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Old Aug 03, 2014, 08:58 AM
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United States, TX, Denton
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HK brushless motor selection for blu baby trainer help!

Hello, i recently built a blu baby with 2" extra added to standard wing length for extra stability bc i m not a very good flyer yet (ive been using a flight sim lately tho)

So the motor ive been using is a 2204 1700kv from a hobby kin g mxs 3d kit i bought a year ago (which is out of commission now). the motor on the mxs and now on my blu baby build works BUT if there is any amount of wind, i cannot fly into the wind! it just hovers there pretty much...

SO can anyone point me to some suggested brushless motor from Hobbyking that will work well on 2cel lipo 500mah or 2cel 800mah (what i have on hand)? ive been using 8x4 props, but briefly tried a 9x5 on my mxs before it became unrepairable. i dont have a watt meter yet. i have yet to try it on my blu baby altho i plan on it next flight i attempt.

my blue baby is around 12oz i believe (***actually 9.2 w/o battery or prop!*** prop is .4oz extra) . my fuslage is a bit wider than the original plan suggested, it is about 1 1/4in wide to fit my 2204 1700kv... also my motor block i secured my motor too is probably more weight than i should have added, i use a 1x2 block bc it was easy to find that morning next build i will use at min a 1/4" ply for my block or thinner if can..

so what motor would gvie me more thrust but similar long flight times (on 2 cel 500-800mah)? i want to have a more sporty plane build when i get basic flight pretty well down. i might go 3 cell eventually which would make this search a lot easier i think. also i fried my 12a plush in a puddle and am now using a 25a plush esc. thanks in advanced and im in love with the diy foamie section on this sight!

***also, is my extra 2in wing length making it harder to fly into the wind? more drag perhaps? i had to reinforce them with 1x3x750mm carbon spars bc of folding.
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Old Aug 03, 2014, 10:37 AM
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You would get much better performance in the wind on 3S and a 7x3 without overloading the motor. You need more pitch speed to improve headway in the wind. You should be able to use a DVM to measure current draw with this motor. Most of them can measure up to 10 amps which should be much more than the Blu Baby would require.

I have a plane of about the same size and weight (11.5 oz) that performs well in a moderate breeze on a 600-800mAh 3S lipo and a 7x3. Motor has a 1400Kv constant and draws just 2-3 amps to maintain level flight. Flight time is well over 10 minutes.
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Old Aug 03, 2014, 12:01 PM
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To answer your last question first; no, the two extra inches of span won't keep you from flying into wind. But it begs another question: which size BB did you build? I assume a 33", that becomes 35" span with the extended tips. If so, then the go to motor is a 24gm 1700kv "blu wonder" 2730, i believe. That motor on 2s with a 7x6sf or apc style prop will fly a 33ish BB fine. You can go to 3s with it if you are careful and stay around half throttle most of the flight. Too much throttle amps will toast the motor windings. It will be a rocket, though!

My first BB was a 42" span which i extended to 48" for reasons similar to yours, later added a 42" wing and they both flew similar. 48 a bit floatier than 42.
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Old Aug 03, 2014, 12:50 PM
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yeah it is the 33" + 2. so i should try a 7x6 then...

i will be getting a watt meter soon.

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Old Aug 03, 2014, 04:48 PM
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Is this your motor? http://www.hobbyking.com/hobbyking/s...tem_combo.html
If so, it is kind of heavy at 39gm, along with the heavy nose block. Did you get the cg to balance at about 25%-30% aft of wing leading edge?
At 1700kv, and a 25amp esc it should spin the 7x6 ok. You might have been overpropping with the 9" one. Motor/esc/batt get hot and don't develop full rpm. When overpropped. You can go higher pitch easier than higher diameter. I would also consider the 800+ mah pack a minimum size for that setup.
If your motor is the donkey, it may benefit from a few drops of 3in1 oil on the bearings. The old j250 needed that frequently (both have plain bearings).
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Old Aug 03, 2014, 09:28 PM
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after further examination, sorry for my stupidity, the original prop was a gws* 8040 EP HD, i tried to gather a 7x6 or similar locally today but ended up with some various prop sizes, but i need to fit the adapter more to find a tighter fitting or adapter for some of them. i did run a apc 8 x 6 sf and a e flight 9 x 4.7 sf, both seemed to fly well and got the motor warm but not scorching my fingers hot. i was also flying in a more wind blocked area today.

my wing likes to try to fold... i need to work that out. i might build the reg width one soon.

also it wasnt the donkey. look a little similar but its the turnigy 2204 1700kv that came with mxs 3d from HK. looks a little more polished looking not that matters much.
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Old Aug 04, 2014, 07:50 AM
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United States, MI, Houghton Lake
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Removing the extra 2" will definitely help with wind penetration! Have you ever heard of a "clipped wing" cub. That is what many Piper Cub owners do to increase flying speed. Reducing wing span will increase flying speed and reduce the "wing folding" tendency. I strong ly recommend you try it. You may even want to go to something less than 33. Try cutting 1 inch off each side and if you want more do it again.

