Engine selection - Ultralight

I'm really dragging up an old post but I've had a renewed interest in getting back to my ultralight project. It's been on hold in the storage side of my shop now for over a year. It wasn't the engine selection that caused the delay just lots of other distractions, some unavoidable, others self induced. I've since bought 2 different 2-strokes and sold one of them and don't really trust the other. Now with my renewed interest in the project I plan to forge ahead with the project and hope to use a 2A042 and prop I also bought. My concern now is that the thrust centerline will be about 4-5" higher than that spelled out on the plans (for the engine spec'd). The 2A042 could be mounted lower but the mount would be more sturdy with it a bit higher, closer to the main tube. I'd expect the plane to try to nose-over a bit under power with the thrust centerline higher. Would that be correct to expect? Any other issues that the higher thrust centerline might cause? The design is a high-wing, with tricycle gear.

I'm really looking forward to getting back to the build.

Lynn

Originally Posted by lake_harley

Would that be correct to expect? Any other issues that the higher thrust centerline might cause? The design is a high-wing, with tricycle gear.

I'm really looking forward to getting back to the build.

Lynn

I don't think it would make any detectable difference. Just my opinion.

we are experimenting with the koheler 27 hp , rotax gear box and our own gears with te ratio under 2 to one , on a kolb fire fly we are getting about 500' pm. climb; and straight and leval at 4200 rpm we are getting 80mph, 450cht. total waight of the engine is 105,lbs. static trust is 220.lbs. our goal is to start using the 40 hp. that we just order since the adapter plate we designed will fit any of the v twin to rotax b box. we will see what numbers we get with the 40 hp v twin in few weeks.

I assume you are getting more power and using a reduction gear because it's souped up?

Originally Posted by lake_harley

Kohler 27 HP
Peak HP 27 @ 3600 with 37 Ft# torque
24 HP @ 3000 with 41 Ft# torque
Peak torque 41.5 Ft# @ 2800 with 23 HP

Briggs 23 HP
Peak HP 23 @ 3600 with 30 Ft#
21 HP @ 3000 with 32.25 Ft# torque
Peak torque 32.7 Ft# @ 2600 with 19 HP

New to the list - Honda 24 HP
Peak 24 HP @ 3600 with 33.5 Ft# torque
21.5 HP @ 3000 with 36 Ft# torque
Peak torque 37.5 Ft# @ 2500 with 18 HP

I'm looking at building a low aspect ratio ultralight and was looking at these engine weights and saw that Briggs has a 35hp Vanguard V twin. It's the same as the others, being an aluminum engine with cast iron cylinder liners but I couldn't find a weight for it but I suppose its getting past the limit for an ultralight.

I need a fair amount of power due to the shorter wingspan.

The 24hp Honda is 670cc, the 27hp Kohler is 721cc, and the 35hp Briggs is 993cc.

I think the 993 would be a good engine for a small plane if it didn't qualify for ultralight, but does anyone here have an idea how much one of them weighs?

I keep looking for 4-stroke alternatives for my U/Ls. But they're all way too heavy. The industrial/generator engines weren't designed with light weight as a priority. Even stripped down, they're too heavy for my U/Ls. They'll be less reliable if you hop them up. So you might as well use a well proven 2-stroke, and keep well ahead on its maintenance. 2-strokes are a snap to rebuild.

I've learned it doesn't take a lot of horsepower. If you use enough reduction to turn a big prop, you can get enough thrust. I keep hoping someone will convert a small motorcycle 4-stroke. The modern dirt and small sport bikes have about the right power, and were designed to be light. The Kawasaki 250 Ninja seems like the ideal engine to convert. But it would take some expensive machining. It's an in-line twin.

I too like a deep sound. But I'm getting that from my 2-stroke by using a lot of reduction and swinging a large prop. The slow turning prop is the predominant sound.

If you've been keeping up with my low aspect ratio UL thread you'll see why I need more power than most ultralights, besides it might not even be ultralight

Anyway, a two stroke engine with dual plugs sounds like a possibility, I want around 40hp so that means a bigger engine or two normal ones, but two engines would need to be linked together somehow.

