So I've been building an ultralight

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danny_galaga:
Funny story 😄

There were two planes at the place I hire from. This engine's hours were nearly up. The engine in the other plane had a lot longer to go. But THAT plane was damaged in a rough landing. Airframe was written off but engine was fine. Hire aircraft engines can't go over 2000 hours. But private can. These Rotax engines are good for over 4000 hours. So I get the engine that was nearly ready to be replaced, and the hire plane I was flying continues on with the lower hours engine

The funny story continues though. They 'borrowed' back my carbies because the butterfly valve shaft in the other engine got bent in the rough landing. So today I hired the plane and I'm STILL flying it with bits of the engine inow own 😄

nitrogen_widget:
so what is the difference between and plane engine and say an engine used for a ground vehicle?
i've got a 20HP engine in a lawn tractor.
I don't think you can put it in an airplane.

intake and exhaust manifolds along with carbs are different i assume?

danny_galaga:
You could definitely put that engine in a small ultralight. How reliable it would be is a different matter. Rotax got into the aircraft game because in the 80's when ultralights we're still a new thing people were using whatever they could get their hands on. From go kart motors to largechsinsaw engines. Rotax made two strokes for snow mobiles etc which turned out to be a good match.

People often say 'that motors not very powerful, my car engine would be much better'. But there are several really important differences that make car engines not as useful as it would first seem. First of all, propellers are most efficient at around 2500 rpm. So a lot of plane engines are designed to develop power around that speed. Rotax design their engines more like a car engine, and then add a reduction gearbox, roughly 2:1. Propellers might be more efficient around 2500 rpm, but it turns out petrol engines are most efficient around 5000rpm.

The other major thing a car engine spends most of its time at a fairly low power setting. To drive on the highway might take 20 per cent of its power for instance. But an airplane might be cruising at maybe 75-85 per cent power, ALL DAY. Not just that, on smaller aircraft especially, on take off you are using 100 per cent power until you level out. That could be five minutes. Imagine a drag race that was five minutes long!

To illustrate how seemingly under rated an aircraft motor seems, I learnt to fly in a Piper Warrior. I think from memory it had a Lycoming 0320 in it. That's pretty close in capacity to a Chevy 327. But the Lycoming only developed 150hp, AND only 4 cylinders! Like I say though, it could run all day near the top of its range.

RandyT:

--- Quote from: danny_galaga on December 18, 2022, 08:04:29 pm ---You could definitely put that engine in a small ultralight. How reliable it would be is a different matter. Rotax got into the aircraft game because in the 80's when ultralights we're still a new thing people were using whatever they could get their hands on. From go kart motors to largechsinsaw engines. Rotax made two strokes for snow mobiles etc which turned out to be a good match.

--- End quote ---

Not sure if it's true, but I also heard that some Rotax engines were shown to be so reliable that they were actually approved by the FAA for ultralight use. I had a couple of rough looking sleds I planned to fix up which had good running Rotax engines. When I decided that I didn't like cold, I seriously considered pulling and selling just the motors because, to the right person, they were worth more than the sleds.

So, has anyone heard of someone crazy enough to use one of the HF Predator 20hp 2-cyl engines in an ultralight? Being a Honda clone, they are surprisingly decent. But you wouldn't catch me leaving the ground with one. I think geared-down 2-strokes would be better. Not much to talk about on the low-end, but when they get up into their power band, they can be insane relative to size and weight. I had a 2-stroke, 3-cyl Kawasaki bike in my younger days, that I wish I kept. Then again, if I did, I might not still be here to talk about it :)

nitrogen_widget:

--- Quote from: danny_galaga on December 18, 2022, 08:04:29 pm ---You could definitely put that engine in a small ultralight. How reliable it would be is a different matter. Rotax got into the aircraft game because in the 80's when ultralights we're still a new thing people were using whatever they could get their hands on. From go kart motors to largechsinsaw engines. Rotax made two strokes for snow mobiles etc which turned out to be a good match.

People often say 'that motors not very powerful, my car engine would be much better'. But there are several really important differences that make car engines not as useful as it would first seem. First of all, propellers are most efficient at around 2500 rpm. So a lot of plane engines are designed to develop power around that speed. Rotax design their engines more like a car engine, and then add a reduction gearbox, roughly 2:1. Propellers might be more efficient around 2500 rpm, but it turns out petrol engines are most efficient around 5000rpm.

The other major thing a car engine spends most of its time at a fairly low power setting. To drive on the highway might take 20 per cent of its power for instance. But an airplane might be cruising at maybe 75-85 per cent power, ALL DAY. Not just that, on smaller aircraft especially, on take off you are using 100 per cent power until you level out. That could be five minutes. Imagine a drag race that was five minutes long!

To illustrate how seemingly under rated an aircraft motor seems, I learnt to fly in a Piper Warrior. I think from memory it had a Lycoming 0320 in it. That's pretty close in capacity to a Chevy 327. But the Lycoming only developed 150hp, AND only 4 cylinders! Like I say though, it could run all day near the top of its range.

--- End quote ---

running all day is probably top of the list of things you want your personal aircraft to do.

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