Go Back  RCU Forums > RC Airplanes > Beginners
Reload this Page >

center of gravity effects?

Notices
Beginners Beginners in RC start here for help.

center of gravity effects?

Old 06-25-2007, 11:40 AM
  #1  
buffmeadows
Member
Thread Starter
 
Join Date: Dec 2005
Location: Eagle Rock, VA
Posts: 96
Likes: 0
Received 0 Likes on 0 Posts
Default center of gravity effects?

I've tried to search for this information on here but the results I get back are largely irrelevant so I apologize in advance if this has already been discussed.

I'm just wondering what the effects are on a plane if the CG is too far forward or if the CG is too far back...

I recently lost a GWS plane to "porpoising" even though I had checked all of the recommended specs for servo center, throw and CG.

It would just continually climb up then dive in a cycle that got increasingly more severe until it finally head-butted the ground.

Any info is appreciated.

Old 06-25-2007, 12:17 PM
  #2  
CGRetired
My Feedback: (1)
 
CGRetired's Avatar
 
Join Date: Sep 2004
Location: Galloway, NJ
Posts: 8,999
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

Well, if it's to far back, the plane becomes tail heavy and you will never have a successful landing, or at least it will be one heck of a hand-ful trying to land.. and fly too.

To nose heavy, well, if at altitude, and you cut the throttle, gravity takes over and landings can be a tad bit fast if not difficult.. if the weight is to much forward.

Stick with the numbers given by the Mfg, varying those only a very small amount, say an eighth to a quarter of an inch at most, at a time to get it the way you want it. Slightly tail or nose heavy is not a problem.. but to far back or forward can really be a handful to fly and could actually make it a tad dangerous.

DS.
Old 06-25-2007, 12:43 PM
  #3  
MadScientist
Senior Member
 
Join Date: Jun 2003
Location: Kissimmee, FL
Posts: 211
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

generally speaking, if you have the CG too far back, the plane will be very pitch sensitive, and uncontrollable. if it is too far forward, you may run out of elevator on landing, and not have adequate elevator response. Most likely you'll run out of elevator on landing, and your landing speed will be higher.

it is generally best to be on the too far forward end of things than the too far back. You can always adjust the CG back if it's too far forward, but most times when you have the CG placed too far back, you seldom have the opportunity to adjust, because you've probably crashed the plane.
Old 06-25-2007, 01:47 PM
  #4  
mscic-RCU
Senior Member
My Feedback: (6)
 
mscic-RCU's Avatar
 
Join Date: Mar 2002
Location: New London, OH
Posts: 1,237
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

Sounds like your GWS was tail heavy, CG too far back. Porpoising is a symptom of that condition.
Old 06-25-2007, 02:01 PM
  #5  
da Rock
Senior Member
 
Join Date: Oct 2005
Location: Near Pfafftown NC
Posts: 11,511
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

As the CG is moved aft, the elevator will be more effective. Less stable? Not as much as more sensitive. If you adjust the elevator throw to match the sensitivity you won't see or feel any difference at all. That is, until the CG gets to the NP and behind.

As the CG is moved forward, the elevator and stab starts having to carry an artificially increased load. The farther forward the CG moves the more leverage that weight has to push the nose down. So the tail has to push down to carry it. And the extra artificial force winds up being doubled and winds up stealing the tail's effectiveness. So the elevator gets less sensitive. And most people think the airplane is getting more stable. Less steerable yes. More stable? Not appreciably.

The porpoising you describe isn't usually what you see from a tailheavy airplane. When a tailheavy starts to pitch what you usually see is your stick movement PITCHING the airplane QUICKLY and HARD whichever way you jam the TX stick. And even when you very gingerly move the TX elevator control, that very sensitive elevator THROWS the airplane too much. And then you see a crash 'cause you can't fly something with that sensitive an elevator.
Old 06-25-2007, 03:17 PM
  #6  
spiral_72
Senior Member
 
spiral_72's Avatar
 
Join Date: Mar 2007
Location: Chesnee, SC
Posts: 511
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

I'd say your plane was properly balanced.... at least slightly nose heavy, or else when it climbed on the porpoise condition the nose probably wouldn't have dropped after it stalled.

