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XTM 5500

Charlie writes:

A quick update on the XTM 5500 twin motor power unit. Since
initially sharing I’ve had four test flights with various improvements after
each flight. Overall I’m very pleased with the results, I believe with further
refinements I should be able to get 200-300fpm climbs with about five minutes of
flight time while maintaining a total weight of less than 15lbs. The extra
weight results in a handling penalty however it’s much more pleasant to fly
power off than the ehelp design.

Details and videos below.

XTM 5500 Footage:

XTM 5500 Mountain Launch:

Carbon Ehelp:

Flight Notes:

1st Flight – Mountain Launch

My Laminar Z9 was in need of parts so the 1st flight was on my 15 year old
Laminar Z8 which I hadn’t flown in 3+ years. In addition, to the first flight
with power it was also the first time I had flow the Z8 with my Tenex 4. I had
also never mounted the unit on the Z8 so had to bring a drill to launch to
mount the motor unit to the keel.

It took about 30 minutes to drill the 2 holes, mount the battery, and get
everything else attached which was a nice proof of concept with respect to
minimal glider modifications. The extra 15.5lbs is extremely noticeable while
ground handling despite being balanced around the CG. Further, changing the
pitch angle on the ground required noticeable extra force and is slower to
respond due to the inertia of the motor and battery units.

I was launching from my neighbor’s property about 400ft AGL above my house which
is the LZ. It’s a flat slope launch and the wind was blowing a steady 15+ mph
gusting to just over 20mph. The launch was very smooth and the extra weight I
suppose could potentially dampen any rapid changes to pitch angle. I launched
into a very nice cycle and kept my speed up given the low altitude and extra
weight. The air was very active and I quickly thermaled to about 3,000 ft over
launch in strong lift. The glider seemed to be flying very well despite the
extra weight but given how active the air was I’m not sure I would have noticed
if it wasn’t.

Once I topped out I started the motors and ran them for only a few seconds, I
wanted to test the folding prop mechanism which failed to work as designed.
When I killed the power the props remained out and windmilled hard. It was like
flying with a drogue chute, the handling was very sluggish primarily when
turning though pitch controls were fine though. I flew around for a little bit
and climbed up in one more thermal before deciding to land. The glide ratio
was abysmal from the windmilling props and the flare window was very tight.

2nd Flight – Aerotow

First test with Laminar Z9 and Tenex 4. I added mechanical brakes to the motors
since the ESC’s don’t have a brake feature. The flight was somewhat uneventful,
the tow was smooth and the glider flew well, the brakes worked and stopped the
props from windmilling. However there was not enough power to maintain zero
sink with full throttle.

3nd Flight – Aerotow

Second test with Laminar Z9 with Tenex 4. I increased the prop size by 1 inch
from 18.7x12 to 19.7x12. I was the first to launch and we had a ~20mile task
called for the day. After getting off tow I thermaled up to 2500ft agl (cloud
base) and started on task instead of waiting for the others. It was an
extremely weak day and I ended up about 2-3 miles miles out at 800ft I decided to
fly back to the field and flew under full power and maintained my altitude most
of the way back. As I neared the tow field the batteries got low, the power
weakened, and I arrived at 400ft. On this flight I also did some shallow turns
with full power and it flew fine albeit a little sluggish.

4th Flight – Aerotow

Third test with Laminar Z9 with Tenex 4. I increased the prop size by another 1
inch from 19.7x12 to 20.7x14 (pitch increased from 12). Didn’t go anywhere on
this flight but at full power I was climbing at 100fpm, however, the amp draw
was at the max for the motors and I ended up frying them both. Despite killing
the motors the glider was flying really well with the extra power, I noticed a
slight pull to one side when one motor died and the other was still going but it
wasn’t unreasonable.
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