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  #1  
Old 03-25-2021, 06:21 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 185
Default Weight & Balance Questions

So I finally got my plane back together enough to weigh today, and while the weight was in line with what I expected, the moment arms (and thus CG) were not. For reference, my -4 is on short legs, has the original non-swivel tailwheel, Concord battery in the box between the rudder pedals, heavy starter, automotive alternator, and a standard weight saber 4 extension with a three blade Catto on the front.

For context, the original W&B performed by the builder had the following measurements, taken with a two blade wood prop and without engine oil. This was on the old 50 datum form.

Right wheel 438.75 pounds, 49.375 aft
Left wheel 445, 49.5 aft
Tailwheel 59.5, 227 aft

Total 943.25 pounds empty, 60.64 empty CG

In my ownership period Ive added some avionics and landing lights, and I also weighed the plane with my normal engine oil level. All the weight I added was at or slightly forward of the CG (instrument panel forward)

With the 60 datum and 7 quarts of oil and unusable fuel I measured the following:

Right wheel 450, 61.5
Left wheel 461, 61.5
Tailwheel 53, 238.5

Total 964#, 71.18 empty CG.

The big difference is my main wheel moment arms. Where previously they were slightly forward of the wing LE, now they are 1.5 aft. I did remove the mount and legs prior to get cracks on my mount addressed, but ended up replacing it with basically the same shims that I took off (I aligned the axles by measuring distance from the LE). That doesnt explain the large discrepancy.

Could I get a sanity check on my numbers? If I somehow mis-measured my moment arms I significantly threw off my CG. Conversely, perhaps the builder didnt have the plane perfectly level? I noted when I aligned the gear during the engine mount reinstallation that with the tail down the axles are approximately 4.5 forward of the leading edge. Speaking with a couple other owners of short leg -4s it seems they also measured the axles slightly aft. However, I note that the sample W&B in the manual from the prototype also list wheel axles forward of the CG. Maybe its as simple as having rolled the plane forward onto the scale vs backwards?
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  #2  
Old 03-26-2021, 11:55 AM
N999BT N999BT is offline
 
Join Date: May 2005
Posts: 215
Default It looks like it could be right

If you move the datum 10 in farther forward, the cg will numerically be 10" farther back from the datum. The acceptable CG range will also numerically be 10" more than the original range.
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  #3  
Old 03-26-2021, 02:20 PM
Mcmark Mcmark is offline
 
Join Date: Aug 2020
Location: Owings, MD
Posts: 25
Default

Mistakes are made.
I found the Smith I raced at Reno had the tail weight doubled on paper.
That'll get your attention when you realize you've been flying out the front by a BUNCH, because he added mount length thinking the tail was heavy.
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  #4  
Old 03-26-2021, 07:15 PM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 549
Default

My long-legged RV-4, wood prop, unpainted, no pants, with oil and coolant:
Aircraft Empty CG Weight Arm from LE Arm from Ref
Right wheel 432 0.7 60.7
Left wheel 445 1.2 61.2
Tail wheel 49.2 175.8 235.8
Total 926.2 70.24

Finn
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  #5  
Old 03-27-2021, 09:51 AM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 185
Default

Seems like perhaps my numbers are plausible. Guess Im getting a weighted crush plate.

A similar question on the topic: gross weight. From my understanding the weak point in the design as far as going heavier than 1500 will be the engine mount and main gear legs. Ive seen some people who have gone to 1600 gross and use 1500 as a max landing weight to try and mitigate the loads on the landing gear.
Has anyone spoken to Vans about this in the past?

The other question on that line is how would one go about changing the paperwork on a plane that already has an airworthiness certificate issued?
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  #6  
Old 03-28-2021, 04:34 AM
Skysailor Skysailor is offline
 
Join Date: May 2011
Location: Atlanta, GA
Posts: 279
Default Gross weight increase unlikely

It is doubtful your FSDO will allow alteration of your original Operating Limitations to provide a gross weight increase with no changes made to the aircraft and no data to support such a change. I think it even less likely Vans will approve the increase. Vans set the gross weight based on their design and is comfortable with that number. People have gotten guidance from Vans on over gross operations for extreme long range operations (ocean crossings) but I am not aware of anyone getting an exception for routine use.
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  #7  
Old 03-28-2021, 12:52 PM
Michael Henning Michael Henning is offline
 
Join Date: Jun 2011
Location: Nashua, NH
Posts: 544
Default Gross weight

I set my Gross Weight at 1750, and have flown it at that weight. When I was doing the operating limitations, my FSDO pushed back on it, but apparently called Vans, and wound up signing off on it. The plane flies great at that weight as long as the CG is in limits.
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  #8  
Old 03-28-2021, 08:53 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 185
Default

Quote:
Originally Posted by Michael Henning View Post
I set my Gross Weight at 1750, and have flown it at that weight. When I was doing the operating limitations, my FSDO pushed back on it, but apparently called Vans, and wound up signing off on it. The plane flies great at that weight as long as the CG is in limits.
Does your plane have short or long legs? How many hours on the plane and any observed issues structurally? Do you try to abide by a lower landing weight?
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  #9  
Old 03-28-2021, 08:57 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 185
Default

Quote:
Originally Posted by Skysailor View Post
It is doubtful your FSDO will allow alteration of your original Operating Limitations to provide a gross weight increase with no changes made to the aircraft and no data to support such a change. I think it even less likely Vans will approve the increase. Vans set the gross weight based on their design and is comfortable with that number. People have gotten guidance from Vans on over gross operations for extreme long range operations (ocean crossings) but I am not aware of anyone getting an exception for routine use.
Is it something that necessarily required new operating limitations? I admittedly would require new limitations for any such change (I have the old verbiage that calls for a reinspection), but under the new verbiage couldn’t the owner just put it back it phase I for a period?

My old operating limitations make no mention of weight, nor does the 8130-6 the builder filled out.

Last edited by 00Dan : 03-28-2021 at 08:59 PM.
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  #10  
Old 03-29-2021, 09:58 AM
Michael Henning Michael Henning is offline
 
Join Date: Jun 2011
Location: Nashua, NH
Posts: 544
Default 00Dan

00Dan,
Over 500 hours on airframe, long legs, no issues at all. I only fly that configuration when Im taking a passenger and a bunch of stuff a long way away. I also have wing tip lockers(the Vans approved mod) to help keep me in my CG limits. I do not do touch and go landings at that weight, but if I had to land heavy, I think as long as you dont drop it in, it will be fine. I still climb out at 1,500 fpm, and the plane flies great still. Ive only flown the plane that way on about 3 trips.
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