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Cermark F16 propjet CG WARNING for new owners

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Old 05-29-2007, 01:18 PM
  #1  
gsoav8r
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Default Cermark F16 propjet CG WARNING for new owners

First off let me say that I have read all the threads pertaining to the F16 and have no idea
why mine is different than the other 98% of them out there flying with no issues.
But it is - so heres the fun stuff.

I bought my F16 in Sept 06, finally got around to completing it about a month ago (April 07).
Its the air up-air down retract version with the nice latch style canopy.
For power I opted to just use the OS 91 FX with the huge stock muffler spinning a 12x8.
After reading all the threads and all the great reports and per the instructions I balanced her
at 8.5 inches on my trusty CG machine. It took about 12oz in the tail (this should be no surprise
to anyone here thats been following this particular model). I thought this to be about par for
the course.

So feeling confident about this bird I went out for the maiden. I got to the field and got grilled
about the model from my friends and went through the preflight routines. A trusted and, more
importantly, experienced flying buddy was their and helped through the taxi test on a paved
runway. Everything being a check up to this point I pointed her down the runway and took off.

To my surprise the F16 was eratic in pitch. Up down, up down... I fought like mad to keep
her in the air. I finally got her up high enough that I could think for a second. Oh #$*%^$* the
CG's off. Having a 7cap radio my friend dialed down the elavator throws from recommeded low
rate to 35% recommended low rate while I kept the model airborne. Interestingly enough this
tamed the model, so we thought. Great I can fly it, lets get it on the ground. I soon learned that
below about 3/8 throttle my beautiful T-bird was once again acting like a rocket ship and
wanting to point to the moon. #%$#&%$#*. Long story short I got it on the ground but knocked
out the landing gear and the bottom fins in the process. Extremely tale heavy.

So after getting the 3rd degree from my friends about what I might have screwed up I came
home and repaired the landing gear and the bottom fins. At this point with the bird back in one
piece I measured the wings and strakes to see what my calculations would say the CG should
be set at.

Heres the most interesting part. I plugged the info into my CAD program and found that the
recommended CG of 8.5 to 9 inches is about the rear most range just for the wing. Now consider
that the strakes that run from just forward of the wing all the way to the tail is basically about a
50 inch wing. Put that into equation and my calculated CG came out to be about 7.25 inches.

So having the model back together I pulled out the 12oz of tail weight and put her back on the
CG machine. I couldnt believe it, the model balanced right around 7 - 7.25 inches. Let me check
again. Yep, it was right. Lets go fly.

I managed to get to the field over the Memorial Day weekend and it flew like a dream with the
recommened setup minus the CG of course. Inverted flight required just slight down pressure so
that told me and my buddy that flew it also that the CG was about right. All in all a very well
behaved model.

I did botch the first landing and had to replace my prop but the second flight was awesome ending
with a beautiful nose high touch down. A welcome sight considering the crash of just a few weeks prior.

( Hope this helps someone new to this model since I dont think Cermark will be discontinuing it anytime soon)

Cheers.
James
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Old 05-29-2007, 01:47 PM
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evan-RCU
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Default RE: Cermark F16 propjet CG WARNING for new owners

Great post James, I wondered how long it would take you to post something... I told you I watch...

Yes the first flight was something and I was thinking of the F-18 before you let me fly it that first flight, wow... still don't belive it survived that flight...

Yesterdays flight was fantastic and I thank you again for the stick time. Great looking and nice flying. Good job!


Hmmm.... now should I get a F-18?
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Old 05-29-2007, 05:49 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Thanks Evan. I knew you would see this pretty quick.

I wish you would get the F18!! Id be more than happy to test fly it for you. HAHAHAHa.
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Old 05-30-2007, 09:13 AM
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mcdonald77
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Default RE: Cermark F16 propjet CG WARNING for new owners

I maidened mine at 8" it seemed ok. At less then 3/4 throttle it seemed to drop the rear, thought it might have been a little tail heavy might have to re-evaluate that and move the cg forward a little and take some weight out if the tail, I have about 6 ounces in the tail and a 2700 6v battery
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Old 05-30-2007, 01:11 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

mcdonald77,
I have to agree with you, you may be slightly tail heavy. Glad your maiden went better than mine.

