aerodynamic question
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no right answers; or perhaps two?
The question in the POST is can I explain the answer. Well, yes and no--the right answer is either doubled or MISSING! Bad question (typical bureaucratic incompetence).
But to amplify Nick (wow, how presumptuous of me!),
The problem is semantic: where is the pitch INcreasing or DEcreasing--that is, where is it CHANGING (note that the answers do not address "where is it greatest" or "where is it least").
And thus two answers are semantically correct: for during the advancing 180 degrees of travel, the pitch decreases then increases; and then during the retreating 180 degrees, pitch increases then decreases.
If we had a correct answer inluded in the choices, it would say "the pitch is greatest somewhere on the retreating side and least somewhere on the advancing side." [Thanks, Frank Robinson, for generating many long discussions of fine-tuning evasion as to exactly where . . .]
Would Nick pass the exam? I suspect so--but for the wrong reasons--he (and many of us) are accustomed to reading between the lines and evaluating what the question-writer had in mind (that obvious lack-of-conceptual-awareness) and choosing the answer the bureaucrat expected. Which is technically wrong, but they don't consider the implications of their wording!
Now what we need to do is form a committee of informed pilots to be so honored by the test-making agencies as to carefully weed out and reword questions like this that are patently unanswerable but the gummint fails to notice or care!
But to amplify Nick (wow, how presumptuous of me!),
The problem is semantic: where is the pitch INcreasing or DEcreasing--that is, where is it CHANGING (note that the answers do not address "where is it greatest" or "where is it least").
And thus two answers are semantically correct: for during the advancing 180 degrees of travel, the pitch decreases then increases; and then during the retreating 180 degrees, pitch increases then decreases.
If we had a correct answer inluded in the choices, it would say "the pitch is greatest somewhere on the retreating side and least somewhere on the advancing side." [Thanks, Frank Robinson, for generating many long discussions of fine-tuning evasion as to exactly where . . .]
Would Nick pass the exam? I suspect so--but for the wrong reasons--he (and many of us) are accustomed to reading between the lines and evaluating what the question-writer had in mind (that obvious lack-of-conceptual-awareness) and choosing the answer the bureaucrat expected. Which is technically wrong, but they don't consider the implications of their wording!
Now what we need to do is form a committee of informed pilots to be so honored by the test-making agencies as to carefully weed out and reword questions like this that are patently unanswerable but the gummint fails to notice or care!
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Nice pa42!
I must say, when I took the FAA aerodynamics test for the CFI, I found a bad question like the one in the thread, and I didn't answer it. Instead, I gave the examiner who proctored the test a detailed written explanation of why the question was bogus, with my name and certificate number on it. I got credit for the question, and the FAA fixed the question in a later version.
I must say, when I took the FAA aerodynamics test for the CFI, I found a bad question like the one in the thread, and I didn't answer it. Instead, I gave the examiner who proctored the test a detailed written explanation of why the question was bogus, with my name and certificate number on it. I got credit for the question, and the FAA fixed the question in a later version.
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Anyhow, did some calcs to satisfy myself of something. The stiffest rotor blade i know is Comanche with 15% effective offset hinge. This gives the blades a natural frequency of 1.12 that of the rotor rotation rate. Depending on blade damping, i get the following pitch phase lead angles:
1.0 * Critically damped: 84.9 degrees
0.5 * Critically damped: 79.9 degrees <-- R22 feels like this.
0.2 * Critically damped: 64.6 degrees
So i guess i've just learnt even more about rotor dynamics from Nick Lappos!
Mart
Last edited by Graviman; 16th Dec 2006 at 10:39. Reason: Correction to Nb/Nr: 1.12 = 1/0.89
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grav,
That angle changes with speed, and the Comanche had a variable gamma as a function of forward flight speed (loch number) and RPM, all tuned by the fly by wire controls.
That angle changes with speed, and the Comanche had a variable gamma as a function of forward flight speed (loch number) and RPM, all tuned by the fly by wire controls.
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Nick, this similar to the 72 degree pitch lead angle on the R22? So from optimum lead angle at 1g, a reduced Nr produces a deeper rotor cone thus producing forward flight starboard roll and "wee-wa" if not corrected. For all the acrimony R22 works very well with "missing 18 degrees".
