AFI Revision (Flapback)
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AFI Revision (Flapback)
Hi all,
I am new here but have been lurking for a while.
I passed my AFI(H) checkride in June of 2000, one of the last under the old CAA rules, so I have grandfather rights in not having to do CPL(H).
I did my AFI under Mike Green, at the time, at Fast Heli's in Thruxton.
However, after I did my AFI, things didn't go my way and things transpired to stop me flying. However, I am trying to get back into it again.
I have not flown (up until recently) for 4.5 years, so have to revalidate or else I lose my rights.
Anyhow, I am trying to get going again. I already have a local school interested in me if I can get my rating back, so I am going for it.
I have done some more flying down at Helicopter Services at High Wycombe, along with some more revision ready for my test.
My flying is good, however, my delivery of information at the moment is not. I am forever missing things. In order to try and bring me back up to speed, I have re-read all my PPL books and re-written my instructor training notes. Still, this wasn't good enough.
Anyhow, I hope you don't mind, but I am trying to revise, so I will be looking for help. I know there is an instructors forum, but that looks like it is mostly plank flyers. I will start a new thread for each distinct question.
Anyway, my first question...
At the advice of the CFI at the local school, I bought a book from R & K. (Some practice lessons are on their website, in fact the one I am asking about is... http://www.rnk.co.uk/himan/ManViewer.htm view lesson 5.
About half way down is some pictures of airfoil and vectors to help explain flapback.
At this point, I looked in the Helicopter Pilots Manual by Norman Bailey. Page 39 has a picture.
At the top, it says Blade High.
At the bottom, it says Blade Low.
On the left, it says...
Min. Velocity
Max. Rate
Flapping Down
Max. Angle of Attack
On the right it says...
Min. Velocity
Max. Rate
Flapping Up
Max. Angle of Attack
So, both left and right have Max Angle of Attack???
Now, look at the ex 5 on the website link above.
The first blade has...
Flight path & relative wind are parallel over the nose and tail.
Second blade...
Advancing blade flaps up causing change in relative air flow and decreasing angle of attack.
Third blade...
Retreating blade flaps down causing change in relative airflow and increased angle of attack.
Now, here is my question. As there is no consistency between the two books, plus the R & K book is opposite to what I thought.
I would have thought that as the advancing blade flaps up, the relative wind would now be lower (greater AoA) than the retreating blade flapping down.
Which book is right? If they are both correct, then what appears to cause the difference?
Thanks for looking.
FP.
I am new here but have been lurking for a while.
I passed my AFI(H) checkride in June of 2000, one of the last under the old CAA rules, so I have grandfather rights in not having to do CPL(H).
I did my AFI under Mike Green, at the time, at Fast Heli's in Thruxton.
However, after I did my AFI, things didn't go my way and things transpired to stop me flying. However, I am trying to get back into it again.
I have not flown (up until recently) for 4.5 years, so have to revalidate or else I lose my rights.
Anyhow, I am trying to get going again. I already have a local school interested in me if I can get my rating back, so I am going for it.
I have done some more flying down at Helicopter Services at High Wycombe, along with some more revision ready for my test.
My flying is good, however, my delivery of information at the moment is not. I am forever missing things. In order to try and bring me back up to speed, I have re-read all my PPL books and re-written my instructor training notes. Still, this wasn't good enough.
Anyhow, I hope you don't mind, but I am trying to revise, so I will be looking for help. I know there is an instructors forum, but that looks like it is mostly plank flyers. I will start a new thread for each distinct question.
Anyway, my first question...
At the advice of the CFI at the local school, I bought a book from R & K. (Some practice lessons are on their website, in fact the one I am asking about is... http://www.rnk.co.uk/himan/ManViewer.htm view lesson 5.
About half way down is some pictures of airfoil and vectors to help explain flapback.
At this point, I looked in the Helicopter Pilots Manual by Norman Bailey. Page 39 has a picture.
At the top, it says Blade High.
At the bottom, it says Blade Low.
On the left, it says...
Min. Velocity
Max. Rate
Flapping Down
Max. Angle of Attack
On the right it says...
Min. Velocity
Max. Rate
Flapping Up
Max. Angle of Attack
So, both left and right have Max Angle of Attack???
Now, look at the ex 5 on the website link above.
The first blade has...
Flight path & relative wind are parallel over the nose and tail.
Second blade...
Advancing blade flaps up causing change in relative air flow and decreasing angle of attack.
Third blade...
Retreating blade flaps down causing change in relative airflow and increased angle of attack.
Now, here is my question. As there is no consistency between the two books, plus the R & K book is opposite to what I thought.
I would have thought that as the advancing blade flaps up, the relative wind would now be lower (greater AoA) than the retreating blade flapping down.
