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

[energo]

Do the bumps on the F-35s underside provide hot spots? Consider the following speculation:

* would it be significant? (technical analysis)
* under which conditions? (operational analysis)
* RAM/RAS; would likely be applied to key areas to minimize unwanted effects
* it's speculated that electrical wires are embedded in the fuselage to absorb or scatter e-m and traveling waves; note absence of traditional saw-tooth features around around access panels, wing/tail trailing edges etc. -- could it be applied to "trouble spots" as well?
* what about the F-35's record breaking skin manufactoring tolerances?

B. Bolsøy
Oslo

[linkomart]

It's the area rule

http://en.wikipedia.org/wiki/Area_rule

http://www.aerospaceweb.org/question/ae ... f102-1.jpg

To minimize the supersonic drag coefficient, the change in area along the length of the aircraft should be smooth (example, having a tubular fuselage with a pair of wings jutting out of the sides is much worse than having a fuselage that narrows along the chord of the wing).

There are a number of ways to control the area distribution. The F-102 used the method I described above, but the inverse it also true; instead of narrowing some section of the plane, you can make a different section wider. The bumps on the underside of the F-35 might make the area distribution more like the "ideal" curve in the F-102 photo (though the ideal shape changes slightly with different Mach numbers). I'm pretty sure that the bumps on the underside of the F-22's wings are there for supersonic drag reduction.

I'd say that slenderness is more important than area ruling, ie it is more important to minimize the max cross section area for low supersonic drag. Also, the area distribution varies with different mach numbers, so it is really hard to get a good area distribution att all speeds, usualy the area distribution looks good at some Mach number, and then you just call it the desired design speed..

[linkomart]

Do the bumps on the F-35s underside provide hot spots? Consider the following speculation:

* would it be significant? (technical analysis)
* under which conditions? (operational analysis)
* RAM/RAS; would likely be applied to key areas to minimize unwanted effects
* it's speculated that electrical wires are embedded in the fuselage to absorb or scatter e-m and traveling waves; note absence of traditional saw-tooth features around around access panels, wing/tail trailing edges etc. -- could it be applied to "trouble spots" as well?
* what about the F-35's record breaking skin manufactoring tolerances?

B. Bolsøy
Oslo

Knowing that LM knows more about LO than me, I'd still say that the RCS contribution from the bumps are significant, especially since they are so small (Wich means they scatter the waves). I guess that the operational analysis made by LM shows that with proper tactics, the most important is the frontal RCS, and that is unaffected by the bumps.
I'll leave the RAM and electrical wires for now, but I'll say that the tolerances are very important for RCS, in my humble opinion.

[exorcet]

I'd say that slenderness is more important than area ruling, ie it is more important to minimize the max cross section area for low supersonic drag. Also, the area distribution varies with different mach numbers, so it is really hard to get a good area distribution att all speeds, usualy the area distribution looks good at some Mach number, and then you just call it the desired design speed..

You can only make your plane so small while carrying X missiles and Y pounds of fuel. A brick is less draggy than a fighter, but it's not going to meet any payload requirements. The F-35 is probably as small as it can get, or close to it. So instead of shrinking it down, they need to change the shape.

Remember, drag is CD*A, there's a component from size and one from shape. You want to minimize both always.

[linkomart]

A brick is less draggy than a fighter, but it's not going to meet any payload requirements. The F-35 is probably as small as it can get, or close to it. So instead of shrinking it down, they need to change the shape.

Don't know about your bricks, but the ones my house is made from is definitely more draggy than any airplane
You're probably right that the F-35 is as small as it can get, I can not comment on the shape of the F-35, it probably looks that way for good reasons (to fulfill the requirements), but it does not look slender to me.

Remember, drag is CD*A, there's a component from size and one from shape. You want to minimize both always.

I'm not an aerodynamics, bu as far as I know drag is devided in to:
*friction drag, depending on reynolds number, if it is laminar or turbulent flow and the wetted area
*induced drag wich is dependent on the lift generated by the object
*shape drag wich is drag depending on the shape of the object
and for supersonic objects there is
*wave drag wich depends on the area distribution trough the mach cone, Mach number and the slenderness.

