I have a Filip 400 and was considering setting it up by mixing the ailerons to also be elevators. has any one tried this or know if it works...

...The ailerons are too close to the CG centerline and will have no moment of arm so they can't be used as elevators...

From : Don Stackhouse

Actually, this can be made to work in some cases, but it gets tricky. There are several factors at work, and their sum can result in the elevator effect working in one sense or the other, or not at all, depending on the circumstances. We have experimented with this approach a while back, with mixed results. "Pitcheron" type models are an extreme example of this concept.

For the skeptics out there, consider the typical reaction of a plane in pitch when flaps are deployed. On some models there is no reaction, while others may tend to pitch up, and still others may pitch down. Obviously the whole issue is complex, and depends on the characteristics of each design (otherwise all airplanes would react similarly, which they don't), but the fact that some airplanes do react significantly in pitch does demonstrate that this idea can have the potential to control pitch to at least some extent.

So exactly how does this work? Ideally we should look at this from the standpoint of the "zero lift line" of the airfoil, but I think it might be easier to follow if we instead use the more commonly understood concepts of chord line and camber.

The basic pitch trim of an airplane is related to "decalage", which is the difference between the incidence angles of the wing and tail. As I mentioned above, the "purest" measurement of this is probably from their zero lift lines (i.e.: an imaginary line through the airfoil that when lined up with the airflow results in a lift coefficient for that airfoil of exactly zero), but we'll use the chord line. The chord line is an imaginary line through the airfoil running from the furthest forward point on the leading edge to the furthest aft point on the trailing edge. If we find the chord line of the wing and the chord line of the horizontal tail, the angle between them is the decalage.

Positive decalage means that the wing is at a more nose-up angle than the tail. For example, if we set the wing on the fuselage with a positive incidence of two degrees and mount the tail with an incidence relative to the fuselage of zero degrees, we have a decalage of two degrees. If we give the wing an incidence of one degree leading edge up and the tail an incidence of one degree leading edge down (or "negative one degree"), we still have a decalage of two degrees. If we mount the wing with an incidence of positive five degrees and the tail with an incidence of positive three degrees, the decalage is still two degrees. The plane will still fly with the wing at the same angle of attack in all three of these examples, and at the same airspeed (all other things being equal), but the fuselage's angle of attack will be different. Incidence between the wing and the fuselage controls the fuselage's flying angle for a given flight condition. The decalage controls the wing's angle of attack for that same condition. If you shim the leading edge of the wing up, the plane will fly slower, not because of the change in incidence on the fuselage, but rather because you changed the decalage.

When you move the elevator, you change the chord line of the horizontal tail, and therefore the decalage. If it's an all-flying tail, then that control movement changed only the decalage, and nothing else.

If you have a conventional elevator + stabilizer ("two element") tail, then you also altered the camber of the tail's airfoil, which changes its zero lift line, in addition to changing its incidence. The change in the zero lift line is in a direction that increases the effect of the incidence change, so that the effective change in decalage is even greater. In most cases (although this can get really tricky in very small, slow, low Reynolds number cases), you've probably also increased the max lift coefficient capability of that airfoil. This is why you can usually get away with a smaller two-element tail than an all-flying tail, if elevator authority is the key deciding factor in the tail size (however, if the key factor is dynamic stability, then they both probably need to be about the same size).

If you change the camber, you also change the airfoil's aerodynamic pitching moment coefficient. For a tail surface this is probably not a significant factor, since tails tend to have relatively small chords and areas relative to the rest of the plane, and therefore the moment caused by this aerodynamic pitching moment tends to be a very minor player in the total sum of the forces and moments acting on the plane. However, even this moment is in the direction that adds to the elevator's effectiveness.

Now, what if we try to put the main surface for pitch control on the wing of a conventional aft-tailed airplane, instead of on the stabilizer? When we deflect that surface, several things happen, and some of them tend to cancel each other.

If we deflect the ailerons/trailing edge flaps downwards, we move the trailing edge location downward, which makes the wing's effective incidence more positive, which therefore increases the decalage. This will tend to pull the nose up.

The increased lift of the wing due to the increased angle of attack will increase its downwash angle. A wing makes lift by accelerating the air downwards, so the air behind the wing is therefore moving downwards relative to the airplane at some angle. If we make more lift, we increase this downwash angle. The increased wing downwash impinging on the tail will tend to push the tail down (i.e.: make its angle of attack more negative), also tending to pull the nose up. This downwash field extends for quite some distance both above and below the wing, so the vertical location of the tail relative to the wing and fuselage is not likely to significantly affect this.

If this is a "pitcheron" airplane, where we just pivot the whole wing for this control input without altering its camber, then this is pretty much the total of what happens. However, if what we defected was an aileron or a flap, leaving the forward portion of the wing fixed, then we also altered the camber of the wing's airfoil. This will change its aerodynamic pitching moment, and in this case that will tend to push the nose down (opposite the direction of the other two effects), and it will probably be large enough to be significant. In that case, the location of the hinge line becomes an important factor. If the hinge line is near the trailing edge, then a given angular deflection of the control surface has a relatively small effect on the total camber and the chord line, but because all that camber is being added near the trailing edge, it has a major effect on the aerodynamic pitching moment coefficient. It could be large enough to overcome the first two effects altogether, causing the plane to pitch nose-down, instead of the nose-up effect the other two factors are trying to create.

