Original Posted By: "walter evans"
I did some research in my extensive aeronautical library and have turned upthe following:Source: Aircraft Design Vol II Aero Structures by Cecil HughLatimer-Needham published 1939 says:"The point of wheel contact with the ground [not the axle centerline] shouldmake an angle with the vertical through the CG in side elevation (aircraftrigging position) of not less than 10 degrees, and may increase with wheelbrakes up to a maximum of 14 degrees. The tendency of aircraft to overturnwhen landing with wheel brakes may be provided against by arranging for thebrakes to be released as the tail skid leaves the ground. It has beenfound, however, that if the angle, made by the line joining the point ofwheel contact [not the axle centerline] to the CG with the vertical throughthe CG when the tail skid is touching, is at least 20 degrees no overturningis likely. An angle much greater than 20 degrees results in excessive tailloads at landing and difficulty in taxi-ing and at takeoff. It may be notedthat the overturning tendency is not great at the moment of touching down,since the aircraft is still largely airborne and skidding of the wheelstakes place. Towards the end of the run both aerodynamic lift and controlare negligible, the braking effect increases and for safety the braking laodshould be decreased."Latimer-Needham is an Englishman.rigging position refers to tail elevated so that upper longereons on thepiet are level.Source: Prelimiary Aircraft Design by R. C. Wilson published 1941 says, inthe context of drawing a side elevation with the tail in rigging position,as defined above:"E. Location of the Landing Gear (Main Wheels and Tail Wheel System) Fromthe assumed CG draw a line downward and forward making the vertical an angleof not more than 20 degrees nor less than 12 degrees. The former angleshould be used when extremely effective brakes are to be used, the latterwhen no wheel brakes whatever are contemplated. A fair normal figure forordinary wheels and brakes is 18 degrees. The axle of the main wheels [notthe point of wheel contact with the ground], with shock absorbers fullycompressed, will lie on this diagonal line."Wilson is from the U.S.I added the comments in the [ ] for emphasis. Note that the Brits definethe diagonal line as that from the CG through THE WHEEL POINT OF CONTACTWITH THE GROUND. The Americans define the diagonal as that from the CGthrough THE AXLE CENTERLINE. The diameter of the wheel, therfore, comesinto play in the British scenario.I hope this helps.Chris Bobka________________________________________________________________________________