Diving Deep on Bearhawk 4-Place Weight and Balance, Especially Arms

[jaredyates]

Like 1 inch or quarter inch?

[zkelley2]

Like 1 inch or quarter inch?

Ya by about an inch when I measured them this time immediately after. I figure the scale numbers are more accurate.
Of big note is how much further aft the baggage is then the stock number. This is a quick build fuselage so I'm not sure how anyone is getting 77" for the cargo area.
Just want to remind everyone to get their own measurements.

[Battson]

I noticed that the exact CG position has a low sensitivity to weight around the datum. That isn't surprising, what it does mean - in practice:

The exact weight & location of the pilot, whether you put your battery under the front seat or on the firewall, or which side of the firewall - this makes a limited difference to CG location. Moving my 15lb battery from under the seat and onto the firewall, this only moved the CG by 9mm (3/8" roughly). Things which put weight aft make a huge difference to CG:

X - Installing a heavy interior
X - Installing a heavy aft bulkhead
X - Installing a baggage tube
X - Extra weight at the trailing edge of the wing or wingtips
X - Any extra weight on the tail, such as elevator counterbalance, or bulky tailwheels.

The above things can really destroy the CG pretty quickly.

To share the values I use these days, measured from the standard datum:
Front seats 0.69m (27.1")
Rear seats 1.45m (57.1")
Baggage 1.96m (77.2") - noting this depends a lot on where you put the weight back there.
The rest I think should be standard, such as fuel location, so I won't repeat them.

[John Bickham]

Just started working on my Weight & Balance. Got an empty weight with moments and then a weight with me sitting in position in pilot seat. Back calculated my front seat arm and got 20.9". Which is a bit different from some of the "published" numbers. Seems to be in line with some of the other results. Just adding to the data.

Scratch-built Model A Fuselage**

[JimParker256]

That's a good way to estimate the arm for a person, but the even better way is what John did: Weight the plane without the seat occupant (in flight attitude, of course), then re-weigh with an occupant. Then it's easy to determine the "actual" arm for that person in that seat.

[zkelley2]

Agreed. Weight it with a person in the seats and a larger known weight in baggage and you can have to the hundredths place while on the scales to get an accurate arm.

[John Bickham]

CORRECTION - Because I was sitting in seat trying to be still, I was dependent on my assistant's note taking and penmanship. The correct arm with me in seat is 20.5 inches.

[zkelley2]

It has come to my attention this last month or so that there's 2 engine mount lengths out there for the 540. One is shorter than the other. I would highly suggest no one use the shorter one if they want full use of their payload. Using math I figured that roughly 3" further forward on the engine would make a perfect empty CG for my airplane and then I find out the other mount is 2" forward, pretty close. Going to have to re-make the cowl and other such things.
Not sure why the short one even exists and how I ended up with one.

[whee]

Dude, that sucks. I thought the short mounts went out of circulation a long time ago.

[Mark Goldberg]

I will chime in here with a little history of why there are two different engine mounts. At first - ALL the engine mounts were Type I and the shorter ones. My BH has the shorter mount. This lasted for several years. Then builders started finishing their planes much heavier than Bob's - so when the Type II mount was developed years later it was extended forward 2". We kept making both sizes for a few years but eventually started making the Type I also the 2" longer a few years later.

So if you have an earlier kit with Type I mount - chances are it is the shorter mount. Bob actually gave two different measurements for positioning the engine mount. So it was not that someone got it "wrong" - making the mount to the longer dimension was a result of how heavy the planes were coming in.

When hauling a moose in from the back country, certainly the longer mount helps some. You guys have studied the CG issue much more than me so I do not know how much difference there would be. But this is the story of why there are two different engine mounts. Like anything that is manufactured, running changes are made in the production when it seems helpful.

I have never felt that the CG aft limits were a problem when flying at gross weight in my BH with the shorter mount. But I do not fly with 600 lbs ( a guess) of moose in the way back either. Mark

[zkelley2]

I will chime in here with a little history of why there are two different engine mounts. At first - ALL the engine mounts were Type I and the shorter ones. My BH has the shorter mount. This lasted for several years. Then builders started finishing their planes much heavier than Bob's - so when the Type II mount was developed years later it was extended forward 2". We kept making both sizes for a few years but eventually started making the Type I also the 2" longer a few years later.

So if you have an earlier kit with Type I mount - chances are it is the shorter mount. Bob actually gave two different measurements for positioning the engine mount. So it was not that someone got it "wrong" - making the mount to the longer dimension was a result of how heavy the planes were coming in.

When hauling a moose in from the back country, certainly the longer mount helps some. You guys have studied the CG issue much more than me so I do not know how much difference there would be. But this is the story of why there are two different engine mounts. Like anything that is manufactured, running changes are made in the production when it seems helpful.

I have never felt that the CG aft limits were a problem when flying at gross weight in my BH with the shorter mount. But I do not fly with 600 lbs ( a guess) of moose in the way back either. Mark

It's not that there is an issue at gross at the aft limit, it's any weight and aft limit. Also, its really hard to get to gross weight within the cg limit. The moose for example was piled against the back of the front seats as much as possible with the rear seats out, two whole packs in the right seater's lap/on the front floor. Then lighter camping gear in the rear. With just 20 gallons of fuel we were a bit over 2500lbs on takeoff, and up against the rear cg limit of 22.5, which I prefer not to do since the airplane doesn't pass longitudinal stability regs at that CG. Not that you can't control it, but flying a neutral to unstable airplane in bad weather is not fun in the slightest, and certainly nothing I'd ever subject an unwitting passenger to.

