Certification of Robinson Helicopters (incl post by Frank Robinson)
Guest
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To: Eurocopter
The reason the pitch horn leads by 72 degrees (approximately) is because the pitch horn can not pass the flapping hinge. On a Bell the pitch horn leads by 90 degrees and there is a small amount of pitch coupling.
That is caused by the fact that under certain pitch settings the pitch horn attachment to the pitch link is not coincedent with the teeter point.
On the Robinson, since the pitch horn is not always coincedent with the flapping hinge, you will get a slight amount of pitch coupling. However, if the pitch horn crossed the flapping hinge the helicopter would be uncontrolable due to massive pitch coupling. That is the explanation for the difference of 18 degrees (approximatly)
I can't say other than what I stated above. Why someone would give the explanation you provided,is either they didn't understand or, they were covering up the truth.
Here is a suggestion to not only Eurocopter but to all of you:
Go on the internet and on the address bar, type in helicopter frontiers-aerodynamics or, http://www.hfi.ca/training/helo_aero.html
Either one will get you to the right place.
Some of what you see on this site contradicts what I say in my report as the web page is run by a Robinson dealer. I will admit it has a lot of good stuff that can't be contradicted. However, there is one point. When they explain the functions of the rotorhead, and they have some good pictures, especially if you have never seen one. they also describe gyroscopic precession. The illustration of swashplate movement relative to gyroscopic precession is for a Bell rotorhead and not a Robinson rotorhead.
They also have a lot of self serving verbage on the safety of Robinson helicopters. It's interesting reading so check it out.
After checking out the website, come on back and let me know what you think.
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The Cat
[This message has been edited by Lu Zuckerman (edited 10 November 2000).]
[This message has been edited by Lu Zuckerman (edited 11 November 2000).]
[This message has been edited by Lu Zuckerman (edited 11 November 2000).]
The reason the pitch horn leads by 72 degrees (approximately) is because the pitch horn can not pass the flapping hinge. On a Bell the pitch horn leads by 90 degrees and there is a small amount of pitch coupling.
That is caused by the fact that under certain pitch settings the pitch horn attachment to the pitch link is not coincedent with the teeter point.
On the Robinson, since the pitch horn is not always coincedent with the flapping hinge, you will get a slight amount of pitch coupling. However, if the pitch horn crossed the flapping hinge the helicopter would be uncontrolable due to massive pitch coupling. That is the explanation for the difference of 18 degrees (approximatly)
I can't say other than what I stated above. Why someone would give the explanation you provided,is either they didn't understand or, they were covering up the truth.
Here is a suggestion to not only Eurocopter but to all of you:
Go on the internet and on the address bar, type in helicopter frontiers-aerodynamics or, http://www.hfi.ca/training/helo_aero.html
Either one will get you to the right place.
Some of what you see on this site contradicts what I say in my report as the web page is run by a Robinson dealer. I will admit it has a lot of good stuff that can't be contradicted. However, there is one point. When they explain the functions of the rotorhead, and they have some good pictures, especially if you have never seen one. they also describe gyroscopic precession. The illustration of swashplate movement relative to gyroscopic precession is for a Bell rotorhead and not a Robinson rotorhead.
They also have a lot of self serving verbage on the safety of Robinson helicopters. It's interesting reading so check it out.
After checking out the website, come on back and let me know what you think.
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 10 November 2000).]
[This message has been edited by Lu Zuckerman (edited 11 November 2000).]
[This message has been edited by Lu Zuckerman (edited 11 November 2000).]
Guest
Posts: n/a
Retraction:
To all that I indicated that the so called missing page should be in the UK Robinson POHs I have to admit that I was wrong. According to the CAA that page does not go far enough. According to the CAA ,much of the material covered in the AD that precipitated the "Missing Page" is MANDATORY AND NOT RECOMMENDED, AS INDICATED IN THE "MISSING PAGE" IN THE USA POHs.
Read the letter referenced above. I sent a copy to Jim Hall of ther NTSB asking him to investigate why there is such a large descrepency between the FAA and the CAA.
Joe pilot if you are out there please read the letter and then please come back on the thread with your comments.
The CAA Letter can be found on the following website:
http://205467.homestead.com/caa.html
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The Cat
[This message has been edited by helidrvr (edited 18 November 2000).]
To all that I indicated that the so called missing page should be in the UK Robinson POHs I have to admit that I was wrong. According to the CAA that page does not go far enough. According to the CAA ,much of the material covered in the AD that precipitated the "Missing Page" is MANDATORY AND NOT RECOMMENDED, AS INDICATED IN THE "MISSING PAGE" IN THE USA POHs.
Read the letter referenced above. I sent a copy to Jim Hall of ther NTSB asking him to investigate why there is such a large descrepency between the FAA and the CAA.
Joe pilot if you are out there please read the letter and then please come back on the thread with your comments.
The CAA Letter can be found on the following website:
http://205467.homestead.com/caa.html
------------------
The Cat
[This message has been edited by helidrvr (edited 18 November 2000).]
Guest
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Lu,
my previous explanation of why the R-22 rotorhead has an unusual swashplate configuration obviously didn't sway your opinion.
Someone else here mentioned that R/C helos sometimes have swashplate offsets other than 90 degrees. This raised my curiosity and I went looking.
Check this out; (Scroll down to PART 11) http://www.lance.co.uk/w3mh/articles/html/csm9-11.htm
It talks about rotating the swashplate when you have offset flapping hinges in a R/C helo but they could be talking about the R-22 from the sound of it.
Try_Cyclic
[This message has been edited by Try_Cyclic (edited 20 November 2000).]
[This message has been edited by Try_Cyclic (edited 20 November 2000).]
[This message has been edited by Try_Cyclic (edited 20 November 2000).]
my previous explanation of why the R-22 rotorhead has an unusual swashplate configuration obviously didn't sway your opinion.
Someone else here mentioned that R/C helos sometimes have swashplate offsets other than 90 degrees. This raised my curiosity and I went looking.
