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War Stories

Too Steep

Early 1968 Bruce Powell

Here is a technical jargon riddled story with added explanation at the end.

The Story:

Two recently trained AH-1G Cobra pilots (Tom Meeks and Bruce Powell) were flying a "UH-1C" (older model Huey) gunship; and at night; in bad weather. They were taught in the new Cobra gunship to use a much steeper attack angle on the target than with the Huey. This became a crew chief and door gunners worst nightmare. 

We were in a relatively vertical dive, firing rockets on a target. Tom realized something was wrong. The gunners had quit firing their door guns. He glanced back to see what was wrong and for a split second was mesmerized by the golf ball size eyeballs of the door gunner and the nearing death facial expression. The gunner was scrambling to hang on with every extremity of his body. I'm not sure what happened to the door gun. It probably was dangling outside of the aircraft on its bungee cord. Tom said something like "I think that's too steep" as we entered the beginning stages of retreating blade stall (a very dangerous and sometimes fatal condition).

Not sure how long we had been exceeding VNE (Velocity to Not Exceed; airspeed). Not sure how high we were; could hardly see the ground anyway with the crummy weather. The dive angle? No doubt that it was well beyond the tested capabilities of a loaded UH-1C gunship, but within the limits of a new AH-1G Cobra.

Some additional degree of panic set in as the aircraft began a stuttering left break on its own. This happens when aircraft is going too fast and the rotor blades approaching the left side (or retreating side) of the aircraft are downwind and not providing lift. On the right side they are into the wind and lifting big time.

The left break, or coming out of the dive to the left, was relatively smooth but was very severe and uncomfortable for those who had not yet experienced a near 3G pull out (Three times the pull of gravity). The fear of death and more large eye balls added to the stress. 

We made it without hitting the ground. Had there been any trees we might have came back with branches in the hull. It was close!

This might have contributed to some of the negative rumors about the insane Cobra pilots.

Maybe someone in maintenance can remember what parts on the rotor head had to be replaced. 

We are anxious to find Paul Delany because he may have been the door gunner that night.

The Technical details:

Fortunately I had some limited experience at entering Retreating Blade Stall (RBS); a very bad thing. As a spirited young pilot I studied some Bell Helicopter info on aerodynamics and had experimented with it a bit in a "UH1-B" model (an earlier model Huey with a substandard rotor system).  

The airflow through the main rotor system is different on the advancing side (the right side) and retreating side (the left side). The airflow over the advancing (right) side is higher due to the forward speed of the helicopter, while the airflow on the retreating (left) side is lower. As the airspeed increases retreating blade has to work harder to hold up its side (lift).
To generate the same amount of lift on both sides, the advancing blade flaps up (bends up) while the retreating blade flaps down (bends down). This causes the angle of attack of the blade to decrease on the advancing blade, which reduces lift on that side, and increases it on the retreating blade, causing increased lift. If the aircraft goes faster, like in a dive, low blade speed on the retreating blade, together with its high angle of attack, will eventually cause a loss of lift (stall).

Therefore Retreating Blade Stall is a major factor in limiting a helicopter’s top forward speed (VNE or Velocity to Not Exceed). The symptoms of initial RBS are a low frequency vibration, pitching up of the nose, and a roll in the direction of the retreating blade (left with Bell Helicopters). The high weight of a gunship (UH1-C), low rotor r.p.m., high density altitude, turbulence and/or steep, abrupt turns can contribute to retreating blade stall at high airspeeds. At higher altitudes it is even worse because the air is less dense.

The book says that correct recovery from retreating blade stall requires the collective (that's the up and down control in the pilots left hand) to be lowered first, which reduces blade angles and thus angle of attack. Aft cyclic (that is the fore, aft, left & right control stick in the pilots right hand) can then be used to slow the helicopter.

The normal reaction, which I did the first time I tried it, is to pull the collective up (tell the aircraft to climb or quit screaming towards the ground). Unfortunately this action will instantly worsen the situation as it increases the stall angle of the already stressed rotor blades (I must not have read that part the first time). In a Cobra, with its faster speed, this could become fatal, quickly. We lost some pilots learning about that. In the case of this story, down collective was not an option since it was already down to induce the high angle dive. My solution was leaving the collective down and providing a steady left turn to ease the left blade stall, then holding the nose from coming up and increasing the G force (maybe even lowering it a bit to unload the blades) to get us out of the vibration phase; then slowly bring in aft cyclic (nose up) to keep the ground from coming up too fast and hitting us. This maneuver is more frightening than playing chicken with an earthmover.

The Analysis:

1. Had one of the pilots not have been "Cobra" trained there would have been immediate input at the start of the gun run: "Holy Shit!!", allowing more time for recovery.
2. Ten demerits to stupid pilots for getting into this situation.
3. Ten merits for pilots having the skill/wherewithall/luck to recover.
4. Some crew members may never be the same.

Bruce Powell, Cobra Pilot