TF-34 Thrust

[Caldera]

Hey All,

 

I know that this has most likely been hashed out and beaten into many dead horse dog food bags.  For that I apologize, the forum search function annoys me to no end.

 

But...

 

As per usual, I am missing something.  I ran tests today trying to determine endurance.  The altitude was 13800 and I used True Airspeed allowing speed stabilization before I recorded data.  The fuel and fan gauges are analog so forgive a bit of in-accuracy.

 

The results I got:

Fan     Fuel    Speed

83      25.0    364         Maximum

80      22.0    348

75     17.0      314

70     13.7      287

65     11.5      263

60     9.0       228  

55      NA      NA         I could not maintain level flight on autopilot as the AOA continued to increase

 

The problem that I have is that if I compare aircraft speed to fan speed then the chart comes out pretty darn linear.  And if I compare fuel flow rate and aircraft speed that comes out pretty darn linear.

 

This is from Code7700.com/aero_range_performance

 

 

 

 

OK.... 

 

So drag equals thrust if velocity (aircraft speed) stays constant.  From the diagrams above, the drag is increasing faster than the velocity past the Minimum Drag point.

 

From my testing, why would a linear change in fan speed (thrust) result in a linear change in speed? 

Does the efficiency of the fan increase with RPM?

Does the A-10 only operate at or near the Maximum Range point?

 

Caldera

 

 

Edited April 14, 2021 by Caldera

[jaylw314]

53 minutes ago, Caldera said:

 

 

OK.... 

 

So drag equals thrust if speed stays constant.  From the diagrams above, the drag is increasing faster than the velocity past the Maximum Range point.

 

From my testing, why would a linear change in fan speed (thrust) result in a linear change in speed? 

Does the efficiency of the fan increase with RPM?

 

Caldera

 

 

 

Clarification--the diagram you're referring to is a FUEL FLOW vs velocity diagram.  Fuel flow often used as a proxy for POWER, not force or thrust.

 

Since Power = Thrust x speed, that means Thrust = Power / speed.  On that graph, your thrust would be the slope of any straight line, and the Maximum Range point would be the point of minimum thrust required for level flight.

 

In ideal conditions, the fan should produce thrust directly proportional to RPM.  It probably doesn't work out that way in real life, though, and I don't know what it's supposed to look like in the A-10

 

Addend:  Oops, formatting issue.  I was referring to the second diagram (Power vs velocity).  The first diagram is actually a Thrust vs velocity diagram, which further adds to the confusion

 

Edited April 14, 2021 by jaylw314

[Frederf]

9 fuel gives 228 speed

25 fuel gives 364 speed

 

160% speed increase required a 278% fuel increase

160% squared is 254% It's looking like that the fuel-speed relationship is to a power slightly more than 2

[Caldera]

Frederf,

 

I am not following you exactly. 

 

Basically what I remember is:

In air        -->    double the speed    -->    square the power required    (airplane)

In water   -->    double the speed    -->    cube the power required       (submarine)

 

That kind of what you are saying?

 

Fuel Flow Rate = Engine Power

 

If I get it right then, I should ignore the linear change in the throttle setting and pay attention to the fuel flow rate changes which are not linear? 

And...

Press the EASY BUTTON.

 

Caldera

[zinhawk]

This is getting in the weeds of engine theory so I'm hoping to learn as well. What I can offer is my practical experience as I am a TF-34 builder and maintainer. Hopefully this answers some questions:

 

1. TF-34 is not a turbojet. Turbojets are single spool (1 compressor, 1 turbine). Pretty much just the core part of a bypass engine so yeah fuel flow means more direct relationship with single compressor speed and thrust. Low bypass turbofan (F100/400 series engines) are a close cousin running dual spool (2 compressors, 2 turbines) and while the low pressure section helps with efficiency and performance, the majority of the thrust still comes out of the core. I don't know enough about those metrics to compare, but I am curious to check that out. A high bypass turbofan (TF-34 and any heavy/civilian application) is the inverse function of the former. ~80% of your thrust equation is directly proportional to the volume of air coming off the Fan (low pressure compressor). A near linear relationship with fan speed and thrust makes sense.

 

2. Faster fan/compressor speed is more efficient due to heat. The blades of the rotors and stators expand to ideally capture all the airflow throughout with zero leakage and increasing discharge pressure. On the turbine side the seals around the rotors do the same thing to capture the outflow and drive the fan to faster speeds with less required fuel flow. Over time the seals wear out and we have to up the fuel flow to achieve the same fan speeds as before. We don't care about fuel flow (as long as it is in limits) or core speed (as long as it is in limits) or ITT (as long as it is in limits), we just focus on fan speed for thrust. The previous three things are a problem when fan speed isn't reached.

