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  #1  
Old 07-13-2017, 12:53 PM
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Bubblehead Bubblehead is offline
 
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Default RV-8 firewall distance from datum

Anyone happen to know the distance from datum for the firewall for an RV-8? I am changing some things around and want to estimate the new cg. Thanks.
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  #2  
Old 07-13-2017, 01:33 PM
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Quote:
Originally Posted by Bubblehead View Post
Anyone happen to know the distance from datum for the firewall for an RV-8? I am changing some things around and want to estimate the new cg. Thanks.
Datum is an imaginary line used for reference only. It can be anywhere you want it. I personally use the leading edge of the wing for the datum because it's very easy to measure anything from there.

Especially on taildragger RVs because in the level flight position, my main landing gear is pretty much exactly on the datum. Makes for VERY simple math. i.e. 0" times anything is 0.
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  #3  
Old 07-13-2017, 02:45 PM
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Bubblehead Bubblehead is offline
 
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I agree it is arbitrary and your choice is a good one. I assume you changed the cg limits accordingly since Van's stated limits are measured from his datum?

For my purposes his datum is convenient because he states the distance back to the rear baggage shelf on the -8. That makes it easy to get the distance from datum to rear battery which I am removing. If someone has the distance from the leading edge of the wing to the firewall on the -8 I can figure the rest out since Vans says the leading edge is 70" back from his defined datum.

The one advantage I can see to Van's datum is all the moments have the same sign - all are CW or all are CCW depending on which side of the plane you're looking at. That eliminates a possible source of error or confusion.
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  #4  
Old 07-13-2017, 07:00 PM
Robert Anglin Robert Anglin is offline
 
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Default You are correct.

Quote:
Originally Posted by Bubblehead View Post
I agree it is arbitrary and your choice is a good one. I assume you changed the cg limits accordingly since Van's stated limits are measured from his datum?

For my purposes his datum is convenient because he states the distance back to the rear baggage shelf on the -8. That makes it easy to get the distance from datum to rear battery which I am removing. If someone has the distance from the leading edge of the wing to the firewall on the -8 I can figure the rest out since Vans says the leading edge is 70" back from his defined datum.

The one advantage I can see to Van's datum is all the moments have the same sign - all are CW or all are CCW depending on which side of the plane you're looking at. That eliminates a possible source of error or confusion.
The 70" from the leading edge is right. Drop a plumb bob there and count down going to the front. Your on the right track. Yours, R.E.A. III #80888
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Old 07-13-2017, 07:11 PM
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Quote:
Originally Posted by Bubblehead View Post
Anyone happen to know the distance from datum for the firewall for an RV-8? I am changing some things around and want to estimate the new cg. Thanks.
Per Vans RV8 W&B pdf it would be 70" minus distance from wing leading edge to firewall.
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  #6  
Old 07-14-2017, 09:21 AM
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Bubblehead Bubblehead is offline
 
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I should have been more specific. I am not near the plane and won't be able to measure from LE to firewall for a couple of days. I was hoping someone had the measurement handy.

Not a big deal. I'll take care of it Sunday at the hanger.
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