CG issues 0-360 vs 540

[svyolo]

I am going to build a QB 4 place, and am interested in the CG issues with a 4 banger vs the 6. Does either engine have huge issues with CG? Putting the battery in the tail with a 0-540 or vis versa with the 360? I am planning on just about a bone stock build, limited instrument panel, very limited electrical system. I will be using an MT constant speed prop.

Any input is greatly appreciated.

[PaulSA]

As I understand it, the 4-cylinder engine mount is longer to compensate for the lighter weight.

[svyolo]

After a bit more research, including Bob Barrows website which I finally found, it seems the CG is best with a heavy, powerful, CS equipped 4 banger, or a light 6 banger. It seems that the designer knew what he was doing.

[Chris In Milwaukee]

After a bit more research, including Bob Barrows website which I finally found, it seems the CG is best with a heavy, powerful, CS equipped 4 banger, or a light 6 banger. It seems that the designer knew what he was doing.

When in doubt, trust The Bob

As with everything, it's a tradeoff. There are no CG "issues" per se, as the plane can be properly balanced with either a 4cyl or 6cyl engine. What it comes down to, really, is how much do you want to carry, versus how much power do you need to do the job that your attempting.

For us flatlanders who stay on solid ground and fly, dare I say, "normal" missions, a 4 cyl of the 180-200hp variety will probably easily exceed the 80/20 rule. However, if I was regularly operating short, if I was on floats, or if I was flying high and hot out west -- or some combination of those -- I'd probably be looking at a larger 6 cyl.

But I'd generally not worry about creating any CG issues when selecting an engine within the design parameters. In this case, the preferred big engines are the parallel-valve (I)O-540 Lycomings and the (I)O-470 Continentals. The Bearhawk can be balanced out using "normal" methods of battery placement, prop selection, and similar. Once you start getting bigger, then you really have to watch what you're doing or get creative.

Those that are using larger-than-normal engines will likely chime in here with the solutions that they chose when putting their planes together. There is one builder here that is putting a custom Continental (I)O-520 variant in his plane, but he's done some really creative things to keep the weight and balance in check. There is one that has an LS-type engine in his as well, which is over the recommended weight range by some degree. Haven't heard from him in quite some time.

Plenty of Bearhawks running with 250-260hp 6cyl engines, though. They're certainly performers!

[svyolo]

I am leaning toward a PV IO-540. Once it is signed off, I doubt it will see much pavement. Or flatland.

[svyolo]

I did a bit of googling and got some hits for several years ago on this forum. Very good info. A heavy 4 cylinder, and maybe pushing it forward a few inches works good. A light 4 cylinder, with a light prop, would be cg limited in its ability to load a lot in the baggage area unless you extend the engine mount forward. A light PV 540 works very good, but it is still 100 pounds heavier.

Since my goal is stock, I will go with a light 50. I will have very little weight in avionics or any other goodies. For me, light is the biggest "goodie". The plane will still be light enough to carry a lot, and won't have any CG issues.

My plane will see very little pavement runways, or fly through busy airspace, its purpose is to take me to the middle of nowhere.

Thanks

[whee]

It depends on how much is "a lot" in the baggage. The very light O360 BH I flew did well and would haul a good load but it did run out of GC before it hit gross weight. If the rear seat is taken out you could load it to a 2500lb gross without running out of CG. But I think your plan is solid. If I were going to install a Lyc it would be a parallel valve 540.

[svyolo]

In some ways I would prefer a 390 or 400 Lyc, and extend the engine forward to compensate. in the real world, I want to put together an airplane correctly, and quickly. I can do it stock that way. If I did it custom, I would spend a month or two, at least, sorting out what to do.

Everything is a compromise. I think the designer did a great job making compromises. I will go with that thought, and build it as he intended.

Thanks for the input.

[svyolo]

i think mine will end up nose heavy, and require that I carry something in the aft cargo to keep me in CG limits if I am flying light, with nothing in the back.. No problem, I will carry 5 gallons of drinking water in the aft cargo, in case I need it. That is an easier and better solution than trying to figure out how to carry 40 pounds in the spinner. And I get more power.

[Mark Moyle]

How much effort or time spent getting the greatest CG range depends upon the type of baggage. If I didn't know what'll find its way into my airplane i would have stuck to The Bob's prop flange distance ahead of the datum line.. and an engine he called for. There are or were two numbers. 56-/12" and 58-1/2" prop flange ahead of the datum line. I believe the preferred number is 58-1/2" due to many builders installing heavy interiors.
My type of baggage looks like this...and this is one half of a moose...and three trips between Russian Mission and Platinum to haul the meat and gear....BTW.....this is Marv's first moose.. Marv has officially converted his Pacer to an Alaskan bush plane.....moose blood.

[Battson]

Mine is outside the CG envelope without a pilot and fuel on board. But you never fly without them!
It still flies very well near the forward limit. You just need a short spurt of power just as you flare, or elevator gap seals + VGs. Either or.
Near the aft limit is a much more annoying way to fly, it takes constant work from the pilot to keep things steady.
I would much rather be at the forward limit, than the aft limit.

[svyolo]

I believe I will make the same choice, as hunting and fishing are the reasons for me to have a bush plane. I will deal with a forward CG when lightly loaded.