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#4703484 - 03/22/18 05:16 PM A Question For Helicpotor Pilots ???
billt460 Offline


Registered: 03/30/15
Posts: 4050
Loc: Glendale, Arizona
I'm not a pilot, so bear with me. When a helicopter is in a dead hover, the main rotor keeps a constant pitch as it rotates through it's 360 degree circle. Now let's say you put the helicopter into forward motion. Say 80 knots. If the main rotor is rotating clockwise, as the blade comes into the 9:00 O'clock position, it will have an extra 80 knots of air blowing over it because of this forward motion. In contrast when that blade gets to the 3:00 O'clock position, it will have 80 knots less air blowing over it. For a difference of 160 knots in just 180 degrees of rotor rotation.

So the main rotor has to increase pitch on the blade when it gets to the dead side, (3:00 O'clock position), of the main rotor in order to make up for that loss of lift, that is created by less airspeed over the rotor blade in the 3:00 O'clock position. This difference in pitch obviously increases as the forward speed of the helicopter increases. And it becomes non existent in a motionless hover. My question is how does the helicopter do that? If the pilot did it with cyclic stick input, he would be pushing the cyclic stick further to the left, (assuming clockwise main rotor rotation), the faster he flew. Is there some automated feature built into the controls that controls this?

And continuing with this, if the pilot were to put the helicopter into a shallow dive, and deliberately over speed it, would the helicopter eventually enter an uncontrolled roll? Because of it's inability to create lift on the dead side of the main rotor, as the speed increased to the point it could no longer counteract the loss of lift on that side with pitch increase of the rotor blade itself.

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#4703531 - 03/22/18 06:00 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
JLTD Offline


Registered: 12/15/17
Posts: 375
Loc: US
The phenomenon you're describing is called retreating blade stall. Before anything like a rollover happens, the helicopter will start to vibrate. This vibration will get worse and worse as speed increases. I am not a helo pilot but have spoken with several in depth. Apparently something will break on the helicopter before a loss of control occurs - which will then result in a loss of control.

Don't know about the dynamics of where they would need to hold the stick at higher speeds, although a progressive move to counteract the ever-changing ratio of lift between the two sides makes sense.
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#4703533 - 03/22/18 06:03 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
CT8 Offline


Registered: 10/09/14
Posts: 10547
Loc: Idaho
By design the retreating rotor increases pitch by design because as you mentioned the air speed is less. I learned that in a Helicopter Maintenance class I took in 1981 Sad though I forgot most everything . I'll bet Cujet can give you a perfect explanation.
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Artificial intelligence is no match for natural stupidity, we shall prevail

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#4703534 - 03/22/18 06:04 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
clarkflower Offline


Registered: 12/27/11
Posts: 712
Loc: US
https://www.decodedscience.org/why-cant-helicopters-fly-fast/22018
When there is any wind at all, or the helicopter moves forward, the advancing blade (the forward moving one) has more air blowing over it – i.e., a higher airspeed – than the retreating blade (the backward moving one), and therefore produces more lift.





To counteract this dissymmetry of lift, we allow the blades to flap up and down, and this flapping equalises the lift across the rotor disc as the blades “flap to equality.” But a side effect of this flapping is that when the cyclic is moved forward to increase speed, the rotor disc tilts forward initially, but then flaps back, and further forward cyclic movement is required in order to continue to accelerate.

This phenomenon, known as “flapback,” occurs throughout the whole speed range of the helicopter. So if we want to increase our speed, the cyclic has to be moved progressively further and further forward. There will come a point at which the cyclic is on its forward limit, and the helicopter cannot fly any more quickly.

Airflow Reversal
However, in practice there are other factors which are likely to play a part in limiting the helicopter’s forward speed. As the helicopter flies faster and faster, there is a progressively increasing difference in the relative velocities of the advancing and retreating blades. To understand this, let’s put in some numbers. If the helicopter is moving forward at 30 knots, assuming no wind, the difference between the airspeed of the advancing blade and the retreating one is 60 knots. But if the helicopter is moving at 150 knots, this difference becomes 300 knots. There comes a point at which the root of the retreating blade, which is the slowest part, has zero airspeed, since the helicopter is moving forward at a faster speed than that section of blade is rotating. When that occurs, this particular section of rotor blade cannot produce any lift.

At first this only happens over a small area of the blade, but as the helicopter speeds up, this “airflow reversal” takes place over a larger and larger area of the retreating blade. The only way the rotor system can compensate for this is for the outer part of the retreating blade to work harder. So the outer section of the blade has to produce more lift, and it does this by operating at a higher and higher angle of attack, which is achieved through more flapping. Although it doesn’t sound terribly efficient, the helicopter can operate quite happily in this condition, and tests have shown that some helicopters at maximum forward speed have 40% of the retreating blade affected by reverse flow.

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#4703557 - 03/22/18 06:27 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
CourierDriver Offline


Registered: 12/27/09
Posts: 4826
Loc: Tn.
I can tell you from my experience in an Air Sea Rescue unit while in the Navy, just twist the throttle on the collective, pull up on the collective, push the stick forward and let her rip and enjoy the ride,,,imho....all we wanted was left off and get going and enjoy the thrill...fly Navy...........
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#4703592 - 03/22/18 06:56 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX
The forward advancing blade has far less angle of incidence, for less lift...

than the retreating blade does.

The is all automatically changed in the main rotor mast each rotation.
The "angle of attack" on each blade changes EVERY revolution.
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"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703594 - 03/22/18 06:58 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX

Here you go:

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"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703597 - 03/22/18 06:59 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX
another

_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703599 - 03/22/18 07:01 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX
MORE


_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703603 - 03/22/18 07:02 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX
with explanation

_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703604 - 03/22/18 07:03 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
rooflessVW Offline


Registered: 12/24/11
Posts: 3662
Loc: North Carolina
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"Zed's dead baby, Zed's dead."

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#4703605 - 03/22/18 07:03 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX
another

_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703606 - 03/22/18 07:04 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX
This isn't for high school dropouts...............

_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703608 - 03/22/18 07:06 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
Linctex Offline


Registered: 12/31/16
Posts: 5694
Loc: Waco, TX
_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4703749 - 03/22/18 10:27 PM Re: A Question For Helicpotor Pilots ??? [Re: billt460]
pkunk Offline


Registered: 03/19/11
Posts: 235
Loc: NM, USA
A slightly out of adjustment of the rotor control arms is what causes the wap-wap wap of (especially a Huey), they are very difficult to adjust.
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