MAD-4A wrote:The stress of a gun firing on a platform is transferred through the air frame to the support,
After that whopper, who needs to read the rest of what you wrote?
Recoil is a Force x TIME = impulse = Delta moment of inertia not stress. How much mass is being moved in a different direction? Depends on the aircraft. Lets ignore deformation/deflection even though we all know a lot of said energy will be absorbed in this fashion.
So, initial mass moment of inertia helo plus delta MMI(mass moment of inertia) will then equate a delta MMI to the rotor system which has its own initial MMI. Lets look at said system. Is it rigid? Nope, not at all. Giant floppy blades, but I said I was going to ignore deflection so...
Does the helo have to stay level? No, what happens is that the helo will move in the direction of the "dip" caused by said delta MMI vector. So the delta MMI delta change of aircraft will be the MMI transferred to rotor is ENTIRELY dependent on how much MASS the aircraft in question weighs minus vibration damping(heat+singing). OF course you have control feedback loops depending on HOW LONG the impulse duration impinges upon the craft.
If it was firing continuously then the engineering problem would relegate back to the need of keeping said helo hovering instead of slightly spinning(sliding[FOV]). There will always be some moment induced. How much depends entirely on the mass of the aircraft, burst duration(impulse imparted), rigidity of the structure(craft+blades), CG distance from rotor system(lift) of said impulse, and finally, rotor shaft diameter and its tolerance for this new strain creating ultimately an additional stress.
What is a typical helo's delta rate of rotational change for normal flight said rotor system endures fatigue Million life cycle point? Hmm? I have not personally seen this side of fatigue analysis as I have only worked on airplanes and their fatigue analysis, but I would be shocked if this rotation point is not mandated at 1 +++million+++ cycles as the rotor will be spinning at 120ish RPM on a BIG bird even at idle(vibrations from wind gusts alone sitting on the tarmac at idle). So each maneuver under normal conditions will require the shaft in question to undergo an enormous number of cycles in short order. So, in hover or direct flight, firing a gun, this additional fatigue induced strain will not even become a blip on the fatigue rotor mast head in comparison to normal flight ops.
Obviously, all things can be taken to the extreme where the MMI of the aircraft is << MMI of the gun continuously firing. Lets not be absurd though.
Long way of saying: You ain't a mechanical engineer.
If you are, do yourself the favor of rough WAG MMI calc with the GAU 8 20,000lb force for 1s burst to a 20,000lb helo with a rotor mass 50 inches from the CG. Assume the rotors are rigid. Assume helo under normal flight ops have rotor rated at infinite stress duty cycle for a rate of change in its MMI of 30 degrees/second(big helo) and rotor spin rate of 200RPM. A HUEY is around 300-400RPM, but obviously is a tiny helo, so... Lets go with Chinook style. Hmm think Ch-46/47 is around 300RPM as well. Call the rotor in question 1000lbs with a radius of 30 feet. In either case, doesn't really matter. So, compare MMI from gun compared to MMI induced by normal flight ops. The flight ops are far greater. I would go so far as to say Vastly greater.
