J-20... The New Generation Fighter III

Originally Posted by latenlazy

So what's making the plane execute the turn then?

Lift

think about it.

Will elaborate tomorrow if you guys are stuck in rut.

Originally Posted by i.e.

Lift

think about it.

Will elaborate tomorrow if you guys are stuck in rut.

Plane rolls into its bank angles, aileron go into neutral, canard pitches to turn angle?, slats sustain turn? So much lift is generated that plane is at risk of over banking, and canards flip the other way to trim? I have no education what so ever in aerodynamics. Not even sure I used all the terms right.

Originally Posted by latenlazy

Plane rolls into its bank angles, aileron go into neutral, canard pitches to turn angle?, slats sustain turn? So much lift is generated that plane is at risk of over banking, and canards flip the other way to trim? I have no education what so ever in aerodynamics. Not even sure I used all the terms right.

Yeah, that'd be kind of my quesion, too.

Originally Posted by latenlazy

So what's making the plane execute the turn then?

Very briefly, when an aircraft makes a banking maneuovre, a lift component goes into the centre of the turn radius and this basically provide the g-force into the turn. The other major component of the lift goes into interacting with the weight of the aircraft.

Originally Posted by paintgun

the maximum lift config would probably look like this (take off and landing config) :



same like siege, also wonder what the trailing edge is (not) doing
can't get my head around this picture, the more i look at it the more confusing

On 11 January 2011 the new Chinese combat jet flew for the first time (in public, at least). The new
airplane is referred to as the Chengdu J-20. Chengdu is the name of the city which houses a few
aviation industry enterprises, including aircraft manufacturing plants producing jetfighters and design
houses developing them. Great many outsiders watched the J-20 fly, as they happened “by chance”
to be around the fence of Chengdu factory’s aerodrome on that day. The flight itself was uneventful.
It took place in the conditions of clear skies allowing photographers to make some good shots.
Before touching off the ground, the pilot
made several passes over the runway
so as to expose his airplane to the cameras
of “aviation admirers” all round the place.
Those took photos of the aircraft from different
angles and depicted everything they
wanted except for doors of internal weapons
bays.
These doors were either thoroughly hidden
or removed from the shots by the picture
takers on the insistence of very competitive
advisers. But it is even more likely that these
doors were not actually fitted to the J-20
first operable prototype. They are not needed
on the very first operable aircraft dedicated
to assessment of flight performance,
flight envelope, various engine settings,
functioning of the essential onboard systems,
proving flight control algorithms. As a rule,
third or even later prototypes are devoted
to weapons testing, but these are yet to be
constructed and outfitted.
The J-20 first public flight occurred just
in time when US defense secretary Robert
Gates was in Beijing on an official visit.
Once there, he was trying to calm down
the Chinese leaders who were much worried
about pending deliveries of modern
US-made weapons to Taiwan. Beijing considers
this island an essential part of China.
A lot of pictures appeared on the Internet
on the memorable day of 11 January.
These shots gave more information
on the new airplane. In particular, they reveal
the shape of the wing and its positioning
in relation to fuselage. This makes it
possible to make some preliminary conclusions
about the aerodynamics layout and
technical characteristics of the J-20, and
make guesses as to the main task the new
jet shall be solving after entering squadron
service.
The J-20 represents a relatively large
tactical jet with the canards (foreplanes)
and large delta wing. The fuselage length
is somewhere between 23 and 25 meters,
wingspan between 13 and 14 meters.
By our estimation the maximum takeoff
weight shall be in the region of 40 tons,
and operating empty weight twice less
than that.
Many aviation experts believe that
the J-20 relies on a pair of Russian engines
or their Chinese copies. In other words,
the J-20’s engines are picked out among
members of the big family uniting the Item
117, AL-31F, WS-14 and WS-10 Taihang.
Two engines together develop in between
30 and 40 tons of thrust. If that is so, then
the capability of the propulsion systemenough for supercruise, or supersonic
cruise flight at military power (highest power
setting without afterburning). We may
also expect that the J-20 with restricted fuel
and combat load (for instance, when flying
air-to-air mission) can fly vertical without
losing speed at subsonic regimes and
low altitudes.
When in-flight photos appeared,
the J-20 became the hottest topic for discussion
among aviation enthusiasts round
the world. But as it appeared, the enthusiasts,
and even world-famous western
journalists, had difficulty in classification
of the new Chinese warplane. Is it a superiority
fighter? Is it a supersonic bomber?
Or, perhaps, it is a multirole, multimode
airplane? Even columnist and experts with
world’s leading aviation magazines hesitated
to give their clear answer to these
questions, — that in the view of them having
good sources in the US and European
intelligence bodies, defense ministries
and the industry. It seems that not only journalists,
but the professionals were in some
state of shock after seeing the new Chinese
bird.
First of all, let’s determine J-20’s center
of gravity position. There are some photos
available of the J-20 taxiing, in which
we can clearly see its long fuselage, wingto-
fuselage connection and landing gears.
The J-20 undercarriage is fighter’s classics:
three-point with a nose gear. And so it
makes it easy to determine center of gravity
position. To do that we take the main
landing gear strut, and attach a line to
it starting on the wheel’s ground contact
point. The line goes up with at an angle of,
say, 15 degrees, leaning towards the nose
of the airplane. The point where it crosses
the fuselage center line is the most likely
position for the airplane’s center of gravity.
Here comes the first surprise: the likely
center of gravity position rests… too far
from the mean aerodynamics chord (MAC)
of the wing. As a first iteration for aircraft
designers, the center of gravity must be
somewhere 25–35% of the wing’s MAC, —
like so is prescribed in the classic aircraft
design books.
But the Chinese airplane appears to
have the center of gravity position somewhere
at MAC’s edge. It is fairly strange
for a maneuverable fighter, since balancing
of the aerodynamic forces and
the gravity will require relatively high deflection
of the control surfaces — canards
in the J-20’s case. Should this airplane try
to execute high-G maneuvers at subsonic
speeds, the deflection of the canards could
be a limitation. All this is rather strange
for a maneuverable fighter… But not for
the J-20, which does not appear to be one
of those !


