Roy Holmes
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Part 1. Start up issues.
Just about everyone complains about the awful starting behaviour of the sim Turboprop engine model.
Tales of airplanes tearing across the countryside because the parking brake was off on start are legion.
Noble efforts have been made to fix the problem, most noticeably by Heretic controlling the corrected fuel flow.
Last February I started working on the FDE for Dino Cattaneo's C-2 Greyhound. Since it is a carrier borne airplane it is more than slightly important that it behaves on start-up.
The airplane has two big turboprops with 8- bladed props. The engines are Allison T56-A-425 turboprops with 4,600 shp each.
Static Prop thrust is 800 lbs at Low speed Ground Idle ,1800 lbs at normal ground idle and 5000 lbs at full power.
So the first and biggest challenge was how to lose that 1000 lbs prop thrust by selecting Low Speed Ground Idle (LSGI).
Initially I adopted Heretic's approach of writing to Corrected Fuel Flow. I used XML Vars to set it to 330 lbs/hr though the actual value was 344.
That gave me idle prop thrust of 650 lbs at zero throttle, but opening the PLA to 33% kept the same fuel flow and increased the prop thrust to 1100, just enough to get the airplane to taxi.
This is where the second problem bites you. As soon as the airplane moves, prop thrust doubles and off you go.
The reason that happens is complex but very interesting and takes a while to explain, so I'll leave that for Part 2 in this post. Hint it, has to do with the prop tables records 511 and 512.
With the fuel flow method I could not match the PLA effects on prop thrust and Dino did not want people to have to install anything other than the airplane, so I decided to control things with mixture settings if I could. So I started on a long series of engine tests varying the mixture and checking its effects on:
N1, N2, Prop thrust, Fuel flow, Prop RPM, PBeta (prop pitch angle), Prop setting percent max torque and SHP.
What I found was that you needed 2.4% mixture to maintain ignition, below 5.4% the Engine held idle and above that value all the power indications began to rise.
With the PLA shut the power increased with mixture increase up to 100%. Above 90% the Prop RPM stabilized at 1105 which was the controlled 100% value.
I also set the propellor pitch control to 60%
Using Mixture and 60% prop pitch I was able to match the manual static prop thrust in LSGI and with LGSI off on take-off and in flight.. The airplane has two LSGI switches on the center instrument panel that affect (L:LOW SPEED GND IDLE LEFT,number) and right.
The code used was in an update section as follows:
<Update>
<Frequency>18</Frequency>
(L:LOW SPEED GND IDLE LEFT,number) 1 ==
if{ 9829 (>K
ROP_PITCH1_SET) 950 (>K:MIXTURE1_SET) }
els{ 16383 (>KROP_PITCH1_SET) 16383 (>K:MIXTURE1_SET) }
(L:LOW SPEED GND IDLE RIGHT,number) 1 ==
if{ 9829 (>KROP_PITCH2_SET) 950 (>K:MIXTURE2_SET) }
els{ 16383 (>KROP_PITCH2_SET) 16383 (>K:MIXTURE2_SET) }
</Update>
With LSGI ON the start behaviour is as follows:
N1 peaks at 80% at 12 seconds after start begins
N1 has a stable idle at 65% and 18 seconds
Peak torque is 50 with 8 at idle
Peak prop thrust is 2400.
Idle prop thrust is 340
Idle prop RPM is 959 which means it is not in constant speed mode, the constant speed algorithm is inactive
With LSGI switched OFF
N1 peaks at 95% at 12 seconds after start begins
N1 has a stable idle at 70% and 30 seconds
Peak torque is 80 with 20 at idle
Peak prop thrust is 2900.
Idle prop thrust is 830
Idle prop RPM is 1105 which means the constant speed mode algorithm is actve, which causes rev instability
In general the effect is a much more controlled start with idle achieved in about half the time. It is done using the prop pitch and mixture controls.
Mixture in this context is variable high pressure fuel feed. It is a bit like what you had to do to start the Meteor Derwent engines, open the HP cock half way and slowly feed it forward as the engine lights.
I checked on the fuel flow gain influence and found the following.
No stated fuel flow gain gives no engine ignition.
