Dissymmetry of lift, flapping and blowback...
Thread Starter
Joined: Feb 2001
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From: UK
Dissymmetry of lift
The CAA recently set this as an engineering question
What is the advancing blade on a helicopter doing
a Increasing in lift
b Going to the highest point
c Increasing in drag
Its option B that is causing the confusion. For the advancing blade which direction is the blade flapping ? After reading Mr Coyles book it is evident that for the advancing blade, two mechanisms are occuring.
Cyclical change of pitch which gives the advancing blade a lower angle and less lift and the retreating blade a higher angle and thus more lift. This is accomplished via the mechanics of the pitch change system. The overall effect here is for gyroscopic precession to put the advancing blade at its lowest point at 180 deg rotor azimuth.
The second mechanism is flapping to equal out the lift on opposite sides of the rotor disc. The advancing blade flapping up and thus reducing in lift and the retreating blade flapping down and thus increasing in lift. The two effects, on opposite sides of the disc help keep the lift equal between the two halves.
These two seem to contradict each other somewhat and after reading the description of flapback/blowback it turns it on its head.
Could someone please explain which way the advancing blade is flapping
Many thanks
GR (fixed wing !)
What is the advancing blade on a helicopter doing
a Increasing in lift
b Going to the highest point
c Increasing in drag
Its option B that is causing the confusion. For the advancing blade which direction is the blade flapping ? After reading Mr Coyles book it is evident that for the advancing blade, two mechanisms are occuring.
Cyclical change of pitch which gives the advancing blade a lower angle and less lift and the retreating blade a higher angle and thus more lift. This is accomplished via the mechanics of the pitch change system. The overall effect here is for gyroscopic precession to put the advancing blade at its lowest point at 180 deg rotor azimuth.
The second mechanism is flapping to equal out the lift on opposite sides of the rotor disc. The advancing blade flapping up and thus reducing in lift and the retreating blade flapping down and thus increasing in lift. The two effects, on opposite sides of the disc help keep the lift equal between the two halves.
These two seem to contradict each other somewhat and after reading the description of flapback/blowback it turns it on its head.
Could someone please explain which way the advancing blade is flapping
Many thanks
GR (fixed wing !)
Iconoclast
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From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Which way is up and other questions
To: Golden Rivet
I may be totally wrong and risk the wrath of NickLappos and Shawn Coyle but this is what I think is happening. The upward flapping of the advancing blade and the downward flapping of the retreating blade occurs during translational lift flight or up to approximately 20 knots. This will result in a roll to the right. The pilot can remedy this situation with cyclic input. If there is a tendency for blowback the pilot can introduce forward cyclic. The lift differential (Dissymmetry of lift) is countered by pitch flap coupling. When the advancing blade flaps up due to the increase of lift the pitch flap coupling will extract pitch from the blade reducing its’ lift. Conversely if the retreating blade flaps down pitch is increased due to pitch flap coupling thus equalizing the lift.
In addressing this situation you must deal with the individual blades and not the disc.
Duck for incomming.
I may be totally wrong and risk the wrath of NickLappos and Shawn Coyle but this is what I think is happening. The upward flapping of the advancing blade and the downward flapping of the retreating blade occurs during translational lift flight or up to approximately 20 knots. This will result in a roll to the right. The pilot can remedy this situation with cyclic input. If there is a tendency for blowback the pilot can introduce forward cyclic. The lift differential (Dissymmetry of lift) is countered by pitch flap coupling. When the advancing blade flaps up due to the increase of lift the pitch flap coupling will extract pitch from the blade reducing its’ lift. Conversely if the retreating blade flaps down pitch is increased due to pitch flap coupling thus equalizing the lift.
In addressing this situation you must deal with the individual blades and not the disc.
Duck for incomming.
Joined: Jul 1999
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From: Warrington, UK
There are two situations to be considered here, Flapping to Equality
and Inflow Roll.
Flapping to Equality.
As the aircraft moves forward, start at a blade tip in the 6 o'clock
position(viewed from above) on the disc. As this blade starts to move
around the disc, due to the forward motion of the aircraft, airspeed
starts to increase. This causes more lift so the blade starts to
rise.
As the blade passes the 3 o'clock postion (US type helos), the
airspeed is at the maximum, induced flow is increased(due to blade
rising), angle of attack is at the minimum, and rate of flap up is at
maximum.
As the blade moves towards the 12 o'clock, airspeed is starting to
decrease, along with rate of flapping up, but the blade is still
rising.
At 12 o'clock, the airspeed is normal and the blade is at it's
highest point.
As the blade moves further, the airspeed is now starting to decrease
leading to less lift, so the blade starts to flap down.
At the 9 o'clock, airspeed is at a minimum, rate of flap down is at
maximum, induced flow is decreases and angle of attack is at maximum.
