Why is the Hornet limited to mach 1.8?

[Vampyr]

Wikipedia says the F/A-18 is limited to mach 1.8, whereas other aircraft of similar size such as the F-16 can go up to mach 2. Why is this? I couldn't find any other aircraft with the same thrust/weight ratio to compare with, but I'm wondering if it has anything to do with the aircraft being heavier due to reinforcements + equipment for carrier ops.

 

Not that I'm complaining; I'm sure that for me at least, going from the slow flying A-10 into an F/A-18 is going to feel fast enough regardless.

Edited May 17, 2018 by BIGNEWY
spelling in title

[Timotsuky]

Wikipedia says the F/A-18 is limited to mach 1.8, whereas other aircraft of similar size such as the F-16 can go up to mach 2. Why is this? I couldn't find any other aircraft with the same thrust/weight ratio to compare with, but I'm wondering if it has anything to do with the aircraft being heavier due to reinforcements + equipment for carrier ops.

 

Not that I'm complaining; I'm sure that for me at least, going from the slow flying A-10 into an F/A-18 is going to feel fast enough regardless.

 

Stop reading Wikipedia

[Vampyr]

Stop reading Wikipedia

 

No.

[Andrei]

I can't say with 100% confidence, but if I recall correctly this is due to the engine inlet design. Hornet is using simple inlet which does not allow engine to operate at high supersonic speeds. You can actually read Wikipedia on the topic.

[SkateZilla]

Intake Design and Drag.

[jojo]

I couldn't find any other aircraft with the same thrust/weight ratio to compare with

 

Rafale is also limited to M1.8, both Air Force an Navy model.

 

So the Rafale has higher T/W ratio than the Mirage 2000 but lower maximum mach (M2.2 for Mirage 2000).

 

But the maximum mach is reached with clean aircraft (F-16, Mirage 2000, Hornet).

Once you add weapons, you add drag and you can kiss good by to your max mach.

 

But for instance, a Rafale with missiles and supersonic fuel tank will accelerate faster than a Mirage 2000.

 

So in the end, the max mach doesn't matter that much :thumbup:

[Speedbrake]

Mach Limit

 

Not sure about the F/A-18C but for the F-16 the limit is structural and pertains to seperation of the canopy amonst other limits such as the engine.

[Andrei]

F-16 is actually pretty fast jet given it doesn't have moving inlet ramps. The intake design with flow separation is quite clever and the bird can reach quite respectable mach numbers. However, as already mentioned, there are other limitations e.g. heat limits. And most importantly - by the time you reach max mach, you'd better be above the airport or have a tanker nearby :)

[Mizzy]

Air intakes create increased drag ..apparently.

 

Mizzy

[Banzaiib]

Air intakes create increased drag ..apparently.

 

Mizzy

 

It's about compressiblilty of air at supersonic speeds. The air needs to be "slowed down" to subsonic speeds prior to contact with the engine compressor blades. The shockwaves that come off the intake geometry accomplish this, but those shockwaves can vary based on mach number, and for some intake designs, varibale geometry is required at higher mach numbers to "slow the air down" appropriately. The F-14 and F-15 both do this, but the F-18 does not. Therefore, it's limited to Mach 1.8.

 

There's a great video about shockwaves slowing air and compressing it for the engine... this is how the SR-71 basically works. Something like 70% of the compression of the air for combustion is a result form the shockwaves, not the actual compressor section (if my memory serves me right).

 

[Akula]

Mach 1.8 is the published speed.... True airspeed is higher and classified info last time I checked.

[Drotik]

The design of the air intakes is for sure a contributing factor but I do not think it is the main reason. You must realise that the desing of an airfoil, especially one supposed to go supersonic, is incredible complex. If it is not designed to reach a higher mach number it won´t accelerate any further. The many forms of drag are stopping it from doing so. It simply is the overall design of the whole aircraft.

And the old saying compressors need subsonic flow isn´t really true anymore. Modern engines are very well capable of operating with airflow in the transsonic region. The complex design of air intakes has more reasons than slowing air down. It is designed to provide the engine with undisturbed air with as little loss in density as possible.

Edited May 17, 2018 by Drotik

[Banzaiib]

Modern engines are very well capable of operating with airflow in the transsonic region

 

This is incorrect. The air must be subsonic in relation to the face of the compressor. The same thing happens to compressors that happened to late generation ww2 fighters' elevators as they approached transonic speeds. Air becomes compressible, therefore the control surface stalls. This means that a compressor will stall if it meets supersonic airflows. The effect of slowing the air down is a pressure increase behind the shockwave.

Edited May 17, 2018 by Banzaiib

[Drotik]

This is incorrect.

 

It is not anymore. You are correct that there is a high risk of flow seperation behind a forming shockwave. But you can handle that if the airfoils, that the compressor blades are, have an appropriate design. We managed to get rid of the problem for wings and controls, and to a certain degree also for turbo compressors. But trans- and supersonic fluid mechanics are such a complex thing, our scientists and engineers just scratched the surface.

