Runaway prop rpm

[jgf]

32 minutes ago, ScottishMike said:

any resemblance to reality is purely coincidental

 

That was my conclusion also.  Often wondered if so much of the data is still "unknown" and MS never released any info regarding it because they basically fudged too much of it just to get the aircraft flying.  In engineering you find "Finagle's Finagling Constant" - that value which, when added to, subtracted from, multiplied by, or divided into, the number you got, gives you the number you want.

 

The RealAir C172 is highly touted as having accurate physics and correct engine specs.  They somehow encrypted their air file so you've no idea what they did in there, but in the cfg file it's easy to see they turbocharged the engine, apparently necessary to get the "correct" performance.  (The only RL turbo 172 I'm aware of is a third party mod using a turbo diesel Continental engine.)

 

So I use the same system I have for race sims (where the best have physics advanced enough you can input real world data for everything ...if you can find it);  input all the hard data I can find and guess at the rest til performance seems "realistic";  of course much of this is subjective since I'm not a pilot.  (At least with race sims I have 35 years of seat time for reference.)

[defaid]

5 hours ago, ScottishMike said:

and Microsoft then also did a lot of fudging to make the other base aircraft work?

 

Lots of that. Record 510, the variable Low Speed Theory Limit is a more fundamental case in point.

 

Some prop maths doesn't work at the start of the roll (or at engine startup) because airspeed = 0... and 'divide by zero' errors are frustratingly familiar so MS made their own maths for very low airspeeds.

[jgf]

Lol, "low_speed_theory_limit=80.000000" is in the cfg of every prop aircraft;  I've yet to find any description of it other than what is in some cfg files, but the value is always 80, for everything from a Piper Cub to a Boeing 377.

 

Likewise "power_absorbed_cf=0.000000 ";  it is either 0.0 or 0.9 for every aircraft I've checked.  But not a clue what it delineates.

[jgf]

On 8/21/2023 at 6:16 PM, defaid said:

The absolute maximium prop rpm to avoid supersonic tips is 60810 / 38.22 = 1574 rpm. To avoid also transonic issues, the practical limit would be rather less.

 

          If the engine speed is 2800 rpm then a 2.0× reduction is about right, giving 1400 prop rpm.

 

 

Both P-47s and the F8f I'm using for reference have the R2800 engine, perhaps 200hp difference in them, all have around 12' props ...and all achieve 2700rpm with prop control at max and a reduction gear of 2.0;  have tried higher ratios and got rpm down, and performance suffered proportionally.  Just using the prop lever, at anything short of full throttle, I can get the rpm down in the 1200-1500 range ...and be in descent mode. 

 

Time to fly the DC-3 again, see how MS did it ...or didn't do it.

[defaid]

On 8/24/2023 at 1:20 AM, jgf said:

 

Both P-47s and the F8f I'm using for reference have the R2800 engine, perhaps 200hp difference in them, all have around 12' props ...and all achieve 2700rpm with prop control at max and a reduction gear of 2.0;  have tried higher ratios and got rpm down, and performance suffered proportionally.  Just using the prop lever, at anything short of full throttle, I can get the rpm down in the 1200-1500 range ...and be in descent mode. 

 

I think your 2700 rpm is engine rpm.

 

I guess that you're looking at the same gauge or variable when you achieve 1200-1500. That would still be engine rpm which may be why you descend. It's the props that should be doing that speed.

 

Use AFSD and the FS9 tooltip popup to monitor. You can check the relationship between engine & prop speeds with AAM.

 

Here's JF's Mosquito, with technology broadly comparable to the P-47, with AAM and AFSD to one side of FS9.

 

          Engine speed / gear reduction ratio = prop speed

                  2811       /              2.38             =     1181

 

As for the low speed theory, the comment in AAM states,

"Propeller thrust is usually defined as follow:

Thrust = Power * Efficiency / Flight Velocity

This simplified propeller thrust model can not be used for calculating static thrust or thrust at very low flight velocity. If flight velocity = 0, the Simulator have to divide by 0. To avoid this, the Low Speed Theory Limit is introduced. Probably the Simulator calculates thrust below this speed only based on Power, RPM and Pitch..."

 

There's confirmation in the FS2020 SDK:

"With the legacy FSX propeller model, the simulation did not work at low airspeeds and a fallback simpler simulation was used at lower speeds."

 

It seems the maths has been updated such that for 2020 native models, the value should = 0

 

[defaid]

On 8/14/2023 at 2:37 PM, jgf said:

at around 35mp the rpm starts increasing along with throttle and the prop lever has no effect at all

 

Going back to your original post, this is because the prop governor has already reached the limit of prop blade angle (prop beta).

