power gauge for turboprops

[Tom Goodrick]

The main feature to note about this article is at the bottom. A universal gauge was developed that shows percent shaft horsepower at all times. In the beginning of this thread it was not eveident that a universal gauge could be made.

Topic: A POWER GAUGE FOR TURBOPROPS (Read 109 times)

Tom Goodrick
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A POWER GAUGE FOR TURBOPROPS
« on: Oct 20th, 2007, 10:15am » Quote Modify Remove

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First we should clarify that we are talking about jet turbine engines spinning props, often called "propjets" ot "turboprops". Their power is measyred as "shaft horse power" or SHP.

The problem of power for turboprops finally makes sense. I have a valid power gauge for turboprops showing shaft horsepower at all times and I have a way of calculating one of the specs that is often unknown.

Allen Peterson showed me this equation that he found both in Air Wrench and in AAM. He has been helping me with calculations and test flights. I have revised the form of the equation somewhat to make it easy to calculate using XML in a gauge:

SHP = R * T * K
Where R = engine RPM (N2)%; T = Torque%;

K = Max N2 RPM/100 * Max TQ/100 / 5252 /gear

where gear = 17.6 for Pratt&Whitney PT6A's.
If you know the max prop RPM, Max N2 RPM = Max Prop RPM * gear.
If you don't know Max TQ but you do know Max SHP (normally known), then
Max SHP = max Prop RPM * Max TQ / 5252 so we can solve for Max TQ:

Max TQ = Max SHP * 5252 / max Prop RPM.

Then we calculate K and put that value into the TPow gauge for that aircraft.

Example: Beech King Air C90GT (New version of the C90).
max prop RPM = 1900 Max RPM = 1900 * 17.6 = 33440 goes into the aircraft.cfg file.
Max SHP = 550 hp
Calculate Max TQ = 550 * 5252 / 1900 = 1520 lb-ft goes into the aircraft.cfg file.
Then K = 334.40 * 15.2 / 5252 / 17.6 = 0.05499 goes into the gauge code for TPow_BC90.xml

There should be an N2 gauge on the panel. This sometimes goes too high. I am still working on the adjustments to the N2 curves in the .air file. See 1503. I have set the far right value of CN2 = 100.0 with success in some cases.

You'll see max SHP at two conditions: 1) On the takeoff at about 30 KIAS and 2) Climbing through 5,000 ft with full throttle. Cruise is normally flown at 60% to 70% max SHP. Climb is normally done at 80-85% max SHP.

Now we can develop turboprop models with a sound basis in power ans then fix the drag and the fuel flow to meet specs, I have data in BRASSEY'S WORLD AIRCRAFT & SYSTEMS DIRECTORY that included max SHP and max prop rpm for all turboprop engines. The results will be very good models.

I have gauges now for the Beech B350, B200 and C90GT and for the Piper Meridian. We will be working on adjusting the FD for these aircraft to match published data using the new "TPow" gauges.
216.180.4.76



Tom Goodrick
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Re: A POWER GAUGE FOR TURBOPROPS
« Reply #1 on: Oct 23rd, 2007, 7:57pm » Quote Modify Remove

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After doing many tests with this power gauge, and using it in an Excel sheet to compute power from recorded data, I am convinced it works fine and can be used with a unity power scalar.

Here is some data on performance of the King Air 350 at fast cruise. The power is calculated as shown above. 1050 is the max SHP.

PERFORMANCE OF KA350 AT HIGH CRUISE

Note: Entire sequence recorded with full data.
Throttle was at 90% all the time.
Prop RPM was at 1500 all the time.

ALTITUDE__KTAS___KIAS___TORQUE%__POWER%__FUEL FLOW pph__ELEV TRIM deg
20,000FT___308.2___231.8_____80.4________75.1________538_____________-8. 03
22,000FT___312.3___227.2_____77.1________72.5________542_____________-7. 77
24,000FT___317.3___223.5_____75.2________71.2________545_____________-7. 55
26,000FT___324.4___221.1_____74.4________71.0________549_____________-7. 41
28,000FT___332.4___219.1_____73.9________70.9________554_____________-7. 30

Before takeoff (630 ft msl) weight was 15,000 lbs with 2300 lb payload and 3610 lb fuel.
CG was at 34.75%

After cruise test (56 minutes, from Huntsville, AL, to 60 nm west of Little Rock, AR,
est 330 nm), weight was 14,014 lbs and CG was at 34.79%. There were 3.28 hours remaining
at that cruise condition.



