Wash-out Question (and answer)

[Chewie]

I showed my project to a buddy yesterday to get pointers on things I should do before closing out the wings. His main observation was that my wings were very straight. TOO straight. He asked if I've ever tried landing an airplane without wash-out. It doesn't end well. I said I haven't (I think even my 140 has a small amount), but even so that would be a huge deviation from the plans which I had no intention of doing. Still, he was surprised the Bearhawk wings do not have any twist. I said I knew it was a sound design, but I couldn't put my finger on why.

So today I called Bob, my annual phone call, to get more backstory. There are a lot of characteristics that factor into stall behavior, but on the Bearhawk the rectangular planform of the wings allows the root to stall first by default, including the LSA (no flaps) and A-model (no Riblett). Rudder and ailerons maintain good authority, and if they are built right BHs exhibit good, predictable and gentle stall characteristics. Older airfoils and elliptical wings... totally different story.

Anyway thought I would mention this in case y'all have any shop visitors who ask similar questions. It's important to remember they are simply looking at the design choices of the BH from the context of what they know. This is generally a good thing, but sometimes it can lead one to question their life choices. Fret not. A thousand (or whatever) flying Bearhawks would suggest otherwise.

PS - if you do have pointers on easily forgotten things to do before riveting the wings, shoot them my way!

[Nev]

Although the BH doesn't have washout as such, the rigging recommendations call for the ailerons to be rigged 3/8 higher than the trailing edge, giving a pseudo washout of sorts. That, coupled with the flaps being deployed for a typical landing, cause a much higher AoA at the wing root compared to the tips when approaching the stall in normal landing configuration. The BH generally exhibits very good slow flight characteristics, with a docile stall. As the inboard wing stalls it tends to develop a higher sink rate. This can also coincide with loss of elevator authority, typically at low speeds and forward CG.

Likewise, with enough elevator authority, a power on full flap stall can result in a very high pitch attitude. Once it exceeds the critical angle with a high pitch attitude, it leads to a wing drop, and spin.

[rkennell]

Nev: The last sentence sounded like it came from personal experience. Can you expand on that a little? Was the wing drop sudden? Was it because you ran out of rudder authority and the aircraft was uncoordinated when it stalled? How many turns did the aircraft do before you recovered and was the recovery accomplish with standard spin recovery technique? Thanks in advance.

[Nev]

Can you expand on that a little?

In my experience, to have a wing drop stall in the BH, I need to go into it with enough power on to retain airflow over the elevators (or an aft CG). This then allows the nose to be raised high enough to fully stall the main wing. i.e. in the approach config, power on. Then, when it goes, it goes quickly, but not unexpectedly. The pitch attitude (from the Dynon) on mine gets to around 28° typically before dropping a wing. I expect with the pitch being that high that the thrust is possibly providing some vertical component. I've never run out of rudder authority, the BH has tons of it. This was all done during the flight testing.

I don't have experience with spinning mine. Grant Bisset spun his to see how it would go, and there's others on YouTube spinning as well. From memory I think it needed a small amount of rudder to induce the spin, and recovered well. Grant might weigh in here.

[rkennell]

Thanks for expanding. I also just discovered you 12-24-2022 video on Grant Bisset's spin test. Very well done. Thanks for your contributions to safe flying.

[Nev]

This video shows Eric Newton and Lee Taylor spinning Eric's 4-place. The rate of spin astounded me ! Recovery is quick.

https://youtu.be/FCFriyJZ15I?si=NHfWEkGk8oTmzdIM&t=90

Here's Grant spinning his 4-place, filmed on a GoPro 360.

https://youtu.be/JjeXoQ0GAGQ?si=OEjkTzN4HBJMY2g-&t=140

[Chewie]

the rigging recommendations call for the ailerons to be rigged 3/8 higher than the trailing edge, giving a pseudo washout of sorts. That, coupled with the flaps being deployed for a typical landing, cause a much higher AoA at the wing root compared to the tips when approaching the stall in normal landing configuration.

Thanks Nev! I did consider the aileron rigging, but wasn't sure about the flaps since the LSA doesn't seem to have bad stall behavior either. Nice video too, crazy to think that was 16 years ago.