Nose "heavy"

[Skippydiesel]

In the not too distant future, I will start using the in fuselage tank/header. This tank, in the Sonex standard tank position, is able to take 35L on ground and potentially 40 L in the air (transferred from wing tanks). This forward load will significantly change the weight distribution - it may be that high speed cruise is not usable, unless I load the baggage compartment.

One reminder point - in an effort (successful) to correct the previous nose heavy condition, the flaps are now slightly reflexed, compared with the ailerons. This is not obvious on the ground but in flight can be seen quit clearly.

[Skippydiesel]

Just had a random thought - The position of the elevator(slightly up) is remaining fairly constant (judged by stick position, slightly aft of centre) - the trim (spring) is just experiencing higher air loads (due to speed) which require more effort (spring tension) to remain in position.
I have already increased the strength of the adjustable (aft pull) spring with quit noticeable impact on stick "feel" so would be reluctant to go stronger again.
The fixed (fwd pull) lighter spring has been adjusted so as to have no pull at full aft stick. It would be impacting on the adjustable spring but not much.
My last aircraft also had spring loaded elevator trim, unlike the Sonex, both springs were adjusted by the trim mechanism - I wonder would this be a better system?
At high speed cruise & when aircraft lightly loaded, I would like the up elevator to be neutral (0 trim up/down) but this may not be possible with my Sonex. I may only get neutral elevator when the baggage compartment is loaded.

[Skippydiesel]

Further musings;
My flaps are slightly raised/reflexed. This was done to address the slightly nose heavy control stick and it worked, at sub 120 knot speeds.
As a consequence of the flap adjustment the ailerons are now slightly down, compared with the flaps (roughly 10-15mm) in level flight.
Might adjusting the ailerons to match the flaps be the solution to my high speed problem?
What would be the effect of such an adjustment on other areas of flight eg stall performance?

[dbdevkc]

I would wait for an answer from someone more experienced/intelligent than me, but... my thought is flaps add both lift and drag when deployed. You can fly slower, and your stall speed is lower. I would guess that when flying with negative flaps your stall speed will be higher/faster. I fly a sailplane that has full length flaperons and I go to -5° when flying fast - that decreases drag somewhat. A difference though is that the ailerons deflect with them so I don't know what the characteristics of negative flaps would be in a Sonex.

[Scott Todd]

We already know the root cause. The wing and tail incidence change from the original builder added effective down thrust. The more power you apply (faster), the more it pulls the nose down.

There are a few simple fixes. Trying to correct the thrust line will just add more variables at this point. Adjust the trim springs as suggested. Add a fixed trim tab to the bottom of the elevator has also been suggested. Don't get hung up on the elevator trailing edge being up during flight. Again, this is a function of the down thrust.

Stop referring to 'nose heavy'. The proper term is something like 'heavy aft stick force'. Nose Heavy implies weights or static forces. This entire situation is dynamic and CG doesn't have anything to do with it. Changing load conditions can obviously affect the trim but its NOT nose heavy.

Once you get the proper trim springs adjusted, or a fixed tab adjusted, all speeds should be trimable.

[Skippydiesel]

Hi Scott,

I take your point on using the term "nose heavy" - it does however describe the symptom, which as you say, could just as well be described as stick heavy.
Dwelling on the subtle changes (I previously said 1.6 when it should have been 1.5 degrees) to the aircrafts main & tail alignment, wont get me further ahead. Nor will dwelling on semantics (although I do agree precise language is desirable).

I now have a flying Sonex - after many small adjustments it is becoming less challenging to fly, however needs further "tweaking" (scientific term) to make it into the nice flying aircraft that most pilots enjoy.

To address your suggestions ;

Thrust line - if I understand (?) what this means, I have never gone down this track, relying instead on the integrity/correctness of the Sonex design.
Trim springs - I think I have gone as far as is sensible in this area. The (pilot) adjustable spring is now much stronger than the fixed. Despite this, I am at full aft trim adjustment at 130+ knots. The aircraft is straight and level at this point but I do not consider this to be an acceptable situation. In this condition I will have insufficient rear (up elevator trim) when I fill the in fuselage 35-40 L fuel header tank . Also this need for continual tail down pressure/thrust is "loading" the main wing, creating additional drag, an undesirable situation in any aircraft.
Fixed trim tabs - will always be the last option in any adjustment that I make. As far as I am concerned, they should only be used when all other fine tuning of the flight surfaces have been considered/tried.

You have not commented on the aileron/flap relationship -

If the flaps (slightly up) have lightened the stick force, will not bringing the ailerons into line with the flaps, further improve this trend?
What might be the effect on other areas of the flight envelope?

