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Indian Military Helicopters

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Re: Indian Military Helicopters

Postby shiv » 09 Oct 2017 21:53

Zynda wrote:I found a few images on the net on how individual rotor blades tilt can be achieved.

Image


Image

Sorry for the multiple images...


To me these two images seem to be showing two different things

The upper image shows the rotor hub tilting separately from the drive shaft/main rotor mast

The lower image seems to show a rigid rotor mast with the swash plate merely controlling rotor blade pitch/angle of attack

Which is correct? I didn't think the rotor disc has a degree of "play/movement" separate from the rotor drive shaft allowing the rotor disc to tilt without also taking the rotor shaft (and rest of helicopter) with it.

The video (to my eyes) just shows rotor blade flexing.

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Re: Indian Military Helicopters

Postby shiv » 09 Oct 2017 22:05

JayS wrote:Changing only AoA will not tilt lift vector in required direction. You could roll or pitch using differential lift but moving sideways or back and forth is not possible that way. Think of it using Newton's third law. Anyway 5 min youtube would get you all the basic info on what are the controls and how exactly they move rotors.

Why would forward and back movement not be possible by changing differential lift? If sideways movement/rolling is possible the same thing could tilt the rotor disc forward and take the helo forward (or back). It there any technical problem in doing that which calls for the added complexity of a rotor disk that tilts separately on the drive shaft?

YouTube and other sources all speak of cyclic but tilting of rotor disc separate from rotor drive shaft is not something that I have read about or seen on YouTube. If you can point me to one such video perhaps I can learn.

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Re: Indian Military Helicopters

Postby vina » 09 Oct 2017 22:26

deejay wrote:No while hovering you have to cater to recirculating air which the wall on the left interferes with thus reducing the benefits of ground cushion. Also the force on the retreating blade acts 90 degrees to where it is applied as it precesses


Ok. Yes if you are in ground effect. But let me put the same question to you .

What will happen if you are hovering very close to a vertical cliff at 1000 ft (ie out of ground effect). How will the Helicopter behave ?

Also related question (same physics) to folks here who have handled boats, ships and stuff, Navy or civilian or just casual sailors etc.

What will happen to the boat when it is close to the wall of restricted waters like in a canal or a shoreline (like say next to the walls of a Fjord) ?

Folks who have been Deck Officers in the Navy, give it a shot. It should be a cinch.

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Re: Indian Military Helicopters

Postby vina » 09 Oct 2017 22:30

Indranil wrote:While hovering, there is no difference in the flow around the advancing or the retreating blade. The wall on the left affects the vortex ring on the left, amplifying it. This asymmetry pushes the heli to the left and back.


Yes. However, do look up the question I posted just before this. What happens is that the Helicopter starts shifting towards the wall (same bank effect that pulls a boat towards the walls of a canal, that suction is due to venturi/Bernoulli effect). That instantaneous movement will change the relative wind and there will be differential lift because there is now a relative wind due to the motion. The helicopter will move forward /back due to the advancing and retreating blades like DeeJay said and there needs to be a control input.

Indranil wrote:This same phenomenon brought the heli down in Abottabad, except there were walls on all sides of the compound. The amplified the vortex ring on all sides and the heli experienced a sudden loss of lift and crashed down.

Abottabad is a different phenomenon. That was settling down under power (the plane sinks in its own rotor vortex) due to ground effect and recirculation of rotor wash.

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Re: Indian Military Helicopters

Postby shiv » 09 Oct 2017 22:31

OK I found a good video. The rotor disc does not need to tilt forward or backwards for forward (or backward) flight. Pitch change via the swash plates do the job. This is what I have been saying. There is no separate forward/backward tilting of the rotor disc on the drive shaft, or even tilting of the entire drive mechanism. The Encyclopedia Britannica pictures are either wrong or badly worded. The only "trick" is that the swash plate has to be angled obliquely (not directly front and back) due to precession. Newton's 2nd law as per the video.

Watch from here onwards
https://youtu.be/2tdnqZgKa0E?t=159
Last edited by shiv on 09 Oct 2017 22:45, edited 2 times in total.

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Re: Indian Military Helicopters

Postby srin » 09 Oct 2017 22:35

vina wrote:
Ok. Yes if you are in ground effect. But let me put the same question to you .

What will happen if you are hovering very close to a vertical cliff at 1000 ft (ie out of ground effect). How will the Helicopter behave ?

Also related question (same physics) to folks here who have handled boats, ships and stuff, Navy or civilian or just casual sailors etc.

What will happen to the boat when it is close to the wall of restricted waters like in a canal or a shoreline (like say next to the walls of a Fjord) ?

Folks who have been Deck Officers in the Navy, give it a shot. It should be a cinch.


Assuming that question wasn't directed solely at deejay, let me give it a shot. Whatever little of physics I remember tells me that due to bernoulli's principle, there'll be a low pressure created and hence the boat will be pushed towards the walls.

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Re: Indian Military Helicopters

Postby vina » 09 Oct 2017 23:02

shiv wrote: The rotor disc does not need to tilt forward or backwards for forward (or backward) flight.