Though I didnt metion it in my original post Im running a high capicity 6v battery as well. Took that tip from the Build It Better thread.
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Old 06-01-2007, 06:11 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Thanks for sharing the info. I called Cermark and Regina does not know about any problems with the F16 CG. She said to talk to Randy next week. I'm about to maiden mine too and would not mind cutting tail weight out if the 7.25" position is the way to go. Anyone else has similar experience like James?
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Old 06-01-2007, 07:35 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

i have the previos version just before yours, Not sure if the cg in the manual are the same. I have mine at 7.75 inches back fron the inboard leading edge. Something else to consider are you doing your cg with the gear down or up. If you are doing it with the gear up. you will be tail heavy with the gear down. I also am running a 2oz header tank and i have that full when i check the cg

ken
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Old 06-01-2007, 08:04 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

I also have the previous version F16, and that one flies well with the CG per instructions. Good point about balancing it with gears up or down. Are you guys marking it at 7.25" or 8" from LE with the gears up or down (extended) for the new version F16? The instructions said to mark it at 8.5" with gears extended (down).
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Old 06-02-2007, 07:02 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Thanks for the great info, I,m watching this post closely, as I to have alot tail weight and it drops the tail when I slow down. I too question the CG as written, and nobody until now confirmed that it is possible that " THIS BATCH" of F-16,s Is different , flies different and as built, tail heavy at 8.5 - 9 inches. I will now drop half my ballest weight. I dead sticked mine into a young corn field with repairable damage to one main retrack mount plate. gear down 8 and 3/8,s cg last flite. Going to aprox 8 inchs next flight and lower if needed after that.
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Old 06-03-2007, 03:41 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Wouldve replied sooner but just got back in town.
I initially balanced her per instructions - 8.5" with gear extended and the bird upright
Probably shoud have put that in original post too.

ORIGINAL: kmtranmd

I also have the previous version F16, and that one flies well with the CG per instructions. Good point about balancing it with gears up or down. Are you guys marking it at 7.25" or 8" from LE with the gears up or down (extended) for the new version F16? The instructions said to mark it at 8.5" with gears extended (down).
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Old 06-03-2007, 03:55 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

With mine still at about 7.25" it doesnt drop the tail at all at any speed.
I will say that my CG could probably be moved back slightly but it would still be no where near 8.5".

ORIGINAL: wylieruneyjr

Thanks for the great info, I,m watching this post closely, as I to have alot tail weight and it drops the tail when I slow down. I too question the CG as written, and nobody until now confirmed that it is possible that " THIS BATCH" of F-16,s Is different , flies different and as built, tail heavy at 8.5 - 9 inches. I will now drop half my ballest weight. I dead sticked mine into a young corn field with repairable damage to one main retrack mount plate. gear down 8 and 3/8,s cg last flite. Going to aprox 8 inchs next flight and lower if needed after that.
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Old 06-03-2007, 04:50 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

I will try it out at 7.5" from LE with gears extended and maiden it next weekend. Will let you guys know. Hope it will have high alpha angle on landing like the previous version.
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Old 06-03-2007, 05:54 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

James, it's perfect where it is. I would not touch the balance.
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Old 06-06-2007, 02:16 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

I've just talked to Alex at Cermark today. He said the correct CG for the new F16 is 7" to 7.5". The incorrect 8.5" per instructions came about from the old version CG location, which does not work with the newer version that is smaller. I will test fly mine at 7-7.25" range this weekend.
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Old 06-07-2007, 01:48 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Im still having trouble believing this has not been an issue for more people.
An inch of descrepency in the instructions (at least the one I recieved) yet many of these jets are flying just fine.
Guess some folks must have gyros in their thumbs.
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Old 06-07-2007, 09:52 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Thanks James for the Great post and Information.