Mart
Mart
Last edited by Graviman; 16th Dec 2006 at 10:40. Reason: Corrected cone angle for reduced Nr
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Mart,
Wee-wa: Is a 'small washout'. It happens when the pilot grabs the big stick between his legs and pushes it forward.
Wee-wee: Is a big washout. It happens when the pilot grabs the small stick between his legs and pushes it forward.
More information:
Wee-wa: A term used by Frank Robinson. The following is an assumption of what it means:~ (small washout) An incident of acceleration cross-coupling. An off-axis tilt that is experienced by the rotor disk as the disk is in the process of on-axis tilting. I.e. during a downward motion of the front of the disk, the angle-of-attack at the front of the disk is increased. This causes the disk on the retreating side to be higher than the height of the advancing side.
Wee-wee:
Wee-wa: Is a 'small washout'. It happens when the pilot grabs the big stick between his legs and pushes it forward.
Wee-wee: Is a big washout. It happens when the pilot grabs the small stick between his legs and pushes it forward.
More information:
Wee-wa: A term used by Frank Robinson. The following is an assumption of what it means:~ (small washout) An incident of acceleration cross-coupling. An off-axis tilt that is experienced by the rotor disk as the disk is in the process of on-axis tilting. I.e. during a downward motion of the front of the disk, the angle-of-attack at the front of the disk is increased. This causes the disk on the retreating side to be higher than the height of the advancing side.
Wee-wee:
Last edited by Dave_Jackson; 16th Dec 2006 at 21:11. Reason: "(small washout)" was previously missing from the explanation.
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Dave, even a rotor with 15% effective hinge offset will need some pre-coning for 1g nominal flight regime. Change of Nr will force flexure to a different cone angle. Don't forget that single main rotor is optimised by increasing Nr with flight speed. I mean Wee-wa in exactly the Robinson sense, but still don't have a coffee just before flying .
Mart
.Mart
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'Pologies for reopening subject...
The more i think about the Robinson "Wee-Wa" explanation for the 18 deg D3 in the R22 teetering head, the less it makes sense. Change of D3 with coning i do get, however.
A teetering gyro is a gyro, which always requires input force 90 degrees to movement. This is true of both the aerodynamic and inertial forces. The exception to this is for the case of some hinge offset, but i got the impression the R22 hub flapwise elastomeric bearing was extremely compliant. Certainly a well tracked and balanced rotor does not vibrate with cyclic input.
Using the figure above for the 15% hinge offset Comanche blade (thus Nblade 1.124 of Nrotor) of damping 0.5 Ccrit, and Prouty's calculations gives a Lock number of roughly 13.9. I would be interested how this compares to the actual machine, since it is higher than generally accepted value of 8 - then again Comanche could perform. Also insert these numbers into another equation in Prouty's book of magic, gives the required phase angle of 76.9 degrees - not far off post #25 estimate.
So the question is: does anyone understand how "Wee-wa" works in the real world?
Mart
A teetering gyro is a gyro, which always requires input force 90 degrees to movement. This is true of both the aerodynamic and inertial forces. The exception to this is for the case of some hinge offset, but i got the impression the R22 hub flapwise elastomeric bearing was extremely compliant. Certainly a well tracked and balanced rotor does not vibrate with cyclic input.
Using the figure above for the 15% hinge offset Comanche blade (thus Nblade 1.124 of Nrotor) of damping 0.5 Ccrit, and Prouty's calculations gives a Lock number of roughly 13.9. I would be interested how this compares to the actual machine, since it is higher than generally accepted value of 8 - then again Comanche could perform. Also insert these numbers into another equation in Prouty's book of magic, gives the required phase angle of 76.9 degrees - not far off post #25 estimate.
So the question is: does anyone understand how "Wee-wa" works in the real world?
Mart
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IMHO
Mart,
The offset flapping hinge gives a faster response to control inputs, whereas delta3 gives a slower softer response to control inputs.
The association between delta3 and phase angle is nowhere near as valid as the offset flapping hinge and phase angle association. However, the phase angle is the only 'adjustment' that a swashplate has.
The relationship between a rotor's offset flapping hinge and a flight control's phase angle is quite straight forward. The reasoning and the math are readily available in Prouty's main book etc.
