J-20 compared to the F-22

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lookieloo

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Unread post19 Aug 2013, 18:58

sprstdlyscottsmn wrote:trolling for a troll lookieloo? classy... :roll:
Hmm, I see your point. Deleting and checking out for awhile.
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Unread post04 Oct 2013, 00:24

I was reading up a bit on canard-delta wing configurations, and came across the claim that the Grippen is aerodynamically stable when it's canards are set loose to free-stream -- which in turn mean that the canards have to provide positive lift in level flight, rather than negative lift. That got me thinking that determining stable-unstable for the J-20 might be fairly complicated. The airframe might be aerodynamically stable without the canards, but unstable with them.
Though it is still shackled with Chinese engines.
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sprstdlyscottsmn

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Unread post04 Oct 2013, 04:38

stability is determined by what and aircraft does without it's pitch controlling surface (canards or h-stabs). If the nose trends down, it is stable. a Stable canard has the same benefit as an unstable tailplane, no trim drag and faster pitch onset.
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linkomart

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Unread post04 Oct 2013, 06:52

sprstdlyscottsmn wrote:stability is determined by what and aircraft does without it's pitch controlling surface (canards or h-stabs). If the nose trends down, it is stable. a Stable canard has the same benefit as an unstable tailplane, no trim drag and faster pitch onset.


IMHO it works a bit different.
Stability is determed by where the center of gravity is compare to the aerodynamic neutral point. At old aircrafts with mechanical controls, stability can be "stick free" where you dont apply forces to your (pitch) control surfaces and with those locked (you hold the stick). In an airplane with servo controlled surfaces the stick free condition does not appy, unless you disconnect the servo, wich is seldom possible to do.

Gripen carries positive lift on the Canard at all (most) of the cg envelope in level flight. Don't know where the idea came from that a unstable canard aicraft have negative lift on the canard in normal flight, that simply isn't the case.

As for the J-20. I don't think it is a blueprint copy of some Western/Russian design. They have thoughti it trough quite a lot. Good or not? Only future will tell.
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em745

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Unread post04 Oct 2013, 08:35

linkomart wrote:Gripen carries positive lift on the Canard at all (most) of the cg envelope in level flight. Don't know where the idea came from that a unstable canard aicraft have negative lift on the canard in normal flight, that simply isn't the case.

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Unread post04 Oct 2013, 11:19

linkomart wrote:Gripen carries positive lift on the Canard at all (most) of the cg envelope in level flight. Don't know where the idea came from that a unstable canard aicraft have negative lift on the canard in normal flight, that simply isn't the case.

This is what happens to an unstable canard-delta when the FLCS has a major meltdown:

http://www.youtube.com/watch?v=o0Q0Qywyd_4

Now then, how is a normally functioning FLCS supposed to keep that kind of inherent unstable behavior in check if not by applying quasi-constant DOWNFORCE on the nose via the canards?
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linkomart

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Unread post04 Oct 2013, 11:43

em, that picture is wrong. A tailed aircraft can have positive lift and still be stable. If you don't believe me, just pull up a w&b sheet for any kind of private plane like a cessna and you will se that the rear center of gravity is behind 25% MAC of the wing.
Same goes for an unstable canard, depending on design and center of gravity, most of the time the canard is loaded with lift. At least on the designs I have worked with.

I think you are confusing centre of lift (usually presented subsonic as 25% MAC) and neutral point (the cg point where the stability of the aircraft is neutral)

As for Gripen crash no 1 and 2, they were not exactly because of a meltdown of the FLCS system. They were caused by PIO because of software that didn't expect the input presented. (Short version)

J-20 probably have lift on the canard at normal cruise, anything else would be strange to my humble experience.
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Unread post04 Oct 2013, 21:51

linkomart wrote:em, that picture is wrong. A tailed aircraft can have positive lift and still be stable.

:bang:

http://bit.ly/1cdXo9F

ANY plane--wing-tail or canard--with its CG behind its CL is de facto unstable.

You through trolling now?
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Unread post04 Oct 2013, 22:40

em745 wrote:
ANY plane--wing-tail or canard--with its CG behind its CL is de facto unstable.



If you by CL means NP, neutral point, we agree. CG must be in front of NP for a stable aircraft.

