Wing tie-down kit

On Sep 18, 6:44*pm, Eric Greenwell wrote:
wrote:

JJ - thank a lot for looking inside the wing - you are one of the few
guys here in a position to do it. I know I'm on my own responsibility
on this - but I feel a bit better knowing that there is some decent
structure in there.

Since the spoilers have to survive opening at Vne, I imagine there is
*very* decent structure in there.

I prefer to use a strap that engages both pivots in the spoiler box,
because it spreads the load to each pivot, and has twice as many straps
to spread the load pressing on the wing surface.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes"http://tinyurl.com/y739x4
* * * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" atwww.motorglider.org

Makes sense Eric. Although the air loads at Vne exert slightly
different forces and moments than the tiedown strap - it is still a
lot of tugging and twisting on the pivots. In a 90-degree dive at
terminal velocity the drag equals the weight. I presume a whole lot of
that drag comes from the flat plate drag of the divebrakes. If you
also consider that you have to be able to open the divebrakes at Vne,
that's a pretty big impulse load.

I have some other practical considerations for tying to one pivot -
but I'm not too worried now.

9B

On Sep 18, 8:52*pm, wrote:
On Sep 18, 6:44*pm, Eric Greenwell wrote:



wrote:

JJ - thank a lot for looking inside the wing - you are one of the few
guys here in a position to do it. I know I'm on my own responsibility
on this - but I feel a bit better knowing that there is some decent
structure in there.

Since the spoilers have to survive opening at Vne, I imagine there is
*very* decent structure in there.

I prefer to use a strap that engages both pivots in the spoiler box,
because it spreads the load to each pivot, and has twice as many straps
to spread the load pressing on the wing surface.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes"http://tinyurl.com/y739x4
* * * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" atwww.motorglider.org

Makes sense Eric. Although the air loads at Vne exert slightly
different forces and moments than the tiedown strap - it is still a
lot of tugging and twisting on the pivots. In a 90-degree dive at
terminal velocity the drag equals the weight. I presume a whole lot of
that drag comes from the flat plate drag of the divebrakes. If you
also consider that you have to be able to open the divebrakes at Vne,
that's a pretty big impulse load.

I have some other practical considerations for tying to one pivot -
but I'm not too worried now.

9B

On further reflection, if you think about the forces involved and how
the divebrake mechanism is constructed, I think each of those pivots
has to be built to take several thousand pounds in shear. That's
because the divebrake is cantilevered on about a 10-12" arm down to a
1" tee that is anchored to the walls of the divebrake box. If the drag
on the divebrake is on a 10-12" moment arm down to a 1" perpendicular
T then 1,000 lbs of drag turns into something like 2,500 lbs of shear
per pivot (1,250 upward at the front mount and 1,250 downward at the
aft mount for each of the four pivots - the force is multiplied 10/1
by the lever of the vertical brake arms themselves). This basically
says that in a tiedown application even the strongest webbing will
snap before the divebake pivots pull out.

I'm not 100% sure on the math - but the basic logic feels right.

9B