Warhawk

[Ivan]

New Propeller - First Attempt

As some folks may already know, a Propeller for Combat Flight Simulator is really composed of two Records:
Record 511 - Propeller Efficiency
and
Record 512 - Propeller Power Coefficient

In my opinion, just about any reasonable Propeller Efficiency Table is a suitable starting point as long as it doesn't have the "Perpetual Motion" curves AND as long as the Propeller Power Coefficient Table matches it.

For a starting point, I decided to use the original Record 511 with minimal but essential modifications.
The efficiency curve for each blade angle now drops to Zero when the Advance Ratio is high enough that the blade cannot maintain a positive AoA. Note that there is also a bit of an allowance for camber and also for the problem that 0.2 is a pretty large change in Advance Ratio but is the smallest increment we can represent in the table and still have a reasonably wide range of speeds.

To cover the practical range of speeds that the P-40 was capable of, the maximum Advance Ratio was extended to 2.8 which at 3000 RPM would be the equivalent of 525 MPH. This would be pretty close to the maximum speed expected in a power dive.

Note that the stock P-51D Record 511 only goes to J=2.2 or the equivalent of 402.6 MPH which doesn't even cover its entire level speed range much less its potential speed in a dive. This is actually a neat way to cheat. At the upper end of the speed range, Propeller Efficiency tends to drop very quickly but since the simulator uses the value for J=2.2 if J is above 2.2, your propeller would be providing much more thrust than it should.

The added columns for 2.4, 2.6, and 2.8 are what I believe are reasonable continuations of the curves.
If there are any mistakes, they should not affect the first round of testing which was at or below J=2.0 (375 MPH).

The original Record 512 had too many odd curves so I decided to use a replacement that I generated using a spreadsheet I wrote for the purpose a couple years ago. The curves are smooth because they are generated by parameters fed into a formula.
The highlighted cells show the places where I tried to match the original Power Coefficient curves.

Test Results to Follow.

- Ivan.

 

[Ivan]

New Propeller - Mod1 Testing

The updated AIR file loaded into CFS without problems.
That was a good sign. Formatting errors in the past have caused load failures.
Engine started - Another good sign.
Idle Speed 496 RPM - Unchanged from original which made me suspect I might be using the wrong AIR file.
Sea Level Military Power (1210 HP)
312 MPH - same.
Propeller Pitch 35 Degrees - same
1230 Pounds Thrust - Higher! - Original was 1225 Pounds.

Sea Level War Emergency Power (1358 HP)
322 MPH - same.
Propeller Pitch 37 Degrees - same
1302 Pounds Thrust - same.....

10,000 feet War Emergency Power (1479 HP)
371 MPH - same.
Propeller Pitch 41 Degrees - same
1333 Pounds Thrust - Original value unknown, I didn't write it down.

12,500 feet Military Power (1351 HP)
372 MPH - Higher. Original was 370 MPH
Propeller Pitch 41 Degrees - same
1242 Pounds Thrust - Higher. Original was 1219 Pounds.

For a first attempt, this was a LOT closer than I was expecting.
I believe the slight differences are due to slight differences in propeller pitch (less than a degree) and my tuning of the Power Coefficient Table was done by eyeball. There are also limitations with trying to match the odd curves of the original tables (not that I want to) using math formulas.

The Efficiency Table definitely needs to have some of the mismatched curves smoothed out and adjusting the efficiency down slightly to reproduce the same maximum speed is pretty trivial.
The question is whether or not the adjustment is really necessary.....

- Ivan.

 

[Ivan]

New Propeller - Mod4 Testing

Yesterday, I had to work with my Son on a writing assignment and didn't get much time with the simulator until fairly late in the evening.

As can be seen by the title of this post, there have been quite a few changes.
I was already pretty close, so the first set of changes (Mod2) was just to make some basic corrections to both Tables.
These changes were simple things such as smoothing out the curves where there were obvious continuity problems and adjusting the efficiency numbers at 0.0 Advance Ratio. (In theory this should be zero but that isn't how CFS works.)
The Power Coefficient Table was also adjusted at the coarse pitch angles to be closer to the original even though it should have no effect at the power levels and speeds reached by the P-40N.

There was quite a bit of flight testing especially for level acceleration during which I found that the low level maximum speed was achieved just past one of the dips in the Propeller Efficiency curve (caused by interpolating between 30 degrees and 35 degrees).
This created an interesting situation which I entirely missed in my previous test runs because I had been using War Emergency Power to get close to the maximum speed before switching back to Military Power to allow speed to climb the last couple MPH.
The WEP was pushing past the dip. Without WEP, maximum speed was barely above 300 MPH - Sometimes....

Why SOMETIMES?
My testing protocol is to record a speed as "maximum" if it has not increased by 1 MPH in 15 seconds.
The timing is done manually so I may be a couple seconds off at times.
Let's say that the acceleration slows at 304 MPH so that to reach 305 MPH SHOULD take 14 seconds.
If my timing is fast, I record it as 304 MPH and quit.
If my timing was a little slow, I might accept it as 305 MPH and then see how long it took to reach 306 MPH.
At certain parts of the Efficiency curve, the acceleration might actually increase and not slow down again until overcome by drag at 312 MPH.

