Yaw stability

[kestrel]

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.

My first solo was in a 65hp J-3 on my 16th birthday.

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.

A 540 Bearhawk is going to have a much larger polar moment of inertia, esp. when loaded to an aft CG. It will have a heavy motor on the front and then a heavy load in the back. This will reduce yaw damping and perhaps noticeably.

Propeller Spacers
...I'm guessing that Bob's prototype had good yaw characteristics.

Bob's prototype, in addition to being very light and having an O-360 always looked to me like the nose was very short/small. It may feel different as a result?

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.

I had not heard about this and would be interested in hearing if others know more about it. However, I would guess that there is very, very little chance that the gear attach points on the fuselage would have been moved. If the wheels were moved forward, it seems more likely that the lower point of the triangle (the wheel) was moved forward and the upper part of the gear leg stayed put. Its location is very tied into the design of the lower fuselage and so is the wing strut. It would be no minor change to move the upper attach points. ...and the upper part of the gear leg represents the majority of the surface area.

Like others, I enjoy the strong rudder authority at low speeds.

[bearhawk2015]

The fins are made from .080” 2024 aluminum and are 5” tall. If I was going to make them again I would make them 6” tall.

[bearhawk2015]

The bolt hole is reinforced by welding a tube inside

[Nev]

That's the part I was after. Thanks very much.

[marcusofcotton]

The fins are made from .080” 2024 aluminum and are 5” tall. If I was going to make them again I would make them 6” tall.

Wonder if they might be more effective on top with the higher pressure up there???

[Nev]

I'm trying to think of a way to try this out without making permanent modifications. I see there are a number of dorsal and ventral mods. bearhawk2015 's horizontal stab winglets seemed quite straight forward, but I now realize it requires a tube to have been welded in place during construction. Another idea is to use the rear flying wire attachment, but it's offset by 190mm.

Any ideas? I'm looking for a modest improvement in yaw stability.

[AZBearhawk272]

Nev,

My experience is that the Bearhawk is stable in yaw.
The issue that I have experienced on several is an issue of the Rudder is not self centering when when all inputs are removed.

Some simple things and a few more complex.
proceed with careful caution.

1. Eliminate friction. Cable fairleads. Pedal assy
Rudder hinge.

2. Eliminate pedal return springs. Inter connect pedal with cable.

3. Gap seal the rudder hinge line.

4. Build up the rudder vertical front hinge line tube so it is approximately 1/8 in bigger than the vertical tube on the vertical fin.

5. Recover the vertical fin, add airfoil shape to structure.
Add a horizontal rib down low about 7 in above the upper fuselage longeron. Stitch the fin fabric tight pulled in to eliminate the bottom “puffyness” of the forward portion of the vertical fin. ( this can still leave the fin looking smooth and curvaceous )

6. Place VG’s on the vertical fin several inches in front of rudder hinge line.

improvements come incrementally…..
no magic one bullet cures all…..

Kevin D
#272

[Nev]

Good ideas there Kev, thanks.

[Battson]

Any ideas? I'm looking for a modest improvement in yaw stability.

Per suggestion on page 1, try increasing the size of the rudder springs. Speak to Graeme P - he's got the largest springs I've flown with, and his plane flew almost like a Cessna in the yaw axis.

It's quick, cheap, and easy to do. Plus it's easy to undo if you don't like the results.

[Nev]

Good idea. I've tried a few different springs but I think I'll revisit this. Thanks.

[jaredyates]

Yes! This is something I was going to suggest also.

[kestrel]

I'm trying to think of a way to try this out without making permanent modifications. ... bearhawk2015 's horizontal stab winglets seemed quite straight forward, but I now realize it requires a tube to have been welded in place during construction.

I can understand if not everyone would be thrilled with this idea, but I wouldn't be afraid of making the "stab winglets" so that they bolted under the front strut and used duct/gaffer's tape down the length. Instead of ending it over 90 degrees to one side, cut the top "flange" where it meets the horizontal stab in sections and bend each section in the opposite direction so that it looks like a T when viewed on end. I wouldn't expect it to stay indefinitely, but I'm quite sure it would stay more long enough to do a few flights.

[Nev]

Yep I get what you mean. That might work.

[Nev]

Incidentally, I'm very interested to see flight test reports on the Companion. With it's shorter CG envelope and smaller engine cowl, it should exhibit a very good control balance, and presumably reduced sensitivity on both the rudder and elevators.

[Utah-Jay]

Incidentally, I'm very interested to see flight test reports on the Companion. With it's shorter CG envelope and smaller engine cowl, it should exhibit a very good control balance, and presumably reduced sensitivity on both the rudder and elevators.

I am doing my best to get to the testing phase

[BravoGolf]

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

The ideal of adding a strake to the lower fuselage is interesting. As I’m just starting to fabricate the fuselage this would be the time to add that capability. Maybe something similar to the wing fence attaching points so the strake could be removed, modified and reinstalled.

I wonder how it would affect the slip characteristics that Bob designed into the series.

[alaskabearhawk]

That would be a good question to ask Mark Scott.

[Nev]

The ideal of adding a strake to the lower fuselage is interesting. As I’m just starting to fabricate the fuselage this would be the time to add that capability. Maybe something similar to the wing fence attaching points so the strake could be removed, modified and reinstalled.

I wonder how it would affect the slip characteristics that Bob designed into the series.

That's a very good idea. Then the capability is there if you need or want to try it, and if you don't it doesn't matter.

