Can low batteries cause crash?

[steve]

I had recently painted my cub and then went to fly it and the plane would barely lift off. It got about 4 feet in the air and stalled hard, cartwheeling in. Now the pretty paint job is history. I don't think the paint added that much weight and it flew great before painting. The only thing I can think of is that I had charged the batteries on the weekend. I flew it this morning in completely calm conditions and crashed. Could the stock batteries drop off so much that there simply wasn't enough power?

[flyer88]

Hi Steve. I'm very, very sorry to hear about your Cub incident. The paint job was a work of art.

I would suspect the battery was the culprit. I'm a believer that Lipos are the solution for Cubs, even 1000mah packs. These cubs can haul a fair amount of weight, so i don't think the paint was the issue. If you took off before the plane had enough lift, then that would be a stall and cause it to drop a tip.

Once again, sorry about the bad news. Rebuild her and get it in the air !!

Cheers
Glenn

Fly it low !

[wjcjr1]

Salvage the parts together and throw it in a scale. I would be interested to see how much extra weight was on the plane compared to stock. Since you flew it before the paint-job is it possible you tried taking off at your earlier lighter weight speed and that caused your stall.

Yes a dead/low battery could cause the low voltage cut-off to activate thus killing your throttle. Are you sure you charged the battery?

Also did the plane go down flat or bank quickly off to the side? If it rolled quickly off to one side and then caught a wing and cart-wheeled it is possible the wing gathered too much paint weight on one side rather than the other. Thus wing was not nuetral. It DOES NOT take much weight to cause the wing to tip.

Just a couple thoughts.
Wayne

[steve]

The plane didn't break-up. It held together fine. The problem is the foam is flexible and the paint isn't. So, wherever the impact caused the foam to flex, the paint came off.

What's weird is that on the weekend I took a partially charged 7 cell battery off the charger, put it right in the pane and it took off fine and acted a little tail heavy but the wind was a little high so I managed to land okay. I added a little nose weight and tried again and had the same stall an cartwheel crash that I had this morning. This makes me think that I was lucky on the first takeoff and the subsequent attempt, the battery had dropped off. The 8 cell pack I used this morning had been fully charged on Sunday.

You make a great point about the wing possibly being heavier on one side then the other. I used a fair amount of sealer and spackle to try and get rid of the foam texture. I really didn't pay attention to side-to-side weight. The crash did have it fall to one side. But it was obvious that the plane was just struggling to stay in the air.

Would having the wing struts too tight impact the dihedral enough?

[flydiver]

Either not charged or having problems. Sounds like LVC kicked in.
Freshly charged cells (good ones) will lose 10% of their charge overnight. After that they lose 1-3%/day with older cells getting worse.
In addition the stock cub will barely really fly on 7-cells. That's why every one soon gets power hungry and moves up.

As cells get old they form crystals/dendrites inside (excessively simple explanation). This increases internal resistance (IR) and degrades capacity and increases self discharge. Yours may be self discharging very quickly or they may have never charged right. This is a common problem with low quality delta-V chargers (Parkzone DC). They 'kick out' prematurely letting you think they are charged when they were not. As cells go bad (abuse, old age, crap cells to start) this problem gets worse.

You may also have damaged/killed a cell.
A PACK of cells is only as good as it's weakest cell. You should NEVER discharge a NiXX pack of ANY SORT completely (zero volts). That old stuff about NiCad memory effect is mostly crap and passed around primarily by uninformed men. A better term is "voltage depression".
A 'functional discharge' cycle is to about 0.9-1.0v/cell (see links below). Nominal voltage on NiXX is 1.2v. A fully charged fresh cell will measure a tad over 1.4v but not hold that long. Below 0.9-1.0v/cell you essentially have NO CAPACITY left. If you push much past there with a PACK there WILL be a weakest cell that goes dead first. Makes sense, right? The rest of the pack tries to continue pushing voltage BACKWARDS through the dead cell. This reverses polarity and may create permanent damage to that cell. Depending on the extent and exposure it may never be the same to totally wasted and interfere with the pack performance going forward.

For a LOT more information than you EVER thought was available and necessary on all kinds of rechargeable batteries I suggest some patient but heavy reading on these 2 sites:

R/C Batteries Tech Tips & NiMH / NiCAD FAQ
www.hangtimes.com/rcbattery_faq.html

Basics every battery user should know
www.batteryuniversity.com/partone.htm

For a short and sweet and slightly insulting education try here:
Dan's Quick Guide to Memory Effect, You Idiots
www.dansdata.com/gz011.htm

Sorry about the plane. You might be surprised at the paint weight, especially if you used a lot of spackle. Buddy just did that to a small foamy that should run on a 5A 19g motor. Won't fly. Tried a 7A 20g motor-won't fly. He's going to have to notch it up. It's all paint and epoxy. Pretty plane but essentially a botched job, won't fly and now will never float like it was supposed to.

I'm of the opinion the Cub is a sow's ear-a good one. No sense trying to make silk purses out of it. There are better candidates.
But it's still a pity-you did a gorgeous job.

[steve]

I bet you're right about the "paint weight". You figure how much a spray can weighs full v. empty. If most of that weight translates to the plane, then it could be a factor.

I agree with the sow's ear comment. I knew when I was doing it that the plane wasn't really worth the effort for a fine finsish job and, it wasn't designed for it either.

I think this one will be one I will experiment with for a while, since I already banged it up pretty good I have no worries. ;D

Then I'll replace the wing and fuse and just spray a light paint coat on the foam directly and forget all of the heavy finish work.

[flydiver]

The Cub will take a fair amount of weight but to really handle it more power is required. Thus, go lipo. You don't need to go brushless or even change electrics - just pull the jumper, put on a GWS 10x6, and go 3S lipo. Power increase is significant. Depending on the size you may need to do some battery box work. Lots of info available here.

That said there is a HUGE amount to learn about lipos and chargers. Do your homework.

Closing remark. Almost all first planes are trashed. Yours is just part way along that path. You have LOTS of crashes and learning to go through yet. Proceed accordingly.

[rcfast123]

Hey FlyDiver, I have no clue what you are saying. He is 48 years old and does not have anymore homework to do. Homework is for children.

[flydiver]

Feb 22, 2009 1:10:05 GMT 1 rcfast123 said:

Homework is for children.

Homework is for everyone, all the time. If you don't know stuff, you need to 'do your homework'. I'm 60 and doing homework all the time. Never too old to learn.