Centripetal Vs. Centrifugal
 

To: All

I caught a lot of flack on this subject and it came from all angles. Some of you quoted College Engineering Texts and others High School Physics Textbooks, all of which said that Centrifugal Force is a non-force. I stated that rotor system designers were more interested in centrifugal force as opposed to centripetal force in order to calculate the stress levels and safety levels on the rotorheads. With that, Nick Lappos called me three kinds of fool and several kinds of idiot. I am willing to take that kind of criticism when I know I am wrong. To either disprove or prove my contention I contacted Sikorsky Helicopters and asked the question. The question was presented to the rotor design group and here is their answer.

“It took awhile, but the response from our Rotorhead Design group is as
follows:

"When you set up a free body diagram of a rotating mass such as a blade, one
force is in the direction toward the center of rotation (centripetal force),
the other force is in the direction away from the center of rotation
(centrifugal force). They are the same force, centrifugal being the more
common term."

We hope that this is helpful.

Regards,
Robert McFarland
WCS HelpDesk
Sikorsky Aircraft Corporation
Ph: 800-946-4337 or 203-386-3001
Fax: 203-386-5983
Email: [email protected] <mailto:[email protected]>

Lu,
You prattle on. Bob McFarland is a Customer Service speacialist, and a crackerjack technician. The quote he gives you is actually quite flawed, but you haven't the engineering background to know it.

In a nutshell, a "free body diagram" (FBD) requires that the object be in an unaccelerated state to study all the true forces on it (I taught Newtonian Physics and had a devil of a time getting this across to budding engineers). Those who put appearant forces due to accelerations onto an FBD are automatically corrected.

The case is like the marble on the merry-go-round I posted above (which I know you don't understand).

Simply said, there is no centrifugal force, there is a centripital acceleration.

Now, I expect you to post at least 75 lines of drivel, and wax on about the Sikorsky Engineer who told you otherwise (he is not an engineer, Lu) and the cousin's uncle's brother who you met in a laundromat.

Doesn't help, you are WRONG. Deal with it. :rolleyes:

To: Nick Lappos

I posed the question to Mr. McFarland and he according to his response in turn posed the question to the rotor design group. His response was that he got it from the rotor design group. Now before you shoot your mouth off about me not understanding anything I would strongly suggest that you ask him where the response came from and then take it up with the individual that made the response.

I await your response.

PS you are still not using the spell check feature on your computer.

I take it up with the individual who posted it. It is wrong, you don't understand it, you merely parrot others.
Its over, Lu.
:D

OK, here goes...

You are both (Lu and Nick) partly right and partly wrong. I've never taught Newtonian physics, but I'm otherwise quite good at it. For review lets look at Newton's 3 laws of motion, which are needed in order to shed the light necessary to understand both the centrifugal and centripetal forces (both of which are real).

Newton's first law of motion, expressed in 3 different ways...

A Every object in a state of uniform motion tends to remain in that state of motion, unless an external force is applied to it.
B An object in motion tends to stay in motion, and an object at rest tends to stay at rest, unless the object is acted upon by an outside force.
C An object in a state of motion possesses an "inertia" that causes it to remain in that state of motion unless an external force acts on it.

Galileo formulated the basic concept of "inertia", which served as the cornerstone for Newton's first law of motion.

Newton's second law of motion, expressed in 3 different ways...

A The relationship between an object's mass m, its acceleration a, and the applied force F, is a = F/m or F = ma. Acceleration and force are vectors. In this law the direction of the force vector is the same as the direction of the acceleration vector.
B Acceleration = Force/Mass. This is usually shortened to A=F/M or F=MA. Since acceleration is the rate at which speed changes, it is usually expressed in units of m/s/s (every second, the object which is accelerating will go that much faster). Force is usually expressed in Newtons (N), wich are kgm/s/s (you can see why they changed the name to newtons!)
C Newton's Second Law is more abstract than the First. The Second Law governs all acceleration and is really very simple -- acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

Newton's third laws of motion, expressed in 2 different ways...

A For every action there is an equal and opposite reaction.
B Every action has an equal and opposite reaction. These actions are forces, so you can remember this law as being every force has an equal and opposite force. Remember that these are two separate forces which act upon two separate objects, and so they do not cancel each other out, and the magnitude of these forces is equal.

There are a few side effects of these laws that are worth covering. Newton's first law implies that any object once set in motion, will stay in motion, continuing at the same speed in the same direction, until an outside force acts on it. This is often called a motion "vector". "Momentum" is a more sophisticated definition of "inertia" that includes values of mass, speed, and direction.

Newton's second law (acceleration equals force divided by mass) is satisfied when vector arithmetic is used to solve the problem of applying multiple acceleration forces to an object at the same time. Vector arithmetic resolves the multiple forces (including the object's original "momentum vector") down to a single (summed) vector, which then shows how the object's motion will change when all the forces are applied. While this is too complicated to get into here, the result will be a simple (and observed) change in the acceleration and direction of the object. It's when there are several forces acting in 3 dimensions (and when the forces are being applied at different times) that this starts to get really complicated.

With this review out of the way, let's cover the subject of centrifugal and centripetal forces.

We often think of a rotating object as having "angular momentum". This momentum consists of both speed and mass, but direction is a problem since the object is rotating. "Coning" rotor blades show that this "inertia" or momentum is preserved by speeding up the rotation or slowing it down as the rotor blades "cone" up and flatten out again.

But all objects in motion obey Newton's first law, since once they are set in motion, they continue in a straight line until acted on by an outside force. Centrifugal force is nothing more than Newton's first law being expressed by the moving object, namely "inertia" or momentum desiring to travel in a straight line.

