Aerodynamics ~ Phase Lag
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heedm
>Physics text books will say don't use centrifugal force because it's not a real force. They're totally right. There is no mechanism for the force, no work done, no energy expended, it is only felt by someone in a non-inertial reference frame.
it doesnt matter what we call centrifugal force, we could call it kelly, but how can it no be a force? is it included in coriolis affect, its simmiler?
if i had a car axle and two retractable measuring tapes and tyed the tapes to the top of the axle (oposite) and put a weight on the end of each tape then spun the axle. the wheight would overcome the spring extending the tape and oposing the accelleration. then as i let it slow down the springs would pull the weights in, oposing the slowing process.
this is centrifugal force in action i think!
dave
yeh, you might be right on the 0deg phase angle, dont think itll ever get there though.
>Physics text books will say don't use centrifugal force because it's not a real force. They're totally right. There is no mechanism for the force, no work done, no energy expended, it is only felt by someone in a non-inertial reference frame.
it doesnt matter what we call centrifugal force, we could call it kelly, but how can it no be a force? is it included in coriolis affect, its simmiler?
if i had a car axle and two retractable measuring tapes and tyed the tapes to the top of the axle (oposite) and put a weight on the end of each tape then spun the axle. the wheight would overcome the spring extending the tape and oposing the accelleration. then as i let it slow down the springs would pull the weights in, oposing the slowing process.
this is centrifugal force in action i think!
dave
yeh, you might be right on the 0deg phase angle, dont think itll ever get there though.
Iconoclast
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Re Centrifugal force:
If centrifugal force is a non-force and it does no work in the rotation of the blades the helicopter would never get off the ground. The centrifugal force balances out the lifting forces on the blades and as such establishes the cone angle. The cone angle can also be effected by gross weight and air density. Without centrifugal force acting ion the blades the blades would achieve a very high cone angle and the helicopter would be sitting on the ground with full collective applied. If in flight centrifugal force could be removed from the equation the blades would cone up and the helicopter would crash. This can also be manifested in low rotor RPM that equates to lower centrifugal force.
Let’s say we remove one blade in flight and in doing so we remove its’ contribution of centrifugal force. The imbalance would be immediate and the rotor system and transmission would immediately depart the airframe.
It’s is very nice to refer to engineering texts as a frame of reference but you have to understand that the author is making his own point and then he is creating the mathematics to describe the condition. However this situation would never be encountered in real life. While in school it gives you the ability to envision various situations but once you leave school you should close your text books and do it the way your employer tells you to do it.
If centrifugal force is a non-force and it does no work in the rotation of the blades the helicopter would never get off the ground. The centrifugal force balances out the lifting forces on the blades and as such establishes the cone angle. The cone angle can also be effected by gross weight and air density. Without centrifugal force acting ion the blades the blades would achieve a very high cone angle and the helicopter would be sitting on the ground with full collective applied. If in flight centrifugal force could be removed from the equation the blades would cone up and the helicopter would crash. This can also be manifested in low rotor RPM that equates to lower centrifugal force.
Let’s say we remove one blade in flight and in doing so we remove its’ contribution of centrifugal force. The imbalance would be immediate and the rotor system and transmission would immediately depart the airframe.
It’s is very nice to refer to engineering texts as a frame of reference but you have to understand that the author is making his own point and then he is creating the mathematics to describe the condition. However this situation would never be encountered in real life. While in school it gives you the ability to envision various situations but once you leave school you should close your text books and do it the way your employer tells you to do it.
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Flapping. Part 1. Natural or Resonant Frequency.
Remember pendulums in school physics? Take a piece of string with a weight on the end, swing it, time 20 swings then work out the period of oscillation. Lengthen the piece of string and repeat the exercise. The result is a longer period, or to put it another way a lower natural frequency. Shorten the piece of string the natural frequency increases.
Now imagine a rotor system. It rotates at a given RPM, which we could equally describe in terms of a frequency. The rotor systems’ natural frequency is therefore equal to the Nr. If you then hinge a blade at axis of rotation (teetering head) it is free to flap in resonance with the rotation and because it is hinged at the point of rotation it will have the same natural frequency (the distance from hub to Centre of Inertia is the same as the distance from the hinge to the Centre of Inertia). Even if the system is damped, by air density, by dampers, springs or whatever the system will remain in resonance.
Now hinge the blade out board of the hub, the natural frequency of the blade will change – just like our pendulum the shorter distance between the point of flapping and the Centre of Inertia will increase the natural frequency of the blade. BUT the NR has stayed the same so the two are no longer in resonance. In aerodynamic speak (and control theory for that matter) one now has reduced the frequency ratio (excitation frequency [NR] / undamped natural frequency). This means if the blade is disturbed instead of returning to its’ original flapping position 360 degrees later it will return less than 360 degrees later. This is without aerodynamic effects it is pure inertia (the same stuff that drive gyros).
