If it quacks like a duck...

In the last post I spent way too much time explaining airfoils and why I selected the one that I did.  Airfoils are important but so are the aspects of plan form, all the other dimensions that make this think look like a wing.  I wish I could say that I had a solid scientific basis for all my dimensions but truth is I don't.  Some of my dimensions are based on the " I like the way it looks" principal.  "Some time you gotta say what the fuck" (are you old enough to name the movie that came from?)  and go for it.  If engineers were the only people that were responsible for most of the things we see and use everyday we would all be driving "boxy" cars and living in boring cookie cutter homes.  There has to be some style in what we create and fly and sometimes there is a compromise between Form and Function.  Life is too short to spend flying ugly wings.

Wingspan

I wanted a good sized wing, one that could take some wind and carry some load.  I have a couple of 36" wings and I think they are a great size for racing and screaming through gaps but I wanted something more substantial.  I thought about 48", it's a great size and I love round even numbers ( don't ask).  I decided early on that this wing would be made out of EPP foam and so I took a look at the available standard sizes from FlyingFoam.com and found that 24" x 36"  was one of their standard sizes.  I found that a 42" wingspan would allow me to build the whole wing from one sheet of foam, so 42" it is. 

Layout

Blunt-nose or not?  I like the way the blunt-nose wings look and it gives a lot more room for batteries and an HD camera.  I think makes it a lot easier to add all the electronics as well so blunt it is.  But now what proportions, how much nose vs wing?  Nothing magical about this for me, I just liked the way it looked.  I'm sure there is some aerodynamic principal that governs this but I was OK with the  "I like the way it looks" method on this one.

I decided that I wanted separate battery and electronics bays for this wing much like Chris Klick's Ritewing Hardcore 44s have.  I eventually changed to a single bay configuration to give myself more flexibility in battery combinations. I wanted to be able to put in a pair of large batteries for long range flying and smaller, higher voltage packs for playing.  I also wanted to configure the battery bay so that I didn't have to worry about CG changes too much with batter configurations.  This was tricky but I am happy with the way it turned out.

I have a wide opening for the prop in the back because I wanted to be able to use a large prop on 3s for more efficiency.  I can use an 11" on 3s  and during testing on an early prototype I was able to get over 45 minutes on a pair of 3s 5200 mah batteries.  

Chord Length

Chord length was a bit of both design principals.  I new the approximate size and proportions I wanted for this wing.  I needed the root cord to be long enough so that the airfoil section was thick enough for the batteries I wanted to use but not so long that it would be difficult to balance the CG.  The tip cord was a little more of a subjective decision.  I needed to make the tip thick enough that it could sustain a good hit and break off, thick enough to be able to hand-cut and I wanted it to look good as well.  I settled on 9" tip and 15" root cords, with the last 2" being elevon.   Had this been a strictly scientific exercise I would have taken my projected weight and wingspan and lift coefficient and calculated the needed wing area. Later I will talk a little about wing loading as well.  There are a lot of charts and graphs on the internet that show the wing loading (weight per square unit of wing area) for different class of aircraft.  This wing has a fairly low wing loading which is great for cruising at lower speeds but can be an issue in strong winds...compromise.

High aspect ratio (AR) wings, ones that are longer with shorter cord lengths are more efficient without a doubt.  Just think of the wings on the Predator Drones, long slender wings have a lower lift-to-drag ratio.  However, there are some downsides, especially for a wing.  They are not as structurally strong especially when being cut from foam, you can't fit as much of your electronics in them and well, they just don't look as good on a wing. 

Sweep Angle

Sweep angle is actually a pretty complex subject.  I will try to keep this as simple as I can because if I think about it too much I confuse the hell out of myself.  Sweep angle has an effect on several aspects.  Because my wing will not be approaching trans-sonic speeds I can ignore a few of these areas.  Sweep angle is very important for flying wings because wings do not have a tail.  Wings will need some type of stabilization along all three axes.  Firstly, longitudinally, the wing needs to have good pitch control and needs to have sufficient reflex to counteract the pitching moment.  To be effective the pitch control needs to be as far behind the CG as possible.  One a flying wing the best way t do this is to sweep the wings back so that the control surface is far behind the CG.

Secondly, the wing needs yaw , around the vertical axis, stability.  This is a problem with nearly every wing in production.  The vertical stabilizer on a conventional tail has a long moment arm because it is so far behind the CG.  For a wing, we need to have a large vertical surface set back as far as we can behind the CG.  The logical choice are winglets on the end of the wing swept back behind the CG.

Lastly, laterally, as one wing dips its angle of attack (AOA) is increased and the AOA of the opposite wing reduced.  Increasing the AOA of a wing will increase the lift of that wing.  This will have a restoring effect on the wing and will try to balance the wing.  The sweep angle can increase this effect thus making the wing more laterally stable.

Based on this discussion you might think it is best to have a lot of sweep, but for every give there is a take in aeronautics.  Increased sweep and has several negative effects as well.  The biggest are reduced lift,  and a tendency to cause tip stalling  With the wing swept back a lesser portion of it is striking the airflow at a right angle thus generating less lift. Also, a large sweep angle can create lateral airflow across the wing, that is air that flows along the wing and not access it.  This lateral airflow increases  along the length of the wing until it gets to the point where the airflow across the wing is so low that the wing can no longer generate lift ant it stalls.  This is very dangerous because when the tip stalls before the root the elevons looses lateral control and the tip stalling wing drops into a spiral.  Too close to the ground and this is an unscheduled landing, tip first.

I selected a sweep angle of 30 degrees, pretty much in the middle of the safe range.  Also, very high angles of sweep are more difficult to hot wire cut especially if you are using washout (more on that later) and 2 templates to bow cut.

OK, in the next post I will get into some of the airfoil and wing analysis I am doing with free aerodynamic software.  This is some pretty cool stuff and a lot of fun for us number people.