Yaw stability

[Steve W]

When I was flying my initial time in the Patrol, I had one wing a bit heavy and needed more left pedal on the rudder. I corrected the heavy wing by adjusting a flap push rod one turn; can't remember which one and which way I moved it, but one turn and wings were level. Now the rudder was a bit different. My plane was a plans project and when I made the vertical stab, the offset was put in at the top and bottom forward tube so the whole fin was offset. I discovered I wasn't the first person to do this. I installed a stiff spring at the rear left rudder pedal to the flap handle assembly with a tab that had attach holes at different lengths so the tension could be strengthened or loosened. Hands off at 145 mph. When I install the amphibs I recently picked up, some of that will change for sure.
I basically didn't do anything for the first 30 hours but take off and concentrate 100% on engine break in. Before I went to work the past May, I was just starting to get into circuits and slow flight and as I found as those that have gone before me, slow the friken plane down!! I was quite happy with what was happening with my progress getting the aircraft on the pavement.
After working the summer on much larger aircraft and sitting 8+ feet off the water for 350 hrs, all that I had gained in the past spring went for a dump. I came home in October and went for a short flight. Needless to say, I was quite happy to see there were no witnesses to my return to the pavement.
The Patrol is the most sensitive aircraft I have ever flown, especially with pitch and rudder. You have to make a strong mental note to slow down and not move anything until your sure you need to. The DeHavilland aircraft I have flown required constant searching for pressure on both sides of control surface forces as this changes so much with reducing airspeed. So with the Patrol, I am over controlling the stick and rudder to the point that must be quite comical to observe from the ground! I know it's only a matter of a couple of hours of getting use to but man, aviation is so humbling!

[Sir Newton]

Nice comment

[whee]

61C42DF9-8935-41DB-B99E-739E889C67C4.gif
Me and many pilots I’ve flown with…

[davzLSA]

That is so cool Whee, are those tomcats or F15s, I cant tell which.

[whee]

Sorry, Dave. This pic is a gif that didn’t work. It’s the scene from Top Gun of Cougars wobbly approach. Seems like some guys fly like that for a couple minutes till they figure out how to use the rudder in my BH. I should probably just delete to post since it didn’t work. Sure wish I had been able to have a career flying Tomcats.

[m.mooney]

I picked up on your theme Whee, Cougar was holding on too tight, he lost the edge. I was one of the lucky ones and flew the Tomcat with the big GE motors for 1100 hours, it was awesome.
Mike

[bearhawk2015]

27F583F6-83AF-4BD1-B3E0-F1703A8832B2.jpg
To really get rid of tail wag you need to install some fins on the horizontal stabilizers and to get rid of the pitch sensitivity install a Ray Allen trim servo in the elevator. Pitch becomes Cessna like and it tracks almost like an arrow.

[whee]

27F583F6-83AF-4BD1-B3E0-F1703A8832B2.jpg
To really get rid of tail wag you need to install some fins on the horizontal stabilizers and to get rid of the pitch sensitivity install a Ray Allen trim servo in the elevator. Pitch becomes Cessna like and it tracks almost like an arrow.

Please tell us more about the fins. Were they to help when on floats and you left them on during the off season and liked them? What’s the size of each fin?

I’ve been thinking about adding fins on the ends of the H-stab in the gap before the elevator.

[bearhawk2015]

They are for when on wheels. They’re made from 1/8 2024 angle aluminum with a sheet of .060 or .080 2024 aluminum riveted to the angle piece. I forget how tall they are but I think think it’s 4 inches and run the whole length of the stabilizer on each side and they could be made larger.

[Sir Newton]

I am planning the very same thing. You have pioneered this modification.

[Nev]

27F583F6-83AF-4BD1-B3E0-F1703A8832B2.jpg
To really get rid of tail wag you need to install some fins on the horizontal stabilizers and to get rid of the pitch sensitivity install a Ray Allen trim servo in the elevator. Pitch becomes Cessna like and it tracks almost like an arrow.

