Two vertical stabilizers on the F-22 Raptor angled

[Cyrus]

I was under the impression that the SR-71 ended up having a huge radar signature due to its exhaust plume.

http://www.aircraftguru.com/aircraft/aircraft-information.php?craftid=80

Several webpages make reference to a mysterious "cesium-based compound A-50" that was used to help mask the exhaust, but I'm not sure that claim is made by reputable sources with references.

Even if there was a compound called A-50 used in the fuel to help mask the exhaust signature, it obviously didn't work very well.

I stand re-corrected (back to my original statement)!

Either way, this is a good discussion with interesting links!

My understanding from the book "Skunk Works" is that they tried to design ad-hoc stealth features into the airplane, but this was years before they had any methodology (mathematics they got from a Russian paper on radiowaves) in the early to mid 80s.

Sure, they tried to make the blackbird stealthy, but that does not mean it was. The book "Skunk Works" also glosses over the big problems they had with the Blackbird engines going through violent restarts.

BTW, those chins were in the way for a sensor that was being developed for the airplane, which is why one early variant had them missing near the nose. The lack of chins caused problem with the stability and so they had to add big fins at the bottom rear of the aircraft. They also developed and fired the pheonix missile specifically for the Blackbird.

but it was eventually used on the F-14 instead.

The book "Lockheed SR-71 Operations in the far East" has many good stories and pictures. There are pictures captured of rusisan missiles being fired at the SR-71 but not being able to reach its altitude. You can see the missile streaks in the picture, which is why I never bought the "SR-71 is stealth" argument put forth in this thread.

Interestingly enough, it was also used exensitvely in the middle east (over Iran).

 

[mgb_phys]

My understanding from the book "Skunk Works" is that they tried to design ad-hoc stealth features into the airplane, but this was years before they had any methodology (mathematics they got from a Russian paper on radiowaves) in the early to mid 80s.

Ufimtsev paper described the effects from the edges of flat surfaces, but IIRC one of the surprising results was that the return from a flat surface was independant of the size.
Other techniques like the non-constant radius curves and avoiding corner cubes (tails and engine inlets) were already obvious in WWII - they are basically optics.

 

[DaveC426913]

...and avoiding corner cubes ...

In exactly the way that boats don't.

http://store.crowleys.com/catalog/DI152.jpg

 

[FredGarvin]

Ufimtsev paper described the effects from the edges of flat surfaces, but IIRC one of the surprising results was that the return from a flat surface was independant of the size.
Other techniques like the non-constant radius curves and avoiding corner cubes (tails and engine inlets) were already obvious in WWII - they are basically optics.

They had the methodology but did not have the computing power to put it to real use at the time.

 

[Cyrus]

Continuous curvature is a pretty well-established principle in stealth, since it softens the radar signature. The father of a good friend of mine was involved in the design and development of the Blackbird, and the chines and constantly-changing curvature of the fuselage were designed to help reduce the radar signature. Any assertion that the Blackbird was not designed with radar stealth in mind should be taken with a grain of salt. Its impressive speed could help get it out of some trouble, but without stealth, its penetration into target areas before detection would have been restricted, reducing its effectiveness.

There was something that just didn't sit right with your post, and now I remember why. The fact that the blackbird was un-stealthly, along with its Ma 3+ speeds, was used to perform fake 'penetrations' into Russian airspace. The radar operators would see the blackbird coming in at high altitude and high speed and turn on all their radars to track it better. The blackbird had a sensor package that would then record the incoming radar signals so the engineers could evaluate what types of radar the Russians were using. Your last comment is inaccurate.

 

[shoehorn]

Stealth doesn't make the plane invisible (the loss of B2s in serbia proved that)

No B-2s were downed in Serbia, or indeed anywhere else. One of them crashed on takeoff in Guam a few years back, but none have ever been shot down.

I went to a lecture by the British radar expert lockheed brought into perform the tests described in skunk works (don't have my copy here - can't remember his name).

They found a number of planes that were surprisingly stealthy, The Avro Vulcan was among the best - despite having a delta wing about the size of Iowa it has lots of non-constant radius curves which scatter the radar return, very deep oval engine inlets and a defrosting heating coating which turned out to be just the correct wavelength to absorb radar of the time.

