Free-fall Autorotate?

[phr8dawg]

So James Bombed and Jinx push the Hughes 500 Notar out of the Antonov and start it in freefall. It was more fun than the roach race in my room. So...

Without hydraulic boost, will a chopper autorotate?

Could you start one in freefall given the airloads?

Where can I get lessons?

 

[Andy Neill]

So James Bombed and Jinx push the Hughes 500 Notar out of the Antonov and start it in freefall. It was more fun than the roach race in my room. So...

Without hydraulic boost, will a chopper autorotate?

Could you start one in freefall given the airloads?

Where can I get lessons?

Yes and Yes.
I don't know about the NOTAR, but the earlier McD versions (OH-6, H500, MD-500D/MD) had no hydraulics anyway. As long as you are flat pitch during startup, I don't see a problem other than trying to keep it from tumbling like a rock.

I went to my friendly Army recruiter to get my lessons.

 

[deadstick]

Sting did that in Airwolf first!

I still think the "tumble like a rock" scenario is likely without some form of stabilization for the first few seconds following the drop.

 

[phr8dawg]

I appreciate the legit answers. What would keep a notar machine in a straight line while autorotating? You'd think it would rotate opposite the main rotor having no counteracting tail rotor.

Aw,its just a movie. Anybody know where I can get a Star Wars speeder bike?

 

[imacdog]

I appreciate the legit answers. What would keep a notar machine in a straight line while autorotating? You'd think it would rotate opposite the main rotor having no counteracting tail rotor.

Aw,its just a movie. Anybody know where I can get a Star Wars speeder bike?

Same reason an autogyro doesn't rotate opposite the blade direction; there's no power being applied to the blades.

 

[stupidpilot]

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

 

[deadstick]

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

Well, the TR drive shaft is still turning because it's connected to the transmission, right? Isn't the NOTAR the same except, intsead of a really long drive shaft and TR, it has a shorter shaft that drives a fan? The fan still turns so there is still thrust being generated, right?

Just some thoughts since I have never seen a real, live NOTAR before.

Just wondering, did the helicopter slide out nose or tail first? I still think with the high CG it would try to flip on its back. If it went out nose first it might porpoise nose up/down, but the nose would still be pointed downward. But what about the blades at that second that they hit the slipstream?

Sounds like a task for the Myth Busters!

 

[stupidpilot]

I believe thrust is being generated, but not sideways. If you look at the tail boom of a NOTAR helicopter, you will notice the end of the boom rotates to vector the thrust. So if thrust is not needed, it goes to a neutral, no-thrust position. (At least I think that's how that works, as I have no experience w/a NOTAR either.) Any one else have anything to add?

 

[Andy Neill]

Well, the TR drive shaft is still turning because it's connected to the transmission, right? Isn't the NOTAR the same except, intsead of a really long drive shaft and TR, it has a shorter shaft that drives a fan? The fan still turns so there is still thrust being generated, right?

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).

 

[mudkow60]

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.

 

[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