What causes "blowback/flapback" when accelerating through transverse flow effect?
Avoid imitations
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Despite extended opposition here (and in my case sometimes via private messages) Lou Zuckerman probably went to his grave convinced that the rotor system was a gyroscope and control inputs were causing precession of said gyroscope, rather than blade pitch control inputs causing blades to fly to a new position.
When I asked him to explain how and why he thought aircraft like those made by Kaman could be controlled by “servo tabs” mounted on the main rotor blades, he never answered. Same with an explanation of why a helicopter was dynamically unstable, rather than possessing the very stable characteristics of a gyroscope.
Phase lag occurs because a blade cannot instantly attain a new position, it has to be made to fly there under aerodynamic forces. It’s similar to gyroscopic precession, but definitely not the same phenomenon.
When I asked him to explain how and why he thought aircraft like those made by Kaman could be controlled by “servo tabs” mounted on the main rotor blades, he never answered. Same with an explanation of why a helicopter was dynamically unstable, rather than possessing the very stable characteristics of a gyroscope.
Phase lag occurs because a blade cannot instantly attain a new position, it has to be made to fly there under aerodynamic forces. It’s similar to gyroscopic precession, but definitely not the same phenomenon.
When we compare rotor system designs....particularly the BO-105/117 series with the rigid rotor system which has been quite successful, against say a fully articulated rotor system.....would the rigid rotor come closer to reacting like a gyroscope than the articulated head?
Is the Rigid Rotor more stable aerodynamically?
My experience in the "Vomit Comet" (un-sas'ed BK) I have to wonder.
Is the Rigid Rotor more stable aerodynamically?
My experience in the "Vomit Comet" (un-sas'ed BK) I have to wonder.
Despite extended opposition here (and in my case sometimes via private messages) Lou Zuckerman probably went to his grave convinced that the rotor system was a gyroscope and control inputs were causing precession of said gyroscope, rather than blade pitch control inputs causing blades to fly to a new position
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They are not cheap as they are scarce.
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Very interesting discussion, and I appreciate everyone's input (and patience!) as I attempt to make sense of all of this. It's surprising how little of this information is in the current helicopter training manuals.
From this discussion, I would expect that the effects of inflow roll would be much more pronounced on takeoff versus flapback, but I've found the opposite to be true in flying and instructing. In theory, while in a no wind hover, we are experiencing no flapback or inflow roll. We move the cyclic forward to tilt the rotor disc and start our transition to forward flight. We need a little left cyclic pressure and some footwork to keep the nose straight, but somewhere around 20-ish knots, we need a distinct forward push on the cyclic to keep everything where it was going just a moment before. What is it that makes this one moment seem so different than any other phase of flight? Or is it just in my head?
From this discussion, I would expect that the effects of inflow roll would be much more pronounced on takeoff versus flapback, but I've found the opposite to be true in flying and instructing. In theory, while in a no wind hover, we are experiencing no flapback or inflow roll. We move the cyclic forward to tilt the rotor disc and start our transition to forward flight. We need a little left cyclic pressure and some footwork to keep the nose straight, but somewhere around 20-ish knots, we need a distinct forward push on the cyclic to keep everything where it was going just a moment before. What is it that makes this one moment seem so different than any other phase of flight? Or is it just in my head?
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Sikorsky had a really good book it published on helicopter aerodynamics.....which I knew as the "Sikorsky Blue Book"....which some evil rascal thought more of than I did and stole it.
The Title is "Sikorsky Helicopter Flight Theory For Pilots And Mechanics". I highly recommend it for use by those wishing to learn more about the subject as it is written in a manner that even Pilots can understand and has drawings for the CFS CFI's to color with wax crayons.
In my quest to find one....I discovered there are three printings of that fine tome on helicopter aerodynamics.....1953, 1964 (the version I carried for years in my helmet bag) and 1994 (which according to John Dixson has a gray cover).
I have found a few at some used book sites.
They are not cheap as they are scarce.
The Title is "Sikorsky Helicopter Flight Theory For Pilots And Mechanics". I highly recommend it for use by those wishing to learn more about the subject as it is written in a manner that even Pilots can understand and has drawings for the CFS CFI's to color with wax crayons.
In my quest to find one....I discovered there are three printings of that fine tome on helicopter aerodynamics.....1953, 1964 (the version I carried for years in my helmet bag) and 1994 (which according to John Dixson has a gray cover).
I have found a few at some used book sites.
They are not cheap as they are scarce.
I don't know who this Lou guy is, but are you sure he truly believed that a spinning rotor "was" a gyroscope, or that it just acted "like" a gyroscope?
