mooney M240V acclaim ultra airplane Interior looks like a comfy place.... BTW: anyone remember the PFM 3200? More Peter Schutz fun & games....My boss VP Marketing at the time (ex-Airbus) was deeply involved in the marketing, and he was not a pilot! But thought to be a good way to put more YPO's in Porsche buckets.... Image Unavailable, Please Login Image Unavailable, Please Login
Nice. I remember when the PFM was intro'd... was supposed to be the greatest thing. Had a Porsche with the same engine (sort of). I think there were a couple of 172's that had them also. (Maybe still do?) I had hoped the Mooney 301 would happen... now the Socata.
Other than having to wear an O2 mask, which they never show pictures of, flying in the flight levels in a single-engine GA plane is fun. Doing it in a Mooney is even better.
The PFM was an unmitigated disaster. Why would one spend more for an airplane that was a few knots slower and had a shorter TBO... Then Porsche pulled the plug and the buyers of the 40 Mooney PFM's owned orphans. I'd guess that most if not all of them were eventually converted back to conventional engines and the buyers took a beating. The concept of gearing an engine to let it run faster than prop speed sounds great until you realize that the prop IS the flywheel and the gears are getting pounded by the combustion pulses. The PFM and the Conti Tiara were notable failures of the concept. The GTSIO 520 has a big damper that lets it work, but if it fails the engine fails in a matter of minutes. The 1600 hr tbo is lower and reflects the higher engine speed (by 30%) when compared to the direct drive versions. No free lunch here, have more combustion events per hour and the life of the cylinder goes down accordingly. No saying the GTSIO is a bad engine, it's not, but by the time you pay the weight and cost penalty of the gearbox, solve the inherent life and vibration problems and add it all up, it's easier to just add displacement and keep the rpm's down. As you go above about 350 hp it starts to make sense since a prop big enough to handle 350 hp is starting to get pretty big and that means a really slow engine speed, but for lower hp systems bigger displacement makes more sense than gearing.
Glad to hear you say that. Not really an airplane guy but it never sounded like a good idea to me. Airplane engines have a very different life than sports car engines and from the oil drain plug up the designs are different for very good reasons. Always thought it sounded like an expensive way to have a short lived airplane engine. Makes me wonder, if it had ever sold big what the history of it would have turned out like. I really have to wonder who thought turning an automotive engine a constant 5300 rpm at high load was a good idea for longevity.
The way it works is pretty simple. The prop swept area is proportional to the horsepower, so for bigger power you need a bigger prop. But the prop tip speed limits the rpm, you don't want to go too fast because the tips go supersonic and that isn't efficient as well as it is noisy. So the prop speed is slowing down as the engine gets bigger. But power is torque x rpm, and torque is proportional to displacement... You'd like to spin the engine faster to make more power, but you don't want too fast a tip speed... This is why you have big displacement (like 540 cubic inches) engines making 250 hp, the engine speed is only 2700 rpm... The classic problem is that you have a hard time getting the heat out of a smaller engine that is turning faster. That and higher frictional losses mean that smaller higher speed geared engines can be done, but as Porsche demonstrated even with their expertise in engines what they came up with wasn't really an improvement compared to the conventional approach of a bigger displacement engine turning slower. It's also easy to get greedy when you are gearing an engine. To be strong enough for the combustion forces means that you can spin it a good bit faster than you would with a direct drive engine. The GTSIO 520 only turns about 30% faster than the direct drive versions, so it isn't screaming, where the PFM had a .44 gear ratio and the Tiara used the cam gear so it spun the engine twice prop speed. But the heat generated and the losses are a function of power generated, make more power and you have to reject more heat, so this tends to limit things like valve seat and valve life, as well as over time cooking the rings and things like that. Basically, aviation engines live a hard life. I once was helping my mechanic take apart a O320 that had gone about 200 hrs beyond tbo or about 2200 hours.. What was really interesting is that just about everything in that engine was tired. The bearing surfaces were just about to the copper, the rings were starting to stick and the lands were full of carbon. The exhaust valves were toast and seats weren't sealing all that well. The lifters and cam showed a good bit of wear. This engine was treated well and oil changed at the recommended intervals, but by the time it got to TBO it was pretty much worn out. There's two types of engines that get run hard and that is airplanes and small boats. Both of those systems run very high percentage of power for all the time and that is totally different than a car that runs at 10 or 15% of rated power for most of the time, with short blasts of high power for 10 or 15 seconds. Even cars on a race track get periods where they get to "breathe" a bit and as Porsche found that is still very different than the expectations of 2000 hours at 75% power that we have for airplane engines. Even 24 hours flat out at Daytona or Le Mans is nowhere near a few hundred hours of life in an airplane at cruise power.
Road race engines almost never see over 10 continuous seconds at 100% power. Even our Bonneville engines do not see it for an entire minute and by the end of Speedweek the salt is covered with stains where motors couldn't take that. Stock car motors built for the speedway events see the worst and they are somewhat detuned to do it.
It's too bad that one cannot "grab" that extra heat with a combined cycle engine and convert the heat into more power.
"Turbo-Compounding" on the Wright engines in the old Connies... Then there was the Napier Nomad turbo-compound diesel engine... one writer said the only purpose for the engine was to produce the exhaust that turned the propeller. It had excellent fuel efficiency, but along came the turboprop... and jet.
Works great if the heat is all coming out in the from of exhaust but too much of it doesn't. Higher heat requires more expensive materials and operation snowball begins. Better avoided if possible.
All true... The heat transferred thru the exhaust ports and valves is more than half of the heat rejected by the cooling system... For an efficient engine like a diesel a good rule of thumb is that about 1/3 of the heat does useful shaft work. About 1/3 of the heat is rejected by the cooling system, and about 1/3 of the heat goes out the exhaust. This means that if you have a 100 kw engine you need a 100kW worth of radiator to dump the heat. IF you do a good waste heat recovery system you can recover about 25% of the 1/3 that is going up the stack, or about 1/12 of the energy in the fuel, and that moves your overall efficiency from 33% to about 41%.. That's not a huge amount of energy when you consider the weight that it takes to make a complete cycle out of it.. The turbo compound approach works because the engine is even less efficient than a diesel and there's a lot more waste to start with. Add to that the fact that the parts are getting decent in size and it clearly worked, but that was the last gasp of big recips, and the reliability of it wasn't that hot. Turbines destroyed the big recips not for fuel consumption reasons, but because they let the aircraft fly higher and faster and that ended up being more efficient overall than flying lower and slower...
I remember how much oil the R2800s leaked on the T-29s I trained in at McClellan AFB. No big deal said the pilots and crew chiefs when they led us through a pre-flight. They just put cans down to catch the oil. Plenty of oil for a 6 hour flight. Then I think of our TF30s that only carried 8 quarts of oil and seldom used any. Big difference in reliability, except when the TF30s did let go, it was not pretty. Especially with the old gasoline based JP-4. One reason I landed at least a dozen times with one engine at idle or shut down. No sense taking a chance.
Porsche supported the engines until 2007, believe it or not. I seem to recall reading that they helped pay for some conversions. Good points about the gear drive. The Porsche engineers didn't seem to understand aerodynamics too well either, and weren't going to take any advice from a bunch of "dumb Texans". They ended up having to tune for 217hp to approach the performance of the original 200hp engine.
