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Lately, I have been preparing for my 4P flight training, and have come
across a section "Airspeed/Mach Limitations" showing the Lancair 4P's
Vne=274 begins to be limited by the Mmo=0.52 limit above 11000'. This
decline as depicted on the graph parallels the well recognized speed of
sound decrease that occurs at altitude. According to these training
materials and confirmed by Lancair, the structural limit of the Lancair 4P
is also pegged at Mmo=.52, and thus (according to these training
materials), Vne=274 KCAS below 11000' changes (declines) to a low 195 KCAS
at FL300. As reported by Lancair, at a more typical FL250, Vne=Mmo=220
KIAS as an important speed limit.
I have recently looked into this topic and have found the explanation of
speed of sound, the decrease in mach with altitude, the complex equations
for calculating mach, and even an elementary school level "Interactive
Atmosphere Simulator" that calculates mach based on your altitude inputs.
See these references below.
From these various sources (from grammar school simulators to college level
calculations), I seem to conclude that Mmo=0.52 NEVER reduces Vne=274
(which would require mach < 527 knots or 606 MPH or 889 ft/sec), which
doesn't really occur even up to 60,000'. I have attached a 40K PICT graph
(title: Mach Vne Mmo.PICT) of these speeds for easy comparison. [Pictures that accompanied this post are on the LML website on the "Select Discussions" page. <marv>]
All calculations are based on AVERAGE temperatures and pressures from long
standing and accepted atmospheric values obtained for independent sources.
Even with calculation variations from KIAS vs KCAS, temperature, pressure,
etc, the numbers are fairly demonstrable.
I would appreciate some sage advice from the experienced many on the
listserv regarding this potentially significant speed limit, especially
with our pursuit of higher engine HPs and higher speeds at flight levels.
Is it TRUE or is it FALSE? Thanks.
Jeff
REFERENCES:
The Speed of Sound
http://www.infoplease.com/ipa/A0004571.html
The speed of sound varies with temperature. At sea level Mach 1 is around
742 mph. It decreases with altitude until it reaches about 661 mph at
36,000 feet, then remains at that speed in a band of steady temperature up
to 60,000 feet.
http://home.iae.nl/users/wbergmns/jetmach1.htm
ALT (ft) TEMP (C) Mach (Knots) Mmo=0.52 (Knots)
Vne 274 KIAS
0 15 660 343 274
10000 -4.8 638 332 274
15000 -14.7 627 326 274
20000 -24.6 614 319 274
25000 -34.5 602 313 274
30000 -44.4 590 307 274
35000 -54.3 577 300 274
Interactive Atmosphere Simulator
http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html
MACH NUMBER and AIRSPEED vs ALTITUDE
http://ewhdbks.mugu.navy.mil/mach-as.htm
Aviation Formulae: Mach Numbers, True vs Calibrated Airspeeds
http://williams.best.vwh.net/avform.htm#Mach
Jeffrey T. Liegner, M.D.
Ophthalmologist in New Jersey
N334P (LIVP 70% done)
http://www.liegner.md/
LML website: http://members.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please remember that purchases from the Builders' Bookstore
assist with the management of the LML.
Please send your photos and drawings to marvkaye@olsusa.com.
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