HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Mar 30, 2012, 12:54 AM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,773 Posts
An update: I flew the C-47 a couple times this week. Tossed it the first flight, no problems, and landed on my new music wire gear, smooth as silk. Then I took off, and let her ROG off the field, and finally got her to take off without the balloon up/stall/crash scenario. I will definately make the new one with smaller elevators to help prevent this issue, as well as some downthrust on the motors. This thing flies so well! Just love it in the air, it really cruises nice!------Metalguy
metalguy is offline Find More Posts by metalguy
Reply With Quote
Sign up now
to remove ads between posts
Old Mar 30, 2012, 09:34 PM
Registered User
Taiwan, 北市
Joined Dec 2010
1,215 Posts
Nice to know you made it !

I'd suggest keeping the stock elevator for its scale looking, and applying smaller travel, ie 75%, instead. I did so apply on my most pitch sensitive bird. May be 80% - 65% D/R can also be helpful, too. Yes, more down thrust on the two motors is also needed.
Chen
twn Chen is offline Find More Posts by twn Chen
Reply With Quote
Old Apr 04, 2012, 12:42 AM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,773 Posts
I will probably still make the new C-47 with smaller elevators. It is just easier than having to limit the throw on the huge one it comes with. After flying this bird with a 2200 mah battery, it really doesn't seem to like all the extra weight. It does penetrate well, and is super-solid, but I will have to move the CG back some to see if I can get rid of the dive on power down for landing. I shall fly it on 1300 next, and see how things go. I would bet it is a bit more spritely, and less of a brick with a little less weight! Still a great flying plane. ------Metalguy
metalguy is offline Find More Posts by metalguy
Reply With Quote
Old Apr 16, 2012, 02:01 AM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,773 Posts
I flew it with the 1300 today, and while it seemed to do well, I had another problem.....My elevator control horn pulled partway out, causing a serious pitch problem! I almost dove it into the ground!! I discovered it after "landing" it...tearing off one wheel, gear, and plastic pocket. I am glad I didn't auger it into the ground, or else it would have been all she wrote for this one. Still not far on the second C-47 build. Been too busy flying, and attempting to fly other models.-----Metalguy
metalguy is offline Find More Posts by metalguy
Reply With Quote
Old Apr 16, 2012, 05:40 AM
Registered User
Derek_TX's Avatar
Joined Dec 2002
201 Posts
Quote:
Originally Posted by metalguy View Post
I will probably still make the new C-47 with smaller elevators. It is just easier than having to limit the throw on the huge one it comes with. After flying this bird with a 2200 mah battery, it really doesn't seem to like all the extra weight. It does penetrate well, and is super-solid, but I will have to move the CG back some to see if I can get rid of the dive on power down for landing. I shall fly it on 1300 next, and see how things go. I would bet it is a bit more spritely, and less of a brick with a little less weight! Still a great flying plane. ------Metalguy
That does not sound like your problem is a too far aft CG. It sounds like you have a problem with the motor thrust lines.

When you cut the throttle, the upthrust is suddenly removed and the airplane pitches down. The cause is more likely the control surface neutral setting or the motor thrust line, or both. The effect of CG is to make it more stable to pitch input rather than there being a 'perfect CG' where a tad in front makes the nose drop, however, a CD too far aft will get unstable in a hurry. The acceptable CG range is likely large, if everything else in the airplane is correct. If you have to trim a ton of 'up' the CG is too far forward (presuming a well designed airplane to start with).

I learned a great deal about the many factors in airplane trim by building scratch hand launch gliders. I found some rules of thumb for flat bottomed airfoil airplane:
1) Lift increases greatly with speed. (IMO this should be the only reason you add downthrust to a flat or semi-symmetric airfoil airplane.)
2) Pitch stability initially comes from the angular relationship (decalage) between the wing and the stabilizer. Decalage resists the natural tendency for a wing to nose under. I found 1-3 degrees of positive decalage is needed, and if you have negative decalage and try to correct with a CG shift, it will simply be unstable and unpredictable. Neutral is fine for aerobatics, but relaxed, hands off flight comes with a little positive angle. You can achieve it either way, a flat wing saddle and a tiny bit of 'up' built into the stabilizer or by having the elevator sit flat and the wing saddle shimmed up a degree or two. If you simply use elevator trim there is extra drag. This should be designed into the model, but check to be sure.
3) Decalage makes the airplane pitch up slightly so weight needs to be added to the nose until the glide is smooth. Minimal effective decalage is needed to keep the airplane nimble. Just the right weight will result in a fast, flat glide. This is handled with a manufactured airplane by giving you a CG range to follow.
4) Once a suitable CG is found, adding weight while keeping the balance point intact will only make the airplane fly faster, it will not degrade the glide ratio. Check it out and see.
4) Confirm a good power off glide by simply gliding the airplane either from a toss in tall grass or killing the motor in flight. If tossed aiming the nose a few degrees down the airplane should not severely pitch up or down but rather gently glide. If your CG is set within the limits, just adjust your elevator trim. If you are toward the back of the CG range, shift it to the front of the range. Then get your trims set. Once you have the airplane gliding nicely, you can experiment with tiny CG shifts if you wish the airplane to be more nimble. Slightly forward CG airplanes are generally much easier to fly.
5) Once the power-off glide is established, observe the behavior when you increase the throttle. If it pitches up at high throttle, you need some downthrust. Finding the right amount can be hard if the mount doesn't allow adjustment. Once set right you should not have a badly climbing throttle response. Too much downthrust will have your airplane pitch up when the motor is cut.

Once trimmed out, the planes should be able to carry a couple pounds of cargo at the preferred CG. The speed will be faster, and you will likely find it flies a great deal smoother with at least a 2200mAh or larger battery than the lightest configuration. They fill gliders with water to achieve optimal weight, lightest isn't always best.

