3rd. lesson, a simple fuselage side view.

It's been a few weeks since the last lesson. I hope you've been able to practise with the first two lessons, we're going to assume that you have the concepts presented in those two lessons down, for this session.

Designing a fuselage is the next best step to consider, as most of us have desired building a new fuse for the surviving wing from a crashed, or damaged airplane. Designing a wing has it's own problems and tricks, some of which may be tougher than designing a new fuselage.

The simplest method, is to use the incidences, and offsets from the original model. Hundreds of successful designs started out as a Nobler, so the practise has merit, and helps keep you away from certain decision making problems you'll run into starting from scratch.

Let us take the Nobler as an example. You've crashed your ARF, and want to re-use the wing for a new plane. The hardest thing we must do here, is to duplicate the airfoil, or at least come close so we can have it represented on our fuselage side view.

I'm hoping you have some sort of plans showing the side view to start from.

Assuming this to be true, we need to first duplicate the airfoil.


Start by laying out a horizontal line, starting about 1/4 of the screen, starting on the left side, and continueing to the 3/4 point on your screen. It helps to have "Ortho on" so that your line is absolutly level, and straight. Use "F8" to toggle ortho on and off.

Draw a line perpendicular to the horizontal line, at about the mid point of the line.

Here's a neat little command to place the line right at the mid point of that horizontal line.

Select the line command. either by clicking on the icon, or typing L, enter.

Type "mid" and hit enter. Notice that a small symbol is now resting on the line when your cursor gets close to the line. This is the exact midpoint of the first line.

Left click on the line, and the symbol changes, then move the cursor up a few inches, and left click again. Now, right click to finish the command.

You now have a line perfectly perpendicular to the horizontal line, located in the exact center of the horizontal line.

Measure the wing chord, in the center of the wing. Measure just to the flap hingeline.

let's assume that theis demension is 10 inches, just to make it simple.

Let's use offset to offset the perpendicular line 10 inches to the left.

OK, here's where you are going to have to transpose some dimensions from the plan to get a good representation of the airfoil.

Do you know how to do this?

Think of the old days, where plans had a grid of squares overlaying the part. The method is similar to that.

You'll need a straight edge, a good ruler, and a good 2H pencil with a sharp point.

Use your straight edge to layout the wing centerline of the root airfoil.

Along this centerline, we want to draw vertical lines at least every half inch, starting at the very front of the airfoil. Lesser line spacing, such as 1/4" make the results more accurate.

Do this on the paper plans first, and then duplicate it on your screen.

On your screen, you can do multiple offsets, or use the array command.

Right now we'll use multiple offsets.

Does the figure on your screen match, for all intents and purposes the figure you've drawn on the plans? It's important that it does.

OK, we have established our vertical datum lines.

Let's use the command "dt" to lab el each vertical line.

Type "dt", and hit enter.Place your cursor at the intersection of the first vertical line, and the horizontal line. Left click, then right click twice. The right clicks are accepting the default text height, and rotation. type, in the case of the first one, "0" (zero)

Now, move your cursor to the next line, and left click. type "1", then move the cursor to the nextline and do it all over again.

At the last line, you will finish up wit an enter on the keyboard.

Enough for now, Tomorrow, I'll add more to this lesson.

OK, here's what you should have at this point.

[photo not recovered: 28519.jpg]


So let's use a move command, from left to right, (remember this only picks entities that are entirely inside the box) Pick the numbers only, and move them down so they are below the horizontal line.


We are now going to transfer the intersecting points to the vertical datum line, where the curved airfoil line intersects the vertical datum lines.

We'll carefully measure the distance at VD (Vertical Datum) 1.

Write down all the distances at each of the VD points.

Now, back to your computer screen.

Use offset, and offset the horizontal line up, the measured distance at VD 1.

Since we are going to have a lot of horizontal lines going up and then back down as we proceed past the high point, let me suggest that you trim the horizontal line where it intersects it's VD line. Trim off the right side of the line, going left to right, up to the high point, and the left side going back down towards the trailing edge.

Using the above techniques, offset the rest of the dimensions, until all are in place.

This is what your drawing should now look like.

[photo not recovered: 28520.jpg]

Notice, I've selected a simple plot, but in reality, the first 4 tor 6 VD lines should be spaced much closer to each other, so the entry curve is more accurate.



We are going to use a special type of line, one that can best be described as a "rubber" line. It's called a Polyline, and we use it to make continuous curved lines.

Select the polyline command. There's an Icon, but I just type "PL", and enter.

Start your polyline at the 0,0 point on VD 0. Connect all the intersection points, and when you reach the trailing edge, VD, finish the command.

You will have a jagged line connecting all the plotted points.

We need to smooth out this jagged line, and it's easily accomplished using one of two commands. If it's absolutly necessary that the line maintain the correct relationship to the intersection points, then use the command, "Fit" or "F". Select the polyline, with a left click on it, then enter. Your line has now curved to fit all the intersection points.

Another smoothing command for a polyline is "Spline", or "S". The resulting curve will be smoother, in most cases, but it won't necessarily retain it's relationship to the intersection points.

You can see this easily, as we haven't yet closed out the command, by simply typing "s". Type "f" to return to the fitted curve.

