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Tuesday, December 20, 2011

E-Flite Cessna 150 Aerobat 250 - Build & Flight Review

This will be a complete build post, including a flight review.
Yes, the wingtips are orange, not red as shown on the box.
The plane comes in one of the original, but rather 
odd, two tone red/orange paint scheme..
I bought this Cessna based on the strength of the real airplane.  With relatively recent challenges from more flashy designs like the Diamond DA-20, and previously Piper's Tomahawk, the little 150 continues to grow in stature.  

The Diamond, well, it looks great but flies like a POS by comparison.  And although I got my civilian wings in a Tomahawk, I never felt it flew particularly well even as a beginning Student Pilot.  Yet, every time I hop in a 150 or 172 I'm treated to a "better than I expected" flying experience.  These aircraft have endured because they fly better than anything else in their class.
The Tomahawk posed a brief challenge to the 150 in the 
early 80s, but it had too little excess power.  Although 
Piper creatively branded the plane "spinnable" 
for training purposes, it is simply not as stable 
or as joyful to fly as a 150.
The little Diamond DA-20 is a charmer, but jeezus those 
seats are rocks, and the bubble canopy with no A/C 
can be a joke.  The wheel pants look nice, too bad 
the nose gear doesn't steer and there is very little room 
to differential brake in the foot-wells. Flying this
underpowered plane is, well, a bit of a chore.
With the strength of the core design not in doubt, $119 minus 20% in illegal store-based coupons sealed the deal.  Horizon Hobby does not permit price competition among it's resellers, apparently not realizing that American retailers will find creative ways around their European socialist nonsense business model seven ways to Sunday.  Without the store discount, probably not, but for under $100 the heavily discounted E-Flite mini Cessna seemed promising enough to try.

I would like this little guy to be a relaxing evening flyer.  Relaxing, to me, means mostly hands-off manners, decent float, plenty of power for sport aerobatics, excellent flight times, and spot land-able for quarters.  To accomplish that, my plan is to disregard E-Flite's instructions entirely and build the plane as it's Chinese designers intended.  With a pair of AS3X micros on deck waiting for good weather, I feel mildly inspired to insert a GWS single axis gyro to keep the mini Cessna's wings on perpetual attitude hold.  My hope is to create a versatile, compact, take-it-anywhere evening flier that, like the real 150, always flies a little better than I expect.  "Hope" is an important four letter word in the previous sentence.

With 196 square inches of Wing Area and an advertised RTF weight of  13.7 oz, there seems to be plenty of Chinese workmanship in the box to work with.  Kudos to Horizon Hobby for leaving the product alone and letting the builder make the design trades, as E-Flight has proven to be one of the worst RC design houses in the business when it comes to analyzing power systems.  They seem to use a De Beers sales model:  creatively package new products to peddle whatever has trouble selling on its own.  Or, said another way: double-down on your worst products to unload them at the customer's expense.   That was certainly the case with their Ascent 450 BL, which include the heaviest brick motor I've tested in a long time--in a motor glider.  Well, if you are going to try to rip someone off, you might as well go all out, I guess.  The bare bones nature of this Cessna seems like a safer bet.

With the airframe by Cessna, the build accomplished in a sweat shop in China, that leaves simple price mark-up in the deal for E-Flight.  And hey, that is just the kind of deal I can strike with an incompetent RC design house.

For the power system, I elected to go with a $24 Super Tigre 370 ($20 after typical discounts) instead of the recommended $40 E-Flight 250.  The reason can be found at the 5:00 minute point in the video clip below:
The ST 370 costs $24 and has a lot more power.  I know from powering my 14 oz 39" GWS Brushless DHC-2 Beaver that a 2S 370 had nowhere near enough power with an 8x4.  A 2S 250 powers my 8 oz 31" E-Flite Jenny without much slack, often coming up short.  So even though the Super Tigre 370 weighs a fraction of an ounce more, this is a no brainer.

