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Mark III weight and balance

 
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ridefst



Joined: 18 Oct 2019
Posts: 10
Location: Ohio

PostPosted: Tue Oct 05, 2021 8:08 am    Post subject: Mark III weight and balance Reply with quote

Getting close to my first flight, and I've found that I need 82lb of steel shot in front of the passenger seat to get my CG around 30% (20.2" back from LE)
Does this seem normal, or is something weird?
Total flying weight would be 748lb (326 left, 371 right, 50 tail)
I'm 195lb / 6', so pretty average size.
Rotax 582, three blade composite prop
BRS in front of engine
Both fuel tanks full (about 10 gal total)
No doors, just windshield
Original mesh seats

Seems to be in good shape like this, just way more weight that I was expecting to need!

Thanks,
Franklin


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neilsenrm(at)gmail.com
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PostPosted: Tue Oct 05, 2021 9:53 am    Post subject: Mark III weight and balance Reply with quote

Franklin
82 lbs does sound excessive. I have a MKIIIC with a redrive VW engine. I seem to recall that my engine is a bit heavier than normal and was a bit behind the CG so I moved my battery to the nose cone. When all weighed and calculated my plane will stay in the safe aft CG range as long as the only person on board weighs at least 160lbs. The forward range couldn't be too far forward with passengers loading well over max allowable weight. 
Revisit your calculations but stay safe. For your first flights anchor some weight on the floor of the passenger area just to be safe.  
Rick Neilsen
Redrive VW powered MKIIIC
On Tue, Oct 5, 2021 at 12:11 PM ridefst <fmgeiser(at)gmail.com (fmgeiser(at)gmail.com)> wrote:

Quote:
--> Kolb-List message posted by: "ridefst" <fmgeiser(at)gmail.com (fmgeiser(at)gmail.com)>

Getting close to my first flight, and I've found that I need 82lb of steel shot in front of the passenger seat to get my CG around 30% (20.2" back from LE)
Does this seem normal, or is something weird?
Total flying weight would be 748lb (326 left, 371 right, 50 tail)
I'm 195lb / 6', so pretty average size.
Rotax 582, three blade composite prop
BRS in front of engine
Both fuel tanks full (about 10 gal total)
No doors, just windshield
Original mesh seats

Seems to be in good shape like this, just way more weight that I was expecting to need!

Thanks,
Franklin




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Richard Pike



Joined: 09 Jan 2006
Posts: 1649
Location: Blountville, Tennessee

PostPosted: Tue Oct 05, 2021 6:13 pm    Post subject: Re: Mark III weight and balance Reply with quote

Something doesn't sound right. I have done a lot of mods to my MKIII but nothing to move the weight fore or aft much, I have a 582 and need no ballast.
I assume you are using the Kolb forms on page 41, 42 & 43 of "The Yellow Book"? If not, PM me, I'll email them to you. FWIW, here is my W&B:


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Richard Pike
Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0

Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing.
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ridefst



Joined: 18 Oct 2019
Posts: 10
Location: Ohio

PostPosted: Wed Oct 06, 2021 3:57 am    Post subject: Mark III weight and balance Reply with quote

My "yellow book" is a small book of builders plans, no w&b info.I've got a blue book though that doesn't have any forms, but does tell me that 25-35% is the range at 9 degrees, and 66" is the average chord, so I just worked off of that.
My measurements for the main arm and tail arm are pretty much identical to yours (I figured the tail wheel was 1/2" farther back)
Have you ever measured weights fully loaded, with the pilot and all?
Rather than estimating moments for the pilot/gas/parachute, I just went the route of weighing the entire craft with me in it.  
I did also weigh it without me, and did some reverse calculations to come up with the pilot/passenger arm measurement, and I decided that it's actually a positive 0.15" for myself instead of the -5" that you used.  That's a change of about 3% forward with no passenger, and nearly 10% with a passenger also - not saying that yours is wrong at all, but if I used that number, I wouldn't have nearly as much weight on the floor either.
I was bumping up against the top limit of my scales, so I couldn't weigh with a passenger also.  I do have a pair of 660lb scales on order though, and will get some actual measurements with those this weekend.  Some of my measurements didn't quite work out right, so I have some concerns about the quality of my scales (nice shipping scales I was able to borrow, but needed to use two per wheel as they maxed out at 150lb)
On Tue, Oct 5, 2021 at 10:19 PM Richard Pike <thegreybaron(at)charter.net (thegreybaron(at)charter.net)> wrote:

Quote:
--> Kolb-List message posted by: "Richard Pike" <thegreybaron(at)charter.net (thegreybaron(at)charter.net)>

Something doesn't sound right. I have done a lot of mods to my MKIII but nothing to move the weight fore or aft much, I have a 582 and need no ballast.
I assume you are using the Kolb forms on page 41, 42 & 43 of "The Yellow Book"? If not, PM me, I'll email them to you. FWIW, here is my W&B:

--------
Richard Pike
Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0

Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing.




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Richard Pike



Joined: 09 Jan 2006
Posts: 1649
Location: Blountville, Tennessee

PostPosted: Wed Oct 06, 2021 4:34 am    Post subject: Re: Mark III weight and balance Reply with quote

Here are the forms:
You don't have to actually sit in the airplane or have your pax sit in it; figure out where the bellybutton is going to be and compute the weight from the datum to that point.


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Richard Pike
Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0

Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing.
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ridefst



Joined: 18 Oct 2019
Posts: 10
Location: Ohio

PostPosted: Wed Oct 06, 2021 4:44 am    Post subject: Mark III weight and balance Reply with quote

Thanks for the pages!I understand that the navel is generally thought to be the cg of the seated person, but that's got to vary by person/build/weight as well.  Seems to me that simply weighing all together takes out that variable (though of course you don't want to weigh the craft again for every new passenger, so an average is probably a good idea)
On Wed, Oct 6, 2021 at 8:37 AM Richard Pike <thegreybaron(at)charter.net (thegreybaron(at)charter.net)> wrote:

Quote:
--> Kolb-List message posted by: "Richard Pike" <thegreybaron(at)charter.net (thegreybaron(at)charter.net)>

Here are the forms:
You don't have to actually sit in the airplane or have your pax sit in it; figure out where the bellybutton is going to be and compute the weight from the datum to that point.

--------
Richard Pike
Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0

Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing.




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jaydub



Joined: 07 Jan 2020
Posts: 30

PostPosted: Wed Oct 06, 2021 8:35 am    Post subject: Mark III weight and balance Reply with quote

On thing I haven't seen mentioned, does the original poster have the aircraft setup according to level flight? What I mean is most tailwheel airplanes need to have the tail raised to a certain position for level flight when using the scales to determine w&b. If the scale is on the floor instead of raised up, it would show more weight on the tail which would make it seem more ballast is needed up front. That leveling process should be spelled out on the w&b sheet in the drawings. If the OP doesn't have the drawing maybe someone can take a photo of that page. Just a thought.

Jaydub


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ridefst



Joined: 18 Oct 2019
Posts: 10
Location: Ohio

PostPosted: Wed Oct 06, 2021 9:14 am    Post subject: Mark III weight and balance Reply with quote

Yeah, I've got a block under the tailwheel to bring the bottom of the wing up to 9 degrees, as specified in the manual.

On Wed, Oct 6, 2021 at 12:40 PM Jay Dub <bearhawk(at)gmx.com (bearhawk(at)gmx.com)> wrote:

Quote:
--> Kolb-List message posted by: Jay Dub <bearhawk(at)gmx.com (bearhawk(at)gmx.com)>

On thing I haven't seen mentioned, does the original poster have the aircraft setup according to level flight?  What I mean is most tailwheel airplanes need to have the tail raised to a certain position for level flight when using the scales to determine w&b.  If the scale is on the floor instead of raised up, it would show more weight on the tail which would make it seem more ballast is needed up front.  That leveling process should be spelled out on the w&b sheet in the drawings.  If the OP doesn't have the drawing maybe someone can take a photo of that page.  Just a thought.

Jaydub
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