Yaw stability

[Nev]

This is a bit of a random one.

If one were too remove one's feet from the rudder pedals for a short period of time, should the aircraft remain balanced, or is it normal for it to yaw away from balanced flight ? I've noticed that mine will yaw to the left at cruise speeds. It does this slowly, but progressively. I've added a small rudder trim tab which has removed the need for continuous right rudder - it's made a huge improvement actually and now it requires only small rudder inputs to "trim" for speed.

Part of the flight test proforma here requires a skid to be introduced and looks at the aircrafts ability to return to balanced flight. Any advice from those of you with flying Bearhawks (and experience in this area) is appreciated - obviously it's something I'd like to correct prior to flying with a lower fuel quantity as it would increase any risk of unporting.

I'm certainly getting much better at using my feet after relying on yaw dampers for many years, (so that's not where I'm going with the above question).

I should stress that I'm only part way through flight testing, with the aim of making any rigging changes needed to result in an aircraft that flies and handles very well and as it is designed to.

[Rollie]

I added a trim tab to my rudder. Then I did about 4 or 5 flights one afternoon where I climbed up a couple thousand feet, levelled off and set cruise power, got everything trimmed out and took my feet off the pedals. Then I went back and landed and hopped out, made a small trim tab adjustment and went out and repeated the process. I got it dialed in pretty well in a few flights.

For the yaw stability you should be able to get the airplane established in level flight, all trimmed up for hands off flight then step on one of the rudder pedals enough to get pretty far out of trim (like the trim ball pretty much all the way to one side or the other - not full rudder) and take your foot off the pedal again. The plane should yaw back and forth a couple times and settle back to where it started. If it started off with the ball centered, it should settle back down with the ball pretty much centered.

The thing about a fixed trim tab is that it will be adjusted for a certain configuration (weight, speed and power setting). It will help all around but if you adjust it for cruise, it will be a bit off at slow speeds. I set mine up for cruise since that is where I spend the most time. Plus I figured at lower speeds I have my feet on the pedals anyway.

[Nev]

That helps a lot thanks Rollie.

[svyolo]

Rollies description of yaw stability is spot on. He described "dynamic" yaw stability. The 540 BH has much more power than most light aircraft. That is going to cause more yaw than a more lightly powered aircraft. I have 90 hours in a 700 hp turboprop trainer, basically a 2 seat Bonanza with triple the power. The rudder and rudder trim were very high workload. I think I would describe it as a characteristic, not a problem. With great power, comes great side effects.

[whee]

Rollie covered it well. My BH recovers on its own but is easy to upset. I think that’s the nature are a short fuselage airplane. For rudder trim I just tightened rudder return spring tension on the right pedal. Keeps the rudder “trimmed” for my normal cruise settings.

[Bcone1381]

This is a bit of a random one.

If one were too remove one's feet from the rudder pedals for a short period of time, should the aircraft remain balanced, or is it normal for it to yaw away from balanced flight ? I've noticed that mine will yaw to the left at cruise speeds. It does this slowly, but progressively. I've added a small rudder trim tab which has removed the need for continuous right rudder - it's made a huge improvement actually and now it requires only small rudder inputs to "trim" for speed.

Is any force applied to the ailerons when you take your feet off the rudder?

Another data point to consider. I had maybe 600 hours of flight time (I was a CFI at the time) when I first experienced adverse aileron yaw. I was flying a Taylorcraft. When I made my first let turn, the bank went left when I applied left aileron and the nose went to the right about 30 degrees, and the ground track didn't change. hahaha!!!

So, I'm curious....in the Taylorcraft if the feet are removed from the rudder, and if the ailerons are used to hold wings level then I might predict that an "out of rig" wing would cause an aileron deflection and then the aileron's adverse yaw would pull the nose off. So this might complicate Rollies test I suppose. I think when you rig an aircraft or set the rudder and aileron trim so it flies straight, you start at the tail and move forward. But this is the internet, and I'm inexperienced and what I say is from my memory, so beware. There is a good authority/expereince on rigging a Bearhawk....I bet Jared will chime in with that source.

A second data point....an extension of Whee's rudder springs....I've wondered if rudder spring tension would increase stiffness of the rudder and improve yaw stability. Again.... another ...internet question that may not be worth exploring.

[robcaldwell]

I installed a rudder tab after about 30 hours of flying. Since the tab is static, I had to find the correct power setting that allows me to remove my foot off the right rudder. But it's never perfect. Outside temps, pressure, winds, aircraft weight, etc. all have an affect as well.