I could not see a measurable difference between a single value and the multiple values recently discussed in the forum. That is not to say the multiple values are not worth considering, just that I did not see a difference.
Part 2. The constant speed issue.
Most of the wild overreactions in terms of power in the sim turboprops come from the issue of constant speed or rather, lack thereof.
To better understand this issue it is first necessary to understand a bit about propellors.
The function of a propellor is to convert engine power into prop thrust.
For a given situation with a certain amount of input power, the propellor will turn at a certain RPM. The power required to attain that RPM is absorbed by the propellor in driving air backwards, creating thrust. The amount of air driven is a function of several things but the main one is the pitch angle of the blade. If the blade angle is increased it takes more power to drive it at a set RPM and it will tend to slow down. Conversely if the blade angle is decreased it takes less power and the prop will speed up. A constant speed mechanism adjusts the blade angle until the prop is turning at the desired RPM.
There are theoretical tables of propellor power coefficients where you can enter with the blade angle and Advance ratio and get the power coefficient required. The sim does just that in table 512 and gets the power required which is then applied to table 511 to come up with the blade angle, and so on keeping the RPM constant.
Now I have simplified that because we have to consider the advance ratio and I skipped over that bit. Advance ratio is essentially how far the propellor moves through the air compared to how far it would move just considering its blade angle alone. The last case could be considered 100% efficiency and the advance ratio is less than that. Advance ratio (called J) is the airplane velocity in ft/second divided by the prop diameter in feet times prop revs/second
Thrust = (Engine Power/velocity) * efficiency. which means that efficiency = thrust/engine power* velocity. The problem with all this is that when the airplane is static advance ratio is zero as is propellor efficiency, so the nice constant speed equations fail. However when static, like on start up, the surge puts the prop equations in a situation when the constant speed tries to work and that accounts for much of the unsteadiness.
Worse still, when you have everything finally steadied and you go to taxi, velocity is no longer zero and efficiency comes back from zero and a surge occurs that send you tearing down the taxiway.
Controlling the mixture by and large keeps the prop at its pitch stop which helps reduce surges.
If anyone finds this interesting, I can expand it in a lot more detail with test results and stick it in the Wiki.
Please let me know
Roy
Just about everyone complains about the awful starting behaviour of the sim Turboprop engine model.
Tales of airplanes tearing across the countryside because the parking brake was off on start are legion.
Noble efforts have been made to fix the problem, most noticeably by Heretic controlling the corrected fuel flow.
Last February I started working on the FDE for Dino Cattaneo's C-2 Greyhound. Since it is a carrier borne airplane it is more than slightly important that it behaves on start-up.
The airplane has two big turboprops with 8- bladed props. The engines are Allison T56-A-425 turboprops with 4,600 shp each.
Static Prop thrust is 800 lbs at Low speed Ground Idle ,1800 lbs at normal ground idle and 5000 lbs at full power.
So the first and biggest challenge was how to lose that 1000 lbs prop thrust by selecting Low Speed Ground Idle (LSGI).
Initially I adopted Heretic's approach of writing to Corrected Fuel Flow. I used XML Vars to set it to 330 lbs/hr though the actual value was 344.
That gave me idle prop thrust of 650 lbs at zero throttle, but opening the PLA to 33% kept the same fuel flow and increased the prop thrust to 1100, just enough to get the airplane to taxi.
This is where the second problem bites you. As soon as the airplane moves, prop thrust doubles and off you go.
The reason that happens is complex but very interesting and takes a while to explain, so I'll leave that for Part 2 in this post. Hint it, has to do with the prop tables records 511 and 512.
With the fuel flow method I could not match the PLA effects on prop thrust and Dino did not want people to have to install anything other than the airplane, so I decided to control things with mixture settings if I could. So I started on a long series of engine tests varying the mixture and checking its effects on:
N1, N2, Prop thrust, Fuel flow, Prop RPM, PBeta (prop pitch angle), Prop setting percent max torque and SHP.
What I found was that you needed 2.4% mixture to maintain ignition, below 5.4% the Engine held idle and above that value all the power indications began to rise.
With the PLA shut the power increased with mixture increase up to 100%. Above 90% the Prop RPM stabilized at 1105 which was the controlled 100% value.