Check this out; (Scroll down to PART 11) http://www.lance.co.uk/w3mh/articles/html/csm9-11.htm
It talks about rotating the swashplate when you have offset flapping hinges in a R/C helo but they could be talking about the R-22 from the sound of it.
Try_Cyclic
[This message has been edited by Try_Cyclic (edited 20 November 2000).]
[This message has been edited by Try_Cyclic (edited 20 November 2000).]
[This message has been edited by Try_Cyclic (edited 20 November 2000).]
Guest
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Not meaning to deliberately complicate things, but isn't there a CAA requirement that all helicopter front seats be fitted with harnesess and not simply 'lap/shoulder belts' (a la R-22)?
I was also pretty sure that the CAA required rotor intertia to be great enough to allow 1.5 seconds of pilot reaction time before rotor stall following a power or drive failure.
I was also pretty sure that the CAA required rotor intertia to be great enough to allow 1.5 seconds of pilot reaction time before rotor stall following a power or drive failure.
Guest
Posts: n/a
To: Try Cyclic.
In reading the entire article, it is a mixture of descriptions of real rotor heads and model rotor heads. In the descriptions of real rotorheads I could find no fault nor,could I find fault in the description of the model heads. However in the model heads section they were talking about models with fly-bars which the Robinson helicopters don't have. It appears that the description of the swashplate off-set was to compensate for the problems of the fly-bar and other mechanical problems in the control system.
I have seen model helicopters fly but I have never really examined the control systems or the rotor heads. The author addresses flapping and leading and lagging. What he says applies to a real rotorhead with offset hinges (Fully articulated) but not having a detailed understanding of model helicopters I don't know if they have fully articulated rotorheads. While working in Germany I saw a beautiful model of a BK-105 that had a rigid rotorhead but to my knowledge all others have fly-bar rotorheads. If wrong I will stand corrected.
Additional Info:
When you have an offset hinge on a rotorhead the swash plate is offset by 45 degrees and the pitch horn leads the blades by 45 degrees which gives a total of 90 degrees. In this case, the swash plate tilts 45 degrees prior to the direction you wish to fly. The advancing blade is 45 degrees ahead of that position and therefore, it has its' lowest point of pitch. Conversely, the retreating blade has its' highest point of pitch and with 90 degrees of precession the disc tips down over the nose and, up over the tail. It has to add up to 90 degrees.
The Robinson rotorhead because of its' design has the pitch horns leading the blade by appproximately 72 degrees and as such the attachment points on the swashplate are positioned under the pitch horns. The swash plate however tips just like a Bell swashplate. So, the lowest point on the swashplate on a Robinson is over the centerline of the aircraft but, the blade pitch horn lags that point by approximately 18 degrees which means that the blade must travel an additional 18 degrees before it reaches the point of maximum down pitch and the same is true for the retreating blade. This means that the disc will tilt down 18 degrees after passing the longitudinal centerline and the helicopter flys left unless the pilot compensates for the 18 degree difference.
Look at this website: http://205467.homestead.com/diagrams.html
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
In reading the entire article, it is a mixture of descriptions of real rotor heads and model rotor heads. In the descriptions of real rotorheads I could find no fault nor,could I find fault in the description of the model heads. However in the model heads section they were talking about models with fly-bars which the Robinson helicopters don't have. It appears that the description of the swashplate off-set was to compensate for the problems of the fly-bar and other mechanical problems in the control system.
I have seen model helicopters fly but I have never really examined the control systems or the rotor heads. The author addresses flapping and leading and lagging. What he says applies to a real rotorhead with offset hinges (Fully articulated) but not having a detailed understanding of model helicopters I don't know if they have fully articulated rotorheads. While working in Germany I saw a beautiful model of a BK-105 that had a rigid rotorhead but to my knowledge all others have fly-bar rotorheads. If wrong I will stand corrected.
Additional Info:
When you have an offset hinge on a rotorhead the swash plate is offset by 45 degrees and the pitch horn leads the blades by 45 degrees which gives a total of 90 degrees. In this case, the swash plate tilts 45 degrees prior to the direction you wish to fly. The advancing blade is 45 degrees ahead of that position and therefore, it has its' lowest point of pitch. Conversely, the retreating blade has its' highest point of pitch and with 90 degrees of precession the disc tips down over the nose and, up over the tail. It has to add up to 90 degrees.
The Robinson rotorhead because of its' design has the pitch horns leading the blade by appproximately 72 degrees and as such the attachment points on the swashplate are positioned under the pitch horns. The swash plate however tips just like a Bell swashplate. So, the lowest point on the swashplate on a Robinson is over the centerline of the aircraft but, the blade pitch horn lags that point by approximately 18 degrees which means that the blade must travel an additional 18 degrees before it reaches the point of maximum down pitch and the same is true for the retreating blade. This means that the disc will tilt down 18 degrees after passing the longitudinal centerline and the helicopter flys left unless the pilot compensates for the 18 degree difference.
Look at this website: http://205467.homestead.com/diagrams.html
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
Guest
Posts: n/a
Lu,
thanks for taking time to read the article.
I think it is correct that R/C helos usually have some form of fly-bar. 2-bladed teetering rotorheads are common. But the article says clearly that the fly-bar actually tends to reduce the need for swashplate offset.
It is not the cause of the offset.
The real cause is that the blades will flap into position more quickly when there is coupling between pitch and flap.
This is the case in the full size R-22
When the R-22 blades flap, pitch is pulled out or addded since the pitch links are not on centerline. This alters the way precession affects the blades. Precession does NOT always work at 90 degrees. It is taught that it is always 90 degrees as a simplification.
The 72 degree swashplate offset exactly compensates for the non 90 deg. precessional response and the result is that the R-22 rotor follows the direction of the cyclic stick in a normal fashion.
Maybe Frank Robinson will post on this sunbject and shed more light on this issue.
Try-Cyclic
thanks for taking time to read the article.