Who ever heard of a gyro with flapping hinges in it? No one, because to be a true gyro the mass muss be rigid, if it can flap out of resonance it will react differently and out of phase and won’t be a gyro.
Coming soon – Phase angle as a function of damping and frequency ratio. (Sorry run out of time for now)
Remember pendulums in school physics? Take a piece of string with a weight on the end, swing it, time 20 swings then work out the period of oscillation. Lengthen the piece of string and repeat the exercise. The result is a longer period, or to put it another way a lower natural frequency. Shorten the piece of string the natural frequency increases.
Now imagine a rotor system. It rotates at a given RPM, which we could equally describe in terms of a frequency. The rotor systems’ natural frequency is therefore equal to the Nr. If you then hinge a blade at axis of rotation (teetering head) it is free to flap in resonance with the rotation and because it is hinged at the point of rotation it will have the same natural frequency (the distance from hub to Centre of Inertia is the same as the distance from the hinge to the Centre of Inertia). Even if the system is damped, by air density, by dampers, springs or whatever the system will remain in resonance.
Now hinge the blade out board of the hub, the natural frequency of the blade will change – just like our pendulum the shorter distance between the point of flapping and the Centre of Inertia will increase the natural frequency of the blade. BUT the NR has stayed the same so the two are no longer in resonance. In aerodynamic speak (and control theory for that matter) one now has reduced the frequency ratio (excitation frequency [NR] / undamped natural frequency). This means if the blade is disturbed instead of returning to its’ original flapping position 360 degrees later it will return less than 360 degrees later. This is without aerodynamic effects it is pure inertia (the same stuff that drive gyros).
Who ever heard of a gyro with flapping hinges in it? No one, because to be a true gyro the mass muss be rigid, if it can flap out of resonance it will react differently and out of phase and won’t be a gyro.
Coming soon – Phase angle as a function of damping and frequency ratio. (Sorry run out of time for now)
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To: Grey Area
“Who ever heard of a gyro with flapping hinges in it? No one, because to be a true gyro the mass muss be rigid, if it can flap out of resonance it will react differently and out of phase and won’t be a gyro”.
While working at Boeing I came across a pictorial text devised to teach gyroscopic principles. It had all kinds of gyros illustrated and showed the application of each type such as those used in gyro stabilized instruments. It also had a gyroscope that was made up of a hub with attaching points for multiple arms. Each arm had a weight at one end and at the other a hinge pin to attach to the hub. If the hub were not turning, gravity would cause the weights and arms to hang from the hinge point. When the hub started to rotate centrifugal force would cause the arms to extend until they were in the same plane as the hub. The text explained that if a perturbing force were applied such as moving the rotational axis from the vertical the entire group of arms and weights would respond as if they were a solid rotor and precess at 90-degrees after the application of the force. In this case the gyro had flapping hinges.
“Who ever heard of a gyro with flapping hinges in it? No one, because to be a true gyro the mass muss be rigid, if it can flap out of resonance it will react differently and out of phase and won’t be a gyro”.
While working at Boeing I came across a pictorial text devised to teach gyroscopic principles. It had all kinds of gyros illustrated and showed the application of each type such as those used in gyro stabilized instruments. It also had a gyroscope that was made up of a hub with attaching points for multiple arms. Each arm had a weight at one end and at the other a hinge pin to attach to the hub. If the hub were not turning, gravity would cause the weights and arms to hang from the hinge point. When the hub started to rotate centrifugal force would cause the arms to extend until they were in the same plane as the hub. The text explained that if a perturbing force were applied such as moving the rotational axis from the vertical the entire group of arms and weights would respond as if they were a solid rotor and precess at 90-degrees after the application of the force. In this case the gyro had flapping hinges.
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vorticey;
"yeh, you might be right on the 0deg phase angle, dont think itll ever get there though."<
I agree with you. In the case of a (hypothetical) absolutely rigid rotor, I believe that;
In all other rotors the 'aerodynamic precession' component will be greater than 0-degrees, with gyrocopters and early Bell helicopters having one of 90-degrees.
This is an overview and is subject to 'fine tuning' by many details.
" also the work being done would be the tentioning of the blades so they dont break of when pitch is applied "<
Agreed, The rotors of the Sikorsky ABC were not even absolutely rigid, and it has been said that the spindle bearings were a concern.
===============
Languages are used to communicate information between forms of life. Mathematics is a very precise language. It would be a little difficult to describe the helicopter's rotor by body language.
[ 27 November 2001: Message edited by: Dave Jackson ]
"yeh, you might be right on the 0deg phase angle, dont think itll ever get there though."<
I agree with you. In the case of a (hypothetical) absolutely rigid rotor, I believe that;
- the 'aerodynamic precession' will be exactly 0-degrees,
- the 'gyroscopic precession' will be exactly 90-degrees,
- and the resultant of the combined precessions will be around 10-degrees.
