F-35A vs B vs C

The F-35 compared with other modern jets.
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zero-one

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Unread post21 Oct 2015, 13:51

I Tried searching the forum, but it looks like there hasn't been any comprehensive discussion between all 3 variants.
If there was one already, then I apologize for not trying hard enough.

Question is, officially all 3 variants have very similar flight characteristics, however I find this quite hard to believe for the following reasons:

1.) The A model is 3,000 - 5,000 pounds lighter than the B and C model respectively, it also has the least amount of drag due to the missing "hump" behind the cockpit like the B model and smaller wings and tail of the C model.

2.) The B model's center of gravity will be different as it carries a second engine and a drive shaft in the fore section, this is replaced by a dynamic load (fuel) on the A and C models which can be burned down. Unless the fuel tank behind the cockpit is used last on the A & C, the B model will always be Fore heavy compared to the other 2.

3.) The C's big wings and additional control surfaces will surely affect maneuvering performance for the better, Test pilots such as Billy Flynn have pointed out that the C model is the best turning variant of all F-35's and may also be the best high AOA performer. In the infamous F-16 vs F-35 document, it was pointed out that at high AOA, AF-02 lost critical energy due to "insufficient thrust and wing area". I wonder if CF-02 (or whatever the flight envelope expansion test bed is) will experience a similar result when tested at high AOA.

It also occurred to me that all 3 may actually have very different flight characteristics when pushed to absolute limits but are programmed to behave similarly via their FCS software.

(i.e, the C model can sustain a 6G turn at X speed and X altitude carrying X weight, but since the A model can only sustain 5.5Gs, the C model's FCS was limited to 5.8 in order to keep it close to A model's performance)
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weasel1962

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Unread post21 Oct 2015, 15:26

There has been more than enough discussions in this forum to address all of the issues many times over. Look harder. Eg view on weight in 1) is incorrect unless the f-35 takes off without fuel. At full fuel, the B is the lightest in the air assuming same weapons weights as it carries 4-5k lbs less fuel. The difference is however negligible.
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zero-one

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Unread post21 Oct 2015, 15:40

weasel1962 wrote:There has been more than enough discussions in this forum to address all of the issues many times over. Look harder. Eg view on weight in 1) is incorrect unless the f-35 takes off without fuel. At full fuel, the B is the lightest in the air assuming same weapons weights as it carries 4-5k lbs less fuel. The difference is however negligible.


but the Center of Gravity would be somehow be affected as the B model is "permanently" front heavy, also, the B model's mediocre 7G limit may also affect it's flight envelope performance.

Were there any discussions on what characteristics are at tactical maneuvering weights
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cantaz

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Unread post21 Oct 2015, 16:39

zero-one wrote:but the Center of Gravity would be somehow be affected as the B model is "permanently" front heavy


That's not necessarily true.

One would require information on F-35's fuel system and fuel ballasting practices.

Let's say the combat maneuvering window for fuel state is between 80-20% of max internal fuel. In other words, we don't care about how the F-35 handles until it's burnt off 20% of its max internal fuel and we don't care about how it handles when it's only got 20% fuel remaining.

The questions are then:

1. What is optimal CG state for the F-35A and B when it comes to combat maneuvering?

2. Does the B have enough volume in its rearward tanks to offset the weight of the liftfan? If so, does it have enough fuel available to do this at all times between 80-20% fuel? If so, is this compatible with the sequence the tanks are emptied?

3. Does the forward central fuselage of the A model have enough fuel at 80% fuel state to reproduce the same or similar CG as the B with its liftfan? If yes, is this reflective of actual ballasting practice for the F-35A? If true, at what point between 80-20% internal fuel state does the F-35A no longer have enough fuel in the forward central tank to share CG characteristics with the B model?
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XanderCrews

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Unread post21 Oct 2015, 16:42

but the Center of Gravity would be somehow be affected as the B model is "permanently" front heavy,


Dude, Seriously?

As for G I would bet the B pulls two less than the A and .5 less than the C. ( just a guess)
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Unread post21 Oct 2015, 16:50

zero-one wrote:
3.) The C's big wings and additional control surfaces will surely affect maneuvering performance for the better, Test pilots such as Billy Flynn have pointed out that the C model is the best turning variant of all F-35's and may also be the best high AOA performer. In the infamous F-16 vs F-35 document, it was pointed out that at high AOA, AF-02 lost critical energy due to "insufficient thrust and wing area". I wonder if CF-02 (or whatever the flight envelope expansion test bed is) will experience a similar result when tested at high AOA.



been said before - if you have exactly the same engine in the CF but are 5000 lbs heavier (thus worse T/W) and have bigger wings (more drag) then energy retention and recovery under similar conditions is going to be worse. However it could still have better ITR and better AOA handling than the A in some cases as the pilot states.
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Unread post21 Oct 2015, 16:59

but the Center of Gravity would be somehow be affected as the B model is "permanently" front heavy, also, the B model's mediocre 7G limit may also affect it's flight envelope performance.


