Is there an RC flying wing that generates the lift required to maintain straight and level flight without having to set it up with some amount of "reflex" with the elevons?

Might it be more efficient to correct it >with adjusting the thrustline..

From : Don Stackhouse

We've gone through the basic concepts involved in this question before, and there is quite a bit more discussion of this general issue in various articles in the "Ask Joe and Don" section of our website.1

Thrust line has nothing to do with it. It's all about stability. Let's start with a grossly oversimplified example, just to get the basic idea:

Consider an airplane with the usual aft-mounted tail. We usually have some "decalage" (i.e.: difference in the incidence angles of the wing and the tail, so that the tail is at a lower angle than the wing) creating a nose-up moment, and a C/G that's far forward enough to create a balancing nose-down moment.

The nose-down effect from the C/G location is pretty much constant for a given flight condition, but the nose-up effect from the tail increases with airspeed. If the airplane goes into a shallow dive, the plane starts to gain airspeed, which causes the nose-up moment from the tail to get stronger, which then brings the nose up and causes the airspeed to head back towards its original setting. Likewise, if something (a gust, perhaps) pushes the nose up, the plane starts to lose airspeed, causing the nose-up effect from the tail to get weaker, allowing the nose-down effect from the C/G location to take over and pull the nose back down.

The amount of nose-down effect from the C/G location depends on something we call "static margin". If we move the C/G back until the plane has exactly neutral static pitch stability (i.e.: you pull the nose up or down a little and it stays exactly at the new pitch attitude, with exactly zero tendency to change back to the original attitude, or to try to continue changing beyond the new attitude), that C/G location is called the "neutral point". It's the place where static pitch stability is exactly neutral. Now, if we move the C/G forward so that the plane now has some positive amount of static pitch stability, the distance from the ne40utral point to the C/G location (typically expressed in percent of the Mean Aerodynamic Chord, or "MAC") is called the "static margin".

Static margin is a measure of how much static stability we have. Move the C/G forward and the static margin and the static stability increase.

Now let's talk about flight trim. The C/G location and the static margin that corresponds to it determine the static stability, but to fly at some specific angle of attack and airspeed we need to have exactly the right amount of lift from the tail to balance the nose-down effect of the C/G location (plus any other effects, in particular the nose-down aerodynamic pitching moment from a cambered, non-symmetrical wing airfoil). The nose-up effect from the tail is the product of the tail's lift force times the tail moment arm. Obviously if we shorten the tail moment arm, we will need more decalage and/or more tail area to make the increased amount of tail lift required to compensate for the shorter tail moment arm, if we still want to balance the same amount of nose-down effect from the static margin.

Alternatively we could move the C/G back, reducing the nose-down effect of the C/G location. Of course, doing this means we've decreased our static margin and therefore reduced our plane's static stability.

OK, so we're going to make the tail shorter anyway, increasing the tail area and decalage, as well as reducing the static margin to compensate for the shorter moment arm. The plane we end up with still flies OK, so in keeping with that great American concept that "If some is good, more is better and too much is just right!", we continue to shorten the tail moment arm some more, increasing tail area and decalage and/or decreasing static margin as we do so. Oh, BTW, we also just happen to have an all-flying tail in this experiment.

Eventually the tail moment arm gets so short that the leading edge of the tail actually touches the trailing edge of the wing! At that point we attach the wing and tail together with some hinge tape, use the control linkage rigging and servo position to set the decalage, and we don't need a fuselage any more to connect the wing and the tail together. Voila! We now have a plank-type flying wing. A flying wing is nothing more than a tailed airplane whose tail moment arm is so short that the wing and tail merge to form a single flying surface.

