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

 

[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!

 

[7777]

Looks like a job for Mythbusters.

 

[sardaddy]

I lost faith in Mythbusters when it comes to anything aviation related when they tried to prove whether an airplane would take off if it was on a treadmill going the same speed and in theory keeping the aircraft stationary.

The answer is obviously no as it doesn't matter how fast the wheels are turning, it matters how fast the plane is moving so that lift is created over the wings. If the plane is stationary, there is no lift. Anyway, they declared it plausible despite a poorly designed test bed.

So if they can't do that simple test, I would hate to see what they would do with this.

 

[imacdog]

Some RC heli guys are doing blade-stop autorotations now. If they wait too long to get the blades going after they stop, and it doesn't take much, the heli will tumble out of control and then it's all over.

 

[phr8dawg]

In defense of Mythbusters, that prop-driven "jetpack" very nearly left the ground. Maybe if they inhaled enough helium......

 

[VNugget]

I lost faith in Mythbusters when it comes to anything aviation related when they tried to prove whether an airplane would take off if it was on a treadmill going the same speed and in theory keeping the aircraft stationary.

The answer is obviously no as it doesn't matter how fast the wheels are turning, it matters how fast the plane is moving so that lift is created over the wings. If the plane is stationary, there is no lift. Anyway, they declared it plausible despite a poorly designed test bed.

So if they can't do that simple test, I would hate to see what they would do with this.

Of course the airplane wouldn't fly without airspeed. Nobody but a moron would argue otherwise. But the whole point is that the treadmill can't stop the plane from moving.

What test bed would you propose?

It's a flawed question.

 

[sardaddy]

What test bed would you propose?

It's a flawed question.

Here is the question posed in one spot:

"A plane is standing on a large treadmill or conveyor belt. The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?"

Here it is posed a bit differently somewhere else:

"Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"


You are absolutely right it is a flawed question and one that has two answers depending exactly how one looks at it. If the wheels and the treadmill are going the exact speed as posed in the first part of the question, then the answer is no it cannot takeoff, as the plane is obviously not moving forward. But if the question is "can the plane overcome the force of the treadmill, move forward and take off," the answer is yes because the plane's thrust can easily push past the minimal friction caused by the treadmill on the wheels since the wheels don't drive the airplane. That is why an airplane can take of on ice easily. But for this to be true the wheels on the aircraft are moving faster than the treadmill (or else there is no forward motion) so the wheels and the treadmill are no longer equal.

That is why I lost faith in their experiment. They only looked at one point of view despite the fact that the question was flawed.

Now back to our physics/bad guy defying question.

 

[VNugget]

Now back to our physics/bad guy defying question.

Allright, anybody wanna design and build a rotor-wing glider? Paging Otto Sikorsky...

Just make sure they stay in high tow...

 

[Andy Neill]

Allright, anybody wanna design and build a rotor-wing glider? Paging Otto Sikorsky...

Just make sure they stay in high tow...

Isn't that what a propless autogyro is?
http://www.youtube.com/watch?v=gP6VnbeWXSY

 

[nosehair]

The rotor blades would cone up and break before they got up enough speed to stay rigid...

 

[sardaddy]

I thought about that but the rotor blades normally cone when a large collective load is applied. If the aircraft is flat pitch, I agree that they will cone up but depending on the rotor system, fully articulated vs rigid vs whatever else, they may not cone to the point where they would break since they are in free fall and not working to lift the helicopter.

There is no science to these ramblings by the way, just thoughts.