Which book is right? If they are both correct, then what appears to cause the difference?
Thanks for looking.
FP.
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I hope this is right because I have my FI test next week.
Advancing blade has maximum rate of flap up on the right, but due to the inertia of the blade it doesn't reach it's highest point until above the nose. As this blade flaps up you get some additional induced flow from the upward movement of the blade.
More induced flow=minimum angle of attack on advancing blade.
Retreating blade flaps down and you get some rate of descent (of the blade) flow, opposing the induced flow which increases angle of attack to max on retreating side.
Take this further on the retreating side and at very high speed the rate of flap down is so high that you get a large rate of descent (of the blade) flow and the angle of attack increases and starts to stall, hence retreating blade stall.
Can't answer the parallel thing.
Advancing blade has maximum rate of flap up on the right, but due to the inertia of the blade it doesn't reach it's highest point until above the nose. As this blade flaps up you get some additional induced flow from the upward movement of the blade.
More induced flow=minimum angle of attack on advancing blade.
Retreating blade flaps down and you get some rate of descent (of the blade) flow, opposing the induced flow which increases angle of attack to max on retreating side.
Take this further on the retreating side and at very high speed the rate of flap down is so high that you get a large rate of descent (of the blade) flow and the angle of attack increases and starts to stall, hence retreating blade stall.
Can't answer the parallel thing.
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hey mr Fanpilot,
assuming forward airspeed is present and a forward cyclic input is applied with the intention of increasing speed the following will occur
advancing blade experiences greater wind velocity therefore generates more lift and flaps up.
this flapping up decreases angle of attack and starts to reduce lift which modulates the flapping up, blade is flapping up at greatest rate at 3 o'clock position and reaches its highest point at 12 o'clock position.
retreating blade experiences reduced wind velocity therefore generates less lift and flaps down.
this flapping down increases angle of attack and starts to increase lift which modulates the flapping down, blade is flapping down at greatest rate at 9 o'clock position and reaches its lowest point at 6 o'clock position.
therefore the net effect is that forward cyclic makes the rotor disc flap up at 12 o'clock position and down at 6 o'clock position which left uncorrected will cause the airspeed to reduce therefore it is neccesary to "push through" the flapback to prevent this occuring.
finally its twin brother "flapforward" occurs when you apply aft cyclic to attempt to slow down and everything happens in reverse.
therefore in my opinion R & K are correct
regards
CF
assuming forward airspeed is present and a forward cyclic input is applied with the intention of increasing speed the following will occur
advancing blade experiences greater wind velocity therefore generates more lift and flaps up.
this flapping up decreases angle of attack and starts to reduce lift which modulates the flapping up, blade is flapping up at greatest rate at 3 o'clock position and reaches its highest point at 12 o'clock position.
retreating blade experiences reduced wind velocity therefore generates less lift and flaps down.
this flapping down increases angle of attack and starts to increase lift which modulates the flapping down, blade is flapping down at greatest rate at 9 o'clock position and reaches its lowest point at 6 o'clock position.
therefore the net effect is that forward cyclic makes the rotor disc flap up at 12 o'clock position and down at 6 o'clock position which left uncorrected will cause the airspeed to reduce therefore it is neccesary to "push through" the flapback to prevent this occuring.
finally its twin brother "flapforward" occurs when you apply aft cyclic to attempt to slow down and everything happens in reverse.
therefore in my opinion R & K are correct
regards
CF
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Jemax - I agree with your explanation, but I think that the examiners would be looking for 'gyroscopic precession' rather than 'inertia of the blade'.
Good luck with your exams,
LP
Good luck with your exams,
LP
Hi FanPilot
Try this explanation; it works in my little mind!
First clear your mind of any terms such as 'gyroscopic precession', ‘Inertia’ etc. they will just cloud the issue at this stage!
Consider a single blade from a helicopter that rotates anti-clockwise when viewed from above. The helicopter is in a still air hover; straight ahead is the 12 O’clock position.
At present, the airflow over the blade is the same, whether the blade is at 12, 9, 5 or 6 O’clock. The blade follows a ‘flat path’.
Now introduce a gentle breeze blowing from the 12 O’clock position and consider the airflow over the blade at various positions.
With the blade at 6 O’clock, the breeze is blowing along the span of the blade from root to tip. It will have no appreciable affect on the lift produced by the blade so the blade will neither flap up nor down at this position.
At 5 O’clock, a small proportion (about half) of the breeze is effectively blowing across the chord of the blade, adding to the airflow created by the rotation of the rotor system. This ‘small’ increase in the airflow will create a ‘small’ increase in lift so the blade will flap up at a ‘small’ rate.
