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

[Don Stackhouse]

Mine has an MP Jet 28/20-7 on two cells, turning a 14-9.5 prop. That prop diameter does make a difference in prop efficiency and climb performance.

If your watts and amps are too high, I'd first try to keep the diameter the same, but back off a little on the pitch. The power absorbed by a prop is proportional to the FIFTH power of diameter, so it's difficult to back off on diameter without backing off too much. Also, for low speed performance (such as climb), you need diameter. Backing off on pitch might actually help efficiency (if pitch is too high, it will put all sorts of load on the motor, but if the blades are stalled, the prop efficiency will be terrible), and in any case, it's less sensitive than diameter, so easier to control.

Of course if your cells vs. Kv setup is way off the mark to begin with, diameter reduction might be the only viable option.

On 180 watts, mine will go vertical, but the airspeed is not enough to provide enough rudder authority to overcome torque. It will sustain a vertical climb if I let the plane corkscrew on the way up. However, the best rate of climb is a little below vertical, about 70 degrees from horizontal. That's with an older battery that was only rated for 20C max when it was new, and was losing amps during the run. It was pulling about 180 amps at the beginning of a run, but would gradually fade off to about 165 watts over about the first minute.

I just bought a new battery for it with a much higher C rating, which might be enough to sustain a vertical climb. However, I haven't had the spare time and weather to test it yet. Hopefully now that I'm back in Ohio and my flying field is back behind our house, it might be easier to make that happen, if I can ever get ahead of things on this project for my new day job.

[vicrider]

Thanks for the advice Don, that lets me know what direction to go in.

[Don Stackhouse]

Vicrider, it sounds like you're getting about 11.4 volts from your battery under load, which with a Kv for your 480 motor equates to 11,600 RPM. That equates to a pitch speed of 80.6 FPS, or 55 MPH. Your rate of climb works out to 1440 FPM (24 FPS), so your prop efficiency is in the ballpark of only about 30%, which is awful. Your prop could be stalling. You're burning up your motor, and getting essentially nothing to show for it. Reduce the pitch, or reduce cells.

Note, all of the above is based on gross extrapolations from the numbers you've given us. Probably a good first step is to measure your actual RPM. Also, if you measure RPM while gradually increasing throttle. RPM should increase steadily as you increase throttle. If it doesn't, your prop could be stalling.

A tachometer is a wonderful thing, that can tell you all sorts of things that the amps and volts alone cannot. If you don't have one, get one!

[vicrider]

Thanks don. I have been looking in to this more and I beleave your right. I think moving down to a 2 cell like you said here and in other threads is the way to go. I have a tach but havnt used it yet, so i couldnt tell you what rpms im turning. But I think im going to stick with the rimfire .10 which is a 1200 kv motor. Ill order a couple new lipos 2 cell 2200, not sure about the prop yet, will order a few differant ones 12x7, 13x6, 13x7 maybe ? I did plug a 1300 2 cell in to it this morning with the rimfire and 12x5 and got 138 watts at 16 amps. A bit of a drop.

[k8zfj]

Originally Posted by Stevephoon

Great to hear Randy,

I had already noticed that the screws connecting the motor to the mount were digging into the firewall ply on mine too. I'll plan on making sure that mine is strengthened as well. I was hoping that just soaking with thin CA will be enough. Maybe you can double check yours after a month or so of flying to make sure nothing else is working loose or breaking down.

Steve

it appears my "fix" for vibration problem is still working. this past weekend, managed a dozen flights in 8-10 mph winds. Used all 3 battery packs (1800, 1500 & 1300ma) with good results...'almost lost it with the 1300ma pack due to height, until l remembered the spoilers arent just needed for spot landings

randy

[Don Stackhouse]

Steve, just heard back from Joe this morning. He discovered he had made a batch of joiner rods that were just a little too short. Looks like about three sets managed to get out before discovering this. We're scrapping the batch, and replacing the ones that got out. Looks like this one was definitely our fault, but thanks for helping us find it.

