h-ray-RCA

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

crasherboy

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.

h-ray-RCA

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

BMatthews

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

khodges

My Feedback: (1)

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.

h-ray-RCA

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

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.

h-ray-RCA

Nope, both are straight wings so it's sounding more and more like the 1.7-2.0" CG is the way to go.

gboulton

My Feedback: (15)

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.

h-ray-RCA

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

gboulton

My Feedback: (15)

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.

BMatthews

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.

rmh

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

gboulton

My Feedback: (15)

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

My Feedback: (15)

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.

rmh

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 -

gboulton

My Feedback: (15)

Then you built yours wrong.

The important question is...

Which eye?