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May 22, 2021, 08:03 PM
Flying again - 4th time
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Carl Goldberg Anniversary Cub Crash Analysis


Note: background info. You may skip this post if you are uninterested in learning about the "how and why."

I picked up a Carl Goldberg Anniversary Cub from a swap meet about 3 years back. It was built, and the wing was covered, but the horizontal stabilizer was missing. An brand new out-of-production nitro .40 and fuel tank were included, as were instructions and a wheels and some other accessories. Price was $50. I sold the motor and tank for $50, so effectively I got the airframe for free.

I finished constructing the missing parts, then covered the airframe in the remaining covering. Over two years, I slowly built out the model, carefully constructing an electric conversion for the aircraft and detailing the plane with many 3d printed pieces.

A few months back, I maidened the aircraft on a 6s system, with a 60 amp ESC, a custom fan system to cool the ESC behind the motor, and a 5000 mAh Turnigy battery. Flight characteristics were fine - the airplane was a little heavy @ 8 lbs flat, and glide with power off was really smooth. It landed in the soccer fields at the local high school effortlessly.
Last edited by FOGeologist; May 29, 2021 at 02:33 PM.
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May 22, 2021, 08:13 PM
Flying again - 4th time
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This post describes the crash. Start here if you are only interested in this part.

I had perhaps 18 or 20 flights on the model, including a couple of ground-handling errors that resulted in a flipped airframe on the tall, soft grass. Right after I ran the aircraft into my tool box at slow speed, the prop must have cracked internally and threw a portion of my blade on takeoff, but I landed the bird without incident.

I flew three or four flights today, with one bad landing that flipped the airframe. I had installed a very heavy duty thick wood prop on the motor, and I was surprised to find it undamaged from this incident. I checked it carefully, and put the bird into the air for several more flights. The airplane was flying beautifully.

I changed the battery and put the plane into the air on a very smooth take off at full power. Then the airframe started pushing over on me - I metered in some aft stick to counter this, and soon found myself fighting a twitchy bird. I resolved to turn it and land as soon as possible, but suddenly the airframe lurched upward, something came off the airframe, motor and control went dead, it flipped over in a hammerhead and plowed nose first into the deck.
Last edited by FOGeologist; May 29, 2021 at 02:35 PM.
May 22, 2021, 08:22 PM
Flying again - 4th time
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This part analyzes the crash and what happened.

When I arrived at the crash site, I was disheartened to see wreckage everywhere. No surfaces save the last 2 feet of the fuselage were spared.

I found the motor with the thick prop still attached, but broken from slamming into the soft earth. Curiously, none of our team could find the battery -it was as if it had been taken to another dimension or something. Examining the fuselage, I could not make an immediate determination on what caused the crash. Could it have been my battery coming loose and moving around in my fuselage? No, there was no physical way for this to occur... unless...
May 22, 2021, 08:41 PM
Flying again - 4th time
FOGeologist's Avatar
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Final analysis and lessons learned:

As I began examining the wreckage, and my team extended their search, I started to put together a theory about the aircraft's last moments. Then, my phone rang - my friends had located the battery, 80 YARDS NORTH OF THE CRASH SITE!

It was at this moment that I put together what had happened.

The original firewall was a rough ply, and I was unimpressed with its condition, so I laminated it with another small square of ply on the motor mount. I had originally planned on placing the ESC in the cabin to make wiring short, and being concerned about airflow, I cut five 1/2" holes through to the cabin. I did not think this would weaken my firewall substantially. I never considered that the firewall might be exposed to repeated stresses of an idiot at the sticks making bad landings.

When I replaced the original skinny 14 x 10 wooden prop with the thick prop, then rolled the airframe, the stress got transferred to the firewall... and cracked it. My later flights were very smooth and the crack wasn't detectable. My full throttle takeoff on the later battery put just enough stress on the firewall to flex it downward, changing the thrust angle, and making me fight the dipping nose with elevator and continued high throttle settings. The moment of failure occured when the motor and firewall ripped off the airframe, yanking the battery out of the nose of the bird and ballistically tossing it 80 yards away. The motor and ESC were still stuck to the airframe by some wreckage or wiring, and the airframe just dropped to the ground.

Lessons learned:

1. if you can place the ESC anywhere you can blow air onto it with a fan, then do so.

2. DON'T PUT HOLES IN YOUR FIREWALL.

3. Don't use a super heavy propeller; the prop should be considered like a "fuse" where it can break before it transfers forces to the rest of the airframe.

4. Lastly, if you get any kind of goofy landing where there is a prop strike, YANK ON THE PROP HUB to see if the mount has come loose.

