Airplane KnowItAlls-Airplane Tails Again: Predatory Practices

Factual Questions
1
      • So I’m looking at the paper just now, and they show a photo of a Predator, and the tail is in an inverted “V”. I seem to remember a small airplane (Bonanza?) from the 70’s that had an upright “V” tail, but that’s it, really.
        And I can’t ever remember seeing any other aircraft that had an inverted tail, except maybe experimental stuff that was drop-launched from a bomber’s wing, or truly antique planes.
  • I can see one disadvantage of any inverted tail myself: that it leaves less rotation angle/room (or requires longer landing gear), but what advantages does it have? - DougC
2

In the case of the Predator, you may note that the working pod containing the cameras and sensors extends down from the nose in a large bulb. I am not privy to the design specs, but I would suspect that both weight balancing and a desire to manage the conturbed airflow from that bulb might have played a part in the design.
(I am mildly surprised at the reverse dihedral, but, presumably, the wing dihedral is more significant.)

The F4 Phantom II, while it did have the standard vertical stabilizer with rudder, also had the “horizontal” stabilizers angled down in a way that gave the appearance of an arrow’s tailfeathers.

3

the Beech Bonanza was in production from 1948 - 1982 (until simply EVEYONE became aware of its nasty habit of falling apart in mid-air. (‘forked-tailed doctor killer’).

The F-117A ‘Stealth’ fighter also uses an upright ‘stabilator’ tail.

My guess is this is used to reduce radar signature - especially inverted, such surfaces would return mush less of the signal.

4

I’ll see if I can dredge this up out of my memory… First a ‘V’ tail is supposedly more efficient than a conventional tail, for the simple reason that there are only two control surfaces instead of three, and there’s only one intersection instead of two. So parasitic and intersection drag are lower.

In practice, the added complexity and weight of the linkages to drive the elevons, plus the added stiffening of the structure erases much of the gain you might otherwise see.

As for inverting the ‘V’, there are a couple of potential benefits. The first is that having the tail lower than the wing increases the amount of tail surface that will be in the downwash from the wing, thus making it slightly more efficient.

Another difference between an upright ‘V’ and an inverted ‘V’ is in stability. An upright ‘V’ has dihedral, which helps the aircraft in roll stability. Imagine the airplane rolled to the right 45 degrees. Now the tail surface on that side would be horizontal, and the other one is straight up. that adds a lifting force to that side of the airplane, which tends to make it roll back to its trimmed position again. This is exactly the way wing dihedral works.

But if you invert the tail, you get the opposite effect - If the airplane rolls to the right, now it’s the LEFT tail that is creating more lift, which makes the airplane want to continue rolling in that direction.

For an unmanned probe with a fly-by-wire computerized system, this might be a benefit, because it increases the manoeverability of the plane. But then you need active computer control to keep it flying straight, or you have to increase roll stability in another way.

In the end, though, the choice of a type of tail often comes down to little more than a series of design tradeoffs and/or the prejudices of the designer. There are other solutions to the same problems. Perhaps the Predator’s wing had to be built longer for high altitude performance, so they decided to invert the tail to gain back some roll performance.

For an example of how one design decision can dictate others, consider a homebuilt aircraft called the ‘Imp’ that uses a pusher prop and an inverted tail. Why the inverted tail? Probably the main reason was so that if the pilot over-rotates the aircraft on takeoff or landing the tail will touch the ground instead of the propellor. Put a small wheel on the tip of the tail, and you eliminate one of the larger problems with pusher propellors. Or you can make the airplane a taildragger, and now the inverted tail does double-duty as a gear strut as well. But now you have to reinforce it to survive hard landings…

Airplane design is a study in tradeoffs. Look at the difference between the Beech Starship and the Beech King-Air - they do similar things, but look radically different. Sometimes the design just leads you in a different direction.