What Airfoil Should I Use?

[Jackson T]

Hey guys, what are some good airfoils for rc gliders? I'm hoping to make a 3m scale glider. I am purposefully not trying to make it light (~1600g), so it will hopefully carry its speed and momentum better. I will probably sheet the whole upper surface and at least to the main spar on the underside, so the airfoil should be fairly accurate. All of the popular airfoils for rc planes I have found so far fly most efficiently at high CL compared to others like the NACA 4 digit series, which I don't want because it makes the plane fly slower and make more induced drag. Any ideas?

 

[Merv]

Here is what Andrew Newton usesfor his 1.2m simple slope soarer

 

[Jackson T]

Here is what Andrew Newton usesfor his 1.2m simple slope soarer

Nice plane! What aerofoil did it use?

 

[Craftydan]

So . . . welcome to the rabbit hole. How far down would you like to go?

So 3m lead-sled (Glider optimized for energy retention in speed rather than floatyness to punch through turbulence and sink). That can be fun, and is a fairly common strategy for several decades, though most builders these days eschew pure balsa for composites if this is their goal . . . but there's some merit in an nice ol' scratch woody.

First, I wouldn't be so hasty to turn your nose up at NACA's so quickly (of any series). They are explicitly all over the map in regards to performance -- the different digits describe parameters of the airfoil itself, with the 3 and 4 describing different geometry, and the 5 series describing lift characteristics. One might be awesome, the next terrible. That being said, I have no direct recommendations among those sets, and few tend to perform in an optimum manner at the low Re RC gliders tend to float at.

A step deeper, let's move to the sweet-spot of the age you're building around -- just before composites came into their own. As I write this, I'm looking at two sets of foam cores cut for S-3071 and S-7032 to be used in a 3m foamcore-balsa sheeted wing on a 25 year old design that lands at the edge of you're golden window. These are both low-RE foils, with the S-3071 selected to be the more-slippery variant of that glider. In either case, this airframe -- the Bob Sealy Laser -- is an uncommon airframe, but every reference I've seen speaks positively about it. (my plans are to carbon-sheet it, add flaps instead of ailerons and electrify it . . . someday). In any case, the S-3071 was a fairly popular foil in this era, the bottom is relatively flat (so a rib and sheeting method should create an accurate foil) and commonly done as constant foil from root-to-tip with the tips hand-sanded to TLAR. Simple to do and good return on little effort.

A touch further down the hole, I'd point to the work of Mark Drela, out of Charles River RC. A skilled airfoil designer, his wings (and entire airframes) were all the rage about 10-15 years ago. Most of his documentation is still available online, and his airfoils are still used on some fairly competitive models. The sweet spot for your current design goals, I'd encourage you to look at the "Bubble Dancer" for a similar span wing. One thing to note is the jump in sophistication -- he didn't use a single airfoil, but but a progressive series of foils along the span. The concept is to change the foil along the wing to better control the lift profile and manage the lateral airflow along the foil to simultaneously improve the slipperiness (penetration and energy retention) and overall floatyness of the wing. He's got full construction plans up, and this is a woody RES, so there's plenty to glean from here . . . and that's from 15 years ago . . .

Even further down the rabbit hole, I'd suggest looking into Gerald Taylor's work (G_T on RCGroups). He's been designing high end airfoils (has three recent competition DLG airfoils to his design credit, the latest of which regularly hits the first place podium in comps), and a few years back transitioned one of his more sophisticated designs -- The Synergy-II -- from the 1.5m span in the F3K class (DLG) up to 3m+ spans to create the SynerJfoil for the F3J class (large thermal duration). The jump in sophistication is not just an improvement in the series of airfoils (optimized over the progressive span of the wing) but it also optimizes for several different camber changes -- adding flaperons/flaps, you can droop/lift the trailing edge to shift the wing between optimum slippery and optimum floaty simultaneously over the whole wing. The result is a ship that has to be flown to be believed -- it will slice through the nastiest of wind at high speed, then a click on your radio, the surfaces droop a few millimeters and the wing morphs into a slow and floaty wing that climbs in a thermal like an umbrella caught in a tornado. Click back and she's ready to hunt through mixed air for the next ride up. He has a few threads and the data is all in there . . . but you will need to dig for it -- his foil data isn't pre-cut into ribs, and the threads go for pages covering details about how it was designed mixed with info you need to make it real. Keep in mind, the airfoils are designed to be of molded composite construction, typically with milled cores -- not even hot-wire cut and bagged, though that does show good performance in spite of the slop. One of the more clever DLG designers used one of his older foils (Zone-V2) in a woody to VERY good results, so while no guarantees the SenerJ foil set will be stellar in a woody, there's still good chance it will perform well. Threads for Gerald's foils: Zone-V2, Synergy-II, SynerJ (2014), SynerJ (2015) (he also has threads on matching airfoils for tail surfaces as well -- this far down the rabbit hole, even the airfoil on your tail makes a difference)

 

[Merv]

Nice plane! What aerofoil did it use?

