Rated Engine Power & Speed

[Zipper730]

I know it sounds like a silly question, but there are ways things aren't always what they seem.

1. If an engine is rated for 15,000 pounds of thrust at military power, does that mean that's the installed thrust, the thrust in a test-cell or some other condition?
2. If an engine is rated for Mach 3: Does that mean that's the speed it can operate routinely at, or the maximum it can operate safely at?

 

[thunderbird]

Engine performance is generally static, uninstalled, with an optimal inlet.

Since aircraft are designed to standard day conditions, Mach number might sometimes be used as the "rating". But jet engines actually are designed to turbine station temperature limits (EGT) and not Mach number. But the turbine inlet temperature is a function of outside air temperature and Mach number, so there is some basis for just saying Mach number. There is probably a steady state limit on EGT and a time limited EGT limit, as well as a do not exceed EGT limit.

By the way, jet engines thrust production changes with speed, altitude and bypass ratio. Jets lose thrust with altitude of course. Mass airflow losses increase with bypass ratio and speed, while ram air compression increases thrust with speed. At some point, depending on bypass ratio, ram air compression starts "winning". As I recall, pure turbojets produce more thrust at 36,000 ft at M 0.9 than they do at sea level static conditions, but they lose some thrust at low speeds to the mass airflow part of the equation. Higher bypass ratio engines lose thrust more quickly with altitude relative to lower bypass or pure turbojets.

Inlet design also plays a part. Simple "shock" inlets start losing ram compression due to the shockwave forming at the front of the inlet, which also increases the incoming air temperature. Centerbody cones and complicated ramp inlets reduce the inlet shockwave losses significantly, such that the engines will continue to produce increasing amounts of thrust as speed increases, up until the material limits of the aircraft.

Sort of "shocks" me that the F-35with its complex inlet geometry and almost 70% more thrust than the F-16, has a Mach 1.6 top speed while an F-16 top speed is Mach 2.

But bypass is part of what gives the new generation engines a higher static thrust to weight ratio and better fuel economy (as long as one stays off the afterburner).

 

[swampyankee]

To a first approximation, a turbojet produces the same thrust at M=2 at 35,000 ft as it does sea level, static. One place to look is Introduction to Propulsion Systems | Aeronautics and Astronautics | MIT OpenCourseWare. Another is to hunt up a good library and look for some books published by AIAA

 

[pbehn]

For aircraft that operate at Mach 3 be prepared for some information to be vague.

 

[davparlr]

Sort of "shocks" me that the F-35with its complex inlet geometry and almost 70% more thrust than the F-16, has a Mach 1.6 top speed while an F-16 top speed is Mach 2.

Inlets may be designed more for stealth than high Mach efficiency, and, of course, cost.

 

[Zipper730]

davparlr & P pbehn

Regarding question #2: If an engine was to be rated for Mach 2.5 or Mach 3.0, basically whatever number is listed. Does that mean it's the maximum speed it can hit safely, or routine operation, as a general rule?

BTW: I was almost ready to start a new thread until I stumbled on this thread.

 

[Engineman]

I know it sounds like a silly question, but there are ways things aren't always what they seem.

1. If an engine is rated for 15,000 pounds of thrust at military power, does that mean that's the installed thrust, the thrust in a test-cell or some other condition?
2. If an engine is rated for Mach 3: Does that mean that's the speed it can operate routinely at, or the maximum it can operate safely at?

Hi,
These are not silly questions but, they need some explanation. All power and performance "ratings" are conditional on many specific conditions. That is why there are industry standards. However, many performance claims are very general or conditional and, it is impossible to be specific unless you know the conditions of the rating. Even buyers of engines in industry have to be absolutely specific in their terms and performance standards. That said, many turbojets and turbofans are/were rated at static thrust on a test bed at ISA, but that is a generalisation, many are rated under other conditions of altitude, temperature and pressure.
Similarly, "rated for Mach 3" is a generalisation. One major factor with high speed is the temperature rise from the intake conditions, it's effect on the engine performance and the limitations in the materials used in the engine. Very simplistically, Aluminium alloy in the first stages of a compressor can exceed their temperature limits, and the same goes for the rest of the engine. So again the conditions of use regarding maximum Mach or IAS, and an engines suitability for the conditions, is totally specific. It is worth noting that many high Mach performing engines are for Military use and, the technology and specific performance is going to be controlled information.

Eng

 

[pbehn]

davparlr & P pbehn

Regarding question #2: If an engine was to be rated for Mach 2.5 or Mach 3.0, basically whatever number is listed. Does that mean it's the maximum speed it can hit safely, or routine operation, as a general rule?

BTW: I was almost ready to start a new thread until I stumbled on this thread.

Dunno why you ask me, i didnt spend much time working on mach 3 aircraft or engines. As I understand it it is the airframe and the engine together. You can put an Olympus engine in a boat or a Vulcan or uprate it with an afterburner and put it in Concorde. It can supercruise in Concorde, but not in a Vulcan. The SR-71 has those tricky inlets to the engine, engines dont cope well with supersonic flow through them.

