Hatz Biplane Association

I would appreciate some wisdom from those of you with flying airplanes, about the welded-in incidence of the horizontal stabilizer.

When I visited Billy Dawson a long time ago he told me that his airplanes always required down trim. He suggested correcting that by adding a little height to the front attach fitting for the horizontal stabilizer (CB-1 plans call for 1/4" higher in front than in back, he suggested a little more). So I reworked mine, adding about 3/16" which changes the incidence about 1/2 degree.

Recently I purchased a set of Hatz Classic plans and have enjoyed studying them. One of the things I was interested to check and compare was this dimension. Hmmm, the Classic plans have exactly the same 1/4" difference as the CB-1. So I guess Billy's advice didn't make it into the drawings. I called Billy yesterday. He remembers advising this, but doesn't remember any dimensions.

So my question, for those of you who are flying, is--do you find yourself flying with down trim most of the time? Would a little more built-in incidence be helpful? I'm wondering now if I should go back to 1/4" offset. I don''t look forward to doing that, but this would be the time to do it

One other idea...Randy Brooks built his airplane with adjustable incidence. It would be very interesting to know what dimensions work out for that airplane. I'm not sure who owns the Bratz now.

Thanks,

Paul

Oh my...

I just found a lot of posts about my question on the archived builders pages.
I guess history does repeat itself!

Paul

I am building mine with adjustable trim using shims and will start with the suggested 1/4" and adjust from there after first flight.

I just realized from my video that I use up trim (which I assume pushes the elevator down) - so would that indicate I need to lower the front of the horizontal stab?

https://www.youtube.com/watch?v=Hunibn6FC5U

By 'up trim', if you mean nose up trim, the tab would be down. If you mean that the trim tab itself is up then you're correct that it's pushing the elevator down and that pushes the tail up. So, if your tail needs an upward lift you would need to raise the leading edge of the horizontal stab. There is too much angle of attack on the stab and is causing the nose to pitch up. That angle of attack needs to be lessened. Mine is the same but, it's such a small amount I don't want to fool with it.

Hi Jim,

I enjoyed your video when you first posted it! What a beautiful wheel landing. I enjoyed watching the control inputs you make during approach, touchdown, and rollout/departure, and also the blips of rudder that were required due to gyroscopic effects as the nose attitude changes. I sent a link to your video to a friend who is a master CFI-aerobatics and highly experienced tail wheel instructor. He wrote back agreeing, and pointing out how little rudder input you used, compared to many people he flies with. Quite nicely flown!

Regarding your trim question, the trim tab in your video is up, so it is pushing the elevator down, which I would call "nose down trim" because the effect is to push the nose down. I think that is exactly what Billy Dawson described to me. I remember Billy saying that most Hatz biplanes end up flying with nose-down trim, and, to fine tune this in the building process, he recommended raising the leading edge of the horizontal stabilizer slightly compared to plans so the airplane would fly closer to neutral trim. I am pretty sure that raising the front of the horizontal stabilizer has the same effect as pushing the trailing edge of the elevator down, like in your video. If the front attach fitting of the horizontal stabilizer were positioned a little higher, I bet your trim would end up neutral on landing. For most of us, though, that would require major surgery! I know mine won't be adjustable. It is welded in one position, and will be covered in place. But it is fun to think about in the building stage. As I wrote in the original post, I raised mine, as Billy suggested. It will still be a few years until I will be ready to fly it and find out.

Thanks for sending this along!

Paul