Specific Fuel Consumption - Bypass ratio - Supercruising - Endurance

[rjmaz1]

Recently I was thinking about the tactical usefulness of supercruising of both the F-22 and the future Russian and Chinese 5th gen fighters. It seems optimising a platform to supercruise dramatically reduces subsonic range and endurance.

I came to the conclusion that the F-35 will have nearly twice the range on internal fuel compared to the F-22 if cruising at Mach 0.8.

While the F-22 can supercruise at Mach 1.6 at 100% MIL power (non afterburning), the drag at this speed is nearly triple of Mach 0.8. So it will still be much more fuel efficient to cruise at subsonic speeds. See the graph below to see drag changes near the Mach 1.0. Mach 0.8 is the ideal transit speed as it has the lowest drag.

Now the specific fuel consumption (SFC) of a turbofan is at its lowest/best around 80% MIL power. As the engines are throttled back to 40% dry thrust the SFC can increase by approximately than 50% making them use more fuel per pound of thrust. Roughly speaking the engine is now putting out half the power but is still using two thirds the amount of fuel. It is not using half the amount of fuel for half the power. See the attached graph to see how SFC changes with Mach and engine speed.

Now with the F-22 and PAK-FA for these aircraft to cruise at Mach 0.8 the engines would have to be throttled right back to where they are not fuel efficient. So these aircraft are not efficient at cruising at mach 1.6 drag wise and aren't efficient at cruising at mach 0.8 SFC wise.

The F-35 with its lower thrust to weight ratio means when fully loaded it has to use a much higher throttle setting to cruise at Mach 0.8. The F135 will most likely be running where it has the lowest specific fuel consumption while cruising at Mach 0.8.

The F-35 is approximately 75% of the weight and would have approximately 75% of the drag of an F-22 while cruising at mach 0.8. They both have the same internal fuel capacity. The F135 has a higher bypass ratio compared to the F119 so the F-35 will have a slightly better specific fuel consumption when both engines are at its most efficient power settings.


The F-35 will have significantly more range as the engines will be at there optimal throttle setting at mach 0.8
The F-35 will fly significantly further due to the lower drag with the same internal fuel.
The F-35's higher bypass ratio will give slightly more range based on commercial engines that have increased bypass ratio using a similar core.

The range advantage is so great that even if a third of the distance had to be covered at mach 1.6 the F-35 could use afterburner and may still have similar range to the supercruise optimized f-22.

Does anyone have any graphs or specific fuel consumptions of various engines to help confirm the above?

This is good news for Australia if a RAAF F-35 pilot even encountered a Russian 5th Gen fighter.

 

[Abraham Gubler]

I came to the conclusion that the F-35 will have nearly twice the range on internal fuel compared to the F-22 if cruising at Mach 0.8.

The tested combat radius of a F-22A without supercruising is around 600 NM or the same as the objective combat radius of the F-35A. There are differences in the type of combat misison, the F-35A mission burns some more fuel via a medium altitude segment, but the difference is cetainly not twice as much if benchmarked.

 

[rjmaz1]

The tested combat radius of a F-22A without supercruising is around 600 NM or the same as the objective combat radius of the F-35A. There are differences in the type of combat misison, the F-35A mission burns some more fuel via a medium altitude segment, but the difference is cetainly not twice as much if benchmarked.

The published F-35 combat radius is the minimum design requirement. The production aircraft has to reach or exceed the design requirements. Previous programs have seen aircraft perform significantly better than the design requirement.

If the F-22 has been tested to have a 600NM subsonic ombat radius then at the same mission format the F-35 will have a combat radius that is much greater, possibly reaching F-15e territory. Unless the F-35 flies with its landing gear down its going to have a very long range.

 

[My2Cents]

The published F-35 combat radius is the minimum design requirement. The production aircraft has to reach or exceed the design requirements. Previous programs have seen aircraft perform significantly better than the design requirement.

Given the ‘cheese paring’ way that the F-35 is being designed and built to control costs, I doubt it will exceed the minimum design requirement by much. In fact it would not surprise me if they end up with a waver because they are just under it. The problem is that the government keep demanding more changes and things added and that frequently the only place they can make up the difference in weight and volume is the fuel tankage.

