Weight and Balance

[Flyhound]

I have an MX7-180C and I love it. I usually fly alone, or with 1 passenger in the front with me. On occasion I have flown with 2 passengers, but recently looked at taking 3 passengers with me. The W&B chart in the POH shows the front seat passengers at station 20, and the rear seat passengers at station 56. With these moment arms, 30 gallons of fuel and 4 180 lb passengers it looks like I am under gross weight, but well aft of the allowed CG. Is this really not a 4 passenger airplane?

Baggage area "B" (just behind the front seats) has a moment arm of 42". It looks like flipping the rear seats so the passengers faced aft would put the same scenario described above into the legal CG envelope. Has anyone flipped the passenger seats in this way to use this as a real 4 person airplane? That would be like the arrangement in the 4 seat Glasair Sportsman.

[Andy Young]

How do you propose flip the rear seat? I assume you have the sling seat in the rear. To me knowledge, there are no anchor points for the sling tubes that allow the seat to be mounted that way.

With the heavier 235 hp engine, the CG issues go away, and you can easily carry four full-sized people. I know that doesn't help you; just maybe some insight into the design.

[Flyhound]

How do you propose flip the rear seat?

Andy: I don't have a proposal, that's why I was asking. I was looking to see if someone else had already come up with a solution. I do have the sling seat, and there is no obvious answer - other than putting a bigger engine in the nose. That would actually cut down on my overall useful load. I do fill this thing to the gills when I go camping. With the rear seat out, the average station for the large baggage area only has a 42" moment arm and that solves the problem. I'm just bummed that I can't get 4 adults in my plane even though I'm under gross weight. Sigh.

[Chris in Milwaukee]

I have an -A model and same problem. I’ll run out of CG before I run out of capacity.

[Andy Young]

How do you propose flip the rear seat?

Andy: I don't have a proposal, that's why I was asking. I was looking to see if someone else had already come up with a solution. I do have the sling seat, and there is no obvious answer - other than putting a bigger engine in the nose. That would actually cut down on my overall useful load. I do fill this thing to the gills when I go camping. With the rear seat out, the average station for the large baggage area only has a 42" moment arm and that solves the problem. I'm just bummed that I can't get 4 adults in my plane even though I'm under gross weight. Sigh.

Yeah, I would find that frustrating also. Be nice if they offered a longer engine mount for the 180, to compensate.

[andy]

I can get 425 lbs of front passengers, 290 lbs of rear sling seat passengers, 73 lbs of baggage and 40 gallons of AVGAS into my MX-7-180 without exceeding weight or balance limits. I doesn't climb well but is your MX7-180C that much different?

Normally, I only carry one person and flight bag(s) in the back seat since it's a bit crowded, but I've carried 4 adults and full main tanks many times.

[Flyhound]

Here's the W&B calculation for my MX7-180C from the POH using the weights described above. I would start and finish my flight aft of allowed CG.

[Flyhound]

The example Andy gave of 425 lbs in the front seat, 290 lbs in the rear seat and 70 lbs of baggage in the rear baggage area with 40 gallons of fuel puts me over gross and aft of allowed CG to start with. Here's the calculation and the graph



My plane weighs in at 1519 lbs empty.

[Chris in Milwaukee]

Hmmm... I can't do that in mine with those same weights.

[montana maule]

I have the same problem when loading my MX7-160. Flyhound my empty weight CG starts out even farther back at 13.5 in. Andy what is your empty weight CG?

[Flyhound]

Looks like Chris and I are getting pretty similar results.

[gbarrier]

I'd be good CG wise on the M6-2235 at 18.3 but would be about 40 pounds heavy.

[Andy Young]

I know this thread is supposed to be about the CG issue with 180 hp planes, but I couldn't resist running the numbers on my 235 hp machine, so I thought I'd go ahead and share my results. Apologies for the thread creep. Stop reading here if you don't want the distraction from the main theme.

Similarly to GBarrier, on my M-6-235 my CG would be fine at 18.95, but I'd be 95 pounds over gross. However, my typical total front seat weight varies between 280 and 360 pounds, depending on who is in my right seat. Rear seat total (with two adults in it) varies between 220 and 340. If I plug in my worst-case scenario, I am still in CG, at 19.37, but also still over gross, by 80 pounds. I'd have to be down to 26.5 gallons to be legal. For me, that's two hours and fifteen minutes of fuel, which is actually much more than enough for many of the places I go.
At a more typical load of 280 pounds in the front seat (me and a female friend) and 330 pounds in the rear (another male/female couple) and still with the 73 pounds baggage and full main fuel tanks, I'm at 19.26 inches and 10 pounds under gross; just within legal. I've actually done this very occupant load out of a 600' gravel bar, but with about 1/3 the baggage, and maybe half tanks of fuel.

Clear to see the CG advantage AND the weight DISadvantage of the 235. The 180 is in the opposite position, of course. Seems that a better-balanced 180 could be ideal in many ways. Even better, how about an IO-390 on a long mount? Similar weight to the O-360, but with 210 hp, and with the long mount, better CG to boot!

[Flyhound]

Thread drift or not, I'm liking the idea of a 390 on an extended mount! Too bad 337s are so hard to come by these days.

[maules.com]

The later Maules have the rear seat position further back than M5 and M6 and early MX7 to get more legroom. Possibly a thick light cushion could move those rear passengers forward, or weld in the forward rear seat position. For a while two position rear seats were an option. Make sure you have the correct Manual and Wt & Bal pages for the airplane.