[m.mooney]

400 hours in a Patrol, assuming the B model is similar…agree with the trim tab advantages for centered ball flight in cruise. That being said, in my experience, you simply don’t fly with your feet on the floor in this thing. It requires more attention to the yaw axis than anything I can recall, it’s very easily displaced from neutral. It’s not problematic, just different, you get used to it. Whenever I take a high time Tailwheel pilot for a fam ride in my Patrol, they are struck by the sensitivity of the rudder. Nev, you’re on the right track, install a trim tab and adjust for cruise, then get comfortable with the light, powerful rudder. The Patrol is a dream to fly, I suspect the B is as well. Enjoy!
Mike

[jaredyates]

I'm low-time in the Patrol, having only flown one and for only about 15 hours. But I don't know that they are as similar as I would have thought. I'd want more data to be conclusive and I wonder if system tension impacts (see below) are greater than the difference between the Patrol and the 4-Place etc.

[jaredyates]

Something that Russ Erb pointed out in his article (that was also a counterpoint to my autopilot whining) is that the yaw stability depends on rudder cable tension. Here is the link:

https://bearhawk.tips/4172

If anyone wants to see the article but doesn't have access, let me know and I can send a pdf.
The summary is that a large portion of the vertical stabilizing area on the Bearhawk is the rudder. Compare that to something like a Stinson 108 where the fixed vertical stabilizer makes up a much larger percentage of the total vertical area. When a yaw displacement occurs, if there is low tension in the rudder system, the rudder will trail. So a yaw left, means the trailing edge of the rudder deflects left, which of course makes the tail go further left. When the rudder deflects, the amount of vertical surface left working to keep the plane straight is small. Russ made up some great graphics to explain this point in the article linked above.
Rudder cable tension varies dramatically from one plane to the next, depending on what size of springs the builder used at the firewall end near the pedals, and perhaps more importantly, how much foot pressure the pilot is applying on the two pedals at once. This also relates to speculation about premature rudder cable wear which could be attributed in part to pilot technique on the pedals.
When there is greater tension in the system, the rudder deflects less during a disruption, which lessens the instability. If you want to have the kind of plane that will act like a spam-can with your feet on the floor, install very tough springs in the system so that the springs can do the job your feet are supposed to be doing. If you want a light-feeling plane that goes where you want without fighting back, use your feet to provide variable pressure as required. You can probably sense which is my preference between these two options, but it's ok if you fall in the other category. But if you want to lean in the direction of "lighter" feeling controls, don't expect the yaw stability to test very favorably with your feet on the floor, since this isn't a feet on the floor kind of airplane.
Tricycles are easier to ride than bicycles, and bicycles are easier to ride than unicycles. Each has a mission and builder choices can point a Bearhawk more towards one philosophy or another.

[Gerhard Rieger]

Hi Jared, may I have the article pls. riegerenterprises1@gmail.com

[Nev]

Thanks to everyone that contributed to this discussion. I think Russ Erbs article summarizes what I was thinking. I'd read this before, and in anticipation of an issue, I had installed two pretty strong springs for rudder pedal retention. Things got all kinda interesting on my first takeoff when the right spring bunched up behind the pedal at full travel, limiting my right rudder input. I think I'll revisit the spring issue and search for some springs that are close to the 5 lbs/inch that Russ uses. The obvious problem is the confined space that they have to fit into.

This morning I did the yaw stability testing, and I'm not yet close to any sort of "stability" in this area. No doubt there probably a trade off because if it becomes too stable in yaw then we might lose some of the desired effect of the big strong rudder for takeoff and landing. There's probably an optimal balance, because if it's less stable then there may be a tendency towards unporting a fuel port when fuel is low.

I also performed quite a number of stalls (on a B model wing) at 2500lbs and 22.2" CG. I'll discuss this on another thread.

[Sir Newton]

Instead of springs. Could elastic bands of some sort be used?

[AZBearhawk272]

All good and valid commentary. I would like to add a few somewhat esoteric comments to the thread.

1. Having minimal friction in the rudder hinges is an important element in the rudder centering properly and providing consistent rudder return to center and in flight feel. Align and lube.

2. Make sure the tail wheel is centering properly in flight and not angled sideways like a little rubber rudder.

3. In conjunction with the above make sure that there is adequate tailwheel chain slack and spring forces are light enough to not interfere with pilot input to rudder while in flight.

4. Poor tailwheel swivel and centering can be intimated as a "rudder issue".

5. I am not a big proponent of strong "return springs" between the rudder pedal and the firewall. If something should "happen" to disconnect the spring cable mechanism on the left side, the right
spring will automatically apply right rudder when you least need it.

Kevin D
#272 KCHD

[jaredyates]

Thank you Kevin for chiming in, always appreciate your insight on these matters.

[whee]

Nev, I’m trying to word this correctly so pardon me if I say something that seems ridiculous or offensive, I don’t mean it that way.

It sounds like you are used to flying heavy iron so perhaps you just need some more time in the BH. IE, you need adjustment, not the airplane. Even guys with only Cessna time can feel like the plane has a rigging issue at first.

