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Tuesday, April 17, 2012

ElectriFly F-20 Tigershark EDF ARF - Flight Review

Update 1:
Two video passes.  The video doesn't seem to do the speed of the jet justice.

Original review follows:

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It's impossible to make the lines of an N-156 derivative look bad, but the red F-20 scheme certainly doesn't accentuate the positive.






 
I collect RC classics. With many thousands of hours flying the classic T-38A/C myself, I couldn't turn down this racy F-20, especially with it priced to move at 70%-Off retail. MSRP $180; Street $120; Paid $55. Yay!

Some might think the F-20 Tigershark, sometimes called the F-5G, was the ultimate development of the T-38/F-5 line. That's because most people don't remember the ol' YF-17 which was also an F-5 development.  The YF-17 Cobra added a second vertical stab to control asymmetric possibilities of its souped up engines and raised the wings to its shoulders to make more payload room, but buried underneath its taught skin are the reincarnated bones of an F-5 E/F Tiger II.
Northrop YF-17 Cobra development of the F-5, 
which later became the F-18 Hornet.
The Cobra lost a fly-off against a vastly superior YF-16 for the USAF light weight Air Superiority Fighter contract, but the YF-17 did not die.  Hardly.  No, it too morphed into one of the most successful fighters in modern history.

Following the USAF flyoff, the USN latched onto the F-17.  They shunned the single engine F-16 in favor of a twin engine safety blanket for ops on the high seas.  McDonnell Douglas, now absorbed into Boeing, acquired the design and developed the 17 into the F-18 Hornet for the Navy.

Recently reborn as a 125% scale F-18E/F Super Hornet, the F-18C/D was always payload-and-range-crippled by the small stature of its T-38/F-5 DNA.  But amazingly, the 1955 N-156 test bed aircraft originally commissioned by the USN, turned USAF supersonic trainer and world-wide light fighter, can still trace an operational lineage straight to our best front-line carrier-based Striker.

Fast forward (or rewind) to the Great Planes rendition of the F-5G/F-20.  To me, the gaudy red Tigershark demonstrator paint job symbolizes the failure of the F-20 to find an export market and ultimately be discontinued.  GD/Lockheed killed the F-20 stone dead by convincing President Reagan to open US-only F-16s to the export market, that was all she wrote for the Tiger II turned Tigershark.

So I decided to return my F-20 to its F-5G roots with an ahistorical paint job.  I think the F-20 looks soo much better in pseudo-scale Aggressor drag:
I decided to go F-5-esque with an Aggressor twist.
SPECS: 
  - Wingspan: 22.5 in (570mm) 
  - Wing Area: 162 sq in. (10.4 sq dm) 
  - Weight: 26.8 oz (760 g) 
  - Wing Loading: 23.8 oz/sq ft (73g/sq.dm.) 
  - Wing Cube Loading: 22.5
  - Length: 37 in (940mm) 
  - Motor:  Ammo brushless 24-45-3790 with HyperFlow ducted fan unit

Target Wing Cube Loading:
Glider - 1 to 4
Trainer - 5 to 7
Aerobatic - 8 to 11 
Scale - 12 to 15

Racer - 16+ 
<< The F-20 Tigershark EDF flies here


On the Ground:

The Electrifly F-20 never really took off, at least I have never seen one flying.  I think that is because Great Planes generally did a good job with the classic lines, but they muffed the round nose.   Combine the round Pinocchio nose with Ferrari red stripes, and this sport scale model just doesn't market well.
There is something annoyingly Patrouille Suisse and non-tactical about the F-20's marketing.  No thanks.  I prefer death and destruction.
It's a shame the blunt nose and funky rendering kept the model from selling in volume. Fortunately, both gaffs are easily fixed. 
I slightly reshaped Electrifly's inaccurate nose cone to better resemble the gorgeous F-20 nose using 100-to-150-to-200 grit sandpaper.  A flat balsa tail antenna also adds flavor.
Great Planes/Electrifly swung and missed on the shape of the nose cone.  The rest of the model looks great.
Another immediate oddity is the aileron design.  The F-20's flaps have turned into ailerons, and the ailerons are fixed.  Maybe moving the ailerons inboard helped keep the wings from tip stalling? Or, probably more likely, the Chinese "designer" didn't know the difference between flaps and ailerons.  Interesting.

