Aerodynamics thread

[Inst]

That's not how it works and this isn't even accurate anyway. Those measures are off. J-20's intake area is not roughly the same as the Flanker's. Also different geometry intakes must have quite an affect on all these claims.

Your conclusion from your theory is not correct even if the theory is.

Edit... sorry people have pointed this out in greater detail.

It doesn't.

Edit: wait, defining it as supersonic incompressible flow is the wrong way to go about it, let me go find the correct terms.

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Look up choked flow. There's no way to design nozzles such that they can bypass a certain surface aperture * speed, because the pressure level of downstream flow cannot increase the pressure of upstream flow in supersonic flight.

 

[ougoah]

It doesn't.

Edit: wait, defining it as supersonic incompressible flow is the wrong way to go about it, let me go find the correct terms.

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Look up choked flow. There's no way to design nozzles such that they can bypass a certain surface aperture * speed, because the pressure level of downstream flow cannot increase the pressure of upstream flow in supersonic flight.

Your theory that inlet area determines what engine is incorrect. There was no difference in inlet design between TF30 engine and F401 or F100 for the F-14. I doubt the Su-57 would have revised intakes when it receives Izd. 30. The J-20 might when it receives WS-15 but still doesn't prove your rule.

Anyway the J-20's intakes look much smaller in area than the Flanker's. How are you calculating the area? The purely perpendicular aspect? Surely a DSI works differently to a variable geo.

 

[Inst]

Your theory that inlet area determines what engine is incorrect. There was no difference in inlet design between TF30 engine and F401 or F100 for the F-14. I doubt the Su-57 would have revised intakes when it receives Izd. 30. The J-20 might when it receives WS-15 but still doesn't prove your rule.

Anyway the J-20's intakes look much smaller in area than the Flanker's. How are you calculating the area? The purely perpendicular aspect? Surely a DSI works differently to a variable geo.

With the F-14, it's notable and known that the F-14's maximum speed is exactly the same whether you're running the TF30 or the F110; i.e, it's inlet limited with regards to speed.

As for the J-20 vs the Su-27, I apologize I haven't taken the time to go measure the inlets again as I said I would, but I'll get to it by the weekend. However, just purely eyeballing it:

Contrary to your assertion, the J-20's inlets are of equal or larger size than the Su-27's when you ignore the DSI. This is considering that the J-20 has approximately a 12.95m wingspan, and that the Su-27 wingspan is 14.7m, or is about 14% greater.

As for the WS-15 vs the WS-10 in terms of mass air flow requirements, it's known that mass air flow requirements are dependent on fan diameter. Knowing that Chinese engine design is behind that of the Americans, we should expect lower maximum temperatures and thus a greater fan diameter to produce the same amount of thrust. While fan diameters for the WS-10 are not available, the rumored fan diameter for the WS-15 is 1 meter, while the Al-31 to Al-41 fan diameter varies from 900mm to 930mm. That's a 16-23% diameter difference. The comparable American engine (F135) incidentally has a fan diameter of around 1.16 meters, which implies that either:

-The WS-15 operates at higher temperature, which implies that the Chinese have passed the Americans in terms of engine metallurgy, or that the Chinese are willing to put up with a lower engine life.
-The WS-15 is going to be significantly more thrust-limited when compared to the F135.
-The WS-15's stated fan diameter is wrong, implying that mass air flow requirements for the WS-15 are even higher than previously expected.

Also, as a preliminary, I get around 0.87m^2 for a single J-20 inlet, using the picture above as an example (650 pixels = 12.95 meters, inlet is 48 pixels wide, 68 pixels tall, assuming it's a parallelogram, then you remove the triangle where the inlet slopes down, and then highlighting the exposed DSI and halving the area, you get about 0.87m^2).

 

[Richard Santos]

Your theory that inlet area determines what engine is incorrect. There was no difference in inlet design between TF30 engine and F401 or F100 for the F-14. I doubt the Su-57 would have revised intakes when it receives Izd. 30. The J-20 might when it receives WS-15 but still doesn't prove your rule.

Anyway the J-20's intakes look much smaller in area than the Flanker's. How are you calculating the area? The purely perpendicular aspect? Surely a DSI works differently to a variable geo.

