From: StarAerospace@aol.com
Message-ID: <64.1b1b7844.29ac92e1@aol.com>
Date: Tue, 26 Feb 2002 02:27:29 EST
Subject: Mach vs. altitude?
To: lancair.list@olsusa.com
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<< "Interactive Atmosphere Simulator" that calculates mach based on your 
altitude inputs...  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. >>

The speed of sound varies only with the gas species and the temperature, NOT 
with pressure or altitude.  The change in TAS of any fraction of Mach with 
altitude is only due to the change in temperature as we climb from msl to the 
stratosphere.  Once there, temperature does not vary with altitude.

Because the stratosphere is (mostly) isothermal, this means that once we get 
to ~FL361 the fall off in TAS for a given Mach limit stops.  We can climb 
from FL361 to FL823 with the same TAS and Mach number while seeing our IAS 
and KCAS decrease with atmospheric pressure.  Above FL 823 (not 60,000 ft.), 
the temperature starts to increase so we could fly even faster for a given 
Mmo if we ever got up that far!

So airliners and business jets like to run up to their Mach limit and climb 
as high as their wings will hold them at said mach limit to get the lowest 
possible fuel burn and therefore the best range on the least fuel.  Often, 
these aircraft are limited in their initial cruise altitude by their high 
wing loading.  Lancairs have less issue here, since what we consider high 
wing loading (30 to 35 psf) doesn't come close to the airliner standard of 
125 to 200 psf.  So, if you've got the rest of the issues of high altitude 
flight ironed out there's lots of range to be had above FL361;  but your 
highest TAS will occur at a lower altitude where Mach 1 is a faster speed due 
to the higher temperature.

Those issues of high altitude flight include (but are not limited to...):

> Cabin pressure differential and emergency depressurization issues;
> Turbocharging limits, both surging and TIT limits;
> Cooling capacity (thin air doesn't cool as much);
> Traffic conflicts (business jets are flying higher now to get slots ABOVE 
airliners...);
> Fuel precipitation (all sorts of things freeze out at high altitudes;  
mostly an issue with Jet A, it will be a real problem once aero-diesels come 
on the market);
> Lack of an accurate Mach meter to compensate for temperature and 
compressability, etc.

So for a Lancair IVP that was never really designed to go faster than M.52 
and 274 KCAS, your best true airspeed with the lowest Mach will occur at sea 
level!  How fast you want to go up against those limits as you climb is up to 
you.  When you are power-limited, altitude gains you speed and range.  Once 
you have power to spare, altitude is mostly for range not speed. 

As to how accurate those limits really are for your particular Lancair:  not 
very.

Martin used some tests, assumptions and simplifications in his initial 
flutter predictions that were not entirely accurate.  Beyond this, the 
structural architecture of the model was not the same as subsequent models of 
Lancair IV production wings.  Last, and certainly not least, the fabrication 
of a Lancair IV wing allows variations in wing thickness and structural 
stiffness that could only be called massive.  One mustang shop I interviewed 
a couple of years back showed me two wings they had produced that had over a 
10% spread in thickness to chord ratio.  They were (and still are) reputed to 
be one of the more knowledgeable and competent shops for Lancair IV's.

So what are the REAL limits?  For your airframe you would need a GVT and 
correlation of the FEA to your specific dimensions and layup variations, or 
you are just guessing.  Going a given speed and introducing doublets is 
irrelevant unless performed at all G, CG, and weights in the aircraft's 
envelope.  Little things like changes in fuel load or a new logo on the tail 
can have drastic effects (we lost an Unlimited because of this and it was 
BELOW the tested dive IAS and Mach when it happened!).  The Columbia 300 and 
400 are held to much tighter tolerances, but it's a different wing and 
they're not kits.

On props, a heavier prop will have higher inertia and dampen torsional 
vibration by  mass;  a composite prop will tend to dampen torsional vibration 
by virtue of the superior dampening of the wood/composite blade structure.

MT props are light and composite;  Hartzell are heavy and metal.  Which is 
better is a question of the engine and prop dynamics combined.  Anyone want 
to make a heavy composite prop?
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