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380busdriver

Member
Hi All

My complete but unpainted RV6A weighed in yesterday at 975lbs, which is not too shabby but the problem is the ARM is 72.21 inches aft of datum. In order to be able to satisfy my requirements I need to move this forward 1.5 inches to 70.71 or so. Can anyone give some guidance and or suggestions on how to achieve this.

Many thanks

Dave
 
Old style starter

Several posters have suggested putting an old fashioned Lycoming starter on the engine. They Weigh about 16 lbs. john
 
If the battery is in the original position, you might try moving it to the firewall if there's room. A heavy crush plate will also help.

Also keep in mind that when you paint, the C/G will move further aft.
 
Moving CG

I have a Landroll balancer I do not plan to use. I bought it for $500 on this site. I do think you could accomplish a lot with the heavier starter recommended earlier.
 
Prop?

Don't know what prop you are using. Sounds like it's wood or something very light based on your total weight. My 6a has an aluminum Sensenich FP which puts plenty of weight right on the nose. John
 
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Thanks for the responses gents. The prop is a sensenich ground adjustable. very light! i also have the light weight starter and pmags, all nice to have but don't help the c of g issue. The battery is already on the fire wall.

Does anyone know if there is a formula for how much extra weight and how far forward I need to place it in order to get the 1.5 inches forward. I would be happy to carry a little dead weight if it would help.

Marvin what is a "Landroll balancer"?
 
380busdriver said:
Does anyone know if there is a formula for how much extra weight and how far forward I need to place it in order to get the 1.5 inches forward. I would be happy to carry a little dead weight if it would help.

Marvin what is a "Landroll balancer"?
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If you computed the weight and balance you already have the formula. If you put your W&B on a spreadsheet it should be pretty easy to experiment with different configurations.

Saber makes a ~20# steel crush plate. I would guess this would go a long ways toward improving your c.g. envelope. The good thing about using a crush plate is that that is the farthest forward from your current datum and therefore requires the least amount of weight to move the c.g. fwd.
 
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alpinelakespilot2000 said:
However, the arm would be a negative number instead of a positive number since it is forward of the datum point.
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Probably not! It all depends on where the datum is. The datum is only a reference point and can be anywhere. However, since he is talking in the 70" range, I assume that his datum in forward of the prop. Therefore all numbers will be positive.
 
Mel said:
Probably not! It all depends on where the datum is. The datum is only a reference point and can be anywhere. However, since he is talking in the 70" range, I assume that his datum in forward of the prop. Therefore all numbers will be positive.
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You beat me to my change, Mel. I realized my mistake and changed my previous post just after submitting.
 
Landroll Balancer

A Landroll Balancer bolts to your prop flange to add weight at a forward CG. If it were me and the light weight propeller were new and returnable, I would consider a heavier prop, a crushplate, heavy starter, or Landroll. They all solve your problem.

Weight x arm = moment

Since you have already done a weight and balance just measure the distance to your prop flange or starter (arm). Increase the weight of the prop or starter until you get the CG you want, that will tell you how much weight you need at the prop flange or starter location. Another route is to move something aft to a more forward location with the same formula.

Good luck,
Marvin
 
380busdriver said:
Thanks for the responses gents. The prop is a sensenich ground adjustable. very light! i also have the light weight starter and pmags, all nice to have but don't help the c of g issue. The battery is already on the fire wall.

Does anyone know if there is a formula for how much extra weight and how far forward I need to place it in order to get the 1.5 inches forward. I would be happy to carry a little dead weight if it would help.

Marvin what is a "Landroll balancer"?
Click to expand...

You can use one of the available Excel spreadsheets to calculate the extra weight you need.

Using someone else's moment arms, I came up with main gear weights of 376 pounds each and a nose gear weight of 223 pounds to get your stated empty CG location.

If these are close, the spreadsheet shows that increasing the nose gear weight by 36 pounds would get the new CG position you need. This gives an idea of the extra weight needed at a specific location i.e., above the nose wheel.

For reference, the arms I used were 84.60 for the main wheels and 30.35 for the nose gear. If your dimensions are different the above calculations will change.
 
No need to over-complicate this. If you know the physical location of the current cg (put an X there!), just measure the distance from the X to where you will hang the weight. Then the cg will move toward the weight by (new weight)x (distance), divided by the new total weight.
example: old empty weight is 975 lbs. new extra weight is 25 lbs, and is to be mounted 40" in front of the X (old cg). New empty weight is 975+25=1000 lbs. New cg is 25*40"/1000 = 1" forward of the old cg.
 
Double check how many Gs your engine can stand with a crush plate. It might not be an issue compared to the weight of a metal prop (I don't know) but the time to find out is before you pull 6 Gs.
 
Once again Gents many thanks for all the feed back and tips. I'm sure the solution lies there somewhere. May be a combo more than 1 will work. Cheers
 
Also remember that the cg will move forward with fuel added. A sensenich 2 blade on the nose makes a really nice difference.
 
I thought I would be tail heavy and my initial measurements bore that out on my 6A. I bought a 19lb. steel crush plate and it moved the CG a little over an inch forward. I recently re-weighed with high quality scales and it turns out I was a bit heavier than hoped, but my CG was much better than I thought. I plan to remove the crush plate and may be selling it. With the new W&B data, I can get my wife and I and 100 lbs of luggage in the envelope with 5 gallons of fuel and still be under my 1800 GW with full fuel. I was a happy guy.

The old starters help as well. I originally had one to put weight on the nose, but it bit the dust in Phase I. Going to the Skytech lost me 9-10 lbs. It makes a meaningful difference given the arm. I have another clunker to replace it, but I really like the high RPM starts and it drains the battery much less. I am too spoiled to go back. Would rather have had the extra weight from crush plate. Someday I'll be able to get my engine to start on one blade, but today it is more like 5-7 when cold. I guess I still need to learn how much priming to do.

Larry
 
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