Scratch Build Glider Light Weight
 

Hi All,

Im new to this Fourm and wondered if someone can help?

I am starting my first glider build and basing the plans on the Centurion from Areofred.

This is a 1972 plan which I plan to modernize and install Ailerons and possibly flaps if beneficial.

To Modernize this, im going to design Ailerons, Flaps, convert tail to V tail, redesign the body and hopefully improve the shape of the wing.

So the wing I was going to use 2 carbon fiber arrows for the spar and maybe get some small carbon fiber tubes for the leading edge instead of balsa.

At 16grams per arrow, I think this would reduce the weight and stiffen up the wing.

Would anyone else recommend any improvements for the wings to get higher efficiency?

What about curved up wing tips at the end and maybe extending the overall length?

Any input would be great.

Thanks

Welcome to RCU for starters.

With all the changes you're making there won't be much of the original design left. But that's OK. It's your project after all.

Along with all the rest of that stuff I hope you're going to update the airfoil as well. To keep it simpler to build and cover and still efficient may I suggest the AG34 to 36 series as used on the Allegro Lite. And use it with all the sheeting that you see on that plan because the performance is based on keeping it smooth over that extent covered by the sheeting.

If you're going with ailerons you won't be able to curve up the tips as that'll foul up the aileron hinging. Unless you mean just the blocks at the ends of the wingtips? As in reverse Hoerner style tips?

Flaps are handy for bringing the model down on demand of you're flying in competitions. But for casual flying you can just go with ailerons and run each with a separate servo and reflex them upwards fairly strongly to kill off some lift and drop the model down a little steeper. Simpler to make at the cost of programming in a reverse flaperon mix.

Will this be electric powered or a pure glider? And again for competition use or just for casual soaring? If competition then your carbon tubes for spars when subjected to a strong winch launch won't be all that great. If that is the style of flying you'll be doing I'll refer you again to the Allegro Lite plans and in particular the strong and stiffly overbulit spar which you'd want for competition.

If you're using electric power or hi starts then the carbon tube can save some time over building up the I beam style spar. But it's still not the best option if you are using sheeted surfaces. What happens is that the upper and lower sheeting being spaced further apart than the carbon tube ends up resisting the load more than the carbon. And because it sees more of the load if it's not supported by a good and proper spar setup it can end up buckling and cracking the sheeting even though the carbon tube spar is still well within its flexibility range. So if using sheeting you're much better off with a proper I beam spar made up of separate upper and lower flat spar caps that are then joined with webbing to form a true I beam.... And suddenly we're back to the Allegro Lite spar setup again.


Hope that helps with some thoughts.

Thankyou BMatthews for your reply.

I really appreciate to detailed reply. Very helpful.

We"ll I was planing to have pure glider as we have large mountain areas near us and also to keep weight down. this will be for casual use and to hone my skills in flying.
Thankyou for your input regarding the wings using carbon. I think I will stick to a I beam setup in that case. In terms of the air foil, I was planing to upgrade this and will probably use one you suggested. My end goal I suppose is to have a light glider which resembles the modern full size gliders but also has good glide characteristics and will give me the ability to gain more skill in aerobatic flying as my skills become better.

In terms of the Wing tips (Most distant part of the Fuselage) does curved pointed tips make a huge difference?

This may be a stupid question, but why do RC glider wings bevel and are not just straight like modern gliders?

Would fiberglass be better for sheeting than balsa if you could mound the shape and glue after?

thanks BMatthews,

Welll... a lot of full size gliders do use some wing tip treatment other than just squared off and given a slight bump of a tip fairing. There's some with winglets and some with Schumann style tips.

The often seen crescent shapes in RC sailplanes is actually a low Reynolds number version of the Schumann tips used on the full size. The idea being to discourage inward flowing air on the upper surface when operating at lower speeds with higher lift coefficients. But as you might see from the old school designs there's a lot of good flying to be had with other options. ESPECIALLY when you're not overly concerned over competition use.

I read you loud and clear on the idea of having a fuselage that appeals. Just say NO! to broom handles with wings is my motto :)

If you're looking at ailerons there's all sorts of options including doing a gull wing. Imagine for just one example if you were to build a scale or semi scale model with a gull wing. Normally the gull dihedral is a problem if using just tail controls. But with ailerons it becomes a viable option for a scale, semi scale or could be scale sort of model. So there's something else to consider.