In all other rotors the 'aerodynamic precession' component will be greater than 0-degrees, with gyrocopters and early Bell helicopters having one of 90-degrees.
This is an overview and is subject to 'fine tuning' by many details.
" also the work being done would be the tentioning of the blades so they dont break of when pitch is applied "<
Agreed, The rotors of the Sikorsky ABC were not even absolutely rigid, and it has been said that the spindle bearings were a concern.
===============
Languages are used to communicate information between forms of life. Mathematics is a very precise language. It would be a little difficult to describe the helicopter's rotor by body language.
[ 27 November 2001: Message edited by: Dave Jackson ]
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Vorticey and Lu,
Reread what I said. Text books will tell you that centrifugal force is an apparent, not a real, force. I acknowledged that while it may be apparent, we have all felt it, and it does make some physics easier. It's a good tool to use, but is not a real force.
"A body in motion tends to stay in motion, in a straight line..." you know who wrote that. The blade is a body. It doesn't want to spin around, it wants to fly off tangentially in a straight line. The hub pulls on the blade (mechanical force) accelerates it so the blade's velocity vector changes in direction. This is what really happens. There is no magical hand that tries to pull the blade away from the helicopter.
Same thing happens with your tape measures, vorticey. They just want to fly off in a straight line tangential to the axle, but eventually the strength of the spring will stop them from flying out and then it will pull them into orbit around the axle.
Coriolis is similiar.
Coning is easily explained without referring to centrifugal force, but the forces are not balanced, something which bothers some. They must be unbalanced because there is a net force that is accelerating (turning) the blade. When you draw a picture, it is often easier to explain it using a centrifugal term and ignore the blades motion rather than showing a force imbalance.
_____________
Grey Area, what you have explained is great. That is the same theory that I've been using to explain gyroscopic precession, aerodynamic precession, and "flying the blades to position". My point (and others) has always been that those theories are all just another way of explaining the same thing. Unfortunately, pendulum-like motion and flapping frequencies are not normally taught to pilots, so some of us keep trying to explain what has been taught.
___________________
"gyro with a flapping hinge"
By definition, it's not a gyro. It does have some properties of a gyro, one of them is that apparent lag. In this case the lag is not going to be 90 degrees. It will be less, actually value is easily calculated. It won't even look like a lag, since the oscillations of the parts outboard of the hinge would be at a higher frequency than the whole system.
A helicopter with offset flapping hinges has a driving force on the blades that is mechanically in phase with the rotor, so you don't see the higher frequency oscillations, instead you see a phase angle of less than 90 degrees (all else being equal).
Matthew.
[email protected]
Reread what I said. Text books will tell you that centrifugal force is an apparent, not a real, force. I acknowledged that while it may be apparent, we have all felt it, and it does make some physics easier. It's a good tool to use, but is not a real force.
"A body in motion tends to stay in motion, in a straight line..." you know who wrote that. The blade is a body. It doesn't want to spin around, it wants to fly off tangentially in a straight line. The hub pulls on the blade (mechanical force) accelerates it so the blade's velocity vector changes in direction. This is what really happens. There is no magical hand that tries to pull the blade away from the helicopter.
Same thing happens with your tape measures, vorticey. They just want to fly off in a straight line tangential to the axle, but eventually the strength of the spring will stop them from flying out and then it will pull them into orbit around the axle.
Coriolis is similiar.
Coning is easily explained without referring to centrifugal force, but the forces are not balanced, something which bothers some. They must be unbalanced because there is a net force that is accelerating (turning) the blade. When you draw a picture, it is often easier to explain it using a centrifugal term and ignore the blades motion rather than showing a force imbalance.
_____________
Grey Area, what you have explained is great. That is the same theory that I've been using to explain gyroscopic precession, aerodynamic precession, and "flying the blades to position". My point (and others) has always been that those theories are all just another way of explaining the same thing. Unfortunately, pendulum-like motion and flapping frequencies are not normally taught to pilots, so some of us keep trying to explain what has been taught.
___________________
"gyro with a flapping hinge"
By definition, it's not a gyro. It does have some properties of a gyro, one of them is that apparent lag. In this case the lag is not going to be 90 degrees. It will be less, actually value is easily calculated. It won't even look like a lag, since the oscillations of the parts outboard of the hinge would be at a higher frequency than the whole system.
A helicopter with offset flapping hinges has a driving force on the blades that is mechanically in phase with the rotor, so you don't see the higher frequency oscillations, instead you see a phase angle of less than 90 degrees (all else being equal).
Matthew.
[email protected]
Iconoclast
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To: heedm
“There is no magical hand that tries to pull the blade away from the helicopter”.
