Blackbird Sims / Milviz Forum Archive

Hi,

I would like to address my observations on torque change when RPM rises or decreases.

I've read the previous topic where this was mentioned (viewtopic.php?p=82867#p82867) but decided to make a new one specific to that single issue. I hope that's ok

Taguilo wrote:There is indeed an increase in torque with prop reduction, between 2.5 - 3.0 psi between 2200 and 2000 RPM. Full governor range is ~ 8 psi.

This change seems incorrect.

Basically - DHC-3 is powered with a free-turbine PT6A, so the gas generator runs independently of prop RPM (no physical connection between the two). Power lever will, therefore, set gas generator power, resulting in the amount of air blown into the free turbine that is connected to prop. Power output on the prop shaft should be similar to the power output of the gas generator. For the sake of the discussion, we can ignore the energy lost in between.

A little more theory: the power is (basically) torque multiplied by RPM. The formula for power would be Power = Torque x RPM. At this point, we it's unitless. Horse Power formula looks like this: Horse Power = Torque (foot pounds) X RPM ÷ 5252. We could calculate PSI to foot-pounds in Turbo Otter, but it's beside the point, and we don't need to calculate the actual power at any point. We just need to see the relation, which is derived from this equation: Power = Torque x RPM.




Leaving specific units aside we can calculate that power settings of 50 PSI / 2000 RPM should produce 100,000 abstract units of power. As has been mentioned earlier - changing RPM does not change the power (free turbine). So when RPM decreases the change in torque should keep the result the same. For example: At 1800 RPM the torque gauge should show 55.56 PSI. It is not so in Turbine Otter.

In my test flight (ISA, approx. 150 kias, 2000 ft, straight and level) I got the following results:
- 2200 RPM, 48 or 49 PSI
- 2000 RPM, 50 PSI
- 1800 RPM, 52 PSI
- 1600 RPM, 54 PSI

That means that moving prop lever from 2000 to 2200 increases power by 10,6%. And there is a corresponding change in speed.

There shouldn't be. No matter what RPM (within the governor range) - power output should be the same and the speed should be the same. Engine wear will differ. Noise will differ. But not the power (nor speed).




If my explanation is not detailed enough I would recommend this article:
http://blog.covingtonaircraft.com/2015/ ... -vs-power/

You can also see the thing I'm talking about in steveo1kinevo's video:
https://youtu.be/GZv-5RBdugM?t=823

Look at how he changes power settings for the cruise. He flies at following power settings: around 86 % / 2000 RPM. Then he reduces RPM to 1900 and gets around 90 %. Do the math - you will get 172000 from both 86*2000 and 90*1900. Actually, you will get 172K and 171K but this difference is far below the accuracy of me reading his gauges.



PS
Let me say that apart of this issue your DHC-3 is a joy to fly and I really love the work you put into correct simulation of this aircraft and it's engine!

Hi,

Thanks for your inputs. However, torque values are the correct ones , confirmed by a real Turbine Otter pilot.

It seems in this aircraft the propeller governor is rigged to deliver a bit less torque than the necessary to keep power constant when RPM are reduced, as to protect from overstressing and or overspeeding situations. That's also the reason for a change in speed. And I guess same reason as to limit RPM reduction to 1800 in flight (green area on tachometer).

Tomas

Thomas,

If the power is constant there is no way in which the propeller could "deliver a bit less torque". By the definition torque results from power and RPM. For any given power output and any given RPM there is only a single possible torque value. You can do nothing to modify it. I would recommend reading this theoretical explanation:
https://en.wikipedia.org/wiki/Torque

Or any other, if you prefer sources with a more reputable scientific background (although I have to say this Wiki article is very well researched).

When you get to the "Relationship between torque, power, and energy" - you will find the relation I wrote in my previous post. It's always Power = Torque x RPM.

And back to the PT6A - a free turbine engine consists of the gas generator and a turbine with the prop attached (plus some gear in between the turbine and the prop). As you set power (of the gas generator) you will have the same power on the prop shaft (minus some losses that are negligible in this argument).

The only thing left is setting your RPM. Torque has to be the result of a calculation - not some arbitrary decision.

Again - I urge you to review available footage which shows the thing I am talking about. There is no video I could find showing a different behavior.

And I guess same reason as to limit RPM reduction to 1800 in flight (green area on tachometer).

