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YokoTsuno
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Starting Helicopter In Free Fall

Sat Jul 28, 2012 6:38 am

Just watched Die Another Day where 007 (Brosnan) starts a helicopter while being dropped from an Antonov.
Is this remotely possible?
 
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Starlionblue
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 7:20 am

Quoting YokoTsuno (Thread starter):

Just watched Die Another Day where 007 (Brosnan) starts a helicopter while being dropped from an Antonov.
Is this remotely possible?

I think that you should be able to autorotate the main rotor to speed while you start the engine(s). But just a guess.
 
LTC8K6
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 7:59 am

Seems like you could have airflow problems with the engines if you are falling "flat".
 
zanl188
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 1:02 pm

A helicopter being shipped by air is typically not capable of flight....

- Blades folded or removed...
- Some types have their rotor head & transmission removed for air shipment...
- Fuel tanks drained or purged...
- Batteries disconnected or removed...
- Intakes plugged...

Of course anything is possible in the movies provided the audience can be encouraged to suspend their disbelief.
 
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Starlionblue
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 1:19 pm

I think for the sake of this thread we must assume that the helicopter is capable of flight when it falls out of the plane. 
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 1:38 pm

Quoting Starlionblue (Reply 1):
I think that you should be able to autorotate the main rotor to speed while you start the engine(s). But just a guess.

Autorotation only works because the rotor is already spinning when the engines fail. I don't think it's possible to start the rotor using autorotation because the entire rotor is stalled when you first start.

Tom.
 
David L
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 1:52 pm

Quoting tdscanuck (Reply 5):
Autorotation only works because the rotor is already spinning when the engines fail. I don't think it's possible to start the rotor using autorotation because the entire rotor is stalled when you first start

And how would you maintain a suitable attitude to even try initiating autorotation without any control from the rotors?
 
nomadd22
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 1:59 pm

Quoting tdscanuck (Reply 5):
Autorotation only works because the rotor is already spinning when the engines fail. I don't think it's possible to start the rotor using autorotation because the entire rotor is stalled when you first start.

I think it could be possible, if there is a mechanism that lets you pitch the blades without the engine or rotor already running. Or, if you meant to do that trick in the first place and have the blades pitched to give you some (whatever you'd call the rotational force) from the blade moving straight down.
Of course, unless you have a plane big enough to drop a C-46, keeping the helicopter from spinning will be a good trick.
 
zanl188
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 2:13 pm

Quoting Starlionblue (Reply 4):

I think for the sake of this thread we must assume that the helicopter is capable of flight when it falls out of the plane. 

Ah a fantasy thread then... In that case it's simple just fire up the rotor tip turbo jets!  
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 2:23 pm

Quoting nomadd22 (Reply 7):
I think it could be possible, if there is a mechanism that lets you pitch the blades without the engine or rotor already running.

The collective works all the time (assuming hydraulic pressure on a hydrauilcally boosted helicopter). The problem is that no rotor can pitch that far over. In order to get a non-stalled AoA on the blades while falling with a stationary rotor, the leading edge of the blades would have to be pointed almost straight down. You need the rotor spinning initially to get at least a portion of the rotor into the driven zone so that you can get enough rotor torque to maintain autorotation. Otherwise the blades are all stalled and you don't get much at all.

Tom.
 
Klaus
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 2:58 pm

Quoting tdscanuck (Reply 9):
The collective works all the time (assuming hydraulic pressure on a hydrauilcally boosted helicopter). The problem is that no rotor can pitch that far over. In order to get a non-stalled AoA on the blades while falling with a stationary rotor, the leading edge of the blades would have to be pointed almost straight down.

But is that really necessary to get the rotor up to speed? Even when the blades are initially stalled and even with a much smaller pitch, the asymmetric force should still provide a rotational moment. Whether it would be sufficient to eventually transition out of the stall is another question, but even a fully stalled surface should react to pitch and provide at least some forward force, even with turbulent airflow on the top side, as long as the pitch is sufficient to produce the required asymmetry in air pressure.
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 3:43 pm

Quoting Klaus (Reply 10):
But is that really necessary to get the rotor up to speed?

Yes.

Quoting Klaus (Reply 10):
Even when the blades are initially stalled and even with a much smaller pitch, the asymmetric force should still provide a rotational moment.