When I flew a BH for the first time I had not flown anything but a Luscombe. I hopped in the plane and went flying. I landed pretty quickly and called a buddy that owned a BH. I was convinced the airplane had an issue; it did not feel stable. He asked something like ‘does it feel like trying to stand on a soccer ball?’ That was a prefect way to describe it. He said it was normal and I wouldn’t notice in another hour. He was right.

If you feel safe flying the airplane I’d give it a couple hours before doing to much adjusting.

[Nev]

It sounds like you are used to flying heavy iron so perhaps you just need some more time in the BH. IE, you need adjustment, not the airplane. Even guys with only Cessna time can feel like the plane has a rigging issue at first.

Whee you're definitely right with that. It does feel quite foreign to me. Even though I did a number of hours in a Cub and an RV beforehand, they're both very "standard" aircraft to fly. The soccer ball analogy rings true with the Bearhawk for sure. I'm up to 20 hours in it now, and I'm gradually getting used to it, but I can see it's going to take a while longer before it has that "strap it on and go flying" feel to it. Also my currency is well down, and as you mention it's been a long time since I was really current in a light aircraft, so I'm just feeling my way.

I spent an hour doing stalling this morning, power off, power on, in turns etc. Currently my approach speeds are 50kts and I'm not confident to go below that yet. Also trying to feel my way with different flap settings.

Certainly appreciate your comments.

[Battson]

I suggest the rudder spring size makes a difference, based on the 5 or so Bearhawk Aircraft I've flown. The ones with more yaw stability seemed to be the ones with bigger rudder springs.

I couldn't get mine to fly perfectly hands-off straight for a long time, yaw and the heavy wing were always connected. Certainly I got rid of the major issues during test flying, but in hindsight chasing perfection wasn't possible during test flying. It took a lot of tweaking, and I needed a lot more experience. I'm still learning how to rig and trim it.

I think what I've realised is there's some basic things that screw everything else up. Some great advice has been given in other similar threads. ie you'll never fully solve a yaw / heavy wing problem with trim tabs, washers under the hinge etc, if the horizontal stabilisers are crooked. The fixes help mask the issue, but I suspect you can only get perfect trim at one airspeed if the plane is crooked.

The other consideration is cruising speed. High cruise speed makes the plane inherently less stable, this I'm sure about, at around 120kts its easier to trim out than at 135. The faster you go the more pronounced any imbalance becomes. Same with aft CG
Ideally find your favourite cruise speed before trimming it out.

If you wait until you're all set with your favourite engine power settings, then you've got one less variable to work with.

[Battson]

I would add, I flew a few months ago for 10 minutes with hands and feet off. Perfect weather. Since doing some work on one of the stablizers, and changing the cable tension, I'm back to a heavy wing and yaw issue.

Cable tension can easily mask underlying issues. As well as wearing out the cables in the fairleads.

[Bissetg]

I think Whee raises a good point. It’s a bit like learning to ride a bike, once you can do it you wonder why you couldn’t do it in the first place. My impressions of the B model are that it’s a very well mannered and delightful thing to fly, albeit mine having less HP than yours. The only yaw dampers I’ve operated are in my shoes so my expectation might be different. Over controlling, and lack of coordination can be an issue in “sensitive” axis and will settle down with experience. If you are worried about it I’d get another Bearhawk pilot to assess it before I started altering anything.

To Jonathan’s point, presumably you’ve beaten any crookedness out of it? I have flown an aircraft that was rebuilt with the fin offset opposite to what it should have been. Rerigging only went so far to address that… ugly.

Have you confirmed the incidence of the tail planes and the tension of the tail plane wires yet? Could be worth a look.

[PaulSA]

As I understand it, the springs should not have anything to do with yaw stability. They are there to stop the pedals falling over, not to hold the rudder neutral. In my RV-10, the pedals are hinged at the top and there are no springs. In the cruise, the cables are effectively slack. I'm still trying to find the "right" springs. From what I can see, you need some short, soft springs which are well extended at neutral. That way, they don't "bunch" and restrict movement when you push the pedal.

I second the comments about the rudder trim. Use a tab and trial and error to get the aircraft in balance at cruise settings. Then in climb and descent, you just need a squeeze in the appropriate direction. That's how I have my -10 set up and it works fine. A lot of people put electric rudder trims in and, in my opinion, it is a waste of time and effort on the -10. I can see why people would want it set up as a yaw damper on the Bearhawk with autopilot, however.

I'm also setting up a fixed aileron trim in the same way.

[jaredyates]

Did you have a chance to read the article above? Doesn't the RV10 get complaints about dutch roll and making passengers air sick certain cases?

[PaulSA]

I've heard of some complaints in the RV forums and many have fitted yaw dampers. I've never had an issue and simply use a fixed tab - guess I just built it better than most ;-)