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Old Jul 07, 2009, 06:03 AM
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Ashford. Kent. England
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Jackerbes...I had problems with a KFm3 ..it would glide ok until you moved the controls, then it fell out of the sky..I put it down to the rear step being too close to the hinge line, but it could have been anything...now I always keep the steps away from the hinge line..Ive not had any more bad ones

heres my latest KFm4 a plank design as in the picture post 3322

problems so far...COG location.. the way I construct my wings has put a fair bit of weight behind the desired location.. I made my nose slightly longer than that in the picture,, but it posibly needs another 1-1.5" to make ballancing easier.. Its marginal at the moment
edit.. all was well in the end
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Old Jul 07, 2009, 07:25 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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"..Jack, I have had similarly mixed results with my KF builds..."

The part that baffled me was that that one build, with the only real change being a transition from KFm1 to KFm3, was the only build that I've had that was troublesome.

Your builds have explored many other parameters (dihedral, wing sweep, drive line types, step combinations, etc.). Sometimes several of the changes were implemented simultaneously, and that might account for your mixed results.

I like to sneak up on change in a more piecemeal manner, starting with something that is already working, because it usually has better chance of working. It also means I can get in some flying enjoyment as I sort things out.

And I'm perfectly happy to leave things alone once they are working right. KFm4 Zagnutz is a plane I'll build again and again with little or no changes.

The only remaining point of "weakness" in the KFm4 Zagnutz at this point is that repeated nose first impacts with the ground will cause the wing to flex forward a little and induce cracks in the FFF center panel. The cracks radiate forward from the corners of the prop slot to the back of the spar.

I changed the right angled corners of the prop slot to rounded corners on the one I'm building now. How's that for a radical departure from last one? Not much innovation there but I'll bet it flies and maybe the cracks won't happen.

"..I also wonder if fewer steps aren't better, for more reasons then thickness and weight?..."

I think you might be right about that. And I think the locations of the KF steps have sort of taken or lead us to different paths.

The planes like the 42" and larger variants (the AP Blu-Babys for example) that are looking for more lifting capacity and stability seem to be better with the thicker KFm3 with two steps on top.

The more sedate flying trainers like the Blu-Baby responded nicely to the KFm1 to KFm3 designs and probably because their flying envelope did not include much demand for speed or aerobatic performance. At the same time the KFm wings added weight, lift, penetration, tracking, and made the trainers fly better when winds picked up.

But as you developed a desire for more speed and maneuverability and moved on sports fliers like the Piranha and Zagnutz, the KFm4 was more likely to be better with it's top and bottom steps and thinner cross section.

It would be interesting to have a set of standards for evaluating KFm performance or for a laboratory with good equipment and smart people that could test some of the variables and maybe describe or quantify the effects or results. As it is now, the results of the various builds as pronounced by the builder and in general terms.

Jack
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Old Jul 07, 2009, 09:43 AM
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Ashford. Kent. England
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jackerbes...
I use tape across the back edge top and bottom, to prevent that stress tearing that you get from sudden stops...rounded corners help a bit..but tape does it better..

cog help..for the plank I am building .. the picture shows a cog position at just about 12%.. .. to me it seems to far forward, isnt 20% a more likely COG for a straight plank design?
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Old Jul 07, 2009, 03:45 PM
Onward through the fog.
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Quote:
Originally Posted by davereap
jackerbes...
I use tape across the back edge top and bottom, to prevent that stress tearing that you get from sudden stops...rounded corners help a bit..but tape does it better..

cog help..the plank I am building shows a cog position at just about 12%.. pic post 3322.. to me it seems to far forward, isnt 20% a more likely COG for a straight plank design?
Hey Dave,

I'm really curious about this little plank plane you found. Please advise when you fly it. It'd be nice to see a critique. Could it work as a pusher or slotted prop?

I currently plan to postpone the latest experiment and try an active canard on a delta with elevon control. I suppose that will be a subject for a different thread. I like the delta but would like to enhance slow flight a bit.

Two days without flying. I told Edna that the plan for today will be flying and/or building. I stake my claim to Wednesday. It's mine and I will have it!

Steve.
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Old Jul 07, 2009, 03:47 PM
PunchDrunk ex-Pug try'n fly'n
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Quote:
Originally Posted by davereap
.... isnt 20% a more likely COG for a straight plank design?
I doubt that Dave, I seem to remember 10-12% on a conventional plank.
I like Steve's idea of a slotted push prop. Should help with CG.
jp
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Old Jul 07, 2009, 05:11 PM
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Ashford. Kent. England
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oh well it feels nose heavy, but I will give it a go at that position first...
edit..it was indeed nose heavy a good start cog is at 18%
cog..needs all the bits well forward so a slotted motor would put even more weight at the rear..

All done and the AUW is 15.25oz...4.2ozsqft
setup 2x9gm servos.. 1250 3 cell .. 4ch corona 2.4 rec... 30A esc... large bell motor form overlander
this one http://www.overlander.co.uk/product....id=93&pid=2581
24oz thrust on a 9x5
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Old Jul 07, 2009, 05:48 PM
Geaux Saints
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Grafton, Il
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Dave

Pull the prop and glide test.
If it glides well then you know you are close.

If the glide is terrible you know your way off.

