Random Griz Aviation Musings

[jmcquate]

How small can turboprops get?

 

[Pags]

There's some truth to that but there's a lot of myth to it too. The debate is way too long for here, but these are some high points:

A lot of ordinary general aviation engines have advanced features like oil squirters under the pistons or sodium-filled exhaust valves. You only get those on very high-end car engines. A lot of was "advanced" for late-WWII era is still state of the art today. FWIW, forged vs cast or billet isn't something I call advanced, it's something else (just selecting the right part for the job).

Aero ignition systems (magnetos) are rudimentary, not much different than a lawnmower, but they do make a powerful enough spark. Fixed timing isn't really an issue when the engine spends most of its time at a pretty narrowly-defined cruise point. There are electronic ignition conversions available now (for certificated airplanes, not just experimentals) but they really only offer single digit percent improvements to the airplane's range and little to no improvement in takeoff power.

Computerized fuel injection, again, not really much value for an engine that's intended to drone for hours and hours and doesn't need to be particularly efficient at maximum power (takeoff and climb is about ten minutes).

Heat rejection is always a big deal for auto engine conversions- there are internal details like I already mentioned and then there is how to get rid of the macro load. Air cooling (good baffles are critical) already solved this problem decades ago. Water cooling works too (the old V engine vs radial engine debate from the 1940s), but it boils down to a different set of engineering challenges (pun intended).

Gear reduction is another problem that got solved decades ago (big radials as well as Vees like the Merlin, and 100hp-class Rotes engines in the modern era) but it's never an easy with a new-design engine. Sorting out vibrations is a lot of work when you put an engine, gearbox, and propeller together. Auto engines are rarely optimized or even optimal for direct drive- the closest ones are some air cooled Volkswagen conversions and the Corvair engine (thrust bearing modifications are available for those, by the way... yet another detail) but those are typically heavier than comparable horsepower aero engines, not to mention the basic designs are from the same era, decades ago.

Geared vs direct drive emotional arguments are funny too. A lot of people used to poo-poo Rotax engines as "not sounding right for an airplane engine" because the exhaust sound and propeller sound were off (they're geared). The radials in the old T-6 are geared, Merlins in the P-51 too, so I'm confused here if a manly-man aero engine is supposed to have a gearbox or what...?

Mechanical strength of the engine parts is one of the myths of auto engine conversions. The reality is that new auto engines get thrashed in a test cell before they're put into production. Some of the "car engines don't hold up" comes from auto racing, I think, where frequent rebuilds are common- and those rebuilds are because they're running then engine very hard, but that doesn't directly translate into how hard an aircraft engine has to be run. A lot of airport bums will tell you that aero engines are designed to run at full power indefinitely... well, sort of, but don't forget aero engines have been known to suddenly throw pistons through the crankcase (usually happens at least once a year in the U.S. general aviation fleet). In reality they spend most of their lives at about 50-75% output- typically a 360 cubic inch/6 liter engine makes around 100-150hp in cruise, at or around 10,000 feet and turning 2400rpm. Compare that with a car engine if you're ripping across western Texas at 80-100mph with the a/c blasting. The car engine is getting around 50hp out of 3~ish liters. Another airport bum-ism is the "slow turning" aero engines. That 2400rpm cruise point in a typical aero engine produces the same piston speed as a Camry engine at 3200rpm and a Fit engine at 3400. So much for the high-revving, "buzzy" car engines. Wanna take a stab at a pickup truck engine pulling a boat or a horse trailer down the interstate in the Appalachians or the Rockies?

Last, fuel delivery combined with engine compartment heat is another deal breaker. Aero engines are well evolved with pretty elaborate systems of blast tubes (cooling), insulated lines (fire sleeves aren't just for engine fires), and where the components sit on the engine.


Anyway, when these discussion come up then you've gotta know your audience. There's no point in getting into it with a so-called airport bum, who is usually an old guy who'd gladly lend you tools, help you push your airplane, or agree to any favor. Now, when you realize a person is either a real gearhead or an engineer with a strong practical background, then you'll probably learn a lot from each other by discussing engineering tradeoffs and numbers.


I did say it was a lot to talk about. This is just the tip of the iceberg.


The devil is in the details though, it really is.

Good super nerdy post. You mentioned some points about auto engine conversion which I take to mean taking an existing car motor and putting it on an airplane. If so, thats not what I getting at as I understand that they're built for different use cases. What I was getting at was taking some technology from auto engines and applying to GA motors. Which I think you kind of addressed, clif notes seems to be that yes, you could but because there are different use cases the between car and GA that there's no large benefit to implementing those technologies on a GA motor. I guess at some point any changes to a GA motor would require more engineering and that would then be passed on to the buyer. Also, it seems that the simplicity of GA motors keeps their ownership cost low.

