Francis Donaldson's thoughts on Evans Volksplane VP-2

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cluttonfred

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It sounds like the actual situation is somewhere between the two extremes...the cockpit bulkheads are not very stiff but not completely floppy either so the stress on the spars is also somewhere between the two scenarios. mcrae0104, how would your calculations change with a torsion box near the middle of the half wing, so just outboard of the strut attachment points, like on the VP-1?
 

robertl

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It sounds like the actual situation is somewhere between the two extremes...the cockpit bulkheads are not very stiff but not completely floppy either so the stress on the spars is also somewhere between the two scenarios. mcrae0104, how would your calculations change with a torsion box near the middle of the half wing, so just outboard of the strut attachment points, like on the VP-1?
I think my head is going to explode, I don't care how or why, I would just like to fly one.
Bob
 

Riggerrob

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V-struts to the forward cockpit bulkhead (instrument panel) reduce differential loads on the aft cockpit bulkhead.
Sure, you would need a stronger compression member across the front bulkhead.
It would also be wise to glue external rails along the cockpit sills.
With no loads on the top of the aft cockpit bulkhead, you might even be able to "step" the right side a few inches aft, to improve shoulder room by allowing shoulders to stagger or over-lap.

I wonder how much 1/4 round plywood fillets would stiffen the cockpit rim. As long as they are larger than human rib radius, they will not impinge on cockpit room.
 

mcrae0104

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It sounds like the actual situation is somewhere between the two extremes...the cockpit bulkheads are not very stiff but not completely floppy either so the stress on the spars is also somewhere between the two scenarios.
Yes, that's a good observation. Consider what happens if the rear bulkhead is completely free to rotate. Aileron goes down, rear spar begins to float, the float reduces the AOA until some equilibrium is reached. With the actual bulkhead, it begins to accept the load proportionally as it twists. So yes, the reality is not quite a completely free bulkhead--if it accepted no load, the ailerons would be completely ineffective as they found equilibrium. I don't know the torsional stiffness of the fuselage so it's hard to say exactly where that balance exists.

mcrae0104, how would your calculations change with a torsion box near the middle of the half wing, so just outboard of the strut attachment points, like on the VP-1?
It wouldn't change much vs. the tip location. The moment would get fed into the spars closer to the root (which is a little better) but the max moment wouldn't change. The reason is that it's just best to start from a good rigid foundation at the root. If you had that, then the VP-1 location for the torsion box makes a lot of sense.
 

BBerson

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The wing can be modified with double drag/anti-drag wires fitted. (one cable set on top of the spars and a second set of wires on the bottom of the spars to make a wire box in each bay)

Also, I am unfamiliar with the "VP-1 wing torsion box". Is this a plywood box or what? Does anyone have a plan or photo?
Why would the VP-2 (with the greater span) not also have a torsion box?
 

jedi

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I am not really into this but wouldn't a set of crossed (drag/antidrag tension) wires between the parallel struts solve the problem of the fuselage twisting between the strut supports?

Little weight but more drag on an already slow draggy aircraft.

You may even find you only need the single wire on each side as the loads transfer thru the fuselage.
 
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BBerson

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I am not really into this but wouldn't a set of crossed (drag/antidrag tension) wires between the parallel struts solve the problem of the fuselage twisting
I think it would. Generally, you want triangles not parallel. Like the struts on a Cub.
 

mcrae0104

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I am not really into this but wouldn't a set of crossed (drag/antidrag tension) wires between the parallel struts solve the problem of the fuselage twisting between the strut supports?
No; you want to get all of the torsion reacted into the forward fuselage. In order to do that, you would need a strut capable of compression.

Generally, you want triangles not parallel. Like the struts on a Cub.
The parallel struts are fine (except for the twisting fuselage) if you remember that the struts are a part of a space frame that includes the compression rib and the drag/anti-drag wires.
 

cluttonfred

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Keep in mind that we are not talking about a structural defect here. The VP-2 wing design has worked just fine and caused no issues other than poor aileron response over almost 50 years. So, what then is the easiest, simplest way to increase the torsional stiffness of the wing enough to improve aileron response? That is the real question here.
 

blane.c

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Wouldn't most anything to increase stiffness increase weight, which is another problem with the VP-2? Maybe the best thing to do with the VP-2 is increase lightness? Provided a person remains within the flight envelope intended by Evans what harm is there in a slow response rate to aileron input or in other words who cares?
 