Let’s take a look at other available photos,
in which the J-20 goes in for the landing
with landing gear down. Apparently,
the canards are set at a rather high positive
angle (leading edge upwards), while
the wing has its leading edge deflected
downwards. The trailing edge surfaces are
also deflected down, at rather a small angle.
Obviously, at the approach for landing
configuration, the wing’s center line
is highly curved by means of the leading
and training edges down, which increases
lift (achieved through altering the camber
of the wing). But not so much as in the case
of classical flaps.
All this is, again, fighter classics for
the delta winged aircraft with foreplanes.
And here lies their limitation: the pilot cannot
move the trailing edge further down,
since the resulting lift force that builds up
on the training edge will be hard to balance
with the canards, in the view of their
limited deflection scope (in the view
of them stalling).
It is well known from the aviation history
how to enable delta-winged airplanes
to generate more of the lift force
at landing. For that purpose the canards
are placed as close to the fuselage’s nose
as possible, to have a larger distance tothe center of gravity. For instance, the Tupolev
Tu-144 supersonic jet liner had foreplanes
that were retracted into fuselage
all the time except landing. But Chengdu
designers did not do this. Rather, they
positioned the canards fairly close to
the center of gravity position, and thus
sacrificed their effectiveness at landing for
some other purposes.
What purposes? Firstly, for non-retractable
foreplanes it is important to have
them within the supersonic cone as it sets
on the top of the airplane’s nose at Mach
numbers
What is the J-20? Is it
a superiority fighter?
Is it a supersonic
bomber? Or, perhaps, it
is a multirole, multimode
airplane? Few columnists
and experts dared to give
their explicit answernumbers exceeding 1.0. This lead to a conclusion
what the Chinese must have been
purposely shaping the J-20 for supersonic
flying.
Why the Chinese shaped the J-20
in the way it is? Perhaps, they are unfamiliar
with the classic solutions for a delta-
winged, canard-equipped fighter? No,
this is not the case knowing that Chengdu’s
previous design was the J-10 light weight
fighter, now in service with PLAAF. On its
first public flight, the J-20 was escorted by
a J-10B twin seater, the operational trainer
version of the baseline J-10 single seat
fighter. This airplane was the star of the Airshow
China 2008 and 2010, when it flew
superbly with the PLAAF display team pilots
at the controls. The J-10 is a very maneuverable
airplane, and this is the testimony
of the Chinese designers’ skills in development
of maneuverable fighter aircraft