I think it is correct that R/C helos usually have some form of fly-bar. 2-bladed teetering rotorheads are common. But the article says clearly that the fly-bar actually tends to reduce the need for swashplate offset.
It is not the cause of the offset.
The real cause is that the blades will flap into position more quickly when there is coupling between pitch and flap.
This is the case in the full size R-22
When the R-22 blades flap, pitch is pulled out or addded since the pitch links are not on centerline. This alters the way precession affects the blades. Precession does NOT always work at 90 degrees. It is taught that it is always 90 degrees as a simplification.
The 72 degree swashplate offset exactly compensates for the non 90 deg. precessional response and the result is that the R-22 rotor follows the direction of the cyclic stick in a normal fashion.
Maybe Frank Robinson will post on this sunbject and shed more light on this issue.
Try-Cyclic
Guest
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Lu,
I see that you went back and added some additional material to your last post which I just read.
What you are doing is repeating the argument that the R-22 swashplate tilts left at 90 degrees, say, but the pitch horns are not at 90 degrees and therefore precession will cause the blades to be at their high point in the wrong position.
I am trying to get you to see that the blades will wind up at the correct position because of the way the flapping response is speeded up in the R-22 by the use of pitch flap coupling or "delta"
Try_cyclic
I see that you went back and added some additional material to your last post which I just read.
What you are doing is repeating the argument that the R-22 swashplate tilts left at 90 degrees, say, but the pitch horns are not at 90 degrees and therefore precession will cause the blades to be at their high point in the wrong position.
I am trying to get you to see that the blades will wind up at the correct position because of the way the flapping response is speeded up in the R-22 by the use of pitch flap coupling or "delta"
Try_cyclic
Guest
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To: Try_Cyclic
The real cause is that the blades will flap into position more quickly when there is coupling between pitch and flap.
This is the case in the full size R-22
When the R-22 blades flap, pitch is pulled out or addded since the pitch links are not on centerline. This alters the way precession affects the blades. Precession does NOT always work at 90 degrees. It is taught that it is always 90 degrees as a simplification.
The 72 degree swashplate offset exactly compensates for the non 90 deg. precessional response and the result is that the R-22 rotor follows the direction of the cyclic stick in a normal fashion.
Maybe Frank Robinson will post on this sunbject and shed more light on this issue.
The coupling effect you describe is common to all helicopters and is referred to as pitch coupling. Under ideal circumstances if the pitch horn on any helicopter were coincident with the teeter hinge or the centerline of the flapping hinge there would be no coupling. However in the real world this is not always true. Yhe pitch coupling is similar in effect to that of a delta hinge tail rotor. When the tail rotor blade flaps the pitch will change fron that input by the pilot. The purpose of this is to equalize the lift on the tail rotor blades and minimize the bending forces on a two bladed tail rotor. The blade going into the wind will flap so as to reduce the pitch and the retreating blade will flap so as to increase the pitch on the blade. On multi blade systems the blades flap individually but not to minimize bending forces. Getting back to one of my original posts about lead/lag being a result of flap, the tail rotor on the CH-37 had a fully articulated tail rotor that could both lead/lag and flap.
Maybe Frank Robinson can better explain how his system works but I also hope that he can explain how he and his engineers were able to defeat the laws of physics. If he can, then I will be the first to say that I was wrong.
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The Cat
The real cause is that the blades will flap into position more quickly when there is coupling between pitch and flap.
This is the case in the full size R-22
When the R-22 blades flap, pitch is pulled out or addded since the pitch links are not on centerline. This alters the way precession affects the blades. Precession does NOT always work at 90 degrees. It is taught that it is always 90 degrees as a simplification.
The 72 degree swashplate offset exactly compensates for the non 90 deg. precessional response and the result is that the R-22 rotor follows the direction of the cyclic stick in a normal fashion.
Maybe Frank Robinson will post on this sunbject and shed more light on this issue.
The coupling effect you describe is common to all helicopters and is referred to as pitch coupling. Under ideal circumstances if the pitch horn on any helicopter were coincident with the teeter hinge or the centerline of the flapping hinge there would be no coupling. However in the real world this is not always true. Yhe pitch coupling is similar in effect to that of a delta hinge tail rotor. When the tail rotor blade flaps the pitch will change fron that input by the pilot. The purpose of this is to equalize the lift on the tail rotor blades and minimize the bending forces on a two bladed tail rotor. The blade going into the wind will flap so as to reduce the pitch and the retreating blade will flap so as to increase the pitch on the blade. On multi blade systems the blades flap individually but not to minimize bending forces. Getting back to one of my original posts about lead/lag being a result of flap, the tail rotor on the CH-37 had a fully articulated tail rotor that could both lead/lag and flap.
Maybe Frank Robinson can better explain how his system works but I also hope that he can explain how he and his engineers were able to defeat the laws of physics. If he can, then I will be the first to say that I was wrong.
------------------
The Cat
Guest
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To: Try_Cyclic
It seems that we were posting at the same time.
I don't seem to be getting through to you.
If you have access to both a Bell and a Robinson please do the following.
Place the blades over the longitudinal center line of both helicopters.
Place the cyclic sticks of both helicopters in the rigged neutral position.
Move the cyclic sticks forward and backward. On the Bell, the blades will not move but, on the Robinson they will. Now, rotate the Robinson blades so that the pitch horn is directly on the Lateral centerline. Move the cyclic forward and backward. In this case, the blades won't move.
Place the blade of both helicopters over the lateral axis of the helicopters.
With the cyclic sticks in the rigged neutral position move the cyclic stick left and right. On the Bell the blades won't move. On the Robinson they will move. Now, rotate the Robinson blades until the pitch links are directly over the longitudinal axis of the helicopter. Repeat the same test. The blades won't move.
On the Robinson the blade must travel in rotation the same number of degrees that you moved them in order to eliminate any movement
in order to get the maximum pitch deflection to effect the maximum movement of the disc.
