Turboprop emmulation with the CFS1 / FS98 jet .air file.

[aleatorylamp]

Hello Folks,

The FS98 (and CFS1) Jet .air file allows a certain margin of flexibility when converting Turboprop Hp to Flb Jet Thrust. Depending on desired aircraft performance, I have encountered .air files using conversion factors ranging from 1.9 to 3.

In my experience, for larger turboprops, the 2.5 x Horsepower = Flb Thrust rule of thumb conversion has worked best for me, and Hp x 3 = Flb Thrust for smaller turboprops.

Large tuboprops like the Lockheed P3-Orion (4 x 6400 shp AllisonT56A14), the DC3 turboprop conversions such as the Braddick Turbo Dakota (2 x 1424 shp P&W Canada PT6A-65AR) and Conroy Tri-Turbo III (3 x 1173 shp P&W Canada PT6A-45A), or the Tupolev Tu-95 Bear (4 x 15000 shp Kuznetsov NK-12MV), will perform admirably using a conversion factor of Hp x 2.5.

In these cases, resulting Flb thrust also accounts for CV Propeller thrust, for which there seems to be sufficient margin in the .air file if engine power lies above 900 or 1000 Hp.

However, with this conversion factor, smaller turboprop engines below 900 Hp show a pronounced lack of punch for accelleration and climbing, even with correct maximum speed, and will require a conversion factor of 3. This extra 0.5 accounts for the CV Propeller thrust, which is lacking in the FS98 jet .air file, and this becomes noticeable on engines with lower power.

Nevertheless, there is a more accurate way of calculating the Hp to Flb Thrust conversion:

For this exercise, I used the Dornier Seastar, which has 2 x 650 shp P&W Canada PT6A-135A), with a top speed of 180 Kt, and a turbine thrust of 1802 flb quoted on the factory´s engine certificate.

Using the formula Flb Torque = (HP * 5252 / Prop. RPM), and we get 1796 flb turbine thrust, which is quite accurate.

Now we have two alternative ways to include the necessary CV Propeller thrust: One simply increases Hp a little, and the other more attractive one alters the jet engine graphs:

1) We can add + (Hp * 0.237) to the formula, in this case 154 flb, and we get a total of 1950 or 1955 flb per engine to enter into the .air file. This will provide a very realistic and satisfactory turboprop FS98 performance curve.

2) Alternatively, we can enter the specified thrust of 1802 flb into the .air file, and proceed to conveniently alter (with AirEd or better with AAM) the following jet engine graphs in a very interesting way, which I actually prefer, as it includes the CV Propeller Thrust inside the engine. One could argue that these graph alterations effectively convert the FS98/CFS1 jet engine into a turboprop:

- Record 601 - Jet Power
- Record 603 - Jet EPR
- Record 604 - Jet N1

Alterations involve making the graph slope on the left, in the lower speed region, a little steeper. This will give us the extra punch needed for accelleration and climbing, without affecting aircraft top speed.

In the enclosed illustration, standard FS98 Jet graphs are shown on the left, and the altered, enhanced graphs, on the right.

Experiments only involving the Power Graph have proven to be insufficient. It seemed that at least the EPR Graph had to be altered as well, but in this case it required an even steeper Power Graph slope on the left, which began to look a bit odd, as it was not so smooth anymore. Thus, best results were obtained with modifications to the N1 graph as well.

I thought this was an interesting and perhaps cleaner way of building a turboprop engine for FS98 and CFS1.

Perhaps this will be of interest to some simmers.
Enjoy!
Cheers,
Aleatorylamp

Attached Images

[hertzie]

Turboprops in CFS1 ?? You have drunk too much, I suppose . Don't think that MS has EVER thought of that..... But what I'm sure of is that YOUR imaginations have no limits . I suppose you let us know what will come out of it one day, maybe an F-27 ?

Lots of succes!!

hertzie

[Ivan]

I have thought about this a bit since we worked on the P-3 Orion way back.
I believe the better way to simulate a Turboprop in CFS would be as a Piston Engine Aeroplane rather than as a Jet.
I don't know all that much about Jet flight models, but the problem as I see it is that there is ONLY thrust and it doesn't vary according to forward velocity. At low speeds, most of the propulsion of a Turboprop is due to the propeller and not the jet exhaust.
The "Horsepower" number only sounds high because a relatively small amount of thrust at a high forward velocity is the equivalent of a lot of rotational HP because of how HP and thrust / velocity are mathematically related.

