Analysis Aircraft Performance Analysis

[Nilgiri]

This is for easy reference going forward on the larger field of aircraft performance analysis.

i.e Comparing design impact/trends on their performance and with underlying data and relevant discussion that pertains to analysis and any salient conclusions.

I didn't elect to commence this in any country or military section.....since this is broad subject at large and not limited to any particular air force.

The intent is for members to see subjects that come up in other threads in broader underlying fashion w.r.t overall design challenges and solutions in this field.

It can be extended to civil aviation (which I have more experience with) with time hopefully.

 

[Nilgiri]

Recent field in this area (4th to 5th gen fuel fraction change inherent to overall design transition)

KF21 is surprisingly bigger than F35, but why has less internal fuel?

F-35 has much higher MTOW and thus much larger mass-range (MTOW - empty weight) to play with.

i.e F35-A:

MTOW = 31.7 tons
Empty weight = 13.3 tons
Fuel capacity = 8.3 tons

Don't let the raw volumetric size of aircrafts fool you.

About 45% of that range is max internal fuel

i.e 8.3/ (31.7 - 13.3) = 0.45

KF-21 fuel ratio/mass range:

KAI KF-21 Boramae - Wikipedia

en.wikipedia.org

Prototype around 30%, and gets bumped up to about 40% in production.

Conventional (4th gen) fighters have this % around 30 - 33% typically.

Other 5th gen fighters (like F-35) tend to have it around 45%*.

It comes intrinsically somewhat with incorporating internal weapon bay on fuselage and RCS blended planform for stealth at same time.

i.e naturally leads to a higher cross-coefficient for fuel capacity compared to conventional fuselage (and external hardpoints only) found in 4th gen and earlier

KF-21 by blending 4th and 5th gen....has RCS-reduction planform but no internal weapon bay...thus comes up around 40% max fuel fraction (of total buffer) for its current production block series. i.e somewhere between the two.

Likely bumps up to closer to 45% in subsequent blocks that go for internal weapon bay and shift to 5th gen design philosophy etc.

*J-20 which is really more 5th gen fighter-bomber gets it to 60% even... given its beefy + long fuselage (21 m long) from the large weapons bay + range req. etc.

I expect TFX somewhere around 45% - 60% for this max fuel/mass range ratio I mention here in F-35 vis-a-vis KF-21:

Indonesian Air Force, Tentara Nasional Indonesia-Angkatan Udara (TNI-AU)

US got some progress but not the French So No Florence parly for indo and malay for this January?

defencehub.live

as TFX current conceptual length is around 21 m, which is similar to J-20 length

So in Layman's term?
In a clean configuration and full of fuel internally, who got more range?

It will be F-35 as it simply has a higher effective fuel fraction (FF) than KF-21 along with F-35 larger Maximum Take-Off Weight (MTOW)

The why though is deeper look than just looking at the raw max fuel capacity on hand (~ 8.3 ton vs 5.4 ton respectively)

To see effect of FF, we can look within a fighter type i.e same empty weight (M0), same MTOW, same aerodynamic envelope (wing design, engine thrust etc) but two FF variants.

I use the FF number ratio relative to mass loading range (MTOW - M0) as the denominator rather than absolute mass, but analysis works out the same either way.


Diagram1: Base case where we start with empty weight (M0), add fuel only first (to eventually reach maximum range) and then get combat ranges by adding payloads last:

NOTE: NOT TO SCALE

Note this base case always considers fuel as the priority (i.e fuel max + range max frontier).

The max range frontiers are represented by the blue vertical lines (max fuel) and arrows (as combat payload starts to get added).

In a variant analysis (which would not produce the range frontier) loaded fuel can be traded off for payload with the consequent sacrifice in range.

The increase of FF (here 0.30 and 0.45 as examples) is essentially what conformal fuel tanks also do within one fighter family.

