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<< Lancair Builders' Mail List >>
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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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Please send your photos and drawings to marvkaye@olsusa.com.
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