#35 Model Airplane Building Clinic >> DJAerotech Chrysalis 2m Electric Sailplane

[Stevephoon]

Lets see if I am laying out the outer left wing correctly. I first laid the trailing edge on the plans. The right most notch in the tailing edge is exactly lined up with rib K2. The notches get further away from the plans as they get closer to the wing tip. I then used push pins to keep the training edge in place. I laid the main and rear spars on the plans (after trimming them to something close, but a little long.) I then placed ribs S and M in place. I allowed the ribs to dictate the positions of the spars. I then placed the shear web over rib M and moved S into position. With the real rib M in parallel to M on the plans. I pin the spars down. Rib S ends up parrallel to the plans, but a little more than the thickness of the rib further towards the wing tip than what the plans show.

I've attached some pictures to help show what I am trying to describe.

Steve

[Don Stackhouse]

Looks good to me.

[FishHawk]

Making progress wing almost done to a point . I'm moving on to the fuse soon and I will finish the wing after the fuse is done. (Spoilers & dihedral )

My next decision is weather to extend the nose or not.
I'll post my motor and battery combo and see what you guys recommend.
FishHawk

[Don Stackhouse]

The MP Jet 28/20-7 in mine weighs 5.7 ounces (162 grams). If your motor is more than a couple ounces less than that, you might want to extend the nose.

[FishHawk]

My motor weights 3 oz and the battery is 11. Oz . So , in general for this plane is the preferred method to extend rather than add weight?
The motor is a Eflite Park 480 and the battery is a 6 cell Venom 2400 MAH.
I could have used a Lipo Battery but I want to see if this combo works.
I'm shy about 2 oz roughly with this set up.
FishHawk

[Don Stackhouse]

11 ounces for the battery ?? !!

That's a very heavy battery. If you can fit that and the motor into the nose, you probably do not need to extend the nose.

However, a 2-cell Li-poly would weigh about 4.5 to 5 ounces, a third of a pound lighter. In that case, extending the nose would probably be a good strategy.

[FishHawk]

I'll test fit the set up and see how it works. If I go with the lighter battery is it a big problem to extend the nose after the outside doubler is put on?
If it is, then I'll have to make a decision which way to go .
FishHawk.

[vicrider]

I used the eflite .480 in mine. With the nose extented 1.5 inches and a 3 cell 2200 in the nose and with the reciever and futaba 3114 servos under the wing it balanced close to perfect.

Going back to lenghten the nose after its built I think would be a real pain. Better to do it now.

[Don Stackhouse]

If you're going to extend the nose, you need to do that before you start assembling the fuselage.

[FishHawk]

Time to make a decision . FishHawk

[Don Stackhouse]

If it was mine, I'd go with a lighter battery and extend the nose. The 480 motor is adequate, but not hugely overpowered, and the plane penetrates fine without extra weight.

BTW, I really do not recommend the lightening holes in the tail just to save weight. We designed them because some customers just like the look of the sun shining through an open framework back there, and we figured it was better to give them some idea of what to cut than to let them just guess at how much to hack away. They really do not save enough to be a significant weight savings, and cutting that material away in the stabs does not help the flutter resistance. The holes in the ruddervators might help flutter a little, but not enough to be worth the trouble.

[FishHawk]

I'm concerned about the safety of the Lipo battery. I've read where they could cause a fire if overheated. What do you guys think about these battries?

FishHawk

[Don Stackhouse]

Been using them for quite a few years. They are safe if you follow the rules and use a little common sense.

The biggest problem is overcharging. Crash damage is also a possible issue. Note, crash damage is an issue with nicads as well. I once saw a Goldberg .60-size Chipmunk burn down to just the wingtips and tail section when the nicad radio battery shorted out in a crash. Even the aluminum engine crankcase was melted, and the radio was just plain GONE.

What used to be a problem was the dumber battery chargers we used to have, that used just the battery voltage to determine how many cells in the pack. The problem is that an almost fully charged 2-cell pack has almost the same voltage as a fully discharged three cell pack. Folks would try to just "top off" an almost charged 2-cell pack, the charger would assume it was a three cell pack, and would try to charge it that way. POOF!!

Today's chargers have much more sophisticated techniques for this, and can tell a 2-cell from a 3-cell. They also make you verify to them how many cells in the pack, as a cross-check.

