Cool a new thread ... so can post again.
In the now locked previous thread LCA News and Discussions
, Cainji asked the following important question that still needs answering viz.
Cain Marko wrote:
Define fuel fraction please. I ask because I am quite confused about this. Also, please tell me why a twin engined bird necessarily has to compromise on FF. Does the flanker have a poor FF? I mean there is reason to believe that at one point F-18s did have better FF than Solah. I was under the impression that a larger internal fuel load (in proportion to empty weight) would contribute to greater FF. And in this regard, I see no such rule that two engines have to compromise on FF, MiG-31 is supposed to have excellent FF.
There-in lies the issue - two engines will have to compromise on fuel-fraction (irrespective of whatever level of engine-technology involved e.g. F18).
First thing first - the defn etc.
As you know, the fuel fraction is defined as teh ratio of the weight of the fuel to the gross take-off weight of the craft (including propellant) excluding external stores and it's a key factor in determining aircraft range. A good write-up is there in Wiki itself : Fuel fraction
and Aircraft Range
It determines the effective range of an aircraft and is dependent on various factors like LD ratio, SFC, amount of fuel available to propel (as opposed to lift the weight itself) i.e. cruise fuel fraction.
Ok, let's try and deconstruct this, one at time and to the best of my ability (which can be wrong, so pls correct me) - let's first look at the comparable stats between a double-engined aircraft (MiG-29A) and a single-engined F-16A (the one with PW engines, F100-PW-200, and not the later GE ones) - apples vs apples as much as we can, pls.
TcWR (Thurst-to-Clean Weight-Ratio) both approx 0.7
So the key indicator/poser is why is that even after having almost comparable Dry SFC figures and a TcWR figures (of around 0.7) why does a twin-engined 29 has lower range than a single-engined F-16.
1) Weight Considerations:
Looking at the weight comparisons, one thing is pretty obvious, both F-16 and the 29s have very comparable weight structure except for the ratio of Internal fuel is to Clean weight (so only with max Internal Fuel) ~ 7-8%.
Ditto with F-18 vs F-16 comparison (haven't got the exact figures, but as Vina-ji says above).
There can be many reasons as to why this gap is there (including other factors like type of material used, weight of subsystems etc- though with around 3% composite usage in F-16 not sure if that's any deciding factor) but a large chunk of this % is also attributable to additional dead weight of plumbing, fuel mgmt and distn across two engines, so on and so forth.
Another important point to consider is the % of engine weight compared to Clean weight (MiG-29 15% compared to 13% of F-16), which means 2% of the 9% is contributed by the addn engine itself.
So, on the ground (i.e. excluding flight characteristics), the proportion of fuel available for propulsion is less in a MiG-29A compared to that of a F-16A - and quite a bit of it is due to the complexities of having to cater for an additional engine etc.
2) Aerodynamic Consideration:
This is where it gets tricky - as Range (and thus Fuel Fraction) is directly proportional to the ratio of Lift Coeff to Drag Coeff (and inversely to wing surface area as well).
Plus SFC as well, but while comparing MiG-29 and F-16 with comparable SFCs, this can be ignored.
IMO (and speculating), any double engined aircraft (with two engines closely fitted) there's an additional penalty to be paid in the lifting-body lift (and drag) compared to a classical single-engined design like a F-16. Having said that, normally lifting-body lift is more dependent on the top surface of the body - which is not effected, except for a proportionate increase in area (than a single-engined plane) due to the constraints of trying to fit an additional engine? But increase in body-area normally increases the body-drag (skin-friction drag) impacting the body lift-to-drag ratio (thus impacting the range and thus the fuel fraction as well).
Experts need to provide some gyan on this.