Aerodynamics thread

[Air Force Brat]

Re: J-20... The New Generation Fighter III

Repeating all those aerodynamic principles here, with some errors here and there, don't really get into the point of the discussion.



By this principle, the J-20 must be very longitudinal unstable since a few of the members here are puzzling over the seemingly pitching down of the canards in most of the maneouvres as seen in the pictures and videos.

Any which way, the article is still wrong in the way it determines the J-20's longitudinal stability by just considering the position of the wing's MAC with respect to CG, and by committing the error of neglecting the overall lift of the aircraft, escpecially the forebody lift enhanced by the few vortex generators at the inlet and LERX as describeed in Dr Song's paper.

I'm going to have to give this one to quikie, mig, especially when it comes to all the generalities about canards, short or long coupled, several of us are aware of those general principles, but they are general and generally about western aircraft and design. Now there are two reasons I believe the forward fuselage is making a lot of lift, Dr. Songs paper,along with Tianlongs paper, both very well written and scholarly, and more importantly the large downward deflection of the canard, the eng stated he estimated AOA at approx 40 degrees in these turn, due to the canard deflection and he contends the canard is aligned with the relative wind. Now both of you guys have opened my eyes here, but the real head scratcher was this large downward deflection of the canard, Indy cars have had front wings with large downward deflection since the late 1960s, to prevent body lift or nose up pitch. While this does cause some drag, it prevents the indy car from "over pitching and loss of control". Thats why Dr. Song speaks of an all flying or extreme angle deflection of the canards, which are pitching the nose down.

---------- Post added at 09:07 PM ---------- Previous post was at 08:56 PM ----------

Not quite. The leading edge flaps allow you to go to a higher angle of attack without stalling and so reach a higher lift coefficient.

I made my statement in response to your #8 post in which you state the leading edge flaps decrease lift, did you mean they pitch the nose down? or am I missing something else. I think I still stand by my statement, but then I'm not the engineer, respectfully brat.

---------- Post added at 09:26 PM ---------- Previous post was at 09:07 PM ----------

I think the fact that canards on J-20 has to pitch down more to hold the nose down makes it potentially more maneuverable. If you want tighter turns all you need is to do is to relax the amount of canard deflection downwards.

From what I saw in the video the J-20 is doing a whole lot of deflection could be due to:

-Turning at low speeds, therefore the rate of pitch of the nose needs to be limited, otherwise you are exposing more of the plane cross section to incomming air flow, increasing drag, makes the plane slow down more and eventually stall.

-Engine not at full power, therefore the aircraft is not accelerating enough and this will limit how fast it can turn. This is interrelated with the previous point.

-J-20 will due to have more powerful engine than it currently has, so with it the canard downward deflection could be relaxed, giving it faster response.

-As far as I see the video of J-20 shows it flying and doing maneuver at slower speeds than the F-22 videos.

-Using low speed performance to deduce supersonic performance may not be on track as the centre of lift shifts with different airspeed


Though I think I'll just wait for high performance revealing test flights like what we've seen with the F-22 demos to convince people

You are on it today Jr., and whats been in the back of my mind is, has anyone around Chengdu heard any sonic booms, I don't think we've even seen the J-20 at high subsonic speeds, now they've made 70+ or so flights, I think its a little odd if the tallman hasn't opened the tap a little, and guys this is my own dumb thinking, so if I'm wrong and you know of mach 1 flights, please straighten me out. Has anyone seen a fast dash say to Mach ,85-90? I may have missed it.

 

[tch1972]

Re: J-20... The New Generation Fighter III

i think you are not understanding the article well, the article is talking about longitudinal static stability, this means, J-20 is rather not very unstable, the videos show a J-20 turning deflecting a lot it canards

compare with the Gripen, watch the gripen it turns tighter and deflects less its canards
[video=youtube;klf2hiix28w]http://www.youtube.com/watch?v=klf2hiix28w&feature=BFa&list=HL1331739815&lf=mh_lolz[/video]

[video=youtube;GcdFjgOMM2I]http://www.youtube.com/watch?v=GcdFjgOMM2I&feature=BFa&list=HL1331739815&lf=mh_lolz[/video]

Not a fair comparison. The J20 we saw had it leading edge slat and flap deploy which alter the aerodynamic configuration. A better way is to compare a J20 doing a turn with it slat and flap retract.

