Free-fall Autorotate?

[Andy Neill]

Please refer to most helicopter manuals that state the importance of maintaining rotor speed.

Quite true if you are interested in generating lift. No so big a deal if not. The airflow will actually assist the start (if upright) since at flat pitch, the majority of the disc is the driven region of the blade.

 

[stupidpilot]

Not quite. There is neither a fan nor a rotor in the tail. Anti-torque is handled through a slit along the tail boom creating a low pressure area on one side (the Coandă effect).

According to McDonnel Douglas it is actually a mix of what you and I are saying.

http://www.mdhelicopters.com/products.php?id=NOTAR

 

[phr8dawg]

As long as we are flogging the same movie: How would that "switchblade" human sized plane fly? Aside from that turbine powered flight suit that crossed the channel, does a stealthy "wearable" high-speed prone glider exist? I want one.....NOW!

 

[skiddriver]

Yup, until you pull pitch to arrest your descent you don't need to use the anti-torque pedals.

Actually, the torque from the autorotating main rotor feeds back into the main transmission, and the friction in the drivetrain actually drives the airframe out of balance in the opposite direction from powered flight. To stay in trimmed flight and reduce your rate of descent, you have to feed in opposite pedal. It happens pretty naturally, the pilot is going to trim to balanced flight as a matter of habit, but there is a pedal input required.

 

[sardaddy]

Aside from the fact you would disintegrate into a massive mess of metal and plastic when falling vertically with no Nr... no, it would not work.

Please refer to most helicopter manuals that state the importance of maintaining rotor speed. Good movie... totally fake helo ending.

Actually, it is feasible. However, the stars would have to align just right.

First things first, you don't need hydraulics to autorotate the rotor as it is a freewheeling unit. If it wasn't, the rotor would slow down in direct proportion with the engines and you wouldn't be able to autorotate in the first place. In other words, unless the rotor brake is on, the rotor will turn. We move the blades on the ramp all of the time without any power applied.

Now, depending on the aircraft, you will need hydraulics to run to flight controls. So eventually you will need to get your hydraulics on line to fly out.

Yes, you could start an engine in a free fall. The only problem might be that the blades would be flapping around alot.

Now to the real question could it be done? Again, possibly. The aircraft won't just start falling apart because it is falling. It will fall apart when it hits the ground. Plus, the helicopter was pushed out of an airplane so you will have some forward inertia changing your descent profile.

The next question is what type of rotor system you have. If you have a very high inertia system (think Huey or OH-58), the possibility of getting the blades turning fast enough to give you enough rotor speed is pretty slim depending on the altitude. The higher the better in these cases. However, if you have a low inertia system (think Eurocopters) you have a pretty good chance of getting the rotors up to a speed that you could effect an autorotation. I have gotten rotor systems to go from about 280 (or less) RPM to 380 RPM in about two seconds correcting for student mistakes during autorotations in the past so that is absolutely possible.

The next thing to consider is the fall. If you go out of the aircraft fairly level, the surface area of the rotor blades wiill keep the aircraft relatively level. Think of a piece of plywood going through wind, the flat surface always wants to be in the wind. it is hard to change its direction once a wind load is on it. The same will happen with the rotors. The wind will aerodynamically start the blades turning even if you are falling vertically. As the blades start to turn faster, you will become even more stable. The airframe itself will naturally point in the direction of the highest relative wind so you will have a relatively stable aircraft. Then as long as the rotor system continues to spool up, you will be good to go as, in most aircraft, your hydraulic pumps will come on line and you will then have control of the aircraft.

So now you are autorotating in a relatively controlled state and need to start the engine. No problems at that point as long as your engines work and you have the altitude to restart it. they start in the air just as easily as they start on the ground. Trust me on that one You don't have to be flat pitch to start them either. You also could have started the engines as soon as you went out the back of the airplane speeding this whole process along.

