Stealth and Aero Shaping: F-35 Versus F-22

The F-35 compared with other modern jets.
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stereospace

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Unread post09 Nov 2010, 11:30

I'm surprised by how differently the F-35 is shaped when compared to the F-22. Beyond the obvious difference of comparing a twin engine aircraft to a single engine, there are a number of differences in plan form and shaping.

One would think, having spent many years and tens of thousands of hours during F-22 development evaluating various designs for stealthiness and aerodynamic performance, LM would apply that (highly successful) model to the F-35 more fully.

Plan view comparison:
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Unread post09 Nov 2010, 11:32

Take a look at these features and compare to the F-22 above:

The one difference I can account for is the air inlet change, which is due to the boundary layer diverter on the F-35. The differences in the wing shaping and horizontal stabs throws me. Ditto for the nose proportions and overall nose shape. I can't account for those things at all.
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Last edited by stereospace on 09 Nov 2010, 12:11, edited 1 time in total.
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Unread post09 Nov 2010, 11:43

Also, compare the bottom views of the two aircraft, in particular the F-22's overall flatness versus the shaping on the F-35:
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shep1978

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Unread post09 Nov 2010, 12:40

Interesting topic, from the top looking down on them they look pretty similar apart from as you've pointed out the intakes and a few other areas. The underside is where the biggest difference seem to be but i'd trust LM knows what they're doing enough to make sure its stealthy down there.
You should throw in some frontal, rear and side shots to compare those. I'm pretty sure Paul Bevilaqua (I think thats the right chap) said the F-35 was designed as a mini F-22 or something like that in a video shown on Av Week not to many months ago but I guess they couldn't scale it down perfectly and still meet all the requiments for the three services as that'd be impossible.

(I wonder if that plonker who claimed the Typhoon has better underside values RCS will show up here to teach us a thing or two...)
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Unread post09 Nov 2010, 15:12

The reason for the differences is pretty simple, two planes with two different missions. The F-22 is going to fly around up high at M 1.5 behind enemy lines looking for fighters. The F-35 will either be loitering at a lower speed looking for fighters, or concentrating on finding ground targets.

I wasn't involved in the design of either plane, so I can't give you justifications for everything, but I can make some guesses (and use some of LM's claims).

The F-35's inlet is a simpler cheaper design than the F-22's. Since the 35 isn't expected to fly as fast as the F-22, there’s no reason to have high Mach inlets.

Lowering wing sweep goes along those same lines, the F-35 won’t spend the majority of its time in supersonic flight, so the wing is probably designed to maximize range at subsonic speed (though the maneuverability is probably strong through all speeds). The lower sweep also gives the F-35 a higher effective aspect ratio, which lowers induced drag (will probably lower cruise drag, but may not lower drag when maneuvering) and increases lift performance, this goes double for the F-35C big wing.

The F-35 seems to have a very “plain” wing compared to the 22. The F-22 has obvious washout and the tip trailing edge is clipped. This is probably in use to control the lift distribution and stall characteristics at high angle of attack. For whatever reason, the F-35 didn’t need these. It could be for costs reasons.

As far as stabilizers go, the F-22 probably has bigger verts because it’s a supercruiser. I don’t know why the horizontal stabs are different. I don’t see one as better than the other aerodynamically, but maybe the F-22’s is stealthier, while the F-35’s is cheaper and easier to make.
I have no idea why the underside is so curvy.
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Unread post09 Nov 2010, 15:46

This is the difference between all-aspect stealth and front focused stealth. Also, the benefits of the F-35 are seen vs. legacy aircraft carrying external stores; not vs. the F-22.
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Unread post09 Nov 2010, 15:48

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Signature management features have evolved over the last 30 years as computing power has increased and understanding has improved...
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Unread post09 Nov 2010, 17:03

I really like the blanket assumption that the F-22 is automatically better... because it's the F-22!!!1

Look at the planform alignment- those wingtip notches on the Raptor create another RCS spike (It's for antennae coverage, the YF-22 did not have them). Look at the horizontal stabs- the F-35's trailing edges are simpler, and the joint gap is much better hidden. The F-35 looks more refined from an aerodynamic perspective, the F-22 fuselage is really just a glorified brick with little wing-body blending to speak of.
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Unread post09 Nov 2010, 17:32

Prinz_Eugn wrote:I really like the blanket assumption that the F-22 is automatically better... because it's the F-22!!!1


I'm pretty certain LM or the DOD (or some official body) have actually said the F-22 is stealthier. Edit:

"According to November 2005 reports, the US Air Force states that the F-22 has the lowest RCS of any manned aircraft in the USAF inventory, with a frontal RCS of 0.0001~0.0002 m2, marble sized in frontal aspect. According to these reports, the F-35 is said to have an RCS equal to a metal golf ball, about 0.0015m2, which is about 5 to 10 times greater than the minimal frontal RCS of F/A-22. The F-35 has a lower RCS than the F-117 and is comparable to the B-2, which was half that of the older F-117. Other reports claim that the F-35 is said to have an smaller RCS headon than the F-22, but from all other angles the F-35 RCS is greater. By comparison, the RCS of the Mig-29 is about 5m2."