Edited May 17, 2018 by Drotik

[jojo]

It is not anymore. You are correct that there is a high risk of flow seperation behind a forming shockwave. But you can handle that if the airfoils, that the compressor blades are, have an appropriate design. We managed to get rid of the problem for wings and controls, and to a certain degree also for turbo compressors. But trans- and supersonic fluid mechanics are such a complex thing, our scientists and engineers just scratched the surface.

 

Any source to sustain your claim than an engine can reliably run with supersonic flow arriving on its compressor stage ?

 

Plus, if these technologies really exist, did they exist back in the late 70’/ early 80’

 

And overall, a fighter limited to Mach 1.8 is cheaper to produce and sustain, and tactically the fighter isn’t likely to go above M1.8 with weapons. So why bother ?

 

Air intake: on Mirage 2000D/ N which are strike variants, the driving engine of the moving devices inside the air intake are removed to turn down maintenance cost.

The max speed goes down from M2.2 to M1.4.

 

So there are multiple reasons to design a plane with lower maximum Mach numbers.

And technically, the limits for such fighters are:

- airframe strength

- temperature

- air intake design

[mkellytx]

Intake Design and Drag.

 

Quite likely, F-18A-C NATOPS Figure 4-1 puts the max Mach at 2.0, if 1.8 is the max level flight Mach the inlets are likely choking and not producing enough thrust to over come drag. FWIW the 2.0 is probably clean in a dive, when setting limit like this usually you take the plane xx% faster to provide a safety margin for engine surges/compressor stalls or various types of aeroelastic nasties like flutter.

[MRSHADO]

This aircraft, along with its Early Release guide and all the other bells and whistles must be ready to drop any second, because we've completely exhausted all relevant topics.

 

Bring it on, so we can keep the peace.

[Vampyr]

This aircraft, along with its Early Release guide and all the other bells and whistles must be ready to drop any second, because we've completely exhausted all relevant topics.

 

Bring it on, so we can keep the peace.

 

we still haven't talked about the ejection seat's max velocity and safe ejection speeds in a hornet.

[Hog_No32]

we still haven't talked about the ejection seat's max velocity and safe ejection speeds in a hornet.

 

IIRC max 600 knots... at least that is what I recall from my NATOPS reading :lol:

[Banzaiib]

IIRC max 600 knots... at least that is what I recall from my NATOPS reading :lol:

 

ouch! I can't even imagine!

 

definitely not a good way to stay in shape...

[MRSHADO]

Or MIDS. Where is the MIDS discussion. Point me in the right direction.

[chief]

Hi all. I'm not a fighter pilot but fly in the airlines. You gotta remember the compressor in an engine can only handleso much air at a time. If you keep increasing this velocity you will choke the compressor. Ok sohow about just speed the compressor up your say? That's possible to a certain point. The compressor blades get too fast and the tips if the compressor will break the sound barrier themselves and disrupt the airflow. So the solution to extreme fast speed is the scramjet. At slower aircraft speeds the engine operates like a turbofan and as it gets faster it converts to a ramjet. Until a few years ago they couldn't design a particular part of the compressor to withstand the extremely high heat this engine required. They kept melting the compressor. Now I believe they can and talks have been made on making a mach 4-5 airliner. But now another problem arises. They said the airliner would have to be designed without windows because the windows would have to be at least 9 inches thick to withstand the heat. So you can imagine the temps created at these velocities. So temperature has a huge say in how fast a plane can travel. Drag obviously has another say. Wing design is a huge factor in how fast a plane can travel. It's s trade off. You want to carry a huge load, keep a straight wing. You wanna go fast you need to sweep em. F111 is s good example of doing both. But of course look what you have to do to the wings if you want a lot of lift and go slower but need the capabilities of going fast. The thing is there areSO many factors in an aircrafts speed and capabilities.

[probad]

So the solution to extreme fast speed is the scramjet. At slower aircraft speeds the engine operates like a turbofan and as it gets faster it converts to a ramjet. Until a few years ago they couldn't design a particular part of the compressor to withstand the extremely high heat this engine required.

scramjet is a truncation of supersonic combustion ramjet. supersonic combustion is the really major hurdle that had to be jumped.

scramjets can't operate "like a turbofan" because like all ramjets there is no turbine compressor... it's kind of fundamental to the concept of ramjets. one would need to specifically tack on a turbine engine unit to get low end thrust like in the j58.

Edited May 19, 2018 by probad

[chief]

You're right. I think it was scimitar engines they called them. My bad. I was reading the article in an aviation mag a few years back.

[chief]

Here I managed find the plane they were talking about.

https://en.m.wikipedia.org/wiki/Reaction_Engines_A2