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Aero Engines

by Jeremy Cox 1. February 2006 00:00
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I consider myself very fortunate to have this corner of the world-wide-web to be able to call my own. Rather like a personal ‘Blog', but not quite. I do however pretty much make the decisions regarding what I write about each month, but unlike a ‘Blog' I have to make sure that what I submit to Jeff for posting here, is reasonably accurate and balanced, unlike most ‘Blogs' out there in the ether of the world-wide-web. This concept of relative freedom leads me in to my chosen subject for this month: Aero Engines; ‘The Weird, the Wonderful, the Not So Wonderful and the Downright Insane in the World of Aero Engines.' The spark for this article actually came from my other interest; Motorcycles.

Probably just like you, I get way too many magazines through my letterbox every month. Getting the time to wade my way through them all is pretty hard (I am genuinely impressed by the faithful souls that regularly read my blithering drivel here), however a small beacon of pleasure winks an eye at me every time I see the cover of one of my Motorcycle magazines amongst the normal rubbish of Bills, Credit Card Offers, Charity Appeals and Catalogs for useless Object d'art. In the February 2006 edition of Cycle World (probably my favorite magazine of the bunch), on page 12 Mr. Kevin Cameron has written a fascinating piece about developing engine technology and the concept of testing engines to destruction. He actually cites the work done by the Army Air Development Center in Dayton, Ohio between 1920 through 1926, when single cylinder test engines were run at full blast until they pretty much blew up or stopped running. This concept tickled Mr. Cameron's fancy as it also did mine. I won't spoil his article for you, but I highly recommend that you seek out a copy of this specific edition so you too can read page 12.

When it comes to today's Aero Engines, we are all very much spoilt in this area of aviation technology. Even though the majority of Piston Aero Engine technology still harks back to the 1940's, aero engines in general, turbines especially along with the ‘piston poppers', are extremely reliable. I remember growing up as a child when my mind was an absolute sponge. I would read every aviation book I could lay my hands on. I have flown gliders and I like them, but you still need some form of engine to get even the most modern glider up into the sky, therefore I have always liked to read about Aero Engines. With this and the issue of reliability in mind, I always laughed at the concept of the first Rotary aero engines like the Gnome and Le Rhône which were produced in France circa 1909 and that powered some of the first truly successful powered aircraft during aviation's birth. These five or seven cylinder contraptions had their hollow crankshafts bolted to the firewall, through which a fuel/caster oil/air mixture passed and was thrown centrifugally out to the cylinders that were spinning wildly around the crankshaft at about 20 revolutions every second, creating immense gyroscopic forces on the engine and the airframe to the order of up to 100g's! Of course there was no oil gallery return, so the un-burnt fuel/caster oil mixture flew out of the small exhaust ‘flapper' valves at the end of each cylinder right into the face of the dashing and intrepid pilot who was trying to prevent his frail contraption from auguring into the ground. It really gives a new meaning to the term ‘running ones engine' as the aviators of that period were pretty familiar with the Sir Thomas Crapper's throne courtesy of the laxative properties of the caster oil. Better yet, the primitive carburetors of that time did not normally feature any type of butterfly valve, throttle control and therefore the engine either ran at full tilt or not at all. The previously mentioned pilot with the bowel problems had to control his power output by employing an ignition ‘blip switch' control, in lieu of a throttle i.e. he controlled the opening and closing of the primary coil circuit in the Magneto. I remember reading that on wet days the poor old chap would enjoy the pleasure of blue sparks tingling his thumb every time he ‘blipped' his switch. This brings a whole new level of appreciation to the single FADEC Power Lever in some modern cockpits.

Another memory that comes to mind from my childhood is that the reliability of the first pure-jet turbine engines left an awful lot to be desired. The notable (eventually Nazi) engineers, Herbert Wagner, Otto Mader and Anselm Franz worked together with a staff of hundreds in Magdeburg, Germany, to develop the 8-stage axial compressor, single-stage turbine and afterburning jet-pipe 004A-0 engines that powered the Messerschmitt ME262 in 1942. I recall that the first versions of this engine had a Time Between Overhaul (TBO) of only a couple of hours. The last of this engine variant, the D-4 had a TBO of 30 hours which is truly a far cry from the ten or fifteen thousand plus hours that modern commercial engines regularly attain. Hell some are even certified straight out of the box with an ‘on-condition' TBO, the BMW/Rolls Royce BR710 is an example of this, as installed on the Gulfstream GV/G500/G550 series aircraft!

