Originally Posted by canard addict

Don, My Gentle lady styled canard glider should weigh 1.5 pound and fly at 25mph. wing area 432 ,canard area 130, Re 80k to 120k and powered by a two ounce Medusa at 185 watts. The ratio of 8% thickness looked good to me last week and I sketched it out. The Allegro glider that John presented must have pushed me toward 9%. I also like the idea of tapering the bottom front of the airfoil UP a bit although it will require a bottom sheet there to hold the shape along the span. The model is meant for lift and not speed.I hope to have the motor up front and a rudder well behind the wing for balance and good yaw control. Some skinny main winglets might look good help a little.

Before you wrote this post, I was thinking that you were going to build a thermal glider without a motor, not a motor glider. Some difference there.

I aggree with Don that 1.5 lb is pretty optimistic. With lipo cells and the 2oz motor, I think a more reasonable target may be around 2 lb if you build light. I wonder if you may have underestimated the weight of the airframe with enough strength for 2m wingspan and longer fuselage. If you can achieve 1.5lb, then thats great.

For the airfoil, it is possible to get slightly better lift to drag ratios if you want to use thinner sections with some undercamber, but once we bring the increased weight figure of 2lb into the equation I think it makes sense to stay with airfoil sections in the range of 8.7 to 9.2%. If you use thinner airfoils on a 2lb model, I think it is unlikely to be a floater. The Allegro-lite wing was designed for a AUW of only 18oz. In the case of a 2lb powered glider, I would lower the aspect ratio to get the wing area similar to the GL design. I have been looking at airfoils such as S9000(9.0%) and SA7036(9.2%) which offer more lift than the AG35. Maybe these would be worth looking at if you want to keep the higher aspect ratio of the allegro-lite 2m.

I have seen a number of designs where winglets are used to improve the the yaw stability. I don't see any major reason why it wouldn't work on a glider. If you have a reasonable amount of polyhedral on the main wing, it might be sufficient to achieve the same outcome without winglets.

John, You may be right about the weight. It tends to creep up and with two wings it will be a challenge. I want it to be light and floaty. Most programs show my models stalling at 15 mph which steers me toward 25 mph level flight. The thin airfoil will reduce drag, add speed and call for heavier construction. I will draw what looks good for strength,lightness and lift. The wing will probably be reduced to 72" and the smaller Medusa with smaller 1320 3 cell lipo used. Since the winglets add weight and tend to get broken, they can be omitted. I want to apologize to you all for the rambling talk off the top of my head for the last several weeks but I have been heavy into this https://www.rcgroups.com/forums/showthread.php?t=777873 It must be finished before sailplane plans are drawn. It has been sketched out and I know what it will look like. I have a full head of steam to build this Sailplane and can hardly wait to get started. Please keep the discussion going because I truly value your thoughts. Charles

Originally Posted by John235

...For the airfoil, it is possible to get slightly better lift to drag ratios if you want to use thinner sections with some undercamber,

Thinner sections, and undercamber, are not the same thing. Undercamber simply means that the camber at some region of the airfoil is more than half the thickness in that same region, causing the lower surface to become concave in that region. You can have thick sections with undercamber, or thin sections with undercamber. However, a thick section can have a greater amount of camber without becoming "undercambered" than a thin section can.

OTOH, a thicker section will have a greater slope on the aft portions than a thin section. Either the top, the bottom, or both will have a steeper slope, therefore a greater adverse pressure gradient. In other words, from the point on the airfoil with the lowest pressure, the air pressure must increase as the flow continues towards the trailing edge, ending up back at the freestream pressure (or close to it) when it reaches the trailing edge. An air molecule carried along in this stream of air along the surface will see that the area it is flowing into has a greater static pressure than the area it's in right now. That rising pressure resists its flow, and it has to muscle its way downstream against this rising pressure, or "adverse pressure gradient".

If the slope of the airfoil is steep enough, the adverse pressure gradient will be more than the airflow can fight, and the flow will separate. An airfoil with too much camber, or with too much thickness, can cause this to happen, basically anything that makes the slope too steep. This can occur on the top or the bottom, anywhere the adverse pressure gradient is more than the airflow can handle. Too much thickness can result in separated flow on the top, or the bottom, or even both at the same time. The lower the Re, the more likely this is.