One like this

 

[Fluburtur]

The selig 3021 should work quite well, it looks very similar to the Eppler 205 who alos glides well and was popular in gliders a decade ago I think, there might be more efficient airfoils now but I know by experience that the E205 glides super well and has tons of lift. Also it is thick enough to easily put spars and servos inside.

 

[Jackson T]

So . . . welcome to the rabbit hole. How far down would you like to go?

So 3m lead-sled (Glider optimized for energy retention in speed rather than floatyness to punch through turbulence and sink). That can be fun, and is a fairly common strategy for several decades, though most builders these days eschew pure balsa for composites if this is their goal . . . but there's some merit in an nice ol' scratch woody.

First, I wouldn't be so hasty to turn your nose up at NACA's so quickly (of any series). They are explicitly all over the map in regards to performance -- the different digits describe parameters of the airfoil itself, with the 3 and 4 describing different geometry, and the 5 series describing lift characteristics. One might be awesome, the next terrible. That being said, I have no direct recommendations among those sets, and few tend to perform in an optimum manner at the low Re RC gliders tend to float at.

A step deeper, let's move to the sweet-spot of the age you're building around -- just before composites came into their own. As I write this, I'm looking at two sets of foam cores cut for S-3071 and S-7032 to be used in a 3m foamcore-balsa sheeted wing on a 25 year old design that lands at the edge of you're golden window. These are both low-RE foils, with the S-3071 selected to be the more-slippery variant of that glider. In either case, this airframe -- the Bob Sealy Laser -- is an uncommon airframe, but every reference I've seen speaks positively about it. (my plans are to carbon-sheet it, add flaps instead of ailerons and electrify it . . . someday). In any case, the S-3071 was a fairly popular foil in this era, the bottom is relatively flat (so a rib and sheeting method should create an accurate foil) and commonly done as constant foil from root-to-tip with the tips hand-sanded to TLAR. Simple to do and good return on little effort.

A touch further down the hole, I'd point to the work of Mark Drela, out of Charles River RC. A skilled airfoil designer, his wings (and entire airframes) were all the rage about 10-15 years ago. Most of his documentation is still available online, and his airfoils are still used on some fairly competitive models. The sweet spot for your current design goals, I'd encourage you to look at the "Bubble Dancer" for a similar span wing. One thing to note is the jump in sophistication -- he didn't use a single airfoil, but but a progressive series of foils along the span. The concept is to change the foil along the wing to better control the lift profile and manage the lateral airflow along the foil to simultaneously improve the slipperiness (penetration and energy retention) and overall floatyness of the wing. He's got full construction plans up, and this is a woody RES, so there's plenty to glean from here . . . and that's from 15 years ago . . .

Even further down the rabbit hole, I'd suggest looking into Gerald Taylor's work (G_T on RCGroups). He's been designing high end airfoils (has three recent competition DLG airfoils to his design credit, the latest of which regularly hits the first place podium in comps), and a few years back transitioned one of his more sophisticated designs -- The Synergy-II -- from the 1.5m span in the F3K class (DLG) up to 3m+ spans to create the SynerJfoil for the F3J class (large thermal duration). The jump in sophistication is not just an improvement in the series of airfoils (optimized over the progressive span of the wing) but it also optimizes for several different camber changes -- adding flaperons/flaps, you can droop/lift the trailing edge to shift the wing between optimum slippery and optimum floaty simultaneously over the whole wing. The result is a ship that has to be flown to be believed -- it will slice through the nastiest of wind at high speed, then a click on your radio, the surfaces droop a few millimeters and the wing morphs into a slow and floaty wing that climbs in a thermal like an umbrella caught in a tornado. Click back and she's ready to hunt through mixed air for the next ride up. He has a few threads and the data is all in there . . . but you will need to dig for it -- his foil data isn't pre-cut into ribs, and the threads go for pages covering details about how it was designed mixed with info you need to make it real. Keep in mind, the airfoils are designed to be of molded composite construction, typically with milled cores -- not even hot-wire cut and bagged, though that does show good performance in spite of the slop. One of the more clever DLG designers used one of his older foils (Zone-V2) in a woody to VERY good results, so while no guarantees the SenerJ foil set will be stellar in a woody, there's still good chance it will perform well. Threads for Gerald's foils: Zone-V2, Synergy-II, SynerJ (2014), SynerJ (2015) (he also has threads on matching airfoils for tail surfaces as well -- this far down the rabbit hole, even the airfoil on your tail makes a difference)

Oh, my goodness!!! I absolutely love aerodynamics!!! I would study aerodynamics if I could (as in more studying than reading every book in the library on the subject and practicing on my rc planes), but I don't want to end up with a desk job. So much info! I'm not very keen on forum digging, because I always get side tracked with other cool threads and have a hard time finding what I'm looking for. My plane will have a wing loading of ~43g/dm2 (~14oz./ft2), so it still probably won't be that fast (according to the lift equation if you double the wing loading, you only increase speed by ~41%). The reason I wasn't sure about the NACA's is because I don't hear of many people using them, and they weren't designed for low reynolds numbers like the other ones you've mentioned. If the best L/D ratio of the airfoil I decide on is found at a CL of ~0.8, my plane will fly ~9.5m/s (~34km/h), so with the mean geometric chord being 125mm the Re will be quite low. I wanted an airfoil that for the above reason perfoms best at the lowest CL I could find to increase speed, Re number, and decrease vortex induced drag. One airfoil I found today that looks good is the HQ 1.5/9 airfoil. It best L/D is at CL ~0.8, which is lower than everything else I've seen except the NACA's, which is why it's on top of the list as of now. What does TLAR mean? I have to admit I've only built two balsa rc planes, one of which is my own design that I haven't maidened yet.

