Allegro lite mod for F3RES - RC Groups
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Sep 19, 2016, 09:10 AM
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Allegro lite mod for F3RES


So the F3RES event seems to be gaining ground here in the states. Me being, well me, I would rather scratch build than import a kit from Europe. Me being me, I also would like something that will go together relatively quickly.

So my idea is to adapt Dr. Drela's AL design for a quicker construction method at the expense of wing strength. This mod is only intended to fly off of a hi start, so don't need the wing to be able to take a winch load.

My idea for this mod is like this: Use 1/16 sheeting on the bottom and 3/32 sheeting on the top. Sheet the bottom first and use two pieces of 1/8" vert grain balsa for a sheer web, with 1/4" space between them (the sheer web would essentially be hollow box). The 3/32" top sheeting would then cap off the sheer webs. The leading edge sheeting would essentially act as the spars.

It seems as though as it would be easy and quick enough to build, but will it be strong enough to handle a typical hi start launch? Any thoughts, ideas, suggestions, criticisms, or other feedback is also welcome.
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Sep 19, 2016, 10:27 AM
Cognitive dissonance
kcaldwel's Avatar
I built a 2m glider in the '70s using that construction. I winch launched it on the 6V winches of the day, and hand towed it. The 3/32" top sheeting eventually failed in compression after several years of flying, just outboard of the wing joiner (centre wing joint). I only had a single 1/16" balsa shear web. Two 1/8" shear webs would be way over kill for the strength of the sheeting "spar caps". The shear loads are quite low compared to the compression and tension loads.

The XRES I have built has very small bass or spruce spar caps, with 1.5mm sheeting top and bottom, and a single 1.5mm shear web. The wing joiners at the dihedral breaks also act as local shear webs. I would be tempted to do something similar in the centre section at least, maybe just for the centre 300mm. The load on the rest of the wing is very low, and even 1/16" sheeting with a single 1/16" shear web would be adequate

Kevin
Sep 19, 2016, 10:31 AM
Registered User
We just completed two X-RES, and like them.

As Kevin pointed out, the spar caps are indeed 1mmX3mm spruce. The bottom spar cap does not overlap the bottom sheeting. The top spar cap is overlapped by the 1.5mm sheeting. The shear web is 2mm balsa, horizontal grain, with notches to accept the ribs. On our hi-start, there is no visible bending of the wing. So, in order, bottom-to-top: 1mm spruce, 2mm balsa shear web (interlocked with 2mm ribs), 1mm spruce, 1.5mm balsa.

The total pull on our hi-start is very close to the F3RES specified pull of 4kg force (more or less eight pounds).

Yours, Greg
Sep 19, 2016, 10:48 AM
Cognitive dissonance
kcaldwel's Avatar
Thanks for the correction Greg, 2mm light balsa, horizontal grain shear webs on the XRES.

I was thinking before that some 1.5mm CF rods would make very light and easy spar caps for one of these RES 2m gliders... 1/16" sheeting top and bottom, and 1/16" shear webs, maybe 3/32" in the centre 300mm of the wing.

Kevin
Sep 19, 2016, 10:51 AM
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rdwoebke's Avatar
CGL, I have no idea if that would work but as a big Allegro lite fan I look forward to reading about your projects.

Ryan
Sep 19, 2016, 11:26 AM
Registered User
This is looking good. It seems that the F3RES models are using less structure than I intend to, and are plenty strong.

Based on what I am seeing in those designs, I am going to modify my plan for the sheer webs, and go with a single 3/16" thick web on the inner panels, and a single 1/8" web on the tip panels. Also going to reduce the top layer sheeting to 1/16".

I have some old arrow shaft that is really light for the boom, and going to build a glass reinforced balsa pod of 1/16".

Should be a fun build. Going to get the balsa within the next few days, and get to building.

Thanks
CG
Sep 19, 2016, 11:41 AM
Cognitive dissonance
kcaldwel's Avatar
Are you going to add some type of spar cap? I don't think 1/16" top sheeting is enough without something else.

3/16" and 1/8" shear webs are 2x more than you need.

A DLG boom will be lighter, stiffer (especially in torsion), and have less mass at the tail:

http://www.hyperflight.co.uk/product...ed-carbon-boom

Kevin
Sep 19, 2016, 11:54 AM
Flying = Falling (Slowly)
dharban's Avatar
Quote:
Originally Posted by cglynn
So the F3RES event seems to be gaining ground here in the states. Me being, well me, I would rather scratch build than import a kit from Europe. Me being me, I also would like something that will go together relatively quickly.