 

[tch1972]

Re: J-20... The New Generation Fighter III

My own view is that turning is like generating lift for the aircraft sideways. (abit with some downward lift to counter gravity.)

In J-20's case the nose area has excessive lift compared to the rest of the aircraft, therefore the canard has to work to 'press the nose down' in line with the rest of the plane, so that the aircraft does not pitch too much and stall.

So it's a balance between how quickly you can change the heading of the plane's nose (which will determine the direction that the enging is pushing the plane forward, as the nose/tail is aligned down the centre of the plane) versus having enough speed in the direction that the nose is pointing to continue to generate the required lift for countering gravity. A nose that moves too fast risk decreasing the velocity of the plane and make it stall.

I think that this problem exists in J-20 and needed to be corrects actually shows that the plane is pretty maneuverable to the extent that it has to be reined in. It is not being corrected because it is a flaw, other plane don't need to correct for this because they didn't have enough nose lift in the first place.

U are only half right. It the wing that have excessive lift at the given airspeed because slats are deployed. (look at the front edge of the wing ) The pitching down movement is due to the effects of flap/slat. In general, a lower pitching attitude is require to achieve the same airspeed when flaps/slat are lower compared to when the wings are clean.

 

[MiG-29]

Re: J-20... The New Generation Fighter III

I'm going to have to give this one to quikie, mig, especially when it comes to all the generalities about canards, short or long coupled, several of us are aware of those general principles, but they are general and generally about western aircraft and design. Now there are two reasons I believe the forward fuselage is making a lot of lift, Dr. Songs paper,along with Tianlongs paper, both very well written and scholarly, and more importantly the large downward deflection of the canard, the eng stated he estimated AOA at approx 40 degrees in these turn, due to the canard deflection and he contends the canard is aligned with the relative wind. Now both of you guys have opened my eyes here, but the real head scratcher was this large downward deflection of the canard, Indy cars have had front wings with large downward deflection since the late 1960s, to prevent body lift or nose up pitch. While this does cause some drag, it prevents the indy car from "over pitching and loss of control". Thats why Dr. Song speaks of an all flying or extreme angle deflection of the canards, which are pitching the nose down.

---------- Post added at 09:07 PM ---------- Previous post was at 08:56 PM ----------



I made my statement in response to your #8 post in which you state the leading edge flaps decrease lift, did you mean they pitch the nose down? or am I missing something else. I think I still stand by my statement, but then I'm not the engineer, respectfully brat.

---------- Post added at 09:26 PM ---------- Previous post was at 09:07 PM ----------



You are on it today Jr., and whats been in the back of my mind is, has anyone around Chengdu heard any sonic booms, I don't think we've even seen the J-20 at high subsonic speeds, now they've made 70+ or so flights, I think its a little odd if the tallman hasn't opened the tap a little, and guys this is my own dumb thinking, so if I'm wrong and you know of mach 1 flights, please straighten me out. Has anyone seen a fast dash say to Mach ,85-90? I may have missed it.

Well airforce, negative deflections also do increase lift, pitch moment is the same, at high AoA, negative deflections of the canard increase lif and have the same effect of positive deflection (since they also increase lift) on pitch moment

So what you see in the J-20 at high AoA and turning is not pitch down, but lifting up the aircraft

See

ALPHA a 34 DEGREES
Kersh's work indicated that the lift coefficient increased as the fixed-canard
deflection angle was varied from 0 to -7 degrees, at this angle of attack. In this
regime, the canard/wing vortex interaction had a positive effect on the flowfield as
flow separation, and thus stall, was delayed.

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As it was mentioned in Volume 2 that small negative deflections had a
beneficial effect on performance at low angles of attack and only minor un-
"* favorable effects at high angles of attack.
In general, the incremental change in
lift due to either positive or negative deflection is relatively constant
with Mach number at each angle of attack for all configurations.

