Ground Resonance
This is a good to know little aerodynamic factoid. Either knowing about it or not knowing anything about it can result in the sudden destruction of your three bladed aircraft with about a one second warning.
Three bladed rotors have blades flying at almost a perfect 120 degrees apart which creates what is known as a three-per-rev vibration. When a helicopter is running it not only shakes itself, but wouldn't ya know it...It shakes other things as well. Like you, and metallic things prone to fatigue, That can of coke on the dash, but yea, that's not important in this example.
The ground is the other thing that is shaking. Now as long as there are PROPERLY SERVICED struts between the rotor system and the ground, you are OK, well, most of the time... But the oleo struts along with struts mounted to the rotor blades absorb a lot of this nasty vibration. And that's why Helicopter pilot's fillings last at least one year these days!
Back to this ground resonance thing. So when the machine is running (Shaking) it is shaking through a lot of things (You, the fuselage, the landing gear, and oleo struts) which are all absorbing this shaking. The fuselage cracks over time relieving it's stress. Pilots have blood vessels breaking, aneurisms and things like that, which also relieves some of this vibration. The landing gear gets replaced often enough so I won't even count that. And those oleo struts get checked on post flights, daily servicing, and pre-flights. No chance they could fail in flight due to say a 2 mile high pressure change or getting banged off a mountain side a couple dozen times. Nope, they SHOULD be OK!
But if they aren't OK and at the end of the day you decide to do something, like, let's say, land! Well if you do that and maybe touch down just a bit funny, you could have a problem. Aerodynamically, I can't quantify "Funny" but as an instructor pilot if you did it, I'd say, "That landing was a bit funny," And you'd understand.
OK so you landed a bit funny and when you did you translated a bit of that humor up through the landing gear, along the fuselage, across your buttox, upward to where all the turning meets all the stationary stuff, and, well if luck isn't with you...or if it's this helicopter's time of the month (They are all female!) well if all that happens then one of those flyin' diving boards might move a tiny bit out of that balanced 120 degree separation.
That isn't a good thing. Nothing good has ever come from that. In all RHH (Recorded helicopter history) there has never been known to be anything good about blades out of phase...nothing!
So the unbalanced and upset rotor systems responds negatively. It pushes back a pulse. Ya know it's like the other night in bed, the wife and I were all cuddled watching shark tank. The mood was good, the show was interesting and it was just nice. As she snuggled into me, and I reached around and pinched a wee bit of fat. Just a small thing really. Wouldn't have registered on the marriage rector scale at all I would have thought, let alone lead to the San Francisco quake of 1908 (Was it 1908??) OK so I slept in the guest bedroom wondering what happened, but really just happy that most of my hearing was still intact!
Rotor blades out of phase are like that. So this push back ends up in the ground which says, "Yo Helicopter, you pushin' on me? You know I don't like you in the first place. Allow me to diverge. In my book I explain the real reason helos fly. Not because of all this mumbo-jumbo aerodynamic hype, Nope that stuff is just not true! The truth is that the earth repels the ugly shaking thing with crew chiefs in the back which are always peeing all over the ground. It repels the helo, pushing it away into the air, ya know to get rid of it, like an unwelcome guest.
Well when this pulse from the outta' phase blade hits terra firma and the ground swings back, and a fight starts. The pulse from the ground pushes that blade a bit more out of phase which generates an even bigger force. Pilots can't react because they have just expired from coronary failure. So the resulting fight between the ground and the helicopter just tears the helo apart and the ground wins...every time.
It all happens in just seconds and survivors of the coronary often regain consciousness and find themselves wondering where their helicopter went, and why did it leave them sitting here in the middle of this smoking scrap heap.
That's ground resonance. Bad for helos, bad for the earth, and as I discovered last night, bad for marriages!
