Posted: 4/12/2010 7:20:47 AM EDT
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I've been watching the "Ax Men" series on the History Channel and I've watched the helicopter logging segments with some interest. However, I've only read about flying helicopters...
I understand there is a "Dead Zone" in the operation of a helicopter where there isn't enough speed or height to autorotate if power is lost. I'm wondering when heli logging how much time is spend in the "Dead Zone". It seems to me as if they spend a lot of time at 150-250 feet with low or no airspeed. Is it the end of the road if they loose power? |
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Been in helicopters for 12 years and 2,500 hours. Never heard of a dead zone.
Long line operations in an OGE hover engine failure are not good, but not a certain death either. First you have to get the line off of the helicopter then while your setteling you need to conserve rotor inertia while gaining a little airspeed to get out of your down draft. It can be done, seeing the skill of the senior logging pilots, I would think it would result a good result. The new guys they are showing are flight school newbies, but the senior guys are normally former military high time/experiance guys with some decent skills. |
I got the name wrong. In this Wiki Article They call it the "Dead Man's Curve or Coffin Corner". The article comes with a nice graphic too.
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I got the name wrong. In this Wiki Article They call it the "Dead Man's Curve or Coffin Corner". The article comes with a nice graphic too. coffin corner is a fixed wing term at high altitude where the stall and over speed V's close in on each other. |
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It's the "Dead Man's Curve" of the height/velocity diagram.
There's kinetic energy stored in your speed and minimal amount stored in the rotor RPM. Potential energy is altitude and you trade that for rotor RPM during an autorotation. At the bottom of an auto you flare, bleed off all your speed / kinetic energy driving the rotor. Then once you're close to zero ground speed and just feet off the ground, you use the remaining energy in the rotor to level the skids and cushion the landing. When you're inside the "dead man's curve", you're too slow or too low to be able to have the kinetic energy to successfully flare and land the aircraft without any damage. If you're above 400-500 ft at zero airspeed, you can dive to pick up the speed necessary for the flare by trading off that altitude. Likewise if you're low, but going really fast, you can trade off that excess speed for a little altitude. If you don't carry at least 45-50 kts going into a flare, you will sink right through, crash the helicopter, a risk rolling it if you have any speed. Everything I mentioned here happens in matter of seconds. Descent rates are 1500-2000+ ft per minute, so if you're flying at 500 AGL at a normal cruise speed, you'll be on the ground in under 30 seconds. With logging, the helicopter is screwed anyway. There's no where safe to land, you're out in the woods. If the engine(s) quits, you're going into the trees anyway, so you just hope you land level. Those helicopters have so much rotor inertia, if they punch the load off, they might be able to arrest the descent rate from 100+ AGL with the stored energy in the rotor. |
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Once the engine quits, you don't owe the helicopter anything. You do whatever it takes to walk away from the crash and the thought of saving the helicopter from damage should not even be a consideration. As mentioned already, heli-logging takes place in some pretty unforgiving terrain and if the engine quits, there is most likely going to be very little left of the helicopter at the conclusion of the landing. Remember, any landing you walk away from is a good landing, and you get extra points if the doors still open when you turn the handle.