Green arc means "normal operation". You can fly safely in areas above and below the red line. As far as I know, there is no "lower" red line for any PT6A-34 (although I've only checked a couple of POHs). From those available to me (unfortunately I could not find a real-world DHC-3T POH) two state that "green arc" starts from 1 (RPM or % depending on the POH). Only Kodiak's states that green arc is 1900-2200 with an additional yellow arc in the range of 450-1050.

It's worth asking - why would there be a lower limit? It's not a piston or a geared turbine. You can have a very slow moving prop and your gas generator runs at its normal RPM. A lower limit would be important for geared turbines and especially for piston engines.



Respectfully I have to say that in this area (torque change on rpm change) the information provided by a real Turbine Otter pilot was inaccurate or simply misunderstood. As you said - it was "confirmed". Maybe the confirmation has just been too broad - as your PT6A is generally very well simulated - maybe this detail had been missed.

I don't believe just because the turbine gives a constant power output, that the actual delivered power output will also be constant. This is something that can only happen in an ideal world with 100% conversion efficiency.
I would imagine that inefficiencies in the conversion of the turbine hot gas output to the prop (torque x RPM) output will vary according to conditions.

Of course, there would be a loss. But there would be a loss always! With higher RPM and with lower RPM. This makes this loss insignificant in this discussion. The argument is not about whether the amount of HP delivered to the prop shaft is the same as generated by the gas generator (it is not - there is a very small loss).

My argument was limited to the fact that Milviz's DHC-3T has a torque gauge working in a different way than any other torque gauge documented on the web.

Torque gauges were installed in aircraft (in the early 40s) for two reasons. To prevent damage caused by putting too much torque on the shaft and to precisely measure the power generated to make planes more fuel efficient. With torque and RPM gauges crews could set the best power and therefore fly further on the same fuel (at first delivering bombs to targets and a couple of years later - flying passengers to destinations).


Now - please just analyze what Milviz engine is doing. In my test flights, the fuel flow was constant. Correctly so - the power was set and did not change. And with the change of prop speed (RPM) the speed rose. It's an evident sign that thrust rose. The conclusion would be - prop was more efficient on highest RPM and that resulted in higher thrust with no change in power generated. But that is obviously wrong. Prop efficiency does not increase with RPM (in many cases it decreases). And you mentioned energy losses yourself. Make your engine run faster and you'll get more heat, more drag and more losses. If there was to be a change - the thrust would be lower at high RPM due to bigger losses.

But that is purely theoretical as the described losses are close to none. In the real world, you can change prop RPM on a free-turbine turboprop and your speed will stay the same, your fuel flow will stay the same. The only thing different will be the noise (hardly a factor here as you don't need a lot of power to generate noise) and torque indication.

Here you have another clear example. 208 with PT6A.
https://youtu.be/-QAt_pOQwZQ?t=381

He goes from 17500 ft*lb and 1900 RPM (actually a little bit less than 1900) to 18200 ft*lb and 1750 RPM (a little bit more actually). The power stays the same. You can multiply this and the result will be within the gauge reading error.

The reason to change prop RPM in a PT6A is to reduce wear (a little) and to reduce noise to comfortable levels. But you don't want to reduce RPM too much - because it will limit power available. That may be why you have "normal operation" green arc starting from 1800 - as it is not practical to fly below this RPM.

CalypteAviation wrote: ↑

Tue Jan 22, 2019 8:43 pm

In my test flight (ISA, approx. 150 kias, 2000 ft, straight and level) I got the following results:
- 2200 RPM, 48 or 49 PSI
- 2000 RPM, 50 PSI
- 1800 RPM, 52 PSI
- 1600 RPM, 54 PSI

I am not an Otter pilot, but I have been flying PT6As for nearly 25 years and those numbers would be what I would expect to see out of a PT6 engine.

Torque is a measurement of engine work and expressed in SHP. Now the -34 seems a little unique as it has two options for getting to 750 shp.

