Hanging one side low
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by [email protected]
The right roll in ACs diagram (opposing TR roll) is a result of the arm between the action of the TR thrust and the vertical position of the C of G. All forces act around the C of G whether it be vertical, horizontal or lateral. I believe that was the point Nick Lappos was making 18 years ago.
I am amazed by the the amount of discussion this has generated. When I was teaching this to student pilots in the 70s it was a five minute item at the end of the lesson on the design and function of tail rotors/fenestrons. The summary is a two-sentence paragraph with a diagram very like Ascend Charlie's.
That is not how physics works. The C of G is only relevant as the point through which the weight (gravity) can be considered to act - hence the name.
Force x arm = moment
I've taught the same as AC to student pilots since the 80s but it is a simplistic and easily explainable version of the truth - as Lofty Marshall used to say- P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by [email protected]
Not sure you are right about that - you need a point for a force to work around or opposing forces to create a couple - a pivot point essentially, and that is the C of G.
Force x arm = moment
This is a matter of static equilibrium and pretty straightforward. If things begin to move then it starts to get complicated and inertia brings the CoG into consideration. Then:
P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.
SAS: “Did anyone bring up Transmission mounting that differs from a true vertical to the Airframe?”
The S-64 with 3 degrees left. If SA still owned the design,I’d not be surprised to learn that the ensuing discussions were still ongoing.
The S-64 with 3 degrees left. If SA still owned the design,I’d not be surprised to learn that the ensuing discussions were still ongoing.
Originally Posted by [email protected]
Not sure you are right about that - you need a point for a force to work around or opposing forces to create a couple - a pivot point essentially, and that is the C of G.
Force x arm = moment
I've taught the same as AC to student pilots since the 80s but it is a simplistic and easily explainable version of the truth - as Lofty Marshall used to say- P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.
Force x arm = moment
I've taught the same as AC to student pilots since the 80s but it is a simplistic and easily explainable version of the truth - as Lofty Marshall used to say- P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.
Rotor thrust is acting from the hub, not the CG. When the T/R thrust tilts the fuselage left, the CG is no longer under the hub, and a restoring moment is created. The 2 moments are continually playing games with each other.
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
When introducing levers in elementary science, a physics teacher as I was, will only use moments about a fixed pivot/fulcrum. That is as far as Galileo went.
This makes the calculations easy to understand and completely hides the existence of the couples in equilibrium. If someone's education goes no further they will continue to think in those terms when faced with rotational effects and be forever looking for a pivot. That is not your situation, but it is that of many that have posted on this topic, including Nick Lappos.
A couple is a rotational effect, a vector, and its size is a moment. They are not the same thing once you advance from the simplifications of elementary science.
If you replace 'moment' with 'couple' above all is well, except that 'are continually playing games with each other' is not a decent substitute for 'are in equilibrium' or 'are in balance'.
I'm sorry to be picky, but these things matter to me.
Well, I also have a science degree with a major in physics, but when dealing with helicopter pilots, I reduce things to the minimum to help understanding. They are simple folk, as you might be aware...
However, the degree was 50 years ago, and I had forgotten the subtle differences between moments and couples.
So, to put things right, we have a couple of couples coupling in the back of the helicopter. Better?
However, the degree was 50 years ago, and I had forgotten the subtle differences between moments and couples.
So, to put things right, we have a couple of couples coupling in the back of the helicopter. Better?
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
However, the degree was 50 years ago, and I had forgotten the subtle differences between moments and couples.
So, to put things right, we have a couple of couples coupling in the back of the helicopter. Better?
If someone's education goes no further they will continue to think in those terms when faced with rotational effects and be forever looking for a pivot. That is not your situation, but it is that of many that have posted on this topic, including Nick Lappos.
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
I am certain that it did.
Mighty Gem and Arm out the window provided simple and accurate explanations at #2 ad #9, only for Nick Lappos to confuse things with incorrect statements at #10 and #26. Q max attempted to put things right at #17, #29 and #38, but the seeds of doubt were already sewn.
Mighty Gem and Arm out the window provided simple and accurate explanations at #2 ad #9, only for Nick Lappos to confuse things with incorrect statements at #10 and #26. Q max attempted to put things right at #17, #29 and #38, but the seeds of doubt were already sewn.
Every couple has its moment, as I said above.
Ah,”simple folk”. Hard to place Mr. Lappos in that group. Second in his class at Georgia Tech,but more to the point, over three decades or so at Sikorsky,I’ve been at any number of meetings with Nick and our resident PhD’s and in those discussions dear friends, Mr Lappos won/loss record remains unbeaten. His main problem was that he was often more than a tad ahead of everyone on the subject at hand,so there was some misunderstanding as a result. Count me in that crowd on occasion.
