Hatz Biplane Association

Basic theory for a craggy relatively slow air frame you need all the traction possible. A shorter prop equates to a higher gear in your transmission. It is always a trade off shorten the prop, gain rpm, to the point of over rev then increase pitch to bring rpm down for increased thrust but poorer climb characteristics. Props are too expensive to do the ideal test required individually.

My ideal would be for red line rpm at WOT straight and level. Unfortunately there is more than one solution to the equation

John

That's correct: our legs have been built before Jeff made the mod for the Rotec version.
Another question to John Kerr: do you recommand that large 84" prop diameter or would you go smaller another time?

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

stearmoth wrote:Hi Hatzers,

Our gear legs are straight to the Classic drawings. Those legs are about 2 or 3" longer than the CB-1. Because we are using large 25" dia Bendix wheels, we will have plenty of clearance for any prop dia with our thrust line also lowered by 6".

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

Just to be clear, your gear legs are the original Hatz Classic legs, correct? You did not build the updated ones designed for the Rotec.

The standard Hatz Classic gear legs are, as you mentioned, 2" or 3" longer than the CB-1 legs. The updated Hatz Classic legs for the Rotec installation are another 2" or 3" longer still (or 4" - 6" longer than the CB-1 legs).

Hi Hatzers,

Our gear legs are straight to the Classic drawings. Those legs are about 2 or 3" longer than the CB-1. Because we are using large 25" dia Bendix wheels, we will have plenty of clearance for any prop dia with our thrust line also lowered by 6".

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

Somewhere in the nether reaches of my mind is the recollection (impression) that the FAA guideline is for a minimum of 9" prop clearance with the ground and in flight attitude. I frankly can not tell you what mine is.

When working with Jeff Shoemake at Makelin we hit on a modest increase in gear length, 2" as I remember it. With all the geometry involved with gear and struts, tread width etc. and along with the compression of the springs the actual increase in clearance is no clear. The impression I have is that that increase might be more than needed. We did lower the thrust line 6" from the plans and we are using a larger prop, 84" than would be expected with an O320.

The plans call for 12" of prop clearance. I believe John Kerr has the lengthened the gear legs, but I'll let him confirm that. Hans & Sam used larger wheels, so that may have addressed their clearance issue.

I expect I'll be doing some sort of compromise between the standard length gear and the longer gear. I want to use a larger wheel so once I settle on which one I'll be using, then I'll adjust the gear length accordingly to achieve the required clearance.

The standard gear and wheels will provide around 8" or 10" inches of ground clearance with the Rotec (I did the math once, but don't recall the exact numbers offhand), so if you wanted to use a larger wheel then you may be able to keep the standard gear.

I just recieved my additional plans from Jeff and he included the R3600 plans he drew up that have lengthened gear legs to give some additonal prop clearance when using a 84" prop. I know Hans and Sam, John Kerr, and Kurt Arnold have fuselages on gear, did everyone lengthen the gear? When I inherited the project from my father, he was working on the gear and it seems he went the with the orginal gear, so now I'm 90% done with a gear that I will not use. If anyone is interested in some of the parts, let me know, I'll let them go.

v/r,
Jud