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What causes "blowback/flapback" when accelerating through transverse flow effect?

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What causes "blowback/flapback" when accelerating through transverse flow effect?

Old 14th Aug 2022, 14:42
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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.
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Old 14th Aug 2022, 15:01
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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.
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Old 14th Aug 2022, 15:38
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Originally Posted by ShyTorque View Post
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
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?
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Old 14th Aug 2022, 15:46
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Originally Posted by SASless View Post
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 ...
I agree. Many of the diagrams in my class notes, that then found their way into AP3456, had their inspiration there.

They are not cheap as they are scarce.
You can look at a copy at https://archive.org/details/sikorsky...-and-mechanics
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Old 14th Aug 2022, 16:20
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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?
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Old 14th Aug 2022, 16:24
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Originally Posted by SASless View Post
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.
Thank you for the recommendation, I just managed to grab a decent '64 version off Amazon.
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Old 14th Aug 2022, 18:07
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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?
ISTR he thought it was 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.
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Old 14th Aug 2022, 18:11
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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?
The phase lag on a Lynx rotor head was quite a way from 90 degrees so No, I don't believe so.

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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Old 14th Aug 2022, 18:29
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Originally Posted by SASless View Post
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.
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.
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Old 14th Aug 2022, 18:45
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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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Old 14th Aug 2022, 19:29
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Originally Posted by ShyTorque View Post
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.
It might be worth emphasising that all the forces that the rotor exerts on the helicopter are transferred through those hinges/supports. For a fully-articulated rotor head that force is just the blade tension [1]. All that aerodynamic forces on the blades are doing is altering the angle at the hinge. I suspect this is largely true for rigid types.

[1] For those interested, this is discussed on p13-14 of that Sikorski manual.
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Old 14th Aug 2022, 20:14
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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.
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Old 14th Aug 2022, 20:29
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Originally Posted by Ascend Charlie View Post
Lu was always asking the question "Where are the missing 18 degrees?" for the R-22, which had a phase lag of 72 degrees.
That's actually kinda funny.
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Old 14th Aug 2022, 21:56
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Oh no, we'll be getting into 'wee-wah' next!
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Old 15th Aug 2022, 00:27
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Originally Posted by [email protected] View Post
Oh no, we'll be getting into 'wee-wah' next!
beats WIWOL!
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Old 15th Aug 2022, 09:26
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Originally Posted by SunofAtom View Post
What is it that makes this one moment seem so different than any other phase of flight? Or is it just in my head?
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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Old 15th Aug 2022, 21:53
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Originally Posted by henra View Post
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.
It certainly could, but I would expect inflow roll to have at least as big of change, since the front of the rotor system is first and the rear is last to realize the airflow changes. However, the roll seems minor compared to the amount of forward cyclic needed to keep the nose where it was.
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Old 16th Aug 2022, 07:11
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However, the roll seems minor compared to the amount of forward cyclic needed to keep the nose where it was.
The roll IS minor, and gets less with increasing airspeed, whereas the flapback increases with airspeed or collective increase.
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Old 16th Aug 2022, 08:44
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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
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Old 16th Aug 2022, 14:29
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Originally Posted by [email protected] View Post
... remember the roll is due to a change in inflow angle, not a massive amount but enough ...
It is also accompanied by vibration, because the inflow angle only changes at the front of the rotor disc and the effect is assymetric until the airflow regime for forward flight is established. In a normal transition from the hover to forward flight the vibration may not be noticed.

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. ;-)
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