OK, we need to clean up our drawing before moving to the next step.

Let's erase all the datum lines, except the horizontal center line.

Now, we'll use the command "Mirror" Or simply "m" if you're typing in the command. and mirror the upper curve, with the mirror, or center point being our horizontal centerline.

Here's what you should have now.

[photo not recovered: 28521.jpg]


We have, in the case illustrated, a crude representation of the airfoil. If I had used closer spacing for the first 4 to 6 VD lines, the entry would have been much closer to the original airfoil.


Let's use this airfoil though and lay out a side view of our plane.

Ther're some questions we need to ask ourselves first though.

It is simplest to use the offsets of the original model the wing came out of. In many cases that will do fine, especially if you liked the performance of the original, and you are going to use the same type of fuselage construction. A profile fuse on a wing that came from a profile design or, a built up fuse for what was originally a built up fuse design.

You could, on the other hand decide to change to a profile from a built up design, or vice versa. In doing so, sometimes we can run into problems with the verical CG.

Perhaps you've wondered why all designs are not inline, all the centerlines of thrust, wing, and stab, on the same line like, for instance, a Twister.

Most of the time, an inline design is not necessarily the best way to go for a lot of reasons, though it often works well for a profile. Why do we suppose this is?

Vertical CG placement, and, Control geometry are a few of the reasons.

Generally, for a profile design, the thrust line can be placed close to the wing centerline, because the weight of the engine and tank, with the fuel, will be located close to the same line the leadouts are on.

Some designers, will raise the thrust line a bit for more ground clearance, and to offset the weight of the wheels below the wing on a profile, but many limit this to about a half an inch.

Having the stab and elevator in-line works for some. Others believe that the stab and elevator can be masked and lose effectivness, because they are in the wing's down wash. I'm one who feels this way, so I almost always raise the stab and elevator up, at least as far as needed.

A rule of thumb can be used to raise the centerline of the stab to a point where it is, at least, even with the top surface of the wing, at it's hieghest point.

Larry Cunningham wrote a piece in Stunt News, some years back, titled "Magic Geometry" which gives a good way of determining the best height for the stab, which also results in giving very symetrical control inputs between the flaps and elevators.

I'm hoping Larry might step in here and either give the date, or a link, or even explain here what this relationship is.

Back to the subject at hand though.

For a Profile, consider placing the thrust line about a half an inch above the wing centerline. Place the stab centerline at least as high as the top of the wings surface. But what about nose legnth, and tail moments?

Using the Nobler ARF as our starting point has a lot of possitives because the Nobler is such a good design, and has continued to perform well for over 50 years now, but, might it be possible to improve it?

Consider the Twister. Does Ted's Fancherized Twister perform better than the original? Many say yes, even though the original also flies pretty good.

For built up designs, the thrust line is often placed a bit higher in the fuselage. Because the weight of the cylinder, piston, and head are below the thrust line, 3/4 to 1 inch seem to be common. and also seem to work well with the vertical CG.

Let's set up some "Rules of thumb" to help us.

First "ROT" (Rule of Thumb)

"You need at least, enough nose legnth to mount the engine, and the tank."

Depending on the engine used, and the resulting tank, this will vary. for a .35 sized design, about 7 to 8 inches seems to be what is needed. You'll need more with larger engines of course.

Most of the older classic designs used a rather short tail moment arm. It was often thought by some of the early designers that shorter moment arms made for tighter turns, and more responce to input.

Later designers starting experimenting with longer tail moment arms, and found that there is more power to start a turn, as well as stopping one from the longer tail moment. It also seems to smooth out the groove in level flight.

Currently, there's a number, mathematically derived, called the "Tail Volumne Coefficient" that is a useful tool for computing the best tail moment for a design. I've never used it myself, rather I've used another "ROT" I'll get into later, so I won't try to describe how you compute the Tail Volumne, but, I hope that some one familiar with the method, will post to this thread, and explain it better than I could.

"ROT" number two.

"The tail and all it's components should balance out the nose."

Since we're not planning on changing the flap area, or the stab-elevator area, if the original balanced well, then let's use that dimension. On the other hand, If you had to add weight to balance, we need to look at how we can achieve balance without adding weight.

We can legnthen, or shorten the nose, or the tail moment to achieve balance.

We can only shorten the nose to the point where we have enough room for the engine and the fuel tank, so, if we are already at that point, we need to look at the rear of the plane.

We have to consider several items when setting up the nose moment in relationship to the tail moment. As an example, consider that we are tail heavy and we don't think that shortening the tail momenbt will be good areodynamically. We need to increase the nose moment obviously. It goes the other way for the opposite condition.

Now we need to figure the moments in inch ounces.

For instance, We have our nose moment as short as we can have it, for the engine and fuel load we intend to use. We've determined the balance point on the wing already, so we can figure the torgue in inch ounces by the following.

(E*D1) = (T*D2)

E= engine weight,in ounces, including prop, spinner, muffler, tank, and fasteners.
D1= distance, in inches, from the CG forward to the center of the mass for E.
D2= distance, in inches, from the CG back to the center of the mass for T.
T= Weight of all components of the fuselage, (behind the CG) Stab, elevator, pushrods, etc.( try for as accurate of an estimation as you can get).

