Go Back  RCU Forums > RC Airplanes > Questions and Answers
Reload this Page >

is it nose heavy or tail heavy or something else ??

Notices
Questions and Answers If you have general RC questions or answers discuss it here.

is it nose heavy or tail heavy or something else ??

Old 09-01-2016, 09:09 PM
  #1  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default is it nose heavy or tail heavy or something else ??

Guys, I have a Cermark 28% Edge, 75.25" span, 71 inches long,, it's some 6 years old and has not flown in 4 years because it's a squirrely flying plane, it just wont fly solid/stable,, I have been told it's Nose Heavy, but I balanced it right at 27% MAC 4-1/4" from the Leading Edge (at the Main Spar) as the manual recommends,

so after 4 years of setting on the bench, and 1 week of re-setting it up I flew it today, when giving Aileron Stick Inputs the plane feels slow to respond then it will over respond, the same for the Elevator (but not with the Rudder) I lowered the EXPO Rate from 25% to 15% and that helped some but still not enough,, again>, if I give it a little Aileron Stick Input the plane will lag to respond then quickly roll to a Knife Edge

Note 1) know that the plane would act the same way with my older JR 9303 TX so I feel the TX is not the problem

Note 2) that all linkages are tight, the plane has only had a gallon of fuel flown threw it, the Engine was broken-in on the bench with 1-1/2 gallons.

Note 3) I had a RC Pilot that has been flying for some 30 years see my plane fly and he told me it's Nose Heavy, but I have to add that he never flew my plane.

Balancing; the plane has a Magnum 180 (30cc) 4-Stroke, when balancing I did used a Balancer and I had to add 4 or 5 OZ of weight to the nose,

BTW, my TX is a JR XG 8

you can see a Cermark 28% Edge in my YT links
https://youtu.be/Bxrf_6ib6ow?t=98

https://youtu.be/Y1VqeudOLE0?t=35

I will answer questions if you ask them
thanks guys !!

Jim

Last edited by the Wasp; 09-01-2016 at 09:31 PM.
Old 09-01-2016, 10:39 PM
  #2  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default

well after reading more about MAC I did the numbers and it came up rather tail heavy,, here are my numbers

19.25 + 9.75 = 29 @ 27% = 3.70"

dang manual, it seems to me I need to move the CG forward by a 1/2 inch to get it balanced at 27% MAC ??



Jim
Old 09-02-2016, 06:12 AM
  #3  
JohnBuckner
My Feedback: (1)
 
JohnBuckner's Avatar
 
Join Date: Dec 2001
Location: Kingman, AZ
Posts: 10,441
Likes: 0
Received 10 Likes on 10 Posts
Default

Sure seems a shame to have missed four years of flying simply because someone said it,s nose heavy when in fact its the other way around at least from the description I just read.

John
Old 09-02-2016, 11:28 AM
  #4  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default

I only said this plane hasn't flown in 4 years, it was my son's plane and he found that girls are more fun than planes,, but thanks for the info

Jim
Old 09-02-2016, 05:47 PM
  #5  
acdii
 
Join Date: Jan 2011
Location: Capron, IL
Posts: 10,000
Received 97 Likes on 88 Posts
Default

CG imbalance can show up at its worst during landing. Tail heavy makes the plane pitchy at slower speeds, nose heavy makes it heavy on landing. I flew my P-47 for the fist time today, balanced on a Vannessa rig to the book, and it is slightly tail heavy. At half throttle, the engine surges and actually causes the plane to porpoise. At higher speeds though, rock steady.

So when landing, does it tend to land hard, or settle in tail down?

You say the rroll rate is slow to respond, so I take it you give a slight left or right aileron input, and then wait for a response, or do you give a little stick, get no response and then give a little more? You mentioned expo, and that is exactly what expo is for, to reduce the amount of input at the stick center. Try it with no expo once you get the balance corrected.
Old 09-02-2016, 05:58 PM
  #6  
Top_Gunn
My Feedback: (6)
 
Join Date: Jun 2005
Location: Granger, IN
Posts: 2,344
Received 6 Likes on 5 Posts
Default

19.25 + 9.75 = 29 @ 27% = 3.70
What dimensions are 19.25 and 9.75?
Old 09-04-2016, 06:27 PM
  #7  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default

