Why This Group is Moribund



Well I went flying today. I had the opportunity to fall into a steep gully alongside where I park the car.... I resisted the impulse.
I clambered up the hillside, and later clambered down again without any displays of falling, tripping, whatever.
What was I flying ? A highly modified little plane from a plan in a magazine. I made a new wing and used a "freehand" curve to draw a nice airfoil shape for the wing ribs. I made an entirely new fin and stab for the back end and fitted a bigger motor than the original design had.
Would my homemade wing work ? Would the new backend have enough control authority ?
It worked a treat and I now have no fear of designing my own models. (I didn't say they would fly... I just have no fear now ! )
If compared against falling down ravines and peering at a model up a tree then I suppose my day was a failure >:-)
Reg
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tux snipped-for-privacy@nowhere.at-all.net wrote:

Oh I fell into a 6' drainage ditch while moving sideways to peer past a tree on finals.
I managed to not break the transmitter, or any bones.
The model went vertically in at around 60mph tho.
Curiously enough, the expensive bits all survived. The model was totalled tho.

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wrote:

Nothing I've read yet is an interesting as Jessica's firm, round - uh - what was I talking about?
It is too hot down here to even think about flying. My new Wing Dragon 4 is lying on the bench, still unassembled. It won't take long to get it going, but what's the point? Can't fly cause it is too hot.
Ed Cregger
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On Thu, 23 Aug 2007 14:43:27 -0400, "Ed Cregger"

Amen, Ed..
David in central Ga.
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| Nothing I've read yet is an interesting as Jessica's firm, round -
... wing tips?
| uh - what was I talking about?
Got me. | It is too hot down here to even think about flying.
Here (Austin, TX) too. So I just go out and do it!
Shorts, sun screen and lots of water ...
--
Doug McLaren, snipped-for-privacy@frenzied.us
I have never met a man so ignorant that I couldn't learn something from him.
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tux snipped-for-privacy@nowhere.at-all.net wrote in 2.news.uk.tiscali.com:

And that's the trick to it. I fly a lot of my own designs; a few fly very well, some fly well, some fly okay, and a few don't fly at all (my twin- tandem springs to mind. Ah, well).
If a new design works, then great! If it doesn't - well, I like designing & building 'em; a good crash just gives me an excuse to start a new one. And I always learn something from a design that doesn't work.
--
"Whatever will have been, will have been."

- Douglas Adams, "Life, The Universe, and Everything"
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-----------
I thank you fellows for reminding me of just how much fun designing, building and flying your own models actually is.
I had completely forgotten.
Ed Cregger
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"Mark Miller" > wrote

I find more than half the enjoyment of this hobby through designing and building my own airplanes. It is what turns my crank.
That is what it is all about. Doing what pleases you. To each his own.
I don't see how the OP came off saying that most in here don't do much flying. It is hard to tell how much they do from the posts, and it seems like a lot do plenty.
--
Jim in NC



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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
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Doing your own design is fun.. This chart by Romey Bukolt gives some basic moments for RC planes. You may find it useful...
See http://www.palosrc.com/instructors/basicdes.pdf
Cheers,
Bill
wrote:

very
designing
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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

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H Davis wrote:

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.

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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.
<http://en.wikipedia.org/wiki/Moment_ (physics)>
"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.)
http://www.sfsite.com/fsf/2000/ben0007.htm
"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:
<http://en.wikipedia.org/wiki/Torque
"Horsepower sells cars; torque wins races" (Carol Shelby):
<http://craig.backfire.ca/pages/autos/horsepower
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
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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.
--
Keith Schiffner
Assistant to the Assistant Undersecretary of the Ministry of Silly Walks.
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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
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Martin X. Moleski, SJ wrote:

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.

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I'm not sure which hairs you are trying to split here, or what definitions you could be using differently.
Perhaps the fact that the center of lift is ahead of the CG, and the tail is providing downward pressure to keep the plane stable, as is true in most conventional light aircraft and models?
--
Jim in NC



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Yes--but it can be achieved by a little down elevator instead of by the native lift of the surfaces.

You are, of course, correct.
When we use the term "center of gravity", we are concerned only with one axis instead of three--fore and aft balance. We ignore the lateral and vertical location of that center for all practical purposes.
"The CG is the point at which the total weight of the aircraft is assumed to be concentrated, and the CG must be located within specific limits for safe flight. Both lateral and longitudinal balance are important, but the prime concern is longitudinal balance; that is, the location of the CG along the longitudinal or lengthwise axis. ...
"The basic aircraft design assumes that lateral symmetry exists. For each item of weight added to the left of the centerline of the aircraft (also known as buttock line zero, or BL-0), there is generally an equal weight at a corresponding location on the right.
"The lateral balance can be upset by uneven fuel loading or burnoff. The position of the lateral CG is not normally computed for an airplane, but the pilot must be aware of the adverse effects that will result from a laterally unbalanced condition. [Figure 1-4] This is corrected by using the aileron trim tab until enough fuel has been used from the tank on the heavy side to balance the airplane. The deflected trim tab deflects the aileron to produce additional lift on the heavy side, but it also produces additional drag, and the airplane flies inefficiently. Figure 1-4. Lateral imbalance causes wing heaviness, which may be corrected by deflecting the aileron. The additional lift causes additional drag and the airplane flies inefficiently. Helicopters are affected by lateral imbalance more than airplanes. If a helicopter is loaded with heavy occupants and fuel on the same side, it could be out of balance enough to make it unsafe to fly. It is also possible that if external loads are carried in such a position to require large lateral displacement of the cyclic control to maintain level flight, the fore-and-aft cyclic control effectiveness will be limited."
http://www.faa.gov/library/manuals/aircraft/media/FAA-H-8083-1A.pdf
                    Marty
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Backwards. The center of pressure is BEHIND the CG for stability. With the pressure behind the weight, the tail's downforce balances it. Airplanes with the CP ahead of the CG are illegal in the real world and require a lifting tail. The last airplanes to do this were built during WWI. If they stalled they were unrecoverable. Canards, of course, are different, but the legal requirement is that the nose falls when the power is reduced so that airspeed is maintained without unusual pilot skills.

Wrong again. That is a most unstable configuration. All aircraft have a CG range that must have the CP behind the aftmost CG, for the above reasons. Many airplanes have gone out of control and crashed when they were loaded too far aft.
See http://www.av8n.com/how/htm/aoastab.html
Regarding "moment": the airplane is longitudinally balanced around its CG. An airplane that turns out nose-heavy, let's say, will need weight added to the tail to balance it. But it doesn't end there: this airplane will now have some less-than-desirable characteristics. If it enters a spin, the tail's weight and the nose's weight are rotating at different levels around a common pivot axis, and they want to come into line with each other: nose up, tail down. Because of this, a balanced airplane that has increased weight at both ends will have a stronger tendency to go flat in the spin and may become unrecoverable.
Dan
Dan
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On Wed, 29 Aug 2007 12:26:55 -0700, Dan_Thomas snipped-for-privacy@yahoo.com wrote in

AH--that's it. I was thinking yesterday that I had gotten it backwards ... I had the right picture of the needed trim (down elevator) and got the reasoning why all wrong.

Great stuff. I don't see the need to use down pressure in recovering from the spiral dives I or my students have gotten into, but it sounds like potentially life-saving information for a full-scale pilot.

                    Marty
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