Great Planes Cherokee 60

August 2010

Scale and Fast

General Specifications

Build Date - August 2010

Airframe Type -	Low Wing Scale		Flying Type -	Sport
Wingspan =	60 inches		Length =	46 inches
Wing Area =	606 sq. inches		All-Up Weight =	95 ounces
Motor -	SK4250-650 OR		Wing Load =	22.5 oz/sq. ft.
Propeller -	12 x 8 Xoar		Power =	1200 watts
Thrust =	130 ounces		Thrust/Weight =	1.37
Battery Type -	Lipoly 6s1p		Capacity =	3000 mah
Speed Control -	Turnigy 60 amp		BEC -	5 amp Switching

Commentary

It was all the great reviews of this plane that got me to buy into it. Here is my post build report on doing this plane with other than recommended gear. Enjoy - Mark

Build Notes
I love how the ARF manufacturers always try and convince us we can get their plane in the air in just a "few" hours. I spent two hours with an iron just getting the covering acceptable. Then there were the little design issues (see text) which always take extra time not to mention the inspection and re-glue of many weak looking joints such as the cowl mount block that literally fell off in my hand. I'm pretty sure I have about 20 hours in the build and an additional 4 to 6 hours in development "re-dos". Of course, had I stuck to the manual using all the recommended gear, it would have gone a little faster.

I will say the manual was pretty good and had plenty of tips and pictures for the electric side although I didn't follow much of this. My plane ended up very light with the changes I made but these changes did not include modifying any structure with the idea to save weight. In fact the only cutting I did was to shorten the battery tray and cut out part of the servo tray. The rest was built per the manual with my equipment choices.

Please note: With regard to my listed weight savings, the comparative values shown came from both actual weight, which I measured, and catalog weight if the component wasn't available.

Equipment Choices

Servos
The fact they are suggesting Futaba 3003 & 9001 servos tells me it's all marketing and has very little to do with optimizing the performance. Here is what I used:

Rudder and Elevator - Hitech 225MG

Aileron - Hitech HS82MG

Flaps - Hitech HS65MG

Since I am using a switching BEC at 6v rather than a separate RX pack, I opted for a much smaller flap servo, the HS65MG, because a full size standard unit is simply not needed given the requirement for torque and duty cycle. The other servos are also lighter, have higher torque and are considerably faster (2X!) than the recomended Futaba units.

All told this set up cost $58 more but I doubled the speed (halved the time) of full throw on the control surfaces. The Hitechs also produce more torque than the Futaba units when operated on 6v. An additional weight benifit of the Hitech set-up is the reduction in rotational inertia from using lighter servos out on the wings.

Total servo weight savings - 4.5oz (128gr)

Batteries
The recommendation here is to use separate flight and reciever batteries. The flight battery is made up of one 3200 3S and one 3200 2S with a series connector.

The total battery weight of the recommended batteries including connectors, ESC and reciever pack is 24 Oz. Here is what I used:

Battery Pack - Zippy 6S1P, 20C, 3000mah, 16.5oz

BEC - Turnigy 6v Switching BEC, .64oz

SuperBrain 60A ESC, 1.8oz

Flight time - 9 - 11 minutes

Total weight of my battery set-up with connectors is 19.5oz, a savings of 4.5oz (128gr)

Motor & Prop

Motor - Turnigy SK4250-650

Prop - Turnigy Wood 12x8

The recommended motor is a Rimfire 42-50-800 which I like, however with the recomended Tower motor mount, it is pretty heavy. Additionally, because I wanted the efficiency of a higher voltage system, I decided to go with 6S and give the Turnigy SK4250-650 a try. It tested very well with the Zippy battery pack and Turnigy speed control and has functioned nicely without a hiccup. Also, given the way I constructed the motor mount, I ended up saving about three more ounces here.

The prop I have ended up with is the 12x8 Xoar Wood. The APC equivalent is fine but the efficiency difference cuts a bit of time off the flight time. I have also tried the Top Flight Power Point and it falls somewhere between the APC and the Turnigy.

Total motor weight savings - 3.2oz (91gr).
Total equipment weight savings - 12.2oz (346gr).

All up weight - 5.91Lb (2691gr)

This was a very pleasent surprise given that the manual states the weight as 6.75 - 7.25Lb (3060 - 3290gr)

The plane balances perfectly at the std 3-1/16" CG location without adding any weight or moving any components other than the main battery.

Power Testing
On the bench (static) numbers using a Medusa Research data acquisition unit:

100% throttle at a 10 second run point

22.7 volts, 49.5 amps, 116oz thrust,1122 watts,11400 RPM,75mph pitch speed

In flight data using the SuperBrain ESC data monitor:

20.8v min, 56.6A max, 1228W max, 11748RPM max, 145.9 deg F

The highest temp I have seen on the motor has been around 112F. The hottest day has been in the mid 80's.

The bench test results are real time at a given run point, the in flight values are minimum or maximum values stored during 5 minutes of flight time. The flight data includes both a 6 second vertical climb and a steady state, level flight run at 100%.

Radio

Spektrum DX6i TX

Spektrum AR6200 RX

I followed the manual on low rates but added some to the high rates.

Control Rods
I did make some changes here as well:

I swapped out the metal pushrods for carbon fiber of my own type on the rudder and elevator

I left the standard set-up on the nose wheel

I chucked the quick clips and used 2-56 all thread with clevises on the wing linkages as a personal preference

I used positive lock connectors inside the fuse on the rudder and elevator servos and spherical rod ends on the tail

Minor Design Issues
Control Rod Tubes
The position of the rudder control rod tube exit (This may be a one time (my kit) deal) does not line up at all with the rudder control horn. This creates geometry problems caused by the angle of the control surface hinge centerline relative to the control rod, this was a challenge to make smooth and reliable.

Wire Routing
Another design kluge is where the exit holes for the servo wires in the wing are placed. If you use these holes you risk having the sevo leads get tangled in the servo control horns of the elevator and rudder servos. The fix is to simply cut a new hole forward about an inch.

Hardware Changes
Would it be too much to ask the manufacturers to stop using #2 - #4 phillips head screws and just use socket head screws in these sizes? I am changing them all.

Flights!

Maiden - In a word, wonderfully un-eventful. On take off the plane tracked solid and needed very little trim. Once trimmed it flies like a much larger plane. It is surprisingly fast, rolls are clean and inverted flight is a breeze. It is one of those rare planes that feels comfortable right away.

Landing this plane is easy and it looks fantastic. With flaps it slows right down and gentle three point landings are a piece of cake.

On low rates it flies scale like very nicely. It is rock solid and very easy to fly. On high rates, it is a bit of a different animal. I like the high rates but generally only fly with aileron and elevator on high with the rudder on low as it gets a litle snappy otherwise.

On a cautionary note, using a 12x8 prop with a 4000mah, 20C battery really brings the plane alive adding 10 - 15 mph to the top speed and making the verticle spectacular. The problem here is the Turnigy motor. The data logger shows I'm hitting 1630 watts on the verticle climbs and this motor is rated at 1150 max. I doubt it will last at this power so be careful out there!. The bigger battery adds 6.3oz to the AUW which wasn't really noticible.

Final
I love it. It is a wonderful plane to build & fly and I would recommend it to anyone.