As angle of attack is increased, there is a reduction in incremental
moment for the positive deflection when compared with the corresponding
negative deflection.

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So a negative deflection at high AoA does not mean less lift, but actually they are trying to increase lift

---------- Post added at 10:24 AM ---------- Previous post was at 10:11 AM ----------

I think the fact that canards on J-20 has to pitch down more to hold the nose down makes it potentially more maneuverable. If you want tighter turns all you need is to do is to relax the amount of canard deflection downwards.

From what I saw in the video the J-20 is doing a whole lot of deflection could be due to:

-Turning at low speeds, therefore the rate of pitch of the nose needs to be limited, otherwise you are exposing more of the plane cross section to incomming air flow, increasing drag, makes the plane slow down more and eventually stall.

-Engine not at full power, therefore the aircraft is not accelerating enough and this will limit how fast it can turn. This is interrelated with the previous point.

-J-20 will due to have more powerful engine than it currently has, so with it the canard downward deflection could be relaxed, giving it faster response.

-As far as I see the video of J-20 shows it flying and doing maneuver at slower speeds than the F-22 videos.

-Using low speed performance to deduce supersonic performance may not be on track as the centre of lift shifts with different airspeed


Though I think I'll just wait for high performance revealing test flights like what we've seen with the F-22 demos to convince people

negative deflection increase lift at high AoA, the J-20 actually is trying to reduce the AoA of the canard.


Kersh's work indicated that the lift coefficient increased as the fixed-canard
deflection angle was varied from 0 to -7 degrees, at this angle of attack
TABLE 4: ALPHA = 34,

Canard Deflection -7 Degrees
This deflection produced a canard angle of attack of 27 degrees

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so as airfleet magazine`s claims

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.

J-20 is deflecting its canard to increase lift at high AoA

 

[Quickie]

Re: J-20... The New Generation Fighter III

Well airforce, negative deflections also do increase lift, pitch moment is the same, at high AoA, negative deflections of the canard increase lif and have the same effect of positive deflection (since they also increase lift) on pitch moment

So what you see in the J-20 at high AoA and turning is not pitch down, but lifting up the aircraft

See

ALPHA a 34 DEGREES
Kersh's work indicated that the lift coefficient increased as the fixed-canard
deflection angle was varied from 0 to -7 degrees, at this angle of attack. In this
regime, the canard/wing vortex interaction had a positive effect on the flowfield as
flow separation, and thus stall, was delayed.

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As it was mentioned in Volume 2 that small negative deflections had a
beneficial effect on performance at low angles of attack and only minor un-
"* favorable effects at high angles of attack.
In general, the incremental change in
lift due to either positive or negative deflection is relatively constant
with Mach number at each angle of attack for all configurations.

As angle of attack is increased, there is a reduction in incremental
moment for the positive deflection when compared with the corresponding
negative deflection.

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So a negative deflection at high AoA does not mean less lift, but actually they are trying to increase lift

---------- Post added at 10:24 AM ---------- Previous post was at 10:11 AM ----------

negative deflection increase lift at high AoA, the J-20 actually is trying to reduce the AoA of the canard.


Kersh's work indicated that the lift coefficient increased as the fixed-canard
deflection angle was varied from 0 to -7 degrees, at this angle of attack
TABLE 4: ALPHA = 34,

Canard Deflection -7 Degrees
This deflection produced a canard angle of attack of 27 degrees

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so as airfleet magazine`s claims

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.

J-20 is deflecting its canard to increase lift at high AoA

The study is on lift enhancement on the wing using closed coupled oscillating canards. It has nothing to do with the canard lift itself. You are just derailing the thread if you continue to post things that are irrelevant to the discussion.

