Questions from Reading Model Aerodynamic Books

[Apivat J]

Hello All,

The reason I started this thread is because I feel there must be a lot of persons like me who is reading or has read Andy Lennon and Martin Simmon books and have soooo many questions that no one near you can reply or explain to you (since you start talking Martian language after that).

So, the purpose of this thread is for everyone who has read the two books to ask any questions you may have about them and hopefully the aerodynamic gurus out there would be kind enough to show us the way. Hey....,those are two great books IF you can understand them thoroughly.

Let me start the first one....

In Chapter 7 and 8 of Basics of R/C Model Aircraft Design by Andy Lennon, the author explains about the forces the horizontal tail has to overcome. He also explains that CG is the point where all forces (weight, pitching moment, etc.) act upon. In Chapter 6, we were thought how to find the CG location which is 25% MAC (Plan view below shows you where this might be). It is here that I was lead to believe that the CG location is on the wing (see side view pic below) but was surprised when the auther said "locate the CG vertically". How do you do that? Vertically from where and by how much?

I would greatly appreciate any answer that would point me in the right direction. Thanks.

[vintage1]

Its easy enogh to find the CG by suspensding the model from various ponts and using a plumb line.

If you look at the model and the plumb lne, and make sure that where you hang the model from, and the plumb line, are visually in the same place, the line of the plumb line MUST pass through the CG.

As far as the CG being 'the place where all the forces act upon' that is a simplistic explanation.

Just because its convenient mathematically to separate forces into 'lift' 'drag' 'thrust' weight' etc etc doesn't mean there are all these spearate 'forces' acting on the model.

Overall a model in steady flight has no forces acting on it......

[Tom Harper]

Vintage,

"Overall a model in steady flight has no forces acting on it"

Or at least they are in balance.

I have a high thrust line model that has a tendency to dive on launch. Would raising the cg decrease this tendency by shortening the moment between the thrust line and the vertical CG?

[slipstick]

In plan view the CG should be located somewhere around the 25% MAC line. In side view the CG is usually located somewhere on the fuselage. I.e. if you hold the plane on its side it balances somewhere. The true CG is the point at where the plane will balance perfectly....at any angle, whether it's flat or knife edge or anywhere in between. It's usually positioned somewhere inside the fuselage just over the 25% MAC line.

BTW vintage1, the stresses in a wing when the plane is in steady flight would suggest there are some forces involved. The vector sum of the forces may be zero but that doesn't mean there are no forces, just that the net effect is equal and opposite. Weight doesn't simply go away in steady flight, nor does the lift supporting it so it's a bit misleading to claim there are "no forces acting" ;).

Steve

[vintage1]

Vintage,

"Overall a model in steady flight has no forces acting on it"

Or at least they are in balance.

I have a high thrust line model that has a tendency to dive on launch. Would raising the cg decrease this tendency by shortening the moment between the thrust line and the vertical CG?

Probably not, since what is relevant is more the center of drag. A high mounted tail might help more.

[Tom Harper]

Vintage,

Once the model attains forward velocity aerodynamics takes over and everything is OK. The problem is the launch. The high thrust line causes a dramatic nose down vector. The thing is difficult to hold in a straight line. I'm using a low mounted stab for structural reasons in an AP design:

[vintage1]

In plan view the CG should be located somewhere around the 25% MAC line. In side view the CG is usually located somewhere on the fuselage. I.e. if you hold the plane on its side it balances somewhere. The true CG is the point at where the plane will balance perfectly....at any angle, whether it's flat or knife edge or anywhere in between. It's usually positioned somewhere inside the fuselage just over the 25% MAC line.

BTW vintage1, the stresses in a wing when the plane is in steady flight would suggest there are some forces involved. The vector sum of the forces may be zero but that doesn't mean there are no forces, just that the net effect is equal and opposite. Weight doesn't simply go away in steady flight, nor does the lift supporting it so it's a bit misleading to claim there are "no forces acting" ;).

Steve

I am so glad you rose to the bait :D

Indeed, but the main point was to open up an appreciation of the difference between vector arrows on a 3 axis plot, and wings folding up in mid dive. :D

Its important I think to realise that splitting forces into lift, drag and thrust, and taking about these 'acting round the center of gravity' is onlya way to make the maths simpler. Its actual physical meaning, if any, is a lot more tenuous.

I have a bit of an axe to grind here. I am rather concerned that most people - including scientists - do not have an appreciaition between the various types of understanding: That at its most basic, we assume there is a world out there, and on top of that, there is the world we think we perceive through our 'senses' around us, and on top of that is an idealised mathematical model of what we think we perceive it to be,that we call science..that is actually twice removed from reality.

Leaving out the first extremely metaphysical an philosophical (or neurological) bit, its patently obvious that if wings break in a dive etc, its not some idealised arrow of force called 'lift' acting 'on the center of gravity of the model' that is causing it.....


