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Bob, Et Al,
Egads! Here we go again on a misunderstanding of the FARs. One does not need any peculiar airspeed markings or POH entries in order to satisfy:
FAR Part 45.29 Paragraph b.iii states "Marks at least 3 inches high may be displayed on ... an amateur-built aircraft when the maximum cruising speed of the aircraft does not exceed 180 knots CAS;" The operative phrase here is "maximum cruising speed" not maximum speed. The FARs fail to define "cruise" so we must rely on the dictionary definition. For example, the Merriam-Webster on-line dictionary defines cruise as a verb and
5. a. of an airplane : to fly at the most efficient operating speed.
My Random House Unabridged Dictionary defines cruise as:
4. to fly, drive or sail at a constant speed that permits maximum operating efficiency for sustained travel.
This means to fly in the altitude span and at power settings the designer meant to yield the least wasteful (most efficient) constant operation over a distance, not the highest sustained speed that can be accomplished. What altitude and power setting is that? Is 55% power the most efficient? In my normally aspirated 170 hp Lancair 320, this means between 7000 and 9000 feet at 2480 rpm and 21 inches MP I typically see 176 IAS (191 TAS), yet closer to the ground I have been clocked at race speeds of 207 knots with a wide open throttle. My 3-inch numbers still satisfy the FARs. If you are really concerned, carry a dictionary, along with your POH, for use at your next ramp check.
And, because of a recent article I read about AOA devices, there should be a mark on the AOA display which reflects the maximum Lift over Drag (L/D) air speed for the current operating conditions such as load and air density. The max L/D is frequently used for engine-out glide speed or best climb speed, but could be used to define the maximum powered "cruise" speed. Today I am going to find out just what that speed is and I will report back to this august group before the end of August.
BTW, I keep my copy of the dictionary reference along side my can of pest repellant and next to my CO/flatus detector.
Scott Krueger
N92EX
EAA Tech Counselor
[Re: the AOA indication for best L/D, it can be found by setting the AOA so the indicator reads the 2nd green bar down, the one with the "X"
in it. On the Sport model (with the LED display), best L/D is attained
when the instrument is displaying the two green LEDs and the first yellow
LED is just blinking. Best L/D identifies 3 serious flight regimes, those
being best angle of climb, the engine out maximum glide, and the maximum
endurance angle. Since Jim's device is reading AOA directly from the
wing it is self-correcting for density altitude, CG, wing loading, and bank
angle. If you were to go through a flight test process to verify that the
AOA was, in fact, telling you the accurate best L/D you would likely find
that it actually occurs at the break between the green and yellow markings
on the indicator... the point at which you pass from normal command to
reverse command (ie, in the yellow and above is "behind the power curve".) Jim's approach with showing it a few bars down decreases the AOA slightly
(perhpas 1 to 1.5 degrees) and provides a bit more penetration. I'm sure
if I've mis-stated any of the above, that Jim will jump in and set the
record straight.
<Marv> ]
LML website: http://www.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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