Runaway prop rpm

[jgf]

Doing an engine swap, donor aircraft is a P-47, recipient is one of those aircraft from a developer who built anything with a propeller on a turboprop model (should have an 18cyl double row radial of 2100hp, near identical to the R-2800 of the P-47).  So far, so good;  low altitude performance exceptional, take off with mere 30mp and no flaps; 30mp, 2500rpm, climbs at around 210kt, 2000fpm, to 30k ft.  And there the weirdness sets in.  Top speed should be around 450mph at 28k ft, so level off there, set simple AP to hold altitude, and start increasing throttle;  at around 35mp the rpm starts increasing along with throttle and the prop lever has no effect at all.  At max throttle, 60mp, rpm is 4500;  speed is in the ballpark, about 420mph.  But shouldn't the governors be holding the prop at 2700 and increasing the pitch?

 

Pertinent data (also swapped engine tables between air files):

 

[piston_engine]
power_scalar = 1.2
cylinder_displacement = 155.5556
compression_ratio = 6.7
number_of_cylinders = 18
max_rated_rpm = 2800
max_rated_hp = 2800
fuel_metering_type = 2
cooling_type = 0
normalized_starter_torque = 0.033
turbocharged = 1
max_design_mp = 65
min_design_mp = 10
critical_altitude = 32500
emergency_boost_type= 1
emergency_boost_mp_offset= 5.14
emergency_boost_gain_offset= 0.59
fuel_air_auto_mixture = 1
auto_ignition = 0
max_rpm_mechanical_efficiency_scalar = 1.1
idle_rpm_mechanical_efficiency_scalar = 1
max_rpm_friction_scalar = 1
idle_rpm_friction_scalar = 1

 

 

[propeller]
thrust_scalar=1.2
propeller_type=0
propeller_diameter=11.200000
propeller_blades=6
propeller_moi=50 //65.000000
beta_max=65.000000
beta_min=10.000000
min_gov_rpm=600.000000
prop_tc=0.010000
gear_reduction_ratio=2.40000
low_speed_theory_limit=80.000000
prop_deice_available=0
prop_feathering_available= 0
prop_auto_feathering_available= 0
min_rpm_for_feather= 700.00
beta_feather= 0.00
power_absorbed_cf= 0.90
defeathering_accumulators_available= 0
prop_reverse_available= 0.00
minimum_on_ground_beta= 1.00
minimum_reverse_beta= 0.00
rotation = -1
fixed_pitch_beta=20.000000
prop_sync_available=0

[Tom Gibson]

I don't see anything obviously wrong (I see 6 blades but I assume that's correct).  Have you tried using the entire AIR file of the P-47 to see if that makes a difference?  Perhaps something else in there?

[jgf]

The engine section is a direct copy from the P-47, the prop section was edited for diameter, number of blades, gear ratio, and moi.  I'm sure some of the flight tuning tables in the P-47 would be way off for the aircraft I'm working on, the P-47 is larger and heavier;  but I'll give it a shot.

[jgf]

Tried two different P-47 files (Alphasim and Tom Kohler, have also tried engine/prop from each), differences with cfg are enough that neither is flyable, but could get to 25k ft and go full throttle.  Same problem but to a lesser degree - prop doesn't start speeding up til 40mp and is at 3800rpm with 60mp.  I suspect an issue with one of the prop tables; possibly 512, which deals with prop rpm. pitch, airspeed, and engine power.  It seems the governor stops working at a certain power level.

[defaid]

I'm not sure how these things work but I think the P-47 had a switch for selecting manual/auto prop pitch control. Perhaps your hybrid needs one too though I've never known the selection to be available in FS9. Tom?

 

My immediate thought is that at some speed at each altitude, the governor may be maxxed out (no more prop pitch trim available) so the prop rpm will increase with throttle. It''s a possibility. I think AFSD shows prop pitch so it should be easy to verify and if it is the case then there must be a table parameter that is applicable.

 

There's also a distinct 'tropopause' type effect in FS9 at around 36 000 feet. That could be having an effect.

 

As an aside, if the 12' 2" prop disk is doing 4500 rpm at fl400 then the tips are doing mach 2.96 

 

D

 

 

[lnuss]

On 8/14/2023 at 7:37 AM, jgf said:

But shouldn't the governors be holding the prop at 2700 and increasing the pitch?

One thing to think about is that, in the real world, there is only so much pitch change available, so that when the blades hit the stops something else has to happen. I don't know how that's represented in the sim, however. So maybe your changes cause it to try to exceed the power that the pitch changes can control. Remember, that control is not infinite.