Originally Posted by Top_Gunn
What dimensions are 19.25 and 9.75?
they are root and tip cord numbers, but it doesn't mater cause I didn't divide "29" by 2,,, ooops,,,

my new numbers

19.25 + 9.75= 29 divided by 2= 14.50 @ 27% = 3.91

Jim

Last edited by the Wasp; 09-04-2016 at 06:33 PM.
Old 09-05-2016, 04:14 AM
  #8  
Top_Gunn
My Feedback: (6)
 
Join Date: Jun 2005
Location: Granger, IN
Posts: 2,344
Received 6 Likes on 5 Posts
Default

Originally Posted by the Wasp
they are root and tip cord numbers, but it doesn't mater cause I didn't divide "29" by 2,,, ooops,,,

my new numbers

19.25 + 9.75= 29 divided by 2= 14.50 @ 27% = 3.91

Jim
The mean aerodynamic chord is not simply the mean chord (except for a hershey bar wing). I ran your numbers (with made up numbers for the dimensions I didn't have, which shouldn't change things) through a calculator and got 4.25 for 27% of the MAC, same as the manual. So I think your plane may be nose-heavy at 3.91. Here's the calculator:

http://adamone.rchomepage.com/cg_calc.htm
Old 09-05-2016, 07:50 AM
  #9  
jetmech05
 
Join Date: Jul 2005
Location: Jacksonville, FL
Posts: 4,865
Likes: 0
Received 0 Likes on 0 Posts
Default

If you can fly it one more time do so. This time put the plane at full throttle, put it in a 45 deg up line roll inverted and let go of the sticks. If it dives it's nose heavy if it climbs it's tail heavy. Adjust accordingly.
put the expo back, you won't feel much less than 25%. Also make sure you have expo the correct way
Old 09-05-2016, 09:10 AM
  #10  
buzzard bait
 
Join Date: Jan 2003
Location: Ithaca, NY
Posts: 3,286
Likes: 0
Received 5 Likes on 5 Posts
Default

Excellent advise from jetmech05. It can't be terribly tail heavy or you wouldn't have been able to fly it, so flight testing is the next step. Another way is to trim for cruise, put into a moderate dive, and let go of the stick. Quick pull out means nose heavy. If it keeps going straight down it is very neutral and you would probably want more nose weight. Jm's method sounds fine too.

And I also wondered if you had the expo the right way because it sounded like you didn't. Most brands require negative expo to soften in the center, but I don't know JR.

Jim
Old 09-05-2016, 11:42 AM
  #11  
JohnBuckner
My Feedback: (1)
 
JohnBuckner's Avatar
 
Join Date: Dec 2001
Location: Kingman, AZ
Posts: 10,441
Likes: 0
Received 10 Likes on 10 Posts
Default

Originally Posted by buzzard bait
And I also wondered if you had the expo the right way because it sounded like you didn't. Most brands require negative expo to soften in the center, but I don't know JR.

Jim
Futaba, Tactic and Hitec utilize negative shift for soft center. JR as well as Spectrum and some of the spectrum clones utilize positive shift for soft center.
Old 09-05-2016, 08:07 PM
  #12  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default

guys, thanks for the replies !!

"fly it again", not the way it is, it's just too hairy to fly, I feel I was lucky I got it down the 2 times I did fly it..

with JR you add (+) Expo.

""It can't be terribly tail heavy or you wouldn't have been able to fly it""

but it did fly terrible, for sure I thought I was going to crash it,, and another indication it was tail heavy is that it was faster than it was before.. so if it was a little nose heavy it would have come in smother to land

as for changing balancing, the ONLY 3 things I did to this plane to get it back up that would have changed the balance,, was>

1) I changed the prop from an APC to a Xroar (both 17X8),, the wood Xroar is much lighter than the plastic APC, I would say the Xroar is about half the weight of the APC, but I didn't think the Xroar was so light it would change the CG too much,,

2) I changed the rudder servo arm from plastic to metal, the metal arm is longer,, the rudder servo is (centered) about 7 &1/2 inches behind the original CG,, again this new metal arm could not weigh more than 1/4 gram more than the plastic arm,,,

3) the original 2000 NiCad was replaced with a 1900 NiCad, the new pack is mounted in the same spot,, the pack is mounted behind the engine under the fuel tank,,, how much lighter can the 1900 weigh than the 2000 pack ?

