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Best spar construction/attachment for strength. Acrobatic wing design

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CLYFFCARLOCK

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What is the best spar attachment for strength for wing load? Also should the wing be one piece, going through the fuselage with a one piece spar going from wing tip to wing tip?

How are acrobatic aircraft wings/spars constructed and attached to allow such a heavy wing load?
 

BJC

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Biplanes spars and wires typically are pinned at the fuselage.

Wooden spar wings, as well as many composite spars, are one-piece, and are pinned through the neutral axis to the fuselage.

Lots of airplanes that are suitable for recreational aerobatics use two or three piece wings and spars that have a variety of bolting patterns.

Are you interested in a specific design%?


BJC
 

cheapracer

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What is the best spar attachment for strength for wing load? Also should the wing be one piece, going through the fuselage with a one piece spar going from wing tip to wing tip?

How are acrobatic aircraft wings/spars constructed and attached to allow such a heavy wing load?

A spar from tip to tip is not impossible, but more difficult to offer wing dihedrel, though that may not be needed for a high, high wing.

You would likely get lots of hits Googling Vans wing builds and center spar builds just through the shear numbers of Vans build logs. Their attachment of bolting to center spar is similar to a few makes of planes.

As for construction, a 6" high, 1/4" thick steel i Beam is going to be strong enough but way too heavy, whereas a sheet of aluminium foil is going to be lightweight, but far too weak, somewhere in between is the right combination of metal, weight and strength. It's called "Engineering", and each application requires different engineering.

One shoe does not fit all.
 

wsimpso1

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What is the best spar attachment for strength for wing load?
Define "best". Weight is the enemy of all things we like in airplanes. We generally will like airplane performance better if what we build is strong enough and low weight for that strength. We also tend to like speed, but that usually comes a little lower in aerobatic ships than strong enough and light for that strength.

Also should the wing be one piece, going through the fuselage with a one piece spar going from wing tip to wing tip?
In a cantilever wing monoplane, the one piece wing generally makes for a lower weight at same airplane weight and g limits. Some aerobatic ships are one piece wings. You will also find two piece and three piece wings for a variety of practical reasons. More later.

How are acrobatic aircraft wings/spars constructed and attached to allow such a heavy wing load?
By designing them to do that. Aerobatic aircraft may run at higher g loads than most other aircraft, but the design efforts are all similar... and should be in order to make whatever strength is required at reasonable weight... More later.

Billski
 

wsimpso1

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Define "best". Weight is the enemy of all things we like in airplanes. We generally will like airplane performance better if what we build is strong enough and low weight for that strength. We also tend to like speed, but that usually comes a little lower in aerobatic ships than strong enough and light for that strength.

In a cantilever wing monoplane, the one piece wing generally makes for a lower weight at same airplane weight and g limits. Some aerobatic ships are one piece wings. You will also find two piece and three piece wings for a variety of practical reasons. More later.

By designing them to do that. Aerobatic aircraft may run at higher g loads than most other aircraft, but the design efforts are all similar... and should be in order to make whatever strength is required at reasonable weight... More later.
So, if I were to define "best" (because you have not) I would say that best is the lightest spar set within a material set for a given airplane. This is because WEIGHT IS THE ENEMY. IDEALLY, we always design to be adequately strong for the mission, then calculate what the total airplane weight impact will be for each option, then choose the option gives us the lowest weight airplane. This is a more sophisticated question than you may realize, because just building pieces to their min weight does not neccessarily mean we will have the lightest airplane. Sometimes you can save enough weight in other places while accepting more weight in one set of parts to overall have a better and lighter plane. Optimize the plane, not the pieces...