19. ABSTRACT (continue on reverse if necessary and identify by block number)
A wind-tunnel study to investigate the effects ofdynamic stall of a close-coupled canard on the canard/wing vortex interaction for increased
lift enhancement was conducted. Two angles of attack of the model were studied: one at the first stall condition of the wing and one in the
post-stall regime where a strong leading-edge vortex was formed. Baseline force and moment parameters were measured at mean canard
deflections based on those determined to be optimum for the static case, as were mean values +/- 3 degrees about the optimum. The
amplitude ofoscillation considered was +/- 5 degrees about each mean; reduced frequencies tested were from 0.046 to 0.232. For most cases,
lift was enhanced beyond the static-canard case at mean deflections equal to those at or greater than the static optimum value. The effective
lift was decreased for mean deflections Jew than those previously determined to be optimum. Lift enhancements were generally 2 to 6
percent higher than the values determined with the static canard. The increased lift was generally independent of reduced frequency and
peaked between k values of 0.1 to 0.2.

LIFT ENHANCEMENT
USING A CLOSE-COUPLED
OSCILLATING CANARD

 

[MiG-29]

Re: J-20... The New Generation Fighter III

The study is on lift enhancement on the wing using closed coupled oscillating canards. It has nothing to do with the canard lift itself. You are just derailing the thread if you continue to post things that are irrelevant to the discussion.

i posted two studies and both say the same, i am not derailing anything, its you that can not understand negative deflections reduce the angle of attack of the canard it self with respect the aircraft AoA

this is the other study

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AERODYNAMIC CHARACTERISTICS OF THE CLOSE-COUPLED CANARDS AS APPLIED TO LOW-TO-MODERATE SWEPT WINGS
VOLUME 3: TRANSONIC-SUPERSONIC
SPEED

you skipped this claiming the oscilation are the reason for that increase, but they are not see

"As it was mentioned in Volume 2 that small negative deflections had a
beneficial effect on performance at low angles of attack and only minor un-
favorable effects at high angles of attack.
In general, the incremental change in
lift due to either positive or negative deflection is relatively constant
with Mach number at each angle of attack for all configurations.

As angle of attack is increased, there is a reduction in incremental
moment for the positive deflection when compared with the corresponding
negative deflection."

 

[Air Force Brat]

Re: J-20... The New Generation Fighter III

ALPHA a 34 DEGREES
Kersh's work indicated that the lift coefficient increased as the fixed-canard
deflection angle was varied from 0 to -7 degrees, at this angle of attack. In this
regime, the canard/wing vortex interaction had a positive effect on the flowfield as
flow separation, and thus stall, was delayed.

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Mig this 1992 paper by Schmidt is for a close coupled canard, and is not relevant for a long coupled canard which is the Primary pitch control in this low speed/high alpha configuration, the downward deflection of the canard reduces angle of attack of the main wing decreasing lift, just like the downward deflected spoiler at the front of an Indy car! Canard angled upwards, AoA increases, Canard angled downward, AoA decreases follow the KISS principle.

 

[Quickie]

Re: J-20... The New Generation Fighter III

i posted two studies and both say the same, i am not derailing anything, its you that can not understand negative deflections reduce the angle of attack of the canard it self with respect the aircraft AoA

this is the other study

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AERODYNAMIC CHARACTERISTICS OF THE CLOSE-COUPLED CANARDS AS APPLIED TO LOW-TO-MODERATE SWEPT WINGS
VOLUME 3: TRANSONIC-SUPERSONIC
SPEED

you skipped this claiming the oscilation are the reason for that increase, but they are not see

"As it was mentioned in Volume 2 that small negative deflections had a
beneficial effect on performance at low angles of attack and only minor un-
favorable effects at high angles of attack.
In general, the incremental change in
lift due to either positive or negative deflection is relatively constant
with Mach number at each angle of attack for all configurations.

As angle of attack is increased, there is a reduction in incremental
moment for the positive deflection when compared with the corresponding
negative deflection."

First of all, you mistakened the wing's lift in the studies as canard lift.

Secondly, the second study is that of a trisurface aircraft i.e. the study has tail elevators as the main pitch control surface. How is this related to true canard delta configuration?

Third, closed couple canards are mainly used for airflow, vortex, downwash control, whose deflections may go counter to the pitch direction but are deflected as such because it increases aerodynamic lift, mainly from the wing. But you got confused with this being an indication that negative deflection of the canards will still give a positive moment!