 

[My2Cents]

While the F-22 can supercruise at Mach 1.6 at 100% MIL power (non afterburning), the drag at this speed is nearly triple of Mach 0.8. So it will still be much more fuel efficient to cruise at subsonic speeds. See the graph below to see drag changes near the Mach 1.0. Mach 0.8 is the ideal transit speed as it has the lowest drag.

Would you happen to have a chart with a scale on the vertical axis? Because if it is not logarithmic the drag coefficient at Mach 1.6 looks closer to 2x that of Mach 0.8. Which would mean the fuel consumption vs. distance are about equal.

Now the specific fuel consumption (SFC) of a turbofan is at its lowest/best around 80% MIL power. As the engines are throttled back to 40% dry thrust the SFC can increase by approximately than 50% making them use more fuel per pound of thrust. Roughly speaking the engine is now putting out half the power but is still using two thirds the amount of fuel. It is not using half the amount of fuel for half the power. See the attached graph to see how SFC changes with Mach and engine speed.

A little more nomenclature would be helpful for those of us that are not aeronautical engineering type. I assume that M is the Mach number, so the chart tops out a M=0.8. I think that we can assume that it refers to the compressor inlet velocity, not at the speed of the aircraft, which makes a lot of sense if you do not want the engine to go into compressor stall before you break Mach 1.

The air intake nozzle design is responsible for keeping the air flow into the engine below sonic velocities, and will have a huge impact on engine performance at different speeds. If it reduces the compressor inlet velocity to Mach 0.8 when the F-22 is doing Mach 1.6, what will the compressor inlet velocity be when the aircraft is only at Mach 0.8, and how will that effect overall fuel economy?

And the same questions apply to the F-35 inlet, except it is optimized for a different speed envelope.

 

[ADMk2]

Given the ‘cheese paring’ way that the F-35 is being designed and built to control costs, I doubt it will exceed the minimum design requirement by much. In fact it would not surprise me if they end up with a waver because they are just under it. The problem is that the government keep demanding more changes and things added and that frequently the only place they can make up the difference in weight and volume is the fuel tankage.

I am certain the design is much more capable than it's KPP's suggest. What is likely to be the case is that it will only be cleared in the Block III configuration "just" beyond it's required performance.

For instance the top speed is going to be cleared just a bit beyond M1.6 to ensure sufficient operating margin, the speed rumoured is M1.67.

Many take this to mean that, that is the aircraft's "top" speed. It isn't, it's just the limit aircraft is being cleared to in combat configuration because that's all the users require of it. It's DSI inlet however provides sufficient capacity all the way up to M2.0 air speeds, as demonstrated on the F-16 during testing in the 90's.

If it's needed some day, no doubt faster speeds will be cleared for the airframe (perhaps in conjunction with engine mods to improve overall supersonic speeds and gulp - even supercrise capability...) :shudder

Code One Magazine: JSF Diverterless Supersonic Inlet

With the amount of thrust the F-35 has and the "clean" airframe the F-35 will be enjoying, there doesn't seem to be any real reason why that top speed envelope couldn't be cleared all the way to that which the F-16 can achieve, it just isn't important enough to do so. M1.6 is fast enough for a tactical fighter in combat configuration, faster actually than many legacy aircraft have ever achieved in real world scenarios...

 

[Haavarla]

There is other aerodynamic challanges to overcome if we look at top speed.
Take a look at the SH and its wing design..

What about the F-35 wing design?
The swept angle wings are not in the same class as F-15 or Su-27.
Is it really design to reach M2.0?
So it might be more than the inlet design alone.

 

[ADMk2]

There is other aerodynamic challanges to overcome if we look at top speed.
Take a look at the SH and its wing design..

What about the F-35 wing design?
The swept angle wings are not in the same class as F-15 or Su-27.
Is it really design to reach M2.0?
So it might be more than the inlet design alone.

I agree, there are other factors that go into the equation, however even wing sweep is just one factor too... Wing thickness is another important one...

The Super Hornet's wing design is excellent, for it's intended role - low speed handling rather than top speed however are you aware that despite this, it still managed to hit M1.84 with it's standard F414 engines in a high speed, high altitude trial during the MMRCA aircraft trials?

I wonder what it could do with EPE engines and an extra 4400lbs of thrust per engine? M2.0 doesn't seem entirely out of the question there, whilst acceleration and climb would be massively increased... Granted it was a relatively pointless high speed run for the sake of achieving a high speed (and likely marketing purposes) but the fact is that the "published" top speeds are usually only that which the aircraft is cleared to do, not necessarily everything it's capable of...

