Since the other discussion is closed, I will attempt to address some issues that I believe would be informative to interested readers.
We still dont know the design or the real capabilities of the engines which are to power the PAK-FA. That includes the shape of the exhaust nozzles. IIRC there was a Su-27/35 test bed with 2D TVC with nozzles like those of F-22. We dont know whether that would be incorporated into the new engines.
Another point that many missed here is the placement of the engines on PAK-FA. If you look closely the engines are aligned at an angle to the central axis with the nozzles pointing lightly outwards. Wrt the explanation that you gave regarding wide placement of engines and their effect on flight control, what do you think about this development? Sukhoi engineers probably had a very good reason to design the placement of eninges in that particular manner.
http://www.defence.pk/forums/650027-post1257.html
He just described the underpowered engines without TVC used in test flights. I am asking about the actual engines that are developed but not yet integrated.
Brief explanation on engine location...
Aerospace/Aviation: Aircraft Structure (Jet Engine's Location), fusilage, single prop
Prime consideration is keeping the top of the wing clean for better lift. Placing the engine on top of the wing would put the thrust above the center line of the aircraft.
Having the thrust below the center line is a compromise.
That's why some aircraft have the engines mounted on the side of the fusilage at the center line. This also aids in relieving in yaw in the event of an engine failure. Keeping thrust as close to the center line both vertically and horizontally makes for good design. Having thrust below the wing pushing up is better than having it above pushing down.
In single prop aircraft, engines were sometime angled down and to the right to offset torgue and produce better climbing angles. Propjet aircraft had engines mounted on top of the wings for prop ground clearance. Again another compromise in design. Any structural advantage would be the ability to drop an engine if the structure failed rather than to have it break into the wing or tail section.
First consideration is the lifting fuselage design. Some may called this 'blended wing body' (BWB) design. The B-2 is blended body. The F-14, F-15, F-16 and F-18 are lifting fuselage. So are top MIG and Sukhoi fighters. Next consideration is internal volume usage, as in how does the designer intend to use the fuselage. Fuel is a diminishing mass, not only that, under maneuvers, liquids moving inside a container create negative force influence on the body. Where to run all the wirings and mechanical contraptions necessary for flight?
Next consideration is physics. The further a mass is from the center of gravity, the greater the rotational kinetic energy required to change the body's state of motion. This requirement is applicable in both
initiating a maneuver and
stopping the maneuver. Take a simple aileron roll, for example, of an aircraft with two underwing engine pods. Because these two masses are noticeably far apart from the fuselage, it will require higher aileron deflection angle and higher aileron deflection rate to initiate a roll maneuver than if the two engines are closer to the fuselage. Once the aircraft is in this roll, it will have a higher roll rate, hence possibly greater maneuverability. The downside is that it will require greater aileron force to stop the roll, as in -- both
initiating a maneuver and
stopping the maneuver. In basic fighter maneuver (BFM) a pilot would be rolling in one direction in very short time before changing to the opposite roll direction, so mass centralization is a positive.
Here is an established example...
Lockheed 322-15/P-38F
The P38 truly shines at high altitudes, where other airplanes start gasping for air. Due to it's long wingspan and high rotational inertia resulting from putting engines out on nacelles, it's roll performance is poor until you get to higher altitudes and higher true airspeeds. Unfortunately, except for hunting strategic bombers or escorting your own bombers, WW2OL air combat takes place down at low altitudes, where the thicker air works against the P38. Even the boosted ailerons that came later in the war didn't help the roll rate below 440kph IAS.
Boosted ailerons means higher aileron deflection angle and higher aileron deflection rate as stated above. For the P-38, each wing contain a supercharged V-12 engine. No wonder powered ailerons did not help the aircraft much.
The Flankedsteak...errr...I mean...Flanker...series has wingspan of 14.7 meters or slightly above 15 meters. So far the PAK-FA's wingspan is little different. The Raptor's wingspan is about 13.5 meters. There are more to wing designs than just wingspan dimensions but it is telling of Sukhoi's consistency across its product line that goes beyond similarity in appearance, which is undeniable. From what is available so far, the PAK-FA's engines are further apart from each other than the Flanker series engines. All these factors must be in fine balance with each other and why decimal points exists in these physical dimensions. Sukhoi must be very comfortable with the basic design of the original Flanker to carry so much of it forward to the PAK-FA.
