Wren 44 TurboProp Users Manual. Copyright Wren Turbines Ltd February 2008 Page 34
Flying the Wren 44TurboProp
Flying a turbo-prop is very similar to flying a normal gas turbine. There is a slight lag in the
acceleration from idle but higher up the rpm range the pickup is faster and less noticeable. It
would be noticed that the propeller thrust comes in much earlier than on a thrust engine and less
is needed to start a plane rolling for taxiing or manouvers. The 44 Turbo-Prop has an extremely
quick acceleration even from idle and even when turning a large propeller and spinner there will
be very little lag apparent.
Throttle Response Curve
The torque of a turbo-prop increases as a function of the engine case pressure increase. This
occurs as an exponential curve meaning that the response curve steepens more rapidly as the
rpm increases. The result of this is that half power in the form of propeller thrust actually occurs at
about ¾ throttle stick and the remaining throttle comes in very quickly at the high end of the stick
travel. This can make flying the unit very sensitive on the stick as only very small movements can
result in large changes in output power.
Flyers might wish to program a throttle curve to reduce the response at the high end now that this
function is standard on all computer radios. The following data may be useful in deciding a
starting point for a suitable curve.
A typical thrust table is shown below for a 610x200 (24x8) 2-bladed propeller.
Engine rpm
Prop rpm
Thrust
Throttle position
55,000
1300
11N
0%
100,000
2820
25N
32%
120,000
3600
37N
46%
140,000
4230
57N
60%
160,000
5070
85N
75%
180,000
6240
128N
89%
195,000
7110
167N
100%
From the chart can be seen that the thrust roughly doubles from an engine speed of 160,000 to
195,000rpm (85N to 167N) which in throttle position terms is from 75% to 100%.
From this it can be seen that the 50% stick position is likely to only produce about ¼ of the thrust.
This means in practice that most flying tends to happen in the 2
nd
half of the stick and therefore a
lot of the useful resolution of the stick is not used for best effect.
Using this data it should be possible to input a suitable curve which allows for a better power
spread across the range of the stick travel. Ideally the response curve should initially rise steeply
and then flatten down towards the top end of the range.
