Why This Group is Moribund

OK, I will be the dummy raising his hand. WTF is MEK?

"Six_O'Clock_High"

Reply to
Fubar of The HillPeople
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On Fri, 24 Aug 2007 11:48:49 -0700, "Fubar of The HillPeople" wrote in :

Methyl ethyl ketone.

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Available from most paint departments for use as a solvent. I'm going to get a can so I can read the label and use it to think rubber cement.

MEK has a very low flash point and is liable to cause intense flaming in this newsgroup. Handle with care. ;o)

Marty

Reply to
Martin X. Moleski, SJ

wrote in

Used daily by plumbers to solvent weld PVC pipe with no apparent ill effects. Mind you all plumbers are brain damaged anyway, so its hard to tell....

Reply to
The Natural Philosopher

Most are on crack.

wait a minute - most SHOW crack.. My mistake..

Reply to
David Hopper

I'm ok, just a little Dr. Pepper up my nose, none on the key board. :) mk

Reply to
MJKolodziej

Careful. More people have died after drinking Dr Pepper than have died after drinking MEK.

(Hint: examine the logic carefully)

Reply to
The Natural Philosopher

I reckon this is THE way to go >:-)) I have graduated from... kit building, to buying plans and making all the bits... to having the incentive now to try and design my own stuff.

The process has been FUN at all stages and I reckon that's what counts.

Reg

Reply to
tux_powered

That's a very good point. I fly alone and in remote places where I don't usually see anyone. I fly both slope and power models. Having been "bitten" on a couple of occasions by a prop I ensure I have enough sterile wadding, tape and bandages with me to staunch a decent blood letting session !

Reg

Reply to
tux_powered

Doing your own design is fun.. This chart by Romey Bukolt gives some basic moments for RC planes. You may find it useful...

See

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Reply to
Bill Fulmer

Ok, that's it. I've had it with the engineering jargon!!! I'm ignorant as hell, and I'm not going to take it anymore. This marketing guy needs some help from the engineers amongst the group.

What does the term "moment" mean in an engineering sense. I've worked with enough engineers to get the general gist of what they mean when they use the term, but the moment passes quickly and I never seem to ask the question. If you have a moment, how about 'splaining the term.

Thanks,

Harlan

Reply to
H Davis

Yo know, I've used it so lon i can't even remember the textbook definition..its a term for things that have an element of rotation implied in them..soe.g.teh resistance of an object to being accelerated in a straight line is 'Inertia' but its resistance to being spun is its 'moment of inertia'

likewise linear FORCE - the weight of an object, becomes 'torque' which is the moment of force I suppose.

In this context the 'tail moment' of a plane is something like the force on the tailplane times the distance it is from the CG, since in flight this is the most convenient axis around which to measure things.

In fact most people use the term to mean the area times the distance from the CG, since at a given speed a given area ant a given angle of attack generates a frce proportional to the area..more or less.

e;g. pitch stability is guaranteed if the first differential of the intergarl of all the pitch moments is negative with respect to airspeed.

Or in words, as long as the faster the plane goes the more the nose tends to rise, you have pitch stability. Excess speed will pull the plane up out of a dive, and too low a speed will push the nose down.

Reply to
The Natural Philosopher

On Sat, 25 Aug 2007 23:46:46 -0500, "H Davis" wrote in :

I'm not an engineer, but some of my best friends are engineers. :o)

Your question arose because of what Bill Fulmer wrote about Romey Bukolt's page on "basic moments for RC planes":

The concept of moment is all about leverage and always includes a pivot point (you can't have a lever without a pivot).

So there is something rotational in it.

A one pound weight one foot from the pivot exerts a moment (tries to give momentum) of one foot-pound. If there is one foot-pound of leverage ahead of the pivot, the system will remain balanced; if there is less than one foot-pound of leverage ahead of the pivot, the lever will descend until it is lifting one foot-pound or until it hits the earth.

If you ever played on a teeter-totter, you know all about this. I suppose they're now outlawed, but they did teach some basic physics. In order for two children of different weights to balance, the heavier child would have to move toward the pivot and the lighter child would have to sit further away. (I was usually the heavier of the two. I seem to remember how uncomfortable it was to sit in front of the handles instead of behind them.)