There absolutely is a torque; it's backwards. The normal flow pattern through a rotor is from above to below. If you take blades that are pitched to produce that pattern, stop the rotor, then put air flow from below to above, the blades want to go the opposite way. Autorotation works by having the driving zone (which wants to go the right way) generate exactly as much torque as the driven zone (that wants to go the wrong way). If you start with a stationary rotor, it all wants to go the wrong way.

Tom.
 
David L
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 4:02 pm

So you'd really want to the helicopter to be dropped inverted from the aircraft, somehow prevent it from tumbling then roll upright when the rotors are up to speed. Hmm...

I'd assumed the rotor blades could be set with negative pitch but I guess that ability probably isn't be necessary in normal circumstances and, on second thoughts, might be quite dangerous .
 
Klaus
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 4:04 pm



Quoting tdscanuck (Reply 11):
There absolutely is a torque; it's backwards.

Good explanation, thanks.

So is the pitch authority too limited to tilt the blades the right way for picking up speed in the proper direction?

[Edited 2012-07-28 09:04:54]
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 6:05 pm

Quoting Klaus (Reply 13):
So is the pitch authority too limited to tilt the blades the right way for picking up speed in the proper direction?

That is my understanding. In theory, there's no reason that you couldn't spin up the rotor if you could get the blades pitched down far enough. However, that would be equivalent to having the leading edge of the blades pointed almost straight down, which implies a rotation of 70+ degrees from their normal position. I don't think any real world rotor has that kind of pitch range.

Tom.
 
Klaus
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 7:02 pm

Quoting tdscanuck (Reply 14):
In theory, there's no reason that you couldn't spin up the rotor if you could get the blades pitched down far enough. However, that would be equivalent to having the leading edge of the blades pointed almost straight down, which implies a rotation of 70+ degrees from their normal position. I don't think any real world rotor has that kind of pitch range.

That is what I don't get. In the stalled case you'd only have to tilt the blades enough so the airflow is directed primarily to the trailing edge, so that a rotational moment is created towards the leading edge.

Why would that require a massive pitch of more than 70 degrees? The blade profile looks far too flat for that.

I would have expected that even within the normal flight and especially maneuvering envelope at least some of the time the blades do already provide neutral or negative lift against their profile.
 
SlamClick
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 7:17 pm

The blades cannot backdrive the engines. There is a clutch mechanism to prevent it, else, with a siezed engine you would not be able to autorotate. A sprag clutch or centrifugal clutch must be installed.

This would not preclude starting the engine from the battery and bringing it up to sufficient speed to "join the needles" as we say. Problem is, a helicopter without a spinning rotor has absolutely no stability whatever. You are as likely to be falling upside down or tail-first when the engine came up to speed.
 
HaveBlue
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 7:38 pm

Quoting SlamClick (Reply 16):
This would not preclude starting the engine from the battery and bringing it up to sufficient speed to "join the needles" as we say.

Glad someone said it! I'm reading this thread going "why do we have to get the airflow moving the rotors.. start the engines!".
 
Klaus
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RE: Starting Helicopter In Free Fall

Sat Jul 28, 2012 7:59 pm

Quoting SlamClick (Reply 16):
The blades cannot backdrive the engines. There is a clutch mechanism to prevent it, else, with a siezed engine you would not be able to autorotate.

Yes, I had assumed that autorotation would be as good as it got without a separate way to start up the engine(s).

Quoting HaveBlue (Reply 17):
Glad someone said it! I'm reading this thread going "why do we have to get the airflow moving the rotors.. start the engines!".

We're way into purely fictional territory anyway.

But the question whether autorotation can be started at all from free fall is still somewhat interesting (if probably not practical).
 
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Starlionblue
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 12:37 am

Quoting ZANL188 (Reply 8):
Quoting Starlionblue (Reply 4):

I think for the sake of this thread we must assume that the helicopter is capable of flight when it falls out of the plane. 

Ah a fantasy thread then... In that case it's simple just fire up the rotor tip turbo jets!

Awesome. 
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 12:54 am

Quoting Klaus (Reply 15):
In the stalled case you'd only have to tilt the blades enough so the airflow is directed primarily to the trailing edge, so that a rotational moment is created towards the leading edge.

Yes. But, in freefall with a stationary rotor, the freestreem is coming straight up at you. You need to get the leading edge pointed enough into that "straight up" flow to get the blade airfoil going...that means the leading edge has to be pointed nearly straight down.