It's interesting however that centrifugal force does not exist unless centripetal force is also being applied. As stated elsewhere, centripetal force draws an object to a center of rotation. Once a centripetal force begins to act on a moving object, centrifugal force acts to try and keep that object traveling in a straight line, and the 2 forces oppose each other. When a centripetal force is relieved (such as when a rotor blade fractures) the part of the blade no longer attached to the rotor will travel in a straight line away from the center of rotation (the hub). This will occur because the "inertia" or momentum of the rotating blade that was previously expressing itself as centrifugal force, will now express itself by causing the blade to travel away from the hub in a straight line (setting aside aerodynamic factors), as the blade's momentum resists changes in its direction.

To put it another way, "inertia" or momentum can express itself in a variety of ways. Aerodynamic drag is a form of "inertia" in that previously stationary (relatively speaking) air molecules are being forced to move out of the way by a passing aircraft. It's the "inertia" of running water (and its tendency to travel in a straight line) that carves out river canyons and turns old fashioned water wheels. So centrifugal force is "inertia" or momentum expressing itself against a centripetal force pulling a moving object toward a center of rotation.

I know this was long winded, but I hope it helps.

(edited for spelling errors and typos)

[ 16 December 2001: Message edited by: Flight Safety ]

Lu,
in all your posts the only thing that I agree with you on is what you said Nick said about you in your initial post on this thread. You can't compete with Nick so don't even try, you only embarass your self. I hope it is not only me, but when I see a post by you on a thread I skip it, mainly because it is usually self serving rubbish. I am sure you occasionally have some very valid things to say, but the way you say them is becoming very tedious and frankly annoying.


I've said enough, sorry to be so harsh but I've had enough. :(

Is the glass half full, or half empty?
Lu will argue both depending on who asked the question.

Lu, you just have to get over this and get on with your life. Just because there are people out there who know more than you about helicopters, you have to take on the system in an attempt to prove them wrong, not because the concept is wrong, but because they told you you were wrong. One day you are telling NL that the S76 had a casting problem in the gearbox,he says you don't know what youre talking about, and you have the gall to believe the rumour you heard from the bloke who ran the canteen!(or wherever you heard it) he test flew the bloody thing Lu, not you. You try and convince everyone that Frank Robinson has deliberately covered up the design of the R22 rotorhead, and that Frank Robinson designed that head wrong. I doubt youve even flown in one let alone fixed one.

About ten years ago I had to ask Aerospatiale about a query on their 365, I got two answers from two different people, both answers different, who did they go to for the correct answer? The test pilot.

Now im not saying theyre always right, but they try to be as close to right as their machine tells them by the data they bring back.

I don't know Nick Lappos, but his credentials are on the board and his experience says alot. Hes posted a few responses where hes said hes wrong, you rarely admit your wrong and then start the argument again in another post.

I don't tell you how to fix helicopters, don't tell engineers you know more about physics, then run off until you find someone who will agree with you or in part qualify some point youre raising.

I view your last post as being nothing but attempting to provoke argument and attempt to embarras NL.

Ive had an absolute gutful of your crap Lu, you hijack posts with your crap, and get everyone off the track of the original post.

My spellcheck works, but this ones for you Lu, antidisestablishmentarianism.
:mad:

To: sling load

Regarding your spellchecker facility on Word my spellchecker detected 12 spelling errors and one
punctuation error in your post but then again, my spellchecker is based on American English.

Now let’s address my comment about the casting error on the S-76 gearbox. I repeated a comment by another member of this forum and it went unchallenged by Nick Lappos. Nick Lappos indicated in one of his posts that in the testing of the S-76 he as the test pilot had to evaluate three different combinations of mixing unit linkages to correct an anomaly in the flight control system. These tests were conducted in order to get the correct movement of the disc in relation to cyclic movement.

Now when I made my comment about the casting error and the resultant testing of the mixing unit set-up he told me that I was defaming the S-76 and that I didn’t know what I was talking about. I don’t know if you have noticed it or not, but in some cases, Nick will accuse me of being an idiot because of my statements. And you and other members of this forum accept his comments about me simply because of who he is but then he never adds any technical content to explain why I am an idiot.

Why don’t you pose this question to Nick asking him if there were any casting errors in the gearbox casting or why the servos were not mounted in a similar manner to other Sikorsky helicopters and then ask him why he had to test different setups on the mixing unit.

“Ive had an absolute gutful of your crap Lu, you hijack posts with your crap, and get everyone off the track of the original post”.


Speaking about the hijacking of posts, this one is about centripetal and centrifugal forces.

[ 16 December 2001: Message edited by: Lu Zuckerman ]

Lu,
I note that you have started your own thread on this (so you can delete it?) despite several requests on the Aerodynamics – Phase Lag thread for you to address the issue there. I have re posted the information here for you.

It is important that you refer to the discussion below by its relevant number so we can look at it in isolation, thus we can find some common ground. As what is presented below is a logic chain (the next point requires understanding of the previous point), all you have to do is stop at the first point you do not understand, and discuss THAT POINT ALONE. Easy.

Simplified, here goes:
1. An object will remain at a steady state of motion (either rest or at a constant speed) unless a force is applied to it.

2. Speed can be thought of as directional. An acceleration is a force.

3. To increase speed in a direction, you must apply an acceleration in THAT SAME direction. Think of a car which accelerates. If the direction of the acceleration was depicted as an arrow, the pointy end of the arrow would be pointing at the front of the car, I.E. in the direction of the speed change.

4. THE LOGIC THUS FAR: Speed is directional. Speed change requires an acceleration (which is also directional). THUS changing direction requires an acceleration, which is depicted as an arrow pointing toward the direction in which the acceleration force is acting.

5. When a car turns a corner, it changes direction, therefore it MUST have an acceleration (or force) applied to it (remember the speed is directional) in the direction of the corner.

6. Which way do we draw the arrow to depict this force? The arrow MUST be drawn toward the centre of the turn because that is the direction the force is acting, just like we did for the car's straight line acceleration.

7. THE LOGIC THUS FAR: when turning, you are changing direction, and an acceleration can be seen to be acting toward the centre of the turn.