Oh, the rotor disc does tilt forwards, backwards, sidewards etc. That is the only way the helicopter can move in whatever direction (other than up and down). That "rotor disc" is the conceptual theoretical part. That is what the Brittanica pictures show and that is what I also said.Achieve it engineering wise is what the video shows.

A theoretical disc assumes uniform lift across the disc which doesn't bother how it does so. In practice because it is made of blades, you have to account for the fact that due to relative wind, the advancing and retreating blades will generate differential lifts (and hence need different angles of attack) and the swash plates do that. The difference engineering implementation wise, is that the helicopter itself changes orientation (ie, puts nose down slightly and hence rotor disc angle change) and moves forward.

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Re: Indian Military Helicopters

Postby Zynda » 09 Oct 2017 23:23

Hakeem saab, a couple of animation videos on how helicopter motion is achieved via controlling pitch of rotor blades.



JayS, the dude on the above video says that differential lift is reason behind movement of helicopters in various directions (If I've listened correctly...he says even forward & backward :) )

One more...this is has more comprehensive explanation than the above.



I am having a headache...will try to watch the above videos again tomorrow.

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Re: Indian Military Helicopters

Postby shiv » 10 Oct 2017 01:18

Zynda it is differential pitch that causes differential lift and makes the whirling disc tilt. But the entire helo tilts and moves with the disc. There is no separate " Disc is made to tilt and therefore causes forward movement". That was the kakoose in the Britannica images.

The linear motion of the helo is exactly the linear motion executed by the disc. The pilot only controls the rotatory movement (whirling) of the disc/main rotor and the blade pitch. He cannot tilt the disc forward. That tilting is only due to differentisl pitch leading to differential lift.

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Re: Indian Military Helicopters

Postby Indranil » 10 Oct 2017 05:06

vina wrote:
Indranil wrote:While hovering, there is no difference in the flow around the advancing or the retreating blade. The wall on the left affects the vortex ring on the left, amplifying it. This asymmetry pushes the heli to the left and back.


Yes. However, do look up the question I posted just before this. What happens is that the Helicopter starts shifting towards the wall (same bank effect that pulls a boat towards the walls of a canal, that suction is due to venturi/Bernoulli effect). That instantaneous movement will change the relative wind and there will be differential lift because there is now a relative wind due to the motion. The helicopter will move forward /back due to the advancing and retreating blades like DeeJay said and there needs to be a control input.

I think the recirculation on the left due to the wall and its effect on the lift would dominate the Venturi effect.
vina wrote:
Indranil wrote:This same phenomenon brought the heli down in Abottabad, except there were walls on all sides of the compound. The amplified the vortex ring on all sides and the heli experienced a sudden loss of lift and crashed down.

Abottabad is a different phenomenon. That was settling down under power (the plane sinks in its own rotor vortex) due to ground effect and recirculation of rotor wash.

Ground effect alone does not form strong enough recirculation. There has to be obstructions to the airflow from dispersing laterally. The Abottabad crew practiced the operation multiple times in a simulated compound. They never faced the problem because that compound's boundary was marked using barbed wires.

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Re: Indian Military Helicopters

Postby shiv » 10 Oct 2017 07:13

vina wrote:Oh, the rotor disc does tilt forwards, backwards, sidewards etc.

Yes but the helicopter tilts with the disc and the disc cannot tilt and rock separately as shown in the Britannica images posted earlier. Those images are wrong.

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Re: Indian Military Helicopters

Postby Shubham » 10 Oct 2017 08:56

Let me take a bite at this.
DeeJee ji question
Q. what happens when you hover a helicopter with big wall on your left ?
Ans. Assuming helicopter is a clockwise rotating main rotors like Russian/french (Chetak), the helicopter will just move forward. No sidewards motion towards or away from the wall. For anti-clock rotors , helicopter will move back. Why - Reason is recirculation and gyroscopic precession.
Any force applied at rotors get shifted and act 90 degree away, ie in clockwise rotors a force applied at 9 o clock point act at 12 o clock point, this is precession.
Now the presence of wall results in the air which was supposed to go down and mostly get lost(not come back on top of rotors to pass through rotors again) after passing the rotors, to be channeled again to the top. This basically reduces angle of attack at point on rotor disc close to wall ( happens due to reduced inflow angle). This is called recirculation and is experienced near obstruction and at times when hovering over tall grasses. Hence the lift at 9 o clock position is reduced. But as we just mentioned about precession, this reduction in lift actually is translated to 12 o clock point, thus helicopter moves forward . The rotor disc will tilt forward and has to be corrected by a back input on the cyclic.

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Re: Indian Military Helicopters

Postby Shubham » 10 Oct 2017 09:19

The other one
Q. How is motion of helicopter effected? Is it differential pitch, tilt of rotor disc? What happens to the fuselage?
Ans. The final reason why helicopter moves in 2D plane is due to rotor disc tilting in that direction ( for forward motion , rotor disc is lowest at 12 o clock point) and so on. This in turn is achieved by pilot applying differential pitch ( he pushes cyclic forward, which is translated to swash plate acting to make 9 o clock point lowest- minimum pitch angle at 9 o clock and maximum pitch angle at 3 o clock, which actually results in 12 o clock part of rotor disc becoming lowest due to gyroscopic precession). The reason why the physical tilt of rotor disc is neither done nor required because the blades are flexible hence the same affect us achieved in an easier manner by giving cyclic pitch change.

