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Critique my design

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Jim Williams

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Sep 17, 2005
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Atlanta ga.
This is design idea have had for a number of years. The aircraft I vision would be a single seat,shoulder wing, all metal aircraft with VW engine light sport aircraft.
One of the problems I am having is the wing. Most light planes with a shoulder wing have there wings swepted forward a few degrees. Why is this?
So please critique my degree. Jim Williams
 

GESchwarz

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Without seeing anything, I'd think that the wing is swept forward so the main spar can pass aft of the pilot. The CG of the pilot (his belly button) must be near or forward of the wing's neutral point, therefore the spar must sweep forward to make that possible.
 
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bmcj

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Without seeing anything, I'd think that the wing is swept back so the main spar can pass aft of the pilot. The CG of the pilot (his belly button) must be near or forward of the wing's neutral point, therefore the spar must sweep forward to make that possible.
I second what GE said. Low wing and high wing planes can have the spar pass below/above the pilot and passenger. With a shoulder wing design, the best you can hope for is to pass the spar between two tandem seats, but that is difficult to do. As a result, the spar for a shoulder winged plane is often placed behind the seats which necessitates a forward sweep in order to get the aerodynamic center closer to the CG (please correct me if I am mis-stating aerodynamic principles). Otherwise, you would have the weight of the engine and passengers hanging too far out there in front of the wing, requiring too much tail trim force for level flight.

Bruce :)
 

GESchwarz

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Yer gonna have to sweep those wings forward...It looks nose heavy. I believe that you are going to have special structural considerations with a forward swept wing. There are others here who can shed more light on that.
 

Canuck Bob

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I have studied forward swept wings on a casual basis. There was a homebuilt called the BA-7 designed by the swede B. Andreasson while working in San Diego. It became the Boelkow 208C Junior

Boelkow 208C JUNIOR: German Oldtimer plane click on the Flight Report tab on the left and you will find some great articles in English that bear a striking resemblence to your plane. Yours is far more attractive.

The forward sweep wing has many positive features. It eliminates any tendency to tip stall and allows the movement of the C of G. The main disadvantage is something refered to as divergence. The forces twist the tips upward under HAO, I think. A wing must be built torsionally rigid. With most GA designs the sweep is usually 3-5 degrees and I was told on another forum on design that up to 7 would work. Universal advice given, make sure the divergence is designed for properly.
 

JMillar

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From reading about the x-29 (I think?) project, I recall that the solution to the divergence problem (positive feedback in deflection) is not torsional rigidity, as you might expect, which would make the wing structure too heavy. Instead, they turned to "aeroelastic tailoring," I think it's called, which is basically planning the fibers in a composite so that the deflection is aerodynamically self-limiting, as it flexes it changes the nature of the forces imposed.

In other words, deliberately flexing in a planned way, to change from positive to negative feedback. But I'd like to hear some confirmation that I'm not totally wrong about this.
 

BBerson

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That design looks cool.
You can sweep the wings forward a bit without any unusual problems, I think, if skinned in metal.
The Blanik glider has quite a bit of sweep.
Another thought: put the main spar farther aft and move the pilot more aft as well. The Piper Cherokee has the spar more aft than usual to fit the front seat ahead of the mainspar.
BB
 

bmcj

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James... correct on both accounts; it was the X-29 and the concept is called aeroelastic tailoring.

Jim... forward sweep can be avoided with proper design. Two examples of shoulder winged craft with straight wings would be the Cassutt and the Sonerai (single and two place). In the single-seat Cassutt and Sonerai, the spar passes in front of the pilot and about where the instrument panel sits. For the two-seat Sonerai (Sonerai II), the spar passes between the front and back seats. Note that the Sonerai's can be built in either low wing or mid wing configuration.

As an added benefit and more pertinent to your original question, the Sonerai has metal wings and is typically powered with a VW engine. In fact, the kits/plans are sold by Great Plains, makers of a popular VW engine conversion. Here is a link to their Sonerai page:

Sonerai Sport Aircraft by Great Plains Aircraft Supply

And to their homepage where you can see their engines:

http://www.greatplainsas.com

Bruce :)
 

Jim Williams

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The benefit of this shoulder wing design is the pilot sit forward of the leading edge of the wing and has greater visibility all around. That is want Iam looking for in this design.
 

Canuck Bob

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Andreasson used titanium skins on his prototype and the production Junior used slightly heavier aluminum skins.

The ARV-2 from England and now offered in the US as a LSA as well as the Wasp from Australia also use sheet metal construction.

The X-29 is not apple to apples when we discuss GA aircraft. A Bolkow Junior is a long way from the X-29. The Citron Bleu design in Europe uses traditional plywood D cell wooden wing construction for a simple plane. For the plane sizes and flight envelopes we deal with a torsionally stiff wing is best. I have no wish to insult anyone but I've researched this layout and always the discussion leaps to the X-29. Its the same as analyzing an RV3 and leaping to an F16.

