Tweet
Hi All,
I find the drag and anti-drag wire routing issue to be an interesting one.
I think that I state correctly that the angle of attack at the wing tips aught to be lower than that of the wing root so that the root stalls first and the wing tips stall afterwards resulting in a more stable stall.
More stable meaning that the nose tends to drop while there remains a bit of wing tip lift on each wing thus reducing the amount of roll that takes place.
I assume that I am correct so far, advise as needed please.
The change in angle of attack(AA) from a greater angle at the root to a lesser angle at the tip is called wash out? I get that that term confused it does not have a physical meaning that I can get glued to my brain.
But the physical result is that the rear spar has to be a bit higher at the tip than at the root.
I did some searching, not much, and find that 2 degree AA reduction at the tip is considered reasonable.
These old planes have a front and rear spar that are separated by about 30" and the length of the spars is about 15' (approx.).
Assuming that the 2 degree AA change is true and that the spars are in the same geometric plane at the root then the rear spar must raise up at the tip by about 1-1/16" ** more than the root to form that 2 degree angle.
**1.05"=30" x sin(2 degrees)
The amount that the rear spar raises up is linear. That is it raises smoothly along it's lenght and if you think about it in terms of how much it raises per inch of run along the rear spar it is; 1.05"/(15feet x 12"/foot)= .006"/inch
The last bay's drag wire will be 1.05" higher at the rear spare than the at the front possible causing the wire push up against the anti-drag wire if the drag wire happens to be below the anti-drag wire.
However the amount of possible inference between wires is not the full 1.05" but rather only a portion of that determined by how far apart they contact the rear spar.
The bay's anti-drag wire connects to the rear spar at a point further inboard of the drag wire and that point on the rear spar is also raised with respect to the front spar but not by as much as the point where the drag wire contacts the rear spar. The difference those two heights is what can cause the possible interference.
To find out how much that difference in height is for any drag/anit-drag wire set find the distance between the points of contact (in inches) at the rear spar and multiply by .006"/inch (see above). The long bay at a T-craft wing outboard end is about 60".
So the diff in heights of the wires at the rear spar 60" x .006"/inch = .36". However if the wires interfere they do so half way between the front and rear spar resulting in half that interference or .18".
I have an uncovered wing on the wing table so I put a digital protractor on the most inboard compression strut and set the protractor to reference that angle. I moved the protractor to the outermost compression strut, unbolted one spar at the lift strut fitting then twisted the wing to get a -2 degree reading as well as +2 degree. reading.
I couldn't see any significant change in the wires look or feel. But I put a small piece of rubber hose on my wires then tape the hose in place so that the hoses are in contact and firmly taped together at the cross point.
The wing on my table is a Champ wing and it's biggest bay is about 40" so it gets a lesser interference.
It is the smart thing to do to put the drag wire on top so that interference is zero. However given the anti chafe used and the slight amount of interference I am not sure it really makes enough difference undo a lot of stuff to change it if you have it "wrong".
Anyhow check out my math and reasoning. Does it makes sense to you?
I find the drag and anti-drag wire routing issue to be an interesting one.
I think that I state correctly that the angle of attack at the wing tips aught to be lower than that of the wing root so that the root stalls first and the wing tips stall afterwards resulting in a more stable stall.
More stable meaning that the nose tends to drop while there remains a bit of wing tip lift on each wing thus reducing the amount of roll that takes place.
I assume that I am correct so far, advise as needed please.
The change in angle of attack(AA) from a greater angle at the root to a lesser angle at the tip is called wash out? I get that that term confused it does not have a physical meaning that I can get glued to my brain.
But the physical result is that the rear spar has to be a bit higher at the tip than at the root.
I did some searching, not much, and find that 2 degree AA reduction at the tip is considered reasonable.
These old planes have a front and rear spar that are separated by about 30" and the length of the spars is about 15' (approx.).
Assuming that the 2 degree AA change is true and that the spars are in the same geometric plane at the root then the rear spar must raise up at the tip by about 1-1/16" ** more than the root to form that 2 degree angle.
**1.05"=30" x sin(2 degrees)
The amount that the rear spar raises up is linear. That is it raises smoothly along it's lenght and if you think about it in terms of how much it raises per inch of run along the rear spar it is; 1.05"/(15feet x 12"/foot)= .006"/inch
The last bay's drag wire will be 1.05" higher at the rear spare than the at the front possible causing the wire push up against the anti-drag wire if the drag wire happens to be below the anti-drag wire.
However the amount of possible inference between wires is not the full 1.05" but rather only a portion of that determined by how far apart they contact the rear spar.
The bay's anti-drag wire connects to the rear spar at a point further inboard of the drag wire and that point on the rear spar is also raised with respect to the front spar but not by as much as the point where the drag wire contacts the rear spar. The difference those two heights is what can cause the possible interference.
To find out how much that difference in height is for any drag/anit-drag wire set find the distance between the points of contact (in inches) at the rear spar and multiply by .006"/inch (see above). The long bay at a T-craft wing outboard end is about 60".
So the diff in heights of the wires at the rear spar 60" x .006"/inch = .36". However if the wires interfere they do so half way between the front and rear spar resulting in half that interference or .18".
I have an uncovered wing on the wing table so I put a digital protractor on the most inboard compression strut and set the protractor to reference that angle. I moved the protractor to the outermost compression strut, unbolted one spar at the lift strut fitting then twisted the wing to get a -2 degree reading as well as +2 degree. reading.
I couldn't see any significant change in the wires look or feel. But I put a small piece of rubber hose on my wires then tape the hose in place so that the hoses are in contact and firmly taped together at the cross point.
The wing on my table is a Champ wing and it's biggest bay is about 40" so it gets a lesser interference.
It is the smart thing to do to put the drag wire on top so that interference is zero. However given the anti chafe used and the slight amount of interference I am not sure it really makes enough difference undo a lot of stuff to change it if you have it "wrong".
Anyhow check out my math and reasoning. Does it makes sense to you?
Comment