Yaw stability

[Enewton57]

I guess can contribute something here since I experienced a sort of runaway elevator trim on my Bearhawk. I had electric elevator trim with up and down buttons on the top of both control sticks.

My wife and I were enroute from south Mississippi to Oshkosh for Airventure. As we neared Memphis, we were cruising along at 8500 feet in smooth, clear air and I was almost dozing off. Without any warning, the nose suddenly pitched up about 20 degrees and I was pushing forward on the stick, it was pretty hard trying to bring it back to level again.

I pressed the down trim button and she leveled out and the control forces were normal. As I was trying to sort out what had happened. I initially thought in my slumber I had somehow hit the trim “up” button, but my hands were not even close to the top of my control stick. What the heck had happened here?

That was when I looked over and noticed my wife had her crossword puzzle book sitting on top of the control stick on her side. She had discovered a word and when she tried writing in the book, she had pressed down hard enough to activate the “up” trim button.

Thankfully, my system had a runaway trim prevention device in line. That trim limiter device only allowed the trim servo motor to run for 2 seconds at a time each time you push the button. Not sure how bad it would have gotten with more than 2 seconds of servo run time, but I think it would have been pretty tough to control unless you can think fast enough to pull back on the throttle real fast.

I would advise anyone looking for electric trim tab control to look into a trim limiter device like I had.

Eric Newton
Bearhawk Tailwheels and Builder Manuals

http://bhtailwheels.com

[schu]

This was a safety trim?

[Pbruce]

“not sure how bad it would have gotten…” As a B737 Max pilot, I think it can get pretty bad. Not trying to make light of this. Quite the opposite. Interesting that your intuition brought you to a similar conclusion to Boeing: best to only allow a limited trim motor run. In your case, you applied the safeguard before an accident; in their case, after two of them. Tread carefully guys

[schu]

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.

The bearhawk is a simple elegant design, but just like any other mechanical device it’s with trade offs. Some of the trade offs the designer made aren’t tradeoffs I’m willing to live with. This is one reason why I’m not flying yet. It takes a tremendous amount of time to design changes that are safe and worthwhile. Many parts I built 4 times.

Anyway, I agree with you, slam the door latches aren’t worth it, but… I’m not okay reaching behind me for quick egress, nor am I okay with a 3-pt seatbelt. In my opinion changing to a 4-pt harness and extending the door latch to be further forward on the inside of the door are changes that do make the simplistic design of the bearhawk better.

Of course I went completely overboard, but I’m having fun, I have a cool airplane to fly in the mean time, and I’ll get exactly what I want. It’s all good.

[N3UW]

So I am trying to understand what some people do not like about the trim on the Bearhawk. Is the Bearhawk trim wheel very sensitive? Does the sensitivity of the trim wheel change with airspeed? Or is it something else?

[Mark Goldberg]

John - the trim wheel on the 4 place is very sensitive. I think that is good although the first time the trim wheel is moved on first flight it often is a surprise how much it does. But once you have gotten used to it - it is much better than going around and around with the trim wheel like on a Cessna.

Trim doesn't change so much as different speeds as it does at different CG loadings. Elevator inputs and trim are much more sensitive at aft CG. It was certainly like that in spades on my RV8 also.

A few BH pilots have done away with the servo system on the trim and make the trim tabs only move with the elevators. They do this because at aft CG the BH four place is twitchy in pitch and loses stability. I know my Bellanca Viking was the same way. Pitch is more stable at aft CG with a straight - non servo'd trim. However, the pitch forces become stiffer all across the CH range. And the light, balanced controls in all axis's that we enjoy changes because of the stiffer pitch forces - even at forward CG. With the modified trim system a few have opted to use. I think Jared put it very well in an earlier post. Mark.

[schu]

So I am trying to understand what some people do not like about the trim on the Bearhawk. Is the Bearhawk trim wheel very sensitive? Does the sensitivity of the trim wheel change with airspeed? Or is it something else?

Compared to a Cessna or Piper the trim wheel is very sensitive and pitch is much faster to react.