The truer picture is one of surface integrals and the calculus of vectors, and is shockingly complicated to even understand the basics of. I know because I am trying to at the moment elsewhere :D Newton's Laws, applied to Euclidian plane spaces, are already extreme simplifications when compared to relativity and quantum effects, are still massively complex and incomputable in their details. Which is why we can't - even with the best computers avbailable - come close to predicting turbulent flow and rely still on things like wind tunnels and test pilots.


In then end, I merely want to issue a warning that these idealised and simplified models are, at best, an aid to grasping a little of what is happening to the plane. They are definitely not 'what is really happening' :D

At a deeply philosophical level, the answer is nobody knows.

Engineers content themselves with thae more limited solution, that their planes appear to fly, and that is a sufficient justfication for their methodology :D

[vintage1]

Vintage,

Once the model attains forward velocity aerodynamics takes over and everything is OK. The problem is the launch. The high thrust line causes a dramatic nose down vector. The thing is difficult to hold in a straight line. I'm using a low mounted stab for structural reasons in an AP design:

Good grief. Is that the Talibans answer to the Warthog? :D

Indeed, you have created a rod for your back. The low mounted tail will be a load of drag that will result in a definite nose down moment until it gets enough speed to develop downforce.

A more rearward C of G might help a bit, if you can keep it stable as speed. It looks like it shares the same problem as the Multiplex jets - you need to roll on full up trim at launch to get the nose up until the model is up to speed. And throw it HARD.

Lifting the prop skywards a bit might help. To angle the thrustline down a bit nearer to the center of drag. If you like the design has massive 'downthrust' in it already.

However that in itself will introduce a thrust vector in exactly the wrong way to help the model climb.

I would suggest launching it upside down frankly. :D

[Tom Harper]

Vintage,

The configuration meets my needs as an aerial photography platform. Compact, rugged and able to carry some weight.

It is roughly based on Jerry Stoloffs YOGI free flight from 1944. Stoloff balanced it at the 50% point and used about 3 degrees difference between the wing and stab. I initially set it up at 0-0 because of the big stab surface. It flies with the CG at 50%. I just removed the stab and reset it negative.

I hope that does the trick because the inverted launch could be iffy.

[Apivat J]

The conclusion drawn so far seems to be "trial and error" kind of method in locating the CG vertically. From a scratch builder point of view, the interest is how do you determine the location of CG, center of pitching moment, and center of drag before you even build the model....Or is it the other way round as Vintage said in reply #7?

Seriously, I really want to know. If you read Basics of R/C Model Aircraft Design Pg 37, 2nd and 3rd column and Pg 38 first column, you'll know what I'm trying to get at.

[Sparky Paul]

Tom. you have yet another nifty idea!
I'd be concerned about the longitudinal stability.. that short coupling won't be good for smooth flight.
As for the launch, I'd add wheels!
This one uses the GP Spirit 2M wing, OS 15. It's sensitive to power changes, but other than that it flies like any 2M.
I use the wheels because my throwing arm has been damaged..

[Apivat J]

Take a look at this Extra EA300L. I'm sure CG, Lift, Pitching moment, and thrust are definitely "in-line" with one another. Center of Drag??? I don't know. But from the look of it, performance should improve over the earlier version with the wing below thrust line.

[Tom Harper]

Apivat,

The idea of design implies that you are varying a configuration with some goal in mind. The vertical location of the CG may or may not be a parameter that impacts your goal. Trail and error is a way to find out.

A model airplane is an analog computer with an infinity of inputs and a single output -> it does what you want.

Varying parameters is tricky because everything depends on the configuration you already have. Moving the CG vertically on one aircraft may have no effect. On another one it may have a dramatic impact. That's why this stuff is fun.

[Tom Harper]

Sparky,

Yeah, you're right, a gear would solve a lot of problems - especially when debugging the early flights. I'm going to rig a steerable gear on the next one. It'l be bolt on so that once the bugs are worked out I can remove the gear and stick in the camera.

Neat airplane - is that 'down' thrust behind the CG?

[Sparky Paul]

Yes, that's downthrust.. I was concerned about the high motor pushing the nose down into the tarmac on takeoff.
It takes off cleanly, and has a very good climb!
The motor idles well, so it can floar around with power once it's up.
The 2 oz tank is more than enough.
It has the tilting camera mount for the Casio, can look horizontally to the side or straight down.
As for vertical c.g.s, the most common intance of the effect of the vertical position is on a high-wing plane, like a Kadet, which will pitch up with power, if there's no downthrust in the motor.
Old timey free-flights with high pylons have lots of downthrust for this reason.

[Sparky Paul]

Couple sketches based on the results of #10...
Using a standard 2M wing, approximately 9"x76".. or more span..
Either glow or electric, with the thrust line thru the wing, to minimize the pitching due to power and speed.

[Tom Harper]

Sparky,

The design I am working on is close to your II-I. I was trying to get a respectable aspect ratio with the cheap foam construction. I didn't want to support booms and I wanted to stick with foam board. One evening I was thumbing through an old Model Airplane News (1944) and ran across Stoloffs YOGI. I did some sketches and rejected the idea at first but then thought that the short coupling couldn't be any worse than a delta.

