Flapless Trim Question

Current advanced aerodynamic thinking on this subject came up with tilting the stab/elevator about 1-2 degrees inboard side low. This was discussed between, Igor Burger, Ted Fancher, Brett Buck and a few others in a previous thread. I think Igor is the only on that has actually tried it on a test plane. E-Mail him and check out his results on the subject.

When flying in a straight line, the lines bow behind the airplane and so the leadout exit point is adjusted to follow the natural curve of the flying lines so that the plane can track properly. This works whether going straight and level, or straight up, or straight down, or whatever. But when we "pop" a corner--most noticeably the lower right coner of the square and triangle maneuvers--we often notice a "banging" in the corner as the outboard tip drops on an untrimmed plane.

Now there are several other "fixes" for this, including removing tip weight, but these aren't solving the cause of the problem. If you imagine the airplane coming straight down in the "square" and approaching the bottom right corner, the lines are raked behind the path of the airplane which is in the "upward" position. Now, as we snap the corner, the airplane turns before the lines have a chance to catch up with the path of the airplane. It is the "upward" pull of the lines that causes the outboard tip to dip. By adding more flap area to the outboard flap--the closer to the tip the better--we are able to give added lift to the outboard wing under these conditions to compensate for the "lift" of the inboard wing caused by the lagging lines. Round loops will experience a must less noticeable effect of this lag, but, then, we give much less control input for the round loops as well.

However, tilting the stabilizer will not help this problem. Your idea of a tab on the elevator might, but it will be far away from the center of lift and might cause some other strange things to happen as well.

One thing, you could try it. If you don't like it, cut if off. But report to us on your findings. (A tab that you could bolt on or take off would probably be better as it would allow you to switch back and forth to see just exactly what effect it does have.

Leonard Neumann

There are effects of the lines, but I think you have a disproportionate view of this particular effect. The effect you describe is that that the lines have to follow a 90 degrees of a ~1" radius circle over the duration of the corner, since they exit behind the pitch axis. This is true, but this little bit of line movement over the period of 1/4 second is pretty much negligible, particularly since we don't usually do much to make sure the geometric Y axis is really a principle axis. It could easily be off by 1/2" at the tip, and that's some serious product of inertia. The majority of the line effect is actually in the opposite direction.

When you turn a sharp corner, the airplane turns but the lines take a while to catch up. In your example, the lines would as their first act drag the inboard wing to the right in the earth frame as the airplane applied some force to change their direction from down to the left. This would be inboard yaw, inboard roll, depending on where you are in the corner. Shortly of course, the lines catch up and whip back past in the opposite direction, rolling and yawing the model outboard. This whips back and forth a few times before it damps out. The natural period, amplitude, and damping of this oscillation depend on the linear density (mass per unit length) of the lines, the line tension, the yaw and roll "plant" of the airplane, the X and Z moment of inertia, and a host of other factors.

This is only one of the roll/yaw effects. The tipweight, unburdended by the lines, tends to roll the airplane in the direction you suggest, for instance. Since everything is balanced for a static sense, the tipweight does more or less compensate for the line weight in steady-state conditions. Unfortunately, this doesn't entirely work in transient conditions. The tip weight is just a solid block that reacts as such, but the lines are a springy element that has an oscillatory effect. The description of the torque I gave above is qualitative. The actual amount of roll and yaw torque depends on how fast the lines change direction. If you suddenly start accelerating around the corner, the tip weight torque is directly proportional to the distance from the airplane CG and teh acceleration, but the opposing torque from the lines is nearly 0 to start, goes up to some large (greater than static) value after half a whip, and then oscillates either side of that needed to balance the torque. The net effect is an outboard roll in the airplane frame.

Put these (and a host of others we didn't discuss here, like the kinematics and the aerodynamics) all together and what do you get? Depends wildly on a whole bunch of parameters and some moderately complex calculus. What's really needed to figure it out is a 6 degree-of-freedom simulation with a bunch of carefully measured parameters. I can figure out the equations, but I don't have a complete set of useful data.

I have fiddled with different parameters and several things become obvious pretty quickly:
***
heavy or long lines, or light line tension, make the amplitude large and the natural frequency low

light/thin lines are great to fix the line whip effects, but are not so good for Y translational oscillations or pitch control feel.

****

This is an exceptionally complex subject.

