Turbine Time JetRanger II Ferry

[aeroscout]

I just started the 206 last night using the minimum N1 method. The engine started 50 degrees hotter, well into the yellow. The start time was about 3 to 5 seconds shorter than the peak technique. Under the same conditions the peak N1 produced a start that approached the edge of the yellow. So in my mind it is a choice between a slightly faster start at a much higher peak temp, or a slightly longer much cooler start. I would think a rapid higher temp would be worse.

[aeroscout]

If the engine is still hot from a previous run, I just motor the starter up to 10% or so and release the starter, letting the airflow cool the engine. You can see the TOT drop even as the engine slows down. I let it come to a complete stop, and then initiate the start. The TOT is usually down to well below 150 by then, and the second run up to lightoff gets it well below any dangerous temperature. IMO that uses less battery, is easier on the starter, and gives a better start. Winding up the starter too high, too often will eventually cause it to fail.

 

That is a start attempt to get to 10%, and another start attempt to get it started. The biggest degradation of starter life is heat. Your way would seem to me to get the starter a lot hotter than the peak method, thereby reducing starter life.

[rotormandan]

If the engine is still hot from a previous run, I just motor the starter up to 10% or so and release the starter, letting the airflow cool the engine. You can see the TOT drop even as the engine slows down. I let it come to a complete stop, and then initiate the start. The TOT is usually down to well below 150 by then, and the second run up to lightoff gets it well below any dangerous temperature. IMO that uses less battery, is easier on the starter, and gives a better start. Winding up the starter too high, too often will eventually cause it to fail.

 

 

I've used that also but usually with the igniter cb pulled. Still though I kinda think it's a false comfort. While that air does help cool down the engine it's still hotter in there then what the gauge is reading. use that method to cool it below 150 after your next shutdown and then check the temp after a few minutes. The gauge will show it warms back up. I don't think maxing out n1 before introducing fuel will hurt the starter if your not getting close to the starter limits. it does put a little extra abuse on the battery. We use a start cart though.

[iChris]

[aeroscout]

It's amazing to me how this thread has become so popular, and is listed as a hot topic in the index. Not that I'm complaining. Just somewhat stunned it has garnered so much attention.

[iChris]

After reviewing my RFM, I found a chart similar to those posted....

 

Use the following guide for desired N1 starting speed versus outside air temperature.

N1 RPM TEMP °C (°F)

15% Above 7° (45°)

13% -18 to 7° (0 to 44°)

12% Below -18° (-1°)

 

I couldn't find some of the other suggestions that were offered in the manual. The one suggestion about the flamefront getting too close to the combustor wall was intriguing, but I couldn't find that in the manual either.

 

 

Behind the scene the combustion chamber must properly mix air and fuel. It must also cool the hot combustion gases to a temperature the turbine section components can withstand. The combustor accomplishes this by taking the compressors output and dividing its airflow into a primary airflow (supporting combustion) and secondary airflow (supporting cooling).

 

Depending on the engine type, primary airflow accounts for 25% - 45% of the total airflow and the remaining 75% - 55% provides a cooling air blanket over the inside and outside surfaces of the combustor liner and centers the flame, preventing it from contacting the metal surface.

 

The flame is contained in an area (flame zone) about the size of a grapefruit. The secondary air enters the liner midstream aiding containment and diluting the mixture to a temperature acceptable for the turbine section. This confinement of the flame allows for a much hotter and efficient burn without damage to the combustor.

 

The last issue is that combustion must take place within a given range of fuel-air ratios vs. combustor gas velocity. They called this a “Stoichiometric Mixture” which means a balanced chemical reaction or the prescribed, calculated stable mixture region.

 

The incoming airflow from the compressor is at a velocity of about 300 – 400 feet per second; however, to support combustion the primary air must be slowed to 5 - 6 feet per second. This is accomplished by directing the primary air through swirl vanes. The swirl vanes create a vortex in the flame area that provides the required mixing of air and fuel at the correct velocity for the flame.

 

From the figure you can see the mixture burns efficiently around a 15:1 ratio. The range is narrow at peak velocity between lean and rich.

 

Rolls Royce holds a patent on a combustor swirl vane designed around creating a “Toroidal Vortex” (basically a doughnut shaped vortex ring) that redirects airflow allowing shorter, hotter flames, more efficient cooling, more complete burning, and reduced smoke.

 

Rolls Royce Patents: US4893475

Edited December 27, 2011 by iChris

[aeroscout]

I love your post, but it doesn't seem to address the start, from what I can see.

[iChris]

I love your post, but it doesn't seem to address the start, from what I can see.