Sea Level Military Power (1210 HP)
313 MPH - Higher
Propeller Pitch 34 Degrees - Finer Pitch
1235 Pounds Thrust - Higher! - Original was 1225 Pounds.

12,500 feet Military Power (1351 HP)
370 MPH - Same.... Back down to original speed
Propeller Pitch 41 Degrees - same
1222 Pounds Thrust - Higher. Original was 1219 Pounds. This is about as close as it gets.

In reality it took more than just three more versions of the Propeller Tables.
It was more like three major revisions and a dozen minor revisions for fine tuning.
I also found out that one of my tools was very unreliable and needs a bit more programming work.

Next comes the Climb and Service Ceiling Tests which are sure to be followed by further revisions.

- Ivan.

 

[Ivan]

New Propeller - Mod 5 Testing

This update really should be described as testing of Mod 5 version 65 or something along those lines.
There have been enough little changes that I haven't bothered to keep an accurate count.
The week has been very busy but most of it has not been related to flight simulators. Real life intrudes sometimes.

The Climb Test was pretty much a complete failure.
The Propeller would select way too coarse a pitch and climb rate was several hundred feet per minute too low.
My original intent was to hold the two endpoints of each curve in place and only adjust the curve as needed but that was not enough to come up with a working set of graphs for Table 512.

Altering the endpoint at Zero Advance Ratio seemed to be the best idea; I could significantly improve the shape of the curves and all that would happen is that the Idle speed would change.
This was done and the Climb Rate was greatly improved. Idle speed went from 496 RPM to 417 RPM.

The minor adjustments of the curves to improve Climb also seemed to make the Speed at 500 feet a bit worse.
The maximum was still 312 MPH most of the time, but Acceleration felt slow. This was not surprising because the Propeller Pitch only reached 33-34 degrees which coincided with where both the 30 degree and 35 degree curves dropped off just past J=1.6.
Adjusting the curves at J=1.6 didn't help much because the real value was J=1.664 and the 30 degree curve dropped to Zero at J=1.8.

The last idea I tried was to use the Power Coefficient Table to reduce the power requirements for 35 degrees at J=1.6 and even out the curves before and after and it seemed to solve the problem.

Test results have now changed to the following:

Sea Level Military Power (1210 HP)
317 MPH - Significantly Higher
Propeller Pitch 35 Degrees - Same
1264 Pounds Thrust - Significantly Higher - Original was 1225 Pounds.

12,500 feet Military Power (1351 HP)
370 MPH - Same.... Back down to original speed
Propeller Pitch 41 Degrees - same
1221 Pounds Thrust - Higher. Original was 1219 Pounds. Basically the same.

Climb Rate at 5000 feet is 2900+ feet / minute or pretty close to previous.
Actual climb rate is probably between 2950 and 3000 feet / minute but autopilot isn't that precise.
Speed is 180-185 MPH IAS.
Power is around 1260 HP.
Climb at WEP was not tried.

Climb Rate at 10,000 to 13,000 feet is about 3000 feet / minute at 175-185 MPH IAS.
Speed wasn't precise because it changes slightly during the climb and a couple MPH difference does not appear to make any significant difference.
Power is around 1323 HP.

Climbs were done with nearly full fuel so rates should be a bit higher with only partial fuel.

Now for a Service Ceiling Test

- Ivan.

 

[Ivan]

Climb Rate & Service Ceiling

The slightly lower climb rate bothered me a little, so I made a small change to the Propeller Efficiency at J=0.8.
I also tried adjusting the Power Coefficients to be able to hit full RPM a little earlier but that did not work out well.
The new climb rates are nearly unchanged but they can be achieved at a wider range of speeds.

Climb Rate at 5000 feet is about 2950 feet / minute and increasing to almost 3000 feet / minute.
Speed is 175-185 MPH IAS.
Power is around 1260 HP.

Climb Rate at 10,000 to 13,000 feet is about 3000 feet / minute at 165-175 MPH IAS.
Power is around 1323 HP.

Climb Rate at 14,000 feet falls to around 2800 feet / minute at 165-175 MPH IAS.
Power is around 1275 HP and falling quickly.

Service Ceiling seems to give a range of values from 32,100 feet to 32,700 feet.
Typical Results:
Service Ceiling: 32,580 feet
Speed: 293 MPH TAS (174 MPH IAS) - Propeller Pitch: 35 Degrees.
Power: 514 HP
Remaining Fuel: 75.4 Gallons.

After the Service Ceiling test, I tried a high speed dive and then concluded with an actual landing without breaking the aeroplane. The wobbly landing tells me I need a LOT more practice.

Attached are the updated Propeller Efficiency Table and a Screenshot of a Warhawk that didn't miss the runway on landing.

- Ivan.