From: StarAerospace@aol.com
Message-ID: <fb.2251192f.29af3d37@aol.com>
Date: Thu, 28 Feb 2002 02:58:47 EST
Subject: Mach limits and manufacuring tolerances
To: lancair.list@olsusa.com
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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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