FSXA Superchargers and engine power

[Dutcheeseblend]

Hi everybody,

Currently, I'm working on the engine performance of my Fokker T.5. It is equipped with Bristol Pegasus XXVI engines (equipped with centrifugal supercharger), with the following ratings:

Few things:

I added the supercharger properties in the piston_engine section of the aircraft.cfg.

supercharged=1
supercharger_boost_low_end=1.0
supercharger_boost_high_end=2.376
supercharger_power_cost=0.05

The number of 2.376 is achieved by checking the boost at sealevel. At throttle 100%, and a sealevel pressure of 29.90 inHg, it should boost 10.18 inHg (equals 5 psi), so the engine manifold pressure should come at 40.08 inHg. This is checked using AFSD and eventually, 2.376 would be the supercharger high end multiplier.

The value of 0.05 (equals 5%) of power cost, is however a random chosen value. Is this realistic? I doubt, but at the other hand I can't even guess the correct value.

Selecting 100% throttle, parking brakes on, sealevel, gives me:
- Engine MP of 40.08 inHg (correct)
- Engine shaft rpm of 2400 (200 too low)
- Engine shaft power of 833.4 HP (within given range)

As soon as I'm take-off (that is, 55 ft AGL), my rpm has gone to 2600, so I wonder why it doesn't come at this value during static full power run?

Things get even worse when I've arrived at 10000 ft, ambient pressure 20.55 inHg:
- MP of 27.44 inHg (should be 30.73 with 5 psi boost)
- RPM of 2599 (well that's OK)
- Power of 596.2 HP (faaaar to low)

So my questions are:
- Is the supercharger power cost assumption correct?
- Why does my engine not get the 2600 rpm at full power runup?
- Why are the altitude results so incorrect?

I just don't get it... Thank you in advance!

Daan

 

[AerobaticsFS]

I got a question about the boost_high_end: should it not be 40.08 / 29.90? (Which is 1,34) At least it is what I did with my Sukhoi Su26. Maybe I'm wrong, but it is what I thought

 

[Dutcheeseblend]

I got a question about the boost_high_end: should it not be 40.08 / 29.90? (Which is 1,34) At least it is what I did with my Sukhoi Su26. Maybe I'm wrong, but it is what I thought

Could be, but I'm not sure. Both the high_end and low_end seem to cooperate in some way: to achieve the same results as with 2.376;1.0, you could also do 1.376;0.0.
Your number 1.34 is however exactly the Boost Gain number that AFSD* gives me during flight tests. But with the entered number or 2.376, the MP will be at the desired value

The boost_high_end variables are multipliers, which isn't very fortunate I guess. Still sitting with engines that do not perform as they should.

Edit: let's state it this way: I don't know how it works, you I'm just guessing

* Very useful tool: http://www.aero.sors.fr/designer_pilot_utilities.html

 

[Dutcheeseblend]

Is it constant speed or fixed pitch?

It's a constant speed prop!

 

[NDORFN]

To get the extra 200RPM at ground test you can adjust the max_rpm_mechanical_efficiency_scalar/idle_rpm_mechanical_efficiency_scalar which I believe are two ends of a constant scale and I think they relate entirely to the effectiveness of the constant speed unit, and/or adjust the max_rpm_friction_scalar/idle_rpm_friction_scalar which again I think are two ends of a constant scale and relating entirely the power of the radials driving the CSU.

What's the max allowable HG at takeoff?

 

[Dutcheeseblend]

To get the extra 200RPM at ground test you can adjust the max_rpm_mechanical_efficiency_scalar/idle_rpm_mechanical_efficiency_scalar which I believe are two ends of a constant scale and I think they relate entirely to the effectiveness of the constant speed unit, and/or adjust the max_rpm_friction_scalar/idle_rpm_friction_scalar which again I think are two ends of a constant scale and relating entirely the power of the radials driving the CSU.

What's the max allowable HG at takeoff?

Thanks, I'm gonna find out about these scalars.

The max allowable during take-off is 101.9 cmHg or 40.12 inHg.

