F-35A vs B vs C

[steve2267]

2000 lb bomb was yet another navy requirement. I've taken this "extreme" position before, (but why not once more in thread that compares all 3 varaints??) the F-35C NOT the F-35B as so many claim is the problem child. Its the most different the Navy desire to have 2K bombs lead to a weight increase that then had to be tamped down with the SWAT effort which lead to huge delays. Irony being what is is the service that was the biggest pain in the butt with the F-35 program is also the service that has the Super Hornet and seems very happy with them (that will change of course)

Ironic then... that without that (Navy) 2000lb bomb requirement, the fuselage might possibly be more streamlined / narrower, enough that LM might have been able to give the Air Force their 8 seconds back on the transonic acceleration KPP. Also, with less weight (e.g. less structure etc), LM might have been able to meet the Air Force & USMC KPP for sustained turn rate. Would be an interesting question for someone on the F-16 design team if LM could have met those KPPs (transonic accel & sustained turn gee) were it not for the 2000lb bomb KPP. If so, that would be an interesting demonstration of the cascading effect requirements can have on performance metrics.

[steve2267]

To be fair, the "requirement" is not what drove up the weight... It was LM's FK up in calculating the weight of "empty space".

This is why all 3 variants were overweight.

I have not heard this before. Spudman can you elaborate or provide some references, or do I need to go fastidiously search the SWAT thread?

Your statement is somewhat surprising to me as LM had built the F-117 and the F-22 both which have a fair bit of "empty space."

[SpudmanWP]

Ironic then... that without that (Navy) 2000lb bomb requirement, the fuselage might possibly be more streamlined / narrower, enough that LM might have been able to give the Air Force their 8 seconds back on the transonic acceleration KPP.

I doubt it since the difference in the diameter from a 1k JDAM and a 2k JDAM is only a combined 8 inches....

[quicksilver]

Anything that would reduce the frontal cross section of the jet would likely be of benefit wrt trans’c accel. Run a plumb line visually from the lower outside corner of the intake to the tail boom and note where 8 inches might matter on the lower fuselage contour along the weapon bays. Perhaps some reduction in trim drag...

Look at a head-on pic of the jet and note how the weapons bays extend like sponsons below the fuselage.

The flip side of ‘weapons bay too big’ is ‘greater weapons flexibility’ and ‘room for growth’ that would eventually need to be accommodated, preferably w/o changing the outer mold line of the jet.

[quicksilver]

To be fair, the "requirement" is not what drove up the weight... It was LM's FK up in calculating the weight of "empty space".

This is why all 3 variants were overweight.

I have not heard this before. Spudman can you elaborate or provide some references, or do I need to go fastidiously search the SWAT thread?

Your statement is somewhat surprising to me as LM had built the F-117 and the F-22 both which have a fair bit of "empty space."

To my understanding, Spud is right, although I might characterize it as a surprise to LM when it happened. The parametrics they used in preliminary design proved to be flawed when they got around to use of finite element models later in the design process. Aiui there also were some features of the design that were intended to enable ease of manufacture and the anticipated high production rates that were problematic as well.

[quicksilver]

Here ya go Stevie --

https://www.airspacemag.com/military-av ... -13117183/

[steve2267]

Here ya go Stevie --

Thanks quick.

Weight Watchers How a team of engineers and a crash diet saved the Joint Strike Fighter.

By Joe Pappalardo Air & Space Magazine November 2006

<snip>
...
The program’s initial focus on affordability also added weight. Off-the-shelf parts cost less but weigh more because they are not optimized for a fighter. To get bulk quantities of replacement parts for a lower cost might require using a heavier component. It soon became obvious that the plan to use common parts among the variants—a strategy that would lower costs and streamline future maintenance demands—was also bulking up the F-35.

Initial estimates of how much a part will weigh are based on its volume and material. But they are just estimates; the actual weight is another matter. A heftier hose, a wider screw, a thicker panel—in dribs and drabs, the weight steadily increases.

Even in a world of precision design tools, weight estimates still depend on data from previous aircraft. That turned out to be a problem as the crowded interior and the demands of the design translated into poundage. “Legacy estimating techniques just don’t work with this family of airplanes,” says R.J. Williams, Lockheed’s vice president of F-35 Air Vehicle Development.

Art Sheridan says that cost, not weight, was the most important measurement during the early history of the program. “The focus was very much on affordability at the time,” he says. “People realized there was a penalty to be paid, and that was included in the weight estimates. It was higher than we thought.”

