31 May 2006

The Aerodynamic Brick

We had a little discussion about rotors in the comments to my post, "When Bad Things Happen", concerning the fatal helicopter crash near Blythe, California.

Let me take a stab at explaining what MAY have happened in that accident to cause that aircraft to crash. (Ya gotta visualize with me now!)

When you fly a helicopter, you are not flying the whole helicopter. You control the rotor, and the rest of the helicopter comes along for the ride because it is supported by the Main Rotor.

The subject helicopter had a Semi-Rigid rotor system. I've discussed the fact that many helicopters have this type rotor........the Huey, JetRanger, LongRanger, Bell 47, Hiller, Robinson......most, if not all, helicopters with two rotor blades have this type rotor system. It is simple, relatively inexpensive, and takes up little space when parked in a hangar.
But it MUST be flown within a certain envelope!

Imagine I have a pail filled with water.
To the handle on the pail, I attach a 4' length of rope. If I lift the rope, I pick up the pail. If I move the rope, the pail hesitates momentarily due to inertia, then follows along. If I stop the movement of the rope, the pail will actually continue to move because of inertia, and will finally settle beneath my hand, holding the rope.

You can compare my hand to the point where the rotor attaches to the helicopter at the Main Rotor Mast. The helicopter itself can be compared to the pail of water.

Now, find yourself a ladder, and, still hanging on to the rope/pail of water, climb 3' up the ladder. Now quickly move your hand downward, and while there is slack in your rope, move your hand left, right, then back to center before the descending, (weightless), pail takes the slack out of the rope. Obviously, under these conditions you did not have control of the pail......there was no direct connection between your hand and the pail while there was slack in the rope.

The same is true with Semi-rigid rotor systems on helicopters. You are in complete control so long as the helicopter is suspended beneath the rotor. But if allow the helicopter to get into a regime where it is near-weightless, you are courting disaster.

Helicopters are complex, wonderful machines. Many parts move and turn on them. When something moves or turns, there is a corresponding and opposite reaction to that action.

Remember that thing at the rear of all the above mentioned helicopters?
That little rotor on the back? Even when the helicopter is weightless, it is still working. When the helicopter is weightless, or near-weightless, the tail rotor has very little mass to contend with, so it becomes super effective. Because it is moving air from one side of the machine to another, it can induce a roll in the helicopter.
The pilot normally stops unwanted movement around the "roll" axis by moving the Main Rotor in the opposite direction by applying opposite cyclic stick.

But here is where he is in danger:
This roll is not being caused by the Main Rotor, and cannot be stopped by moving the rotor in the opposite direction. This movement is being caused by the tail rotor thrust acting on a helicopter that weighs very little in this scenario.

There is a finite amount of space between the Main Rotor and the Main Rotor mast. Most of these machines have what is called a "droop stop" or "static stop" that is a reinforced part of the Main Rotor hub that bumps the rotor mast while the rotor is turning slowly or stopped. Most of us have seen a Huey or JetRanger at rest on the ground, rotor untied, slight breeze turning the rotor into a "teeter-totter". This is normal. This slight bumping causes no damage.

But the rotor system is not designed for that "static stop" to make contact with the mast at normal rotor operating speeds.

Remember the pail beneath my hand?
Under normal circumstances, the rotor moves, and the helicopter follows along. The distance between the static stop and the main rotor mast is never compromised.
In a near weightless condition, the helicopter fuselage (pail), will not follow along beneath the Main Rotor attachment (my hand). So the rotor can use up all the space between it and the mast......and the static stop can make contact with the rotor mast with tremendous force. This force can sever the main rotor mast, causing the main rotor to depart the machine.
When the main rotor departs, it can (and has), cut off the tail cone, or cut the cockpit in half.

Instructors have been aware of this phenomenon for almost 40 years.
It's one of the things we teach our students to avoid.

That's why it was such a puzzle to me that a pilot with the amount of experience this gentleman had would crash and die under conditions that he should have been able to control.

I'll keep an eye on the investigation and let you know how it progresses.



GB, thanks for the detailed description of this accident. I appreciate it.

Now, how about a block of instruction on gimble lock? I'm up for it.

Mike Poole said...

OK I am a nerd, but here goes courtesy of Wikipedia:

"n gyroscopic devices controlled by Euler mechanics or Euler angles, gimbal lock is caused by the alignment of two of the three gimbals together so that one of the rotation references (pitch/yaw/roll, often yaw) is cancelled. This would require a reset of the gimbals using an outside reference. It may also be described as the situation when all three gyros hit the limits of their ability to move within the sensing mechanism - they hit hard stops and stop moving around."