An example,

Your power package, E, weighs 14 ounces, and the D1 distance is 12 inches.

So, 168 = (T*D2)

We can estimate fairly accuratly, what T equals, so our estimate is 10 ounces.

We now have,

168 = 10*D2

To solve for D2, we divide both sides by 10, and our solution is

16.8 inches

16.8 inches is the tail moment distance from the CG to the center of all the tail mass.

"ROT" number 3, I usually estimate the center of all the mass aft of the CG as about right at the leading edge of the stab. Though this location may need to be re-thought on occasion.

Since the above estimations could reult in a rather long tail, we can make changes, such as legnthening the nose moments slightly, or varying the weight of some of the components to achieve balance.

This is all relevent to how accurate your estimations are for the elements aft of the CG.

Let us touch on some additional refinements, and possibly set up a few more "ROTs" for them.

"ROT" number 4, Relativly lower aspect ratio stab and elevators, in relation to the wing, seem to work better.

"ROT" number 5, Generally the area of the stab and elevator, as a percentage of the total wing area, will be equal to, or extemely close to the actual best percentage of wing chord for the CG.

In other words, as espoused by Ted Fancher, and others, a stab and elevator that is 25% of the total wing area will support a wing that will balance at 25% of the MAC. (Mean Aerodynamic Chord)

"ROT" number 6, Generally, elevators, at less percentage than half of the total stab elevator area works better than setting the ratio at 50-50.

What does this mean? I like to use a 60-40 relationship. 60% of the area will be stab, the rest, 40% will be movable elevator.

"ROT" number 7, Flap percentages are connected to the efficiency of the design elements, and vary considerably with total weight, power, and moments as the main contributing factors.

What can I say, this is a big subject that would take a lot of time and space to explain. Let's accept, for the time being, that 17 to 23% of total wing area, (including the flaps themselves) works well in most cases.

There are more "ROTs", but for now, we'll go with these.

Back to the drawing.



Let us extend the horizontal centerline.

There's a neat little function, in AutoCad, and I assume in other systems as well. It's called Grips.

If you place your cursor on the horizontal centerline of the airfoil drawinng we have, and left click, it will change to a dotted line, and have small blue boxes, or grips show up at points along the line.

When your cursor is set close to one of these box's, it will snap into the blue box, and the color will change. At that time, left click again, and the cursor now grips the box and line. If you do this at the front of the airfoil, you can extend the line, further to the left. Do this, when it's where you want it, left click again, and the grip is released. Right click, or enter, and you are finished.

Shall we make a profile fuse, or a built up?

Let's do a profile first.

We need to establish the thrust line, in relationship to the wing first. We'll use offset to do this.

Offset the chord centerline up .5 inches.

Extend, using grips, the thrust line to the left about 8 inches. In the same way, also using grips, shorten the line to about even with the leading edge of the airfoil.

Now, we'll set the height of the stab-elevator relative to the wing.

Offset is again used.

We are going to use a little time saving trick to set the offset to the top surface of the airfoil.

Enter the offset command, notice the command line is asking several questions? One of these questions is distance, and you can either enter the distance via the keyboard, or by setting your cursor to the high point of the airfoil, left click, and the ending point, in this case perpindicular to the chord centerline. Left click once more, and the distance is set.

Now, left click on the chord centerline, move your cursor above the centerline, showing the program the direction you want to offset, and left click again. You now have a line the exact height of the top of the airfoil. Right click to finish the command.

OK, we have established our incidence lines for the thrust, and the stab-elevator.

For the next moves, we will need to zoom out so we have more room.

Let's do something a little different. we are going to set up the extents of our drawing.

On the upper pull down window, 5th. from the left, is the format pulldown. Left click and a drop down menu appears. There are a lot of neat items we haven't used yet, but the one we want to click on right now is "Drawing Limits". Find it and left click.

Look down at the command box, in the lower left portion of the screen. It's asking you to define the lower left portion of the drawing extents.

The default is 0.000,0.000

Notice the apostrophy, it's important.

Hit enter on your keyboard to accept the default.

The command line is now asking you to accept, the default, or to specify a new upper limit.

Let's use a D sized drawing tablet by specifying 48.000,36.000 and hit enter on your keyboard.

By the way, you don't have to use the decimals if you are using whole numbers. We now must do a Zoom All command to see our new screen. You can use the Icon, or simply type Z enter, A enter.

Now you have a lot more room, but the work we've done may not be where we want it.

Use the move command, and place it where you want it.


We are now ready to begin really laying out or fuselage side view.

All CG COMPUTATIONS ARE FIGURED USING THE MAC.

So, for this illustration, we'll consider a Nobler wing. Place the CG at the shown position on the plans, on to the airfoil we have on our screen.

Since I don't have a Nobler wing to work from, I'm now going to try and establish the MAC and the CG position.

To simplify, I'll figure a leading edge taper of 1.5 inches, and the flap trailing edge will also taper 1.5 inches. My airfoil is 10 inches long to the flap line, and the root flap chord is 3 inches, for a total of 13 inches.

