Calling all airplane savvy oppos!

"jsmizira" (jmizira) 01/07/2014 at 01:03 • Filed to: None

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I have made friends with an older gentleman who owns a small Cessna. I know nothing about prop airplanes of this size.
How do these engines work that's different than a regular pistion engine? Ls7 swap? What is Optimum prop rpm? Does a faster rpm = a faster plane? Educate me. I know nothing yet!

PanchoVilleneuve ST > jsmizira 01/07/2014 at 01:05

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They're basically overbuilt, extra-reliable internal combustion engines. Since in a car, if the engine dies, you're on the side of the road, but if you're in a plane, that's some bad news.

For Sweden > jsmizira 01/07/2014 at 01:08

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Car engines aren't reliable enough for aviation.

jsmizira > For Sweden 01/07/2014 at 01:13

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Please elaborate.

HammerheadFistpunch > jsmizira 01/07/2014 at 01:16

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small aircraft engines are very large, very torquey and very simple. usually the engineering is from the 1960's because each engine has to be faa certified which is too costly and lengthy a process to do often. they are so old school they actually run on leaded fuel. LL100

as for prop speed, the practical limit on tip speed is subsonic, so you can't spin a prop faster than the tips will reach supersonic speeds (you can but its complicated and causes problems) so usually added thrust is done with pitch, which is why torque is important.

HammerheadFistpunch > For Sweden 01/07/2014 at 01:17

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depends, subaru ej engines are approved for a class of small light planes.

jsmizira > PanchoVilleneuve ST 01/07/2014 at 01:17

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So does that mean that the horsepower ratings could be higher? What if someone had a de tuned ls7 rated for 300 hp instead of 500? Does the rpm have a practical use ceiling? Can you in theory turn a prop, i dont know, 15k rpm if you had an overdrive on the front of a torquey engine?

For Sweden > jsmizira 01/07/2014 at 01:18

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Aircraft engines must demonstrate a certain level of reliability before certification. Automobile engines aren't built to and can't meet that standard.

jsmizira > HammerheadFistpunch 01/07/2014 at 01:19

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Thank you so much! So what would be the maximum pitch on a prop? What is the norm? Does it depend on material used?

HammerheadFistpunch > jsmizira 01/07/2014 at 01:20

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rpms aren't critical, blade pitch and swept are are. increasing thrust is done by pushing a larger blade at a steeper pitch. thats why turboprops (jet engines with props on the front of the shaft) are geared down to reduce the prop speed but increase the torque. I should also point out that engines loose ~3% power for every 1000 feet up you go, so a 300 hp engine in a car for an engine meant for 10,000 feet for flying is only rated as 200 or so hp.

jsmizira > HammerheadFistpunch 01/07/2014 at 01:23

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That makes sense. It's a More surface area would equal more thust thing right? What is meant by "swept"? Is that the length of the blade?

HammerheadFistpunch > jsmizira 01/07/2014 at 01:30

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swept area = surface area that passes the wind

HammerheadFistpunch > jsmizira 01/07/2014 at 01:33

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its a curve, obviously 0 pitch means no air is being pushed by the blade, but 90 degree pitch isn't biting into the air either, just beating it up. props have a built is variation in the pitch but on more sophisticated small aircraft the pitch of the prop is adjustable, like a cars transmission.

Someone Else's Projects > jsmizira 01/07/2014 at 01:37

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There seems to be a higher priority on smoothness in aircraft engines, as boxer engines are very common in light aircraft, and V-12 engines were the only inline (as in non-radial) configuration used by air forces in WWII. Also explains why some exploration has been done with Wankels, behaving as a sort of weird stopping point between reciprocating engines and gas turbines.

jsmizira > Someone Else's Projects 01/07/2014 at 01:49

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Yeah I noticed that too. But you could make a inline 3 smooth enough with a flywheel that is weighted right

KSUENGINEER > jsmizira 01/07/2014 at 01:55

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Your typical GA single prop aircraft is overly engineered. Redundancies are key in every aircraft. Ignition systems have two magnetos (='s two spark plugs for every cylinder). These are tested before every flight. AVGAS (typically 100LL in the USA), is held to much higher standards than typical pump gas at your local gas station. Most aircraft like the 172 are air-cooled engines. Since weight is critical in aircraft having a cooling system would add a weight disadvantage (then you have to worry about the huge temperature changes in flight for the coolants). These engines aren't high reving either. Redlining in most of these Cessna's is around 2500 rpm. In aircraft engines you want you peak torque curve around your cruise rpm. This is typically around the redline. So having an LS7 wouldn't be beneficial. The prop wouldn't be able to handle it. In fact the prop efficiency decreases are the tips of the blades approach Mach 0.85. The key to making a prop airplane to go faster is to climb to a higher altitude. Air is less dense, thus you can achieve a greater speed. More horsepower/torque/rpm isn't what it is all about. It is all about efficiency.

911e46z06 > jsmizira 01/07/2014 at 02:05

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I think there's an LS1 swap kit for Seabees

Someone Else's Projects > jsmizira 01/07/2014 at 02:08

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Yeah but then you run into the age-old problem of weight prioritization. If you're going to shove a big chunk of metal onto your engine, might as well have it provide more power rather than just providing smoothness through inertia. Of course you could easily solve the straight-3 smoothness problem by making it a 2-stroke, but I've never heard of them being used in full-sized airplanes.

FlipperB > jsmizira 01/07/2014 at 08:16

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If the plane is pressurized, it can climb to where ambient temperatures are very hostile (think -30F). Ergo, most piston-powered aircraft have an air-cooled engine. Your theoretical LS swap would probably have issues operating in those temps with moisture/ice.

Jayhawk Jake > jsmizira 01/07/2014 at 08:32

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Most people have covered it.

On smaller planes people have used VW engines, but modified. You need to add a second spark plug to each cylinder and a second magneto system.

Prop RPM varies from plane to plane, but something like a Cessna usually runs between 2 and 3 thousand RPM.

For an FAA certified airplane, you can't use an engine that isn't certified. Certification is really expensive, so many airplane engines today are basically the same as they were in the 50s.

Jayhawk Jake > KSUENGINEER 01/07/2014 at 08:33

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I wouldn't say it's over-engineered. The FAA requires certain systems in the engine for safety

Jayhawk Jake > HammerheadFistpunch 01/07/2014 at 08:35

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For a certified airplane, car engines aren't reliable enough. Experimental planes do occasionally use them, like the Seabee or the Hummelbird

PostOMatic2000 > jsmizira 01/07/2014 at 10:55

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I seem to recall that marine engines need to run at high torque at all times, and aircraft engines need to run at high rpm at all times. Car engines are a compromise between the two. Also, cooling is an issue because coolant and radiators are generally too heavy. That's why Porsche has made quite a few aircraft engines: air cooled. And VW too.