Pushing the nose over to "get out of your down wash" is a waste of potential energy and RPM. Unless you are working on a totally calm day, which almost never happens in the mountains, your down wash is already being blown behind the helicopter. Additionally, when the engine quits and the pilot enters autorotation, there is no more down wash. Unless you can gain enough airspeed to effect a flare, even that tiny little push of the cyclic forward is only going to cause a drop in RPM and an increase in rate of descent for no useful gain. The most important thing in a vertical autorotation is RPM, and anything that results in it's lose before you can use it to your benefit should be avoided. If you absolutely feel the need to "get out of your own down wash", pull a little aft cyclic, because once you have lost that little bit of RPM and increased your rate of descent for no gain and realized that you are going to hit the ground really hard, at least having the tailboom low and absorbing the impact first will probably save your spine in the end. |
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Thanks for your answers guys. You've confirmed what I thought... In the event of power failure you better have the Medevac folks on speed dial.  In that scenario, yes. In normal cruise, at 500 ft + altitude, over "forgiving" terrain it's actually pretty easy once you get the hang of it. After you get the collective down, the rest is just to save the airframe. |
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Once the engine quits, you don't owe the helicopter anything. You do whatever it takes to walk away from the crash and the thought of saving the helicopter from damage should not even be a consideration. As mentioned already, heli-logging takes place in some pretty unforgiving terrain and if the engine quits, there is most likely going to be very little left of the helicopter at the conclusion of the landing. Remember, any landing you walk away from is a good landing, and you get extra points if the doors still open when you turn the handle. Pushing the nose over to "get out of your down wash" is a waste of potential energy and RPM. Unless you are working on a totally calm day, which almost never happens in the mountains, your down wash is already being blown behind the helicopter. Additionally, when the engine quits and the pilot enters autorotation, there is no more down wash. Unless you can gain enough airspeed to effect a flare, even that tiny little push of the cyclic forward is only going to cause a drop in RPM and an increase in rate of descent for no useful gain. The most important thing in a vertical autorotation is RPM, and anything that results in it's lose before you can use it to your benefit should be avoided. If you absolutely feel the need to "get out of your own down wash", pull a little aft cyclic, because once you have lost that little bit of RPM and increased your rate of descent for no gain and realized that you are going to hit the ground really hard, at least having the tailboom low and absorbing the impact first will probably save your spine in the end. High Hot and heavy, and you wana have a high rate of descent with little to no forward airspeed and arrest that at the bottom? Good luck with engines doing that, much less power off. Once you go ring state you had a better change of jumping at altitude. |
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High Hot and heavy, and you wana have a high rate of descent with little to no forward airspeed and arrest that at the bottom? Good luck with engines doing that, much less power off. Once you go ring state you had a better change of jumping at altitude. Not going to be heavy anymore without the load. "Ring State"?? You cannot get into VRS when there isn't any power applied. 1-2-3 rule: Less than 10 kts A/S, greater than 20% power, and greater than 300 fpm descent rate. How much airspeed do you think you're going to get from a 100 ft hover? Maybe 10 kts? Surely not enough to have any kind of flare. As soon as you drop the nose, the RPM is going to drop like crazy and the ROD goes way up. You're going to be lucky to get it level by the time, much less flared by the time of impact. The lowest I ever practiced these was 500 ft and you were lucky to hit 50 kts at 75 AGL. And that was looking straight down for several seconds. Anything below 100-200 ft you're better off dumping the collective and saving what little RRPM you have left to cusion the impact. Land level and the skids and collapsing seats will absorb the some of the impact. |
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+1 what delorean said. Regalrocket, I certainly hope you never end up in this scenario because you are going to end up killing yourself simply do the the fact that you do not have a realistic understanding of energy management in an autorotation. Airspeed does magically appear out of thin air. Think of the three critical factors (altitude, RPM, and airspeed) as three glasses. In a hover, the airspeed glass is empty and at 250' the attitude glass isn't very full either. You have to pore contents of the altitude glass into the airspeed, simple fact, and there isn't enough in the altitude glass to fill the airspeed glass to the level needed in order to flare. The result is going to be a huge increase in ROD for no effective gain in airspeed - if you think a helicopter comes down fast in a vertical autorotation, just try pushing the nose over - you are going to almost double your ROD. Not to mention you are now going to impact the ground slightly nose down with much less of the airframe to absorb the impact. Level attitude or even a little tail low is going to offer a much more effective crumple zone. The important thing is to dissipate all the impact energy before your spine breaks.