 

I experimented with Alpha's Bristol Beau and discovered while checking some stuff that its airfile doesn't have a section 510 -- the prop stuff is already covered in the aircraft.cfg file. If FS9 uses aircraft.cfg in preference to the airfile then it makes sense to leave it out. The maximum blade angle is 65° (beta_max) and once that has been reached then changes in throttle just change the rpm.

 

In any case, I wonder if the problem may be simpler that we've all thought... What is the weight of the hybrid and what is the weight of the P47? If, at full chat, your prop beta is at its limit and you're still not getting up to speed then possibly your hybrid plane is simply under-powered. Drag and aircraft weight can be altered but it may be simpler to go to the [piston engine] section in the aircraft.cfg file and increase power_scalar.

 

[jgf]

Spent some time with other aircraft then returned to this project and started over last night, stopped trying to do things logically and "by the book".  Still using the F8f air file as a base;  its size, weight, and performance are very similar to the J7W1 (base weight just over 8000lb, hp 2100-2200, top speed in the 450 range, though at different altitudes, only 20sq.ft. difference in wing area.   Engine, prop, and geometry sections of F8f used in cfg file (eventually will try changing all geometry to match J7W1 specs, originally left all this default and had aircraft so unstable in pitch it was unflyable).

 

Gauges used are from a CFS2 A6M5 Zero and an old MAPE set of Japanese WWII gauges;  "Americanized" cockpit (actually done by USN in preparation for flight testing in the US) consists of gauges from an AAF WWII set, a MAPE WWII US Bombers set, and a WWII US set.  Only concession to modern equipment, other than GPS and radio, is a dual needle RMI for convenience (repainted to match the rest of the panel).

 

Typically I consider the scalars as diagnostics, increase or decrease something and see if that has the desired effect, then reset to 1.0 and edit the appropriate parameters;  unfortunately this doesn't always work, the parameters may be embedded in a difficult to edit table or be so interactive with other parameters as to be almost as difficult to edit.  So rather than take a course in FS2004 aerodynamics and spend ages tweaking an obscure aircraft in which no one else is interested, scalars it is.

 

Increasing the engine power scalar exacerbates the prop issues, so left it at 1.0.  Reduction gear works oddly, but I've made progress there.  With the default setting of 2.0, flying level at 28k ft, 35 mp, and rpm 2500;  minimize FS and edit gear "ratio" to 2.4, this should lower rpm.  Back in FS, reload aircraft, nothing else changed, rpm now 3500.  Repeat, eventually settled on reduction gear of 1.53, though will probably go taller with further testing.

 

Changed specs:

induced drag scalar - 0.72

fuselage drag - 0.27 (in air file);  both seem reasonable as the J7W1 is a much sleeker, slimmer aircraft than that portly Grumman

prop thrust scalar - 1.3

max mp - 70

max_rpm_mechanical_efficiency_scalar=1.1

critical_altitude=31000

reduction gear - 1.53

 

Performance is quite close to what I want, and in keeping with what you would expect from a high altitude interceptor (though I've yet to test its ceiling, 39k ft, only flown to 32k so far).  Top speed at 28k ft is 385kt/443mph (design is 407kt/465mph), though get overspeed warning at 381kt (must be in air file).  Take off around 130mph with 40mp, 2500rpm, and no flaps;  once gear is up, drop to 30mp or you'll be doing 300kt before you know it;  will climb 2000ft/min, 200kt, to 30k ft, with 35mp and 2300-2500rpm.

 

Still experimenting with the prop issue;  if I understand correctly this prop should never be turning more than around 1400rpm, so at 28k ft, full throttle, max speed of 465mph should be attained with that 1400rpm.  Currently, at 28k ft, 60-65mp, I can lower the prop to around 1500 and get 300kt.

 

I suspect I have misconstrued all I've read previously, which implied the rpm gauge reads prop rpm.  But if it is showing engine rpm (which explains the ubiquitous 2700rpm references), it's a whole new ballgame.  While I understand how a CS prop functions, for tweaking an aircraft I must consider the entire system - the interaction of engine rpm, mp, prop rpm, blade angle, and gearing.  Design the system so at full throttle, full prop, altitude of max speed, you never overspeed the prop.  And do this on a system with a tenuous relation to reality.

 

(And of course I may still be way out in left field.)

 

 

 

[jgf]

Well, the more I read the more turgid becomes the solution.  Does the rpm gauge show engine or prop rpm?  One article said engine rpm, later in the same paragraph said prop rpm.

