Technicalities: We’d Just Like to Ask You A Few Questions

Where it concerns center of gravity, you can think of the horizontal stabilizer like a chunk of the wing that has been detached and moved aft. Lucas Sevilla/Alamy Stock Photo

From time to time, I get letters suggesting topics for future articles. I welcome them. After several decades of writing “Technicalities,” I sometimes feel panic welling up as I try to think of something new to say. Some readers, sensing that a question may not be large enough, send several. One, who identified himself only as Mark, sent five. Here they are:

Q: Horizontal stabilizers can be huge, but rarely do we see winglets on them. I see it claimed on the Web by some that the down forces are not so large as to require them. If that was true, then why make the horizontal stabilizers that big?

A: Winglets are really useful for drag reduction only when a lifting surface is operating at 40 percent or more of its capacity. That is true of horizontal stabilizers only during landing, when trim drag is of no concern. (By the way, “lift” in this context just means the force acting at a right angle to the direction of flight — no distinction between upward and downward.)

As to why they can be big, see below.

Q: What are the factors determining the acceptable range of CG position and control authority at the limits? Any differences in Experimental versus certificated?

A: The size of horizontal stabilizers reflects two requirements: stability and control.

Stability means a tendency to return to a trimmed ­attitude after a disturbance. When its center of gravity is forward of the aerodynamic center, or center of lift, of the wing, an airplane is naturally stable and does not require a large horizontal stabilizer. But there is also a question of control: The stabilizer has to be big enough to hold the nose up at the minimum speed with the flaps down. Flaps increase the nose-down force, and so airplanes with powerful flaps need large stabilizers.

Conversely, when the CG is behind the center of lift, the horizontal stabilizer is needed to supply stability. Think of the horizontal stabilizer as a chunk of the wing that has been detached and moved aft. It shifts the center of lift aft with it so that the CG is still forward of the center of lift of the two surfaces — wing and tail — combined. The ­larger the stabilizer, the farther aft the CG range can ­extend. Since a wide CG range is desirable for most types of aircraft, stabilizers can get quite large.

On weight-and-balance charts, fore and aft CG limits are given in terms of inches from the datum, a reference point usually located somewhere around the nose. This convention has probably confused a great many fledgling pilots, who naturally suppose that forward and aft limits should be measured from some middle point, presumably the center of lift of the wing.

Perhaps it would make the most sense to think of the CG range in terms of distance from the neutral point. The neutral point is the place at which, if the CG were there, the airplane would become unstable. For most general aviation airplanes, the aft limit of the published CG range is 5 or 6 inches ahead of the neutral point. As the CG moves forward (until the limit of low-speed control is reached), stability increases, speed holding becomes more positive, and the stick forces needed for pitching maneuvers ­become greater.

Are there differences between Experimental and certified airplanes? Yes. Certified airplanes are required to demonstrate a certain degree of longitudinal stability in all configurations. In the United States, at least, ­amateur-builts face no such requirement; they need only demonstrate that the pilot is still alive at the end of the 40-hour (or whatever) initial test period.

Q: Airlines are adding circular bumps at the top of the fuselage back toward the tail to house the wireless ­Internet antenna. One pilot for a major airline claimed that the drag caused by that bump offset the gains of a winglet. You buy that?

A: Nope.

The key point is the phrase “back toward the tail.” If the antenna bump were near the nose, its drag would be significant because there the airflow close to the surface is moving at high speed. As air flows back along the fuselage, however, friction slows it down. By the time you get near the dorsal fin of an airliner-size fuselage, the layer of ­decelerated air is several inches thick, and within a couple of inches of the surface it is very decelerated indeed.

For that matter, if the drag reduction from a winglet were no larger than the drag increase from an antenna, airlines, notorious penny pinchers that they are, would not be spending millions of dollars to equip their fleets with winglets.

Q: I recently flew on a B777 and noticed that, in addition to the conventional ailerons, there was a stubby aileron very close inboard. It seemed to move first before the conventional outboard ailerons, or at least for fine corrections it moved, but I did not see the outboard ailerons move. As well, the spoilers seemed to be deployed as a third stage. Presumably efficiency considerations?

A: No, structural ones. Outboard ailerons can make the slender outer portion of the wing twist in a way that cancels or, in rare cases, even reverses the effect of the aileron, so they are locked at high speed. The inboard ailerons, smaller and located on a thicker, broader portion of the wing, do not have this twisting effect and are sufficient for the lateral-control needs of cruising flight. Spoilers, because they are hinged farther forward and farther inboard on the wing, aren’t subject to control reversal and can augment the effect of inboard ailerons.

Q: What are the structural and aerodynamic implications of V-tails versus T- and conventional tails?

A: V-tails seem like a good idea because, for the same apparent areas in side view and top view, they are smaller and lighter than the usual horizontal-plus-vertical arrangement. With fewer intersections, they have less intersection drag. Trouble is, the damping they provide is not as good. Their performance can be improved by making them of unusually high aspect ratio, but there’s a weight penalty for that. There’s also a control power problem: The effectiveness of hinged surfaces like rudders and elevators diminishes beyond about 15 degrees of deflection, so it’s hard to get the full effect of both when they’re combined in a single surface.

Now, as for T-tails —

Oops, out of space. Well, maybe next month. Thanks, Mark!

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, "Technicalities" and "Aftermath," with FLYING readers.

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