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

CG of a Bipe?

Reply

Old 04-26-2008, 11:50 AM
  #1  
h-ray-RCA
Senior Member
Thread Starter
 
h-ray-RCA's Avatar
 
Join Date: Jun 2003
Location: Bellingham, WA
Posts: 194
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default CG of a Bipe?

Hi all,

Bought an ARF bipe, but the location of the CG just looks wrong (based on several years worth of rc flying.)

Any way to figure out the cg and double check their figure?


TIA,

Steve
h-ray-RCA is offline  
Reply With Quote
Old 04-26-2008, 12:05 PM
  #2  
crasherboy
Senior Member
 
Join Date: Oct 2006
Location: Bryant Pond, ME
Posts: 113
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

I think that there is a thread on this subject,but from what I remember of it is you add the two wingspan cords together and go from there. I would imagine that about 25% of that would be about right. Depending on what the airfoil is makes some difference. You do know to test the balance over some tall grass ,that way you could get some feel as to how it will fly without a wipeout.
crasherboy is offline  
Reply With Quote
Old 04-26-2008, 12:15 PM
  #3  
h-ray-RCA
Senior Member
Thread Starter
 
h-ray-RCA's Avatar
 
Join Date: Jun 2003
Location: Bellingham, WA
Posts: 194
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

Thanks. I should have done a search first.

Looks like the answer is to add the effective chord of all the wings (don't include any overlap) and go from 22%-25% back from the LE of the forwardmost wing.



Any other thoughts, I'm listening....



Steve
h-ray-RCA is offline  
Reply With Quote
Old 04-26-2008, 12:48 PM
  #4  
BMatthews
 
BMatthews's Avatar
 
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,337
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

That's the short and quick method and seems to be proving out nicely. However I think you'll find that unless the tail area is either a bit low or the tail length really short you'll be just fine with a more normal 25 to 28% as a starting point and be ready to move it back a little more based on flight testing. Use the RCU Search funtion to look for dive test
BMatthews is offline  
Reply With Quote
Old 04-26-2008, 01:21 PM
  #5  
khodges
Senior Member
My Feedback: (1)
 
Join Date: Jul 2003
Location: newton, NC
Posts: 5,443
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?


ORIGINAL: h-ray-RCA

Looks like the answer is to add the effective chord of all the wings (don't include any overlap) and go from 22%-25% back from the LE of the forwardmost wing.

Steve
20-25% of Mean Aerodynamic Chord. Measure horizontal distance from leading edge of forwardmost wing (usually th top one) to trailing edge of rearmost wing (usually bottom one). Divide by 5 or 4 to get 20 or 25%, then go back this distance from the top wing L.E. (if it is the forward wing). This is a good starting point. If you use 20%, it will be more likely to start out nose heavy, which, as you know, is much better than tail heavy.

On a plane with a lot of stagger, such as a WACO, the balance point may be well over half way back on the top wing's chord. On a plane with little or no stagger, such as a SPAD, the balance point will be around the main spar, usually.
khodges is offline  
Reply With Quote
Old 04-26-2008, 02:01 PM
  #6  
h-ray-RCA
Senior Member
Thread Starter
 
h-ray-RCA's Avatar
 
Join Date: Jun 2003
Location: Bellingham, WA
Posts: 194
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?



Ok gang, have a look at these numbers and see if they make sense:


Upper wing: 5 3/8” chord
Lower wing: 4 ¼” chord
Overlap: 1 ½”

Effective span: 6 7/8”

25% of 6 7/8= 1.71” should be the actual CG.

Mfg lists cg as being 3 ½” from LE of upper wing.



Thoughts?


TIA,

Steve



Attached Thumbnails Click image for larger version

Name:	Lj21332.jpg
Views:	18
Size:	69.9 KB
ID:	937417  
h-ray-RCA is offline  
Reply With Quote
Old 04-26-2008, 03:26 PM
  #7  
Rodney
Senior Member
 
Join Date: Dec 2001
Location: FL
Posts: 7,769
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

If the wings are not swept back and both wings the same span, I'd go with the 1.7 to 2 inch back point. If the wings are swept then you have to find the mean aerodynamic chord and go from there.
Rodney is offline  
Reply With Quote
Old 04-26-2008, 03:48 PM
  #8  
h-ray-RCA
Senior Member
Thread Starter
 
h-ray-RCA's Avatar
 
Join Date: Jun 2003
Location: Bellingham, WA
Posts: 194
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?