8. David's sling is wrapped around the rock, and the rock is traveling at speed. but the rock is not travelling in a constant direction - it is traveling in a circle. Thus the rock is constantly changing direction. Thus a constant force MUST be being applied to make it constantly change direction.

9. We can depict the force acting on the rock by drawing an arrow. Just as we did with the car, the pointy end MUST BE pointing in the direction that the force is acting, I.E. toward the centre, I.E. toward David's hand.

10. THE LOGIC THUS FAR: There is a force that can be seen to be acting toward the centre of the circle.

Physicists decided to give this force a name so that we could refer to it without saying "that force that acts on a mass that is changing it's direction"
They named it centripetal.

11. . When the sling releases the rock the force trying to change it's direction is gone, therefore the rock will travel in a straight line again, along a tangent. If the rock were to fly out of the sling in any other direction, an acceleration must be applied (because we know that a change of direction can only be accompanied by an acceleration).


Lu, you have mentioned several times the “force” that you feel is flinging objects outward through your marble example and your FAA question. I believe that that feeling of being thrown outwards is central to the difficulty in understanding centripetal, and the reason centrifugal is used as an easy substitute to promote understanding.

12. Going back to the logic steps I used above to describe centripetal, I used a car accelerating along a straight line as an example of a change of speed. Because speed had direction, and acceleration does too, when we depict the force, we can draw an arrow in the direction the car is traveling. in other words, to where the car is accelerating TO. Thats why centripetal was IN toward the centre of the circle: because it was an acceleration TO that direction.

13. So why do we feel flung outwards? When the car accelerates in a straight line do we feel a force? You betcha. We sink into the seat. We would get "flung backward" if the seat was not restraining us. In other words - we "feel" the acceleration in the opposite direction to which it is commonly thought of as being applied. In this case, the acceleration is going forwards, but we feel it going backwards.

14. Thus, when we are turning, we feel the opposite direction to the acceleration. I.E. we feel "centrifugal" because of the application of centripetal. Thus the force is centripetal (same as acceleration forward) and the sensation we feel is the REACTION (Thank you Mr Newton) to that force, but is NOT in itself a force.

15. THE LOGIC SO FAR: Centripetal acts like ALL acceleration forces – in the direction of that acceleration. When an acceleration force is applied, we “feel” it in the opposite direction but that “feel” is NOT a force of its own – it is a reaction to the acceleration.

16. SUMMARY: Centripetal acts toward the center. Centrifugal is NOT a force – it is the term applied to the reaction of centripetal.

I guess they gave centrifugal a special name to make it an easy to grasp concept, but at the end of the day, it is the same force we feel as we are resisting the change in speed (acceleration) of a car in a straight line. I dont know why they neglected to name that feel too??
Wait...what about "g forces"?


Remember Lu,
Stick to the logic chain. Stop at the step that does not ring true and discuss that step BEFORE going on to the next.

Good luck.
I must drink heavily now.....
:D :D

Let's try another merry-go-round example.

Suppose you are located on the edge of a merry-go-round and with the left hand you hold on to the metal bar handle thing, and with the right hand you hold a 20lb dumbbell. Now suppose someone else spins up the merry-go-round for all that it's worth.

Once in motion, your left arm is applying the centripetal force necessary to keep you on the merry-go-round. Your right arm is supplying the centripetal force necessary to keep the 20lb dumbbell in your hand. Pretty soon however things start to change as the muscles in your right hand and forearm (which are supplying the grip on the dumbbell and hince the centripetal force that keeps the dumbbell moving in an arc) start to get tired.

But wait, there's another complication. Since you started this strange experiment, a rather large crowd has gathered around the merry-go-round to look at this unusual sight. You soon realize that if you relax your tired grip on the dumbbell, somebody is going to get seriously hurt. You call out to your companion to slow down the merry-go-round, but for some strange reason (the noise of the crowd?) he can't hear you.

Now you have a real delimma on your hands, and this situation (like all serious situations) starts to focus and clear your mind. While your muscles are getting weaker and weaker and the dumbbell is trying to pull your arm out of its socket, you have to decide if centrifugal force is really an actual force or not. You know your arm is really starting to feel the "pull" of the dumbbell and the pain is starting to become extreme. You also know that if you let go, somebody is going to get a stomach full of fast moving dumbbell, which you know will hurt a great deal.

Quickly you decide that centrifugal force, which acts against centripetal force to try and cause an object to travel in a straight line, is real enough. Therefore as a skilled pilot trained to think quickly and to act in an emergency, you hastily decide to yell out to the gathered crowd to quickly move away from the merry-go-round. As soon as they do, you release the grip on the dumbbell (to the great relief of your arm) and watch as centrifugal force instantly transforms the circular course of the dumbbell into a linear course of free flight through the air. Then you watch as the centripetal force of gravity pulls the dumbbell towards the earth's center of rotation, and causes it to crash into the ground a good number of feet in front of you.

Thus this wonderful experiment demonstrates that centrifugal force is in fact a real force.

(edited for spelling errors and typos)

[ 17 December 2001: Message edited by: Flight Safety ]

Here is some more proof that Lu gets his information from rumor, not from facts, and that Lu knows little of what he posts.

We have seen Lu repeat his false assertion that the main gearbox of the S-76 was made by "mistake" and that the servos are in the wrong place as a result of this mistake. He even said that the designers "screwed up" in one post. That is an illustration of the kind of crap that Lu has floating in his brain, all the rumor and inuendo that a simple mechanic picks up in 50 years of hanging around the snack bar as real pilots and real engineers talk. All that drivel then circulates around for years, gets confused, and then appears in his posts.

I was on the design team 2 years before we flew the first flight of the S-76. I helped make the decision to incorporate 17 degress of Delta-3 into the main rotor to create a better ride for the occupants in cruise flight in turbulence. This decision was made after the main gearbox castings were designed, and after the supplier of these castings was making his casting tools. If we changed the gearbox, we would have delayed the whole program, so we decided to use the upper deck controls to correct the cyclic phasing (all this is too much for Lu, it is exactly like the legendary 18 degrees of the Robinson that causes him to foam at the mouth).