The effect on fuselage - does it tilt forward when helicopter is moving forward ?
It depends on type of main rotor system. In underslung or teetering type of main rotors , there is a very prominent lag in fuselage following the rotor disc ; in rigid/semi rigid rotors (ALH) it is quite fast. So for any significant rotor disc attitude change, there is a fuselage change, the time lag depends on type of rotor system.

But for quick inputs, in a say teetering rotor, the fuselage will not be affected.

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Re: Indian Military Helicopters

Postby vina » 10 Oct 2017 09:24

shiv wrote:
vina wrote:Oh, the rotor disc does tilt forwards, backwards, sidewards etc.

Yes but the helicopter tilts with the disc and the disc cannot tilt and rock separately as shown in the Britannica images posted earlier. Those images are wrong.


It is more complicated than that. My understanding is that it depends on the type of Rotor Head . Whether it is a fully articulated hinged type, a rigid rotor or a teetering rotor head.

Hari Nair (when he used posted here), talked about that wrt to Dhruv (which has a fixed rigid rotor). Yes, when you change the cyclic, the pitch changes individual blades and (differential lift generated), blade flexes /moves up/down and the rotor disc orientation changes.

My guess is whether the hull itself aligns with the rotor disc attitude will depend on the rotor head type. I would think yes for a rigid head (like Dhruv) and maybe NOT for fully articulated rotor heads.

Deejay / others would obviously know these as experts & people who have operated this first hand. They can possibly answer it better.

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Re: Indian Military Helicopters

Postby shiv » 10 Oct 2017 09:29

vina wrote:
It is more complicated than that. My understanding is that it depends on the type of Rotor Head . Whether it is a fully articulated hinged type, a rigid rotor or a teetering rotor head.
.

Fair enough - you have used better terminology than I have and I will look out for more info

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Re: Indian Military Helicopters

Postby vina » 10 Oct 2017 09:36

Shubham wrote:The other one
Q. How is motion of helicopter effected? Is it differential pitch, tilt of rotor disc? What happens to the fuselage?
Ans. The final reason why helicopter moves in 2D plane is due to rotor disc tilting in that direction ( for forward motion , rotor disc is lowest at 12 o clock point) and so on. This in turn is achieved by pilot applying differential pitch ( he pushes cyclic forward, which is translated to swash plate acting to make 9 o clock point lowest- minimum pitch angle at 9 o clock and maximum pitch angle at 3 o clock, which actually results in 12 o clock part of rotor disc becoming lowest due to gyroscopic precession). The reason why the physical tilt of rotor disc is neither done nor required because the blades are flexible hence the same affect us achieved in an easier manner by giving cyclic pitch change.

Yes exactly. Very well explained. The reason why the rotor disc changes orientation is because either the blades flex or move due to the flapping hinges , and the rotor cone changes orientation .

You a pilot Sir ?

The effect on fuselage - does it tilt forward when helicopter is moving forward ?
It depends on type of main rotor system. In underslung or teetering type of main rotors , there is a very prominent lag in fuselage following the rotor disc ; in rigid/semi rigid rotors (ALH) it is quite fast. So for any significant rotor disc attitude change, there is a fuselage change, the time lag depends on type of rotor system.

But for quick inputs, in a say teetering rotor, the fuselage will not be affected.

Oh thanks. I guessed right then. I would suggest that a fully hinged rotor (teetering is semi rigid ) the hull would be rather well isolated from the rotor attitude.

Shubham wrote:Q. what happens when you hover a helicopter with big wall on your left ?

Lets rephrase this question to a 1000 foot vertical cliff instead of wall, so that you don't have ground effect. What will the helicopter do ? From your previous answer just hover in the same spot without any motion ?

Remember, due to the cliff, the rotor downwash will be moving faster downwards between the cliff and helicopter than on the other side.
Last edited by vina on 10 Oct 2017 09:45, edited 2 times in total.

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Re: Indian Military Helicopters

Postby shiv » 10 Oct 2017 09:38

Shubham wrote:The effect on fuselage - does it tilt forward when helicopter is moving forward ?
It depends on type of main rotor system. In underslung or teetering type of main rotors , there is a very prominent lag in fuselage following the rotor disc ; in rigid/semi rigid rotors (ALH) it is quite fast. So for any significant rotor disc attitude change, there is a fuselage change, the time lag depends on type of rotor system.

But for quick inputs, in a say teetering rotor, the fuselage will not be affected.

Interesting information. Will look for info on teetering rotors

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Re: Indian Military Helicopters

Postby Shubham » 10 Oct 2017 10:49

vina wrote:
Shubham wrote:The other one
Q. How is motion of helicopter effected? Is it differential pitch, tilt of rotor disc? What happens to the fuselage?
Ans. The final reason why helicopter moves in 2D plane is due to rotor disc tilting in that direction ( for forward motion , rotor disc is lowest at 12 o clock point) and so on. This in turn is achieved by pilot applying differential pitch ( he pushes cyclic forward, which is translated to swash plate acting to make 9 o clock point lowest- minimum pitch angle at 9 o clock and maximum pitch angle at 3 o clock, which actually results in 12 o clock part of rotor disc becoming lowest due to gyroscopic precession). The reason why the physical tilt of rotor disc is neither done nor required because the blades are flexible hence the same affect us achieved in an easier manner by giving cyclic pitch change.