The J-10 is a classic design with “proper”
positioning of the center of gravity, like
in the books. This is clear to tell looking
at the main landing gear struts attached
to the fuselage somewhere near 15–30%
of the wing’s MAC. So, let us ask ourselves
the same question again, why the Chinese
designers shaped the J-20 in the way it is?
Here are some suggestions.
First, to achieve smooth airflow with desirable
parameters at the entry to the engine’s
fan, the J-20’s designers have to
make the air intakes rather long. This was
an important consideration at design stage.
Second, they also needed to make
the air channel S-shaped, so as to hide
the fan blades from the radio waves emitted
by enemy radars. The latter is needed
for a lower visibility of the airplane. It
is worth to notice that the J-20’s air intakes
resemble those first tried on the Lockheed
Martin F-35 Lightning II. This gives move
ground to assert that the J-20 is optimized
for supersonic regimes and supercruise,
much like the F-35.
Third, let us make distribution diagram
for the airplane’s cross section along
the J-20’s fuselage centerline. We need to
take account of the thickness of the wing,
canards and tailplanes. The diagram is
very smooth, — exceptionally smooth! It
comes without a peak, running smoothly
at approximately the same height from
the tops of the air intakes all the way to
the engine nozzles.
This seems to be the main thing for
the Chengdu designers. Apparently, they
wanted to make the airplane’s equivalent
body of rotation as narrow as possible.
And they needed to make provision
for internal carriage of weapons, which
is a characteristic feature for fifth generation
fighters. In actual fact, the J-20 has
much smoother cross section distribution
diagram than the F-22A Raptor, the F-35
Lightning II and the Sukhoi T-50 (PAK FA or
FGFA). Apparently, it required quite an effort
from Chengdu designers and so made
them go for compromises on other things.
Should the designers from Chendgu
have made it “classic”, they would not
have moved the wing all the way towards
the engine nozzles. But they did because it
was the only effective way to make the airplane
as narrow as possible, with the need
for big air intakes, air-supply channels and
internal weapons bays.
Again, this is the main thing about
the J-20 design, and it sets it apart from
all other known next-generation fighters.
Other designs have “peaks” in some
60–70% down the way from the fuselage
nose tip to the engine nozzles.
A smooth cross section distribution diagram
is important for transonic drag. Supersonic
aircraft are being designed
in accordance with so-called “area ruling”.
For high Mach numbers (M>2)
the distribution diagram is not so important
as for transonic regimes, M=1…1.5. It
seems the Chinese designers optimized
their new jet for transonic regimes and
moderate supersonic speeds.
Our impression from the J-20 is that it
is an uncompromised airplane for supercruse,
for flying at moderate supersonic
speeds corresponding to Mach M=1.3–
1.6. Such speeds can be achieved without
afterburning. Surely, the J-20 can accelerate
to M=2 and faster, but this would
require engaging afterburners. In turn,
the fuel burn will go high, lowering operational
range of the aircraft and enlarging
its heat signature.
In our view the Chinese designers optimized
their new jet for M=1.3–1.6.constructed
Here comes the clue: the J-20 is a missile
launching platform able to evade enemy
interceptors by high cruise speed.
The J-20 may prove a good interceptor, —
very possibly. But its main task seems to
be anti-shipping: firing missiles at enemy
warships while denying their air defense
cover.
It may well be that one day the new
Chinese jets would be used in anger. And
it would probably be PLAAF sending their
pilots to attack warships off the coast
of a freedom-loving island not far from
the mainland China.
The history of the powerful US Navy
can be traced back to the famous
duel of the USS Monitor and VSS Virginia
(Merrimack) on 9 March 1862,
the first-ever battle of ironclads. Although
the Confederacy gunners scored hundreds
of direct hits, shells bounced off
her armor: the Monitor seemed to have
impunity to enemy fire. The USS Monitor,
a 987-ton armored turret gunboat,
was built at New York, with a large single
cannon turret on a low freeboard. After
the battle, the North Americans constructed
fifty monitors modeled on their
namesake and made them the backbone
of their navy. For their rather strange
looks, these ships were called “cheese
boxes on rafts”. Since the memorable Battle
of Hampton, the North Americans never
lose at sea, and now their cheese boxes
sale when and where they want. China
prepares a ram for them.
Vladimir Karnozov