 

This thread is interesting, ultimate conclusion .... it depends on the specific aircraft-

https://www.pilotsofamerica.com/community/threads/does-rpm-refer-to-the-prop-or-the-engine.57414/

 

In the context of FS, who knows what a particular gauge is designed to read.

[defaid]

13 hours ago, jgf said:

ultimate conclusion .... it depends on the specific aircraft...

... In the context of FS, who knows what a particular gauge is designed to read.

 

Truly.

 

It depends on which gauge is installed. I'm amused by the article which says both; that's not uncommon on the internet. Different people will tell you different things and sometimes the same person will tell you different things. They've seen one aircraft panel (their favourite) and have decided that all other planes must be just the same. They'll also pass off their supposition as incontrovertible fact, even when their understanding is so incomplete as to result in such contradictions.

 

I assume you already use FSUIPC. I'll suggest again that you download and install AFSD by Hervé Sors. You can run multiple instances to observe concurrently several groups of data relating to different aspects of the aircraft. It's so useful that in flights where fuel economy is critical, particularly with Air Hauler, I have it running alongside FS9 in the role of a more precise set of digital gauges.

 

https://www.aero.sors.fr/designer_pilot_utilities1.html

 

It will show the propeller speed for whichever engine you've selected, so eliminating any confusion.

 

Does it show the same rpm as your gauge?

Yes? - gauge shows prop rpm

No? - gauge shows engine speed (or fan speed in %)

 

Three a/c below:

1. The cub has a 1:1 engine:prop ratio because it's just a couple of blades on the end of the driveshaft. It also has no record 510 in the airfile.

2. The Mosquito's gauge is very well coded -- the popup tells you it's showing engine rpm, which is confirmed by AFSD and AAM.

3. Rick Piper's HS.748 not so good but given the gear ratio in the airfile, it's unlikely to be showing prop speed. Again, AFSD is definitive.

 

 

 

 

 

[tiger1962]

45 minutes ago, defaid said:

3. Rick Piper's HS.748 not so good but given the gear ratio in the airfile, it's unlikely to be showing prop speed. Again, AFSD is definitive.

 

 

The HS.748 is a turboprop so the 9394 rpm is turbine rpm, while the prop 'speed' will be indicated on the torque gauges in percent.

[jgf]

10 hours ago, defaid said:

I assume you already use FSUIPC. I'll suggest again that you download and install AFSD

 

Oh yes.  Not familiar with that util but just d/l'd it.  (thanks for link)

 

I am assuming (and we know what happens when we do that) the rpm gauge I'm using reads engine rpm since it's max is around 5k, can't imagine any prop useful for RL aircraft needing that range.

 

And all this got me to thinking (I know, I should leave that to qualified people), if you had to fly an aircraft with no knowledge of engine speed or gearing, and given that improper operation can have dire consequences, what criteria would you use for safe operation?  Experience would give you an idea of prop rpm just from its size, but what else?

10 hours ago, defaid said:

not uncommon on the internet

 

I long ago decided the internet is the perfect example of Sturgeon's Law.

[lnuss]

3 hours ago, jgf said:

if you had to fly an aircraft with no knowledge of engine speed or gearing, and given that improper operation can have dire consequences, what criteria would you use for safe operation?  Experience would give you an idea of prop rpm just from its size, but what else?

Sound and feel would play a big part, along with whatever I could understand of the instrumentation available. Trial and error, taxi a bit, see how it feels, gradually adding more power a bit at a time, perhaps move the controls around a bit when speed is up somewhat. The slow down, stop, start again, adjust for whatever might seem needed from the aircraft behavior.

 

And I'd always keep in mind that, typically for recips, 75% power seems to be max cruise for normal operations and usually it's full power for takeoff, often applied gently with higher-powered aircraft, smoothly with all aircraft.

 

 

[ViperPilot2]

On 8/19/2023 at 10:01 AM, tiger1962 said:

 

I've been futzing with the aircraft.cfg and .air files for almost 20 years now and I have to agree with you. MSFS2020 has actually simplified this diabolical state of affairs by eliminating the .air file, and good riddance!

The best tool for adjusting .air files that I have ever used (and still do) is AirWrench. It's payware (from Flight1), it has a steep learning curve, and it only does half the job - which is to set all the tables for you automatically. The fine tuning is done mainly in the aircraft.cfg file. It really is trial and error to begin with, but if you persevere you'll get a 'feel' for what needs adjusting and where after a while. A top tip for you - create a .air file which is empty except for the aircraft title: this is all that AirWrench needs to populate all the other parameters from the aircraft.cfg file.

