Q:
Many older aircraft designs such as the Cub and the Champ are tougher to fly than more modern designs such as the metal high wing Cessnas. This has nothing to do with the tailwheel, since I'm talking only about in-flight handling, and the comparison is as valid between a Super Cub and a Cessna 170 as it is between a Champ and a Cessna 152.
So what are the factors that make it tougher to fly the older designs? Hint: It has nothing to do with flaps, or the lack thereof. (Steve, Ernie- wait a while, please)
Larry N.
Ed. Note: This was from a Feb. 2003 thread in the Outer Marker forum, around which time there were a number of informative questions and posts about
flying, aircraft responses, techniques and other things about real aviation. Marvelous discussions ensued. The answer below condensed many, many posts by many knowledgeable people into one explanation.
A:
OK, folks, it looks as if we have pretty well got all the pieces together. Although I saw a couple of misconceptions, for the most part there were a lot of contributions that added one or more pieces to the puzzle. So let me put it all together.
There are really two main factors that made the difference in ease of flight. One is
aileron design and the other is the "
washout" that Larry A. mentioned above, and that others offered in different words.
As mentioned above, modern light aircraft utilize a differential
aileron travel, that is, the ailerons only travel about a third as far in the down direction as they do in the up direction. In older designs such as the Cub and Champ, the ailerons traveled the same amount both up and down.
The reason this makes a difference is because of
adverse yaw (the MAJOR reason for
rudders). Remember that there are two main types of
drag, parasite and induced. Parasite drag comes from things hanging in the breeze, and induced drag is "induced" by the wings creating
lift, the more lift the more drag.
Now think about what an aileron actually does. It changes the lift on the portion of the wing where it is installed. The upgoing aileron decreases lift over its portion of the wing, thus causing a slight drag reduction. But the downgoing aileron increases lift by a lot, thus increasing induced drag. So the two working together generate a
yawing motion in the direction of the downgoing aileron: in other words, if you move the stick to the left in order to lower the left wing, you've raised the left aileron, reducing drag slightly on that wing, but you've also moved the right aileron down, causing a considerable increase in drag, thus the aircraft
yaws to the right. This is why you need left
rudder to maintain a
coordinated turn.
But with the reduced downward aileron travel in more recent designs, there is less drag on (in the above example) the right wing, so there is less yaw, so you don't need to apply as much rudder.
The other factor is
washout. Basically this means that the outboard portion of the wing is twisted so that the leading edge is slightly lower than on the rest of the wing, and the remainder of the wing shape on that outboard section is accordingly twisted. This reduces the
angle of attack, compared to the rest of the wing, in all flight regimes. Remember that a
stall is caused by too high an angle of attack, such that the air no longer flows smoothly, therefore cannot create lift. The result is that the inboard portion of the wing stalls first, leaving the outboard section unstalled, thus allowing the ailerons to retain some effectiveness, even in a stall. It also has a side effect of, all else remaining equal, reducing the severity of the stall. In practice, the
airfoil shape is often (not always) changed, as well as twisting the wing.
Most pilots are familiar with the coordination exercise often mis-called "dutch rolls." For you non-pilots, this exercise consists of establishing the aircraft in level flight a bit below cruise speed (or even in somewhat
slow flight) and
rolling constantly and smoothly from roughly 25º to 30º bank one way to the same bank angle the other direction. If you're not doing this correctly, the aircraft winds up
yawing all over the place, so you must stop, stabilize the aircraft, then start the exercise all over again. It is MUCH more difficult in a Cub than in a Cessna, and even requires different amounts of right and left rudder.
Also, when near
MCA (
minimum controllable airspeed), swinging the ailerons back and forth without rudder actually
rolls the aircraft back and forth, but in so doing also generates a lot of adverse yaw, so the nose is swinging back and forth in the opposite direction from which you are applying stick. This is in a Cessna or Cherokee, for example, the more modern aircraft.
However, doing the same thing in a Cub or Champ when at or very near MCA will generate the yaw, but little or no roll, and may even put you in a
spin. What happens is that the increased lift from the downgoing aileron increases the
angle of attack of that wing section, and on the older aircraft there is no washout. Since you were already almost at the stalling angle of attack, the aileron going down increases that angle of attack beyond the stall, so the outboard section of wing stalls. So now the lift is gone on that wing section, and you've got a LOT of yaw from the extra drag, so that wing drops and the aircraft yaws in that direction, and if you don't relax
back pressure, you're on the way into a spin.
What this means is that you'd better keep the wings level with rudder (how this happens is another story), rather than ailerons-- a hard thing to teach, but necessary.
Now there's another factor that often confuses pilots, too. Because of the thousand and one compromises in design, the steeper the bank, the more the aircraft wants to bank, especially once you pass 30º to 35º of bank. So you wind up holding a little opposite aileron in a stabilized steeper turn, in order to maintain a steady bank angle. This is MUCH more pronounced in a lot of the older aircraft, so that I'm in a 40º bank to the left, with a bit of left rudder and a very noticeable amount of right stick, plus holding considerable back pressure to keep the nose up.
Since we're talking here about in-flight handling, I won't go into
heel brakes, tailwheels, and other bits and pieces that come into play on the ground.
However, I will mention one more thing. Someone above mentioned different sized rudders from old to new. The larger amount of adverse yaw in the older aircraft required more rudder available to offset it.
I hope I haven't left anything out at this point, but this is such a long post that I might have overlooked something.
Larry N.
See also the FAA's
Airplane Flying Handbook,
Chapter 13,
Transition to Tailwheel Airplanes.
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