64.5 at 2000 RPM ((64.5 x 30.57)x2000)/5252 = 750.9 SHP
58.7 at 2200 RPM ((58.7 x 30.57)x2200)/5252 = 751.6 SHP


49 at 2200 RPM ((49x30.57)x2200)/5252 = 627 SHP
50 at 2000 RPM ((50x30.57)x2000)/5252 = 582 SHP
52 at 1800 RPM ((52 x 30.57)x1800)/525 = 544 SHP
54 at 1600 RPM ((54 x 30.57))x1600)/5252 = 502 SHP

Even if I set maximum torque at 1600 RPM I still won't get maximum SHP

64.5 at 1600 RPM ((58.7 x 30.57)x1600)/5252 = 600 SHP

So for a given torque setting as propeller RPM decreases SHP will decrease. The only way to get maximum SHP out of the engine is either 64.5 at 2000 or 58.7 at 2200.

So why don't we just fly around wide open and max propeller RPM? One the propeller is a big noise maker and two for a given airspeed there is a most efficient Propeller RPM that will give you the best specific fuel consumption.

That is just how a PT6A works. Now you can argue how much torque increase based on propeller RPM change, but you are splitting hairs. Heck our FFS aren't that exact.

So for a given torque setting as propeller RPM decreases SHP will decrease.

That is exactly what I wrote in my previous posts - just in different order.

Now you can argue how much torque increase based on propeller RPM change, but you are splitting hairs

Just wanted to suggest a change that would bring Turbo Otter closer to real aircraft behavior. The problem is that in Turbo Otter there is a significant change in power at various RPM.

Check the first post:

CalypteAviation wrote:- 2200 RPM, 48 or 49 PSI
- 2000 RPM, 50 PSI
- 1800 RPM, 52 PSI
- 1600 RPM, 54 PSI

Power had been kept at the same level. So based on your post the Turbo Otter gets:
627 SHP @ 2200 RPM
582 SHP @ 2000 RPM
544 SHP @ 1800 RPM
502 SHP @ 1600 RPM

All with the same fuel consumption. That is like saying that 125 SHP went out of the window just due to RPM change in PT6A. In theory - this is just impossible. Could you do this on a plane you fly? Set the power lever and without changing its position get a change in power like this with only the RPM change?

there is a most efficient Propeller RPM that will give you the best specific fuel consumption

Correct me if I am wrong, but that is not the case for most PT6A engines. Let me cite from TBM 850 manual:

TBM 850 manual wrote:Propeller RPM utilization between 1600 and 2000 RPM is possible without changing performance

If that is not a big problem - would you mind taking a short video of gauges in your plane to show what happens on RPM change? I wonder if it will be different from the videos posted above.

CalypteAviation wrote: ↑

Sat Jan 26, 2019 3:10 pm

The problem is that in Turbo Otter there is a significant change in power at various RPM.

I am sure if you have manufacture's charts that show exactly how much torque should increase versus propeller RPM change at a given altitude and airspeed this can be incorporated.

I have not seen anything like this in the public domain and people that have this information (manufactures and FFS builders) don't share that information.

The SHP decrease is exactly per the information that Pratt and Whitney provide. I am not an engineer with P&W so I can only provide you with the information they provide their training partners. If you choose not to believe the information then you need to take that up with P&W.

From what you have written in your last post - I feel that you have not understood what I tried to describe. I'm sorry to take your time - that is probably due to my not so good English.

I am sure if you have manufacture's charts that show exactly how much torque should increase versus propeller RPM change at a given altitude and airspeed this can be incorporated.

No. I do not have manufacturers charts. I referred real-live videos showing exactly the situation tested in the sim. I can easily find more such videos - showing a different behavior than in the sim, and I could not find a single video showing PT6A behaving the way it does in the sim...

64.5 at 2000 RPM ((64.5 x 30.57)x2000)/5252 = 750.9 SHP
58.7 at 2200 RPM ((58.7 x 30.57)x2200)/5252 = 751.6 SHP

And how it works in the sim?
64.5 at 2000 RPM = 750.9 SHP at 98% Ng at 77 GPH
58.7 at 2200 RPM = 751.6 SHP at 94% Ng at 70 GPH

The question is simple - where is this energy from extra 7 GPH and where is this air from extra RPM of gas generator going? In the sim it somehow gets lots between the gas generator and the power turbine.

CalypteAviation wrote: ↑

Sun Jan 27, 2019 11:13 pm

The question is simple - where is this energy from extra 7 GPH and where is this air from extra RPM of gas generator going? In the sim it somehow gets lots between the gas generator and the power turbine.

Seven Gallons per hour for a 200 RPM propeller change is excessive. If fuel flow changes you might only notice one or two pounds per hour difference.

