Why are airplane engine TBO hours so low?

Dan Thomas said:
Totally different animal. The turbine runs smoothly with little vibration. The pressures in the system are constant, not changing from 800 psi to less than zero 1300 times a minute. There are no reciprocating loads. The metals used in the turbine are pretty exotic. All of it adds up to many more dollars per horsepower compared to a piston engine.
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Exactly, and much higher fuel consumption per passenger too, at least for smaller aircraft.

I remember chatting with an acquaintance who flew bizjets (not his own), and we figured out that he burned about the same amount of fuel on long taxi to the runway as I burned flying my family 2½ hours from Ottawa to NYC in my 160 hp Piper — about 23 gallons in both cases. For short hops at low altitude, it's hard to beat our old piston engines for efficiency.
 
Tantalum said:
Why does everyone say this? Your car engine is probably 2.5 liter and 180 hp. The engine in the club rental is 6 liters and 180 hp. According to the Ultragauges I've had in my car it takes about 30 percent power to stay at a 72 cruise setting.. (30% X 180)/2.5 = 22 hp per liter. In the plane (75% X 180)/6 = 22..and most people run plane engines closer to 65% power. Per displacement they're being asked to do the same thing

A lyco 360 weighs about 300 lbs
A Honda civic engine weighs about 400 lbs.. but the block of the 1.6 weighs a mere 150 lbs (roughly)

..anyway.. I've resolved to "it is what it is" and just accept it as part of life.
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LS7 is also about 400lb. Even with accessories it is a difference in weight that can be very easily overcome in airframe design given extra power available. I think it's pretty clear that you can't just stick an auto engine into an airplane and expect a good outcome. They are designed for different purposes. But it's also pretty clear to anyone impartial that most of GA piston innovations were done before the 80s(Rotax and deisels being an obvious exceptions). Since then the automotive engines design, manufacturing, and build quality have taken huge leaps and bounds. What was state of the art in the 70s looks pitiful now. And that's without lead and with a host of restrictive emission controls. Even a small amount of that technology and engineering would dramatically improve the GA engines.

But alas, there is no money in it. No money to spend on engineering and no money to recover in sales. So it's not that it cannot be done, it's just not feasible.
 
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Tantalum said:
That's fair, but if cost and regulations were no object we could undoubtedly find ways to improve the engines. Maybe we've reached the realistic max of what a piston ga engine can offer, but certainly not a theoretical max
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If cost were no object, we could do almost anything, anywhere. Even a tiny sliver of the research money that goes into rich-world diseases, for example, could prevent millions of deaths from poor-world diseases, if most research institutions weren't forced to chase the money so that they can pay their lab techs and make rent.

Everything in aviation is about forced tradeoffs among cost, performance,and safety. GA is a tiny market, and we wouldn't even get the little bit of R&D we do if it weren't for the endless supply of devoted fools lining up to convert their large fortunes into small ones. I can easily see myself falling for the imaginery glamour of an aviation startup, if I had a fortune to lose.
 
I'm sure if the GA gods could go back in time to the 30's/40's when the transition from radials occurred (know what we know now), they would have started designing diesel aircraft engines and we'd be fine. Diesel engine tech, while it has improved, hasn't changed a ton over the decades, and we'd all be on Jet-A with no lead necessary. We'd have FADEC liquid-cooled all-aluminum turbo diesels that are perfectly suited for the job. However, there's not much money into converting things at this point, so we get what we get.
 
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SoonerAviator said:
I'm sure if the GA gods could go back in time to the 30's/40's when the transition from radials occurred (know what we know now), they would have started designing diesel aircraft engines and we'd be fine. Diesel engine tech, while it has improved, hasn't changed a ton over the decades, and we'd all be on Jet-A with no lead necessary. We'd have liquid-cooled all-aluminum turbo diesels that are perfectly suited for the job. However, there's not much money into converting things at this point, so we get what we get.
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ha! yeah :). Ironically it's the very thing that keeps most of us flying(ability to buy very old planes and fly them for generations) keeps all of us from getting any kind of upgrades to engine designs. Catch-22
 
TCABM said:
I’d hazard to guess TBO hours are driven by the data submitted to the FAA for certification and probably includes some type of MTBF analysis.
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I'd hazard to guess that someone runs a couple engines on a dyno and then pulls the number out of their butt. :)
 
The only new modern engine to take to the market the Rotax.Yeah, I know small displacement primarily in LSA's and the Sling 4.

