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Saturday, March 5, 2011

$5.99 Air Hog Titan -- 50" Foam Glider Conversion

Walking out of an arts and crafts store, I couldn't resist this simple 50" Air Hog foam glider, sold in a bag for $6. 
How hard could it be to convert to RC?  And it's about time some of those spare parts get airborne.  Turns out, this conversion took a little more time than I thought, nearly a day's work in total, but it came out nicer than I thought too.

The basic idea is a 3 channel powered glider with elevator, rudder, and a light motor on throttle.  The motor is a C10 brushless pusher installed at a prop-protected height, right in the middle of the tail.  It eats about 5A for roughly 6 oz of installed thrust, using a 5x3.8 spinning at about 12,800 RPM on a 2-cell, 300 mAh battery.

The left and right horizontal stabs were sliced off whole, connected with a 1/8" carbon fiber rod, and glued to function together.  I toyed with the idea of using only elevons, but decided to go a more tradition route instead.  The rudder was similarly sliced at the base, a span of metal piano wire poked through the hinge point, through the bottom of the plane, and bent into a steerable tail wheel.  A thin recessed main wheel finishes off a bicycle gear config.  I might add two wire wing skids for upright taxiing if I ever get around to it.  A rubber spinner adorns a chopped nose.

I wasn't in love with the notched canopy's crookedness, or the supplied black stickers to emphasize it, so I devised a simple two tone blue paint scheme to make the bubble look a little more aerodynamic and elliptical.

Total flying weight with battery is 10.3 oz, heavier than I hoped, but the base foam airframe was also heavier than I had imagined at exactly 6 oz in stock form.  Wing loading is a very reasonable 4.4 oz/sqft.  Final T:W is around 60%  Not bad for a sailplane. 

Test flight tomorrow, winds permitting.
Flying:

Not surprisingly, this thing flies great.  The motor was plenty strong and it climbed right out on an arrow-straight, upward arc.  The pusher config was perfect for this plane, though the tiny high RPM motor was a tad ducted fan-esque.

The strong dihedral and wing sweep makes the plane very stable in forward flight.  It also allows the plane to fight through the wind without bouncing astray of the fundamental platform.  Even though it is quite light, it refuses to be tossed beyond a reasonable bank angle.  How refreshing to fly a plane actually frolicking in today's 10G20 winds!

This airplane definitely has the characteristics of a slope soarer, and on a day with a few more thermals and a little less bluster, a skilled glider pilot could probably keep the Hog airborne for quite some time.  Today with a heavy foot on the gas, I was a little disappointed to run dry after 6+ minutes.

Control setup: I like to err on the side of too much control throw rather than too little, so I was surprised to learn that my elevator and rudder throws were not excessive.  I think I got it just right by building-in what I thought was 30% to 40% too much throw.  In turns, rudder-only was certainly adequate, but the nose was forced lower, so significant coordinated elevator was a must.  That was surprising to me, I was expecting more of a level floater.   Ailerons, or maybe better still, differential spoilers, would have helped, especially during today's bumpy, direct-crosswind landing.  Balance was spot-on using the unmodified glider's CG point, the center of the wing chord measured at the root.

The most surprising thing in a flight filled with mostly pleasant surprises, was how big the Air Hog turned.  I was hoping this project would yield a be a really big, slow, cul-de-sac floater to supplement my micros.  Not so much.  I was very happy to have tested this plane in a larger setting.

Loops were easy from full power, level.  With elevator all-in, the loop radius wasn't much larger than the plane is long (ok, maybe I had more than enough elevator throw :).  With consistent throw and power, the back half of the loop dished out, F-22 style.  I think the elevator has a slight thrust vectoring effect on the effective thrust angle.   Barrel rolls were more difficult to accomplish, but certainly doable after losing perhaps 30 ft of altitude.
I was glad I flew two mistakes high for my TP stall series, since my first mistake was assuming the glider would float through the set without drama.  I was wrong.  I arced the nose into the strong winds and slowly took the power off.  The Hog got about 30 degrees nose high in level flight, then wham, the nose fell symmetrically 90 degrees down and the plane accelerated straight to hell.  Exactly one mistake later, the plane was level again.  Phew that's a stall for you!

Landing was U-2-like.  This Hog doesn't want to quit.  And I had no good way to force it lower.  I was a little wary of getting slow in the traffic pattern, for obvious reasons.  You know what it needs?  Thrust reversing.  Let me think on that one for a while.  In the mean time, setting up a long graceful final is prudent, with a long graceful overrun to match.

Overall, I couldn't be happier with this buy.  After a few misfit pieces and parts, my Hog was essentially a free airplane.  It thrives in the wind, but I can't wait to better assess its handling nuances and soaring ability on a more steady morning or evening.  I think I'm going to leave it aileron free for now, to keep the weight down, it seemed to fly heavier than it actually weighs.  Maybe the best use of that second servo is differential spoilers instead of rudder, but I'm not sure without flight testing that slightly out-of-the-box idea.

Overall: a solid C+ flier with great wind tolerance and a few intermediate handling traits just beyond the edges of playful flight envelope.  An A+++ on price rounds to about a B+ overall.

Look for some video this weekend.
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