Rudder Trim Gurney Flap

[noema]

I recently trimmed out my Bearhawk Patrol for hands-free (and feet-free) flying. It's remarkable how little modification it took to go from okay-ish to dialed in.

I started with a light 6" angle extrusion taped to the rudder. This was comically too much trim. After reducing it to about 2 inches, the airplane completely balanced out. The issue was never concerning, but the plane would drift into a slowly increasing right turn. I couldn't tell whether it was mostly rudder or ailerons/wing twist causing this. With slight pressure on the left rudder pedal, the airplane wouldn't fly well on autopilot, making very uncoordinated turns and taking long phases to get coordinated again. I basically couldn't tell if it would self-stabilize after an AP turn because of my manual correction. The manual correction in coordination with the AP is curiously much more awkward than with your own hand on the stick.

This had me concerned it would be a difficult issue to solve, and I questioned whether skipping a rudder servo was a mistake.

I'm happy to report that a little trim goes a long way, and a gurney flap accomplishes this beautifully and simply. My guess is there's a tipping point where the dihedral can no longer stabilize the airplane. It starts to feel off and the autopilot struggles with turns. Once you get it within what the dihedral can handle, the hands-free and autopilot flying normalizes.

At any rate, autopilot turns are completely normal now. The airplane automatically regains coordinated flight within three seconds. Also, when I place the airplane into a coordinated bank (20-30*), there's no auto-leveling and no tendency to increase bank. It is surprisingly sticky to the bank angle I set it to, hands-free. I really like it.








Thanks @way_up_north for his walk around and reassurance that gurney flaps are effective. They are a bit unintuitive.

https://youtu.be/kmw-Z-yymRI?si=J5zP54U81z9KGKyS&t=1290

[Battson]

Great report, glad to hear you've made an improvement and are feeling good about it!

I recall similar experiences. After finally getting it flying hands off - and much testing, I have found that aircraft loading, and apparently even the ambient conditions affect the exact trim. Some days it just flies better hands off than others!

I have found that over the years, I have gone back and forth a few times on how my trim tabs are set. That's the gripe with easily adjustable bendable trim tabs, they are sometimes too easy to move.
In the end, I discovered that the rudder and aileron trim was actually fighting one-another, and now both tabs have almost no deflection (practically they are almost doing nothing) but I haven't removed them yet, as they are going to take paint off when I do.

This was a surprise, as I definitely needed both tabs to begin with, for hands free flight. So what changed in the meantime?

I found that most of the error was coming from how I set up my horisontal stabilisers. While they measured, then set straight and level compared to the ground plain, and compared to the elevator horns, the elevator horns were actually a bad point of reference. Now I have them truly co-planar (no twist), I barely need trim tabs at all. There is still some small variance coming from the elevators themselves, which deflect unevenly both on the ground and during flight (but air loads change the defection). But is is hardly noticeable and I doubt it has much effect in flight. But this whole rigging area .

[noema]

I can see how touching a few bolts on the horstab changes things. Also good point on the adjustability of a normal trim tab.

[zkelley2]

Great report, glad to hear you've made an improvement and are feeling good about it!

I recall similar experiences. After finally getting it flying hands off - and much testing, I have found that aircraft loading, and apparently even the ambient conditions affect the exact trim. Some days it just flies better hands off than others!

I have found that over the years, I have gone back and forth a few times on how my trim tabs are set. That's the gripe with easily adjustable bendable trim tabs, they are sometimes too easy to move.
In the end, I discovered that the rudder and aileron trim was actually fighting one-another, and now both tabs have almost no deflection (practically they are almost doing nothing) but I haven't removed them yet, as they are going to take paint off when I do.

This was a surprise, as I definitely needed both tabs to begin with, for hands free flight. So what changed in the meantime?

I found that most of the error was coming from how I set up my horisontal stabilisers. While they measured, then set straight and level compared to the ground plain, and compared to the elevator horns, the elevator horns were actually a bad point of reference. Now I have them truly co-planar (no twist), I barely need trim tabs at all. There is still some small variance coming from the elevators themselves, which deflect unevenly both on the ground and during flight (but air loads change the defection). But is is hardly noticeable and I doubt it has much effect in flight. But this whole rigging area .

This was also almost all of my hands off roll. The horzontal stab wasn't really level. Once I finally got that dialed in, most of the roll went away. I have taken my gurney flap off my ailerons. I still have a small bendable tab on my rudder that has a tiny bit of bend to it.