 

[delft]

Re: J-20... The New Generation Fighter III

AFB wrote:

I made my statement in response to your #8 post in which you state the leading edge flaps decrease lift, did you mean they pitch the nose down? or am I missing something else. I think I still stand by my statement, but then I'm not the engineer, respectfully brat.

When you deflect the leading edge flap you change the flow all around the wing. In the first place you increase the pressure on the upper side of the flap, decrease it on underside so reducing the lift from that part of the wing. That is only partly compensated by the air speed increase on the upper side of the wing aft of the flap and the corresponding air speed decrease below.

 

[MiG-29]

Re: J-20... The New Generation Fighter III

First of all, you mistakened the wing's lift in the studies as canard lift.

Secondly, the second study is that of a trisurface aircraft i.e. the study has tail elevators as the main pitch control surface. How is this related to true canard delta configuration?

Third, closed couple canards are mainly used for airflow, vortex, downwash control, whose deflections may go counter to the pitch direction but are deflected as such because it increases aerodynamic lift, mainly from the wing. But you got confused with this being an indication that negative deflection of the canards will still give a positive moment!

The canards were tested at several positions
and deflected from -10 to +10 degrees. In addition,
configurations consisting of a horizontal tail and a
canard with horizontal tails are analyzed.
The results of the analysis inidcate that

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DO you understand what the sentence in addition means? it means the study was done also to triplane configuration, not that the study was done only to triplane configurations, also the position was studied, which means they moved aft and forward the canard

---------- Post added at 05:16 PM ---------- Previous post was at 04:48 PM ----------

ALPHA a 34 DEGREES
Kersh's work indicated that the lift coefficient increased as the fixed-canard
deflection angle was varied from 0 to -7 degrees, at this angle of attack. In this
regime, the canard/wing vortex interaction had a positive effect on the flowfield as
flow separation, and thus stall, was delayed.

Please, Log in or Register to view URLs content!

Mig this 1992 paper by Schmidt is for a close coupled canard, and is not relevant for a long coupled canard which is the Primary pitch control in this low speed/high alpha configuration, the downward deflection of the canard reduces angle of attack of the main wing decreasing lift, just like the downward deflected spoiler at the front of an Indy car! Canard angled upwards, AoA increases, Canard angled downward, AoA decreases follow the KISS principle.

If you see Figure 1 -Sketch of Models, you can see the studied the horizontal separetion of the canards, which is confirmed in page 11 by this
"The effect of longitudinal canard position on the aerodynamic characteristics
of the 50-degree model had been well established by the time
the transonic wind-tunnel program was run. "

This was done in the previous study true but the position also have horizontal separetion and figure 100 shows it with P1 and P2 having the largest horizontal separation and gap bewteen wing and canard , but here is where the study becomes clearer, if the J-20 has a canard long coupled canard, but the canad position is near the center of gravity, which is detrimental as pitch control, plus the J-20 has not a very large horizontal gap between the wing and the canard, plus J-20 has not a vey large horizontal gap to say the canard has not effect on vortex lift.


Song said the canard was mover forward to increase pitch lever arm, why? simple simple,if you have seen X-36 JAST or any american stealth fighter with canard, you will see they set the canard and wing at the same level, this is detrimental to lift, but benefitial for stealth so what the chinese did is add a LERX.

This is in part confirmed by the paper

"The effect of canard longitudinal position on the incremental pitching
moment is shown in Figure 25. Data are for Mach numbers from 0.6 to 1.10
and the canard positions are P 1 and P As expected, moving the canard
forward increases the incremental pitching moment"


However if you see the distance of X-31`s canard and the wing you see J-20 has not such a large horizontal gap, not even compared to Eurofighter, it only has a large gap compared to Rafale or IAI lavi

He even claims canard and LERXes increase lift at high AoA, which means some vortex lift exists.


And my point was negative deflections do not mean pitch down at high AoA, i do not agree with you because a car and indy car in this case is not flying at high AoA but straight at 0 degrees