The Chief Designer of the S-76 was a great engineer named Al Albert, who knew more about helicopters than Lu Zuckerman knows about ANYTHING. Al designed the S-67 Blackhawk, which was the fastest helicopter ever for a period of about 10 years, the S-76 which is a legend, the UH-60 Black Hawk, and the S-69 ABC (the absolutely fastest helicopter ever). To say that Al Albert, who died last year, made a "mistake", and "screwed up" is easy for an ignorant blowhard like Lu, and is simple proof that Lu knows only about half of what he posts on this newsgroup. The problem is that Lu does not know which half of his utterances is fact and which half is bull, and when someone does know, Lu refuses to admit it, either.

Remember a few months ago when Lu would post drivel about how he knew that the designers were making mistakes and that he was a hero for helping save lives with his proclamations? Thank the Lord that garbage from Lu has died down, mostly because this group now knows what those design teams found out (the hard way for all of us). Lu only knows about half of what he says, and he doesn't know which half. When told which half, he argues and fumes, and then blames someone else.

Flight Safety,
Regrading CF, the best way to describe what we are discussing is to note that the fellow's arm in your discussion is actually getting tired because it is continually throwing the weight around the circle. It is tired not because there is a force the weight is exerting on him. If he let go, the weight would merely continue on its path. If he holds on, the weight is pulled into the circle he is making. In other words, the inertia of the dumbell is what he is fighting.

There is an appearant CF, but that is only needed to accelerate the weight around the circle. :cool:

[ 17 December 2001: Message edited by: Nick Lappos ]

FS:

In light of your example, would you care to comment on the logic chain posted immediately prior to your post? From what I gather, you have a disagreement with step 13 onwards. Accordingly, I have a further question for you: if you were sitting on a rollerskate (rather than a seat) when you accelerated your big V8 RV, you would have to hold on tight to avoid being flung backwards. Is this a force, or a reaction?

As neither of these forces is mentioned in the AS332 manual the french must have found a way of doing away with them; :confused: ; but rest assured these things can fly rotary way :D :D

Helmet fire, I hope I have time to cover this.

I actually agree with step 13 in the logic thread, but step 14 is where I disagree. It's interesting that both Lu and Nick are missing it at exactly the same point, which is blending (or merging) 2 separate forces together. They are blending the centripetal force with an acceleration force.

Let me try it this way by covering your rollerskate and V8 RV example, but I'd like to change the RV to a van with no seats (except the driver's) and a flat floor, so the rollerskates can work.

Suppose the van were stopped on a road with a driver inside and you were standing outside the van on the road next to the driver's window, with the rollerskates on. Then suddenly the driver hit the gas and accelearated the van down the road. The van would now be in motion, but you would be left standing on the road in the same spot, since the state of your motion would not have been changed by the acceleration of the van.

Now suppose you are inside the van (instead of being outside) next to the driver with the rollerskates on. Again the driver suddenly accelerates the van. At first, due to the vastly reduced friction between yourself and the floor of the van (caused by the rollerskates), you will remain in the same spot (over the road) since no acceleration force has been applied to you yet. In other words your "inertia" or momentum has not changed without acceleration.

But this will last only briefly until the back of the van accelerates to the same spot over the road that you occupy. Then you'll be accelerated in a very abrupt manner as the back doors apply the acceleration force to your body that the floor could not, because of the rollerskates.

Now here's where Lu and Nick are missing it...gosh, I just ran out of time. Bad timing, I have to get back later.

[ 17 December 2001: Message edited by: Flight Safety ]

FS - I think your explanation just about sums it up, any difference from my understanding of the subject, I suspect, come down to semantics.

Helmet - I have to say that the logic chain falls apart a bit at point 2.
a) Speed can not be thought of as directional! - A body moving in a circular path may well have a constant speed, in which case it will have a constant acceleration and constantly changing velocity. A car travelling at 30mph on a roundabout has a constant speed, i.e. it covers 30 miles in every hour. It is, however, accelerating constantly towards the centre of the roundabout.
b) "An acceleration is a force" not in my book!

Flight safety , not to be pedantic . Wouldn't the dumbell after beeing released actually hit the ground several feet behind you , unless of course your assistant managed to stop the roundabout the instant after you let go of the dumbell. I guess if the roundabout also managed to do one full rotation while the dumbell was airborne that would count too. In fact id you are going in a circle what is behind you is in front of you as well. Dohhhh.

This thread appears to be going in circles.

The following web page should greatly simplify and clarify Newton's second law of Motion.

___________

Just joking.

To: Nick Lappos

You do have a novel way of showing everybody how dumb I am by telling everybody how smart you are. I have mentioned this on several occasions asking you not to personalize your comments about what I say whether I am right or wrong. On these various threads I have been told by many pilots that my theories are pure bunk and when they offered me their theories on the subject and much of it was pure crap and it indicated that they did not know what they were talking about. Much of the criticisms from the pilots regarding my theories were based on how they were taught as opposed to how I was taught. I accepted those differences as an alternate theory to mine and hopefully those contributors to this forum felt the same way about my theories. In every case there was technical discourse and no name-calling. That is until you started to post and peppered your comments with accusations about my mental capacity, that I was an idiot or better still a total fool. With your comments towards me, and the positron you occupy, they felt that they could also join in.