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cluttonfred

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Absolutely, reducing weight is critical to making the VP-2 work, however the aileron response is the one aspect of VP-2 handing that I have heard criticized by many that have flow them. If adding small torsion boxes, some lattice strips, or a narrower-chord aileron (adding a false spar and aileron spar but reducing the balance weight required) would significantly improve that aileron response, I think it would be worth it. What I don't want to do is redesign the whole wing and center fuselage, I am just looking for a small modification for a modest improvement.
 

BBerson

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The parallel struts are fine (except for the twisting fuselage) if you remember that the struts are a part of a space frame that includes the compression rib
The wing/strut space frame on VP-1 is a prism, with three sides that are made of rectangles. These rectangles need a cross brace to be rigid in torsion. The wing panel rectangle is made rigid by the wing drag wires but the strut rectangle has no wires. All three rectangle sides need a cross brace to transfer torsion.
A Cub space frame is a pyramid with only one rectangle.
 

cluttonfred

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The wing/strut space frame on VP-1 is a prism, with three sides that are made of rectangles. These rectangles need a cross brace to be rigid in torsion. The wing panel rectangle is made rigid by the wing drag wires but the strut rectangle has no wires. All three rectangle sides need a cross brace to transfer torsion.
So are you suggesting that aileron response would be improved by adding a wire bracing X between the wing struts on each side? That's an interesting idea but would add four more turnbuckles and complicate cockpit entry quite a bit, plus since the struts are in compression in flight that would tend to slacken the wires.

vp wing brace wires.jpg
 

BBerson

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So are you suggesting that aileron response would be improved by adding a wire bracing X between the wing struts on each side? [/QUOTE
No, it wasn't my suggestion. I simply was agreeing with jedi who suggested it as an idea.
Actually, my suggestion from several posts back, was to use double sets of drag wires inside the wing for torsion. That is, upper and lower wires, as explained in ANC-18 for cantilever wings covered with fabric. Would only need the double wires on a portion of the outboard cantilever section, I think.
 

cluttonfred

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Actually, my suggestion from several posts back, was to use double sets of drag wires inside the wing for torsion. That is, upper and lower wires, as explained in ANC-18 for cantilever wings covered with fabric. Would only need the double wires on a portion of the outboard cantilever section, I think.
I wonder if some light wood X strips top and bottom between the outboard rib bays would accomplish much the same thing? Or maybe light geodetic lattice strips outboard of the stock bracing wires only?

Put a proper wing on it, there are many examples out there. A stiff wing would take care of the fuselage twist issue as well.
It's already got a "proper" wing that has been flying safely for almost 50 years, I am just exploring ideas to address one known weakness of the design.
 

mcrae0104

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The wing/strut space frame on VP-1 is a prism, with three sides that are made of rectangles. These rectangles need a cross brace to be rigid in torsion. The wing panel rectangle is made rigid by the wing drag wires but the strut rectangle has no wires. All three rectangle sides need a cross brace to transfer torsion.
A Cub space frame is a pyramid with only one rectangle.
Look again at the VP-2 structure. It is a complete space frame and does not lack bracing. It resists deflection in the direction of the blue arrows, and consequently, can transfer torsion--if the fuselage is stiff.
scan_1888.jpgscan_1888-2.jpg

So are you suggesting that aileron response would be improved by adding a wire bracing X between the wing struts on each side?
This would not help unless the fuselage were stiffened. Descending aileron -> rising rear spar -> slack cable from lower rear to front upper. You could make that cable a strut instead, and then delete the rear strut.

A stiff wing would take care of the fuselage twist issue as well.
A torsionally stiff wing still needs to be attached to a stiff fuselage unless it is externally braced in some other way. A stiff wing will not stiffen a flexible fuselage.
 

poormansairforce

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It's already got a "proper" wing that has been flying safely for almost 50 years, I am just exploring ideas to address one known weakness of the design.
If it had a proper wing with a D cell, lattice, or sheeting we wouldn't be having this conversation. ;)
Sorry, not trying to offend but the solution seems obvious. Carry on.
 
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