@
gambit
Just wanna ask about launch rails, JF-17 launch rail looks odd as compared to the launch rails of F-16. What do you think about launch rail of JF-17, would it put more stress on wing as compare to the launch rail of F-16 ?
You touched on a not very glamorous but very important subject.
We all know what is centripetal force so am not going into details on that here. Because of centripetal force, the weight limit on the F-16 missile rail launcher is about 160 kg or 350 lbs. But that is not all there is about these devices.
What is the center of mass of an object ? And how important is it ? In high performance motorcycles, the idea of 'mass centralization' is important, it can mean winning/losing races.
Mass Centralization
Motorcycles are naturally mass centralized because of the vertical “sandwich” of rider, fuel, and engine. The only masses that are not centralized are the wheels, but the developmental events of the past 40 years have greatly lightened wheels, tires, and brakes. No one company has led this process; really, racing has accomplished it.
If you look at the recent street motorcycle designs, many of them moved towards this idea, most noticeably having the exhaust silencer, often several kgs, moved to underside of the motorcycle. In the past, we have this mass off center: to the rear and to either port or starboard, affecting stability and controllability during rapid maneuvers.
Missile designs are designed similarly, as in we want to have as much 'mass centralization' as possible, and it is even more important when the missile is carried on an aircraft. Rocket fuel, solid or liquid, is an expendable mass. But solid fuel have an innate advantage over liquid fuel -- slosh. Or rather solid fuel do not slosh -- move -- under intense maneuvers. Motorcyclists know very well the different behaviors of their bikes between empty, half empty, and full fuel tanks.
Here is an example of the importance of center of mass in rocket design...
Rocket Control
The
motion of any object in flight is a combination of the
translation of the center of gravity and the
rotation of the object about its center of gravity, which is also called the
center of mass.
So what does this have to do with air-air missile rail launcher designs ?
Here is a reasonably accurate breakout of the AMRAAM...
http://fas.org/man/dod-101/sys/missile/amraam-5.jpg
Look at the propulsion section. Once the missile is in flight, the missile have to deal with the forces of physics on its own according to the NASA lesson above. However,
UNTIL the missile is launched, it is the parent aircraft who has to deal with the forces of physics that acts upon the missile while it is still attached to the aircraft, all 160 kg of it.
It is important that the rail launcher have the missile's center of mass in line with the wing's strongest supporting members going all the way back to the wing root. Remember centripetal force. We have a 160 kg mass attached to the outer most reach of the aircraft -- the wing tip. This body is not uniform like a ball, but like a tube and the internal volume of this tube are not uniformly filled.
We can have the rail long enough to hold the missile, but if the missile's center of mass if in front or towards the rear of the wing tip, we can do serious structural damages, such as deformation due to twisting forces, to the wing itself. And if the maneuvers are intense enough, as in a 9g capable aircraft, we may even rip the entire contraption off the wing tip under those intense maneuvers. Not only that, we
WILL launch the missile while under those intense maneuvers, and if the missile's attachment to the wing is not stable because its center of mass is not supported by the most powerful member of the wing, missile launch
WILL be erratic, the missile may lose flight stability which will affect targeting which will result in a miss.
As far as the rail launcher for the JF-17 goes, I am confident that Pakistani engineers have all the physics and the math figured out and designed an appropriately robust device. So the issue, not problem, to me, is more about potential future upgrades. If the physics and the math says the wing cannot handle more the so-and-so mass at the wing tip, the JF-17 will be limited to whatever current load it has today.