Pitch authority at low speeds

[Nev]

Many of us with an IO540 or similar weight engine up front experience loss of elevator authority at low airspeeds when at reduced thrust and in the forward CG range.

Kevin D mentions a potential fix on this thread HERE :

Then I lowered the horizontal leading edge about 3/16 of an inch after installation of tail VG’s no stability degradation but notable improvement in low speed elevator authority.

I thought this would be a very interesting discussion. Has anyone else made similar adjustments? Can we collate the results on this thread ? If there is a way to reduce or eliminate this issue then it would benefit all of us.

[jaredyates]

Hi Nev, this is a great topic and there are a few things to unpack, including VGs on the horizontal stab, the impact of engine thrust/propwash as an induced airflow over the tail, center of gravity location, and the angle between the horizontal stab and the fuselage/wing.

First, as you for sure know, but to review, why are airplanes stable in pitch? The center of lift is behind the center of gravity, and the airfoil contributes a nose-down pitching force. These two factors make the whole contraption always want to pitch nose-down. So we add a horizontal stabilizer, and make it produce enough pounds of down-force to balance out the forward-pitching factors. To do this, we give the HS an angle relative to the wing which places the front of the HS lower than the back (or, we deflect the elevator in the nose-up direction, either will accomplish the same thing, but with different costs). We're using the upside-down lift of the horizontal stab/elevator to pull the tail down. Because this tail down force is aerodynamic, the faster we go, the more effective it becomes.

That yields pitch stability. If the airplane speeds up the tail goes down, and, the airplane pitches up. If it slows down, the tail goes up, and the airplane pitches down.

So, the more pronounced the natural nose-down pitch of the airplane is, the more work the horizontal stabilizer/elevator combo has to do to keep the forces in balance. If we want to improve elevator authority at low speeds, we need to realize that there are physical limits to how much work the HS/elevator can do. In the Bearhawk, I'd propose that the weak link is a flow separation (stall) that happens at the hinge line between the horizontal stabilizer and the elevator. Basically, it becomes too sharp of a turn for the airflow to make. There is a little more to talk about here with regards to the Model B design changes but this is already going to be way too long so I'll try to stay focussed. If you'd like to visualize this, here's a video:

https://youtu.be/NB0kh3Z4OVM?si=3CFjlw1UDKaHnUjG

In the video, I only compare with and without a gap seal. It would be a good test to repeat with and without VGs, but theory would support the idea that VGs will help reduce the flow separation, including at high elevator deflections. You can apply everything you know about wing stalls to this HS stall, and for the same reason that they can help with wing stalls, VGs can help here because they are introducing energy into the boundary layer and keeping the airflow stuck to the surface longer.

Moving on from VGs, if you want to keep the airflow attached, two variables worth considering are reducing the AOA of the lifting surface, and increasing airspeed. Applying this to the HS/Elevator, we can increase speed flying faster, but also by increasing engine thrust. These surfaces are in the propwash, which creates an induced airflow that makes the airspeed over those surfaces locally higher than the airspeed indicated by the pitot system and experienced by the wing. This is very easy to demonstrate and feel when flying a Bearhawk. Try a stall at idle, and try one with an inch of throttle, and the difference is huge. Two things are happening here and both are good. Because we are wanting the HS/Elevator to yield a certain number of pounds of tail down force, increasing the airspeed of the surface allows us to reduce the AOA to get the same lift. We do that by forward stick deflection. That reduces the angle that the airflow has to turn, thus mitigating the separation. The second factor is that assuming the tail isn't stalled, any increase in airspeed at a fixed AOA (stick position) will also increase tail down force. So an excellent way to increase elevator effectiveness is also to add a little engine thrust.

