>>
>>Daydreaming some more.....
>Modifying my original suggestion, more effective than a "ramp bracket"
would
>be a pivoting, slightly over-center lever with a small bucket-like end to
>capture the end of the gas strut and unload the door as it's closed. This
>would get rid of any friction of sliding up the 'ramp bracket' when
closing
>the door, and would actually pull the top corner of the door inwards
>slightly when fully closed (due to being over-center). The lever would
need
>to be lightly spring loaded to position it outwards to receive the end of
>the gas strut when the door is open.
>
>I could sketch it out if need be to explain a little more clearly.
>
>Cheers,
>Pete A239
>>
We will be shortly approaching the time to fit our gas strut and have been
watching the comments re this with interest. We thought the above an
excellent idea and it was our favourite until we studied the geometry of
the door hinge & strut pivots. It would appear to us that the direct
line of force exerted by the strut when the door is closed is almost
exactly through the hinge pin and, although there is a slight twisting
moment on the door frame due to the length of the lug the strut is attached
to on the door, there is almost no vertical bending component on the door
which would tend to lift the rear as has been reported. When the door is
held in the open opsition, however, there is an enormous force bending the
door frame. Even although the strut has reached the end of its travel it
is still supporting the door on a very small moment arm. One only has to
pop the end of the strut off its ball joint at the door end and try to
support the door in the open position by pushing on the exposed ball with
your thumb to appreciate the force involved. By alternately holding the
door up with your left hand on the door handle and pushing on the pin with
your left thumb you can see the bend occurring in the door. From this we
are suggesting that the bend is being induced only when the door is in the
open position and, as it gradually takes up this set, it becomes more and
more visible when the door is closed.
We have now given this problem some thought and our solution (which is only
in the mockup phase at the moment made out of 2 lengths of scrap 1" X 3/4"
aluminium angle ) is to use a simple folding lever strut, as has already
been suggested, but to incorporate the gas strut within it. The gas
strut is pivoted on the lower pivot point of the folding lever on the
fuselage and the other end of the gas strut is pivoted about 1 1/2 " from
the folding point of the levers and on the top lever. The top lever is
then pivoted on the door about 1/3 way round its curve.
This method gives the following advantages:-
1. The lever pivot points are a reasonable distance from the door hinge
line and therefore only exert a low force to hold the door open.
2. The line of force exerted by the lever is almost in the same plain
as the door centre of gravity.
3. The gas strut is totally contained within the folding levers and
exerts NO FORCE IN ANY DIRECTION on the door or fuselage when the door is
in the closed position.
4. The gas strut offers the maximum lifting force when the levers are at
right angles to one another and this occurs when the door is about half way
up.
5. The force produced by the levers lifting the door gradually decreases
---From the half way point upwards just as the force required to lift the
door decreases as it approaches the fully open position.
6. The levers are designed to reach their fully open position at 150
degrees and are held in this position by the force in the gas strut,
therefore there is no necessity to unlatch the levers to close the door nor
is there any possibility of a gust of wind causing the door to crash down
as would be happen with a simple, over centre, lever strut.. All that is
required is to reach up and pull down the door.
7. The damping action built into the gas strut slows the opening of the
door and, as it is expanding twice as fast as the door is opening, its
effect is even more evident.
8. As the force produced by the strut reduces to zero as the door is
closed there are none of the distorting forces which prevent the rear of
the door from seating properly with the possibility of the rear shoot bolt
not engaging. (this has resulted in the door being sucked off in flight
on at least two occasions - - one of which I videoed)
With the desire to utilize the existing gas strut the resulting lever
mechanism is rather large and it is a tight squeeze in the recess in the
door aperture but it looks as if it could be made to fit. The ideal
solution would be to scrap the existing gas struts and purchase the next
smaller size. This would allow greater leeway in positioning the pivot
points to produce the greatest mechanical advantage and possibly allow some
of the gas to be released from the strut by means of the Schroeder valve in
its base to fine tune the balance for the door.
We will shortly be putting some pictures of the mockup strut on our web
page.
Anything we have overlooked ?
A Guid New Year to One & All
---From
Ted & Justin
G-ZTED
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