If I was to guess I'd say your angle of incidence on the main wing or horz stab was wrong, generating too much lift or creating a nose up and a stall, or your elevator was improperly adjusted causing a nose up and stall.

Moving your cg forward (nose heavy) won't fix the problem. For what it's worth.
Old 06-25-2007, 05:12 PM
  #7  
daveopam
My Feedback: (9)
 
daveopam's Avatar
 
Join Date: Oct 2002
Location: ELK CITY, OK
Posts: 7,493
Received 2 Likes on 2 Posts
Default RE: center of gravity effects?

I have tried to fly a few tail heavy planes over the years. They felt like this. After takeoff the plane would start to climb out normally. The plane would then start to roll one way or the other. When I tried to correct with aile the plane did nothing for a split second. Then it snapped in the direction of the aile I was giving. The second one I was able to push it out inverted. After I got rolled back upright I was able to cut the power and kind of glide it down. I say kind of because it did porpoise on the way down. Just a touch of elevator would bring the nose way up.

David
Old 06-25-2007, 06:29 PM
  #8  
Dave trimmer
Senior Member
 
Join Date: Oct 2005
Location: Hamilton, ON, CANADA
Posts: 496
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

also think... If you have a nose heay plane you can correct it by giving up elevator to fly straight and level. On a tail heavy plane you need to give down elevator to fly straight and level, now your comming in for landing and need to keep the nose down to prevent a stall, and your a few feet and need to give down elevator, that can't be good[&:]

Dave trimmer
Old 06-25-2007, 08:51 PM
  #9  
firestone1121
Senior Member
 
Join Date: Jun 2007
Location: Reading, PA
Posts: 277
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

heres a question i wanna know- why does the fuel tank have to be empty?? the cg will be off once the tank is full right?? more times than not the plane will be full of fuel too. so why not balance with fuel?? im new to planes too and am curious as to why this is.
Old 06-25-2007, 09:25 PM
  #10  
Dave trimmer
Senior Member
 
Join Date: Oct 2005
Location: Hamilton, ON, CANADA
Posts: 496
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

after a 10 minute flight your tank will be empty, and at proper cg for landing. If you did it with the tank full tour plane will be tail heavy on lnading which will make it tough to land. Tail hevy= [:@]

Dave trimmer
Old 06-25-2007, 09:49 PM
  #11  
firestone1121
Senior Member
 
Join Date: Jun 2007
Location: Reading, PA
Posts: 277
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

Well that makes sense. never thought of it that way. thanks dave
Old 06-25-2007, 10:13 PM
  #12  
Dave trimmer
Senior Member
 
Join Date: Oct 2005
Location: Hamilton, ON, CANADA
Posts: 496
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

no problem, thats what were here for.
Old 06-25-2007, 10:38 PM
  #13  
Bad_Daddy
Senior Member
 
Bad_Daddy's Avatar
 
Join Date: Jan 2007
Location: Tracy, CA
Posts: 529
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

I've only flown a couple of GWS planes (Pico Stik, Slow Stik), but both of those have a very high angle of attack on the wing. That makes them very forgiving planes, but it also makes the flight very sensitive to throttle speed, where more throttle makes the nose pitch up, until the plane stalls. It can be negated by adding 1/4" spacers under the trailing edge of the wing, in the saddle area.
So, which GWS model were you flying?
Old 06-25-2007, 10:43 PM
  #14  
ArCeeFlyer
Senior Member
My Feedback: (1)
 
ArCeeFlyer's Avatar
 
Join Date: Jul 2005
Location: York, PA
Posts: 315
Likes: 0
Received 0 Likes on 0 Posts
Default RE: center of gravity effects?

The O.P. didn't mention if he was at full power, half power, no power, etc., the whole time it was porpoising. He did say the c.g. was at the recommended spec. Assuming that is the correct location for said plane, then something else would have to be afoot. Possibly no down thrust in the motor, or even up thrust. Maybe the wing incidence somehow got positive. Then when at power the plane would climb up and if it doesn't have enough power to pull it over the top, then I could see it diving back down for another climb. Of course, if the c.g. was truly too far back, then you have pretty much the same results. It sounds like the plane has a lot of dihedral or polyhedral to keep it from snapping out during the porpoising if it was actually an extreme tail heavy situation. Just some ideas. Really need more info about the plane.

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 - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.