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<< Lancair Builders' Mail List >>
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I got a couple of questions on my last post and thought the answers might be
useful to everyone planning to go high and fast.
<< ... but why is the percentage of the speed of sound the true structural
limit for an aircraft? >>
I didn't quite put it that way, sorry for the misunderstanding. The Mmo of
an aircraft is typically a given percentage of its controllable Mach limit
speed. As you near the aircraft's Mach limit, a supersonic "bubble" of flow
forms as the air is accelerated over the airfoils. The higher the thickness
to chord ratio, the lower the Mach number at which this occurs. This bubble
collapses in the pressure recovery region in a shock wave that disrupts the
boundary layer behind it. As you go faster, the velocity in the bubble
increases and the shock becomes strong enough to cause boundary layer
separation. If (when) this separation blankets a control surface, loss of
control in that axis can result regardless of the dynamic pressure.
This separation can also be unstable and cause oscillations in the control
surfaces leading to failure at well below Vne. The early 20 and 30 series
Lear Jets were infamous for Mach overspeed shock induced separation causing
rapid oscillations in the ailerons which led to aileron failure and abrupt
departure from controlled flight. Often, the last thing ATC heard was the
aircraft flipping over hard enough to key the mic with each tumble...
So Mach is more of an aerodynamic stability and control limit; although
going divergent in any axis can lead to structural overload even at less than
Vne IAS.
Vne has always been general aviation's limit metric because we tend to not
fly too high or fast in our Cessnas to worry about Mach. Now that we have
Lancairs that have half the drag of a 182 and 2 to 4 times the power, we have
a problem. Hollman has actually admitted that he has a range of flutter
limits for the Lancair depending on which codes he uses and which layup
schedules he assumes. The range is quite broad, from over 360 KIAS to lower
than most have already flown!
<< How would you recommend to actually measure the shape of the completed IV
wing in order to make aerodynamic improvements (longer laminar flow?)? We
are now at that stage. In other words what are the priorities in wing shape?
>>
How to guarantee your wing is accurate? Jeez, no offense here but we're
talking about one-off wood assembly tooling, hand carved ribs, slop layup
flanges, and a blind closeout with no control of the bondline displacement or
thickness. 10% variance is pretty good. The only thing I can recommend is
to take an accurate plot of the airfoils off the plans, laser level the
tooling, cut your ribs accurately, do a practice section closeout to find the
actual bond line thickness, repeat it all at each rib so that they are all
the same, etc.; do everything possible to drive that manufacturing tolerance
down to 2 or 3%. It takes machined tooling to do it right to 1%. Hand laid
up and finished plugs, splash tooling, hand cut ribs, etc., just doesn't lend
itself to manufacturing accuracy. Unless the ribs have premolded flanges and
everything comes out of CNC gantry milled tooling and is made to the kind of
tolerance that the Thunder Mustang, Nemesis NXT, and others (including us)
have or will use, its simply a crap shoot.
On the plus side, the original Lancair IV design was radically overbuilt for
the limited power available. Now that power is being introduced to push the
limits of the design, we need to know where each aircraft stands in that wide
range of manufacturing tolerance. It would be easier and cheaper to machine
new tooling to insure accuracy than it would be to try to measure an already
built wing.
I had planned some aerodynamic improvements to the Lancair IV, there are
three obvious areas that create about 90% of the available drag reduction.
IM<HO, we can pull about 10 to 15% of the drag off a Lancair IV, mostly in
separation drag. I gave guidance on the second most important area to a
customer of one of the new big V-8's and he did a good job. I'm no longer
working on that program, but it should be fairly obvious what was done when
he flies it. As to the rest of the drag reduction, I'm working on other
projects right now and it's not a priority. These mods may also increase the
Mdd of the airframe by M.02 to .04 and provide for more warning before loss
of control. Unfortunately, we'd have to equip a few aircraft with ejection
seats and destroy a couple to be sure.
Anyone wonder why I'm focusing on all new aircraft design that takes all of
this into account from the start?
Increasing laminar flow up to the design limit is simply a function of
reducing surface waviness and increasing smoothness to the maximum possible.
Sailplane drivers and tuners have great info on what to do. Beyond this, any
given airfoil will have a given percentage of design laminar run for a
specific Reynolds number and lift coefficient. It's really not possible to
go farther than this design value without changing airfoils or going to
active boundary layer control (suction).
Be happy with your L-IV. It's a fine aircraft with performance that the spam
cans can only dream of. All of these hot engines going in them have great
potential to go right past every limit Lance ever conceived of. Use your
power wisely.
Eric
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Please send your photos and drawings to marvkaye@olsusa.com.
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