With Propeller driven flight models, the extra jet thrust can be addressed as additional propeller efficiency.
Many if not most of the stock propeller tables already do this to take into account engine exhaust thrust.
One of the rather annoying tasks I am currently working on is a new set of Propeller Tables for my A6M2 Reisen.
Its current propeller tables are much too efficient for an engine that does not have exhaust thrust so I need to build a set of tables that not only are low enough in power coefficient but also low enough in efficiency and then correct all the other things this will break.

I know that messing with the efficiency tables does work because I did that when I was working on an alternative "Drive Model" for Hubbabubba's Jeep many years ago. It was the only way that I could get the proper acceleration with the very limited Horsepower of the Jeep's engine.

- Ivan.

[aleatorylamp]

Hello Hertzie,
Well, Lapsang Souchong tea mostly, and in my younger days I very much liked Newcastle Strong brown Ale.

I suppose you are right about Turboprops being a bit out of place in CFS1, but this would not apply if one were not to limit CFS1 to the WW1 / WW2 Era, because after the wars, turboprops started finding extensive military use.

Fine examples are the Embraer Super Tucano, the Grumman OV-1 Mohawk, the A-400M, the P3 Orion, the C-27J, the CASA C-295 and the Hercules. I believe the Fokker 27 was also used as a troop transport!

Thanks for your encouraging comments anyway! ;-)

Hello Ivan,
Interesting, that you should mention our CFS1 venture with the wonderful Lockheed P3-Orion that we built a model of some years back, with your valuable counsel in achieving correct engine nacelle shapes.

As you say, in turboprops, most of the turbine power goes into the propeller thrust, and I believe pure turbine thust only accounts for about 8% of the power.

I was just experimenting a bit further with the Jet Power graph, so as not to have to alter the Jet EPR and Jet N1 graphs. I tweaked an extra column, so as to include the 180 Kt Speed, and it worked very well. This would make the turboprop performance envelope even more realistic, as it leaves the other two turbine graphs untouched.

When this graph modification started working, I thought that it was really cool to be able to improve turboprops performance in the jet .air file, as it effectively "puts in" the propeller thrust for the lower speed ranges, the principal characteristic of the turboprop.

Two other characteristic would be fuel efficiency as compared to both jets and piston aircraft, and also a much improved altitude performance over the piston engine, which leads me to an answer to your idea of using a supercharged CFS1 piston engine .air file for a turboprop, by improving the propeller efficiency.

I had also experimented with this idea a while back, but found that despite a very strong supercharger, there was still considerable power loss at altitude, something that a real turboprop doesn´t suffer from, so I had to stick with the habitual use of a jet .air file for turboprops, although perhaps there is a way to get round this that I haven´t found yet.

Anyway, with the tweaking of the Jet Power Graph, the performance curve is very realistic.

Cheers for now,
Aleatorylamp

[Ivan]

Turboprops in CFS1 ?? You have drunk too much, I suppose. Don't think that MS has EVER thought of that..... But what I'm sure of is that YOUR imaginations have no limits.

Hello Hertzie,

I don't believe MS actually did a whole lot of planning and detail work before releasing CFS1. That is why we keep finding new things we can do with a 20+ year old simulator. The stock aircraft are pretty good proof that the developers really didn't really understand what many of the pieces of the AIR file really were for.
We don't necessarily understand how things work either (at least I don't) but we keep experimenting and finding things that seem like they have the results we want and learn a bit about aerodynamics in the process.

- Ivan.

[Ivan]

Hello Aleatorylamp,

I believe that although you may be able to get a solution to the problem you are working on, you first have to break down the two parts of the propulsion of a Turbo Prop.
The jet thrust versus shaft HP for each engine is probably going to be a bit different.
The conversion of shaft HP to propulsive thrust is going to be the most complicated part and perhaps your calculations already take that into account and perhaps they do not. Propellers tend to also behave differently at low and high altitude. Efficiency (propulsion) also varies quite a lot with forward velocity.
Since at most practical speeds (remember this phrase), propellers are the major propulsive force, I believe that is the factor that needs the most consideration.