How can you discount the presence of centrifugal force on a helicopter rotor system? If there were no centrifugal force involved the designers of the rotorhead would not have to build them to take these applied loads. They could only build them strong enough plus a safety factor to accelerate the blades to speed. This would save hundreds of pounds and grossly reduce the cost of the helicopter. When I worked at Sikorsky they were placing the CH-37 Helicopter into service. One of the tests performed involved the installation of strain gages to measure applied and reactive loads. The tensile load on the spindle was 72,000 pounds (approximately). That was for one blade.
If there were no centrifugal forces acting on the blades and reacted by the rotorhead the helicopter would never get off the ground. As I stated previously the centrifugal force balances out the lifting force on the blade and the resultant vector is the cone angle for the rotor system.
As an aside, I once sat in on a training class on the CH-37 at Ft. Eustis, VA and the instructor was asked what would happen if a blade were thrown during flight. His response was that the rotor system being fully articulated the other blades would move to take up the space occupied by the thrown blade. Each blade is balanced to Ľ inch ounce and the rotorhead is balanced to a level close to that. The entire assembly has to be in balance or the centrifugal loading would cause the rotor system to oscillate. A small imbalance would produce small oscillations and a larger imbalance would cause more sever oscillations. The loss of a blade producing 72,000 pounds of centrifugal force would result in total and immediate destruction of the helicopter.
Ask anybody that had the experience of having water collect in a tip cap and then it freezes overnight.
[ 28 November 2001: Message edited by: Lu Zuckerman ]
“There is no magical hand that tries to pull the blade away from the helicopter”.
How can you discount the presence of centrifugal force on a helicopter rotor system? If there were no centrifugal force involved the designers of the rotorhead would not have to build them to take these applied loads. They could only build them strong enough plus a safety factor to accelerate the blades to speed. This would save hundreds of pounds and grossly reduce the cost of the helicopter. When I worked at Sikorsky they were placing the CH-37 Helicopter into service. One of the tests performed involved the installation of strain gages to measure applied and reactive loads. The tensile load on the spindle was 72,000 pounds (approximately). That was for one blade.
If there were no centrifugal forces acting on the blades and reacted by the rotorhead the helicopter would never get off the ground. As I stated previously the centrifugal force balances out the lifting force on the blade and the resultant vector is the cone angle for the rotor system.
As an aside, I once sat in on a training class on the CH-37 at Ft. Eustis, VA and the instructor was asked what would happen if a blade were thrown during flight. His response was that the rotor system being fully articulated the other blades would move to take up the space occupied by the thrown blade. Each blade is balanced to Ľ inch ounce and the rotorhead is balanced to a level close to that. The entire assembly has to be in balance or the centrifugal loading would cause the rotor system to oscillate. A small imbalance would produce small oscillations and a larger imbalance would cause more sever oscillations. The loss of a blade producing 72,000 pounds of centrifugal force would result in total and immediate destruction of the helicopter.
Ask anybody that had the experience of having water collect in a tip cap and then it freezes overnight.
[ 28 November 2001: Message edited by: Lu Zuckerman ]
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Lu,
The load on the blades is not a centrifugal force, it is centripetal. The blades don't try to stay at the rotor hub and get pulled away by a force, they want to fly tangentially from the head in a straight line. The loads at the hub are what are required to keep the blades from flying off. They pull in towards the center, thus are centripetal forces.
You're right that you can draw a picture with a force acting away from the hub balancing the other forces that demonstrates {how coning works}, but as I stated, that is only a way of explaining it...it doesn't prove that any such force exists.
The proper drawing is complete with a minimum of a vertical lift component, an equal and opposing weight, and a horizontal component of the lift that "supplies the required centripetal acceleration". The forces are out of balance because the system is not in equilibrium, the rotating blades are always being accelerated.
Don't think I fault anyone for using centrifugal force, I use it myself. That doesn't mean it's a real force...just a tool.
I agree with what you said about losing a blade. One of our aircraft ran into a crane about 10 miles offshore (fortunately), lost about 1m from each of the forward rotor blades. Even though they all lost about the same amount the imbalance was still enough to make the helicopter just barely flyable. They made it to the beach and shut down.
[ 28 November 2001: Message edited by: heedm ]
The load on the blades is not a centrifugal force, it is centripetal. The blades don't try to stay at the rotor hub and get pulled away by a force, they want to fly tangentially from the head in a straight line. The loads at the hub are what are required to keep the blades from flying off. They pull in towards the center, thus are centripetal forces.
You're right that you can draw a picture with a force acting away from the hub balancing the other forces that demonstrates {how coning works}, but as I stated, that is only a way of explaining it...it doesn't prove that any such force exists.
The proper drawing is complete with a minimum of a vertical lift component, an equal and opposing weight, and a horizontal component of the lift that "supplies the required centripetal acceleration". The forces are out of balance because the system is not in equilibrium, the rotating blades are always being accelerated.
Don't think I fault anyone for using centrifugal force, I use it myself. That doesn't mean it's a real force...just a tool.
