On Sunday, March 20, 2016 at 11:52:47 AM UTC-7, Andy Blackburn wrote:
On Sunday, March 20, 2016 at 9:53:48 AM UTC-7, DaleKramer wrote:
I am not saying the mechanical dynamics behind the broomstick analogy is different, just the visualization of it is a little different.

You are correct - it was a crude analogy. The main point (which you clearly understand) is that whole thing in vertical flight is quite likely statically unstable and if it tips over beyond a certain angle of vertical it is likely to flop over nose down. Unless you have enough altitude when this happens to get flying speed and pull out...well it could be a problem. Ideally you want to keep it stable so you never get to that angle until you have flying speed, which entails tipping over with differential electric thrust enough to get flying speed through the deep stall where the wing and elevator can operate. Yes indeed adding big controls on the tail and big old Fowler flaps on the wing could help you get stable at a higher angle of attack and lower speed. Lots of the hybrid aircraft-helicopter designs I've seen resort to tilt-wing to facilitate the transition more easily but all of that adds weight and complexity.

A simple calculation would be (without benefit of the dimensions on your plans) 220 lbs of thrust from the bottom tail motors produces a nose-up moment of 880 lb-ft based on a 4-foot offset from the center of mass. If the center of mass is 5 feet in front of the lift of those two motors that are trying to hold the nose up and you have a TOW of say 500 lbs you'd end up with a nose-down moment of 2500 lb-ft which would overwhelm the ability of the lower motors to right the aircraft as you approach horizontal. Now calculating the nose-down moment for totally horizontal is not realistic as you'd be accelerating before you ever got to horizontal but a little trig would tell you roughly what kind of angle off vertical the voters have the juice to recover from. Obviously you also have the other motors pulling as well and the prop wash over the tail adds a bit of moment, but the motors on the vertical centerline mostly just reduce the effective mass feeding the nose-down moment for reasonably vertical orientations. They don't help you at all as you approach horizontal. I expect there is somewhere around 30-40 degrees off of vertical where things get interesting and you better have some forward velocity and altitude before you let the nose get that tipped over. Sounds like you have a computer program to figure it all out but my gut feel of is that the last half of the transition to forward flight could get pretty sporty - wing still stalled but the two bottom motors have run out of ability to add enough nose up moment. The reverse maneuver could be even more exciting - a zoom has been mentioned.

Of course with enough thrust almost anything is possible. :-)

Again, thanks for sharing. Interesting design. All in all I think I'd rather have one of these than those scaled-up quadcopter drones people are promoting for personal transportation. Yikes!

Andy

People have been known to climb aboard rockets. Rockets have the exact same requirement: you have to keep them pointed upward -- at least until you get airspeed for meaningful wing lift.

One could argue that relying upon motors to keep working is pretty routine for flying machines. A helicopter needs to have it's one motor keep working when it's taking off vertically to avoid a dire consequence.

Back to battery life... there must be enough for an aborted landing scenario. That means electrics on as you're screeching to a stop through deep stall deceleration, then maneuvering to the desired landing spot in vertical and letting down sufficiently gradually. If there is too much wind or if the landing site didn't end up in the right place or something is wrong at the site, there must be enough power reserve to blast back up vertically to flying speed and perhaps a horizontal landing elsewhere.