Fun or Fire 2012

A few reminders for safe Turbine operations

By Bob Violett

A little History

     BVM has been operating model size gas turbines since 1995 when the first JPX propane fueled engines became available.  We designed and manufactured correct flow path components such as the first ByPass duct that coupled the inlets to the stainless Steel Tailpipe while helping to keep the engines running cool and protect the model’s internal structure from heat. Kerosene fueled engines from RAM and AMT were next on the market followed shortly by JetCat.  All of these early engines required an external source of compressed air to spool-up the engine.  Some manual manipulation of fuel valve, propane valves, and ignition required training, practice and some technique to get the engine started.
     Personally, I thought this was fun and a rewarding experience for an R/C Jet Pilot. Since it was a “hands on” operation and proper technique was required to not over-temp the engine, one’s attention to the turbine temperature during start and first throttle-up was absolutely required.  If the temperature rise looked like it was going to exceed the published limit (~800°), the pilot in command simply closed the fuel valve(s) and continued the application of the compressed air to cool things down.  If it wasn’t improper technique, inadequate air pressure, or too much propane pressure that caused the problem, a visual inspection of compressor and turbine wheel blades was in order before another attempt to start.

Fast Forward to Automation

  Completely automatic ECU (Engine Control Unit) control of engine start, throttle control, and shut down are now the norm and offers convenience, some safe guards, and less equipment on the field.
   My guess is that the new technology works perfectly 95% of the time which has lulled many “Turbine Wavier” qualified pilots into complacency.
   During the last few years at several jet events, we have witnessed pilots bring their jet models to the start-up area with the hatches closed on the model, fuel valves open (or not installed), no ground service unit (GSU) connected to the ECU and no fire bottle close by.  Proudly they stand back, advance the transmitter throttle and watch their jet come to life.  And 95% of the time, it does as commanded. It is that 5% of the time that they are flirting with possible sever damage to the inside of the model, endangerment to bystanders, and/or catastrophic in-flight airframe failure.
  The two most important parameters of turbine operation are temperature and vibration.  The complacent operation described above monitors neither.  It is a formula for the inevitable small problem to become a very big problem.  Multiple hot and flaming starts cause accumulating damage to tailpipes and ByPass components until eventually the aft end of the model suffers structural failure and uncontrolled flight.  Nobody wants to see this happen, but it is occurring more now than earlier in our 17 year old sport.  At the very least, it is an expensive adventure for the airplane owner.  At the extreme, it is a risk of personal injury and severe property damage.  This behavior is not reflective of the extra responsibility required by AMA Turbine Wavier Pilots.

Engine and Airframe Inspection

  As a young Naval Aviator, I never strapped into the ejection seat of a Navy Fighter/Attack aircraft without first visually inspecting the compressor and turbine blades with a flashlight during the “Walk Around” preflight inspection.
  Be assured that when you ride on an airliner, a crew member or mechanic has done the same.  Ultimately, in both scenarios, it is the pilot’s responsibility for safe operation.   
   If the model has had an off-the-runway excursion, a failed nose gear landing etc. one should remove the hatches and use a small mirror and light to inspect the compressor wheel and turbine blades.  Of course, a more thorough inspection of the blades can be accomplished by removing the engine from the airframe.
  Your engine manufacture’s Operation Manual deals with failed, wet, or hot starts.  It is expected that you know and follow these procedures.  If your engine is full of fuel from a failed start, unless you purge it properly, expect a tailpipe fire.
  Besides the “start-up” being a challenge to turbine temperature limits, “throttle-up” from idle during taxi causes temperature spikes.  Add the extra rolling resistance on a grassfield, and wind up the tailpipe, and it is easy to understand these added stresses.  One method to counter this is to adjust the engine idle RPM up 2-4000 RPM. Refer to the engine Operation Manual to accomplish this.
   Another warning sign that requires corrective action occurs if the engine “stutters” on throttle-up for taxi.  This can be a sign of engine compressor damage or an indication that some fuel pump voltage parameters need adjustment.

Tailpipe wear and tear and the need to inspect it

  Besides hot and/or flaming wet starts potentially damaging the tailpipe, a smoke system ignition can also stress this spot welded stainless steel component.  It is good practice to inspect the tailpipe with a flashlight. Besides checking that the engine exhaust nozzle is still centered, you will want to look for popped rivets or more than a slight bit of rippling.  Discoloration towards a bronze color is normal.  A crimson color indicates exposure to higher temps.  A blue color indicates very high temps and possible metal fatigue. Do not fly a turbine powered jet with a compromised tailpipe, it will only get worse and eventually cause a serious problem.

Tailpipe Attachment and Support

  Models equipped with a properly designed Bypass duct generally have good support for the front of the tailpipe.  Redundancy in how the two are affixed is good design.  “Dump Diffuser” design installations, i.e. no Bypass, sometimes rely on just two stainless steel straps spot welded to the tailpipe at the 3 and 9 o’clock positions and then screwed to a plywood engine mount 3-5 inches forward of the tailpipe.  This arrangement offers little resistant to positive “G” forces that are most common in flight and on firm landings.  We suggest a shaped former (see photo below) that cradles the forward end of the tailpipe transferring positive “G” loads to the fuse bottom. Note that the steel straps are spot welded to the tailpipe.  Inspect these welds often.
  Know that some of the products made in China are not thoroughly tested.  But, this is the marketplace now and what consumers choose because of price.  So as consumers, you need to know enough to guard against the above mentioned issues.
  Think about it, you are operating a high performance machine that is powered by fire breathing 110,000+ RPM engine. We are simulating real fighter jet power and performance so we must also accept the periodic inspections, maintenance, and responsible behavior to keep it operating safely.

The tailpipe support former shown is part of a BVM upgrade to a Skymaster Cougar.

  A better understanding of what’s going on inside our ultimate performance machines and attention to the details of the proper flow path for the hot gases will allow us to enjoy the “fun” part of our sport and avoid the calamity of a fuselage fire associated with poor design and complacent operation.

  I maintain my “turbine qualification” with BVM jets like this 7 year old Ultra Bandit with JetCat P-200SX power.  We call it “Pilot Proficiency Training” while having fun.


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