Hello guys,

I have a question about airplane longitudinal stability in those common foamy with compensators built into elevator. For compensator I mean that control surfaces that extends ahead of the hinge line - i.e. the control surface found on nearly all the 3D planes. I don't know how they are called precisely... I hope you understand!

With my foamy and other small 3D planes I notice a weird pitch behaviour: they cannot be trimmed for level flight. In other words, this is what happens to me.

You trim it for level flight. Ok... but after some manouvers, you level the plane at the same throttle and speed, and now it dives slightly. Ok, let's give a little up-trim... and after some time you'll discover that it has a tendency of climbing up slightly...

I assure you that the CG is not the cause - I fly pretty nose heavy. So, after some inspection, I found little play in my linkage, so I fixed it. Now the wear is gone, the surface is solid, and the little play is only due to the geartrain (VERY small, almost nonexistant). The problem still exists though, even if it is of very less entity.

So... I arrive to the question: can that compensator be the cause of this little unstability in pitch? Remember that I'm referring to HS-55 servos, they are not the best as far as precision goes... What I'm thinking about is that the compensator can destabilize the control surface.

A side question: the fact that they are not used in pattern airplanes, makes me think that they are the cause.


Let me know what do you think!

well --sorta
if the world were perfect there would be no problem with those little things - called aerodynamic counterbalances.
The idea is to reduce the force it takes to move it (the elevator) and to increase effectiveness of the elevator (bigger is better).
You end up with less stabilizer and more movable elevator That means the force to hold the movable part must be very precise. which in the case of most small servos (10 15 inch ounce) is not very good. also hinging must be tight and the surfaces should not bend.
Pattern planes don't have because up till recently they need only very small elevator power - basically flying on big trim tabs.

The only thing I will add to Dick's comments is hat if the horn is too large, you can experience "snatch".

The term comes from full size aircraft when, with too large a horn on the control, a slight deflection can result in the horn moving the rest of the control surface, thus "snatching" the stick out of the pilot's hands.

The servo is not going to let go of the control surface, but too large a horn may cause a twist in the elevator (in either direction) which may be the reason for what you describe.

Ok guys, thanks.

Dick, referring to pattern planes, some time ago ZN produced a plane (Extreme) with counterbalance (thanks for the definition!) on rudder... but on news planes that has been removed.

So we can say that the control is more precise without the counterbalances, even with powerful servos?

I would add another thing.

I think the issue depends on counterbalance surface - or the ratio between surface area and counterbalance area. I say this because that weird behaviour happened to me even with a .40 3D plane, which I tried today. It had the same problem, and the counterbalance area of the elevator was quite big with respect to elevator area...

So with a better mechanical advantage the problem might be reduced?

If you check the control system and there's no slop in it then what you are experiencing is not related to the elevator but to the flat airfoils.

Flat plate airfoils create a separation bubble just behind the leading edge on the low pressure side of the wing. The overall airflow passes over that stagnant bubble (think of it as a back eddy in a stream) and then sticks back down to the surface behind the bubble. So the bubble is actually becoming a flexible part of the airfoil shape. As the angle of attack changes the bubble changes too. But at higher speeds and lower angles of attack this bubble becomes very small and tends to bounce from the upper to the lower side of the wing. This really messes things up as you can imagine and the model will seem like it's hunting up and down as the bubble snaps from one side to the other in flight.

The pizza box flyer guys notice this all the time if they try to fly too fast. It can also be exagerated by the flexibility of the wing. And the PBF's and flat foamies DO have fairly flexible wings.

If you slow down and fly so that the wing has a more or less easily noticable angle of attack you'll probably find it's fine. Only if you go faster does it show up.

PS: I seem to remember reading that the area ahead of the hinge line should not be larger than 15 to 25% of the area behind the hinge line. If it gets to be more than that you will have problems with the balance area trying to fly the surface.

Very interesting Bruce!

Anyhow, my foamy has a profile wing [&:] ... the stab and elevator are flat though... this could apply to them?

"stick snatch" was a nasty thing that happened when the elevator effectively "self servoed " that is you added an input and the thing continued to move in the same direction - overly large counterbalances can easily cause that
Some yokels on their large Extra models , radically increased the counterbalance area -to gain more rudder effect - BAD idea .
I saw one go into lock to lock throw and the pilot did not shut down fast enough ----ka boom!
same idea.
the flat airfoil out of foam is not good at speed-- it bends too easily BUT for rapid fire direction reversals and just fun dodge n dart aerobatics -it can't be beat. one outstanding effect is that you can rapidly pitch to vertical whilst killing power and it simply stops - low mass -- another one is the blindingly fast snap rolls it can effect even at low airspeed
If you ain't into this you will likely dismiss the flat foil as "quaint".

If you are experiencing this on a foamie, check your hinge tape. Did you use the fiber reinforced, or thick clear tape? Some of the brands tend to not center well. You get it all trimmed, and then you give up or down, and it wont go back to exact center. It will always favor the side you just gave. Example- You trim it for neutral and then give full up elevator. The servo will then go back to center and the control surface will stick up just slightly. I do mean SLIGHTLY.
Try 3m "scotch" tape. The really clear stuff with no fiberglass.