The often egg shaped fuselages on such models could be done in a little more simplified manner by using octagonal shaped sections. You'd build up short sides and top and bottom caps then plane and sand bevels to add on the four angled parts and finally sand most of an egg shape out of the resulting sheet parts.

Then again with all the changes you're considering how about just go with your own design right off the bat? To avoid mistakes with wing to tail area ratios and the like copy a known good performing design for the outlines and fuselage length But start out with a model that has the wing and tail shapes you want and give it a new fuselage to suit your tastes.

Or perhaps there's something out there like the very nice looking old Graupner Cirrus that you like. The Cirrus in this case being from the old rudder and elevator days. So just drop the wing dihedral down to roughly scale for the real Cirrus and add your ailerons. And the original came with a plastic fuselage. But there's a plan out there for a wood fuselage. Or just "square up" the side view and give it some nice lines with thicker sheet balsa on top that you carve to a pleasing shape and leave the belly with mostly flat lines with just a thick block in the bottom of the nose. You can get some pretty nice shapes that way.

Here's a couple of doodles I've done over the years. The structure would be done using thicker sheet then get glassed for skin strength. Surface areas and tail moments are based on proven designs of the time these sketches were done. Plagiarism CAN be a sort of compliment after all... :)
https://cimg9.ibsrv.net/gimg/www.rcu...2e99f22d83.jpg
https://cimg3.ibsrv.net/gimg/www.rcu...5674118f1d.jpg
https://cimg4.ibsrv.net/gimg/www.rcu...eae9efc82e.jpg

You have really helped me. Thanks. The Graupner Cirrus is a great looking glider and is along the design I am looking to build. Nice and slim body and seems good in the air from the videos on YouTube. Can only imagination how ailerons would affect the handling.

Very interesting how Reynolds number changes when you scale.

So I think I will use the plans for the Graupner Cirrus to make the changes to.

If I use the AG34, install flaps and Ailerons and Convert to V-Tail, I think this would be a great glider.

Thankyou for all the time you put in your replys. You really have great input.

What i'll do is bang the Graupner Cirrus plans in Autocad and start modeling it. Hopefully the laptop doesn't freeze when modeling.

In terms of the V-Tail, does air foiling them make much difference to the efficiency or is it more time than its worth in your opinion?

I will post the plans once I finish it, will probably take me awhile to draw up in 3d.

Very nice designs you have there. What program you using?

Thanks

I learned my CAD on an early copy of TurboCAD. And much like playing a music instrument I've stuck with it rather than try to learn new keystrokes with any other option. It's been maybe 20 years of using that program at this point. I'm so comfy with it for 2D drawing that I can work faster in TCAD than I can with a pencil, paper and ruler at this point. No 3D for me but I've started playing with the free version of Fusion. But haven't developed the dedication to learn more than the basics so far.

There's much to be gained by using a little more shaping than simply rounding the leading edge. And there's a lot of drag to be saved by tapering the elevators to a sharp and fine edge. In between some flatness isn't much of an issue. But for that last couple of %? Then you might want to consider the HT05 from Mark Drela again. If you look at the link you'll see that it has an odd bump about 3/4 of the way back. The bump is where you're supposed to put the hinge line. And the idea of the bump is that when the elevator or ruddervator surface moves through a slight angle that there's a curve for the air to follow. Sort of like this sketch for an easy to shape from flat stock "cheater" HT05. If you make the tail surfaces from solid sheet then I'd try to achieve something closer to Dr Drela's HT05. But if you make them built up with strips for leading and trailing edges and hinge line spars with ribs then I'd say it might be easier and stronger to go with something like this option since it would be pretty easy to shape the wood while still maintaining a reasonable thickness for strength.

https://cimg6.ibsrv.net/gimg/www.rcu...a2a2e36d07.jpg

Fusion is a great program offering many good tools but I think I will stick with what I know. I've been using Autocad 360 for about 7 years now mainly working with 3D but my third student licence is going to expire soon may look into an alternative as AutoCad is way too expensive.

Thanks for all your help.

Now the fun part is to draw it up and see how she looks. Thanks for everything. I will keep you updated.

:)

Nearly finished the design and just wanted to check a few things.

So the wing is going to be 2.160M including tips. Body is 0.845M excluding slanted back V tail. I have used the AG34 air foils and designed a I beam spar design.

The wing is going to have a 3 deg slant from the horizontal and a following 2 degrees around 1/3rd of the wing.

Ailerons are going to be the 1/3rd of the wing and im not sure about the flaps.

