This is where having been an airline captain for so long does help. To the studs we've added 4" posts on the topside which will eventually support the flight-deck, whilst on the underside I've added four off-cuts to form legs of around 13" length. All of these include M6 threaded inserts as previous, in order to mount skids and a flight-deck.
The posts up top need be a minimum of 3" incidentally to allow for those threaded inserts beside 2" for the actual stub. The lower legs intended to support the skids will be reduced to nearer 9" or 10" however, a length based on a combination of both flight and structural requirements. The legs are the Achilles' heel of the construction, not so much fore and aft as regards side loads imposed on them and though steel-reinforced they may yet require pressed-steel brackets as well.
During take-off and landing maneuvers a bank-angle warning is a commonplace with airliners, the 737 for instance risking an engine-scrape during crosswind landings. As the primary flight instrument is calibrated however in tens of degrees, bank angles of say 25° or 35° are unmarked. As a consequence I have targeted 20° as a mark beyond which 'there be dragons' for the incautious pilot during those critical phases of flight.
The 40" propeller is suspended 3" by the underslung T-motor U15 and so we need to add that to the clearance calculation (though the skid-connectors will provide an inch and a quarter toward that). Meanwhile as the propeller projects 20" from its axis, then 20" and 20° is a memorable combination of figures.
The calculation on the right of the picture calls for 7.25" from which we subtract 1.25" for the skids, however, so that we need a 6" drop and 3" to allow for the displacement by the motor: this therefore requires undercarriage legs of around 9" apiece.
I may drop that to 8" or increase it to 10" instead, depending on how I feel on the day...