That would make my tip chord, including flaps 10 inches. (13 - 1.5 -1.5 = 10)

So, simplifying further, my MAC should be very close at the half span point, where the root 13 inches, + the tip 10 inches/2 = 11.5 inches. My MAC then is 11.5 inches, so if I want to use "ROT" number 5, and my stab-elevator area = 25% of total wing area, I should be able to multiply the MAC by .25 to find the distance from the leading edge to the CG at MAC.

The number is 2.875 but does this help me yet? Of course, but I can't use this number yet because I need to figure the position of the leading edge at half span with all the tapers.

This is easy enough to do, as half of the 1.5 inch total leading edge taper is .75.

By adding .75 to 2.875, I have my 25% MAC balance point where it will be at the root of the wing.

The number is 3.625.

Let's get this information onto our drawing.

Draw a vertical line at the leading edge of the airfoil.

Offset that line 3.625 to the right. This is your balance point at 25% of the MAC. We can now layout the nose and tail moments from the balance point, but there's a little kicker to watch out for.

Remember "ROT" number one?

"You need enough nose legnth to mount the engine, and the tank."

Just keep it in mind that you are measuring from the CG, not the leading edge.

As an example, let's say you need 8 inches to mount your tank and engine. You'll have to add thaqt 3.625 to the 8 inches when you offset the line on the CG to the left for the nose moment.

The dimension to offset is 11.625 in this case.

After you offset for the nose moment, you may need to do a Zoom All so you can see all that you've done.

OK, let's use "ROT" number 2 and find what our tail moment needs to be.

We are going to use an engine that weighs 8 oz's, a spinner that weighs 1.5 oz's, a tongue muffler that weighs .8 oz's, a plastic RC clunk tank that weight 1.5 oz's, a prop that weighs 1.5 oz's, and fasteners that weigh .2 oz's. for a total weight of 13.5 oz's. We are not going to be concerned with the structure weight, as it usually comes close to balancing out, due to the heavier nose construction used to absorb the stresses from the engine.

Let us estimate that all the weights listed above average out to the center of mass being about 2.5 inches behind the prop, or 11 inches from the CG.

Our formula is still (E*D1)=(T*D2)

E= 13.5 oz's
D1= 11 inches

13.5*11 = 148.5 inch oz's = T*D2

OK, we need to estimate T.

The flap horn and pushrod weigh 2 oz's, the elevator pushrod weighs 1 oz, the stab-elevator, weighs 2.5oz's the vertical stab weighs 1.5 oz's. We'll use an estimate for the weight of finishing here for about 25% of the total estimated weight of the finish. Let's use 10 oz's for the total weight of finish, so 25% of that is 2.5 oz's.

Let's see what wwe've got.

2+1+2.5+1.5+2.5=9.5oz's

Now our formula looks like this.

148.5"oz's = 9.5" * D2

Solve for D2, equals 15.63 inches to the leading edge of the stab.

Our stab has a chord of 3 inches, so the CG to hingeline dimension would be 18.6 inches after rounding off.

Let's plug this into our drawing.

Offset the vertical line at the CG to the right 18.63 inches.

Now, using grips, extend the thrust line forward a few inches to the left of the prop line. In the same manner, extend the stab centerline to the right a few inches past the elevator hinge line.

Here's what your drawing should now look like.


[photo not recovered: 28522.jpg]

OK, my disclaimer:

I am not the last word, or the end all be all of model airplane design. The items discussed are some that have worked for me to get close to where i wanted to be, but I am always open to discussion, and learning better ways of doing things.

It's also important to know that I am not, in any way shape or form a mathematition. I'm at best math challenged, but capable of simple algebra, and some forms that are formula based, and simple. Please excuse any errors you may find, and offer constructive critism and helpful advice should you find occasion to do so.

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BEAR WITH ME PLEASE AS I GO FURTHER INTO THE MATH MAZE.

It occurred to me last night that I should explain the full method for summing the moments, used in figuring the dimensions and weights to achieve balance. What I’ve explained previously is a simplified method, using estimations to come close. It can be fairly accurate, but only as accurate as the estimations.

The formula for figuring the moments, taking into account as many of the variables as possible is:

(A1*T1) + (A2*T2) + …….. (for as many variables as exist on the left of the equal sign. The equal sign is related to the CG position for our use.) = (B1*D1) + (B2*D2) + …….

(A1*T1)+(A2*T2) ….. = (B1*D1)+(B2*D2) …..

A1 = the weight, in oz’s, of the furthest left component.
A2 = the weight, in oz’s, of the next component , to the right.
……. = each occurring component in descending order to the CG point.

T1, T2, ….. is the distances, in inches relating to the component it is coupled to.

B1, B2, ….. Is the weights, in oz’s Starting at the furthest right side component Each occurring B is the next component, in descending order, up to the CG point.

D1, D2, …. Are likewise the distances, in inches of each component in descending order, as they are coupled to the weights.

So, for an example;

We have a spinner and an APC prop that weigh 3 oz's and the center of mass lies 11.5 inches to the left of the CG.

(3*11.5) + (our engine and muffler weigh 10.5 oz's and the center of mass lies at 10 inches to the left of the CG.)

(3*11.5)+(10.5*10)+ (Our tank, empty, weighs 2 oz's and the center of mass lies 8 inches to the left of the CG.

