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Jul 20, 2020, 12:26 PM
Dynamic RC
FlyingTyger's Avatar
Thread OP
Build Log

CAP 2021 - Frog Diana x 1.5


My entry for the Vintage Glider Build-Off will be Frog's Diana. I am scaling it up 150% to a wingspan of 54". Plan is to power it with a nose mounted electric motor with rudder and elevator controls. I have a 250 sized motor and 8" folding prop that I think will do quite nicely on a small 2 cell pack.
I have traced the plan in AutoCAD so that I can start making the RC mods. I also think I will need to make a few small adjustments for the increased size, but my hope is to keep it as close to the original design as possible.
While I was playing around in CAD, I threw a couple color options at it. Not set on one yet but narrowing it down
Last edited by FlyingTyger; Jan 01, 2021 at 12:45 AM.
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Jul 20, 2020, 12:47 PM
B for Bruce
BMatthews's Avatar
I looked at the Diana for a similar size enlargement and felt that all would be well other than I wanted to increase the number of ribs to better support the covering and keep the fidelity of the airfoil. Or another option would be a turbulator spar or two.

The other thing that put me off and which could affect performance when loaded down with RC stuff is the leading edge. The flat shape that produces a sharp edge between the ribs is OK for light stuff like the Diana would be as a pure free flight. But with the added weight we want a more controlled airflow around the leading edge. My plan was to alter it to something as shown below. Or you could simply go with a more regular solid square strip leading edge. Which isn't a bad idea either as the stock and modified versions don't have a whole lot of wood to resist impact damage.

The colour schemes you've come up with a so very classic looking. Either is a lovely match to the Diana design. I actually prefer the black and white option for the strong contrast. But having done a few models way back with black finishes and seeing and mostly FEELING how hot they get in the sun I opt for lighter colour options now. Nothing darker than royal blue. And even that get's pretty warm.

But no doubt about the shapes of the trim. They look great!
Jul 20, 2020, 03:30 PM
I like real wooden aeroplanes!
Sundancer's Avatar
Love Frog models, subscribed. My approach to the leading edge would be to make the flat component somewhat wider and sit a square piece - 3/16"? - on top at the front. And yes, turbulator spars are a good idea, two or maybe three - this is what I did on my Lulu which has a similar sort of section.
Jul 20, 2020, 04:23 PM
Dynamic RC
FlyingTyger's Avatar
Thread OP
You both have used a term I am not familiar with. What is a turbulator spar?

I do plan to change the LE section, my initial thought was to use some full height square or rectangular stock. Though I do like Sundancer's idea of the flat LE with a smaller square stacked on top. I also planned on adding a subspar between the main spar and LE similar to your Lulu.
Jul 20, 2020, 04:37 PM
B for Bruce
BMatthews's Avatar
Those subspars, as you call them, are what are also often referred to as turbulator spars. Same things. Just depends on what you're hoping that they will do...

The thought on the "turbulator" designation is that the sudden ridge on the covering will hopefully produce enough roughness that the flow changes from the frequent laminar flow at "our" Reynolds numbers to a turbulent boundary layer which is more "sticky" to the airflow.
Jul 20, 2020, 11:42 PM
I like real wooden aeroplanes!
Sundancer's Avatar
Sorry FT - it is a term much used in F/F circles in the vintage era and simply refers to sub spars on the upper surface of the wing in the first third of the chord. As Bruce says these were used to promote turbulent flow which could improve the glide performance of some of our wing sections. As structural members they have the side benefit of reducing covering sag between ribs and thus improving the accuracy of the wing section, which was my main aim in using them on the Lulu. (Some designs which had sheeted leading edge sections used doped on thread to produce the same effect.)
Jul 21, 2020, 12:10 PM
Dynamic RC
FlyingTyger's Avatar
Thread OP
Thanks for explanations. I have built several planes that have had these, I just never knew that they had a name. I never would have guessed that they had an aerodynamic effect either. I always assumed they were just a simpler/lighter alternative to sheeting. I would have guessed that the sheeted wing option would always be preferred, but it seems that some designs might benefit from having turbulators instead.
Jul 21, 2020, 12:17 PM
Dynamic RC
FlyingTyger's Avatar
Thread OP
So moving forward with the wing design. I started playing around with a revised wing section but nothing seemed to look correct. So, trying not to reinvent the wheel, I decided to borrow a wing section from another design. I built a Stevensaero 4charlie a few years back. It was a delightful little flier and had a wing design featuring these forward sub-spars. So I pulled out the plans, and low and behold, they are actually called out as turbulators. I guess I would have been familiar with these had I studied my plans better... Anyway, I measured the chord of the 4charlie and compared it to the Diana. A perfect match! So I am just going to steal this wing section and use it. With that, I now have the wing design nearly finalized.
Jul 21, 2020, 12:36 PM
B for Bruce
BMatthews's Avatar
For the size of the enlargement and higher aspect ratio the main spars shown are going to be quite "iffy" if you have to haul back hard at some point. The size isn't bad but I'd slightly modify the main spar caps to lay flat instead of on edge as shown and to make them from spruce instead of balsa.

It also looks like the joiner is just a thin blade of plywood between the two spar caps. I'd want to change that to go with the 50 something span as well. Through the center and a couple of ribs to each side I'd replace the webbing with 1/16 ply spar joiners that act as webbing and joiners in one. And to taper the effect of the joiners one of them to extend to the first rib out from center and the other to extend two ribs out. That way there's no major stress riser.