I agree with Mike that a 7X6 prop should be about ideal. You can make one by cutting 1/2 inch off the tips of your 8X6.

Have fun.

12 Oz. sounds awful heavy for a 33" BB???? Are you sure it is that heavy? From your description of the gear I would expect more like 6-8 OZ??
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Old Aug 04, 2014, 09:18 AM
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i will try to re-weigh it asap. i probably use a lot of glue. i weighed the motor also, i need to take better notes, it was 19-20grams.

is there some sort of conversion factor when going from ep to sf props when trying to keep the same amprage draw?
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Old Aug 04, 2014, 09:46 AM
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Not withstanding Don's comments about how less span equals less drag and more speed, as a new flyer i would suggest you stay with at least the 33" span to keep it floaty, and focus on getting power right.

If the motor is in fact a 20 gram one, then it may be marginal powerwise if your plane came out heavy. A 1700kv BW will solve that issue.
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Old Aug 04, 2014, 09:53 AM
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Found the motor on hk site. Yeah, it is a bit small for a portly 33" BB. 7amp max current.
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Old Aug 04, 2014, 10:59 AM
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Contrary to my friend Mikes suggestion I again suggest that you clip the wing tips to increase wind penetration. As he said slow and floaty is generally better for beginner pilots, but you said the problem was being too floaty when going up wind. Reducing the span will help that. Since you reported the problem in going upwind I assume you are a good enough pilot to be flying in some breeze. If you are a real nubee then keep the plane floaty and just fly only in calm winds.

Also the fact that the motor is 20g vs 24 is not at all a factor unless the motor is getting too hot. Any 1700 Kv motor will turn a 7X6 prop at the same speed. And that means produce the same power. The only difference is smaller motors will be running hotter than larger ones. As long as you can comfortably hold your hand on the motor it is not running too hot and replacing it with a bigger one will just weight the plane down.

Finely, be aware under camber wings like the BB almost always require significant down elevator to fly up wind in any kind of a breeze. So, they tend to float/balloon up when you increase power to fly up wind. then you just need to input down elevator to keep the nose down and make progress against the wind.
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Old Aug 04, 2014, 12:11 PM
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I might just make a 33 and a 31 for various conditions... What about the KFm3 wings? are they harder to fly with? Would the offer any advantage for cutting thru the wind OR the folding issues?
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Old Aug 04, 2014, 12:18 PM
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I was just going to comment that It's easy to try the shorter wing, can cut it, try out, then just glue tips back or or build another~ the advantage of Foam! Go for it, you can try the various methods of reinforcing the wing center and span, and pick the ones you like best.

KFM3 won't be as floaty, but not harder to fly. KFM2 is simpler and just as good on a 33" BB. You will want to change the wing saddle on fuse to remove the positive incidence. (Undercamber wings like a couple of degrees of positive incidence, which on the 33" BB is about 1/8" higher at Leading Edge compared to a line parallel to the Horizontal stabilizer) KFM wings like zero incidence, so make it parallel to H Stab. Way back in my blog, I showed a picture for a saddle that accomodated both wings. click on my user name to find it.
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Old Aug 04, 2014, 08:04 PM
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KF and other flat wings will fly faster and penetrate the wind better. But the faster flight speed means thing happen faster and two mistakes high is significantly higher for beginners. You should definitely learn to fly both. But it is easier to learn the basics with an under camber wing. That is why the BB was designed with one and why it has been so successful.
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Old Aug 04, 2014, 09:36 PM
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yesterday i was i was definitely in love with my blu baby, im started to get flying a lot better and it really self corrects pretty well for me . also ive crashed it several times and it still looks pretty solid ! the mxs 3d foamie was not so forgiving, and i made a small delta prior that was too hard for me. i think the sym over the last week has really helped a lot also.
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