Also decided to use elevons on the stab to remove any control burden from that foam wing and to avoid linkages to the twin rudders. So far so good - it all seems to work. The model is not as rugged as a delta but it does not hit as hard either. It has survived a couple of landings in the creosote bushes.

I'll put some time on it this weekend but I think I'll shelve it then. It was an experiment and I have a mod 2 on the way. This one uses a left wing aileron combined with an elevator. Also, I took your advice and built a removable gear dolly. The wing is from a New Ruler that I built in 1967:

[Apivat J]

Tom,

Certainly, I have some design objectives in mind. On the top level, the model must easy to handle and enjoyable to fly. Actually, I'm working on a 1/10th scale P-40E project and I was wondering why a true-to-scale model don't fly that well. As we speak, the prototype has already been flown but I'm not too happy with it.

In the other post (http://www.rcgroups.com/forums/showthread.php?t=319011) our fellow friends in the forum told me it has weight problems. Thanks guys. That's got me started review all model design concepts again. Then, I found there are so many things I don't understand or misunderstood completely. Well, it's my first time exploring the flight theory. I guess it has to be painful before you'll learn somthing.

By the way, after searching over the past few days for the answer to my own question of finding vertical CG location, I found this article to be useful
http://www.steenaero.com/articles_detail.cfm?ArticleID=59

I don't know if you guys agree but verticle CG do have its significance at least in the proper placement landing gear as indicated by the article.

[Tom Harper]

Apivat,

It looks like a tenth scale P-40 would be:

...............Model................Original

Span........3.73 ft...............37.3 ft
Area.........2.36 sq ft...........236 sq ft
Weight......5.5 lbs...............5500 lbs
Power........33 HP...............1040 HP
Wing Ldng...2.33 lbs.sq ft.....233 lbs sq ft

So that's 37 oz per square foot for scaling the P-40 at it's unloaded weight. To deliver an honest .33 horsepower to the prop takes something between a .40 and .51 engine. The scale take off velocity should be around 44 FPS. The climb rate should be about 15 ft per second max.

The question is - what's scale flight? Real airplanes do not bore holes in the sky like models. And, a P-40 is not an endurance free flight. By comparison real airplanes are modest performers. If your model rolls down the runway, climbs out, makes shallow turns and returns safely it is performing in a scale manner.

It would be interesting to have a scale event where power and weight were scaled rather than only dimension and features. I have been at the field when three models of similar size showed up - a Piper cub, an AD1 and a P-47 - all of similar linear dimension, give or take 8", all with some kind of .60 engine and all with a 12X6 prop. Any comparison of 'scale' performance was superfluous.

The CG question is really one of flight dynamics not aerodynamics. The grouping of forces is discussed at length in some of Zaics yearbooks. Also, in the old texts by C.H.Grant. His views on CLA have been argued against but his discussion of CG position may be the best you are going to find.

[Tom Harper]

Arpivat,

When I try to build a display model I find that by the time I get it to fly it is repaired and modified to the point where it is no longer 'display'. The best solution is to build a profile approximation to 'debug' then go for the finished product. You can start with something like this:

[Ollie]

Paramotoring & Powered Paragliding get their pitch and lateral stability low CG far lower than the wing. The stability doesn't come from dihedral, reflex airfoil or tail.

[kdwzagi]

I am so glad you rose to the bait :D

Indeed, but the main point was to open up an appreciation of the difference between vector arrows on a 3 axis plot, and wings folding up in mid dive. :D

Its important I think to realise that splitting forces into lift, drag and thrust, and taking about these 'acting round the center of gravity' is onlya way to make the maths simpler. Its actual physical meaning, if any, is a lot more tenuous.

I have a bit of an axe to grind here. I am rather concerned that most people - including scientists - do not have an appreciaition between the various types of understanding: That at its most basic, we assume there is a world out there, and on top of that, there is the world we think we perceive through our 'senses' around us, and on top of that is an idealised mathematical model of what we think we perceive it to be,that we call science..that is actually twice removed from reality.

Leaving out the first extremely metaphysical an philosophical (or neurological) bit, its patently obvious that if wings break in a dive etc, its not some idealised arrow of force called 'lift' acting 'on the center of gravity of the model' that is causing it.....


The truer picture is one of surface integrals and the calculus of vectors, and is shockingly complicated to even understand the basics of. I know because I am trying to at the moment elsewhere :D Newton's Laws, applied to Euclidian plane spaces, are already extreme simplifications when compared to relativity and quantum effects, are still massively complex and incomputable in their details. Which is why we can't - even with the best computers avbailable - come close to predicting turbulent flow and rely still on things like wind tunnels and test pilots.


In then end, I merely want to issue a warning that these idealised and simplified models are, at best, an aid to grasping a little of what is happening to the plane. They are definitely not 'what is really happening' :D

At a deeply philosophical level, the answer is nobody knows.