http://www.gudko.ru/Airfleet/airfleet_2011_1.pdf

Originally Posted by i.e.

leading edge devices only provides higher stall alpha, doesn't provide a high lift at a given alpha. nor does it provide the pitching moment.

trailing edge elevarons (elevator-and-aileron) on a pure delta provides pitching moment by dumping lift at outer edge of the large delta, which happens to change pitching moment. because out board lift dump moved the total center of pressure fwd.

anyways all of you are wrong


as for using elevarons AND canard. dassault rafale uses it but not the main pitching control, partly because its canard are sized pretty... conservatively. for a large canard aircraft like J-10/20/Typhoon/Gripen there is really no need to use for pitch because the canard is big enough. but some do uses it for one reason or the other (Gripen for example...as the famous and widely published Pilot-induced-oscillation or PIO event shows the inner workings of Gripen's control law. anyways, way beyond the realms of amature forum boys but into the realm of pros )




by itself the 1st order function of canard is nothing but a fwd pitch moment device...just like a elevator but fwd of cg.

---------- Post added at 09:23 PM ---------- Previous post was at 09:18 PM ----------



The 2nd picture is a momentary picture.

could simply be that...
The pilot could be pulling back on the stick to ask for more alphas(Gs) but Flight COntrol System is deflecting the canard surface down to arrest the extra pitch rate so the airplane don't overshoot its maximum alpha and stall.

all in day's of work of a full authority flight control law.

dassault rafale uses it but not the main pitching control, partly because its canard are sized pretty... conservatively.

The Rafale's canards have a shorter moment-of-arm compared to the others above which explains why it may need help in pitch control. The J-20 also have canted tail control surfaces (or ruddervators or whatever you call it.) for pitch control.

could simply be that...
The pilot could be pulling back on the stick to ask for more alphas(Gs) but Flight COntrol System is deflecting the canard surface down to arrest the extra pitch rate so the airplane don't overshoot its maximum alpha and stall.

Woudn't that indicate a less than optimum FCS? A good FCS wouldn't have caused the extra pitch rate in the first place. My guess is, looking at the pitch angle of the canards, the J-20 was already reducing alpha. If only we have a video of that J-20 in action to solve the puzzle.

Originally Posted by latenlazy

So what's making the plane execute the turn then?

Tie a toy aircraft at the end of the string and make sure the toy is right-side up. What is keeping the toy in the air? Your hand at the other end of the string, which is providing lift. Now, swing the whole apparatus in such a way so the string is slanted and the toy moves in a circle parallel to the ground. What is causing the toy to turn? Again, your hand at the other end of the string.

Originally Posted by Engineer

Tie a toy aircraft at the end of the string and make sure the toy is right-side up. What is keeping the toy in the air? Your hand at the other end of the string, which is providing lift. Now, swing the whole apparatus in such a way so the string is slanted and the toy moves in a circle parallel to the ground. What is causing the toy to turn? Again, your hand at the other end of the string.

What makes an airplane turn is the centrifugal force and it acts like an invisible string. Once the plane banks, it will naturally turn. Having said that, load factor increases with bank angle. The tendency is the nose will drop during a turn and the pilot needs to pull back the control to maintain the angle of attack.

Originally Posted by MiG-29

...

http://www.gudko.ru/Airfleet/airfleet_2011_1.pdf

Thanks for that file....