In terms of flaps, it will be very hard to design in but if I installed spoilers it would be very easy to do.

is there any pro's and cons of each?

In terms of the I beam, is 8mmx2mm Sitka spruce OK? I can machine any size as I already have a 2"x2" 16ft length.

I've decided im going to make the fuselage from fiberglass as I already have some.

In terms of the air foil, does 5 degrees seem OK for angle of attack?

Unfortunately I cannot post any pictures/PDF's until I have ten posts unless there is another way?

Thanks,

Plans so Far

PM'd about the picture....

Here's my own thinking when it comes to designing such a model....

If I'm using ailerons anyway then I need a sub trailing edge for the section of the panel where the ailerons go so I can close the wing structure and also have a place to support the hinges. And if I'm doing a sub trailing edge anyway then I might as well make the sub TE extend for the whole span and use flaps.

Flaps offer up a wider range of use than simply adding drag and killing lift. And especially if you have ailerons and flaps you have the option of full span camber changing which provides the ability to slightly lower the flaps for aiding with slightly reducing the sink rate when in lighter lift. Or for slightly reflexing the whole span of the wing to reduce camber and allow the model to fly faster at the same rate for aiding with getting back upwind on stronger wind days. All in all flaps offer a lot more than simply giving you a way to bring the model down quicker than it would otherwise.

On the other hand spoilers are only good to bring the model down quicker than it would otherwise........

I do feel that some dihedral in the wing even with ailerons aids with making the model more self correcting. Plus it can aid with reducing the need for "top aileron" to avoid the model wanting to steepen up the bank angle in a turn if not corrected. But without seeing the drawing I'm thinking that 3° at the center line dihedral joint and 2°more at the outer panel breaks might be a touch too much? Or not.... It won't be far off at any rate. Mind you I'd be more likely to do it with 2° at the center line break and 3° at the outer breaks. Depending on where you put the outer breaks it can aid with making the whole wing as seen from the front follow more of an elliptical arc for dihedral. And that's not only a more pleasing shape but also puts the dihedral out at the tips where it does the most good. That's why the 5 panel wings are so popular. It puts the higher angles out at the tips where it provides the most roll response to rudder inputs on RES models.

For spruce spars you'll want to go with more like 8x3 or even 8x3.5mm. If the wood has much grain runout this avoids any weak spots that would otherwise occur in the very thin 2mm size you asked about. Plus if you will go the "easy way" and add the shear webbing as front or back or box in the spar with front AND back glued on webbing the 3mm provides more gluing area to the front and rear edges of the spar caps. And that's an important factor. The glue is more than strong enough for our use. It's the lack of joint area on the 2mm wood though. There's so little joint area that it can easily rip away the wood fibers. Going up to 3mm thickness aids this by providing a 50% increase in the glue joint area. The outer panels is a different story. For those if it's not too much trouble you could get away easily with 8x2mm since the loads are a lot less.

This also assumes an electric launched glider. If you will be flying off a winch then for sure increase the spar caps to more like 10x3 or 10x3.5mm. You'll need that for dealing with the loading off a winch or stronger high start.

Small point but you don't really mean angle of attack. Rather you're describing the angle of incidence. The AoA is something we control in the air and which changes with changes in flying speed and due to higher loading in turns. The angle of incidence is what we secure the wing at so the fuselage is pointed more or less into the airflow to give a lower drag value. OK, terminology done..... :)

We want the fuselage to be at a low frontal area aspect when we are flying at a blend of the best L/D and slightly faster speeds. And at those speeds we are generally going to be running a glider at rather low lift coefficients and thus lower angles of attack. Like down around 0 to +2°. 0° you ask? Yep, on airfoils with camber the zero lift angle is actually a negative angle. The AG35 airfoil for example with 2.3% camber only provides zero lift at around -3°. At an AoA of 0° it is operating at a Cl value of up around 0.4. And that's roughly the value you'd be flying with during your best L/D flying. And at higher cruise speeds for penetrating back upwind you might well end up at more like -1° or so. We only fly at more like 0.8 to 1.0 Cl values and angles of attack at the point just before stalling. So even when flying in a thermal we're likely only up around a Cl of 0.6 to 0.7. And at that point the actual angle of attack with the airflow is only up around +3°. So as odd as it seems the right choice for setting the wing's angle to the rough centerline of the fuselage is going to be down around 0°. If you proceed and set it at +5° then it will end up flying around looking like it's diving and holding the tail surfaces well above the obvious glide path line. And that means you're making the fuselage frontal area exposure bigger than it needs to be and making more drag than you want.