Engineers content themselves with thae more limited solution, that their planes appear to fly, and that is a sufficient justfication for their methodology :D


my 5 year old made a paper plane yesterday and it flew very well

some so called professors with multi million budgets still fail to design a working prototype

sometimes you can look too hard and see nothing

play with some toys and have fun :D ;)

kdw

[vintage1]

Isn't that what we are all doing anyway?

[kdwzagi]

Isn't that what we are all doing anyway?


sa :D

[slipstick]

my 5 year old made a paper plane yesterday and it flew very well

some so called professors with multi million budgets still fail to design a working prototype

sometimes you can look too hard and see nothing

play with some toys and have fun :D ;)
If all you want to do is play without worrying about the "why" of things good luck to you and your 5 year old. But if you have no interest in Science as applied to Modelling there's probably not much point you reading the Modeling Science forum. That'll then save you the trouble of talking down to those of us who are interested ;).

(OTOH even I draw the line at vintage1's eccentric mix of science, pseudo-science and pop philosophy ;))

Steve

[vintage1]

I resent that. I've studied all three to far more than the pop level mate!

Its not easy to try and present a simple picture of stuff that is weird and hard to come to grips with.

However I will leave yo with two gems from a friend who has a doctorate in philosophy.

"90% of the problems in French Philosophy would have been solved if they had written in German"

"Popper said that nothing is proveable, and science is merely unsdisproved hypotheses"

The former is amusing, and almost true. The latter is deeply relevant to the continuous discussions that go on here between e.g. Bernouilli/Newtonian stuff.

Its is really really important to distinguish between what happens in the real world - 'stones fall' -and the 'existence of gravity', which is merely a mathematical description of the WAY in which stones fall. For example.

Gravity doesn't explain WHY stones fall. It simply replaces the question 'why do stones fall?' with the question 'why is gravity there?'

Gravity doesn't 'exist' at quite the same level as a stone does either.

All science is,. is a predictive map we lay over the world, that allows us to predict the future, sometimes well, sometimes poorly.

It is a mistake to confuse that map, with the world though. Maps are limited, don't show everytbng, and are drawn on paper that is flat, whereas the world is not. To extend the analogy.

Unversities are full of people who went looking for what turned out after all, to be no more than a rip in the map, or a coffee stain on it...:D

So I repeat my warning: What you learn in science is not fact. It is the use of predictive maps and mathemetical tools that work well enough to help you build e.g. toy planes that fly better.

I particular the use of orthogonal axes in vector algerbra does NOT mean there is 'lift' 'drag' 'weight' 'thrust' in real terms. These are mathematical conventions only. A wing does what a wing does: Its convenience ONLY to seperate the force(s) on it into thrust, lift, drag and weight. Not to mention 'induced drag' and 'profle drag'.

As long as this is appreciated, that these are semantic and mathematical conventions, then there is no harm in talking about them loosely as 'facts' .

[slipstick]

Yeah, yeah. And quantum mechanics need to use really funny shaped spanners ;).

Having a sort of mathematical background I can assure you that mathematics is indeed factual and entirely reasonable. Applying it to an unreasonable universe can be a bit problematic. Fortunately at the level of detail that we need almost all the universe is content to behave as though the theories were actually correct. The numbers that come out reflect what happens.

Sure it's only a representation but so's everything else. I'm not really seeing this screen as I'm typing I'm seeing my brain's interpretation of a bunch of nerve impluses generated by light scattered from....etc. There doesn't seem to be much point getting all worked up about the fact that all of a plane's weight isn't actually at the CG. Everybody knows that same as no-one actually thinks that all the lift is generated at the CG and all the rest of the wing doesn't do anything. It's too obvious to waste your time labouring.

And all of that in a vain attempt to defend your initial statement that there were NO forces acting on a wing in steady flight which is basically gibberish at any level of abstraction :(.

Steve

[vintage1]

Yeah, yeah. And quantum mechanics need to use really funny shaped spanners ;).

Having a sort of mathematical background I can assure you that mathematics is indeed factual and entirely reasonable. Applying it to an unreasonable universe can be a bit problematic. Fortunately at the level of detail that we need almost all the universe is content to behave as though the theories were actually correct. The numbers that come out reflect what happens.


Then you haven't studied enough cutting edge physics, or mathematics :D




Sure it's only a representation but so's everything else. I'm not really seeing this screen as I'm typing I'm seeing my brain's interpretation of a bunch of nerve impluses generated by light scattered from....etc.


or is it? That in itself is just another theory. And it gets logically very hard to defend a theory that depends on the results of the very same theory. Namely that we exist, can think, and make models of a world 'out there' that actually in some way correspond to what it really is.

It's equally plausible (though unncessarily complicated in an Occamian sense) to consider the solution prpsed in the Matrix. For example. You cannot actually prove that it is not so...but Popper can relieve is here by defining a theory that cannot be disproved as 'non-science'. Science can have nothing to say about it.


