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  #1  
Old 10-01-2022, 06:51 PM
Jvon811's Avatar
Jvon811 Jvon811 is offline
 
Join Date: Mar 2014
Location: GA80 Whispering Pines Airpark
Posts: 250
Default Empty W&B CG Range

A friend/neighbor is getting close to first flight with his RV-8. Doing the W&B and lining up a DAR.

The empty W&B came out to 1118lbs and 77.4". IO-360, Hartzell C/S prop, nothing crazy.

The DAR says that it's too far forward (Van's published range is 78.7"-86.82" inches aft of datum) and is saying that weight needs to be added to the tail to bring it into range before he'll sign off on it.

Isn't that the CG range is for a loaded, ready to fly airplane, not the empty? For example, by this thread here, many of the posters are forward of the CG limits according to this particular DAR and he wouldn't sign them off.

Even on the EAA's website, in the "Building Articles" section, the example aircraft (an RV-8 fortunately) is forward of the limit when empty. Scroll about 3/4 of the way down this page.

My neighbor is going to call into Van's on Monday for the official word.
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RV-4 N249P
O-320, Dual P-Mags, Warnke Prop, Short legs, Manual Flaps, GRT Sport EX

Last edited by Jvon811 : 10-01-2022 at 07:00 PM.
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  #2  
Old 10-01-2022, 06:58 PM
Kyle Boatright Kyle Boatright is offline
 
Join Date: Aug 2005
Location: Atlanta, GA
Posts: 5,054
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The DAR is mistaken. The CG range only applies for the aircraft loaded for flight.
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Marietta, GA
2001 RV-6 N46KB
2019? (actually 2022) RV-10 N46BX
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  #3  
Old 10-01-2022, 07:02 PM
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Carl Froehlich Carl Froehlich is online now
 
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Location: Dogwood Airpark (VA42)
Posts: 3,938
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CG only means anything for stated conditions. Here creating a table of examples is the best way to demonstrate the CG and total weight will be in spec. Examples include:
- Solo, full fuel and empty
- Pilot and passenger, with baggage, full fuel and empty
- Pilot and passenger, with baggage at gross.

I keep this table in my POH so if ever ramped checked and ask “where is your W&B for this flight” I can just point to the examples.

Carl
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  #4  
Old 10-01-2022, 07:02 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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You are correct. The allowable CG range is for the purpose of flight not what the empty CG is. I suggest that your friend do a couple of sample scenarios showing the worst case load situation at each end of the CG range. Example… Do one for a very light weight pilot and full fuel tanks, and another with a lot of baggage and heavy passenger in the rear at maximum gross weight. These will show where the CG position will be for flight at the opposite extremes of possible load conditions.
I can’t help but wonder about the thoughts this DAR has, and whether he has an understanding of what adding ballast weight at the extreme aft end of the tail can do regarding other performance factors. Changing the CG is not the only influence it would have. I’m not saying it would for sure have a negative impact, but it could.
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Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
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  #5  
Old 10-01-2022, 07:08 PM
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Jvon811 Jvon811 is offline
 
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Thanks for the quick replies. 3/3 (so far) for verifying our thought process...

I'm not a DAR nor am I that acclimated with their manuals for signing airplanes off (yet), but it was related to me that this DAR was describing two types of CG ranges for certification (empty and loaded) and Van's published CG ranges "Takes that into consideration" so therefore the empty CG must be in their published range...

I'm still scratching my head on where he's getting that info from...
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RV-4 N249P
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  #6  
Old 10-01-2022, 07:22 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Jvon811 View Post

I'm still scratching my head on where he's getting that info from...
I have no idea because it’s not from anything published by us.

With as many RVs as get certified each year I would be surprised if this was this DAR’s first, so I hope he hasn’t been forcing people to ballast their airplanes for an empty CG adjustment to land within the range.
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Opinions, information, and comments, are my own unless stated otherwise.
You are personally responsible for determining the suitability of any tips,
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Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
FAA/DAR
RV-6A (aka "Junkyard Special ")
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  #7  
Old 10-01-2022, 07:38 PM
BobTurner BobTurner is offline
 
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One thing more: the aircraft should always be within cg in flight. With my -10 that means checking the cg anticipated at landing, as it can move past the limits as you burn gas.
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  #8  
Old 10-01-2022, 11:53 PM
RV8JD RV8JD is offline
 
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Wow, how can DAR make that kind of mistake?

Here is a W&B for my RV-8. Note that the BEW is 1108.2 pounds with a CG of 78.08". And that is forward of the forward limit of 78.70", and as others have said, this perfectly OK.

This may help with the DAR. Below is page 14-4 from the Van's RV-8/8A Construction Manual. Note that in their W&B sample calculations, the Empty Weight CG of 76.26" is well forward of the forward limit. Have your friend show the DAR the pages of Van's sample W&B calculations from the Construction Manual (Samples 1 through 5). Of course, your friend will have some W&B examples for his particular airplane.

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Last edited by RV8JD : 10-02-2022 at 12:09 PM.
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  #9  
Old 10-02-2022, 07:30 AM
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Toobuilder Toobuilder is offline
 
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The airplane will never be flown empty, so the empty CG numbers are meaningless for flight. They serve only as the baseline from which you derive the flight condition. The crew, fuel and bags are the "ballast" that brings the aircraft into the correct CG range.

Time to find a new DAR. If he cant get this right, I'd question every other decision he's made in the inspection. How is he even qualified to be a DAR?
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

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1940 Taylorcraft BL-65
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Last edited by Toobuilder : 10-02-2022 at 07:33 AM.
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  #10  
Old 10-02-2022, 07:39 AM
Desert Rat Desert Rat is offline
 
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How can a DAR have such fundamental misunderstanding of something so basic? That would be like a DPE not understanding how stalls work...
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