[Stevephoon]

Originally Posted by k8zfj

it appears my "fix" for vibration problem is still working. this past weekend, managed a dozen flights in 8-10 mph winds. Used all 3 battery packs (1800, 1500 & 1300ma) with good results...'almost lost it with the 1300ma pack due to height, until l remembered the spoilers arent just needed for spot landings

randy

Great to hear Randy! I'm still also planning on getting flat screws and/or small washers as well to make sure I don't dig into the firewall too much....

Originally Posted by Don Stackhouse

Steve, just heard back from Joe this morning. He discovered he had made a batch of joiner rods that were just a little too short. Looks like about three sets managed to get out before discovering this. We're scrapping the batch, and replacing the ones that got out. Looks like this one was definitely our fault, but thanks for helping us find it.

It's nice to find out I'm not 100% crazy... (just 99.9%)



No real progress yet.... A little sanding and added some light-weight spackling to the center wing section to smooth out the epoxied area. I've also figured out that I'll be a little tight on the amount of covering I have for my planned scheme... and that the stuff I bought is now out of stock... (HobbyPartz Trans Violet) With a little planning I should be OK, but it will be tight. I only have 1 roll and it's 2M x .65M (79 x 25 in) It's not a very wide roll.

Steve

[Don Stackhouse]

That's cutting it awfully close. Don't forget, you need some extra around the edges to pull on to get the wrinkles out.

[Dimeflyer]

Frame work looks good guy !!
Now make it look pretty and you will be a great builder !!
George

[vicrider]

Originally Posted by vicrider

Thanks don. I have been looking in to this more and I beleave your right. I think moving down to a 2 cell like you said here and in other threads is the way to go. I have a tach but havnt used it yet, so i couldnt tell you what rpms im turning. But I think im going to stick with the rimfire .10 which is a 1200 kv motor. Ill order a couple new lipos 2 cell 2200, not sure about the prop yet, will order a few differant ones 12x7, 13x6, 13x7 maybe ? I did plug a 1300 2 cell in to it this morning with the rimfire and 12x5 and got 138 watts at 16 amps. A bit of a drop.

I changed the prop to a 13x6.5 with 2200mah 2 cell, now drawing 27 amps and 214 watts and its turning 5850 rpms. It pulls decently but the motor gets hot in a matter of seconds, esc is fine. Im guessing I have to much prop.

[Don Stackhouse]

That's usually what that means. What is that motor rated for?

A motor wants to run at an RPM corresponding to the supplied voltage (remember, most batteries will lose some voltage under load if the current is too high) times the motor's Kv (the "voltage constant", in RPM per volt). It will pull whatever current it needs to try to get to that RPM.

Therefore, if your prop has too much pitch and/or diameter, it puts too much load on the motor, which then pulls too much current trying to get to an RPM of Kv times volts. Since watts (power) is volts times amps, too many amps results in too many watts, and an overloaded, overheated motor.

Diameter is much more sensitive than pitch in most cases. The power absorbed by a given prop is proportional to the fifth power of diameter, so a small change in diameter can make a big difference in amps and watts.

Going down in the number of cells reduces the voltage, so the RPM the motor wants to run at goes down, and with it the load the prop puts on the motor at the new, lower RPM.

[Stevephoon]

Baby steps...

Started on the covering of the center wing section. This is a couple of days of part time work... The Violet is from HobbyPartz and is about the weight and strength on MonoCote/Ultracote... The Black and White are Ultracote from a LHS that I bought a while ago.

I am trying to not waste any of the Violet, so I am piecing things together. I started with the largest piece on the bottom, then the matching piece on the top. Then the bottom front and the top front. I reduced the wrinkles with a little heat on just the viloet at this point so the black on top would not be applied over wrinkles. Next came the bottom black spar covering piece and then the tips and center. There is still some additional stretching to do, but that will wait untill all of the covering is applied.

The plan is to do the same on top with white as the other color. I'll need to hinge in the spoilers as well. I'm planning on using covering hinges per the plans.

Steve

[Don Stackhouse]

Caution - you can put Ultracoat/Oracover over top of other brands and it should stick OK. However, if you put other stuff on top of Ultracoat/Oracover, it will not stick.

[vicrider]

Its lookin great steve.

[FishHawk]

Hi I'm FishHawk and will be starting my Chrysalis build soon. I am newbie so I got a lot to learn . Vicrider sent me here , thanks for the link.
FishHawk.