But maybe you are in a situation where extra engine thrust is not available or desirable for some other reason. The other big variable in keeping the airflow attached on the tail is the local AOA of the horizontal stab/elevator combo. I would suggest that the biggest things we can do to alter this, as Bearhawk pilots and not as airplane designers, is to move the CG aft. As the CG moves aft, the natural forward pitching moment is reduced. That means we are asking less of the horizontal stab (fewer pounds of downforce) in our mission to hold a pitch angle. Asking less of it ultimately means reducing its AOA. AOA is the difference between the chord line and the relative wind, and for the HS/elevator assembly, the chord line is drawn between the leading edge of the stab and the trailing edge of the elevator. The amount of stick deflection (and, elevator deflection, and, HS/elevator AOA) required to maintain the desired nose-up position will become less aft as the CG moves back. This is of course limited in that once the CG gets too far back, then we lose pitch stability, which is generally though of as a bad thing for what we are trying to accomplish here. When the CG moves aft, we are shortening the effective length of the lever arm of the horizontal stab, but keep in mind we are really talking about 10" or so, and that detrimental effect is easily overshadowed by the other benefits.

So all of that is to say, a fantastic way to increase the elevator effectiveness at forward CGs, is to move the CG back. In the short term, you can do this with ballast, tool bags, etc. Though this does have limits if you are on your way to or from carrying a heavy load.

Going back to the very beginning of the discussion, we established pitch stability by having the tail produce a constant down-force while in flight. What we are doing here is basically pushing a wedge through the air. The more forward the CG, the stronger the pitch stability, and the "blunter" the wedge. Air racers, and some airlines, have figured this out, and aim to move the CG aft to improve efficiency. Did you guys ever do that on the big ones? I know that at work our loads are optimized with this as one of many factors, but it's all transparent to the pilots.

Now we are in a position to talk about the structural angles between the horizontal stab and the wing and the fuselage. As I understand it, we generally call the wing/fuselage angle "incidence" and the wing/HS angle "decalage". For the purpose of this discussion the two are pretty much interchangeable. Differences between the two ultimately determine the resting pitch attitude of the fuselage in flight. For example, crop dusting airplanes are built so that the fuselage sits quite upright to improve forward visibility while spraying. The angle between the fuselage and the wing is Bob's domain and it is fixed, so we can disregard that. But the ultimate final thing that matters for your question is the decalage, or the angle between the wing and the horizontal stab, and that can be adjusted with shims where the HS assembly bolts to the fuselage.

The way that I adjust this is to establish cruise flight at a single CG and power setting of my choosing. Once established, I look back at the tail, and see if the elevator is deflected relative to the horizontal stab. If it is deflected nose-up, then I would lower the leading edge of the stab, or vice-versa. The goal is that in my most common load and power setting, I would like for there to be zero elevator deflection. At any speed less than that, the elevator will be naturally deflected nose-up in unaccelerated flight. If I'm going faster than my ideal speed, then the elevator will be pitched nose-down in unaccelerated flight. The amount of deflection will vary based on how far forward or back the CG is. The farther forward the CG is, the more pronounced pitch trim (and, elevator deflection) changes are with speed. What I'm trying to do here is to maximize cruise flight performance and efficiency by minimizing the hinge-line turning of the air, even if it is only in my one single optimum configuration. If the elevator is not deflected, we have a more efficient tail surface, probably approaching something like the full-flying horizontal stab that is used on some other GA airplanes. My understanding is that Bob did not select that for its complexity and structural implications, but I'm not speaking on his behalf and may be wrong about that. Suffice it to say that he knew about that option and selected the one we have.

So, does changing that angle impact elevator effectiveness? Maybe. If the leading edge of the HS was lower, you could get the same number of total pounds (kgs) of tail down force with fewer degrees of up elevator deflection at flare speed. If you wanted to optimize the airplane for elevator effectiveness, you could do that, but, you'd also be flying around in cruise with your elevator deflected nose-down all the time. It just depends on the mission goals. If you were making one of those ridiculous stol competition airplanes that is only good at takeoffs and landings, then you aren't worried about cruise efficiency. But for me, the slight gain that might be realized in the flare is not worth the loss of cruise efficiency. I'm flaring for a few seconds each flight, and cruising for many minutes. It doesn't matter to me if I can land in 500 feet vs 450 feet, but if I'm loosing a knot or two of airspeed, that gets very expensive at scale. If the goal is a well-rounded airplane, I would look to the other methods to improve elevator effectiveness before allowing this adjustment to be dominated by anything other than cruise speed, especially because those other methods are highly effective and available.