Let's once again step back in time and continue our next exploration into antiquity, by now remembering the much loved (especially on this side of the Atlantic) Curtiss OX-5 aero engine from 1910? This 320Lb 90HP 1,400 RPM water cooled V8 engine powered the Curtiss JN-4 ‘Jenny' primary trainer. When I first came over to the United States I met a man that proudly wore the OX-5 Club medallion. I had no idea what he was blathering on about until he explained that the OX-5 Club is becoming more and more exclusive every year as members pass away. This is because to be eligible for membership, you must have flown behind and commanded the venerable OX-5 Engine, which was likely mounted on the front end of either a Curtiss JN-4 Jenny or a DeHavilland DH6! We are talking pre 1925 here for these aircraft so unless you are a very fortunate private collector or a lucky pilot at a museum that owns an original or replica, you will probably not becoming a member of this exclusive group anytime soon.

Staying with Piston Engines we are about to make a leap to the other end of the power spectrum (or should I say ‘band'?), i.e. from 90HP to 3,700 HP. Well I have always had a soft spot in both my heart and my head when it comes to ‘Round' Engines. None come any finer in my opinion, than the 18 cylinder Wright R-3350 Turbo Compound Engine, as installed on the ‘Super G' Model Lockheed Constellation. Picture this if you can, you start with the basic layout of a R-3350 as installed on a postwar Boeing B29 Stratofortress, i.e. this baby has Wright Superchargers and Direct Fuel (and Water for take-off boost) Injection. Next you add 3 ‘Blow Down' Turbines spaced 120 degrees apart and feed them from 6 of your 18 cylinders. The rotational power created by the force of the exhaust gasses acting through the Turbines was fed to back to the engines Crankshaft by way of an Inconel X disc, a Stainless Radial Shaft and a Fluid Coupling which was directly geared to the Crank. Wow! These 3 Turbines coupled with some other tweaks here and there, resulted in the basic power output of 2,700 HP being elevated to the great heights of 3,700 HP. If ever you happen to be over in Kansas City, Missouri during daylight hours, you have got to stop at the TWA Museum on the North side of the Downtown Airport. If you are really lucky, you will get to see and hear one or maybe even four of these behemoths' being started (I mean belching smoke and flames while coating the ramp with a generous donation of Mineral Oil) and run, on the museums' restored and flying example of a ‘Super G.' Okay I can almost hear some of you Pratt & Whitney aficionados out there saying what about the P&W R-4360 Compound that was designed and built for the ill-fated B-36? Well I'm sorry to not spout about its 4,000 HP power output. Unfortunately I have never spent any time around this engine therefore it hasn't made the deeply monstrous impression on me like the R-3350 has!

Thinking and writing about the quintessential ‘round engine' has almost made my eyes moist, and therefore a trip down the emotional memory lane of Aero Engines would never be complete if we didn't mention the Royalty of all piston engines, the Rolls Royce Merlin and Griffon series. If you happen to be standing in the garden when one of the following aircraft passes overhead: A Supermarine/Mitchell Spitfire, a North American P51 Mustang, a Hawker Hurricane, a Fairey Seafire, a DeHavilland DH98 Mosquito, an Avro Lancaster, or an Avro Shackleton, you would have just heard the best sounding ‘Vee' 12-cylinder package ever built! Beethoven may rule the sound waves on the ground, but as far as I am concerned, Messer's Henry Royce and Charles Rolls certainly rule the sound waves in the sky! In no small way did this company and its engines prevent me and everyone else like me, from having to speak German as my first language (I hope that my friends from Prussia won't take offence at this remark. Remember how we kicked Napoleon's bottom together in the early 19th Century. Unfortunately Wilhelm and then Hitler in the 20th Century, are the ones that screwed up this age-old relationship). A stock, late-model Merlin will kick out about 2,540 HP at sea level, and will maintain 2,000 HP to at least 20,000 feet barometric. That's 200 HP per cylinder, which is about the same as the R-3350 Turbo Compound engine. Better yet the nutters, a'hem, I mean the highly interesting people who get together every year out in Reno, Nevada, are able to breathe another 1,100 to 1,500 HP into these high-bred works of art which is just mind-blowing to me.