Dick, it's that sudden braking effect that seems to provide the Flat Foamies with their appeal for me. I had watched a lot of videos of the larger profile 3D'ers and the flat foamies and found that "something" was missing on the larger ones. Then it hit me. The FF's just STOP when you flick them up while the larger airfoiled 3D stuff tends to just loop very tightly unless care is taken to get it right.

Stek, re-reading your original post again I'm thinking the guys that are saying you may have a "wandering neutral" are right. Typically a 3D model is set with the CG right on the neutral point or super close to it. In fact many of the Flat foamies run the CG behind the neutral point by a little bit. But this all means that there is no built in pitch stability at all or, in the case of the CG behind the NP a tendency to diverge from the flight path.

For starters I'd use some form of indicator needle to check that the elevator returns to a postive nuetral within less than 1/64 at the trailing edge from both directions. I suspect you'll find that you don't have an accurate enough return to neutral.

From the sounds of it this happens at pretty much "regular" speeds so it's not that likely that you're getting any speed related flexing. But if your trim change shows up as the speed changes then it may be that you have some speed related twisting happening. Or you may have some speed related pitching variation happening due to being on the ragged edge of the CG placement.

I think with the large throws on the foamies the servo neutral deadband is much more exaggerated. Having a ball with a pizza box type homebuilt with about 60 degree throws and has between 1/8 and 1/4 in. of play at neutral. but these type of planes don't stay at neutral very long anyway

Guys, thank you all for the suggestions.

Anyhow, my foamy is a bit different. I set it up for a pattern style flying, not 3D. I used 3M scotch as my hinges, and it works really GREAT. No single corner has detached since when I put it. Regarding elevator linkage histeresys, I have to check but it is definitely out of consideration: hinge is solid and my elevator has 10° of throw! CG is well forward: I have a mix for hands off downlines, otherwise a pitch to the canopy will occur - well forward the neutral point I guess!

The strange fact is that it has absolutely no play, it flexs a little due to the thin linkage, but no free play. That linkage is the best I have on my plane! My ailerons have some amount of play, the rudder also, but the elevator is absolutely ok. I was surprised when I saw that the elev servo didn't have any play! I've been lucky here!

Anyhow I'm speaking about very small trim differences: I put it hands off at half throttle at the beginning of the field, and I watch it flying for several dozens of meters...

Perhaps am I demanding too much? [&:] I'm a pattern guy...

A side note: last time I flew (yesterday) I lowered the trim amount on my 8U Futaba, now it is the min. I have to try it! I don't know if HS-55 are precise enough to keep such small trim differences though...

Bruce, I was thinking about what you said, about counterbalance surface.

Here is a link with some pics of my plane (this is not a site of mine) , there is a picture from which you can see the amount of counterbalance on the elevator:

http://spazioinwind.libero.it/estival/aero/arlechin.htm

What do you think?

Guys,

I've checked elevator centering with my throw meter, it is perfect! I have 9.5° of up throw, 11° of down throw!

I tried to maximize mechanical advantage, even though I have a switch for 3D throws - not 45° stuff though, I biased it towards precision: I think I don't reach 25° deg of max throw !!!

If you want max performance from th flat foam -in a "pattern" type flying - use 6mm foam for stiffness
the Shock flyer type stuff is for adventure flying
The FAI Shock Flier is prety good in dead calm with GOOD servos and proper hinging and linkage but this is not what these models were intended for.
we have gone thru easily-- 100 small foam models this year -(Guy and I) of all types from 4 ounce all up to 20 ounce all up - sort of a "crash" program .

Nice airplane Dick!

Mine has all surfaces of 6mm depron thickness. The fuse is made by 3mm depron, but it is very stiff due to its "square" construction.

What kind of servo do you use Dick?

servos -- a million of em!
so far --the JR 241 and the new servo by EFlight-EFLRS75 seem as good as any - these are about 17 in oz torque.
Some others we have tried are also good but so far I have seen none better.

Ok I have a Parkzone Typhoon that is very unstable. I did change the servos to Hitec 81MG due to stock servo stripping. No matter what I do I can't get this thing to level out. Any help is welcome. As I pretty new to RC planes some of the lang is new. but I did learn the best way to learn is ask. Thanks

Very unstable??

Where is your CG? And where is the suggested one?

The more aft it is, the less stable the plane is.

Stek79, is the horizontal stabilizer on your airplane in the same horizontal plane as the wing, i.e. directly behind it, not above or below (or very close)? I only ask because I don't remember anyone bringing up that possibility.

On these little buggers -99% of the problem is pitch stability ( tail heavy)
OR
just plain ol slop and wiggling and bending. any or all of these are common here.
Trying to get a good setup which is very controllable with a lot of power can be a tall order
Start with as much sevo travel as you can get --which is hooked up to give very little control surface travel
ideally the system will have a lot of resistance to flex or wiggle if you try to move the surfaces by hand
(be careful)
Then -if you hold the model and simply move the trim levers on the TX--- you should be able to see the surfaces move -within 2 -3 clicks of trim either direction.
On large models - I setup for one click sensitivity -this is likely not possible with your setup -just the nature of these tiny buggers.
The CG must be slightly "nose heavy - then learn to fly it -- expo is very handy but improperly setup - you will not like it
first try the setup I suggested.
use just enough power to make it feel stable
save the theatrics till you get a feel for control at lower speeds.
this is where computer flying and the real world part ways.