Re: J-20... The New Generation Fighter III
Not according to the americans
Results of the analysis indicate that the canard is effective in increasing lift and decreasing drag at Mach numbers from subsonic to high transonic speeds by delaying wing separation. The effectiveness of the canard is, however, decreased with Increasing Mach number. At supersonic speeds the canard has little or no favorable effects on lift or drag. It is further shown that the horizontal tail is a superior trimming device than the close-coupled canard at low-to-moderate angles of attack and
that a configuration consisting of canard, wing, and horizontal tail is superior in performance, to either canard or horizontal tail at high angles of attack.
Nope. The bold part of your quote says that a tri-plane configuration is superior to one that either has a horizontal tail or a canard. From
:
TRISURFACE CONFIGURATION
...
A sketch of this configuration is shown in Figure 20. The rationale behind the configuration is to use the horizontal tail for trim at low-to-moderate angles of attack and to supplement the tail trim power with the canard at higher angles of attack when the negative deflection of the tail causes large lift losses.
Thus, the above paper does not in anyway agree with you that tailplane is better than canard Not only so, but the use of canard to supplement the tailplane in trimming shows the tailplane is ineffective at high angle-of-attack. This is in agreement with another
which mentions tailplane is ineffective at high AoA:
A concentration of characteristic curves C[sub]m[/sub] for the tailplane setting angle φ[sub]t[/sub] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.
Thus, like Dr. Song's has pointed out, canard is superior to tailplane at high AoA:
Control surfaces placed in front of the center of mass, like the canards, are negative load control surfaces. Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions. Figure 7 compares the pitch down control capabilities of the canards and horizontal stabilizers. From the high AOA pitch down control stand point, it will be wise to use canards on the future fighter.
Now, the problem with your
is that its findings are not relevant to the J-20. To see why, let's look at one of its statements:
The canard, if in proper position for favorable interference, is not as efficient a trimming device for a stable configuration.
The first reason is that the paper analyzed a stable configuration, not an unstable one as in the case of J-20. The second reason is that the paper analyzed close-coupled canards as indicated by "in proper position for favorable interference", but J-20's canards are not close-coupled in the traditional sense. From Dr. Song's paper regarding canard position:
Canards on close coupled canard configuration aircraft have relative short lever arms. Employing the LERX canard configuration can increase the canards’ lever arms while retaining the benefits of positive canard coupling. Considering the overall lift enhancement effect and pitch down control capabilities, we can set the canards’ maximum relative area to around 15% and the maximum canard deflection to 90 degrees.
J-20's canards are placed so that they have a long moment arm for trimming. The LERX helps to enhance the vortices generated by the canards, thus retaining the benefits of close-coupled canards. Thus, Dr. Song solved the conflict of lift vs. trim in canard placement. The trimming difficulty of a closed-couple canard as described
simply doesn't exist on the J-20.
---------- Post added at 03:58 PM ---------- Previous post was at 03:32 PM ----------
Brat.
The aerodynamics of J-20 have been studied in the US long time ago, in fact the X-31 was studied with different tail arragements and the canard leading edge and trailing edges deflections are almost identical.
In fact i have just read a paper that shows it, i have not posted it yet because i am studying it and i will post it once i have understand it well its meaning.
Wrong. You may say that benefits and short comings of canard-delta configuration have been studied long ago, but that doesn't equate to J-20 aerodynamics being studied long ago. Even for aircraft that look very alike, such as the B-1B and Tu-160, you cannot claim one team studied the aerodynamics of another because the two aircraft has different aerodynamics -- tiny difference makes a huge difference in terms of aerodynamics.
The F-22 could have been fitted with canards if they had wanted, but in the US stealth was more demanding than anything so this complicated the configuration so going for a aft tail meant simplifying the design.
The Chinese had some limits that forced them to chose different solutions.
The Chined forebody, canard and wing commingling was studied in the US long time ago in fact in the 1980s.
This is what Song calls lifting body of J-20 since the vortices shed by the chined forebody and canard vortices interact with the wing allowing for excellent lift drag coefficients at low to medium AoA.
