The last two sessions were both 2hr ground training blocks focused on Aerodynamics of Hazardous Flight. Sounds kind of intense, but the point of it all is to make sure that when (not if) something unexpected happens I know what to do. Makes good sense - be prepared.
These are the topics we covered:
Dynamic Rollover:
Description: Uncontrollable roll of helicopter under power.
Requirements:
1) Pivot point
2) Rolling moment
3) Main rotor (MR) or tail rotor (TR) thrust
Example: During a hover your skid gets caught on a runway landing light.
Static Rollover
Description: Uncontrollable roll of helicopter.
Requirements:
1) Pivot point
2) Rolling moment
Example: Landing on a slope where the settling angle of the helicopter exceeded the critical rollover angle. Basically, falling over.
Retreating Blade Stall
Description: Unintended and dangerous motion in forward flight at hight speed. Because the advancing and retreating blades will have different relative airflow, certain conditions can cause turbulence in the outer region of the retreating blade.
Factors:
1) Too hight of an airspeed. Above helicopter design limitations.
2) Over / High total weight
3) Low rotor RPM
4) High density altitude
5) Turbulence
6) Abrupt turns
Example: High speed causes low frequency vibration, pitch up and roll.
Low Rotor RPM / Blade Stall
Description: Total failure of main rotor blades resulting from low RPM
Factors:
1) too high of a pitch angle on MR blades
2) governor failure
3) high density altitude
4) pilot error
5) heavy, hot, humid & high
Example: The engine is under too much stress because of the force needed to drive the rotors and drop below acceptable ranges.
Recovery: Lower collective & Roll up throttle
Settling with Power / Vortex Ring States
Description: Full power cannot prevent descent. Due to the MR blade tip vortices, that occur normally, the helicopter gets caught in it's own downdraft and cannot maintain altitude.
Requirement Triad:
1) Power of at least 18' manifold pressure
2) Airspeed less than ETL
3) Descent rate > 300 feet / minute
Example: Coming in for a landing and not maintaining appropriate airspeed with too fast of a descent.
Recovery: Lower collective to reduce vorticies and increase airspeed (to get out of down-wash)
Low G
Description: When climbing a sudden pitch down removes all weight load from the main rotor causing excessive roll to the right. Significant possibility for mast bumping and main rotor separation.
Recovery: Re-load the main rotor with aft cyclic.
LTE (Loss of Tail-Rotor Effectiveness)
Description: The anti-torque rotor no long is providing the thrust to prevent intense yaw.
Types:
1) Main Rotor Disc Interference: If a wind is blowing at the helicopter from the front left, the turbulence generated from the MR can be directed into the TR - reducing its thrust.
2) Weather-Vane Effect: If there is a strong tail wing, the helicopter will tend to act like a weather vane (just like on top of a barn) and point head-in to the wind.
3) TR Vortex Ring State: Similar to "Settling with Power", a strong wind opposite the thrust of the TR will force it's own turbulent air into its path... decreasing the overall thrust.
Lots of bad things here... obviously. There will be plenty of test situations during real flight to practice the recovery procedures. Kristie mentioned that at points later in the training she will covertly be bringing power down and looking for me to recover. That will be interesting.
1 comment:
Good stuff to be talking about.
On vortex ring state. I disagree with the part about 18" MP. I believe you can enter a VR state at lower power settings than that. The guy who does our recurrent training teaches the 1-2-3 rule for conditions to get into VR state. 1 = 10 KIAS (ie. below ETL), 2 = 20% power (on a turbine, the point being that you must have some power), 3 = 300 fpm or more descent rate.
In the flight portion of the Robinson Pilot Safety Course we practiced recovery from VR state. As you start to get into it, the helicopters tells you by shaking fairly violently and then starting to drop like a rock. We did it from 2000 AGL as I recall!
They show a film clip at the course taken during a shoot for a commercial in Malaysia. It shows an R44 making an approach to land at the top of the skyscraper. The pilot started to get into a VR state, recovered and would have made a hard landing on top of the building. Instead of accepting that and flooring the collective, he tried to save the helicopter and, in doing so, did a dynamic rollover with a railing as the pivot point. My recollection is that he fell about 60 stories. There's a moral in that: if something bad is happening accept damage to the ship (that's why you have insurance) and worry about saving your butt instead!
On low RPM rotor blade stall. I was taught that if fully developed, this is an unrecoverable condition. I've seen a home video taken in Britain of an R22 Mariner crashing because of low RPM blade stall. It's a sobering video. Legend has it that it was this particular crash which prompted Frank to start installing governors. The aftermath of such an accident has a sign: the blades are almost always bent upward because there's insufficient RPM to generate enough centrifugal force to support the weight.
On retreating blade stall. Fly enough and it will happen to you. My first encounter was bringing our Raven II back from California. Robinson insists you follow I10 to avoid the mountains. Somewhere past Phoenix on the way to Midland Texas, I had the pedal to the metal, so to speak, pulling 23" MP and making 120 KIAS at around 6000 MSL in high temperatures. At some point the helicopter started to shudder. When helicopters shudder they are always trying to tell the pilot something. In this case the shudder was due to incipient retreating blade stall. It's not the end of the world -- I just lowered the collective a bit and brought in a little aft cyclic. And shed off about 10 KIAS. No harm, no foul.
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