Fist paragraph of my last post is o.k., except for the prop's effects. The propshaft, if at AoA, will produce a yawing moment, which would require a rudder trimtab to null out, so there's another reason for re-setting wing incidence when going to a different airfoil shape, with its different zero-lift AoA. Cruise altitude, speed, and weight play into this, so the designer has to pick some representative cruise point for setting wing incidence.
The cruise AoA of the fuselage is not that important, within bounds, of course. More important might be retaining the ability to fold the wings, That may be what drove the incidence change when going to the new airfoil. But be careful when considering new "higher tech" airfoils. Design assumptions could trip you up in practice. Here's an example: Years ago a colleague of mine used the then-latest boundary layer theory and even invoked variational methods to determine the optimum shape of the Cp distribution for maximizing single-element lift, and achieved a sectional CL of about 2.2 at 5 million Reynolds number, more-or-less where we fly. Homebuilders were calling him regularly asking if his airfoil was appropriate, and if they could use it on their airplanes. With CL max of 2.2, man, could you shrink the wings (in principle, anyway!). His standard answer was "try NACA4415." This guy was well known in the aerodynamics world, and became a senior technical fellow of the corporation. His airfoil was tested in a low-turbulence wind tunnel and performed substantially as predicted. Of course, being optimized for high lift, it had super-nasty stall characteristics, and really fell out of the sky when it let go. (Can we say "snap-roll", boys and girls?) There were other considerations that amateur armchair aerodynamicists might be hard-pressed to understand, that had to do with surface finish and the need (and technology) to carefully orchestrate the boundary layer state (laminar vs. turbulent). Those and other considerations made his airfoil poorly suited for general application to experimental aircraft. NACA 4415, however, has test data over a wide range of operating conditions, and is relatively immune to small surface roughness (say, due to fabric weave) and boundary layer state. It just works, reliably. Oddly, with its essentially flat bottom, it looks a lot like USA35B. I spent most of my career not doing CFD, but instead developing the technology for keeping the boundary layer in the low-friction laminar state on large, swept, transonic, and even supersonic wings. This culminated in the design of the successful active laminar-flow system on the production Boeing 787 -9 and -10 tailfeathers, after which I retired. A rule of thumb I use is: if there's distributed tactible roughness, like fabric weave, assume the boundary layer is turbulent. This has certainly worked on my windsurf fins. All things equal, ones with a matte finish tend to work better than perfectly smooth ones. Laminar boundary layers are notably wimpy. Turbulent ones are way more robust in the face of rising pressures in the downstream direction.
My first line, last post was for CR127, who put up some impressive graphics. That discourse seemed too long to quote. Leni, I think your intuition serves you well. I expect Scrappy will show everybody up, but like Mike Patey says, it's not a practical airplane. My Cp plots on the Avid wing (next thread, unfortunately, as the site wouldn't let me post my plots as a new post inside this thread!) show the airflow at cruise grossly oversped on the lower surface outboard near the tip and just behind the leading edge. The rapid deceleration just downstream of the leading edge might just separate the flow on the lower surface; we can't know without more calculation, or even better, wind-tunnel testing. Tufting that area of the wing would tell the tale, and it's visible from the cockpit, being on the underside. But that slowdown and pressure-rise for sure beats the sh#@$t out of the poor boundary layer! The tried and true USA35B_mod will serve you well, I think. Fancy new section shapes are out there, but unless you can compare apples-to-apples, i.e. wind-tunnel data to wind-tunnel data (not wind-tunnel data to computation), and are sure you know which will be better for your specific conditions, I'd stick with tried-and-true. Building or rebuilding a wing is a lot of work! So what's my read on John Monnett's choice of NACA 64_415 on the Sonex? Too much camber! With a different airfoil choice, even that rockin' little bird could be faster!
I don't quite understand. As AoA changes, so does lift. How can you fix lift while changing AoA? What velocity are you talking about? Yes, the first ~10% chord of that airfoil doesn't appear to produce lift for my cruise case (3kft, 911lbs, 85mph, wing CL=0.40). Others defend Dean's choice of airfoil, but I think it has way too much camber. Why effectively be stuck flying around with the flaps down? (that's what it's like with all that camber!) For a simple Hershey-bar wing, it's important that the airfoil not be operating at high drag, up on the side of its drag bucket, and we all know the CL falls off going outboard towards the tip. So the airfoil should work well over the range of CL the wing experiences at all span stations, meaning at low CL too, like out at the tip. It's much better if the camber level is set appropriate to the important flight conditions - like cruise. I say the slight shortening of ground run all that camber provides is a bad trade, compared to how it kills cruise. It would be interesting to see how much faster the bird could be with NACA 23012 on it! Yes, takeoff ground run would be longer, but by how much? I think all that washout was intended to protect us hosers from ourselves!
Take a look at my first post in the thread "Avid C STOL - Some Calculations". That should clarify things. I would have added this post to that thread, but couldn't insert the two plots. It seems the site's software got buggered so one can only add a pic or plot if you're starting a new thread.
I totally agree. There's a local guy, a CFI with experience in type, who is going to go around the patch with me a few times before I start the long trek back home to the frozen north. And oh, forgot to mention that various pilots who have flown the Sonex corroborate the factory's claim that the airplane is easy to land. Nice to hear!
Thanks for the correction. Great ideas in both cases, I think. I never liked the tailwheel springs on my Avid, despite swapping the stiffer one to the port side, where the thrust center is while taxiing, due to the prop-shaft AoA, and left-turning prop. That helped, but still left the back end laterally smooshy (There I go ranting technical again!). Of course, tightening up the springs helped too. Will do. I have heard of some Avid copycats that were almost impossible to keep straight. With the higher landing speeds, the Sonex had better be easier to control! Months ago I got seriously panned for suggesting fast taxiing as a way to increase comfort level with taildraggers, but I am convinced it was helpful to me. The key was to keep it on the ground. But that moment when the rubber first meets the tarmac on landing was, for me, for many landings, one of near-panic.
From everything I've heard, the Sonex is easier to land than many other taildraggers, in part due to the direct connection between the rudder and tailwheel. It took me a surprisingly long time to be comfortable landing the Avid, even at those comparatively low speeds. I conjecture that the aft-swept landing-gear legs of the Sonex contribute to this. We'll see! We steer boats from the rear, and pitch airplanes from the rear (although not on tandems and canards). Yet we steer our cars, bikes, and other traction vehicles from the front. Why? I used to back my sailboat using the rudder like a canard. But the keel was huge. In a traction vehicle the rear follows the front absolutely. But try putting the outboard on the front end of a canoe. I think it relates to lateral stiffness of the un-steered end; its ability or lack thereof, to follow. On the Sonex, I suspect that the aft-swept titanium-rod gear legs contribute to a slight amount of lateral smooshiness (scientifically speaking), making tailwheel steering more manageable, as it's stable to small lateral disturbances. John Monett's brilliance! Just my 2 cent theory on this. On pitch, I was forever overcorrecting until I discovered that flaring per se was unnecessary; the airplane does it all on its own! With the go-cart stance of the Sonex, I gotta think it too will flare itself. Talk about ground effect!
I have no data on H. Riblett's airfoils, so can render no opinion. However, computational fluid dynamics (CFD) has come a long way since the USA 35B was designed. I do tend to believe good wind-tunnel data over CFD predictions, though, and would be hesitant to make comparisons between the two. CFD is a great design tool, though.