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Salutti tutti,
Finally my copy of Martin Hollman's book Modern
Aircraft Design has arrived. Here's some excerpts that are relevant to the
recent LML discussuion about the LIV wing design and strength and G loads.
From Chapter 4, Loads per 14CFR, Part 23, page 61
paragraphs 2&3.
"We must make a decision as to what category our
aircraft will be designed to and we have three categories to pick from. For the
normal category, 14 CFR, Part 23 specifies that the maximum g limit load factor
is 3.8gs. For the utility category 4.4gs is specifed, and for acrobatic, 6.0gs
is required however we have been using 8.75gs in the design of acrobatic
aircraft. Pilots in acrobatic aircraft commonly pull 10gs"
The positive limit load factor, n1, is the g load
that we design the aircraft to. It is defined as positive if it is pushing the
pilot into the seat. By the way the Ercoupe and Cessna line of aircraft are
designed to the normal category. If we intend to perform mild maneuvers such as
loops and rolls at gross weight we design to the utlity category. Most composite
aircraft such as the Stallion and Lancair IV are designed to the normal category
with n1 = 3.8gs"
From Chapter 10, Wing Structural Testing, page 133,
paragraph 1.
"In my past literature I have advocated using a
safety factor of 2.0. However since the certification of the Starship in which a
safety factor of 1.5 was used by the FAA, I have been using a safety factor of
1.5. The practice of using a safety factor of 2.0 comes from the Aerospace
industry which requires a safety factor of 1.33 in addition to a factor of 1.5.
Multiplying 1.33 times 1.5 gives 2.0. The factor of 1.33 is used to account for
such things as temperature effects and is normally thought of as an ignorance
factor. If elevated temperature material properties have been used a safety
factor of 1.5 is considered to be adequate"
(elevated temperature material = high
temperature cured prepregs such as wing skins, fuselage mouldings
etc)
From Chapter 6, Structural Sizing of a Composite
Wing, Table 6.4 Lancair IV Wing design Data, page 106.
"Wing Span = 30 feet. Tip chord 30 inches.
Root chord = 50 inches. Washout = 2.0 degrees. Sweep = 0 degrees. Airfoil moment
coefficient = -0.05. Spar cap width = 5.0 inches. Fuselage to wing BL
= 25.5. Limit load factor = 4.4 gs. Gross weight less wing weight = 2,900
- 310 = 2,590 lbs. Limit flight maneuvering speed , Va @SL= 200 mph.
Design dive speed, Vd @ SL = 360 mph. Safety factor = 1.5 for ultimate
loads.
(There's more in this chapter but not relevant for
this discussion.)
From Chapter 6, Figure 6.13, page 111.
This is an engineering drawing reproduction scaled
to fit the page. The notes in the bottom right hand corner read:
Notes:
1) Gross wt = 3,000 lbs. Less wing panel wt =
160 lbs/panel
2) Limit load factor = 4.4 gs, Safety factor
1.5.
3) Vd = 360 mph, Va = 200 mph
4) (....about adhesives, not
relevant)
Like the wing area you don't have but always
thought you did, now you know the safety factor is 1.5 ONLY for a max gross
weight of 2,900 or maybe 3,000lbs.
Be carefull, be safe,
Ciao,
Roberto d'Italia.
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