Turbulators can help. At low Re's, the flow is laminar, meaning there is no mixing between layers. Therefore, the energy that the bottom of the boundary layer flow has to fight against an adverse pressure gradient is just the energy it had with it at the leading edge, minus what it lost through skin friction as it flowed across the airfoil surface getting to that point. By the time it approaches the trailing edge, it's probably getting quite "winded". By turbulating the flow, we mix faster air from the layers above into that tired boundary layer flow, infusing it with some fresh energy, maybe just enough for it to make it the rest of the way to the trailing edge, without separating. Yes, a laminar boundary layer generally has less drag than a turbulent one, but a turbulent boundary layer is much less draggy than separated flow.

but once we bring the increased weight figure of 2lb into the equation I think it makes sense to stay with airfoil sections in the range of 8.7 to 9.2%. If you use thinner airfoils on a 2lb model, I think it is unlikely to be a floater.

Not that simple. At these Re's, a thicker section (like 8.7-9.2%) could possibly have separated flow due to too much adverse pressure gradient, which will increase drag and decrease lift. Yes, at high Re's (like the ones typical of full-scale general aviation aircraft), generally speaking a thicker section usually has more lift than a thin one. However, at our Re's, the reverse can be true. For example, I've found that at RCHLG Reynolds numbers, sections more than about 7% or so start to see a DECREASE in max lift. This is why historically I've lobbied against some of the misguided attempts to legislate a minimum thickness or camber for indoor models.

In addition, it's not valid to say that a wing with a fast, low-drag, racing airfoil can't be a floater (our Chrysalis 2-meter has float similar to a Gentle lady, but a lot more range and penetration, using a low wing loading plus a thin, racing airfoil), or that a higher lift section can't go fast. Our high aspect ratio Spectre series sailplanes had outstanding penetration despite a very light flying weight. The low span loading and high max lift airfoil gave them excellent float, while the high wing loading (area-related, not span) gave them excellent penetration. Both approaches can be made to work if you do it right. However, it doesn't happen by accident, you have to do your homework.

...I have seen a number of designs where winglets are used to improve the the yaw stability. I don't see any major reason why it wouldn't work on a glider...

For it to help yaw stability, the winglets have to be behind the C/G, so that forces acting on them can make the airplane yaw. If he's using a long tail boom for the vertical fin, that will be farther behind the C/G than the winglets, and therefore generate more yaw stability per square inch than the winglets. The exception to this is that in a turn, the outside winglet will be flying faster than the inside winglet, trying to yaw (and due to dihedral, roll) the airplane away from the turn and back to level flight. However, a vertical tail on a long tail moment arm gets help from curvature of the airflow due to the turn (with a long enough tail moment arm in a tight enough turn, the relative wind at the tail could be over 15 degrees different from the angle at the wing), which helps improve its effectiveness.

So, yes, you can get yaw stability from winglets. You also get it from a well-aft conventional fin. The question is whether a given number of square inches invested in a winglet is better or worse than those same square inches invested in a conventional fin. If the wing is operating at limited lift coefficients, so the induced drag benefits of the winglets are limited, and the winglets are near the C/G (while the conventioinal fin is well aft of it), the fin might be the better investment. If the winglets are among the farthest aft items on the airplane (such as with the Rutan Eze series), then they may make sense regardless of whether or not they provide any induced drag benefits.

Regarding induced drag benefits, the other thing you have to consider is the plane's mission profile. Even a "floater" sailplane can't spend all its time "floating". The days of the "gasbag" sailplane are long gone. Today's sailplanes have to be able to float in very light lift, but they first have to launch to a high enough altitude to get a reasonable ability to search upwind for that lift (no point searching downwind - because thermals drift with the wind, searching downwind means you're looking for thermals in the same air you just searched), climb in that lift when you find it, but then (after that thermal has drifted you way downwind while circling in it) you have to be able to penetrate back upwind to get home. The size of the usable speed range of a good sailplane is one of the widest in the entire field of model aviation.

Winglets (if well designed and built) will help you at min sink (thermalling) and at best L/D speed (searching for lift in still air). However, you always have to launch, and you will rarely fly in zero wind, so the high speed end of your performance envelope is still very important. Winglets tend to be a detriment there, unless they serve some other function (such as yaw stability), and serve it better than the alternatives. Each design situation will be different, and it's important to objectively evaluate each case on its own merits.

Originally Posted by canard addict

... I want it to be light and floaty. Most programs show my models stalling at 15 mph which steers me toward 25 mph level flight. The thin airfoil will reduce drag, add speed

Be careful here. Reducing drag does increase your top speed (penetrating ability), but it does not directly control your low speed performance ("float"). The way to reduce your minimum efficient flying speed (float) is by increasing lift. Lift is what controls how slow you can fly, not drag

and call for heavier construction.

