by dougm » Tue Aug 13, 2013 7:50 am
I've been looking into the CG question on the Classic for a few months and I've made a couple interesting findings.
I started looking at this more closely since I'm building an F-2 variant of the Classic and I was interested in moving the Rotec a few inches closer to the firewall. Obviously, this affects CG location so before I did this I wanted to see what the effects would be. I decided to contact some classic owners and see if they would be willing to supply their CG & WB info and several were kind enough to do so. I've reviewed the info on Mehlin's Warner Hatz and four Hatz Classics and what I found is that all the Classics were flying with their empty weight CG location approximately 4.25 inches to 6 inches FORWARD of the Empty Weight CG location specified on the plans. Now, as a disclaimer, all these planes are flying just fine so there doesn't appear to be any control issues. However it might explain some discussion of control feel differences between the CB-1 and the Classic since most CB-1s are probably more "balanced" feeling than some Classics might feel.
After checking my numbers numerous times I decided to contact Jeff Shoemake about this and he confirmed what I was beginning to suspect... The heavier engine that the Classic uses moves the CG forward so, unless the builder compensates for that in some way, you will end up with an EW CG forward of the plans location.
For me, this works out great as it means I can move the Rotec closer to the firewall to achieve the look I'm after with no CG penalty. Actually, it will help bring it in line where it should be.
You're right that 25% - 30% of MAC is what you are aiming for. If you do the calculations you'll find that the EW CG location on the plans works out to be approximately 24.5% MAC. So, what I would suggest is weigh your Hatz and determine its current EW CG location. Compare this with the location specified on the plans (I believe its 37 inches aft of Station 0.00) and adjust weight accordingly to get as close as possible to that location. Once you start playing with the numbers you'll see that unless you overload the aft baggage area, its hard to get out of the CG range.
I've been looking into the CG question on the Classic for a few months and I've made a couple interesting findings.
I started looking at this more closely since I'm building an F-2 variant of the Classic and I was interested in moving the Rotec a few inches closer to the firewall. Obviously, this affects CG location so before I did this I wanted to see what the effects would be. I decided to contact some classic owners and see if they would be willing to supply their CG & WB info and several were kind enough to do so. I've reviewed the info on Mehlin's Warner Hatz and four Hatz Classics and what I found is that all the Classics were flying with their empty weight CG location approximately 4.25 inches to 6 inches FORWARD of the Empty Weight CG location specified on the plans. Now, as a disclaimer, all these planes are flying just fine so there doesn't appear to be any control issues. However it might explain some discussion of control feel differences between the CB-1 and the Classic since most CB-1s are probably more "balanced" feeling than some Classics might feel.
After checking my numbers numerous times I decided to contact Jeff Shoemake about this and he confirmed what I was beginning to suspect... The heavier engine that the Classic uses moves the CG forward so, unless the builder compensates for that in some way, you will end up with an EW CG forward of the plans location.
For me, this works out great as it means I can move the Rotec closer to the firewall to achieve the look I'm after with no CG penalty. Actually, it will help bring it in line where it should be.
You're right that 25% - 30% of MAC is what you are aiming for. If you do the calculations you'll find that the EW CG location on the plans works out to be approximately 24.5% MAC. So, what I would suggest is weigh your Hatz and determine its current EW CG location. Compare this with the location specified on the plans (I believe its 37 inches aft of Station 0.00) and adjust weight accordingly to get as close as possible to that location. Once you start playing with the numbers you'll see that unless you overload the aft baggage area, its hard to get out of the CG range.