I have been working on helicopters since 1949 and I have only seen one helicopter that did not (on a continuous basis) have a 90 degree phase angle. That helicopter was the Cheyenne and it was eventually cancelled because of that problem.
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The Cat
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
It seems that we were posting at the same time.
I don't seem to be getting through to you.
If you have access to both a Bell and a Robinson please do the following.
Place the blades over the longitudinal center line of both helicopters.
Place the cyclic sticks of both helicopters in the rigged neutral position.
Move the cyclic sticks forward and backward. On the Bell, the blades will not move but, on the Robinson they will. Now, rotate the Robinson blades so that the pitch horn is directly on the Lateral centerline. Move the cyclic forward and backward. In this case, the blades won't move.
Place the blade of both helicopters over the lateral axis of the helicopters.
With the cyclic sticks in the rigged neutral position move the cyclic stick left and right. On the Bell the blades won't move. On the Robinson they will move. Now, rotate the Robinson blades until the pitch links are directly over the longitudinal axis of the helicopter. Repeat the same test. The blades won't move.
On the Robinson the blade must travel in rotation the same number of degrees that you moved them in order to eliminate any movement
in order to get the maximum pitch deflection to effect the maximum movement of the disc.
I have been working on helicopters since 1949 and I have only seen one helicopter that did not (on a continuous basis) have a 90 degree phase angle. That helicopter was the Cheyenne and it was eventually cancelled because of that problem.
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The Cat
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]
Guest
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Lu,
you say you are not getting through to me.
That isn't the problem. I understand your position rather well, but don't agree with it.
Now getting to your last example:
Yes, if I were to do the experiment that you suggest, then when the stick is moved fore and aft, the robby blades will move, etc, etc.
Yes, all this is true on the ground.
Unfortuneatly the blades are not rotating during this demonstration.
If you do the demonstration when the rotor is up to speed and THEN look at the way the rotor tilts you will see no difference betwen the Bell and the Robinson.
Magic? No it is called rotor dynamics.
There are forces on the rotor that come into play only when it is moving. Specifically I refer to the pitch-flap coupling.
The blades don't flap when they are standing still, so all you see is the effect of cyclic tilt. But that's not the whole picture.
Mr Robinson did not change the laws of physics. He simply understood them.
Lu, instead of twiddling the stick on the ground, perhaps you should actually fly in a R-22. I fly them and I see no indication of the ship not flying where I point it.
you say you are not getting through to me.
That isn't the problem. I understand your position rather well, but don't agree with it.
Now getting to your last example:
Yes, if I were to do the experiment that you suggest, then when the stick is moved fore and aft, the robby blades will move, etc, etc.
Yes, all this is true on the ground.
Unfortuneatly the blades are not rotating during this demonstration.
If you do the demonstration when the rotor is up to speed and THEN look at the way the rotor tilts you will see no difference betwen the Bell and the Robinson.
Magic? No it is called rotor dynamics.
There are forces on the rotor that come into play only when it is moving. Specifically I refer to the pitch-flap coupling.
The blades don't flap when they are standing still, so all you see is the effect of cyclic tilt. But that's not the whole picture.
Mr Robinson did not change the laws of physics. He simply understood them.
Lu, instead of twiddling the stick on the ground, perhaps you should actually fly in a R-22. I fly them and I see no indication of the ship not flying where I point it.
Guest
Posts: n/a
To: Try_Cyclic
This is getting to be like my arguments between myself and Joe Pilot.
Log onto this web site and read the diagrams.
http://205467.homestead.com/diagrams.html
These diagrams explain the argument I am making about the differences between Bell and Robinson.
Read them and come back so we can be talking from the same piece of paper.
So far, the people that have requested my report and have seen the diagrams have not countered my argument.
------------------
The Cat
This is getting to be like my arguments between myself and Joe Pilot.
Log onto this web site and read the diagrams.
http://205467.homestead.com/diagrams.html
These diagrams explain the argument I am making about the differences between Bell and Robinson.
Read them and come back so we can be talking from the same piece of paper.
So far, the people that have requested my report and have seen the diagrams have not countered my argument.
------------------
The Cat
Guest
Posts: n/a
Hello all,
I've been reading this thread for a while and would like to add a bit.
Lu, I think Try_Cyclic has the idea, dynamic rotors do not react simply like the blade angle. On certain systems the behavior of the stationary blades is not the same as the behavior of the rotating rotorhead. The phase lag is defined by
delta(phi) = 90° - arctan((nu^2-1)/(gamma/8))
where nu is the dimensionless natural frequency of the rotating rotor, and gamma is the Lock number, which defines the ratio of aerodynamic to inertial forces on the rotor blade.
For a simple articulated rotor with no root spring, nu=1, meaning the natural frequency of the rotor is 1/rev and the phase lag is just defined by the first term of the equation, 90°. This seems to be the case on almost all fully-articulated rotors and many teetering rotors like the 206. But if either a spring is introduced at the root, such as in many modern hingeless rotors, or (I assume), as pitch-flap coupling is introduced, nu becomes slightly greater than 1 and the phase response of the rotor is reduced. Hence, a hingless rotor is quicker to respond, and the phase lag between control input and the rotor disc motion is less than 90°.
Whether this is the case for the R-22 and R-44 I am not sure, but if I understand Try_Cyclic, he is saying that the delta hinge is reducing the phase lag to less than 90, possibly 72°.
As others have echoed, I await the official response from Mr. Robinson.
- Kyrilian
P.S. If interested, formulae above come from 'Helicopter Theory' by W. Johnson, pp 225
I've been reading this thread for a while and would like to add a bit.
Lu, I think Try_Cyclic has the idea, dynamic rotors do not react simply like the blade angle. On certain systems the behavior of the stationary blades is not the same as the behavior of the rotating rotorhead. The phase lag is defined by
delta(phi) = 90° - arctan((nu^2-1)/(gamma/8))
where nu is the dimensionless natural frequency of the rotating rotor, and gamma is the Lock number, which defines the ratio of aerodynamic to inertial forces on the rotor blade.