Terrific engine with FEDEC with models including fuel injection and turbo-normalized. Wish other manufacturers would take this lead and innovate.
 
Folks were all hot and bothered about the Dyna-Cam, Axial Thrust, Roto Cam etc. a few years ago. What happened to that?
 
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Tantalum said:
Why does everyone say this? Your car engine is probably 2.5 liter and 180 hp. The engine in the club rental is 6 liters and 180 hp. According to the Ultragauges I've had in my car it takes about 30 percent power to stay at a 72 cruise setting.. (30% X 180)/2.5 = 22 hp per liter. In the plane (75% X 180)/6 = 22..and most people run plane engines closer to 65% power. Per displacement they're being asked to do the same thing

A lyco 360 weighs about 300 lbs
A Honda civic engine weighs about 400 lbs.. but the block of the 1.6 weighs a mere 150 lbs (roughly)

..anyway.. I've resolved to "it is what it is" and just accept it as part of life.
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Respectfully, I think this analysis is flawed. Specific power output per unit displacement isn't very relevant to TBO. The entire bottom end of the engine (where TBO is really focused), has no exposure to the displacement at all.

The best metric for comparing aviation engines to automotive engines would be something like (torque * hours) / mass

Lets compare an IO-360 being run at 65% power over its lifetime. A rough calculation of its output would be:

HP = T * RPM/5250
180 = X * 2700/5250 = 350 ft lbs
350 * 65% = 227.5 ft lbs

227.5 * 2000 / 300 lbs = 1516 ft-lb-hours per pound


And now for a 'typical' 180 HP car engine. We'll make some assumptions and lets say it hits peak torque at 5000 rpm and weights about 400lbs. And on average is asked to produce 35% power.

180 = X * 5000 / 5250 = 189 ft lbs
189 * 35% = 66 ft lbs

66 * 2000 / 400 lbs = 330.75 ft-lb-hours per pound


So each pound of engine weight in the Lycoming is being asked to create about 4x more units of force hours over that same timeframe.

-G
 
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Tantalum said:
The germans did a bunch of stuff with diesels in their planes, no? I seem to recall Junkers had a couple diesel engined planes
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Yes.
But one needs to not confuse ignition systems with durability.
Truck / Stationary /Marine diesels are super durable. And Super Overbuilt and Heavy.
And, then, there was the GM 350 cid diesel from the '70s...

Piston speeds are a big deal. Bearing sizes are a big deal.
 
Tantalum said:
The germans did a bunch of stuff with diesels in their planes, no? I seem to recall Junkers had a couple diesel engined planes
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The Germans in the Second World War were wizards with diesel -- their diesel-powered E-Boats (Snellboots) were all metal and heavier, but could still outspeed the British light wooden MTBs and MGBs with petrol engines, and outrange the American petrol-powered PT boats. Despite all that, they didn't do much with diesel in their aircraft, whatever the reason (it wasn't a lack of capability). Maybe they never managed to deal with the vibration. (?)
 
David Megginson said:
The Germans in the Second World War were wizards with diesel -- their diesel-powered E-Boats (Snellboots) were all metal and heavier, but could still outspeed the British light wooden MTBs and MGBs with petrol engines, and outrange the American petrol-powered PT boats. Despite all that, they didn't do much with diesel in their aircraft, whatever the reason (it wasn't a lack of capability). Maybe they never managed to deal with the vibration. (?)
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Diesels are hard on metal propellers.
 
Tantalum said:
..anyway.. I've resolved to "it is what it is" and just accept it as part of life.
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If that were the case we wouldn't keep reading your posts about how "50 year old engine technology is crap" over and over again, would we?

Just saying...
 
George Mohr said:
Respectfully, I think this analysis is flawed. Specific power output per unit displacement isn't very relevant to TBO. The entire bottom end of the engine (where TBO is really focused), has no exposure to the displacement at all.
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It was back of the napkin, most people who claim the "but airplane engines run at full power all the time" don't really think about the whole picture..

donjohnston said:
If that were the case we wouldn't keep reading your posts about how "50 year old engine technology is crap" over and over again, would we?
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I know :( I shouldn't have taken the bait!
 
