Engine Performance Tuning Tutorial - Page 2
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Thread: Engine Performance Tuning Tutorial

  1. #26

    Flight Performance versus Engine Performance

    This tutorial is about tuning Engine Performance, but the end result is to make our Flight Simulator aircraft perform as the actual aircraft did. There are factors in the simulator that we cannot control and there are also non-linear factors and interactions between parts of the aircraft that are not captured by a purely mathematical model.

    So far we have been discussing Engine Output in Horsepower and I believe we have proven that we can tune that as precisely as we have patience for. One factor that we have not taken into account at all is the Engine's Exhaust Thrust which is shown at the bottom of the Jumo 213A graph.

    This is usually on the order of several hundred pounds of extra thrust added to that of the propeller and is non trivial.

    As an illustration, we can take the case of the Japanese Type Zero Fighter. The A6M3 Model 32 and A6M5 Model 52 have almost exactly the same airframe but for wing tips and minor details. The Engine is the same. The Model 32 uses an exhaust collector manifold while the Model 52 uses ejector stacks for exhaust thrust. The maximum speed of the Model 32 is about 335 mph. The maximum speed of the Model 52 is 350 mph or around 15 mph higher.

    I don't know of a "correct" way to simulate exhaust thrust which has its greatest effect at high speeds.
    The two ways I can see to do this are:
    1. Decrease Airframe Drag which would have its greatest effect at high speeds.
    2. Increase Engine Power.

    In this case I chose to increase Engine Power.

  2. #27

    Matching Level Speed

    Guessing the amount of Engine Power required wasn't difficult. I simply used the MW50 War Emergency Power and throttled back until I was getting a bit over 426 mph. Perhaps my joystick (Sidewider Precision Pro) was acting strangely, but the actual power output seemed to jump around a bit. A touch over 1500 HP seemed to work.

    As a guess based on the previous tests, I used 3.24 for a new Supercharger Boost Gain:
    1511 HP gives 428 mph.

    There are three issues that this test brings up:

    1. I usually try to get 2-3 mph better speed than the documented tests show. The reason for this is because my testing is done with an autopilot while the original test would be done by a pilot with a kneeboard and only holding altitude and direction by hand or adjusting trim. If we Simulator pilots tried the same thing, we would probably also lose a bit of performance because we can't hold as steady as the autopilot can. There also tends to be some variation between aircraft on a production line. We want to give our pilots the best of the lot.

    2. In reality, MW50 would not have done a thing to increase power at this altitude. This is a problem with the simulator and we can't fix it.
    MW50 with the Focke Wulf and Higher Octane Fuel with the Spitfire increase the ALLOWABLE Boost of the Engine. They allow the Engine to use excess supercharger capacity where the throttle limits are because of temperature or detonation. At altitudes where the Engine is already using all of the supercharger's capacity, they obviously can't increase the supercharger's pumping ability.
    GM1 or Nitrous Oxide injection on the other hand is basically a consumable supercharger. It works at any altitude where the supercharger cannot supply enough boost.

    3. Engine WEP and Combat settings are mostly procedural "Paper" restrictions. Except for cases in which there is a limited supply of anti-detonant (Water Injection, Methanol-Water Injection), there isn't anything mechanical other than a shorter operating life to the next overhaul or temperature limits that prevent continuous full power operation. Running out of Water Injection while running manifold pressures that require it will almost certainly kill your engine, but that infamous 5 minutes 10 seconds of WEP in the Mustang before the Engine is crippled is a myth.
    The maintenance people probably don't want to be replacing spark plugs after every flight or pull an engine after three flights for an overhaul, but bringing back a tired engine beats not bringing the engine back at all.


  3. #28

    Testing Engine Power

    Now that we have proper engine power at Critical Altitude and Sea Level, the next step is to see how the entire curve looks.
    I generally test Engine Power at 2500 foot intervals. It is hard to test at Sea Level so the "Sea Level" test is actually at 500 feet.
    Getting these numbers took about 5 minutes.

