Help with Sagitta 900

[Dale Bradly]

Chaps, can you lend me your knowledge please.

I have a kit built electrified Sagitta 900, which i very much like the look of, and am very pleased with it in general. 

However, it's a dog, but I really really want it to be a good flyer.

It has two issues which i want to sort as best i can:

a. It's overweight. Currently tips the scales at 1450g, ready to fly. According to the book, should be in the region of 1200g, so it's about 20% overweight. as soon as it's off the throttle, its on its way down, i've never had it airborne for more than a couple of minutes, as in thermals are for other people, not me.

b. It is an absolute pig to control on throttle.

 

re (a), i built it as light as i could, and there isn't a whole lot i can do now, i'm contemplating cutting holes in the ply fuse sides and covering over them, but realistically this will only remove a handful of grams anyway, and make it look ugly. I am more seriously considering extending the wing to say 3m, which won't add 20% in area, but will probably help more than any other mod in this regard, although changes to point b here might also have an impact.

(b), is running a 2836 1120kV, spinning 11x6 folding prop, from a 3s 1300. Just measured 180w/20A on a store charge level batt. I mistakenly didn't put any downthrust on the motor during build, i understand e-gliders perform more comfortably with a not insignificant amount. Under throttle, it hauls it's nose up and is very very difficult to control, to the point i don't enjoy it and am convinced it will be the end of things one of these days. I am most certainly going to change this, and aim for something in the order of 6° unless someone suggests otherwise.

I would much prefer to shed some weight at the expense of a longer/shallower climbout, so can someone suggest changes that will tame this somewhat, and also make it lighter? happy to replace any and all components at the front end to achieve this.

Is the CG on the plan acknowledged to be the place to fly it at?

Cheers

 

 

 

 

 

[Shaun Walsh]

Not sure about how to lose the excess weight but regarding the extreme climb angle have you thought about mixing in some down elevator with the throttle? I have done this on one of my gliders, as the throttle is opened increasing amounts of down elevator are applied. The mix is on a switch so I can turn it on and off as required.

[Outrunner]

Hi Dale. Sorry to hear about the problems you have with your Sagitta, I can't help you with weight reduction, but from what you describe the climb out sounds normal. I have a couple of electric gliders and neither have any downthrust but climb up very steep, almost vertical which is what I want them to do. The motor is just to gain height then after shutting down the motor I soar around trying to find thermals, usually in vain.

[Martin Hodgson]

A bit late but here are some ideas for you

Some down thrust is obviously needed, and/or mixing some down elevator as the throttle is opened.

Regarding the poor soaring. The Sagitta was never a floater. The E205 (I assume you built it to plan) likes to move. Try flying it faster and keep it moving.

Another option could be to add turbulators. 1 or 2 layers of car trim tape at about 20% chord back from the LE ( rough guess but it worked on my E205 wing). This should improve the lift generation at low speeds but will hurt the top speed.

Experiment! 

Lastly, and this needs some work, strip off the top covering and extend the top sheeting to 40/45% of the chord. Wind tunnel testing has shown the E205 works much better when the sheeting is extended.

 

Weight is not always a problem. I have quite happily thermalled soarers with 12 to 15oz/sqft wing loadings, but they didn't like flying the same way as my 5oz/sqft loaded Bird of Time.

 

Hope that helps

[Dale Bradly]

Thanks guys, will consider all points when it's next on the workbench.