No matter the reason, when weight became the enemy, the SWAT team concentrated its effort on reducing it, as well as reducing the bureaucratic hoop-jumping that can slow a redesign. “The number one commitment was to remove obstacles and make quick changes,” Sheridan says.

...
<snip>

https://www.airspacemag.com/military-aviation/weight-watchers-13117183/?all

I had previously read that article, but did not recall the quote about legacy estimating techniques not working with the F-35 program. One large example of that affordability focus is seen in the initial concept of the quick-mate joints which added a lot of weight. IMO, the reason they did not work has more to do with the initial focus on affordability with which I doubt the F-117 and F-22 had to contend; they certainly did not use quick-mate joints. The second aspect of the F-35 program for which the F-117 and F-22 did not have to account was a family of aircraft, initially designed to be as common as possible. The concept of cousin parts grew out of SWAT. The third difference between the F-35 and the earlier Nighthawk and Raptor was the fact that one F-35 was a STOVL aircraft and had a lift fan. However, I did not observe in this article that the STOVL design or its "empty spaces" caused the problems as much as the initial emphasis on affordability which resulted in using non-weight-optimized COTS parts and quick-mate joints.

Thanks again for the link. Twas good to read it once again.

[spazsinbad]

Same URL as above but ALL pages for ease of reading/making PDF pages - just FOURTEEN YEARS ago now. YIKES.

Weight Watchers How a team of engineers and a crash diet saved the Joint Strike Fighter.
Nov 2006 Joe Pappalardo Air & Space Magazine

"...On the F-35 STOVL variant, the F-35B, the weapons bays must share internal space with an enormous lift-fan engine, which enables the vehicle to hover and land vertically, and with the engine’s ducts. The wide cavities demanded for these components contribute to weight gain because they compromise the best layout for the aircraft’s load-bearing structure. Creating an airplane around these systems is akin to designing a human skeleton after the organs have been installed. It forced the airframe team to adopt a heavier design….

...By the end of February 2006, Lockheed Martin had paid out more than $1.2 million to employees for ideas....

...In October 2004, the Defense Acquisition Board signed off on more than 500 recommendations, officially making the STOVL weight loss attack team a success. In eight months, the Lockheed engineers cut a total of 2,700 pounds from the F-35B. The effort also trimmed 1,300 pounds from the other variants. Comfortable with that legacy, SWAT faded, with accolades, into company history, but an estimated 20 ideas a week still turn up in the Weight Improvement Program office...."

Source: https://www.airspacemag.com/military-av ... 17183/?all

[zero-one]

Ironic then... that without that (Navy) 2000lb bomb requirement, the fuselage might possibly be more streamlined / narrower, enough that LM might have been able to give the Air Force their 8 seconds back on the transonic acceleration KPP. Also, with less weight (e.g. less structure etc), LM might have been able to meet the Air Force & USMC KPP for sustained turn rate. Would be an interesting question for someone on the F-16 design team if LM could have met those KPPs (transonic accel & sustained turn gee) were it not for the 2000lb bomb KPP. If so, that would be an interesting demonstration of the cascading effect requirements can have on performance metrics.

They still might. Will Block 4 have any significant weight gains? The Thrust increase will be 10% if I remember correctly, and if the weight stays the same or maybe even drops, we could meet the original performance KPPs.

[spazsinbad]

Can you itemize these 'original performance KPPs' & how there are now 'different KPPs'? Why were original KPPs changed?

[zero-one]

Now that you say it. I'm not sure they were changed in the first place. If I remember the statement in that infamous 2013 spec change. It went along the lines of, the JPO expressed interest to reduce the transonic acceleration and sustained G rates of all 3 models.

Key word was "expressed interest". I'm not sure if those interest were actually implemented

[spazsinbad]

Why not find out? Good to be accurate with your words/ideas - rather than inaccurate.

[zero-one]

Here we go:
viewtopic.php?t=21926

The program announced an intention to change
performance specifications for the F-35A, reducing turn
performance from 5.3 to 4.6 sustained g’s and extending
the time for acceleration from 0.8 Mach to 1.2 Mach by
8 seconds. These changes were due to the results of air
vehicle performance and flying qualities evaluations.

Were those intentions actually implemented?

[spazsinbad]

Can't you find out? Go to thread - search that thread or another or the INTERNET using 'intention' for example....

[zero-one]

If you go to the thread you'll see that the general consensus is. Without a specified weight, altitude and speed, the parameters cannot be used to draw a conclusion. No mention if the spec change was actually implemented in the first place.

Buddy, if you already have the answer then please post it, if not, then what you're doing isn't helping the discussion which was actually raised by Steve in the first place not me. I'm just trying to help.