Brodhead Pietenpol Association

Original Posted By: Greg Cardinal
Subject: Pietenpol-List: Strength of Various Woods>Hello Brent!>As promised I dug out my copy of ANC-18 Design of Wood Aircraft>Structures, 2nd ed. 1951, which is 234 pages of wonderful details on>wooden aircraft construction methods. This manual was put together for>joint Air Force, Navy and Civil use. I'll provide some basic numbers>comparing Sitka Spruce, Douglas Fir and Western Hemlock based on a>typical 15% moisture content from Table 2-6, p. 22.> Density (lb/cu ft) Spruce 28> Fir 33> Hemlock 30>> Static Bending Spruce 5,300> Fiber Stress at Fir 5,900> proportional limit Hemlock 6,200> (psi)>> Compression parallel Spruce 3,530> to grain fiber stress Fir 4,220> at proportional limit Hemlock 4,080> (psi)>> Shear strength Spruce 990> parallel to grain Fir 950> (psi) Hemlock 860>> Tension strength Spruce 9,400> parallel to grain Fir 10,900> (psi) Hemlock 11,000>>So what does all this mean? There are other physical properties in>ANC-18, but I think these are enough to show that the hemlock, properly>selected for quality and adhering to the standard grain runout of no>more than 1 inch in 12 (preferrably 1 in 15), is an acceptable>substitute for spruce in the Air Camper. The Air Camper is a pretty>conservative design and is not intended for aerobatics, so the hemlock>should provide you with the robustness Bernie Pietenpol intended. You>might want to make up some small test samples of your wood to pull and>bend with your adhesive of choice for your own peace of mind and the FAA>inspector.>>In case you were wondering, I am a design engineer for the Lockheed>Skunk Works. Just so you knew the data didn't just come off the>street. Hope this helps.>Mike List>________________________________________________________________________________

Original Posted By: Brent Reed
Hello Brent!As promised I dug out my copy of ANC-18 Design of Wood AircraftStructures, 2nd ed. 1951, which is 234 pages of wonderful details onwooden aircraft construction methods. This manual was put together forjoint Air Force, Navy and Civil use. I'll provide some basic numberscomparing Sitka Spruce, Douglas Fir and Western Hemlock based on atypical 15% moisture content from Table 2-6, p. 22. Density (lb/cu ft) Spruce 28 Fiber Stress at Fir 5,900 proportional limit Hemlock 6,200 (psi) Compression parallel Spruce 3,530 to grain fiber stress Fir 4,220 at proportional limit Hemlock 4,080 (psi) parallel to grain Fir 950 Tension strength Spruce 9,400 parallel to grain Fir 10,900So what does all this mean? There are other physical properties inANC-18, but I think these are enough to show that the hemlock, properlyselected for quality and adhering to the standard grain runout of nomore than 1 inch in 12 (preferrably 1 in 15), is an acceptablesubstitute for spruce in the Air Camper. The Air Camper is a prettyconservative design and is not intended for aerobatics, so the hemlockshould provide you with the robustness Bernie Pietenpol intended. Youmight want to make up some small test samples of your wood to pull andbend with your adhesive of choice for your own peace of mind and the FAAinspector. In case you were wondering, I am a design engineer for the LockheedSkunk Works. Just so you knew the data didn't just come off thestreet. Hope this helps.Mike List________________________________________________________________________________

Original Posted By: michael list
Your the man, Mike! Great info! Just so I'm clear, grain run out of lessthan 1 in 12 would mean that as it runs lengthwise it is going to one sideof my board less that one inch for every 12. Is that right?I found Hemlock at Home Depot. It's graded C or better. If I selectstraight grain (as above) knot free 1"x 6" for my ribs am I choosing wisely?This group is invaluable! Thanks.Brent ReedKent WA-----Original Message-----

Original Posted By: "McNarry, John"
Brent Reed wrote:> Your the man, Mike! Great info! Just so I'm clear, grain run out of> less> than 1 in 12 would mean that as it runs lengthwise it is going to one> side> of my board less that one inch for every 12. Is that right?>> I found Hemlock at Home Depot. It's graded C or better. If I select> straight grain (as above) knot free 1"x 6" for my ribs am I choosing> wisely?>> This group is invaluable! Thanks.>> Brent Reed> Kent WAJust one thing that might not be so obvious about grain run out. Youmust make sure that the you take into consideration the combined*angular* runout on both faces so that it is not more than 1 in 12.Sometimes you will have adequate runout specs on the face of the boardonly to be countered by unacceptable runout on the edge. I think theformula for determining the sum of both sides is A squared plus Bsquared = C squared (this is just off the top of my head so may not beright) Granted however, if most boards are really straight on the face(mine was 1 in 40 or 50") you will be fine.Stevee________________________________________________________________________________