When the original comment was made by another contributor regarding the bumps on the S-76 one contributor said it was to accommodate the servos. A second contributor stated that the casting of the gearbox and the servo attachments were offset from where they should have been. In other words what he was implying was that Sikorsky screwed up. This went completely unchallenged. Some time later in one of your posts you indicated that you had to perform tests to develop the ideal control linkage but at that time you did not explain why. What you have to understand is the nature of PPRuNe (Rotorheads) in that it is not designed for technical discourse on the finite principles of engineering. Technically I should not be able to contribute because I am not a helicopter pilot. The nature of this forum is that it allows individuals to pose questions and get answers, to offer opinions and await comments from the other contributors and most threads will deviate from the original thought that was posted and although it might aggravate the original poster it is the nature of the beast.

In several of my posts I made statements dealing with control lead of the pitch horn and the servo adding up to 90-degrees on most Sikorsky helicopters except the S-76. This too went unchallenged.

I will state the following and then I will ask a few questions. Please do not personalize your comments and disable the capital letter I on your keyboard.

Question: In reference to the longitudinal axes of the S-76 what are the positions of the fore and aft and lateral servos? Are they offset 45-degrees from the control axis of the servo or, are they disposed in a different manner from other Sikorsky designs?

Question: If the servos are not disposed in the same manner as other Sikorsky designs what was the design theory that caused this difference?

Question: In a previous post you explained the testing of the mixing unit linkage to provide a better ride at speed. Was this developmental testing related in any way to the possible mispositioning of the servos?

I will accept any answers you provide but please do not attempt to obfuscate the facts.

I personally believe that you are entirely too defensive about Sikorsky products. I firmly believe that they are the best and Sikorsky products built by Agusta are even better. However Sikorsky is not immune to screwing up and one example of that is the blade design on the S-76. If you want other examples I will gladly provide them.

One final note. About my comments about the design of the Robinson rotorhead and control system you accused me of defaming Frank Robinson and his design. Many Robbie pilots told me that since I had never flown in one or had actually piloted one that I had no standing on the subject. I asked you to familiarize yourself with the control system, the rigging procedures as well as the rotorhead design. If you have not done that then you have no standing on the Robinson helicopter no matter how many other helicopters you have flown.

[ 18 December 2001: Message edited by: Lu Zuckerman ]

Sorry to butt in on your Sikorsky Gearbox thread Lu, but an answer to FS: I know you have not yet finished, so please forgive me for interjecting.

Your van model is at the critical stage for point 14. As the back of the van now applies the full acceleration to you, it is acting in a certain direction (in this case, in the same direction as the van is going). But all you feel a "force" pushing you back into the back doors of the van. This is not a force however, it is the reaction to the acceleration the van doors are attempting to place on your body. Proof of this is the fact that assuming a frictionless rollerskate/floor interface, you would have not moved from over the same point on the road until hit by the van doors, i.e. the acceleration needs to overcome your inertia. Your inertia does not apply a force to the doors. Here we are exploring Newtons action/reaction. The reaction is the resistance of your body to the acceleration (i.e. its inertia), but it is not a "force", it is a reaction to the force applied by the van doors in trying to make you change speed.

I think, however, that you would argue that your body would be placing a force on the doors, and it would certainly feel like that. Using your previous formula of F=ma, you are the mass, and the acceleration is provided by the doors. Getting back to the critical issue of direction now (i.e. which way does the force act), as TeeS quite rightly points out, acceleration is not a force per se, it is more correct to say that it becomes a force when it acts upon a mass (F=ma as you pointed out). But acceleration has a direction. Mass doesn't. Therefore force must have a direction because it is a product of acceleration, and certainly the convention of vector analysis would depict that direction. Thus the direction of the force is said to be acting in the direction of the acceleration, Hence the validity of point 14.

To TeeS: you are technically correct, appologies for any confusion. I have answered your acceleration/force relationship above. As for speed Vs direction, I believe that I was probably trying to say "velocity" and express it as a vector, and have (incorrectly) over simplified the explanation by using speed. Try and divorce speed from the simple speedometer exmple you used (which is quite correct) and substitute velocity in my examples. Alternatively, I could go back and edit the points to clarify this, your choice. Does that get you past step 2?

Mind you, I am no physicist, or aerodynamicist, and I have never been mistaken for one! Heedm may be able to offer a better more technical explanation of speed/direction/velocity.

Good luck :)

Edited for (some) of the spelling mistakes!

[ 18 December 2001: Message edited by: helmet fire ]

Flight Safety, you have a very good understanding of a lie that's been told to us for some time.

The question is not "Can you ever feel a centrifugal force?" (we all know that it can be). The question is, "Is centrifugal force a real force?"

For a centrifugal force to be felt, the "feeler" must be rotating and off the axis of rotation. That means the feeler is accelerating. Although the force is felt, there is no effort that creates it. The dumbell in your example is not pulling on your muscle, your muscle is pulling on the dumbell. This is where many get confused and say that centripetal and centrifugal are equal and opposite. A/ They can't be otherwise their would be no net force and thus no acceleration, and B/ They can't be measured in the same reference frame.

If you weren't aware that you were spinning on a merry-go-round, then you would be very perplexed on why the dumbell seems to repel your body. You wouldn't see an object firmly attached to the floor of the merry-go-round and think, "If I pick that up, I must supply the required centripetal force in order to keep it in it's current radial position."

If you were aware that you were spinning on the merry-go-round, then you're actually thinking from the stationary reference frame. In that frame, you're not surprised that the dumbell wants to pull away from your body, you realize it wants to fly in a straight line, and you must pull on it to move it in a circle.


Sum up:
- Centrifugal can be felt but only in a reference frame where the laws of inertia don't hold.
- Centrifugal force isn't created by any effort.
- Centrifugal is an apparent force.


One more thing, Newton's second law according to Newton is that a force is the instantaneous rate of change of momentum. The F=ma comes out of that for non-relativistic physics.