The LHS green arrow illustrates the trade-off on payload capacity i.e what the extra fuel carried internally displaces in payload capacity.

i.e M0 and MTOW change very little but you get larger FF and larger combat ranges as result within say the same 4th gen class by adding conformal fuel tanks. The F-16 is famous example of this:


Diagram 2: Regular F-16 vs F-16 with conformal fuel tanks

Photograph credit: open source USAF.


So this concept (of FF impact on range) continues across different fighters as well (arising by intrinsic design approach/consequence w.r.t fuel cap "baked in" and ready to harness) as long as they are not drastically different sizes* or types of aircraft.

In a way you can say 5th gen lends itself naturally to being more "conformal" from the get go w.r.t 4th gen due to weapon-bay volume combined with RCS reduction strategy.

In the following 3rd diagram for example, consider how close the F-15 and F-35 are for their M0 and MTOW respectively, yet how much physically larger the F-15 is.

Alternatively consider the same physical sizes of the F-22 vis a vis F-15 and the F-35 vis a vis F-16, but how the 5th gen in each case is far heavier than its 4th gen equivalent in both M0 and MTOW and has the much larger Fuel Fraction (FF) as well.


Diagram 3: 4th Gen vs 5th Gen Sizes, Masses and Fuel fractions

Diagram components are open source


The conclusion is by way of significantly higher FF, we gain higher ranges and higher range per payload kg by the very intrinsic design approach of 5th gen, given more (potential**) fuel relative to it in each case.

It is for example why J-20 most likely achieves a very big (still unclear + classified IIRC) max range and combat range (compared to most other 5th gen a/c) given its FF is at very large 60% derived from its 12 ton fuel capacity.

==============================================================================================

* i.e M0 similarity (and assuming ceterus paribus), meaning similar available envelope/scope for aerodynamic performance and trade-off analysis between platforms

** I say potential as this can be traded off with payloads, but this impacts all cases the same anyway too (i.e you diminish the maximum range by some equivalent factor).

N.B If members spot errors, please let me know and I will discuss and correct them.

Depending how this thread evolves w.r.t aircraft performance discussion in general and time pending, I might create article based on it at later date.

@MisterLike @T-123456 @Cabatli_53 @Test7 @Anmdt @AlphaMike @Gessler @Yasar @Bilal Khan(Quwa) @Stuka Dive @Paro et al.

Edit: pictures re-attached

 

[TR_123456]

Recent field in this area (4th to 5th gen fuel fraction change inherent to overall design transition)









It will be F-35 as it simply has a higher effective fuel fraction (FF) than KF-21 along with F-35 larger Maximum Take-Off Weight (MTOW)

The why though is deeper look than just looking at the raw max fuel capacity on hand (~ 8.3 ton vs 5.4 ton respectively)

To see effect of FF, we can look within a fighter type i.e same empty weight (M0), same MTOW, same aerodynamic envelope (wing design, engine thrust etc) but two FF variants.

I use the FF number ratio relative to mass loading range (MTOW - M0) as the denominator rather than absolute mass, but analysis works out the same either way.


Diagram1: Base case where we start with empty weight (M0), add fuel only first (to eventually reach maximum range) and then get combat ranges by adding payloads last:

NOTE: NOT TO SCALE

View attachment 38970


Note this base case always considers fuel as the priority (i.e fuel max + range max frontier).

The max range frontiers are represented by the blue vertical lines (max fuel) and arrows (as combat payload starts to get added).

In a variant analysis (which would not produce the range frontier) loaded fuel can be traded off for payload with the consequent sacrifice in range.

The increase of FF (here 0.30 and 0.45 as examples) is essentially what conformal fuel tanks also do within one fighter family.

The LHS green arrow illustrates the trade-off on payload capacity i.e what the extra fuel carried internally displaces in payload capacity.

i.e M0 and MTOW change very little but you get larger FF and larger combat ranges as result within say the same 4th gen class by adding conformal fuel tanks. The F-16 is famous example of this:


Diagram 2: Regular F-16 vs F-16 with conformal fuel tanks

View attachment 38971
Photograph credit: open source USAF.