Now if you are rolled out of the back of the aircraft, you will have a lot more to deal with but the aircraft could stabilize and the process will be the same. In fact you could potentially start the engines while you are rolling and eventually regain control depending on the aircraft.

So, yes it is feasible. However, it is not advisable

 

[Andy Neill]

it is not advisable

I'll drink to that!

 

[stupidpilot]

Actually, the torque from the autorotating main rotor feeds back into the main transmission, and the friction in the drivetrain actually drives the airframe out of balance in the opposite direction from powered flight. To stay in trimmed flight and reduce your rate of descent, you have to feed in opposite pedal. It happens pretty naturally, the pilot is going to trim to balanced flight as a matter of habit, but there is a pedal input required.

I guess so. Geez its been so long that I'd forgotten!

 

[mudkow60]

I will be contacting MD and asking their opinion of the plausibility.

 

[7777]

I have not seen the movie but do not think it is feasible. Lift is keeping the helicopter upright in flight. Without it, it is a rock with nothing providing a stablizing force for a pilot to use in keeping it upright.

As far as the NOTAR goes; the slot that runs along side the tailboom is called a Coanda slot. There was supposed to be a fan off the transmission that provided 15psi or so of pressure out the side of this slot that would provide a counter torque effect in conjunction with the downwash created by the main rotar that would increase/decrease static and dynamic pressure and make it work well, in flight, as an antitorque force. However, one of the weaknesses that was found was the ability of the downwash to provide this differential pressure in an autorotation. In an autorotation the helicopter roter blade is free wheeling (sprag clutch) and it is not creating a downwash in an autorotation. The Coanda slot wasn't providing enough antitorque capability and they were having control problems with it. So they decided to go to the H or V style stabilator and apparantly it was working better. However, I still heard there are some problems with it when you load the aircraft and perform the same manuevers. Not only control issues but power issues as well.

I have alot of Army helicopter time but never flew the NOTAR (No Tail Rotor). It was supposed to be so easy to fly that the FAA was considering making it a different rating altogether. Not sure if it ever went through or not.

Even James Bond couldn't make that stunt work. I would bet money if anyone could do it, the little bird drivers of the 160th Night Stalkers could. If they couldn't do it, NO ONE could.

Just my .02. Helicopters kick ASS!!!!

 

[sardaddy]

I understand your humble opinion Matt but you also said "Aside from the fact you would disintegrate into a massive mess of metal and plastic when falling vertically with no Nr." and that is unlikely to happen until it hits the ground.

Let's go smaller scale, and we won't even use a cat. Have you ever seen a parachutist jump out of an airplane wearing a snowboard? Or have you ever seen them drop a car out? In the case of the snowboard, the parachutist is automatically pushed underneath the large flat panel of the snowboard and he hangs very stable underneath it. He isn't even a cat!!! When they drop a car out, it is also a very flat surface and it often stabilizes in the air. Many times the car will remain stable for a very long time. Look at some of the vidoes on youtube. Sometimes the car rolls. That is why I say it is feasible.

There is a lot of potential that it could happen. on many rotor systems you can get the rotor to spool up almost instantaneously during an autorotation so I have no doubt given enough time the blades would spin fast enough. There is also more potential that you could start a helicopter in the air even if it is not wings level or even upright as it is falling allowing the rotors to spool up. Turbine engines don't care what direction they are facing when you start them unless of course you are ramming about 120 kts of wind in from the back which of course could hapen.

So I still say it is feasible as the large surface area of the rotors would act like out riggers as the aircraft fell stabilizing it in the process. I am not saying it would work every time or even most times but it is plausible that it could work.

Remember, the laws of physics say a bee cannot fly.

I've flown helicopters for 18 years, hold a degree in aeronautics and am currently staying at a holiday in express!! Actually I am in the BOQ at ATC but its just as good. So there.

Of course now that you deleted your post none of this post especially the smarmy experience comment makes sense to anyone else as it related to items in your post. Just great!