[gammaxy]

Thrust line - the trouble is the builder of your airplane already went down this track by rotating the wings relative to the thrust line. Exactly how much affect, I really don't know. I'd experiment with adjusting the throttle setting significantly at high speed to see if the nose tends to rise or lower when you increase throttle. If it lowers, that's probably a good sign your thrust line is low. Maybe others will share what theirs does. I suspect my nose rises, but would have to do the experiment to confirm, but from memory it seems mine yaws a little and the nose pulls up a bit, but I haven't given much thought to it in a while.

Trim springs - I'm not convinced you've really gone as far as sensible, but without actually seeing your installation it's hard to speculate. I haven't heard of anyone else with this difficulty and the stick forces in the Sonex are pretty light. I feel like I could trivially add way more up trim to mine if I wanted to. You seem to have no problem trimming it by hand and the elevator is close to centered so it's not like there's crazy forces involved. There may be a balance where you can't simultaneously satisfy the highest speed and lowest speed simultaneously, but it sounds like you're pretty close already. Curious if your installation and springs were the same as everyone else's.

Continual tail down pressure - This is just the way things are. I feel like your emphasis on this is misplaced. My explanation is a little simplified, but the torque of the CG being ahead of the wing's lift vector, causes the nose to drop. The tail produces negative lift to counteract this torque. As the CG moves backwards to be coincident with the lift vector, there's no longer any torque rotating the nose, so the tail can be streamlined with zero lift. As the CG moves aft of the lift vector, it tends to rotate the nose up and the tail can counteract by providing lift as you desire, but less efficiently than the wing would have. Somewhere around here, though, you start running in to stability problems where, when disturbed upwards in pitch, the wing provides more nose up torque than the (comparatively smaller) tail immediately counteracts. There's some window where the time constant is long enough that you can still maintain control, but this is way outside of the published CG range. If you wish to keep moving the CG aft, you need a larger and larger tail. Eventually you have tandem wings, and as you keep moving back you end up with a canard. Due to the nature of 3d fluid flow, there's efficiency tradeoffs in all these designs. The continual down pressure "problem" you are trying to solve is simply a feature of choosing a design with a small tail in the back and operating towards the front of the CG range.

By streamlining the elevator behind the horizontal stabilizer like you really want to, you are probably slightly reducing drag, but you won't be changing the fact that the horizontal stabilizer is still producing negative lift--it is not neutrally aligned into the relative wind. The only way to get the control surface streamlined is to change the incidence of the horizontal stabilizer, fly with the CG further aft, or drastically affect the pitching moment of the rest of the airplane to compensate, probably wasting any efficiency you're hoping to gain. The most efficient way to produce pitching moment is at the tail (canards on the cowl would be cool though).

Adjusting pitch trim with aileron/flaps--this is probably the least efficient way possible to change pitch trim unless you're flying a flying wing and you have no other choice. You're effectively rotating out a fraction of the 1.5 degrees the builder built into the wings, but you're going to have to change the comparatively smaller control surfaces by many degrees to have a noticeable impact.

Still seems clear that the only things to do are figure out the dial-a-trim, install a small fixed trim tab on the elevator, or install the adjustable tab.

[Skippydiesel]

So Chris, no observations on the ailerons being slightly lower than the flaps???

AND

Might raising the ailerons, to the same level as the flaps, lift the nose slightly at above 130 knots????

If I was to do this the movement ratio up: down would change - would you care to speculate on how this might effect handling????

As things stand, the aircraft can be put into a shallow turn, without rudder, and the ball remains centered.

[Bryan Cotton]

I used to own a 1-35C glider. You could reflex the flaps for cruise but it lowered Vne. This is because it moves the lift outboard on the wing and gives higher gust loads at the wing root. Schweizer had an optional system that would also reflex the ailerons. Then you got your Vne back.

I'm not saying this will fix your woes. My Waiex is stock and I can trim it through the entire speed range. Stock is really the way to go in my opinion.

[Scott Todd]

I have over 1000 hours in Kitfox's, most of it in Classic 4's. The models use flaperons to trim pitch. Granted, its only 100 mph but it works. Its EXTREMELY sensitive. I have also flown them with an added trim tab to the elevator. The elevator tab is much nicer and has better resolution for higher speeds such as the ones with larger motors and all the fairings. You can do it with the ailerons and flaps but I suspect it will be less efficient than doing it at the tail as Chris points out.

You are worried about drag when its just not that much. I'm a retired Aerospace Engineer, have over 2000 hours test flying over 40 years (BTW, NOT an airline pilot), have built 3 airplanes, and done first flights on 13 including all the initial trimming. Plus I've helped sort out flying issues on dozens of other homebuilts. Add a trim tab and you will be much happier.

I'll try too get out to the airport today and get some pictures.