Source is that Global Security site, I might go try and find the actual reports if i've got the time but I doubt global security would lie in the first place.
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Unread post09 Nov 2010, 18:09

So it's stealthier from the front but also ten times bigger? Open sources aren't going to be that helpful here. The golfball thing was always just a convenient invention for explaining a complicated concept the non-technical people that control the money.

The truth is that both designs are compromised somewhat for various reasons, and were designed to completely different requirements, so "better" is not a relevant term. Your original post might as well be asking why McDonnell-Douglas didn't just make an F-4 with bigger engines for FX (F-15) back in the day, since the F-4 was the best fighter of its time. Different requirements and different technology equal different airplanes.

In fact, we should all be more surprised the F-22 and F-35 look more alike than not.
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Unread post09 Nov 2010, 19:47

I don't understand, why is it so incomprehensible to think that the F-22 has better 'stealth' levels than the F-35?
Sure the F-35 may be newer but perhaps the powers that be didn't want the F-35, which will be sold to foreign nations, to have better stealth than the F-22 which is perfectly understandable.
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Unread post09 Nov 2010, 20:28

shep, the term is 'exportable stealth' often confused to mean that F-35 has different levels of stealth for different countries, when all F-35 variants have the same level of stealthiness for reasons you state.
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Unread post09 Nov 2010, 21:18

Prinz_Eugn wrote:I really like the blanket assumption that the F-22 is automatically better... because it's the F-22!!!1

Look at the planform alignment- those wingtip notches on the Raptor create another RCS spike (It's for antennae coverage, the YF-22 did not have them). Look at the horizontal stabs- the F-35's trailing edges are simpler, and the joint gap is much better hidden. The F-35 looks more refined from an aerodynamic perspective, the F-22 fuselage is really just a glorified brick with little wing-body blending to speak of.


It’s not assumed, it’s been stated publically. I’m fitting what I know to the public info. On the 22’s clipped wings, the YF was a prototype, so expect shortcuts. Just because the YF didn’t have the clips does not make them aerodynamically superior. The YF-15 lacked sawtooth stabilizers, and it was a grave weakness. That said, I’ve confirmed the use for antenna coverage, though they may also be there for aerodynamic reasons.

I can see some of your points on the horizontal stabs, but you comment about the F-22 being a brick is a bit absurd. Care to explain? I don’t really see anything on the F-35 that looks “more refined”.

Prinz_Eugn wrote:So it's stealthier from the front but also ten times bigger? Open sources aren't going to be that helpful here. The golfball thing was always just a convenient invention for explaining a complicated concept the non-technical people that control the money.

The truth is that both designs are compromised somewhat for various reasons, and were designed to completely different requirements

Who says the F-22 is only stealthier up front? The golfball isn’t only what you say, but it’s also a way to tell someone something without actually telling them, so to speak. The F-22 seems to be stealthier, we just don’t know where/when/how.

You are completely right on these designs being made to fit their specifications. On that front, we think the same. But not, a plane may be required to be worse than another, apparently, this is the case with the F-35’s stealth vs the F-22’s.
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Unread post09 Nov 2010, 22:22

It would not be terribly surprising the F-35 has a lower overall RCS, but you also talk about aerodynamics in your post. That is easily addressed by the different regimes in which they fly. Making it more like the F-22 would also make it worse at what it's supposed to do.

One would think, having spent many years and tens of thousands of hours during F-22 development evaluating various designs for stealthiness and aerodynamic performance, LM would apply that (highly successful) model to the F-35 more fully.

Actually, they did, but adapted the basic design to JSF requirements. Look at the X-32 to see what a truly different design would be like.

The F-22 block fuselage was a deliberate design decision early on in the program, mostly for simplicity reasons (cheaper to produce, get to RCS target). Take a gander at the YF-23 or even the PAK FA and see how blended those designs are. Pretty much every non-stealth design in recent memory has had more wing-body blending than the F-22, which should tell you there is a reason for it. . Getting away with more complex fuselage features like that (better airflow at normal flight regime) and still meeting RCS requirements is why I think the F-35 shouldn't be thought of as a half-baked F-22 design.
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Unread post09 Nov 2010, 23:03

Well, in the end, the X-32 wasn't that different

http://en.wikipedia.org/wiki/File:Boein ... tarmac.jpg

I'm still not seeing what's wrong with the F-22's fuselage, which is what I was aksing you about. Where is the F-35 more refined specifically?

Other than a few bumps on the underside, they're pretty similar, ie wide flat lifting bodies. Where they truly differ is wing and control surface design.

On blending, from the top, the YF-23 and PAK look pretty blended, but so does the F-22. From the bottom, you can see where the wings meet the fuselage.

http://upload.wikimedia.org/wikipedia/c ... flight.jpg

http://static.panoramio.com/photos/original/4634883.jpg

http://tinypic.com/view.php?pic=241jl11&s=5
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