He was a very clever guy but sometimes simple things escaped him - I had a long argument about pitch change rods with him - he couldn't grasp that, much like a piston in an engine, they didn't have constant rate of vertical movement as they followed the swashplate and actually stopped moving vertically at the top and bottom of each stroke before they started moving again in the opposite direction.
When we compare rotor system designs....particularly the BO-105/117 series with the rigid rotor system which has been quite successful, against say a fully articulated rotor system.....would the rigid rotor come closer to reacting like a gyroscope than the articulated head?
Another factor is the relationship of blade inertia to aerodynamic damping (Locke Number if memory serves) where a heavier blade will flap further than a light one given the same aero forces or the same blade will flap more when aero forces are reduced (high DA for example).
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When we compare rotor system designs....particularly the BO-105/117 series with the rigid rotor system which has been quite successful, against say a fully articulated rotor system.....would the rigid rotor come closer to reacting like a gyroscope than the articulated head?
Is the Rigid Rotor more stable aerodynamically?
My experience in the "Vomit Comet" (un-sas'ed BK) I have to wonder.
Is the Rigid Rotor more stable aerodynamically?
My experience in the "Vomit Comet" (un-sas'ed BK) I have to wonder.
The “rigid” description is in that there are no specific hinges on the head, ie there are no leading/lagging, flapping or pitch change hinges. The blade support system is all one piece, which can twist/bend to allow relative blade movement and pitch changes. The Lynx type of head was/is semi rigid in that it does have conventional pitch change hinges.
Both the BO 105 and the Lynx exhibit some unusual pitch/roll coupling as a result of the phase lag not being 90 degrees.
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So called “rigid” rotors still work by flying the rotor blades to their required positions, rather than by gyroscopic precession.
The “rigid” description is in that there are no specific hinges on the head, ie there are no leading/lagging, flapping or pitch change hinges. The blade support system is all one piece, which can twist/bend to allow relative blade movement and pitch changes. The Lynx type of head was/is semi rigid in that it does have conventional pitch change hinges.
The “rigid” description is in that there are no specific hinges on the head, ie there are no leading/lagging, flapping or pitch change hinges. The blade support system is all one piece, which can twist/bend to allow relative blade movement and pitch changes. The Lynx type of head was/is semi rigid in that it does have conventional pitch change hinges.
[1] For those interested, this is discussed on p13-14 of that Sikorski manual.
Lu was always asking the question "Where are the missing 18 degrees?" for the R-22, which had a phase lag of 72 degrees.
The BK had feathering hinges but no flap or drag hinges. Without SAS, it could be uncomfortable to fly, with the slightest puff of wind or teensy cyclic movement passed straight into the cabin. Our Chief Pilot was known as Chuck, because most of his crewmen did.
The BK had feathering hinges but no flap or drag hinges. Without SAS, it could be uncomfortable to fly, with the slightest puff of wind or teensy cyclic movement passed straight into the cabin. Our Chief Pilot was known as Chuck, because most of his crewmen did.
Oh no, we'll be getting into 'wee-wah' next!
Originally Posted by [email protected]
Oh no, we'll be getting into 'wee-wah' next!
Could it be due to the relatively instantaneous change of the airflow from static hover with its specific airflows through the disc to 'normal flight' airflow which happens when going through ETL? The Rotorblades 'suddenly' getting clean air from the front. This could potentially lead to some 'overshooting' of the blade track, somewhat similar to a gust hitting the disk.
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Could it be due to the relatively instantaneous change of the airflow from static hover with its specific airflows through the disc to 'normal flight' airflow which happens when going through ETL? The Rotorblades 'suddenly' getting clean air from the front. This could potentially lead to some 'overshooting' of the blade track, somewhat similar to a gust hitting the disk.
However, the roll seems minor compared to the amount of forward cyclic needed to keep the nose where it was.
What AC said 
remember the roll is due to a change in inflow angle, not a massive amount but enough - the pitch is due to velocity differences between advancing and retreating sides of the disc and is V squared so much bigger
remember the roll is due to a change in inflow angle, not a massive amount but enough - the pitch is due to velocity differences between advancing and retreating sides of the disc and is V squared so much bigger
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Originally Posted by [email protected]
... remember the roll is due to a change in inflow angle, not a massive amount but enough ...
However, if you have spent endless hours in the hover in a SeaKing over the Atlantic with a strong breeze (25+ knots) blowing you cannot ignore it. That is firmly in the vibration bracket and it manifests itself by shaking the body so that the fleshy tip of the nose wobbles and tickles almost unbearably. ;-)