CalypteAviation wrote: ↑

Sat Jan 26, 2019 3:10 pm

there is a most efficient Propeller RPM that will give you the best specific fuel consumption

Correct me if I am wrong, but that is not the case for most PT6A engines. Let me cite from TBM 850 manual:

TBM 850 manual wrote:Propeller RPM utilization between 1600 and 2000 RPM is possible without changing performance

I seem to have missed answering this. As a propeller spins through the air it has two velocities, the radial velocity Vr and the Aircraft Velocity Va/c. These two velocities provide the Freestream Velocity and make up the total angle of attack the the propeller has. If you track the tip path through the air you have what is referred to as the helix. That is the propeller advance per RPM. At lower TAS a smaller advance is more advantageous for producing maximum thrust. At higher TAS a larger advance is more advantageous for maximum cruise speed.

Thus for every propeller there is a most efficient helix for a given TAS. On a airplane with a narrow range of RPMs available versus overall horsepower of the engine, such as the TBM the difference is not appreciable enough to derive different cruise charts. On other airplanes, such as the straight KA 200 charts were provided for maximum cruise with propellers at 1900, 1800, and 1700 RPM. The difference was maybe 50pph fuel over both engines and 5 KTAS. Hardly worth the cost of all that paper to print the charts.

There is an aeronautical engineering paper through ERAU that goes through all of the math explaining the above. I am a pilot, not an engineer, so I have to keep my discussion simple and I probably over simplified the above.

The difference was maybe 50pph fuel over both engines and 5 KTAS

Consider one important factor. Are you sure you are comparing performance charts for the same power settings?! I made this mistake. But we can also draw some interesting conclusions. So stay with me, even if the first example is not perfect.


I'm reading Super King Air Performance charts. I have these:
http://www.avialogs.com/index.php/en/ai ... 2todo.html

Let's say the KA is cruising at 16000 ft. ISA. Weight: 10000lbs.
7-85 Recommended Cruise 1700 RPM: 2230 ft-lb / 397 lbs/hr / 279 KTAS.
We can calculate: Power = 722 SHP / Fuel flow per 1 SHP = 0.549 lbs/SHP / Power per 1 KTAS = 2.588

7-93 Recommended Cruise 1800 RPM: 2130 ft-lb / 399 lbs/hr / 281 KTAS.
We can calculate: Power = 730 SHP / Fuel flow per 1 SHP = 0,546 lbs/SHP / Power per 1 KTAS = 2.598

7-101 Max Cruise 1900 RPM: 2077 ft-lb / 409 lbs/hr / 284 KTAS
We can calculate: Power = 751 SHP / Fuel flow per 1 SHP = 0,545 lbs/SHP / Power per 1 KTAS = 2.644

(keep in mind that I all calculations are based on a single engine)


I think that the conclusion is clear. It is exactly what TBM's POH stated:
"Propeller RPM utilization between 1600 and 2000 RPM is possible without changing performance". In the case of KA, we could write: evidence shows that propeller RPM utilization between 1700 and 1900 RPM is possible without changing performance. If you look at the numbers - the performance does not change considering some slight power changes between the charts. Sure - it would take more SHP to keep higher speed (due to higher drag). And lower fuel flow per 1 SHP is easily explained with the gas generator efficiency rising with Ng.


Now I noticed my mistake - I looked at 16000 ft where the power settings differ between all three charts. So let's look at 20000ft. Again ISA and 10000lbs.

7-85 Recommended Cruise 1700 RPM: 1921 ft-lb / 343 lbs/hr / 276 KTAS.
We can calculate: Power = 622 SHP / Fuel flow per 1 SHP = 0.551 lbs/SHP / Power per 1 KTAS = 2.253

7-93 Recommended Cruise 1800 RPM: 1826 ft-lb / 344 lbs/hr / 277 KTAS.
We can calculate: Power = 626 SHP / Fuel flow per 1 SHP = 0,549 lbs/SHP / Power per 1 KTAS = 2.26

7-101 Max Cruise 1900 RPM: 1801 ft-lb / 357 lbs/hr / 282 KTAS
We can calculate: Power = 652 SHP / Fuel flow per 1 SHP = 0,548 lbs/SHP / Power per 1 KTAS = 2.312

Do you see the consistency of KA's performance?