 

[Jackson T]

The selig 3021 should work quite well, it looks very similar to the Eppler 205 who alos glides well and was popular in gliders a decade ago I think, there might be more efficient airfoils now but I know by experience that the E205 glides super well and has tons of lift. Also it is thick enough to easily put spars and servos inside.

I've had a look at that one, but I am hoping for something that performs best at a lower CL to increase speed and Re number, and decrease vortex induced drag.

 

[Craftydan]

Oh, my goodness!!! I absolutely love aerodynamics!!! I would study aerodynamics if I could (as in more studying than reading every book in the library on the subject and practicing on my rc planes), but I don't want to end up with a desk job. So much info! I'm not very keen on forum digging, because I always get side tracked with other cool threads and have a hard time finding what I'm looking for. My plane will have a wing loading of ~43g/dm2 (~14oz./ft2), so it still probably won't be that fast (according to the lift equation if you double the wing loading, you only increase speed by ~41%). The reason I wasn't sure about the NACA's is because I don't hear of many people using them, and they weren't designed for low reynolds numbers like the other ones you've mentioned. If the best L/D ratio of the airfoil I decide on is found at a CL of ~0.8, my plane will fly ~9.5m/s (~34km/h), so with the mean geometric chord being 125mm the Re will be quite low. I wanted an airfoil that for the above reason perfoms best at the lowest CL I could find to increase speed, Re number, and decrease vortex induced drag. One airfoil I found today that looks good is the HQ 1.5/9 airfoil. It best L/D is at CL ~0.8, which is lower than everything else I've seen except the NACA's, which is why it's on top of the list as of now. What does TLAR mean? I have to admit I've only built two balsa rc planes, one of which is my own design that I haven't maidened yet.

I feel ya'. I'm a bit of an armchair aerodynamicist myself -- fluid dynamics is fascinating stuff if your job doesn't depend on it. I am an engineer by profession, just not that kind

At these scales Re is always painfully low. It is the fundamental linchpin in the principal "Air does not scale". The work characterising laminar flow lift just works better at larger scales, but shrinking down the boundary layers get painfully thick and our speeds are just so slow in this relative soup we fly in. If you think this is bad, you should check out indoor freeflight with weight budgets tracked in the micrograms and flight speeds at a slow walking pace -- fun stuff.

TLAR is the time honored Aerodynamic design process of "That Looks About Right". It's shocking how effective it is. Generally if it looks like it will fly, and a few flight parameters are respected (balance is close, throw is reasonable, structure is acceptably rigid, thrust is sufficient) most plane designs can't help but fly. They may have some nasty habits, but that is what separates the quality of the designs.

I must apologize for above -- the S-3071 I mentioned above is a typo. I was referring to the Selig 3021 that Fluburtur mentioned . . . but it is an old airfoil designed to be a constant foil across the wing. Good for it's simplicity and about as good a performer as you can expect at these Re, but not designed with the mindset of drag reduction across the entire wing. The HQ 1.5/9 looks interesting (the undercamber may be difficult to reproduce accurately with ribs) as a single slice, but optimizing down the vortex drag requires the progression of airfoils, not a single all-purpose cross-section, and in some cases requires a progression of incidence changes as well. It might be a fair illustration to download a copy of Fusion360 and open the Prantl-D models in it to view how the foil shape and twist progresses across the span in a wing optimized to reduce drag -- it's not practical for this project, but the concept is well demonstrated in it.

If the wing you're creating will be at most spoiled (no ailerons, no flaps), then look in detail at the bubble-dancer I linked above. Mark Drella designed it with such a progression. Any one airfoil in the AG series he's designed are good on their own, but taken as a family in the right progression produces performance a single airfoil slice alone cannot provide. He did not, however, further optimize it for changing camber. if you don't mind the wing loosing a touch of efficiency in more cambered modes, it can still work fine, jut keep in mind it will only perform at it's best when clean.

The SynerJ/y and Zone wings, however were designed with such camber optimization included. The Zone-V2 does not perform as well as the SynerJ/y's, but that's partially because it was designed in discrete sections, not as a continuous of progression. You should be able to find wire-cutting templates, which would convert well to rib sections. do keep an eye on any twist on the wing -- I don't recall much if any on the one's I've seen, but a subtle washout wouldn't be much and dramatically impacts the wing's performance at the lower Re ends of the envelope.

