The age-old engine question: 540 vs 390 vs 360 for 4-place

[tailwind]

Here is a real world illustration of an aircraft with the O-360, and an MT constant speed propeller. You can see that the cg is aft and limits the load capabilities of the craft. We will see how well it serves my family at this time in our lives. It came out heavier than I would have liked but I know where the additional weight is and I do not wish to remove it at this time. It has a full interior with carpets. We expect to do most of our traveling with two people and luggage. When we arrive we will probably give rides and there will be no luggage to be concerned with. If it does not do what I want I can always install the larger engine. My hanger neighbor has a Glassair. Every time he come over to look he is envious of the room to work on the engine. The six cylinder has a lot more packed into the same space.

[tailwind]

I see that the photo is very poor quality. To get a better picture:

step 1 open the picture. Step 2 copy the picture. step 3 paste the picture into microsoft word.

[Zzz]

I see that the photo is very poor quality. To get a better picture:

step 1 open the picture. Step 2 copy the picture. step 3 paste the picture into microsoft word.

It's easier than that, actually. Just click the thumbnail so the preview comes up. Right mouse click and select Copy Image URL --or-- Open Image in New Tab. These features of course depend on the browser you use, but in my case I use Google Chrome. What you get is: http://bearhawkforums.com/filedata/fetch?photoid=6950

The raw image will appear, and is usually auto-zoomed to fit within the browser window, but hover over it and the cursor will switch to a magnifying glass. Click once to see the actual size image.

[whee]

I wasn't going to respond because Z already knows my opinion. My expectations are probably lower than most and my tolerance for getting close to terrain is probably higher so I was happy flying a O360 BH in the idaho mountains at or near gross weight with my family. I have no desire to wear oxygen so getting to the altitudes I wanted to cruise at was no big deal. Most of my time in the BH was spent flying with the field DA over 8k and frequently bumping 10k. Loaded light you only need 1000' of runway even at those DAs. Loaded heavy and you needed 1500'. CG issues would be my main reason for going with a larger engine. It was easy to load the O360 bearhawk near the aft limit and believe me it is not a fun ride.

[Zzz]

Or we could go the opposite direction...

Wow, heavy! And hard on the wallet...

[Battson]

It would a nice "future proof" option though, be interesting to run the numbers on weight and dollar savings on fuel burn. But all that has been had-out on BCP already.

[mswain]

Couldn't you mount the 360 a bit farther from the firewall to get the CG envelope where you want it?

[Zzz]

Couldn't you mount the 360 a bit farther from the firewall to get the CG envelope where you want it?

Conceivably, yes, you could hang it way out front, but I don't really want to mess too much with the lines of the cowling. The forward length of the engine cowling is the same I believe in the plans, whether you use a 360 or 540, and the length of the engine is made up for in the engine mount. I'm just going to find the lightest weight 540 I can. That's a whole new discussion...

[jaredyates]

I'd like to explore the CG issue a little more. I haven't had any issues with the CG getting aft with a real-world 2500 pound yet, but I have not been doing the sort of flying that some of you have. Could some of you 540 operators please post your empty moment so that I can make some meaningful comparisons? Mine is 15024 at 1330 lbs.

[Battson]

Running your numbers Jared, assuming the differences in our builds are negligible:

-360 I can get 4x 80kg people (175lbs), full fuel, and 5kg of baggage (11lbs).

-540 I can get 4x 80kg people (175lbs), full fuel, and 35kg of baggage (77lbs).


Like I say, for my mission I use every bit of aft CG regularly, so I am probably more biased than others.
For instance, I am considering moving my battery to the firewall (also I can strip out a bunch of heavy cable and get better starting power too).

[Battson]

An afterthought, I guess you don't have a heavy baggage tube included in your moment?
I way overbuild my baggage tube and it easily adds 15lbs way back in the tail, the arm is huge. I can take that out and get about 25lbs more gear in the cargo area, trouble is I need the extra room the tube provides!

Without the tube:
-540 I can get 4x 80kg people (175lbs), full fuel, and 46kg of baggage (102lbs).

[whee]

Interesting numbers Battson. Like you I will be using the aft CG a lot.