NOTE) now that I am typing I do remember I did change the rudder servo 3 years ago and it was never flown, same size standard servo, I can't remember what the old one was but I do know that the old servo had nylon gears while the new one has Titanium gears,, the weight can't be much of a change..

I never had a thought that these small changes would have changed the CG sO much that the plane would fly sO crazy, so I will re balance it..

my only question is do I balance it right side up, or upside down ??, I can't remember how I did it 6 years ago, and my manual doesn't tell me..

my guess is to balance it right side up because all servos/receiver and battery pack are below the wing tube and the engine of course is inline with the wing tube and the heaviest part of the fuselage is below the wings, so with that I would think to balance it right side up ???

thanks again guys

Jim

Last edited by the Wasp; 09-05-2016 at 09:43 PM. Reason: wel, I edited my post because I wanted to, and I wanted to because there was something wrong with my post
Old 09-06-2016, 05:29 AM
  #13  
acdii
 
Join Date: Jan 2011
Location: Capron, IL
Posts: 10,000
Received 97 Likes on 88 Posts
Default

Sounds like you found the issue. It is amazing how little it takes to change CG. All those little changes you mention can add up to it being just a hair tail heavy which, if you flew it prior at the aft end of CG, it would put it over the CG enough to be a handful. Rule of thumb I follow to balance a plane, and I use the Vannessa rig, is if its a low wing, upside down, a high wing, right side up, and a mid wing depending on mass. If its top heavy, upside down. Sounds like yours should be bottom heavy, so right side up.


Case in point on CG sensitivity, my H9 P-47 150 has a dead on CG of 6 1/8". Not a range like most planes, but an exact setting. I balanced it at exactly 6 1/8" and found it to be just a touch tail sensitive, so I rebalanced it and move CG forward just a slight bit to reduce that sensitivity. All it took was 1/4 ounce on the nose. It was so sensitive that at just under half throttle, the DLE30 was surging, and every time the engine revved up the nose would pitch up, rpm would drop, nose would go down. It made for an interesting first flight. Need to lean out the engine just a hair to smooth out the midrange. Hopefully my next flight will be rock solid, would rather it be a touch nose heavy than tail heavy. Great flying plane though.
Old 09-06-2016, 06:18 AM
  #14  
Top_Gunn
My Feedback: (6)
 
Join Date: Jun 2005
Location: Granger, IN
Posts: 2,344
Received 6 Likes on 5 Posts
Default

If I understood the original post correctly, he re-balanced it after making the changes at the same point (4.25" back from the LE) where it was balanced before. So if it wasn't tail-heavy before the changes, it wouldn't be tail-heavy after the changes and re-balancing.
Old 09-06-2016, 06:51 AM
  #15  
buzzard bait
 
Join Date: Jan 2003
Location: Ithaca, NY
Posts: 3,286
Likes: 0
Received 5 Likes on 5 Posts
Default

Yeah, but it's not clear to me that he re-balanced. For sure a switch from APC to a wood prop could throw off the balance. First thing is to re-check to see if it still balances according to the manual. This is an aerobatic design, so the recommended CG is probably not far from the neutral point. The means a small change in balance can make a big difference in handling.

Being nose heavy would not make it hairy to fly, it would just not be as aerobatic and it would land at a relatively fast speed. Therefore I would assume it was tail heavy. Pitch response is where you would see a tail heavy condition. When you give up, it diverges and keeps going up too much. Then you try to correct with some down and it diverges the other way, and the dive quickly gets steeper. It's hard to stop, and you're lucky if you can get it landed safely. That's tail heavy. Hard to tell from the description if that's what was going on. It's confusing that the aileron and elevator seem to have the same problem. Aileron problem is not a CG problem.

Also, decreasing the expo should have made those problems worse, not better. That's why I thought he had the expo the wrong way. I would have INCREASED the expo for the aileron problem. Maybe the elevator too, but the first thing is to be certain the CG is correct.
Old 09-06-2016, 08:50 AM
  #16  
Top_Gunn
My Feedback: (6)
 
Join Date: Jun 2005
Location: Granger, IN
Posts: 2,344
Received 6 Likes on 5 Posts
Default