(3*11.5)+(10.5*10)+(2*8)+(Our main landing gear, with the wheels, weighs 4 oz's, and the center of mass lies 1.5 inch to the left of the CG.

(3*11.5)+(10.5*10)+(2*8)+(4*1.5) = There is the first half of the equation. The number will be in inch ounces, a compound number that relates to the sum of all the balance forces acting on the left side of the CG. Obviously, to achive balance, the sum of all the forces, to the right of the CG, will have to total the same compound number.

Let us look at the components to the right.

We probably do not have to include the bellcrank, as it usually stradles the CG. The flap pushrod though is to the right of the CG, as is the flap horn. The flaps themselves are to the right of the CG, so we'll include their weight. The elevator pushrod assembly is to the right, along with the tail wheel assy, and the vertical stab. The largest contributing factor to the right of the CG though is undoubtably the stab and elevator. I also like to add a percentage of the total finish weight to the right side, because the finish weight affects the rear more because of the distance from the balance point.

We can get very technical here, and figure in the percentage of structure weight to the left and right of the CG, and use the computer drawn parts to find the center of mass, but with all the variables, we could miss absolute balance, but using the information we have selected above, we can get very close. Close enough that we should not have to add much weight to achieve balance.

So, let's look at the right side of our equation.

(our stab and elevator, including the elevator horn, weighs 3 oz's, and we don't know how far to the right of the CG it must be.)

= (3*D1)=(4*13, This is 40% of the finish weight. The center of mass for the rear part of the structure is estimated to be 13 inches to the right of the CG.)

=(3*D1)+(4*13)+(2*18, this is the weight of the tail wheel assembly with the center of mass located 18 inches to the right of the CG.)

=(3*D1)+(4*3)+(2*18)+(1.5*18, this is the weight of the vertical stab, with the center of mass, also at 18 inches to the right of the CG.)

=(3*D1)+(4*3)+(2*18)+(1.5*18)+(1.5*11, this is our elevator pushrod assembly, with the center of mass at 11 inches to the right of the CG,)

=(3*D1)+(4*3)+(2*18)+(1.5*18)+(1.5*11)+(3.5*6.5, this is the weight of our flaps, including the flap horn assekmbly, with the center of mass at 6.5 inches to the right of the CG)

=(3*D1)+(4*3)+(2*18)+(1.5*18)+(1.5*11)+(3.5*6.5)+(.75*3, this is the weight of our flap pushrod with the center of mass located 3 inches to the right of the CG.)

Our complete equation to sum all the moments is:

(3*11.5)+(10.5*10)+(2*8)+(4*1.5)=(3*D1)+(4*3)+(2*18)+(1.5*18)+(1.5*11)+(3.5*6.5)+(.75*3)
+
Let's solve the equation.

34.5+105+16+6 = (3*D1)+12+36+27+16.5+22.75+2.25

161.5 = (3*D1)+ 116.5

Subtract 116.5 from both sides of the equation.

161.5 - 116.5 = (3*D1)+116.5-116.5

We now have all the variables on one side of the equation. remember these numbers are compound nubers, in inch/oz's.

45 = 3*D1

Divide both side by 3 to solve for D1.

45/3 = 3*D1/3 = 15 inches (the oZ's are divided out)= D1

The center of mass for the stab and elevator has to be located 15 inches to the right of the CG.

Now, all the weights I used were numbers I picked from my head, so most likely will not be representative of the actual numbers derived from a model plane.

OK, I don't want my stab and elevator that close to the CG. I like a hinge line to hingeline dimension of 16.5".

using the same weights and dimensions, I can figure where some of the components should be to balance out the longer tail.

First, I'll need to translate the hingeline dimensions to the CG to hingeline dimensions. we'll call this 25.125 inches.

Let's simplify the formula a bit for the ease of typing.

We'll make the engine location the unknown, and figure the prop and spinner as part of the engine weight.

(3+10.5*T1)+(2*8)+(4*1.5)=(3*25.125)+(4*3)+(2*18)+(1.5*18)+(1.5*11)+(3.5*6.5)+(.75*3)

(13.5*T1)+16+5.5=75.375+12+36+27+16.5+22.75+2.25

(13.5*T1)+16.5 = 191.875

13.5*T1= 191.875-16.5

13.5*T1 = 175.375

T1 = 175.375/13.5

T1 = 12.99 inches or let's round up to 13 inches.
The center of mass for the engine, prop, and spinner, in this case should be located at 13 inches, or perhaps slightly less if the hingeline to hingeline distance is to be 16.5 inches.

Now, let's estimate where the center of mass might be for the engine, prop, and spinner.

The engine and muffler weigh 10.5 oz's.

The spinner and prop weigh 3 oz's.

The center of mass will be where the two balance each other out.

We could try to sum the moments, or, we could consider a fast and dirty method, such as, The spinner and prop are 28.6% of the total weight of the engine, muffler, prop, and spinner.

From the tip of the spinner, to the back of the engine, is 5.5inches in this case.

28.6% of 5.5 inches would be 1.573 inches

The center of mass should lie at a point 1.573inches to the left of the back of the engine. we can simplify a bit and call it 1 9/16".