Sorry for the wall of text and I hope I haven't added more confusion than help.
Derek_TX is offline Find More Posts by Derek_TX
Reply With Quote
Old Apr 16, 2012, 08:05 PM
Registered User
Taiwan, 北市
Joined Dec 2010
1,215 Posts
Hi Derek,

Very good points ! I flew scratch built F/F gliders many, many years ago, too, ie. hand launched, rubber powered and tow-line gliders. Yes, I did toss some of my R/C planes in tall & deep grass to see if the CG was located well. I even did the same testing my scratch built Alula last summer. THis way works well on light airplanes, the heaviest plane I did such test on was with AUW of 1200 grams or 42.38 oz powered by a .20 glow engine.

Metalguy told me he'll add more downthrust on his new C-47 earlier. Probably he already did the same on his old C-47 as well.

Chen
twn Chen is offline Find More Posts by twn Chen
Reply With Quote
Old Apr 16, 2012, 08:49 PM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,773 Posts
No confusion here. The CG with the larger, heavier battery in the same location is too far forward. I don't think you read my post. The plane will dive when power is off with the larger 2200 mAh battery in the same location as the lighter, 900 mAh battery I was using prior. I was going to use the 1300mAh in place of the 2200mAh for a happier medium in CG. The battery area is only so big, so I am limited as to where I can place the various batteries without cutting foam. I am well aware of why, and how an airplane flies, but thanks anyway.------Metalguy
metalguy is offline Find More Posts by metalguy
Reply With Quote
Old Apr 19, 2012, 12:01 AM
Registered User
Taiwan, 北市
Joined Dec 2010
1,215 Posts
Metalguy,

Derek was very kind and smart telling his findings.
You might have repaired up the contol horn on your old C-47. Let me know how it flies now with the lighter 1300mAh lipo.

Chen
twn Chen is offline Find More Posts by twn Chen
Reply With Quote
Old Apr 24, 2012, 03:24 PM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,773 Posts
Chen, I will let you know when I get back home. I am in Texas for the next three weeks, attending a school. It figures that at home right now they are having great flying weather!!!-------Metalguy
metalguy is offline Find More Posts by metalguy
Reply With Quote
Old Apr 24, 2012, 03:47 PM
Registered User
kensp's Avatar
Darwin Australia
Joined May 2002
6,960 Posts
Quote:
Originally Posted by isvana View Post
Ouch - mine has come out at a porky 19oz without LiPo (say 22oz AUW). Pretty much stock but with the additional weight of a GWS 600 ESC, motor heat sinks, and a (slightly bigger) HS81 aileron servo.
My GWS C-47 was bult using lightweigh low power brushless motors so it did not need a large capacity battery. As well originally no undercarriage was fitted. Nowdays it has a larger battery and wheels so it now weighs 23 oz.

Ken
kensp is offline Find More Posts by kensp
Last edited by kensp; Apr 24, 2012 at 04:00 PM.
Reply With Quote
Old May 12, 2014, 05:55 PM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,773 Posts
I will be building my second GWS C-47 kit soon. I got it as a replacement for my old, and ugly one that was a good flyer once in the air, just a handful on the ground. I have decided on retracts (Servoless) that I will use from a Durafly Bf-110, and I will also utilize the motor system from it. Should be an easy build. I will post pics as I progress.... ------Metalguy
metalguy is offline Find More Posts by metalguy
Reply With Quote
Old May 12, 2014, 09:17 PM
Registered User
United States, MI, Kingsley
Joined Jan 2013
74 Posts
I will be looking foreword to your posts. I have one to build that I've been saving until I improve my flying skills. It would also be great to get brushless power info for your build as well, I have yet to try a twin build yet. Thanks and best of luck, good flying, Matt
Mattkingsley is offline Find More Posts by Mattkingsley
Reply With Quote
Old May 12, 2014, 09:41 PM
MINE!
Mick Molloy's Avatar
Australia, VIC, Point Cook
Joined Jul 2003
3,186 Posts
Quote:
Originally Posted by metalguy View Post
I will be building my second GWS C-47 kit soon. I got it as a replacement for my old, and ugly one that was a good flyer once in the air, just a handful on the ground. I have decided on retracts (Servoless) that I will use from a Durafly Bf-110, and I will also utilize the motor system from it. Should be an easy build. I will post pics as I progress.... ------Metalguy
Mick Molloy is offline Find More Posts by Mick Molloy
RCG Plus Member
Reply With Quote
Old May 13, 2014, 12:31 PM
always something to learn...
metalguy's Avatar
Tacoma, WA
Joined Jan 2011
2,773 Posts
I will get you a box of kittens......------Metalguy
metalguy is offline Find More Posts by metalguy
Reply With Quote
Old May 13, 2014, 08:28 PM
MINE!
Mick Molloy's Avatar
Australia, VIC, Point Cook
Joined Jul 2003
3,186 Posts
Quote:
Originally Posted by metalguy View Post
i will get you a box of kittens......------metalguy
lol
Mick Molloy is offline Find More Posts by Mick Molloy
RCG Plus Member
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Building C 47, slight problem Mopar92 GWS (Grand Wing Servo) 1 Aug 17, 2004 11:48 PM
DAW Extra - Building Tips? Sal C Foamies (Kits) 27 Sep 26, 2001 09:08 PM
Simple 400 Building Tips? 99esq High Performance 4 Sep 10, 2001 07:07 PM
Wattage F-22 Building Tips / Pictures rcfanatic Foamies (Kits) 9 Sep 08, 2001 02:25 PM
IFO Mark3: Any last minute building tips ? volt flyer Parkflyers 13 Jun 21, 2001 03:33 PM