Aft C of G

[ZU-RVZ]

Hi,

I'm getting ready for maiden flight later this week.

RV-7A equipped with Dynon D100 and EMS D120 glass panels, XP-360 motor with Fixed pitch prop.

The aircraft has weighed in extremely light at 1074 lbs. My concern though is that this is at an arm of 81.72" or 37% from foreward.

My weight and balance calcs show that with 2 x 220lb crew and only 50lb baggage, the C of G goes out of aft limits with anything less than half tanks.

Is this a problem for the 7, and how would you propose I correct it? (weight in the nose?)

Is it common for the 7?

Your advise will be appreciated.

 

[dan]

What was the weighing procedure? Are you confident in the scales' accuracy? Scales zeroed properly?

Was the airplane level? In an RV-7A this means the scales under the mains need to be elevated roughly 2" higher than the scale under the nosewheel.

Your empty CG of 81.72" is 3.5+" aft of where a typical RV-7A weighs in. Something must be wrong in the measurements or calculations (sorry to be blunt, just being objective).

If you wouldn't mind, can you post the data you collected, i.e:

- weight of mains
- weight of nosewheel
- distance leading edge to left/right main gear
- distance leading edge to nose gear

I recently got into the business of weighing airplanes & producing W&B spreadsheets for my customers, so I'd be happy to sanity check your data if you'd like.

 

[Mel]

If this is a slider, it sounds like the canopy might have been open during weighing.

 

[ZU-RVZ]

Thanks Dan and Mel.

Tip up and it was closed.

We did NOT level the plane. I can see how this can make a difference.

Also, as we do not have accurate scales available, we used bathroom scales 2 each on the mains and one for the nose.

We operate out of a small strip with no facilities. My intention is to fly the plane to a decent FBO as soon as she's flying to have her weighed by the pros.

Also, I did not measure the distances but instead used distances measured previously on a sister ship.

Will advise.

 

[tacaruth]

Dan,

You wrote

"Your empty CG of 81.72" is 3.5+" aft of where a typical RV-7A weighs in."

In my documentation of my 7A, Van's recommends a forward CG of 78.70". If you took 81.72-3.5 you get 78.22 which is in front of the recommended forward CG limit. Am I thinking incorrectly?

Tom

 

[dan]

Dan,

You wrote

"Your empty CG of 81.72" is 3.5+" aft of where a typical RV-7A weighs in."

In my documentation of my 7A, Van's recommends a forward CG of 78.70". If you took 81.72-3.5 you get 78.22 which is in front of the recommended forward CG limit. Am I thinking incorrectly?

Tom

Common misconception. Empty CG doesn't need to fall within the range. Just your "operating" CG.

 

[Norman Stapelberg]

ZU-RVZ Where are you based in SA, if you are close to Springs PM me you can use my scales to do your Mass and balance. my scales were built for the use by the experimental chaps and can take 500Kg per wheel.

cheers
Norman
FASI
norshel AT mweb DOT co DOT za

 

[David-aviator]

Common misconception. Empty CG doesn't need to fall within the range. Just your "operating" CG.

Absolutely true. The MD80, low on fuel and empty of pax, will tip on its tail when the flight deck crew leaves for the motel. More than once we had to remain on board until maintenance could transfer remaining wing fuel to the forward fuselage tank.

An empty RV will not tip but same applies regarding CG. The CG calculation of 81.72 could be correct considering the FP prop. Chances are though, with airplane level and on accurate scales, it will come out closer to 80". If the nose was just a bit high for the initial weigh in, that would throw the number aft for sure.

Considering that the 7 was designed with the IO360 and CS prop in mind, your 81.72 could be very much in the ball park. You may have to install a larger, heavier battery.