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Friday, August 26, 2011

E-flite Jenny JN-4 Slow Flyer Flight Review

Update: Videos added.

They don't make 'em like they used to.
WCL of 2.4 for $55?   Heck yeah it is coming soon...

- Wing Span: 32 in
- Overall Length: 26 in
- Wing Area: 304 sq in
- Flying Weight: with Battery 7.5–8 oz; without Battery: 6.9 oz
- Motor Size: 250-2200 Kv
- Radio: 3 channels with 2 super sub-micro servos
- Servos: S60 Super sub-micro
- Prop Size: GWS 6x5 slow flyer
- Speed Control: E-flite 10A Pro Brushless
- Recommended Battery: 480 2-cell Li-Po
The colors are stunning.
On the Ground:

The E-flight Jenny is an interesting foamie.  The ARF is made of 1/8" thick foam sheeting.  The fuselage is pre-built from flat and curved panels already assembled and painted, all you have to do is foam-safe-glue-on the horizontal and vertical tail with stamped, pre-hinged surfaces.  Even the pushrods are pre-installed in plastic tube casings.  The painted wing is stamped with molded rib detail that strengthens the chord and span.  Wooden braces quickly glue into place.  The wing box is further strengthened with supplied thread rigging.

The resulting airframe is quite strong for it's size and silly light weight.  At 32", the plane certainly isn't small and carries over 300 square inches of Wing Area.  At 7.6 oz RTF with a 5 minute battery, my finished sample has amazingly light Wing Loading at just 3.6 oz/sqft.  It's Wing Cube Loading is even more impressive measured at only 2.5.  These numbers are hard to beat, and that fact showed up loud and clear in the flight tests.

For electric guts, I selected a 0.52 oz Common Sense RC Brushless Outrunner 250 and 8g SkyRC 10A ESC.  I elected to put a longer than stock shaft and prop adapter on mine for easy prop changes.  But the instructions call for a prop-saver with O-ring type of prop adapter, which is how the motor comes set up by default. I used two 4.4 gram Turnigy sub-micro servos and a Hitec Optima 6 Rx with built-in voltage telemetry.I removed the hard plastic case to lighten the Rx to 18 grams. The system worked perfectly with no installation issues.

The Park 250 Brushless Outrunner Motor has a small round bearing case that fits snugly into an included carbon fiber tube engine mount, which then slides into matching circular holes in the firewall an first former, making it very easy to install and align.  The downside is that the changing the thrust angle is very difficult.  I elected to go with the Common Sense RC motor instead for better power and durability.

For ease of motor installation, is simply zip-tied the motor's three mounting feet to the Jenny's thin plywood firewall using the pre-cut holes.  I slid a small plastic shim under the top motor foot, to adjust the thrust angle about 2 degrees down from center-line.  This turned out to be perfect.

The instructions are pretty good, and make it clear that the most important aspect of the DIY part of the aircraft build is warp-free wing box.  I was very careful to align the wings with triangles while assembling and glueing.  Then, sure enough, when the glue dried it turned out to be slightly warped, with more incidence on the left than right wings.  Fortunately, I was able to twist the wing into perfection by installing the included string rigging.  I glued the string in place under just the right amount of tension to straighten the warp.  Finally, I soaked the string with CA for good measure.  The string rigging is not shown in my pictures.
Classic lines make the JN-4 a convincing barnstormer, 
but at 3.3 oz Wing Loading, Jenny is rarely in a hurry.
In the Air:

My Jenny now has two flights under it's belt.  The first flight was in perfectly still air, and although my thrust angle was off a little (more on that later), I was able to explore the entire flight regime and a little beyond.  The second flight tested a new thrust angle and started in 3-5 mph winds, progressing to 5-10 mph, and eventually forcing an early landing to keep the plane from being blown off site.  The second flight is captured in the video below.

Let's discuss the first flight then the second.  I originally tried a neutral thrust angle in the vertical, with about 2 degrees right vector.  The Jenny lifted off wonderfully controlled after a 3-5 foot take-off roll, and tracked perfectly into the blue.  Once trimmed for a light criuse, the WWI relic was a joy to fly.  The plane's nose pulled upward with throttle, which is possibly ok, but in idle the plane dove for the ground indicating that the up-thrust-vector was holding the nose up during cruise with a nose-heavy CG.