I agree with what you said about losing a blade. One of our aircraft ran into a crane about 10 miles offshore (fortunately), lost about 1m from each of the forward rotor blades. Even though they all lost about the same amount the imbalance was still enough to make the helicopter just barely flyable. They made it to the beach and shut down.
[ 28 November 2001: Message edited by: heedm ]
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Actually, every action has an equal and opposite reaction.
If you sit on a sled on some ice and throw a brick, the action is the brick moving. The reaction is you moving. The only force involved is the one your muscles created. That force is directed from your hand towards the brick. It is not directed from the brick towards your hand.
On anything that spins, the action is pulling inward to prevent outboard masses from flying off tangentially. The reaction is an equal and opposite pull on the hub. This is balanced on spinning things whenever possible. The force is the mechanical force the hub exerts (tension) on the masses.
Compare these two definitions from Encarta:
Encarta's Definition of Centripetal Force
Encarta's Definition of Centrifugal Force
Check out Encarta's Encyclopedic entry on centripetal force.
Encarta Encyclopedia: Centripetal Force
Here's one I don't fully agree with because it confuses a few terms, but it still supports what I'm saying.
Encyclopedia.com: Centripetal Force
You may not trust those sources, but surely you'll trust PBS.
PBS.org: Centripetal Force
There are many more reliable sources, if this isn't enough. I haven't yet found a reliable source that contradicts.
Matthew.
[ 29 November 2001: Message edited by: heedm ]
If you sit on a sled on some ice and throw a brick, the action is the brick moving. The reaction is you moving. The only force involved is the one your muscles created. That force is directed from your hand towards the brick. It is not directed from the brick towards your hand.
On anything that spins, the action is pulling inward to prevent outboard masses from flying off tangentially. The reaction is an equal and opposite pull on the hub. This is balanced on spinning things whenever possible. The force is the mechanical force the hub exerts (tension) on the masses.
Compare these two definitions from Encarta:
Encarta's Definition of Centripetal Force
Encarta's Definition of Centrifugal Force
Check out Encarta's Encyclopedic entry on centripetal force.
Encarta Encyclopedia: Centripetal Force
Here's one I don't fully agree with because it confuses a few terms, but it still supports what I'm saying.
Encyclopedia.com: Centripetal Force
You may not trust those sources, but surely you'll trust PBS.
PBS.org: Centripetal Force
There are many more reliable sources, if this isn't enough. I haven't yet found a reliable source that contradicts.
Matthew.
[ 29 November 2001: Message edited by: heedm ]
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To: heedm
1) Centripetal force (plural centripetal forces) noun
force pulling towards centre: a force that pulls a rotating or spinning object towards a centre or axis
2) Centrifugal force (plural centrifugal forces) noun
force pulling away from centre: an apparent force that seems to pull a rotating or spinning object away from a centre. (Seems?)
3) Centrifugal, from the Latin Centrum (center) and fugere which means "fleeing" literally means moving away from the center and is considered by SCIENTISTS to be a fictional force. It causes no acceleration.
When traveling in a circle, it seems as if we are slung out away from the center. This is not the case. There is no force directed away from the center of the circle and therefore there is acceleration (change speed or direction) in that way.
Hopefully I don’t offend anybody but if you believe items 1 and 3 you have swallowed a lot of pap put out by SCIENTISTS. They can generate all types of formulae to prove their point but if you look at all of the illustrations provided to include the definitions above.
Centripetal force is not a force at all. It would not exist if there were no centrifugal force. Regarding the movement of the ball when it the string breaks all we have to do is look at David when he slew Goliath. His slingshot was rotated in a circle and his hand restrained any outward movement of the rock in the slingshot. It was the mass of the rock that created the centrifugal force and when the slingshot was let go the rock went out in a straight line and not at a tangent. They say that centrifugal force is a fictitious force and should not be considered because it does no work. In those devices that are constructed to employ centrifugal force the release of the spinning object projects it outward and that is the work.
On a rotor system the designers calculate the strength requirements for the rotorhead by calculating the centrifugal loading caused by the spinning blade. They incorporate a safety factor and build the rotorhead and then they test it to see if it meets the requirements. It is the resistance to the centrifugal force that is considered to be centripetal force and the only work it is doing is to keep the blade from flying off. In fact it is not a force at all. It is resisting the real force
1) Centripetal force (plural centripetal forces) noun
force pulling towards centre: a force that pulls a rotating or spinning object towards a centre or axis
In the example above the rotorhead is restraining the blade but it is not pulling the blade towards the center of rotation. The centripetal loading on the head if that is the correct term is the result of the centrifugal loading caused by the mass of the blade.
Once again I must caution that anybody that resorts to college texts to prove a point will have a hard time in any industry having learned about centrifugal and centripetal forces by swinging a ball in a circle. If you get in to the helicopter industry you will have to change your way of thinking. When you get into an engineering slot you will use that companies design handbook as your bible and you can throw your textbooks away.