We now have the spot to place the engine on that line 13 inches from the CG.

Rough calculations would now place the nose ring at about 11 inches, ahead of the leading edge of the wing.

It's really better to get used to the idea of figuring nose and tail moments from the CG, don't you think?

Actually, as you get used to these simple calculations, it gets to be kinda fun, and several useful solutions can be found, such as the following.

My plane took 2 ounces of lead in the tail to balance. How heavy of an engine/muffler will it take to balance without the lead in the tail?

OK, enough fun for today.

Hmmmmm, on a new design I usually just leave the motor mounts overlong and shift the engine until it balances...

>Hmmmmm, on a new design I usually just leave the motor mounts
>overlong and shift the engine until it balances...

You could also move the stab and elevator back and forth until the plane balances.

Isn't that the way it always goes. There's always a simple way to do something that will work fine for most people. What you do is very workable, but, Isn't it nice to be able to predict, within a reasonable margin, what your offsets should be?

Also, isn't it helpful to be able to figure just how much longer one moment needs to be to balance out another, after you change the weight, or position of an element?

When designing, and then drawing a new plane, You can set up the shapes more to your liking when the moments are close to where you need them to be?

It's not necessary to do all the math, you can simply use some preset numbers that work on other designs, but what if you are wanting to go the extra effort.

Anyway, the Math was offerred as an aid. You can use it if you want, or not. I just thought it would be helpful to aid in the design process.

Keep in mind that balancing before applying the finish, will almost always result in a tail heavy plane, when working with the average stunt plane.

John, I'm not knocking your system. I actually didn't know such formulas existed. You're right of course, it's another design tool that could be very useful.

When balancing "in the bones" it has to be set a little noseheavy to compensate for the weight of the finish. Still something of a guess. I figure if you get it close, then you can fine tune it with wheels, spinner, prop, etc. of different weights. Then you get it perfect, then fill the tank and make it noseheavy again! Ultimately, the final adjustments have to be based on how it actually flies anyhow.

Very interesting stuff. Keep it coming!

--Ray

>John, I'm not knocking your system. I actually didn't know
>such formulas existed. You're right of course, it's another
>design tool that could be very useful.
>
>When balancing "in the bones" it has to be set a
>little noseheavy to compensate for the weight of the finish.
>Still something of a guess. I figure if you get it close,
>then you can fine tune it with wheels, spinner, prop, etc. of
>different weights. Then you get it perfect, then fill the tank
>and make it noseheavy again! Ultimately, the final adjustments
>have to be based on how it actually flies anyhow.
>
>Very interesting stuff. Keep it coming!
>
>--Ray
You are absolutly correct Ray.

The final adjustments do come after you get it trimmed out. All this is just to try and get it as close as we can before it's ever flown.

Thanks for your support, and never be worried about making a comment..

Let’s get back to our drawing now. I apologise for all the math stuff, but it’s good stuff to know, and will help you out more often than you may think.

Does your drawing look like the last one I posted? If it does, you’ve done good.

I like to draw a spinner to place at the front first. One of the neat things about using Cad for your drawing is that you’ll only have to draw often used parts, one time. Later you can insert the part into your next drawing. We’ll later set up a drawing, named “Parts”, into which we can put a copy of these often used parts, which will simplify the process.

Get out a spinner, the size you intend to use.

Offset the front line the distance from the backplate to the point of the spinner.

Extend, if needed, the thrust line so it ends slightly to the left of the above line.

Offset the thrust line ½ the dia. Of the spinner. In my case, I’ll be using a 2” spinner, so I’ll offset 1 inch.

Use a zoom window to enlarge the area we are going to work on.

Draw a polyline starting at the front, we’ll use four segments, getting as close to the curve of the spinner as we can. End the polyline at the point where the offset thrust line intersects the vertical line that represents the back plate of the spinner.

We now have a jagged looking line that we will smooth using “polyline edit”, or “PE”. Once into the PE command, set your cursor on the line, and left click. The command line, at the bottom left of your screen has changed, and now asks you what you want to do. Before, when we used this command, we used “Fit”, or “F”. This time, we’ll use the “Spline”, or “S” command.

The “S” function will smooth the lines curve, but doesn’t force the curve to pass through the defined points we entered.

We can adjust the curve to more closely represent the line we want, by using “Grips”.

After grips is brought up, on the line, you can select the grip box closest to the portion you want to adjust, and move it while watching the line’s shape. When it’s where you want it to be, release the grip, and there you have it.

Your drawing should now look like this.





[photo not recovered: 28560.jpg]

Just for your information, the rest of the drawing still exists, to the left, off screen. I simply zoomed in to be able to work on this portion of the drawing.

Here's the entire drawing after a Zoom all.




[photo not recovered: 28561.jpg]

Shall we draw in the stab and elevator?

Let's use a nice little technique I like to use to avoid messing up my drawing, especially when the drawing is very detailed, and has lots of different lines and shapes involved.

We will draw the side view of the stab and elevator seperately, and copy it over to the correct position later. This also leaves us a side view to later work up the top view from, seperate from the main drawing.

Some where, at a point several inches, at least, from your main drawing, draw a horizontal line with "Ortho" toggled on. so it is absolutly straight.

