Two recent incidents have highlighted the dangers of applying the brake
in circumstances that could tip a mono-wheel Europa forward and damage
its propeller.
In the first incident, a mono-wheel Europa with 2 on board and normal
mid-range c of g was run up to full power against the brake on its Rotax
912 with fixed-pitch Warp Drive prop, facing slightly off directly into
a 15 knot wind, with full "up" elevator applied. The aircraft started to
roll forward, and the handling pilot attempted to arrest this movement
by further application of the brake. The aircraft started to tip forward
onto its nose, despite both pilots being certain the stick was hard
back. The prop sustained damage before the aircraft settled back to its
normal ground attitude.
In the second incident, another mono-wheel tipped forward and damaged
the propeller when the aircraft went off line after landing, and the
brake was applied while the aircraft was turning.
The basic cause of the first problem was that the braking force of the
tyre at ground level was opposed by the thrust of the prop and the
inertia of the aircraft trying to keep moving. This inertia force of
deceleration can be considered as acting through the centre of gravity,
which is well above ground level, as is the thrust line. This
combination of horizontal forces provided a strong couple acting to tip
the aircraft forward, which was opposed only by the vertical forces of
aircraft weight (with only a small moment arm by which the cg is behind
the main wheel) and the negative lift on the tail generated by the
combination of slipstream and headwind (with a much larger moment arm,
but a much smaller force).
Once the tail started to lift, the only way to stop it would have been
to throttle back or release the brake, or both.
Note that it is possible with some engine and propeller combinations to
apply sufficient thrust to raise the tail against the brake.
In the second incident, during the turn the sideways force on the main
wheel put some weight onto one outrigger. Since the outriggers are to
the rear of the cg, a much lower braking force would have caused the
aircraft to tip forward over the line joining the contact points of the
main wheel and the outrigger than would have been needed to tip forward
with the aircraft moving straight ahead.
There are several lessons here.
Cg position:
Always be aware of the cg position at each stage of flight or ground
operation.
After installing an expensive VP prop (or carrying out any other
modification to the aircraft) make sure that the weight and balance are
recalculated or re-measured.
Even if the new values are within permissible limits, consider
repositioning equipment such as the battery if the cg is toward the
forward limit -- otherwise you are much more vulnerable to tipping
forward.
Remember that the crew are ahead of the cg, so that with two heavy
people and little baggage extra care should be taken.
The fuel tank is only just behind the cg, so the fuel state will not
have a great effect.
During a run-up:
Always park into wind for maximum elevator effect.
Always leave room in front so that the aircraft could safely roll
forward if the brakes do not hold.
Always ensure the area behind is clear of anything that might be damaged
by the slipstream.
If the tail starts to lift, throttle back immediately.
If the aircraft starts to roll forward, do not attempt to reapply the
brake until you have throttled back.
Be prepared to release the brake and then throttle back if the aircraft
starts to roll forward.
Avoid full-power run-ups against the brakes without a tie-down unless
there is a good reason (eg required as part of the flight test schedule,
but not called for during normal operation except when using mogas and
checking for vapour locks).
When taxying:
Apply sufficient power to get moving, then use minimum throttle to keep
moving.
Throttle back before applying the brake.
(Remember, it is possible for brakes to lock on suddenly).
Take great care when applying the brake during a turn.
On take-off and landing:
(The following advice is repeated from Europa Aircraft News, Issue No
23, Sep 99, p.10)
"With the Monowheel Europa it is very important to keep the aircraft
straight on take-off and landing, to avoid getting into a ground Loop
situation.
"The straighter you keep the aircraft, the less work there is for the
pilot. If things do get more out of shape than is comfortable, DON'T GO
FOR THE BRAKE, but steer out of the problem. Braking the aircraft when
in a turn will only worsen the situation. In fact, at taxying speed, it
is possible to deliberately "ground loop" the Monowheel using the brake,
such that you can turn around in a narrow space. This must be done
carefully as, if you're too fast or the cg is towards the front of the
range, you could still damage the propeller. Better to avoid narrow
spaces.
"The Europa lands at a speed slow enough that you shouldn't be thinking
about touching the brake until the very end of the landing roll, let
alone transferring your hand to the brake lever from the throttle while
still airborne in the flare."
Keep safe
Mike
Europa Club Safety Officer
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