Reading all that's been posted on the wing thickness thread so far, I believe we may be focusing too narrowly.

WARNING.....What follows is a long design exercise as I understand it.

For instance, wing thickness is not the only parameter influencing the design of a competitive CL stunter.

Take two designs, identical in every way, except airfoil thickness, and, if the wing loading is light, they will likely fly very similar, as long as the percentage is between about 14% to 18%. This idea comes from comparative flights, and logic, not pure scientific data, but the plots seem to hold up for our use.

Now, increase the wing loading, 30%, and what happens? Does the thinner winged plane now have to fly faster to avoid stalling in the hard maneuver's? Which plane will be able to perform within the envelope easier? The thicker wing? I think it may be true, because the added spacing creates a better distribution of forces over the structure, and increased lift, at the given, or designed airspeed.

Does the thinner wing now have enough reserve strength to survive the higher G load from trying to fly faster through the square maneuvers, or will the wing be more prone to folding under the higher load?

Will it be necessary to design in more structure to carry the added loads, in this example, to help avoid that possibility? If so, would this negate some, if not all, the weight savings that may, and I'm not sure that it will amount to more than a few extra grams, be gained with a thinner wing?

Thinking about the above questions, leads me to believe that we are possibly dancing on the head of a pin, considering airfoil thickness, and any possible weight penalty only.

Looking at what may be a more complete design question, relating to the original question in the wing thickness thread, the following questions need to be considered, IMO.

What power are you planning on using.

We must consider how much power is going to be available to fly the plane through the pattern. This is important, because much of the rest of the design problems will relate directly to this choice.

The next item to consider, is how much fuel, in the case of an IC power plant, will be needed to fly through the pattern at sea level. The size of your tank will be governed by the answer to this question, and therefore, the total length of your nose moment.

Once you know the nose moment, you can determine what the tail moment will have to be to balance your design, except, you don't really know the balance point yet, until you design your wing.

Some will use a proven wing from a successful design. If so, then you are going to have to consider the total weight of the plane, and try to keep the new design within the limits imposed by the wing they have chosen. This is probably the most often use method of stunt wing design.  If you use a Nobler wing as your basis, and many have done this, then certain elements are already selected for you. Your wing loading will necessarily have to fall within certain known brackets for the plane to truly be successful

Using a wing from a heavier successful design, and you'll be able to pack more weight, all other items being equal, but you'll also have to select most of the moments of the contributing design.

Answering the question at the top of that thread, will require more information, but trends and pertinent ideas can still be, and have, been discussed.

Erik asked some rather detailed question relating to specific flight envelope desires he wished to address.

This, IMO, makes the study of airfoils, with the tools available to us, such as Profile, very valuable to our discussions.

Now, let me add something that I feel some of us may not be taking into account. Airfoils are in related families. The NA63A is still an NA63A at either 6%, or 30%. The E-169 is still an E-169 at 6% or 30%. The NACA 4 digit is the easiest to see this relationship because the last 2 digits are the percent of thickness. Their graphs, though, will often be different, according to the percent of thickness. This may make a particular percentage better at a specific speed, or wing loading, than another in the same family, so perhaps we should consider which airfoil we want to use based on the wing loading, and the speed we want to fly the pattern at.

How big a wing we want is another factor to consider.

We can go with a low aspect ratio wing, though generally not recommended, or to an efficient high aspect ratio, with mixed recommendations, or somewhjere in the middle, which is what is most often seen. Let's for the sake of this discussion, dismiss the low aspect wing right now because of drag and lift problems.

High aspect ratio wings are more efficient and make great lift, with reduced drag, but are often trouble in turbulence. Such a wing can use a thinner wing than one at a more normal aspect ratio, and still develope the lift needed due to increased efficiency. Turbulence, or wind is the bane of such a wing though, and you haven't lived until you try to fly one in a 25 mph, or higher, wind.

The more normal wing will have an aspect ratio of somewhere between 4.5 through 5.5 to 1. Sometimes a little more, sometimes a little less.

Let's decide to use a normal aspect ratio for stunt ships. let's use 5 to 1.

We'll also use a leadout sweep of, say, 1.5 inches, so we can have the stabilizing effect of the slight dihedral effect from a swept back leading edge.

In most cases, we prefer to keep our flap hingeline straight, and at a right angle to thee fuselage centerline, so our flap trailing edge will sweep forward unless we use constant chord flaps. (not recommended)  let us figure a flap root chord of 3 inches, and a tip flap chord of 1 inch in this example, so our parameters are being set.

Now, we've already determined the power we want to use, so we next select the wing area needed to carry our estimated weight with the power available.

Let us assume, in this example, that we think we want 600 square inches of area.

Let's plug some numbers in and see what our winspan will be to match our desires.

We'll use 13 inches for our root chord, including the flaps

We'll have 9.5 inches tip chord, also including flaps.

Simply figuring, we'll have 11.25 inches at MAC.

Dividing the desired sq inches by the MAC, we wind up with 53.3 inches of wing span, and a total wing area of 600 sq inches.

OK, so what actually will be the proper dimensions of the flaps?

