Hi Area 51
Yes you turn off the power to the TCU to lock the waste-gate servo in whatever
position it happens to be in. There is a series normally closed switch in series
with the waste-gate that is a momentary switch, it's used to reset TCU if boost
pressure is rising and dropping. Maximum off of switch is 5 seconds, see
page 4-3.
As far as idle airbox pressure, looking at figure 6 on page 7-10, Rotax by using
the throttle position sensor, closes the wastegate between 0 and 10% of throttle
movement (they are using 115% movement). It's not just the cracking of butterfly
off of idle, it's the TCU opening the wastegate when the lever is at 10%.
10% equates to just about 1/2" in the throttle slot. I'm thinking perhaps
the reason Rotax closes the wastegate, is to prevent carb ice if the airbox was
at ambient pressure with butterfly very closed??? Then looking at figure 6,
they open the wastegate from 10% to a little under 50%.
You were not comparing apples with apples when you said the 914 can only make
74hp
at 5,000 rpm. You were probably looking at performance data for CS prop page
5-4. That shows a 75% (that is of 100%) setting of 31" and 5,000 rpm. 31" is
not the maxium MF pressure, it just happens to be what is required for 75%.
If you look at Manifold Pressure graph page 5-2 and Performance graph for
standard
conditions page 5-2, you will see that 100% continuous manifold pressure
and 5,00 rpm horse power is just under 90hp. Using 115% manifold pressure and
5,000 rpm, horse power is just under 100hp.
For a D)178 B take off If the TCU seems to be working, the manual has you have
the TCU on, set 115% power, then turn off TCU. If the TCU isn't working
properly,
personally I would just leave the wastegate opened till I could fix it, but
if I really needed to have some extra power, close wastegate then turn off TCU
and CAREFULLY add throttle and take off, then open wastegate and turn off TCU.
Ron
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