As the blade moves back towards the 6 o'clock, the airspeed is
starting to increase, rate of flapdown is decreasing.
Back at the 6 o'clock, the airspeed is normal and the blade is at
it's lowest point.
During translation from the hover, the above produces flapback and is
countered by moving the cyclic forward.
Inflow Roll.
This time, we start off in the 3 o'clock position. As the blade
moves forward, induced flow is decreasing, so angle of attack is
increasing, so the blade flaps up.
At the 12 o'clock, induced flow is at a minimum, angle of attack is
maximum, as is the rate at which the blade is flapping up.
As the blade moves towards the 9 o'clock, induced flow is starting to
increase, so angle of attack is starting to decrease, but the blade
is still flapping up.
At the 9 o'clock, induced flow is still increasing, angle of attack
is decreasing, the blade is at it's highest point.
As the blade starts to move towards the 6 o'clock, as the induced
flow is still increasing and angle of attack decreasing, the blade
starts to descend.
At the 6 o'clock, induced flow is at a maximum, angle of attack is
at a minimum, and the rate of flap down is at a maximum.
As it starts to move towards the 3 o'clock, induced flow starts to
decrease, angle of attack starts to increase but the blade is still
flapping down.
As the blade gets back to the 3 o'clock, it is now at it's lowest
point.
Inflow roll wants to roll the aircraft to the right, opposed by left
cyclic. As this is being combined with flapback, theaircraft wants
to pitch up and right.
In answer to the question, as increase in lift gives an increase in
drag, all three answers could be right.
Phew, now I need a rest!
and Inflow Roll.
Flapping to Equality.
As the aircraft moves forward, start at a blade tip in the 6 o'clock
position(viewed from above) on the disc. As this blade starts to move
around the disc, due to the forward motion of the aircraft, airspeed
starts to increase. This causes more lift so the blade starts to
rise.
As the blade passes the 3 o'clock postion (US type helos), the
airspeed is at the maximum, induced flow is increased(due to blade
rising), angle of attack is at the minimum, and rate of flap up is at
maximum.
As the blade moves towards the 12 o'clock, airspeed is starting to
decrease, along with rate of flapping up, but the blade is still
rising.
At 12 o'clock, the airspeed is normal and the blade is at it's
highest point.
As the blade moves further, the airspeed is now starting to decrease
leading to less lift, so the blade starts to flap down.
At the 9 o'clock, airspeed is at a minimum, rate of flap down is at
maximum, induced flow is decreases and angle of attack is at maximum.
As the blade moves back towards the 6 o'clock, the airspeed is
starting to increase, rate of flapdown is decreasing.
Back at the 6 o'clock, the airspeed is normal and the blade is at
it's lowest point.
During translation from the hover, the above produces flapback and is
countered by moving the cyclic forward.
Inflow Roll.
This time, we start off in the 3 o'clock position. As the blade
moves forward, induced flow is decreasing, so angle of attack is
increasing, so the blade flaps up.
At the 12 o'clock, induced flow is at a minimum, angle of attack is
maximum, as is the rate at which the blade is flapping up.
As the blade moves towards the 9 o'clock, induced flow is starting to
increase, so angle of attack is starting to decrease, but the blade
is still flapping up.
At the 9 o'clock, induced flow is still increasing, angle of attack
is decreasing, the blade is at it's highest point.
As the blade starts to move towards the 6 o'clock, as the induced
flow is still increasing and angle of attack decreasing, the blade
starts to descend.
At the 6 o'clock, induced flow is at a maximum, angle of attack is
at a minimum, and the rate of flap down is at a maximum.
As it starts to move towards the 3 o'clock, induced flow starts to
decrease, angle of attack starts to increase but the blade is still
flapping down.
As the blade gets back to the 3 o'clock, it is now at it's lowest
point.
Inflow roll wants to roll the aircraft to the right, opposed by left
cyclic. As this is being combined with flapback, theaircraft wants
to pitch up and right.
In answer to the question, as increase in lift gives an increase in
drag, all three answers could be right.
Phew, now I need a rest!
Thread Starter
Joined: Feb 2001
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From: UK
Cyclical change of pitch gives the advancing blade a lower angle and less lift and the retreating blade a higher angle and thus more lift. This is accomplished via the mechanics of the pitch change system. The overall effect here is for gyroscopic precession to put the advancing blade at its lowest point at 180 deg rotor azimuth.
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From: AB, Canada
The advancing blade has less pitch but is moving down, increasing angle of attack, and it's seeing a higher airspeed. Flapping to equality is when the flapping (moving down) is sufficient to balance lift on either side. Of course, the opposite happens to the retreating blade.
I can't see any conflict here.
I can't see any conflict here.