BTW, I think you've got some relationships reversed. The heavier your plane, the faster it must go before stall. This is particularly true of gliders where your energy is from burning altitude. Cleaning your wing and fuselage to reduce drag will only increase the upper end of her speed envelope. I am also wondering, from your description it appears you're attempting to minimize Cl in an attempt to make the wing more slippery, or am I misunderstanding you? I'd expect your goal would be quite the opposite -- reducing the liftiness of the wing may have a coupled effect of reducing the drag (Cd, which I believe is your real goal), but it's an indirect relationship, not a direct one. For instance, a brick has a fairly low Cl (honestly, I haven't tested it, but, I think I'm safe on that claim), but has a high Cd. Minimizing the Cl in that case has a negative impact on Cd . . . so don't hunt that way. Optimize the Cl v Cd chart if you can (push the vertical line to the left and make it as long as you can), but don't do it at the cost of a depressed Cl. Maximize Cl v Alpha and Minimize Cd v Alpha for a more balanced approach. With too low of a Cl for your wingloading, a slippery plane will only fall more quickly out of the sky.

 

[Merv]

I've been building plane for 20+ years and have always used the TLAR approach. How much better are the "engineered" airfoils to the FT ones? I'm looking more for what scale would they fall on. One to two percent better. Ten to twenty percent better. Or one hundred to two hundred percent better?

 

[Jackson T]

I feel ya'. I'm a bit of an armchair aerodynamicist myself -- fluid dynamics is fascinating stuff if your job doesn't depend on it. I am an engineer by profession, just not that kind

At these scales Re is always painfully low. It is the fundamental linchpin in the principal "Air does not scale". The work characterising laminar flow lift just works better at larger scales, but shrinking down the boundary layers get painfully thick and our speeds are just so slow in this relative soup we fly in. If you think this is bad, you should check out indoor freeflight with weight budgets tracked in the micrograms and flight speeds at a slow walking pace -- fun stuff.

TLAR is the time honored Aerodynamic design process of "That Looks About Right". It's shocking how effective it is. Generally if it looks like it will fly, and a few flight parameters are respected (balance is close, throw is reasonable, structure is acceptably rigid, thrust is sufficient) most plane designs can't help but fly. They may have some nasty habits, but that is what separates the quality of the designs.

I must apologize for above -- the S-3071 I mentioned above is a typo. I was referring to the Selig 3021 that Fluburtur mentioned . . . but it is an old airfoil designed to be a constant foil across the wing. Good for it's simplicity and about as good a performer as you can expect at these Re, but not designed with the mindset of drag reduction across the entire wing. The HQ 1.5/9 looks interesting (the undercamber may be difficult to reproduce accurately with ribs) as a single slice, but optimizing down the vortex drag requires the progression of airfoils, not a single all-purpose cross-section, and in some cases requires a progression of incidence changes as well. It might be a fair illustration to download a copy of Fusion360 and open the Prantl-D models in it to view how the foil shape and twist progresses across the span in a wing optimized to reduce drag -- it's not practical for this project, but the concept is well demonstrated in it.

If the wing you're creating will be at most spoiled (no ailerons, no flaps), then look in detail at the bubble-dancer I linked above. Mark Drella designed it with such a progression. Any one airfoil in the AG series he's designed are good on their own, but taken as a family in the right progression produces performance a single airfoil slice alone cannot provide. He did not, however, further optimize it for changing camber. if you don't mind the wing loosing a touch of efficiency in more cambered modes, it can still work fine, jut keep in mind it will only perform at it's best when clean.

The SynerJ/y and Zone wings, however were designed with such camber optimization included. The Zone-V2 does not perform as well as the SynerJ/y's, but that's partially because it was designed in discrete sections, not as a continuous of progression. You should be able to find wire-cutting templates, which would convert well to rib sections. do keep an eye on any twist on the wing -- I don't recall much if any on the one's I've seen, but a subtle washout wouldn't be much and dramatically impacts the wing's performance at the lower Re ends of the envelope.

BTW, I think you've got some relationships reversed. The heavier your plane, the faster it must go before stall. This is particularly true of gliders where your energy is from burning altitude. Cleaning your wing and fuselage to reduce drag will only increase the upper end of her speed envelope. I am also wondering, from your description it appears you're attempting to minimize Cl in an attempt to make the wing more slippery, or am I misunderstanding you? I'd expect your goal would be quite the opposite -- reducing the liftiness of the wing may have a coupled effect of reducing the drag (Cd, which I believe is your real goal), but it's an indirect relationship, not a direct one. For instance, a brick has a fairly low Cl (honestly, I haven't tested it, but, I think I'm safe on that claim), but has a high Cd. Minimizing the Cl in that case has a negative impact on Cd . . . so don't hunt that way. Optimize the Cl v Cd chart if you can (push the vertical line to the left and make it as long as you can), but don't do it at the cost of a depressed Cl. Maximize Cl v Alpha and Minimize Cd v Alpha for a more balanced approach. With too low of a Cl for your wingloading, a slippery plane will only fall more quickly out of the sky.