If you by CL means total center of lift, we disagree, total center of lift goes trough center of gravity, if thrust and drag can be disregarded.

If you by CL means center of lift on main wing, we disagree. Center of gravity can be behind center of lift on main wing for a stable Aircraft.

One example is this CG range on an MCR 4S (P19)
http://www.ecofly.ch/cms/upload/dokumen ... 270105.pdf

As I said, my objections were against those statements, as I find them wrong:

1: A stable Coventional (tailed) Aircraf talways have downlift on the tail.
2: A unstable canard Always have downlift on the canard.

Feel fre to try to educate me, no one is to old to learn something new.

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cola

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Unread post04 Oct 2013, 23:48

em745 wrote:You through trolling now?

Actually, are you?
Linkomart said that Gripen carries positive lift on the Canard at all (most) of the cg envelope in **level** flight.
At low (cruise) alphas, plane's Cp is somewhere around wing's trailing edge, which is still way behind your ordinary CG.
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Unread post04 Oct 2013, 23:56

As I said, my objections were against those statements, as I find them wrong:

1: A stable Coventional (tailed) Aircraf talways have downlift on the tail.
2: A unstable canard Always have downlift on the canard.

Right. It would be more correct to say that they have to provide less uplift at a given angle of attack than the main wing, not necessarily any downlift.

I don't know about anyone else, but I realized that I had a couple of misconceptions about this subject. For one thing, trying to phrase it in terms of "center of lift" and "center of gravity" is probably an oversimplification. For another, aircraft generally don't fly at a zero angle of attack.
The whole airframe is subject to torque forces around the center of gravity -- torques that balance when ever the rate of pitch in constant. What is important is how the torques change with angle of attack -- and that can be complicated.
Take that example of the Grippen. With it's canards fixed at a level flying trim, an increasing angle of attack increases the pitch-up torque making it unstable. With it's canards free to weather-vane, the pitch-up torque decreases as the AoA goes up, making it stable.
What I find interesting about this is that, (I think) you could actually build a functional "unstable" canard aircraft without a flight control computer if you had a mechanical way of exerting a constant torque on the pivot of the canard while otherwise letting it weather-vane.[/quote]
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linkomart

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Unread post05 Oct 2013, 06:29

One of the links provided by em745 is illustrative if you wants to calculate stability for different configurations.
http://adamone.rchomepage.com/cg_canard.htm for canards and
http://adamone.rchomepage.com/cg_calc.htm for tailed Aircraft.

It is a simple way of calculating NP, but IMHO it works pretty well.

As for the J-20 Center of gravity, at first glance the main wing looks to be waaaaay aft, but when you take the ruler and draw the lines it looks fairly "conventional". It is probably the relatively wide fuselage that fools my eye.

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Unread post05 Oct 2013, 11:21

linkomart wrote:One of the links provided by em745 is illustrative if you wants to calculate stability for different configurations.
http://adamone.rchomepage.com/cg_canard.htm for canards and
http://adamone.rchomepage.com/cg_calc.htm for tailed Aircraft.

It is a simple way of calculating NP, but IMHO it works pretty well.

As for the J-20 Center of gravity, at first glance the main wing looks to be waaaaay aft, but when you take the ruler and draw the lines it looks fairly "conventional". It is probably the relatively wide fuselage that fools my eye.

my 5 cent

The problem there is that the calculators only look at wing lift, not body lift.
If you look at pictures of the J-20, the main landing gear seems to be very near the leading edge of the wing root. You would think that would have to make it stable, but the amount of body lift it gets isn't something we can just spit-ball.
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Unread post17 Jan 2014, 22:55

J-20 has been changed slightly, namely in the intakes (which now more resembles the F-35, and brings the LERX in line with the canards when viewed from the side) and the actuator fairings on the wings, which are smaller and no longer make 90 degree angle with the wing. New paint scheme looks pretty neat, and the edges resemble the F-22's. Ventral fins are here to stay, it seems. Overall, not too shabby I suppose...

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Unread post17 Jan 2014, 23:41

I don't see how this bulky thing will be able to supercruise or even turn effectively. It's the size of a bus and not even aerodynamically unstable.
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