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Hans_Petter
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Re: A POWER GAUGE FOR TURBOPROPS
« Reply #2 on: Oct 24th, 2007, 10:34am » Quote Modify Remove

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That's great news! A general question regarding the turboprop modeling of MS -- does it seem sound? I'm asking because there's been a few discussions on runaway throttle, ground idle and cruise idle. Most casual simmers won't know anything about the two latter. What they know is that turboprops tend to want to start moving before you move the throttle lever / tap F3.
84.49.151.53

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best regards,

Hans Petter


Tom Goodrick
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Re: A POWER GAUGE FOR TURBOPROPS
« Reply #3 on: Oct 24th, 2007, 11:42am » Quote Modify Remove

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There is more data posted in the thread on FS performance issues under tutboprops.

Yes, I can say after a fair study of FS performance and a study of detailed pilot reports, FS does a good job of modeling the turboprops. But that is only true if the values for prop gear ratio, for N2 and for Max Torque in the aircraft.cfg match the specified shaft horsepower for the engine using the equation in the first note above.

When switching from a prop or jet aircraft to a turboprop, you have to stop the engines and restart them to get the Condition lever at the right setting. (This is exactly 40% of the "mixture" lever travel but is hard to set.) The starting process sets it correctly (using Ctrl-E). Then do not forget to turn the generator swithces to "GEN" or you will get alights-out about 2 or 3 minutes after takeoff. All flying can be done with the condition lever at Low Idle. In a real aircraft High Idle is sometimes used on final before landing. But often it is not used. This procedure varies between aircraft.

Beechcraft aircraft are generally flown with reduced prop RPM. This makes the cabin quieter but also keeps N2 from going too high.

The characteristic of turboprops always trying to creep forward on idle thrust is realistic. The standard procedure after getting some forward motion is to cut to idle and use slight negative pitch to control taxi speed. This can be done easily in FS using keys to control the throttle because one key tap is enough in either direction - positive or negative from zero.

In the climb data I just posted on the other thread, it is very interesting to see how power, torque and blade angle beta vary during a steady climb on autopilot. N2 also varies although the prop RPM is held constant.
« Last Edit: Oct 24th, 2007, 11:51am by Tom Goodrick » 216.180.4.249



Tom Goodrick
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Re: A POWER GAUGE FOR TURBOPROPS
« Reply #4 on: Oct 27th, 2007, 11:01pm » Quote Modify Remove

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All along I have been looking for a power gauge that was universal - one gauge useable with any aircraft of the same type. By accident I have finally found it. The equations shown in the top note on this thread boil down to a constant when you make a percent power gauge. With a little algebra, all the parameters specific to a praticular engine cancel out and you get a value of K=0.01 for a percent gauge.

So the power displayed as a percent of max simply
= N2 (%) * TQ (%) * 0.01

for all turboprops of every size.

Sometimes math makes things seem more complex and sometimes it simplifies things.

I call the new gauge simply "TPower".
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Tom Goodrick
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Re: A POWER GAUGE FOR TURBOPROPS
« Reply #5 on: Oct 29th, 2007, 7:12pm » Quote Modify Remove

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Note that, instead of multiplying by 0.01, you can divide by 100 and get the same result. That's what I do in this source code for the TPower gauge:

<Gauge Name="TPower" Version="1.0">
<Image Name="Box.bmp" ImageSizes="64,48"/>
<Element>
<Position X="14" Y="11" />
<Visible>(A:Circuit general panel on, bool)</Visible>
<Visible>(G:Var2) 0 == (A:Avionics master switch, bool) &&</Visible>
<Element>
<Text X="40" Y="14" Bright="Yes" Length="6" Font="Arial" Color="#00FFFF" Adjust="Center" VerticalAdjust="Center" Multiline="No" Fixed="No">
<String>%((A:TURB ENG1 CORRECTED N2, percent) (A:TURB ENG1 MAX TORQUE PERCENT, percent) * 100 /)%!6.2f!</String>
</Text>
</Element>
</Element>
<Element>
<Position X="11" Y="32" />
<Element>
<Text X="50" Y="12" Bright="Yes" Length="8" Font="Arial Bold" Color="#114444" Adjust="Left" VerticalAdjust="center" Multiline="No">
<String>TPower</String>
</Text>
</Element>
</Element>
<Mouse>
<Tooltip>Turbo Prop Power shp</Tooltip>
</Mouse>
</Gauge>


