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W&B Sanity Check Needed

Toobuilder

Toobuilder

Well Known Member
I have performed an actual weight and balance on the RV twice ? once before the rebuild two years ago and once just recently. These two results conflict with my expectations so I?d like to appeal to your collective experience to try work out the problem.

The airplane is an RV-8 with a 200 HP (angle valve), horizontal induction engine, Hartzell BA prop, loaded glass panel, and big rear mounted Concorde RG battery. I'd expect this airplane to be nose heavy.

The old CG was calculated at 78.36 (1162 lbs), while the ?new? is 80.25 (1143)

Since I did not remove enough weight from the right places to cause anywhere near a 2? shift in CG, I have to conclude that one of these calculations is wrong. Given the above aircraft configuration (engine, prop, battery) which is the more reasonable CG?


Thanks
 
I agree with you Michael - this airplane should be nose heavy, even with the battery aft. The closest exmaple I can thin kof is Danny King's "Beautiful Doll" - same engein and prop, rear mounted battery (athough he may have an Odyssey). Perhaps he'll see this and respond with a number.

Paul
 
It's a mystery Paul... The heck of it is, the overall weight difference is close to expectations, but the single biggest factor that drives the CG aft is the significant difference in the tailwheel scale reading (was 55, now 66) - and that measurement is the one that I have the most confidence in!

Yet if I am to believe the new numbers, it is physically impossible to drive it out of the forward CG range. I even ran the numbers on a 100 pound pilot and 100 pounds in the fwd baggage, and still came out in the green.
 
Splits the difference

Just a data point, but my numbers are 1162 and 79.40 for the same configuration, 200HP angle valve, Hartzell, but with a vacuum system and steam panel. The battery is rear mounted Odyssey. Hmmm, what are the odds on the weights being the same? :rolleyes:

John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
Thanks John,

I'll call that one vote for the validity of my latest calculation. Once I take 10 pounds off my battery (to simulate your Odyssey), I get 79.52.
 
Last edited:
Makes sense

Those numbers do make sense. The weight of the aircraft went down but the tail wheel loading went up significantly...a 2 inch shift with that type of load distribution change would be expected...
 
Playing the devil's advocate here:

Was the plane perfectly level? Was it a good/reliable level?

Did you measure the actual arms of the main gear? Tail wheel?

How did you empty the tanks? Was it different from the initial?
(did you drain it from the sump drains or pump it out till the boost pump lost suction?)

Just something to consider?
 
Berchmans said:
Those numbers do make sense. The weight of the aircraft went down but the tail wheel loading went up significantly...a 2 inch shift with that type of load distribution change would be expected...
Click to expand...

Well, yes and no... At first blush it makes sense. I did remove a bunch of weight during the rebuild, and most was FWF which drives more weight to the tail wheel... Where it goes off the tracks is that even if I put all of the 19 "lost" pounds back on the engine, it still does not move the tailwheel scale the 11 pounds I see between the first and second measurement. So I'm right back to questioning the validity of the earlier measurement.
 
AltonD said:
Playing the devil's advocate here:

Was the plane perfectly level? Was it a good/reliable level?

Did you measure the actual arms of the main gear? Tail wheel?

How did you empty the tanks? Was it different from the initial?
(did you drain it from the sump drains or pump it out till the boost pump lost suction?)...
Click to expand...

Yes, those are good points and they were checked. The same equipment was used, it's high quality; the mains and tail wheel arms were measured as a baseline the first time, then reused. Fuel was drained through the boost pump the first time, but unusable fuel was found to be insignificant, so this time was drained through the quick drains.
 
I know it has been mentioned already, but if the plane was not level-----fore and aft.

If the first weighing was with the tail up, and second with tail on the floor, I can easily see an 11 pound difference.
 
No mention to this point about the scales. There are scales and there are scales, some accurate, some not so accurate.

Lever arm bathroom scales, if that is what was used, can be very inaccurate.
 
Good point about the scales. I know the bathroom types are very poor (and I have used them in a pinch in the past), but for the record the scales used both times on the RV were fairly high dollar, 4 pad digital race car scales. They were tested before use with known weights, and were dead on. I tested them to only partial capacity however, but it seems to me that if 1000 pound capacity scales can accurately measure a five pound gym weight , that's a pretty good indicator of performance.
 
