Originally Posted by nickchud

Sakis..

Here's a really useful website. If you scroll about 2/3 down the page you'll see some diagrams that might help. Plus it's worthwhile reading all that stuff about stability. You'll see that it's usual to find the AC and then allow a "static margin" by putting the CG about 10% in front of that.

I think you'll find that several people on this thread would recommend moving the CG a little further back for KF type airfoils. Not too far, maybe for your model about half an inch at first. It should be perhaps 5% in front of the AC IMHO.

Good luck

Nickchud, thanks a lot for the link.
Very interesting reading material.
I am thinking of loading around 250gr of deadweight at the nose and toss it in the wind.
If it is nose heavy, I land it, remove 50gr and try again.
Hopefully this way I will avoid destroying it beacause I flew it tail heavy.
Thank you again.

Sakis

Install a long boom out in front of the nose and attach your ballast weight on the end of that.
This will need much less weight to move the CG to the correct position.

Don't make the boom out of something that will break on the first hard landing in case you get the C/G too far forward. Ask me how I know not to do that.

ok a flight video and measurements of the wing and its all fiber glass rods used for sparing no cf

swept wing bomber fight video (3 min 7 sec)

build info (6 min 12 sec)

Originally Posted by A Useless Geek

Don't make the boom out of something that will break on the first hard landing in case you get the C/G too far forward. Ask me how I know not to do that.

True! The boom can be nice and strong because you need a bit of weight up forward anyway.

That is an excellent idea!
Thank you Whiskers and Geek.
I must install the boom in the correct place so if I nose crash it will not penetrate the foam body destroying my electronics.
I keep my hopes up that the kfm2 step will need a more backwards located CG.
Thank you again.

Sakis

You might be making this too hard. Get a piece of broom handle or something and lay the plane on that, then you can roll it fore and aft and see exactly where the CG is at (where it tips down).

With that and this CG calculator you'll be OK:

http://fwcg.3dzone.dk/

Here is the advanced version of the calculator:

http://wingcgcalc.bruder.com.br/en_US/

That calculator has always been right on the money for me.

Jack

Jack,
thank you for the help (again). : )

Your links are the calculators I used to get a CG 20 to 25cm from the nose.
And I already used a plastic solid rod underneath my wing to see where it balances as it is.
The reason I am worried ( and afraid I wil destroy it during maiden) is that that my CG is waaaaaay back from 25cm (from the nose).

Fellow builders thank you again.

Sakis

with that first calculator of jack's go for the 15% location calculation..that has always resulted in good flights for me.... go any further back and you will have an oversensitive wing...

Originally Posted by sgrouts

Jack,
thank you for the help (again). : )

Your links are the calculators I used to get a CG 20 to 25cm from the nose.
And I already used a plastic solid rod underneath my wing to see where it balances as it is.
The reason I am worried ( and afraid I wil destroy it during maiden) is that that my CG is waaaaaay back from 25cm (from the nose).

Fellow builders thank you again.

Sakis

Most of mine have been 102cm/40" to 48"122cm in Wing span and the CG has been falling quite a bit forward of yours. What are the dimensions you entered in the CG calculator for:

Wing Span
Root chord
Tip chord
Sweep

And are you including the elevons in the chord measurement at the tip? If not, you should includge the elevons as they are effectively wing area in flight.

I usually use a point where the elevons would intersect for the corcd measurement, even though they do not meet there.

Your's being a delta with not sweep on the back of the wing is changins things but I wouldn't expect it to change that much. Do you have two batteries? And used them both for setting the CG?

I would consider a no power test glide to see what it does too, before I added the boom. I do those over tall weeds and it is gentle on the plane. If a full lvel arm push launch from shoulder level produces nose up or nose down that will tell you where you have it.

When the CG is right, the test glide will leave you hands, stay level for a moment or two and settle to the ground. It will fall off to one side or the other probably but if it starts settling without pitching the nose up (tail heavy) or diving sharply (nose heavy) it is probably ready for a low power test launch. When that happens, use only enough power to keep it moving forward and accelerating slowly. Too much power will make it much harder.