In absolute, qualitative terms, yes. In quantitative terms, the actual amount of increase might not be very significant. Also, if you go with an airfoil that's thick enough to suffer premature flow separation (which at the Re's where we work much of the time is not difficult to do), then you will need extra wing area to make up for the lost lift from that, and more wing area means more wing weight and more skin friction, both of which hurt performance.

I will draw what looks good for strength,lightness and lift. The wing will probably be reduced to 72" and the smaller Medusa with smaller 1320 3 cell lipo used.

Which means you're carrying the same radio gear on a smaller span, so the savings in weight might not be enough to offset the induced drag penalty of the smaller span. Induced drag is linear with weight, but inversely proportional to te square of the span. OTOH, the effects on bending moments in the wing structure are pretty profound as well.

The important thing is to look at the net result of all the parameters together, and not get tunnel-visioned on any one parameter alone.

Originally Posted by Don Stackhouse

Thinner sections, and undercamber, are not the same thing. Undercamber simply means that the camber at some region of the airfoil is more than half the thickness in that same region, causing the lower surface to become concave in that region. You can have thick sections with undercamber, or thin sections with undercamber. However, a thick section can have a greater amount of camber without becoming "undercambered" than a thin section can.

The point I wanted to make is the airfoils that perform best at low reynolds numbers tend to be thinner, and the ones with higher lift co-efficients are often undercambered. Examples such as S4083, AG25 to AG27.

Originally Posted by Don Stackhouse

Not that simple. At these Re's, a thicker section (like 8.7-9.2%) could possibly have separated flow due to too much adverse pressure gradient, which will increase drag and decrease lift. Yes, at high Re's (like the ones typical of full-scale general aviation aircraft), generally speaking a thicker section usually has more lift than a thin one. However, at our Re's, the reverse can be true. For example, I've found that at RCHLG Reynolds numbers, sections more than about 7% or so start to see a DECREASE in max lift. This is why historically I've lobbied against some of the misguided attempts to legislate a minimum thickness or camber for indoor models.

When I have modified some airfoils such as AG25 -AG27, I have found that in Profili simulation with Re around 70k, incresing the thickness to around 9.0% increased the maximum lift co-eficient (see the attached polars). Maybe with more highly cambered sections the opposite may occur due to seperation as you say. I understand there are a lot of variables involved at the various parts of the airfoil, so I am not saying it won't happen, but it hasn't generally been my experience so far. I am interested to know of airfoils where this phenomonen occurs.

Originally Posted by Don Stackhouse

In addition, it's not valid to say that a wing with a fast, low-drag, racing airfoil can't be a floater (our Chrysalis 2-meter has float similar to a Gentle lady, but a lot more range and penetration, using a low wing loading plus a thin, racing airfoil), or that a higher lift section can't go fast. Our high aspect ratio Spectre series sailplanes had outstanding penetration despite a very light flying weight. The low span loading and high max lift airfoil gave them excellent float, while the high wing loading (area-related, not span) gave them excellent penetration. Both approaches can be made to work if you do it right. However, it doesn't happen by accident, you have to do your homework.

I agree that both approaches are valid. What do you think is likely to be the optimum aspect ratio in the case of the 2lb, 2m glider, assuming we want to build a floater?
Do you think the AG35 airfoil is a reasonable choice for either approach?

Originally Posted by Don Stackhouse

Winglets (if well designed and built) will help you at min sink (thermalling) and at best L/D speed (searching for lift in still air). However, you always have to launch, and you will rarely fly in zero wind, so the high speed end of your performance envelope is still very important. Winglets tend to be a detriment there, unless they serve some other function (such as yaw stability), and serve it better than the alternatives. Each design situation will be different, and it's important to objectively evaluate each case on its own merits.

Don, it is interesting that the broad winged hawks (incl. eagles) which splay (spread out) their large tip-feathers to help them soar at minimum sink in thermal lift and probably at best L/D, which I seem to remember is probably just a bit faster, to work on the main tip vortices (in their case by breaking it up into smaller ones, as they have by definition a broad wing with a broad vortex). But at high speed, especially with their wings slightly retracted and swept back, the tip feathers are held such that the wingtip is more like that of a gull, swift, and namely high-performance modern R/C sailplanes with raked tips.