Heh. That the Vulcan was stealthy is one of those odd myths that have never gone away in spite of being contradicted by almost everyone who was ever involved with the aircraft. I was sitting in the control tower last September for an airshow to celebrate 100 years of Marshall's here in Cambridge. One of the highlights of the show was a refurbed Avro Vulcan that flew an air display. You wouldn't believe the radar returns from it: we compared signal intensity from a variety of aspects at 10, 20, and 50 miles and they were an order of magnitude larger than anything else that flew that day. Even in clean configuration with gear up the engine inlets and tail bounce a large amount of radar energy back to ground.

The Vulcan was a lot of things, but stealthy it ain't.

 

[shoehorn]

If you read the history about only one airplane, read about the blackbird. It's an amazing machine.

The U-2 is a far more interesting and impressive aircraft, imo, particularly in terms of its contribution to science over its lifespan. For instance, it's doubtful we'd have gone to the trouble of launching COBE and collecting such detailed data on the CMB were it not for the fact that NASA allowed Alvarez, Muller, and Smoot to put a differential radiometer on a U-2 all those years ago.

 

[turbo]

There was something that just didn't sit right with your post, and now I remember why. The fact that the blackbird was un-stealthly, along with its Ma 3+ speeds, was used to perform fake 'penetrations' into Russian airspace. The radar operators would see the blackbird coming in at high altitude and high speed and turn on all their radars to track it better. The blackbird had a sensor package that would then record the incoming radar signals so the engineers could evaluate what types of radar the Russians were using. Your last comment is inaccurate.

How many Blackbirds were shot down by the Russians? Give us a ball-park number.

At the same time, you might want to give us a ball-park number of how many Russian high-altitude aircraft invaded US air-space during the same period. An order of magnitude or two error is OK, if you can cite some reliable sources.

 

[Cyrus]

How many Blackbirds were shot down by the Russians? Give us a ball-park number.

Zero, but what does this have to do with what I said?

At the same time, you might want to give us a ball-park number of how many Russian high-altitude aircraft invaded US air-space during the same period. An order of magnitude or two error is OK, if you can cite some reliable sources.

Again, why is this relevant? The answer, is at least one. The Russian built a spy space station that flew over the US every time it orbited over and took photos using a high resolution camera. This was linked by (Ivan?) in the GD thread a while back to a PBS special (maybe it was BBC). It was really interesting, I wish I kept the link. If you search you'll find it. If you have any interesting links to contribute, please share!

 

[mgb_phys]

No B-2s were downed in Serbia, or indeed anywhere else. One of them crashed on takeoff in Guam a few years back, but none have ever been shot down.

Sorry, It was the other one (F117?), the stealth bomber we are calling a fighter cos bombers aren't as cool.

The Vulcan was a lot of things, but stealthy it ain't.

Great list of Vulcan stories here.
http://www.pprune.org/aviation-history-nostalgia/111797-did-you-fly-vulcan-merged.html

Including the optomistic plans to issue the crew with walking boots because after nuking Moscow they wouldn't have enough fuel to make it out of Russia and were supposed to crashland and hike to Turkey.

 

[Cyrus]

Sorry, It was the other one (F117?), the stealth bomber we are calling a fighter cos bombers aren't as cool.


Great list of Vulcan stories here.
http://www.pprune.org/aviation-history-nostalgia/111797-did-you-fly-vulcan-merged.html

Including the optomistic plans to issue the crew with walking boots because after nuking Moscow they wouldn't have enough fuel to make it out of Russia and were supposed to crashland and hike to Turkey.

You're right, I didn't realize one had been shot down before!

http://www.serbnews.com/f117/f117_2.jpg​

[/URL]

 

[russ_watters]

I read somewhere that the SR-71 used some "stealthy" design features, but nevertheless had a massive radar signature.

The plane itself did not: it had a rediculously small radar cross signature given its size. In Skunk Works, they said it looked like a Piper Cub on radar despite being as large as a WWII bomber.

I tend to consider the exhaust plume question a separate issue...

 

[FredGarvin]

I have read the stories of the SR simply out running SAM launches, but I have never read anything that said it had a "huge" RCS. I'm going to have to dig out some more books on this one.

I got to see the Vulcan fly about 15 years ago. What a cool airplane (if you weren't one of the crew). I can't imagine, with that HUGE vertical stabilizer, how it could be stealthy. I'd say someone looking at it from an ATC standpoint pretty much squashes that rumor.

 

[FredGarvin]

Including the optomistic plans to issue the crew with walking boots because after nuking Moscow they wouldn't have enough fuel to make it out of Russia and were supposed to crashland and hike to Turkey.