Hmmmm, let's use 18% of total wing area to figure the flap size. we need 108 sq inches of total flap area. 54 sq inches each flap, (if flaps are equal legnth)

So, in our figures for flap size above, considering removing material for the fuselage width, we come up with about 52 sq inches per flap.

We are starting to establish our wing fairly well, but, here comes another decision.

We like the 53.3 inch wingspan, it fits our transport, but we wonder if we can really hit the weight limit? Here's where we take another look at percentage of wing thickness. We want to fly at a particular airspeed, and are limited to no more than 70 feet of line length. We'd like to use no more than 65 feet of line length, rather than the maximum so we have some room for trim considerations. Line length and lap times determine flying speed.

We can run the options through programs, such as profili and test out various thicknesses of our favored airfoil family. By doing so, or by using experience, we decide to go to a 20% airfoil compared to an 18%, and find that we gain enough lift, and the tradeoff for drag, in level flight, and in the hard turns are acceptable.

The very slight increase in weight is offset by increases in strength, and available lift at the speed we want to fly. In some cases, we may even see a reduction in weight considering building to surviving the same G loads, due to the increased strength of the thicker wing.

OK, let's try and establish where the CG should be for our design.

A good rule of thumb has been offered by none other than Ted Fancher. Whether it's his original idea or not is immaterial, but he's the one I learned it from. It's simply the idea that Cg at MAC will equal the percentage of total horizontal stab and elevator area as a percentage of total wing area.


For the sake of our example, let's use 25% for this area. With our wing as defined above, our balance point will be about 6.4 inches forward of our straight flap hingeline.

Now, we can figure the legnth of our nose moment. I prefer to relate to the CG, rather then the often used back of the prop to the leading edge at the root, because it's easier to figure the balances relating to the CG.

So, back to the nose.

We need a total of 10 inches to the leading edge of the wing to comfortably fit the engine and a tank large enough for the pattern. Our CG should be 3.25 inches behind the root leading edge, for a total nose moment of 13.25 inches.

At this point,, we know enough, that we can estimate the weights and their moment arms in the nose, relating to the CG. By summing the moments, we can establish the tail moment needed to balance the plane. By plugging in our best estimated weights for aft portion of the wing and flaps, behind the CG, the weight of the center of mass for the fuse, and the weight, at the center of mass for the stab and elevator, along with other sources of weight and moment arms behind the CG.

There are other factors, such as gear and gear location, as well as wheel size, prop, spinner type and weights etc. to be considered, but we've got most of our dimensions for our new world beater.

A lot more than airfoil thickness for sure.

Good design considers the following to determine the various factors used.

The mission.
The power, and incidentals such as fuel load.
The estimated weight.
The wing sizes, including percentage of thickness, is established by considering the above, as related to weight, and desired flying speed.
The strength needed to survive the G forces that will be encountered within the flight envelope.
Now, we may need to look back and tweak some of the above, as we build our data base.
The size of the rear empanage can be determined.
Once the above is close to final, we can begin to figure tail moments.

We now have the rough basics for our design.

These are the numbers. The rest is mostly aesthetics.

 

Edit: I'm sure there's a lot more that could be added, and invite you to do so.

Have you just designed an Impact 600?  By todays standards, that new plane needs to be using a PA 75 or RoJett 76 for power

But we want tomorrow's standard. And that is a hummmmmmmmmm.......

A PA 1.50 for power!!!!!!!!

I really think tomorrows standard is going to be what electric motor and how much power to throw in with what batteries.  It makes me sad, but is probably quite correct.

Jim Pollock   

My problem is that I really like designing my own planes, both radical and more conventional. 'a problem? Well, yeah, because I spend altogether too much time thinking about changes during construction. I just cannot seem to stick to the plan I've drawn. The present one has been changed structurally in three or four (major) ways already, and I'm kicking myself for one of those changes, a bright idea that I sort of "bought" from a good builder on the forums. There are several wonderful kits in my basement that would have gone togther much faster, without all that "thinking" and fabricating time. Last night I drew up the final, final (final???) proportions for a really bizarre design, when I should have been rigging up my neat new RSM CF bellcrank and its installation - IOW, building! So, what am I doing here now?......Oh, well.

Well, I like John's topic here, and my approach is similar to his, but not really the same. Perhaps I'll write out an example later, but basically I'm pretty systemmatic, using the more basic aero-engineering principles, mostly with high-school math, and some "rules of thumb" pertaining to things too complex for me to model mathematically. I do expect my planes to fly pretty well right off the bench - as the few I've completed have done - but I'm such a klutz at taming tank problems that this doesn't seem to matter as much as it should... so far. Over in the other forum's archives, there are a couple examples/descriptions of my approach. But for now, I haven't even gotten to the Profili wing section thing yet, since I haven't quite finished my little MAC/a.c./NP problem. Anyway, I'm still "easily entertained", but there are still four more Christmas concerts to play. 'gotta practice.

SK

Howard,

Just go over to Paul's and copy his setup,  I'm sure you will be right on the cutting edge of electric technology - wherever that is at now!

Jim Pollock   

Thought so!

Jim