Regarding the Cirrus Jet you mentioned, it's a center of gravity issue combined with an elevator effectiveness issue - go get AirWrench and have at it!

Thanks for the suggestion! I have an older copy of Airwrench, so I will find it, give it a try, and let you know! 

[jgf]

1 hour ago, lnuss said:

Sound and feel would play a big part

 

Experience.  Something the sim cannot provide.

 

And the same reason my performance in race sims is less than stellar;  in real life you receive so much tactile information from the car, which you process subconsciously, and adjust your driving accordingly.   I suspect it is the same with FS, especially with smaller aircraft.

 

 

[ViperPilot2]

The Cirrus SF50 now gives me an FS9 Error Message and shuts down. No changes made.

 

I knew there was a reason why I didn't have that A/C in my Hangar! 

[jgf]

Well, as a result of everyone's help, and that utility, I'm back on track.  My initial problem stemmed from ... a mislabeled gauge (and some erroneous conclusions on my part).  Hovering the cursor over the rpm gauge gets a popup blatantly stating "propeller rpm", but it is actually reading engine rpm.  Thus when I set the reduction gear to its specified setting I got an increase in rpm;  my guess (probably wrong) is that FS9 pays no attention to the engine redline and will continue raising engine rpm to match prop settings.

 

Specs for the Mitsubishi Ha-43 are:  2536ci, 7:1 compression, 2200hp at 2900 rpm with 10.1psi (.69bar) boost;  1800hp at 22k ft.  Prop rpm is a hair under 1200, .412 engine speed;  so a 2.4 reduction.  Now I must convert that boost figure to the required max mp and I should have the basic specs as required by the cfg file.  (Using this for reference - https://eqtuning.com/blogs/technical-write-ups/manifold-absolute-pressure-vs-boost-and-more   - it's for auto turbos but fluid dynamics do not change.)

   

Critical altitude is next, I know what it means but no idea where to set it in regard to this aircraft (everything I read seems in reference to a normalized boost, not to a supercharged military engine).  The F8f, the airfile I'm using as a base, had it set for a miserly 10k ft, I currently have it at 30k ft.  The Shinden's top speed is at 28k ft and its ceiling is 39k.

 

Once I'm satisfied with all this, I am back to the original problem - loss of prop control at high power settings.  It seems the pitch is reaching max long before hp/mp/speed, so has exceeded the governed range and further throttle increase causes the prop rpm to increase along with engine rpm.  The above edits should bring things more in line, then it looks like I must deal with those prop tables;  they correlate prop speed, pitch angle, airspeed, etc.  Being insufferably lazy, I will first search for other piston aircraft with similar power, prop size, and performance, and try their prop tables; if that doesn't work, well, bite the bullet and experiment.  At least someone else faced this eleven years ago, though in FSX -

https://www.fsdeveloper.com/forum/threads/constant-speed-recip-uncontrollable-rpm-at-high-speed.325361/

 

 

BTW, found this in an article on mp (italics mine):

"...since most MP gauge-equipped airplanes have constant-speed propellers, the RPM will not change as a result. Eventually you’ll get to low enough a throttle setting that the propeller is below its governing range and from there throttle controls RPM as well as MP…but that’s a function of prop mechanics and not the physics of manifold pressure."

 

[lnuss]

For what it may be worth, I just came across this page related to the P-47 power curve:

https://ww2aircraft.net/forum/threads/performance-analysis-p-47.17937/

 

 

[jgf]

Very interesting.  Thanks.

[ViperPilot2]

On 8/23/2023 at 6:20 PM, jgf said:

 

Both P-47s and the F8f I'm using for reference have the R2800 engine, perhaps 200hp difference in them, all have around 12' props ...and all achieve 2700rpm with prop control at max and a reduction gear of 2.0;  have tried higher ratios and got rpm down, and performance suffered proportionally.  Just using the prop lever, at anything short of full throttle, I can get the rpm down in the 1200-1500 range ...and be in descent mode. 

 

Time to fly the DC-3 again, see how MS did it ...or didn't do it.

If you haven't flown the Default, give the Manfred Jahn model a try; over the years I've been flying it for fun, on DC-3 Airlines hops and in the now defunct DC-3 World Rally, at Published Performance specs it flies very realistically, and at Cruise settings of 23"/2050RPM she's burning 91Gal./Hr. which is right down the middle of the Performance curve. During the World Rally you could tell who was flying what Model by the large variation in Fuel Burn for a given Flight

(MSFS Default, MJ, MAAM-SIM, FSX Default/Awesome4Some).