Wow. Jimmy Doolittle was alive and well in those planning meetings.

 

[FredGarvin]

Another hijacking of this thread:

NASA's Lessons Learned from the SR-71 program. This paper has TONS of really good information. It does have a blurb on page 36 about meeting the customer's requirements for RCS reduction, but it doesn't really go into detail.

http://www.scribd.com/doc/18029452/NASA-SR71-Blackbird-Challenges-and-Lessons-Learned-2009

 

[Danger]

Anyone here ever fly a T-bone? I haven't, but I'd like to see how it handles. I can't say as I find it aesthetically pleasing, but it certainly is distinctive and a true classic. The 'V' tail in this instance had nothing to do with radar, since it was strictly a civilian aviation design. The first prototype flight was in 1945, and it hit the market a couple of years later. They dropped the 'V' tail in '82, but the Bonanza itself is still in production.

 

[Cyrus]

Another hijacking of this thread:

NASA's Lessons Learned from the SR-71 program. This paper has TONS of really good information. It does have a blurb on page 36 about meeting the customer's requirements for RCS reduction, but it doesn't really go into detail.

http://www.scribd.com/doc/18029452/NASA-SR71-Blackbird-Challenges-and-Lessons-Learned-2009

sweeeet, this is turning out to be an A+ thread guys!

 

[mugaliens]

Why are the two "vertical" stabilizers on the F-22 Raptor angled sort of like a V-Tail as opposed to being completely vertical?

Right angles form what's known as a http://en.wikipedia.org/wiki/Corner_reflector" .

 

[Cyrus]

The link above that I gave directly states that they were canted in for reduction in RCS. Whether the aircraft was stealthy or not is a moot point (which I disagree with). They were canted because they were thinking about making the aircraft smaller on radar.

http://books.google.com/books?id=fVQRmQo-bfUC&pg=PA17&lpg=PA17&dq=kelly+johnson,+canted+vertical+tails&source=bl&ots=eUPlC_78Qq&sig=WOYbMSciQuyGYzaqH686TVnHObU&hl=en&ei=5C4gS4qtO5XSMo78ucUC&sa=X&oi=book_result&ct=result&resnum=3&ved=0CAsQ6AEwAg#v=onepage&q=kelly%20johnson%2C%20canted%20vertical%20tails&f=false

Turn to page 17:

Ultimately, the low-RCS technology on the A-12 proved less useful than expected. By 1962, U.S. intelligence had detected the Soviet Union's massive P-14 early-warning radar, code named Tall King, and its computer based control and display system. The P-14s performance undermined the blip-scan theory. The level of RC reduction achieved on the A-12 would not prevent an adversary from detecting it.

In other words, nice try, but no cigar.

Side: I got the book in yesterday from Amazon for only $15 bucks. It has lots of full page images - very nice.

 

[FredGarvin]

I think I still have the cigar: Whether they worked or not is not the issue (I do concede that argument). The sources shown all stated Lockheed said the intent of the inward can't was for stealth. OK, it didn't work but there is nothing saying any other intent. Your wind tunnel source mentioned nothing about the SR-71. Do we know that that reasoning applies, especially since the SR was a delta wing operating in a much different regime?

So we still have no directly stated source stating the contrary to the stealth argument.

 

[FredGarvin]

I had to laugh when I found this:

Stealth

There were a number of features in the SR-71 that were designed to reduce its radar signature. The first studies in radar stealth seemed to indicate that a shape with flattened, tapering sides would reflect most radar away from the place where the radar beams originated. To this end the radar engineers suggested adding chines (see below) to the design and canting the vertical control surfaces inward. The plane also used special radar-absorbing materials which were incorporated into sawtooth shaped sections of the skin of the aircraft, as well as caesium-based fuel additives to reduce the exhaust plumes' visibility on radar. The overall effectiveness of these designs is still debated, but since the aircraft did not include other elements of today's stealth technologies, it was still easy to track by radar (and had a huge infrared signature when cruising at Mach 3+).

http://www.aircraftguru.com/aircraft/aircraft-information.php?craftid=80

 

[Cyrus]

I think I still have the cigar: Whether they worked or not is not the issue (I do concede that argument). The sources shown all stated Lockheed said the intent of the inward can't was for stealth. OK, it didn't work but there is nothing saying any other intent. Your wind tunnel source mentioned nothing about the SR-71. Do we know that that reasoning applies, especially since the SR was a delta wing operating in a much different regime?

