Nose "heavy"

[Skippydiesel]

Hi Scott,

The trim tab you are referring too - FIXED or INFLIGHT ADJUST ?

[Skippydiesel]

I still don't seem to be able to get any comment/conversation on the aileron matter - why the reluctance?

Given the span of the ailerons, even a small change in adjustment (10-15mm up) would (I would think) have a significant impact on what I call trim (may be "attitude" is better). Any such adjustment will change the up:down movement ratio - what might this do to handling?

This Sonex is currently in its TEST FLYING phase. I would expect all manor of "tweaking" would be required and anomalies addressed. All I am trying to do here is aim for the best level of trim (flight tuning) that this Sonex can achieve (with all its idiosyncrasies) in the hope that it will be an nice (not too challenging/tiring) economical, cross country tourer.

[Dave Wolfe]

Most aircraft need nose down trim at high speeds and nose up trim at low speeds. Yours seems opposite to that which raises my eyebrow.

As speeds go up, so do control surface forces. You are finding your trim springs are sufficient at lower speeds where control surface forces are lower, but at higher speeds the elevator aerodynamic centering forces are overpowering the springs.

You need to get the aerodynamic forces more in balance. You can add a fixed elevator trimtab that deflects the elevator upwards, or you can adjust the incidence of the horiz stabilizer by moving the leading edge downwards compared to the trailing edge.

[Skippydiesel]

Most aircraft need nose down trim at high speeds and nose up trim at low speeds. Yours seems opposite to that which raises my eyebrow.

As speeds go up, so do control surface forces. You are finding your trim springs are sufficient at lower speeds where control surface forces are lower, but at higher speeds the elevator aerodynamic centering forces are overpowering the springs.

You need to get the aerodynamic forces more in balance. You can add a fixed elevator trimtab that deflects the elevator upwards, or you can adjust the incidence of the horiz stabilizer by moving the leading edge downwards compared to the trailing edge.

Yes Dave - it would seem that my horizontal stabiliser is generating a bit too much lift, relative to other forces acting on the aircraft, hence the need for aft stick/trim to overcome this.

If you read back on this thread, this was most pronounced when I first put the aircraft into the air. Reflexing the flaps up a little, has dramatical changed things for the better (to about 120 knots).

Some of this improvement is lost at higher speeds above 120 knots.

The ailerons trailing edges, are notably lower (10-15mm) than where the flaps are now.

I understand about putting a trim tab on the elevator but consider this to be a last resort (accepting an uncomfortable compromise).

I also understand that it may be possible to change the horizontal stabiliser angle of attack (big job) to a less lift position, by raising the trailing edge edge, thus reducing the lift forces at high speeds (may also negatively change the stall characteristics).

I am trying to find out if adjusting the ailerons to match the flap angles may (1) further reduce the aft stick/trim required ? (2) what impact this may have on flying qualities?

Can you help ?

[Onex107]

I would like to add two cents to this discussion. My experience is with a Onex w/ Aerovee and a nose wheel. But, I didn't like the way the tail moved around in straight and level. Elevator trim was not a problem using the Sonex system, but the tail did not want to stay in line. So I evened out the air flow on the rudder/vertical stab by adding a gap seal to the rudder hinge line. That calmed the tail down so well that I went ahead and added gap seals to the bottom of the horizontal stab/elevators also. On the first flight I had to turn the trim down about two turns to go back to hands off. I don't change the trim when landing so it was set to the previous cruise setting. I have to believe that I gained a lot more down force with the gap seals. Besides, they look a lot better too. It's very easy to do, only adds ounces of weight and if you don't like them only two rivets holds everything in place.

[Skippydiesel]

I would like to add two cents to this discussion. My experience is with a Onex w/ Aerovee and a nose wheel. But, I didn't like the way the tail moved around in straight and level. Elevator trim was not a problem using the Sonex system, but the tail did not want to stay in line. So I evened out the air flow on the rudder/vertical stab by adding a gap seal to the rudder hinge line. That calmed the tail down so well that I went ahead and added gap seals to the bottom of the horizontal stab/elevators also. On the first flight I had to turn the trim down about two turns to go back to hands off. I don't change the trim when landing so it was set to the previous cruise setting. I have to believe that I gained a lot more down force with the gap seals. Besides, they look a lot better too. It's very easy to do, only adds ounces of weight and if you don't like them only two rivets holds everything in place.