 

3. No the A-10 does not fly around at max the whole time. The DCS engine is 4% too low at max on the average. How that scales at the other settings I do not know for sure.

Edited April 15, 2021 by zinhawk

[Frederf]

Parasitic drag should go up by speed squared but induced drag goes down as speed increases. Total drag should be semi-linear. There is a slight >1 exponent above LDmax but it would take careful graphing to see it.

[jaylw314]

10 hours ago, Caldera said:

If I get it right then, I should ignore the linear change in the throttle setting and pay attention to the fuel flow rate changes which are not linear? 

And...

Press the EASY BUTTON.

 

Caldera

 

Yup.  with high-bypass turbofans, turboprops and even piston prop motors, fuel flow is pretty good approximation of engine power output, and a good way to cross check your other engine performance instruments

[Caldera]

Zin,

 

From what I can figure out and please let me know where I miss.

 

Chuck's Guide page 138.

 

 

 

The TF-34 has, of course, two rotors.  The High Pressure (HP) Rotor drives the compressor.  It has 14 compressor stages and 2 turbine stages, rotating some where near 17000 rpm at full power.  It is attached to the outer shaft.  The Low Pressure (LP) Rotor has 4 turbine stages and rotates about 7000 rpm at full power..  It is attached to the center shaft and drives the Fan through a reduction gear.  Both rotors are free wheeling and the rpm of each is directly proportional to fuel burn rate.

 

The compressor does produce super heated air.  In my experience, it could be 800 to 900 F and 200 psia or more.  The compressor itself may consume as much as 65% of the total horse power produced by the the HP turbine.

 

I do not know if the the engine has Inlet Guide Vanes (IGV) or Variable Stator Vanes (VSV).  Those would be present to increase compressor efficiency, but increase weight.  With the fan out front, probably not?

 

The combustor has 18 annular fuel nozzles (cans). 

 

Indicated - The rpm of the HP rotor is the core speed

Indicated - The rpm of the LP rotor is the proportional to the Fan speed

 

The rpm's are expressed as percentages of maximum rpm for the purpose of cross aircraft pilot simplicity?

 

In this engine, the HP rotor essentially functions as a gas generator.  The Interstage Turbine Temperature (ITT) is essentially the Exhaust Gas Temperature (EGT) for the engine.  That means the the 4 LP rotor turbine wheels are going to consume a vast majority of the energy contained the high velocity high temperature gas exhausting from the HP rotor.    In doing so, converting that energy into driving the fan.  In my opinion, the 4 turbine stages would be designed to be quite efficient.  And that would be extremely relevant to the overall fuel economy.

 

It is my guess, that the fan is producing the vast majority of thrust from this engine.  As you say, at least 80%.

 

 

Frederf,

 

Thank you!

 

My goal was to find the most efficient endurance.  Basically, the Maximum Range on the charts above.  Do you happen to have those numbers?

 

Zin and I have had engine discussions before.  Have you ever looked into or performed testing to determine the "simulated" engine thrust?

 

I can only think of doing some kind of acceleration test.

 

Caldera

Edited April 15, 2021 by Caldera

[jaylw314]

13 hours ago, Caldera said:

My goal was to find the most efficient endurance.  Basically, the Maximum Range on the charts above.  Do you happen to have those numbers?

 

Zin and I have had engine discussions before.  Have you ever looked into or performed testing to determine the "simulated" engine thrust?

 

I can only think of doing some kind of acceleration test.

 

Caldera

 

 

Maximum range occurs near best glide speed -- 140 KIAS +/- 2 KIAS for every 1,000 lbs +/- 30,000 lbs

 

Maximum endurance will occur slightly below that speed

[Frederf]

Endurance is minimum fuel per unit time which is LDmax. In level flight endurance case L = 1 x weight of airplane so LDmax is really Dmin. For level constant speed flight Dmin occurs as a specific AOA which occurs at a specific speed.

 

Range is an optimization of distance traveled for fuel burned so in general it depends on winds, but you can take zero wind case. On fuel flow v speed graph every efficiency is a sloped line. Long distance on little fuel is a shallow line while little distance on much fuel is a steep line. Endurance optimum is the shallowest sloped line which is actually possible. The drag curve tangent point is that best possible.

 

AOA gauge has marks for (in increasing AOA) range, endurance, and approach. With that tool you can find the particular speed for your situation. The only caveat is that range is maximum air distance not ground distance. You'll go faster into a headwind and slower with a tailwind to max range.

[zinhawk]

Sorry was out a few days trying to merc bears.