The vertical tail position you see in the J-20 was studied too on canard aircraft models and models of X-31.
For the american design philosophy, canards are not better than tailplanes and you can see it in that F-35 and F-22 do not have them.
Canards being superior to tailplane at high AoA is a fact supported by many papers, not a design philosophy. What is design philosophy is how designers of the aircraft go around the inferiority of a tailplane. In F-22's case, its designers employed powerful engines with thrust-vectoring nozzles to compensate. In J-20's case, engines cannot be relied upon so complicate aerodynamics are needed. In both case, the designers face the same problem, which is tailplane's inferiority at high AoA.
says the following:
A concentration of characteristic curves Cm for the tailplane setting angle φ[sub]t[/sub] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.
---------- Post added at 04:34 PM ---------- Previous post was at 03:58 PM ----------
Everything is design parameters and requierements.
I will tell you why i think in the US found tailplanes are better for their needs (your needs since you are american)
First, they say tailplane deflection is lower than canard deflection at trimming at low AoA, so it improves stealth.
Regardless of canard or tailplane, the deflection is going to be negligible at cruise phase where stealth matters. When large pitching-moment is needed, the aircraft would either be in WVR or evading missiles where stealth would be meaningless. Furthermore, the Americans have considered employing canards on their stealth aircraft numerous of times, so stealth isn't the driven factor that makes the US to go for tailplanes.
Second tailplanes can be used at poststall so there is no such superiority of canards.
In fact i will say to you that Chines as well as canards generate excesive pitch up at high AoA and this is regarded as undesired (at least in the american thinking) if you can not generate a good pitch down force plus canard requiered more deflection to achieve same pitch force.
That's your opinion which has no basis in facts. Tailplane cannot be used at post-stall. Take for example
. It is explicitly stated that tailplane losses effectiveness at high AoA:
It can be explained by loosing of effectiveness of control surfaces... In the range of AoA up to 35[SUP]o[/SUP] the normal increases approximately linearly, then stabilises and practically the tail surface losses its effectiveness.
, and says the following with regard to the effectiveness of tailplane at high AoA:
A concentration of characteristic curves Cm for the tailplane setting angle φ[sub]t[/sub] being varied at post-critical AoA (i.e. very low sensitivity of pitch moment with respect to the tailplane setting angle) reflects the loss of effectiveness of a horizontal tail at higher AoA.
Thus, tailplane becomes ineffective at high AoA, which means little to no pitch force is generated regardless of how the tailplane deflects. As explained by Dr. Song, canard works differently and remains effective, making canard superior to tailplane in high AoA situations. The exact statement from Dr. Song's paper is as follow:
Control surfaces placed in front of the center of mass, like the canards, are negative load control surfaces. Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions. Figure 7 compares the pitch down control capabilities of the canards and horizontal stabilizers. From the high AOA pitch down control stand point, it will be wise to use canards on the future fighter.
This debunks your claims that "tailplane can be used at post-stall" and "there is no such superiority of canards".
Third the US since the 1980s has experimented and studied TVC nozzles allowing the design to have a simplier aerodynamic configuration with no ventral fins, no canards and simplier FCS.
This. A readily accessible TVC is the main reason why US goes for traditional configuration rather than canard configuration. As explained in
:
Hence the reason that the post-stall region has only been a fairly recent area of study: T/W ratios needed to increase, C[sub]L[sub]max[/sub][/sub] values needed to increase, and non-aerodynamic controls (such as TV) had to be developed before an aircraft would be capable of controlled flight in this very adverse aerodynamic region.
The use of tailplane on the F-22 does not support your claim that tailplane is not inferior. It only means tailplane losing effectiveness at high AoA won't be an issue when thrust-vectoring is employed. However, the problem is still there as explained
:
if the aircraft can fly at angles of attack of 80[sup]o[/sup] - 120[sup]o[/sup] with the ability to maintain stability in all channels. In this flight regime the ability for conventional control is usually lost.