Yes exactly. Very well explained. The reason why the rotor disc changes orientation is because either the blades flex or move due to the flapping hinges , and the rotor cone changes orientation .

You a pilot Sir ?

The effect on fuselage - does it tilt forward when helicopter is moving forward ?
It depends on type of main rotor system. In underslung or teetering type of main rotors , there is a very prominent lag in fuselage following the rotor disc ; in rigid/semi rigid rotors (ALH) it is quite fast. So for any significant rotor disc attitude change, there is a fuselage change, the time lag depends on type of rotor system.

But for quick inputs, in a say teetering rotor, the fuselage will not be affected.

Oh thanks. I guessed right then. I would suggest that a fully hinged rotor (teetering is semi rigid ) the hull would be rather well isolated from the rotor attitude.

Shubham wrote:Q. what happens when you hover a helicopter with big wall on your left ?

Lets rephrase this question to a 1000 foot vertical cliff instead of wall, so that you don't have ground effect. What will the helicopter do ? From your previous answer just hover in the same spot without any motion ?

Remember, due to the cliff, the rotor downwash will be moving faster downwards between the cliff and helicopter than on the other side.


The explanation that I have considered is not considering ground effect. The ground effect affects the rotor disc uniformly ( whole 360 degree, all part of rotor disc have reduced inflow angle thus higher angle of attack).
In free air hover next to a cliff, the 9 o clock portion of the rotor disc is now experiencing more amount of downwashed air which is redirected to flow back from bottom of rotors, way up along the cliff and back down to the rotors. To use technical terms the tip vortex are intensified. This reduces lift. This phenomenon is called recirculation since downwashed air is circulated back.
Just for understanding, let us consider a plane 0.5 m about rotor disc. Previously the sector of say 11 o clock to 7 o clock was freely receiving air towards the rotors; now that volume becomes restricted due to presence of cliff, so the air has to flow in this sector at higher speeds since we have the helicopter maintaining position and height. This as per Bernoulli, the presence of cliff creates a more lower pressure region here wrt when there was no cliff. This will cause larger amount of air to be sucked up from below the rotors from the sector of 7 o clock to 11 o clock.
In my understanding the conditions which results in max lift (actually higher AoA) is generated by rotors are - fresh, undisturbed, no velocity air on top of rotors ; opposition to downward flow of air after passing down from rotors.
can't answer first one.

So either in ground effect or not helicopter will move parallel to obstruction.


Effect on fuselage.
I Re read about this. It is called as Control Power - how much effective cyclic is in changing fuselage attitude.
It is highest in Rigid rotors(ALH), followed by fully articulate (Chetak,mi 17), least in teetering (Sioux). Another way to say is that, the fuselage attitude change depend on the distance between main rotor shaft and point where cyclic force is effective being applied ( flapping hinge in case of articulate rotors).

The interesting thing and the importance of this question- fuselage attitude change is that; more the fuselage change capability,more is the manoeuvrability as well as speed range, this is why ALH is rigid rotors , allows great manoeuvrability.
Last edited by Shubham on 10 Oct 2017 12:52, edited 1 time in total.

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Re: Indian Military Helicopters

Postby JayS » 10 Oct 2017 12:08

shiv wrote:
JayS wrote:Changing only AoA will not tilt lift vector in required direction. You could roll or pitch using differential lift but moving sideways or back and forth is not possible that way. Think of it using Newton's third law. Anyway 5 min youtube would get you all the basic info on what are the controls and how exactly they move rotors.

Why would forward and back movement not be possible by changing differential lift? If sideways movement/rolling is possible the same thing could tilt the rotor disc forward and take the helo forward (or back). It there any technical problem in doing that which calls for the added complexity of a rotor disk that tilts separately on the drive shaft?

YouTube and other sources all speak of cyclic but tilting of rotor disc separate from rotor drive shaft is not something that I have read about or seen on YouTube. If you can point me to one such video perhaps I can learn.


Only moment is produced if you have differential lift, if there is not change of rotor disc plane, i.e. no tilt of lift force. This moment can only let you roll or pitch but no sideways of front-back movement. That's what I wanted to say (I didn't take into account gyroscopic effect). I think others have already explained more than I could have. But the key point that I would have added if Shubham had not already added it is the flexible rotor blades. When you increase pitch on one side using cyclic, the lift force is increased on blade passing through that point, which makes the blade deflect more upwards. While on the 180deg opposite side the blade is less deflected because of less AoA. This effectively makes the rotor blades to rotate in slightly inclined plane.

I misunderstood initially what you were trying to say I think. But now I get what you wanted to say. You were trying to point out that there is not relative mechanical motion between the rotor plane and the fuselage. The answer for that is already given I suppose - it depends on type of mounting system of rotor. I was not aware of all the details about it. I thought all helis had flexible mounting.

Also the videos posted by you and zynda shows that there is rigid body roll or pitch of entire helicopter and rotor together due to gyroscopic couple (this is 2nd law) which tilts the rotor plane with respect to fixed reference frame (say earth). There may or may not be relative motion between the rotor and fuselage.