Last edited by heedm; 23rd November 2003 at 03:01.
Avoid imitations
Joined: Nov 2000
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From: Wandering the FIR and cyberspace often at highly unsociable times
A isn't true because of flapping to equality.
B would occur (flapback) if there was no pilot or automatic trim input but not where the aircraft is in controlled and steady flight.
I think answer C is most correct
B would occur (flapback) if there was no pilot or automatic trim input but not where the aircraft is in controlled and steady flight.
I think answer C is most correct
Joined: Jun 2001
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From: Australia
golden rivet
an increase in air speed, makes more lift and drag and would make the blade fly to a higher point, if the pilot did nothing. but that would change the disc attitude (flapback) and the helicopter would slow down. the pilot must hold the disc level (slightly forward) with cyclic, to fly strait and level, so we cant let this happen. all these things that are mentioned earlier are compensated for, by the pilot. in forward flight there is less pitch on the advancing blade to stop it increasing in lift with the extra air speed , and more pitch on the retreating blade to produce the same lift at the reduce air speed (on that side, in forward flight).
advancing blade - faster air flow, less pitch, less induced drag but higher parasitic drag.
retreating blade - slower airflow, more pitch, more induced drag but less parasitic drag. same lift! theorfore same total drag (if lift is uneven the disk tilts and you start to fly that way)
i dont think its any of the answers but questions like that dont usualy consider the pilots reactions so i would have locked in (D) all of the obove!
advancing blade - faster air flow, less pitch, less induced drag but higher parasitic drag.
retreating blade - slower airflow, more pitch, more induced drag but less parasitic drag. same lift! theorfore same total drag (if lift is uneven the disk tilts and you start to fly that way)
i dont think its any of the answers but questions like that dont usualy consider the pilots reactions so i would have locked in (D) all of the obove!
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From: AZ
Vorticey- "advancing blade - faster air flow, less pitch, less induced drag but higher parasitic drag.
retreating blade - slower airflow, more pitch, more induced drag but less parasitic drag. same lift! theorfore same total drag (if lift is uneven the disk tilts and you start to fly that way)"
Blades do not have parasitic drag since they are lift producing bodies and parasitic drag deals with non lifting producing bodies. Perhaps you meant profile but then I believe that the decrease in AOA and the increase of velocity would be close to equaling out...dont know the exact numbers.
retreating blade - slower airflow, more pitch, more induced drag but less parasitic drag. same lift! theorfore same total drag (if lift is uneven the disk tilts and you start to fly that way)"
Blades do not have parasitic drag since they are lift producing bodies and parasitic drag deals with non lifting producing bodies. Perhaps you meant profile but then I believe that the decrease in AOA and the increase of velocity would be close to equaling out...dont know the exact numbers.
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From: truely asia
Ideally an aerofoil section should not have the parasitic drag but in reality it will surely have some parasitic drag.
Parasitic drag is the sum of profile drag and interfernce drag.
Profile drag is sum of Skin friction and pressure drag
1. Usually the aerofoil is designed to give the greatest decrease in Skin friction drag at low lift coefficients or in other words at high speeds
2. Pressure drag arises due to overall presure didtriburion of an aircraft, it the diffrence between forces caused by high pressure on the forward portion and low pressure on the aftportion of the aircraft
3. interference drag is caused by regions of turbulence at junctions
All the three factor above may be small for heli blades, will they be increasing more than reduction in induced drag ?
Hard to answer that i suppose.
Parasitic drag is the sum of profile drag and interfernce drag.
Profile drag is sum of Skin friction and pressure drag
1. Usually the aerofoil is designed to give the greatest decrease in Skin friction drag at low lift coefficients or in other words at high speeds
2. Pressure drag arises due to overall presure didtriburion of an aircraft, it the diffrence between forces caused by high pressure on the forward portion and low pressure on the aftportion of the aircraft
3. interference drag is caused by regions of turbulence at junctions
All the three factor above may be small for heli blades, will they be increasing more than reduction in induced drag ?
Hard to answer that i suppose.
Joined: Apr 2003
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From: USA
The problem is the question is badly flawed. In trimmed level flight, the advancing blade is doing none of those things, because the cyclic controls have angled the swashplate (because there is a pilot somewhere around) to make the blade produce roughly the same forces (lift) and moments (control) as it advances or retreats.
In an uncontrolled helicopter rotor, the advancing blade gets increased speed (the sum of forward spoeed and rotational speed) and it has an increase in lift and drag. This makes it rise, and peak at the full forward position. The handy-dandy pilot forsees this because he pushes the cyclic forward as the nose starts up (the nose up is a product of the blade rising and the disk tilting backward).
In Canada, there is no pilot, and the blade flaps uncontrolled, causing the helo to crash, of course.
