Airplanes normally have a download on the horizontal tail. Many of the Old Timer free flight models have horizontal stabilizers with a wing-like airfoil section, intended to give an up load on the tail. Is this what is required of these models?

If so, how is pitch stability maintained? Finally, would it be OK to substitute a flat airfoil for the curved one in the horizontal?

From : Don Stackhouse

I've seen big discussions of this topic in the past on various forums. Quite honestly, I knew of many reasons why a lifting stab would be bad for performance, and couldn't think of or find (including in consultations with old-time free-flight experts) any reasons why it would be desireable. Eventually it came out (as has been mentioned elsewhere in this thread) that some of the old AMA free-flight classes restricted the wing area, and using a lifting stab was a way to get around this rule. Well-intentioned rules are often the root cause of some truly bizarre design features.

As was so well explained on the R/C Soaring exchange recently in one of Blaine Beron-Rawdon's posts about tandem-winged models (of which lifting stabs are a special case), the lowest induced drag comes from generating as much of the lift as possible from your primary lifting surface (the wing), and trying to keep the other lifting surfaces (and the lift they generate) as small as possible. The largest lifting surface will generally also be the most efficient lift-maker, and any work done by the other (less efficient) surfaces, be it a lifting stab, a canard, or whatever, will end up hurting the aircraft's overall efficiency. The optimum generally comes from a small tail surface on a long moment arm, with a slight download on the tail surface.

An additional factor involved here is downwash. This is what really puts the nails in the coffin of the myth of canard (and lifting tail) efficiency. A wing (or other lifting surface) makes lift by grabbing chunks of air and accellerating them either downward (for positive lift) or upward (for negative lift). It's Newton's third law, the one about action and reaction. Shove the air one way, and it shoves you back the other way. All that business about Bernoulli, and low pressure on top of the wing and higher pressure under the wing, is merely an explanation of HOW the wing grabs hold of the air in order to shove it downwards. The actual shoving is what makes the lift.

The induced drag of the flying surface (the drag that results as a by-product of the lift-making process) depends on how big a chunk of air the wing is grabbing (which is determined mainly by the span and by the air density, NOT the aspect ratio) and by the amount of lift the flying surface has to make.

If the air is shoved downwards, then the air behind the wing is moving downhill. A wing with lots of span (and therefore grabbing BIG chunks of air) compared to the lift it's making will have a shallow downwash angle, while a shorter wing (grabbing smaller chunks of air) making lots of lift will have a greater downwash angle.

A tail flying behind the wing is flying in this downdraft induced by the wing. This is why a plane with a downward-lifting tail can be in trim with the wing and tail incidences both at zero. The downwash of the wing puts the tail at a negative angle of attack, as if it had some decalage.

If the tail is making negative lift (such as with a conventional downward-lifting tail), then this downdraft from the wing HELPS the tail's efficiency. If the tail is lifting upwards (as with a lifting stabilizer), then its efficiency is HURT by this wing-induced downdraft. Likewise, the wing of a canard aircraft is constantly flying in the canard's downwash, negating any benefit of the "positive" lift that the canard is making.

The bottom line is that for a given amount of stability and net positive lift, the drag penalties of a canard, lifting stab and conventional downward-lifting stab are all about the same.

As far as airfoils on stabs and canards are concerned, consider that a tail surface follows the same basic design rules as a wing. The required airfoil depends on how much lift you're planning to make from that flying surface. If the lift required is very small, and/or the maximum amounts of "up" lift and "down" lift during various maneuvers are about the same, then a symmetrical airfoil might be appropriate. A wing on a lightly-loaded aerobatic model, or the stabilizer on a model with its C/G near the "neutral point" (i.e.: the C/G location that results in neutral static pitch stability, more on that in a moment) are examples of this.

If the lift is significantly large and in one direction most of the time, then it may make more sense to use a cambered airfoil. Most wings, canards, and lifting stabs fit this description. Likewise, a downward-lifting tail on an airplane with a way-forward C/G (such as an airliner under certain loading conditions) may need the extra downward lift of a negative-cambered stabilizer to make enough downforce (without stalling the tail) to raise the nose during landing flare.

Some full-scale sailplanes also have downward-lifting tails. Tail area depends primarily on two requirements; the tail must provide sufficient area for adequate dynamic stability, and it must provide enough control force (without stalling itself) for whatever maneuvers are required. If the requirements for dynamic stability are low (most modern full-scale saiplanes have only a bare minumum of both static and dynamic stability, since stability usually hurts efficiency), and there is not very much control authority needed in the "down elevator" direction, then the size of the tail needed for a given amount of "up elevator" control authority is reduced if a downward-cambered airfoil is used.

Another reason to use a cambered stabilizer airfoil is to reduce the effects of airfoil hysteresis. This is particularly often a factor with models. Many airfoils, especially at low Reynolds numbers ("Re"), such as on the tails of models, can have a different lift coefficient ("Cl") at a particular angle of attack ("alpha") if that alpha is approached from above, vs. from below. Usually the range of alphas where this happens is at or near zero Cl.

For example, assume you have the model trimmed for stable cruise. Give it a blip of up and release the stick, and note the speed and climb rate you get. Now give it a blip of down and release, then note the speed and climb angle. If the airfoil suffers from hysteresis at around that angle of attack, you will see a different speed and climb angle after the "down" blip than you did after the "up" blip. This can leave you with a model that likes to fight you in pitch; you try to pull the nose up a bit and it comes up too much, try to push it back down and it goes down too much, it just refuses to go quite exactly where you tell it to. This can especially be a problem with all-flying tails (which is one reason why I rarely use all-flying tails on my designs; they also are structurally trickier, and usually have to be larger than a stabilizer-elevator combo to achieve the same amount of control authority).

By adding a small amount of camber (such as the downward camber on the Selig 8025 airfoil, which is the same otherwise as the symmetrical Selig 8020), the band of hysteresis can be shifted to one side, so that it's out of the normal flying range. The Selig 8025 has about the same amount of hysteresis as the 8020, it just happens at a range of alphas that you usually just pass through while maneuvering, but don't normally fly within.

In part 2, we'll discuss the other part of Bob's question, how pitch stability is maintained.

Don Stackhouse
DJ Aerotech