So there's much to be said

Hi BMathews, I have sent via We transfer as i cannot send pictures yet.

From the plans I sent,

So in regards to the sub TE extend for the whole span, do you mean have a the flaps like how the Ailerons are designed as apposed to a thin flap which is under the wing. This would make it much more easier to make and less compilated.

Thanks for your input regarding the flaps. seems like the flaps would allow much more flying adjustment and as it wouldn't be too hard to add, would allow a slightly simpler wing stricture.

We my biggest concern is regarding landing, is actually landing and not just gliding over the ground which as I have not flown a glider, im probably worrying about nothing. Round me, most the land is hilly and flat areas are rare. lol

From the plans, I can change the angle easily enough. In terms of the wing tips, I was going to have the top and bottom of the I beam Spar extend slightly and steam a curved section of spruce where it will glue into the extended I beam spars.
In terms of the plans, do they look OK other than the 3/2 degree angles being swapped?

The great thing with a table saw, any size is possible so this would be easy to change to 8mmx3/3.5. In tegards to the boxing, do you mean add balsa between the ribs and on both sides of the I beam to allow more gluing area. Of do you mean remove the centre section of the I bean and turn it into a box with spruce on the top/Bottom and balsa on the sides?

In terms of a winch, I think it will be wasted on me as I have loads of hills around me and am around 1150ft above sea level.
Very detailed explanation and somehow I understood it. Thanks. So the angle of incline should be around 0 degrees. I knew at 0 degrees, the air foil produces lift but I never knew at 0 degrees it would produce enough lift.

Thanks for your explanation. Very good feed back. Hopefully the plans only need slight correction.

Thanks for everything.

Really helpful information.

Thanks.



.

So this will be a slope soaring model primarily? In that case you don't really need the outer dihedral break at all. But the original 3° at the center will make the model act a touch more groovy and self leveling by enough for use with ailerons on a mostly slope soaring model. And not having the outer dihedral angle makes building the wing even easier.

Landing with the flaps down CAN be an issue. For this reason a lot of folks prefer to buy servos with metal gears since they are a bit more durable. The other option is to retract the flaps just a slight moment before making contact with the ground.... Or hillside bushes in your case.... And in fact with the loss of the lift from doing so it ensures that the model drops down smartly right at that point.

The first option. It's also the easy option since you maintain the wing ribs as one piece. It's also easier to build. And we do like easy as long as it's still effective. And for a model which sounds like it'll be primarily just tossed off the tops of a slope using the separate pieces between the ribs is easily enough to make the wing very strong.

There's no need to fit the pieces so closely that you can glue the webbing to the ribs as well as the spar caps. It won't hurt if you do though. Since the real goal is to box in the spar caps and I like "easy" I tend to make the webbing pieces in these cases about 1/32" less than the rib spacing so it's easier to fit them. And that means I don't bother gluing the webbing to the ribs.

Well Considering I have mountains all around me, Means a slope soarer will make the most sense. Not having to drive to fly makes more sense to me. This is why I was looking to build a glider as there is always wind.

Well luckley I have a load of unused towerpro MG92B which would come in really handy for this. I think they will be more than capable for all of the controls. Plus I don't need to buy any for the time being. Well if I can bring the glider down in doing so, it would be great and from your input, seams flaps are the way to go. Also a bonus would be the ability to adjust the flaps to suit the flying.

In terms of the wings, I was going to have 4mm holes so I could build the ribs on 2 4mm bars to ensure that the wings were straight. If changing to a box section would make things easier to build, i will give it a shot. I think if the plans are OK with minor adjustments, I will go ahead and see how things go. In terms of the dihedral, does this generally make the glider better? Would there be any pros to having it?

I hope you managed to get the plans I sent you.

I have changed the angle of the angle of incline back to 0 degrees.

The fuseulage shouldn't be too hard as I've some experience with fiberglass for boats.

Thanks for all your help in designing this. Will be a great project to build.

If you have a lot of slopes that you can access to fly from then this shifts the goals a fair amount. Up to now my posts and suggestions have centered on the idea that you were working on primarily a thermal model that would occasionally be used on a promising looking hillside. But if you have more hills than flat areas and have access to the slopes at their tops or well up the side of the hills that see the oncoming breezes that alters things a lot. At this point I'd suggest a shift to a slightly more compact model with a more moderate aspect ratio that is geared to slope flying. Such a model would be less likely to be damaged during the often rough arrivals that describes "landings" with slope soaring models in brisk conditions.