There doesn't seem to be much point getting all worked up about the fact that all of a plane's weight isn't actually at the CG.

until it breaks up in flight, because the weight and lift is distributed throughout the structure, not at the CG...:D

Everybody knows that same as no-one actually thinks that all the lift is generated at the CG and all the rest of the wing doesn't do anything. It's too obvious to waste your time labouring.


That point maybe, but how many peple think a wing generates lift and suffers from drag, and thats a hard fact, ratheer than perhaps thinking about a resultant vector that is angled back from vertical?




And all of that in a vain attempt to defend your initial statement that there were NO forces acting on a wing in steady flight which is basically gibberish at any level of abstraction :(.

Steve

I think if you bother to read what I wrote, that there are no net forces acting on an aircraft in level flight, and bother to apply newtons laws of acceleration to it

F=mA. A plane in steady level flight or a steady glide is not accelerating. Therefore, unless its mass is infinite, there are no net forces acting on it.

Perhaps its time to study some more maths texts?

And even if you consider the wingtips have lift on them, why aren't they accelerating upwards?

A little knowledge is a dangerous thing. A little knowledge possessed by someone who thinks he has it all, is deadly...;)

[hwhall]

It is important to recognize a difference between forces and net, or sums of, forces. Gravity still exerts force upon the plane's masses, but if sum of all lifting forces exactly oppose sum of all gravity forces the net force (considered for the whole plane) is zero along the vertical & thus no net acceleration. That both lift & gravity forces still exist independently is seen in the mechanical stresses upon plane components (e.g., wing root, etc.). That is, the net zero force is not valid if you look only at individual parts of the plane, it holds only if everything is summed.
--Wayne

[slipstick]

Tell you what, I'm really bored with this so why not just go back and edit your earlier message so it actually says what you've now decided you should have written ? Then you can claim you were right all along.

A little knowledge is a dangerous thing. A little knowledge possessed by someone who thinks he has it all, is deadly...;)
Recognising that you have a problem is the first step on the road to a cure. Good luck with your further steps ;).

Steve

[Thomas B]

Sparky, I usually use a good bit of upthrust in high pylon mounted motor situations.

Rigged correctly, you can get rid of the vast majority of power change/pitch change issues with high pylon mounted motors.

Note the upthrust in my little Skimmer parkflyer depron flying boat. This upthrust makes a HUGE difference in the takeoff performance as well as how it reacts to power changes....

[Sparky Paul]

My thingy doesn't have any really awful flight characteristics with the downthrust.
Performance wise it's as capable as an electric "normal" plane both of which I flew Sunday.
I'm looking at building the pusher version, with either glow or electric.. interestingly, the glow power pod and electric power pod weigh the same, 11 oz, but the 19 to 22 oz battery for the electric is not a plus. :)

[Apivat J]

Tom,

Your estimate on the dimension was quite close to the modified version of my P-40. From calculation, I find it hard to faithfully scale it down without some alteration to the dimensions of the plane. It'd probably fly with original scaled down dimensions but it'd fly better if modified. But that's only in theory...I need to try it out and I'll follow your advice on the profile build. That should save me lots of time and effort.

Thanks

[vintage1]

Tell you what, I'm really bored with this so why not just go back and edit your earlier message so it actually says what you've now decided you should have written ? Then you can claim you were right all along.


No need, since I said what I meant right there .

Overall a model in steady flight has no forces acting on it.

The integral of all the supposed force vetors over the whole aircraft equates to zero? Rather see that?

It was, I admit, troll bait to see who understood it, and whose knowledge was limited enough that they would immediately scream 'wrong!'.


Recognising that you have a problem is the first step on the road to a cure. Good luck with your further steps ;).

Steve

Indeed it is. The more you learn the more you realise how limited and slippery the so called 'scientific facts' we claim to know really are...

If everybody hasd a masters in science and philosophy, there simply would not be a debate about creationism versus Darwinism etc.

In the same way that Bernuilli and Newton present no final conflicts, or supert string theory verss quantum field theory etc.

[adam_one]

Overall a model in steady flight has no forces acting on it.

The aircraft has forces acting on it even when it is standing still on the ground!

Vintage1: you have forces acting on yourself even when you are sitting motionless in your chair!

It's obvious that an aircraft in flight is always subject to aerodynamic forces even if the net sum of those forces is equal to zero, regardless it is on a steady flight or just standing still in the air... :rolleyes:

[Wingin' Wayne]

Can't we all just get along! You experts are confusing us rookies. I know engineers and scientists don't always agree on theory, when I was a modelmaker I heard it all the time. It's okay to voice your opinion, it's good to get more than one point of view, but is mudslinging needed? I think part of the problem with listening to experts is they know the subject matter but don't know how to teach! We are seeking your advice, when someone asks a newbie question, it would help to get an answer a newbie can understand! Save your engineer-speak for brandy and cigars in the library.

[slipstick]

Wayne, this thread was started by someone who was reading books about aerodynamics theories and wanted help understanding them. If you ask questions about theories you get answers about theories. The Universe does not become simpler just because you don't like complex answers. Sorry ;).

