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Saturday, July 9, 2011

Yak vs. Yak, Round 4

The "Monster" is in the house ...let the next round of flight testing begin.

You know your Yak is a monster when...
Its four wing bolts are four times longer than the Carbon Z's two wing bolts
You can land a .10-size Triplane in the battery hatch
The embedded carbon fiber wing tube is longer than the smallest Yak's wingspan
After you unzip it, your girlfriend just wants to stare at it for a while

The 1.5M Monster Yak 54 from E-Do Model (not "Models." E-Do sell lots of RC Airplane) is a lot of fun to get in the air. It's even more fun to get it flying properly. Initial testing was interesting, as the plane is severely tail heavy as shipped, and correspondingly misbehaved.  Adding to the fun, is motor that could be a lot happier.  Nevertheless, the plane seems to have a lot of potential, and for $135 for 680 square inches of wing area, it seems worth my time.

The plane has a lot of beef for a foamie. Not only is the span about five feet, the girth of the fuselage impressive. This big Russian thug is not light, either, especially after you fix the factory CG with the requisite 4500-5000mAh battery. A 3200 4S is recommended, but with the battery located directly behind the motor, that's majorly tail happy. Even with a 4000mAh, my Yakolev wanted to climb strongly (that is, to dive in the 'up' direction) when inverted.  A 5000mAh battery brings the CG to the manual's recommended location, which is conveniently marked with a panel line in the foam at the max thickness of the airfoil (see above photo).  And with that big battery for CG placement (plus the unwanted bonus of 12+ min flight times) comes a more massive behemoth, bringing the plane's electricity-free 69 oz mass up to a portly 88 oz flying weight.  Ouch.  This motor has work to do.

Unfortunately, the motor is a place the manufacturer skimped a little to save yuan.  With the included 14x7 prop, there is a distinct "dinner is almost ready" smell when the motor is really cooking.  Propping down to 13" seems prudent, but the cowling is massive and presents a formidable obstacle to thrust production.   My original plan was to go with a MAS 13x6 K-series, to get more blade area out around the cowl boundary.  Unfortunately, the motor has a bit of difficulty spinning that taper profile, much like a true 14 incher, and the thrust was not significantly better than a 13x6 sport profile.    So to relieve the motor of extraneous workload, given the up-hill battle it's already facing, I decided to try the 13x6 first.

The big Russian fireplug felt spirited with a 5000mAh lipo for balance and a 13x6 on the nose.  Inverted flight was perfect, the nose stays up but the plane no longer wants to climb skyward when upside down.  The price to be paid is thrust-to-weight ratio of just over 1:1 until 15.2 Volts or so, then the Yak slips beneath 1:1.  By normal aerobatic standards, the Monster 54 has decent power, completing big loops without fuss and even holding a knife edge well, but by an RC 3D standard, she's reluctant to put out.

First things first, during flight testing, I found the Yak's overly-complex color scheme difficult to orient, with its multicolored scheme on a white base, both top and bottom.  So I took a few liberties with the Yak's top scheme.  Now it is dominant warm on the top and cool on the bottom:

Most of the white on top is gone to eliminate confusion with the
primarily white bottom.  Plus I rounded a few jagged edges.
No changes except the thin sheet aluminum on the wingtips, chin
and air scoop.  I also added a cowling vent (plus more firewall 
ventilation) for better motor cooling.
With the plane visible, on to the fixes.

Shenzen's E-Do 1.5m Yak 54 website claims a 1:1.3 max Thrust-to-Weight ratio for the monster HKY54.  It is unclear if they are correct, since I think they meant 1.3:1 based on the contex.  1:1.3 might be more like it toward the end of battery life.  Fortunately, I found a few easy to fix T:W problem areas.  In rough order of importance:

Easy to find thrust issues:
  1. (Major) The included 14x7 knock-off prop doesn't have a proper twist profile.  The prop twists too quickly, to near 0 incidence at the tip (see photo).  Combined with the flower pot cowl, this flaw robs most of the installed thrust from the motor.  The stock prop pulls 56A, and most of that electricity is producing noise, not thrust.  At speed, any portion of the prop near 0 incidence produces a negative AoA (relative wind = prop rotational vector summed with the aircraft's forward vector; as the speed increases, so forward motion swings the wind vector to the negative-AoA-side of shallow incidence) and the prop tips actually push backwards until the blade reaches zero- thrust equilibrium.
  2. (Minor) The spinner and motor mount facade blocks all cooling airflow.
  3. (Major) There is very little exit airflow from the cowl.  There should be twice as much exit area opening as inlet area.  Combined with 1 and 2 = overheat waiting to happen.
  4. (Major) 14x7 is too much prop for the engine power.
Chinese knock-off 14x7 prop on top, APC 14x7 on bottom
Easy fixes:
  1. Switch to a real prop.  See #4 for more.
  2. Change spinners.  As a creative solution that looks ok, I used a PZ Focke Wulf 190 spinner.  It is a front mounted spinner (goes under the prop nut, not behind the base) so there is no blockage of the motor coil inlet area.
    A front mounted spinner lets the motor breathe.  
    See paint scheme photos, above, for the overall look.
  3. Used a Dremel sanding drum to open firewall exit area around the ESC and between the decorative muffler downpipes.  I also intend to change the decorative pipes to real aluminum tube, shortly.
  4. After concluding that the stock 14x7 is too much prop for the motor, and trying a bunch of 13" props with too weak T:W (after the big battery to fix the CG problem) I finally found an excellent match:  the APC 14x6 Pattern Competition Propeller.  It is a bit pricey at around $12, but to fix this big mama it is so worth it.  This prop pulls the plane, including the 5000mAh 4S, nearly 40% harder than the stock knock-off prop.  Better still, as a scimitar it pulls a cooler 47A. Combined with 1,2, and 3, I hope this is a 100% thrust fix for the HKY54.
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