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

Where would I put my CG?

Notices
Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Where would I put my CG?

Old 02-07-2008, 01:39 AM
  #1  
skylark-flier
Thread Starter
 
skylark-flier's Avatar
 
Join Date: Sep 2002
Location: VA, Luray
Posts: 2,226
Received 15 Likes on 13 Posts
Default Where would I put my CG?

Hey guys, I've seen several threads concerning CG but never for a biplane. I'm restoring an old Scale Line Models kit of the Great Lakes 2T-1A and I'm lost as to where to try my initial CG.

The bottom wing is straight but the top wing is rather seriously swept. As opposed to the bottom wing, the top wing rear tips are maybe 10% in front of the bottom wing rear at the wingtip, but at the center point of the aircraft the top wing trailing edge is at about 30% behind the bottom wing leading edge.

Now guys, keep in mind that I've built lots of planes over the years (50+) and most have come out just fine with my limited aerodynamic knowledge, but this is a new one - I've never done a biplane before, let alone one with staggered wings. Wouldn't trust myself with that CG calculator I saw on another thread either, I need to keep it simple if possible.

I'm attaching a 3-view of the plane, just for reference. My mind kinda tells me that the CG should be in the neighborhood of the rear of the center-brace, by the front cockpit.

Any opinions on just how close this might be?

Tks,
Dave W.
Attached Thumbnails Click image for larger version

Name:	Om33830.jpg
Views:	19
Size:	47.5 KB
ID:	872097  
Old 02-07-2008, 02:06 AM
  #2  
pimmnz
 
Join Date: Nov 2005
Location: Auckland, NEW ZEALAND
Posts: 1,961
Likes: 0
Received 1 Like on 1 Post
Default RE: Where would I put my CG?

Cg will be where it's always sposed to be, on this sort of model 25~30% MAC. There is a calculator as a 'Sticky' on this forum somewhere, but if you use your drawing, measure 1/2 way out on the combined wing plan from the centre (treat the top and bottom wing as one with a straight trailing edge and swept leading edge) draw a line parallel to the fuse centre line from L/E to T/E and measure between a 1/4 and 1/3 back from the L/E, this point, when projected back to the centre will be pretty close.
Evan WB # 12.
Old 02-07-2008, 04:12 AM
  #3  
highhorse
My Feedback: (2)
 
highhorse's Avatar
 
Join Date: Nov 2006
Location: Memphis, TN
Posts: 2,564
Received 93 Likes on 49 Posts
Default RE: Where would I put my CG?

Pimmnz's method may well be an effective shortcut, but it's not one I'm familiar with so let me ask you something:

Are you comfortable calculating the exact CG location for each wing independently? If so, you can simply "average" the two locations PROVIDED the two wings are of the same area. If they are not, each CG will have to be weighted according to it's area.

If you prefer, I have no reservations about calculating it for you. I just need dimensions.

Regards, Don.
Old 02-08-2008, 02:19 PM
  #4  
skylark-flier
Thread Starter
 
skylark-flier's Avatar
 
Join Date: Sep 2002
Location: VA, Luray
Posts: 2,226
Received 15 Likes on 13 Posts
Default RE: Where would I put my CG?

Don, I really appreciate the offer, and I'm going to take you up on it. This is the first biplane I've ever even been near (other than a ride in a full-size 2T1A when I was 16 - 44 years ago) and with this being swept-wing, I'm lost. If it was a straight wing (like a Jenny or something) I'd just go with my normal practice of averaging the 2 center spars (which normally gets me pretty close), I'm sure that's not going to work with this lady though.

Everything I've gotten on her so far: Wingspan (both) 56.75" constant chord (both) 8"

TOP WING (swept)
Nose to LE @ center = 5.75"
Nose to TE @ center = 13.75"
Nose to LE @ tip = 9.5"
Nose to TE @ tip = 17.5"

LE @ center to vertical stab LE = 25.25"
TE @ center to v-stab LE = 17.25"
LE @ tip to v-stab LE = 22.5"
TE @ tip to v-stab LE = 14.5"

LE @ center to horizontal stab LE = 24"
TE @ center to h-stab LE = 16"
LE @ tip to h-stab LE = 23.5"
TE @ tip to h-stab LE = 15.5"

BOTTOM WING (straight)
Nose to LE = 10.5"
Nose to TE = 18.5"

LE to h-stab LE = 19.5"
TE to h-stab LE = 11.5"

V-stab LE to rudder TE = 7.75"
H-stab LE to elevator TE = 8.75"
V-stab LE is 1" behind h-stab LE, TE's are at the same distance (she'd stand vertically on her tail)
Landing gear axles stand about 1.5" behind top wing LE

My numbers may be off a tiny bit but I'm pretty sure they're all within 1/8" of being right. Hope they help you. Once again, I appreciate your taking the time to do this.