Front heavy? I think your neglecting the fact that the 3BSM itself weighs a crap ton more than the standard nozzle and has a very aft moment arm.
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Unread post21 Oct 2015, 17:15

XanderCrews wrote:
Dude, Seriously?

As for G I would bet the B pulls two less than the A and .5 less than the C. ( just a guess)


Well the question is, would the B model always be 2Gs below the A and .5 below the C in every part of the envelope, or does the difference only show itself once both the A and C models reach their parts of the envelope where they can go past 7Gs?
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Unread post21 Oct 2015, 17:19

basher54321 wrote:


been said before - if you have exactly the same engine in the CF but are 5000 lbs heavier (thus worse T/W) and have bigger wings (more drag) then energy retention and recovery under similar conditions is going to be worse. However it could still have better ITR and better AOA handling than the A in some cases as the pilot states.


I get that, but what I was looking for was a more in depth analysis into the characteristics of each variant.

For example, the F-22 and F-35A are both rated at 9Gs, but we all know that the Raptor's G envelopes are far far larger than the F-35As. The Raptor can still pull above 5Gs at 30,000 feet around supersonic, not sure the F-35A can do the same.

With the F-35C's larger wing area, are there parts of the envelope where it can reach higher Gs than the A model?
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Unread post21 Oct 2015, 17:21

According to 'LM Fast Facts' since it began the A has a 9G limit; B has a 7G limit whilst the C has a 7.5G limit.
https://www.f35.com/assets/uploads/down ... 3q2015.pdf

F-35 Stores Testing: An Observation
21 Feb 2012 SMSgt Mac

“...Everyone doesn't have access to AIAA archives, but everyone on the web should know what a frickin' search engine looks like. Bevilaqua also notes in more than one of his AIAA papers that the mid-mission CG of the B model is effectively the same as the A model. This is a critical point because one seeks to design the aircraft to have the best mass properties for maneuvering at mid-mission weights because that is the time when a combat aircraft needs its best maneuverability....”

Source: http://elementsofpower.blogspot.com.au/ ... ation.html

At 0330 my time I'm not going to read this four part series again for the good bits - becuz it is all good bits:

http://elementsofpower.blogspot.com.au/ ... -spec.html
&
http://elementsofpower.blogspot.com.au/ ... -spec.html
&
http://elementsofpower.blogspot.com/201 ... ec_26.html
&
http://elementsofpower.blogspot.com.au/ ... -spec.html
RAN FAA A4G Skyhawk 1970s: https://www.faaaa.asn.au/spazsinbad-a4g/ AND https://www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/
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Unread post21 Oct 2015, 17:36

zero-one wrote:
With the F-35C's larger wing area, are there parts of the envelope where it can reach higher Gs than the A model?

In short, yes. Any time the A is lift limited to less G than the C is structurally rated for then the C will have better turning both ITR and STR.
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Unread post21 Oct 2015, 17:38

i think the relationship between the A and the C version would be similar to the relationship between F-16 and F-18?
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Unread post22 Oct 2015, 12:24

sprstdlyscottsmn wrote:In short, yes. Any time the A is lift limited to less G than the C is structurally rated for then the C will have better turning both ITR and STR.


Thank you Sprst, can you make a comprehensive comparison between the 3 like what you did with the F-35A, F-16C, F-16I and F-15E before? :mrgreen:
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Unread post22 Oct 2015, 13:45

zero-one wrote:
sprstdlyscottsmn wrote:In short, yes. Any time the A is lift limited to less G than the C is structurally rated for then the C will have better turning both ITR and STR.


Thank you Sprst, can you make a comprehensive comparison between the 3 like what you did with the F-35A, F-16C, F-16I and F-15E before? :mrgreen:

I am working on just such a comparison. My models get the known performance stats to within +-1% of the actual values but I did have to make one huge assumption on the Acceleration spec, the loadout. I was left to assume that it was done with 60% fuel and two AIM-120 (same as the sustained G spec). I am trying to find a way to properly model a Rutowski (sp?) profile to see it's effect on the acceleration spec.

In a separate thought experiment I also did a high level analysis of F-15C, F-16C. F/A-18E, and F-35A on a take-off to landing bomber-intercept mission to see how fast they can climb, accelerate to speed, and maintain their max speed before having to fire 6-8 missiles and turn around to cruise home. Let me say now that While the F-35A has a lower dash speed than the F-15 or F-16 it can hold it for a LONG time and at higher altitudes.
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Unread post22 Oct 2015, 14:46

sprstdlyscottsmn wrote:In short, yes. Any time the A is lift limited to less G than the C is structurally rated for then the C will have better turning both ITR and STR.


Quick question, if the A and C models are traveling at the exact same speed and altitude, pulling the exact same G, will there be any difference in the turn rate?

Or will the F-35C's larger wing cause it to pull the turn at a higher AOA thus having a higher turn rate?
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