Here are some spreadsheet graphs I generated from an woldofkrauss database:

Interpret these how you like. Half of the outliers either have typos or really jagged polars. These are at Re = 100,000, and include 1200 airfoils. The database copies very well into spread sheets. I only did thicknesses above 8% and L/D above 30 and camber above 1%, and I cut off some of the extremes so the graphs would be zoomed in. I checked out the extremes first, and it did not look like you'd be missing much. Most of the horizontal values are in % of cord. I'm not completely sure how they define trailing angle.

Note: the Cl is Cl max, not Cl @ L/D max.























You can't estimate the drag from the Cl and the L/D max, since the Cl is the Cl max, not the Cl @ L/D max. Cl is included only for stall speed. The strongest correlation here is that increasing camber increases the coefficient of lift. As for airfoil thickness and Cl, I think the only reason we see a correlation at all is that most of the thicker airfoils also have higher camber as a percentage of cord.

 

I should have posted this in the Light Stuff forum.

The reason I did this was to try and figure out a way to make a thick airfoil with good L/D.

Slow ultralights are blown around by the wind. I want one that takes off slow, flies slow, and glides very well, so I can feather the prop and glide at 20 mph without getting blown all over the place. To me, that means we need a shorter cord that produces the same lift and similar drag as a longer cord Clark Y.

Yes, thick airfoils have more drag per cord. But what really matters is how much drag it has per spar depth.

High camber airfoils make more lift, but they tend to do so with more drag. Fast airplanes don't need more lift, and really don't need more drag, and so this is one of the reasons their designers put them down.

High cambered airfoils also tend to have their Cl at L/D max much closer to Cl max than do less cambered airfoils. This could make them easier to stall if they need to cruise higher. But maybe a bigger leading radius will help with that, as would some non-moving front slats.

Making the airfoil thicker seems to have a bigger effect on lift than on drag, but maybe only because the thicker airfoils tend to have bigger leading radius and more camber. Still, thickness does not seem to be a big penalty for a slow airplane that needs way more power to climb than it does to cruise level at its maximum speed. Most ultralights have wires and don't retract or fare their landing gear.

The goal is to levitate around but not be blown around by the gusts. I think the key is to use the correct mean camber line generation method, expanding from a clark Y, and increase the camber the correct way, and increase the leading radius, and increase the thickness distribution so that the bottom stays kind of flat.

Look at the YM 15 vs the YM 18. The 18 has more drag, but not more lift except maybe on the top end. They made the 18 thicker, but they did not increase the camber to match. I wonder if the camber was increased with the thickness, would it have both higher lift and higher drag and similar flight characteristics but at 80% the speed.

My goal is to design an airfoil between 24 and 30% thick, a max Cl with flaps of 4.0 or more, and a cruize Cl of around 2.5. That would allow a cord that is less than half the size, and protect against gusts. The pitching moment will be higher, but that will not be as much of an issue with the lower moment arm of the smaller cord. The other issue is that if the ultralight gets slow enough with a smaller cord and higher Cl, will a separation bubble occur just like as happens with models? And would turbulators help?

Induced drag depends on span loading, not wing loading. As long as I can fit a deep spar in the wing, I can keep the span. It might even have a bigger effective span, depending on which equation is applicable, since the aspect ratio would be higher.


Less skin, less ribs, and less paint also means less weight and less cost. I suspect wing flutter would be less with the lower moment arm. Lower speed also means less power needed, or a steeper climb angle. I don't want a 50 mph wind in my face. I just want to fly and climb where I please.


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The real question is, if I have twice the wing loading of most legal ultralights, does that mean I'll be safe to fly on a somewhat gusty day? Actually I'll have the same wing loading as most ultralights, but a higher cruise coefficient of lift so I can land and fly 50% slower.