On reading it again, it looks like he didn't re-balance, so tail-heavy seems right.
Old 09-06-2016, 10:46 AM
  #17  
baronbrian
My Feedback: (13)
 
baronbrian's Avatar
 
Join Date: Nov 2002
Location: Grand Forks, ND
Posts: 890
Likes: 0
Received 0 Likes on 0 Posts
Default

fly and trim it again, then roll inverted. if the nose drops quickly, nose heavy. if the nose rises, tail heavy. For a aerobatic plane, you want it to just barely fall, and you will be balanced.
Old 09-06-2016, 12:14 PM
  #18  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 9,527
Received 177 Likes on 152 Posts
Default

What servos are being used? And what voltage is the RX pack? CG has absolutely nothing to do with the described roll rate. Also when tiring to troubleshoot a poor handling airplane its best to know the thrust angles and incidences as well. Never listen to local guys on CG placement unless it's somone who flys IMAC or pattern in Intermidiate class or better. Most sport pilots don't quite understand CG and correct trimming.
Old 09-06-2016, 09:29 PM
  #19  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default

sorry guys, I'm stupid LOL, I did not re-​balance it after making the changes, I didn't think they would make such a drastic change,
to add the plane flew fine before the changes, but that one pilot that's a rather good pilot with some 30 years of flying said he thought it was nose heavy so that also made me feel the changes wouldn't hurt,

I just got finished balancing it, I balanced it as the manual recommends at 4 &1/4, and wow, it took 7.75oz of lead to get it to balance, it balances level and it will balance with the tail slightly high, with the weight I use if I try to balance it with the tail slightly lower than level the tail will slowly drop,

my god my god I don't know why it took so much lead to balance it, but then again I do have the lighter wood prop on it, and that new rudder servo must weigh more than the old one, plus the new metal servo arm,

speedracerntrixie,, everything I have read about an overly heavy tail plane has said a plane will (also) try to spin over and do a flat spin,, the thrust angle have always been fine, even the other day it didn't pull left or right on climb-outs or while landing,, yes it flew so bad I found myself doing unwanted climb-outs, so I just went with the flow and did them,, as for the wing servos, same servos, the only thing I changed is the battery pack, it was a 2000mah NiCad 4cell, and the new pack is a 1900mah NiCad 4cell, but as I said I did buy a new radio, so maybe I did get something set wrong somewhere,

if any one is wondering how I mount the weight, as I said the plane has a 180 4ST, so it has a Glow engine type engine mount, and I melt the lead into a stick and mount it right to the 2 tips of the engine mount,

thanks again guys !! you are all great for trying to help !! I took the advice and balanced it where the manual recommends, maybe in a couple days I can take some photos and post them, but I can not test fly the plane this week end, got things to do, maybe I can fly it Monday

Jim


Last edited by the Wasp; 09-07-2016 at 12:20 PM.
Old 09-07-2016, 04:52 AM
  #20  
buzzard bait
 
Join Date: Jan 2003
Location: Ithaca, NY
Posts: 3,286
Likes: 0
Received 5 Likes on 5 Posts
Default

If a plane is tail heavy the principal effect is to reduce pitch stability to the point of being difficult or impossible to control in pitch. Yaw stability can also be reduced because if the plane is tail heavy then less side area is behind the CG, and more side area is ahead of it. Once a plane is uncontrollable in pitch, other bad things might happen, like tip stalling and spinning. I can't think of any secondary effects from a tail heavy condition that would cause your aileron problem. As I noted above, the aileron problem is not a CG problem.

If your rudder servo is in the tail, near the rudder, then a small weight change can affect the CG noticeably. The prop is likewise at the farthest-forward point. Both are at the extremities of the airplane where they will have the most effect.

I don't know your pilot and this may not apply to him at all, but a common error of pilots, even some with decades of experience, is to mistake a trim problem for a CG problem. The plane tends to swoop up so it must be tail-heavy...not true. The plane pitches down so it must be nose heavy...also not true.
Old 09-07-2016, 08:27 AM
  #21  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 9,527
Received 177 Likes on 152 Posts
Default

BB and others. I will keep my replies focused on a light aerobatic type airplane as that is the thread topic. The Edge is a very stable airplane, one reason it remains popular. It will fly with the CG considerably aft without major stability issues. You have to remember that when the CG is moved aft ( note I did not say tail heavy ) the elevator becomes more effective ( note I did not say sensitive ) so less throw is needed. Most guys use too much throw in the first place then factor in an aft CG condition and the result is an over controlling situation that many will mistake for instability. The fact that in many of these cases the airplane is saved by the pilot simply flying with small inputs illustrates this. When setting up this type of airplane I will start off by setting the CG to 30% of MAC. That usually gets me 95% there and adjustments are simply a matter of moving the RX battery. If that alone is not enough then I look for ways to remove weight.