I was very keen to use the trim servo in the elevator. Can you tell me where your empty CG ended at ? Did you add additional balance weight to the elevators? Really like both of your ideas.

[Mark Goldberg]

I just spoke with Bob and mentioned this discussion. Hopefully I can accurately recount Bob's comments to me. He said that the BH has very good yaw dampening, but not as good yaw stability. And this is by design. The vert stab is not so big as this inhibits the ability of the plane to slip hard. Bob views the ability to slip hard as important. He said the Maule has good yaw stability with its big vert fin, but does not slip like a BH. He said the original Maule 4 had a much smaller vert fin and flew well compared to the later versions with the bigger vert fin. I have flown a fair amount with Bob in the left seat, and he slips dam near every landing.

The flow fence on the upper wing skin Bob shows on the Patrol plans he ended up not liking because they inhibited the Patrol's ability to slip. Mark

[m.mooney]

Definitely agree with this. The Patrol slips very well, I wouldn’t want to give that up either.

[Bissetg]

That is very good info Mark. I agree that Bob has nailed this design well and I love the controllability of the Bearhawk as opposed to the Maule … I really never bonded with the Maule. Crosswind landings are an example where this design feature shines as well as slipping approaches. I think Whees point about time on type and adapting to the nuances of Bearhawk will resolve the issue … unless there is a rigging error.

[bearhawk2015]

I checked my elevator balance after installing the servo and I didn’t need to add extra weight. After this whole rebuild of the tail feathers I reweighed the airplane and my empty cg is now at 10.5. It actually moved forward from the last time I weighed it.

[bearhawk2015]

I forgot when I first made the fins they were 4” tall but I made them bigger and I believe they’re 6” tall now.

[Sir Newton]

Awesomer :-)

[zkelley2]

27F583F6-83AF-4BD1-B3E0-F1703A8832B2.jpg
To really get rid of tail wag you need to install some fins on the horizontal stabilizers and to get rid of the pitch sensitivity install a Ray Allen trim servo in the elevator. Pitch becomes Cessna like and it tracks almost like an arrow.

Interesting. I made a set of those for my float install, but never tried to use them on wheels. They've got quite a bit more vertical area though.

And I figured as much with the ray allen servo taking the servo action out of the trim, I've always meant to do that, but at some point I just learned to live with the pitch instability. I also really don't like the idea of electric only trim in this airplane. If that should fail and leave you in 1 trim position, you're going to have some incredibly high pitch control forces at any other airspeed.

[zkelley2]

Ya, that'd be even worse. If you didn't get it right away, I'm not sure it'd end very well.

In airplanes I've flown with only electric trim, which is mostly very large stuff, there are multiple ways to stop the trim from actuating and there are more than 1 trim system so that you can have a runaway trim, stop it and then recover control of the airplane via the standby system.

You could do something like that in the bearhawk, but it's a lot more than just a servo. At that point, I think I'd put the effort into making it a moving horizontal stab and get rid of the tabs.

[schu]

zkelley2 this isn’t as hard as you think. Using a trim controller like the safety trim or the garmin GAD-27 that is fed into the garmin GSA-28 pitch servo gives you a fair bit of redundancy. The safety trim gives you run away protection and multiple buttons to command trim from, and even if that completely fails, you can switch the AP on, and it will command trim independently.

Wiring this up and building a mount for the servo is much easier than sorting out a jack screw that drives the stab lift strut, or engineering a totally new horizontal stab.

Even then, if you want electric trim, you are in many ways starting over again.

[zkelley2]

Ya, to really get there, you'd need something limiting input time, plus a servo on each trim tab that are controlled, wired and switched redundantly. To in effect have two entirely separate trim systems. A main trim and a standby trim. I'm rather sure this is the standard for certification, not that it matters here, but I think it's a good standard of safety for an all electric system.
I've never seen all electric trim in a certified single engine airplane. Even a 737 has manual, though I don't know how connected those wheels are to the trim tabs, could just be the facade of having manual trim.
On my plane we have electric only, but what is essentially 4 independent ways to move the horizontal stab.