Quoting Klaus (Reply 15):
Why would that require a massive pitch of more than 70 degrees? The blade profile looks far too flat for that.

Most airfoils stall out at something like 20 degrees. I figured if you could get to 70 degrees nose down you'd have enough angle of attack to the flow coming straight up at you from the ground that it would be enough to generate sufficient lift to start rotation.

Quoting Klaus (Reply 15):
I would have expected that even within the normal flight and especially maneuvering envelope at least some of the time the blades do already provide neutral or negative lift against their profile.

Absolutely. But normal rotor speeds mean that the wind due to rotation is at least as large, if not larger, than the vehicle's speed through the air. As a result, "neutral" lift means the blade is about flat.

Tom.
 
nomadd22
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 3:41 am

How about a nosedive? A vertical stabilizer could even counter the torque from the rotor then. (Assuming the valet attendant left the radio on all night and killed the starter battery)
 
vikkyvik
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 4:08 am

Did someone say "fantasy"?  

You guys are all forgetting that the Antonov would be carrying, along with a helicopter, somewhere around 100,000 pigeons (depending on payload, obviously). Given that some of the pigeons would certainly be dropped along with the helicopter (due to the Antonov's load getting significantly lighter, therefore requiring less pigeons....similar to dumping fuel), all you need is the pigeons to start flying in a circle, pushing the rotor up to speed as they fall.

It's really quite simple....not sure why everyone's harping on about all this "autorotation" baggage.
 
Klaus
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 10:28 am

Quoting tdscanuck (Reply 20):
Yes. But, in freefall with a stationary rotor, the freestreem is coming straight up at you. You need to get the leading edge pointed enough into that "straight up" flow to get the blade airfoil going...that means the leading edge has to be pointed nearly straight down.

Ah, I see. You meant the pitch required to immediately get the bades into laminar flow on both sides of the profile.

But why would that even be necessary right away? Even fully stalled when the upper surface doesn't contribute anything due to its turbulent flow, asymmetrical flow on the lower surface should already provide lateral acceleration at a much smaller pitch, getting the rotor into motion initially and with increasing rotational speed lowering the pitch necessary to eventually capture laminar flow on the upper side as well.

I'm not talking about the desired end state of having achieved laminar autorotation already (assuming an engine start is not possible to actively drive the rotor), but just about getting the rotation started at all from stall, before eventually transitioning into laminar autorotation once the rotational speed is sufficient.
 
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airmagnac
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 11:03 am

Quoting vikkyvik (Reply 22):
Antonov would be carrying, along with a helicopter, somewhere around 100,000 pigeons

And IIRC, the helicopter is ejected from the Antonov by means of a conveyor belt...
but maybe we shouldn't go there 
 
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SAAFNAV
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 11:44 am

Quoting vikkyvik (Reply 22):

Did said Antonov take off from a conveyor belt? With the pidgeons flapping their wings?
 
vikkyvik
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 11:54 am

Quoting Klaus (Reply 23):
Ah, I see. You meant the pitch required to immediately get the bades into laminar flow on both sides of the profile.

I think he means attached flow, not laminar flow.

Turbulence actually delays flow separation. Most aircraft won't have laminar flow all the way across the wing.

Of course, I don't really know much about helicopters.  
 
Klaus
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 12:01 pm

Quoting vikkyvik (Reply 26):
I think he means attached flow, not laminar flow.

Could the flow be fully attached without being laminar?
 
MD11Engineer
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 1:11 pm

I mean the Antonov is HUGH, but how will you get a decent sized helicopter into the aircraft without folding the rotor blades first? With rotor blades folded the helicopter would just tumble down like a brick.

Jan
 
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glen
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 1:42 pm

Quoting Klaus (Reply 23):
But why would that even be necessary right away? Even fully stalled when the upper surface doesn't contribute anything due to its turbulent flow, asymmetrical flow on the lower surface should already provide lateral acceleration at a much smaller pitch, getting the rotor into motion initially and with increasing rotational speed lowering the pitch necessary to eventually capture laminar flow on the upper side as well.