Aah, so that's what TLAR means! As I said, I don't have that much experience building, so I'm going to use just one airfoil section across the entire span. The reason why I like the HQ 1.5/9 is because it has a decent L/D, but even better, it flies at its best L/D at a lower than average CL (it has a lower CL but also a lower Cd). This means I still get a good glide slope, but it flies a bit faster because of the lower CL. I know it will increase sinking speed by flying faster, but I'm OK with that. Also, induced drag is proportional to the square of the lift coefficient, so that further improves the HQ 1.5/9's overall L/D compared to others. Do you think the covering film will be able to stick to the ribs to make the undercambered part on the underside?

About the aerodynamics stuff, I meant that for a given model at a given CL (CL for best L/D), doubling the AUW will make the velocity sqrt2 times the original speed (~41% increase).

Thanks for all the tips and advice!

P.S. I am planning to have a removable winglet system to try out different winglet designs.

 

[Craftydan]

Merv,

I've yet to see a sim or wind tunnel run on a FT wing, so . . . Dunno. Performance from just an airfoil itself is a bit over the map. A Clark-Y is an easy foil to build and performs all-around well, but high performance? No. How inferior is the venerable Clark-Y to a well optimized wing? A lot. If I hadn't gotten sucked into DLGs and seen what the difference a foil can make between meh, impressive and unbelievable, I'd have a hard time believing how good it can get way out on the tail of that bell curve . . . which is a fancy way of putting "the more I know, the more I know I don't know", and "how good it can get" is way out out into unbelievable.

Balsa flies better . . . but how much of that is the better foil from cut ribs vs. generally stiffer design? Still hard to say, but all things forced to equal, on the low end of balsa (sporting a TLAR or Clark-Y airfoil) I expect it'll fly "noticeable but only a little better" than a folded foam wing. Start optimizing the airfoil to the task, and it progresses down that tail adding performance and multiplying the cost.

In the end, if I take a folded foam DLG wing to a contest it would be as a gimmick (which I'm not above 3 ), but it will give me more than a few enjoyable flights. An old-school DLG sporting dated foils will push past it with ease . . . and be put to shame by the latest up-and-comers. Each step has it's joy in flight, and each is roughly 5-10 times the last in cost. How far you want to push it will be tempered by your tolerance for the price of entry.

. . . or in other words, how much better you want it, always depends on how much more you're willing to pay. Personally I find FT has found a sweet spot in that price/fun ratio, and I expect that's a rather popular opinion around here

 

[Craftydan]

Aah, so that's what TLAR means! As I said, I don't have that much experience building, so I'm going to use just one airfoil section across the entire span. The reason why I like the HQ 1.5/9 is because it has a decent L/D, but even better, it flies at its best L/D at a lower than average CL (it has a lower CL but also a lower Cd). This means I still get a good glide slope, but it flies a bit faster because of the lower CL. I know it will increase sinking speed by flying faster, but I'm OK with that. Also, induced drag is proportional to the square of the lift coefficient, so that further improves the HQ 1.5/9's overall L/D compared to others. Do you think the covering film will be able to stick to the ribs to make the undercambered part on the underside?

About the aerodynamics stuff, I meant that for a given model at a given CL (CL for best L/D), doubling the AUW will make the velocity sqrt2 times the original speed (~41% increase).

Thanks for all the tips and advice!

P.S. I am planning to have a removable winglet system to try out different winglet designs.

The removable wiglets is not a bad idea. It'll make the wing more modular and potentially save a bit of hangar rash if you pick something fancy or low-slung for your final set. That and you seem to want practice building removable sections . . . that would do it

So I think you're missing the mix of what the Cl v Alpha chart is saying. For a given airspeed (in formal design the cruise speed is a design goal, in model building it tends to be more of a point of interest) you will need to attain a given Cl for the downward velocity to stay constant (Lift = Weight). For a given airspeed, this value depends more on your wing-loading than your airfoil selection. The Cl v Alpha chart tells you at what Alpha -- your AoA -- the wing will need to maintain to get that lifting force. now whether you can maintain that velocity or not depends on your engine . . . which is weird for gliders, but optimize away drag for an engine and you'll do well in glide. The point is, if you have a higher Cl curve, you can keep a lower AoA for the same amount of lift -- that generally means you're also on a lower spot on the Cd curve as well.

All that being said, I'm not saying the HQ 1.5/9 is a bad choice, just that your search methodology is flawed. It's a decent foil according to the charts, and I'll agree, it's a better performer than the S3021 . . . but lets take a look at the chart comparisons at a low Re between the HQ (yellow), the 3021 (blue) and for reference, I'll toss in the AG18 (purple), one of Drela's foils:

Just looking at these two, It's fairly easy to see over most of the positive Alpha, HQ lags in Cl (you'll need more speed -- fall faster -- or higher AoA for the same lifting force) , but I'd like to point out the Ag foil over a reasonable cruising range of AoA beats them both in Cl and Cd. down-selecting in Cl would have eliminated the Ag in favor of the Hq, where the Ag is a better performer in each. How about the other plots?