Tantalum said:
they haven't had innovation. My friend's partner nearly went for a swim in Long Island sound because one of the plastic magnetos failed in a peculiar fashion that was causing bad misfirings, limped home and the engine died on the ground. I've got the email from a mechanic somewhere. That's inexcusable that we still rely on tech like that. Our spark plug gaps are so itsy bitsy that the slightest bit of fowling ruins it. Even a 50 year old car engine runs better, check out the guys at Corvair..

..and they are cheaply built, sand casting a mold with crummy metallurgy is cheap, requiring careful break ins.

..and they are low volume

I don't know why some engineers take this so personally. There's no defendable objective argument that can be made to justify the "low tech" nature of these engines.
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Plastic magneto gears fail because they don't get the specified inspections. The gears are plastic beacuse they carry high voltage, and a metal gear would short everything. They used to be made of micarta, a fabric-reinforced urea-formaldehyde, a really ancient plastic that had plenty of its own problems.

Spark plug gaps are small because a magneto can only generate so much voltage. You want more gap, you need way more voltage, which means battery-fed electronic ignition, and now we have another failure point: the airplane's electrical system. Just look at the endles stories on POA about alternator failures.

Corvairs in homebuilts, with something like 14 broken crankshafts so far.

Sand casting has NOTHING to do with break-in. Where do you get this stuff?
 
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Tantalum said:
Why does everyone say this? Your car engine is probably 2.5 liter and 180 hp. The engine in the club rental is 6 liters and 180 hp. According to the Ultragauges I've had in my car it takes about 30 percent power to stay at a 72 cruise setting.. (30% X 180)/2.5 = 22 hp per liter. In the plane (75% X 180)/6 = 22..and most people run plane engines closer to 65% power. Per displacement they're being asked to do the same thing

A lyco 360 weighs about 300 lbs
A Honda civic engine weighs about 400 lbs.. but the block of the 1.6 weighs a mere 150 lbs (roughly)

..anyway.. I've resolved to "it is what it is" and just accept it as part of life.
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We could easily get a small engine to put out 200 HP, but it requires high engine RPM that simply cannot be used to spin any useful propeller. So now we need gearing of some sort to reduce the RPM and raise the torque for the prop, and that adds weight, cost, complexity and technical issues. Direct drive is far more reliable. Homebuilders have been fooling with reductions for many years (60 or more) and it's a difficult problem. There have been numerous failures, some fatal.
 
NordicDave said:
The only new modern engine to take to the market the Rotax.Yeah, I know small displacement primarily in LSA's and the Sling 4.

Terrific engine with FEDEC with models including fuel injection and turbo-normalized. Wish other manufacturers would take this lead and innovate.
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Again - rotax is a modern engine. Has a 1500 hour TBO. HP is 100. Cost $25000. Ancient Continental 200 hp engine is $45,000, has a 2000 hour TBO? Rotax is not giving you multiples of extended time.

If rotax could make a 200 hp engine with 3 times the TBO they would have done it. It’s just not that easy.
 
Airplanes with propellers seem to work well with slow rpm, slow piston speed, high torque engines. Which means large displacement.

Torque X RPM gives HP. Smaller displacement engines have to spin faster. To get a 200 hp rotax with its higher rpm you will need more number of and more heavy pistons than the current 912, all moving faster than your ancient continental big displacement.

It’s just not easy.
 
WDD said:
Again - rotax is a modern engine. Has a 1500 hour TBO. HP is 100. Cost $25000. Ancient Continental 200 hp engine is $45,000, has a 2000 hour TBO? Rotax is not giving you multiples of extended time.

If rotax could make a 200 hp engine with 3 times the TBO they would have done it. It’s just not that easy.
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not sure to which rotax you are referring. 915is is 150hp, around 200 lb, tbo 2000, $20k. It’s also turbo, so it can keep the power to mid teens.

http://paramotoraviation.com/products.php?product=Rotax-915-iS-Turbo-
 
WDD said:
Referring to the 912.

Love the 915i. You found a site with better prices than what I found. BTW the 915is on your link is 135 hp. But point still is that you don’t have double or triple the TBO
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True, but as discussed, tbo is just a suggestion. This is a new engine, they may be very conservative. This engine is also hard to hurt with improper operation.
 