Very Interesting! Onex107

My last aircraft used, from memory, two hinge points per moving surface, with a long narrow gap between wing edge & moving surface, so the gap seals that I installed, cleaned up the air flow quit a bit, especially at low speeds/high angle of attack. She was fully controllable right through her sub 30 knot stall and could pick up her skirts to achieve 120 knots indicated at 500ft ASL all on Rotax 912ULS power/fixed pitch prop.

I would have thought that on the Sonex, the full length/width piano hing on all moving surfaces, would have acted very much like a gap seal - you say not.

Any chance you would post some photos?

[Dave Wolfe]

So you want you elevator to be a lifting surface like a canard rather than a traditional elevator that provides downforce?

This will kill you! I wont help you achieve this. Conventional aircraft need downforce on the elevator to achieve pitch stability.

There seems to be a fundamental misunderstanding of aerodynamics going on here. The elevator MUST provide downforce in flight, this is a fundamental part of conventional aircraft design.

The way you make the elevator a lifting surface is by moving the center of gravity way aft of the approved limit. Then you have an aircraft that is unstable in pitch, unpleasant to fly, and WILL ABSOLUTELY KILL YOU AND YOUR PASSENGER!.

This is why people are hesitant to talk about aileron position with you. Because theres another huge elephant in the room not making sense.

If you suddenly stop posting here, Ill look for your tail number on kathyrns report.

[Scott Todd]

Dave is SO right. If readers go back thru the thread, a few of us have used the proper engineering terms to describe whats going on but I think it keeps getting missed. The horizontal stabilizer is NOT generating too much lift. Just like its NOT nose heavy. The plain simple fact is it has too much down thrust due to mods by the original builder.

In the time it takes to respond to these posts you could have turned those aileron balls a turn and tried it out. That's what a Test Pilot/Engineer would do during Flight Testing. That's not really my suggestion/answer but I'm just saying....And as soon as you do that, you are changing the effective airfoil of the wing so other things will change. Welcome to Aircraft Design and Test Flying :)

FWIW, the gap seal doesn't really change the down force. It changes the stick free position of the elevator which changes the airplane pitch trim. Its basically doing the same thing as a fixed trim tab. It changes the elevator effectiveness, reduces drag slightly, and obviously looks better :) But adding them should involve going back to Phase 1 and doing some extensive testing.

I'm headed to airport this morning. I'll take some pictures of trim tabs.

[dbdevkc]

I don't recall you stating if you modified the thrust line to compensate for the modification of the wing/horizontal tail angle of incidence. If not, I could see how that by itself could cause 'nose down' force. And another thought although it might not have any effect, the Sonex fuselage shape could be considered a 'lifting body', so the builder's mod also changed the angle of attack of that lifting body.

After all this thought, discussion, debate, and effort... I personally would have begun thinking about what it would take to make the modifications to the aircraft to return the wing and tail angle of incidence back to plans/factory spec. I would think it is fair to say that is a rather significant departure from the plans with potential unforeseen effects, despite what logical understanding of the aerodynamic forces involved might indicate.

[daleandee]

Thanks to Kevin & Dave for shouting the warning that needs to be applied here. I've followed this thread with amazement at what I don't see.

When I reported about the Corvair install on my Sonex I was excoriated by quite a few people as this being the way to kill myself and bring shame upon the fine line of Sonex aircraft. Even though there was much thought, engineering, & empirical evidence as to the safety of what I had done there were many voices that came and shouted the warning (even though I gave several disclaimers during the thread itself).

Where are all of those voices now?

Read the thread and see:

viewtopic.php?f=9&t=4273&hilit=justification

Larry also pointed out the concerns that many have with the Aerovee engines in the thread "Sonex High Wing".

But here we have a Sonex that was built with admitted changes that make the airplane unstable and possibly a danger to both pilot & passenger.

Why hasn't Sonex themselves raised their voice as to what is happening here? They sure were available when Corvairs were in the discussion!

It's just hard to understand all the vitriol over a Corvair engine on a Sonex (which has been proven to work for many years with multiple examples flying) and no one is screaming the warning about this unstable aircraft that was built and purposely changed from the plans and now appears to be exhibiting some dangerous characteristics.

This is nothing against Skippydiesel as he seems a really good guy but builders and pilots on this forum should be forthcoming about the degree of danger that is happening here as Kevin & Dave have done ...

My $00.02 and change ...

Dale
3.0 Corvair/Sonex Taildragger