Sunday, February 7, 2021

Motor mounting comparison to other eGoats

I've decided not to mount the motor in this traditional location behind the seat.  That location is closer to the CG, so it makes sense for very heavy engines to have a smaller impact on balance.  That location is also large enough to eat a reasonably sized propeller (~49in).  But, it is almost entirely shielded by the pilot's body, meaning there is a lot of turbulence that the propeller has to eat.

Here's the Lima Goat (Pelican) with the typical motor mount location.  From what I can find, this setup is ~20kg according to Alex Ferrer in a YouTube comment (6.4kg motor Freer 202/80 27, 1.9kg KBL ESC 72V-144V/400A, 6.68kg six CNHL 6S 8.0Ah =1kWh, ~2kg wood prop 49x30in == ~30kg thrust, ~6min WOT or 10min sustain for 170kg TOW)

Instead, I'm going out front on a post ahead of the forward nose tube.  This is a traditional location for human powered aircraft (e.g. MIT's Daedalus), and the Romanian Goat was the first Goat that I've seen to do it.  It puts the prop in the cleanest air possible, allowing getting the most out of the propeller efficiency (prop selection matters too, but so is getting the most from that installation by putting it in clean air).  A typical downside is the forward location doesn't match well with heavy internal combustion engines without a long driveshaft.  Even human-powered aircraft usually have a long driveshaft arrangement to make it work.  For me, I went for a minimally sized motor, so the balance works out just fine, and I really need to maximize the propeller efficiency.

Here's the Romanian Goat with the forward motor mount.  From what I can find, this setup is 28kg (4.44kWh battery, 20kW motor (2350rpm, 78kg thrust), 1.3m Helix 3-bladed prop, 45min runtime).

Here's the first mock-up of my selected mount location.  The motor is mounted on its tube (yes, the prop is on backwards) and clamped very roughly in the right place.  I'm planning to pin the mount to the nose tube and support it up from the nose tube roughly parallel to the prop.  If it does not feel sturdy enough, I can add guy wires from near the motor to the leading edge of the wing, which should be more than sturdy enough.


Key weights so far come to 9.3kg (2.5kg motor, 0.4kg ESC, 3.2kg for four 6S 5Ah = 0.4kWh).

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