For the jackscrew on the bearhawk, ya I'd imagine you'd be designing a cantilevered horizontal. Or stealing one off something.

[Utah-Jay]

K.I.S.S.
Keep it simple stupid

Seems the BH design is a simple and proven design. I was asking my build mentor just yesterday about the BH door latches as opposed to the “slam shut” door latches that are referenced in the Bearhawk Tips page about doors. Bill (mentor) quoted some French dude along the lines of “It’s not what you can add, but what do you need?”. My point is, you can add a lot of things, but does it make the simplistic design of the Bearhawk better?

The above it not about Yaw dampening, but it is on point to some of the posts/comments above.

The more I get into my build the more the genius of the design is apparent.

[Steve W]

I talked to Bob about about eliminating one side of the trim servos. He felt that you probably wouldn’t be able to control the pitch with trim if you ever experienced a loss of elevator control.
I do have an electrical trim installed in my Patrol but it just drives the linkages in the tail of the fuselage rather than the push pull cables. The servo action is still there of course. The CB for the trim is sitting at the far left of the panel and is a “pull type” in case of a runaway situation.
I understand the reasoning for the servo to be installed in the elevator or stab like the Rebel and Glastar that I built.

[Nev]

At that point, I think I'd put the effort into making it a moving horizontal stab and get rid of the tabs.

Yeh the moving horizontal stab idea has merit. It could also utilize the current trim wheel.

I think any change to the trim system that removed the servo action would also have a positive effect of the feel of the pitch.

[jaredyates]

I'm not wanting to say that the non-servo action is absent any benefits, but rather that the costs outweigh them. I haven't flown a non-servo system at the aft limit, and I wouldn't be ready to say that there's no part of the envelope that non-servo elevators are better. I have flown the as-designed systems to the aft limit, both with and without correct elevator trim cable tension, which makes a very big difference for some of the same reasons as the rudder cable tension (there's an article about that too). It may be that at the aft limit non-servo action is better, but is it worth giving up the good qualities in the first 8 inches just to have better feel in the last two? There's not a right or wrong answer to that question because it will vary based on the builder's mission and needs, but my biggest concern is that someone who perhaps has not ever even flown a Bearhawk might read a statement like Nev's and think the servo action is a problem that has to be solved. That's not a consensus view.

[schu]

Great feedback that begs more questions, and a comment:

What is the proper elevator trim cable tension?

Would you launch into IFR with a servoing trim system at the correct tension at 21”?

How much flight time do you have with the non-servoing system?

I know of 3 people that switched with more than 50 hours with and without servoing. None of them that I’m aware of regrets the decision.

I’m not saying that the bearhawk must have this change, it certainly works as default, but I’m for sure in the camp of pitch stability for a bush/cruising rig that might end up in IMC when the weather changes or FU.

[jaredyates]

The spec for the trim cable tension is 60 pounds:

http://bearhawksafety.com/4-place/march2019.html

I have not been able to test a non-servo system beyond a short flight.

[svyolo]

Is the BH uncontrollable if the trim failed full up or down, or just unpleasant? I would think just slowing down to reduce control forces would be all that is required. I also think the electric trim is simpler and lighter than stock. I am starting out with stock trim.

[zkelley2]

Not uncontrollable. Just wildly high control forces. If it failed in cruise, you'd need both arms to fly for any period of time at a slower airspeed, and the landing would be difficult.

You can go try it. Everyone should. Go up, trim out for cruise and then pull power to 70kts, never touching the trim.
Or trim for 70 kts then speed up to 120+
Or to the extreme you said. Go up higher and put the trim fully down/up and try to fly it.

[Sir Newton]

Can I say thank-you to all the contributors of this thread. Everyone's opinions & ideas really got my thinking gears turning on this subject matter. Build on BearHawkers Build on!