Yes you have some lateral forces, but they will drive the rotor in the wrong direction. As Tom mentioned earlier:

Quoting tdscanuck (Reply 11):
The normal flow pattern through a rotor is from above to below

So in freefall you have a reverse flow through the rotor and it would start turning in the wrong sense. To overcome this you would need to have a rotor where you can set a negative pitch. As far as I know there are only a few marine helicopters (like the Lynx) which are able to do so, but only on ground in order to keep the helicopter on board. (And I think a lot of RC-Helicopters are capable of negative rotor pitch, therefore they are able for inverted flight.)
 
SlamClick
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 1:58 pm

I don't understand why anyone is thinking along the lines of airflow starting the rotor spinning. It wouldn't. There would be no reason for it. In rotary wing flight, autorotation would never happen if the rotor wasn't already at operating RPM at the moment of engine failure. Pushing a helicopter or an automobile or a lathe out the back of an airplane are, aerodynamically, the same act.

By the way, during autorotation a typical helicopter will still have a positive angle of incidence (chord line measureed against plane of rotation) The only reason the airflow up through the rotor makes it keep spinning is that the collective pitch is lowered enough that the net of all those little lift-vector arrows is slightly forward of the axis of rotation. Some helicopters have weights near the blade tips to give a little more inertia during the transistion from translational lift to autorotation.

Answer to the o/p question: Not in a million years.
 
Klaus
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 2:06 pm



Quoting glen (Reply 29):
So in freefall you have a reverse flow through the rotor and it would start turning in the wrong sense. To overcome this you would need to have a rotor where you can set a negative pitch.

That's what I was referring to, and apparently that is indeed part of the rotor design with autorotation capability if wikipedia is correct:

http://en.wikipedia.org/wiki/Autorotation_(helicopter)

Quote:
Autorotation is permitted mechanically because of both a freewheeling unit, which allows the main rotor to continue turning even if the engine is not running, as well as curved main rotor blades such that when the collective pitch is fully down the inner part of the blade has negative pitch relative to the horizontal plane and can be spun up by the relative wind.

 


I had always been wondering how autorotation could even work, but this makes it a whole lot more plausible to me.

If this is correct it would follow that it should also be possible to accelerate the rotor from a fully stalled start if there was sufficient altitude.

Quoting SlamClick (Reply 30):
In rotary wing flight, autorotation would never happen if the rotor wasn't already at operating RPM at the moment of engine failure. Pushing a helicopter or an automobile or a lathe out the back of an airplane are, aerodynamically, the same act.

How so? If the wikipedia explanation is correct, the driving part of the rotor is just windmilling like any turbine within an airstream, supplying the driven part with enough power to produce enough lift to slow the descent and storing enough rotational energy to allow for a final flare on landing.

If autorotation could only ever work from the driven state, the rotor's inertia should at some point be exhausted and a crash would be inevitable. But apparently autorotation can be kept up indefinitely as long as there is sufficient altitude:

Quote:
The longest autorotation in history was performed by Jean Boulet in 1972 when he reached a record altitude of 12,440 m (40,814 ft) in an Aérospatiale Lama. Because of a −63°C temperature at that altitude, as soon as he reduced power the engine flamed out and could not be restarted. By using autorotation he was able to land the aircraft safely.

Without the rotor being driven by the airstream how could this have been achieved?

[Edited 2012-07-29 07:17:02]
 
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larshjort
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 2:19 pm

Quoting MD11Engineer (Reply 28):
I mean the Antonov is HUGH, but how will you get a decent sized helicopter into the aircraft without folding the rotor blades first? With rotor blades folded the helicopter would just tumble down like a brick.

Jan

You just need a helicopter with a 2 bladed rotor like the Bell 206. But in the movie they use a MD600N with a 5 bladed rotor and a diameter of 8,38m compared to the width of the AN-124 cargo compartment of 6,4m

/Lars
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 2:48 pm

Quoting Klaus (Reply 23):
Ah, I see. You meant the pitch required to immediately get the bades into laminar flow on both sides of the profile.

Vikkyvik beat me to it:

Quoting vikkyvik (Reply 26):
I think he means attached flow, not laminar flow.

It doesn't matter if it's turbulent or laminar, just that it's (mostly) not separated.

Quoting Klaus (Reply 23):
But why would that even be necessary right away? Even fully stalled when the upper surface doesn't contribute anything due to its turbulent flow, asymmetrical flow on the lower surface should already provide lateral acceleration at a much smaller pitch, getting the rotor into motion initially and with increasing rotational speed lowering the pitch necessary to eventually capture laminar flow on the upper side as well.