Now take a look at the part in bold font because this is important for my argument about Milviz's Otter. Almost the same power. One can safely assume that power lever position is nearly the same in both cases (I said "nearly" but actually all these values are probably beyond the precision of levers and gauges).

Now take a look at RPM and torque values, please. There is consistency between the drop of RPM and rise of torque. The same power will give 1700 RPM at 1921 ft-lb or 1800 RPM at 1826 ft-lb.

The problem (that I think could be easily corrected - with all the amazing work Milviz put into the PT6A already) is this:
At the same power, Otter shows these values.
49 at 2200 RPM ((49x30.57)x2200)/5252 = 627 SHP
50 at 2000 RPM ((50x30.57)x2000)/5252 = 582 SHP
52 at 1800 RPM ((52 x 30.57)x1800)/525 = 544 SHP
54 at 1600 RPM ((54 x 30.57))x1600)/5252 = 502 SHP

So basically - a huge drop of power due to prop RPM change.

I applaud your efforts but you continue to equate SHP as a liner constant and not consider propeller efficiency in you conclusion that RPM has no effect on performance. There are in fact many variables in the overall performance of the aircraft which is why, as expected, the SHP caculations do not remain static. They should not as they only represent what the reduction gearbox is doing.

Even though it is a free turbine, it is not a frictionless environment inside the engine. Nor is power transfer between the N1 and N2 turbines liner. The turbine blades are airfoils and the hot gas connecting them a fluid. As the N2 section slows down the N1 section is effected. Since the fuel control system governs N1 the FCU will automatically adjust. So even if you don’t move the power levers, that does not mean the N1 section is static. The entire engine is a highly dynamic environment. Changing Propeller RPM effects N2 and N1.

The cruise charts were never meant to be extrapolated to try and find a constant SHP setting nor do they ever say that N1 remained constant across 1900, 1800, and 1700 RPM. That is an incorrect assumption that you made, most likely seeking confirmation bias.

If I seek chart performance I will allow the airplane to stabilize after arriving at my cruise altitude, adjust propeller RPM to the desired setting and then adjust torque. I would then expect engine performance to be +10 / -25 ppl on fuel flow and TAS to settle in close to book. Remember these are performance planning charts, not the engine specification charts. There is some slop in actual aircraft performance. Most pilots will complain when the airplanes performs noticeably less than book. Most happily accept an airplane that is slightly faster, burns a little less fuel, or runs a little cooler.

To truly see how the engine is performing versus specifications you need that charts out of the Pratt and Whitney maintenance manual. That will allow checking for minimum acceptable performance under very specific conditions.

On the DHC-3T: You have only stated that you dislike the change in torque versus RPM thinking that it is too aggressive. That is fine and as I told you before MV would be amicable to changing this if provided with data that provides specific performance output from a PT6A-34. All I have heard is you expect there to be an higher increase in PSI so that power is nearly liner.

The other piece of this is how does the airplane perform when compared to performance charts. Does it burn close to the fuel charter and cruise at close to the expected TAS when flown to chart specifications. Is that performance repeatable across multiple DA and weight configurations. If the answer is yes then they got the performance correct as far as what can be pulled from the performance planning charts. Everything else is subjective. Does the airplane generally feel right or the engines generally look to perform correctly. Without the charred data it becomes guesswork. The software engineers need to recreate the curves.

We need something more specific. ETM data, charts, a video from a turbo Otter, or even experience flying this airplane.

As Ken has stated, power measured in SHP does not remain constant when RPMs are reduced/increased. Extreme example of this would be an engine running at max N1 (Ng) RPM with the propeller locked; as prop RPM will be 0 SHP will be 0 as well no matter whatever torque value obtained.

For the Turbine Otter the only reference values we have for SHP are the already described for max torque at 2000 and 2200 prop RPM.
As we lack of complete performance charts, we have to rely on information provided by people with experience flying this aircraft.
Bear in mind performace in this case is associated to the characteristics of the propeller modelled, number of blades, max pitch angle, min pitch angle, blade size, etc. Same aircraft with a different propeller (4 blades, etc) would yield different data.

So, until we can find (if ever happens so) an official source of performance data for this engine in this aircraft, with same propeller, we must stick to what we have been told by Turbo Otter pilots. Flight characteristics have been tested in a whole by them and found correct within acceptable limits, compared to the real plane.

Now I will lock this thread as it makes no sense to keep discussing theorical numbers.

Tomas