What I see, however, is line drag, not weight, affecting the plane. Tip weight is there to balance the weight of the lines while flying straight and level (where line weight does enter in) but it, also, does a bunch of other things, too. So it all gets complicated, as one trim change will affect a lot of others.

If we do an inside loop, however, (or several consecutive loops) any increase in line weight caused by the increase "g" load is negated by the same increase in load on the tip weight. But the plane is constantly pulling against the lines in such a way that there is a slight tendency for the plane to roll in. On an outside loop there would be a slight tendency to roll out. This is very minor, however, but control deflection is very minor, as well, so the increased size in the outboard flap works equally effectively here to balance this out.

On the sharp square corner, however, we have an abrupt change in direction and a greater effect in pulling against the lines. And we have more control deflection to compensate. However, as soon as we make that change in direction for the airplane, we are, also, immediately moving away in the next direction, dragging the lines behind us in the air and largely negating any oscillating effect (as I see it) that might have taken place had we simply turned and stopped. The forces might still be there, but there are now other forces that tend to overshadow them.

I see it a little like, if a plane is flying slow and light on the lines, the effect of inadequate tip weight becomes quite noticeable. The weight of the lines will cause it to roll in. If, however, the plane is screaming around the circle and pulling quite hard (perhaps like some combat planes) the lack of proper tip weight might not become quite so noticeable while flying straight and level. So the gravitational forces of the lines are going to be less in this case, but the forces caused by line drag are still going to be there. (an extreme example would be a speed plane with no outboard wing at all. It still flies level because of the extreme pull against the ines.)

I don't have the math. I am just trying to make observations. But what you say about line size and weight will definitely enter in, with thicker lines causing more drag (and weight) to muddy up the equations.

I guess that is why it is often said that trimming on a plane never stops until its last flight. We were still making trim improvements on Matt's plane right up to the FAI trials, even though that plane had over 1000 flights on it at the time.

Leonard Neumann

The reasons for the need for a larger outboard flap are not entirely clear to me, but I don't think it has anything to do with precession. The larger outboard flap would have the effect of yawing the plane more outboard (due to additional drag) and rolling it more inboard (due to aileron effect) in both inside and outside corners than it would have otherwise.

I think, but can't prove just yet, that the larger outboard flap is the result of a CP offset in the stab/elevator due to the airspeed differential. This results in a rolling moment outboard in both insides and outsides that is compensated for by the larger outboard flap.

I think this is the reason for the dreadful roll performance of the All-American - the CG required to fly level in level flight is probably 1 1/2" inside of the fuselage, but the tail is centered. When you start maneuvering the tail torques it dramatically ouboard.

My theory is that the wing and tail both require assymmetry to put the CP's on the center of the fuselage, and the need for the larger outboard flap will go away. We'll see.

I still subscribe to the line drag theory as being the most obvious as to what is happening. We do know the effect of moving the leadouts back and forth, but have no method of moving the leadout location up and down during flight to compensate for the affects of line lag in the corner. The best we have is to build in a larger outboard flap (or add a "wart").

Matt and I have built several planes where the wing was not in the vertical center of gravity and this caused a noticeable roll (one high wing, one low wing) in the plane as a result. It appears to me that this is the effect we are inducing on the plane in the corners, as we effectively move the location of the lines "up" and "down" during the maneuver.

It would be interesting, however, since this was Crist's original question, for him to try the offset stab on a flapless plane to see how this works. There can be more than one solution to a problem (adjust tip weight, make the outboard flap larger, make the inboard stabilizer larger?)

Leonard Neumann

LAST EDITED ON Nov-06-01 AT 10:14 AM (CST)

I build the Gieseke Bear. The wing is offset. The inboard flap is larger than the outboard flap. I use the thin light lines and the roll is reduced. I built a Saturn it has the larger outboard flap. The yawing and rolling was always present and I could never get it really trimmed well. I am not sold on the larger flap idea.

As far as a The Jamison Special. Give it alot of power and fly it pretty fast and all this roll and yaw stuff won't really apply. Get the speed up and have some fun.

DMoon

I agree with this. The Jameson Special is like an All-American with all the horrific mistakes corrected. If I sat down to design a model to do the OTS pattern, it would look a lot like this.

The issues of roll/yaw trim that lead to larger outboard flap, and the other micro-adjustments and half-psychological "problems" that I and others neurose over are really in the noise for OTS, with it's grand total of 12 corners. And even the slightest tiny misalignment, weight distribution difference, etc, washes out these effects.