I agree that minimize drag is always a priority, at least if you want to have a high performance airplane.
My five cent.

[exorcet]

Well, actually a brick should be less draggy than a fighter, as in less drag force generated. It's because it's so small. In terms of CD, a brick is far, far worse, which is what you are probably referring to.

You're correct on drag classifications, but all of that is usually condensed into a CD (you can approximate this as constant over a small range of conditions).

[linkomart]

Well, actually a brick should be less draggy than a fighter, as in less drag force generated. It's because it's so small. In terms of CD, a brick is far, far worse, which is what you are probably referring to.

Ah, you managed to fool me on that one..

You're correct on drag classifications, but all of that is usually condensed into a CD (you can approximate this as constant over a small range of conditions).

Well, I'm mostly interested in the whole envelope so I need to divide the drag, a single Cd don't work. (Actually, the computer does everything for me, but still...)

[hornetfinn]

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.

It has been almost eight years since this information came about and I think it's time to revisit it. I think that by November 2005 there had been no real RCS tests done with F-35 and the metal golf ball reference was only speculative or maybe apprioximate RCS threshold size. F-22 RCS was definitely well known by then by the USAF but F-35 RCS was not. First production F-35 flew about year later than this report and a lot of things have changed since then.

I think there has been statements that F-35 has exceeded most expectations, including RCS: I think it's very probable that F-35 RCS is not significantly larger than in F-22. Of course RCS is very complex issue overall and there might well be certain areas (radar band and viewing angle) where either one is superior to the other. I see F-22 having certain advantages when it comes to stealth (no cost compromises, bigger jet with comparatively smaller internal fuel and weapons volume) while F-35 certainly has its own advantages (later design, newer materials, lessons learned from F-22).

[Corsair1963]

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.

It has been almost eight years since this information came about and I think it's time to revisit it. I think that by November 2005 there had been no real RCS tests done with F-35 and the metal golf ball reference was only speculative or maybe apprioximate RCS threshold size. F-22 RCS was definitely well known by then by the USAF but F-35 RCS was not. First production F-35 flew about year later than this report and a lot of things have changed since then.

I think there has been statements that F-35 has exceeded most expectations, including RCS: I think it's very probable that F-35 RCS is not significantly larger than in F-22. Of course RCS is very complex issue overall and there might well be certain areas (radar band and viewing angle) where either one is superior to the other. I see F-22 having certain advantages when it comes to stealth (no cost compromises, bigger jet with comparatively smaller internal fuel and weapons volume) while F-35 certainly has its own advantages (later design, newer materials, lessons learned from F-22).

Well, said and I am sure one day we will see F-35's flying with the Finnish Air Force!

[hornetfinn]

Well, said and I am sure one day we will see F-35's flying with the Finnish Air Force!

I'm sure too, because Finnish Air Force is planning to replace the our C/D Hornets in 2025-2030 time frame after 30 years of service. At that time there are not many worthwhile alternatives as Typhoon, Rafale and Super Hornets are on the very end of their service lives. Chinese or Russian aircraft are a very remote possibility and JAS Gripen NG is not very competitive with F-35.

[munny]

Perhaps these can help with understanding side aspect stealth a little better.



RCS polar charts of stealth features of the 3 aircraft shown. As you can see, due to the more shallow surface angles of the F-22's side panels, the main RCS lobe is angled further downward than the F-35 and a LOT further down than some features on the PAK FA. You can see the cross sections of the F-35 airframe were quite lumpy and bumpy, but because of the consistent adherence to the 65 degree surface angle rule, the lumps have little effect on RCS at the angles that matter.

The T-50 on the other hand is a god awful, horrible mess. Its has roughly 5 separate main lobes on it bottom hemisphere which Carlo Kopp acknowledges in an RCS test he did as well. Keep in mind, the component models used for the above polar charts were all to scale. The scale on the rear chart for the PAK FA is 10dBSM different to the others due to the massively higher RCS. A little issue with APA's analysis (complete garbage actually) is that they say that the T-50 profile may be "preferable" to the f-35's.

