Nose "heavy"

[Skippydiesel]

Last adjustment, of flap up, resulted in nil gain, will undo the last 1/2 turn. Still have about 100 mm up on trailing edge . I may have to consider weight redistribution (if possible) or a addition of ballast to tail.

[Scott Todd]

When a 'normal' or straight wing like ours makes lift, it creates a negative pitching (nose down). The horizontal stab produces a down force to counter this. Its simple airplane physics. Look it up. In order to create this downward force, the stab usually has a negative incidence compared to the wing. Many airplane designs don't account for this properly so some Up elevator trim is required. Sonex's fuselage also contributes to the pitching moment. John and Pete did a good job getting this harmony correct. Once the subject airplane wing and tail incidence changed, the overall pitch trim of the airplane changed.

The last Sonex I owned had a fixed trim tabs on the underside of elevator. So does my Little wood Biplane. Its a simple solution to a simple issue. The spring trim system in a regular Sonex does the same thing. It merely takes some of the stick force out to 'trim' the airplane. I adjust it to put the movable trim tab in its center for hands-off cruise flight.

I would suggest the airplane is NOT nose heavy. Look at the CG on high performance airplanes like the Glasiar and Lancair. They are in the teens and your Sonex is nowhere near that. We get concerned about nose heavy if we are running out of elevator at flare. The 5 Sonex's I have flown were not even close to this. PLEASE don't add tail weight without consulting with some experienced Aeronautical engineers. It can have detrimental effects including added difficulty in spin recovery.

[gammaxy]

Still have about 100 mm up on trailing edge.

Are you saying the elevator trailing edge in cruise is raised 100mm? Is this a typo? How do you measure it in flight? Looking over your shoulder? 100mm seems extremely excessive to me. Here's Michael Smith's video that shows it to be negligible. https://youtu.be/mh6Ys6MwTiY if yours isn't similar, there's something so messed up with your airplane it should be obvious. I'd question whether the incidence was really set as described. There's no magic to this--if the cg is in the ballpark and the theres nothing dangling off the plane creating a bunch of torque then your incidence is wrong. By the way, from your description, I assume the thrust line wasn't adjusted along with the incidence.

[GraemeSmith]

As Scott Todd says - the horizontal stab provides down force. It's an "upside down" flying wing. It 'lifts" the other way from the man wing that levitates the plane off the ground. Some of it somes from the angle of incidence. Some from "negative lift".

Spitballing - as I don't know. I bought a built airplane. Are the horizontal stabs actually airfoils? With one surface having more camber than the other? If yes. Anyvchance they got put on the wrong side and are "upside down" and generating "lift" the wrong way that is countering the angle of incidence? Like I say - spitballing.

--

And to reinforce what Scott said - if the plane IS in CG. It's NOT the weight distribution that is the problem. It's something else. Find the something else - don't redistribute the weight - failure to spin recover is a big risk here.

[Bryan Cotton]

Are the horizontal stabs actually airfoils? With one surface having more camber than the other? If yes. Anyvchance they got put on the wrong side and are "upside down" and generating "lift" the wrong way that is countering the angle of incidence? Like I say - spitballing.

Graeme,
Same ribs for both the left and right sides of my Waiex. I assume the pedestrian straight tail Sonexes are the same. So they are symmetrical, and not super-airfoiley. After the leading edge they are pretty flat.

[Area 51%]

If you're confident the wing incidence is correct, I would take a close look at the engine thrust line.

Full disclosure; This Sonex has had its wing/horizontal stabiliser, angle of attack increased by 1.6 degrees from factory/plans specifications.

Engine thrust line is defined by Sonex (factory) engine frame, with latest Sonex/Rotax "bed" adapter - I would hope that this would be well within design parameters.

Back to my original thinking......The design thrust line is set according to the wing's angle of attack. The design wing-to-fuselage incidence allows the use of the fuselage as the datum to set the thrust line. A 1.6deg. change in wing incidence also changes the thrust line by 1.6deg. In this case the thrust line is pointed down the 1.6 and must be compensated for by up elevator.

Question: Does the stick pressure relax if the throttle is retarded? If not, please ignore the above.

[Skippydiesel]

My apologues not sure how an extra 0 was added to my elevator up figure (100) - it should be 10mm (well under your 1/2 in).

How was this determined? - In straight/level flight, a small piece of board was placed on the instrument panel and the position of the control stick marked on it. On landing the stick was moved to the same position, as in flight and the elevator trailing edge position, in relation to the horizontal stabilizer outer "horns," noted (10mm up).

The same board was used to determine a further change to flap settings had no discernible benefit to elevator position.

Trim of any sort will not remove the 10 mm up - trim is used to remove (lighten) stick forces not change the position of the control surface.

Adding ballast - definitely a last resort. No one wants to carry around dead weight, but if here is no other way (eg moving the battery ) to get the elevator to a level with stabilizer position it may have to be done.

At this early stage of test flying ("tweaking") it would seem that at 135 knots the above elevator position is constant, reduction in power/speed will inevitably result in further up elevator as the pilot ties to keep the aircraft from descending (maintaining straight & level).

[Scott Todd]

As Area51 points out, the thrust line is now more down. This is why its a bad idea to change airplanes designs unless you REALLY know what you are doing. Its just going to require more nose up trim because of this. You could try shimming the stab but the net down force on the tail in flight is going to be roughly the same to compensate for the added down thrust.

You could try shimming the engine but I suspect this is a no-go because of the cowl fit. It would require some major cowl work to make it look natural again. But this of course just adds more off-design variables to the system.

Its probably only costing a few knots. You may just have to live with it. Adding ballast is not really a last resort. It should be considered a Non resort.

[gammaxy]

it should be 10mm (well under your 1/2 in).

Okay, that doesn't sound as crazy as what I was picturing :-) I suspect most of us haven't measured our elevator deflection to that level of precision, but it's an interesting data point. Your measurement seems reasonable to me and I question whether you'd be able to measure the drag reduction of getting it perfectly streamlined. Do you have your wheel pants and fairings installed? Those have some affect on the pitch trim, but I don't know how hard it is to measure. As you explore more of your CG range, it will be interesting to see what that does to your elevator deflection.

[Skippydiesel]

it should be 10mm (well under your 1/2 in).

Okay, that doesn't sound as crazy as what I was picturing :-) I suspect most of us haven't measured our elevator deflection to that level of precision, but it's an interesting data point. Your measurement seems reasonable to me and I question whether you'd be able to measure the drag reduction of getting it perfectly streamlined. Do you have your wheel pants and fairings installed? Those have some affect on the pitch trim, but I don't know how hard it is to measure. As you explore more of your CG range, it will be interesting to see what that does to your elevator deflection.

Wheel pants & fairings installed.

I would not be so concerned about the elevator up position, if the aircraft did not have a strong tendency to nose down, when stick released.

The spring type elevator trim just doesn't have the power to overcome the elevator pressure - when I flew yesterday I had full aft trim for almost the whole flight. The only time stick forces lightened, was on base/final with 2 stages of flap at about 60 knots. Yes I can further adjust the spring lengths (or install heavier springs) but even if I get it to "hold" in flight this will just be masking the problem.

It seems to me that in straight level, high speed cruise, when not carrying baggage, the elevator trailing edge should be level with the stabiliser trailing edge.

I am seriously considering changing the, nearly 12kg Airmaster CS, prop for lighter ground adjust or inflight adjust.