Of course while they have this wonderful shape-changing abilty to help their flight, they are also forced to use it to spend a lot of time trying to catch dinner, which is successful only 10% of the time on average. So they are not to be totally envied.

Originally Posted by John235

The point I wanted to make is the airfoils that perform best at low reynolds numbers tend to be thinner, and the ones with higher lift co-efficients are often undercambered. Examples such as S4083, AG25 to AG27.

Adding camber has the effect of taking the entire plot of Cl vs alpha (lift coefficient vs. angle of attack) and shifting it upwards, as long as you don't trigger separation in the process. It also increases the moment coefficients.

Also, note that absolute max lift (just below stall) is really not a usable parameter for normal flight, other than for looking at the case of whether or not the canard stalls before the wing. For performance and for general behavior in normal flight, including thermalling, you actually have to look at what's going on below stall, and design for the amounts of lift you can get before the drag starts to increase near stall due to the beginnings of separation. That generally starts at about the same point where the lift plot starts to round off at the top. In fact, it's that increasing separation and the lift loss it represents that causes that rounding off.

Besides the separation issues, the other problem with adding camber is the onset of separation at low lift on the bottom. This limits your high speed performance, such as during launch or penetration. Adding camber is usually a good thing up until you can't get back to around zero lift without triggering lower surface separation.

A number of years ago, Selig came out with an airfoil he thought would be an optimum for RCHLG's. It had a lot of undercamber, and a lot of aft loading (camber in the rear portions of the airfoil). It had an amazing max L/D, but massive lower surface separation turned the airfoil into a drag chute at any lift coefficients less than about 0.7 or so. It thermalled great, was very competitive in the hands of someone like Joe Wurts who could "read" the lift, run to that part of the field and throw the model right into the thermal every time. However, searching ability was limited, and launch was poor.

When I have modified some airfoils such as AG25 -AG27, I have found that in Profili simulation with Re around 70k, increasing the thickness to around 9.0% increased the maximum lift co-eficient (see the attached polars). Maybe with more highly cambered sections the opposite may occur due to seperation as you say. I understand there are a lot of variables involved at the various parts of the airfoil, so I am not saying it won't happen, but it hasn't generally been my experience so far. I am interested to know of airfoils where this phenomonen occurs.

How about the ones you just used as examples? The L/D's of those two airfoils dropped by 9% to 11% in your attached plots. That's a pretty significant loss, and probably due to the beginnings of separation. That higher max lift coefficient is lift you can't use (other than maybe for steeper landing approaches in smooth, still air, IF you aren't worried about flying final approach right on the edge of stall), because the drag rise associated with it will kill your performance.

... What do you think is likely to be the optimum aspect ratio in the case of the 2lb, 2m glider, assuming we want to build a floater?

Depends on what you want to accomplish. In general I can squeeze a wider usable speed range and more peak performance out of a high lift airfoil and very high aspect ratio, such as we used on our Spectre series. However, for that to work, the airfoils still have to be able to get to slightly negative lift coefficients without lower surface separation and the associated drag rise, in order to get decent high-speed performance.

But, there are other things to consider.

I can do best in terms of absolute performance and width of the usable speed range with the high lift airfoil + high aspect ratio approach. Some of the Spectre series airplanes had aspect ratios in the range of 16 to 20. They also had thin, but very high lift airfoils (but that could still get to zero lift with low drag).

OTOH, if done just right, I can come close with the low aspect ratio + racing airfoil approach, such as on the Chrysalis series. The 2-meter Chrysalis has an aspect ratio a little less than 9:1, and a root chord of more than 10" (more than a number of open class airplanes), but very thin, low camber airfoils. One of the most important parameters for visibility is root chord, and a lot of our customers for the Chrysalis want an airplane that's easy to see. As far as performance, it has a lower aspect ratio and much thinner airfoils than a Gentle Lady, about the same weight, a wing spar that's about 5 times stronger and stiffer, and a wider usable speed range. One of our customers described it as having "the float of a Gentle Lady, but twice the range."

Do you think the AG35 airfoil is a reasonable choice for either approach?

Can't answer that without a lot of analysis. The last thing I define when I'm designing a wing is the airfoils. I'll get a rough idea of what's reasonable to expect, but then I go through the entire planform analysis and aerodynamic (not geometric) twist optimization, one of the results of which is a map of the required lift coefficients along the span at the various flight conditions, including the effects of turns. Using that, I can then design the various airfoils (and its almost a certainty that the best airfoil for the root will not be the best for mid-span or the tip), and use their individual characteristics plus the required aerodynamic twist to define the geometric twist.

