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Pumpkin drop event

Advice and links for my first twin engine build

Headbang

Well-known member
#22
Yes, that would work. A 3,000 mAh 40C battery will supply 120amps (3000/1000 x 40).

@Headbang makes a good point, make sure you are using the continus C rating not the burst rating. A burst rating is ONLY for a few seconds. It's always a good idea to leave yourself a bit of headroom, we don't live in a perfect world.

I use Turngey/Zippy brand batteries, their ratings seem to be about right. I routinely draw 45 amps from a 2200 mAh 25C battery. I cannot speak to other brands, if their C rating is inflated.
I know zippy is good. Nanotech's seem lacking in practice. Gens ace are solid. I have recently had issues with leapord power 3200 50c 3s and 4s batteries sagging badly at only 58amp draw. Just depends on the brand. I have bench tested many, many are seriously inflated on specs. Most you can tell in advance they are inflated, if they weigh noticeably less then a known good brand at the same spec, chances are they are fudging the numbers.
 

Vimana89

Well-known member
#23
Figured I'd probably want the 20a ones. Quick side question: Is a 75c battery generally overkill for a simple 3s single prop plane with a motor like I listed above(either the graupners or 2205)? I have tried 550,650,and 850 mAh batteries with my two scratch builds and didn't look too much at c rating when I bought the first ones, figuring they were right. Turns out most planes that use a motor like that and 3s seem to recommend a battery around 45c instead. These 75c batteries work, but they run out a bit too fast and leave things a bit warm under the hood. I think it might not be efficient.
 

Vimana89

Well-known member
#25
I would love it if someone would team up with you and make a set of planes that can be posted on here so we can build it too. that is a great looking plane and id love the build challenge.
Once I get a model flying well, I'm sure somebody will be able to take my measurements and rough sketches and make a plan, perhaps even printable templates. This project seems to have gotten a lot of interest and attention here. It's just such a nice looking plane and there's not enough airliner style stuff being built, so I'm sure if I get it flying the community will help make it a finished product with a definitive plan. There is one other person working on one of these and I'll have to check how they are doing. Anyone itching to design their own Dash should do so and we can compare and trade info, anyone just wanting to build and fly one shouldn't have to wait too terribly long for mine.
 
#26
Figured I'd probably want the 20a ones. Quick side question: Is a 75c battery generally overkill for a simple 3s single prop plane with a motor like I listed above(either the graupners or 2205)? I have tried 550,650,and 850 mAh batteries with my two scratch builds and didn't look too much at c rating when I bought the first ones, figuring they were right. Turns out most planes that use a motor like that and 3s seem to recommend a battery around 45c instead. These 75c batteries work, but they run out a bit too fast and leave things a bit warm under the hood. I think it might not be efficient.
The higher C rating the better, after you get a battery with high enough C rating to supply your motors of course.

So if a battery is able to supply 70 amps and you only ask 30 amps from it you'll put less of a strain on the battery, which could allow it to have more cycles.
 

jpot1

Well-known member
#27
@Vimana89 - interesting build. I would love an FT style airliner type build but the rounded fuse pushes my sketchup abilities past their limit. I’m actually working on something similar as it has the T-tail and the fuse in the a-10 style to simulate a rounded appearance. Narrow the fuse at the rear, change the wing shape and raise it with two motor pods and not too far off.

94804BC8-C2ED-412D-A924-CEDF21DF1857.jpeg
 

Vimana89

Well-known member
#28
@Vimana89 - interesting build. I would love an FT style airliner type build but the rounded fuse pushes my sketchup abilities past their limit. I’m actually working on something similar as it has the T-tail and the fuse in the a-10 style to simulate a rounded appearance. Narrow the fuse at the rear, change the wing shape and raise it with two motor pods and not too far off.

View attachment 130949
Like I mentioned above, unlike some much more experienced builders, I don't intend to do a rounded fuse! I intend to do a somewhat boxy, FT style fuse with some curving up front maybe, that captures the look and profile without being exact. Think like the FT viggen, or pretty much the cool T tail jet you are building(y). I want to make this as easy and accessible as possible, not to mention within the limits of my own skills which are still somewhat raw. No advanced techniques will be needed, just basic A folds and probably some small curved pieces in the very nose. I'm a hands on guy who doesn't use rendering programs, so I'll have to sit down and see what I come up with for a fuselage, but it will be as basic as possible.
 

Vimana89

Well-known member
#29
The higher C rating the better, after you get a battery with high enough C rating to supply your motors of course.

So if a battery is able to supply 70 amps and you only ask 30 amps from it you'll put less of a strain on the battery, which could allow it to have more cycles.
Exactly what I needed to know, thanks for clearing that up. something I probably should have learned much earlier. The batteries I'm using are pretty ideal then at least in theory. These 550s are just probably not good quality, a half ounce heavier than( my dead over-discharged) tattu 650s and run out in probably under 4 minutes. The tattu 850 I have gives some good run time. All are 75c.
 

Vimana89

Well-known member
#30
Thanks everyone for the help so far. I'll be ordering parts soon and starting an official build thread once I've got sort of a rough draft as far as a sketch or templates. I'll brush up one last time with all the basics of the wiring and all that, but I've got a good hold on how to do it and know what parts I'll need. The thing I haven't researched yet is programming thrust differentials, but it seems to be a frequent question so I'm sure I'll find many good guides.
 

L Edge

Well-known member
#31
Here are some pointers about multi-engines planes(think I have had a total of 11) and what I have gleaned over the past moons of flying. For electrics, the direction of rotation turning towards the fuse or outward to the wing, really doesn't have that much difference in stability, but where I (and others in another group) found when the props turn outwards, the touch of the ailerons are crisper (not rolling the plane over, for turns) than if props are turning inward. I know some are laughing at this, but you should try it. Once you have flight time under the belt, you can even feel a stall coming on where you become one with the plane. The less expo, the more you can sense it.

For batteries, I use a 3300 mah with 2200kv motors and 10 x 3/8 props.. You will find that you can cruize around at 1/2 throttle easily and get flights over 10 min. Remember, the lighter the plane, the lower the stall speed. Also, where you position the battery makes a difference.

Now for thrust differential. Imagine this, you are in your 30 degree turn and the lower wing motor craps out. What is going to happen
a: if you have a moving rudders and you don't know what to do?
b: if you just have differential thrust and no moving rudder?
c: if you have both?
Now, what happens if you are straight and level and your right motor craps out if the plane is flying towards you?

Most pilots never address or think about that until it happens. So what is your emergency procedure(s) and what do you want to do for your build? My first multi lasted a total of about 4min before it was totally destroyed. Now, I have an emergency procedure for almost everything that is possible. Hey, foam is a lot cheaper for the learning process.