 

I recall the Stearman I used to fly that had a 300 HP Lycoming and constant speed prop, where after level off I'd set the prop to 2000 RPM (max was 2300), and once we got in a slight dive (and especially the steeper dives) the RPM would increase beyond the 2000 and coming out the back side of the loop, it would actually go past the 2300 RPM "limit."

 

So those controls have limits in the real world, and maybe in the sim as well.

[jgf]

7 hours ago, defaid said:

As an aside, if the 12' 2" prop disk is doing 4500 rpm at fl400 then the tips are doing mach 2.96

 

Exactly. (though my tests are at 28k, where top speed is rated, ceiling is 39k)

 

Recently flew both P-47s again (first time in ages), neither exhibited any anomalies, just a bit sluggish and neither reaches its specified top speed (even when as lightly loaded as possible).  I have about a half dozen aircraft using the r2800 engine and similar props, from 11' to over 13' diameter;  no appreciable differences in engine or prop parameters in the cfg file, but the prop tables in the air file are noticeably different.  So keeping the same engine specs and prop specs in the cfg (adjust for diameter and number of blades ...not sure if the latter even affects overall performance), I've tried three different prop tables, all exhibit the problem but to different degrees, so more experimentation to determine in which direction the parameters are changing when the rpm shift lessens, then seek a prop table further in that range.  I do not feel competent to edit this table manually.  And there remains the possibility of interaction with some other table.

 

What a can of worms;  was hoping for some simple adjustment I had overlooked.

 

 

[lnuss]

8 minutes ago, jgf said:

just a bit sluggish and neither reaches its specified top speed (even when as lightly loaded as possible). 

What speeds ARE you seeing vs rated speeds?

[jgf]

The P-47M is rated "Max speed: 475 mph @ 32500 ft", by removing all extraneous weight (the loadout even allows removal of guns and ammo) and taking off with minimal fuel, I barely get 400mph at 32k;  the P-47D is rated "Top speed, 25000 ft: 420 mph", I get about 375mph.

 

This has carried over to the aircraft i'm working on, which had a design speed of 465mph at 28k,  I'm getting around 410.

[lnuss]

OK. Have you checked induced (and parasite) drag (1204 in AirEd, flight tuning in aircraft.cfg) or sections 510-512? Or maybe you changed the parameters in aircraft.cfg -- I'm not sure whether the .air file takes precedence over the "new" fields in the .cfg file.  You probably did check at least a couple of things in 510, since you made changes. And 511 and 512 are tables that I'd hate to mess with. Also, what were the temperature at your altitude and the barometric pressure when you tested? I ask because those published speed figures are for standard conditions*, or density altitude, and will vary in other circumstances, though the difference in real life might not be as great as you have found. 

 

This might not be much help, but I wanted to be sure you are aware of these things. It's been years since I messed with this stuff (my buddy was intensely into this), but I know it takes lots of patience and experimentation and that so many FS aircraft (both default and add-on) haven't had the time and effort spent to make best use of the FS capabilities, so you might be stuck with less accuracy than you want unless you're willing to spend days experimenting.

 

Good luck.

-----------------------------------------------------------------------------

Standard conditions= 29.92 Hg" at 59º F (15º C), with standard lapse rate.

[jgf]

I did play around with drag briefly, after reading what I could find regarding it in FS.  Other than a couple of warnings about not setting it too low (though no description of what would happen, nor even what constituted "too low"), I found little useful.  I did try setting each drag figure, one at a time, about 10% lower;  one produced little noticeable effect in any phase of flight, the other had little effect til I started a normal descent - where it plummeted like a rock (over 4k ft/min and overspeed warnings).  So both back to default for now.

 

Had no specific weather selected, so default.  OAT was around -40C, but had pitot heat and carb heat on (carb air temp showing +13C).  Is there somewhere in the world with 59F at 30k ft?

 

One of the most frustrating aspects of dealing with aircraft physics in FS is that there is no consensus, even among very similar aircraft, of some values.  One of my first forays into this was tweaking a small single engine, GA aircraft;  I compared a parameter among seven or eight different planes of basically same size, same weight, same engine, same performance ...and values ranged from -27000 to +34000!   This is still the case with AAM, for some parameters it merely lists the values for the default aircraft and you're on your own making sense of it.

 

For these reasons I believe there will never be completely accurate aircraft for FS2004, I don't know if FSX or later have improved on this regard, but knowing MS I doubt it.  So you plug in the hard data and fudge the rest to get realistic, if not entirely real, performance.  I'm satisfied if I get correct, within a few percent, top speeds at correct altitudes, correct ceiling, correct stall, correct fuel consumption, and if the overall handling and performance seems "right" for that aircraft.  Is the roll rate too fast or too slow?  I wouldn't know unless it were extreme (if your C172 rolls like a B-52 it's a good assumption something is wrong.)