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
Ah,”simple folk”. Hard to place Mr. Lappos in that group. Second in his class at Georgia Tech,but more to the point, over three decades or so at Sikorsky,I’ve been at any number of meetings with Nick and our resident PhD’s and in those discussions dear friends, Mr Lappos won/loss record remains unbeaten. His main problem was that he was often more than a tad ahead of everyone on the subject at hand,so there was some misunderstanding as a result. Count me in that crowd on occasion.
While I have been lurking here, I have read back over a number of threads to which NL has contributed and I have assured myself of his very high credentials. I, too, have been impressed by his lucid explanations and I understand how he came to earn guru status. But gurus can be mistaken. This is just plain wrong:
You are exactly right, the relationship [of the tail rotor] to the CG is what counts. Somehow, the idea that the height above or below the main rotor is important has been surfaced from time to time. It is not correct.
This is just plain wrong:
Don't get me wrong, I'm not equipped for an academic argument with you on physics but I have flown helicopters for 38 years and the height of the TR from the MR really makes no difference to the hover attitude.
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by [email protected]
and your experience of testing and designing helicopters is????
Don't get me wrong, I'm not equipped for an academic argument with you on physics but I have flown helicopters for 38 years and the height of the TR from the MR really makes no difference to the hover attitude.
A high tail rotor cancels some of the needed left roll, a low tail rotor, on the centreline, needs more left roll.
Last edited by HissingSyd; 22nd Mar 2020 at 19:31.
When I said that we helicopter drivers are "simple folk" I wasn't casting nasturtiums at Nick or John. I was referring to the way that things need to be simplified for students to understand them, but the simplification makes the student believe the wrong thing.
Many years ago, Nick put out a list of Helicopter Urban Myths, and among the myths were the misconceptions such as:
"The rotor system IS a gyroscope", rather than "it sometimes behaves LIKE a gyroscope."
"There is an area of high pressure air under the hovering disc, and this makes a cushion of high pressure air that the aircraft sits on". Bong! Wrong!
"Flapping to equality is happening all the time, so the advancing blade is flapping UP to equalise the lift."
O. M. G.......
Many years ago, Nick put out a list of Helicopter Urban Myths, and among the myths were the misconceptions such as:
"The rotor system IS a gyroscope", rather than "it sometimes behaves LIKE a gyroscope."
"There is an area of high pressure air under the hovering disc, and this makes a cushion of high pressure air that the aircraft sits on". Bong! Wrong!
"Flapping to equality is happening all the time, so the advancing blade is flapping UP to equalise the lift."
O. M. G.......
I am certain that it did.
Mighty Gem and Arm out the window provided simple and accurate explanations at #2 ad #9, only for Nick Lappos to confuse things with incorrect statements at #10 and #26. Q max attempted to put things right at #17, #29 and #38, but the seeds of doubt were already sewn.
Mighty Gem and Arm out the window provided simple and accurate explanations at #2 ad #9, only for Nick Lappos to confuse things with incorrect statements at #10 and #26. Q max attempted to put things right at #17, #29 and #38, but the seeds of doubt were already sewn.
OK Syd, a genuine question for my education:
We have been discussing couples that create rotation of a body - how do you identify the axis of that rotation? Is it along the line between (in this case as in ACs diagram) MR head and TR and, if so, where along that line?
My point about height of TR having little effect is that I have flown helicopters with both high and low TR (of both clockwise and anti-clockwise rotation and differing head designs) yet the roll angle in the hover is always between 2 and 4 degrees with that upper limit being on a semi rigid MRH with a large effective hinge offset.
We have been discussing couples that create rotation of a body - how do you identify the axis of that rotation? Is it along the line between (in this case as in ACs diagram) MR head and TR and, if so, where along that line?
My point about height of TR having little effect is that I have flown helicopters with both high and low TR (of both clockwise and anti-clockwise rotation and differing head designs) yet the roll angle in the hover is always between 2 and 4 degrees with that upper limit being on a semi rigid MRH with a large effective hinge offset.
Interesting question, Herr Krebs.
The left-rotation couple has the rotor hub at one end, and the tail rotor at the other - well above the CG. No idea where the axis would be.
The right-rotation couple has the rotor hub at one end, and the CG at the other end. I don't think the axis could be at the extremity, at the CG.
But does it really matter, other than giving you something to think about, and divert your mind from the lack of Lou Rawls and the closure of Dan Murphy's?
The left-rotation couple has the rotor hub at one end, and the tail rotor at the other - well above the CG. No idea where the axis would be.
The right-rotation couple has the rotor hub at one end, and the CG at the other end. I don't think the axis could be at the extremity, at the CG.
But does it really matter, other than giving you something to think about, and divert your mind from the lack of Lou Rawls and the closure of Dan Murphy's?