    Getting the graph into a Spreadsheet took a couple hours.

    The Red Line represents the power output we are trying to achieve.
    Notice the big dip in the Red Line?
    That is the switchover between the Low gear and High gear of the blower.
    The Blue Line is what we are actually achieving.

    The big difference here is that CFS doesn't handle multi-speed superchargers.
    We are always tweaking a single speed supercharger.
    If the numbers match up with an actual multi-speed supercharger at altitude, they will be pretty far off at some intermediate altitude.

    We are around 300 HP or 20% too high at 15000 feet.
    Any CFS Aircraft with a multiple speed supercharger will have this issue. It is just a matter of degree.
    For CFS, the maximum level speed of an aircraft is generally quite predictable: it will be either at the altitude the engine generates maximum power or at the next higher altitude.

    For this FW 190D, we get the following:
    00500 feet ==> 1721 HP - 364 mph
    15000 feet ==> 1971 HP - 437 mph
    17500 feet ==> 1823 HP - 437 mph
    20000 feet ==> 1632 HP - 432 mph
    22500 feet ==> 1454 HP - 426 mph

    Note that we are only a little over 1% off at Sea Level and nearly dead on at critical altitude, but around 10 mph off at medium altitudes.

    A quick Service Ceiling test was also done in the following manner:
    1. Put the aircraft in the simulator at 36500 feet with an expectation that the actual Service Ceiling will be around 37400 feet.
    2. Throttle back to see what the minimum flying speed will be before the aircraft stalls.
    3. Use full throttle to put the aircraft back to about 10 - 15 mph over that stall speed.
    4. Set the autopilot to maintain a 500 feet per minute climb.

    The low speed eliminates the possibility of a zoom climb.
    Gradually the climb rate should drop off.
    Note that altitude at which the aircraft can maintain at least a 100 feet per minute climb.

    I conduct this test with a full internal ammunition load and with at least 50% fuel because weight greatly influences the result.
    The results from this test were the following:
    Service Ceiling: 37100 feet
    Engine Power: 626 HP
    Fuel Remaining: 120 gallons (Full Load is 150 gallons)
    True Air Speed: 218 mph

    I don't know that this is a terribly rigorous way of doing things, but the results are pretty repeatable and it seems to work well enough for me.


  4. #29

    Starting Over Again!

    Now that we have gotten this far, believe it or not, we are about to start over again.
    The difference is that this time we have a VERY good idea of the values we need to achieve the intended flight performance.
    The current results are not bad, but are way too high at medium altitudes such as 12500 feet to 17500 feet.
    Since we are making specified power at 500 feet and still have a few mph too much speed, we can probably afford tune the power below specifications and still have reasonable performance. This should also work to reduce the power at medium altitudes.

    There are three Power @ Altitude combinations we are trying to match
    At 500 feet, we are currently achieving 364 mph on 1721 HP.
    Because speed is proportional to the Cube Root of the power difference, we should be able to achive 360 mph on 1665 HP.

    But wait!
    Kurt Tank's test run was at 300 meters altitude which is 985 feet.
    At that altitude, we are currently going 365 mph on 1730 HP.
    If the speed increase 365 / 360 is proportional to the Cube root of 1730 / (New Power), we can predict we should need 1660 HP at 985 feet.

    At 500 feet, we are looking for 1660 HP to 1665 HP.
    (Yeah, I know the range starts a few HP lower, but I like nice round numbers.)

    At 21650 feet, we are still looking for 1511 HP.
    (Nothing has changed there.)

    We are hitting the Service Ceiling at 37100 feet with around 626 HP.
    We want the Service Ceiling at 37500 feet where we are currently making 607 HP, so....
    At 37500 feet, we are looking for 626 HP.

    With this information, re-adjusting the necessary parameters is easy....


  5. #30

    Re-Tuning the Engine

    Adjusting the Torque down a bit (0.555 to 0.54) gets us
    1661 HP @ 500 feet with a speed of 359 mph

    Adjusting the Supercharger Boost Gain up a bit (3.24 to 3.33) gets us
    1511 HP @ 21650 feet for a speed of 428 mph

    Horsepower was still
    607 HP @ 37500 feet which is a bit below the 626 HP that we need.