So we have total 3 things going on here when cyclic is applied -
- 1 tilting of entire heli due to gyroscopic couple
- relative tilting of rotor wrt fuselage depending on what kind of mounting system is used. This is mechanical motion
- Aero-elastic tilting of rotor wrt fuselage.

The rotor disc tilts wrt a fixed frame of reference. There is some relative tilt also present (either only aeroelastic or mechanical + aeroelastic) between the rotor and the fuselage. This tilting (whichever way its achieved) tilts the effective lift vector of entire rotor and a part of it acts as thrust and pushes the heli in that direction. This is 3rd law I was referring to.But good discussion. Added a lot to my understanding.

Zynda:
I did not consider gyroscopic movement. I was only thinking of aerodynamic tilting.

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Re: Indian Military Helicopters

Postby negi » 10 Oct 2017 12:38

vina wrote:
shiv wrote: :eek: What happens?

My guess, you crash into the wall, because of the suction effect that will be created by the air flowing faster between the helicopter and the wall compared to the other side due to Bernouli effect.

My guess is that the rotor has to be tilted right to counter that.

Is the answer of this question not dependent on additional factor i.e. is there land between the wall and immediate surface over which helicopter is hovering ? If there is land on the section that defines the space between the chopper and the wall then what is that height/depth ?

The way I see it without talking about articulation of main rotor there are at least 3 clear effects in play here

1. Bernoulli effect (in my opinion it is perhaps not the strongest one in play)
2. Ground effect depending on height of the chopper from the ground below it
3. When adjacent to the wall is the tail rotor pushing air into the wall or away from the wall
Last edited by negi on 10 Oct 2017 12:49, edited 2 times in total.

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Re: Indian Military Helicopters

Postby deejay » 10 Oct 2017 12:45

Well the wall on the left has caused the same issues here as everywhere else. :mrgreen: Look the seals even crashed. We did better.

Anyways, that was not the intent. To briefly understand, try constructing diagrams. I am pasting some images from a website which lays out rotordynamics in non technical terms (i.e. no formulae). It has handy diagrams and just keep clicking on next or previous to reach other pages. They do cover forces in a hover with ground effect and in free air. For technical guys and experts this may not be the best resource.

Image

Image

Image

http://www.copters.com/

Also ground effect is considered insignificant for exact calculations at height = 2/3 of rotor dia.

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Re: Indian Military Helicopters

Postby Shubham » 10 Oct 2017 13:07

negi wrote:
vina wrote:My guess, you crash into the wall, because of the suction effect that will be created by the air flowing faster between the helicopter and the wall compared to the other side due to Bernouli effect.

My guess is that the rotor has to be tilted right to counter that.

Is the answer of this question not dependent on additional factor i.e. is there land between the wall and immediate surface over which helicopter is hovering ? If there is land on the section that defines the space between the chopper and the wall then what is that height/depth ?

The way I see it without talking about articulation of main rotor there are at least 3 clear effects in play here

1. Bernoulli effect (in my opinion it is perhaps not the strongest one in play)

Bernoulli is in effect and it is as usual.

2. Ground effect depending on height of the chopper from the ground below it.

Assuming just two cases here a. No ground effect of ground below. b. Full ground effect, like helicopter just hovering after pickup in a tarmac next to the wall.
Both cases helicopter will move parallel to wall; forward for clockwise main rotor, rearward for anti-clock rotors.

3. When adjacent to the wall is the tail rotor pushing air into the wall or away from the wall

Any affect of tail rotor will cause yaw, not any translative motion.

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Re: Indian Military Helicopters

Postby shiv » 10 Oct 2017 20:01

Shubham wrote:Effect on fuselage.
I Re read about this. It is called as Control Power - how much effective cyclic is in changing fuselage attitude.
It is highest in Rigid rotors(ALH), followed by fully articulate (Chetak,mi 17), least in teetering (Sioux). Another way to say is that, the fuselage attitude change depend on the distance between main rotor shaft and point where cyclic force is effective being applied ( flapping hinge in case of articulate rotors).

The interesting thing and the importance of this question- fuselage attitude change is that; more the fuselage change capability,more is the manoeuvrability as well as speed range, this is why ALH is rigid rotors , allows great manoeuvrability.


I found a great article on flapping hinges which I hope AM Matheswaran will also read

http://www.rotorandwing.com/2013/10/01/hinge-offset/

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Re: Indian Military Helicopters

Postby Indranil » 11 Oct 2017 00:24

Shubham sir,

I want some clarification. I don't think that the wall + ground effect has the same effect as a cliff. According to my understanding. Let us take a heli with a rotor moving clockwise (as seen from top) and the wall is to the left (9 o' clock as seen from the top).

1. The cliff restricts the movement of the air from the left and hence the airflow at 9'o clock is both faster and at higher effective AoA. This will decrease the lift on the left hand side. Due to precession, effectively a loss of lift will be felt at 12 o'clock, hence pushing the heli forward. I also agree that a secondary effect will also be that the tip vortex at 9 o'clock will also be enlarged, further aggravating the effect. Re-circulation although higher at 9 o'clock than 3 o' clock will not be greatly effected, as there is space for the air to "escape" from the bottom.