I design my own airfoils, because I've found I can do better that way than using public domain airfoils. Yes, when I'm designing a new wing I do compare the best available sections including the public domain sections, and if one turned out to be better for what I was trying to accomplish, I would use it. Hasn't happened yet, but if it did, I would. All that said, yes, the AG35 has a good track record.

What really matters is not so much the section itself, but how well it works in combination with the other parameters. If you design an airplane with a wing that at min sink, cruise, etc. has the individual regions along the span operating at lift coefficients that correspond to where the AG35 does well, then the resulting wing should do well.

That AG35 looks like my kind of airfoil and it is the smooth middle of the road performer as I read it. That is a super bit of info, John and at Re 70k. I must fight the tunnel vision per Don and go for what my abilities allow. Charles

Don, thanks for taking the time to answer my questions. Your answers are thorough and will be a help in my approach to designing model gliders.

I am still inclined to look towards sections with around 8.7 or 9.0% thickness for the main wing of a 2m canard glider because they seem to allow good max lift co-efficient, and what appears to be acceptable drag figures across the range. The point about it being preferable not to use the maximum lift co-efficient of the aifoil due to efficiency penatly is very relevant to this application. Maybe I consider this is the preliminary airfoil study as part of the design process you talked about.

It is interesting to hear your preference for higher aspect ratios to achieve the best peak performance. Maybe there is a need to compromise between the high and low aspect ratios. I wonder if the high aspect ratio path is going to be more demanding in terms of pilot skill. Medium to low aspect ratios might make a better floater, and don't require high lift airfoils that can be difficult to build for this range of Re.

I can understand there are many factors involved, so its very hard to get absolute answers. Thanks again for your comments so far.

Don, Now I realised exactly what you mean by 9 to 11% penatly in lift/drag ratio. If I look at airfoils that are designed for 9.0% thicnkess eg S9000, it retains the high max lift co-efficients of the 9.0% AG25-AG27 sections but without the compromising the lift/drag ratio at Re = 70k.

Sorry if this is getting too far off-topic for this thread.

Originally Posted by John235

...It is interesting to hear your preference for higher aspect ratios to achieve the best peak performance. Maybe there is a need to compromise between the high and low aspect ratios. I wonder if the high aspect ratio path is going to be more demanding in terms of pilot skill. Medium to low aspect ratios might make a better floater, and don't require high lift airfoils that can be difficult to build for this range of Re.

In my experience none of that has been the case.

Remember, "high lift airfoils" means with the additional constraint that they be able to get back to whatever the minimum lift coefficient is within your mission profile, typically about zero for most airplanes, including sailplanes. If you don't abide by this, then you've just created a one-speed airplane. Even in a case where you THINK you have a one-speed mission profile (such as planes for some of the SAE weight-lifting competitions I've consulted on or judged), under closer scrutiny you'll usually find things are more varied than you realized.

Given that constraint, and assuming (as with the vast majority of sailplanes) that lower Cl requirement is around zero, then one of your big battles will be keeping the flow attached on the underside at high speed. That rules out most undercambered and even most truly flat-bottomed airfoils. What you normally end up with is a slightly semi-symmetrical section, usually flat enough from about the max thickness point aft that you can treat it as a flat bottom for most building purposes.

Also, don't forget, the wing of a canard aircraft never gets to use that max lift coefficient. The airfoil needs to have enough margin there to make things easier for the canard design, but in terms of actual performance, what the wing airfoils do in their mid-range is more important.

As far as the effect of high aspect ratio on handling, usually it helps. One of the biggest factors in getting good dutch roll damping and good, crisp roll and yaw control response is the mass in the wingtips. High aspect ratios naturally reduce the amount of mass in the tips. Designing an efficient low aspect ratio structure, with minimal mass in the tips (like what I had to do with the Chrysalis series), is actually more difficult. Thin airfoils on at least the outboard portions of the wings can be a big help, but that also means thin airfoils inboard, but that's still good at our Re's in terms of USABLE max lift coefficient. There are some other tricks in low-Re airfoil design and some other "sacred cows" in the common knowledge about this (one of my favorite engineering dishes is barbequed sacred cow), but I don't want to give away all of my secrets, I'm still trying to make my living at this.