Jack

Jack

Guys, you are giving me more space and time that I would ever hope for...
Thanks.
Proceeding to numbers (metric but I have everything rounded in metric beeing Greek )
Span 160cm
Root chord 60cm
Tip chord 30cm
Sweep 30cm
Elevon is 7,5cm and it is included in the above dimensions as it should
(So the root chord you see in the images is actually 52,5cm because there are no elevons on the root chord)
Total wing Surface 72dm2.
Kfm2 step is located at 50% of chord. ( including elevon both at tip and root )
Wing thikness is 2cm white foam as base and 2cm white foam forming the step. At center I added an extra 2cm layer resulting to a total 6cm of thikness where I installed the lipos and autopilot.
AUW less than 2.000grams including camera, bec, normal servos, twin 3mm pushrods soldered side by side, 5,8ghz 500mw video TX with cloverleaf in one wingtip, frsky telemetry RX with dual 60 cm baloon dipole antennas on the other wing tip, FY31AP autopilot, Remzibi OSD with current sensor, artificial horizon and GPS antenna shared with autopilot, GSM-GPS stand alone locator, turnigy plush 80A esc.
Lipos used are 2x 2.200 3s for NTM 35-48 motor with 12" prop and one 1.300 3s for all electronics.
The funny thing is that my friends at the club insist that I should go ahead and buy a fying wing (X8 ) but they don't understand the satisfaction you get when you make everything work from scratch... .

So, if I try a hand launch from shoulder height shouldn't I give a little bit of reflex to the elevons? I think from my other wings experience that this is mandatory for any wing, any size, any weight. Right?
My problem is that with this specific build I don't want to smash it to many times on the ground testing required reflex because I want to protect the stuff I have embedded in the wings. So this is why I triple check everything before launch day...
Sakis.

I am Newbie!

...so i read a LOT of different things, as my own questions come up...

And here on this thread, testing of a KFm pusher wing has come up...

Jack is posting nice info on CG and it relates to a nice ez toss to determine initial flite...

and this relates to a catapult thread I just read...

I didn't much care for the result of the catapult.
Everyone else viewing the thread seemed to like it. Yet to me, it wasn't even as good as a hand toss, with the exception that it used a FOOT to trigger the launch.

Then a fellow came along in the thread and expressed his appreciation for the simple design and described his reason for wanting to build one.

And this relates directly to testing ANY pusher design, not just KFms...

He had chucked a pusher wing and the prop had lacerated his hand so badly that he bled severely on his way to the hospital, and did incur permanent damage to his hand from the incident. No more hand launches for him!

I am generally casual to the point of reckless and not known to exercise suitable caution.... when doing things I am accustomed to.
Probably a recipe for disaster, yet I've survived it so far.

Never-the-less, as a newbie, beginning to experiment with designs I will be very cautious of spinning props! Especially with a model plane that is awkward to hold on to!
rustycase

So true.
This is the reason I always wear a heavy glove to my launching hand and trained myself to do the launch while holding my wings from the leading edge and not from underneath, near the pusher prop.

Ritewing ZII Launch (6 min 39 sec)

Sakis

Nice Zephyr, Sakis!
and a good explanation of how to launch without injuring yourself!
I had been thinking more of a DLG style launch, myself... but this looks good.

...yah, I had a good heavy glove and chicken sticks when I used to mess with the nitro planes. Them little props would slice yer finger quick!!! I learned that real fast! lol

Side arm launches, discus style, holding the wing out and away from props is easy and safe...with our models having so much power you only have to swing it round and up at 45 degrees and away it will go
Keep you fingers well away from props, even with the smaller motors you can get your hand sliced up..

sgrouts....with a non swept wing type you will have to be accurate for your COG, they are more sensitive... If you have too many problems cut a V out down the middle and sweep the wings back ...