For a simple articulated rotor with no root spring, nu=1, meaning the natural frequency of the rotor is 1/rev and the phase lag is just defined by the first term of the equation, 90°. This seems to be the case on almost all fully-articulated rotors and many teetering rotors like the 206. But if either a spring is introduced at the root, such as in many modern hingeless rotors, or (I assume), as pitch-flap coupling is introduced, nu becomes slightly greater than 1 and the phase response of the rotor is reduced. Hence, a hingless rotor is quicker to respond, and the phase lag between control input and the rotor disc motion is less than 90°.
Whether this is the case for the R-22 and R-44 I am not sure, but if I understand Try_Cyclic, he is saying that the delta hinge is reducing the phase lag to less than 90, possibly 72°.
As others have echoed, I await the official response from Mr. Robinson.
- Kyrilian
P.S. If interested, formulae above come from 'Helicopter Theory' by W. Johnson, pp 225
Guest
Posts: n/a
To: Kyrillian
There is no spring moment in the flapping of the Robinson rotor. Once the blades are coned and are free to flap there is absolutely no restriction in the flapping outside of any friction on the hinge caused by centrifugal loading.
Pitch coupling which Try_Cyclic is referencing is explained in my post above.
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The Cat
There is no spring moment in the flapping of the Robinson rotor. Once the blades are coned and are free to flap there is absolutely no restriction in the flapping outside of any friction on the hinge caused by centrifugal loading.
Pitch coupling which Try_Cyclic is referencing is explained in my post above.
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The Cat
Guest
Posts: n/a
Lu,
While there is no mechanical spring acting on the stationary rotor, a delta-3 hinge creates an aerodynamic spring that comes into play when the helicopter rotor is turning and the blades are moving fast enough to produce lift. The formula I mentioned earlier can include this (though I didn't include it), where the delta-3 angle is subtracted from the phase lag:
delta(phi) = 90° - arctan((nu^2-1)/(gamma/8)) - (delta-3 angle)
So even if a rotor has no mechanical spring, the delta hinge can reduce the phase lag to less than 90°
As you recall, Hookes law states (something to the effect) that a spring applies a restoring force proportional to the deflection. If you have a rotor with a delta hinge and the rotor is turning, as the blade is perturbed upward the angle of attack is thus reduced (made more negative) because of the coupling, and the blade is aerodynamically forced downward. This in essence, is a spring.
Until I looked around, I wasn't sure if this was the case in your examples, but I found a page that shows both the rotor head of a JetRanger, and that of an R-22.
http://www.ai.mit.edu/projects/cbcl/...h/mr_semi.html
While the photos aren't exactly lined up, you can see that the point at which the horn attaches to the pitch links is on a line through the teetering hinge on the 206 (the photo doesn't show this well but I'm guessing this is the case). This would mean that there is no flap-pitch coupling and the phase lag is exactly 90°. However, if you look at the R-22, the delta-3 angle (between the teetering hinge axis and the line running through the vertex of the teetering and feathering axes out to the pitch link attachment) is greater than zero, and quite possibly 18°.
Next time I go out to fly an R-22 I'll have someone move the stick directly forward and watch from outside to see in which direction the rotor disc tilts. I'm guessing it will be in the exact same direction as stick movement.
I understand your argument Lu, and have read all your posts on this forum. I've also looked at your diagrams. I commend your tenacity but think you haven't looked at all factors that affect a dynamic rotor system--it's not quite as simple as a motionless one.
I'm learning and thus cannot be 100% certain of the validity of my assumptions. A little knowledge is often dangerous... However, I firmly believe the aforementioned text is worthy of citing--Dr Johnson has probably the most expertise in the science and engineering behind rotary-wing flight.
- Kyrilian
While there is no mechanical spring acting on the stationary rotor, a delta-3 hinge creates an aerodynamic spring that comes into play when the helicopter rotor is turning and the blades are moving fast enough to produce lift. The formula I mentioned earlier can include this (though I didn't include it), where the delta-3 angle is subtracted from the phase lag:
delta(phi) = 90° - arctan((nu^2-1)/(gamma/8)) - (delta-3 angle)
So even if a rotor has no mechanical spring, the delta hinge can reduce the phase lag to less than 90°
As you recall, Hookes law states (something to the effect) that a spring applies a restoring force proportional to the deflection. If you have a rotor with a delta hinge and the rotor is turning, as the blade is perturbed upward the angle of attack is thus reduced (made more negative) because of the coupling, and the blade is aerodynamically forced downward. This in essence, is a spring.
Until I looked around, I wasn't sure if this was the case in your examples, but I found a page that shows both the rotor head of a JetRanger, and that of an R-22.
http://www.ai.mit.edu/projects/cbcl/...h/mr_semi.html
While the photos aren't exactly lined up, you can see that the point at which the horn attaches to the pitch links is on a line through the teetering hinge on the 206 (the photo doesn't show this well but I'm guessing this is the case). This would mean that there is no flap-pitch coupling and the phase lag is exactly 90°. However, if you look at the R-22, the delta-3 angle (between the teetering hinge axis and the line running through the vertex of the teetering and feathering axes out to the pitch link attachment) is greater than zero, and quite possibly 18°.
Next time I go out to fly an R-22 I'll have someone move the stick directly forward and watch from outside to see in which direction the rotor disc tilts. I'm guessing it will be in the exact same direction as stick movement.
I understand your argument Lu, and have read all your posts on this forum. I've also looked at your diagrams. I commend your tenacity but think you haven't looked at all factors that affect a dynamic rotor system--it's not quite as simple as a motionless one.
I'm learning and thus cannot be 100% certain of the validity of my assumptions. A little knowledge is often dangerous... However, I firmly believe the aforementioned text is worthy of citing--Dr Johnson has probably the most expertise in the science and engineering behind rotary-wing flight.