Alright now that we have gone to the extremes with piston aero engines, let's trip the light fantastic into the realm of Turbo-Props and Turbo-Jets/Fans. The most powerful Turbo-Prop engine ever built is Russian. The Kuznetsov engineering group established in 1948 with hundreds of German Prisoners of War and headed up by Comrade Nikolai Dmitriyevich Kuznetsov managed to design and build an engine, the NK-16, which is capable of delivering 14,995 SHP. It powers the Tupolev TU-95, TU-114 and TU-142, along with the Antonov AN-22. Each installation drives massive contra-rotating propellers that are geared down to rotate at an amazingly low, 750 RPM in cruise flight. A totally awesome Powerplant, but statistically the United States and Canada has these engines beaten when judged on profligacy. What I mean by that is that a North American company, Pratt and Whitney (United Technologies Corporation) currently holds the record for producing the most Turbo-Prop and Jet engines of one type, specifically the PT6A series and the JT-8D series. The PT6A, the true workhorse of the worlds varied prop fleet was first introduced as a Helicopter engine in the late 1950's. And even though the Hiller Helicopter Company never did use this engine in any of their helicopters, there have now been more than 33,000 copies of the PT6A series engine delivered new, with new ones still rolling down the production line and there being no end in sight. Additionally Pratt and Whitney also are still just holding on to the lead in the Jet engine arena with worldwide in-service production since 1964 holding a little above 14,000 engines. The gentlemen or should I say the Monsieur's at Snecma/General Electric in France are closely nipping at Pratt's heels with the CFM-56, which is currently at 13,500 engines and counting.

I have researched the business aero engine industry and I have concluded that the Honeywell (Garrett/Allied Signal) TFE-731 series is the most prolific business aircraft engine. Versions of it power Sabreliner, Jetstar, Dassault Falcon, Bombardier Learjet, Cessna Citation, Raytheon Hawker, IAI/Gulfstream and Jet Commander Aircraft models. According to Honeywell's website, so far 11,718 TFE731 engines have been built/delivered, with 51,000,000 operational hours accumulated fleet-wide, while 1,200 of the TFE-731-20/40/60 engines have collectively accumulated more than 1,000,000 operational hours. These are pretty impressive statistics by anyone's standards.

Okay now for the pièce de résistance: the engine that I would like to close with is the Super Daddy of them all. Can you guess what it is? Alright, I have taken quite enough of your time already so I won't delay any longer. The true ‘King of the world' is another engine from the Royal Family of Engine Manufacturers: Rolls Royce. This article would be incomplete if I did not write about the new RR Trent 900 Turbofan. This is the launch engine of the Airbus A380. There are four of these ‘big-daddy's mounted under the wings of the worlds largest passenger aircraft (second only to the Antonov AN225). The A380 has a Maximum Gross Take Off Weight of 1,300,000 Lbs. Just think of that! This is the equivalent of more than 46 individual Hawker 800XP aircraft at their Gross Weight! Each RR Trent 900 has been cleared to eventually certify at 81,000 Lbs of Thrust which is more than 75,000 HP. It is currently being run at 76,500 Lbs, about 71,000 HP. Even though this is a massive lump of metal and plastic that is propelled through the air somewhere in the mid 300 to low 400 Flight Levels at a cruise speed of around Mach 0.85, the fuel consumption in cruise is going to be minimal. The A380 aided by the RR Trent 900 engine, will be the first long-haul aircraft to consume less than three litres of fuel per passenger over 100 kilometers (approximately 81 miles per US gallon) – a fuel burn comparable with the best of the small modern turbo-diesel cars. Wow! To finish with, I actually got some very interesting facts from the Rolls Royce website as they pertain to the RR Trent 900, they are as follows:

  • At take-off, the A380's four Trent 900s will deliver a thrust equivalent equal to the power of more than 3,500 family cars.
  • The engine's hollow, titanium fan blades suck in over 1.25 tons of air every second and could empty four squash courts per second.
  • The fan operates at nearly 3,000rpm with tip speeds at 1.5 times the speed of sound.
  • By the time the air leaves the exhaust it has been accelerated to a speed of nearly 1,000 miles per hour.
  • Temperatures in the engine core are half those on the surface of the sun.
  • The blades in the engine's high pressure system rotate at 12,500 rpm, with tip speeds reaching 1,200 miles per hour.
  • Each Trent 900 has around 20,000 individual components.