So my idea is to adapt Dr. Drela's AL design for a quicker construction method at the expense of wing strength. This mod is only intended to fly off of a hi start, so don't need the wing to be able to take a winch load.

My idea for this mod is like this: Use 1/16 sheeting on the bottom and 3/32 sheeting on the top. Sheet the bottom first and use two pieces of 1/8" vert grain balsa for a sheer web, with 1/4" space between them (the sheer web would essentially be hollow box). The 3/32" top sheeting would then cap off the sheer webs. The leading edge sheeting would essentially act as the spars.

It seems as though as it would be easy and quick enough to build, but will it be strong enough to handle a typical hi start launch? Any thoughts, ideas, suggestions, criticisms, or other feedback is also welcome.
It is worth remembering that Dr. Drela designed the Allegro Lite to withstand a 150# wing load. I think when we extrapolate backwards from that we are probably overthinking the problem.

It is my understanding from his writings on spar design that he was relying, at least computationally, on the spar strength. Two carbon strips separated by a full width shear web. These were wrapped with Kevlar essentially to ensure that the top spar would not fail in column. No "credit" was included in the calculation for the monocoque construction of the leading edge.

I to have been designing a plane based on the Allegro Lite. I think you are on the right track, but could probably simplify further by using a single 1/4 inch shear web and 1/16 inch balsa D-tube sheeting top and bottom. The only thing that you might gain by using two 1/8 shears in a "box configuration" is a little torsional strength. But the 1/16 balsa sheet monocoque will provide much more torsional strength than you will ever need. The success of this would depend on proper selection of wood and careful gluing technique.

The bottom sheeting for the entire wing bottom leading edge could probably be 1/16 contest balsa. The top sheeting for the center section could be a medium weight (8-10 pound) or even C-grain if you can find it. I would use 15 minute epoxy to attach the ribs and shear webs to the bottom sheeting and would use a careful system of weights and tension and sparing application of Gorilla Glue to attach the top sheeting to the shear webs and ribs. And I would use contest balsa for the top sheeting on the tips.

I just pulled up my plans for the original Esprit, a 2-meter design by Lee Renaud and it uses a relatively thin 6409 airfoil with 1/8 x 1/4 spruce spars top and bottom and 1/16 shear webs BUTT glued to those spars. It has no sheeting, top or bottom. It was more than adequate for hi-starts of the day and careful use with 6V winches.

One of the construction features I really like on some of these planes is the use of 1 mm laser cut plywood trailing edges. One of the things I found in building various Bubble Dancers and the Allegro Lite is that it is fairly difficult to get straight trailing edges using the wood and dimensions that Drela specified.

If we stick to the suggested 4 Kg pull limit, i really doubt that we will see failures on these lightly constructed wings. But wood selection, precision fits and careful glue applications will be essential.
Sep 19, 2016, 12:17 PM
Flying = Falling (Slowly)
dharban's Avatar
Quote:
Originally Posted by kcaldwel
Are you going to add some type of spar cap? I don't think 1/16" top sheeting is enough without something else.

3/16" and 1/8" shear webs are 2x more than you need.

A DLG boom will be lighter, stiffer (especially in torsion), and have less mass at the tail:

http://www.hyperflight.co.uk/product...ed-carbon-boom

Kevin
The use of something like a single 1/4" shear provides not only prevents buckling on the top sheeting, but adds to the compressive strength on the top of the wing. Admittedly, the compressive strength across the grain of the shear web is not as much as the compressive strength with its grain, but its bulk AND careful gluing practice will provide more than adequate strength.

My calculations suggest a safety factor in excess of 2.5 at the wing root. If there is any question, doubling the shears for the first two or three rib bays will more than mitigate any concerns.

Happy Landings,

Don
Sep 19, 2016, 12:50 PM
supreme being of leisure
ZAGNUT's Avatar
cheap 0.5x3mm carbon strip spar caps would add only 10g to the wing and you could save that by using 1/16 sheet on top. durability for bad landings would be much better.
Sep 19, 2016, 01:02 PM
Flying = Falling (Slowly)
dharban's Avatar
Quote:
Originally Posted by ZAGNUT
cheap 0.5x3mm carbon strip spar caps would add only 10g to the wing and you could save that by using 1/16 sheet on top. durability for bad landings would be much better.
The 3/32 sheet on the top is not necessary whether or not you use the carbon cap.