Let's go through the primary options. Before we do that, let's understand that the wing outboard of the connection points is the same for all three options I will present below:

One-Piece Wing - The spar is one continuous piece from somewhere near the tip on one end to the same spot on the other end. It usually mounts with two pins very near the fuselage walls. The main beam accumulates all of the lift outboard of the any spot in question, culminating in the pinned location. The drag spar typically is picked up by a fitting on the fuselage that reacts off torsion in the wing the results from pitching moment generated by the wing. Between pinned locations, the shear in the main beam is essentially zero while the moment is equal to the moment just outboard of the pin location and constant. This requires all of the spar cap size required to the pin location, and usually results in the shear web from just outside the pins being carried between the pins as well. Pins loads are only as high as the one-half the wing lift. This is a minimum material requirement for the spar designed to any shear and bending moment.

The one-piece wing has the disadvantage of having to build the entire wing from tip to tip in one piece. This requires a long very true build table, an ability to execute all of the pieces for both wing and their tooling with excellent symmetry, and then be able to finish it symmetric too. For production aircraft, this is doable and has advantages. For a homebuilders, it becomes rather difficult to achieve.

Two Piece Wing - The nominal way to do a two-piece is with the same pin locations as for a one-piece wing, but now you have two separate wings with two spars that overlap each other through the fuselage and both engage both pins, so the spars have to be slightly different for port and starboard wings. The amount of material in the spar caps is approximately the same as for the one-piece, but now you do have two shear web sets and both are carrying shear between the pin locations. The adverse weight impact is relatively small as the shear webs are usually quite a bit lighter than the caps. The big advantages are that the separate wings are more easily built, transported and stored, particularly in sailplane length wings, and the individual wings can be built separately using many of the same fixturing pieces in first one wing, and then in the other for symmetry assurance. At a cost of a little more than doubling the shear web, the build, transport, and storage advantages combined with closer approach to symmetry may be considered appropriate.

Three-Piece Wing - This is common. The center section of the wing is either entirely within the fuselage or the break occurs outboard of the fuselage on the wing. The center section spar must be every bit as beefy as in the other versions, but then we either have the outer panel spars overlapping major portions of the center section spar or have very beefy fittings at the breaks in the wing to transmit the shear and bending moments between outer panel spars and the center section. At same strength, this adds more weight over the Two-Piece and One-Piece wings through: More hard points; Duplication of shear webs, duplication of spar caps. This scheme too has practical advantages. The biggest practical advantages com in allowing considerable airplane structures to be built into the fuselage, such as landing gear, fuel tanks, and hardpoints for connections. This also allows the homebuilder to build shorter wing segments at any one time, facilitating the build process and requiring less precision in build per amount of precision in finished project.

In some airplanes, you may find four- or five-piece wings. Many sailplanes use four-piece wings to make for easier transport and assembly, with the inner panels attaching like in a two-piece wing, and then with the outer panels attached to the inner panels like in a three-piece wing. Some light airplanes use a "hollow" structure in the inner panels (cored skins and ribs) for fuel stowage and/or landing gear stowage and equipment bays, but transition to massive core for the outboard panels, which tends to be lighter than the "hollow" structure at the modest wing chords we use in homebuilts.

I hope that this helps to understand the usual suspects in wing design and why some designers may opt for one of several versions.

I shall get into design of the pieces in a later response.

Billski
 

CLYFFCARLOCK

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Biplanes spars and wires typically are pinned at the fuselage.

Wooden spar wings, as well as many composite spars, are one-piece, and are pinned through the neutral axis to the fuselage.

Lots of airplanes that are suitable for recreational aerobatics use two or three piece wings and spars that have a variety of bolting patterns.

Are you interested in a specific design%?


BJC
Nah, just curious of how the construction/materials/attachment of planes that can handle extreme wing loads
 

wsimpso1

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Now to structural design. Aerobatic airplanes may seem to carry huge loads. While they do tend to carry larger g's than utility and transport aircraft, the design of them all is similar. They all are designed to carry the intended loads at something approaching minimum weight to do so. We have some threads here describing design, beam theory, and composite theory.

https://www.homebuiltairplanes.com/forums/threads/beam-theory-explained-how-spars-work.28953/

https://www.homebuiltairplanes.com/forums/threads/read-first-composites-faq.8888/

https://www.homebuiltairplanes.com/forums/threads/mechanics-of-composite-plates-beams-and-bigger-structures.29030/

No magic, just engineering.