[lnuss]

12 hours ago, jgf said:

Is there somewhere in the world with 59F at 30k ft?

 

No, that 59º is at sea level. The standard lapse rate is a reduction of 2º C (3.5º F) per 1,000 ft of increased altitude, up through 36,000 ft. From there up to 80,000 ft the temp is somewhat constant around -65º F or so. So under standard conditions the temp at 30,000 ft would be about -46º F.

 

12 hours ago, jgf said:

One of the most frustrating aspects of dealing with aircraft physics in FS is that there is no consensus, even among very similar aircraft, of some values.  One of my first forays into this was tweaking a small single engine, GA aircraft;  I compared a parameter among seven or eight different planes of basically same size, same weight, same engine, same performance ...and values ranged from -27000 to +34000! 

Unfortunately the problems you run into seem to be what everyone sees (to the best of my knowledge) --  certainly my buddy kept telling me about that, and he spent the better part of twenty years chasing these things down and learning about them and it still took many, many hours of experimentation to get an aircraft to behave right, though some he never did get quite right. Apparently the default aircraft were not really properly figured out, but rather just adjusted (strangely it would seem) until they sort of worked (my impressions, not established fact), with no consistency, leaving others a mess to deal with.

[jgf]

7 hours ago, lnuss said:

though some he never did get quite right

 

Knowing my luck I have one of those.

 

I have spent hours perusing threads and downloaded pdfs on the topic, with the conclusion that no one knows, or even agrees.  For every thread complaining that the default 172 is underpowered there is a thread complaining that it is overpowered.

 

Now, I hate math (so naturally became an engineer  ....electronics, not railroad) but found a pdf with aerodynamics equations as related to FS2004.  This led to a humorous (if it weren't so irritating) circle of logic:

Parameter A is critical to the dynamic balance of the aircraft, this is easily obtained from manufacturer data. (Lol, not if the aircraft is eighty years old.)

But this parameter can be computed by a string of equations requiring mostly physical dimensions of the aircraft.  Except,

the first equation requires parameter B, which we don't have; but this is computed by another equation, which requires data C, which we don't have; but C can be determined by another equation, which requires parameter D, which we don't have;  but D is derived from a simple equation which requires ..... parameter A.

 

Anyway, last night I decided to start over.  Wipe out the files in the root folder, copy the original cfg back, change engine type and fuel, correct a few dimension issues, copy the engine and prop data from my new donor - an F8f (same engine, very similar size, weight, and performance), changing only the prop diameter and number of blades.  Then copy the F8f air file over and rename it.

 

Low altitude performance was comparatively pitiful;  took full throttle, one click of flaps, and a good bit of nose up trim just to get airborne.  Gear up, flaps up, full throttle, 2500rpm, and a leisurely 170kt climb at around 1200ft/min;  above 5k ft it picked up, throttle back to 40mp and climb 1800ft/min, around 200kt.  Level off at 28k ft, set alt hold, rpm at 2700 and prop control does nothing;  slowly advance throttle and rpm increases also, peaking at 4k rpm at 65mp, speed around 300kt.

 

Obviously nothing endemic to a particular air file, so either some arcane interaction between the cfg and air file, or some erroneous cfg parameter.  And probably trial and error (and luck) to find this.  (Also tried changing the prop to four, even three, blades;  made no difference I could discern.  I wonder what would happen with a setting of one blade?  engine would shake itself to pieces before takeoff? lol)

[lnuss]

Yes, making an F aircraft fly well is an arcane art.

 

Just in case you haven't come across it, the Aviation Formulary is an interesting site:

http://www.edwilliams.org/avform147.htm

 

 

[ViperPilot2]

Some say the .air file controls the 'parameters' of the airplane, some say the Aircraft.cfg does, some say a little bit of both. It seems that after twenty years plus and all of the 'documentation' out there, no one's willing to 'spill the beans' about what and how exactly all those Values and Boolean figures are supposed to correlate. It's like some Top Secret data that no one in the know is willing to divulge, or else they seed the answer with doublespeak and other mumbo jumbo, usually ending in "Well, you'll just have to experiment and figure it out for yourself." To be honest, I don't think the guys on the ACES team ever figured it out, either.

 

So everyone's left with plugging guesstimate figures into AirEd, futzing with the Aircraft.cfg in the hopes of 'getting close', and living with the results. That's a shame.