In the case of the F-35 what's more important, getting it's currently planned flight envelope fully cleared and moving onto mission systems testing as quickly as possible, or spending more time and money proving for chest beating purposes, that it can or cannot do the extra M 0.33 it needs to reach the "magic" number?

However this may be, the point is they clearly aren't spending the money or time to push the F-35 to it's design limits and see what it can really do. That may come later and it will be interesting to see the reaction on the internet if the F-35 ever does demonstrate a speed beyond M1.0 without using afterburner...

I imagine there will be some red faces, hidden away behind those keyboards...

 

[rjmaz1]

Would you happen to have a chart with a scale on the vertical axis? Because if it is not logarithmic the drag coefficient at Mach 1.6 looks closer to 2x that of Mach 0.8. Which would mean the fuel consumption vs. distance are about equal.

That transonic drag curve provided is only an example. The height and width of the transonic peak is mainly determined by the shape and sweep of the wing. A brick can supercruise if enough thrust is provided. The F-22 combat radius figures take a large hit when there is a supercruise section added. This proves that drag at mach 1.6 is well over double that of Mach 0.8.

If it reduces the compressor inlet velocity to Mach 0.8 when the F-22 is doing Mach 1.6, what will the compressor inlet velocity be when the aircraft is only at Mach 0.8, and how will that effect overall fuel economy?.

That is correct, stealth inlets are no longer variable with ramps so have to be tuned for a designed speed range. When the F-35 and F-22 are both flying at the same subsonic speed the F-22 may have a lower inlet speed at the face of the engines due to its more restrictive supercruise optimised inlets.

The main effect of fuel comsumption will be the F-22 and PAK-FA having to throttle right back its engines to cruise at the efficient drag friendly Mach 0.8 transit speed. You want the air at the tips of the fan blades to be near supersonic. Throttle right back will significantly reduce engine efficiency. The F-22 will without any doubt be burning more fuel per pound of thrust than the F-35 at mach 0.8. The F-22 will also require more thrust to maintain Mach 0.8 due to more size/weight/drag compared to the F-35. So it will be burning more fuel per minute while cruising at mach 0.8.

As both aircraft have the same internal fuel supply the F-35 will travel much further.

And the same questions apply to the F-35 inlet, except it is optimized for a different speed envelope.

The F-35 inlet will no doubt be better suited to Mach 0.8, where the drag is lowest (on a log scale) and its engine the most efficient. When added up it will give a huge range advantage.

 

[colay]

IMO just go by the latest range figures emanating from the JSF program. They are in the best position to know, realizing of course that we have no way of corroborating these figures. Otherwise, any attempt range estimates are based on multiple and assumptions and don't take into account all the variables that must be considered. As such, any result will be highly speculative.

 

[My2Cents]

I am certain the design is much more capable than it's KPP's suggest. What is likely to be the case is that it will only be cleared in the Block III configuration "just" beyond it's required performance.

For instance the top speed is going to be cleared just a bit beyond M1.6 to ensure sufficient operating margin, the speed rumoured is M1.67.

Many take this to mean that, that is the aircraft's "top" speed. It isn't, it's just the limit aircraft is being cleared to in combat configuration because that's all the users require of it. It's DSI inlet however provides sufficient capacity all the way up to M2.0 air speeds, as demonstrated on the F-16 during testing in the 90's.

If it's needed some day, no doubt faster speeds will be cleared for the airframe (perhaps in conjunction with engine mods to improve overall supersonic speeds and gulp - even supercrise capability...) :shudder

Code One Magazine: JSF Diverterless Supersonic Inlet

With the amount of thrust the F-35 has and the "clean" airframe the F-35 will be enjoying, there doesn't seem to be any real reason why that top speed envelope couldn't be cleared all the way to that which the F-16 can achieve, it just isn't important enough to do so. M1.6 is fast enough for a tactical fighter in combat configuration, faster actually than many legacy aircraft have ever achieved in real world scenarios...

Speed, that they will meet because the aerodynamics and thrust are pretty well fixed at this point. Range is a different matter, fuel tankage is easy to cut to free up weight and volume for other uses.

 

[ADMk2]

Speed, that they will meet because the aerodynamics and thrust are pretty well fixed at this point. Range is a different matter, fuel tankage is easy to cut to free up weight and volume for other uses.