Billski
 

TFF

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Same with an Extra 300.
The mid size regional jets I worked on only had either 6 or 8 bolts total holding the wing on, total.
 

wsimpso1

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The whole airplane seems to be flying on those two flimsy pins.Amazing.
Just engineering, and I assure you, they are not flimsy. They are properly sized and the hardpoints they engage were designed to be more than adequate too.

I know folks with hundreds or thousands of hours in Cessnas and then visit the maintenance shop while the covers are all off. Some have been freaked out by the fact that each wing is hung on one little bolt at each spar and two little bolts at each end of the struts. Works great, fantastic safety record, why would you need more?

Two pin systems like that shown are used in many one-piece and two-piece wings. Works great when sized correctly.

Billski
 

cheapracer

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The whole airplane seems to be flying on those two flimsy pins.Amazing.
On the Thorpe T18, there's 3 x "flimsy" 8mm (5/16th) bolts holding the entire landing gear on.

John Thorpe actually designed it with 3 x even flimsier 6mm (1/4") bolts but recognized that his customers would not believe they were strong enough, so went to the next size up!
 

BoKu

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The whole airplane seems to be flying on those two flimsy pins.Amazing.
Well, let's see. I believe that they're 3/8" AN6 bolts, but for the sake of argument let's say they're 5/16" AN5s. AN hardware is good for 125ksi in tension, and probably 55% of that in shear, so each bolt is good for 5/32^2*pi=5270 lbf in single shear. However, each bolt is in double shear, and there are two bolts (one each right and left), so the pair is good for about 21,000 lbf worth of lift. Of course that disregards asymmetrical lift from roll inputs, but even so probably makes for about 20g of capacity with a handful of margin.
 

pictsidhe

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On the Thorpe T18, there's 3 x "flimsy" 8mm (5/16th) bolts holding the entire landing gear on.

John Thorpe actually designed it with 3 x even flimsier 6mm (1/4") bolts but recognized that his customers would not believe they were strong enough, so went to the next size up!
Rutan also upsized the wing bolts on one of the EZs purely for TLAR appearance.
 

Hot Wings

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The whole airplane seems to be flying on those two flimsy pins.Amazing.
and probably 55% of that in shear, so each bolt is good for 5/32^2*pi=5270 lbf in single shear.
AN-5 = 5750# single shear (75Kpsi min) per my spec sheet. But the bolt isn't what I'd worry about, it is the wood. Notice the large diameter aluminum plug surrounded by what is probably Garolite/Phenolic? That is there to create enough bearing area in the wood and to make sure the bolt isn't bending and deforming the wood ... or the other way around depending on if you are the wood or the bolt. We have the same kind of problem with composite structures as well.
 

User27

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Aluminium plug is epoxied into spar with thickened resin with fuselage vertical to set the incidence of the wing - zero, so parallel to the fuselage longerons. Spar is quite heavy as it is solid being laminated from several planks. It is relatively straight forward to build but does result in a relatively heavy wing - difficult for 2 people to lift. Extra 300 wing is not very much heavier (sorry don't have numbers), but would be much more difficult to build in your garage.
 

BJC

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The whole airplane seems to be flying on those two flimsy pins.Amazing.
On my Pitts, all bending loads from the wing are carried by four (two per side) 1/4” clevis pins.

The Glasair wing is attached with two 3/8” bolts at the spar. It also has 1/4” bolts at the rear spar. When my wife saw them, she said that they looked too small to rely on, so I spent two dollars and bought two bolts that are typical of what is used in the One Design that I referenced earlier, as well as the Lasers and early Extras. She said that they looked adequate for the Glasair wing. Photo of the bolts below.

Also in the photo is a 6 a @120 v miniature switch. I have hundreds of them, still available for anyone who needs some, at no cost.

BJC
08F49263-1EE2-4966-88B8-34C8921F6203.jpeg
 
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