Dave W.
Attached Thumbnails Click image for larger version

Name:	Qo40240.jpg
Views:	15
Size:	124.8 KB
ID:	873189  
Old 02-11-2008, 07:56 PM
  #5  
mred33
Senior Member
 
mred33's Avatar
 
Join Date: May 2006
Location: Glenwood, GA
Posts: 360
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Where would I put my CG?

Any plane has a CG that it likes and may in part be due to your flying style. The quick and dirty methed of 25% to 33% holds true for a starting point on any plane be it single wing or bi-plane. On a bi-plane you treat both wings as one and go from the LE of the top at center to the bottom TE and figure 25% to 33% and mark your CG. After you fly it, you may want to move it forward or back depending on where it is and your feel of the plane. I uasually start at 25% and go from there. That is a safe place to start and will not give you a touchy plane in pitch. Starting at 25% is safe and gives you a chance to see how it flies and you can adjust it after flying it to you likes. You can go through the math and still wind up having to change the CG on most planes because there are to many things envolved in determining the GC to say it is always going to be this point. It won't. It will be where the plane likes to fly and you are at ease with. It depends to much on to many thing to say it will always be this point and never change. It will change from one plane to the next even if they are the same kind. With a two wing plane it is going to look further back on the top wing then you will see on a single wing, but that is because of the bottom wing. Start with 25% and you will have a plane that is easy to fly, but probably a little nose heavy. Safer then tail heavy and not as hard to control.
Have fun with the new bi-plane, they are loads of fun.
Ed
Old 02-11-2008, 08:33 PM
  #6  
mred33
Senior Member
 
mred33's Avatar
 
Join Date: May 2006
Location: Glenwood, GA
Posts: 360
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Where would I put my CG?

http://www.geistware.com/rcmodeling/cg_super_calc.htm

If you want to have some fun playing around with numbers, go there and have a ball. You can get your CG figured out by inputing a few numbers.
Ed
Old 02-12-2008, 05:01 PM
  #7  
skylark-flier
Thread Starter
 
skylark-flier's Avatar
 
Join Date: Sep 2002
Location: VA, Luray
Posts: 2,226
Received 15 Likes on 13 Posts
Default RE: Where would I put my CG?

Hi Ed, you're right in everything you said, most definitely. Personally, I like them a bit nose-heavy, me not being the greatest flier ever to take to the air. I like the plane being able to remember where the ground is and how to get back to it - all I have to do is enjoy the approach and flare at the right moment. It's always worked for me.

I'm having a lot of fun restoring this beautiful old plane. Once I got the covering off and REALLY took a look at her I found that many (most) of the glue joints in the wings needed a bit of re-enforcement but that didn't take long and there's no "creak creak" anymore when I flex the wings a bit.

I've also broken-out one of my old engines, a 30-yr old Enya .35 that I think I'm going to put in the nose - assuming I can still find a prop extension for it somewhere. I'm going to have a couple issues with that particular engine (or any engine, for that matter) but we'll see what we'll see. One issue is that that particular engine has NEVER had a muffler attached (she was long before mufflers came into very widespread use), and the plane ALSO has never had a muffler mounted. The entire nose section is carved balsa, including the rather large cowling/engine area cover, and on the inside of the cowl right now is a brass plate that I assume was for heat dissipation "way back when...". Exhaust simply went out the louvered sides, along with engine heat. My intention is to take a stab at running the engine up as-is to see just how much that total cover around the compartment will actually help muffle sound, as it was "back in the days...". If I HAVE to re-cut and install a muffler I will, but I'm hoping I don't really have to.

Hey guys, don't panic here!! I know about mufflers!! My big advantage here is that I'm living in the sticks (central Shenandoah Valley, Virginia) and I fly out in the field with our horses, where the nearest house is 1/4 mile away, and it's mine. Next house is another 1/4 mile away or so. So, I'm not all that worried about noise.

Dave W.
Old 02-13-2008, 01:01 PM
  #8  
mred33
Senior Member
 
mred33's Avatar
 
Join Date: May 2006
Location: Glenwood, GA
Posts: 360
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Where would I put my CG?

Hay Dave;
One thing you can do if you can't find an extension for the engine is to put an extension for the motor mount such as a box extension on the firewall of the correct length to get the enging out to where you should be. As for the muffler, you can always get a custom muffler made to fit the engine, although that is a little extreem and costly, but you may be able to make one yourself. Make a strap-on type and wrap it around the back of the engine so it is not sticking out the side. Something like the Top Flight muffler that mounts to the firewall and then make an exhaust manafold with a rubber extension to go to the muffler itself. Yo may find one already made and adapt it to your engine. There are many things you can do to put a muffler on the engine.
Good luck with your rebuild. It's a fine looking plane and worth the effort to get it fixed up.
Ed

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 -

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