BTW this is a discussion group for anyone to discuss subjects they're interested in. It's not a kindergarten. If you don't like the discussions you are entitled not to read them ;).

BTW Vintage good luck with the "Deliberate bait" excuse....even a basic grasp of English allows one to work out that "The vector sum of all the many forces acting on the plane is zero" and "There are no forces acting on the plane" are very different statements. One of them is reasonably true (depending on the reference point), the other is not. I suspect you understand that perfectly well but you've now dug in and are simply unable/unwilling to rephrase your original statement. Oh well :(.

Steve

[vintage1]

The aircraft has forces acting on it even when it is standing still on the ground!

Vintage1: you have forces acting on yourself even when you are sitting motionless in your chair!

It's obvious that an aircraft in flight is always subject to aerodynamic forces even if the net sum of those forces is equal to zero, regardless it is on a steady flight or just standing still in the air... :rolleyes:


Try looking up the meaning of 'overall' in a dictionary..

That is what the 'net sum equals' (zero) means..

The point of the point was to illusrtrate the exact differnce youi have pointed out. That a diagram in a book with the odd arrow from the CG downwards and an upwrads arrow called 'lift, and a couple of other arows called 'thrust' and 'drag' is a gross oversimplification and takes no account of exactly the pont you made.

I merelty carried the gross oversimplification one stage further, and declared that since the arrows all cancel out in steady flight, why bother to have any arrows at all?

:D

The fact that some people are prepared to accept one over simplification but not another, sorts the sheep from the goats..

[vintage1]

BTW Vintage good luck with the "Deliberate bait" excuse....even a basic grasp of English allows one to work out that "The vector sum of all the many forces acting on the plane is zero" and "There are no forces acting on the plane" are very different statements. One of them is reasonably true (depending on the reference point), the other is not. I suspect you understand that perfectly well but you've now dug in and are simply unable/unwilling to rephrase your original statement. Oh well :(.

Steve

Steve. I never said there are no forces acting on the plane., I used the qualifying adverb OVERALL.

I did not have sex with that plane! :D

And in the context, of the overall acceleration of the plane, that can be seen to be a completely true statement.


If you think that three years spent getting a masters in engineering means I didn't carefully choose my words with a very careful purpose in mind, then I assure you, you are wrong.

[Wingin' Wayne]

Slipstick, I realize we are dealing with complex topics! My life and career are founded in math and science. Not to the degree yours is. Part of my job is teaching apprentice machinists. I use different terminology and theory with different students. I teach at a level they understand. Again, this should be an educational forum, not a pissing match between engineers. If I want that I'll go to work. Maybe you guys should have a rugby match and have it out ounce and for all!

[adam_one]

Try looking up the meaning of 'overall' in a dictionary..
That is what the 'net sum equals' (zero) means..

Anyway, it doesn’t make your assumption correct, regardless if it is with or without "overall”.
The arrows are shown there simply because those forces actually exist and are acting on the plane, which make the plane behave that way.

[vintage1]

You still don't get it do you?

[adam_one]

Vintage1.
(for the last time)
One should not say that in a steady flight "the plane has no forces acting on it" even if you put the word "overall" because such statement is incorrect and misleading.
This because despite the sum of all forces are zero at that moment, it does not mean that the plane has no forces acting on it.
I merelty carried the gross oversimplification one stage further, and declared that since the arrows all cancel out in steady flight, why bother to have any arrows at all? Again, you show how you are misunderstanding this issue, because if you take away all those arrows, the plane is not flying anymore, this because the arrows show (albeit in a simplified form) why the plane is in the air with a certain constant speed and altitude.

I regret, but it seems to me that you've only got the ability to confuse yourself after those years spent getting master engineering.

[vintage1]

Mmm. You STILL don't get it.

According to Newton

A body will remain at rest or at a constant veliocity IF THERE ARE NO FORCES ACTING ON IT.

It seems your comprehension has not even reached the 18th century, let alone the 20th, and here we are in the 21st...

[Sparky Paul]

Mmm. You STILL don't get it.

According to Newton

A body will remain at rest or at a constant veliocity IF THERE ARE NO FORCES ACTING ON IT.

It seems your comprehension has not even reached the 18th century, let alone the 20th, and here we are in the 21st...
.
There are forces acting on a plane in trimmed level flight.. All of them are balanced, but they're there. :)
Change any one of them, and the delta change changes the level flight parameters....

[slipstick]

Mmm. You STILL don't get it.

According to Newton

A body will remain at rest or at a constant veliocity IF THERE ARE NO FORCES ACTING ON IT.

It seems your comprehension has not even reached the 18th century, let alone the 20th, and here we are in the 21st...
That's just your crude paraphrase of Newton's first law. It is of course true that IF there are no forces there is no acceleration. But unless you imagine that gravity, drag, lift etc have all been repealed for the special case of a plane in steady flight then there are forces acting on the plane.

Newton was actually a lot brighter than that and a more accurate statement of his first law is
"An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force."