I ran some numbers based on the O-360 BH I was flying and expected the baggage load to be much less. I need to go over my spreadsheet and make sure I didn't screw something up.

4x 175lbs, full fuel (52gal), and 85lbs of baggage puts me at the aft limit.

[Battson]

What moment and weight did you work with Whee?

[whee]

1270# and 15475. I checked my spreadsheet and it looks ok and I checked it against the W&B app I used to use and it gave the same numbers.

[Enewton57]

Here is my W&B numbers web page for my O-360 equipped Bearhawk - I get two 200 lb people up front, two 175 people in back plus 100 lbs baggage. http://mybearhawk.com/finish/weight.html Eric Newton - Long Beach, MS http://mybearhawk.com

[jaredyates]

I don't have any baggage tube at all, so I have neither the utility of one or the aft contribution. As it is I can load 200 pounds in each seat, full fuel, and 50 pounds of baggage in the back and be at 21.64 and 2480 pounds. Or I can leave out the baggage and do 450 in the front seats and 420 in the back seats to be right at 2500 pounds and 20.67. In a 2-place configuration I can do 450 in the front seats, 325 in place of the back seat, 125 in the baggage section, and that puts me at 2500 and 21.84, a little further aft than I like, but still within Bob's limits. Our usual family-hauling configuration is for 400 in the front seats and 150 in the rear seat area (which includes little girls and car seats, tools/spares, and luggage in the rear seat foot well). That gives us up to 190 pounds in the area behind the rear seat before we reach my comfort limit with the CG (21.55 in this case), but I've not had an opportunity to carry a real-world load that weighs that much but could still fit in that area yet.

All of the above numbers are with full fuel. I'd like to hear from other 540 operators to increase the sample size, but so far it doesn't sound like CG considerations are as important in engine choice. Like any airplane, if you start carrying lots of stuff particularly in the back, it is going to be aft. And as others have said, Bob's limit is not conservative at all. In my flight testing with progressively aft concrete ballast loads, I decided that at 21.5 (as I measure and calculate it) is as tail-heavy of an airplane as I care to fly.

I have attached the spreadsheet that I use for calculations. It works fairly well in Apple Numbers also on the ipad. Feel free to customize it for your own use, just use it at your own risk, and don't blame me if you bend up your airplane while using it!

[Battson]

Further comment - following some analysis.

With all respect, I don't agree with some moment arms used in this spreadsheet - particularly the front seat, to a lesser degree the rear seat. They are short, perhaps measured with both seats rolled all the way forward, or with the aircraft in the three point attitude rather than level flight attitude, either of which would explain the reducing error as the measurement points move further back? Unsure, but it's a moot point.

The point is, these short arms cause the spreadsheet to give overly optimistic CG locations. This explains the 21.5 limit you imposed Jared, by my calculations your real CG position is 23.7 inches under that loading situation, well beyond the aft limit Bob imposes. I do not recommend using these arm lengths as this will result in unstable flight conditions.

[jaredyates]

Very interesting, we should investigate this. What do you figure your arm is for the front seat, and at what point in its adjustment?

[Battson]

My front seat arm is 690mm (27"), rear seat is 1450mm (57"). I probably measured that with the front seat fully aft, as the most conservative option because the front passenger often leaves their seat back, also my legs are unusually long

[jaredyates]

And as far as climb rates go, I don't have enough data to be really reliable, but a climb to 10,000 feet at mid-range weight is going to take 15-20 minutes at 80-90 knots, with a fuel flow that starts around 16gph and drops as the altitude goes up. So the 540 definitely wins far a fast climb to high altitude goes. .

[Battson]

Hey Jared,

I would be interested to know how you determined your back seat CG location? I am wondering if I have overstated mine.

Incidentally, I have been told by Bob that you can mount the rear seat backwards to move the CG further forward.

[jaredyates]

You can put whatever arm you'd like for the aft seat arm in cells D11 and D12. What are you using? I see that Eric used 52" in his calculations, and that's also what's in the 2007 Beartracks. I think I got my number of 53.5 by using a measuring tape to the center of the seat, but it has been a while since I did that and I'd have to look back at my logs to be sure.