The pilot instructions for T.5 with Pegasus XXVI gives:

- Maximum during take-off (2600 rpm): +5 psi, +352 gr/cm2, 101.9 cmHg.
- Maximum during climb (2475 rpm): +3.5 psi, +246 gr/cm2, 94.1 cmHg.
- Maximum during 5min full power horizontal flight (2600 rpm): +5 psi, +352 gr/cm2, 101.9 cmHg.
- Maximum during cruise (2250 rpm): +3.5 psi, +246 gr/cm2, 94.1 cmHg.
- Maximum during leaner cruise: +0 psi, +0 gr/cm2, 76 cmHg.

 

[Dutcheeseblend]

To get the extra 200RPM at ground test you can adjust the max_rpm_mechanical_efficiency_scalar/idle_rpm_mechanical_efficiency_scalar which I believe are two ends of a constant scale and I think they relate entirely to the effectiveness of the constant speed unit, and/or adjust the max_rpm_friction_scalar/idle_rpm_friction_scalar which again I think are two ends of a constant scale and relating entirely the power of the radials driving the CSU.

What's the max allowable HG at takeoff?

Ok, modifying the max_rpm_mechanical_efficiency_scalar and idle_rpm_mechanical_efficiency_scalar to 1.2 and 0.75 respectively, results in the engine stabilizing at 2600 rpm full throttle and 600 rpm idle (as mentioned in the handbooks).

Err, somehow your 'constant speed or fixed prop' post has disappeared? I remember faultly pressing 'Report' instead of 'Reply', if that's the case...

 

[NDORFN]

Ah ok. The reason you'll be getting a low reading in HG at 10000ft is because the air is thinner and you have no way of increasing throttle. My understanding is that supercharged airplanes are able to throttle higher than max for takeoff but the pilot either stops at max or there are notches in the levers. That way, as a climb is made and the manifold pressure drops, the pilot can keep increasing the throttle to maintain max for climb until critical altitude is reached and no more throttle can be added, and manifold pressure drops off after that.

So I THINK, and I emphasize "think" here because I'm no exert, you need to find out what it's critical altitude is, it's probably the 10000ft altitude stated in the references you're using, then increase the supercharger_boost_high_end to match 30.73 at that altitude. In theory, this should mean that at sea lever it will take slightly less throttle percentage to make 40.08 inHG but you'll have the extra to maintain pressure as you go up. If that's correct, then the extra 3.29 inches will provide an exponential amount of HP, not the 300 horses you're short of but it'll help.

How are you measuring you HP output? And how do you find out what the ambient pressure is?

 

[NDORFN]

Ok, modifying the max_rpm_mechanical_efficiency_scalar and idle_rpm_mechanical_efficiency_scalar to 1.2 and 0.75 respectively, results in the engine stabilizing at 2600 rpm full throttle and 600 rpm idle (as mentioned in the handbooks).

It makes sense that you've subtracted from the idle what you've added to the max, because if you don't it messes up the the scale of you RPM range. The 0.05 difference accounts for the extra 200RPM you've achieved. So I guess it's a solution.

Do you happen to know what it's like on the prototype ie at 40.08 inHG does it only just make 2600RPM or does it slam against the constant speed unit (so-to-speak) and make it work hard not let it go over? I'm interested because if you want to make realistic sounds later, the PROP and PROPFEATH sounds are governed by how hard the CSU works. So you can end up with some strange sounding stuff if the mechanical and friction scalars aren't right. Which could be what really happens on the prototype... it's something I haven't been able to find out.

 

[Dutcheeseblend]

Ah ok. The reason you'll be getting a low reading in HG at 10000ft is because the air is thinner and you have no way of increasing throttle. My understanding is that supercharged airplanes are able to throttle higher than max for takeoff but the pilot either stops at max or there are notches in the levers. That way, as a climb is made and the manifold pressure drops, the pilot can keep increasing the throttle to maintain max for climb until critical altitude is reached and no more throttle can be added, and manifold pressure drops off after that.

The T.5's throttles have four 'sections' (dents probably, but that's not mentioned): DICHT (closed), KRUISEN (cruising), MAX:SNELH: (maximum speed) and START (take-off).