So this concept (of FF impact on range) continues across different fighters as well (arising by intrinsic design approach/consequence w.r.t fuel cap "baked in" and ready to harness) as long as they are not drastically different sizes* or types of aircraft.

In a way you can say 5th gen lends itself naturally to being more "conformal" from the get go w.r.t 4th gen due to weapon-bay volume combined with RCS reduction strategy.

In the following 3rd diagram for example, consider how close the F-15 and F-35 are for their M0 and MTOW respectively, yet how much physically larger the F-15 is.

Alternatively consider the same physical sizes of the F-22 vis a vis F-15 and the F-35 vis a vis F-16, but how the 5th gen in each case is far heavier than its 4th gen equivalent in both M0 and MTOW and has the much larger Fuel Fraction (FF) as well.


Diagram 3: 4th Gen vs 5th Gen Sizes, Masses and Fuel fractions

View attachment 38972
Diagram components are open source


The conclusion is by way of significantly higher FF, we gain higher ranges and higher range per payload kg by the very intrinsic design approach of 5th gen, given more (potential**) fuel relative to it in each case.

It is for example why J-20 most likely achieves a very big (still unclear + classified IIRC) max range and combat range (compared to most other 5th gen a/c) given its FF is at very large 60% derived from its 12 ton fuel capacity.

==============================================================================================

* i.e M0 similarity (and assuming ceterus paribus), meaning similar available envelope/scope for aerodynamic performance and trade-off analysis between platforms

** I say potential as this can be traded off with payloads, but this impacts all cases the same anyway too (i.e you diminish the maximum range by some equivalent factor).

N.B If members spot errors, please let me know and I will discuss and correct them.

Depending how this thread evolves w.r.t aircraft performance discussion in general and time pending, I might create article based on it at later date.

@MisterLike @T-123456 @Cabatli_53 @Test7 @Anmdt @AlphaMike @Gessler @Yasar @Bilal Khan(Quwa) @Stuka Dive @Paro et al.

Edit: pictures re-attached

What about changing the fuel?
Like liquid gas,is that a possible option?
Is it ever been tested?
Just asking.

 

[Nilgiri]

What about changing the fuel?
Like liquid gas,is that a possible option?
Is it ever been tested?
Just asking.

In the future if liquid hydrogen i.e LH (for example) becomes feasible*, it offers a far higher energy density than current aviation fuel (kerosene).

It is likeliest potential contender going forward given its energy density is around 140 MJ/kg (~ 3 times higher than regular hydrocarbons)

It would however need a lot of re-design of all the (regular**) engine classes to handle it as its stochiometric (i.e optimal with pure oxygen) combustion is around 3000 Celsius compared to 2400 Celsius for Kerosene.

The other approach is fuel cell hydrogen (and use electric transfer propulsion), but this is new area along with batteries in general right now.

We are having challenges and problems with our current knowledge and application of materials in handling Kerosene as it is. So that would likely be the limiting factor well before Hydrogen and other cryo-liquids problems (production and insulated storage).

Current "more available" liquid fuels like LNG (which solve production side somewhat, but still face same storage issue) offer little advantage in Energy density compared to kerosene and gasoline

They are about 55 MJ/kg vs 46 MJ/kg.

* Their (LNG, LH, LOX) storage and handling costs (given they have to be cooled and kept cool otherwise they evaporate to gas) also make them unfeasible right now compared to fuels that are just liquid at room temperature.

** Internal combustion engines. Non conventional propulsion is different matter. For example the "best" rocket engines already employ cryogenic fuels+ liquid oxygen given their advantages, but their propulsion problems (related efficiencies and explosiveness) and issues render them not feasible for mass civil and military scale use.