The correct answer is (z) Why ask a government flight examiner to write a test on aerodynamics when we have Dave Jackson?
In an uncontrolled helicopter rotor, the advancing blade gets increased speed (the sum of forward spoeed and rotational speed) and it has an increase in lift and drag. This makes it rise, and peak at the full forward position. The handy-dandy pilot forsees this because he pushes the cyclic forward as the nose starts up (the nose up is a product of the blade rising and the disk tilting backward).
In Canada, there is no pilot, and the blade flaps uncontrolled, causing the helo to crash, of course.
The correct answer is (z) Why ask a government flight examiner to write a test on aerodynamics when we have Dave Jackson?
Joined: Oct 2002
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From: US...for now.
Nick, you put it so simply and elegantly.
I'm always amused when pilots talk about how a blade flaps as it goes round and round the mast. I suppose that in a rotor system without cyclic control, the advancing blade would surely flap upward.
But in a real helicopter, the advancing blade will not flap up- otherwise how could forward flight be maintained? Obviously, the flapping that the books talk about *must* be in equilibrium. That is, the advancing blade "flaps" up exactly the same amount as the retreating blade "flaps" down: none. Why? Primarily because the pilot is pushing forward on the stick, making the pitch of the advancing blade lower and the retreating blade higher.
Thanks to Nick for putting it in a way that makes it so clear.
I'm always amused when pilots talk about how a blade flaps as it goes round and round the mast. I suppose that in a rotor system without cyclic control, the advancing blade would surely flap upward.
But in a real helicopter, the advancing blade will not flap up- otherwise how could forward flight be maintained? Obviously, the flapping that the books talk about *must* be in equilibrium. That is, the advancing blade "flaps" up exactly the same amount as the retreating blade "flaps" down: none. Why? Primarily because the pilot is pushing forward on the stick, making the pitch of the advancing blade lower and the retreating blade higher.
Thanks to Nick for putting it in a way that makes it so clear.
Joined: Apr 2003
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From: Vancouver, BC, Canada
Hi Nick
Talking of aerodynamics, how about a discussion on Sikorsky's 'Reverse Velocity Rotor Concept'?
` . ` . ` . ` . ` .
You might wish to assess the craft's ability to transition from forward flight to autorotation before saying yes.
Talking of aerodynamics, how about a discussion on Sikorsky's 'Reverse Velocity Rotor Concept'?
` . ` . ` . ` . ` .
You might wish to assess the craft's ability to transition from forward flight to autorotation before saying yes.
Joined: Apr 2003
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From: USA
I posted a web page with the film of the rotor blade in high speed flight.
http://www.s-92heliport.com/rotor.htm
http://www.s-92heliport.com/rotor.htm
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From: Australia
nick
is this old footage nick. is the blade made of wood or fiberglass? or is this footage of a helicopter blade going supersonic? is there resent footage like this? surely the pilot could see this happening by looking at a portion of the disc (it wouldnt look flat) please expain!
Joined: Apr 2003
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From: USA
vorticey,
That blade has an aluminum spar, and is shown in high speed level flight. I doubt that the track looked too bad, as the movement at the tip would not be apparent to the crew. Each blade would pass through about the same space, so the tip path would look reasonably sharp, it just would not be flat, but we usually don't measure that do we?
This is my guess, perhaps some ppruner could translate the German narration, which might shed some light (although it might have been made by someone who was misinformed, ofcourse).
I know of no more recent such footage, although the weight and size of tv cameras should make this kind of video very easy to get. Certainly the size of the deflections we must induce in our fatigue tests of blades make me think these motions are fairly normal, even on today's blades.
That blade has an aluminum spar, and is shown in high speed level flight. I doubt that the track looked too bad, as the movement at the tip would not be apparent to the crew. Each blade would pass through about the same space, so the tip path would look reasonably sharp, it just would not be flat, but we usually don't measure that do we?
This is my guess, perhaps some ppruner could translate the German narration, which might shed some light (although it might have been made by someone who was misinformed, ofcourse).
I know of no more recent such footage, although the weight and size of tv cameras should make this kind of video very easy to get. Certainly the size of the deflections we must induce in our fatigue tests of blades make me think these motions are fairly normal, even on today's blades.
Joined: Aug 2000
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From: UK
Same footage, dubbed in English was shown on an HORIZON programme in 1989!! I still have the footage.
Can't recall anything more recent.
NL: Correct me if wrong, Vorticey: rotor blades cannot and nust not go supersonic, because of the stresses involved. hence one of the reasons why we are limited in fwd speed.
Can't recall anything more recent.
NL: Correct me if wrong, Vorticey: rotor blades cannot and nust not go supersonic, because of the stresses involved. hence one of the reasons why we are limited in fwd speed.