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#1
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Testing Stick Ribs
Rib Testing
The rib is supported upside down by a jig simulating the wing spars. The load, consisting of a single mass suitably divided by balance beams, is applied to the upper edge of the rib (as positioned in the jig), distributed by pallets of wood as needed to prevent stress concentration. The test load is divided according to the manner in which lift is distributed across the chord of the wing. For the purpose of testing airfoils having a thickness ratio of 18% or less intended for airspeeds of 150mph or less, the following load distribution has been standardized. As measured from the nose of the airfoil: Zone 1 = 0 to 19.1% of the chord Zone 2 = 19.1% to 46.2% of the chord Zone 3 = 46.2% to 90% of the chord Note that no load is placed on the extreme trailing edge of the rib. The test weight is distributed according to the following schedule: Zone 1 = Half the weight Zone 2 = One-quarter of the weight Zone 3 = One-quarter of the weight Testing Procedure The mass representing the test weight is to be positioned on a tray or pallet below the test jig, suspended from the mid-point of a balance bar, one end of which applies its load to Zone 1, the other end being divided by a second balance bar so as to apply its load equally to Zones 2 & 3. When the testing jig has been assembled and balanced, weights are applied to the tray or pallet according to a schedule provided by the designer. The objective is to increase the weight in a graduated manner beginning with large amounts then tapering off with small amounts until the rib fails, at which time the last amount added to the pallet is subtracted from the total. Practical Aspects. Ensure you have enough weights on hand. A properly designed rib weighing only a few ounces is usually capable of supporting several hundred pounds. Lead in the form of pigs, bars or bags of shot has proven to be the most practical form of weights. Each weight must be individually weighed and marked. The weight of the balance bars, pallet and the stays connecting them must be included in the total weight. Proof of Concept & Quality Control. For a new design at least ten samples should be tested in order to define the minimum acceptable standard for strength. For a proven design, three ribs, randomly selected from each production batch should be tested. Should any of the samples fail to meet the minimum acceptable strength, the entire batch must be condemned. - - - - - - - - - - - - - The information above has been extracted from static and dynamic testing procedures found in the Civil Air Regulations, Part 04 (circa 1936) and the structural test section of the ‘Handbook of Instruction for Airplane Designers,' Air Corps, U.S.Army (circa 1937) -R.S.Hoover |
#2
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This test procedure is overly simplistic and may not test the critical loading condition of the ribs. All ribs should be tested for at least two conditions - in some cases possibly three (a negative g case). I will just stick to two here. 1. High Angle of Attack (Cp forward) - this appears to be the test condition that you have outlined. 2. Lower Angle of Attack / High Speed (Cp rear i.e. Max Pitching moment). This will result in a more uniform load over the rib with a more constant distribution. The test schedule you have outlined will definitely not test the mid to rear section of the ribs to the maximum loads that they may see in flight. I would recommend the test procedure outlined in AC21-3 Basic Glider Criteria as a more concise and thorough load test schedule. This method tests the different flight conditions mentioned and also make allowance for the variations in pressure distributions over airfoils of different shapes. I have used this schedule for many years on a variety of aircraft .... it is actually amazing just how much load a rib built from 1/4" sq spruce sticks can take ! This publication also provides very detailed instructions on how to mount and load the rib. You can of course also calculate your own load schedules from the airfoils pressure distribution rather than relying on this cook book approach. BTW ... is there a web site where you can download 'Handbook of Instruction for Airplane Designers,' Air Corps, U.S.Army (circa 1937) or are you working from a paper copy. "Bob Hoover" wrote in message om... Rib Testing The rib is supported upside down by a jig simulating the wing spars. The load, consisting of a single mass suitably divided by balance beams, is applied to the upper edge of the rib (as positioned in the jig), distributed by pallets of wood as needed to prevent stress concentration. The test load is divided according to the manner in which lift is distributed across the chord of the wing. For the purpose of testing airfoils having a thickness ratio of 18% or less intended for airspeeds of 150mph or less, the following load distribution has been standardized. As measured from the nose of the airfoil: Zone 1 = 0 to 19.1% of the chord Zone 2 = 19.1% to 46.2% of the chord Zone 3 = 46.2% to 90% of the chord Note that no load is placed on the extreme trailing edge of the rib. The test weight is distributed according to the following schedule: Zone 1 = Half the weight Zone 2 = One-quarter of the weight Zone 3 = One-quarter of the weight Testing Procedure The mass representing the test weight is to be positioned on a tray or pallet below the test jig, suspended from the mid-point of a balance bar, one end of which applies its load to Zone 1, the other end being divided by a second balance bar so as to apply its load equally to Zones 2 & 3. When the testing jig has been assembled and balanced, weights are applied to the tray or pallet according to a schedule provided by the designer. The objective is to increase the weight in a graduated manner beginning with large amounts then tapering off with small amounts until the rib fails, at which time the last amount added to the pallet is subtracted from the total. Practical Aspects. Ensure you have enough weights on hand. A properly designed rib weighing only a few ounces is usually capable of supporting several hundred pounds. Lead in the form of pigs, bars or bags of shot has proven to be the most practical form of weights. Each weight must be individually weighed and marked. The weight of the balance bars, pallet and the stays connecting them must be included in the total weight. Proof of Concept & Quality Control. For a new design at least ten samples should be tested in order to define the minimum acceptable standard for strength. For a proven design, three ribs, randomly selected from each production batch should be tested. Should any of the samples fail to meet the minimum acceptable strength, the entire batch must be condemned. - - - - - - - - - - - - - The information above has been extracted from static and dynamic testing procedures found in the Civil Air Regulations, Part 04 (circa 1936) and the structural test section of the 'Handbook of Instruction for Airplane Designers,' Air Corps, U.S.Army (circa 1937) -R.S.Hoover |
#3
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"smjmitchell" wrote in message . au...
This test procedure is overly simplistic and may not test the critical loading condition of the ribs. I would recommend the test procedure outlined in AC21-3 Basic Glider Criteria as a more concise and thorough load test schedule. This method This publication also provides very detailed instructions on how to mount and load the rib. BTW ... is there a web site where you can download 'Handbook of Instruction for Airplane Designers,' Air Corps, U.S.Army (circa 1937) or are you working from a paper copy. Is there a place where one could down load this "more concise" manual? A quick Google didn't turn it up. =============================== Leon McAtee "Bob Hoover" wrote in message om... Rib Testing The rib is supported upside down by a jig simulating the wing spars. The load, consisting of a single mass suitably divided by balance beams, is applied to the upper edge of the rib (as positioned in the |
#4
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Yeap ... go to the FAA web site and then big down till you find a link to
the historical Advisory Circulars (AC's). This will then link you to the DOT Digital Special Collection or somthing like that. Just search for AC21-3 when you get there. "Leon McAtee" wrote in message om... "smjmitchell" wrote in message . au... This test procedure is overly simplistic and may not test the critical loading condition of the ribs. I would recommend the test procedure outlined in AC21-3 Basic Glider Criteria as a more concise and thorough load test schedule. This method This publication also provides very detailed instructions on how to mount and load the rib. BTW ... is there a web site where you can download 'Handbook of Instruction for Airplane Designers,' Air Corps, U.S.Army (circa 1937) or are you working from a paper copy. Is there a place where one could down load this "more concise" manual? A quick Google didn't turn it up. =============================== Leon McAtee "Bob Hoover" wrote in message om... Rib Testing The rib is supported upside down by a jig simulating the wing spars. The load, consisting of a single mass suitably divided by balance beams, is applied to the upper edge of the rib (as positioned in the |
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