It's still recommended that you charge Li-poly batteries in a fireproof container, just to be safe. Ammo cans are very popular for this.

I've been using these batteries for over a decade, and I have only burned up one, a little 250 mah 2-cell pack. Back then one of the biggies in the industry was recommending putting them in the oven while charging. I put the battery in the kitchen oven and started charging it. Fully charging a Li-poly takes about 1.5-2 hours. A while later I remembered I needed to bake cookies for the kids' lunches the next morning, and absent-mindedly turned the oven on to 400 degrees to pre-heat it. A couple minutes later, just about the same instant I realized what I'd just done, I heard a muffled "PFFFT !!". The battery was a little pile of ashes in the bottom of the oven, and the charger was a melted lump of useless plastic. Fortunately it was one of those older ones that needed to be retired anyway.

The other sure-fire (no pun intended) way to flame a Li-poly is to try to charge it with a nicad/NiMH charger. The charge methods of the two are absolutely incompatible. There are some charger, such as the Hitec, that can safely do both types, but in general you need a Li-poly charger to charge Li-poly batteries.

Your speed controller needs to have its cutoff voltage set properly for the number of cells in your pack. Generally, Li-poly batteries should not be discharged below 3.0 to 3.2 volts per cell (different cell manufacturers vary in their recommendations).Nominal voltage is 3.7 volts per cell, and they should NEVER be charged above 4.1-4.2 volts per cell. If you go above about 4.5 volts per cell, you should expect some fireworks.

If you are still afraid of Li-poly batteries, better get rid of your cell phone. That's a Li-poly battery you're carrying around in your pocket! Most laptops also use them, so better get rid of that as well, along with any Nooks, Kindles, Ipads, tablets, etc..

[FishHawk]

Thanks of the tutorial on the Lipo battery. I think that the Lipos in our cell phones and laptops have dedicated chargers and are also not subjected to violent crashes.
Boeing has had problems with these batteries in their new airliner.
You make a good case for using them. I'll probably go this route.
I will follow your recommendation of 1.5" extension.
This has been an interesting model to build ...lots of challenging problems to solve which makes it interesting.
I hope you don't mind my newbe questions.
FishHawks

[Don Stackhouse]

The Li-poly batteries we use ALSO have dedicated chargers. In many cases they are much more sophisticated than the ones for our cell phones.

In any case, the charge philosophy for a Li-poly charger is more of a "constant voltage" system, although the first of the two phases of the cycle is also current-regulated. One of the key differences is that nicads and NiMH batts actually show a slight voltage drop when they get completely full. Their chargers sense this drop ("peak-detecting") and use that to determine when to end the charge. Lithium batteries just continue to gain voltage, never giving the charger a voltage drop, just continue to gain voltage until they blow up (or as the AMA says "vent with flame"). That's why you NEVER charge a Li-poly with a nicad charger.

For good advice and good products at good prices, I like Dave Thacker's "Radical RC" (www.radicalrc.com). Yes, Dave is a good friend of mine, but that's not why I recommend him. He knows his stuff, stays on top of changes in the technology, and sells it at excellent prices. For chargers, if you want to go "Cadillac", I like the Hitec X4 charger:
http://www.radicalrc.com/category/Ch...MH-NiCad-PB-68
which comes in a variety of versions, depending on details. If you go with the basic one, you will also need a car battery or a DC power supply to feed it. The X4 Plus DC/AC version can operate off a car battery or AC wall current directly.

Yes, these are an investment, but they handle both Lithium and nicad/NiMH batteries, include cell balancing, and read out diagnostics on the battery as part of the charge operation. They can charge four different batteries at a time of any mix of types and sizes on their list.

You can get away with something cheaper and simpler of course, but if you stay with the hobby, sooner or later you will want something like this.

As far as Boeing, one of the guys I work with at my new "day job" worked on that airplane, and we have had discussions about where they went wrong. He blames it on the present corporate culture and management philosophy there, that believes you can design an airplane entirely on computers, with an absolute minimum of prototyping and lab testing during development. Engineering is not that cut-and-dried, and now they are paying the price for that.

As far as questions, the only "dumb" question is any question you should have asked, but didn't!