I assume you are basing your opinion on this?

F-35's Range Falls Short of Predictions - Defense News

That figure has come up based on a range of assumptions. It has not been borne out through flight testing (or if it has that info has not yet been made public).

It is also not taking into account the fact that a 5% buffer margin is built nto the F-35's fuel capacity. Taking this 5% buffer into account, each variant comfortably exceeds it minimum range requirements.

As to your idea that L-M will be cutting fuel internal capacity, the reverse is actually true. They are researching ways to fit more fuel in...

F-35A may need mods to fix range shortfall

 

[Abraham Gubler]

Now is also a good time to once again show the specifics of that F-35A KPP combat radius mission. This is very different to that of many other aircraft. As can be seen by this representation of the original requirement it include two combat phases and 100 NM of ingress/egress flight at 20,000 feet at 540 KTAS which is a significant fuel burner that many other aircraft don't have in their 'combat radius' missions.

 

[rjmaz1]

Air Force Association estimates that use of supercruise for a 100-nautical-mile (190 km) dash as part of a mission would cut the F-22's combat radius from about 600 nautical miles (1,110 km) to about 450 nautical miles

That shows how much drag Mach 1.6 has compared to Mach 0.8. Three times the drag is probably accurate. With supercruise it even says a lower 410nm at this link: F-22 Raptor Team Web Site: Technology - Flight Test Data

The F-22 has a published combat radius of 540nm or only 410nm if a 100nm supercruise section is added.

The F-35 mission profile in the above image has a combat radius of 700nm. That last 50nm combat section is probably burning fuel at twice the rate of the transit section. So no doubt it will have a greater subsonic range than the F-22's official data. Again thats the design requirement, most aircraft exceed the design requirement once they enter service. The F-22 exceeded it designed range and supercruising requirement by 14% and 15% respectively.

I would assume the F-35 will exceed its design requirement, though it will be hard to compare the different mission profiles to that of the F-22.

If the F-35 added a Mach 1.6 100nm dash section to its mission profile using afterburner it may still have greater range than the 410nm provided in the F-22's supercruising mission profile.

So anyone that says the F-22 can shoots its missiles further or avoid missiles easier due to its higher speed is uninformed. The F-35 can do exactly the same simply by using a bit of afterburner as it has the fuel to burn.

This is great news for any country that plans to use the F-35 for its frontline fighter.

 

[Abraham Gubler]

The F-35 mission profile in the above image has a combat radius of 700nm.

That's not the F-35's mission profile. It was the paper profile for the JAST requirement. JAST became JSF and with further development towards a real plane the overall radius was reduced from 700 NM to 590 NM, still keeping the 100nm of ingress/egress flight.

 

[My2Cents]

I assume you are basing your opinion on this?

F-35's Range Falls Short of Predictions - Defense News

That figure has come up based on a range of assumptions. It has not been borne out through flight testing (or if it has that info has not yet been made public).

Yet the 2nd article you quote backs up the 1st article you dismiss.

Actually, while I have probably read both, my comment was based entirely on my own experience with ‘fixed price contracts’ with creeping design requirements. It almost always ends in tears and no one is satisfied with the result, but the project usually keeps going because the client is afraid to admit they screwed up and start over, so they negotiate the requirements down to what can be delivered.

It is also not taking into account the fact that a 5% buffer margin is built nto the F-35's fuel capacity. Taking this 5% buffer into account, each variant comfortably exceeds it minimum range requirements.

As to your idea that L-M will be cutting fuel internal capacity, the reverse is actually true. They are researching ways to fit more fuel in...

F-35A may need mods to fix range shortfall

They are researching ways to fit more in, but that does not mean that they will find any practical ones.

And you have to love the statement “But programme officials are also debating whether to change how the range of the F-35A is calculated, the source said.” I wonder who those ‘programme officials’ are, the DoD, L-M, or both? If you cannot meet the requirement, just redefine it! I guess it is a more politically acceptable form of a waiver. :hul

As for the 5% reserve, that is their cushion for last minute design changes. To use it up now could be seen an act of desperation.

 

[colay]

That's not the F-35's mission profile. It was the paper profile for the JAST requirement. JAST became JSF and with further development towards a real plane the overall radius was reduced from 700 NM to 590 NM, still keeping the 100nm of ingress/egress flight.

I was wondering for a while why it's a Boeing slide..