But wait a minute! The airfoil created by the combination of the now-merged wing and tail looks awful. The trailing edge is all bent up to provide the required decalage, and almost certainly has to be hurting the airfoil's ability to make lift! Also, the tail moment arm is so short that there isn't much static margin possible before things become totally impossible in the decalage department (if the trailing edge is reflexed too much, the airflow will separate from it), and the tail moment arm is so doggone short that the dynamic stability (the ability to damp out oscillations) is awful as well (dynamic stability is linearly proportional to tail surface area, but proportional to the SQUARE of the tail moment arm, so extremely short tails tend to have poor dynamic stability). Is there some way we can still eliminate the fuselage, but end up with a tail moment arm that's at least a little bit longer?

Of course there is. First we add some sweep to the wing. This moves the tips aft of the root. Now, instead of mounting the horizontal tail to the trailing edge at the center, we cut it in half and stick one piece on each wingtip. Since the sweep moved the tips aft of the root, it increased the effective tail moment arm. In addition, we don't need that bent-up trailing edge. Instead, we keep "normal-looking" non-reflexed airfoils all along the span (just like a conventional wing and tail), and mount the tips twisted down by the amount of decalage required (we call twisting the tips down like this "washout"; the wings on the majority of airplanes have some washout anyway just to try to make the root stall before the tips, but not as much washout as in a typical swept flying wing).

The end result is a swept flying wing, with a longer effective tail moment arm that a plank type flying wing, and high-performance non-reflexed airfoils along the entire span.

However, there are some drawbacks to this setup as well. In particular, since the tips are now the horizontal tail and are probably lifting downwards, the effective wingspan of the airplane is considerably reduced. Instead of an effective span nearly equal to the geometric wingspan, the effective span is only going to be about 80% or so of the actual span. That's right, your 3 pound 100" swept flying wing will probably make induced drag like a 3 pound model with only an 80" span. In addition, since the amount of twist required and the aerodynamic efficiency are both fairly sensitive to even minor changes in weight, G-load, flying speed and C/G location, it's likely to be efficient only over a fairly narrow operating range. Altering twist in flight is structurally a tricky proposition at best, so that option tends to be limited. Also, the twist is set for positive-G flight, so negative-G performance tends to be compromised. At least with elevons used to provide the necessary reflex, the direction of the reflex can be changed for flying inverted.

Typical combat wings like the Zagi use a hybrid approach. The use of some sweep helps the yaw stability and the dynamic pitch stability, but the use of reflex instead of washout for static pitch stability helps preserve their inverted flying ability. The sweep also provides a better moment arm for the vertical fins. Unfortunately, the use of a reflexed airfoil requires that sweep be kept fairly low, or else you will have problems with spanwise flow channeling along the leading edges of the elevons, which can cause all sorts of problems.

Sweep also tends to create tip stall problems. It causes the lift distribution along the wing to shift outward, loading up the tips and unloading the center section (causing what's called a "lift valley" in the center section). The washout required to provide pitch stability can help this, but if it's a swept-with-reflex airplane like the combat wings, that benefit goes away.

Then there's the case of forward sweep. This has the opposite effect, causing a "lift peak" in the center that tends to make the middle of the wing stall first. However, to get positive pitch stability you generally need to add wash-in to the wing, which offsets some of this benefit. In addition, the forward sweep hurts the yaw stability, generally requiring an enormous vertical fin.

There are other ways to make a flying wing stable, without using twist or reflex, and without resorting to "fly-by-wire" tricks such as gyros. I have some flying wings I've developed that have essentially no twist, conventional non-reflexed airfoils and a C/G located at 52% of the MAC. No, I won't tell you how I did it.

In general, though, for most aircraft designs (including flying wings) it's not too difficult to make them efficient at one particular flight condition. It's far more difficult to make them efficient over a wide range of flight conditions. If it's a flying wing, it becomes exponentially more difficult to make them efficient over a wide range of flight conditions. However, if the designer is clever, truly understands the problems and what drives them, and does his/her homework, a flying wing can have superior performance over that of an equivalent tailed aircraft. However, that generally does not happen by accident. In the history of aviation, blind luck has NEVER been a reliable design tool.

Don Stackhouse
DJ Aerotech