At 4 O’clock, over three quarters of the breeze is blowing across the chord. The lift is increased more, so the rate of flapping up is increased.
At 3 O’clock, the breeze is blowing across the chord, at this point lift is at a maximum; it is flapping up at its maximum rate.
At 2 O’clock, we are in more or less the same situation as at 4 O’clock, the lift has reduced from its maximum. It is still flapping up, but not as fast as it was at 3 O’clock.
At 1 O’clock, it’s back to just half the breeze across the chord, but that is still an increase over the no breeze situation, so the blade is still flapping up!
Finally at 12 O’clock, the breeze is blowing along the span of the blade from tip to root. No increase in lift, so no flapping! The blade is at its highest point now and it is down hill from here to 6 O’clock
Explain how this ties in with an increase/decrease in airspeed and I think that is all you need to discuss about Flap Back.
Cover Inflow Roll next (when you think the student can absorb it) but leave ‘Flapping to Equality’ and ‘Dissymmetry of Lift’ etc. to a later date.
It’s over 15 years since I did any instruction so I am quite happy to be shot down in flames if this does not tie in with normal teaching practice!
Cheers and Good Luck
TeeS
Try this explanation; it works in my little mind!
First clear your mind of any terms such as 'gyroscopic precession', ‘Inertia’ etc. they will just cloud the issue at this stage!
Consider a single blade from a helicopter that rotates anti-clockwise when viewed from above. The helicopter is in a still air hover; straight ahead is the 12 O’clock position.
At present, the airflow over the blade is the same, whether the blade is at 12, 9, 5 or 6 O’clock. The blade follows a ‘flat path’.
Now introduce a gentle breeze blowing from the 12 O’clock position and consider the airflow over the blade at various positions.
With the blade at 6 O’clock, the breeze is blowing along the span of the blade from root to tip. It will have no appreciable affect on the lift produced by the blade so the blade will neither flap up nor down at this position.
At 5 O’clock, a small proportion (about half) of the breeze is effectively blowing across the chord of the blade, adding to the airflow created by the rotation of the rotor system. This ‘small’ increase in the airflow will create a ‘small’ increase in lift so the blade will flap up at a ‘small’ rate.
At 4 O’clock, over three quarters of the breeze is blowing across the chord. The lift is increased more, so the rate of flapping up is increased.
At 3 O’clock, the breeze is blowing across the chord, at this point lift is at a maximum; it is flapping up at its maximum rate.
At 2 O’clock, we are in more or less the same situation as at 4 O’clock, the lift has reduced from its maximum. It is still flapping up, but not as fast as it was at 3 O’clock.
At 1 O’clock, it’s back to just half the breeze across the chord, but that is still an increase over the no breeze situation, so the blade is still flapping up!
Finally at 12 O’clock, the breeze is blowing along the span of the blade from tip to root. No increase in lift, so no flapping! The blade is at its highest point now and it is down hill from here to 6 O’clock
Explain how this ties in with an increase/decrease in airspeed and I think that is all you need to discuss about Flap Back.
Cover Inflow Roll next (when you think the student can absorb it) but leave ‘Flapping to Equality’ and ‘Dissymmetry of Lift’ etc. to a later date.
It’s over 15 years since I did any instruction so I am quite happy to be shot down in flames if this does not tie in with normal teaching practice!
Cheers and Good Luck
TeeS
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And just in case your head really needs messing with, have a look at the last time precession was discussed !
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Some interesting replies, MightyGem is correct, however an easy way to explain it is that flapback is the resultant of the blades flapping to equality. Advancing and retreating blades must create the same amount of lift or the a/c will roll. to do this the blades are hinged to allow flapping this changes the AoA due to the induced flow change, this balances out the velocity difference created by the blades advancing and retreating.
Lift=Velocity and AoA (basicaly)
If velocity goes up then AoA must come down to keep lift as a constant.
I thank you
Lift=Velocity and AoA (basicaly)
If velocity goes up then AoA must come down to keep lift as a constant.
I thank you
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Thanks all for your help.
I understand why flapback happens, the problem I initially had understanding was why the advancing blade that was flapping up had small angle of attack and the retreating one had a large angle of attack.
The way I was visualising it was that as the blade is rising, it was riding on a higher angle of attack. When an aircraft is climbing, the angle of attack is what helps to give it the lift. This is what I trying to visualise when I can see that the blade is lifting.
FP
I understand why flapback happens, the problem I initially had understanding was why the advancing blade that was flapping up had small angle of attack and the retreating one had a large angle of attack.
The way I was visualising it was that as the blade is rising, it was riding on a higher angle of attack. When an aircraft is climbing, the angle of attack is what helps to give it the lift. This is what I trying to visualise when I can see that the blade is lifting.