The reason is because the trim torque tube is fixed in the h-stab. Because of this when you add up elevator trim, the tab goes down which causes the elevator to go up, but because the end of the pushrod doesn’t go up with it, the trim tab goes further down which causes the elevator to go up, which…..

Many many people fly the bearhawk this way. Some prefer it, others have gotten used to it, and others have gone through the effort to make it more Cessna like.

Reading through this thread should make it clear the pros/cons of the system and who is in what camp.

[N3UW]

Mark and schu…..thanks for the explanation

[Sir Newton]

I have not flown a BearHawk. Your comments are thought provoking.

[Battson]

I guess can contribute something here since I experienced a sort of runaway elevator trim on my Bearhawk. I had electric elevator trim with up and down buttons on the top of both control sticks.

My wife and I were enroute from south Mississippi to Oshkosh for Airventure. As we neared Memphis, we were cruising along at 8500 feet in smooth, clear air and I was almost dozing off. Without any warning, the nose suddenly pitched up about 20 degrees and I was pushing forward on the stick, it was pretty hard trying to bring it back to level again.

I pressed the down trim button and she leveled out and the control forces were normal. As I was trying to sort out what had happened. I initially thought in my slumber I had somehow hit the trim “up” button, but my hands were not even close to the top of my control stick. What the heck had happened here?

That was when I looked over and noticed my wife had her crossword puzzle book sitting on top of the control stick on her side. She had discovered a word and when she tried writing in the book, she had pressed down hard enough to activate the “up” trim button.

Thankfully, my system had a runaway trim prevention device in line. That trim limiter device only allowed the trim servo motor to run for 2 seconds at a time each time you push the button. Not sure how bad it would have gotten with more than 2 seconds of servo run time, but I think it would have been pretty tough to control unless you can think fast enough to pull back on the throttle real fast.

I would advise anyone looking for electric trim tab control to look into a trim limiter device like I had.

Eric Newton
Bearhawk Tailwheels and Builder Manuals

http://bhtailwheels.com

I've had this happen several times now, with various unwanted pressed of the co-pilot stick buttons from passengers, things they were holding, etc. I have no trim-safety installed.
Invariably it's not hard to get the plane back to straight and level - always under control, although it does increase excitement in an unwanted way.

[zkelley2]

I've found most my pax really prefer not to have the stick in their way. Makes it easier to get in and out.
Also originally I didn't have anything covering the rudder cable on the right front side. I found that for whatever reason lots of people like to put their foot on that cable, essentially giving a bit of right rudder.

[schu]

@zkelly2 pictures of your cable cover would be nice. Did you just bend something in the brake and blind rivet to the floor skin?

[zkelley2]

I used some clear vinyl tubing same as I did in the rear. Can't remember the ID off hand.

[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.

bearhawk2015

Any chance you could post some more pictures of this, showing the aft attachment?

[alaskabearhawk]

Mark Scott added a ventral fin on his aircraft. You can see it on the bottom aft of the fuselage.

8EC5176B-13B7-440C-A676-F52553AAB174.png

[Nev]

That'd work well too. I'm looking for something that could be retro-fitted without too much trouble. After 200 hours I still feel it's "twitchy" at cruise speeds.

[bearhawk2015]

I’ll try and get some more pictures in the next couple of days.

[kestrel]

After 200 hours I still feel it's "twitchy" at cruise speeds.

Nev, I can't say that you're wrong because your experience in your airplane is the truth. ...but I don't find any issues with yaw stability. I would be very surprised if our planes behaved differently on the yaw axis. I wonder what makes our experiences so different?

[whee]

I can't say that you're wrong because your experience in your airplane is the truth. ...but I don't find any issues with yaw stability. I would be very surprised if our planes behaved differently on the yaw axis. I wonder what makes our experiences so different?

I’m keen to read what Nev has to say. He’s obviously an experienced pilot and I enjoy hearing his perspective.