In terms of pilot skill, again not a problem if the plane is designed well. Higher aspect ratio means a smaller tail, and less mass in the extremities about all three control axes. The amount of ballast needed for extra wing loading on windy days is less, and therefore less increase in induced drag. You do have to be careful about aeroelasticity issues, and design of the wing spar, especially in the inboard portions, can be more of a challenge. The other thing that definitely is an issue is visibility, a narrow wing is harder to see (which is one of the big reasons for taking the low aspect ratio/racing airfoils approach for the Chrysalis series).

That's a case where good planform analysis capabilities as well as being able to at least fine-tune your own airfoils can be an advantage. You can use a planform that normally would not be considered optimum from a lift distribution standpoint, compensate for that in the airfoil designs (so you won't need to use lots of washout to correct things, which would have a detrimental effect on your usable speed range), and end up with a wing that's easier to see than the aspect ratio would suggest, but that still has an efficient lift distribution at all speeds, and a sufficient margin at the tips for avoiding tip stall. That's what I did on the wing for our Spectre 120 open-class ship, with its almost 20:1 aspect ratio. I'm using that technique again on another sailplane design I'm currently working on.

here's a bunch to analyze! I find that one slip of the sandpaper and all perfection is lost!

Don, thanks for sharing your views on high aspect ratios. In reference to your comments, does it assume the wing loading is going to be the same or lower when the aspect ratio is increased?

There is basically the physics involved that if you reduce the wing area without changing to airfoils with higher lift co-efficient, then the weight must be reduced to fly at the same speed. There is also the issue of the Reynolds number being lower, so I figured that may be one reason why model sailplanes with higher aspect ratios need to be flown a bit faster.

I was looking for some information on the Spectre series sailplanes, especially the recommended flying weights, but I wasn't able to find these models on your webpage.

Thanks for the airfoils, Captain. The S3021 and S2027 look great to me for model building. The S2027 looks like the one in one of my canard designs. My Graupner 2M sailplane was flown twice today. It must weigh near 3 pounds with the plastic fuselage and probably balsa covered foam wings. It seems faster in a dive with the motor off and the prop folded back. It thermaled with a Red Tailed Hawk on the first flight and landed well into the wind. A strange thing occured on landing in calm air at sunset. It came in hot with neutral elevator trim and when it touched down, the nose popped up and it did a 30 degree right banking U turn with the right tip skimming the ground. The nose glanced off the ground and the wing was twisted some on the mount. One fellow said "ground effect". That baby needs a new owner and I need to try a canard sailplane. Can anyone comment on what happened on the landing? Charles

Originally Posted by John235

... In reference to your comments, does it assume the wing loading is going to be the same or lower when the aspect ratio is increased?

In general, the wing loading should be the same or higher. The possible aspect ratio is the combination of a number of factors, including how much lift you can get from the airfoils.

There is basically the physics involved that if you reduce the wing area without changing to airfoils with higher lift co-efficient, then the weight must be reduced to fly at the same speed.

Exactly. At which point, you're keeping wing loading constant, but reducing the span loading. Penetration is mostly a function of wing loading, so penetration is still about the same at the reduced weight. Also, it takes less ballast to make a given increase in wing loading for more penetration. However, induced drag is a function of span loading (not wing loading), so induced drag goes down.

There is also the issue of the Reynolds number being lower,

It's a tradeoff. Reducing Re hurts drag, but reducing induced drag helps low speed performance, and reducing whetted area improves high speed performance. What you have to find is the point at which the gains from those last two no longer exceed the losses from the lower Re.

so I figured that may be one reason why model sailplanes with higher aspect ratios need to be flown a bit faster.

Not necessarily true. It depends on how good the designer is with lower Re airfoil design.

I was looking for some information on the Spectre series sailplanes, especially the recommended flying weights, but I wasn't able to find these models on your webpage.

We had to stop production of all of our composite construction airplanes in September 2001 as a result of market changes directly related to the terrorist attacks (it's a long, sad story, which I'll spare you the details of unless you really, really want to hear them). That included all of the Spectres, which included the 2 and 4 channel HLG's, the 2-meter, the Spectre 100 and the Spectre 120.

The 1.5 meter HLG versions had wing areas of 289 (the original 2-channel Spectre, typical flying weights between 5.5 and 6 ounces) and 220 sq. inches (Spectre VR, originally a 5.5-6 oz 4 channel and later a 5 oz 2-channel). The Spectre 120 had an aspect ratio just shy of 20:1, and a flying weight of 45 ounces. None of them had airfoils anywhere near 8% thick.