F-14s have variable air intakes with movable ramps that divert air entering the intake away from the engine and expel it through bleed doors on upper side of the plane. The ramp movement can be programmed to alter the amount of air bleed at in each flight regime. So the F-14 intakes can accommodate a wider range of engine mass flow requirements than aircraft with fixed intakes.

Photos of the Su-57 taken from right ahead show large flat triangular area on both inside walls of the intake tube near the intake. These look like they are there to clear movable ramps on the upper side of the intake tube. The presence of movable ramps that require a significant length of the intake ducting near the intake to be straight may explain why the Su-57 is unable to adopt F-22 and J-20 style, heavily curved, stealthy S intake ducts that shield the engine fan faces. But the presence of a variable ramp would also allow Su-57 intakes to accommodate a wider range of mass flow requirements than fixed intakes.

 

[Atomicfrog]

With the F-14, it's notable and known that the F-14's maximum speed is exactly the same whether you're running the TF30 or the F110; i.e, it's inlet limited with regards to speed.

As for the J-20 vs the Su-27, I apologize I haven't taken the time to go measure the inlets again as I said I would, but I'll get to it by the weekend. However, just purely eyeballing it:

View attachment 73906

Contrary to your assertion, the J-20's inlets are of equal or larger size than the Su-27's when you ignore the DSI. This is considering that the J-20 has approximately a 12.95m wingspan, and that the Su-27 wingspan is 14.7m, or is about 14% greater.

As for the WS-15 vs the WS-10 in terms of mass air flow requirements, it's known that mass air flow requirements are dependent on fan diameter. Knowing that Chinese engine design is behind that of the Americans, we should expect lower maximum temperatures and thus a greater fan diameter to produce the same amount of thrust. While fan diameters for the WS-10 are not available, the rumored fan diameter for the WS-15 is 1 meter, while the Al-31 to Al-41 fan diameter varies from 900mm to 930mm. That's a 16-23% diameter difference. The comparable American engine (F135) incidentally has a fan diameter of around 1.16 meters, which implies that either:

-The WS-15 operates at higher temperature, which implies that the Chinese have passed the Americans in terms of engine metallurgy, or that the Chinese are willing to put up with a lower engine life.
-The WS-15 is going to be significantly more thrust-limited when compared to the F135.
-The WS-15's stated fan diameter is wrong, implying that mass air flow requirements for the WS-15 are even higher than previously expected.

Also, as a preliminary, I get around 0.87m^2 for a single J-20 inlet, using the picture above as an example (650 pixels = 12.95 meters, inlet is 48 pixels wide, 68 pixels tall, assuming it's a parallelogram, then you remove the triangle where the inlet slopes down, and then highlighting the exposed DSI and halving the area, you get about 0.87m^2).

Pretty hard to compare size from different inlets designs. Part of the air ingested could be used solely for cooling arround the engine for IR suppression or some planes having bypass doors bleeding out excess air. The inlet with variable ramp of Su-27 is quite harder to compare to a fixed one for J-20...

 

[Richard Santos]

Not so fast... more thrust only equals more mass flow if specific thrust does not increase from altered engine cycle parameters. Between the WS-10 and WS-15 these certainly will change though, because to enable supercruise the latter is going to have markedly lower BPR and higher turbine inlet temperature. Depending how much lower/higher, the WS-15 could easily get 10% more thrust out of the same mass flow as the current engines (which are in the 13 to 14tf ball park) from the BPR decrease alone. For reference, IIRC the Izd.30 is said to have 25% more specific thrust than the AL-31F - that sort of improvement would take WS-15 into the 17tf region without any increase in airflow whatsoever. If a minor mismatch with the current engines were accepted (it's always a compromise between low speed and high speed requirements anyhow), the existing intake could suffice no problem at all.

Fair point. The WS-15 probably has much lower bypass ratio than the WS-10. So while the hot section of the Ws-15 undoubtedly has a much higher mass flow requirement than the hot section of the WS-10, proportionally much less air would de diverted around the hot section in the WS-15 than would be the case with the WS-10. So the total mass flow requirement of the WS-15 is not necessarily dissimilar to that of the WS-10.