Conversely, an well-forward hinge line location will tend to minimize the effects on the aerodynamic pitching moment coefficient, and maximize the other two effects. On some of the models Joe and I experimented with, we used hinge line locations around 20% of the chord back from the LEADING EDGE of the wing. Aerodynamically this can have all sorts of interesting and mostly beneficial effects, but structurally it can be an engineer's worst nightmare!

The vertical location of the wing can be a factor. If the wing is low relative to the aircraft's C/G, then the increased drag from that extra lift we're now making will tend to pull the nose down. A high wing's drag will tend to pull the nose up. Dihedral is also a factor here, since the key location of this drag is at the Mean Aerodynamic Chord ("MAC") location, which on a typical tapered wing is a little bit inboard of the middle of the panel. If there's a lot of dihedral, it raises the MAC higher relative to the rest of the plane, making it act more like a high wing.

Another issue is pitch rate. So far we've discussed steady-state forces and behavior, when the plane is flying in equilibrium in a straight flight path. However, there are times such as during a loop, or pulling up during landing flair, or when the plane is in a steeply-banked turn, when we want the plane's flight path to be curved relative to the pitch axis.

This is where tail moment arm becomes an issue. To understand it, let's look to an automotive analogy.

Consider the front and back wheels of a car while it's driving in a straight line. Extend a line through the axis of the front wheels, and another through the back wheels. As long as these two lines remain parallel, the car will continue to move in a straight line.

Now, turn the steering wheel. The line through the axles of the front wheels is now angled aft on one side. The front wheels' axle line is no longer parallel to the one for the back wheels, and they now cross each other at some distance off to one side. Since the wheels roll in a direction perpendicular to their axle lines, the car's path will now be in a circular arc, with its center at the point where the axle lines intersect.

Now let's look at the similar situation of a plane doing a pitch-up maneuver, such as the pull-up into a loop. We're looking at a transient case here, so the airspeed hasn't changed yet to create a new equilibrium of lift forces and moments that puts the plane back on a straight flight path. However, there is still an equilibrium of sorts going on, because we still have to satisfy that most fundamental of engineering principles in Newtonian physics, that the sum of all the forces and the sum of all the moments (i.e."twisting forces") acting on the plane must always add up to exactly zero. If they don't, then somewhere there is a force or a moment that you haven't accounted for. This rule is right up there in stature with the Law of Conservation of Mass and Energy, the one that says you can't get something for nothing (which, by the way, is a pretty good law in economics as well!).

When we initially pull back on the stick, the angle of attack increases, so the wing starts making more lift, which accelerates the mass of the plane upwards. However, that upward movement changes the direction of the relative wind at the wing, reducing its angle of attack until the extra lift and the upward acceleration of the plane's mass is cancelled out. The wing's lift is now equal to what it was before plus whatever is needed to balance the centrifugal force from the plane's now-curved flight path, but less than what it was at the instant of the initial pull-up because the new relative wind direction at the wing has reduced the wing's angle of attack a little, part of the way back to what it was before the initial pull-up.

Meanwhile the plane's now-curved flight path has changed the relative wind at the tail in the opposite direction. The local airflow blowing on the tail is not in the same direction as the airflow's direction at the wing! How much different? Generally it's a small angle (exactly enough to cancel out the effects of the change in the effective decalage from the deflected controls), but in the some cases it can be surprisingly large. For example, in an R/C hand-launched sailplane it can exceed 10-15 degrees in a very tight thermal turn. We found on our own RCHLG designs that the amount of up elevator needed just to compensate for this effect in a very steep turn was about twice what was required to bring the plane to a stall in level flight!

So, getting back to the car analogy, the relative wind at the wing that results in an equilibrium of lift is in one direction, and the relative wind at the tail that satisfies that equilibrium is in a different direction. If we draw a line perpendicular to the wing's relative wind at the wing's location and another line through the tail perpendicular to the tail's local relative wind, the point where the two lines cross will be approximately the center of the arc that represents the plane's curved flight path.

If the two lines are far apart at the wing and tail (i.e.: a plane with a long tail moment arm), then the radius to the center of this arc will be bigger than if the plane has a short tail moment arm.

BTW, this whole line of thinking applies even better to the question of aileron response and roll rate, but we'll save that one for another discussion, unless someone really wants to go into it now. This treatise is already getting way too long!

This radius of turn concept is partly where the idea of short-tailed planes being more maneuverable than long tailed airplanes comes from, an idea that has some truth to it but that does not cover all the factors. The amount of mass in the extremities is also a major factor, and there are a number of other factors as well. It is possible to have an extremely maneuverable plane with a very long tail moment arm. For example, the Spitfire has an unusually long tail moment arm, as does the famous Bucher Jungmeister,which literally "wrote the book" on modern aerobatics (the Aresti dictionary of aerobatics).

Having a short tail moment arm will help the turn rate, or the radius of the "turn" about the pitch axis, but it also makes some of the other factors, especially the effect of the wing's aerodynamic pitching moment coefficient, more critical.

In summary, regarding using flaps and/or ailerons as elevators, yes, it can be done. In our experience it's tricky to get it to work well, the direction of response for a given control input can be uncertain until you actually fly it ("up elevator" could result in a downward pitching response if the factors added up the wrong way), and in general the pitch control authority tends to be weaker than what you can get with a lot less trouble from a conventional elevator (if this were not so, then we would have stopped bothering to run control linkages all the way back to the tail a long time ago!). However, if there is some valid reason for going to all the trouble of sorting it out (for example, in some of our experiments we actually built proof-of-concept models with adjustable-length tail booms), it can be a viable alternative.

Don Stackhouse
DJ Aerotech