Mike
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Old Jul 07, 2009, 07:40 PM
PunchDrunk ex-Pug try'n fly'n
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United States, MN, Minneapolis
Joined Jan 2009
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Quote:
Originally Posted by davereap
....AUW is 15.25oz...4.2ozsqft
..... 1250 3 cell .......... large bell motor from overlander
Dave, chances are your battery weighs more then your motor. Therefore, placing the battery in front of the motor should help move the CG forward as in a prop in slot pusher.
Your wing loading is "out of sight". Great as that is, at times I have found it necessary to run a heavier battery in order to achieve proper CG. That of course would be mounted maximum forward.
I'm sure you would agree, the challenge is usually to get the CG forward enough.
I'm sure you will figure it out, and of course please keep us filled in! I love the straight wing. It should soar like an Albatross!
Kudos for going with a tractor prop, I don't have the courage to go there anymore. jp
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Old Jul 08, 2009, 04:02 AM
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Ashford. Kent. England
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I am going down my usual route and building a small scale glider to check the cog.. By doing it this way setting a few degrees of up on the elevator, I find the point with the nicest stable flatest glide ...this is always the most rearward point for the COG any less weight and it would go nose up..
I translate this point scaled up to the full size and go forward 1" or so for a safe starter..

test glides on scale model...22% gives a flat glide which is ok ...18% much more stable and faster

edit maiden flight went well at 18% and the cog can go back some more
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Old Jul 08, 2009, 06:10 AM
Onward through the fog.
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Bohol Philippines
Joined Aug 2008
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Quote:
Originally Posted by davereap
I am going down my usual route and building a small scale glider to check the cog.. By doing it this way setting a few degrees of up on the elevator, I find the point with the nicest stable flatest glide ...this is always the most rearward point for the COG any less weight and it would go nose up..
I translate this point scaled up to the full size and go forward 1" or so for a safe starter..

test glides on scale model...22% gives a flat glide which is ok ...18% much more stable and faster
When can we expect test flight report and video?
Steve.

PS. Nice build!
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Old Jul 08, 2009, 07:35 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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davereap wrote:

"..I use tape across the back edge top and bottom..."

I've generally been taping that area but thanks for the reminder, I'll redouble my efforts there.

Let me ask another question. And answers are invited from all of course.

I'd like to see if I can tighten up the Zagnutz's roll axis a little bit (the rolls are very barrel like and also accompanied by quite a bit of altitude loss). I want to experiment with it by inducing some differential.

I have the servos on top of the wing, directly in front of the elevons, and have been starting with the servo arm in a vertical position. If I rotate the servo arm forward or backwards one or two spline notches, it will put the arm at an angle and induce some differential.

On the aileron movement if I rotate the servo arm forward the up (wing down) aileron side has less deflection than the down (wing up) side. And rotating the arm backwards gives me the inverse situation.

Just as an opinion or guess, which of the servo arm angles (forward or backwards) do you think I need to use to get the roll axis to move towards the center line?

I'd think that you would want to use less up deflection on the wing down side and more up deflection on the wing up side but that is a guess.

Jack
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Old Jul 08, 2009, 07:52 AM
PunchDrunk ex-Pug try'n fly'n
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United States, MN, Minneapolis
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Quote:
Originally Posted by jackerbes
..... which of the servo arm angles (forward or backwards) do you think I need to use ......
Jack
Great question, I wish I had a definitive answer.
I have heard it said to add more down throw on a wing.
I however, have gone with more up on most of my builds. This was because my EasyGlider asks for 5mm down throw and 20mm of up on the ailerons!
I obviously am of the "more up" school of thought. Just wanted to also mention, there are those who disagree with that opinion.
As always let us know, this is how technology moves forward.
jp
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Old Jul 08, 2009, 08:05 AM
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jackerbes try moving the cog back a bit.. then you need less elevator up trim and the rolls should improve.. if it gets a bit sensitive reduce the elevator movements

No video yet, but Ive just flown the Plank..in windy conditons..
I set about 5mm up on the elevons, the COG for the first flight was just in front of the 18% mark.. Engine thrust set at 0 degrees..

results

A half power launch saw it floating steadily away.. full power and it climbed strongly (the effect of the up elevons).. rolls fast, loops nice...every thing was just great,.. apart from the strong climb on power up..
I took the COG back to the 18% mark, reduced the up on the elevons and had another go..this was better, much less climb on increased power. all else nice and controlable right down to very slow speed..
My thoughts are that the COG can go further back, the glide is a bit fast still, with a nice hands off slope.. I think it can go to a flatter glide slope..The flare out requires fair amount of up, so its not yet too sensitive..
Inverted no problem but it reqired a fair push to keep it level, another cog indicator

In all ways this is a keeper...its certainly flies as good as the zagnuts shape but it is a much simpler build. Having the prop at the front makes hand launches no problem, although the side launches we use are no trouble as well..

This is another instant feel good plane..I even did a touch and go.. not bad with no wheels
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Old Jul 08, 2009, 08:34 AM
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Great news Dave! Apparently 20% of MAC is a good CG starting point for all types of aircraft. I'll bet she rolls fast just like my Delta. I also bet She glides like She has a mind of her own. I think we were due for a great first flight test!
jp
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Old Jul 08, 2009, 10:06 AM
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Ashford. Kent. England
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After the very successful maiden I had to draw the plans up for you. It really does fly extreemly well and is so simple to make

Here is a construction plan and the sketchup file in a zip for you to use

note the steps top and bottom are at 45%.

Laminate all using UHU por...1.5 " wide along the front of the layers..Shape the LE same top and bottom.. add the tape covering top and bottom.. note all of the elevons is covered.. also the wing tips.. see finished pics

servos are just let in through the top two layers, and cool glued in place.
ESC and REC on velcro strips...ditto the battery, the battery has an over fastner using velcro to hold it secure. Push rods are from 3mm CF rods.

steve ...if you try one make sure you downrate the ailerons and use lots of expo..it rolls fast with a lot of movement ..I have 35mm up and down for both elevator and ailerons..measured just moving the sticks on one axis at a time
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