The same Turboprop engine that makes "6500 HP" equivalent at 450 MPH is not going to make anywhere near that much HP when installed in an aeroplane that is only capable of 250 MPH. If you want some really amusing numbers, look at the claims of how much Horsepower the early jet fighters were putting out.

Representing jet thrust and engine exhaust thrust as additional propeller efficiency isn't always accurate either especially if the RPM isn't maximum (odd Advance Ratios) but I believe it has better potential under the typical maximum throttle combat conditions.

- Ivan.

[aleatorylamp]

Hello Ivan,

I fear that the problem with using a piston engine .air file to simmulate a turboprop, is going to
be more an altitude problem than anything else. However strongly turbo/supercharged the piston
engine may be, it still seems to lose more power at altitude than a jet engine, and this shortcoming
will most probably not be possible to compensate with a more efficient propeller.

I was conducting some trials with .air files for a supercharged 650 hp piston engine, and a jet engine of equivalent power - 1802 flb thrust. The engine certificate for the P&W Canada PT6A-135 turboprop quotes engine power both in Hp and Flb thrust, so I chose to use this engine as a basis for testing.

Results were that while both had the same 180 Kt top speed at 1000 ft, lack of power at altitude
was the problem for the turbo/supercharged piston engine, and weak accelleration and weak RoC
was the problem for the jet.

However, I could remedy the jet´s problem very well by steepening the slope of the Jet Power Curve in Record 601 (increasing the power for slower speeds without affecting top speed), but I couldn´t manage to get the piston engine to perform very much better at altitude, either by increasing supercharger values or by incrementing propeller efficiency.

Maybe it is just that the air is too thin further up for a propeller to be made to work more efficient,
and only a turbine can do the job because of its 39000 RPM.

This just gave me an extra idea: How about a smaller, faster (geared-up) propeller? I wonder...

Nevertheless, as you know a great deal more about propellers than I do to make a more efficient one, possibly you could indeed manage to get better results than mine!

Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,

I´m glad to say your idea does work!

I was having trouble setting up the supercharger, and it looks like I was obviously doing
something quite wrong there...

At first it wasn´t working at all, and then suddenly it started giving me 3 times the power at
altitude, which was of course no good reference whatsoever, so eventually had to do a reset
from start, as it were.

I already had a decent 8.5 ft CV propeller, with finely adjusted thrust and efficiency, and set
up my 18-cyl Double Wasp again, to give 650 Hp and 180 Kt at 500 ft without the supercharger.

Just to make sure, I tested it at 30000 ft, where it gave a very low 98 Hp, as was to be expected.

With that set, I put in a decent supercharger, giving at 61 inches of mercury, and started
adjusting Boost Gain, and finally, with a setting of 2.35, I got my 650 Hp at 30000 ft.

So, it works, I´m glad to say, and with the decent propeller, it gives a nice push for accelleration
and climbing. We can argue that with a properly turbocharged engine and a decent propeller
in the .air file, we can emmulate a turboprop engine in CFS1 very well!

Consequently, my simpler Jet Power Curve adjustment for Turboprops would be more indicated
for FS98 .air files, as there is no provision for superchargers there.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I was a bit busy earlier today so didn't have a chance to get to a computer until now.
I will read posts on my cell phone, but I am not going to try to type any significant posts there.

For what it's worth, if the Propeller Tables work the same way in FS98 as they do in CFS1, I don't believe it matters if there are superchargers or not. In fact, I believe you will have an easier time building a Turboprop equivalent powerplant with a very minimal or no supercharger at all.

I actually have very good confidence I can make this work and build an "Engine / Propeller" with constant thrust up to some arbitrary altitude. The "constant" would tend to waver a bit as it changes from one "step" to the next but it should not change much.
The important thing with this "Propeller" is the Propeller Coefficient Table. It would be pretty much a bunch of flat lines which has no relationship with reality but.... I do believe it would work.
The Propeller Efficiency Table is used to take care of Altitude power losses. It is in effect the new supercharger.