Question:

On many Sikorsky rotorheads they have stops installed that limit flapping in a static or low rotational speed and droop stops that limit the downward travel of the blades when they are at rest. These stops are mechanically linked to the rotorhead so that when the rotorhead is spinning the stop assemblies rotate with it. However the stops are free to move outward in a radial direction and as the speed builds the anti cone (flap) stops will move outward against a spring. This frees the blades to cone with applied collective pitch. Once the blades come off the droop stops they too will also move outward against a spring load. As the blades slow down after landing the droop stops will move inward thus limiting the downward movement of the blade and shortly thereafter the anti cone (flap) stops will be drawn inward limiting the ability of the blade to sail at low speed. The inward movement is effected by a spring(s)

Here is the question: What force causes these stops to move outward against the springs?

Would anyone disagree with the following thought process?

1. I lie on the floor, on the equator, which is part of the Earth's surface rotating at impressive speed about the Earth's axis (and in a multitude of other directions through space but lets ignore that!)
2. Luckily gravity sucks me towards the centre of Earths mass (actually towards all the individual components of its mass)and in fact I attract the Earth with exactly the same force. If it were not for this mutual attraction I would disappear from the Earth's surface in a direction tangential to the rotation of the Earth's surface at this point (Rather like some of these threads!).
3. Gravity is providing a force on my body, towards the centre of the Earth, rather greater than the centripetal force required to accelerate me in a circular path along with the surface of the earth. This extra force can be felt, while I am lying here I can feel sharp stones pressing into me (Of course they are not, I am pressing onto them and I feel the REACTION to that).
4. The extra force is rather useful, it means that if I jump over a fence I return to earth the other side and if an engineer drops a spanner he is likely to find it at the bottom of the engine bay or on the floor.
5. Now let's carry out an experiment of the mind - Let's, in a major mining exercise remove a vast quantity of the Earth's mass and send it off into space where it will not have a significant gravitational effect on us. As the Earth's mass reduces our mutual attraction reduces until at some point the gravitational force is exactly that required to maintain the centripetal force. I am now IN ORBIT on the Earth's surface, I feel weightless and if I jump up I will leave Earth's atmosphere.
6. At this point there is only one force acting on me - gravity - it is acting on my body, towards the centre of mass of the earth, this is centripetal force.
7. Where does centrifugal force come into this? My belief is that centrifugal force is the REACTION to centripetal force and is the force that I exert on the earth, attracting it to me. If you measure very accurately you will find that I am causing the Earth to 'wobble' on it's axis, even though I still feel weightless.

Thats it for now, I would be interested in your thoughts on this, I'm off for a lie down!

[ 18 December 2001: Message edited by: TeeS ]

Lu
As the rotor head starts to rotate the springs provide the centripetal force to accelerate the weights towards the centre. As the rpm increases more force is required by the springs and so they stretch. The weights themselves are just trying to do there own thing and travel in a straight line.

To: TeeS

Remove the springs from the stops. The weights will be accelerated in a circular path and as a result will still fly outward and the only restraint is their attachment to the rotorhead. Now this could be the same as one of the definitions of centripetal force in that it is the force exerted in order to move a body through a circular path. In another definition it states that it is the inward force exerted in order to restrain a body that is moving in a circular path from flying outward.

I know it is not just me that finds this concept difficult to absorb although I have, thanks to the many posts on this and other threads but non educated individuals like myself find it difficult to view the restraining action of the rotorhead as a force tending to move the blade inward when there is no inward movement.

I think the best non-engineering explanation is that the centripetal force is opposing centrifugal force. That way, both forces are recognized.

Lu

Take the springs away and gravity on it's own does the job. I suspect that if you put your rotor head into a gravity free, frictionless environment (I know it's getting silly now!) then as soon as you rotate the shaft the balls will extend to the horizontal. Not because they are being 'thrown out' but because that is the closest to a straight path that they can travel in. The only thing stopping them traveling in their desired path (a straight line)is the centripetal force applied by the shaft.

Lu,
I request you go back to my logic chain and discuss the point at which you lose understanding. Otherwise, we will continue to go around and around (absolutely no pun)with your latest example. I chose David & Goliath because that was the example you first asked about, then I mentioned the car after the second series you asked about. We then explained the alcohol pump in the FAA questionnnaire to you, and the marble on the merry go round.

Instead of introducing yet another example by trying to answer your questions on droop stops, shouldn't we nut out the principle so they can be applied to all of your examples?
Please?

Again: I request you go back to my logic chain and discuss the point at which you lose understanding.

TeeS, you're bang on except for the part about centrifugal force. Centrifugal force is what we call the force that we feel when we're rotating. We feel it, it is apparent, it is not real.

Lu, you said, "...[some] find it difficult to view the restraining action of the rotorhead as a force tending to move the blade inward when there is no inward movement."

In fact there is inward movement. The "natural" movement is a straight line. To move in a circle, you must move from the straight line towards the center...inward movement.

You seem to consider all the diagrams to be stationary. When you do this, you are in a rotating (non-inertial) reference frame. In that frame, only a centrifugal force appears to be acting. Describing the droop stops as being moved by a centrifugal force is accurate when you're confined to that reference frame. That doesn't mean it's a real force.

If you think of the two as equal and opposite forces, then you end up with a net force of zero. In a rotating frame a net force of zero means you feel nothing. In an inertial frame a net force of zero means the object's path doesn't curve.

