But that is different from a ramp that only adjusts the throat area.
SDF Aerospace and Aerodynamics Corner
- Thread starter paintgun
- Start date
But that is different from a ramp that only adjusts the throat area.
MiG-29
Banned Idiot
the ramp is called variable throat intake, F-14 is a good example, its ramps not only generate oblique shocks but also control throat area.
at subsonic speeds they open wide the intake and the throat is open wide, at supersonic speeds the intakes deploy the ramps and reduce throat area
The intakes are of multi-ramp wedge configuration and offer a straight path for the air entering the engines. Each intake has a pair of adjustable ramps attached to the upper part of the inner intake. Hydraulic actuators in the upper part of the intake adjust the positions of the first and second ramps in the upper surface of the inlet and of the diffuser ramp located further aft, reducing the inlet air to subsonic velocity before admitting it to the engine. A gap between the back edge of the second ramp and the leading edge of the diffuser ramp allows bleed air to escape from the inlet, passing overboard via a bleed-air door in the outer surface of the inlet. The inlet ramps are under the automatic control of a computer, which calculates the optimal position for the ramps based on engine speed, air temperature, air pressure, and angle of attack. At supersonic speeds, the hinged panels narrow down the throat area while diverting the excess airflow out of the ducts through aft-facing spill doors at the top of the intakes. At low speeds (especially during takeoff) when more engine air is needed, this airflow is reversed and extra air is sucked in via the spill doors
Why open at low speed the throat area? why reduce the throat area at high speed and bypass the extra mass flow?
11. At higher supersonic speeds, the pitot type of air intake is unsuitable due to the severity of theshockwave that forms and progressively reduces theintake efficiency as speed increases. Amore suitabletype of intake for these higher speeds is known asthe external/internal compression intake (fig. 23-8). This type of intake produces a series of mild shockwaves without excessively reducing the intake efficiency.12. As aircraft speed increases still further, so also does the intake compression ratio and, at high Mach numbers, it is necessary to have an air intake that has a variable throat area and spill valves to accommodate and control the changing volumes of air (fig. 23-9).The airflow velocities encountered in the higher speedrange of the aircraft are much higher than the enginecan efficiently use; therefore, the air velocity must be decreased between the intake and the engine air inlet.The angle of the variable throat area intake automatically varies with aircraft speed and positions the shockwave to decrease the air velocity at the engine inlet and maintain maximum pressure recovery within the inlet duct. However, continued development enablesthis to be achieved by careful design of the intake andducting. This, coupled with auxiliary air doors to permitextra air to be taken in under certain engine operatingconditions, allows the airflow to be controlled withoutthe use of variable geometry intakes. The fuselageintakes shown in fig. 23-10 are of the variable throat area look at the F-14 intake at max supersonic it is
at subsonic speed
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...The point is throat area and capture area are two different things. Capture area determines the air mass flow, and throat area controls the shocks.
MiG-29
Banned Idiot
totally wrong, the ramps do control de mass flow as do control the oblique shocks.
you can have a fixed intake that generates 2 oblique shocks and one normal shock or a intake that has variable geometry ramps that generate 2 oblique shocks and one normal, the difference is only the ramps that are moveable
Engineer
Major
There is nothing wrong with what I said. I pointed out that the graph shows mass flow performance changes with respects to speed, and this is indicated on the graph quite clearly. The graph does not show change in capture area, as capture area is A1 which stays invariant even in your own diagram.
The vertical value of the graph shows A0/A1, which changes because the angle of the oblique shock wave changes. Capture area A1 is unchanged.
You are now substituting your own definition to mass flow performance, and this definition is totally wrong.
Engineer
Major
haha here the only one saying unaccurate things is you, the illusatrations show two ramp positions and the graph shows their respective mass flow which are different.
First of all, mass flow is not capture area. Secondly, the plot shows mass flow ratio, as opposed to mass flow. The former value has no units, the latter value has units.
So, nothing changed and you are still wrong in claiming that capture area changes on the inlet of F-14 and aircraft in the Su-27 family. 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.
Engineer
Major
The change in throat area is a consequent of having to adjust the ramps to optimally position the oblique shock wave. The concept of throat area is not the same as capture area, and you are wrong to use them interchangeably to argue F-14 and aircraft of Su-27 family can vary their inlets' capture area.
By , the restricted throat area will simply cause air to move faster at the throat to compensate. So, mass flow is unaffected by the narrowing of the throat. What affect the mass flow are position of the shocks, amount of air spillage, and amount of air bypassed.
Also, the presence of bypass doors tells you that the ramps cannot regulate the mass flow. If you claims were true, then bypass door won't be present.
Your own reference disagrees with you. Here are some important extracts from the above paragraph:
It says specifically that the ramps position the shock waves, but no where does it say that the ramps control the amount of flow.
This is applicable to the Caret inlets on the F-22.
Engineer
Major
latenlazy is correct in pointing out capture area is not throat area. Your argument that capture area changes on the inlets of F-14 and aircraft in Su-27 family is based on your twisted redefinition of what capture area means, which is wrong.
Change in oblique shocks as indicate in the first diagram results in change of mass flow ratio in the second diagram. However, the capture area A1 remains unchanged.
Wrong. Fixed intake and DSI are two different types of inlets, the former has diverter while the latter is diverterless.
The three shocks variable-geometry inlet that you mentioned is the same type on the F-4D. It has lower pressure recovery ratio compared to DSI on the J-10B. At Mach 1.8, DSI pressure recovery ratio is 0.91 whereas the inlets on F-4D has a ratio of 0.89. At Mach 2.0, the pressure recovery ratio of both are similar. This is according to 2005 figures.
MiG-29
Banned Idiot
reead this in contet and you will see how wrong you and latenlazy are
the variable ramp inlet will reduce excess airflow about 13%
gracias feliz 2012
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reead this in contet and you will see how wrong you and latenlazy are
the variable ramp inlet will reduce excess airflow about 13%
gracias feliz 2012
The problem is you assume ramp to mean adjustment of capture area, when a ramp can be used to either adjust both throat and capture area, or just one of the two.