 

[BoraTas]

With the F-14, it's notable and known that the F-14's maximum speed is exactly the same whether you're running the TF30 or the F110; i.e, it's inlet limited with regards to speed.

As for the J-20 vs the Su-27, I apologize I haven't taken the time to go measure the inlets again as I said I would, but I'll get to it by the weekend. However, just purely eyeballing it:

View attachment 73906

Contrary to your assertion, the J-20's inlets are of equal or larger size than the Su-27's when you ignore the DSI. This is considering that the J-20 has approximately a 12.95m wingspan, and that the Su-27 wingspan is 14.7m, or is about 14% greater.

As for the WS-15 vs the WS-10 in terms of mass air flow requirements, it's known that mass air flow requirements are dependent on fan diameter. Knowing that Chinese engine design is behind that of the Americans, we should expect lower maximum temperatures and thus a greater fan diameter to produce the same amount of thrust. While fan diameters for the WS-10 are not available, the rumored fan diameter for the WS-15 is 1 meter, while the Al-31 to Al-41 fan diameter varies from 900mm to 930mm. That's a 16-23% diameter difference. The comparable American engine (F135) incidentally has a fan diameter of around 1.16 meters, which implies that either:

-The WS-15 operates at higher temperature, which implies that the Chinese have passed the Americans in terms of engine metallurgy, or that the Chinese are willing to put up with a lower engine life.
-The WS-15 is going to be significantly more thrust-limited when compared to the F135.
-The WS-15's stated fan diameter is wrong, implying that mass air flow requirements for the WS-15 are even higher than previously expected.

Also, as a preliminary, I get around 0.87m^2 for a single J-20 inlet, using the picture above as an example (650 pixels = 12.95 meters, inlet is 48 pixels wide, 68 pixels tall, assuming it's a parallelogram, then you remove the triangle where the inlet slopes down, and then highlighting the exposed DSI and halving the area, you get about 0.87m^2).

It is not that simple. The WS-15 will probably have a bypass ratio around 0.30 which means the core will make a significantly larger portion of the engine compared to the F-135. This means faster exhaust gas speed which means higher trust. Also, airflow is not only affected by the diameter. The rotation speed and shape of the fans are equally important too. We know the WS-10 is heavier than the F-135 by around 10%. It is very likely that the WS-15 will have a similar weight to the WS-10. The extra weight probably enables beefier blades that can sustain a faster rotation speed. With these factors, the WS-15 may very well outperform the F-135 while having a smaller diameter. The cost for this is probably extra weight and higher fuel consumption.

 

[Inst]

It is not that simple. The WS-15 will probably have a bypass ratio around 0.30 which means the core will make a significantly larger portion of the engine compared to the F-135. This means faster exhaust gas speed which means higher trust. Also, airflow is not only affected by the diameter. The rotation speed and shape of the fans are equally important too. We know the WS-10 is heavier than the F-135 by around 10%. It is very likely that the WS-15 will have a similar weight to the WS-10. The extra weight probably enables beefier blades that can sustain a faster rotation speed. With these factors, the WS-15 may very well outperform the F-135 while having a smaller diameter. The cost for this is probably extra weight and higher fuel consumption.

You're pincered between the F135 and F119. The F135 is a high bypass ratio engine, compared to the low or relatively low bypass ratios of the WS-15 and F119.

As for rotation speed, choking / ventruri effect (inability for fans to pull air from the supersonic stream, only have it pushed in) means that you're limited by the mass air flow available. F = MA, i.e, you can increase the acceleration all you want, but you're decreasing mass so thus decreasing force.

 

[Inst]

@Deino

to clarify, I never said the inlets had to be enlarged, only that the DSI bump would likely be adjusted for the new engine.Enlarging the inlets is a major structural redesign, whereas adjusting the DSI has, first, been done before, and second, is relatively cheap.

 

[Inst]

@plawolf

Which is nonsense, because you train how you fight and you fight how you train. If pilots don't regularly push the airframe toward its design limits in peacetime, they will not have experience with how the plane at its design limits work in wartime.