    To address this, we need to reduce the Friction Loss and also reduce the Torque multiplier.
    A single adjustment is unlikely to get the values we want:
    1660 HP @ 500 feet
    626 HP @ 37500 feet

    so we see where the values are off, adjust and re-test.
    If the Power at Low altitude is too high, we adjust the Torque down.
    If the Power at High altitude is too low, we adjust the Friction down.
    Eventually the incremental changes get smaller and we get the results we want.
    It took me about 15 adjust and test cycles.
    The end result was the following:
    1663 HP @ 500 feet
    1516 HP @ 21650 feet
    627 HP @ 37500 feet

    A few more cycles and more decimal places could get us closer, but these values are close enough for me.
    Besides, these kinds of repetitive edit and test cycles are BORING!

    I still have not re tried the speed and service ceiling tests, but a couple HP should not make a significant difference.

    Attached are the Table and Graph of updated Engine Power output.

    - Ivan.

  6. #31

    ReTest Complications

    Re Testing the flight performance should have been easy.
    It was.... Up to a point.

    The Speed Run at 500 feet altitude gave 359 mph
    The Speed Run at 21650 feet gave 427 mph with 1516 HP.
    The Highest Speed was 437 mph at 17500 feet

    So far so good....

    The Service Ceiling test was impossible to reproduce reliably:
    One run would give 36900 feet. The next run would give around 38000 feet.
    The Engine Power was as expected, but the power required and speed were not consistent.
    When the Service Ceiling was too low, I even tried (successfully) to boost the engine power from the predicted 626 HP to 660 HP, but each attempt would end differently and unpredicably.

    It finally occurred to me that the wacky looking Coefficient of Lift Graph might be to blame.
    See the attached screenshots.
    I believe that the original P51D CL graph is such that the Autopilot can not reliably detect the Stall.

    I edited the graph a bit at the higher Angles of Attack which should not affect the speed tests.

  7. #32

    Service Ceiling Test

    The end result of the edit to the CL Graph was that the Service Ceiling test worked out pretty much as expected.
    The airspeed is a touch higher, the Power required is a touch lower and the Service Ceiling is less than a hundred feet higher than predicted, but everything is in the "acceptable" range.

    I expect that a test by a human pilot instead of autopilot would Stall the aircraft a couple hundred feet lower because this aeroplane is VERY unstable with so little power at this altitude range.

    - Ivan.

  8. #33

    Tuning Idle RPM

    Generally I prefer the Idle speed to be somewhere between 450 RPM and 600 RPM.
    The easiest way to set this is to used Record 506 to specify what a closed throttle means.
    The original value here was 0.2
    After a couple experiments, 0.11 for 521 RPM seemed to be a reasonable setting.
    I believe adjusting Torque would also work as well.
    Adjusting Friction also would work, but doesn't seem like a good idea.

    - Ivan.

  9. #34

    Propeller Animations

    I like the Propeller(s) on a AF99 / CFS project to behave in the following manner:
    At Idle RPM, the Blades are completely visible
    As RPM is increased, the Blades become Transparent and a Transparent Propeller Disc appears.
    As RPM increases further, the Blades disappear to be replaced by a Propeller Blur (Transparent Triangles)
    As Full RPM is reached, the Propeller Blur slowly rotates in the direction of Propeller Rotation.

    I found a while back that this is apparently controlled by the Maximum and Minimum Governed RPM.
    I have found no other effect from changing these parameters.

    In this case, with a little experimentation, I found that
    Maximum Governed RPM == 3350
    Minimum Governed RPM == 1400
    seems to work though with the Propeller Blur moving a touch too fast.

    And so concludes the Engine Performance Tuning Tutorial.
    Good Night.
    - Ivan.