The method with using a Supercharger boost setting to get adequate HP at altitude has the problem that HP at medium altitudes will be too high. (See Engine Tuning Tutorial.)
I am going to run into that problem in a pretty serious way when I try to tune the Jumo 213F engine for a Dora-13.
The actual HP reading for this Turbo Prop is pretty much meaningless; it is the propulsion that is important.

Remember: There is Physical Reality and then there are Flight Sims. Sometimes the two aren't all that closely related.

- Ivan.

[aleatorylamp]

Hi Ivan,
Goofed again. I can eat my words from my previous e-mail...
No wonder I was having troubles before. They only get worse.

Stupid of me... when I´d got my 650 hp without supercharger at S.L. and 98 Hp at 30000 ft, I put in the supercharger and set it for 650 Hp at 30000 ft, forgetting to check it at S.L. - and now it turns out I´m getting 2435 Hp at S.L.

That won´t do at all... I´d forgotten about what you just said:
"The method with using a Supercharger boost setting to get adequate HP at altitude has the problem that HP at medium altitudes will be too high. (See Engine Tuning Tutorial.)". My memory is getting totally useless with time...

So it´s back to the drawing board! "Player! Go to Square One!"

For now, it seems that the FS98 Jet .air file with my Jet Power Curve tweak is still much better!

Interesting, your mentioned plan of action:
"The important thing with this "Propeller" is the Propeller Coefficient Table. It would be pretty much a bunch of flat lines which has no relationship with reality but.... I do believe it would work.
The Propeller Efficiency Table is used to take care of Altitude power losses. It is in effect the new supercharger."

It looks a bit daunting... Nothing wrong with further experimenting though...
Cheers,
Aleatorylamp

[aleatorylamp]

Hi Ivan,

I just did some trials with the propeller curves as
per your last post, but I´m still getting nowhere fast...

Cheers,
Aleatorylamp

[aleatorylamp]

Hi Ivan,
Just out of curiosity, and in answer to your comment:

"For what it's worth, if the Propeller Tables work the same way in FS98 as they do in CFS1, I don't believe it matters if there are superchargers or not. In fact, I believe you will have an easier time building a Turboprop equivalent powerplant with a very minimal or no supercharger at all."

Thanks to AAM (Aircraft Airfile Manager), one gets to see inside the FS98 Engine Torque and propeller graph tables. It looks like FS98 reads them in a similar way to CFS1, but sets them up and presents them differently.

Now... would there be a gladiator brave and daring enough to try and re-work these propeller tables?
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I believe you are doing something SERIOUSLY wrong here.
First of all, if you tune an engine for 650 HP at SL and then adjust the Supercharger Boost so that you get a critical altitude of 30,000 Feet, you should NOT affect the power at Sea Level at all. <---- IMPORTANT!!!!!
If you are affecting Sea Level power, you have done something very wrong.

Where you will see some ridiculous power output is probably around 17,000 Feet or so -- The "Middle Altitudes" I was describing.
This is hard to avoid with a Single Speed Supercharger that CFS implements.
Maybe others know how to solve this problem. I do not.

For what its worth, for this particular task, there is no reason to mess with much beyond Records 511 and 512 once you have a basic engine with some "reasonable" HP value at Sea Level. Reasonable can be almost anything here, but something that registers on Jerry Beckwith's Test Gauges would be a really good idea.
I don't happen to know what the specifications and performance for a reasonably useful Turboprop would be. The only one I have even a mild interest in is the Allison T56 installed in the P-3 Orion and that is because I intend to finish that project eventually.
Details:
4910 Shaft HP
473 MPH Maximum Speed
28,000 Feet Service Ceiling - I would use 30,000 Feet just to keep the numbers uncomplicated.
The claim is 6500 HP Equivalent....

Regarding Daring Gladiators.... All my Technicians and Gladiators are occupied at the moment.
The lazy ones and procrastinators are the ones currently assigned the light research and experimental work on Alex Simon's Focke Wulf 190D-9 conversion to a D-13. Some of the others are in studying and trying to figure out how to program a Fuel Selector Gauge that I need. Others are working on finishing texturing jobs and a few are looking at evolving my old FW 190A-8 into something different.

- Ivan.

[aleatorylamp]

Hi Ivan,
Not to worry! I wasn´t hinting at any of your gladiators to try anything with the FS98 prop graphs, The CFS1 ones are clearer, and difficult enough already!