Sorry for the length of this.
May 23, 2021, 12:55 AM
ITTD you're Clear for Takeoff!
IntheTubeDeep's Avatar
Wow thats too bad- any video!?? (this is why I video every flight)
May 23, 2021, 01:53 AM
Flying again - 4th time
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Negative, amigo. No video. I do kind of wish I had something to go on other than my memory, but I think I have mostly hit all the right points. The firewall gave way and shot the motor out of the fuselage, dragging the battery out and throwing it away from the airframe - all because I had cracked it previously from several dumbass moves on the ground.

I may put a piece of delrin into the next airframe - which is again going to be a Cub. My flying buddy is giving me his old gasser airframe, which I will use to convert to electric.
May 23, 2021, 01:09 PM
Registered User
sorry to hear it!

great analysis and write up. reminds me of my first r/c experience 55 years ago, goldberg shoestring.
straight up stall on take off, nose into frozen tundra, went home with a bag o parts

that was a beautiful airplane
May 25, 2021, 10:33 AM
Registered User

I will make a personal guess about batteries in ANY type of model.


Most are not secured tightly enough !!!!!!!!!!!! I have for decades have used glued in place foam blocks, to prevent any movement in the compartment.

Velcro pieces to get the COG balance & glide perfect. Then measure and put in the blocks.

EDIT...........I DO NOT ......glue in the friction fitted foam blocks. .............Done for battery wear out & changes.
May 25, 2021, 02:38 PM
Registered User

Let's put a heading then


Quote:
Originally Posted by cyclops2
Most are not secured tightly enough !!!!!!!!!!!! I have for decades have used glued in place foam blocks, to prevent any movement in the compartment.

Velcro pieces to get the COG balance & glide perfect. Then measure and put in the blocks.

EDIT...........I DO NOT ......glue in the friction fitted foam blocks. .............Done for battery wear out & changes.
Finish down here
May 25, 2021, 02:39 PM
Registered User
Said no one ever
May 30, 2021, 05:55 PM
Build more, websurf less
FlyingW's Avatar
Quote:
Originally Posted by FOGeologist
Negative, amigo. No video. I do kind of wish I had something to go on other than my memory, but I think I have mostly hit all the right points. The firewall gave way and shot the motor out of the fuselage, dragging the battery out and throwing it away from the airframe - all because I had cracked it previously from several dumbass moves on the ground.

I may put a piece of delrin into the next airframe - which is again going to be a Cub. My flying buddy is giving me his old gasser airframe, which I will use to convert to electric.
Nice write-up and good lessons learned part. I think it is ok to make holes in the firewall, especially when you need to admit air for cooling. If the holes are not too big to take away too much of the firewall material then you should be ok.

You are absolutely right though, the most important thing is to always do a pre-flight check the parts of the plane that are supposed to be connected to other parts. Well done.

Good luck with your next Cub.

Paul
Jun 14, 2021, 10:09 AM
Registered User
E-Challenged's Avatar
I fly a 1/6 scale electric powered Sig J-3 Cub, weighs 5+ pounds. Power is just adequate for loops from level flight and Cub-type aerobatics. Model flies mostly " on the wing" at scale-like speeds. Landings and takeoffs are mostly gentle using scale-like tail-dragger techniques in relatively calm air. Cubs and similar high wingers are not really "easy to fly" and need scale-like piloting skills as shown on You Tube. IMHO, many Cub-like high wing models are overweight and overpowered , tail-heavy, and clumsily flown at much higher than scale-like speeds. Takeoffs and landings are often poorly done with prop strikes , flip-overs and excursions off the runway. Standard advice follows:

Do not fly tail-heavy. Let model lift off when it is ready with very little up-elevator at about 3/4 throttle. Use gentle nudges of right rudder to keep straight during takeoffs. Use up-elevator at start of takeoff to keep tailwheel on the ground and steering until rudder becomes effective, then release elevator to let tail rise. Coordinate rudder with ailerons to bring tail around in turns and prevent "adverse yaw" . Cubs need a fair amount of rudder and aileron throw and moderate elevator throw. I use rudder aileron mix and switch it off when not wanted.. Land with some power on , don't fly too slowly and risk stalling during landings. Land on main wheels, reduce power and steer with rudder until tail wheel settles to the ground and is steering. I avoid flying in gusty, windy or crosswind conditions, dislike making repairs!! Always check carefully for damage after a mishap.
Last edited by E-Challenged; Jun 14, 2021 at 10:30 AM.
Jun 18, 2021, 04:54 PM
DHG
DHG
Kinetic Sculptor
Sorry for your loss. Sounds like an accurate analysis. If we could all sell our experience for what it cost us, none of us would have to work for a living.


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