From Wiki
The air inlets allowed the SR-71 to cruise at over Mach 3.2, with the air slowing down to subsonic speed as it entered the engine. Mach 3.2 was the design point for the aircraft, its most efficient speed.[31]​ However, in practice the SR-71 was sometimes more efficient at even faster speeds—depending on the outside air temperature—as measured by pounds of fuel burned per nautical mile traveled. During one mission, SR-71 pilot Brian Shul flew faster than usual to avoid multiple interception attempts; afterward, it was discovered that this had reduced fuel consumption.[47]​

At the front of each inlet, a pointed, movable cone called a "spike" (inlet cone) was locked in its full forward position on the ground and during subsonic flight. When the aircraft accelerated past Mach 1.6, an internal jackscrew moved the spike up to 26 in (66 cm) inwards,[48]​ directed by an analog air inlet computer that took into account pitot-static system, pitch, roll, yaw, and angle of attack. Moving the spike tip drew the shock wave riding on it closer to the inlet cowling until it touched just slightly inside the cowling lip. This position reflected the spike shock wave repeatedly between the spike center body and the inlet inner cowl sides, and minimized airflow spillage which is the cause of spillage drag. The air slowed supersonically with a final plane shock wave at entry to the subsonic diffuser.[49]​

Downstream of this normal shock, the air is subsonic. It decelerates further in the divergent duct to give the required speed at entry to the compressor. Capture of the plane's shock wave within the inlet is called "starting the inlet". Bleed tubes and bypass doors were designed into the inlet and engine nacelles to handle some of this pressure and to position the final shock to allow the inlet to remain "started".

Schlieren flow visualization at unstart of axisymmetric inlet at Mach 2
​

In the early years of operation, the analog computers would not always keep up with rapidly changing flight environmental inputs. If internal pressures became too great and the spike was incorrectly positioned, the shock wave would suddenly blow out the front of the inlet, called an "inlet unstart". During unstarts, afterburner extinctions were common. The remaining engine's asymmetrical thrust would cause the aircraft to yaw violently to one side. SAS, autopilot, and manual control inputs would fight the yawing, but often the extreme off-angle would reduce airflow in the opposite engine and stimulate "sympathetic stalls". This generated a rapid counter-yawing, often coupled with loud "banging" noises, and a rough ride during which crews' helmets would sometimes strike their cockpit canopies.[50]​ One response to a single unstart was unstarting both inlets to prevent yawing, then restarting them both.[51]​ After wind tunnel testing and computer modeling by NASA Dryden test center,[52]​ Lockheed installed an electronic control to detect unstart conditions and perform this reset action without pilot intervention.[53]​ During troubleshooting of the unstart issue, NASA also discovered the vortices from the nose chines were entering the engine and interfering with engine efficiency. NASA developed a computer to control the engine bypass doors which countered this issue and improved efficiency. Beginning in 1980, the analog inlet control system was replaced by a digital system, which reduced unstart instances.[54]​

 

[davparlr]

External to the engine, which is well explained above, at the speeds greater than Mach 2.5 or so, the friction-stagnation temperature exceeds the level that aluminum can maintain strength. Above that speed, exotic and expensive material, like titanium. needs to be used and cost goes up. The under-appreciated aerodynamics of inlet and exhaust design, made significantly more complex with stealth requirements, requires significant effort. As mentioned above, typical turbojet engine inlet air velocity must be less than Mach one at the compressor face. Shock wave generation can destroy an engine. Fixed inlets, like the F-100, F-5, and F-8 are limited to around Mach 1.8. Faster aircraft, F-4, F-104, f-106, etal, utilize adjustable ramps, or cones, e.g. F-104, Mig 21?, and SR-71, to control inlet shock waves and reduce inlet velocity below Mach 1.

 

[Zipper730]

Inlets may be designed more for stealth than high Mach efficiency, and, of course, cost.

Since the 1970's, I'd say that's correct. In the past (from early supersonic flight to then), it seemed the matter was efficiency combined with adequate low speed performance at the same time.

These are not silly questions but, they need some explanation. All power and performance "ratings" are conditional on many specific conditions. That is why there are industry standards.

What would an industry standard be? It seems that generally, for sea-level power at least, for a commercially rated engine, would be on a test-bed at sea-level, 15C/59F.

Even buyers of engines in industry have to be absolutely specific in their terms and performance standards.

To ensure that the engine maker and the buyer both know what they want from each other?

Dunno why you ask me, i didnt spend much time working on mach 3 aircraft or engines.

You responded earlier

The SR-71 has those tricky inlets to the engine, engines dont cope well with supersonic flow through them.

Actually, that I understand. I know little about propeller driven aircraft, but I understand a little bit about jet aircraft.

I wasn't talking about the SR-71 in particular. Regardless, I was surprised that it wasn't known that flying faster than the normal speed would reduce fuel consumption (test pilots usually push the plane right up to the point they can get away with and factor in performance all the way up there).