[FishHawk]

Well doing my research on these batteries you can get ac adaptors for some of the chargers but the cost of a good charging system is about the price of the model. I don't want to sink in a lot of money intothe project right now to see if I like the hobby.
This hobby is .... " A hole in the Sky where you pour money."
The other issue with Lipos is storage and fire prevention.
In one of the threads the molder broke the cardinal rule and left the room while charging his battery and his house almost burnt down.
Stupid think to do but it happens.
A lot of guys store the batteries in ammo cases.
I guess for me how stable is the battery when left alone?
There are a lot of threads on these batteries and careless mistakes could be costly.
Thanks for your input . FishHawk

[Don Stackhouse]

There's a lot of hype and exaggeration about these, although not quite as much as there used to be. For power on electric models Li-poly batts are pretty much the norm these days, very few folks using nickel-based batteries for that any more. Essentially all "park flyers" use Lithium batteries now, it would not be practical to fly them on nickel batteries, just too heavy.

As far as stable while sitting around, doesn't seem to be an issue. They do not just spontaneously decide to blow up. When they misbehave, it's for a reason.

There are cheap and simple chargers on the market, that don't cost all that much and that get the job done safely.

Yes, models can be expensive, depending on your definition of expensive. However, if you look at the accumulated costs of things like bowling or golf, models are relatively cheap. If you want a real eye-opener, look at the cost of a typical pack-and-a-half per day smoking habit. A friend of mine was into that, and we figured out that for the money he was converting into toxic smoke every year, he could have several top-of-the-line aerobatic gas-engined helicopters, including support equipment. His bar-hopping on Friday nights was even worse.

No, models are cheap.

[FishHawk]

Yes, try fly fishing.If I added up the costs of my flyrods it would be well over
$2000. Another solution , is to try my first model with the Nicads and the buy a fuse kit and go Lipo. FishHawk

[Don Stackhouse]

It will fly with the nicads, but thermalling ability will suffer a small but noticeable amount. Penetration will be great, but unless you live where it's really windy all the time, it will be more penetration than you need. This is based on my tests of a 7-cell RC2000 nicad pack in mine, which was 10.5 ounces heavier (48 oz flying weight) than my 4.5 ounce 2S 2000 mah Kokam Li-poly pack (37 oz flying weight, including a 270 mah NiMH radio battery). It flies OK with the nicads, but it's significantly happier with the Kokams. If it's a weak-lift day, the difference in thermalling ability is fairly pronounced.

You might consider extending the nose anyway, then mounting the nicad pack further aft. On my prototype I got it to balance with the battery pack centered over the C/G, so I could try a variety of different battery setups without altering the C/G. It can be done.

[FishHawk]

Should I extend the nose 1.5 inches as you have shown shown on your cad drawing?
FishHawk

[Don Stackhouse]

It's a wild guess, no firm answer (a lot of other things that are also involved), but that should be in the ballpark.

[FishHawk]

Ok , I will use your 1.5 inch nose extension recommendation.
FishHawk

[Stevephoon]

Originally Posted by FishHawk

...This hobby is .... " A hole in the Sky where you pour money."... FishHawk

All hobbies allow you to do this. You can spend as little or as much as you want. It's always a balancing act.

Do you already have some existing NiCad batteries? If so, you maybe able to use them. I started with a HobbyZone Firebird which had NiCads. I reused those batteries in my next plane and then finally started to switch to LiPos and the needed charger.... It's always good to make sure that you are planning on continuing to stay in this hobby before spending lots of money. If you do want to start with your existing batteries, then you will need to select your power system based on the capabilities of those existing batteries.

Good luck with the rest of the build!

I still have not been finding much time to build. My second 1/2 of the wing just now has the ribs installed. I did order some additional covering from Skyward Hobbies that I hope is the same that I started with.

Steve.

[FishHawk]

I have a NIMH battery that I will use and extend the nose .Perhaps , I' ll go with the Lipo later on as I get into the hobby .I'm taking it slow and see how it goes.
FishHawk

[FishHawk]

Test fitting my wing sections for the Dihedral, I assume you keep the washout shims in place . Correct?
First, I glue in the outer panel to the inner panel then both inner panels together together with washout shims in place place.
FishHawk