Again, E-Flight is in La La Land recommending a 2S 250 driving a 7" prop for a 37" plane.  I guess they have 250 motor inventory they need to move.
The 370 fit perfectly with a few 1g, custom cut, thin brass tube spacers.  I selected a 10A Common Sense RC Swift ESC, which like the motor, is less than half the price of E-Flight ESCs which have the same specs, but without E-Flite ESC's lousy track record of burning up on me.  A $45 Hitec 6 Lite tops off the internals.  Commonly available store coupons brought the deal down another 20%.  But this plane is anything but cheap, after store discounts: $96 (airframe) + $19 (motor) + $11 (ESC) + $36 (Hitec 6L with built-in V telemetry) = around $162 before $36 in servos that I have stocked.  I used a metal gear servo for the rudder given its double-use to steer the nose gear.

Total cost came out to just under $200 as a Hitec Rx BNF with built-in V telemetry.  As an after thought, I added a GWS wing leveling gyro that I had laying around new in the box, which tacked on $27.

My thrust scale read a preliminary 12.6 oz of installed thrust operating at full bore while spinning a GWS 8x4 HD.  Once better aerodynamics are in place, namely the cowl and windscreen, that number should read closer to 14 oz.  Max Amps on the Wattsup were 5.49A from an 800 mAh 3S, suggesting 18 minutes flight time at 50% throttle or 9 minutes at 100%.   Nice.

I later switched to a 8.5x5 prop (clipped from a 9x5) for 18 oz of thrust at 8.5A, more on that below.
In the Air:

This Cessna is an excellent flier.  From the realistic/scale rotation on takeoff roll, to the stable puttering cruise, the 150 lives up to its venerable reputation.

So to save time gushing over the good things, I'll jump right to a few rough edges.

First, the airframe is expertly built and covered.  It is exceptionally light and quite sturdy.  But E-Flite managed to screw up the plane anyway by under-powering it substantially.  Even with my decision to jump straight to a 370 spinning an 8x4, the prop really wasn't big enough to overcome the 1000 kV RPM limit and generate plentiful thrust.  The Cessna had enough thrust for better than scale flight, and most people would probably consider the 370/8x4 combo sufficient, but for the way I like to fly and if there is any wind to fight, the 37" Aerobat needs a little more pull.

E-Flite's recommended 250/7x3 power system should not even be considered, as it would be way too weak for comfort.  I found that even an 8x4 prop spinning near the kV max limit wasn't quite strong enough.  It was reminiscent of, bad not as bad as the old Select Scale Super Cub's way to weak stock 8x4 prop.

I tried an ST 400 to see if the bigger motor generated more thrust, since the mount's form factor is the same.  It didn't, because the prop shaft to wheel bottom limits you to an 8.5" prop diameter, and with such a small paddle, a larger motor's kV limit dominates.  Since the 400 is 950 kV vice 1000 for the 370, the 370 actually pumps more air.

After a good amount of motor+prop+thrust scale+Wattsup experientation, I think the best prop and motor combo for the 150 is the lightweight and powerful Super Tigre 370, a GWS 9x5 clipped to 8.5".  The extra .5 inch and steeper pitch actually adds 50% more thrust, at 18.2 oz after installation losses.  The chosen motor/prop pulls 8.5A at max throttle, which seems very reasonable for my 10A (continuous) / 12A(burst) 0.3 oz Common Sense RC Swift ESC.  The final power system comes in at a little better than 1.1 early in the battery, with mostly-aerobatic flight times in the 10-12 minute range from an 800 mAh 3S.  Super!

Next, the wing is somewhat prone to tip stalls after crossing the ragged aerodynamic edge beyond critical AoA.  I think my particular setup bears a third or more of a the blame, but not all of it.  I decided to put an a GWS PG-03 Gyro on the ailerons, just to use it and do something interesting.  It worked tremendously well to stick the wings where you left them.  But I think the gyro works so well that it makes tip stalls seem a little more pronounced by fighting bank until there isn't anymore air left to support the plane.  Then whoosh, down comes a wing.

On the flip side, the presence of the gyro probably reduces wing drop, right up to the limit.

Another issue I caused, or, well, didn't preempt, is the vice of fully symmetrical ailerons.   Because of the gryo, I opted for a Y harness to connect the ailerons.  I knew it would hurt, but I decided the gyro route anyway, since it isn't like this plane is hard to fly.  The downside of the Y harness is aggravated stalls near the wings tips because neutral aileron doesn't offer any washout, and when wing drop is countered, the down-going aileron moves as far as the up going aileron, making the lifting-aileron-wing prone to stalling first and stalling hard.