Anyone who believes that there are no forces acting on a plane in steady flight has only to modify one of the non-existent forces, e.g. by switching the engine off, to discover that the other non-existent forces soon show up and make the plane change its behaviour ;).

Steve

[adam_one]

A body will remain at rest or at a constant veliocity IF THERE ARE NO FORCES ACTING ON IT. That statement is only valid if the body is located at the outer space outside the hearth's gravity force and atmosphere...
The structure of any plane that is moving through the air and subject to gravity is always subject to forces, regardless it is in steady flight or not.
In fact, the plane's structure may indeed collapse in case it is not able to stand those forces even if the plane is actually hanging still in the air during strong winds… :)

Vintage1, you never give up your confusing tacticts, do you?
It seems your comprehension has not even reached the 18th century, let alone the 20th, and here we are in the 21st...I think you owe us all an apologise or at least you should show us your gratitude for our patience in taking the time trying to bring knowledge into your confused mind… :rolleyes:

[vintage1]

Adam. I am discontinuing reposnses to the thread.

I am sure the hundreds and thousands of practicing scientists and engineers will be delighted to be informed by you that Newtons Laws only work in outer space, when they have been religiously using them to design earth bound machinery for years.

Get a life, or better still, a science education ;)

[biber]

The typical engineer doesn't care about philosophy but does the needed simplifications to get a result of acceptable exactness within a reasonable expense. That is not correct maybe from philocientific point of view, but will do for us as long as we are still designing, building and flying airplanes. So please calm down. There is no need to understand the whole world, but it' s helpfull to deal with the physical conditions around us and to be able to calculate and figure out something in order to get a good working flyer :p . If there are no net forces or no forces at all is equal for some certain calculations, so in that cases i don't bother if it's reality or just imagination ;) . Hope nobody feels attacked after that. I would like the discussion to recover, not to excess.

Biber

[adam_one]

I am sure the hundreds and thousands of practicing scientists and engineers will be delighted to be informed by you that Newtons Laws only work in outer space,...I notice you're using now the word “laws” (plural), while you only have brought up some statement that might be related to one of the Newton’s laws…

Newton's first law of motion:
An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

You are simply misinterpreting the Newton’s law.

[slipstick]

Adam I'm afraid you're wasting your time. You have to realise that you're not actually discussing physics here. The problem is that vintage1 has convinced himself that the statement "Overall there are no forces acting" has the same meaning as "There are many forces acting but they are in balance".

It's not so much that he doesn't understand the physics it's an inability to express this understanding accurately in English. Well actually he probably is able by now but there's no way he's going to concede that there was anything wrong with his original statement. It's probably some sort of macho Master of Engineering thing ;).

Steve
(no qualifications worth continually harping on about, sorry :().

[vintage1]

Well then lets say that there are an infinite number of forces actng then shall we? Or at least as many as there are molecules in the surface covering...and then there's all the internal forces, acting between the molecules of the structure,....so our diagram should be covered in an infinite number of arrows, not just 4...

What I obejct to is the simplistic assumption that there are (just) four forces acting. So I make a suggestion that in fact there are no overall forces acting at all, and get involved in 3rd grade arguments. That's cool - It identifies the third graders.

I think Roger Penrose summed it up - we tell 'em lies in school, and more complicated lies at college, and by the time they get to the cutting edge, we have a really complicated set of lies to tell them.

:D

[adam_one]

I’m afraid I have to agree with you Steve. :)
Vintage1 is just too proud to admit that he has chosen the wrong statement.
Now he's using the tactic: if you can’t beat them, confuse them… :D

[biber]

i think vintage, you are quite right at that, but this discussion maybe wouldn't have been exploded, if you just had written:

"If mathematical simplified like there are only four different forces acting on it, mathematics of course allow this to be simplifyed to: Overall a model in steady flight has no forces acting on it......"

That's because of mathematics doesn't care much about reality or physics or something, but as a tool it is able to describe the coherences of some things. Even pysics just describe nature but is not reality! So that's a discussion about the difference between reality and its description. Thats about as helpfull as to discuss about if something is "the same" or if it's "equal". Sorry, but that doesn't take us anywere.

No one of you is completely wrong but you are talking about what is reality and thats a philosophical topic, not scientific, nor does it lead to better model airplanes, am i wrong?

[vintage1]

Not at all. I am - and always have been - trying to make the point that all mathematical analysis of force vectors is a gross oversimplification, and is not 'what is really going on' but an idealised picture that you adjust to make the mathematics soluble and address a particular issue of flight you are trying to get a handle on.

And you draw as many arrows as are necessary to solve the particular problem you have.

If you are for example, analysing flutter, it gets to be an awful lot of arrows and a fiendish bit of computation.


However in steady level flight, you don't need any arrows at all really, until you start to fiddle with the airplane.

Then you draw a few in some useful directions. Then as you start to analyse structural strength, you need more, then as you start to analyse laminar flow over the wings, you need even more, then when it gets to turbulent flow...you need millions.

