Message text written by Henk Roelofs
>Also the touchdown speed seems to be important, high speed somehow
influences the behaviour later in the roll out. Tailwheel first with
a little bit of power seems to be best.
We're also planning to bring the CG further aft. This seems
contradictory but in comparison with another Europa which doesn't
have this problem our CG is more forward.
<
My Europa has not flown yet so I do not have any experience in this area.
However I remember having a long conversation a couple of years ago with
Martin Stoner ( an ex Red Arrows pilot, CFI on Hawks and subsequently in
retirement test pilot and instructor on Europa). He made two comments that
supports the above.
1. Like most taildraggers the Europa is much easier to land if touchdown is
delayed until the aircraft almost stalls onto the ground. This helps to
reduce the risk of bounces and the slightly higher attitude means that
there is weight on the tailwheel straightaway. Risk of loss of directional
control on both take-off and landing is much higher when there is little or
no weight on the tailwheel as all directional control is then reliant on
rudder control (which reduces with speed). This means that directional
control is potentially at its weakest at speeds where the aircraft is too
slow for full rudder authority and too fast to have much weight on the
tailwheel. It is therefore helpful to spend as little time as possible at
these intermediate speeds. On take-off, higher power engines (eg 912S) and
VP or fine set props, ensure a more rapid acceleration. On landing
directional instability is only initiated once the mainwheel has touched
down. If this is delayed to a speed where the tailwheel not only touches
down simultaneously but also has a significant proportion of the weight on
it immediately the potential for loss of directional control is minimised.
Once on the ground full aft stick helps to increase the download on the
tailwheel.
2. As a result of the above comments it is easy to see how an aft C of G is
helpful. Although this slightly reduces overall stability in the air it
more than makes up for it on the ground. Europa have set the aft C of G
limit to give safe handling so there is no reason why we shouldn't trim our
aircraft near to that limit. The risk of ground loops and prop strikes are
much reduced if it is more difficult for the tailwheel to lose traction.
One way of considering this problem is using an analogy from the drag curve
graph (which combines induced and form drag) and substituting a measure of
directional authority on the Y axis (combining tailwheel and rudder
control). The dip in the graph then represents the speed of lowest
directional authority. Moving the C of G aft has the effect of lifting the
bottom of the curve and making the back of the curve shallower. Unlike the
drag curve we wish to stay away from the bottom of the curve as much as
possible.
I emphasise I am no expert in this area but it might stimulate some
thought.
Nigel Charles
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