IMO, 2000tbo is fine if I get new engine. For most private operators it’s 15-30 years. I think that’s more than enough. It’s the all the things around legacy engine operations that’s annoying and often leads to premature problems due to pilot errors and outdated accessories
 
David Megginson said:
If cost were no object, we could do almost anything, anywhere. Even a tiny sliver of the research money that goes into rich-world diseases, for example, could prevent millions of deaths from poor-world diseases, if most research institutions weren't forced to chase the money so that they can pay their lab techs and make rent.

Everything in aviation is about forced tradeoffs among cost, performance,and safety. GA is a tiny market, and we wouldn't even get the little bit of R&D we do if it weren't for the endless supply of devoted fools lining up to convert their large fortunes into small ones. I can easily see myself falling for the imaginery glamour of an aviation startup, if I had a fortune to lose.
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Sort of like Alan Ruud and the DeltaHawk engine? Will that thing ever be certified? God bless someone with money like him to buy it up and try to keep advancing the industry...but man, it'd be nice to see something tangible. They've been expecting certification for what feels like more than 15 years and every year it "by the end of the year". People have said that since Ruud's ownership the engine has changed significantly and it may actually get certified. Time will tell.
 
Bill said:
I'll take the bait, what the hell, it's Friday!

Because aircraft engines are built with 1940's technology and metallurgy. Look at car engines from that era, they weren't models of long term durability. Car engines have evolved with science and engineering advances, piston AC engines have not.
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This is a simple answer. Aircraft engines are historically air cooled. That's all there is to it.

The tolerances and above mentioned metallurgy in air cooled, large displacement engines that sit for months and collect rust and carbon and are run at full power on every flight will take their toll on the lifespan of these engines.

The newer generation of liquid cooled engines have much smaller reciprocating masses and run with higher compression and tighter tolerances. They are built to run all out for years and take the abuse of power changes and a wide operating temperature range.

I submit that any modern automotive engine would far outlast an air-cooled aircraft engine from the mid-20th Century in the same application.
 
genna said:
True, but as discussed, tbo is just a suggestion. This is a new engine, they may be very conservative. This engine is also hard to hurt with improper operation.
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Hope you’re right. Would be amazing to get 4000 or 6000 TBO!
 
IK04 said:
I submit that any modern automotive engine would far outlast an air-cooled aircraft engine from the mid-20th Century in the same application.
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It's working out pretty good in the Raptor...
 
David Megginson said:
Seventh, you're right that our piston engines are old tech. The industry could probably do better if there were 10× as many of us with 20× as much money to spend, but there's not (we're getting older on average and gradually dying off), so everyone who tries to introduce new engine tech just loses their money. My prediction is that we'll have improved battery tech (e.g. 4–5 hours endurance) for electric planes before we have modern gas engines in most piston planes, because we'll benefit from all the R&D on electric cars.
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Or gas-electric hybrids.

Once you have a super-reliable Electric motor, you can put in an automotive-grade gasoline engine to power it, and then provide a battery backup. You just need enough battery backup to cover an engine failure at low altitude. Maybe 30 mins. Combination of those would likely yield on par or better than the safety record of existing piston aviation engines.
 
mondtster said:
It's working out pretty good in the Raptor...
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Peter creating a poorly cooled boosted monster should not be an analog to what modern pistons are capable of.. to the contrary, it should demonstrate what kind of torment a modern automotive engine can tolerate
 
WDD said:
Hope you’re right. Would be amazing to get 4000 or 6000 TBO!
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Hope so. But even rotax isn’t exactly new. 912 is from the 80s. 915is is an evolution of 912. I’m not sure, but I suspect it is a derivative of their other small engines. All this means that the family never intended to compete with 360+ legacy engines. yet it is now coming close to 360 performance and on par with 320. At lighter weight, lower cost, far easier operation, better high altitude performance and significant fuel cost savings. If they chose to design something competing with 200hp 360, I bet it would be a significant improvement over lyco/conti
 
Honest question, what's the technical challenge that makes the proposition of a rotax an elusive unicorn at a meager 200-230hp, but not at 140HP? As to weight, the a supersized 915 has a lot of weight to go before it starts encroaching on the latest diesel offerings fighting weights (CD-265) of the same desired output (mid 200s).
 