Now on thrust vectoring (TV) and how it relate to a flight control system (FLCS), specifically -- flight control laws (FCL).
Flight control laws (FCL) are finalized through five distinct phases:
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Off-line design. This is where the basic principles of flight theories are laid out. System architecture variations must be defined. A glider have no need for pneudraulics but a fighter would. Air data requirements specified -- extract only the information necessary for the functioning of the system architecture. The variations list is considerable. The Space Shuttle's FCL? We are talking about a vehicle that changes environment, going from one that give air data to one that does not. Gravity does have an effect on gyroscopes. This vehicle goes from a gravity environment to one that effectively has none. Everything at this stage is simulated.
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Pilot-in-the-loop. Also simulated and self-explanatory. This phase simulate the system's responses to pilot inputs, which contain both predictable to unpredictable commands.
User Stories - Gulfstream Aerospace Develops Pilot-in-the-Loop Aircraft Simulator with MathWorks Tools
Gulfstream engineers needed to build a flexible pilot-in-the-loop aircraft simulation facility, including a six-degrees-of-freedom simulation of the aircraft, in preparation for a scheduled flight test of a modified Gulfstream G550.
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Iron bird. This phase actually insert the flight control system (FLCS) hardware into the so far theorized system architecture. The FCL for a delta winged aircraft do not have horizontal rear stabilators in its equations, for example.
Aerospace Test Systems - Iron Bird Full Scale Testing | Moog
Iron bird solutions can be used to study and test flight controls, landing gears, and hydraulics of an aircraft system. Both the correct functioning of different aircraft systems, as well as endurance testing can be supported.
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Clearance. This phase is where the entire FCL, from theory to hardware integration, is demonstrated to be functional, at least within certainty limits, such as those coming from virtual air data inputs, in other words, we are giving the FCL a range of crosswind wind speed, for example, and demonstrate how the FCL can compensate the aircraft in that situation.
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Flight test. Self-explanatory. Regarding the crosswind example from above, the flight test is where we will experience the greater range of environmental spectrum.
The advent of FCL enable us to study closely stages of flight with the environmental conditions and pilot inputs within each stage. Take-offs and Landings do have variations in environmental inputs they are far narrower in range. Barometric pressure, for example, do not varies much from ground level to several hundreds meters altitude. Same for pilot inputs. Barring collision avoidance or engine out compensatory maneuvers, pilot commands variations are also narrow in range. The more we know about these variations and their ranges inside each stage of flight, from source to destination, the better we can incorporate their automation into FCL and naturally reduces pilot workload.
But here is the complication...And we will take mass loss, for example...
Military aircrafts, from fighters to transports, but especially fighters, can have large variations of mass, center of gravity and inertia. Transports usually behave the same as airliners. On the other hand, a fighter-bomber may approach a runway as if he is going to land, except that his speed is far higher than normal, and in a few seconds, his mass is considerably lighter, aka 'bomb delivery'. The FCL for fighters must be able to compensate for these great variations in environmental and pilot inputs. Not only are they great but can also be sudden as in a bomb delivery. If an airliner do experience as great and as sudden a loss of mass of several thousands kilos, it would be considered catastrophic. For a bomber or fighter-bomber, such a loss would be considered routine. The
Clearance phase is where the FCL must demonstrate adequate ability to cope with such a great loss of mass, changes to center of gravity and inertia. What if a bomb or missile failed to leave its rack, aka 'hung ordnance'? Indeed such a situation would create an off-balanced center of gravity and possibly off-center aerodynamic drag. The FCL must be able to adjust the entire FLCS to compensate for this abnormal condition, probably until the aircraft made it home.