[ 19 December 2001: Message edited by: heedm ]

An addition to TeeS' post. Rather than imagining you're in orbit at the surface of the Earth, think of geosynchronous satellites. If there were any such thing as Centrifugal force, they'd be charging off across the Solar System with great abandon. These beats are doing around 7000mph, the only thing holding them in orbit is the Centripetal force of Gravity. To quote Marshall Brain's HowStuffWorks site on Satellites:
"Without gravity, the satellite's inertia would carry it off into space. Even with gravity, if the intended satellite goes too fast, it will eventually fly away. On the other hand, if the satellite goes too slowly, gravity will pull it back to Earth. At the correct orbital velocity, gravity exactly balances the satellite's inertia, pulling down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface, rather than flying off in a straight line."
Note the use of the word "inertia". Another point: "Centrifugal Force", if it existes, operates in line with, but opposite to the Centripetal force holding an object in a circle; take away the Centripetal force (David's sling breaks, you let go of the dumb-bell, the Earth disappears) and the object would fly off radially - in the direction of the "Centrifugal Force". It actually flies off tangentially, in the direction its inertia takes it.

a tractor hooks a chain on to a spike wich is driven into a stump. the ground is boggy and the tractor slips on the surface not able to move the stump. some say a force would be pulling the tractor backwards, i say it is a reaction that the tractor stops when it has'nt got the force to pull the stump out. the stump needs a curtain amount of force to remove it. now the tractor turns to drive perpendicular to the stump, the chain spinning on the spike. the tractor gets faster and faster around the stump using its weight more efficiently. eventualy the stump comes out.
some say a force pulling the tractor to the stump causing a direction change is what causes the stump removal. i say that the centrifugal force acoplished by the tractor over comes the force the stump needs to be moved. you could easily say the stump had centripital force holding the tractor, but when the tractor wasnt spinning around the stump what would this be called?

vorticey:
If you dont mind, I will use the logic chain as per our last discussion on the other thread, to answer your question/statement.

If you go to step 3 & 4 re the fact that changing direction requires an acceleration, and thus a force. The the tractor is (like Davids rock in the sling, the car turning a corner, the girl on the merry go round, the alcohol in the pump, the marble on the circular plate, and the droop stops of a rotor system) changing direction because it is going around a circle. Thus, in accordance with step 5 & 6 it requires a force to keep changing it's direction. No change to the logic chain thus far.

What seems to confuse the issue here is that the thrust is coming from the tractor at the edge of the circle, not from the center such as Davids hand in the rock throwing exercise. But what force is the tractor doing? I assume here that the tractor is attempting to create tension on the chain, therefore it cannot be turning around the stump because the chain would then be limp, i.e. it must be trying to go straight ahead. To get it to change direction and go around and around, we need to apply an acceleration (as per above).

We recall from step 3 & 4 that you depict an acceleration as acting in the direction you are accelerating to, such that if you accelerated in a straight line, you would depict that acceleration as toward the front of the tractor. But this tractor has an acceleration causing it to go around and around toward the center of the stump. Therefore the force is acting TOWARD the stump, i.e. centripetal. Thus the fact that the tractor is providing the thrust does not change the direction the force is acting. Indeed the only difference between the car turning and the tractor is that the centripetal is being provided by the tyres on the car, and the chain/stump for the tractor.

The next confusing thing is that the stump gets pulled out of the ground, and it looks like there has been a force pulling it outwards. It is easier here to think of a straight line acceleration equivalent, lets say like the van used in the posts above, with a passenger inside the van on rollerskates. The van accelerates, and the guy inside gets slammed into the back of the van, and continues to get "pushed" into the back of the van as long as it accelerates. We know that the acceleration force is acting toward the front of the van, but if the force is high enough that the van doors can no longer transfer that force to the guy on skates, the van doors break open and the guy on skates becomes sore. Why? Becuase the accelerative forces were more than the than the van doors could handle. Back to the stump. We saw that the tractor's constant directional changes were due to a constant acceleration, and as force equals acceleration X mass, and the tractor has much mass, there must be a large amount of force being applied. As in the van doors, it is simply a matter of the at rest inertia of the stump being overcome by the amount of force to continue changing the direction of the tractor.

Now to you question: you say >>you could easily say the stump had centripital force holding the tractor, but when the tractor wasnt spinning around the stump what would this be called?<<

This is the bit where it gets really confusing (well it did for me when I learnt it). The stump is NOT exerting centripetal force. Remember from above, the cenripetal force acts toward the stump. Therefore the question is not really answerable, but I think I can see where you are comming from. Remember that the van doors busting open above? Well that was because the force required to accelerate the mass of the guy on skates became greater than the ability of the doors to stay closed. Just as in our tractor case, the force required to move the stump was overcome by the acceleration (changing direction) of the tractor's mass. So what are the van doors without the guy being accelerated and what is the stump without the chain? They simply adhere to Newton's law that a body will remain at rest unless acted upon by a force (see Step 1 of the logic chain). How mush force is required to move them is dependant upon mass, friction with the ground, etc. Thats what the stump is before the chain - a big mass with huge friction which would therefore require someone to exert heaps of force to move it!!

Too tired to make the above brief. Sorry vorticey.

Is it not correct to say that the centripetal force on the 'rotating object' is opposed by an equal and opposite centripetal force on the 'central object'?

Which of the two objects is rotating about the other is dependent upon the perspective of the observation.

Assume that the difference in mass between the two objects is very large. If the person is located on the item with the smaller mass, he will feel the centrifugal effect. Alternatively, if the person is located on the item with the larger mass, he will not feel the centrifugal effect.

<img src="confused.gif" border="0"> <img src="confused.gif" border="0"> <img src="confused.gif" border="0">

When the tractor is driving around the stump, the tension in the chain supplies the required centripetal force. The tension in the chain also pulls on the stump. The stump isn't moving in a circular path so it doesn't need or feel centripetal nor centrifugal forces.

When the tractor pulls on the stump in a straight line, there are equal and opposite forces on the tractor: tension in the chain, and the driving force developed by the tractor. The system is in equilibrium so there is no acceleration.

If centripetal and centrifugal added up to zero, then there would be no circular motion since an object in circular motion is not in equilibrium.

The stump doesn't create any force...it reacts. Consider doing a chin up on a chin up bar. The bar doesn't pull you up, your muscles do.
_________________

Dave, there is no centripetal force acting on the central object. A force pulls on it, but if the object is truly central then that force cannot be centripetal. When the moon orbits the earth, neither body is central, they both orbit a point in between. (it's easy to calculate ratio of masses equals ratio of distances from the point...and it's closer to the heavier object).

Hi heedm

I would beg to differ; <img src="eek.gif" border="0"> and submit the following argument.

