Van's Air Force
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RV-10 Firewall Aft of Datum Station?
- Thread starter boearl
- Start date
Of course, this is an EAB airplane, so where you set zero arm length is up to you. That said, Vans recommended (in the construction manual, I think) setting arm zero at a specific distance forward of the leading edge of the wing, said point being slightly forward of the spinner. I’m not at the hangar so I forget the actual number. But once you have that, it’s easy to measure the arm for the firewall.
Thanks Bob, According to Van’s the datum is 99.44 forward of the wing’s leading edge. I can do a couple of measurements and determine a number (+or - my measuring error). I’m hoping , somewhere there is a definitive number for the firewall. I can’t be the first person that needed that number. It shall be interesting to see what folks use.
That sounds right. Just measure from the wing leading edge to the firewall, subtract from 99.44 (like, anyone can do this to.01" accuracy!). My numbers for other points are:
112.3" to the junction of the pilot's seat cushion and back cushion, seat in full forward position.
189" to the aft end of the baggage area (where the corrugated bulkhead is).
You should be able to figure the firewall arm with either of these two numbers, and a tape measure.
Bill Boyd
Well Known Member
The 99.44" does appear to be arbitrary - maybe tongue in cheek (see: pure as Ivory Snow) - any datum station would work as long as it's held constant for all calculations. If you are going to sweat the datum to the hundredth of an inch, as Vans does, then you need both the weight and the CG of each item you plan to attach to the firewall that you are trying to run CG numbers for ahead of time. Example: perhaps the panel-mount radio you plan to mount weighs 4.25 pounds. Its CG is not at the faceplate (panel) but somewhere further back towards the rear heat sink (so, forward of the panel, as mounted). Good luck with getting all these calculations correct to the 100th decimal place!
The true CG contribution of each passenger varies greatly with their body fat distribution and where they elect to position their seat in flight. Ditto for suitcases jammed into the hold: did you put that bourbon bottle in the front or back of the bag? Beach chairs in front of the luggage or behind? That baggage weight is almost never centered at the baggage datum number that the mothership gives you - it's a WAG. Not trying to be pedantic here, just remembering when I was over-thinking the same things...
My suggestion: WAG it, then measure the finished aircraft and use that as your empty weight and station. Calculations ahead of time during the build will get you a ballpark number and keep you from building something way out of range for your intended use, but only a final weight and balance will get you the true numbers, because the true CG station of each additional part is difficult to know and allow for. A lot of us tried to make our -10's on the nose-heavy side to allow full gross weight without running the cg out the rear limit, but none really knew how successful (or overdone) that effort was until final weigh-in. And now we get to carry ballast when we're solo
The true CG contribution of each passenger varies greatly with their body fat distribution and where they elect to position their seat in flight. Ditto for suitcases jammed into the hold: did you put that bourbon bottle in the front or back of the bag? Beach chairs in front of the luggage or behind? That baggage weight is almost never centered at the baggage datum number that the mothership gives you - it's a WAG. Not trying to be pedantic here, just remembering when I was over-thinking the same things...
My suggestion: WAG it, then measure the finished aircraft and use that as your empty weight and station. Calculations ahead of time during the build will get you a ballpark number and keep you from building something way out of range for your intended use, but only a final weight and balance will get you the true numbers, because the true CG station of each additional part is difficult to know and allow for. A lot of us tried to make our -10's on the nose-heavy side to allow full gross weight without running the cg out the rear limit, but none really knew how successful (or overdone) that effort was until final weigh-in. And now we get to carry ballast when we're solo
Maybe a head nod to Ronnie Milsap's song "Pure Love"? You know, 99 and 44 one hundredths percent pure love...The 99.44" does appear to be arbitrary - maybe tongue in cheek (see: pure as Ivory Snow) - any datum station would work as long as it's held constant for all calculations. If you are going to sweat the datum to the hundredth of an inch, as Vans does, then you need both the weight and the CG of each item you plan to attach to the firewall that you are trying to run CG numbers for ahead of time. Example: perhaps the panel-mount radio you plan to mount weighs 4.25 pounds. Its CG is not at the faceplate (panel) but somewhere further back towards the rear heat sink (so, forward of the panel, as mounted). Good luck with getting all these calculations correct to the 100th decimal place!
The true CG contribution of each passenger varies greatly with their body fat distribution and where they elect to position their seat in flight. Ditto for suitcases jammed into the hold: did you put that bourbon bottle in the front or back of the bag? Beach chairs in front of the luggage or behind? That baggage weight is almost never centered at the baggage datum number that the mothership gives you - it's a WAG. Not trying to be pedantic here, just remembering when I was over-thinking the same things...
My suggestion: WAG it, then measure the finished aircraft and use that as your empty weight and station. Calculations ahead of time during the build will get you a ballpark number and keep you from building something way out of range for your intended use, but only a final weight and balance will get you the true numbers, because the true CG station of each additional part is difficult to know and allow for. A lot of us tried to make our -10's on the nose-heavy side to allow full gross weight without running the cg out the rear limit, but none really knew how successful (or overdone) that effort was until final weigh-in. And now we get to carry ballast when we're solo
Many Thanks
Thanks. I pretty much agree with all you say. The plane is complete and flying. While I intend to re-weigh the airplane in the near future, I need to perform an interim calculation until obtaining scales. I figure if I start out with an accurate station for calculations, any additional slight inaccuracies will not be compounded. I’ve sent an email to Van’s support. I ‘ll report back what comes of it.The 99.44" does appear to be arbitrary - maybe tongue in cheek (see: pure as Ivory Snow) - any datum station would work as long as it's held constant for all calculations. If you are going to sweat the datum to the hundredth of an inch, as Vans does, then you need both the weight and the CG of each item you plan to attach to the firewall that you are trying to run CG numbers for ahead of time. Example: perhaps the panel-mount radio you plan to mount weighs 4.25 pounds. Its CG is not at the faceplate (panel) but somewhere further back towards the rear heat sink (so, forward of the panel, as mounted). Good luck with getting all these calculations correct to the 100th decimal place!
The true CG contribution of each passenger varies greatly with their body fat distribution and where they elect to position their seat in flight. Ditto for suitcases jammed into the hold: did you put that bourbon bottle in the front or back of the bag? Beach chairs in front of the luggage or behind? That baggage weight is almost never centered at the baggage datum number that the mothership gives you - it's a WAG. Not trying to be pedantic here, just remembering when I was over-thinking the same things...
My suggestion: WAG it, then measure the finished aircraft and use that as your empty weight and station. Calculations ahead of time during the build will get you a ballpark number and keep you from building something way out of range for your intended use, but only a final weight and balance will get you the true numbers, because the true CG station of each additional part is difficult to know and allow for. A lot of us tried to make our -10's on the nose-heavy side to allow full gross weight without running the cg out the rear limit, but none really knew how successful (or overdone) that effort was until final weigh-in. And now we get to carry ballast when we're solo
You could easily measure forward from one of the main wheel locations (axle center). Those locations relative to the datum were established during the initial measuring/weighing of the plane. Drop a plumb bob from a main wheel axle and then from the firewall. Measure that difference and subtract from the wheel location to get the firewall location.
Thanks Bob, Excellent idea! As always, your insight is spot on!
You could also use a laser level. Most of the current ones will project orthogonal lines and it is really easy to move them around to each station.
Good point. Knowing me, I’ll try them all!