The higher the performance envelope, the greater the complexity level of the FCL for a particular design. Relaxed stability is a requirement these days for fighters. The problem for the
Clearance phase is that even now we are unable to anticipate the effects of the full environmental spectrum, from air data to aerodynamics, upon current known aircraft designs with relaxed stability. Once we input what we know, depending on the computational power available to us, of course, the
Clearance result will give us
Flight test conditions. This is why we see flight tests where the aircraft just make a go-around and land and he did with full gear down. The next test will have gear up, but still just a go-around and land. The next test may have the pilot do a few aileron rolls and land. And so on. The data collected in each test flight in the
Flight test phase is analyzed and compared against the
Clearance theoretical result that allowed that particular test flight. There would be adjustments in
Clearance to create new
Flight test and each adjustment remove some restrictions. Hopefully the entire project is approved for acceptance by the customer, be it Delta Airlines or the USAF. So it is imperative that as much of work should be done and be successful in
Clearance as possible. More adjustments mean more test flights and that drive up cost -- in time and patience.
2nd Workshop on Clearance of Flight Control Laws
This workshop is organized within the activites of the EC STREP-project COFCLUO (Clearance Of Flight Control Laws Using Optimisation) and is intended to bring together researchers and practitioners with interest in the variety of problems and techniques related to the clearance of flight control laws. A first successful workshop was already held in Siena in 2008. We expect that this second workshop will delineate the state-of-the-art and will cover emerging trends in this research area. The program will include overview presentations of invited speakers illustrating their research activity, as well as talks on the achievements of the COFCLUO project, which at the time of the conference will be terminated.
This is serious business when an international workshop is created just for the purpose of theorizing something as esoteric as 'flight control laws'. How recent is this workshop indicate that computational power is still inadequate at the
Clearance phase, hence we still need test flights. It also hint at the computational power required to create something as exotic as the F-22 or the B-2, as far as avionics goes. The
Clearance phase is an extremely time, money and patience consuming effort.
According to US sources, the F-22's FLCS, or FCL, have automatic controls of the aircraft's thrust vectoring system...
How Things Work: Thrust Vectoring | Flight Today | Air & Space Magazine
They simply point the airplane where they want, and the onboard systems automatically coordinate the right combination of flaps, rudder, elevator, and nozzle angle. "The F119's vectoring nozzle is integrated into the F-22 flight control system" so that "the pilot doesn't control the nozzle independently," says Chris Flynn, Pratt & Whitney's F119 director.
When the US modified existing F-16 and F-18 to have TV capability, each aircraft's FCL were rewritten from the
Off-line design phase. We are introducing an alternate avenue of changing the aircraft's attitude and one that we want to exclude from pilot's control. The word here is 'exclude' meaning the pilot never had control in the first place. To 'remove' mean the pilot did have control and now we are denying him that control. The FCL that contain both exclusion and removal of control from the pilot will be more complex than one that only 'exclude'. The reason is that even though the FLCS is currently allowing the pilot controls of the nozzles, it still must maintain vigilance of nozzle movements under all flight atttitude and conditions so that the system can smoothly reassert its own controls whenever the pilot decide to relinquish controls. In other words, there is no 'neutral' status for nozzle controls. There is 'neutral position' when the nozzles are in alignment with the aircraft but that position is still under control by someone.
Just as current high performance aircraft cannot fly without computer assist, I doubt that the Russians would have the PAK-FA's flight control laws (FCL) allow independent pilot nozzle control provision. Here is why...And we will revisit the in-flight emergency (IFE) of UA 232...
Propulsion Control of Airplanes
In July 1989, the tail engine of the DC-10 of United Airlines Flight 232, enroute from Denver to Minneapolis, sustained a "catastrophic uncontained failure" that created a hail of shrapnel, slicing the hydraulics lines of all three independent systems, leaving the aircraft "marginally controllable" at 37,000 feet. Contrary to the realistically motivated consensus at that time that this flight should have ended in disaster, Captain Al Haynes, with the help of United Captain and DC-10 Flight Instructor Dennis Fitch, quickly improvised a way to keep control of the aircraft by maneuvering the throttles of the remaining wing engines. To the great amazement of aviation officials, the crew managed to bring the aircraft to a crash landing in Sioux City, Iowa, saving the lifes of most of those on board.