  10. #35

    Test Flight

    Hello All,

    Attached is a copy of the "FW 190D" Test Aircraft that resulted from the basic P-51D after modifications.
    Note that I also edited the Horsepower from down to 1500 so that the sound effect will be full power at the critical altitude.

    I am including this so that folks who have been following along can confirm that these modifications do work as explained.
    Note that this is NOT a completed FW 190D.
    The AIR file does not reflect anything other than the Engine modifications and other changes needed for Engine Testing.
    The visual model is also an obvious mismatch.
    The DP file should be fairly close
    The Panel has been converted to use the stock FW 190A because I do not know how others may have configured their Test Panels.

    - Ivan.

  11. #36

    Exhaust Thrust - Version 2

    The problem with doing a tutorial and the reason I am generally quite reluctant to actually do one is because they are generally monologues with no sanity checks of any kind. They hopefully represent the best information the author has at the time. Very seldom do more knowledgeable people critique or comment when the information being presented is outright incorrect.

    Occasionally, we learn new things as time goes on.

    This tutorial is no exception.

    Regarding ways to simulate Exhaust Thrust, I had listed the following:
    1. Decrease Airframe Drag.
    2. Increase Engine Power.

    A third idea seems at the moment to be better than the other two:

    3. Increase Propeller Efficiency at high speed (Higher Advance Ratios)

    This appears at the moment to have the fewest side effects on flight performance.

    - Ivan.

  12. #37

    More Supercharger Tuning

    Up until a week or so ago, I was certain that the implementation of Superchargers in Combat Flight Simulator was incorrect.

    There are two factors in the implementation that appeared to be less than ideal:

    The first is that only Single Speed Superchargers are simulated.
    With a multi-speed supercharger, there are shift points.
    With a two speed supercharger, the Low gear is engaged at low altitude.
    At sea level, it supplies all the boost that the engine is capable of handling.
    As altitude increases, the Low speed supercharger is less able to provide full boost and power drops....
    ....Until at a predetermined altitude, the supercharger shifts to High gear.
    At that predetermined altitude, the High blower is able to supply all the boost that the engine is capable of handling.
    At lower altitudes it could not be used because it would have over boosted and possibly damaged the engine.

    The second is the issue of War Emergency Power.
    Often a anti-detonant is used to reduce combustion temperatures and allow more boost to be used than the engine can normally tolerate.
    Sometimes, no anti-detonant or power adder is used at all and additional boost can be used for a very short time.
    In general, it is excess heat that cannot be removed from the engine that causes damage or causes detonation that causes damage.
    It takes a certain amount of time for the heat to build up and before that happens, hopefully no lasting damage is done.
    In reality, this has a tendency to reduce the engine's time to the next overhaul and may even require an inspection to be sure there was no damage done.

    Above the engine's critical altitude, using anti-detonant WEP makes no difference because there is no additional supercharger capacity that is available.
    Some aircraft have a throttle lever restriction or gate which limits full throttle movement.
    The last bit of throttle travel is considered to be WEP.
    To use it may require that the throttle lever break through a safety wire.
    With this type of throttle, that last bit of throttle travel should increase power regardless of the altitude (because the critical altitude is really a manifold pressure limitation).

    In working with Aleatorylamp on his Martin Baltimore, his testing showed some odd effects on supercharger performance.
    This AIR file parameter:

    WEP Pressure Change Rate (0.528 or Zero)

    seemed to reduce WEP effects above "critical altitude". (Critical Altitude in the AIR file is not as obvious as it might appear.)

    I did some detailed testing with a P-40N that is in development and found that this number is really a modifier to

    SuperCharger Boost Gain

    when WEP is engaged.

    If this number is Zero, then WEP adds no power above critical altitude.
    If the number is greater than Zero, it is added to SuperCharger Boost Gain to calculate the NEW critical altitude.

    This was very misleading because no value other than 0.528 or Zero is ever seen in a stock AIR file.
    If the stock AIR file has working WEP, the value is 0.528. If it does not have WEP, it is 0.000.

    - Ivan.

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