Hatz Biplane Association

Grüezi Joerg,

While attending the great Hahnweide Oldtimer Fly-In two weekends ago, I had the pleasure of meeting Hans and Sam Gautschi. They kindly answered our many questions and allowed us to try out the cockpit of their stunning Hatz Classic.

I understand that you have a more lengthy report concerning your structural analysis? If possible I would appreciate receiving this. I am an aerodynamicist, so my interest in aircraft design ends mostly beneath the skin. Fortunately, my friend in Germany is an aerospace structures engineer and he is helping me on a number of topics like this. We both worked at Dornier on Lake Constance back when it still made airplanes.

Besten Dank im voraus - Many thanks in advance,

Kent Misegades
Seven Lakes, North Carolina, USA
Hatz S/N 954

I'm going to run this by my engineer friends at coffee but to continue this conversation I have a couple questions. I can understand the lift forces of the bottom wings transferred through the N struts to the rear spar of the upper wings. And that spar has compression forces transferred to the center section. I can also imagine that the lifting forces being held for the most part by the flying wires and the center section cabanes. The fittings and bolts holding the lower wings to the fuselage have a lesser lifting force on them.

So my question. Why does the each lower wing have a 3/8" bolt and a 5/16" bolt holding them while the cabanes which hold both the upper wings, center section structure and part of the lifting force of the lower wings only have four 1/4" bolts attaching them to the fuselage? The larger bolts in the lower wings seem to be overkill. Also, the landing wire lug bolts are 5/16" while the clevis pins for the landing wires are only 1/4". It would seem that 1/4" bolts in the lugs would suffice.

The flying wires seem to be well matched throughout. The wires, lugs, and clevis pins are all 5/16". The shear strength of the bolts and pins are 40% more than the tensile strength of the wires themselves.

Hello Juerg!

You have very clearly explained the issues. I agree with your conclusion - the Hatz as designed, both the CB 1 and Classic, has no structural concerns regarding the wing structures within the load limits specified.

Many years ago I did an analysis of the wing structure with the help of a dear friend who is a retired chief engineer at Boeing. We looked at the wing spars, N struts, Cabane struts, center section spars, flying and landing wires, and attaching bolts. Your analysis differed from ours in also including other elements of the center section. We did not look at that.

For a big picture view of the analysis, imagine standing at the wingtip and looking at the upper and lower wings. The upper wing is forward of the lower wing by 23 inches, the stagger. Now begin by imagining the loads during upright flight. Visualize the lift forces generated by each wing, and the structural elements that keep the wings from folding up. The N struts are the major outboard connections between the lower wings and the upper wings. As lift is generated by the wings, the wings are kept from folding upward by the flying wires which are loaded in tension, and by the spars of the upper wings which are loaded in compression. The compression loads from the left and the right upper wings meet in the middle through the center section spars which connect the wings.

Remember the stagger? Because of the stagger, the lift loads from the lower wings flow mostly through the N struts to the rear spars of the upper wings. Thus the rear spars of the upper wings are highly loaded, carrying loads generated by both the upper wings, and the lower wings. Considering only the spars, our analysis showed that the loads were greatest in the rear spars of the upper wings, and greater still in the rear spar of the center section. Juerg’s analysis accounted also for the load carrying capacity of other center section elements, particularly the plywood floor. We agree, I think, that the load paths through the rear spars of the upper wings and the center section are highly loaded. And, as he so well describes, we agree that existing structures are adequate for these loads.

I learned so much about the Hatz by doing the analysis. My engineer friend, now in his middle eighties, is patiently waiting for me to finish my airplane so we can fly it together. Neither he nor I will worry about the wing structure when we make those first flights!

Juerg, thanks! It is great to hear from you. Come visit again!

Paul Uhlig
Wichita, Kansas

Thank you very much for your clearly understanding explanations! We appreciate very much your engineering efforts, which made our Hatz configuration in this country to become a reality and let us live our dreams!
I am feeling very save in our Hatz and Sam is frequently performing sound gentlemen aerobatics.
Best wishes to Juerg and all Hatz Buddies
Hans & Sam, H.C.#78, Switzerland

Hi Bernie,

I am the Swiss engineer. It has been quite a while since I did my analysis in order to fulfill one of the requirements for acceptance of the Hatz by the Swiss Airworthiness Agency (FOCA). My own Hatz project is still in the works. But in the meantime the report allowed the Gautschis to get the permit to fly for their beautiful Rotec powered Vintage Hatz.

My analysis is not the only one around. But I believe it to be the most complete one available for the Hatz. An earlier analysis was done by an engineering friend of Dr. Paul Uhlig in Wichita. If I remember correctly, the main findings were published in an early Hatz Newsletter. I believe Paul frequents this forum and may have more information. Later there was another study performed under contract from the Hatz Biplane Association by Streamline Designs LLC. These two studies only looked at the wing structure. The statement that the rear center section spar is the weakest point comes from the latter. My analysis actually showed the upper rear spar to be the weakest point.

We see that depending on the chosen structural model, the results will differ. Engineers try to keep the calculation effort minimal by using the simplest model, which is representative of the real structure and which will give conservative results. If the calculations result in a positive safety margin (meaning strong enough), there is no need to go further into details and there is nothing to worry. The goal of all three analyses was not to determine which is the actual weakest point or which parts may be over-designed, but to show that the structure is adequate for the expected loads. For example in the case of the rear spar, it makes a big difference if the plywood center section floor is taken into account or not. But if the calculation show that the structure is strong enough even when the plywood is neglected, why go further? Thus it is very easy to draw wrong conclusions from statements such as "the weakest part is so and so".

The bottom line is that the Hatz structure as designed is perfectly fine, as long as the airplane is operated within its operating limitations (I have assumed MTOW 1600lbs, +4.4g). Note that this statement is true for both the CB-1 and the Classic since they are essentially the same from a structural point of view. I think there is enough operational evidence to confirm that the design is sound, considering the number of Hatz' built and flying,.

Juerg
Hatz S/N-622

Is anyone modifying their Hatz during construction to accommodate the recommendations of the Spar Analysis. From the limited structural analysis it appears that the rear spar of the upper center section is the weakest part to the plane. I'm not sure what part of the CS is referenced in this Swiss engineers analysis. I'd appreciate any comments and ideas on this subject.

Thanks,
Bernie