 

Case in point... Kaz Ito's Cirrus Jet. It seems there ain't enough Elevator Trim in the entire Solar System to get this airplane to Climb. It's a good looking Model, but if you can't trim it properly to fly, why even have it installed? How many thousands of entries in the Library are out there in the same predicament?

 

If anyone has a fix for the Cirrus Jet, please let me know... 

[tiger1962]

28 minutes ago, ViperPilot2 said:

Some say the .air file controls the 'parameters' of the airplane, some say the Aircraft.cfg does, some say a little bit of both. It seems that after twenty years plus and all of the 'documentation' out there, no one's willing to 'spill the beans' about what and how exactly all those Values and Boolean figures are supposed to correlate. It's like some Top Secret data that no one in the know is willing to divulge, or else they seed the answer with doublespeak and other mumbo jumbo, usually ending in "Well, you'll just have to experiment and figure it out for yourself." To be honest, I don't think the guys on the ACES team ever figured it out, either.

 

So everyone's left with plugging guesstimate figures into AirEd, futzing with the Aircraft.cfg in the hopes of 'getting close', and living with the results. That's a shame.

 

Case in point... Kaz Ito's Cirrus Jet. It seems there ain't enough Elevator Trim in the entire Solar System to get this airplane to Climb. It's a good looking Model, but if you can't trim it properly to fly, why even have it installed? How many thousands of entries in the Library are out there in the same predicament?

 

If anyone has a fix for the Cirrus Jet, please let me know... 

 

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!

[jgf]

Interesting you mention Mr. Ito, it is one of his WWII aircraft I'm working on;  he made beautiful models, but anything with a propeller is a turboprop.

 

In my slow plodding way I am making progress.  The giveaway was "Gear Reduction", what the heck is it for?  Logically (and I realize computers have absolutely nothing to do with logic) why do you need a gear reduction between a 2800rpm engine and a 2700rpm propeller?  I had set this according to the ratio computed for the Mitsubishi engine (2.4), and had the issues delineated in the first post;  setting it even lower (2.6, even 3.0) exacerbated the problem greatly;  setting it back to 2.0 (all the supercharged WWII engines I checked used 2.0-2.1) helped, but I still lost prop control before hitting max mp.  Setting it higher (1.8, 1.5, 1.1, even 0.8) did nothing, it seems the sim accepts nothing lower than 2.0; couldn't get my head around this, time for a walk (where I do my best thinking).   Back home twelve miles later, I think i have it.  The gear reduction has nothing to do directly with the prop, it must be the drive to the supercharger;  at 2.0 the supercharger turns at half the engine rpm, increasing that makes it even slower.   If I can get this lower than 2.0 I think the prop control and mp will fall right into line (is this why some of these WWII aircraft do not meet their published specs?) The 2.0 is possibly an arbitrary figure MS decided for modern pressurized engines (there seems no differentiation between turbo charging and supercharging;  and a "gear ratio" is easier to compute than the complex exhaust pressure and turbine equations) and is either hard coded or controlled by one of the tables in the air file;  so my current project is investigating this, also what relation between critical altitude and gear reduction might be evident.  (Am I anywhere close with this?)  I may never untangle the interactions of these, and if I do I may be sick of this aircraft by the time i get it flying decently.

 

[jgf]

23 hours ago, ViperPilot2 said:

Some say the .air file controls the 'parameters' of the airplane, some say the Aircraft.cfg does, some say a little bit of both.

 

My understanding is anything in the cfg file overrides the equivalent entries in the air file;  what remains in the air file are mostly the tables and some entries directly related to them.  You can actually delete about 70% of the airfile now and it still works.

[Tom Gibson]

Hi,

 

No, the gear reduction is between the engine and the prop.  When they mention a 2700 rpm prop that means engine rpm, not the rotation speed of the prop.  I.e. the constant speed mechanism limits the rpm to 2700 engine rpm.  The reason they have to slow down the prop is the tips would go supersonic at 2700 rpm and the prop would lose efficiency.

 

Hope this helps,

[jgf]

1 minute ago, Tom Gibson said:

Hi,

 

No, the gear reduction is between the engine and the prop.  When they mention a 2700 rpm prop that means engine rpm, not the rotation speed of the prop.  I.e. the constant speed mechanism limits the rpm to 2700 engine rpm.  The reason they have to slow down the prop is the tips would go supersonic at 2700 rpm and the prop would lose efficiency.

 

Hope this helps,

 

Aware of the prop limitation.   But if the max rpm of the engine is 2800 what is being reduced?  A 2.0 reduction gear would be giving a 1400rpm output from the 2800rpm input.  So it seems the engine and prop would be on a common shaft with rpm limited to max 2700 and just the prop pitch varied, no reduction gear necessary.  What am I missing?