LUDS96

Revor 52
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Old 06-10-2007, 05:10 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Awaiting more feed back on changed CG and flite behavor at various CG,s All info welcome Be as specific as possible. If you don,t believe this bird is different look at your manual. It shows rudder linkage on right side on page 12 mines on the left side.
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Old 06-10-2007, 07:53 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

I had mine at about 7.25 and I didnt use any elevator trim. Flew pretty level. Landings were nose high and a little hot but pretty good. I m having trouble with it pulling right. I imagine its from the engine being side mounted right. I never did balance the wings. I wish I would have cause third flight I stacked it in the dirt on takeoff. Wants to roll right on me real bad when the motor is turning up. To much weight to the right combined with all that thrust must be making it pull reall hard. Ive just about got it fixed. It's not showroom anymore but wont look to bad. Get a long runway at least if you have a 11x8 on it. I can tell it's going to be a blast to fly though once I work out the bugs.
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Old 06-11-2007, 05:15 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Thanks for the info, left some good info on another post thread concerning your pulling right, checkit out. Called Alex at Cermark this afternoon. He said CG was 7 inchs, but sounded kind of iffy to me, Quote "I think we said it was 7 inchs on RCU or somewhere." END quote. So I patiently wait for some additional feed back.
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Old 06-11-2007, 05:19 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Thanks bob27 for fast reponse to 60LX questions. Tried to E-Mail Dub , no luck with that computer kept trying to set me up an account I didn,t want, not sure e-mail went thur. Computer just kept bogging me down. Will go the the Jett engineering.com home page and get a phone # to call in the next couple of days, or find it on a old invoice.
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Old 06-11-2007, 11:36 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Maidened my new version F16 today. CG at 7.25" seemed a tad noseheavy, plane has tendency to dive coming out of the bank at 1/2 throttle. Will try again this weekend with adding 1 oz lead in the tail, guessing will bring it closer to 7 3/8" CG. But it's not anywhere near the 8.5" per instructions. I like to fly my propjets fast and low, so don't like idea of bird dropping the nose coming out of the bank. It will help get the high alpha attitude on landing too. Bird did land a little hot with OS 91, apc 11x9 prop idle set at 2700rpm. Will try again with rigging for some flapperons. I'd rather not drop the prop pitch and sacrifice top speed. My OS 91 with jettstream muffler peaked at 14Krpm on the ground, giving static thrust of 10.6 lbs. That's 112% Thrust to Weight Ratio.
Only scary thing was the plane had high speed stall and nearly lost it after take off. Roll out was smooth, no excessive pulling, just some rudder adjustment for usual engine torque and 5mph cross wind. When I brought it around, I banked probably too steep with high rates on the elevators (more than recommended) and the bird had a high speed stall, rolled hard to the right and flipped upside down. Lucky to straighten it out, reduce elevators to low rates and no more problems. By the way, I put in 1/8" up trim from neutral on the elevators per instructions and did not have to make any inflight adjustments. Take home message: don't put in excessive elevators travel and make sharp turn after take off. That's my 2 cents. Will give more feedback after making above adjustments.
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Old 06-12-2007, 05:55 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Good stuff GI DOC , keep sending the killer info!
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Old 06-17-2007, 06:47 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Here is another update from further flight testing on the F16. The addition of 1oz in the tail was positive, the plane now does not drop the nose much coming out of a 45dg bank with addition of 1oz tail lead after getting the CG balance at 7.25". I will now say the CG range should be 7 1/4" to 7 3/8" range. That's my 2 cents for my style of flying propjets. Plane still landing a little hot with my OS91 having trouble staying lit below 2700, would be much better/slower when I can get it down to 2200 idle. But bird can come in nose high easily. Elevators rates now fixed at 7/8" full deflection and 50% expo more than enough for nice landing flare and jet-like takeoff from grass. Be careful of too much elev as noted above. First attempt of using flaperons was not good. 1/2" deflection at half throttle caused plane to dive, not climb as expected. Will try again with flaperons on dial instead of switch to test different deflections. Will not need flaperons to take off from grass. Plane sits nose high already, easier to take off than, say the Patriot XL. Oh yeah, it's a little faster than my older/bigger version F16 with Jett90L combo. Just my 2 cents.
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Old 06-17-2007, 07:38 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Try an OS F plug to get your idle better...
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Old 06-18-2007, 04:48 PM
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Default RE: Cermark F16 propjet CG WARNING for new owners

Thank you for the good info, I will now set my cg around 7 3/8 - 7 1/2 inchs with no more than 7/8 deflection on elevators, And no flaprions to start.
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