Unless you can get the blade pitch *extremely* negative, the torque will be the wrong way and the rotor will want to go backwards.

Quoting Klaus (Reply 27):
Could the flow be fully attached without being laminar?

Yes. That is the normal state of things for most airliner wings, and probably a lot of helicopter blades.

Quoting SlamClick (Reply 30):
I don't understand why anyone is thinking along the lines of airflow starting the rotor spinning. It wouldn't. There would be no reason for it. In rotary wing flight, autorotation would never happen if the rotor wasn't already at operating RPM at the moment of engine failure.

Because the question got asked if you could autorotate down if you couldn't get the engine started. I'm arguing that you can't because you can't get autorotation going from a stopped rotor.

Quoting Klaus (Reply 31):
If this is correct it would follow that it should also be possible to accelerate the rotor from a fully stalled start if there was sufficient altitude.

Only if you can get the blades pitched over (i.e. considerably negative) to get some lift in the right direction. Most real world rotors can't do that. If you have any positive pitch on the blades at all and you're falling straight down the rotor does want to windmill but in the wrong direction. To get it going in the right direction you need the lift force on the blades to be in the right direction. If you don't already have rotation, that means a huge negative pitch on the blades.

Quoting Klaus (Reply 31):
How so? If the wikipedia explanation is correct, the driving part of the rotor is just windmilling like any turbine within an airstream, supplying the driven part with enough power to produce enough lift to slow the descent and storing enough rotational energy to allow for a final flare on landing.

The part I think you're missing is that the angle of attack that the blade actually sees is a strong function of the rotor speed. With an already spinning rotor, pushing the collective all the way down gives you enough negative pitch on the driving region to get positive torque on the rotor in the right direction. This is possible because the rotor is already spinning so the angle of attack is very shallow even if the helicopter is descending.

If the rotor isn't already spinning, the angle of attack on the blade is just relative to the freestream of the falling helicopter...i.e. it's coming straight up from below. A "flat" rotor has an angle of attack of ~90 degress in freefall.

Tom.
 
Klaus
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 4:58 pm



Quoting tdscanuck (Reply 33):
If the rotor isn't already spinning, the angle of attack on the blade is just relative to the freestream of the falling helicopter...i.e. it's coming straight up from below. A "flat" rotor has an angle of attack of ~90 degress in freefall.

Yes. And at exactly 90° the airstream will be diverted towards both edges equally, resulting in an equilibrium and zero torque.

As soon as you tilt the blade even just a little in either direction, however, the airstreams towards both edges will become asymmetrical and a force towards the lowered edge should result (apart from drag/lift); Not just at a very large pitch deviation but right away, even while the airflow over the upper surface is still fully stalled.

When eventually the combination of rotational speed and pitch (resulting in the respective angle of attack) gets the blade out of its stalled condition, it should become much more efficient since the upper surface will go from shedding energy through its turbulent wake to efficient conversion of airspeed to additional torque, while the lower surface will just continue to contribute its share of torque as it has from the beginning.

As far as I'm aware you can fly even a fully stalled aircraft straight and level if you have sufficient engine power just based on the lower-surface lift alone (even without having enough thrust to actually lift the aircraft just with that) – it is just horrendously inefficient since the upper surface of the wing does not participate efficiently in producing lift and instead dumps most of your expensively produced propulsion power into a big turbulent wake. But the lower surface still produces lift on its own, just not nearly as much as both surfaces together in laminar flow as far as I understand.

Quoting tdscanuck (Reply 33):
Unless you can get the blade pitch *extremely* negative, the torque will be the wrong way and the rotor will want to go backwards.

Why? Only the exact 90° angle should result in a complete absence of any torque. Even just a slight pitch deviation in either direction should immediately result in a moment towards the respective direction, even if it becomes really efficient only when the upper surface eventually achieves laminar flow.

And if the wikepedia explanation is correct, autorotation-enabled rotors already have a spanwise-variable pitch built in which even becomes negative along the inner span in the lowest collective range.

If I read this correctly, multi-engine helicopters may be built with rotors which are not capable of autorotation, effectively replacing one emergency backup with another (and probably being more efficient during normal operation).

Quoting tdscanuck (Reply 33):
Quoting Klaus (Reply 27):
Could the flow be fully attached without being laminar?