In this plot you can get a feel for how strong and how sharply the lift/drag tradeoff as you change the AoA. The fatter the humps, the more forgiving the AoA shift is. The taller the humps, the more favorable the tradeoff is. As you can see here the Ag is a happier wing, where the 3021 will run better at higher AoA, but not better than the HQ in it's sweet spot . . . the problem is it's sweet spot is fairly narrow -- it likes 6 degrees of AoA, and rapidly becomes surly as you move away from it. Keep in mind, this effect isn't very pronounced -- they're all still pretty close.

In this plot, optimum is farther left (lower Cd) and longer stretch of the vertical (CD kept low over a wider CL) This chart is independent of attitude. In this case, the Ag stays cleaner until we get into negative Cl (the wing is dragging us down at this point) whereas the 3021 excels only at it's highest Cl (either in climbing or low velocity) The HQ again, hits the middle ground most everywhere else.

And finally . . . bleck. this chart relates how much the Cg and the aerodynamic center move from each other as you shift AoA (normalized for airspeed, naturally) It's rarely a linear chart (it generally has a flat spot with dimples at mid-range Alphas), and the lower the Re, the more chaotic it gets . . . signaling the airfoil is right at the edge of it's envelope in Re. At this Re, I'd declare that all three of these airfoils are going to be a bit of a handful . . . but running a quick Re calc for the 50,000 on an 8" chord, that works out to 3.5m/s . . . ~8mph. This is pretty darn slow. (yes, I shoulda picked the next higher up Re, but I ain't redoing it )


As for the Induced drag (Cdi) vs Cl relationship . . . think about it. For a given airspeed, you will need to adjust your Cl to be a fixed value to maintain altitude (weight = lift). If you want your cruising speed to be a particular speed . . . say 18.32m/s . . . your Cl from the wing will be the same REGARDLESS of the airfoil (assuming the wing can create that Cl at that airspeed). In the end, Cdi would be impacted more by your wingtips than the foil you chose . . . and here's where I buried the lead: Cdi is theoretical. Cd is empirical. Cdi is baked into Cd. if you increase Cdi by increasing Cl, but Cd goes down when measured, your total drag has still dropped. Other contributing factors to Cd have dropped faster than Cdi went up. Again, don't throw away valuable Cl in search of less drag. you'll tend to loose that bet in the end.


So . . . after all that, am I recommending the Ag18? over the HQ 1.5/9, well, yeah, but won't say it's the "best", and none of these are in any means "bad". Look again, watching each of the graphs, thinking in what Alpha means to you in flight. They all have a story to tell, and laid one over the other they'll not just say A is better than B, but it can warn of the quirks A will provide, and the spots B may actually be better.

 

[TooJung2Die]

I got confused right after you said "So I think".

 

[Jackson T]

The removable wiglets is not a bad idea. It'll make the wing more modular and potentially save a bit of hangar rash if you pick something fancy or low-slung for your final set. That and you seem to want practice building removable sections . . . that would do it

So I think you're missing the mix of what the Cl v Alpha chart is saying. For a given airspeed (in formal design the cruise speed is a design goal, in model building it tends to be more of a point of interest) you will need to attain a given Cl for the downward velocity to stay constant (Lift = Weight). For a given airspeed, this value depends more on your wing-loading than your airfoil selection. The Cl v Alpha chart tells you at what Alpha -- your AoA -- the wing will need to maintain to get that lifting force. now whether you can maintain that velocity or not depends on your engine . . . which is weird for gliders, but optimize away drag for an engine and you'll do well in glide. The point is, if you have a higher Cl curve, you can keep a lower AoA for the same amount of lift -- that generally means you're also on a lower spot on the Cd curve as well.

All that being said, I'm not saying the HQ 1.5/9 is a bad choice, just that your search methodology is flawed. It's a decent foil according to the charts, and I'll agree, it's a better performer than the S3021 . . . but lets take a look at the chart comparisons at a low Re between the HQ (yellow), the 3021 (blue) and for reference, I'll toss in the AG18 (purple), one of Drela's foils:
View attachment 121705 View attachment 121706

Just looking at these two, It's fairly easy to see over most of the positive Alpha, HQ lags in Cl (you'll need more speed -- fall faster -- or higher AoA for the same lifting force) , but I'd like to point out the Ag foil over a reasonable cruising range of AoA beats them both in Cl and Cd. down-selecting in Cl would have eliminated the Ag in favor of the Hq, where the Ag is a better performer in each. How about the other plots?

View attachment 121708

In this plot you can get a feel for how strong and how sharply the lift/drag tradeoff as you change the AoA. The fatter the humps, the more forgiving the AoA shift is. The taller the humps, the more favorable the tradeoff is. As you can see here the Ag is a happier wing, where the 3021 will run better at higher AoA, but not better than the HQ in it's sweet spot . . . the problem is it's sweet spot is fairly narrow -- it likes 6 degrees of AoA, and rapidly becomes surly as you move away from it. Keep in mind, this effect isn't very pronounced -- they're all still pretty close.