So we still have no directly stated source stating the contrary to the stealth argument.

Oh, his discussion with me was not about stealth, it was purely about straight twin tails vs canted twin tails from an aerodynamics point of view. Interestingly enough, I saw a picture of the latest variant of the F-15 Silent Eagle. It now comes with canted vertical tails, probably for the exact reasons mentioned on vortex shedding.

http://themoderatevoice.com/wordpress-engine/wp-content/dr-e/f-15-silent-eagle-boeing.jpg

​

You are right that they were canted inward for stealth on the SR. That I don't argue.

 

[FredGarvin]

Oh, his discussion with me was not about stealth, it was purely about straight twin tails vs canted twin tails from an aerodynamics point of view. Interestingly enough, I saw a picture of the latest variant of the F-15E strike eagle. It now comes with canted vertical tails, probably for the exact reasons mentioned on vortex shedding.

You are right that they were canted inward for stealth on the SR. That I don't argue.

I thought the F-15 was a done deal. When did they come out with a canted tail version?

 

[Cyrus]

I thought the F-15 was a done deal. When did they come out with a canted tail version?

See my edit above.

Source:

[1] http://themoderatevoice.com/27219/b...5-silent-eagle-with-fifth-generation-add-ons/

 

[mugaliens]

I thought the F-15 was a done deal. When did they come out with a canted tail version?

"http://en.wikipedia.org/wiki/F-15_Silent_Eagle#Design_and_development"."

Canted in or out doesn't matter.

The F-15SE's vertical stabs are, as Cyrus mentioned, canted out to reduce vortex issues, as delta or pseudo-delta's tend to "sit" in slow flight, and inward-canted vert stabs exacerbate the problem. The '-71 experienced this issue with the 'Q during refueling, but it wasn't insurmountable.

 

[Cyrus]

"http://en.wikipedia.org/wiki/F-15_Silent_Eagle#Design_and_development"."

Canted in or out doesn't matter.

The F-15SE's vertical stabs are, as Cyrus mentioned, canted out to reduce vortex issues, as delta or pseudo-delta's tend to "sit" in slow flight, and inward-canted vert stabs exacerbate the problem. The '-71 experienced this issue with the 'Q during refueling, but it wasn't insurmountable.

I think the main lesson is, don't have them vertical: but can't them inward or outward depending on what the aerodynamics favors (for a similarly obtainably RCS in either configuration).

Edit: While I don't necessarily doubt the claim by Boeing that the angle decreases RCS, I also don't put it past them to lie and say this to boost sales of the aircraft. Without actual comparison of RCS of the E and SE models to actually show if the tail works - be skeptical.

 

[mugaliens]

I think the main lesson is, don't have them vertical: but can't them inward or outward depending on what the aerodynamics favors (for a similarly obtainably RCS in either configuration).

Edit: While I don't necessarily doubt the claim by Boeing that the angle decreases RCS, I also don't put it past them to lie and say this to boost sales of the aircraft. Without actual comparison of RCS of the E and SE models to actually show if the tail works - be skeptical.

It's not that they're vertical, but that they're not at 90 deg angles to the wings. That said, canting inward is better for deflection against ground-based radars, but worse for low-level operation against lookdown radars.

The SR-71 did not employ radar absorbant material, so canting was their only option given the high heat-resistant requirements, and given their high-altitude profile, canting inward was the better stealth choice, even if it resulted in slightly degraded slow flight performance (drivers said "it wallowed").

The F-15SE's operating environment is less heat-restrictive, allowing the use of far better radar-absorbing materials (though some are simply translucent or transparent to radar).

I agree Boeing is in the market, but given their engineering expertise, they have little need of stretching the truth. I think they're main goal is to sell it as a much lower-cost but nearly capable alternative to the far more expensive F-22 and F35.

Besides - we have lots of potential targets around the world that don't require "the best of the best of the best" in technology. Just look at how admirably Sandy's (A-1 variants) and similar aircraft performed in Vietnam. It was a 40's-era plane that kicked butt twenty years later!

Interestingly enough, the plethora of variants created somewhat of a maintenance nightmare. Fortunately, most of the versions involved ancillary equipment to the basic airframe and powerplant. Still, it was a lesson in how not to modify an aircraft into every conceivable role.