On Tuesday, July 21, 2015 at 9:39:20 PM UTC-7, wrote:
There are many errors in the FAA's "Glider Flying Handbook". In particular Chapter 3, the aerodynamics/ theory chapter, seems to average about one major (embarrasing!) error every couple of pages. S

Thanks for posting, Tom, I am interested to read your version of the book.

Here are some of the errors that caught my eye in the FAA book--
(FAA-H-8083-13A, "Glider Flying Handbook", 2013 edition, available on-line here https://www.faa.gov/regulations_poli...ider_handbook/)

Figure 3-1 on page 3-3: The L-D-W vector diagram is horribly mangled. The lift vector is drawn as being inclined forward relative to the flight path, and is much too large. There is also a Thrust vector included! The vectors included in the diagram cannot be arranged into a closed polygon-- the net force therefore cannot be zero. A correctly drawn diagram would allow the student to instantly understand why the L/D ratio is also the glide ratio through still air. This diagram does not.

Page 3-7: discussion of planform-- the statement is made "The rectangular wing is similar in efficiency to the elliptical wing". This would be better said of the tapered wing, not the rectangular wing.

Page 3-8: discussion of glide ratio-- no explanation is made of WHY the glide ratio through still air is the same as the L/D ratio. This is very easy to explain with a properly drawn L-D-W vector diagram.

Page 3-11: discussion of pitch stability-- flutter is included within the discussion of pitch stability-- it ought to be addressed elsewhere.

Page 3-12 through 3-13: discussion of lateral stability-- the fallacious "the wing that is more horizontal creates more lift" argument is used to explain how dihedral contributes to roll stability: "As one wing lowers, it becomes closer to perpendicular to the surface and level. Because it is closer to level and perpendicular to the weight force, the lift produced directly opposes the force of weight." The real truth is that we can't explain how dihedral really works to contribute to roll stability, without talking about sideslip. Oddly, page 3-12 does address sideslip: "When a glider is rolled into a bank, it has a
tendency to sideslip in the direction of the bank." Yet instead of going on to talk about how the sideslip interacts with dihedral to create a roll torque toward the high (downwind) wingtip, the authors veer off into a discussion of roll damping. Figure 3-23, entitled "lateral stability", is really an illustration of roll damping! Then, having raised the subject of roll damping, the authors miss the chance to point out that when a pilot is trying to roll an aircraft into a turn, if he allows the aircraft to adverse-yaw and sideslip, then any dihedral that is present will create an unfavorable roll torque that will tend to reduce the roll rate. The authors miss the chance to point out proper rudder coordination will boost the roll rate, compared to the same aileron roll input with no accompanying rudder input.

Pages 3-15 through 3-16-- discussion of sideslips-- the authors state:

"The shape of the glider's wing planform can greatly affect the slip. If the glider has a rectangular wing planform, the slip has little effect on the lift production of the wing other than the wing area being obscured by the fuselage vortices. The direction of the relative wind to the wing has the same effect on both wings so no inequalities of lift form. However, if the wing is tapered or has leading edge aft sweep, then the relative wind has a large effect on the production of lift. If a glider with tapered wings, as shown in Figure 3-14, were to begin a slip to the left with the left wing lower, the left wing will have a relative wind more aligned with its chord line and effectively higher airflow (airspeed) that generates more lift as compared to the higher right wing with angled relative wind, resulting in lower effective airflow (airspeed) over that wing. This differential in airflow or relative airspeed of the wings when taken to the extremes of the flight envelope results in the higher wing stalling and often an inverted spin."

Where to start with this? This is extremely problematic. The wing in figure 3-14 has equal taper on the leading and trailing edges-- the reverse sweep angle of the trailing edge is the same as the sweep angle on the leading edge. Should this be considered a swept wing? Traditionally, sweep is measured at the quarter-chord line. It is true that the quarter-chord line of the tapered wing in figure 3-14 is very slightly swept. Are we being told that due to this very slight sweep angle, during a sideslip, the "upwind" wing will generate much more lift than the "downwind" wing? That would be absurd. With most sailplanes, any slight dihedral-like effects or anhedral-like effects due to wing planform will be utterly dwarfed by the effect of the actual dihedral that is present in the wing geometry. During a sideslip, the dihedral will always generate a strong "downwind" roll torque-- the "upwind" wing will fly at a higher angle-of-attack, and generate more lift, than the "downwind" wing-- and that's why we have to maintain an aileron deflection during a sideslip. In other words an imbalance in lift between the two wings is an extremely normal thing during a sideslip-- every pilot is familiar with the need to maintain an aileron deflection during a sideslip. The only exception would be in a glider with a nearly flat wing, like Fox. What exactly are we being warned about here? Are there really specific glider types that are extremely prone to tip-stalling and spinning during sideslips? If so, which ones?

Page 3-17: There is a heading "sideslip", and a paragraph about sideslips, and then the authors insert a paragraph about dihedral-- yet fail to explain why the left and right wings experience different angles-of-attack during a sideslip due to the dihedral. In fact the authors imply that the only significant cause of a difference in angle-of-attack between the left and right wings is a rolling motion! The authors suddenly start talking about roll damping again, and describe again how a non-zero roll rate creates a difference in angle-of-attack between the ascending wing and the descending wing, still under the heading of "sideslip"! They are echoing the very same confusion that we saw in pages 3-12 through 3-13. Nowhere is the reader told how dihedral really works to create a roll torque during sideslip, and every time the subject of dihedral comes up, the authors veer off into a discussion of roll damping!

Pages 3-17 through 3-19-- spins-- nowhere is it noted that slipping turns generally do not invite spins.

S