Here is information on Flap Hinge Offset
Here is information on Phase Lag.
The relationship between a rotor's delta3 and a flight control's phase angle is quite complicated.
My posting, just above your two, is meant to be humorous, however, the response to 'wee-wa' is serious.
Here is information on Delta3 The [Sketch of Pitch Angle:] on this page should explain Robinson's use of delta3. And, this page has more information on Delta3 and Phase Angle
Dave
The offset flapping hinge gives a faster response to control inputs, whereas delta3 gives a slower softer response to control inputs.
The association between delta3 and phase angle is nowhere near as valid as the offset flapping hinge and phase angle association. However, the phase angle is the only 'adjustment' that a swashplate has.
The relationship between a rotor's offset flapping hinge and a flight control's phase angle is quite straight forward. The reasoning and the math are readily available in Prouty's main book etc.
Here is information on Flap Hinge Offset
Here is information on Phase Lag.
The relationship between a rotor's delta3 and a flight control's phase angle is quite complicated.
Frank Robinson said on PPRuNe;
"...the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms."
"...the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms."
You said;
So the question is: does anyone understand how "Wee-wa" works in the real world?
So the question is: does anyone understand how "Wee-wa" works in the real world?
Here is information on Delta3 The [Sketch of Pitch Angle:] on this page should explain Robinson's use of delta3. And, this page has more information on Delta3 and Phase Angle
Dave
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Nick says:
But i think he is wrong (...again. Respectfully nothing personal):
as he flys faster (forward) the pilot PLACES the stick further forward to create less pitch on advancing side.
He PUSHES to the right to create the FORCE required at the front of the disc to cause the pitch INCREASE at the front.
(assumptions for simplicity: US rotation direction, no assisted force controls, 'ideal' 90 deg 'phase lag', disregard 'inflow roll', MightyGem...are those CFS notes, EPTS or yours?)
All: the pitch INCREASES at the front DECREASES at the back, IS HIGH on the advancing side IS LOW on the retreating side.
91205: The explanation is the frequent misuse of ENGLISH concerning any description of rates of change. - Well described by Pa42
In defence of the Exam Question setting person, he would have intended to ask how the pitch was different on the advancing side in forward flight compared to not being in forward flight. - Then the pitch HAS DECREASED on the advancing side!
ichris, Nick, all. Question: is the rate of pitch change constant per angular progression around the disc as shown in ichis's diagram?
.......so the pilot places the cyclic to the right as he flys faster (US/UK convention) and this makes the geometric pitch of the blade increase in the forward section of the disk, and decrease in the aft (90 degrees before the desired blade motion).
But i think he is wrong (...again. Respectfully nothing personal):
as he flys faster (forward) the pilot PLACES the stick further forward to create less pitch on advancing side.
He PUSHES to the right to create the FORCE required at the front of the disc to cause the pitch INCREASE at the front.
(assumptions for simplicity: US rotation direction, no assisted force controls, 'ideal' 90 deg 'phase lag', disregard 'inflow roll', MightyGem...are those CFS notes, EPTS or yours?)
All: the pitch INCREASES at the front DECREASES at the back, IS HIGH on the advancing side IS LOW on the retreating side.
91205: The explanation is the frequent misuse of ENGLISH concerning any description of rates of change. - Well described by Pa42
In defence of the Exam Question setting person, he would have intended to ask how the pitch was different on the advancing side in forward flight compared to not being in forward flight. - Then the pitch HAS DECREASED on the advancing side!
ichris, Nick, all. Question: is the rate of pitch change constant per angular progression around the disc as shown in ichis's diagram?
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Dave,
Taken me a while to convince myself but i agree that d3 is not the same thing as
90` - PhaseAngle
Looking at the R22 M/R hub from this superb Rotorhead Image Site it can be seen that the pitch link does not have to be a vertical installation. It could attach to swashplate 90` ahead of blade while the pitch horn is only 72' ahead of blade.
I also agree with you about +ve d3 (pitch cone coupling) reducing the response of the rotor to cyclic input. This probably explains the possibility for reduced "pilot gain factor" in R22 hovering (ie subconciously filtering out fast cyclic inputs (~0.5 sec time constant)).