As delorean pointed out, once the engine quits, there is no VRS or settling with power, as the direction of airflow through the rotor system is UP! Once the load is punched off the cargo hook, the weight of the helicopter is almost cut in half, and at the very least, a third, which is going to reduce the ROD and keep RPM from falling off quite as fast. Definitely a bad situation to be in none-the-less. |
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Not intending to pour gas on a fire here... I assume if you are going into the trees you execute the flair just before treetop level since once you are below treetop level there won't be enough left of the rotors to perform the flair? Once you hit the trees you could try to do a barrel roll and it wouldn't matter |
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Not intending to pour gas on a fire here... I assume if you are going into the trees you execute the flair just before treetop level since once you are below treetop level there won't be enough left of the rotors to perform the flair? Yep...........flare above the trees, come to a complete stop, fall vertically, through them and hope like hell a branch doesn't grab a skid or tailboom and roll you left, right, on pitch nose down. You want to hit skids level since that's where the crash protection is. One manufacturer's safety school (can't remember which), used to recommend that you don't level out of the flare––almost stand the helicopter on it's tail just above the trees. Their thinking was that you had a better chance of surviving because the helicopter you probably who maintain that pitch, not roll, and put as much of the helicopter as you could behind you to absorb the impact. No one ever tried it and it surely would be different everytime......so it was a good theory at best. The Army published a 10-20 page document on how to crash in the forest. It was pretty descriptive on the different types of trees––how they would break, bend, etc. Basically, aim for coniferous trees if you can since they are dense and don't break as easily. I'll try to find that publication. |
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A good read of some logging incidents. http://depts.washington.edu/sky2001/proceedings/papers/Manwaring.pdf |
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Clonehead spells this out pretty well in my opinion. From a fixed wing only pilot's perspective, the idea that all of the down wash disappearing immediately after flameout in calm winds seems a bit off the way I envision it. Tons of air moving that speed seems likely to keep going that direction for a few seconds and then it's over anyway. Not that cyclic input to leave the cylinder is possible either....Good info...
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A twin engine Chinook can hover OGE ( Out of Ground Effect) one one engine at max gross weight. Are you sure about that??? Not a chance. It might be able to maintain altitude at MGW at certain speed, but there's no way it can hover OGE at MGW on one engine. Since it has such a large useful load, it probably can hover OGE on one engine––but it would have to be fairly empty at that point. |
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I was a military and civilian helo pilot for about 35 years, flying Jet Rangers, Hueys, Bell 212s, Sikorsky S-76A, BO-105, and lastly UH-60 Blackhawks. I've never done any logging, but there are a couple things to discuss here. 1. The big Sikorsky's that typically do this type of logging are multi-engine - the aircraft I've seen on that particular series are S-61s, or H-53(Sikorsky S-54, I believe) H-61s have two engines, H-54s have two engines. Unless you're dealing with fuel starvation, the chances of losing both engines simultaneously are pretty slim. At 250', if you manage to punch off the load, you can probably fly away if you can get to flyaway airspeed quickly enough. If you can't punch off the load, you're probably going down anyway. 2. If you're flying a helicopter with a high-inertia rotor system (like most older Bell products), you don't need to accelerate as fast to get into your autorotation envelope (which maintains your rotor rpm within safe limits so you have energy for a soft landing at the end). If you're in a low-inertial rotor system aircraft (MD 500, UH-60 Blackhawk, BO-105) you need to do whatever is necessary to get the airspeed into autorotational limits to keep your rotor rpm up; it will decay swiftly. When I got my BO-105 checkout, we would hover to 75', cut one engine, point the nose 20 or 30 degrees down, and dive for the ground. If we could get to flyaway speed before reaching the ground (about 55 knots, depending upon aircraft gwt), we would pull out and establish single engine climb airspeed power. If we couldn't get to flyaway airspeed, we would make a runon landing. That being said, I don't know if 250' would be sufficient for a helicopter of that size to get to flyaway speed. However, I don't think any pilot would attempt a vertical autorotation from that altitude. |
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A twin engine Chinook can hover OGE ( Out of Ground Effect) one one engine at max gross weight. Thats a HUGE negative, comming from a chinook guy. First it depends on DA. But at Max Gross, your always hovering above 50 percent torque per engine, which is a combined over 100, now you can exceed 100 percent tq for a few seconds, but in every situation I have been in at max gross (50,000 lbs for the .mil ,54,000 for the BV-234) we didn't have IGE hover power, much less OGE. If we could hover OGE single engine, why would we ever practice roll on landings? If we are REALLY light, we will have IGE power availiable, and maybe OGE at low DA's, but thats about it. When we are heavy, and go single engine, like a plane, or any helicopter, any altitude we lose we won't be able to recover, and we must maintain 70 knots or greater (approx) until the landing flare. |
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That being said, I don't know if 250' would be sufficient for a helicopter of that size to get to flyaway speed. However, I don't think any pilot would attempt a vertical autorotation from that altitude. Depends on the type of rotor system. In a high inertia system, like a Huey or 58, 250' is plenty of altitude for a vertical autorotation. With a low-inertia system, I'd want 1000'. |