement of the Su-57, or T-50, was Mach 2.35 which is identical to that of the Su-27. While the maximum Mach was reduced to around 2 due to structural and material considerations, they may still want to validate the aerodynamic modeling to the original specifications, hence the CFD figure. Some of the changes to the mechanical design were avoid perhaps to control costs as the spec change occurred when detailed design was well underway (~2006), and the shape had been frozen since 2004.
 
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LMFS

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The original maximum speed requirement of the Su-57, or T-50, was Mach 2.35 which is identical to that of the Su-27. While the maximum Mach was reduced to around 2 due to structural and material considerations,
You talk about that as if the TTZ of the plane were public. Do you have the sources? The other reference to 2 M as max speed, was it official? Is it the be all, end all evidence that cannot be false, outdated or misleading? And again, why the variable intakes? Such a silly question, that nobody has an answer for it...
 

icyplanetnhc (Steve)

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Do you have the sources? The other reference to 2 M as max speed, was it official?

The change was directed in 2006 by the head of the Russian Air Force at the time, Viktor Mikhaylov. I would consider that to be rather official.

Has it changed then? Who knows, as there haven’t been any announcements to the contrary. It would be of rather limited tactical value. Even the Su-30SM and MiG-35 with variable inlet ramps have a maximum speed of Mach 2; the Su-35’s is Mach 2.25 despite variable inlets and more powerful engines than the Su-27.

 
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