Yes. That is the normal state of things for most airliner wings, and probably a lot of helicopter blades.

(my emphasis added)

Is the airflow still fully attached in that case or just mostly attached, except in certain areas under certain circumstances?

[Edited 2012-07-29 10:02:42]
 
SlamClick
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 5:21 pm

.

Quoting Klaus (Reply 31):
If this is correct it would follow that it should also be possible to accelerate the rotor from a fully stalled start if there was sufficient altitude.

To test this, get an RC helicopter. Stop the rotor and drop the model. Repeat as many times as you like. You will grow old and die before it ever falls in the upright position and remains so until the rotor begins turning. Not in a million years of testing. The helicopter without the rotor turning is no more aerodynamic than a washing machine. There is simply no force that would act to turn the blades in the correct direction. It will tumble randomly and the blades will not start turning. If you add some outside force to keep the thing in the flying attitude then maybe if the atmosphere was about ten thousand times deeper there might be enough time for the spin to get up to a usable RPM. I would bet my life on each and every drop that it would fail to turn the rotors up.

Quoting Klaus (Reply 31):
If the wikipedia explanation is correct, the driving part of the rotor is just windmilling like any turbine within an airstream

I haven't read their article but if that is a correct conclusion from it then the article is not correct. A turbine intended to be rotated by fluid flow (air, gas, water, whatever) will have the blade angle set for correct impingement. It will even have stators upstream from the blade to give the desired angle. A helicopter does not.

Quoting Klaus (Reply 31):
storing enough rotational energy to allow for a final flare on landing.

The successful outcome requires (usually) that the collective remain on the bottom stop, giving the smallest possiible angle on the blades. It is possible to pull a bit of pitch (e.g. trying to "carry" the helicopter a little farther over the ground) to the point where RPM is lost, blade energy is depleted and there is not enough to arrest the descent at the bottom. A friend of mine burst his aorta and a couple others broke their backs attempting exactly this.

Quoting Klaus (Reply 31):
If autorotation could only ever work from the driven state, the rotor's inertia should at some point be exhausted and a crash would be inevitable.

The entire point of my statement is this: It was necessary to have enough rotor RPM at the beginning of an autorotation for the relative wind passing UP through the rotor system to be able (due to the slight forward-of-axis angle) to KEEP the rotor turning. Absent that initial rotation it is a dead certainty that the helicopter would begin to tumble randomly like a Simonized beer bottle long before the "upward" wind would begin to turn the rotors. Furthermore, because the rotor is at operating speed at the beginning of an autorotation you have cyclic and antitorque control to keep the helicopter positioned to encounter the relative wind at the correct angle. (long way of saying you can keep it upright)
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 5:32 pm

Quoting Klaus (Reply 34):
As soon as you tilt the blade even just a little in either direction, however, the airstreams towards both edges will become asymmetrical and a force towards the lowered edge should result (apart from drag/lift);

Yes, absolutely. My point is that you physically can't tilt the blade so much that you've got enough net torque to get the rotor going the right way if the rotor isn't already turning.

Quoting Klaus (Reply 34):
And if the wikepedia explanation is correct, autorotation-enabled rotors already have a spanwise-variable pitch built in which even becomes negative along the inner span in the lowest collective range.

Yes. It's negative *relative* to the outer blade. It assumes the rotor is at speed when you start autorotating. If the rotor is stationary the angle of attack to the blade is something like 70 degrees higher. I don't think there's anyway to get the driving region torque to overcome the driven region torque.

Quoting Klaus (Reply 34):
Is the airflow still fully attached in that case or just mostly attached, except in certain areas under certain circumstances?

Fully attached. Separated/not-separated is a completely different thing than laminar/turbulent.

Tom.
 
vikkyvik
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 8:34 pm

Quoting Klaus (Reply 34):
Is the airflow still fully attached in that case or just mostly attached, except in certain areas under certain circumstances?

As I stated earlier:

Quoting vikkyvik (Reply 26):
Turbulence actually delays flow separation.

It's not always desirable to have completely laminar flow across a wing.
 
User avatar
glen
Posts: 368
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 9:17 pm

Quoting tdscanuck (Reply 36):
Yes, absolutely. My point is that you physically can't tilt the blade so much that you've got enough net torque to get the rotor going the right way if the rotor isn't already turning.

