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JF-17B Updates, News & Discussion

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Are the "B"s still grounded or cleared for flight again?
You just need to search and read on the forum.

 
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Just some stuff for nerds to geek out on:
1. The aircraft is flying at a lowish speed in this picture. This is evident from the increased angle-of-attack (AoA) - nose pointing slightly upwards relative to the direction the jet is flying in. Furthermore, the downward deflected leading edge flap is also indicating this (this is done by the flight control system to allow flight at high angle-of-attacks).
2. The horizontal stabilizer is deflected down. Assuming that the aircraft is picture in trim this actually suggests that the aircraft is stable in pitch because the hor-stab is deflected to produce downward force (dynamic stability can be "imagined" by imagining a disturbance that increases angle of attack and then seeing if the aircraft naturally decreases its AoA or increases it).
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At least that's what I think. I might be wrong - wrote this in a hurry.
Just to add some more (possibly unnecessary) detail to @The Raven's excellent post:
Things get even more complicated for fighter jets, which have to fly both subsonic and supersonic. Basically the center of lift shifts significantly (moves aft) as you transition to supersonic flight.
View attachment 753916
If you want to be stable throughout the envelope (as I suspect JF-17 is) you have to design the subsonic center of lift to be just behind CG but this means supersonically the center of lift moves quite a lot futher back. The moment generated by the lift (about the CG) must be balanced by a tail. Of course the tail must be designed for the worst case (supersonic). Notice tail generates downforce (not lift) so it's wasting energy in cruise.

As you can probably imagine, you can have the situation where your jet is stable supersonic but unstable subsonic.
View attachment 753917
Here your tail can be smaller and actually produces lift subsonically. BUT now you require active stabilization in subsonic flight.



Finally you can have
View attachment 753918
This is the kind of situation you have in aircraft like X-29 (nevermind that it has a canard instead of a horizontal tail). In this design you are always unstable but you never produce unecessary downforce so you are aerodynamically efficient.


So in addition to the advantage in turn rate, being unstable makes you cruise more efficiently (that is you can fly further on less fuel) because you don't produce downforce and your tail can be smaller sometimes.

So why aren't all jets unstable then?
1. You require active stabilization through FBW system - this can be expensive and/or heavy and/or out of your technological ability.
2. Being unstable imposes fundamental limitations on robustness to off-nominal behaviour. So an unstable design stabilized by a control system is more sensitive to changes in weight distribution etc than a stable design, and might be less versatile.

Sorry if this is too much detail lol.
 

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