View attachment 121709

In this plot, optimum is farther left (lower Cd) and longer stretch of the vertical (CD kept low over a wider CL) This chart is independent of attitude. In this case, the Ag stays cleaner until we get into negative Cl (the wing is dragging us down at this point) whereas the 3021 excels only at it's highest Cl (either in climbing or low velocity) The HQ again, hits the middle ground most everywhere else.

View attachment 121712

And finally . . . bleck. this chart relates how much the Cg and the aerodynamic center move from each other as you shift AoA (normalized for airspeed, naturally) It's rarely a linear chart (it generally has a flat spot with dimples at mid-range Alphas), and the lower the Re, the more chaotic it gets . . . signaling the airfoil is right at the edge of it's envelope in Re. At this Re, I'd declare that all three of these airfoils are going to be a bit of a handful . . . but running a quick Re calc for the 50,000 on an 8" chord, that works out to 3.5m/s . . . ~8mph. This is pretty darn slow. (yes, I shoulda picked the next higher up Re, but I ain't redoing it )


As for the Induced drag (Cdi) vs Cl relationship . . . think about it. For a given airspeed, you will need to adjust your Cl to be a fixed value to maintain altitude (weight = lift). If you want your cruising speed to be a particular speed . . . say 18.32m/s . . . your Cl from the wing will be the same REGARDLESS of the airfoil (assuming the wing can create that Cl at that airspeed). In the end, Cdi would be impacted more by your wingtips than the foil you chose . . . and here's where I buried the lead: Cdi is theoretical. Cd is empirical. Cdi is baked into Cd. if you increase Cdi by increasing Cl, but Cd goes down when measured, your total drag has still dropped. Other contributing factors to Cd have dropped faster than Cdi went up. Again, don't throw away valuable Cl in search of less drag. you'll tend to loose that bet in the end.


So . . . after all that, am I recommending the Ag18? over the HQ 1.5/9, well, yeah, but won't say it's the "best", and none of these are in any means "bad". Look again, watching each of the graphs, thinking in what Alpha means to you in flight. They all have a story to tell, and laid one over the other they'll not just say A is better than B, but it can warn of the quirks A will provide, and the spots B may actually be better.

Gee, that AG18 is a real performer! Sorry, I think there has been a slight misunderstanding on both sides. I want to fly my plane at it's best L/D, and I want an airfoil that has it's best L/D at a smaller CL, so that it flies faster at its best L/D. Yes, I know it will sink faster that way, but I'm OK with that. I'm not trying to make it a soarer. Do you think the covering film will stick to the undercambered part of either airfoil strong enough?

 

[Craftydan]

It's nice . . .I knew it was a good foil, so it's a bit of a ringer, but it's not by any means the best in every case . . . and there are reasons you'd skip it, like buildibility -- it's pretty slim and strongly undercambered. Fine for foam core, tricky for ribs.

For undercamber, It depends on how strong the curve is and how aggressively you shrink the film. Having a wider TE strip and lower spar-strips will help the plastic have good areas to grip spanwise to follow the curve. Those add weight, but you'll pick up both strength and stiffness while you're at it. If you're a glutton for punishment, Geodetic ribs will do all the same, but personally, I would't touch that nonsense without a good 3D modeling package and a laser cutter Cool, and the result is worth the effort, but it's a lot of effort.

In the same vein, Take a look at the AG03 -- not as high of a performer as the AG18, but should hold it's own and was designed to be flat bottomed to simplify construction. Should be easy to build accurately with a rib/spar structure, without giving up much in performance.

 

[Jackson T]

It's nice . . .I knew it was a good foil, so it's a bit of a ringer, but it's not by any means the best in every case . . . and there are reasons you'd skip it, like buildibility -- it's pretty slim and strongly undercambered. Fine for foam core, tricky for ribs.

For undercamber, It depends on how strong the curve is and how aggressively you shrink the film. Having a wider TE strip and lower spar-strips will help the plastic have good areas to grip spanwise to follow the curve. Those add weight, but you'll pick up both strength and stiffness while you're at it. If you're a glutton for punishment, Geodetic ribs will do all the same, but personally, I would't touch that nonsense without a good 3D modeling package and a laser cutter Cool, and the result is worth the effort, but it's a lot of effort.

In the same vein, Take a look at the AG03 -- not as high of a performer as the AG18, but should hold it's own and was designed to be flat bottomed to simplify construction. Should be easy to build accurately with a rib/spar structure, without giving up much in performance.

If I sheeted the whole wing with balsa top and bottom, and used the HQ 1.5/9, do you think the covering film would stick to the under cambered part? I don't think I could use the AG18 because it won't be thick enough to accommodate the two piece wing mechanism, seeing as the root chord is only 150mm. Speaking of wing joiners, do you think a piece of aluminium 7.5x10x400mm (200mm each side of the wing) will be strong enough?