- Kyrilian
Guest
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Regarding the two pictures:
On both the Robinson and the Bell the blades are shown in the low pitch or, full down collective. On the Robinson head, the pitch horn attachment to the pitch link is almost coincident with the axis of the flapping hinge. On the Bell the same point is almost coincident with the axis of the teeter hinge.
When the rotor is turning and the blades are in an aerodynamic state , and the pilot pulls in collective the points referenced above are now above the respective axis points. Whenever the pilot introduces cyclic there is in effect a coupling caused by the delta hinge effect. This is common to all helicopters and is called pitch coupling. Because of this coupling the pilot must make a correction in his cyclic stick position. The coupling also tends to restore the individual blade when some external force tries to perturb it from the normal tip path just like the tail rotor.
Regarding your observation of the Robinson rotorhead and the relationship of the pitch horn to the teeter hinge, the pitch horn has to be in this position because if the horn was in line with the teeter hinge the coupling would be so massive as to render the helicopter uncontrollable.
This rotorhead is totally unlike any other rotor head and the rules in your textbooks don't apply. If you read the report I sent to you, you would have seen the reference to the fact that the FAA required special tests in order to certify the design. My question in the report was ," Was this testing done?"
It is my contention that the rotor design and the control rigging procedures are the root cause of the rotor incursion incidents and loss of control incidents.
Go back to your basic aerodynamics and determine if pitch coupling influences phase angle. Then, diagram a Bell rotor system and a Robinson system to see the differences.
Oh yeah, I forgot I sent those diagrams to your email site along with the report.
Talk to your professors and present the problem to them. As I said to you in the email, "don't always believe your text books".
Text books define the problems under ideal conditions and they don't always cover the total picture.
You will find this out when you get your first job. Just like lawyers making new law you will on occasion make new engineering in order to meet design requirements and/or in order to correct a design problem.
Do your text books cover rotor blade design where the blades have a negative twist and a constantly changing camber from the root to the tip or, do they just deal with negative twist and a constant camber. The blade described above was created to overcome a design restriction that limited the rotor span. The design was created by Ray Prouty who is one of the best aerodynamicists. Much to his chagrin, the design was a failure and caused a lot of problems from an aerodynamic standpoint. Two helicopters were lost due to this design. One had the blade hit the fuselage and partially destroy the wind tunnel at the Ames Research Lab. and the other had a blade hit the cockpit due to divergence and killed the pilot. The problems were so severe that they cancelled the program. This was mentioned posts above and, it was the Cheyenne.
The Robinson has the same problem of rotor blade divergence. In fact there have been 32 cases to my knowledge where the rotors hit the fuselage. What do they teach about divergence and what do they teach about how to correct it?
I have stated this in a previous post, " if I am proven wrong like a good dog I will drop this bone and look for another bone".
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 November 2000).]
On both the Robinson and the Bell the blades are shown in the low pitch or, full down collective. On the Robinson head, the pitch horn attachment to the pitch link is almost coincident with the axis of the flapping hinge. On the Bell the same point is almost coincident with the axis of the teeter hinge.
When the rotor is turning and the blades are in an aerodynamic state , and the pilot pulls in collective the points referenced above are now above the respective axis points. Whenever the pilot introduces cyclic there is in effect a coupling caused by the delta hinge effect. This is common to all helicopters and is called pitch coupling. Because of this coupling the pilot must make a correction in his cyclic stick position. The coupling also tends to restore the individual blade when some external force tries to perturb it from the normal tip path just like the tail rotor.
Regarding your observation of the Robinson rotorhead and the relationship of the pitch horn to the teeter hinge, the pitch horn has to be in this position because if the horn was in line with the teeter hinge the coupling would be so massive as to render the helicopter uncontrollable.
This rotorhead is totally unlike any other rotor head and the rules in your textbooks don't apply. If you read the report I sent to you, you would have seen the reference to the fact that the FAA required special tests in order to certify the design. My question in the report was ," Was this testing done?"
It is my contention that the rotor design and the control rigging procedures are the root cause of the rotor incursion incidents and loss of control incidents.
Go back to your basic aerodynamics and determine if pitch coupling influences phase angle. Then, diagram a Bell rotor system and a Robinson system to see the differences.
Oh yeah, I forgot I sent those diagrams to your email site along with the report.
Talk to your professors and present the problem to them. As I said to you in the email, "don't always believe your text books".
Text books define the problems under ideal conditions and they don't always cover the total picture.
You will find this out when you get your first job. Just like lawyers making new law you will on occasion make new engineering in order to meet design requirements and/or in order to correct a design problem.
Do your text books cover rotor blade design where the blades have a negative twist and a constantly changing camber from the root to the tip or, do they just deal with negative twist and a constant camber. The blade described above was created to overcome a design restriction that limited the rotor span. The design was created by Ray Prouty who is one of the best aerodynamicists. Much to his chagrin, the design was a failure and caused a lot of problems from an aerodynamic standpoint. Two helicopters were lost due to this design. One had the blade hit the fuselage and partially destroy the wind tunnel at the Ames Research Lab. and the other had a blade hit the cockpit due to divergence and killed the pilot. The problems were so severe that they cancelled the program. This was mentioned posts above and, it was the Cheyenne.
The Robinson has the same problem of rotor blade divergence. In fact there have been 32 cases to my knowledge where the rotors hit the fuselage. What do they teach about divergence and what do they teach about how to correct it?
I have stated this in a previous post, " if I am proven wrong like a good dog I will drop this bone and look for another bone".
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 21 November 2000).]
Guest
Posts: n/a
Lu, you are trapped by your own words.
You state that all helicopters have some pitch coupling and you say...
"This is common to all helicopters and is called pitch coupling. Because of this coupling the pilot must make a correction in his cyclic stick position. "
But you already stated that the Robinson pilot is correcting for the unusual phase angle in the head.
So is the pilot now correcting for two problems?
I suggest the pilot does not make any corrections because the phase shift is cancelling the delta coupling effect.