2. In the case of the heli hovering IGE close to a wall at 9 o'clock recirculation is most dominant effect. This is because the air from the rotor has no where to go. It moves along the ground to the wall, up the wall and into recirculation. The effect is much more drastic. The loss of lift at 9 o'clock is significantly higher than in the case of the cliff. The tip vortices are also significantly higher.

In both cases the helicopter will be pushed forward with loss of lift (if collective is not applied), but the effect is much more pronounced in the case of the wall on the ground than the side of a cliff.

P.S. My knowledge about heli aerodynamics is very limited. Just casual discussions with friends. One of them was quite knowledgeable about recirculation. He was studying how to minimize recirculation of air in tandem rotor helicopters. He was not concerned with loss of lift but with limiting whiteout conditions and damage to blade while operating from dusty environments.

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Re: Indian Military Helicopters

Postby Shubham » 11 Oct 2017 09:03

Indranil wrote:
1. The cliff restricts the movement of the air from the left and hence the airflow at 9'o clock is both faster and at higher effective AoA.

It should be lower AoA



Thank you Indranil ji for indulging me, but no saar for this nanha mujahid.
Generally I can't disagree with you for any point. Recirculation ( which refers to phenomena in which downwashed air comes back on top of rotor) is leading to intensified rotor tip vortex. Loosely speaking for the purpose of their resultant effect both are the same. In cliff case consider two points just above and below 9 o clock of rotor disc. In absence of cliff there is some pressure difference between them that is causing rotor tip vortex, when cliff comes into picture the pressure difference between these two points increases leading to it attracting more air from everywhere including air trying to go down along the cliff. So recirculation/tip vortex increase loss of lift.

As you say the similar thing happens in IGE. Intutively the effect should be more pronounced here but I would not say conclusively because IGE Power itself is less than OGE hover power ( say upto 10%), other factors like distance from cliff, hover ht may balance things out.

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Re: Indian Military Helicopters

Postby Indranil » 11 Oct 2017 10:25

Agreed. I will also ask a heli pilot friend of mine for his thoughts as well. He flew helis where the rotors spun the other way. But, I am pretty sure he will agree with us.

Also, no "ji" for me please. Me too nanha mujaheed only.

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Re: Indian Military Helicopters

Postby Zynda » 11 Oct 2017 11:47

I was reading an article about LUH printed on Economic Times supplement I think. The entire saga of LUH is moving very slowly. Apparently, the Ka-226T weight is higher than what was quoted in RFP by Russians. Also I think there has been a surge in pricing by the Russians again due to which the negotiations are still on-going. The sudden & exorbitant price hikes by Russians on almost all items is surprising. No wonder there are reports out there about Indo-Russian relations is at its nadir. Supposedly the IAF is not happy with S-400 as well. The amount of S-400 capability watering down is higher than expected or what Russians initially mentioned (?). Even that file is stuck at MoD. Apparently most major Russian purchase program files are either stuck or moving slowly at MoD.
Last edited by Zynda on 11 Oct 2017 12:42, edited 1 time in total.

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Re: Indian Military Helicopters

Postby deejay » 11 Oct 2017 12:12

Indranil wrote:Agreed. I will also ask a heli pilot friend of mine for his thoughts as well. He flew helis where the rotors spun the other way. But, I am pretty sure he will agree with us.

Also, no "ji" for me please. Me too nanha mujaheed only.


While I may not be the friend you wanted to ask :) , this is what I have reconfirmed from someone who I am sure isn't shooting from the hip.

"Whichever side the wall is, it cuts off re-circulation, so lift increases on the blade next to the wall. The lift increase moves forward (rotates) by 90 degrees and affects the helicopter.

Wall to the left, increases lift on the blade nearest to the wall, which moves ahead (since rotating clockwise when viewed from the top) by 90 degrees and acts. This causes the aircraft to pitch up and move backward."

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Re: Indian Military Helicopters

Postby Philip » 11 Oct 2017 12:20

Russian files slowdown could be due to two reasons.First,the pro-West-read US lobby has captured the corridors of power in the MOD/PMO and is trying to turn us into a US lackey. The second,partly due to the above and great Russian disquiet.Remember BK's report on Ru suspecting the IN/IN officers to pass on Akula N-sub tech and wanting certain officers not to serve aboard its N-subs, Ru/Putin's wariness after so much of US bum-chumming by India ,causing it to ply a waiting game on Indian guarantees orPV

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Re: Indian Military Helicopters

Postby Shubham » 11 Oct 2017 13:26

deejay wrote:
Indranil wrote:Agreed. I will also ask a heli pilot friend of mine for his thoughts as well. He flew helis where the rotors spun the other way. But, I am pretty sure he will agree with us.

Also, no "ji" for me please. Me too nanha mujaheed only.


While I may not be the friend you wanted to ask :) , this is what I have reconfirmed from someone who I am sure isn't shooting from the hip.

"Whichever side the wall is, it cuts off re-circulation, so lift increases on the blade next to the wall. The lift increase moves forward (rotates) by 90 degrees and affects the helicopter.

Wall to the left, increases lift on the blade nearest to the wall, which moves ahead (since rotating clockwise when viewed from the top) by 90 degrees and acts. This causes the aircraft to pitch up and move backward."

saarji you are killing me :) , I guess quoting prouty might settle it, since all three of us (you, me, your a2 friend) holds it true.