And I don't need this because my airplane is heavy. I'm under 1400 empty. It's just the rear seats and baggage are far enough aft that you have to have a forward CG to get all the weight into the airplane.

The fix is simple. The empty CG needs to be forward of 10.5in. 3 inches further forward with the engine accomplishes this. 2 gets close. If I was heavier and had more junk inside the cabin, I'd probably need more.

[Battson]

It has come to my attention this last month or so that there's 2 engine mount lengths out there for the 540. One is shorter than the other.

That is interesting.

That would explain why many of the earlier aircraft have a shorter-looking nose than many of the newer ones.

[zkelley2]

It explains why my bendix mags didn't fit when they do on others as well.

[Battson]

With just 20 gallons of fuel we were a bit over 2500lbs on takeoff, and up against the rear cg limit of 22.5, which I prefer not to do since the airplane doesn't pass longitudinal stability regs at that CG. Not that you can't control it, but flying a neutral to unstable airplane in bad weather is not fun in the slightest, and certainly nothing I'd ever subject an unwitting passenger to

If you are flying a truly unstable aircraft, and this is subjective because it's not black and white, then you are probably outside the published limits a little bit.
I view the CG limit like a stall. It's not a sudden occurance, it's a transition from controlled flight to loss of control.
There will be some difference between calculations and reality.

It took me a few years to figure this out, however I am sure of it now as far as the Bearhawk is concerned. I had unwittingly flown a number times with the plane well outside the aft limit, because of measurement and calculation errors. You can tell where the CG is, once you have the experience, just from the feel of the plane's controls.

[Battson]

And I don't need this because my airplane is heavy. I'm under 1400 empty. It's just the rear seats and baggage are far enough aft that you have to have a forward CG to get all the weight into the airplane.

The fix is simple. The empty CG needs to be forward of 10.5in. 3 inches further forward with the engine accomplishes this. 2 gets close. If I was heavier and had more junk inside the cabin, I'd probably need more.

Totally agree - the lighter the plane's empty weight is, the further forward the empty CG must be.

[zkelley2]

If you are flying a truly unstable aircraft, and this is subjective because it's not black and white, then you are probably outside the published limits a little bit.
I view the CG limit like a stall. It's not a sudden occurance, it's a transition from controlled flight to loss of control.
There will be some difference between calculations and reality.

It took me a few years to figure this out, however I am sure of it now as far as the Bearhawk is concerned. I had unwittingly flown a number times with the plane well outside the aft limit, because of measurement and calculation errors. You can tell where the CG is, once you have the experience, just from the feel of the plane's controls.

Ya, I've flown it enough now to know when I'm aft of where I want to be. I can only see a loss of control happening in IMC where you get behind the airplane. It's completely controllable throughout the envelope and recovers from a stall at the 22.5" limit just fine.

It's not unstable, it's neutral - longitudinally. Aft of roughly 20.0", you can put it into a dive or climb and it does not recover in the oscillations required if it was certified and by 22.5" it never recovers to it's pitch/airspeed ever. That is, trim to 90kts, without touching the trim, pull it back to 80kts and it just sits there at that pitch attitude as the speed bleeds off.
It's not that you can't fly a plane like that. There's lot's of aerobatic airplanes and high performance/military aircraft that display the same flight characteristics, but they're either not certified or have a computer to aid in control. It's just rather undesirable for anything but a nice VFR day so the airplane doesn't try to really get away from you on instruments. In bumpy weather it's a handful like that because you have to correct every little upset, the airplane won't return to where it was. Anyone that's flown a 182 or 206 accidentally past the aft limit will recognize the same phenomena.

I'm rather confident of my arms since I went through the trouble of getting the arms via the scales. I'm quite sure I have the furthest aft arms of anyone I've seen post them, so I'm pretty sure I'm flying around with the probably the most forward true CG.

[Bcone1381]

What Prop is installed? I'm wondering if you have a composite prop how much difference a heavier aluminum propeller might make.

[zkelley2]

It's a 2 blade composite whirlwind. Indeed I would not recommend that. 3 blade composite or 2 blade aluminum. I'm not going to put an aluminum one on because I'm not going to start adding weight to fix the issue, but adding 20lbs to the the prop AND moving the engine forward makes a really awesome empty CG.

[AKKen07]

Since I am close to covering the fuse I took advantage of the naked tubes to get some early weight and balance notes. I measured a few locations with a plumb bob relative to the leading edge datum (I measured 13" fwd of the front attach fitting). For anyone pondering weight and balance ideas... this might make juggling some numbers easier. I estimated a bunch of numbers in the spreadsheet too (indicated by a "~") then just played with the spreadsheet to see how things worked out. Then I got bored and added a metric version for anybody outside 'Merica. It also has a section to help calculate the arm for seat locations etc. when you've got 'er on the scales. Hope it helps/entertains someone. If anybody finds errors or whatnot - please lemme know.

[rv8bldr]

Very nice! Thanks for sharing that. I will be fitting the wings to my 4pl here in a month or two so it will be the perfect time to use your template and fill in my specific aircraft's arms and do some noodling.

Very timely indeed...

[RatherBFlying]

Is there an easy way to tell if I have the short mount or the long mount? I've got a mount from an early kit, not sure what #. No engine yet.

[jaredyates]

Here is the short one.