I know, the Y harness isn't really an excuse, but I was too lazy to dwell on wingtip aero with this plane, just in case it stalled gently without wing tip magic.   It didn't.  So I'll go back and dial-in mechanical washout and differential.

The Aurora 9 has me spoiled, it is so easy and configurable to do this stuff via computer, and a bit of a pain to set it up manually.  Spoilerons can be hardwired, or set to a switch or roller.

To set up mechanical washout, two options come to mind.  One is to physically twist the wing tip incidence angle a few degrees lower than the root, then melt it there with a heat gun.  The Aerobat is beautifully covered, so this method would be no problem.  The other way is to increase the length of both aileron push-rods (assuming they are under-wing), which deflects both ailerons up a little.  As long as the ailerons are not full wing, this method works well and allows adjustments in knowable increments.    I'll use method 2.

To dial-in differential ailerons mechanically, I'll need to open the aileron servo compartments and adjust both servo arms to forward-of-center when the stick is neutral, then again, lengthen the push rods to compensate.  By adjusting the servo arm forward-of-center when the aileron stick is centered, when the aileron gets pushed up the servo arm's arc is more tangential, which generates more linear travel than when it pulls on the aileron.

Still, I want to emphasize that the 150 is a superb flier even without perfect dial-in.   The video below shows a typical flight with the final power system described above, but still with no mechanical wing tip /aileron wasout/differential adjustments.  The 8.5" prop has min clearance and strikes a few pebbles, but overall the tricycle gear protects it pretty well.  The wind was swirling at a turbulent 5-10 mph, and you can see the aileron gyro working hard on some of the closer passes (see minute 1:20, you can sort of see some large flags strongly billowing, below the plane and behind the tress, at the beginning of that pass)--what fun!

Since my HD camera is arbitrarily limited to 10 min clips thanks thanks to bad decisions made by Canon, I tacked on a landing from the next flight after the first flight video stopped filming at 10:00:
As you can see, aerobatics are very spirited.  Loops can be about as big as you want, but due to the high camber airfoil, you have to push around the top to avoid an egg.  Aileron rolls are a little strained due to the basic airframe design, but with blended controls on each axis the plane as no problem completing a roll with no altitude loss.

I really like the effect of the swept rudder, and I don't know why more floaters don't employ them.  By sweeping the vertical stabilizer including the rudder hinge, deflecting the rudder control surface also has some up-elevator and aileron effect.  As such, the Cessna is an excellent plane to steer with the rudder.

Inverted flight is surprisingly stable (gyro enhanced, for sure), but total down-elevator travel is too limited after subtracting the amount the plane requires to hold level flight, in order to confidently transition into an outside loop.  I need to take a better look at increasing elevator down travel.  The lazy exit also could have been due to a near-dead battery. 
Landing couldn't be much easier.  The plane slows down nicely even without pushing stall speed.  I popped a few wheelies on the taxi back to illustrate how easily prop wash rotates the airplane.

At 16-20 mins aloft with mixed aerobatics, flying time is probably too high using an 800 mAh of 3S pure vertical-capable power.  A 500-600 mAh 3S would be ideal and allow 8-12 mins of spirited flying.

I like giving grades like these, mostly due to layering my own design on top of the fine craftsmanship of the Chinese builders.

Appearance: A+
Classic lines.  Funky but cool two tone red/orange vintage paint scheme. 

Airframe: A-
Beautiful flier; glides well.  Nice aerobatics in pitch/yaw; roll is strained.

Power System: F (E-Flite)A+ (Z8RC)
Super Tigre 370.  Prop clearance limited to 8.5".  10-12 mins aerobatic power w/800 mAh 3S.

Build Quality/Durability: A+
Strong airframe--nothing else included.  Great details: sturdy pants, cowl, hatch.

Value:  C-
High China mark-up at $120, minus internals.  Needs other-brand electronics to score avg-minus.

Overall Grade: A+ (with Z8RC power system)
Great looking and great flying classic.  Braves wind very well with a gyro or two.
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