FP
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Any "automatic" change in angle of attack is normally due to variations in induced flow - as the advancing blade flaps up, its induced flow is added to, which increases the induced angle, leaving less room for the angle of attack. And vice versa.
Phil
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I am always amazed at the use of angles to explain rotor dynamics. I find it confusing, and non-intuitive, frankly. Also, a quick flick of the wrist makes the cyclic change that angle vastly, and confuses the whole picture. Seldom is a flapping/lagging blade behaving like the diagrams, when the pilot is in the loop making control inputs. Also, the pristine logic of a teetering hinge head is a special case, any real rotor head has no ability to flap without making a roll or pitch moment input to the helo. For example, when we think that flapping relieves the differential lift, we are dead wrong, because that flapping also creates a large roll moment that the pilot must cancel with cyclic.
It is much easier to think of the delta velocity as the cause of a back-flap, (which is also the prime speed stability term for the rotor.) Increased speed makes the upwind blade a bit faster and the downwind blade a bit slower. This makes the rotor flap up due to phase angle, and usually roll slightly left. The extra pitch up requires the pilot to push the stick forward to put the nose back down, and this is perceived as a forward stick trim for increased speed. We call that Longitudinal Static Stability.
Backflap is the reason why you have a horizontal tail on all helos. If you didn't have one, there would be no way for the nose to stay down when a slight upgust hits the aircraft. The horizontal tail sees the nose up as an increase in its angle of attack, so it increases its lift, and presses the nose back down to help the pilot keep the aircraft from flipping. Without a horizontal tail (or with one that is too small) the backflap can make the helo very unstable in pitch, especially at high speed. In some helos, a big nose up can lead to loss of control, because the backflap is more powerful than full forward stick. This is usually a sign that the horizontal tail is too small, or the CG is too far aft (because the nose up is a combination of backflap and the net increase in lift applied forward of the CG, which also makes the nose kick up.)
It is much easier to think of the delta velocity as the cause of a back-flap, (which is also the prime speed stability term for the rotor.) Increased speed makes the upwind blade a bit faster and the downwind blade a bit slower. This makes the rotor flap up due to phase angle, and usually roll slightly left. The extra pitch up requires the pilot to push the stick forward to put the nose back down, and this is perceived as a forward stick trim for increased speed. We call that Longitudinal Static Stability.
Backflap is the reason why you have a horizontal tail on all helos. If you didn't have one, there would be no way for the nose to stay down when a slight upgust hits the aircraft. The horizontal tail sees the nose up as an increase in its angle of attack, so it increases its lift, and presses the nose back down to help the pilot keep the aircraft from flipping. Without a horizontal tail (or with one that is too small) the backflap can make the helo very unstable in pitch, especially at high speed. In some helos, a big nose up can lead to loss of control, because the backflap is more powerful than full forward stick. This is usually a sign that the horizontal tail is too small, or the CG is too far aft (because the nose up is a combination of backflap and the net increase in lift applied forward of the CG, which also makes the nose kick up.)
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I'm looking forward to that book of yours Nick, but you're going to have to be gentler on your audience than that !
We Wagtendonk grads are all sitting happily on the assumption that flapping relieves dissymmetry of lift. Throw us a bone if you're going to knock that one down.
I'm trying to read between the lines of your second para. By delta velocity do you mean the increasing differential air speed seen by the blades caused by the increasing forward speed of the helicopter ? If so aren't you just restating that higher advancing airspeed causes the blade to flap up, etc etc. per the other posts on this thread ?
Happy to believe there are more secondary issues on a fully articulated rotor than are explained by our basic model, but not sufficient surely to chuck it out the window ?
We Wagtendonk grads are all sitting happily on the assumption that flapping relieves dissymmetry of lift. Throw us a bone if you're going to knock that one down.
I'm trying to read between the lines of your second para. By delta velocity do you mean the increasing differential air speed seen by the blades caused by the increasing forward speed of the helicopter ? If so aren't you just restating that higher advancing airspeed causes the blade to flap up, etc etc. per the other posts on this thread ?
Happy to believe there are more secondary issues on a fully articulated rotor than are explained by our basic model, but not sufficient surely to chuck it out the window ?
Last edited by puntosaurus; 30th Nov 2005 at 07:46.
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The problem with Principles of flight is that there are so many of them and they all in reality blend into each other. Each topic, Flap-back, inflow roll is just a snapshot of what the disc is doing at that moment in time not what the pilot does to counter it or any other effect. It obviously helps to know what causes the effect eg inflow roll following on from translational lift.
Tees i did explain to somebody once that there was a one bladed helicopter and he wouldn't believe me
Tees i did explain to somebody once that there was a one bladed helicopter and he wouldn't believe me