I got the drawings and as promised have posted them below. I have a couple of thoughts. Overall the design looks fine but for two main things. First is that the V tail surfaces are far larger than they need to be. in particular the top view. They could easily be made about 2/3 their size and it would still be enough to provide a good degree of stability.

The other issue is the low number of wing ribs. The spacing in the third picture below looks great. Might even be more than is really needed. But the other wing plans have what I feel personally is too few ribs to best support the covering and limit the distortion of the wing section due to the shrink "tenting" that occurs between the ribs. For wings where I'm after the best performance possible with a film covered open frame structure I like to go with no more than 2" spacing. Your setup looks more like it is pushing towards 3" spacing.

If I'm right and there's more slope than flat land and more slope updrafts than there will be thermals then I'm going to suggest something less long legged than your current plan. Something around 60 inch span and light but sturdy. Something like these suggestion links....

I got these suggestions from a search on Outerzone for RC gliders in the 50 to 70 inch span range. Some of these would be at their best in brisker conditions. But there's also a lot of simpler and lighter models that could work well in the lighter lift days. Anything in this list or on the search page results catch your eye?

There's certainly nothing wrong with the design you're working on but I do feel that it's focused more towards flat field thermal flying than to slope flying. The long aspect ratio wings are going to tend to be more susceptible to landing damage in any but the lighter conditions.

Oz : Airster plan - free download (outerzone.co.uk)
Oz : Avenger plan - free download (outerzone.co.uk)
Oz : Coaster plan - free download (outerzone.co.uk)
Oz : Compact 2-50 plan - free download (outerzone.co.uk)
Oz : Jubilee plan - free download (outerzone.co.uk)
Oz : Kema 90 plan - free download (outerzone.co.uk)
Oz : Little Plank plan - free download (outerzone.co.uk)
Oz : Little Plank III plan - free download (outerzone.co.uk)

If none of these or any of the other models really nails it for you then you could take their shape and general arrangement and do much like you're doing now and "borrow" what you will and come up with a shape of your own. But I'm thinking that a shift away from the nice looking thermal model towards a nice looking slope model would be a good shift in goals from how you've described the area




https://cimg8.ibsrv.net/gimg/www.rcu...06de99d8c4.png
https://cimg9.ibsrv.net/gimg/www.rcu...51f363210e.png
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https://cimg1.ibsrv.net/gimg/www.rcu...a8f78b96f9.png
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Thankyou for all your input. So what you helped me designed is infact a thermal glider? If we say for arguments sake, if I was to continue with this design for thermal gliding, I should have more ribs and look into the areas of the tail? In terms of everything else, does it seam like it would be a good flyer? I know vague question. lol. As I plan to build more than one type of glider, I would like to see this through for those days which are not as windy.

In terms of the hills near me, I have very steep hills and gradual slopes. Getting to the crest can be a little difficult as it gets very steep and you cant drive there. Most of the slopes are 3/4 of the way up the mounting. At the lower spots, the hills are gradual with some large fields with grass. The area I live is Cahir, An Chathair, Aharas, Co. Cork Ireland. We have lots of hills and cliffs 3/4 of the way up the mounting.

Thank you fro your suggestions, there are a lot of suggestion there I can borrow from.

I think the ribs were spaced 70mm in the larger plan and 35mm for the bottom plan.

I think what I will do is probably go ahead with this one for the time being and build another for another project. The plans you suggested really open up what you can build for a slope glider.

Thanks.

Aaron Moat

Well, it was you that picked the thermal glider to use for the basis for your modified design. So it started as more of a thermal glider right off the bat.

There's certainly nothing at all wrong with flying a thermal style glider at a slope. It's just that you'll want to use a bit of discretion with the wind and how the wind complicates and can increase the "arrival energy" during slope landings. And mostly that is the issue. In stronger conditions it is good to have a smaller and more durable model such as many of the suggestions above. But your current plan will be fine for light to moderate slope conditions provided you have a place to touch down which won't destroy the model each time.

There are some specific rules for moving between a standard and V tail. The rules and easy match equations show up at a number of web sites. HERE IS ONE SUCH SITE that provides some good reasoning.

I was always under the impression that the flying broomsticks were thermals and had to be extremely light . We lean every day.

From what I have learned from you, I think I will go ahead with this project and also make a smaller one on the side. Can never have too many.

Il do the calculations on the VTail and get the parts cut.

Il post when I have some update.

Thanks