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Thursday, December 30, 2010

Update: Beaver Goes BL

Cost: 40" GWS Beaver ($30) + Super Tiger T 370 ($15) = $45
(reflects 20% Tower Hobbies member email discount)

Luv that Beaver. I plan to run this as a child's plane with a 10x4.7 SloFlyer and 1000 mAh 11.1V LiPo on a buddy box.  Switching to the ST 370 saves 0.9 ounces (10% of the no-battery aircraft weight), doubles max prop RPM, and allows the use of a same weight but triple capacity and 50% higher voltage 3S Lipo.   Hopefully, the BL motor doesn't tear the wings off.   Toning the throttle curve down should give 45-60 minute flight times with about 1:1 thrust to weight ratio.  I sandwiched all leading edges, the CG line, all all tail surfaces with clear mailing tape for a little more strength. 

More here when the BL Beav shoots skyward:

Houston we have a problem.  33 ounces of thrust from the ST 370 w/10x4.7.  12 ounce plane.  Time for 2s.

First test flights:

The first flights reveal what a beautiful little floater this air plane is.  It is so light that it is very sensitive to small changes in weight distribution and flight characteristics change accordingly.  Balance is gong to be a bit of a trick.  Even with a 7.2V 2S battery instead of an 11.1V 3S, the plane wants to pitch up to vertical under full power.  Elevator effectiveness is changed dramatically by thrust output (rudder effectiveness is unchanged).  At the same time, the plane is so airy, it is easy to trim out a broad range CG points.  So there needs to be a delicate balance between the trimmed elevator position, the best CG location, and the amount of power applied from glide to mil.  I am searching for the best battery position and may need to alter the recommended thrust line given the tremendous output of the feather light BL S/T 370 motor.  When I get it sorted to perfection I'll post the info/photos here.

Overall, it is easy to see that the plane will putt along like a dream at 20-30% throttle. The unique power to weight ratio of the ST 370 is presenting some solvable challenges to otherwise carefree handling.  Shouldn't be long to find the right CG placement and thrust line, if required.

Winds are not friendly to a 12 oz model, so we have a little longer development cycle than desired.  This morning I was able to float airborne in moderate swirl and identify the need to lower the thrust angle.  In order to balance the Beaver with the S/T 370 in the nose, I have pushed a 1000mAh battery all the way forward to the lip of the cowl cowl.  It is actually a very convenient spot since the wing can stay in place while changing batteries.   I was thinking 2S but for now at least, I'm going with 3S due to (1) I don't have a heavy enough 2S battery to balance the light engine and (2) the plane can barely hover with 2S.  The Beaver is perfectly balanced with the 1000mAh.

Update - Full Flight Review

Final verdict up front:  A

This little Beaver is a amazingly slow super fun flier, but because it can go so slow, it can be quite unforgiving if you fully stall it out.   If you have a small field and need short take offs and landings, it is exceptionally capable if not a bit tricky, a lot like the real thing.  It can glide incredibly far and is often advertised as a slope soarer.  In fact it can be difficult to spot land because it catches air so well and only has rudder and elevator--no slips. 

I wound up using a 3-cell brushless Super Tigre 400 ($19) that weighs less than the chunky stock brushed 2-cell, the larger coils are a better match to a smaller, low pitch prop than the 370 was at half throttle, which is where the Beaver thrives.  An 8x4 is the perfect prop size to keep both thrust and speed under control while still climbing out 80 degrees nose high in a steep torquey spiral, or straight ahead with half right rudder. The increased low rpm low prop load efficiency of this motor is impressive, average flight duration with a 1000 mAh 3-cell is about 20 minutes with mixed performance.  Practical throttle range spans from around 40% (DX8) for an easy max endurance cruise, to 60% for spirited over the tops.  A larger low pitch prop might increase cruise flight times even more at lower RPM, but full throttle thrust would probably need to be ESC travel limited to keep the plane under control at all times.

GWS Beaver  --  Super Tigre 400  --  GWS 8x4 Prop  --  1000 mAh 20C 3-cell
Flight Regime
Power Setting
Amp Draw
Flight Time
Extended glide
Max Endurance Cruise
50 minutes
Fast Cruise
28 minutes
Over the Top Aerobatics
16 minutes
10 minutes
Based on installed static thrust  - Airborne Amp draw will be lower

The plane could easily haul a 30% larger battery with the high T:W ST 400 up front, but with the above flight times, there is no need, and being an Induced Drag machine it might not fly significantly longer.  A smaller battery would likely require some lead up font to counter any weight reduction, so it may not make sense either.  I'm very happy with the overall power system/flight regime match.

The wing design of the Beaver is actually an under-cambered pocket, which provides good strength for the very light construction and an incredible amount of lift, as it is 2/3rds airplane wing and 1/3rd para-sail. The end result is an ultra light foamie with classic ($25) Beaver looks that easily flies backwards in a steady headwind. Winds are not kind to any 12 oz floater, and the GWS Beaver is no exception.

The build is more like a foam kit plane than ARF. The fuse and wings ship in halves and need to be glued up, it takes a full day or two to complete. The fuselage is tough. The wing is not very thick, but well supported with functional struts--I reinforced mine with clear mailing tape from tip to tip. The supplied push rods don't work without a secure sleeve around them (I used carbon fiber tubing pinned with foam safe CA), they bend way too easily making the plane difficult to control.  The landing gear is thin wire that flexes a lot during medium bounces, so much so, that you can strike the prop.  I solved that problem with small collars on the piano wire gear struts, close to each wheel, then I stretched a long thin rubber band over the two to act as a support and shock absorber.  The steerable cheap plastic tail wheel functions well, but the plastic has no grip--replace with a conventional rubber or foam tire for functional steering.  The plastic pinwheel mains work great, and are darn near weightless, but are the low hanging fruit to be replaced for scale looks.

Even though this plane is an exceptional slow/fun flier and excellent glider, it's not always the easiest plane to fly near the edges of its extreme performance envelope, and that will delight intermediate to advanced fliers.  The elevator can be overly sensitive to high throw due to the short distance from the CG. The wing design is pocket under-camber, which catches the air beautifully and floats just like an air-hockey puck (same principle). It has plenty of dihedral and is largely self correcting in flight, but if you push the rudder to far or too hard too fast, the wing can display some pretty unstable and asymmetrical, falling leaf stall characteristics. Loops, Hammerheads, Barrel Rolls, and 3 foot radius cornering turns are possible. It can make a really wonderful slow flying trainer after an intermediate flier trims it out and most importantly, limits the throws before handing it over.

As long as one realizes that the Beaver's amazingly short take-off rolls (5-10 feet), steep climb outs, near motionless slow flight, and glider-like engine-out performance comes at the cost of being quite tricky on the edge of those same generous limits, it will be a sure fave that can do uncanny looking stuff with the simplest maneuvers.  Commonly available for $30 or less, it's a lot more fun than ordering pizza.

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