That's the point where I can't follow your arguments, and I think that's also the problem Klaus has:

Quoting Klaus (Reply 34):
Even just a slight pitch deviation in either direction should immediately result in a moment towards the respective direction

From experience I know that it is possible to bring a stopped fix propeller into the windmilling state, only by adding enough force, i.e. by a high enough diving speed.* Such a propeller isn't tilted very much and it can even overcome the compression of the standstill piston engine.
So - given you could tilt the rotor into a slight negative pitch (and you could solve the problem of stabilising the helicopter, e.g. by some stabilising surfaces) - why shouldn't the rotor start to turn into the desired direction?

* too low idle RPM setting resulted in a engine failure during aerobatics (turn over with zero forward speed) in an oldtimer plane without battery start possibility (starting only via external elec. power)
 
tdscanuck
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 9:42 pm

Quoting glen (Reply 38):
So - given you could tilt the rotor into a slight negative pitch (and you could solve the problem of stabilising the helicopter, e.g. by some stabilising surfaces) - why shouldn't the rotor start to turn into the desired direction?

Because I don't think any real world rotors exist, absent perhaps some unique military applications, that can get the whole blade into negative pitch. I do not think the tiny bit of negative that you get on the inboard portion of an existing autorotating rotor is sufficient to overcome the positive that you've got on the outboard portion. There is just too much difference between the angle of attack for a spinning rotor and a stopped one.

There's no aerodynamic reason you can't design a rotor that will work, I just don't think you'll find one in practice.

Tom.
 
MD11Engineer
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RE: Starting Helicopter In Free Fall

Sun Jul 29, 2012 9:43 pm

Quoting Larshjort (Reply 32):

You just need a helicopter with a 2 bladed rotor like the Bell 206. But in the movie they use a MD600N with a 5 bladed rotor and a diameter of 8,38m compared to the width of the AN-124 cargo compartment of 6,4m

/Lars

The problem is that helicopters are extremely top heavy. When the rotor is running they are hanging from the mast, and are therefore stable, but if the rotor is not providing lift they tend to topple over.
If you ever are a passenger on a helicopter, which has to ditch in water, expect the helicopter to turn upside down within seconds.

Jan
 
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glen
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RE: Starting Helicopter In Free Fall

Mon Jul 30, 2012 7:04 am

Quoting tdscanuck (Reply 39):
Because I don't think any real world rotors exist, absent perhaps some unique military applications, that can get the whole blade into negative pitch.

Sorry, all clear now. Thank you for clarification. I misunderstood your statement:

Quoting tdscanuck (Reply 36):
My point is that you physically can't tilt the blade so much that you've got enough net torque

I was thinking of it as a theoretical impossibility to tilt te blade so much as to generate enough torque instead of seeing the statement as a remark about real world helicopters. I was thinking about "physically" meaning "by natural laws". In complex matters sometimes a single word misunderstanding in a foreign language can make quite a difference...
 
nomadd22
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RE: Starting Helicopter In Free Fall

Mon Jul 30, 2012 11:26 am

You can get whatever blade pitch you want. Nobody said the aircraft had to be level. Just tilt the whole thing.
Of course, it might have to be upside down to get the rotor going in the right direction. This might require more detailed engineering analysis and beer to figure out.

[Edited 2012-07-30 04:30:16]
 
SlamClick
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RE: Starting Helicopter In Free Fall

Mon Jul 30, 2012 2:16 pm

Quoting nomadd22 (Reply 42):
Nobody said the aircraft had to be level. Just tilt the whole thing.

If we are adding magic to the problem then anything can happen including what the o/p saw in the movie. The pilot of a helicopter without the rotor turning cannot tilt anything but the swashplate, you cannot "tilt" the blades. In normal flight the cyclic stick tilts the swashplate and the blades follow and "fly" to the desired path.

Furthermore, I cannot think, offhand, of a single helicopter on which hydraulic pressure wouldn't also be required. Typically the hydraulic pump(s) would be driven off the transmission rather than the engine so as to be available during autorotation. No rotor spin, no hydraulics.

So if this is a helicopter on a conveyor belt with magic rules then his answer is yes. If we are constrained to what would happen in the real world - never in a million years.
 
HaveBlue
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RE: Starting Helicopter In Free Fall

Mon Jul 30, 2012 4:39 pm

Quoting SlamClick (Reply 30):
Some helicopters have weights near the blade tips to give a little more inertia during the transistion from translational lift to autorotation.