Use a Zoom window to close in on the line, so it's taking up most of the screen.

Connect a verticle line perpendicular to the horizontal line.

Let's make it simple for this design, and make the stab chord 3 inches to match the most common wigth of balsa used.

Offset the vericle line, 3 inches to the left.

We are going to make the stab 3/8" thick, so let's offset the horizontal line 3/16" above the original.

We now have a box to use to define the top half of the stab's side view.

Let's take a few minutes and discuss the shapes that are commonly used for the stab's cross section.

Some have used a flat plate style of construction. Basically this means that the leading edge of the stab is rounded, but remains essentially flat. The trailing edge is often treated the same way, or left squared off. It's fast and dirty, and will work. I've seen some decent flying Shark 45's with this type of treatment.

Personally, myself, I wouldn't use it. There have been some good studies in the recent past that seem to conclude that this style has problems with tracking. The fix has been the use of a turbulater wire at the center of the leading edge to create a sharper entry.

There are as many shapes for the stab entry as there are ideas, but for my use, I've settled on an airfoiled entry with a relativly sharp entry as my stab cross section of choice.

Follow me, please while I lay out my preffered style on our drawing.

We will use a "Polyline" once more.

Zoom in tight on the area that will become the leading edge of the stab.

Start your Polyline, (PL) at the intersection of the original horizontal line, and the verticle line you just offset 3 inches to the left.

Move your cursor up, and about the same distance to the right, and left click to establish the first PL segment.

Continue to do so until you reach a point where you will want to join the new horizontal line, 3/16" above the original horizontal line. You may notice, that the end of the PL wants to jump to an intersection point or some other place than where you tell it to go. You can use escape, the key at the top left of your keyboard to get out of this situation and back to the previous command.

If you use the near command, either from the icon, or by typing in the short cut "nea", the PL will stop trying to jump to some other place, and accept the location you want it to attache to.

Your draw should resemble this.



[photo not recovered: 28562.jpg]

Now, use the "Poly edit", or "PE" command, and "spline" to smooth the curve.

It should look like this.


[photo not recovered: 28563.jpg]

We can now clean up our drawing by erasing the fron and top defining lines, but wait, part of the top defining line is a part of the stab's shape from the point where our polyline attaches to it. Let's use "Trim", or "TR" to trim the line.

Sometims this can get a little tricky when trimming a regular line, with a polyline. Make the "PL" your trim line, but, zoom in real close, so when you select the line to be trimmed, you can select a point, as close as you can get, to the "PL". See what sometimes happens if you want to, by just selecting somewhere on the line to be trimmed. Undo will get you back, so don't worry.

After the top line is properly trimmed, erase the front verticle defining line. Next, use "Fillet", Or "F", set the radius to 0, (Zero), and square off the hingeline.

Mirror the resulting shapes along the original horizontal line, and your stab is defined. It should look like this.



[photo not recovered: 28564.jpg]

Next, we can define the cross sectional shape of the elevator, but, first, let's discuss some of the various shapes that can be used.

The Flat Plate occurs hear as well. It usually has the trailing edge radiused in a similar manner as the Flat Plate leading edge. It works, and I've seen this style used on some designs. I personally don't like it much, so I don't normally use it.

The leading edge of the elevator is most often shaped to a 45 degree point defined by the center of the elevator's leading edge. others sometimes radius the leading edge.

Some will continue the airfoil shape to the trailing edge, myself I like to use a straight taper back to the trailing edge.

Some like to taper the trailing edge of the elevator to a sharp point. Other's, myself included, prefer to make the trailing edge about 1/8" thick, and squared off. Tests have shown that this is a very good, and much stronger way to shape the trailing edge.

There's been long standing debate over the concept of making the elevator thinner in cross section, that the stab. The idea is to reduce the elevators effectiveness, near nuetral, to improve the planes ability to groove in level flight. Tapering the elevator has a smaller, but similar effect.

The practise does work, but comes at a cost, in my opinion. By softening up the controls around nuetral, the intersections can have a lag time, which makes it more difficult to make good intersections on the Eights, when flying the pattern.

Good design principles, and accurate building practises, can, once more in my opinion, almost eliminate the problems that the use of a thinner elevator is used to correct.

So, on my drawing, the elevator will be the same thickness as the stab.

Another consideration, when laying out the elevator, is the percentage of elevator to stab. Many use a 50-50 relationship. I prefer having a bit more stab, in relationship to the elevator, more like a 60-40, but we'll use 50-50 in this case.

So, back to the drawing.

Offset the verticle line that forms the trailing edge of the stab, 3 inches to the right.

use the command "extend", or "EX", and extend the top line of the stab to the elevators trailing edge.

Now, "Offset" the original horizontal line up 1/16". Our defining box, for the elevator should now look like this.




[photo not recovered: 28566.jpg]

We are going to use a 45 degree line at the elevator leading edge. There are 2 easy ways to do this. First, we can simply off set the stabs trailing edge 3/16" to the right, and draw a line intersecting the points of interest, or we can use a new tool.

Look at the bottom left of your screen. Right above the command line there should be some numbers that change as you move your cursor. These are the coordinates, that define the points on yur screen. All lines and entities are defined within this coordinate system.