 

[Craftydan]

If you sheet top and bottom, the plastic will stick to the curve. may not be pleasant getting/keeping the wrinkles out, but it should work. That amount of sheeting is a bit overkill . . . but I've seen crazier. Just to state the obvious-but-easy-to-miss, don't forget to subtract out the width of the sheeting from the rib templates.

If you're still set on a center joiner . . . don't skimp. AL is a fine material but you really should be considering steel for joining two 1.5m wing halves. Having witnessed a Bird-of-Time fold on a 1/4" steel joiner rod (~6mm) like it was taffy has given me a strong respect for the forces in the center wing-box. That rod would be an appropriate size for your wing (the BoT was well over-speed and on it's way down at the time), but bumping up a few mm in diameter wouldn't hurt. It's heavy . . . but you have the weight budget for it.

If you're going with or steel rod in a mid-span break, you'd be surprised how little thickness you need. 1/4" cold steel rod (~6mm) becomes overkill, but it's generally not hard to find it or the brass tube to mate with. Add in an indexing pin near the TE and wrap the joint in electrician's tape for the flight and you're golden.

If you prefer a square AL joiner for a mid-span, 10mm seems . . . Wide to me. if you could get a pair of joiner pieces cut from 3mm or so strips then epoxy a 3-10mm strip of Basswood/hard blasa wood between them, you'll get a nice strong joiner suited to the mid-span.

Center-wing, 200mm/side should be plenty. If you haven't tied your joiner into the wing's web by that far in, you probably won't do it by 1500. for a mid-wing joiners, half that would be a little short, but still acceptable.

 

[Jackson T]

If you sheet top and bottom, the plastic will stick to the curve. may not be pleasant getting/keeping the wrinkles out, but it should work. That amount of sheeting is a bit overkill . . . but I've seen crazier. Just to state the obvious-but-easy-to-miss, don't forget to subtract out the width of the sheeting from the rib templates.

If you're still set on a center joiner . . . don't skimp. AL is a fine material but you really should be considering steel for joining two 1.5m wing halves. Having witnessed a Bird-of-Time fold on a 1/4" steel joiner rod (~6mm) like it was taffy has given me a strong respect for the forces in the center wing-box. That rod would be an appropriate size for your wing (the BoT was well over-speed and on it's way down at the time), but bumping up a few mm in diameter wouldn't hurt. It's heavy . . . but you have the weight budget for it.

If you're going with or steel rod in a mid-span break, you'd be surprised how little thickness you need. 1/4" cold steel rod (~6mm) becomes overkill, but it's generally not hard to find it or the brass tube to mate with. Add in an indexing pin near the TE and wrap the joint in electrician's tape for the flight and you're golden.

If you prefer a square AL joiner for a mid-span, 10mm seems . . . Wide to me. if you could get a pair of joiner pieces cut from 3mm or so strips then epoxy a 3-10mm strip of Basswood/hard blasa wood between them, you'll get a nice strong joiner suited to the mid-span.

Center-wing, 200mm/side should be plenty. If you haven't tied your joiner into the wing's web by that far in, you probably won't do it by 1500. for a mid-wing joiners, half that would be a little short, but still acceptable.

What would you recommend, the two piece wing or three piece? Whether I use steel or AL, two or three piece, the material I have available is 3mm flat plate. I can obviously glue them together to make thicker sections, but that's the material I have. The reason I shyed away from the three piece wing was because I would have to sort out something for the aileron connections.

 

[Craftydan]

Having had both 2 and 3 piece 3m wings, I'd prefer 3 given the option. 1m sections are easier to work with, connections are lighter and the hardest loaded part of the wing is kept strong. Sure, weight moves out toward the wingtips, but a 3m wing will never be nimble in roll.

For 3 segment wings, AL joiners are fine. I'd cut the 3mm plate into two strips and sandwich a piece of hard balsa or basswood between them and epoxy the stack together. This would be mounted in the wing with the AL oriented vertically. You should have an I-beam effect in the direction you need, and will be more than strong enough.

Don't forget to include an indexing pin about 3/4 the way to the TE to absorb the twisting forces on the joint.

As for aileron connections, with servos mounted in the wings, you'd need extensions anyways. Get a pair just a bit longer than center to the break, and leave a small pocket right at the joint on each side for the connectors to live. With open bays between the ribs, the wire will have plenty of place to go as the joint is closed up.

Be sure, regardless of the number of joints to face each side of the joint with a thin sheet of harder wood than the typical rib (might get away with hard balsa, but I'd recommend thin ply or basswood). 1mm or thinner, if you can. Set the angle of the join with the balsa end-rib, and when you're happy with that, glue on the harder facing strip, precut to match the rib+sheeting. Naturally, you'll cover over these, so it'll look clean when you're done.

For joining, I'd recommend wrapping each side with a strip of packing tape (to protect the covering) then after joined wrap the joint with a strip of electricians tape along the joint -- the vinyl tape has just a bit of give and good grab. Lateral forces on the wing aren't that strong, so vinyl tape is plenty.