 

That’s correct. That’s the point. There is no documented support for the start procedure you quoted (Peaking N1 and/or long cool starts). The Bell manual doesn’t address it nor does the Rolls Royce manual support it. Must all of the other advice posted here can be supported, like the intriguing flame front getting too close to the combustor wall. However, your start procedure lacks support, other than you’ve tried it and it works.

 

The objective here is to hopefully insure the procedure you quoted does not spread any further and hopefully never again is taught. At best it’s misinformation.

 

Reviewing the Bell and Rolls Royce manuals the start procedure you quoted (Peaking N1 and Long cool starts) is almost the direct opposite.

 

I always let N1 peak and stabilize before lightoff, ensuring the coldest start.

 

I disagree with your assessment of not wanting the coolest possible start. Cooler starts will ensure the longest engine life. There will be added load on the battery achieving peak N1, but not by much, and to me it's worth it. If you have never heard of peak N1 prior to lightoff I can only conclude you haven't been around turbines too long. No offense.

 

I just started the 206 last night using the minimum N1 method. The engine started 50 degrees hotter, well into the yellow. The start time was about 3 to 5 seconds shorter than the peak technique. Under the same conditions the peak N1 produced a start that approached the edge of the yellow. So in my mind it is a choice between a slightly faster start at a much higher peak temp, or a slightly longer much cooler start. I would think a rapid higher temp would be worse.

 

This is what's documented:

 

“Use the following guide for desired N1 starting speed versus outside air temperature.”

N1 RPM TEMP °C (°F)

15% Above 7° (45°)

13% -18 to 7° (0 to 44°)

12% Below -18° (-1°)

 

“As N1 rpm accelerates through 12–15% N1, move the throttle to the IDLE detent to begin fuel flow.”

 

“NOTE: Do not wait for N1 peak out. Introduce fuel immediately upon reaching desired N1 speed. Delay in moving the throttle to the idle detent may diminish battery capacity early in the start cycle.”

 

“CAUTION: INCORRECT FUEL CONTROL STARTING ADJUSTMENT MAY BE DETRIMENTAL TO THE GAS PRODUCER TURBINE LIFE. CONSISTENT LONG, COOL STARTS, AS WELL AS OVERTEMPERATURE STARTS, ARE POTENTIALLY DAMAGING TO THE GAS PRODUCER TURBINE.”

 

“The start–derichment (derich) adjustment can be used to correct lean (slow) start or ric h(hot) start conditions.”

 

That is a start attempt to get to 10%, and another start attempt to get it started. The biggest degradation of starter life is heat. Your way would seem to me to get the starter a lot hotter than the peak method, thereby reducing starter life.

 

Motoring the engine is not an additional start.

 

“Residual TOT should be no more than 150°C (302°F) when light off is attempted. (Residual TOT can be easily reduced to or below 150°C (302°F) by motoring engine with the starter.)”

 

 

Again, the objective here is to hopefully insure the Peaking N1 procedure you quoted does not spread any further and hopefully never again is taught. At best it’s misinformation.

Edited December 28, 2011 by iChris

[aeroscout]

You might be right on all counts, but for motoring. ANY PRESS OF THE STARTER IS A START ATTEMPT. Starter duty cycle limits should not be exceeded with "motoring". By the way, I never "quoted" a "Topping procedure". I only shared what I was taught, and what has worked for me for a 30+ year career flying turbines. If I had a procedure that I could reference I certainly would. But i don't. By the way, notes are only essential to highlight.

[rotormandan]

I wouldn't think it would be a start attempt unless the fuel is introduced. While there is the starter time limits to be observed still, I've always been under the impression that the limits to how many starts can be done on a particular engine are due to the rapid temperature change. With no fuel and no rapid temp increase I wouldn't count it as an attempted start. I only have about 800 hours in a 206 so I'm all ears(or eyes in this case).

[iChris]

You might be right on all counts, but for motoring. ANY PRESS OF THE STARTER IS A START ATTEMPT. Starter duty cycle limits should not be exceeded with "motoring". By the way, I never "quoted" a "Topping procedure". I only shared what I was taught, and what has worked for me for a 30+ year career flying turbines. If I had a procedure that I could reference I certainly would. But i don't. By the way, notes are only essential to highlight.

 

You're correct, you never said "Topping". You said "Peak" N1. I'll make that correction.

 

A lightoff is necessary for a true cycle. Engine start counters have confused many. No need to count compressor washes and motoring.

 

"A cycle is defined as a start or start attempt. The operator is assisted in maintaining a cycle count by means of an engine furnished counter which will record the number of times the ignition exciter is energized. (It is considered that the number of times the ignition exciter is energized and lightoff does not occur is negligible.)"

