Nose "heavy"

[Skippydiesel]

My Sonex/Rotax 912 ULS requires constant back pressure (up elevator) to fly straight & level.

I have checked the aileron/flap settings and all are within factory specifications - an ability to adjust the horizontal tail, reduce angle of attack, would correct the problem but I cant see how this might be done.

Any suggestions?

[Bryan Cotton]

What is your CG?

[Skippydiesel]

Hi Bryan,

W & B data:

Aircraft datum - Wing leading edge
Aircraft moment index - 105935 Kg.mm (5932086.in)
Aircraft empty weight - 340 Kg (749.572 lb)
Aircraft empty weight C of G - 311.34 mm (12.258 in)

[Bryan Cotton]

I converted your numbers to work in my spreadsheet. The wing leading edge is 52" aft of the standard Sonex datum, the tip of the spinner. If I assume you are 150 lbs and have full fuel, that puts you at 64.38" which is within the CG range, although forward. My worst case forward CG at my weight (200 lbs) is about 65.12". I am working at getting to 170 which would put me at 64.75". At the 65.12" I'm not seeing any trim issues in cruise.

Do you have trim installed? I need to trim throughout the speed range.

[Skippydiesel]

Spring type longitudinal trip - Full Up for duration of test flight(s) - Stick back (& heavy) for most of flight.

[sonex892.]

Skippy. Your non standard fuel system of main fuel being in the leading edge close to the c of g is quite different to the normal sonex. It wont alter the c of g as much as the standard fuel tank does. That is actually probably a good thing. With the standard tank there is a significant trim change required from full to empty. Providing yours is within normal c of g range it shouldnt be a problem and you will never need as much forward trim as the normal sonex requires with a full tank.
With the spring trim, I would simply experiment and add more tension to the system by either changing or shortening the spring to get more nose up trim. Personally I prefer the trim tab and adding that could be another option. The only time I run out of trim is at low speed. On landing I always go to full aft trim, its still never quite enough but not enough to be a problem.
Steve

[Skippydiesel]

Hi Steve,

Trim (as in the using spring or tab methods of maintaining the position of a control surface) is, I believe, not the issue. The problem is having so much up elevator (stick back) all the time. The effect is to load the main lifting surface/wing, creating drag, using energy , slowing the aircraft. When in cruise, I would like to see the elevator level with the horizontal stabiliser, only coming into play during ground opps, take off, climb out and slow flight

[GraemeSmith]

Elevator anti-flutter bungee in place? It applies some back stick pressure.

[T41pilot]

My "B" model nose pitches down right away if any flap at all is applied. You might check your flap rigging again just to make sure.

[Bryan Cotton]

My "B" model nose pitches down right away if any flap at all is applied. You might check your flap rigging again just to make sure.

Good point! My classic A model does the same.