Concerning the four stroke adaptations of motorcycle engines, why do you say that they will need a lot of machining? To cut off the transmission? I have the worlds biggest angle grinder that could cut a car in half in 60 seconds (exaggerating)

For my needs it seems one of the 500cc water cooled singles off a dirt bike would be powerful enough, but what kind of weight are we looking at for something like that?

Starman,

Converting a M/C engine might be fairly straightforward for a machinist. You can buy the whole motorcycle for less than the cost of an U/L engine. It would come with the complete charging system, carbs all tuned in, proven ignition system, electric starter, and you could probably use parts of the exhaust system. (Then you could sell the parts you don't use on eBay.)

Even these race tuned M/C engines are probably a little heavier than the common 2-stroke U/L engines of the same power. But they're way lighter and smaller than the industrial/generator engines. Bailey Aviation's web site suggests considering the weight savings of carrying less fuel for the flight, because of the 4-strokes' much better fuel efficiency.

I wouldn't recommend having two engines. Maintaining more than one would be an awful lot of maintenance and problems. And I think one larger engine would be lighter and more reliable than two small ones.

I guess they're probably not reliable, but the model airplane guys now have radials that are claimed to spin large props at reasonable rpms with obscene (for a model) amounts of power:
RCS 400cc Radial engine with electronic ignition. (RCS400R) - troybuiltmodels.com
They also have an 800cc model. Big bucks, tho.

On the Briggs, the vertical shaft ones with single cylinders seem to have better specs, at least if you could get them to run horizontal, or else have some weird redrive that changed the axis. The hovercraft guys use long belts with this kind of engine, sometimes with pulleys turning the belt 90 degrees.

Just the thing for Starman's project:
UH-TW-1 Tandem Wing : Universal Hovercraft, The World Leader in Hovercraft Technology

Geo engine with one redrive for drive prop and another with 90 degree turns for lift prop:
http://www.hovercruiser.org.uk/sev/sevtec/Geoin.jpg

appears to be Vanguard with right angle drive:
http://www.hovercruiser.org.uk/aird1.JPG
He did have some issues with the belt jumping off! But I don't think this setup is unusual so I suppose there's a way of dealing with it. I suspect that with the long belts, the resonant frequency, rotationally at least, is so low that the crankcshaft and prop are safe. But it would be very odd to see a vertical shaft engine driving a horizontal shaft prop!

Originally Posted by lr27

I guess they're probably not reliable, but the model airplane guys now have radials that are claimed to spin large props at reasonable rpms with obscene (for a model) amounts of power:
RCS 400cc Radial engine with electronic ignition. (RCS400R) - troybuiltmodels.com
They also have an 800cc model. Big bucks, tho.

My goodness....that 400cc radial "model" engine puts out 25HP and only weighs 20 lbs.

Originally Posted by lr27

On the Briggs, the vertical shaft ones with single cylinders seem to have better specs, at least if you could get them to run horizontal, or else have some weird redrive that changed the axis. The hovercraft guys use long belts with this kind of engine, sometimes with pulleys turning the belt 90 degrees.

Just the thing for Starman's project:
UH-TW-1 Tandem Wing : Universal Hovercraft, The World Leader in Hovercraft Technology

Geo engine with one redrive for drive prop and another with 90 degree turns for lift prop:
http://www.hovercruiser.org.uk/sev/sevtec/Geoin.jpg

appears to be Vanguard with right angle drive:
http://www.hovercruiser.org.uk/aird1.JPG
He did have some issues with the belt jumping off! But I don't think this setup is unusual so I suppose there's a way of dealing with it. I suspect that with the long belts, the resonant frequency, rotationally at least, is so low that the crankcshaft and prop are safe. But it would be very odd to see a vertical shaft engine driving a horizontal shaft prop!

I'm glad I refound this and those pictures, thanks for posting. Gearboxes I want cost around $450 each and weigh 24 pounds in cast iron (I could make aluminum bodies for them though) so I'm looking at going to an all belt drive.

That last picture is a little small to see what is going on, but if the distance between pulleys is too small the belt will want to turn sideways and that will cause it to jump off.

28hp Kawasaki 4-stroke industrial engine
listed weight 92lb, could be made lighter I think
$1500
Arrowprop - Skooter Airboats