I've just obtained a set of prints on the All American Senior
and was about to set about the construction of one. Could you please give us a quick outline of "the horriffic mistakes"?

I am drawn to the rather funky aestheics of the thing... does funky flight follow?

Thanks...Nitro Fumes!.......Hib

>Ouch!, Mr. Buck!,
>
>I've just obtained a set of
>prints on the All American
>Senior
>and was about to set about
>the construction of one. Could
>you please give us a
>quick outline of "the horriffic
>mistakes"?
>
>I am drawn to the rather
>funky aestheics of the thing...
>does funky flight follow?

I was greatly exaggerating. The two real issues with the airplane are the grossly excessive assymetry and the short tail moment.

The CP of the wing is roughly 1.5" inboard of the fuse and the tail is centered. To get it to fly level in level flight, the CG has to be 1.5" inboard of the fuse. The CP of the tail is well outboard of the CG. The net result is that the same force that pitches the airplane around in manevuers also causes a large rolling torque. This is problem 1

Problem 2 is that the thrust line, aligned along the centerline of the fuse, is of necessity well outboard of the CG. This means that anytime the airplane accelerates, there's a large inboard yaw torque. The highest thrust occurs on the ground at takeoff. The result is that the airplane has a strong tendency to yaw in at you right at takeoff release. The yawing in flight is pretty negligible compared to this.

The short tail moment creates a situation similar to most OTS airplanes. The maximum pitch rate is high, and the pitch acceleration is relatively low. That means that it can be driven to high pitch rates and stall, but still not be very easy to stop and start.

The partial solution to the roll issue is to build it light and fly it fast. This raises the roll and yaw restoring forces while reducing the moments of inertia, greatly increasing the roll and yaw natural frequency and reducing the amplitude. Consider this to be "stiffening up" the lines so they have better ability to keep the wing aligned.

The "yaw in at you" problem is best solved with flying technique. The best thing is to hold the airplane level with the tail well off the ground at release. That way it just flys away at the right pitch angle, avoiding any GP from having to pick the tail up. Also, make sure the airplane is tangent to start with. If it's yawed out when you release it, the resulting nose-in swing will exacerbate the problem. And get ready to run. One other possible solution would be to put in a large amount of engine offset to get the thrust line to aim through the CG, but this is a large angle.

The way to help the short tail moment problem is to set the controls up so slow that the airplane simply can't be driven to high enough pitch rates to stall. My relatively heavy All-American (low 30's) with the CG at 15% ended up with about +-1/2"-3/4" of elevator motion with full hand motion. This is exceptionally slow even by current standards. It still turned plenty quick enough. The idea that you have to have it flap back and forth 45 degrees for "better" turns or emergencies is completely fallacious. It turns a lot tighter if the wing *isn't* stalled. The All-American is much better in this regard than the Ringmaster. The airfoil and wing loading is so superior on the All-American that it's pretty tough to get into too much trouble, unlike the Ringmaster, which stalls if you look at it funny.

The airplane is Ok as is, as long as you know the limitations. Besides, fixing it makes it invalid for OTS

But compare this with the Jameson Special. The wing is nearly identical except for the fixed flap, with the asymmetry removed. This solves all the problems with roll and yaw trim. The tail moment is much longer, which limits the max pitch rate but greatly improves the pitch acceleration. It takes off well, it flies nice and square without too much problem, and it's easy to fly with nice control feel. You can fly it as slowly as line tension allow. The Jameson is legal for PAMPA OTS, but not GSCB, so if you live in New Jersey, don't try to fly it in a contest.

Mr. Buck, please elucidate my fuzzifiction. If the CG has to be 1.5" inboard of the fuselage for proper level flight then isn't the thrust line located on the fuselage centreline outboard of the CG rather than inboard as you state in Problem 2? It seems to me that the thrust line would have to be outboard of the CG to develop an inward yaw moment.

Best, Hoovie

Best, Hoovie

Somebody actually DOES read this stuff!

You are of course correct. I will edit the response.

Brett

Well then Ty I guess you'll be needing a digital gram scale for rubber, a high resolution torque meter, a winding stooge, a motor blast tube, a trick billet winder, a clockwork multi-function dethermalizer, a Gizmo-Geezer adjustable thrust button/freewheel, an electronic plane finder module, a bubble lift detector, and a mini-bike for retrieval. Ah the simple life.

There is only one thing required for any enterprise to be carried to the uttermost extremities of esoteric complexity--a human bean. Ain't it grand?