Copyright ©2011 by Paul Niquette. All rights reserved. |
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| Reminiscence
Alert: Sharing technical
features
of
flight with a first-timer in the cockpit provides
great entertainment
for every old-time pilot. Excuse me for dropping
into the first person
singular here, but I especially enjoy giving elementary
demonstrations
aloft. I always begin in straight and level
flight, exulting about
the cockpit's distinguished view of the world. I
gesture at the control
wheel and gently pull back, raising the nose and
making the passenger
in the seat beside me feel momentarily heavier.
Pushing forward results
in wide-eyed weightlessness. Next, I bank
the plane with the control wheel left and right; no
surprises there.
Finally, I point at the rudder
pedals on the floor and demonstrate left and right
skids.
“Go ahead, try it yourself,” I say with a smirk. I ceremonially withdraw my hands from wheel and my feet from the pedals. Parenthesis. The first thing first-timers observe is that the flight controls seem to be interconnected in crazy ways. Whereas the plane responds to the control wheel with banking as expected, at the same time, there is a surprising effect called adverse yaw. It’s as if an invisible foot has mischievously trounced a rudder pedal. Likewise for the rudder itself, which does indeed cause the plane to yaw as expected, but at the same time, the plane wants to bank the wrong way.Of course, during my little flight demonstrations, I cheat. I deliberately coordinate aileron and rudder control pressures, concealing these strange doings in the sky and setting the stage for surprises for -- and discoveries by -- the hapless first-timer. Pertinent to the question in the puzzle is the plane's counter-intuitive response to the rudder. When one applies pressure on either pedal – the one on the right, say – the rudder itself is moved toward the right, which makes the tail of the aircraft swing left, such that the nose yaws to the right as intended. Are we on the same page here? Meanwhile, without corrective action by the pilot, the aircraft incongruously banks toward the left. Hey, and what if there is no pilot to do the corrective action? Excuse me but I am getting ahead of my story. The explanation is elementary: The conventional empennage is built upside down! (Exclamatory punctuation merely signifies an astonishing assertion.) The rudder stands upright on top of the fuselage. It should hang below the fuselage, where it would induce rolling motion consistent with yawing. By the way, on the planes that have them, V-Tails are likewise mounted atop the fuselage, where they can produce the same anomaly.
Take a close look at the Predator's wings. They are glider-like, slender and long. High aspect ratio, it's called. Great for efficient, long endurance flight, which is what the Predator is designed to do. You will see a slightly curved camber line as well, which assures adequate lift at low speed. What you do not see are ailerons! (Exclamatory punctuation merely signifies an astonishing observation.) Apparently with its underslung empennage, the Predator does not need draggy old ailerons near the wintips flapping up and down for routine coordination of turns. The inverted V-Tail all by itself will naturally manage turns in both roll and yaw. Orville and Wilbur would have admired that kind of systems integration, since it facilitates an exceptionally simple design for the Predator’s wings. Lighter weight, too. With neither bladder nor bowels onboard to limit time aloft, our solution can be...
...which suggests a new, emphatic definition for the expression wet wing, plus...
Almost forgot, here is the response to my query...
Reaper and Avenger Not all models of UAVs built by General Atomics Aeronautical Systems feature the inverted V-Tail. As can be seen in photographs of the Reaper and the Avenger, their ruddervators resemble butterfly-wings placed atop the fuselage. Solvers will see something else, too: ailerons! (Exclamatory punctuation merely signifies an apparent affirmation of our solution.) Both the Reaper and the Avenger are larger, heavier, and complexer than the Predator. Their respective empennages feature separately hinged control surfaces. Nota bene, the Reaper has an underslung rudder, but the Avenger does not. For good reason. One version is called the Sea Avenger, and it is designed to operate from an aircraft carrier. Tailhook, anybody? Which reminds me: How does the Predator take-off and land with only 5.3 degrees of ground clearance? First Principles coerce us to assume that for taking off fully loaded, the Predator must be given a long paved runway -- surely at least a couple thousand feet. For landing with nobody on board, the Predator is apparently designed to be mashed onto the ground with essentially no flare, much like a wheel landing in a tail-dragger.
Epilog: Design Criteria for Unmanned Aircraft Systems
Solvers will find in ETL 09-01 more technical information than most people might ever want to know about the Predator, with its inverted V-Tail, along with confirmation of our obervations about other UAVs with upright V-tails (Fire Scout, Global Hawk, Reaper, Shadow, Warrior). Detailed technical specifications in
the document for
the Predator include (emphasis added)...
...which seem to support our horseback
estimate
"for taking off fully loaded, the Predator must be
given a long paved runway
-- surely at least a couple thousand feet."
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Except
on the 