It's not a bad idea to cut generous tapered "fish mouths" into the ends of the joiners too. That lets the spars start to flex naturally instead of just connecting to the joiners in a hard inflexible manner and causing a stress point.

Sounds like a lot but the changes are really very minor. The 3 center ribs will need to be cut in two parts with the spar box missing and the slots for the spars altered but that's really about it.
Jul 22, 2020, 11:01 AM
Dynamic RC
FlyingTyger's Avatar
Thread OP
Quote:
Originally Posted by BMatthews
For the size of the enlargement and higher aspect ratio the main spars shown are going to be quite "iffy" if you have to haul back hard at some point. The size isn't bad but I'd slightly modify the main spar caps to lay flat instead of on edge as shown and to make them from spruce instead of balsa.

It also looks like the joiner is just a thin blade of plywood between the two spar caps. I'd want to change that to go with the 50 something span as well. Through the center and a couple of ribs to each side I'd replace the webbing with 1/16 ply spar joiners that act as webbing and joiners in one. And to taper the effect of the joiners one of them to extend to the first rib out from center and the other to extend two ribs out. That way there's no major stress riser.

It's not a bad idea to cut generous tapered "fish mouths" into the ends of the joiners too. That lets the spars start to flex naturally instead of just connecting to the joiners in a hard inflexible manner and causing a stress point.

Sounds like a lot but the changes are really very minor. The 3 center ribs will need to be cut in two parts with the spar box missing and the slots for the spars altered but that's really about it.
You are about right on with my plan. I am figuring spruce spars and a ply joiner along the back side of the spars extending out the first two rib bays.
I also plan to wrap the center joint with fiberglass cloth. The spars in the remaining rib spaces will get a shear truss (versus a web) like I did on my giant Livewire. This will also help stiffen the wing further out from center.
Jul 22, 2020, 01:36 PM
B for Bruce
BMatthews's Avatar
Like minds and all that eh?

This will sure be a cutie.
Jul 23, 2020, 03:43 PM
Registered User
If you haven't already started, I suggest that you use the original airfoil with turbulator spars. On a glider of this size, the chord is relatively narrow, so the Reynolds number is low. This favors thinner foils, particularly with RC, where you may find yourself fighting a headwind to return to the field. Speed is also useful when you're trying to get out of sink. If you really want to scoot, you might also have the forward bottom surface sweep up a little the way it does on the Clark Y, the Selig 3021, or the NACA 4412. But keep it thin. Without sheeting, this wing won't be precise enough to label, so there probably isn't much magic in using a world beating airfoil.
Jul 23, 2020, 05:35 PM
Registered User
You may already know this, but many other types of wood are acceptable for spars. Mayne a little bigger or a little smaller depending on the stiffness and strength. Boxwood, Douglas fir, other kinds of spruce or fir, sugar maple, etc. I'd expect Douglas fir to be somewhat stiffer, stronger, and heavier. I'd expect sugar maple to be quite a bit stronger, stiffer, and heavier, which would justify a somewhat smaller size. I used to have a 2 meter RC glider with fat maple spar caps that would bog down many winches. Straight grain is just as important as the species.

You can, of course, taper spar caps to save weight, because the load drops off quickly as you move toward the tips. I've thought about doing this and then filling in with balsa to maintain the size.

Do you prefer the diagonal truss because of tradition or aesthetics? If not, I imagine a balsa shear web, installed between the spar caps, would be as good or better, and less work. Even less work would be thin ply glued to the fronts and/or backs of the spar caps. 1/64 might be thick enough not to buckle. Gluing balsa shear webs on the front or back of the caps loads them in rolling shear, where balsa isn't very strong. At least if you have the grain running vertical. If I had a time machine I would tell myself this just before the first time I built an RC glider.

More info than you probably want about selecting and inspecting different species of wood for aircraft use can be found in ANC-18. Info on shear web design, as I recall, can be found in ANC-18 or maybe it was ANC-19. I think you can find both at a site called something like West Coast Piet, and in other places on the web. Full scale methods for shear web design would probably generate complex designs using plywood that's thinner than anything available retail. Come to think of it, I have plans for a full scale ultralight that uses blue foam for shear webs, except at the wing joints and where the center section attaches to the fuselage. At those points, wood reinforcements are added.
Jul 23, 2020, 05:36 PM
Registered User
Quote:
Originally Posted by BMatthews
Like minds and all that eh?

This will sure be a cutie.
Yes it will.
Jul 24, 2020, 10:27 AM
Dynamic RC
FlyingTyger's Avatar
Thread OP
Thanks Lincoln for all the advice. I will have to do some more research before I commit to an airfoil. For now I have moved on to the fuselage and started adjusting that basic design for RC.

Quote:
Originally Posted by lincoln
Do you prefer the diagonal truss because of tradition or aesthetics?
Both? Truth is that I find cutting and fitting shear webs frustratingly putsy. I saw this truss technique years ago on a sport/pattern plane by Direct Connection called the Tsunami. I have since used it on my small and sport type models because it is fast to build and strong enough for these applications. Though on the Diana, which I am hoping to cover with transparent film, it will also contribute to the aesthetics.


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