There's no doubt about it these LiPoly's open up a whole new ballgame with the RFFS-100. We figure that we may be able to build a Skeeter that would have unlimited vertical (take off the landing gear etc). Will have to see. Even so with the MASSIVE (at least at these weights) weight reduction you get with the LiPoly's at the very least we will end up with a very high performance Skeeter.

Dan
P.S. The Guided Mite that Phil pointed me too is even lighter then the Skeeter and flys well on two 50mah Nicads and a KP00.

If you are able to get the Li-Poly with its own protective circuit (which has a hard cutoff), you can use this output for driving the motor, and a direct connection from the battery (before the protective circuit) can be used for the control system, provided your system allows such a connection..

I don't know the details of your receiver/ESC/motor connections, so this idea may be useless..

regards,

Bulent

That sounds like a good idea. If the esc is switching to ground it will work perfectly. How much does the circuit weigh?

Graham

Dave,
To get some ideas, you might check out Dave Robelen's Pixel. It is 13-inch span, uses a geared M20, has 60-sq.inches area, and weighs 26g with 50x3 NiCd's, and 2.2 oz/sq.ft loading. With a LiPoly you can easily beat this weight. I think the key is to scale to wing area, not span. I made that mistake with my Fokker D-VII. It has a 13-inch span, but ony 47 sq.inches because the lower wing is shorter and has a narrower chord. The LiPoly cell may be able to save its wing loading problem, but it might still be a fast flier.

Where I am right now with semi-scale is thinking that somewhere between 65-80 square inches is a good range (I know the Aeronutz go smaller, but this is just where I'm at). For my next scale plane I'll probably start with the equipment including this new light cell, and work backwards to get a wing loading of 2.2 oz/sq.ft or less and then derive size from that. This is what I did with my Dr1 except that I wanted lots of wing area so I scaled the chord by an extra 30%. I should get 1.45 oz/sq.ft loading.

I think with this new cell it's possible to make a semi scale model with some details and still come in with a nice low wing loading, without having to be a super light builder. In otherwords, there is sufficient margin for less than perfect building skills and for scale details like color. I'm guessing if you scale your SE5 to 14" you'll get something around 65-70 sq.inches (don't take my word for it). At this span, 1/32 balsa for the wings will be just fine.

So, try
* 65-80 sq inches
* Geared M20, 6:1 and 5" prop
* Try for 2.2 oz/sq ft or better loading
* Increase landing gear length to allow slightly larger prop
* Mount all your equipment as far forward as possible to get it to balance
* Use a relatively flat airfoil, scaling down the airfoil from your plans may have too much camber (remember our conversation with Dave Robelen?)
* make your own balsa wheels to save weight
* use paint or printed tissue to add color since you can afford it
* Promise the Aeronutz that you really, really are going to make a Depron plane at some point.

Bulent and Graham,
I like that Idea. I've only seen the protective circuit in pictures but have wondered about canibalizing it. Now, how do we find out if the RFFS-100 switches to ground? Of course, it's accademic since none of my cells have a protective circuit.

one of the motor terminals will route directly to ground or positive, you can probably tell. The one that isn't direct is switched. Or Dan could tell us.

Graham

Oh and Gordon, you had better!

Then again depron is a bit "2001", blue foam now theres a material. And it makes you look like a smurf (holding a dremel)which can only be a good thing.

Graham

Graham,
Really, really, I have a box of "Zepron", which is the closest we can get to Depron over here. I just can't help myself. I actually cut a Dr1 wing out of three different thicknesses of Zepron (which is I think somewhat heavier than Depron) weighed them, cut one out of contest grade balsa, found it was lighter, and there I went down the sawdust route again. But, I have an idea for a plane to use two LiPoly's and an Astro Firefly with the new gearbox or the MTM planetary geared DC5-2.4. I plan to do that one with Zepron. But, at this stage it's still in my head.

Graham, thanks for the input.

Gordon, dito, and yes, I do remember that talk with Dave, good point to keep in mind. I wish he could be present on this board, he is a wealth of information, maybe under a "pen name".
I just talked to Clarence Hurd and ordered some coils....Fill will be very dissapointed.....I still enjoy making them, but it was more fun when they were $20. Point is, if I am willing to buy coils then Depron may be next....though in regard to your later post, I have also found good balsa to be lighter than foam. The real trick is to find out who in NASA supplies Dave Robelen with the helium injected balsa he must use, I keep waiting to see one of his planes lashed to the table to keep it from floating away.

Dave W.

Dave Robelen is an amazingly light builder. The only way I can come close is with LiPoly cells. But, then he will get his hands on some and build something even lighter.

Dave,

At the new prices of the coils, I can't blame you. It is no longer worth the time to save a few bucks. After that being said, would any one like to buy a couple pounds of magnet wire??? Just kidding, I still like my 44 awg wire.

Philip

Woe is me, another lost art. Like weaving baskets out of boy scouts

Graham

Philip,

I think there is still a place for homemade coils, like the tiny ones that you and Ralph have been using. Glad I didn't quit my day job to go into the coil winding business.

Dave W.

Originally posted by gordonjohnson
I think my charger may use a variable charge rate, but I'm not sure.

Gordon,
I have that same charger, and it is a constant rate charger: 450 ma for lithium cells and 850 ma for Ni-XX cells, the manufacturer claims.

But, this may be a bit misleading for Li cells, since, to the best of my understanding, Li cells must be charged at a constant voltage ( which is 4.2 volts for Li-Ion ), that is, this charger puts out 4.2 volts ( per cell , this charger can do up to 4 on AC power ( maybe only 3? ) ) and makes available 450 ma, but the actual transfer of energy is: that current ( 450 ma ) times the DIFFERENCE in voltage between 4.2 and the current voltage of the battery.
So when the battery is at 2.5v, the energy transfer rate ( really power ) is
( 4.2v - 2.5v) x .45a = (1.7x.45) watts = .756 watt
but when the cell gets near full, say at 3.9v, the energy transfer rate is only
.3v x .45a = .135 watt

This is why Lithium cells charge to 80% or so pretty quickly, but take a long time to get to full charge.

But, I may be wrong about the way energy flows into a battery from a charger.

Rian Mullins

Rian,
I had the same concern after reading the manual that came with the charger. I asked (emailed) Skyborne about this and they said that the whole purpose of this charger was for it to reduce the charge rate, and that it was appropriate for the 135mAh cells. These cells are so new that what we know about them is still growing. I do trust the guys at Skyborne. They have backgrounds in the defense electronics industry and assured me that this charger was a very good charging solution for the LiPoly cells. I'll ask them for some more clarrification.

As a side note, Bob Selman has designed a small charger specifically for the 135mAh cells for $25. Also, he will have these cells for sale within a couple of weeks.