ORIGINAL: Rodney

If the wings are not swept back and both wings the same span, I'd go with the 1.7 to 2 inch back point. If the wings are swept then you have to find the mean aerodynamic chord and go from there.

Nope, both are straight wings so it's sounding more and more like the 1.7-2.0" CG is the way to go.
h-ray-RCA is offline  
Reply With Quote
Old 04-26-2008, 05:58 PM
  #9  
gboulton
My Feedback: (15)
 
gboulton's Avatar
 
Join Date: May 2005
Location: La Vergne, TN
Posts: 3,743
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

Couple of thoughts, having scratch built a bipe or two

First, hard to tell from the picture, but it LOOKS like you're measuring that 3.5" from out near the wing tip. Unless specified otherwise, it's fairly common to measure the CG from the CENTER of the top wing on most bipes. Not saying either way is 'right or wrong", just common, and possibly worth checking. Looks, by rough guess, that the 3.5" back in the center would be close to your 1.7" figure out near the tip.

Second, when all else fails, aside from it being time to get a new all else, I've had consistent good luck with bipes using the top wing's spar as a starting CG point. Again, this is at the center of the wing.

And, finally, in general, any CG that falls between the front and rear cabane strut attach points (on a bipe so equipped) will usually result in at least a flyable machine. I've never come across one whose "safe and comfy" CG was outside of that range, but obviously, that's a fairly subjective judgment.

Hope some of that helps!

**EDIT** Just re=read above, and saw you'd said both were straight wings...if that's the case, then clearly suggestion 1 above is moot...however, that top wing LOOKS to be swept back a bit in the pic. Could just be an illusion.
gboulton is offline  
Reply With Quote
Old 04-26-2008, 06:10 PM
  #10  
h-ray-RCA
Senior Member
Thread Starter
 
h-ray-RCA's Avatar
 
Join Date: Jun 2003
Location: Bellingham, WA
Posts: 194
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

ORIGINAL: gboulton

Couple of thoughts, having scratch built a bipe or two

First, hard to tell from the picture, but it LOOKS like you're measuring that 3.5" from out near the wing tip. Unless specified otherwise, it's fairly common to measure the CG from the CENTER of the top wing on most bipes. Not saying either way is 'right or wrong", just common, and possibly worth checking. Looks, by rough guess, that the 3.5" back in the center would be close to your 1.7" figure out near the tip.


**EDIT** Just re=read above, and saw you'd said both were straight wings...if that's the case, then clearly suggestion 1 above is moot...however, that top wing LOOKS to be swept back a bit in the pic. Could just be an illusion.
Thanks gboulton. Yup, they are both straight wings, here's a better pic taken while I was painting it.

Like you suggested, I measure at the center of the wing root. I just put a sticky note on the wingtip to show the CG in crossection.



Steve
Attached Thumbnails Click image for larger version

Name:	Bz76895.jpg
Views:	15
Size:	52.9 KB
ID:	937571  
h-ray-RCA is offline  
Reply With Quote
Old 04-26-2008, 06:20 PM
  #11  
gboulton
My Feedback: (15)
 
gboulton's Avatar
 
Join Date: May 2005
Location: La Vergne, TN
Posts: 3,743
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

For what it's worth...

Looking at it, I'd be willing to risk MY plane on balancing that bird on the top wing spar, and heading off to fly. Might not be ideal, but I'd be comfortable that it'd be flyable.

Obviously, pretty easy for ME to say, since it ain't my airplane.
gboulton is offline  
Reply With Quote
Old 04-26-2008, 06:22 PM
  #12  
BMatthews
 
BMatthews's Avatar
 
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,337
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?


ORIGINAL: h-ray-RCA
....
Upper wing: 5 3/8” chord
Lower wing: 4 ¼” chord
Overlap: 1 ½”

Effective span: 6 7/8”

25% of 6 7/8= 1.71” should be the actual CG.

Mfg lists cg as being 3 ½” from LE of upper wing.
The overlap should be listed as the lower exposure I think. Otherwise the numbers don't add up.