EVERYTHING the aircrew did that day to land that aircraft constitute a set of flight control laws (FCL). It is irrelevant if those actions are mathematically recorded or not. Each action working either independently or in concert with other actions in response to aerodynamic forces and air data inputs violated no laws of physics. What we may called 'miraculous' is simply our emotional response based upon our inexperience and ignorance of aviation, airmanship and awe of someone's abilities in an extreme situation. Test pilots are very much like UA 232's crew in that test pilots stored within their minds, their muscle memory and their instincts a great deal of knowledge, conscious and subconscious, of many sets of FCLs. In an extreme situation, the test pilot will put together a superset of FCL comprised of many subsets of FCLs to try to recover the experimental aircraft. The human mind remains supreme in this ability despite our accomplishments in computer science. Once the emergency is over, the superset of FCL that allowed the recovery -- !?!? poof !?!?. We are left with the human and hopefully we can mathematically quantify some of his techniques into something more concrete.
Thrust vectoring (TV) is no different than UA 232's selective asymmetric thrust in that we are introducing a new influence into the current basic FCL. The difference here is that UA 232's asymmetric thrust is
ad-hoc while TV is deliberate. UA 232's asymmetric thrust was working pretty much alone since the FLCS was barely available. But we demanding that TV works in concert with a fully functional FLCS in an aircraft that will go to extreme maneuvers. In effect, by allowing the pilot independent control of TV nozzles we are asking for a test pilot in every PAK-FA fighter. The US is not making that request. Good luck to the Russians and the Indians. Yer gonna need it.
I may have poke fun at the Russians but am also willing to give them much leeway with the PAK-FA at this time. Keep in mind that this is the first test flight under the 5 phases of FCL development explained above. The next
Clearance will have greater allowance for the next
Flight test. If the Russians do decide to make hardware changes they may have to go back as far as the
Iron bird phase and move forward again. This is the reason why it is an extreme rarity that any aircraft, civilian or military, have the latest and greatest technology. The further the regression, the more time, money and patience it will cost. This is why that there is a good probability that what we see here today will be very similar to the final product given the delays the PAK-FA project have experienced so far. The engineer must obey the laws of physics. The project manager must obey the laws of economics. The manager can have parallel FCL developments with different hardwares but when everything is totaled, the manpower hours will still tell the same tale -- that the more complex the design, the more complext the FCL, so the more money it will cost.
Finally...This is also why every indigenous aviation program should be applauded. I chuckled every time I read someone spout 'reverse engineer' a bought aircraft. A modern high performance aircraft can have its hardware reverse engineered. But without sufficiently complex FCLs to manage the hardware, all you will have are expensive and pretty looking static displays as the FCL codes are quite closed source. There must be complete mastery of the five phases of FCL development. Else all you can achieve are WW II level fighters. Can Russia fully 'reverse engineer' an F-22 in all aspects? Yes. Can China? May be. Can Japan? Better odds than China. Can South Korea? Same as China. Can India or Pakistan or Iran? Sorry. Hope none will take offense. The Iranians cannot even reproduce an F-14. Once a country is determined to have an indigenous aviation program, there should not be any pauses. The collapse of the Soviet Union was disastrous for Russian aviation, hence so much attention and hopes are upon the PAK-FA.
is historically high, though not unprecedented. However, the extensive application of Resin Transfer Molding (RTM) technology and high temperature bismaleimide (BMI) composite materials directly resulted in the high weight/performance efficiency the Raptor demonstrates. The use of metallics technologies such as titanium Hot Isostatic Pressed (HIP) castings and electron beam welding allowed the airframe designers to incorporate complex features into a single component without the weight of fastened assemblies. The continuing challenge is to reduce material and component costs through a constant reassessment of emerging technologies. Recently developed machining technologies, for instance, have allowed the inlet canted frame lip to be changed from a casting to a lower cost machined component with no appreciable weight penalty.
