# Purrpose of short struts

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#### stanislavz

##### Well-Known Member
HBA Supporter
I try to reinwent excel files for primary wing calculations, and explain it for myself. All was ok for strut less wing till i did stuck to draw proper spar cap load, if struts are placed not in middle of wing lift point.

But not pk for different position. As an.example. 10m wingspan, square load curve of 300kg per each meter. If we place struts at 2.5m. Position - load of spar will be zero at wing tip and wing root. strut transmit all load.

If we put strut at 2m from wing tip - strut transmit 1200kg of force, 300 kg - directly via wing root.

But if we put strut at 1 meter from wing root - how do all curves do look ? Found some old glider with even bigger ratio of strut to wingspan. What is the purpose of struts in it ? PIK-5 - Wikipedia

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#### TFF

##### Well-Known Member
If it’s not attached or strong enough to be cantilevered, it has to have the strut.

Since it is cantilevered outside the strut mount, to me that is as strong as the wing is.

Don’t have to design a cantilevered joint.

If the wing is removable, it’s less hassle to design a with a strut.

Because you can do it today with confidence does not mean they could back then. Look at the timeline of history before judging.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Not home now, will be back in a week. I have some example wing shear and bending moment diagrams for cantilever and strut braced wings. I will try to remember and post them.

Strut bracing greatly reduces the maximum bending moments along the spar, but increases compression in the same spar. Usually strut braced wings are lighter, but have drag penalties.

Billski

#### WonderousMountain

##### Well-Known Member
All the Carbonfiber was under war rations.

#### stanislavz

##### Well-Known Member
HBA Supporter

I am referring this sheet as base :

Strut at the ~middle of load :

Closer to fuselege, but still allow ok entrance :

And ladder wing style (yes, it is missing jury struts , but shows the tendency..) :

And the question is - do tube connecting all three types of wing would be the same for all types ?

Limiting factor is shear load ?

#### Dana

##### Super Moderator
Staff member
Remember that anything outboard is the strut is cantilevered. A wing with struts far inboard is essentially a cantilever wing, but with attach points farther apart (and thus not as heavily loaded) than attachments at the root on the top and bottom of the spar.

#### WonderousMountain

##### Well-Known Member
If you look at original Pik 5 there's a tube spar with the strut a double pinned axial joint.
In torsion the tube will do well, in strain the strut will do well. Tube is also good at sheer.
Thin sheer webs can and do fail. Large cap wings, with poorly secured webs, can go to ruin.

#### stanislavz

##### Well-Known Member
HBA Supporter
Thank you for reply. I do found another example - Jim's Monarch glider. Even his was using small spars. And he is a pultruded carbon rod guy..

And back to my topic - i have asked this, because i did buy (just now) my first airplane. I was looking for front part of fuselage only + all controls - but for small amount of more money, i do buy whole skyranger frame.

Some damage to wing leading edge , and landing gear. But - it is easy to change, due to bolted construction. Will do a nynja like skin for fuselage. And will think that do with wings. Here is its structure:

Cantilever wing is a no option here.

But - one could built an composite wing (or BoKu style or Jim style) and use short strut to make loads of attach point lower...

As in this example of my baby excel :

If you attach strut at 1.5 metre from wing root, you have weight of spar 2.6 kg + 3.3 kg (from wing root to strut same as most loaded station, but thiiner from strut to tip of the wing) ~ 6kg in place of 9kg cantilevered wing + much less load on attach points. And more head room. For pultrusion numbers are even funnier 1.6 kg for cantilevered wing ~ 1 kg for strut braced..

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#### stanislavz

##### Well-Known Member
HBA Supporter
Attached excel too. But it is plain simple ,with uniform wing load, and for untapered wing only.

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#### dana62448

##### Member
This topic is of interest to me as well. My goal is is a lightweight easily detachable wing panel that can be then trailered.
My wing is composite d-tube, single strut, with a drag spar from the rear root attach point to the strut attach point.
My question is how to handle the loads at the cabanes.
Is it essentially a cantilever wing with the fuselage hanging from the bottom wing joining fitting;
Or, are they two separate half spans where the top cap loads transfer through the shear web to the bottom spar cap fitting and then into the cabane?
In other words, is there an advantage to carrying the loads through to top fitting, as though the wing was cantilevered

#### stanislavz

##### Well-Known Member
HBA Supporter
Have not done any more math yet, but where are tons of short strutted wings, especially in older glider area.

And yes, it is a debate what is better from aerodynamic point. Extra (18-20 cm / 7-8" ) in fuselage height due to full spar above your head, or two short struts, but long enough for ok entry and exit..

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Schweizer 2-33 is prolific in USA. Strut is about one third semispan.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Enclosed is a comparison of an example wing in both strut braced and cantilever wing forms. It is an idealized wing mounted at the centerline of the airplane. Note much lower shear and bending moment in the strut braced version. Also note that I positioned the strut attachment to minimize bending moment in the spar - moment is close to being identical in both the inner and outer portions of the spar in this case. This also shows why many strut braced power planes use a straight and fairly light spars with little or no tailoring, while almost all cantilever planes have a lot of tailoring of the spar. I did not include the compression on the wing due to the strut as this is a function of both the where on the wing the strut is attached and what the angle of the strut is to vertical.

Specifics: Max Shear in the strut braced wing is 32% of the cantilever wing, while max bending moment in the strut braced wing is only 9% of the cantilever wing. In conventional materials the weight reduction allowed by using struts can be substantial and in some cases still produce very fast airplanes, with Steve Wittman's Tailwind being an excellent example.

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#### Riggerrob

##### Well-Known Member
This topic is of interest to me as well. My goal is is a lightweight easily detachable wing panel that can be then trailered.
My wing is composite d-tube, single strut, with a drag spar from the rear root attach point to the strut attach point.
My question is how to handle the loads at the cabanes.
Is it essentially a cantilever wing with the fuselage hanging from the bottom wing joining fitting;
Or, are they two separate half spans where the top cap loads transfer through the shear web to the bottom spar cap fitting and then into the cabane?
In other words, is there an advantage to carrying the loads through to top fitting, as though the wing was cantilevered
Perhaps you could use struts as part of your wing-fold mechanism (ala. Stitts Playmate). If the wing only folds, constantly-attached control cables can reduce/simplify assembly time.

#### dana62448

##### Member
The folding is possible but at this point, my intention is to create a wing with no openings or control cables/arms. The Junkers Flaps plug in automatically on assembly. Three connections to remove.

#### WonderousMountain

##### Well-Known Member
Thicis from one of our stickied articles entirely worth reading.
With the 3 point braced spar, your structure need are minimal.

#### stanislavz

##### Well-Known Member
HBA Supporter
With the 3 point braced spar, your structure need are minimal.
I know. using carbon you have only skin with some layers of ud as spar.

But the main question - do short spar will have benefits if wing is more on cantilever side for high wing aircraft...

#### wsimpso1

##### Super Moderator
Staff member
Log Member
I do not know why some designers have built wings with short struts that only go out to a spot a little ways out.

There is weight savings to be had in wood and aluminum spars using shorter spars. I just ran the strut to 33% of semispan - max shear load is 70% of cantilever and max bending moment is 38% of cantilever. It is worth doing, but as long as you are accepting the interference drag of four intersections and a length of the struts, you might as well take more length and get much bigger weight reductions... unless there are other practical issues like supporting the front of the wing or facilitating a folding mechanism...

Billski

#### WonderousMountain

##### Well-Known Member
Theory:
Struts stayed the same, wings were made longer.