I don’t take other pilots flying very often but there are few that I have taken and all had a bit of trouble with yaw control. All of them were trained in Cessnas and pretty much only had Cessna experience. I think that is a big part of it. You can fly a Cessna with your feet flat on the floor, at least every Cessna I’ve flown is that way: C150, 152, 172, 182, 180, 206, 207. A BH is a “rudder” airplane, you can’t fly it without rudder.

For example, I’ve flown a few times with a pilot who’s first ride in an airplane was in my BH. I let her fly quite a bit and she did a great job, no tail wagging or anything. She love it so much she enrolled in an accelerated program. After earning her CFI and some 600hours she came back to fly the BH to gain some TW experience. She couldn’t fly it to save her life. Literally was like Cougar trying to land after getting shook up; tail was dancing all over the place while she was chasing ailerons and rudder. After a while she did ok but if the plane got upset I had to correct it. It was horrible. A few hundred hours later and flying 20 or so other models she came back again and was able to fly reasonably well.

On the flip side, my pops and I learned to fly in Luscombes. We each hopped in a BH and went flying, zero issues. We are definitely not super pilots, just regular Joe’s. Luscombes are rudder airplanes just like a BH. In fact, I think the BH flies very similarly to a Luscombe and therefore think if you can’t get training on a BH try to get some training in a Luscombe.

[Nev]

Nev, I can't say that you're wrong because your experience in your airplane is the truth. ...but I don't find any issues with yaw stability. I would be very surprised if our planes behaved differently on the yaw axis. I wonder what makes our experiences so different?

There's a few different perspectives on the BH yaw. I've noticed that some pilots who have a background on the older style taildraggers manage it very well.

However the common theme I've found after taking many pilots flying with a vast range of experience levels, is that most struggle with the yaw stability (as I did initially). Recently I've discussed it with two very experienced GA examiners - both who own and fly their own taildraggers. Both commented on how sensitive the yaw was and how much attention it required.

Theres one or two factors that affect this.

CG
I notice when I'm loaded to a more aft CG that the yaw is more sensitive. On mine it starts to become noticeable around 18" (as do the elevators) and requires just a little more attention.

Propeller Spacers
I also think that the arm of the propeller could be having an effect. My understanding is that some Bearhawks have longer prop spacers than others. Those with the longer prop spacers will presumably have a larger cowling area that will be countering the effect of the vertical stabilizer to some extent. I'm not sure where mine falls in this regard, but it may explain why some of us see it as an issue and some don't. I'm guessing that Bob's prototype had good yaw characteristics.

Landing Gear
My understanding is that the main landing gear was moved slightly forward early in the evolution of the BH. There's a reasonable "sail area" to the gear struts and I wonder if moving it forward has also had a small effect on the yaw stability at cruise speeds. Not sure about this.

Like others, I enjoy the strong rudder authority at low speeds. Although I don't side slip often (haven't needed too) it's a good tool to have in the tool kit, particularly for improving visibility in a crosswind landing. But if I could tone the sensitivity down just slightly I reckon I'd have the best of all worlds - strong rudder authority at low speeds and a stable aircraft in the cruise. I want to have my cake and eat it too !

[kestrel]

I’m keen to read what Nev has to say. He’s obviously an experienced pilot and I enjoy hearing his perspective.

Agreed!

I don’t take other pilots flying very often but there are few that I have taken and all had a bit of trouble with yaw control. ...

I've seen the same thing and it is always about the adverse yaw, not stability. I will state that a typical Cessna has more yaw stability than a Bearhawk, but I don't find the Bearhawk lacking at all. I've said it before, and I'll say it again, "I LOVE the rudder authority of the Bearhawk and how it handles crosswind landings."

Though I'd love the chance to get a ride from Nev in the amazing backcountry of NZ, I've never flown with him, but I'd be shocked if adverse yaw were still an issue after a couple hundred hours?

For example, I’ve flown a few times with a pilot who’s first ride in an airplane was in my BH. ... She couldn’t fly it to save her life.

Wow! That is quite a story/experience! It says a lot about how people adapt.