The concept of a force itself anyway is pretty unreal. Go down to the sub molecular level and its hard to see where mass, force, space and time all fit in.

One theory has it that everythig starts with gravity, which bends the spacetime locally and causes energy to appear as mass. Weird. Forces don't actually exist, just minimal energy states in a curved spacetime...or something.

[biber]

Ok vintage, i agree with that. Maybe this difference between reality and its description by science isn't clear to everyone in here. Science has not just the one and only way to describe nature but is all the time a more or less simplification. If more or less simplifying depends on what special problem you are actual dealing with. Many problems are solvable in a sufficient accuracy even if they are strongly simplified. And, yes, every prob has to be simplified to some level, to become able to be calculated at all.

An example to those who want to be really accurat: every mass (e.g. an electron) has gravity working on every other mass in this universe. Now please calculate that!

See the problem?

[Wingin' Wayne]

I quit arguing machining principles years ago. Half the time we were both saying the same thing using different terminology, the other half we were splitting hairs. The end product is what counts! He may have a different approach but it works for him. A plane is a balancing act! There are many forces that need to work together and cancel each other in order for a plane to fly. I read a book by Andy Lennon, it may be fine for engineers but not practical for the average flyer. I didn't have much trouble understanding the principles discussed, but I don't have that kind infoemation available to me. Reynolds numbers, stall angles laminar flow are for the guys at NASA, I know I've worked there. If I bend up a foamie fully symetrical wing with a 4/1 A.R. that's 16% percent thick, what's the correct angle on the ground. That's what I was trying to find out. Does a plane wit unlimited vertical really have a stall angle? Well at 5 degrees it took forever for it to get off the ground! Andy was no help. So I used the Wright brothers method. I slowly increased the angle until I was happy with the result. I know ther are a lot of considerations here. A parkflyer with 50 watts a pound and too much angle will jump into the air before it has enough airspeed and stall if the angle is too great! We need a more relavent book!

[adam_one]

Overall a model in steady flight has no forces acting on it. I'd rather say:
a model in steady flight has no unbalanced forces acting on it :)

[adam_one]

Well then lets say that there are an infinite number of forces actng then shall we? Or at least as many as there are molecules in the surface covering...and then there's all the internal forces, acting between the molecules of the structure,....so our diagram should be covered in an infinite number of arrows, not just 4...When some people can't understand something they may say: oh, there's not enough arrows…
What really happens is that they don't have the whole story, and just want to blame on something else... :rolleyes:

[vintage1]

well I can agree with THAT in a wry sort of way.

I've been tring to get a grip on modernquantm theory, insofar as my limited maths is able, and that is pretty much where its at. A LOT more arrows, all pointing in different directions, and all making up a picture that is so weird as to be inconceivable except in terms of a unversed shaped in such a complex way...

I mean, its not even CLOSE to being understandable in terms of one and one only set of 'rules' . So hey keep adding another layer of reality behind it all, and fiddle with the maths to see if it comes out right, and then look at the equations they have arrived at, and frankly go nuts trying to understand what it might mean.

I think it was Terry Pratchett who remarked that 'the chief problem in phsyics is trying to explain quantum mechanics in language designed so that one monkey could tell another where the next banana tree was'

[biber]

banana tree!:D That's it! :D

But seriously, Vintage, you're right and Terry Pratchet is right too.

Nevertheless i'm prefering Douglas Adams :D .

[adam_one]

To quote an old stand up comic:
Wha'da bunch of chameleons! :cool:

[Murocflyer]

Reading through this post, trying to learn something, at least about CG anyway, I have come to the conclusion that the original question didn't get answered, or did it?

Vertical CG is simply an implied fuselage station line measured vertically, or simply put, slices down the A/C fuselage, normally starting out forward of the nose of the aircraft. Isn't that what the original poster was asking about?

[Sparky Paul]

The "vertical c.g." will be located on a vertical line thru the horizontal c.g., plane leveled.
It's a result of the placements of the aerodynamic shapes and installed equipment just as the horizontal c.g. is.
Suspending the completed plane from two points... one ahead of the wing, the other behind, and dropping a line using a plumb bob from each suspension point to the floor will locate the vertical c.g. where the two lines cross.
Sometimes this location will be inside the wing, which is awkward to mark. :)
It's seldom a parameter of any importance, the horizontal c.g. is always a matter of extreme importance.
The vertical c.g. is important usually if there's ballast or cargo involved.
The effect on "pendulum" stability can be something to consider then.

[kristianb]

Centre of gravity is the sum of a lot of small gravity arrows, with a location in one point. Depending on the number of dimensions You live in it is located somewhere in space relative something, for example on the fuselage centerline, 60 mm behind the leading edge of the wing and 15 mm below the wing center line (at 60 mm behind the leading edge). NB this was valid today at noon, but changed after that because of fuel consumption.
Thanks to the impropability drive, forces sorted out and resulted in a nice dinner at the restaurant at the end of the universe...