Here are two examples:
Sikorsky Bluebook says “It can be seen that because centrifugal loading is the predominant force acting upon the blade, it will assume a position closer to horizontal than to vertical. The vertical force (lift) is considerably less than the horizontal force (centrifugal)”. It goes on to provide an example of the lift and centrifugal forces balancing forming the cone angle. It also addresses the centrifugal loading of one blade as being 50,000 pounds.
FAA Rotorcraft Flying Handbook says, “ As vertical takeoff is made, two major forces are acting at the same time. Centrifugal force acting outward and perpendicular to the rotor mast, and lift acting upward and parallel to the mast. The result of these two forces is that the blades assume a conical path instead of remaining in the plane perpendicular to the mast.
Every helicopter company I ever worked for always addressed centrifugal forces and the engineers calculated how strong the rotorhead had to be to react those loads. Maybe some wienie in the dynamics department dealt with centripital forces but he most likely was isolated from the real engineers.
[ 29 November 2001: Message edited by: Lu Zuckerman ]
1) Centripetal force (plural centripetal forces) noun
force pulling towards centre: a force that pulls a rotating or spinning object towards a centre or axis
2) Centrifugal force (plural centrifugal forces) noun
force pulling away from centre: an apparent force that seems to pull a rotating or spinning object away from a centre. (Seems?)
3) Centrifugal, from the Latin Centrum (center) and fugere which means "fleeing" literally means moving away from the center and is considered by SCIENTISTS to be a fictional force. It causes no acceleration.
When traveling in a circle, it seems as if we are slung out away from the center. This is not the case. There is no force directed away from the center of the circle and therefore there is acceleration (change speed or direction) in that way.
Hopefully I don’t offend anybody but if you believe items 1 and 3 you have swallowed a lot of pap put out by SCIENTISTS. They can generate all types of formulae to prove their point but if you look at all of the illustrations provided to include the definitions above.
Centripetal force is not a force at all. It would not exist if there were no centrifugal force. Regarding the movement of the ball when it the string breaks all we have to do is look at David when he slew Goliath. His slingshot was rotated in a circle and his hand restrained any outward movement of the rock in the slingshot. It was the mass of the rock that created the centrifugal force and when the slingshot was let go the rock went out in a straight line and not at a tangent. They say that centrifugal force is a fictitious force and should not be considered because it does no work. In those devices that are constructed to employ centrifugal force the release of the spinning object projects it outward and that is the work.
On a rotor system the designers calculate the strength requirements for the rotorhead by calculating the centrifugal loading caused by the spinning blade. They incorporate a safety factor and build the rotorhead and then they test it to see if it meets the requirements. It is the resistance to the centrifugal force that is considered to be centripetal force and the only work it is doing is to keep the blade from flying off. In fact it is not a force at all. It is resisting the real force
1) Centripetal force (plural centripetal forces) noun
force pulling towards centre: a force that pulls a rotating or spinning object towards a centre or axis
In the example above the rotorhead is restraining the blade but it is not pulling the blade towards the center of rotation. The centripetal loading on the head if that is the correct term is the result of the centrifugal loading caused by the mass of the blade.
Once again I must caution that anybody that resorts to college texts to prove a point will have a hard time in any industry having learned about centrifugal and centripetal forces by swinging a ball in a circle. If you get in to the helicopter industry you will have to change your way of thinking. When you get into an engineering slot you will use that companies design handbook as your bible and you can throw your textbooks away.
Here are two examples:
Sikorsky Bluebook says “It can be seen that because centrifugal loading is the predominant force acting upon the blade, it will assume a position closer to horizontal than to vertical. The vertical force (lift) is considerably less than the horizontal force (centrifugal)”. It goes on to provide an example of the lift and centrifugal forces balancing forming the cone angle. It also addresses the centrifugal loading of one blade as being 50,000 pounds.
FAA Rotorcraft Flying Handbook says, “ As vertical takeoff is made, two major forces are acting at the same time. Centrifugal force acting outward and perpendicular to the rotor mast, and lift acting upward and parallel to the mast. The result of these two forces is that the blades assume a conical path instead of remaining in the plane perpendicular to the mast.
Every helicopter company I ever worked for always addressed centrifugal forces and the engineers calculated how strong the rotorhead had to be to react those loads. Maybe some wienie in the dynamics department dealt with centripital forces but he most likely was isolated from the real engineers.
[ 29 November 2001: Message edited by: Lu Zuckerman ]
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To: heedm
"Pretty easy to tell when Lu realizes (not admits) that he's wrong".
Where is it that I realized I was wrong?
"Pretty easy to tell when Lu realizes (not admits) that he's wrong".
Where is it that I realized I was wrong?