&gt;"... there is no centripetal force acting on the central object."&lt;
There is no object that can truly be considered central.

&gt;"... if the object is truly central then that force cannot be centripetal."&lt;
The concept of centrality is only a point of reference.

I agree that the earth and the moon are rotating about a central point. They rotate about this point because there is an equality of opposed centripetal forces.

Using the merry-go-round as an example;
If this merry-go-round could be in absolute balance, then its very center is the point of reference. There is no mass (object) at this point, only opposed centripetal forces.
.
If the merry-go-round is out of balance, because there is only one rider, then this point has shifted slightly. It must now be said that the earth, which is attached to the physical center of the merry-go-round is experiencing an infinitesimal amount of centripetal force about the rider.
_____________-

Got little to do with helicopters but it's fun. <img src="smile.gif" border="0"> http://www.UniCopter.com/7up.gif

To Dave and vorticey:

I think I have put my finger on part of the confusion. I note that you have both spoken of Newtons law in terms of each force has an equal and opposite force. But the law actually states each action has an equal and opposite reaction relating to a body in equilibrium. Not semantics, but quite crucial to our discussion here.

When pushing on an object that stays at rest (i.e. in equilibrium), it can be said that the object has an equal an opposite reaction to that force therefore it doesn't move. Using the chin up bar from Heedm, you are applying a force upwards, and the bar remains stationary (in equilibrium) but you wouldn't say it is applying a force downwards would you? As you lift your self off the ground (assuming you are not as fat as me) your speed has changed (upward). Accordingly you have applied an acceleration upward. But still, the bar has not applied any force or acceleration has it? The bar is still at equilibrium, therefore you are still getting an equal and opposite reaction from the bar. But not a force. See the distinction?

<img src="smile.gif" border="0">

OK helmut fire, here goes


&gt;"But the law actually states each action has an equal and opposite reaction relating to a body in equilibrium. Not semantics, but quite crucial to our discussion here."&lt;

A couple of separate sources say;

" Newton's second law of motion can be formally stated as follows:
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object."

"By Newton's Second Law, such acceleration implies that there must be a force acting on the object, directed at the center of the circle."

Even if the word 'action' is substituted for the word 'force', there must be a reaction to the action.
___________________

&gt;"Using the chin up bar from Heedm, you are applying a force upwards, and the bar remains stationary (in equilibrium) ."&lt;

&gt;"As you lift your self off the ground your speed has changed (upward). Accordingly you have applied an acceleration upward."&lt;

As the athlete accelerates upward, the earth accelerates downward (damn little of course).
____________________

&gt;"But still, the bar has not applied any force or acceleration has it? The bar is still at equilibrium."&lt;

Neither the bar (and the world which is attached to it) nor the torso of the athlete have applied any force, but the arm muscles located between the two have.
__________

Your serve <img src="smile.gif" border="0">

It seems so strange that members of this forum are attempting to finesse each other by publishing their interpretation of Newton three laws. Each has his own interpretation and tries to convince the other guy that his version is right. It is analogous to my arguments with the Brits and Ausies about gyroscopic precession. My reason for making this post is to state that last night I heard a report that scientists have found a way to accelerate light beyond light speed. If this is true many new scientific breakthroughs are on the horizon and Isaac Newton’s laws will go along the wayside along with other scientific concepts. All of this most likely in an alternate universe.

Lu

Some people enjoy having non-confrontational arguments.

As you know, there's a pleasure in contributing knowledge to others. There's also the greater pleasure of broadening one's knowledge, thanks to the contributions of others.

Its fun provoking and being provoked into reassessing one's beliefs. In fact, doesn't helmut fire have an unanswered question pointed in your direction. <img src="wink.gif" border="0">

[ 21 December 2001: Message edited by: Dave Jackson ]</p>

Dave said, "There is no object that can truly be considered central." and "The concept of centrality is only a point of reference.".

Those statements contradict each other. Since 'centrality' is only a point of reference, then by choosing your point of reference appropriately, an object can truly be considered central.


Just because two bodies are gravitationally attracted, it doesn't mean they are orbiting each other. To call something a centripetal force, the body on which the force acts must be moving in a curved path.

___________

Of course, now we're getting really picky. At this level of detail we should be considering relativistic mass of the rotor tips and how that affects tracking. <img src="wink.gif" border="0">

"The greater our knowledge increases the more our ignorance unfolds." --John F. Kennedy

heedm

To continue a hypothetical argument, in the hope that there may be a practical conclusion. <img src="smile.gif" border="0">

&gt;" Dave said, "There is no object that can truly be considered central." and "The concept of centrality is only a point of reference.".

Those statements contradict each other. Since 'centrality' is only a point of reference, then by choosing your point of reference appropriately, an object can truly be considered central. "&lt;

Going into deep space ( or getting into deep sh-t), assume that two planets of equal mass are rotating about each other; or to be more exact, rotating about a central point. If the mass of one planet is increased visa-vie the other, the central point will shift toward the planet of heavier mass. They still should be considered as rotating about each other.

No matter how great the difference in mass becomes;
1/ they will still be rotating about a central point (rotating about each other).
2/ this central point will never be concentric with the large planets center of mass.


&gt;" To call something a centripetal force, the body on which the force acts must be moving in a curved path. "&lt;

From a remote perspective, both the above planets are moving in curved paths. For the perspective of one of the planets, only the other planet is moving and that is with a curved path.


Could not the above be said about the helicopter, in respect to two (or more) blades? <img src="confused.gif" border="0">
__________

Lu: If you've read this far and are answering questions.

Leonardo da Vinci designed a vehicle that was intended to 'screw' its way up into the atmosphere. Do Newton laws disprove Leonardo's thinking, or do they compliment it with a higher level of technical understanding? <img src="frown.gif" border="0">

[ 21 December 2001: Message edited by: Dave Jackson ]</p>