As a side note I found fellow Englishman, Mr. Bill Gunston's third edition of his book from 1995, the World Encyclopaedia of Aero Engines, ISBN 1 85260 509 X, extremely useful in the preparation of this article.

So what are your favorite or memorable aero engines?

What's your Good, the Bad and the Ugly List?

Any input that you care to make will be of great interest to all of the readers here at Globalair.com. So please don't be bashful and go ahead and write your comments and suggestions here. Please don't forget that whatever your write here, can be seen publicly by everyone that visits this page, so "be nice."

Service Bulletin: What is an Aircraft Owner to Do?

by Greg Reigel 1. February 2006 00:00
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If you own an aircraft, you no doubt have received a "service bulletin" from the manufacturer of your aircraft or one of its components (e.g. the engine, avionics or accessories). Depending upon the manufacturer, a service bulletin may also be called a "mandatory service bulletin," "technical service bulletin," "service letter" or "service instructions." Service bulletins are automatically sent to the owner of an aircraft by the aircraft or component manufacturer. However, aircraft owner's should be aware that service bulletins are not automatically sent to maintenance providers.

A service bulletin contains a recommendation from the manufacturer with which it believes the aircraft owner should comply and that often reflects a safety of flight issue that the manufacturer believes should be addressed within a certain time frame. It may result from an improvement developed by the manufacturer. Or it may address a defect in its product or published documentation.

The manufacturer responds to one of these situations by issuing a service bulletin that recommends a certain type of inspection, replacing certain components, performing maintenance in a specific manner or limiting operations under specified conditions. Sometimes, compliance with a service bulletin may be triggered by the occurrence of a particular event (e.g. the lapse of time or operation under certain types of conditions).

Although a service bulletin may be labeled or characterized by the manufacturer as "mandatory," it is important to know that compliance with a service bulletin is not specifically required under the Federal Aviation Regulations ("FAR's") unless the service bulletin is accompanied by or includes an Airworthiness Directive. Airworthiness directives affect safety of flight and compliance is mandatory. However, a review of FAR Part 43, Appendix D, which details the maintenance required in connection with an aircraft's annual or 100-hour inspection, will confirm that an aircraft may be returned to service without complying with a manufacturer's service bulletin, except where an airworthiness directive is applicable.i

So, simply because the FAR's do not specifically require an aircraft owner to comply with a service bulletin does this mean an aircraft owner can ignore service bulletins? We know that an aircraft owner will not invoke the wrath of the FAA if he or she does not comply with a service bulletin (unless, of course, the service bulletin contains an AD). But does this mean that the inaction will not come back to haunt him or her at some point in the future? Not necessarily.

We all want the aircraft we own and fly in to be safe. And we want other aircraft in the sky at the same time as us to be safe as well. The manufacturer issues a service bulletin because it believes compliance will make the aircraft or its components safer. (The manufacturer may also be trying to limit its exposure to products liability, but that is a discussion for another day.)

However, for an aircraft owner, compliance with a service bulletin typically translates into higher costs. Whether it is requiring replacement of a component or performance of a more elaborate and detailed inspection, a service bulletin's recommendation usually means that the aircraft owner is paying more money in either parts or labor. As a result, some aircraft owners will defer or reject compliance with a service bulletin to save money.

This is especially true if the aircraft owner believes that the aircraft is still safe without compliance. After all, if a service bulletin does not contain an airworthiness directive, the FAA apparently does not deem its recommendations to be necessary or mandatory. So why should the aircraft owner? And why should the owner spend additional money for parts or maintenance that may or may not actually make the aircraft safer?

The obvious answer is safety. But how will an aircraft owner know whether the service bulletin really does address a safety of flight issue? Unless the aircraft owner is a maintenance provider, he or she will only be able to make that determination by thoroughly discussing the service bulletin and its requirements with a maintenance provider. If a service bulletin addresses a safety of flight issue, compliance should be without question.

