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Prop W&B Arm for RV-8?

Ironflight

VAF Moderator / Line Boy
Mentor
I’m getting ready to do some prop testing on the Valkyrie, and because it will be on/off/on/off would rather not reweigh every time - does anyone have an accurate measurement of the C/G Arm for the C/S prop on An O-360 on an -8?

I can do a bunch of plumb bobbing, but if anyone has the number, it will save some time in a hot, closed-up shop....

Paul
 
I’m getting ready to do some prop testing on the Valkyrie, and because it will be on/off/on/off would rather not reweigh every time - does anyone have an accurate measurement of the C/G Arm for the C/S prop on An O-360 on an -8?

I can do a bunch of plumb bobbing, but if anyone has the number, it will save some time in a hot, closed-up shop....

Paul

Won't have to weigh it every time, just twice. once before removal and once after. Those two measurements will let you calculate the moment arm of the prop and you can use that for all the future testing.
 
Won't have to weigh it every time, just twice. once before removal and once after. Those two measurements will let you calculate the moment arm of the prop and you can use that for all the future testing.

Yes, aware of the process - but I’m trying not to have to borrow a set of scales. I’ll just do the measurements unless someone has the number...

Paul
 
I so wish I knew!

Man, if I still had my RV-8 in the hangar I would be out there doing the plumb bob thing just so I could say that "I once told Paul Dye something he didn't know." Opportunities like this don't come along but once in a lifetime!

That would be even better than telling Michael Jordan that his shoe came untied! :D
 
I'll be interested to know the final value. I put together a little empty W&B calculator in an attempt to quantify the effects of equipment choices - different props and engines in particular. I did best-guess estimates of the engine and prop arms, and I wonder how close I got.
 
And the answer is....12” (plus or minus 1/4” as best I can tell). Plumb bobbed it, laid out squares on the ground, measured a couple of times, and did the math. The potential error is guessing exactly where the CG of the prop might be, but I think the center of the prop blades is probably pretty close.
 
Interesting - that's significantly off from my numbers (which are, to be fair, semi-educated WAGs). I ended up with an arm of 3.7" for either a Hartzell metal or Whirlwind prop. To get these numbers, I started with an estimated prop flange arm of 8.3", and then assumed (as you did), that the CG of the prop would be at about the midpoint of the blades. I used dimensional drawings from the prop manufacturers to get an estimate of prop arm relative to the flange, and then summed the two numbers to get the final arm relative to the datum stated in the construction manual.

My best guess here is that my estimated prop flange location is way off. I don't recall where I got that number from, though - I did a whole ton of searching while trying to put this together and it may be that I just took someone's word for it. Based on Paul's measurement and my estimation of prop CG relative to the flange, seems like ~16.6" is a more accurate estimate of the flange location.

I guess it's a good thing I'm only using my little utility for "advisory purposes," as the phrasing goes.
 
Interesting - that's significantly off from my numbers (which are, to be fair, semi-educated WAGs). I ended up with an arm of 3.7" for either a Hartzell metal or Whirlwind prop. To get these numbers, I started with an estimated prop flange arm of 8.3", and then assumed (as you did), that the CG of the prop would be at about the midpoint of the blades. I used dimensional drawings from the prop manufacturers to get an estimate of prop arm relative to the flange, and then summed the two numbers to get the final arm relative to the datum stated in the construction manual.

My best guess here is that my estimated prop flange location is way off. I don't recall where I got that number from, though - I did a whole ton of searching while trying to put this together and it may be that I just took someone's word for it. Based on Paul's measurement and my estimation of prop CG relative to the flange, seems like ~16.6" is a more accurate estimate of the flange location.

I guess it's a good thing I'm only using my little utility for "advisory purposes," as the phrasing goes.

16.6” for prop flange is probably close, just thinking about it. My measurements are actual measurements on the airplane from the leading edge of the wing, adjusted for the 70” Datum offset.
 
CG Arm for RV-8 prop

Thanks to all who responded especially Paul Dye

I ran the numbers based on 12” Arm, IO-360, metal Hartzell C/S prop and there wasn’t a big change in the CG. Ready to fly with full fuel and me on board with the metal Hartzell C/S prop (53 lbs) is 80.64 CG. Changing to the MT prop (45 lbs) shifted the CG aft a little but not much to 81.34.

Here’s some CG calculations for the MTV12B-183-59B prop for reference:

- All of the above with full 5 gal smoke oil tank in the fwd baggage compartment shifted forward to 79.87
- All of the above plus 25 lbs tool bag on the aft baggage floor 80.74
- All the above (minus the 25 lb tool bag) and a 180 lb pass is 83.74
 
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RV-8 prop CG

Thought of an easier way to compute/measure the prop CG location. I taped a measure upside down to the left side of the fuselage and put the 70” mark at the leading edge of the wing. Then taped it on the fuselage all the way fwd. That way I don’t need to use plumb bobs or level the airplane. Looking at the prop location measures somewhere between 11-15” aft of the Zero datum depending where the “center” is. Based on the pictures below where is the exact prop CG?





 
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