The impact of those main lobe angles is as follows. When flying past ground or low flying radar at each aircraft's typical operating altitude, this is how exposed they are within a 10-15 degree sector perpendicular to their flight path.

From left to right:

F-22 at 55kft
J-20 or F-35 at 30kft
J-20 or F-35 at 55kft
T-50 at 55kft


This image is not as to scale as the others below.

If you look top down on the aircraft, this 15 degrees of bad beam RCS (in straight and level flight) exposes the aircraft like so. A bad result here equates directly to being exposed to radar for longer periods of time = able to be engaged by networked interceptors from a longer range.

Top to bottom:

F-22 at 55kft
J-20 or F-35 at 30kft
J-20 or F-35 at 55kft
T-50 at 55kft



Something interesting I just worked out. I did a comparison of the T-50 vs the F-35 in a networked environment where any radar can guide any weapon. A couple stats based on the depression angle of their high RCS lobes being illuminated by ground radar from maximum range as they fly past.

F-35 at 35kft: -25 degree lobe extends to 22km and can be tracked for a maximum of 6km of its flight ( 25secs at M0.8 ). Can be engaged by M4.0 missiles (9sec accel time) within 23km of its location at the end of the 25secs.

T-50 at 55kft: -7 degree lobe extends to 136km and can be track for a maximum of 35km of its flight ( 152secs at M0.8 ). Can be engaged by M4.0 missiles (9sec accel time) within 164km of its location at the end of the 152secs.

F-35 and PAK FA at M0.8 Passing radars at the widest part of their worst side aspect. The circles roughly show the range at which the aircraft can be engaged from by networked launchers.



Seems it doesn't pay to cheapen up on your stealth in a networked environment and this is why conventional fighters are soon to be 100% obsolete against any medium tier IADS.

Carlo Kopp
The F-35 JSF exhibits similar, but in some respects more severe beam aspect specular RCS behaviour than the T-50. This is a direct consequence of the use of multiple complex double curvature convex and concave shaping features in its lower fuselage design, and lower wing root area, and a much shorter fuselage. The ventral shaping features were introduced in the SDD aircraft and were not part of the X-35 demonstrator design. Another unfortunate feature of F-35 shaping is the depression angle of the slab sides of the engine inlets, which is shallower than the F-22 and J-20 designs, and similar to the T-50, as a result of which the associated mainlobe peaks at a lesser depression angle, in turn degrading performance against long range surface based threats.

Tsk tsk Mr Kopp, so many mistakes in one analysis.
1. The F-35 side features are nowhere near as impacting as the T-50's as shown above.
2. The J-20 has the same angles as the F-35.
3. The double curves you mention have very little effect on specular return from the side aspect. All aircraft are radar beacons when they bank.
4. The T-50 has numerous surfaces a lot more vertical than the F-35's including some corner reflectors. Factored with the differences in operating altitude, it is FAR less exposed.

Except for the sweep angle of the wings, there's no reason the F-35 can't have a similar or better frontal RCS than the F-22 as far as shaping.

[SpudmanWP]

Dear Lord....

Are those simplistic shapes on the bottom of the image what was used for the RCS measurement?

[sferrin]

Dear Lord....

Are those simplistic shapes on the bottom of the image what was used for the RCS measurement?

Well it is Carlo Kopp we're talking about.

[SpudmanWP]

Is it just me, or does it look like Carlo has a boob fetish (lowest series of three images, especially the center one)?

[munny]

Dear Lord....

Are those simplistic shapes on the bottom of the image what was used for the RCS measurement?

No they are mine in the absence of model design ability. Kopp has a report on the PAK FA on his web site which pretty much shows the same results but they used a proffessional built T-50 model.

The angles of the surface were taken from numerous reference images. The shapes give an idea of where the main specular reflections are going to be.

....and what's wrong with B(o)(o)bies?