On the flip side, my pops and I learned to fly in Luscombes. We each hopped in a BH and went flying, zero issues. We are definitely not super pilots, just regular Joe’s. Luscombes are rudder airplanes just like a BH. In fact, I think the BH flies very similarly to a Luscombe and therefore think if you can’t get training on a BH try to get some training in a Luscombe.

I learned in a J-3. When I bought my Bearhawk I had been flying our RV-4 which is a "feet on the floor runs" airplane. I hadn't flown anything with much adverse yaw for quite a while, yet jumped in the Bearhawk and flew from FL to NH without yaw issues. ...Pitch was a different story because of the rigging of the trim tab and its servo behavior.

EDIT: The reason I'd be shocked if Nev is having trouble with adverse yaw because he has posted video proof that he's quite good with that airplane!

[Bissetg]

I don’t think it’s a matter of Nev not being able fly it in balance, more a case of him wanting to improve what he sees as a handling quirk.

[Bissetg]

From a safety perspective there are compelling reasons why the ball should be in the middle. Fuel imbalance and the associated risk of Unporting a tank and stalling in a skidding turn on to final are things that could make your day very exciting, albeit briefly. These aren’t Bearhawk peculiar issues but present in many designs to some extent.

Personally, I like my Bearhawk the way it is. It has great control authority and I find it delightful to fly, having said that I’ve had 40 years of flying things that require your feet to be in good working order. However, a number of reasonably experienced pilots have initially struggled to fly my Bearhawk in balance so I can see where Nev is coming from. Most quite quickly reprogram their mental yaw dampers and adapt after a short while.

In discussing this matter over the weekend with another scratchy old dinosaur we agreed that pilots who learn to fly in modern designs that have had a lot of the fun designed out of them, or long time jet pilots, can struggle with the Cub, Bearhawk etc.

The biggest risk I see with out of balance flying is the skidding or over ruddered low speed turn onto final. It’s important not to confuse a very benign stall characteristic in balanced flight with the reality of physics and a stall in a skidding turn. If you’re not familiar with the risk of stalling in a skidding turn as opposed to a balanced or slipping turn I’d suggest you go and try them at altitude with your CFI, it might save your bacon.

[kestrel]

My thoughts on the yaw behavior of a 4 place Bearhawk with a 540 hung on the front:

It has lots of adverse yaw. If the stick moves left, the rudder moves left and vice versa.

It has a fair bit of yaw inertia when compared to the yaw damping, even when the aux tanks are empty. Mine has 25 gallon "aux tanks", so it might feel a little different here, esp. when compared to Bearhawks with no aux tanks. A consequence of this is that if the ball is out to the left (needs left rudder) but the nose is already "swinging" to the left, I will apply right rudder. This is not what you would expect for where the ball is. I do it to prevent overshoot. If I don't, the ball will end up out to the right. I am acting as an active yaw damper.

To me, "yaw damping" is how it responds to momentary disturbances and what is the behavior of the resulting oscillation.

To me, "yaw stability" is how well the airplane responds to a fixed disturbance. Does it track well in yaw when only minor disturbances are inflicted and If I push and hold a small amount of rudder. Does it diverge? I find the yaw stability just fine. If I leave the ailerons alone, I can leave the rudder alone. If I apply and hold a small amount of rudder, I get a small amount of slip and nothing more.

Someone else may have better descriptions for yaw damping and yaw stability. ...or perhaps better terms for what I'm trying to describe.

To have it here in context, even though I've already mentioned it, I love the rudder authority of the Bearhawk. It is very useful for slips and crosswind landings (which are slips). Adding additional fins to the tail will reduce this, though perhaps not enough to matter. I'm wondering if the the Five is as good in a crosswind as the 4 place. It has a longer tail and a larger vertical fin.

One time that my Bearhawk surprised me was the first time I crested some tall trees (120-150 ft?) on short final and then applied some aileron and kicked opposite rudder to slip it down. The combination of yaw authority and yaw inertia resulted in a massive swing of the nose! That got my attention...