Others will have much more experience with slipping, but so far after 900 landings (very slow learner ), I've seldom felt a need to slip the aircraft. The flaps are huge and they work very well. Placing the aircraft on the back of the drag curve allows very steep approaches - so steep it's hard to believe sometimes. The one area that I have used a side slip is landing with a crosswind from the left (and flying from the left seat) when a gentle side-slip allows better forward visibility.

A small increase in yaw stability would absolutely get my vote and in my opinion would not detract from the ability to side-slip, should it actually be desired.

[Battson]

A small increase in yaw stability would absolutely get my vote and in my opinion would not detract from the ability to side-slip, should it actually be desired.

Hmm. It would depend how you increased stability, I suspect the two things are coupled.

You need 100% of the rudder's authority to get a decent slip in a 4-place Bearhawk, pedal to the wall. Any loss of rudder authority or increased resistance to a slip would hinder this ability.

While accepting that some people prefer to avoid slipping - the aircraft was designed to slip. I certainly find it very useful and regularly slip on approach.

[Mark Goldberg]

While we all have our personal flying habits that suit us - Bob designs the planes to slip well even with full flaps. Bob slips almost every landing. I once saw him do a constant slip in his LSA from about 3,000 AGL and pull out so close to the ground I was starting to worry. He considers the ability to slip the plane to be very important and useful. Mark

[AKKen07]

Battson, do you slip to increase descent rate or for visibility? Both? Other? I’d like to understand why the entirety of the slipping ability is important. I’ve flown a handful of GA planes that all were more stable than the BH and still slipped quite well. I feel like I may be used to more modern designs (like post 1930s) and am struggling to appreciate the need for the low yaw stability. I also think my skis might be affecting my yaw with their forward surface area.

[Battson]

Hi Ken,

For me? Mostly to increase descent rate when going into very difficult spots - or because I misjudged the wind conditions or angles during final approach - this isn't something which will suit everyone. I just use it for whatever is required really, sometimes I like to see behind me But it may not suit some people. Passengers are usually surprised by an aggressive slip, even pilot passengers.

To get a really aggressive descent rate in my machine, I either need to have the throttle fully closed with the nose very high and be quite close to the stall (V1.1 or less) - which is hard on the engine - the cooling rate is aggressive, and not possible in gusty conditions. Or I can use maximum rudder to get a decent slip going. If I use anything less than full rudder, the plane just doesn't achieve a useful descent rate in my experience, and the slip is not useful. I might as well not slip in that case, and fly a normal approach. There is a price to pay when I am finishing the slip - at least with my unpolished technique. I usually gain airspeed as I return to neutral rudder, and I need to account for that in the approach profile. Perhaps a well timed increase in back pressure on the stick would correct that, but I am too waring of a stall out of balance to do it "in anger". For this reason, the slip should be aggressive enough to get below my intended glide path, so I can then re-attain the correct stabilised approach profile when I exit the slip. I want to be able to attain the correct profile before the touchdown phase, to ensure I am in a stable approach at touchdown.

If you don't like to slip in your aircraft and would prefer more yaw stability, that sounds like a case for making a change.

I would encourage discussing it with an aircraft designer to understand the affects of any change. Say you decided to reprofile the top of the rudder - targeting the rudder horn area, for argument's sake. Rudder pedal forces may go up quickly as horn area reduces (for example purposes only), which could result in needing to redesign other parts of the system for the higher forces and wear / tear. I am not saying it's going to be a hard change to make, I am just highlighting the "law of unintended consequences" which seems to pervade aviation.

[AKKen07]

Thanks Battson, if it takes the whole of the rudder authority to slip the plane and increasing yaw authority would eliminate that potential I am less sure I want to go that route just yet. I really am not considering changing the size or shape of the rudder so much as adding a fin/fins/strakes, of some sort to the structure. I have used slipping enough that I want the tool available, so I’ll wait till I have more time in the BH.
Maybe stronger springs and more foot discipline will be my best solution.

[Battson]

I have a suspicion that stronger springs at the pedals would help. It's on my "to do" list.

[AKKen07]

So, I switched to 31" bushwheels today and the stability was noticeably better! Much more enjoyable to fly. The skis must have enough forward surface area to have that effect.

[ThreeD]

Vortex Generators

in 2019 I was at SZP ( Santa Paula,California) doing some aerobatic Training with Chris Olmsted

tucked away in one of the hangars of Ray’s aviation was the Pitts S1 that Spencer Suderman set the world record for inverted flat spins and the vertical stab forward of the hinge line had vortex generators installed to increase rudder authority

the plane was on Barnstormers last week after a bit of a rebuild….

Spencer successfully deadsticked when the crank shaft broke at the flange and the prop departed the aircraft….this was earlier in 2019


anyhow, Ray said the vortex generators really helped Spencer maintain rudder authority at altitude to maintain the inverted flat spin…

I have a picture of it somewhere but cannot find it tonight

my 2 cents

Dano

[AKKen07]

I actually had to stop and think for a second to get you point, haha. I suppose if I added something to add stability, I could then add VGs to regain rudder authority and have a little of both. Nice thought and cool story!

[Battson]

Hard to say what would happen without testing. Some other forum users have said that having VGs under the horisontal stabiliser made the elevator twitchy. Others said they noticed no difference.

[Nev]

I've got VG's on the underside of my horizontal stabiliser. I didn't notice any change in "twitchiness". It did give a small increase in elevator authority at high AOA/low airspeed.