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Nov 27, 2018, 05:21 AM
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Discovered because I didn't use a stick motor mount and instead justed glued firewall direct to the foam a hard bump will wrip the motor out. 2nd time it's happend.

So reinforced with a thin piece of ply to stop it twisdting the foam.

Also to stop the fuse. Twisting froint to back ran cf flats on the sides. Much stiffer now.


Tried it outside yesterday as the winds were low but nobviously not low enough almost impossible to get. It turned back into the wind. The best I could do was keep it high Alpha into the wind and slow fly it forward. It had a bad tendancy to wing rock
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Nov 27, 2018, 04:08 PM
flyin' fool
goldguy's Avatar
tail heavy = pitch problems and rockin'

Flat LAR designs have a very sensitive center of gravity. Tail heavy is OK for indoor and high alfa/prop hanging, but not outdoor in the wind.

Move the CG forward and check the motor thrust line. PBFs seem to like a little down thrust on the motor. 'Little' is the word of the day. Meaning ......… adjust only small amounts at a time.

I mark two battery mounting locations on my models, one for no wind and one for wind.
Nov 27, 2018, 06:26 PM
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OK I'll try moving forward. At our ikndoor I did notice that I had to had down elevator trim. Did I calculate my CG wrong? Using wing CG calc have me 5" but where is that from. The CG cslc doesn't take into account the motor being built in to the wkng so maybe that's where I went wrong. I just used a 20x20.

5" seemed a bit reasrward to me
Nov 30, 2018, 06:14 AM
gpw
gpw
“There’s no place like Foam”
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Baz, 25% seems reasonable on a 20X20 ? … no matter where you put the motor … We always carry a lump of modeling clay in the flight box , just for such delicate CG adjustments …
Latest blog entry: Lost plans
Nov 30, 2018, 12:24 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Quote:
Originally Posted by bazsound
OK I'll try moving forward. At our ikndoor I did notice that I had to had down elevator trim. Did I calculate my CG wrong? Using wing CG calc have me 5" but where is that from. The CG cslc doesn't take into account the motor being built in to the wkng so maybe that's where I went wrong. I just used a 20x20.

5" seemed a bit reasrward to me
Here are three screenshots of a flying wing CG calculator for a 20 x 20 flying wing ↓ .
Dec 01, 2018, 01:59 PM
Registered User
OK that's the same results I got. Where am I measuring from those as the motor and prop are cut into the foam. Resulting in part of nthe wing arewa being non existent.
Dec 01, 2018, 02:00 PM
Registered User
Oh and I moved the bstterry all way forwqsrd. It was mutch better outside but still had a tendancy to point up into the wind and would get into rocking by itself.

I wlknder ifg my elevons arnt stiff enough
Dec 04, 2018, 01:09 PM
gpw
gpw
“There’s no place like Foam”
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The more nose heavy you get as per the diagrams , the more elevator trim you’ll need to keep the plane level …The more trim you carry , the more sensetive it becomes at different speeds … just sayin ( it wasn’t noted on the charts)
Latest blog entry: Lost plans
Dec 05, 2018, 11:12 AM
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here is a 15" traced over/drawn in sketchup.. for a 10-15gm motor and lighter electrics..
working the area numbers using sketchup, for a COG, I find the position marked is at 29% area .. which is too far back for my flying...lol..
so likely something is wrong with my numbers..
Dec 05, 2018, 01:01 PM
Registered User
No wind days are rare.

And as this is under 200g and light on power I'm gonna try testing it indoors tonight. I did a non powered glide outside and it seemed to drop like a rock.

I've increased the throws which should make it be able to roll in doors
Dec 06, 2018, 02:33 PM
Registered User
Tried indoors with battery all way forward. Didn't like it and ended up moving it back. I triedf to do rolls but it would get stuck inverted?
Dec 06, 2018, 03:30 PM
flyin' fool
goldguy's Avatar
You need to mark the CG as recommended and always compare each change to that, 'small' changes with each adjustment.

I mark the CG locations by poking a pen in the foam on each side of the root cord and then put a small blob of hot glue there. Apply the glue, pull it to a long string and when cool cut with scissors so you have a sharp point. Now you can find the location easy with the tip of you fingers. That tiny point of glue is way more accurate then the ends of your fingers.

Once you have the CG sorted, you can change the location of the points accordingly.

I've built loads of PBF and all were rock solid flyers. I always started at 25% exactly and adjusted from the to suit my flying style.

So, start from them beginning again with the CG …………………..
Last edited by goldguy; Dec 06, 2018 at 03:37 PM.
Dec 06, 2018, 03:36 PM
flyin' fool
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Or …………………… https://www.rcgroups.com/forums/show...postcount=1638
Dec 07, 2018, 03:38 AM
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davereap's Avatar
The 15" was a bit too fast and too responsive ..
And after a few hard hits it has split back to front.
Ive been using EPP for so long I had forgotten to reinforce the trailing edge with tape..
Impact stresses will split the XPS foams from/at the rear just as easily as they damage the leading edges.
The speed of the 15 would have been OK if I had got it sorted.
but I would say go to a 20" to reduce the wing loading and have a slower flight.
So far this doesnt look like it will be a preferred model.

gear... gartt 2204-1800..8x4.3.. orange dsmx mini rec..turnigy 1440a servos.. 500 2s lipo..

PBF15 6 dec 2018 (2 min 12 sec)
Last edited by davereap; Dec 07, 2018 at 03:48 AM.
Dec 07, 2018, 05:55 AM
gpw
gpw
“There’s no place like Foam”
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Ouch !!!
Latest blog entry: Lost plans


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