Are you using multiple scales? If so what happens if you swap them with each other?

If you're confident the scales are accurate and you've got the plane level, then I'd be inclined to believe the new measurement. Its quite possible that the old measurement was wrong, for reasons that are now lost.
 
Alan Carroll said:
...Are you using multiple scales? If so what happens if you swap them with each other?...
Click to expand...

Multiple scales? Sort of. These are 4 individual wheel pads tied to a central display.

I just went out and redid the tail weight to see if it would repeat with a random pad. Despite the fact that it's now full of fuel (which I factored in), it fell within a few pounds of where it should of on paper. Considering that fuel changes weight with temperature, and I don't know exactly how much is in the airplane, I think I'm well within the range of calling the test repeatable. I also tested this pad up to 30 pounds in 5 pound increments and it was dead on each time, and finally, I even grabbed the scale out of the bathroom and IT agreed.


Alan Carroll said:
...Its quite possible that the old measurement was wrong, for reasons that are now lost...
Click to expand...

Yes, it looks that way. I'll call it good and simply have to wonder what happened the first time.

... It still seems odd that that I ended up with a bird that should be nose heavy, yet weighs in with a 80.25" arm. I guess it beats flying around with a 50 pound toolbox in back!
 
Mike S said:
Density, maybe a little, volume also, but a pound of hot fuel should weigh the same as a pound of cold fuel.
Click to expand...

True, but you don't buy it by the pound...

It's 6.02 pounds per gallon at standard temperature, increases to 6.4 at -40, and decreases to 5.9 when hot (~100).

Close enough to 6.0 though...
 
The Doll's "CG"

Ironflight said:
I agree with you Michael - this airplane should be nose heavy, even with the battery aft. The closest exmaple I can think of is Danny King's "Beautiful Doll" - same engine and prop, rear mounted battery (all though he may have an Odyssey). Perhaps he'll see this and respond with a number.

Paul
Click to expand...

The Doll's aft mounted battery is a 25 amp full size aircraft battery. The original empty weight was 1159, (I'm afraid to weigh her again), so we differ by only three pounds. The weight and balance paperwork is in the Doll, so I will check it tomorrow and let you know were the CG is.
 
Toobuilder said:
True, but you don't buy it by the pound...

It's 6.02 pounds per gallon at standard temperature, increases to 6.4 at -40, and decreases to 5.9 when hot (~100).

Close enough to 6.0 though...
Click to expand...

Yep, but the volume has expanded a bit, so the total fuel weight should not change---------unless some is puked overboard or evaporated.
 
Danny King said:
The Doll's aft mounted battery is a 25 amp full size aircraft battery. The original empty weight was 1159, (I'm afraid to weigh her again), so we differ by only three pounds. The weight and balance paperwork is in the Doll, so I will check it tomorrow and let you know were the CG is.
Click to expand...

Thanks Danny. Looking forward to seeing your numbers.
 
Michael,
This may be a long shot but take the emp. faring off and look in the tail section. May be a critter made a home back there or a lost tool.
Good Luck.
 
W&B

Did you roll the airplane back and forth a few inches on the scales to remove any preload from the position of the rubber tires on the scale pads before you read the scale weights?
 
Rich,

I did a good FOD check before closing out the aft fuselage, but it's certainly worth another look.

Tom,

Good guess, but this time the canopy was closed, flaps up, elevator and trim neutral - Flight configuration.
 
terrye said:
Did you roll the airplane back and forth a few inches on the scales to remove any preload from the position of the rubber tires on the scale pads before you read the scale weights?
Click to expand...

Yes. Rocked the wings and rolled it around to remove preload. It was free.
 
When we weigh large Jet aircraft for 135 operators we rotate the scales three times so each scale provides a reading from each gear location. Average the three numbers for each gear location for your CG calculations. this will average out a scale that may be reading off. If you are in doubt reweigh the plane and try this method.
 
Welcome to VAF!!!!

Terry, welcome to VAF:D

That is a great idea about the scale rotation/averaging, thanks for sharing with us.
 
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