The X-29 was a modern jet aircraft the size of a fighter plane spec built by NASA and used to test forward sweep (FS) for experience with the benefits of forward sweep and supersonic compressability. The sweep was also very severe in comparison to GA aircraft. Using data from that experiment for a small GA design is inappropriate. There are many succesful FS GA aircraft to base a design and construction analysis on that are far more appropriate. The structural problem of divergence in our types of aircraft is a minor problem that has been overcome easily in this aircraft range many times and many ways.

The design we are critiqueing is an excellent design and very attractive. Also the designer seems to want the pilots eyes slightly ahead of the leading edge. The visibility would be outstanding. I also like the idea of the powerplant arriving at any crash scene before the pilot does. Much better than an angry Continental pushing the pilot to the scene while the pilot cushions the blow for the engine. The aesthetics of the vertical tail are very pleasing, hope its ok to borrow your tail profile.

I have no engineering but I have looked hard and long at this very configuration. Biggest practical question is how does one place the pilot and passengers and engine ahead of the front spar assuming an app. 25% of chord location. The forward swept 5 degree wing of the Cygnet, a very similar rag and tube layout, only moves the C of G range 4 or 5 inches at most (from memory).

Most designs swept or not historically taper the roots back to roughly the spar point for downward visibility. It doesn't seem ideal but it may not be a big penalty. For me looking at the top of the wing with the shoulder location wing and no taper is unacceptable.

Your design is excellent and if acheivable will give the safest level of visibility one can get from a tractor engine plane in this size range. In my risk analysis this is the safest set of design parameters. Outstanding visibility in the pattern, front engine crash protection. The shoulder wing keeps the tips out of snow banks and runway lights, offers a little more ground effect compared to the average high wing but retains pendulum stability. If fuel is carried in the wings one can look into thier tanks as they should, fueling is easier yet gravity feed is a possibility.
 

DaveK

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Have to agree with what Canuck Bob has to say, there is no reason that a small amount of forward sweep can not be done. It's not like there is some magical thing about going past straight out from the side of the fuselage. Yes the torsion becomes an increasing concern with increased sweep, but at low speeds and only a few degrees of forward sweep this isn't a big problem. It also avoids the rather touchy weight and balance you would have to have with the tandem layout and tractor engine.
 

Canuck Bob

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I looked into some other designs and found the folowing.

Many trying to use this style have the pilot in an upright sitting position to keep thier rumps as close to the rear of the cockpit as possible.

Some also design the firewall so the engine sits rearward and the feet actually extend a fair distance forward. If you look at pictures of the Boelkow Junior it is quite obvious. The 0-200 rear accessories sit above the feet which are close to the oil pan. When I compare it to my Fly Baby's A65 and conventional J3 mount I could gain at least a foot. The Vw is flat on the bottom and might lend itself to moving back safely.

The single seat config will allow you to tailor your balance because the varying weight of passenger or no passenger is no problem. It should be possible to use a straight wing but it will be a challenge which makes it fun.

One other thing, with a cantilever wing I think forward sweep would be real complicated to achieve. It adds an element to spar design and attachment requiring a high level of engineering experience I would think.

Would you consider putting up a 3 vue if you have a sketch. The more I look at your drawing the more I find your design to be very appealing.
 

wsimpso1

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Structural divergence in a forward swept wing works this way:

G loading from whatever source drives the tip up. Being as the tip is forward, the leading edge of the tip is driven upwards too, which drives tip some more, which causes the tip to deflect further, which drives up alpha at the tip... You get the idea.

Most metal skinned wings on little airplanes have excess stiffness. And most little airplanes only have small amounts of forward sweep, giving only small amounts of this tip lifting tendency. That probably is why the forward swept wings are done successfully. Add in a tendency to put the shear center (the center of torsional stiffness) forward a bit, and it tends to be no big deal.

Structural divergence in the X-29 was written about in composite materials texts. Composite structures of this type are assembled from pre-preg tapes. The fibers run only in the length of the tape and they are held together with resin, which flows and then cures in an autoclave when the part has been assembled. Anyway, they can put the tapes on with the fibers in any direction that they want, so they can put the tapes on so that there is coupling between bending and twisting. If you do it the right way and with right fraction of the total layers, wing bending will induce the same amount of twisting opposite to the basic twist that is being induced aerodynamically. The profs I have asked have said that no tailoring of this sort was actually done on the X29.

You can also tailor the wing by moving the torsional center of the wing forward as you move outboard by shifting more and more of your resistence to torsion forward as you go outboard on the wing. This is simply a matter of applying more tapes forward, making more of them at an angle to the span, and either eliminating the spar, making it very thn, or moving it forward.

Both methods are tough to do with metals. But with the amount of sweep you should need, it is no big deal either.

Billski
 
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