Originally Posted by Air Force Brat

Seige, leading edge devices are a passive device that increases the camber of the wing, they may be manually deployed, flight control system deployed or as they were in the early days employed by aerodynamic loads. For example, with the aircraft on the ground the leading edge extensions would be fully deployed, during the takeoff roll and in the low speed regime they remained deployed until increasing airspeed increased pressure on the leading edge and pushes them into their low drag, fully retracted position, where they were flush and smoothed out high speed airflow. In high angle of attack manuever they would deploy as needed to increase lift in the low speed or high aoa condition. They are not a pitch control surface, per se and yes the F-15s pitch is controlled by stabilators, once again if memory serves me correctly the max aoa of the F-15 is around 35 degrees. If I'm wrong we will both hear about it.

I think I understand now. So in a canard delta aircraft the canards control pitch in a turn and in a pure delta aircraft the rear control surfaces take the place of the horizontal stabiliators?

Originally Posted by latenlazy

Plane rolls into its bank angles, aileron go into neutral, canard pitches to turn angle?, slats sustain turn? So much lift is generated that plane is at risk of over banking, and canards flip the other way to trim? I have no education what so ever in aerodynamics. Not even sure I used all the terms right.

two concept in flight mechanics.

1. Nose pointing, or how your physical nose point relative to an inertial frame ( earth). how high your nose is below or above horizon for example.

2. where the airplane is going, which is where your airplane's velocity vector is pointing. or the flight path angle. the angle between where the airplane is Going relative to an inertial frame ( earth).

now, turn, what is it? it is simply the change in flight path vector / more specifically, flight path angle.

so, how do one change flight path angle.

how does one change velocity vector espcially its angular component? by apply acceleration ofcourse. just like in any object.

more specfically, applying acceleration perpendicular to its velocity vector.

Lift, just happens to be perpendicular to the airpalne's velocity vector.


So, a turn is nothing but changing the airplane's velocity vector.

you do that by using lift, which is perpendicular to your airplane's velocity vector.

So,
to change #2, one has to change #1.
why? #1 yank the airplane nose higher temporarily so that a high aoa is achieved higher aoa (*aoa, angle between velocity vector and physical airplane) usually = higher lift
how do one change #1?
your use a pitching moment device, which is either elevator, elevarons or canard.


elavons is not necesary on a non-tailless configuration.

leading edge slats are not necessarily, it is just but a tool to extend the point where no more additional lift can be generated by going to a higher alpha.

Originally Posted by tch1972

The tendency is the nose will drop during a turn and the pilot needs to pull back the control to maintain the angle of attack.

Well, that'd be what my common sense or instinct tells me, too. And that's why I feel the canards should deflect the air in order to lift the nose. Since with J-20, the canards are doing the opposite (I do understand or remember i.e.'s explanation), what's the thing the pilot pulls back to maintain the angle of attack?

Originally Posted by Player99

Well, that'd be what my common sense or instinct tells me, too. And that's why I feel the canards should deflect the air in order to lift the nose. Since with J-20, the canards are doing the opposite (I do understand or remember i.e.'s explanation), what's the thing the pilot pulls back to maintain the angle of attack?

Because in a turn, the wing is generating less lift(part of the lift is channel to centrifugal) as compare to flying straight and level. Hence the nose will tend to drop if the pilot does nothing to correct. To maintain the same flight altitude in a turn, the pilot needs to pull the control in order to prevent the nose from dropping. This resulted in greater AOA and lower airspeed. The steeper the turn, the more he needs to pull.

Originally Posted by tch1972

Because in a turn, the wing is generating less lift as compare to flying straight and level. Hence the nose of will tend to drop if the pilot does nothing to correct. To maintain the same flight altitude in a turn, the pilot needs to pull the control in order to prevent the nose from droping. The steeper the turn, the more he needs to pull.

Hehe, I think you took my "what" for "why." I understood what you said but was asking you what control surfaces he's pulling in order to prevent the nose from dropping? We saw the photos that the canards were doing the opposit for the purpose that i.e. says. And I don't recall seeing other control surfaces doing much during a turn to lift the nose...