The picture tells a 1000 words and those words are saying "THEY ARE NUTS! ! !". You're right to question that location. I don't know where they got that number but it would be a disaster looking for a place close by to happen at takeoff time. I'd go with 1.7 to 1.75 from the upper leading edge and see how it works out for the test flight. That would be a safe starting point for sure.
BMatthews is offline  
Reply With Quote
Old 04-26-2008, 07:57 PM
  #13  
rmh
Senior Member
 
rmh's Avatar
 
Join Date: Dec 2001
Location: , UT
Posts: 12,630
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?


ORIGINAL: BMatthews


ORIGINAL: h-ray-RCA
....
Upper wing: 5 3/8” chord
Lower wing: 4 ¼” chord
Overlap: 1 ½”

Effective span: 6 7/8”

25% of 6 7/8= 1.71” should be the actual CG.

Mfg lists cg as being 3 ½” from LE of upper wing.
The overlap should be listed as the lower exposure I think. Otherwise the numbers don't add up.

The picture tells a 1000 words and those words are saying "THEY ARE NUTS! ! !". You're right to question that location. I don't know where they got that number but it would be a disaster looking for a place close by to happen at takeoff time. I'd go with 1.7 to 1.75 from the upper leading edge and see how it works out for the test flight. That would be a safe starting point for sure.
when in doubt -make a paper drawing of the plane -in planform this will show the both wings as just part of a fat chord
use that "fat chord" as basis for the cg location
rmh is offline  
Reply With Quote
Old 04-26-2008, 08:07 PM
  #14  
gboulton
My Feedback: (15)
 
gboulton's Avatar
 
Join Date: May 2005
Location: La Vergne, TN
Posts: 3,743
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?


ORIGINAL: BMatthews
The picture tells a 1000 words and those words are saying "THEY ARE NUTS! ! !". You're right to question that location. I don't know where they got that number but it would be a disaster looking for a place close by to happen at takeoff time. I'd go with 1.7 to 1.75 from the upper leading edge and see how it works out for the test flight. That would be a safe starting point for sure.
Gotta agree there. Whatever CG the OP decides, I think he's very wise to question that 3.5" measurement...it looks light years too far rearward.
gboulton is offline  
Reply With Quote
Old 04-26-2008, 08:10 PM
  #15  
gboulton
My Feedback: (15)
 
gboulton's Avatar
 
Join Date: May 2005
Location: La Vergne, TN
Posts: 3,743
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?

BTW, slightly off topic, but I never miss a chance to toss it out there:

When determining where the CG of a bipe IS (as in, checking to see where it balances) the "Vanessa CG Machine" is, perhaps, the world's greatest invention. I've used it to find the CG of a .25 size trainer, a 33% Pitts Bipe, and even a 44% Extra 260.

if you're unfamiliar, http://home.mindspring.com/~the-plum...%20Machine.htm has the details. I'll be happy as well to share pictures, what-have-you, if anyone wishes to PM.

We now return you to you regular CG Determination thread.
gboulton is offline  
Reply With Quote
Old 04-26-2008, 09:46 PM
  #16  
rmh
Senior Member
 
rmh's Avatar
 
Join Date: Dec 2001
Location: , UT
Posts: 12,630
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?


ORIGINAL: gboulton

BTW, slightly off topic, but I never miss a chance to toss it out there:

When determining where the CG of a bipe IS (as in, checking to see where it balances) the "Vanessa CG Machine" is, perhaps, the world's greatest invention. I've used it to find the CG of a .25 size trainer, a 33% Pitts Bipe, and even a 44% Extra 260.

if you're unfamiliar, http://home.mindspring.com/~the-plum...%20Machine.htm has the details. I'll be happy as well to share pictures, what-have-you, if anyone wishes to PM.

We now return you to you regular CG Determination thread.
that's one way but yougotta shift things around
I use the old close one eye n squint - then fly it then change it if it feels wrong to me

seriously- this works with a wee bit of experience .
Deltas fooled me -once -
rmh is offline  
Reply With Quote
Old 04-28-2008, 08:33 AM
  #17  
gboulton
My Feedback: (15)
 
gboulton's Avatar
 
Join Date: May 2005
Location: La Vergne, TN
Posts: 3,743
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: CG of a Bipe?


ORIGINAL: dick Hanson
that's one way but yougotta shift things around
Then you built yours wrong.

I use the old close one eye n squint - then fly it then change it if it feels wrong to me

seriously- this works with a wee bit of experience .
Deltas fooled me -once -
The important question is...

Which eye?
gboulton is offline  
Reply With Quote

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