Forum: F-22A Raptor

I've been wondering, why does the F-22 have vertical (diagonal) stabilizers? I know that they are typically used for yaw control and for directional stability in flight, but hasn't modern technology allowed for them to be eliminated?

Take the X-36 for example: http://www.fas.org/man/dod-101/sys/ac/x-36.htm

This tailless design utilizes thrust-vectoring technology and split ailerons for control in the yaw axis. It eliminates the needs for vertical fins. According to the above mentioned website, the lack of a tail can lead to a reduction in radar cross section, weight and drag as well as an increase in range, maneuverability and survivability.

I think I read somewhere that the vertical tails on the F-22 house antenna arrays or other instruments, so I guess they could have some use. But still, the above-mentioned advantages of a tailless design seem very tempting to explore.

I know that the X-44 MANTA and FB-22 both were proposed tailless derivatives of the F-22, but so far as I know, nothing has come of them. Is there any reason for this? Cost? No need for them?

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My guess would be survivability. I think they're talking about survivability as in harder to see. I'm talking about it from the point of being shot up. X-36 gets shot in a split aileron/drag-rudder and it's probably "buh-bye".

sferrin wrote:

My guess would be survivability. I think they're talking about survivability as in harder to see. I'm talking about it from the point of being shot up. X-36 gets shot in a split aileron/drag-rudder and it's probably "buh-bye".

The thrust-vectoring nozzles alone could still be sufficient without the ailerons. I believe that I read that a computer simulation was run of the X-31 and it was found that it would have been controllably stable without its vertical stabilizer (due, no doubt, to its thrust-vectoring nozzle). I can't confirm that, though.

Since the F-22 has pitch-only thrust vectoring, that would not help with yaw control. I doubt that split ailerons would provide sufficient yaw control at low airspeeds, especially when they are also being used as roll control devices at the same time. If you are in a full roll with ailerons maxed out, any attempt at yaw control using split ailerons will reduce available roll power.

Kryptid wrote:

The thrust-vectoring nozzles alone could still be sufficient without the ailerons. I believe that I read that a computer simulation was run of the X-31 and it was found that it would have been controllably stable without its vertical stabilizer (due, no doubt, to its thrust-vectoring nozzle). I can't confirm that, though.

A single engine aircraft would not be able to control roll with thrust vectoring. And the F-22 uses pitch only TVC (whereas the X-36 had yaw only). A tail-less fighter is too vulnerable to damage. The advantages of not having a tail just aren't enough. It's not the same with a strike aircraft (where the whole idea is NOT to get shot at).

With the progress in flexible array tech and with the way the Raptor's goin' vis-a-vis the electronic battle space, the rudders (esp. the LE's) are great places to mount contoured conformal arrays.

Well, of course the idea I'm proposing is to modify the Raptor to incoporate 3D thrust vectoring in place of 2D thrust vectoring nozzles. That's how it would remain stable.

It is my understanding that the reason for the large vertical tails is the requirement that the Raptor be supermaneuverable at all angles of attack. Absent that requirement, no vertical tails may be doable. Also, the more thrust you have to use to actually control an airplane the less thrust is directed exclusively to propel that airplane forward, making the Raptor, therefore, less efficient in supersonic cruise, another principal requirement.

Obamanite wrote:

It is my understanding that the reason for the large vertical tails is the requirement that the Raptor be supermaneuverable at all angles of attack. Absent that requirement, no vertical tails may be doable.

Until you have an engine out or you get a MANPADs in the nozzles. . .

I doubt thrust vectoring could keep up with the reduced yaw stability at high Mach numbers. Thats why the Tom cat and Eagle both had twin tails (and the 'Vark had such a HUGE vert)

Roscoe,
In my experience the main reason for the twin vertical tails was improved yaw stability at high angle of attack, not high mach. Whereas a single tail can be blanked out by the fuselage at high AoA, twin tails are out there in the breeze to provide yaw stability. Compare the AoA limits of the F-16 with the F-18 and you'll see what I mean. But thinking further I can see that two relatively short tails might be stiffer in twist than a tall single tail of the same area. Why is torsional stiffness important? Because lack of tail stiffness is what causes loss of directional stability at high mach. Remember aileron reversal at transonic and supersonic speeds? Same thing happens to the rudder and vertical tail. So I'll agree that you are partially right.