I should find out more about the throttle quadrant.

So I THINK, and I emphasize "think" here because I'm no exert, you need to find out what it's critical altitude is, it's probably the 10000ft altitude stated in the references you're using, then increase the supercharger_boost_high_end to match 30.73 at that altitude. In theory, this should mean that at sea lever it will take slightly less throttle percentage to make 40.08 inHG but you'll have the extra to maintain pressure as you go up. If that's correct, then the extra 3.29 inches will provide an exponential amount of HP, not the 300 horses you're short of but it'll help.

The documentation mentions an 'automatic manifold pressure control' which is native to the Bristol Pegasus. This system limits the maximum MP to 101.9 cmHg or 40.12 inHg. Probably, this is also of importance.

How are you measuring you HP output? And how do you find out what the ambient pressure is?

I use AFSD (mentioned above). The HP output is the shaft power.

 

[Dutcheeseblend]

It makes sense that you've subtracted from the idle what you've added to the max, because if you don't it messes up the the scale of you RPM range. The 0.05 difference accounts for the extra 200RPM you've achieved. So I guess it's a solution.

Do you happen to know what it's like on the prototype ie at 40.08 inHG does it only just make 2600RPM or does it slam against the constant speed unit (so-to-speak) and make it work hard not let it go over? I'm interested because if you want to make realistic sounds later, the PROP and PROPFEATH sounds are governed by how hard the CSU works. So you can end up with some strange sounding stuff if the mechanical and friction scalars aren't right. Which could be what really happens on the prototype... it's something I haven't been able to find out.

If I understand you correctly, it is just making 2600 rpm (slight overshoot during stabilization). The resulting prop rpm is 1731 and beta blade is 15.3 deg.
Sound is still doing OK, higher pitch but no strange effects.

 

[Roy Holmes]

The value of 0.05 (equals 5%) of power cost, is however a random chosen value. Is this realistic? I doubt, but at the other hand I can't even guess the correct value.

The power cost would have been much more than 5%
A supercharger takes power from the engine because it is driven by the engine.
The RR Merlin supercharger on an early version took 150 HP. Without supercharging the engine developed about 750 HP and with a supercharger it produced about 1000. This gave it an increase in HP from 600 (750 minus 150) to 1000. for a net gain of 250. So the power cost was 150 to gain 400 which is 37.5%
Roy

 

[NDORFN]

The resulting prop rpm is 1731 and beta blade is 15.3 deg.

Is it geared?

Sorry for pestering.

 

[Dutcheeseblend]

The power cost would have been much more than 5%
A supercharger takes power from the engine because it is driven by the engine.
The RR Merlin supercharger on an early version took 150 HP. Without supercharging the engine developed about 750 HP and with a supercharger it produced about 1000. This gave it an increase in HP from 600 (750 minus 150) to 1000. for a net gain of 250. So the power cost was 150 to gain 400 which is 37.5%
Roy

Thank you Roy! So this is another variable influencing the testing process.

Increasing the scalars as suggested earlier in this thread, does not only increase the RPM but also the HP.

Is it geared?

Sorry for pestering.

Yes, it's a reduction of 0.666!

 

[NDORFN]

According to AFSD data, the supercharger_power_cost parameter should be zero. I took a turbocharged 450HP at 36.5 inHG aircraft with turbo, and replaced it with the supercharger parameters and it comes to 450HP at 36.5 inHG at zero supercharger_power_cost. So long as the supercharger_boost_high_end is set high enough to make 36.5 or higher, it produces that figure, even if I set the high_end to make 75 inHG... still get 450HP at 36.5. I might be missing something here, but I think the supercharger function is basically exactly the same as the turbo but without lag.

 

[Dutcheeseblend]

According to AFSD data, the supercharger_power_cost parameter should be zero. I took a turbocharged 450HP at 36.5 inHG aircraft with turbo, and replaced it with the supercharger parameters and it comes to 450HP at 36.5 inHG at zero supercharger_power_cost. So long as the supercharger_boost_high_end is set high enough to make 36.5 or higher, it produces that figure, even if I set the high_end to make 75 inHG... still get 450HP at 36.5. I might be missing something here, but I think the supercharger function is basically exactly the same as the turbo but without lag.

