Go Back  RCU Forums > RC Airplanes > Golden Age, Vintage & Antique RC
Reload this Page >

What is it and where he cg

Notices
Golden Age, Vintage & Antique RC Want to discuss some of those from the golden age, vintage rc planes or even an old classic antique vintage rc planes, radios, engines, etc? This is the place for you. Enjoy!

What is it and where he cg

Old 08-02-2020, 09:48 PM
  #1  
cress
Thread Starter
 
Join Date: Sep 2002
Location: Newburgh, IN
Posts: 18
Likes: 0
Received 0 Likes on 0 Posts
Default What is it and where he cg

Remake of vintage freeflight plane. Fringing to figure what it is and where to start to balance it. It has a very under chambered wing and a flat bottom thick airfoil elevator stab. Thanks

Old 08-03-2020, 12:04 AM
  #2  
Lifer
My Feedback: (1)
 
Lifer's Avatar
 
Join Date: Aug 2008
Location: Kansas City, MO
Posts: 4,327
Likes: 0
Received 8 Likes on 8 Posts
Default

With a lifting stab the c/g will be somewhere between 50 to 60% of the chord measured from the leading edge. I have no idea what the name of the aircraft might be.
Old 08-03-2020, 03:24 AM
  #3  
rgburrill
 
rgburrill's Avatar
 
Join Date: Dec 2010
Location: Dallas, Tx CT
Posts: 2,407
Received 22 Likes on 21 Posts
Default

A nose heavy plane flies poorly, a tail heavy plane flies once. You put the CG at 50-60% of chord and you won't fly long.

The CG should be at or forward of the CL, center of lift. Now you can go through all the calculations to determine the center of lift or you can do the simple thing like has been done since the beginning of flight, experiment. Start with the CG at 1/4 chord and move it back until the plane stops flying better each time. With that big tail you can likely get away with start at 30%.

Over 100 years of aviation has shown that aerodynamics is all about experimentation - all the design in the world just gets you a stating point. I used to get paid to do this stuff. My boss and I at Boeing disproved some "hard and fast" rules during wind tunnel testing.
Old 08-03-2020, 04:28 AM
  #4  
Lifer
My Feedback: (1)
 
Lifer's Avatar
 
Join Date: Aug 2008
Location: Kansas City, MO
Posts: 4,327
Likes: 0
Received 8 Likes on 8 Posts
Default

Check out some Free Flight designs that have similar moments and lifting stabs. Note the C/G on those designs and then decide for yourself.
Old 08-03-2020, 05:01 AM
  #5  
UStik
 
Join Date: Oct 2006
Location: Augsburg, GERMANY
Posts: 989
Likes: 0
Received 1 Like on 1 Post
Default

Very good advice to start with more forward balance, no doubt. In this case, I wouldn't just start at 25% from leading edge. The "lifting stab" will not lift before the C/G is set far backwards, but its camber and especially the wing's camber make for a lot of nose-down pitch (airfoil moment). That's why such models have a lot of decalage (say 4 incidence difference between wing and stab) and a rearward C/G. This model has a very small elevator, which is undoubtedly enough if balance is OK but may be not powerful enough in case the C/G is very far forward.

The attached pictures show a glider I built in the sixties. It has similar configuration and undercambered wing airfoil. Unfortunately, the plan had to be repaired with tape but I drew a red arrow where the C/G should be. The instructions specify 69% from leading edge. Another well-known example is the Telemaster with (not even undercambered but) flat-bottom airfoil where the neutral point (aft limit of C/G) is at 50%. I would use at least that as a starting point.



Last edited by UStik; 08-03-2020 at 05:05 AM.
Old 08-03-2020, 02:07 PM
  #6  
wnewbury
 
Join Date: Dec 2004
Location: Durant OK
Posts: 156
Received 3 Likes on 3 Posts
Default

Dragonfly by Tex Newman. Bill Northrop's Model Builder Magazine. Published 1971 and October 1996. Mine flew well - super easy. Radio influenced rather than radio controlled.
Old 08-03-2020, 02:11 PM
  #7  
wnewbury
 
Join Date: Dec 2004
Location: Durant OK
Posts: 156
Received 3 Likes on 3 Posts
Default

CG on plans looks like 55% or 60%.
Old 08-03-2020, 02:19 PM
  #8  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 7,838
Received 50 Likes on 48 Posts
Default

This should help.


Looks to be in the 50%-60% range to me.
The following users liked this post:
R/C Pilot 304 (08-03-2020)
Old 08-04-2020, 07:24 AM
  #9  
rgburrill
 
rgburrill's Avatar
 
Join Date: Dec 2010
Location: Dallas, Tx CT
Posts: 2,407
Received 22 Likes on 21 Posts
Default

Well I'll be
Old 08-06-2020, 12:11 AM
  #10  
Lifer
My Feedback: (1)
 
Lifer's Avatar
 
Join Date: Aug 2008
Location: Kansas City, MO
Posts: 4,327
Likes: 0
Received 8 Likes on 8 Posts
Default

It's always a good idea to advise based on knowledge or experience. Those requesting help depend upon accuracy rather than a guess or an adage. Free flight is an excellent training ground that unfortunately isn't as widespread as it once was.
Old 08-06-2020, 09:14 AM
  #11  
Truckracer
My Feedback: (19)
 
Join Date: Apr 2004
Location: Des Moines, IA
Posts: 5,084
Likes: 0
Received 14 Likes on 14 Posts
Default

Originally Posted by rgburrill View Post
A nose heavy plane flies poorly, a tail heavy plane flies once. You put the CG at 50-60% of chord and you won't fly long.

The CG should be at or forward of the CL, center of lift. Now you can go through all the calculations to determine the center of lift or you can do the simple thing like has been done since the beginning of flight, experiment. Start with the CG at 1/4 chord and move it back until the plane stops flying better each time. With that big tail you can likely get away with start at 30%.

Over 100 years of aviation has shown that aerodynamics is all about experimentation - all the design in the world just gets you a stating point. I used to get paid to do this stuff. My boss and I at Boeing disproved some "hard and fast" rules during wind tunnel testing.
You apparently don't have any free flight experience. Many of the old designs did indeed fly with the CG at 50 - 60% MAC. With that said though, if this plane is being flown with RC assist, I would move the CG to a more normal location though still rearward of what one would call normal for a traditional airframe. Reason being this plane is meant to fly very slowly and any excessive use of elevator (which could mean almost no movement) could result in an instant stall that will be followed by a series of scallops that could be very hard to correct unless you have experience with this sort of plane. Other than during powered climb when down elevator will be necessary, most of a flight including the landing can be performed with little or no elevator use. Even rudder inputs should be kept to a minimum. Design for a very slow glide, not aerobatics. Can be fun but challenging if horsed around.

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.