The delta-3 or pitch coupling as you call it
was put in for the same reasons it is used in tail rotors- it limits the flapping angle and gives better rotor clearance at the fuselage.
Now as you say, pitch coupling requires that the pilot correct with cyclic.
I suggest that Robinson decided to put the "correction" in the controls and that is the purpose of the phase shift in the head. The pilot moves the stick normally abecause delta-3 coupling is automatically compensated for with the 18 degree swashplate phase offset from 90 degrees.
You also mention divergence of the rotor.
The Robinsom blade has twist like any other typical hekicopter. It also incorporates pitch-flap coupling like a tail rotor has.
Why should it be divergent? Are tail rotors dvergent? What is the design feature that you think is making this rotor divergent?
And where is the evidence that accidents are happening because of divergence?
By the way, the hinges are called coning hinges in the Pilot's handbook, p. 7-2 You call them flapping hinges and that is incorrect.
The the blades flap as a unit around the teetering pivot. The coning hinges allow for coning of the blades and that's why we don't call them flapping hinges. These are semi- rigid teetering rotors and not fully articulated.
You state that all helicopters have some pitch coupling and you say...
"This is common to all helicopters and is called pitch coupling. Because of this coupling the pilot must make a correction in his cyclic stick position. "
But you already stated that the Robinson pilot is correcting for the unusual phase angle in the head.
So is the pilot now correcting for two problems?
I suggest the pilot does not make any corrections because the phase shift is cancelling the delta coupling effect.
The delta-3 or pitch coupling as you call it
was put in for the same reasons it is used in tail rotors- it limits the flapping angle and gives better rotor clearance at the fuselage.
Now as you say, pitch coupling requires that the pilot correct with cyclic.
I suggest that Robinson decided to put the "correction" in the controls and that is the purpose of the phase shift in the head. The pilot moves the stick normally abecause delta-3 coupling is automatically compensated for with the 18 degree swashplate phase offset from 90 degrees.
You also mention divergence of the rotor.
The Robinsom blade has twist like any other typical hekicopter. It also incorporates pitch-flap coupling like a tail rotor has.
Why should it be divergent? Are tail rotors dvergent? What is the design feature that you think is making this rotor divergent?
And where is the evidence that accidents are happening because of divergence?
By the way, the hinges are called coning hinges in the Pilot's handbook, p. 7-2 You call them flapping hinges and that is incorrect.
The the blades flap as a unit around the teetering pivot. The coning hinges allow for coning of the blades and that's why we don't call them flapping hinges. These are semi- rigid teetering rotors and not fully articulated.
Guest
Posts: n/a
Regarding the virtual Delta 3 hinge: this will have a significant and beneficial documented effect on disk response to pilot input and will effect the Lock number consequently modifying the aerodynamic phase lag. The collective/pitch couple is a red herring and unrelated to the Delta 3 question. The aerodynamic collective/pitch couple is a function of airspeed (advancing Vs retreating blade), it does not occur in a still air hover. Any pitch of the aircraft in the hover attributable to collective inputs will be due to downwash or similar effects.
There cannot be a comparison between the Bell and Robinson heads because the Bell does not have “coning” hinges. In the case of the Bell the two blades are, for all intents and purposes, linked and will provide mutual damping and can be expected to have a natural frequency of 1R. The Robinson is a different design as Lu points out, but the text books still have validity. You must take the virtual Delta 3 component and the relationship between the 3 hinges (teeter and coning x2) into account when calculating your phase lag.
Any modern non pure teetering head is likely to have a phase angle other than 90 degrees. You can rush out with your protractors and pencils and check. I have flown teetering, articulated, semi-rigid and rigid headed helicopters and believe me you can’t tell what the phase angle is until you measure it, and then it will change with density altitude. Even then it is academic; you look out and fly the aircraft, let test pilots decide if it has adequate handling qualities and believe me if they don’t like it you won’t get to fly it.
The only point I would make, and this is purely my opinion, is that during low G manoeuvres the dynamic relationship between the teetering and coning hinges may become confused which is a good enough reason not to enter a low G regime. However without flight testing (which may have already been done) the answer is pure conjecture.
There is a lot of supposition dressed as fact in some of the posts. We have heard a lot from Lu and I eagerly await Mr Robinson’s’ reply, although I do expect “Lock”, “Virtual Delta 3” and “Teeter and coning hinges” to be mentioned!
There cannot be a comparison between the Bell and Robinson heads because the Bell does not have “coning” hinges. In the case of the Bell the two blades are, for all intents and purposes, linked and will provide mutual damping and can be expected to have a natural frequency of 1R. The Robinson is a different design as Lu points out, but the text books still have validity. You must take the virtual Delta 3 component and the relationship between the 3 hinges (teeter and coning x2) into account when calculating your phase lag.
Any modern non pure teetering head is likely to have a phase angle other than 90 degrees. You can rush out with your protractors and pencils and check. I have flown teetering, articulated, semi-rigid and rigid headed helicopters and believe me you can’t tell what the phase angle is until you measure it, and then it will change with density altitude. Even then it is academic; you look out and fly the aircraft, let test pilots decide if it has adequate handling qualities and believe me if they don’t like it you won’t get to fly it.
The only point I would make, and this is purely my opinion, is that during low G manoeuvres the dynamic relationship between the teetering and coning hinges may become confused which is a good enough reason not to enter a low G regime. However without flight testing (which may have already been done) the answer is pure conjecture.
There is a lot of supposition dressed as fact in some of the posts. We have heard a lot from Lu and I eagerly await Mr Robinson’s’ reply, although I do expect “Lock”, “Virtual Delta 3” and “Teeter and coning hinges” to be mentioned!
Guest
Posts: n/a
I have to reply to this, as much as it pains me to add to the cacophony:
Lu wrote:
Regarding stick input during zero G flight let us assume that the pilot has corrected for the 18 degree offset and the cyclic is placed forward and right of the rigged neutral position. As such, the helicopter is flying forward and not to the left. If the helicopter enters the zero G condition with the cyclic in this position the following can happen:
1) If he pulls straight back he will not add to the right roll set up by the tail rotor thrust.
