Can we try and figure out what this airplane needs to look like?
More on Constraints and Goals
The best way to get any design done is by identifying the constraints (hard and soft) and goals (must haves and would likes). In the previous article of this series I talked about constraints and goals. Here are some more thoughts on that.
My primary goal for the power system is to select a good all-around power system for a two pound (1 kg) airplane. There are many airplanes designed to use the E-flite Park 480 motor (or its equivalent). Several of the 3D airplanes I chose for my comparison are designed specifically for this power system.
I like to use the appropriate tool for the task at hand. So far I have not felt the need to run an RCadvisor calculator analysis of the power system. I already have plenty of evidence to convince me that the power system I chose will work well with the airplane I’m designing. I may rely on the calculator later to optimize the choice of propeller and battery, however.
Because of the shipping costs involved, I do not like to rely on foams that are only available to me via mail order. That limits my choices, but makes experimentation a lot less expensive (both in time and money).
I drive a small car with openings that are only about four feet (1.2 m) wide. That is a real world constraint that I cannot ignore. For added strength and to save weight, I like to design airplanes with permanently attached single-piece wings.
Another thing. I round numbers out. I once designed a model airplane where I was trying to measure 4.22515 inches. No more. Now I do 4.25 inches and I’m done with it.
As much as possible, I use simple tools to help me design an airplane. I like to use graph paper, spiral notebooks, and a hand-held calculator. I like to do my more advanced calculations using a spreadsheet on a computer. Click here for a copy of the spreadsheet containing my Kilo3D design calculations. I will be updating it as the design progresses.
Wing Design Trade-offs
For most practical purposes, the wing is the airplane. It’s shape, size, and location determine more than anything else how the airplane will behave in the air. I typically spend over half the entire design effort figuring out what the wing should look like. Not surprisingly, I have spent several hours already doing calculations for the Kilo3D wing design.
The hard decisions have to do with the wing span, wing area, and taper ratio. It was an easy decision to go with a mid-mounted wing without dihedral and with minimal sweep. It’s a 3D design, after all.
I decided to use a straight (unswept) leading edge. I don’t like using wing spar joiners because of the weight and complication. I plan to use a small wooden dowel on the leading edge for crash resistance. This idea worked beautifully on the ModiFly design. If the leading edge is straight, then I can use a single dowel for the entire wing span and have it also reinforce the wing.
For better strength, faster rolls, and overall good looks, I want to use taper on the wing. That left me with a problem. If the leading edge is straight and the wing has taper, that means that the main spar would need to be swept forward. But who says that the main spar has to be at 25% of the chord for the entire span? Not me! The solution I settled on was having the main spar start at 20% chord at the root and migrate to 33% chord at the tip. Voila! With 0.6 taper, I had an unswept main spar. The wing has about four degrees of forward sweep.
I had another constraint I had to worry about. The sheets of Cellfoam 88 are a maximum of 11.5 inches (29 cm) wide. With strip ailerons of about 25% of the chord, that meant that the biggest chord I could use was about 15 inches (38 cm). With a reasonable taper (0.6), that gave me a tip chord size of 9 inches (23 cm).
I have used round dowels in the past as wing spars, but I really want to use the square dowels in this design. They are only 36 inch (91 cm) long, so that limited my wing span. Will also make for easier loading and unloading onto my car!
Stepped vortex airfoils are a great choice for wings made out of flat pieces of foam. Modelers often call them Kline-Fogleman (KFm) airfoils. They work mainly by acting as turbulators and delaying flow separation (i.e., stalls). Their main disadvantage is added drag, but in this design that is a non-issue. Flying fast is not a goal.
Given the aerobatic nature of the design, a fully symmetrical airfoil is best. At the low Reynolds numbers that we fly model airplanes, airfoils thicker than 6-8% experience early flow separation as the angle of attack increases. This is to be avoided because it would hurt our ability to fly slowly.
A technique I have used in the past with much success involves building a wing out of multiple layers of foam that disappear as the wing chord decreases towards the wing tip due to the wing taper. That way I can make the airfoil thickness be what I need it to be throughout the wing span.
The tail is easy. The horizontal stabilizer is about 20% of the area of the wing. The vertical stabilizer is about 10%. I chose 3/16 (5 mm) foam for them because it felt right. Based on the required areas, I chose an aspect ratio slightly lower than the main wing.
The fuselage is even easier. For simplicity, I plan to use a flat fuselage. It is built very similar to the wing. Make it just as long as the wing span. Use a thick piece of foam and a spar just like the wing. Put a thin dowel on the bottom to protect it, just like the wing leading edge. That will make the bottom of the fuselage a straight line from front to back. I chose to make it 5 inches (13 cm) tall, because it felt right.
As I was trying to balance all these constraints, I also needed to keep a close eye on the weight budget of the airplane. The densities of the foam and dowels are well-known quantities, so it was mostly a matter of adding up the weights of the power and control systems and estimating the volumes of the dowels and foam pieces.
Although the airplane could be up to 32 ounces (900 g) given my overall constraints, a 3D airplane needs a really good power to weight ratio. Since 25 ounces (700 g) looked like a realistic goal weight-wise, I decided to target that.
I should mention at this point that since I always fly from grass, I’m planning on leaving out the landing gear. You are of course free to add one to your version!
A Happy Compromise
Despite starting from a blank sheet of paper, you can see that I quickly built up a long list of constraints and goals. There’s still a lot of room for personal style and preferences, but not nearly as much as you might have thought.
Note that the wing design has steps going back towards the trailing edge like a traditional KFm design, but also going out towards the wing tip. I decided to put the span-wise step one third of the way out from the root. I’m going to use a combination of different thicknesses of foam to get the right airfoil shapes where I want them.
I normally use graph paper to lay out my designs, so that is what I used this time, too. See the attached drawing. Each square represents one inch. The blue area is five layers thick, orange is three, and the other colors are single layer.
And before you ask, yes, I wanted to make changes to the design as soon as I finished the drawing. But at this point I’m down to tweaks, not major overhauls.
I worried for a long time about avoiding tip stalls. But the unusual nature of the airfoils used made that too hard to predict. I will have to wait until I build a prototype to see if that is a problem in practice.
Finally, how do I know for sure that the wing will be strong and stiff enough to meet the goals? I don’t. Well, that is where experience and research come into play. Adding another wing spar would be easy enough to do, if it turns out to require one.
Am I on the right track? What have I overlooked? Let me know!