To: Lu,
I can scarcely believe that you think all physicists have got it wrong, but you said it! Are you sure? Perhaps this is another example of the fact that some instructional books simplify discussions so that intellectual “weenies” can understand enough to do their jobs without having to get deeper (aka gyroscopic precession). Perhaps a formal qualification course may have cleared this up for you. Although perhaps Nick is wrong – you are no crackpot, the world’s scientists however, must be!
Lets look at your factual story of David and Goliath:
You said (and we will use numbering for specifics here):
1. Lu said: >>Centripetal force is not a force at all. It would not exist if there were no centrifugal force.<<
My response: I love a good double negative as a proof. You are saying that if there was no centrifugal force there would be no centripetal force. Yet you are arguing that there is a centrifugal force. Therefore, using YOUR OWN logic, there must be a centripetal force. Accordingly you do not agree with your own opening sentence that centripetal is not a force! Wonderful stuff.
2. Lu said: >> His slingshot was rotated in a circle and his hand restrained any outward movement of the rock in the slingshot<<
My Response: No Lu, his hand created the torque to propel the rock. As an acceleration has a direction, there must be a force constantly changing the direction of the rock (because it is going around an arc). You have correctly identified that there is such a force, but it is the sling restraining the rock from flying out. Scientists (though they may all be crackpots) term the force exhibited by the sling as centripetal force, in other words: a force acting toward the center to that continues to change the direction of the rock.
3. Lu said: >> when the slingshot was let go the rock went out in a straight line and not at a tangent<<
My Response: A straight line from a circle IS a tangent. But if you mean it flew at off at right angles to the circle, you need to do a vector analysis. In order for it to fly off at right angles, you must have instantly stopped all momentum about the circle at the precise moment of release, leaving only the force exhibited by the sling which now removed allows the ball to fly outwards. I can hear you nodding Lu. Here is the clincher though: the outward momentum as you call it, is entirely generated by the directional change of the rock about the circle, therefore, the instant you stop the momentum around the circle at release point, you would also stop any momentum outwards, and the rock would not fly off at right angles, or any other direction. I cannot wait for you vector analysis here.
4. Lu said: >>Once again I must caution that anybody that resorts to college texts to prove a point will have a hard time in any industry <<
My Response: Perhaps then, it would be better if we all stopped going to college and began reading simplified texts. Perhaps we should have stayed believing in the fact that the earth was flat because it was simpler, and allowed us to do our job. Or maybe they should never college qualify engineers because they will have a hard time in industry. I am sure we would still have all the incredible surgical machinery, all the amazing laboratories that allow the manufacture of life saving drugs, etc, if we had not sent any of them to college. What amazing thing have you brought to the helicopter world thus far Lu?
5. Lu said: >>When you get into an engineering slot you will use that companies design handbook as your bible and you can throw your textbooks away<<
My response: To me, this is the crux of the problem. Company design books are designed such that they are the most easily understood text for all levels of qualified and non qualified personnel to follow so that there is NO room for misinterpreting procedures which may cause accidents, etc. They are not college text books aimed at helping design of the aircraft, or systems. Get it?
Same point being made over and over and over and over………
Dont mean to be so harsh Lu, but calling all scientists "weenies" is very offensive.
I can scarcely believe that you think all physicists have got it wrong, but you said it! Are you sure? Perhaps this is another example of the fact that some instructional books simplify discussions so that intellectual “weenies” can understand enough to do their jobs without having to get deeper (aka gyroscopic precession). Perhaps a formal qualification course may have cleared this up for you. Although perhaps Nick is wrong – you are no crackpot, the world’s scientists however, must be!
Lets look at your factual story of David and Goliath:
You said (and we will use numbering for specifics here):
1. Lu said: >>Centripetal force is not a force at all. It would not exist if there were no centrifugal force.<<
My response: I love a good double negative as a proof. You are saying that if there was no centrifugal force there would be no centripetal force. Yet you are arguing that there is a centrifugal force. Therefore, using YOUR OWN logic, there must be a centripetal force. Accordingly you do not agree with your own opening sentence that centripetal is not a force! Wonderful stuff.
2. Lu said: >> His slingshot was rotated in a circle and his hand restrained any outward movement of the rock in the slingshot<<
My Response: No Lu, his hand created the torque to propel the rock. As an acceleration has a direction, there must be a force constantly changing the direction of the rock (because it is going around an arc). You have correctly identified that there is such a force, but it is the sling restraining the rock from flying out. Scientists (though they may all be crackpots) term the force exhibited by the sling as centripetal force, in other words: a force acting toward the center to that continues to change the direction of the rock.