 

On 4/15/2021 at 10:03 AM, jaylw314 said:

 

Yup.  with high-bypass turbofans, turboprops and even piston prop motors, fuel flow is pretty good approximation of engine power output, and a good way to cross check your

other engine performance instruments

 

I don't see this being the case with high bypass or at least this engine if I am understanding correctly in that you are associating X pounds per hour with Y pounds of thrust. Newer/well built engines will run with much less PPH at max than worn engines but have essentially the same thrust because of tuning to the same fan speed. And there are different levels of worn depending on how good the builder was. We often want to slap pilots silly who align PPH for fuel burn calculations and then complain of asymmetric thrust. "Well, dummy, you throttled back your weaker engine!". Of course this point is moot for the purposes of DCS with 2 clones. 

 

On 4/15/2021 at 10:42 AM, Caldera said:

Zin,

 

From what I can figure out and please let me know where I miss.

 

Chuck's Guide page 138.

 

 

 

The TF-34 has, of course, two rotors.  The High Pressure (HP) Rotor drives the compressor.  It has 14 compressor stages and 2 turbine stages, rotating some where near 17000 rpm at full power.  It is attached to the outer shaft.  The Low Pressure (LP) Rotor has 4 turbine stages and rotates about 7000 rpm at full power..  It is attached to the center shaft and drives the Fan through a reduction gear.  Both rotors are free wheeling and the rpm of each is directly proportional to fuel burn rate.

 

The compressor does produce super heated air.  In my experience, it could be 800 to 900 F and 200 psia or more.  The compressor itself may consume as much as 65% of the total horse power produced by the the HP turbine.

 

 

Super heated yes but compared to chamber and turbine temps it is considered rather cool. I'm not 100% positive on compressor temps since we do not measure that but those numbers sound about right. Discharge pressure is in the 300s. I don't know the balance of power between the turbines but sure that makes sense.

 

On 4/15/2021 at 10:42 AM, Caldera said:

I do not know if the the engine has Inlet Guide Vanes (IGV) or Variable Stator Vanes (VSV).  Those would be present to increase compressor efficiency, but increase weight.  With the fan out front, probably not?

 

The combustor has 18 annular fuel nozzles (cans). 

 

 

1 set of IGV and the first few stages of the compressor are VSVs that are adjusted within fine limits closed and opened. These actuate from the fuel control based on a complex analog system of throttle input and mechanical feedback. 

 

A nitpick on vocabulary as it means totally different things. These are cans:

https://engineering.purdue.edu/~propulsi/propulsion/jets/basics/burner.html

 

This is annular:

 

 

On 4/15/2021 at 10:42 AM, Caldera said:

Indicated - The rpm of the HP rotor is the core speed

Indicated - The rpm of the LP rotor is the proportional to the Fan speed

 The compressor, HPT, gearbox and accessories run at the same RPM. For the gauge this is taken from one of the accessories.

 

Fan Speed is read directly from the fan shaft.

On 4/15/2021 at 10:42 AM, Caldera said:

The rpm's are expressed as percentages of maximum rpm for the purpose of cross aircraft pilot simplicity?

 

In this engine, the HP rotor essentially functions as a gas generator.  The Interstage Turbine Temperature (ITT) is essentially the Exhaust Gas Temperature (EGT) for the engine.  That means the the 4 LP rotor turbine wheels are going to consume a vast majority of the energy contained the high velocity high temperature gas exhausting from the HP rotor.    In doing so, converting that energy into driving the fan.  In my opinion, the 4 turbine stages would be designed to be quite efficient.  And that would be extremely relevant to the overall fuel economy.

 

It is my guess, that the fan is producing the vast majority of thrust from this engine.  As you say, at least 80%.

 

 

Yes.

 

On 4/15/2021 at 10:42 AM, Caldera said:

Frederf,

 

Thank you!

 

My goal was to find the most efficient endurance.  Basically, the Maximum Range on the charts above.  Do you happen to have those numbers?

 

Zin and I have had engine discussions before.  Have you ever looked into or performed testing to determine the "simulated" engine thrust?

 

I can only think of doing some kind of acceleration test.

 

Caldera

 

 

Real world thrust checks we throttle up to max and compare to GE values on an OAT chart. That is how I came up with the discrepancy and why it is important to thrust. Even went so far as to recreate local environments on real test (installed) days. The answer is always the same. Now that is not overly helpful to those who do not have access to cross check. How to go about the real nerdy engineering way like GE did in the 60s I don't know, but I'm real keen on learning how. Acceleration checks sounds good to my limited Physics 101 brain.

[Caldera]

Zin,

 

Thanks!  We always called the burners cans, maybe it was bad slang.

 

Caldera