The plane ships in a few big chunks that go together very quickly and there are strings installed to help thread the servo wires through the fuselage.    The pre-finished cockpit serves as a magnetic battery hatch for a 2200 4S.  The instrument panel and old school white-helmet USAF pilot look pretty nice.  The box advertises 85+ mph.  I hope it stays put. 
Nice cockpit details are pre-finished.  The pre-built cockpit doubles a magnetic battery hatch
The included fan and motor install easily after shifting the upper tail section backward, then up and off. All servos are mounted in external, molded bays.  There is no rudder control according to the stock plan.  I might add that later.

Presumably in a quest for maximum speed, Electrifly designers forgot that airplanes take off and land.  I guess I agree in concept, but my vision of successfully ditching a supersonic airframe in the grass is still a little sketchy.  Time to tape up the leading edges and most of the bottom.  What's another quarter ounce when you have no wings?

All said, this plane fell together in a hurry.  The most time consuming part of my build was a voluntary re-paint.  I think this F-20 turned F-5G will be a real head turner, if for no other reason than rarity.  Let's hope the rarity is all about dorky marketing and not the result of aerodynamic fundamentals.   I expect great things in the jet handling department.

One final thought before lift off.   I don't think the pictures do the specs justice.  This model has the same wingspan as a Hobbyzone Champ!   Ok, the fuse is 2.6 times longer, and it certainly doesn't feel like a micro, but removable wings?  Hah!  What wings?   This is definitely a good model for those driving a small car to a big field.

In the Air:

OMG!  This thing is so much fun!  Where to start??  I guess I'll go from launch to ditch.

Hand launches were easier than expected.  With a firm toss, the F-20 climbs out with surprising ease and control.   Given 23 oz Wing Loading and Cube Loading and a very heavy general feel given the wing size, I expected the plane to want to settle and sink on the slow speed climb out--but not so much.  The motor is so thrusty and the low speed nose authority is so good, holding the nose above the horizon is not an issue while the motor powers up hill seemingly effortlessly.

I visually aligned the ailerons, and there was no rolling tendency, or fighting to keep the wings level on the initial climbout.   In fact, the plane was amazingly stable in roll from start to finish.   Like a real F-5, the small wingspan and high wing loading is a double edge sword.  On one hand, rolling inertia is minimal, so the airplane rolls very easily.  On the other, the control surfaces are in tight to the CG, so they don't generate large moments.  The basic idea is to enjoy the inherent stability of a stubby wing, then overcome the lack of a rolling moment arm with large control throws.

It works.  The F-20 was dart-stable, hands off.  Oh, and did I mention winds were highly variable at 15G25 mph?   I was utterly impressed by the general stability of the plane, and in pretty crazy wind.

But there's more than stability here.  The little F-20 handled exactly like the full size F-5 series.

Set up a high-speed turn circle entry.  Bank 90 degrees and pull.  The stick comes back into your lap with a lot more required stick travel than most might be used to, but the nose tracks with authority and the angle of attack builds predictably and under complete control.  The plane pivots like a see saw on the CG, causing a tighter and tighter turn.  High AOA itself builds turning ability, as thrust is vectored downward (pilot perspective) with the nose jacked up high as you carve the circle. 
The jet continues to dig deeper until the wing approaches critical Angle of Attack and a stall begins to onset.  Unlike a low performance wing, reaching critical AoA is not an acute event where lift suddenly goes away and the nose falls abruptly or a wings drops.  A high performance wing has a very gradual and predictable stall onset.

