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Salutti tutti,
Time for some myth busting:
...data data Data! where to start? How
about with tonight's LML Digest and and Brent R's assertion that "you could load
a IVp to 28,500lbs and it would still fly" Well, sorry Brent, it won't.
(Brent, if you were Kelly Johnson and I was Ben Rich I'd collect a quarter from
you. Allowing for extreme inflation lets call it $100 and I'll post you my
address....)
It won't fly because the wings have fallen off.
Actually that's not technically correct, the wings have clapped...at 18,500lbs
of wing lift because that's what the wings were designed to lift and that's
what they will lift....well, sort of. We'll get to that little "gotcha"
shortly.
Lets look at Data, from the horse's mouth. The best
source of information on LIVs I know of are Martin Hollman's book "Modern
Aeroplane Design" vols I, II, and also
"Boogie with a Turboprop". Should be required reference books for ALL LIV
builders/owners/pilots but I do digress.... In "MAD" vol I, Chp 6, is a
section describing the LIV wing. Page 106 gives some pertinent
information, (I'll only quote that which is pertinent for the purpose of
this discussion). Limit Flight load factor = 4.4gs. Gross Weight = 2,900 lbs,
minus the wing weight of 310 lbs = 2,590 lbs. Limit Flight maneuvring speed, Va
= 200 mph. design dive speed Vd, 360 mph. (both speeds at SL). Safety Factor =
1.5 for ultimate loads.
Page 111 of Vol I shows a copy of an engineering
drawing of the Lancair IV wing layup schedule. In the bottom right hand corner
in the notes section are notes I & II, as follows: 1) Gross wt 3,000lbs less
wing panel wt 160lbs/panel. 2) Limit load factor = 4.4 gs, Safety Factor
1.50. (interesting is it not? ...Safety factor of ONLY 1.5 Kinda blows the
200% Ultimate Load Factor myth out of the water...no?)
Let's skip to page 113, Fig 6.15, Wing Deflection,
skin shear stress, etc. Quote "An actual wing structural test was performed for
the load condition of Vd = 360 mph. The wing failed under a total load of 9,250
lbs and a deflection of 15 inches. The wing load at 4.4 Gs is: Load = 2590 x 4.4
x (154.5/360) = 4891 lbs". Therefore with 4.4 Gs load equaling 4891
lbs wing panel lift we can easliy calculate the max G load at the wing
panel point of failure of 9,250 lbs = 8.32 Gs.
What a great design! Nearly 200%
ultimate load factor...lot's of margin built into this wing... let's
up the gross weight and our wing can carry it for sure!
Yeah right...sucker! ....myths based on assumptions
based on wishfull thinking....
Let's backtrack to page 107, top of the page
"...Structural Dimensions for an airspeed of Va = 200 mph and an angle of attack
of 14 degrees, and for an airspeed of Vd = 360 mph and an angle of attack of
2 degrees are determined". Then para 1: "Figures 6.10 & 6.11 show
that the maximum bending moment is expected for Va = 200 mph since the tip is
loaded more than at Vd = 360 mph" So what's this then? what does it mean?
Let's go look at Figs 6.10 & 6.11... a couple of graphs and a
couple of tables of, wait for it.... DATA!!!
The graphs on page 108 are self explanatory and
show clearly that at Va the wing tip is loaded more than at Vd. Then page 109
gives numbers...lots of them but it easy to make sense of it. Our
interest are the lines (wing station) WS2 on both tables. Firstly for
Va, 200 mph, 14' AoA, the wing panel lift = 4952 lbs outboard of BL 18
ie. and wing bending moment at BL 18 is 28,554 ft-lbs. For Vd, 360
mph, 2' AoA, the wing panel lift is 4709 lbs outboard of BL 18 and the wing
bending moment at BL 18 is 22,436 lbs. In other words, for nearly identical
wing panel lift for Va and Vd conditions, the wing bending moment at Va,
... ie the force that breaks the wings, is 28,554/22,436 = 1.27 times greater
than at Vd. Got your attention now I'll bet!
To tie it all together, to know the G load
achievable at the point of wing failure at Va, divide the 8.32Gs
achievable at Vd by 1.27 = 6.55Gs
So there you have it: At 2,900 gross weight at Va
your wings will clap at 6.55 Gs or
thereabouts. Perhaps you lead sled builders might want to play in the
margins and in the edges of the bell curve of the wing failure test (a test
sample of 1 to date), maybe your wing are stronger...but maybe not, maybe
Lancair Company has redesigned and strengthened the wing since this data was
published...or maybe not, maybe you think fuel load in the wing alleviates the
wing bending moment and these numbers don't apply to you...but for the purpose
of calculating wing bending moments weight of fuel in the wing is not permitted.
If you think that way you're relying on lots of "maybes" Best of luck to
you and I hope the "maybes" line up in your favour as I don't wish to read more
Lancair accident reports thank's very much.
In this recent once again iteration of Lancair IV
weights on LML others have covered pretty much all the negative points of
increasing gross weights but I'd just briefly like to put some numbers to
increasing stall speeds with increasing weights. Stall speed increases with the
square root of the load factor. If the stall speed @2,900lbs is 66 knots,
increase the weight to 4,000lbs and the increase in load factor is 4,000/2900 =
1.379. Square root it = 1.174. multiply by the original 66 knots
stall speed = 77.5 knots. Lets look at the clean configuration and a stall speed
of lets say 80 knots... multiply by 1.174 = 93.92 knots. Pull a 2 g turn (clean)
and the stall speed = 132.8 knots.
Build 'em light, Really Light!! (it's not
hard) ....and straight...
ciao
Roberto d'Italia.
ps, lead sled builders...don't forget to redo
the white/green/ arcs on your ASI.
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