 

[Griz882]

How small can turboprops get?

The Czechs make an turboprop engine called the TP1100 that runs 135 lbs is 16” wide, 13” tall and about 36” long and makes 241 hp!

Also I think they make the tiny jet engine that goes on the Sonex.

 

[Pags]

How small can turboprops get?

They make little jet motors for cruise missiles. A quick Google showed that there's an emerging markets for small turboprops...I found someone who has an RV-10 with a turboprop.

 

[JTS11]

Here's some small hand jets.

Wouldn't you lawn dart pretty fast if you lose a motor?

 

[Griz882]

Here's some small hand jets.

Wouldn't you lawn dart pretty fast if you lose a motor?

What percentage of SNAs get hand jets?

 

[Gatordev]

I’m getting better at hot starts but damn who would have thought they would be so finicky! I haven’t quite figured out the primer valve thing. However, my engine (IO-360) does good with throttle open and mixture closed.

The number of pumps is up for internet debate, but basically, if the engine is hot, don't prime. But if it's warm, you can shoot a squirt in there and if it's cold, shoot <insert number of internet approved squirts>.

My -360 likes 3 squirts when cold (although the first shot isn't a full shot). I also shutdown making sure the RPMs are at 1,000 and then don't touch the throttle until after starts. It's essentially the same as the 1/4" of throttle that most checklists say, which who knows how much that really is.

They make little jet motors for cruise missiles. A quick Google showed that there's an emerging markets for small turboprops...I found someone who has an RV-10 with a turboprop.

The problem is they're still thirsty, by design. Contrast that with the Diamond DA-62's diesel motor that cruises at ~8 GPH, but also can put out 180 HP. Like Jim was saying, applying some modern technology helps with that efficiency. And it runs on Jet A, which is a huge plus nowadays.

 

[Pags]

The number of pumps is up for internet debate, but basically, if the engine is hot, don't prime. But if it's warm, you can shoot a squirt in there and if it's cold, shoot <insert number of internet approved squirts>.

My -360 likes 3 squirts when cold (although the first shot isn't a full shot). I also shutdown making sure the RPMs are at 1,000 and then don't touch the throttle until after starts. It's essentially the same as the 1/4" of throttle that most checklists say, which who knows how much that really is.



The problem is they're still thirsty, by design. Contrast that with the Diamond DA-62's diesel motor that cruises at ~8 GPH, but also can put out 180 HP. Like Jim was saying, applying some modern technology helps with that efficiency. And it runs on Jet A, which is a huge plus nowadays.

Right on. I'm admittedly way out of my swim lane but it's been an interesting conversation. Good points on turbine gas consumption.

 

[Griz882]

So....I wonder if @wink got his engine started?

 

[wink]

What kind of engine is in that, @wink ? Is it a small Continental? (A popular family of engines that I don't know much about.)

All I know about GA pistons and hot starting is that the plain vanilla Lycomings, in spite of any other shortcomings, are such easy hot starting engines- the combination of an updraft carburetor and an impulse magneto.

Not trying to turn it into a Ford vs Chevy argument either, both brands are very popular for a lot of good reasons, just curious.

You can stand down the Alert 30 for Wink SAR. I made it back. Had to wait about 40 minutes or more. Got back to home plate and it wouldn't start after fueling. So I tied it down and went to work. Will go back to the airport this evening to start and taxi to the hangar.

I am rock'in a Continental A65. AWESOME little motors. Funny the posts about 1950s technology in contemporary engines. My engine is a 1930s design and the very one powering my Luscombe was built in 1945. It is 75 years old .

When I left this morning it started on the first throw of the prop. I never go beyond 3 attempts, unless I have a hot start issue. The Luscombe I owned ( same motor) in the 80s never had a hot start issue. Never had any hot start issues with my Cessna 170B, which had a 6 cylinder Continental 145. It actual is rare on this plane. Done it three times now. It has a Stromburg carb, which is an odd duck. It won't go full cut off and kill the engine. So there is a procedure for shutting it down with the mags only. I think maybe I am not getting it right sometimes and with a hot engine I am screwed. Remember, I am hand propping, so procedures that are undertaken while turning over the engine or require setting a high power setting are not possible or are down right scary when hand propping a plane. Everything is at idle power settings. Even trying to clear it by setting switches off, shutting off fuel and mixture lean can be hard on the hands after 10 or 12 throws. In the end it wasn't so bad. Didn't really have to be anywhere and it was a nice morning. A guy in a beautiful Carbon Cub with huge tundra tires dropped in to see if I needed help.

 

[Jim123]

@Pags , some other random thoughts about auto engines technology and aero engines.