I also set the propellor pitch control to 60%
Using Mixture and 60% prop pitch I was able to match the manual static prop thrust in LSGI and with LGSI off on take-off and in flight.. The airplane has two LSGI switches on the center instrument panel that affect (L:LOW SPEED GND IDLE LEFT,number) and right.
The code used was in an update section as follows:
<Update>
<Frequency>18</Frequency>
(L:LOW SPEED GND IDLE LEFT,number) 1 ==
if{ 9829 (>K
ROP_PITCH1_SET) 950 (>K:MIXTURE1_SET) } els{ 16383 (>K
ROP_PITCH1_SET) 16383 (>K:MIXTURE1_SET) }(L:LOW SPEED GND IDLE RIGHT,number) 1 ==
if{ 9829 (>K
ROP_PITCH2_SET) 950 (>K:MIXTURE2_SET) } els{ 16383 (>K
ROP_PITCH2_SET) 16383 (>K:MIXTURE2_SET) }</Update>
With LSGI ON the start behaviour is as follows:
N1 peaks at 80% at 12 seconds after start begins
N1 has a stable idle at 65% and 18 seconds
Peak torque is 50 with 8 at idle
Peak prop thrust is 2400.
Idle prop thrust is 340
Idle prop RPM is 959 which means it is not in constant speed mode, the constant speed algorithm is inactive
With LSGI switched OFF
N1 peaks at 95% at 12 seconds after start begins
N1 has a stable idle at 70% and 30 seconds
Peak torque is 80 with 20 at idle
Peak prop thrust is 2900.
Idle prop thrust is 830
Idle prop RPM is 1105 which means the constant speed mode algorithm is actve, which causes rev instability
In general the effect is a much more controlled start with idle achieved in about half the time. It is done using the prop pitch and mixture controls.
Mixture in this context is variable high pressure fuel feed. It is a bit like what you had to do to start the Meteor Derwent engines, open the HP cock half way and slowly feed it forward as the engine lights.
I checked on the fuel flow gain influence and found the following.
No stated fuel flow gain gives no engine ignition.
I could not see a measurable difference between a single value and the multiple values recently discussed in the forum. That is not to say the multiple values are not worth considering, just that I did not see a difference.
Part 2. The constant speed issue.
Most of the wild overreactions in terms of power in the sim turboprops come from the issue of constant speed or rather, lack thereof.
To better understand this issue it is first necessary to understand a bit about propellors.
The function of a propellor is to convert engine power into prop thrust.
For a given situation with a certain amount of input power, the propellor will turn at a certain RPM. The power required to attain that RPM is absorbed by the propellor in driving air backwards, creating thrust. The amount of air driven is a function of several things but the main one is the pitch angle of the blade. If the blade angle is increased it takes more power to drive it at a set RPM and it will tend to slow down. Conversely if the blade angle is decreased it takes less power and the prop will speed up. A constant speed mechanism adjusts the blade angle until the prop is turning at the desired RPM.
There are theoretical tables of propellor power coefficients where you can enter with the blade angle and Advance ratio and get the power coefficient required. The sim does just that in table 512 and gets the power required which is then applied to table 511 to come up with the blade angle, and so on keeping the RPM constant.
Now I have simplified that because we have to consider the advance ratio and I skipped over that bit. Advance ratio is essentially how far the propellor moves through the air compared to how far it would move just considering its blade angle alone. The last case could be considered 100% efficiency and the advance ratio is less than that. Advance ratio (called J) is the airplane velocity in ft/second divided by the prop diameter in feet times prop revs/second
Thrust = (Engine Power/velocity) * efficiency. which means that efficiency = thrust/engine power* velocity. The problem with all this is that when the airplane is static advance ratio is zero as is propellor efficiency, so the nice constant speed equations fail. However when static, like on start up, the surge puts the prop equations in a situation when the constant speed tries to work and that accounts for much of the unsteadiness.
Worse still, when you have everything finally steadied and you go to taxi, velocity is no longer zero and efficiency comes back from zero and a surge occurs that send you tearing down the taxiway.
Controlling the mixture by and large keeps the prop at its pitch stop which helps reduce surges.
If anyone finds this interesting, I can expand it in a lot more detail with test results and stick it in the Wiki.
Please let me know
Roy