Yep, and Hueys are one of them (as I know you know). My father flies for a company that takes H model Hueys blades and engines and puts them on B or C model frames, which requires them to cut 2 feet off of each blade on account of the difference in fuselage length (so you don't chop off the tail rotor!). Anyhow I have one of those 2' sections in my house and man is that thing heavy! And it has the endcap removed and you can see the 2 bolts that they slap the weights onto that you were talking about, and that they add or remove the amount of weights for tracking and balancing of the rotor blades. My dad and the A&P's get those Hueys tweaked sooo smooth, very nice.

Slam you used to fly Hueys didn't you? I know you have helo time but seem to recall Hueys were one of them.
 
SlamClick
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RE: Starting Helicopter In Free Fall

Mon Jul 30, 2012 5:23 pm

I taught Huey mechanics (crewchief-door gunner MOS 67N20 at the time) before I went to pilot training. Later I did fly Hueys, mostly D and H models.

Once picked up a brand new H from the factory and ferried it about 1500 miles. There was not vibration one in that thing. You could let go of the cyclic for a minute at a time and it would fly along straight and level. That with the "force trim" OFF. Got it to the unit and some colonel out to get his 4 hours a month oversped it and it was never the same again. That Huey is on a stick in another state.
 
HaveBlue
Posts: 2178
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RE: Starting Helicopter In Free Fall

Mon Jul 30, 2012 5:44 pm

Quoting SlamClick (Reply 45):
I taught Huey mechanics (crewchief-door gunner MOS 67N20 at the time) before I went to pilot training. Later I did fly Hueys, mostly D and H models.

Right on, thought so. My dad mostly flies the Bell 205 A-1 (D/H equivalent) but the company he flies for does that conversion I mentioned earlier. Usually its the 13 seater for him though. The B/C model with the bigger blade/engine is a performance model.

Quoting SlamClick (Reply 45):
Once picked up a brand new H from the factory and ferried it about 1500 miles. There was not vibration one in that thing. You could let go of the cyclic for a minute at a time and it would fly along straight and level. That with the "force trim" OFF. Got it to the unit and some colonel out to get his 4 hours a month oversped it and it was never the same again. That Huey is on a stick in another state.

Shame what one idiot can do to a good machine. Don't think I've seen a Huey on a stick.. we got a Cobra right down the street though. Whereabouts is the Huey if ya don't mind?
 
SlamClick
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RE: Starting Helicopter In Free Fall

Mon Jul 30, 2012 6:16 pm

Quoting HaveBlue (Reply 46):
Huey on a stick

I can see one from my bedroom window.

Yeah, the 205 has a tailboom extension to accomodate the longer blades. Surprises me that it is worthwhile to trim the blades. Perhaps the A/B blades are hard to find. BTW I am pretty sure but not 100% that the C is an entirely different blade from the A or B. It did have the "540" rotor system like the AH-1 Cobra. I think that came with a blade of much greater chord but it has been a very long time.
 
tdscanuck
Posts: 8573
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RE: Starting Helicopter In Free Fall

Tue Jul 31, 2012 1:35 am

Quoting glen (Reply 41):
I was thinking of it as a theoretical impossibility to tilt te blade so much as to generate enough torque instead of seeing the statement as a remark about real world helicopters. I was thinking about "physically" meaning "by natural laws". In complex matters sometimes a single word misunderstanding in a foreign language can make quite a difference...

Sorry about that..."physically" is a bad word for double meanings in English. I meant the "no real world rotor" sense, not the "theoretically impossible" sense.

Quoting nomadd22 (Reply 42):
This might require more detailed engineering analysis and beer to figure out.

Less analysis, more beer, and all will be fine.

Quoting SlamClick (Reply 43):
Furthermore, I cannot think, offhand, of a single helicopter on which hydraulic pressure wouldn't also be required.

The Museum Of Flight in Seattle has a ~1960's general aviation helicopter that has, to all appearances, about 4 moving parts and no hydraulic system. It's a tiny two-seater powered by what looks like a water-cooled version of a VW engine.

Tom.
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Starting Helicopter In Free Fall

Tue Jul 31, 2012 3:59 am

Yeah, I also flew the Hiller 12E and if memory serves, it didn't have a hydraulic system. The comparable Bell 47 series did.

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