2) If he pulls back and slightly to the left he will add to the right roll component.
3) If he pulls back and further to the left he will compensate for the 18 degree offset and not add to the right roll component
4) If he pulls back and even further to the left he will introduce a left roll component causing high flapping loads and possibly lose his rotor system or cause a rotor incursion.
With the intensity of pucker factor being so high during a zero G condition which direction would you move the cyclic if you were in that situation. Refer to my point 1 above and think about the sense of aircraft movement relative to cyclic stick displacement. Does this sound right? What if these same conditions existed in your automobile?
REGARDING YOUR LAST QUESTION ABOUT CHANGING THE RULES RELATIVE TO RMS Vs. ENGINEERING I WOULD STAND A BETTER CHANCE OF CHANGING THE BIBLE TO RECOGNIZE GOD AS BEING FEMALE.
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Lu, it doesn't matter what you do with the cyclic in a zero-G situation, the cyclic controls nothing but the disc. The right roll of the FUSELAGE is caused by tail-rotor effects, aft cyclic merely reloads the disc by changing the disc attitude relative to the prevailing airflow, once the disc is once-again loaded (even slightly) do what is required to maintain aircraft control.
Left cyclic applied in the mistaken belief that it will correct the right roll is what kills so many pilots, especially low-helo, high-aeroplane time pilots who have not yet learned adequate respect for low G in helicopters.
With reference to earlier comments I didn't copy, rig whatever pitch you want in the blades, it will not affect overpitching one iota, only exceedence of engine power will do that. Pull it in the hover, do a hover check (if you didn't you are't a helo pilot) and you will know whether it is grossly out of rig in collective pitch.
Now Lu, are you telling me that the 60 knot (.6 VNE) full application of pedal is/was a normal procedure? You are most certainly insinuating this when you state that there is "now" a restriction preventing this maneuver.
By the logic you have presented here all helicopters so certified should have a demonstrated crosswind capability at least equal to .6 VNE? I don't recall any.
The R22 was designed to the limits of what could be done in its weight and cost class. It was not designed as a trainer, it was used in this manner because it was economical. Everyone then assumed it was just a rotary C-150. It isn't.
I read earlier that people were stating that it had been taught that it is impossible to recover from an engine failure on climbout. I have had a student mistakenly roll the engine off on me in the climb. I got on the controls, lowered the pole, and recovered quite fine thank-you, despite being in the middle of talking with my hands.
I will be reviewing my old R22 manual tonight to see exactly what it says in section 4, not the snippets I see here.
Please send me this report, I would very much like to read it:
[email protected]
And I have NEVER seen a helicopter that smoothly transitioned to forward flight without lateral cyclic input, rotary wing aerodynamics aren't that simple.
Lu wrote:
Regarding stick input during zero G flight let us assume that the pilot has corrected for the 18 degree offset and the cyclic is placed forward and right of the rigged neutral position. As such, the helicopter is flying forward and not to the left. If the helicopter enters the zero G condition with the cyclic in this position the following can happen:
1) If he pulls straight back he will not add to the right roll set up by the tail rotor thrust.
2) If he pulls back and slightly to the left he will add to the right roll component.
3) If he pulls back and further to the left he will compensate for the 18 degree offset and not add to the right roll component
4) If he pulls back and even further to the left he will introduce a left roll component causing high flapping loads and possibly lose his rotor system or cause a rotor incursion.
With the intensity of pucker factor being so high during a zero G condition which direction would you move the cyclic if you were in that situation. Refer to my point 1 above and think about the sense of aircraft movement relative to cyclic stick displacement. Does this sound right? What if these same conditions existed in your automobile?
REGARDING YOUR LAST QUESTION ABOUT CHANGING THE RULES RELATIVE TO RMS Vs. ENGINEERING I WOULD STAND A BETTER CHANCE OF CHANGING THE BIBLE TO RECOGNIZE GOD AS BEING FEMALE.
-------
Lu, it doesn't matter what you do with the cyclic in a zero-G situation, the cyclic controls nothing but the disc. The right roll of the FUSELAGE is caused by tail-rotor effects, aft cyclic merely reloads the disc by changing the disc attitude relative to the prevailing airflow, once the disc is once-again loaded (even slightly) do what is required to maintain aircraft control.
Left cyclic applied in the mistaken belief that it will correct the right roll is what kills so many pilots, especially low-helo, high-aeroplane time pilots who have not yet learned adequate respect for low G in helicopters.
With reference to earlier comments I didn't copy, rig whatever pitch you want in the blades, it will not affect overpitching one iota, only exceedence of engine power will do that. Pull it in the hover, do a hover check (if you didn't you are't a helo pilot) and you will know whether it is grossly out of rig in collective pitch.
Now Lu, are you telling me that the 60 knot (.6 VNE) full application of pedal is/was a normal procedure? You are most certainly insinuating this when you state that there is "now" a restriction preventing this maneuver.
By the logic you have presented here all helicopters so certified should have a demonstrated crosswind capability at least equal to .6 VNE? I don't recall any.
The R22 was designed to the limits of what could be done in its weight and cost class. It was not designed as a trainer, it was used in this manner because it was economical. Everyone then assumed it was just a rotary C-150. It isn't.
I read earlier that people were stating that it had been taught that it is impossible to recover from an engine failure on climbout. I have had a student mistakenly roll the engine off on me in the climb. I got on the controls, lowered the pole, and recovered quite fine thank-you, despite being in the middle of talking with my hands.
I will be reviewing my old R22 manual tonight to see exactly what it says in section 4, not the snippets I see here.
Please send me this report, I would very much like to read it:
[email protected]
And I have NEVER seen a helicopter that smoothly transitioned to forward flight without lateral cyclic input, rotary wing aerodynamics aren't that simple.