In our application, carbon caps are not remotely necessary on the bottom because the bottom sheeting has more than enough tensile strength for the loads we have.

My calculations indicate that the 1/16 sheeting on the top with a 1/4" shear web would also be more than adequate.

The carbon is nice, but really adds nothing that is needed in our application -- especially if nothing is done to mitigate the possibility of failure by buckling on the top spar.

Happy Landings,

Don
Sep 19, 2016, 01:18 PM
supreme being of leisure
ZAGNUT's Avatar
maybe not needed but cheap insurance both cost and weight wise. IMO something more than balsa as spars is always welcome when accidents happen.

and the only time i've seen carbon arrow shaft used as a tail boom the plane had hideous yaw characteristics that instantly got better once the boom was stiffened. and IIRC the boom was shorter than what would be used on a 2m today.
Sep 19, 2016, 01:53 PM
Cognitive dissonance
kcaldwel's Avatar
Quote:
Originally Posted by dharban
The use of something like a single 1/4" shear provides not only prevents buckling on the top sheeting, but adds to the compressive strength on the top of the wing. Admittedly, the compressive strength across the grain of the shear web is not as much as the compressive strength with its grain, but its bulk AND careful gluing practice will provide more than adequate strength.

My calculations suggest a safety factor in excess of 2.5 at the wing root. If there is any question, doubling the shears for the first two or three rib bays will more than mitigate any concerns.

Happy Landings,

Don
Don is allergic to carbon fibre.

The 1/4" really is the spar then. Only the very outer part of the 1/4" is acting as the spar caps, with way too much for the shear web in between.

A 1.5mm CF rod on the top in the centre section only, a 1/16" balsa shear web, and 1/16" sheeting top and bottom is an alternative.

Lots of ways to build these wings, the loads are very small compared to something that will survive a modern winch or a heavy duty high start.

Kevin
Sep 19, 2016, 03:52 PM
Flying = Falling (Slowly)
dharban's Avatar
Quote:
Originally Posted by kcaldwel
Don is allergic to carbon fibre.

The 1/4" really is the spar then. Only the very outer part of the 1/4" is acting as the spar caps, with way too much for the shear web in between.

A 1.5mm CF rod on the top in the centre section only, a 1/16" balsa shear web, and 1/16" sheeting top and bottom is an alternative.

Lots of ways to build these wings, the loads are very small compared to something that will survive a modern winch or a heavy duty high start.

Kevin
Actually, I am not allergic to carbon. I probably have more carbon stock, booms and cloth in my shop than everyone else here combined. And I have probably built at least as many carbon and carbon reinforced planes as anyone here.

Over the years, I have also built many dozens of all wood planes. My experience with them suggests that an awful lot of people have fallen so in love with composite reinforcement that it has become massively overused. The fact is that, at least planes flying under the German rules, are significantly under stressed relative to the first planes we flew back in the '70's -- at least until the advent of the 12V winch.

The pull restriction and the 2 meter limit result in bending moments at the root which are way less than bigger planes with stronger launches.

I was trained as an engineer and designer and simply have a penchant for form and function to be appropriate for the task. And when I sit down to design a plane, I actually do the calculations before I decide how I want to build the plane.

Happy Landings,

Don

I am in the process of building a test section to determine how much of a load my design will withstand.
Last edited by dharban; Sep 19, 2016 at 04:23 PM.
Sep 19, 2016, 05:32 PM
Cognitive dissonance
kcaldwel's Avatar
If we have to resort to rolling out diplomas, I too was trained and worked my entire career as an engineer. I have my old copy of Bruhn "Analysis and Design of Aerospace Structures" from university days sitting in my bookshelf. As an engineer, I find a material that has 1.5 times the specific strength of balsa, much higher stiffness, uniform properties (no sorting through piles to find straight grain, right density, etc.), is ready availability in a variety of shapes, and pretty low cost because you need so little of it, all make CF the obvious choice for structural elements in an airplane.

I grew up building balsa airplanes in the '60s and 70's, but I love the strength and stiffness of CF, and will use it whenever possible. It makes so many types of RC sailplanes (F3J, DLG, DS) possible that could not be made without a material with out CF's outstanding properties.

Yep, if you really like balsa, a RES sailplane can be built entirely without CF because the loads are very low. You could re-design it to be lighter, stronger, and stiffer with a bit of CF though.

Kevin
Last edited by kcaldwel; Sep 19, 2016 at 05:45 PM.