If you could sit inside the plane you would feel light buffet at maximum sustained turning performance.  If you demand more turn, the buffet turns to a rumble.  More still and the entire plane starts to shake.  Finally, in the full stall you'd feel violent, arrhythmic bangs like someone was hitting the tail plane with a sledge hammer.  Throughout the stall progression the nose track stays predictable and even accelerates, the wings stay generally level (unless the plane is out of rig) and the clouds keep rating by.  Once the violent full stall settles in, which is marked by a very fuzzy gray line, the nose rate becomes very erratic or stops altogether and some wing rock usually sets in.  Nothing more to give.  The airspeed needle is stabilized well into double digits and the jet is dead in the water, but for relaxing the stick + afterburners + time.

That is how an F-5/F-20 is supposed to act.  And that is exactly how the Electrify model does act.

I love this plane!  It is so faithful to the real thing.  That is good because the plane performs amazingly well for the dart aerodynamics.  It is bad because demanding too much turn without enough power will open up the turn radius instead of clamp it down.  That's a tough thing to judge without firsthand fighter experience, and its tough even with fighter experience since you can't feel the plane talking to you.

The good news is that riding the edge is not a sharp line.  If you know what to look for, and keep a power reserve on hand, you can power out of most bad situations with time before they fully develop.  It's not like the plane wants to drop a wing and spin in. 

Aerobatics are equally faithful to the F-5 series.  Huge powerful loops are not only possibly, they are required.  Remember that as my turning diagram illustrates, achieving level flight out of the backside of a loop requires a nose-hi pitch attitude if you are still pulling G.

Rolls are blurred and effortless, but slightly coupled without some aileron differential or a little push on the nose.  Nothing a computer radio can't fix.  Or you can dial-in geometric differential by tilting the servo arms forward when the control surface is neutral.

Inverted flight is easy and stable.  Like the real thing, pitch changes require a lot of stick travel so precise control is natural.

F-5 series flight controls take a little getting used to because the "T" (the pattern made by stick movement) is highly elongated.  Lots of required elevator stick travel; hardly any required aileron stick travel.

Two less than 100% faithful things.  There is a little bit of wing camber, but the airfoil is so long and thin it doesn't detract much from a symmetrical feel.   But I think that does cause a very slight rolling tendency out of a full stall, not too bad.  It also causes more level landings than I expected.   Maybe I just need more practice slowing the jet down.

High speed passes are thrilling--incredible for a stock motor, these days, which tend to be weak.  Not this motor, the plane has > 1:1 Thrust:Weight out of the box.  Yay again!   The manual includes a great matrix of Great Planes "Ammo" brushless Inrunner motor options and even a brushed option, including battery voltage, amps, watts, RPM, and associated top speed.  The ARF ships with the top end motor in the matrix.  That 4S row states 32 amps at 48,000 RPM and 85 mph.

I think the data reflects older battery technology.  My 35C dumps 38.9 Amps at 570 Watts, so I'd estimated the jet is hustling past 100 mph.  Radar gun to follow.

A quality 4S-2200-35C lasts 6:30 minutes to LVC (Hobbyking 40-50A ESC w/Castle BEC).  

Ditching (sort of like landing) was not half as fun as flying.  Approaches are long and the jet takes its sweet time slowing down.  Find a big grass field.  Landing in medium (~6") grass seemed easiest on the airplane and most predictable.  I might add carbon fiber skids and try a paved runway next time.

Time for grades:



Appearance: B+ 
Round foamie nose hurts smokin' F-5G lines.  Much better with tactical paint.  

Airframe: A-
Superb stability + agility.  Flies like an F-5G.  Small.  Gear-up emergency landings. 

Power System: A

Great push for an EDF.  Challenging to launch without gear.  Runs at design limits.

Build Quality/Durability: D
High quality electronic components.  Gets torn up by fast grass landings.
 
Value: A-

Awesome speed and performance for money.  Too short foam half-life w/o gear.

Overall Grade: A
True Sunday afternoon shocker.  F-5 flight characteristics need to be respected.
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