Helicopter Aerodynamics - R W Prouty, chapter 15 Accidents waiting to happen, para - Operations near obstruction
Quoting relevant part
" for example, if, because of recirculation, the download is stronger on the left side, helicopter will tend to move backwards because of the 90 degree lag in flapping. Not only is the cyclic pitch affected but the increased down flow looks like a climb condition- which requires more power to the main rotor and therefore to the tail rotor also. This represents a decrease in ground effect and might keep a heavily loaded helicopter grounded."

Just to clarify prouty has mostly considered anti-clock main rotors in his books, hence moving back mentioned here, otherwise rest of the effects are self-explanatory. The relevant diagram is such that there is adequate wall-rotor clearance to let recirculation, unlike say the clearance of compressor and turbine blades in an engine ( which I feel is the condition where your friends explanation might be sort of relevant).

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Re: Indian Military Helicopters

Postby Kashi » 11 Oct 2017 13:28

Philip wrote:Russian files slowdown could be due to two reasons.First,the pro-West-read US lobby has captured the corridors of power in the MOD/PMO and is trying to turn us into a US lackey. The second,partly due to the above and great Russian disquiet.Remember BK's report on Ru suspecting the IN/IN officers to pass on Akula N-sub tech and wanting certain officers not to serve aboard its N-subs, Ru/Putin's wariness after so much of US bum-chumming by India ,causing it to ply a waiting game on Indian guarantees orPV


Of course, it's all our fault, poor Russians, are completely innocent and extremely generous...they want so badly to give all the cutting edge equipment and technology at throwaway rates, but the yeevil GoI is conspiring with the bloody Amreekans to turn us into an American lackey.

Have you completely stopped filtering your bias?

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Re: Indian Military Helicopters

Postby Aditya_V » 11 Oct 2017 14:21

And the Best part- certain pro US Navy officers. Suggesting these Indian Naval Officiers are US spies without any basis and casting doubts on the Indian Navy is unwarranted.

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Re: Indian Military Helicopters

Postby Philip » 11 Oct 2017 17:21

Russian files slowdown could be due to two reasons.First,the pro-West-read US lobby has captured the corridors of power in the MOD/PMO and is trying to turn us into a US lackey. The second,partly due to the above and great Russian disquiet.Remember BK's report on Ru suspecting the IN/IN officers to pass on Akula N-sub tech and wanting certain officers not to serve aboard its N-subs, Ru/Putin's wariness after so much of US bum-chumming by India ,causing it to play a waiting game until India guarantees the security of its weapon systems.

The foll. report below speaks for itself."Cultivate" is a polite word for blackmail/inducement.

https://theprint.in/2017/09/20/no-forei ... rs-centre/
Wednesday, 11 October, 2017
ThePrint

No more foreign govt-funded courses for Indian military officers: Centre
PRANAB DHAL SAMANTA 20 September, 2017

No more foreign govt-funded courses for Indian military officers: Centre
Many middle-rung officers have been seeking clearances for course offers from foreign countries (representational image) | Source: Wikimedia Commons
Uncomfortable with other countries cultivating Indian military officers, government says it’ll pay for an international course if justified.

The defence ministry has barred armed forces officers from pursuing training courses abroad that are sponsored by foreign governments. It has said that the Indian government would pay for an international course if it is justified.

The decision was taken after it came to light that a large number of middle-rung officers were seeking clearance for course offers from foreign countries, particularly Western nations.
:mrgreen:
*(Western nations,read US)

Typically, an official said, many of these countries would send out invites offering short, fully-funded courses or academic familiarisation tours to officers, who would then seek clearances on the ground that it did not burden the government financially.

An official in the know of the details told ThePrint that while many of these courses may have been professionally relevant, it was a matter of discomfort that these countries were using such opportunities to cultivate younger officers.

Also, it was argued that it did not suit India’s stature to accept such assistance given that New Delhi itself was offering courses for militaries in its neighbourhood.

This led to an official audit of permission granted for such cases, which was followed by an exercise to determine the cost the government would incur if it would have funded these officers.

The estimated amount, an insider said, was about Rs 12 crore. It was then decided at the highest levels that the government would rather fund these officers from than allow them to take benefits from a foreign country. A small part of the defence budget will now be earmarked for this purpose, the officials said.

Going forward, military officers can identify courses in foreign institutions and seek government funding, which would be put through the same approval process.

The only exception will be for courses that are agreed on a reciprocal basis, where India will also take in a specified number of foreign military officers for courses it offers


PS:Why younger officers? Becos they're more susceptible to the "honey-trap",like ex-RAW top officer,Unnikrishnan.There was even a case a few years ago of a naval officer who had a relationship with a Russian woman while on deputation there.With Western and European morals are very different from ours,where mutual relationships between the sexes is not seen as any great crime,our officers come under serious threat from blackmailing should they indulge in relationships,with the threat of being found out and exposed back home. There's a great story about how the israelis tried to compromise an Arab prince.After showing him the pics of him engaged with his lady in various poses,he reportedly asked for dozens of copies to send to his friends to show-off his virility! However,it is when money is also offered as inducement that trouble begins. A freebie course with "bonuses" attached ,does have an impact upon the receiver.It is all part of a well-trodden strategy to "cultivate" the officer in Q.The huge kickbacks that def. majors are also willing to pay ,could really tempt some individuals.Catching them young is great as it is an investment for the future as they would inevitably rise in the ranks and become decision-makers later on.More serious is that they would in turn pick selected juniors for the same freebies so that a chain is established for networking.