[defaid]

5 hours ago, jgf said:

 

Aware of the prop limitation.   But if the max rpm of the engine is 2800 what is being reduced?  A 2.0 reduction gear would be giving a 1400rpm output from the 2800rpm input.  So it seems the engine and prop would be on a common shaft with rpm limited to max 2700 and just the prop pitch varied, no reduction gear necessary.  What am I missing?

 

This thread is one of the most interesting I've read for a while: not only the coffee-break maths but the vagaries of the airfile & config file too. I had guessed that the aircraft.cfg data supersedes the airfile data and that is mentioned in the notes appended to data in both Aired and AAM.

 

What is being reduced is the engine speed down to prop speed. Transmission is not via a fixed shaft but through a gearbox. If you have seen an unadorned rpm quoted somewhere with a value of 2700 then it's surely engine & not prop rpm because at 2700 prop rpm, the tips will be supersonic.

 

          The 12' 2" prop disk has a circumference of 38.22 feet

 

          Speed of sound at fl280 is 1003 feet / second, which comes in at 60180 feet / minute

 

          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.

 

This C441 below being a turbo prop, AFSD shows N2 rather than engine rpm but the aircraft.cfg gives the 100% N2 = 29920 rpm. The tips of the 7' 6" dia prop will go supersonic at around 2550 prop rpm. The constant speed of the prop is fixed at 2000 rpm through the 14.96× reduction gear (29920 / 14.96 = 2000).

 

D

 

[jgf]

Interesting.  Logic dictated 2700 could not be an across the board limit since obviously the tips of a 12' diameter prop will be moving much faster than those of an 8' prop at the same rpm ....yet nearly every radial engine I checked had a reduction of 2.0-2.2  while prop sizes varied 9'-13'.  Of course I labor under the fallacious assumption all these designers have done their homework.  (Since the default C172 has a "reduction" of 1.0, that must mean the prop is bolted to the crankshaft ...and that the sim accepts ratios taller than 2.0;  I could not get anything taller than 2.0 with the radial.)  This was also clouded by the fact most superchargers run at a fraction of engine speed, via gears or belt drives.

 

So now i work backward from the 11.2' prop on my plane then curse, er tweak, the other parameters.   (I could go the easy route and make a "Brundlefly" combination of the Shinden and the F8f, both are very similar in size, weight, power, and performance;  but there is an old engineering adage - "you learn nothing from success".)

[ScottishMike]

You may like to have a look at the cfg. and air files for Samdim's Tupolev TU-114 (the world's fastest prop aircraft) the props are 18 feet 4 inches (5.6m) in diameter and the tips did indeed go supersonic making the aircraft one of the noisiest ever; reportedly 110 Dbs INSIDE the passenger cabin!

[ScottishMike]

The point I am trying to make is that what we expect from the .air file and aircraft.cfg is in some ways misguided (one could say naïve?).

The example of the TU-114 is that it flies well and close to its' published specs, yet if you look at the aircraft.cfg the engine is not even defined as a turboprop (it's defined as a jet) so its' distinctive props are not defined at all.

We have an expectation that the equations used to join all the variables would, if all variables are entered correctly, perfectly reflect the real aircraft. As others in the previous posts have pointed out it does not work out to be true.

Pragmatically it's best to look at the two files as means to make the simulated aircraft fly as we wish. Forget what the variables are called, any resemblance to reality is purely coincidental.

Without the equations being published it becomes more of an art form than science.

We are back to the: “tinker around and fly, re-tinker and re-fly” till the performance resembles what we want.

I wonder if Asobo (MS2020) were given the equations? Or did they develop their own from scratch? Is there someone at Microsoft who developed the algorithms and he/she has long since left the company. Or do the maths only work well for the Stock Cessna 172 and Microsoft then also did a lot of fudging to make the other base aircraft work?

[jgf]

That's akin to the Republic XF-84H, which earned the nickname "Thunderscreech" due to the outer two feet of the prop going supersonic, even at idle speeds.  During engine runup on the runway it could reportedly be heard twenty miles away, generated a visible sonic boom whose shockwave could knock people down, nauseated and disoriented ground crew, and even interfered with control tower equipment.  One of the test pilots walked away from the program after one flight, stating, "You aren't big enough and there aren't enough of you to get me in that thing again".

 

FWIW the official record for fastest prop driven aircraft is a Piaggio P180 at around 576mph;  a Tupolov bomber is reportedly capable of over 600mph in level flight but this has never been done under official circumstances.  For piston engines it is held by a highly modifed F8f racer, "Rare Bear", at around 530mph.