What your d3 web page does not directly account for is the effective hinge offset of a rigid rotor. The set up in this case would be the flap hinge tipwise of the pitch bearing, but with the pitchlink at the root. The closest would be the twisted spline geometry ( Method C/ ), with most rigid rotors having no pitch link coupling.
The practical upshot is that in a rigid rotor head phase angle is as per the Prouty calculations (which consider Lock number - my original prompt), and rigid rotor d3 could only be achieved by the pitch cone coupling in the flexural compliance of the blade.
I definately can understand why alter the phase to compensate for coning in forward flight at 1g loading. However, this is the bit i don't get, unless considered for a rotor which is being "forced" by non aerodynamic/gyroscopic forces:
So perhaps a reasonable explanation might be: 
That said the R22 is a superb flying machine, so this is really more an academic exercise in understanding rotorcraft dynamics.
Mart
Taken me a while to convince myself but i agree that d3 is not the same thing as
90` - PhaseAngle
Looking at the R22 M/R hub from this superb Rotorhead Image Site it can be seen that the pitch link does not have to be a vertical installation. It could attach to swashplate 90` ahead of blade while the pitch horn is only 72' ahead of blade.
I also agree with you about +ve d3 (pitch cone coupling) reducing the response of the rotor to cyclic input. This probably explains the possibility for reduced "pilot gain factor" in R22 hovering (ie subconciously filtering out fast cyclic inputs (~0.5 sec time constant)).
What your d3 web page does not directly account for is the effective hinge offset of a rigid rotor. The set up in this case would be the flap hinge tipwise of the pitch bearing, but with the pitchlink at the root. The closest would be the twisted spline geometry ( Method C/ ), with most rigid rotors having no pitch link coupling.
The practical upshot is that in a rigid rotor head phase angle is as per the Prouty calculations (which consider Lock number - my original prompt), and rigid rotor d3 could only be achieved by the pitch cone coupling in the flexural compliance of the blade.
I definately can understand why alter the phase to compensate for coning in forward flight at 1g loading. However, this is the bit i don't get, unless considered for a rotor which is being "forced" by non aerodynamic/gyroscopic forces:
Originally Posted by Frank Robinson
In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as "wee-wa." This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive.
Originally Posted by Chuck Beaty
Mr. Robinson is not only president and chief engineer of Robinson Helicopter but is also chief promoter and head salesman. When forging dies for blade grips and pitch arms are bought and paid for, it's sometimes expedient to make theory fit practice.
Mart
Last edited by Graviman; 17th Dec 2006 at 11:33. Reason: Typos
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Mart,
The subject of delta3 is taking this thread off topic. In addition, a lot has been previously been posted on this subject; on PPRuNe threads and on this copy of another forum's thread
A couple of hundred hours have been spent studying delta3, plus even more time has been spent attempting to understand the Robinson rotorhead. I believe that my understanding of delta3 is correct. However, being a cynical engineering type I have never been totally comfortable with Frank Robinson's association between delta3 and phase lag.
Chuck Beaty's comment [in your posting], may have been the appropriate response to Frank Robinson's "This is a highly technical subject which can only be fully explained using very technical engineering terms."
If there is any desire to continue this topic, perhaps it should be done as a continuation of an earlier related thread.
Dave
The subject of delta3 is taking this thread off topic. In addition, a lot has been previously been posted on this subject; on PPRuNe threads and on this copy of another forum's thread
A couple of hundred hours have been spent studying delta3, plus even more time has been spent attempting to understand the Robinson rotorhead. I believe that my understanding of delta3 is correct. However, being a cynical engineering type I have never been totally comfortable with Frank Robinson's association between delta3 and phase lag.
Chuck Beaty's comment [in your posting], may have been the appropriate response to Frank Robinson's "This is a highly technical subject which can only be fully explained using very technical engineering terms."
If there is any desire to continue this topic, perhaps it should be done as a continuation of an earlier related thread.
Dave
Last edited by Dave_Jackson; 17th Dec 2006 at 23:51. Reason: boredom
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Originally Posted by DaveJackson
If there is any desire to continue this topic, perhaps it should be done as a continuation of an earlier related thread.
Originally Posted by NickLappos
That (phase) angle changes with speed, and the Comanche had a variable gamma as a function of forward flight speed (loch number) and RPM, all tuned by the fly by wire controls.
Mart