I've been reading a lot about high lift devices, and except for the ones that extend the wing cord, it seems like Cl max = 2.9 to 3.2 is the best that can be achieved. That sounds sad. But the fact is all these airfoils are designed for airplanes that fly 600 mph at low Cl, and just change when they come in for landing. If my goal is to cruise at Cl = 1.8+, and I don't mind a higher drag coefficient at my low speed, I wonder if it still is possible to make an airfoil that will achieve this.


Until I design a new airfoil, I think my best bet is to a thicker clark Y with flaperons that are collectively slightly down, or an 18% FX MP 180. Since I'd be flying close to Cl max, I'd need some turbulators to increase the maximum Cl so I can land safely or perform basic maneuvers without stalling all the time. The flaperons should be the simplest, most aerodynamic improvement I can make for legal landing speed with my reduced wing area.

There is a chance I won't be able to make a 24% thick airfoil like I want, so I'll probably be relying on flying wires and maybe even a king post. But as long as I can land at 20 mph and cruise at 30 without being blown around that much, I'll be happy.

Another way to get stall speed down is to reduce the weight as much as possible. I'd like to primarily take off and land with small wheels, but have the option of foot landing and launching if I have to land in an unplanned area.
I can also reduce weight by not taking a full 5 gallons of gas. If I do run out of gas, at least I'll have less weight on my knees when I land in the bushes.

For fun, I'm more concerned about minimum sink rate than minimum sink angle. For safety, there certainly are trade offs between being able to get to a clearing vs being able to land in a tight spot. So I think my light weight design and wires may be the way to go.

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If my 3D Cl = 0.85 2D Cl, and my all up weight is 230 pounds, then I can have a 4 pound per square foot wing loading and fly at 30 mph with a 2D Cl = 1.456, and land at 22 mph with a 2D Cl = 2.7. I would still be capable of flying faster than that. And if I can get the L/D up to 20:1 with a feathered prop, I might even get some soaring in. If my airfoil gets its camber only from the flaperons, then it could fly upside down too.

My plan is a 67 square foot wing, span of 25 feet, and streamlined support wires for everything including the tail boom. The half weight, 2/3 speed plane should need 1/3 the power and thus 1/3 the engine weight.

 

my opinion?

you should step away from the theory for a bit. build a model, see if there's any merit to the idea. if it shows promise, refine it, build a bigger model. lather, rinse, repeat.

that's my opinion.

 

You can make an RC flying law mower, and the aerodynamics do not scale either.

My plan is to fly it with XPlane, and then have an RC robot pilot fly the full sized plane with sand bags in the seat. If it crashes, it was cheap anyway. I'll use a propeller that will shatter on impact and leave the engine undamaged, with other protection around the engine. The gas tank will be on empty. A wing this small will not cost much. My first engine will probably be used. I'll upgrade to a new one later. But we are talking several months from now before I even do a detailed design. Right now I'm just looking at a wide range of options and reading about stuff. I just don't want to have to choose between a fast windy plane vs a plane that needs lots of power or a plane that can be blown around easily.

 

How can this blunt ended airfoil have such decent L/D?

Airfoil Investigation Database - Showing SIKORSKY DBLN-526

I like the thickness, and the L/D max is at the Cl I want to use for the cord I could have with that thickness.

I'm afraid to use that foil even if it is that good, because I don't know the tolerances needed for the shape. Looks like it is good at many AoA's though.

It sure out performs this pointer trailing edge airfoil:

http://www.worldofkrauss.com/foils/show_compare/?id[]=1950&id[]=1566&chord=6.5


This one has almost the same front half, but then gets pointier in back, and it still does much worse:

http://www.worldofkrauss.com/foils/show_compare/?id[]=1950&id[]=1244&chord=6.5

But if I try to build it, I might have to worry about trailing edge sheeting just as much as leading edge sheeting.

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One down side of my high cruise Cl plane is it won't be able to take off in as short of a distance or low a speed as it could with a bigger wing. Maybe that is a worthwhile trade for being a bit safer from sudden gusts up there.

I can only run 15 mph. So if I try to land at 22 mph, I wonder if I'd fall on my face, or if the lift I was getting would make it possible to run 22 mph until I slow down.