Tantalum said:
Peter creating a poorly cooled boosted monster should not be an analog to what modern pistons are capable of.. to the contrary, it should demonstrate what kind of torment a modern automotive engine can tolerate
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Modern or not, aircraft or not, engines are far more durable than you give them credit for. But each engine was designed for a specific purpose/application, and putting a car engine on an airplane is taking it out of its element. Does the Raptor have problems? Yes. Can they be overcome? Probably, but at what cost? Will it be durable? We'll see.

You always throw around the term "modern" but I don't think what you perceive as a "modern" engine really has relevance in discussing an adequate clean sheet aircraft engine design. How many engine development programs have you been part of? What do you think drove the automotive industry to develop the engines we have now? I'll give you a hint, it wasn't because those companies wanted to innovate.

Have the typical aircraft engines in service been modernized? Yes, to a point.

Could aircraft engines be better? Yes, and some of the developments made elsewhere could be implemented, but I think a properly developed engine for aircraft applications would not look like what you think a "modern" engine should look.

Will they get better? Not likely. I say this every time these threads come up, but everyone whining about lack of modernization in aviation seems to conveniently forget that they're flying 50 year old airframes and have the technology to match. The people that should be put off with the current state of aviation engines are the folks buying new airplanes, yet you don't seem to hear about those guys complaining too much. It always seems to be the guys flying the old clapped out stuff.
 
hindsight2020 said:
Honest question, what's the technical challenge that makes the proposition of a rotax an elusive unicorn at a meager 200-230hp, but not at 140HP? As to weight, the a supersized 915 has a lot of weight to go before it starts encroaching on the latest diesel offerings fighting weights (CD-265) of the same desired output (mid 200s).
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I'd guess that the largest detriment to a larger, 225ish horsepower Rotax is a lack of a reason to build one. How many people would actually buy it?

The best thing Rotax could do, assuming they kept the final pricing under control, would be to develop a 150-180hp engine and obtain STCs to install the engine on common airframes such as the 172 and Cherokee lineup. I suspect that they'd gain a lot of customers if a brand new engine could be bought for a competitive price and had some durability improvements over the competition.
 
Dan Thomas said:
Corvairs in homebuilts, with something like 14 broken crankshafts so far.
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I follow you well enough to know that you don't make off hand statements. I fly a Corvair and follow them closely but I can't recall that many breaks. Yeah I know Langford had three in his KR. Seems adding the fifth bearing for the prop loads has eliminated that concern. In addition to that Dan Weseman at SPA now has billet cranks for stock and stroker engines.
 
Coming at this from a completely different angle.
I put 100 to 150 hrs/yr on my plane.
That means the engine will last 13-20 yrs.

I’m good with my engine lasting ~15 years.

If I ran my car as hard as I run my plane, I’d be OK replacing the engine after 15 years too.
 
Daleandee said:
I follow you well enough to know that you don't make off hand statements. I fly a Corvair and follow them closely but I can't recall that many breaks. Yeah I know Langford had three in his KR. Seems adding the fifth bearing for the prop loads has eliminated that concern. In addition to that Dan Weseman at SPA now has billet cranks for stock and stroker engines.
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A guy local to me has broken at least one. I don't know the details beyond that though.
 
hindsight2020 said:
Honest question, what's the technical challenge that makes the proposition of a rotax an elusive unicorn at a meager 200-230hp, but not at 140HP? As to weight, the a supersized 915 has a lot of weight to go before it starts encroaching on the latest diesel offerings fighting weights (CD-265) of the same desired output (mid 200s).
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Double the displacement; larger diameter pistons and/or longer stroke and/or another two cylinders. But that would be a lot more piston mass running at high RPM's; all sorts of problems with that, including new crankshaft, bearings to handle the load. Probably need a new valve design to handle the air mass / velocity. Unlike the lower RPM Continentals, the higher RPM Rotax might need 4 valves per cylinder valves to breath. Cooling would have to be re-done. New design on oil circulation / lubrication / oil cooling. Reconfigure the case to handle the higher propeller load.

As much as I'd like to see it, I wouldn't think it would be that easy.
 
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