Ok, thanks!

I must admit that I'm still learning about turbo- and superchargers. Some other fellas at another forum told me that the given boost numbers (+5 and +3.5) should not be added to the ambient pressure, but to the sealevel pressure. Therefore, the manifold pressure at altitude should be 101.9 cmHg (40.something inHg). So, if I am correct, then the supercharger_boost_high_end should be adjusted to achieve 101.9 cmHg at 10000 ft. Is this correct? Still trying to understand everything, but probably I'm just trying to join anywhere mid-learning-curve

So, if I then have achieved 101.9 cmHg at 10000 ft, AND the corresponding RPM and HP, I should make sure the max numbers at sealevel are also correct. Difficulty is that the throttle levers should be at 100% position.

Now I have a few questions: is this such a different engine? Am I thumb in understanding? Or is FSX so whacky in simulating engines?

Thank you.

 

[Roy Holmes]

I think the supercharger function is basically exactly the same as the turbo but without lag

To the extent that they both provide the ability to maintain sea level power up to their rated altitude, the function is the same. That may be all you need to consider for modeling performance.
The essential difference is how that function is achieved.
For illustration, consider a Spitfire or Mustang with an inline V-12 engine. The exhaust pipes are really short and each cylinder has one.
Then consider a radial engine with equivalent performance. The exhaust system connects all cylinders and then it exhausts through a single pipe.

In the first case you have to use a supercharger for obvious reasons. That supercharger by its design takes power from the engine to drive its compressor, so there must be a power cost. The stock P-51 has supercharger_power_cost = 0.22.

In the second case you just put a turbo compressor in the exhaust system and what would otherwise be wasted energy is used to generate boost, so there is no real power cost.

I guess if you tell the sim that the BHP is X and then change the type from turbocharged to supercharged, the sim might not take account of the power cost. Be interesting to do it the other way, start as supercharged with say 20% power cost then change to turbocharged and see what happens
Roy

 

[Paul Domingue]

The power cost would have been much more than 5%
A supercharger takes power from the engine because it is driven by the engine.
The RR Merlin supercharger on an early version took 150 HP. Without supercharging the engine developed about 750 HP and with a supercharger it produced about 1000. This gave it an increase in HP from 600 (750 minus 150) to 1000. for a net gain of 250. So the power cost was 150 to gain 400 which is 37.5%
Roy

Roy, I'm working on the early Merlin III and later on the XX. I have not been able to find the specifications on those engines. You wouldn't happen to have them would you?

 

[Roy Holmes]

"Spitfire" by Jeffrey Quill ISBN 0 09 937020 has a lot of engine specs and some speed/altitude charts
"Sigh for a Merlin" by Alex Henshaw has a lot of detail in the text, but no charts
Avialogs has Pilot's Notes for about 10 Spitfire variants, but they have little performance data in them.
The Merlin III had 1030 HP and the Merlin XX had 1280.
Roy

 

[NDORFN]

Difficulty is that the throttle levers should be at 100% position.

Throttle is where I get confused. On the supercharged set up I've done, it is the throttle position that determines critical altitude ie. you reach critical altitude when you run out of throttle extension, but I don't think that's how it's supposed to work. So I think it may be that a naked engine in the sim is effectively set to have a critical altitude of sea level. Adding the turbo increases boost to specified MP (which also determines throttle limit) and also allows you to set critical_altitude so that takes care of that. The supercharger set up doesn't use the critical altitude parameter as far as I can tell, so maybe it has to be tuned in the air file using entries like Torque Factor vs Fuel/Air Mixture and worked out by trial and error. If that's the case and I set it up correctly, I would need to use a high supercharger_boost_high_end combined with a supercharger_power_cost to make the correct figure, which in your situation would allow you the throttle position/power correlation you need.

I'm confused... I'd probably have lost my mind by now if it wasn't for Roy.

I'm going to experiment and come back with some results.