3. Lu said: >> when the slingshot was let go the rock went out in a straight line and not at a tangent<<
My Response: A straight line from a circle IS a tangent. But if you mean it flew at off at right angles to the circle, you need to do a vector analysis. In order for it to fly off at right angles, you must have instantly stopped all momentum about the circle at the precise moment of release, leaving only the force exhibited by the sling which now removed allows the ball to fly outwards. I can hear you nodding Lu. Here is the clincher though: the outward momentum as you call it, is entirely generated by the directional change of the rock about the circle, therefore, the instant you stop the momentum around the circle at release point, you would also stop any momentum outwards, and the rock would not fly off at right angles, or any other direction. I cannot wait for you vector analysis here.
4. Lu said: >>Once again I must caution that anybody that resorts to college texts to prove a point will have a hard time in any industry <<
My Response: Perhaps then, it would be better if we all stopped going to college and began reading simplified texts. Perhaps we should have stayed believing in the fact that the earth was flat because it was simpler, and allowed us to do our job. Or maybe they should never college qualify engineers because they will have a hard time in industry. I am sure we would still have all the incredible surgical machinery, all the amazing laboratories that allow the manufacture of life saving drugs, etc, if we had not sent any of them to college. What amazing thing have you brought to the helicopter world thus far Lu?
5. Lu said: >>When you get into an engineering slot you will use that companies design handbook as your bible and you can throw your textbooks away<<
My response: To me, this is the crux of the problem. Company design books are designed such that they are the most easily understood text for all levels of qualified and non qualified personnel to follow so that there is NO room for misinterpreting procedures which may cause accidents, etc. They are not college text books aimed at helping design of the aircraft, or systems. Get it?
Same point being made over and over and over and over………
Dont mean to be so harsh Lu, but calling all scientists "weenies" is very offensive.
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Lu,
I assumed you were just trolling for a heated argument. There was no admission of guilt, but it appeared from your ludicrous but humerous response that you had realized the truth of what centrifugal force really is.
That wasn't the case. Sorry for making assumptions about what you had realized.
I'll respond specifically to what you have said.
You said, "In those devices that are constructed to employ centrifugal force the release of the spinning object projects it outward and that is the work."
Nope. If the force is perpendicular to the displacement then no work is done.
Also, "In the example above the rotorhead is restraining the blade but it is not pulling the blade towards the center of rotation."
The rotorhead must be pulling the blade towards the center of rotation, otherwise the blade will continue in a straight line.
"Once again I must caution that anybody that resorts to college texts to prove a point will have a hard time in any industry..."
I learned from college texts, and continue to resort to them. I didn't have a hard time fitting in when I was doing research in particle physics, I joined the military and fit in, topping all my aerodynamics classes, and I'm involved in the helicopter "industry" in my capacity as a flight safety investigator. Oh yeah, I didn't "have to change [my] way of thinking."
Your two examples of books that refer to using centrifugal forces and your comment that engineers use centrifugal forces in their calculations doesn't dispute anything I've said. I said that it is an apparent force, but is a useful tool.
I don't fault anyone for using centrifugal force. I just find that some of these tools confuse people too much, so should be avoided. You are one of those who have been confused. If you listen and think, you will be set straight. Close your mind and you will remain being wrong.
I assumed you were just trolling for a heated argument. There was no admission of guilt, but it appeared from your ludicrous but humerous response that you had realized the truth of what centrifugal force really is.
That wasn't the case. Sorry for making assumptions about what you had realized.
I'll respond specifically to what you have said.
You said, "In those devices that are constructed to employ centrifugal force the release of the spinning object projects it outward and that is the work."
Nope. If the force is perpendicular to the displacement then no work is done.
Also, "In the example above the rotorhead is restraining the blade but it is not pulling the blade towards the center of rotation."
The rotorhead must be pulling the blade towards the center of rotation, otherwise the blade will continue in a straight line.
"Once again I must caution that anybody that resorts to college texts to prove a point will have a hard time in any industry..."
I learned from college texts, and continue to resort to them. I didn't have a hard time fitting in when I was doing research in particle physics, I joined the military and fit in, topping all my aerodynamics classes, and I'm involved in the helicopter "industry" in my capacity as a flight safety investigator. Oh yeah, I didn't "have to change [my] way of thinking."
Your two examples of books that refer to using centrifugal forces and your comment that engineers use centrifugal forces in their calculations doesn't dispute anything I've said. I said that it is an apparent force, but is a useful tool.
I don't fault anyone for using centrifugal force. I just find that some of these tools confuse people too much, so should be avoided. You are one of those who have been confused. If you listen and think, you will be set straight. Close your mind and you will remain being wrong.
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To heedm, helmetfire and Dave Jackson:
I am in awe of you three. This is keen, I am warming my hands by holding them up to the monitor!
At Georgia Tech, if you used centrifugal force in any discussion or test, many professors gave an automatic "F". Too bad Lu wasn't there to set them straight.
I am in awe of you three. This is keen, I am warming my hands by holding them up to the monitor!
At Georgia Tech, if you used centrifugal force in any discussion or test, many professors gave an automatic "F". Too bad Lu wasn't there to set them straight.