Thanks for your indications! I was just re-reading your engine tuning tutorial, for a first step in my endeavour to set up my CFS1 prop .air file, so that it will more or less keep my 650 Hp at 30000 ft from dropping to 98 hp, without distorting the 650 Hp at S.L. That´s the first thing...

The next thing, if I understand correctly, is that it may be a good idea to stick to the propeller tables, to avoid the power/altitude distortion inherent to the CFS1 supercharger.

It will be a very interesting experiment. Nose back to the grindstone!

Update: Well... after trying out different Manifold Pressure and Boost Gain combinations, most of which produced exaggerated power all over the place, a close approximation to what I was aiming for came with a Manifold Pressure of only 31 inches of mercury, and a Boost Gain of 4 or 5, which was necessary to keep power up at altitude. However, there was over 200 Hp more at 30000 ft than there was at S.L., so possibly this means that Manifold Pressure has to be even less.

Update 2: Interesting findings testing with very low Manifold Pressure entries:
At 23 inches, and power at S.L. starting at 653 Hp, as altitude was gained, power dropped to
503 hp at 7500 ft and then went up again until it was at 654 Hp at 30000 ft.
At 24 inches, power loss on the way up was 38 Hp lower, but at 30000 ft, there were 698 Hp.

Update 3: Leaving MP at 24, but reducing Boost Gain from 4 to 2.644 got rid of the excess 50 Hp or so Hp at 30000 ft, and now it´s at 650 Hp there.
I tried increasing MP to 25 inches to get rid of the -118 Hp power slump on the way up, (the lowest is 532 Hp at 5000 ft), but power surged to over 800 Hp higher up around 25000 ft. Using only 24.5 for MP gave a 50 Hp power surge, so that might be the solution. This improved the power slump around 10000 ft a little.

Very interesting, nevertheless! This would be the practical explanation to your comment as to the possible usefulness of a very small supercharger, if any.

At any rate, this is a great improvement on the non-supercharged engine, that started out with 650 Hp at S.L. and ended up with only 98 Hp at 30000 ft!

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Have you ever observed the Manifold Pressure Gauge when the Engine is OFF and you are just sitting on the runway?
What does it read and what does that number mean?
What does a Maximum Manifold Pressure of 24 inches Hg really mean?
What kinds of conditions would cause this? Is this considered a "Normal" condition?
What does a Supercharger do?
What is Standard Temperature and Pressure?

If you have been able to answer these questions, do you really think your experiments thus far have actually represented a "practical" supercharger?

I already commented that the Propeller Tables 511 and 512 were what I believed would be a workable solution but that THAT solution would not have a whole lot of connection with reality. You seem to have a different idea in mind with you "engine tuning" and I have no clue what you are actually trying to do at this point.

I also gave you sufficient information about the Engine on a P-3 Orion to have a pretty good idea where to go. Do you have equivalent information for the particular Turboprop you are interested in simulating? Keep in mind there was no unnecessary information there. Everything listed is necessary for calculations.

- Ivan.

[aleatorylamp]

Hello Ivan,

- At S.L., MP reading with a switched off engine is 29.8, (almost one atmosphere).
- A max. MP of 24 means that my supercharger will give an MP of 24 in.Hg upto critical altitude.
- I don´t know if it is "normal" or not, but entering any more, power gets too high as altitude increases.
- A supercharger blows more air into the cylinders.
- Standard temperature and pressure is one atmosphere at zero degrees centigrade.

I don´t know if my experiments represent a practical supercharger or not, but I can´t make a more useful one. As you already announced, there are variations in power with increasing altitude.

Why these basic questions? Am I stupid?
You have no clue as to what I´m trying to do? I´d say it´s pretty obvious.

Perhaps you could suggest better MP and Boost Gain values to put in, unless you mean a supercharger is not the answer here, and only the propeller tables are.

Anyway, I can´t do any more. Upto now neither my supercharger nor my propeller tables have been good enough, so I´ll stop my experiments.

My own conclusion for the moment, is that steepening the left side of the graph curve in Record 601 of
a jet .air file is a clean and practical way to produce a convenient and regular turboprop performance curve.

Cheers,
Aleatorylamp