About ten or fifteen years ago, Honda figured out something with the bearing surfaces of their crazy high revving car engines, something about the crystal alignment and how it holds the oil film. If that benefits an 8000+rpm redline engine with small diameter bearings in an Acura then it would benefit any engine. Worth the cost? I dunno. But Honda did that because they built that engine to be run very hard.

Let's see what else... multivalve engines. Probably some benefits here too. I don't think their general reliability and durability is in question- lots of road cars have had them for decades. Of course it goes directly against the old engineering tradition that fewer moving parts is more reliable, but if a machine meets a reliability standard and it can be built to an affordable cost, then the results are the only thing that matters.

Talked about electronic fuel injection and electronic ignition already, mixed benefits of these.

Other things like how the engine is internally laid out, like pushrods and rockers in the valve trains vs overhead cams, there were OHC aero engines in both world wars. A lot of people think of OHC engines as a high tech, modern car engine thing but it's really not, it's just another tradeoff. General aviation piston engines usually have timing gears (crankshaft pinion touches the camshaft gear), no chain or belt- simplicity, compactness, and reasonable efficiency.

Manufacturing processes? Some car engines have lightweight plastic or composite manifolds; some have had terrible problems but other have been successful in service. Some experimental airplanes have these, those are kind of on the fringes and I don't know about any type certificated airplanes or STCs (conversation/modification paperwork), but I think this technology is probably worthwhile in the long run.

Overall the way aero engines are out together, it's actually pretty crazy. Each cylinder is removable, the crankcase is usually in two parts, and that's a lot of extra nuts and bolts- chances to get something wrong and extra weight compared to an integral casting. I've replaced a single cylinder. It was fun in a "get your hands dirty" sense and it's nice to be able to service and replace the single bad component/subassembly instead of an entire engine, but at the same time I've never had a cylinder go "bad" on a car engine, which begs the question, WTH? Anyhoo...

More food for thought, the BSFC of small Continental four cylinders (100-125hp) gets down close to 0.4 when you dial them back in cruise. Thats a really good number and there aren't many car engines that significantly improve on it.

 

[nittany03]

And didn't realize this was right under the airway. Ended up flying over it completely by chance. Insert various movie memes here.

 

[jmcquate]

DC Flyover canceled. Low ceiling. Scheduled for the same time tomorrow.

 

[ChuckMK23]

@Pags , some other random thoughts about auto engines technology and aero engines.

About ten or fifteen years ago, Honda figured out something with the bearing surfaces of their crazy high revving car engines, something about the crystal alignment and how it holds the oil film. If that benefits an 8000+rpm redline engine with small diameter bearings in an Acura then it would benefit any engine. Worth the cost? I dunno. But Honda did that because they built that engine to be run very hard.

Let's see what else... multivalve engines. Probably some benefits here too. I don't think their general reliability and durability is in question- lots of road cars have had them for decades. Of course it goes directly against the old engineering tradition that fewer moving parts is more reliable, but if a machine meets a reliability standard and it can be built to an affordable cost, then the results are the only thing that matters.

Talked about electronic fuel injection and electronic ignition already, mixed benefits of these.

Other things like how the engine is internally laid out, like pushrods and rockers in the valve trains vs overhead cams, there were OHC aero engines in both world wars. A lot of people think of OHC engines as a high tech, modern car engine thing but it's really not, it's just another tradeoff. General aviation piston engines usually have timing gears (crankshaft pinion touches the camshaft gear), no chain or belt- simplicity, compactness, and reasonable efficiency.

Manufacturing processes? Some car engines have lightweight plastic or composite manifolds; some have had terrible problems but other have been successful in service. Some experimental airplanes have these, those are kind of on the fringes and I don't know about any type certificated airplanes or STCs (conversation/modification paperwork), but I think this technology is probably worthwhile in the long run.

Overall the way aero engines are out together, it's actually pretty crazy. Each cylinder is removable, the crankcase is usually in two parts, and that's a lot of extra nuts and bolts- chances to get something wrong and extra weight compared to an integral casting. I've replaced a single cylinder. It was fun in a "get your hands dirty" sense and it's nice to be able to service and replace the single bad component/subassembly instead of an entire engine, but at the same time I've never had a cylinder go "bad" on a car engine, which begs the question, WTH? Anyhoo...

More food for thought, the BSFC of small Continental four cylinders (100-125hp) gets down close to 0.4 when you dial them back in cruise. Thats a really good number and there aren't many car engines that significantly improve on it.

I always describe an aero piston engine as not just a single engine but 4 or 6 small engines since failure modes from something other than fuel starvation are often partial power scenarios - the engine is incredibly resilient even after chewing up a valve or a disintegrating cam shaft - and it usually tries to tell you before it gives up.

 

[jmcquate]

Flyover scrubbed again. Permit expires today. Everybody is fling home.