Hatz Biplane Association

Surprisingly we didn't find this topic, hence set it newly: the center of gravity range on the Classic

To our biplane understanding we expect the CG at 25 to 30% MAC (mean aerodynamic chord). Simplificated: draw a line from top wing leading edge to the bottom wing LE and make a line half between the the position of both wing profile bottom line. At the cross go back 25% for the front and 30% for the back position. Based on this we made our first calculations in order to set the position for the Rotec engine.
Now comes the time to verify if it's correct at the finished ship.
Is this o.k. or does anybody has other opinions?

Your input would most be apppreciated!

Beste regards
Hans & Sam, H.C. #78, Switzerland

I used these instructions for my CB-1, and it seemed to work very well: http://www.hatzbiplane.com/BratzCG.html

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.

My observations - Correct if I am wrong. The EW CG for the CB 1 is 8" forward of station 10. Which appears to put it very near the leading edge. On my classic the EW CG is -2.3" which is a little behind the CB1. (Leading edge being 0) and my fuel loaded and a pilot CG is +1.8". Note I used a 0-360 which weighs about 29 pounds more than a 320 but went with a wood prop that weighs a lot less than the aluminum prop. The plane flies very level with no major trim required.

I would like to know the EW CG for other classics

Michael
N9648B

mmcgrew wrote:The EW CG for the CB 1 is 8" forward of station 10. Which appears to put it very near the leading edge.

This sounds correct, but I can't make a direct comparison since the CB-1 plans (which I don't have a copy of) use "Station" identifiers and the Classic plans do not. The Classic plans simply use measurements starting at the firewall and moving aft. However, the EW CG location ends up being 0.5" forward of the lower wing LE so this aligns with the CB-1 location. I understand from Jeff Shoemake that the EW CG location was not changed with the Classic so it is in the same location as the CB-1.

mmcgrew wrote:On my classic the EW CG is -2.3" which is a little behind the CB1. (Leading edge being 0) and my fuel loaded and a pilot CG is +1.8".

Can you clarify this? If I read it right, your EW CG location is 2.3" closer to the firewall than the plans location. Is that correct?

Yes. The neg numbers are towards the Fire wall (or forward of the LE)

OK. that's what I thought. That means your EW CG is similar to the others I researched - i.e. forward of the plans location... just not as far forward as others.

Curious, where did you place your battery?

I was doing the preliminary calculations for the weight and balance and don't agree with the Empty CG at Sta 37.0 shown on the plans. Correct me if I'm wrong.

A few weights on this forum show about ~1000 lbs on the front wheels (Sta 23.0) and ~50 lbs on the tail wheel (Sta 188). If you plug that into the formula:

Empty CG = (Main Gear Weight * Arm + Tail Wheel Weight * Arm) / Total Weight. I get Sta 30.9 as the Empty CG location.

All the rest works out:

The LE of the MAC = (LE Bottom Wing + LE Top Wing) / 2 = (14.5 + 37.5) / 2 = Sta 26.0
Forward CG = 18% MAC = 26 + 0.18 * 50.0 = Sta 35.0
Aft CG = 35% MAC = 26 + 0.35 * 50.0 = Sta 43.5

Loading the plane just about any way I can think of keeps the CG within the forward and aft CG limits.

Oops - mistake already. Top wing has more area so LE MAC should be:

The LE of the MAC = LE Bottom Wing - (LE Bottom Wing - LE Top Wing) * 54.5% = 37.5 - (37.5 - 14.5) * 54.5% = Sta 25.0

Forward CG = 18% MAC = 25 + 0.18 * 50.0 = Sta 34.0
Aft CG = 35% MAC = 25 + 0.35 * 50.0 = Sta 42.5

what is all these numbers I'm looking at. make a few drawings, some pictures. will it fly? will it stall out and recover, can you fly her a slow speeds or do you need to apply power to keep the tail down. What is the question that no one ask. how do set the flying wires.