The F-16 stuck with a single tail for reduced drag, weight, and cost. Then they added the short, stiff ventral fins to improve high mach directional stability. Although it doesn't have as good AoA capability as the F-18, it's evidently good enough.

Have you ever seen the preliminary design for the F-111 vertical tail? It was nothing like the final design, as it resembed a B-58 tail, much taller and less chord. In order to fit on a carrier hangar deck (F-111B) it would have required a fold mechanism. (Didn't the RA-5 and maybe the S-3 have folding tails?). To save weight and cost (and to keep the Navy pacified), the final design was a compromise, with larger area but much shorter. Area had to be increased to maintain AoA capability. Then of course, The Navy cancelled the F-111B and the USAF was stuck with Navy-required compromises all over the airplane.

johnwill wrote:

Then of course, The Navy cancelled the F-111B and the USAF was stuck with Navy-required compromises all over the airplane.

Have a feeling another John Will some 40 years from now will speak about the F-35 in similar terms...

Quote:

Compare the AoA limits of the F-16 with the F-18 and you'll see what I mean.

I thought that may have been due to the LERX on the F-18?

If possessing vertical tailfins is needed to fulfill these requirements, then why not give the F-22 the V-tail of the YF-23 but with dihedral-controlling mechanisms in order to give the aircraft the advantages of having no vertical tailfins as well as those of having them. Just alter the dihedral of the tail as needed. You could lay them flat in order to hide them behind the wing for decreased RCS in the frontal hemisphere, or you could raise them to form a V-tail for extra directional stability and yaw control. Put some 3D TVC nozzles in place of the 2D TVC nozzles that the Raptor now has in order to produce yaw control when the tailplanes are laid flat.

Quote:

If possessing vertical tailfins is needed to fulfill these requirements, then why not give the F-22 the V-tail of the YF-23 but with dihedral-controlling mechanisms in order to give the aircraft the advantages of having no vertical tailfins as well as those of having them. Just alter the dihedral of the tail as needed. You could lay them flat in order to hide them behind the wing for decreased RCS in the frontal hemisphere, or you could raise them to form a V-tail for extra directional stability and yaw control. Put some 3D TVC nozzles in place of the 2D TVC nozzles that the Raptor now has in order to produce yaw control when the tailplanes are laid flat.

see now you are breaking the cardinal rule of maintenance

"The more complex the system, the easier it is to break and the harder it is to fix."

johnwill wrote:

Roscoe,
In my experience the main reason for the twin vertical tails was improved yaw stability at high angle of attack, not high mach. Whereas a single tail can be blanked out by the fuselage at high AoA, twin tails are out there in the breeze to provide yaw stability. Compare the AoA limits of the F-16 with the F-18 and you'll see what I mean. But thinking further I can see that two relatively short tails might be stiffer in twist than a tall single tail of the same area. Why is torsional stiffness important? Because lack of tail stiffness is what causes loss of directional stability at high mach. Remember aileron reversal at transonic and supersonic speeds? Same thing happens to the rudder and vertical tail. So I'll agree that you are partially right.

I would up that to "mostly right". Lot's of single-tailed aircraft had to have their vertical stabilizer enlarged because of loss of directional stability at high speed. Some even had folding ventrals (whos utility at high AOA would seem to be dubious). F-100- had to have it enlarged, Crusader III- had folding ventrals AND had to have it's tail enlarged, F-108 would have had a giant tail and FOUR ventral fins, Tomcat originally was designed with a single tail and folding ventrals. In all cases the primary thought was directional stability. You'll notice that neither the Gripen, Typhoon, nor Rafale have multiple tails and I'd assume they considered the problem of high AOA flight.

Forum: F-22A Raptor

The F-22 in light of the X-36 program...