The other thing that almost always gets overlooked is that CG will greatly affect control cross coupling between the rudder and pitch. With an aft CG when rudder is applied the airplane will pitch down. This is a complicated explaination so to simplify its a function of the down trim used to keep the airplane flying level. So while you are fighting an overly effective elevator you see a pitch down every time you apply rudder. It's easy to see where guys mistake all this for instability.

Prior to test flying an airplane it is important to take an hour and measure the trust line and incidence of the wing and stab. If you know where they are or better yet have adjusted them to where they should be then there is no way they can mask a required CG adjustment. For this type of airplane you will want to start with the engine up thrust at zero, right thrust at 2 degrees, wing at + 1/2 degree and stab at zero. For your maiden, you want the elevator perfectly neutral then take note what trim was applied. If up trim was needed then you are nose heavy, if down elevator is required then you have an aft CG. This is where you need to have everything else set correctly prior to test flight. Once you have the CG set to the point where no elevator trim is required, you can move onto other tests such as the inverted 45 degree upline and knife edge. Usually unless slightly on the nose heavy side the airplane will have a very slight tuck to the gear in knife edge. I tend to employ a mix of 2% to 4% to eliminate.

I had asked about servo's and battery voltage because this is where I beleive the slow aileron response is at. For one I'm concerned about the use of 4.8 V especially Nicad. Nicad batteries experience the largest voltage drop under load of all battery types we use on the larger airplanes. Then the unknown brand and model of the servo's may not be up to the task. For this size and type of airplane I would use a 2000 mah LiFe battery and servos with a minimum of 120 oz of torque.

Of of course one this all is sorted out comes the actual task of trimming the model completly which will include fine tuning throws and expo, setting aileron differential, fine adjusting thrust line and maybe a mix or two.
Old 09-07-2016, 12:55 PM
  #22  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default

EDITED,

it flew fine with these servos before so I don't believe the servos are the problem, they are 80oz Hitech's Digitals, my son and I are not 3D pilots and I had the same type servos on my old PacificAro 82" span GeeBeeY (MVVS 35cc) and it flew fine for zipping around, looping and rolling,

so as I said I balanced it last night and the balance was way-off, I am now wondering if the first time I balanced it (6 years ago) I did it upside down, I don't remember,,

I have seen on You Tube people are flying this plane with 40cc to 56cc engines, and all I have in this one is a (much lighter) 30cc 4st, I mean if a 71" long plane is out of balance by 8.75oz then it has a problem, right ?

thanks for the info, I am learning a lot !!

Jim

Last edited by the Wasp; 09-07-2016 at 12:59 PM.
Old 09-07-2016, 02:02 PM
  #23  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 9,527
Received 177 Likes on 152 Posts
Default

The servos may be 80 oz but only with the correct voltage/current delivery and linkage geometry. If you have poor linkage geometry and a voltage drop of just .5V under load you now have a 40 oz servo.
Old 09-16-2016, 07:59 PM
  #24  
the Wasp
Thread Starter
My Feedback: (1)
 
Join Date: Sep 2003
Location: VT
Posts: 5,395
Likes: 0
Received 41 Likes on 39 Posts
Default

well she is ready to fly again, maybe the 2 words I should use are "try again" LOL,

I had to re balance her 3 more times, I was unhappy with the shape of the lead weight so I remolded it and I was still unhappy so I took a hunk of wood and milled a mold on the mill/drill, the shape was still not correct so I modified the mold and it came out great, I noticed that the slower I poured the lead the better the end result was with no pitting, plus some of the lead had zinc in it so it came out pretty LOL, as for the balance it balanced level at 1/8th inch forward of what the manual recommends, better than tail heavy for starters,

I also did a load check on the batteries, my vote meter put a 0.50 load on the pack plus at the same time I physically loaded the two 80oz elevator servos and the rudder servo (250oz HS-7955TG) and I got a drop of 0.03 in 20 seconds, voltage dropped to 5.36, I would have got a better result IF I charged the pack first, I think I will be fine for the way I fly, as I always had success with these SCR NiCads.

I hope to fly next week

Jim

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Manage Preferences - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.