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Re: Indian Military Helicopters

Postby shiv » 11 Oct 2017 18:47

deejay wrote:"Whichever side the wall is, it cuts off re-circulation,


Shubham wrote: The relevant diagram is such that there is adequate wall-rotor clearance to let recirculation,.


Pardon the ignorant intrusion but aren't the two of you speaking of two different situations - i.e "closer to wall" and "some distance from wall"?

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Re: Indian Military Helicopters

Postby negi » 11 Oct 2017 21:59

Proximity to the wall will induce re-circulation as downwash from the rotor has been obstructed ; the side of the chopper which has been exposed or on opposite side of the wall will encounter less/no re-circulation . I was completely unaware about the concept of Gyroscopic precession but reading about it (minus the math) if the wall is on the left side of the chopper when you look at the chopper from behind then re-circulation on left flank should lead to reduced lift on that flank and that would mean in a chopper where rotor moves clockwise when viewed from the top the chopper will move forwards because the right flank at 3'o clock is now producing higher lift as compared to the blade when it is at 9'o clock position.

Long story short Shubham's explanation to me makes perfect sense.

Reading more on this gets interesting as when chopper transitions from hover to forward motion the blade on 3'o clock will produce even more lift as it is cutting against the incoming wind whereas the blade at 9'o clock is actually moving backwards (again applies to clockwise main rotor).

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Re: Indian Military Helicopters

Postby Shubham » 11 Oct 2017 22:43

negi wrote:as when chopper transitions from hover to forward motion the blade on 3'o clock will produce even more lift as it is cutting against the incoming wind whereas the blade at 9'o clock is actually moving backwards (again applies to clockwise main rotor).


Actually it is opposite, for clockwise rotor in forward flight, the relative velocity is highest at 9 o clock and lowest at 3 o clock , ie if we just consider effect of gust of wind from front for a hovering helicopter ( equivalent to effect of forward flight with no pitch variations cyclic considered) , the highest AoA is at 9 o clock and lowest AoA is at 3 o clock. This is what also causes right tilt in forward flight for clockwise rotor helicopter.

http://www.helistart.com/Figures/aerodynamicforces.jpg
Refer diagram above, AoA = pitch angle - inflow angle.
As relative velocity increases inflow angle reduces.

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Re: Indian Military Helicopters

Postby negi » 11 Oct 2017 23:24

^ I got mixed with clockwise direction there :)

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Re: Indian Military Helicopters

Postby Indranil » 12 Oct 2017 06:40

deejay wrote:While I may not be the friend you wanted to ask :)

That you consider me as your friend is an absolute honour. I cannot tell you how much I am attached to airmen. The guy I was speaking of is a tennis buddy. He is on the older side, very fun to be with.

deejay wrote:this is what I have reconfirmed from someone who I am sure isn't shooting from the hip.

You don't shoot from the hip either. Because we are all guesstimating, O am getting all kind of second opinions. :D By the way, I just realized something, we can test this model heli. Let me see if I can entice a friend to fly his enough close to a wall. He is a great pilot and foolish enough to risk his thousand dollar machine :P

deejay wrote:"Whichever side the wall is, it cuts off re-circulation, so lift increases on the blade next to the wall. The lift increase moves forward (rotates) by 90 degrees and affects the helicopter.

Wall to the left, increases lift on the blade nearest to the wall, which moves ahead (since rotating clockwise when viewed from the top) by 90 degrees and acts. This causes the aircraft to pitch up and move backward."

Your friend is correct only if the helicopter is really really close to the cliff. So close that the cliff stops the tips vortex from completing. We are speaking of a few tens of inches here. In this situation, the lift will increase at 9 o'clock pretty much on the same principles as how lift increases in IGE, or addition of winglets on fixed wings.

But as soon as the heli moves far enough where the vortices can complete, the effect will completely switch as explained by Shubham above. As I read more, I am becoming increasingly sanguine about it. Also, my respect for heli pilots who land on the edge of cliffs to load/offload is skyrocketing.

Although this is visualization at very low Reynolds number, you can see how the vortices would move in ground effect. If there was a wall/cliff to block this airflow, it will move up along the wall and back into recirculation.

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Re: Indian Military Helicopters

Postby shiv » 12 Oct 2017 09:55

Wonder which book this pilot was consulting?
https://twitter.com/rotormagic/status/9 ... 1301079040
Image

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Re: Indian Military Helicopters

Postby deejay » 12 Oct 2017 10:00

^^^ Thanks Shivji. That is the exact image that I wanted to bring in discussion here.

Shubham you are right, my friend is an A2 but this is part of the info base since in helicopter flying this is a regular occurrence.

Indranil, rotor tip vortices are getting impacted here at 45 degrees point. Also the Ground Effect is marginal and differential since the floor is sloping away. The helicopter is also partially on ground because it is still at hover power. Interesting case this.


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