 

The copy & paste from Mac's Safari defaulted all the quotes to bold highlight since your post # 42 was bold-highlight.

 

 

Edited December 28, 2011 by iChris

[aeroscout]

Probably semantics, but I'm not talking about cycles. I'm talking about starter duty cycle limits.

[Gomer Pylot]

Yes, the time used to motor the engine to cool it down counts against the duty time of the starter. But the 5 to 8 seconds of motoring don't make much practical difference. It doesn't take long to get up to 10 percent, and you don't have to go that high most of the time. You still have plenty of time to make the start within the RFM limits.

[Tarantula]

I only shared what I was taught, and what has worked for me for a 30+ year career flying turbines. If I had a procedure that I could reference I certainly would. But i don't.

 

All it takes is one misinformed instructor teaching a handful of people, who then go out and teach more for misinformation to spread. You have no support for your method other than your anecdotal experience. You are willfully ignoring the recommended procedures for your helicopter. You are also teaching that this is an accepted method, which it is not. I am concerned that you might be more willing to ignore other procedures or regulations. Sounds very anti-authority to me.

 

iChris,

Thank you for your very informative posts. I always learn something when you post in a thread.

[iChris]

I only shared what I was taught, and what has worked for me for a 30+ year career flying turbines. If I had a procedure that I could reference I certainly would. But i don't.

 

Here’s a procedure you can now reference:

 

Allison Commercial Service Letter CSL 1176 and the Air Beat magazine article on the subject:

 

CSL1176 - OPTIMIZED START TECHNIQUE FOR ALLISON 250-C20 SERIES AND C20R SERIES ENGINES (in part, see reference for full text):

 

Various misconceptions exist about what is a good start and how to obtain an optimal start for an Allison 250-C20 or C20R Series engine. This service letter addresses Bendix controlled engines specifically, but most of the principles expressed also apply to engines utilizing CECD controls. Allison recommends judging starts by the performance of a first start of the day situation utilizing a fully charged battery. Do not attempt to adjust fuel control systems based on cold soaked engines in low ambient temperatures or by using an APU/GPU power source. An optimized start primarily focuses on starting in the least possible time without exceeding any temperature limits.

 

Allison defines a good first start as one taking less than 25 seconds from the introduction of fuel until the engine reaches ground idle. To obtain an optimized start, move the throttle to the ground idle position to begin fuel flow as the N1 RPM accelerates through 12-15% N1. Do not wait for N1 RPM to peak out before initiating fuel flow, as this will unnecessarily utilize battery capacity early in the start cycle. Peak observed turbine outlet temperature may reach 820·C for C20 Series engines or 860·C for C20R Series engines during the optimized start. However, start temperature limits remain as defined in the appropriate Operation and Maintenance Manual, not to exceed 810·C for more than 10 seconds and never to exceed 927·C.

 

During starts Pilots should note temperature peaks, the drop between the peaks as well as the total start time. This information will assist maintenance personnel in making appropriate fuel control adjustments.

 

It does not matter which observed peak is the higher. At times the two peaks may be so close together that they appear as one peak. In an optimized start the temperature drop between the two peaks, known as the start notch, is usually 50·C or less. However, the prime determinant of the quality of the start is the length of time from introduction of fuel to attaining 58% N1 and releasing starter assist. Flight personnel should provide maintenance personnel with accurate time and temperature information in order to exact the most effective start adjustments.

 

Consistent long, cool starts (35 seconds or more) can be detrimental to the gas producer turbine life. Remember that the temperatures indicated on the TOT gauge are interturbine, between the gas producer and power turbine, and may not reflect the temperatures being experienced by the first stage turbine nozzle and first stage turbine wheel. The quick, warm starts recommended by Allison actually increase cooling air flow in the combustion section to help insulate the gas producer turbine.

 

REF:

 

Link: Allison Commercial Service Letter CSL 1176

 

 

“I (like many of you) learned to fly in the military. During the latter stages of flight school I had transitioned into the OH­58 helicopter. My instructors all showed me their way of starting this engine. It was the typical: Hit the start button; wait for Nl to spool up past 15% then let it peak out (usually around 18%) while tapping the gauge (to make sure it's not sticking). Next, open the throttle and watch the gauges. Sound familiar?

 

Well, it wasn't until many years later I found out I was doing it all wrong.”

 

By: Chuck Hurdleston, Air Beat Magazine

 

Link: Air Beat Magazine: Let's get off to a good start

Edited December 30, 2011 by iChris

[aeroscout]

I have to admit, the law of primacy did have me in it's grasp. But thanks to iChris, I have been persuaded to re assess my habits for a new and improved habit. Much appreciation to iChris !