If any of that doesn't make sense or seems wrong, let me know so we can talk more about it!

[svyolo]

Was there a measurable increase in cruise speed when you altered the decalage?

[jaredyates]

Excellent question, unfortunately I don't know. That was the last plane and I didn't collect data. It is entirely possible that I'm overestimating the impact.

[Nev]

That's an excellent explanation and a great contribution there thanks Jared. Yep we used to optimise for CG at work, and the 747 had a 10 ton fuel tank in the horizontal stabiliser to help with that.

I guess one way I am viewing this discussion is along the lines of the pitch authority during an engine failure at forward CG. Currently I would need to maintain a minimum of 60 kts into the flare in that scenario. In Bearhawk's with an IO360, the approach speed can be safely reduced while still maintaining pitch authority, due to their forward CG limit being more favourable.

Very interested to hear of others experience and whether others have changed the incidence of the horizontal stabiliser.

[jaredyates]

Back when we were testing 619MS, I learned that Bob sets his forward CG limit based on there being enough elevator effectiveness to flare without any help from the propwash. By his method, you would be forward of the limit. I would look to equipment relocation or reconfiguration if the various ballasting strategies are off the table.

[AZBearhawk272]

Jared,

You have captured well the dynamics of the situation. My adjustment was made to accommodate my forward CG with min fuel and my low mass body.
I do carry some ballast of survival gear, tools and water to move the CG aft for normal recreation. Full fuel or 2 on board there is plenty of elevator to flare.
Interesting that for normal operations I have found the sweet spot to be 60 kts right into the round out and flare..........

Kevin D
N272DG

[svyolo]

One of the changes to the "B" model was less incidence in the stab. 3 degrees less if I remember right. Adding that back might increase trim drag. If you are flying around on big Bush Wheels or floats, that increase in trim drag may not be that noticeable. Who knows, for some it might be a good tradeoff.

I built mine as nose heavy as I could without purposely adding any weight, just keeping everything I could out of the tail. It worked out good, I am at 7.1" cg. I always planned to carry ballast. Toolkit, survival/camping gear, and plenty of water. I will dump the water if I need to use the weight. I did most of the first 40 hours at around 13" cg, no VG's of any kind or gap seals, and so far have not had pitch issues in the flare. I sitll need to do heavy weight and aft/forward CG stuff before I take my plane out of phase 1. I would expect the same pitch issues at forward CG.

[Battson]

The reason for the change in incidence in the model-B is the aerofoiled profile horizontal stabiliser. This mod is often retrofitted to original 4-place, if a profiled stabiliser is installed. I have this mod, and reduced the incidence about 1 degree.

While the 4-place does need a LOT of up elevator during a slow approach at forward CG locations, it's easily controllable provided there's prop wash. This issue is common to aircraft with a wide CG range and the same elevator trim tab design. A fully trimming stabiliser would be the ideal solution, but I have only seen a couple of people who were looking to retrofit.

With the CG as forward as I can get it, flying as slow as I can (under 40 knots), the elevator has complete control (albeit limited travel) with just a tiny trickle of power. It's more than enough control.

If you close the throttle, the nose will pitch down beyond control range. This is still easy to control with throttle.

I have never wanted more from the aircraft. Nothing is perfect!

[Nev]

Slight thread drift here, but on the subject of the kitset evolution, I had the opportunity to view a brand new 4-place kitset during the week. There are so many neat factory improvements to the kit, including new seat rail design, ball bearings on the seat rails, rod end bearings on both ends of the landing gear struts and pitch trim push rods, miscellaneous steel tabs on the fuselage, additional seatbelt harness attachment points, a plethora of match drilled rivet holes on the boot cowls, redesigned fuel selector and gascolator position, and redesigned flap handle position with new cable pathway. I'm sure there were more too that I didn't immediately notice.

The kit is continually evolving, with Virgil and the team eking out incremental improvements to help streamline the build experience.

[predragvasic]

The kit is continually evolving....

Makes you want built another one!