My estimated fix was simple: move the ESC farther backward under the cowl and above the servos, then move the Velcro battery mount back to just in front of the low wing leading edge.  The plane balances only about an 1/8th inch behind its recommended CG location this way, as my first flight was slightly nose heavy by intent.  I also added about 2 degrees of down thrust, which by the nature of the required mounting shim location, took out most if not all of my right thrust.  I decided to do the second flight this way, since the plane tracked perfectly during the first flight, from idle to mil, and I wasn't sure if it would track perfectly regardless of a slight left/right thrust angle.

During the second flight, CG and vertical tracking were absolutely perfect.  The plane didn't budge from level flight as the power came out.  The Jenny's climbing tendency under high power was better controlled, and given the copious thrust available, just about perfect.   Generous right rudder was required at slow speeds and high power settings, indicating that the plane needs the 2 degree right thrust angle put back in.  That is all set for flight number three.
Second flight with the Jenny.  Down thrust line is perfect 
but needs the 2 degree right thrust vector 
put back in. Loops at 4:03.
All that said, it was easy to evaluate the JN-4's flight characteristics while ironing out the thrust angle.  I'll state it succinctly:  this WWI vet is fabulous.  Feather-light Wing Loading gives this plane a floating speed very comparable if not slower than an Ultra Micro class high wing trainer.  Stability is perfect, not hard to turn, but easily finding a hands-off arrow-straight line through space.

My stall series went much better than I expected.  I expected the under-cambered wings to hold on to lift til the very end, then dump the nose all at once.  Not the case.  Stall onset is as gentle as can be, and as long as you don't change very much very fast, the plane only shudders almost imperceptibly, plowing along on its intended flight path.  The wing tips stay level.  Wing rock is initially all but absent, then slowly develops into a steady cycle that I would classify as "slight."  What a wonderful surprise.  The plane can be coaxed down to explore its eerily slow lower airspeed limits, and it won't snap at you when over stimulated.

Not perfect, but to be expected from an ultimate floater:  top speed is with an 8x 4 Slo-Fly prop is pretty good, faster than a UM, but requires significant down elevator and that makes it little hard to balance the pitch inputs during short "high speed" bursts.  Although rudder control authority is just right, ailerons would definitely take control to a new level.  The weight of the additional servo might steal of tick of the low speed fun, but there is so much margin there to sacrifice that I think it would be a good trade--and I love floaters.

Loops are uncanny-tight for a 32" span airplane, completing in approx 5 feet of altitude.  Rolls are limited to very positive Barrel Rolls, and resemble one turn of a gentle spin on 30 to 40 degree forward-down vector, unless entered about that much nose-high.  General flight control is outstanding for a 3-channel.  4-Channels would make an interesting upgrade.

Landings are cake in calm winds, easily flared and floated into a wheel-dancing 5-10 foot roll out.
In too much of a hurry, E-flite used the top photo to erroneously 
conclude the tail marking was "FUI" instead of "FUL"
Jenny's gorgeous profile view gets better with age
One minor quibble is the beautiful color scheme.  It is pale yellow on top and white on the bottom.  While highly visible (yellow is generally the best color for vis), the translucent nature of the wing foam effectively tints the ground-up view in sunlight to almost the same pale yellow color as the top.  The insignias also bleed through the foam to an extent, making the plane rather hard to orient against any bright sky.

Of course, this plane becomes a handful above about 5 mph of wind, but at 7-8 oz it is more wind tolerant than an Ultra Micro and flies in about the same space.  I incorporated a wind test into my second flight, by flyingmy most wind-tolerant micro plane, the UMX Sbach just a few minutes after landing the Jenny for winds.  The Sbach was a lot worse and virtually uncontrollable in the same winds, as shown in the (short) video clip below:
Same wind was too strong for my most wind-tolerant micro
I love flight reviews like this one: "Inexpensive plane promises the world and delivers the Moon, too."  Horizon's  E-Flight JN-4 Jenny deserves high marks for shacking the supreme floater target, dead center.

Appearance: A
Beautiful pastel scheme.  Classic lines.  Highly visible, but difficult to orient in the air. 

Flight Performance: A
Masterful slow flight .  Stalls as gentle as can be.  Limited as a 3-ch.

Build Quality/Durability: C+
Couldn't ask for more at this price point.  Foam gets more brittle with age.

Value:  A
A fair price for the ARF level of completion and workmanship.  Great scale for the money.

Overall Grade: A
3x Bulls-eye and a reasonable price to mitigate the last.
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