[Don Stackhouse]

Welcome aboard! Good luck, and remember, the only "dumb" question is any that you should have asked, but didn't!

[bluzjamer]

Had a great day today flying my E-Bot which I bought from my good friend Alex....but what made the day even better is when Alex sold me his Chrysalis....just need to power it. It is a superb flyer set up with spoilers and built perfectly by someone that is a perfectionist. Not like me at all, I'm so lucky and humbled. Flying in December in T-shirts is another huge + with low wind to boot. Just a great all around day here in little Rhody.

[Stevephoon]

Welcome aboard FishHawk. You will find that there are lots of helpful people here on Wattflyer.

I have added the white trim on the top. 1st added the trim around the spoiler opening, then the bottom of the spoiler while attaching the underside covering hinge. Then one large piece on top that I cut the spoiler free from. The idea was to minimize the pieces for looks and keeping it smooth. A couple of addition white pieces and the center wing is complete!

I still need to hook up the spoilers, but that's for another day.

Steve

[pd1]

The covering looks great.

[Dimeflyer]

Looking good guy

[FishHawk]

Enjoy watching you guys building the Chrysalis. I am getting ready to build and have gotten some good tips from the photos and questions posted.
FishHawk

[FishHawk]

Broke one of the shear webs when trying to align the ribs. My trailing edge notches don't line up that well but I guess this is normal, which makes alignment difficult.
What I did for the sheeting is I used tracing paper and traced the sheeting from the plans.This will help me when I cut the sheeting .
FishHawk

[Dimeflyer]

It usualiy makes it easer than freehand cuting unless you have a totaly steady hand with an exacto or singleedge razzerblade guy I have been making part paterns out of styreen sheet that a friend gave me a few years ago and I want to use it up to get
it out of my shed and figured if I want to keep the paterns I should be able to store them easer if they are made of this stuff instead of balsa or cardboard , at least they will not break or bend as easely I hope ?
George

[Dimeflyer]

another thought - I have several sheets of this stuff in the shed , if any one wants a sheet or 1/2 sheet I may be willing to part with some quite reasoneabley as in for the cost of shipping only !!!! they are 4'X8'X aprox. 1/8" thick .
I don't have a way to waigh them in the sheet or 1/2 sheet size so i would have to take one to the post office get it waighed and get a price for the shipping cost to
an indavidgual adress .
Sofar I have used it for fuselage and wing former paterns and it
seems to work well for them !
George

[Don Stackhouse]

Originally Posted by FishHawk

Broke one of the shear webs when trying to align the ribs. My trailing edge notches don't line up that well but I guess this is normal, which makes alignment difficult.

By that I assume you mean you broke that web free from the carrier strip along the top edge. Just tape it back onto the carrier strip with a little swatch of masking tape on the side that will face away from the spar caps.

As has been discussed many times in many places, the folks who print our plans have a problem with the calibration of their $10,000 digital printer. We have worked and worked and worked with them on this, but the problem keeps coming back. Just align the notches in the trailing edge stock with the plans in a way that "splits the differences", then make the ribs parallel to the lines on the plan, but let the notches in the trailing edges and the slots in the shear webs determine their exact spacing.

What I did for the sheeting is I used tracing paper and traced the sheeting from the plans.This will help me when I cut the sheeting .
FishHawk

Probably not as much as you think. The sheeting gets bent into a 3-D shape, and a pattern traced directly from a 2-D drawing is going to end up short.

Measure the width of the sheeting from the plans, then make up a strip of cross-grain balsa sheeting a little but wider (about 1/16" to 3/32", so you can sand it flush with the ribs later, and don't have to worry about getting it exactly positioned side-to-side when gluing it on) than the width across the ribs, and maybe about 15" or so long. Cut the panels in sequence from this strip. When the cross-grain strip starts getting a little short, glue some more segments to it and keep going.

At the leading edge, cut the panel so it overlaps the leading edge dowel about half the width of the dowel, then bevel the underside of the sheeting along the leading edge for about half the thickness of the sheeting, at about 30 degrees from the surface of the sheeting. This will let the sheeting nestle down into position on the dowel, with enough left on top to sand and blend the upper surface of the sheeting smoothly into the leading edge.