You can copy this source code text into a blank Notepad file and call it TPower.txt. Then change the extension to .xml and you have a gauge to hang on the panel of any turboprop. You'll need the little Box.bmp figure Coleman designed for stand-alone gauges.
I have now used this successfully in a variety of aircraft from the single-engine Piper Meridian to the quad-engine Douglas C-133B with four 7500 shp engines.

I have typed out the derivation of this equation. If anyone wants to see it, I'll email it to you.
« Last Edit: Oct 29th, 2007, 7:14pm by Tom Goodrick »

[Tom Goodrick]

Hans-Petter raised a second issue in this thread that was not answered. One was about proper partial power for the turboprop and one was about the difficulty of keeping a turboprop from creeping or moving significantly while supposedly stopped.

The partial power problem is answered this way. Many turboprop engines have a condition lever that sets either "high" or "low". This looks like a mizture control but has nothing to do with mixture although it uses the same controls in FS9. Some engine installations have no such lever except for the secondary use which is to shut off fuel or turn it on. That can be done with a small switch marked "Fuel ON/Off". Even in aircraft with this condition lever such as the King Airs, the lever is used only at the start where it is set at low and then on final approach where it can be set to high before landing. It is set back to Low after landing and before taxiing. In FS9 when you start the engine of a turboprop with CTRL-E, the condition lever is moved to Low automatically. I leave it there all the time. IT use on final seems to be questionable.

Another feature of turboprop engines in flight at partial power, is that the RPM or prop control has limited use. Its main use is to reduce the noise level. But it also reduces the power available with the throttle so using it may not be important. The Beech King Air series are now built with tuning forks in the walls to cancel some of the pro noise in the area of 1700 Hz. So it is customery to set 1700 RPM in the King Airs when in cruise.

The Piper Meridian single-engine turboprop has eliminated the prop lever in an act of simplification for the non-professional pilot. The engine is set at 2000 rpm and this is not changed.

Most turboprop engines today are controlled by computer for best operation in all conditions.

Turboprops are famous for creeping at idle throttle and taxiing too fast. On many the brakes must be used often, almost continuosly. On others pilots have developed the practice of running with a slight negative beta pitch to prevent creep. You can do this in FS with a single tap on F2 from the idle position.

You should also check your throttle control and verify that it really has a true idle setting. Many do not. I use the F keys (F1,F2,F3,F4) all the time for throttle control.

[Tom Goodrick]

I developed power gaues for piston engines that read the actual power value set within the sim and display it as a percent power. Unfortunately you need a different gauge to match the max power of each engine. There is no universal power gauge for piston aircraft. These power gauges fit on the panel so you can actually control the power of the engine directly. This is common today on modern panels with engine management computers even in small aircraft.

For the purpose of understanding a particular aircraft, you can download a "POWER PANEL" and a "TPOWER PANEL" from my site that are pop-up panels showing various propulsion-related parameters for trouble-shooting aircraft problems. These are usinversal and are not intended for use in flight. The piston Power panel shows total horsepower and thrust in addition to other useful items.

[Tom Goodrick]

Unfortunately, the TPower_Panel gauge was NOT available on my web site. Somehow I had forgotten to post it. But it is available there NOW. I have included in that zip the general Tpower gauge that gives percent power as you fly. Only one universal gauge is needed for power of turboprop engines. The TPower_Panel gauge is a pop-up you can install that will also show thrust, blade angle, condition lever setting, pressure ratio and fuel flow. This is good for trying to understand what the different controls do. I used it to prove that the Condition lever can just be left at the low idle setting it is left in when you start the engines with ctrl-E.

See the thread "How to Fly Turboprops in the FS flight performance section.

I have also posted the revised version of the Avanti as discussed in the Avanti threads.