My engineering buddy calculates inch-ounces when he is modifying a plane whose CG he already knows. To preserve the CG, every inch-ounce added to or subtracted from the nose has to be balanced by an inch-ounce added to or subtracted from the tail. To make the math easy, imagine adding an engine that is one ounce heavier than the previous engine and imagine that the engine is 10" ahead of the CG. We have changed the nose moment by 10 inch-ounces. To compensate, we need to add 10 inch-ounces to the tail moment. If the place where the weight will be added is 30 inches from the CG, 1/3 ounce of lead there will add 10 inch-ounces to the tail moment, leaving the CG where it used to be. The teeter-totter along the fore-and-aft axis of the fuselage will balance at the CG.

"In physics, the moment of force (often just moment, though there are other quantities of that name such as moment of inertia) is a quantity that represents the magnitude of force applied to a rotational system at a distance from the axis of rotation. The concept of the moment arm, this characteristic distance, is key to the operation of the lever, pulley, gear, and most other simple machines capable of generating mechanical advantage. The SI unit for moment is the newton meter (Nm)."

That reminds me of the sad story of the loss of the Mars Climate Orbiter in 1999. Both "foot pounds" and Newtons (the force exerted by one kilogram at one meter) measure the amount of leverage exerted on a pivot point, but one has to go through conversion of feet to meters and pounds to kilograms (or vice versa) to move information about moments of force from one system to the other.

Someone forgot to do this. The American team used foot pounds and the British team used Newton meters. (According to the folks who control this stuff, I should actually talk about pound feet and ounce inches instead of foot pounds and inch ounces--but I can safely ignore them when playing around with my small unmanned aircraft.)

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"Everybody now knows that 'the' cause was a failed translation of English units of thrust (foot-pounds; the force needed to move a pound a foot in a second) to metric (Newtons, which move a kilogram a meter in a second). True, but not the whole story. The metric conversion mistake was a classic hand-off goof, one side believing that the other was thinking the same way, but not checking."

Our ordinary language preserves the meaning of the physical term:

"The discovery of a wine is of greater moment than the discovery of a constellation. The universe is too full of stars" (Benjamin Franklin).

When we compare two moments to each other around the same pivot, the greater moment will prevail over the lesser moment. The heavier kid will sink and the lighter kid will rise.

Now that you realize that you have understood this all along, you're ready to get into debates about torque vs. horsepower:

"Horsepower sells cars; torque wins races" (Carol Shelby):

I don't think you will go too far wrong if you substitute the word "leverage" for "moment" when you're trying to decipher something someone said. "Tail moment" would then become "tail leverage."

Marty

Reply to
Martin X. Moleski, SJ

Marty if I recall another point of massive confusion is that the CG is not always the same as the BP. IIRC the BP is what you figure moments from an CG is used to stabilize flight.

I apologize if a mash that up it's been several years since I last did any of this and having teenagers has eroded my memories to the consistency of too thin tapioca.

Reply to
Keith Schiffner

Thank you Marty and TNP for educating me about a concept that I guess I did kind of understand without really realizing it. I feel less ignorant today, and that is a step forward.

Harlan

Reply to
H Davis

Thanks for that Bill, will come in handy and save a bit of head screatching.

Reg

Reply to
tux_powered

On Sun, 26 Aug 2007 08:32:49 -0600, "Keith Schiffner" wrote in :

I'm not familiar with that distinction.

There is a difference between CG, which we calculate on the fore-and-aft axis, and the center of pressure, which takes into account the sum of all of the lift produced by lifting surfaces. I think I heard that when the center of pressure is ahead of the CG, you have an airplane that can be trimmed and flown.

There is also a center of mass which is where all three axis are balanced. Folks who are trying to figure out what a thruster will do to a space vehicle have to worry about things like that.

It's the end of the Last Day of Summer for me. I was out with the club at a fun-fly & picnic, then drove to my family's camp for a little quick maintenance. No energy to google terms tonight ...

Marty

Reply to
Martin X. Moleski, SJ

Reply to
Paul Ryan

In article , Martin X. Moleski, SJ wrote: ...

| >What does the term "moment" mean in an engineering sense? ...

|

Yup, there's the answer.

And as for the rest, that's possibly the most rambling answer I've seen to a question in a very long time! Yes, thread drift happens, but not usually that much in a single post! :)

Reply to
Doug McLaren

Doug:

Sometimes rambling is a good thing. I'd hate to be part of a world where everything is linear. How boring it would be!

Ignorance is not a permanent condition, and as long as there are people who can supply both questions and answers. Is it blissful? Who cares as long as its interesting.

Harlan

Reply to
H Davis

wrote in

If the aircraft is in stable flight the CG and the center of pressure HAVE to be at the SAME point.

Thats teh center of gravity, by another name.

Reply to
The Natural Philosopher

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