Use the line command, and start a line right at the intersection of the original horizontal line, and the verticle line that defines the trailing edge of the stab.

Look at the coordinates as you move the mouse. They will change as you move the cursor, but, you can toggle between straight coordinates for the second set of numbers, or the angle of the line from the start point to the location of the cursor. It's evident as you move the mouse, which one is toggled on. If you are in the wrong one, pressing F6 will toggle to the other. Move the cursor until you are reading an angle of 45 degrees.

Start another line from the intersection of this angled line, and connect the other end, at the intersection of the line you offset 1/16" above the original horizontal line.

Trim the section of the line spanning the trailing edge of the stab.

Erase the line that was offset 1/16".

Use Fillet with the radius set to 0 and trim up the rest of the lines.

Now, mirror the shape we've just drawn, and your drawing should look like what I'll show you in the next post. I've run out of room for any more attachments on this post.

Attachment #1, (jpg file)
Attachment #2, (jpg file)
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Attachment #6, (jpg file)

OK, here's what the drawing should look like.


[photo not recovered: 28567.jpg]

Now, we have defined the shape for our stab and elevator, so let's now place a copy on our fuselage drawing.

Use the "Copy" or "CP" command to enter the copy mode. Use a left to right window to select the stab and elevator shapes, but do not include the original horizontal line. Make your selection, and define your base point at the point where the trailing edge of the stab intersects the original horizontal line.

By not selecting the original horizontal line, we are not going to make a copy of that line, on top of the stab centerline we already have on our fuselage drawing. Piling lines up on top of each other is bad practise, and slows down the plotting, makes for larger files, and is a general Pain In the posterier.

Zoom back out so you have your fuselage on your screen, zoom back in to the stab area so you can make a clean movement of your copy onto the fuse.

Select the intersection that is on the stab centerline, and finish up the command. Your copy should now be in place on your fuse.

It should look similar to this.



[photo not recovered: 28568.jpg]

OK, enough fun for today. Enjoy and let me know if ther're any questions.

Attachment #1, (jpg file)
Attachment #2, (jpg file)

Hey John
Did not mean to take so long. Was looking for a shareware NACA generator, the one that I found is at
http://www.pagendarm.de/trapp/programming/java/profiles/NACA4html The trouble is getting your airfoil into a .jpg. What I do is a Shift Print Screen of the airfoil from the internet, open the word processor and Paste the airfoil there. Now check your Print Preview, if your airfoil is not centered Right Click in your airfoil this will open a window and by selecting Format Picture or Format Object you can move your airfoil around. Now you print your airfoil. Then I scan the printing into PhotoShop, there I get a .jpg copy of the airfoil. Save it where you can access it from TurboCAD.

Tracing JPG Pictures
While in a clear sheet in TCW
Start by opening a new Layer
Menu: / Insert / Picture / From File
At the bottom of this widow next to the word cancel Click the little down sign.
At the bottom of this window select All Files
Open the Folder where the JPG picture is.
Select the JPG File Click Open
Make sure you are not on Layer O
Move the Cursor to the Top Left corner of the window.
Hold down the Right Click and Drag the box to the Bottom Left
Menu: / Tools / Trace / Trace by Rectangle
Make sure you are on Layer 0
Move the Cursor to the Top Left corner of the window. Right Click Hold down and Drag the box to the Bottom Left
Your drawing will turn Red,
Click on the checkered flag in the Inspector Bar at the Bottom Left.

The tracings are not very good but at least you have a drawing you can work with in TurboCAD and save as an .tcw file.

David Craft
[email protected]

Try this Hyoer Link ?????

Ok so you want to use Paint instead of PhotoShop.
Go to the Hyperlink, http://www.pagendarm.de/trapp/programming/java/profiles/NACA4.html
Ajust the airfoil the way you want it.
Shift+Print Screen to capture the picture.
Push the Start Key,All Programs, Accessories, and Paint
Push the Edit button and Paste
In top of the tool box at the left is the Select button, press it move your cursor to the top left of the section you want to cut and drag it to the bottom right.
Press Edit and Cut
Now you have your airfoil on your Clipboard.
Press the Start Key, All Programs Accessories, and Paint
Press the Edit button and Paste
Press File, Save As
Here Paint will want to save your airfoil in My Pictures, but it has a long path and is difficult to get to from TurboCAD,

I have a folder C\: CL Pic
You may want to open a folder for these files that has a short path.

In the Directory Window I press the Scroll Down Pointer and bring up the Local Disk C click on it from there select the folder you want to save your file in.
Scroll down to the bottom and at the Save as Type Window
Press the Scrol Down Pointer and select the .JPG option.
Give the File name.
Press the Save button.


David Craft
[email protected]

John,

I missed something some were in the translation. Would you mined starting over.

Thank you very much,
Gordy

>John,
>
>I missed something some were in the translation. Would you
>mined starting over.
>
>Thank you very much,
>Gordy


AAAARRRRRRGGGGGHHHHHHHH

glad I checked in on this one, John do you want me to elaborate on the TVC math for you,, since your already having all these poor guys twisted around with all them thar formulas?



Gordon, well done,, well done,, keep him in check snicker

Sure

bump