When safety of flight is not necessarily an issue, an aircraft owner may then want to perform a cost benefit analysis to compare the cost of compliance (How much will the labor or parts required by the service bulletin cost?) with the benefit obtained by complying with the service bulletin (Will compliance enhance the safety or value of the aircraft or limit the aircraft owner's liability exposure to third-parties?). This analysis and the answers to these questions should assist an aircraft owner in deciding whether he or she will comply with a particular service bulletin.

At the end of the day, it is the aircraft owner's responsibility to decide whether or not he or she will comply with an applicable service bulletin. By talking with a knowledgeable maintenance provider, an aircraft owner can understand not only the requirements of compliance, but also the costs and benefits associated with compliance. Only then can an aircraft owner make an informed decision as to what to do with a service bulletin.

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Greg Reigel

Evaluating New Aircraft

by David Wyndham 1. February 2006 00:00
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Personal relationships develop not because of a pre-determined set of conditions has been consciously met, but because you just "hit it off." We tend to go into our aircraft relationship in much the same way. However aircraft are not people (or pets), but machines with a set of qualities that can be measured and evaluated.

You need to be aware of what your true requirements are, and be brutally honest in what the benefits are to you of those requirements. So many times I hear "we want a jet to fly coast to coast non stop." However, 95% of the time their trips are two or three hours in length. My job is to then guide the discussion into what the true needs are, and to acknowledge the coast to coast capability. But, when the discussion gets around to costs, I am usually able to help folks realize that you get what you pay for in terms of capabilities – more capabilities cost more to acquire.

Be realistic in your expectations. All aircraft are a complex set of trade-offs. No one aircraft can do 100% of your trips 100% of the time. What do you truly need in order to successfully complete your primary mission? If you need to carry three to five passengers then a six-passenger aircraft will do the job. Yes, eight seats or more is nice, but when you get to the costs, what will you be willing to pay to get that two extra (empty) seats?

Keep the discussion in terms of quantifiable criteria. Six seats, 1,200 NM range, able to operate from a 4,500 foot runway are all measurable. Big and fast is not. Quantifiable criteria helps remove emotions from the analysis and you are much more likely to find success in selecting an aircraft based on measurable facts.

If you routinely fly trips that are a certain distance, do you really need to fly twice as far non-stop? If your requirement is a 1,000 NM trip with four passengers, then an aircraft that can fly 1,110 NM can be just as successful in completing your mission as one that can fly 2,000 NM. It all comes down to this, capability costs and excess capability costs extra.

Don't forget to plan ahead in your requirements. You may need to fly different trips next year than you currently fly today. Again, be realistic and don't over estimate what those needs will be. I rarely find people underestimating their requirements. Must be the "bigger is better" syndrome.

Separate required and desired criteria. Your required criteria may be six passengers, 1,200 NM, from a 4,500 foot runway. Desired may be eight seats and 2,000 NM from the same runway. You need the former, but the latter is "nice to have." This enables you to not only compare costs, but value as well.

It all ties into costs. More range requires more fuel which requires a bigger, heavier structure to carry the fuel which requires more powerful engines to propel the heavier aircraft. Bigger engines burn more fuel and so forth.

We recently finished a report for a client who had a variety of needs which resulted in aircraft ranging from mid-size business jets to the super mid-size category. We also included one large cabin jet as well. Guess what? The smaller jet met all their needs at the lowest cost. It didn't offer much excess capability and didn't meet the "nice to have" criteria. The large cabin business jet met all their needs, including their "nice to have" ones. It was also the most expensive to own and operate. The super mid-size jets again met all their needs but were lacking in some of the nice to have features. They also ranked in the middle in terms of costs. So their decision was going to involve value – was the extra capability in meeting their "nice to have" criteria worth the additional monies?

Value is the final determinant. What cost is the extra capability? Will you really use the extra capability? If you may want the extra capability but only infrequently, are their other alternatives (like charter or fractional time-sharing)? Be realistic in defining your needs, separate out required from desired (or "nice to have") criteria and then look at what each option costs you in terms of ownership and operations.

You are much more likely to have long term success with your aircraft if you focus in on the requirements first.



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