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Oct 22, 2019, 11:17 AM
Purdue Engineering
Rocketman1092's Avatar
Thread OP
Discussion

CG Calculation - Don't Ignore the Fuselage?


Hi all,

So I've designed a new airplane and it's getting close to the maiden flight. But I'm finding it difficult to determine the proper CG location. Typically I'd ignore lift/moment contributions from the fuselage, plug the wing/tail geometry into an online calculator, pick a static margin, and be good to go. However, the fuselage of this model presents quite a bit of horizontal area and most of it is ahead of the wing. So I think I need to consider its effects in determining static stability but I'm not sure how.

Thoughts?
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Oct 22, 2019, 11:37 AM
Sink stinks
Montag DP's Avatar
There are a few methods, none of which will be exact, but can give you a rough estimate.
  • Build a "chuck glider" of your model's planform out of flat balsa.
  • Use a vortex lattice code like AVL and model the fuselage as a flat wing section. Note: inviscid panel codes may give a neutral point that is too far aft for low-Re models.
  • Use a CG/neutral point calculator that can account for the fuselage contribution, like this one.
Oct 22, 2019, 04:59 PM
Purdue Engineering
Rocketman1092's Avatar
Thread OP
Thanks! I've given your calculator a try and the results seem reasonable. Though curiously, I get pretty different results when I include the portion of the wing that passes through the fuselage (but with "Remove intersecting wing area from calculations) than when I exclude it and offset the wing panels outwards. Which would you imagine makes more sense?
Oct 22, 2019, 09:46 PM
Sink stinks
Montag DP's Avatar
Quote:
Originally Posted by Rocketman1092
Thanks! I've given your calculator a try and the results seem reasonable. Though curiously, I get pretty different results when I include the portion of the wing that passes through the fuselage (but with "Remove intersecting wing area from calculations) than when I exclude it and offset the wing panels outwards. Which would you imagine makes more sense?
It's been so long that I wrote it that I had to look it up. It looks like when you tell it to remove the intersecting wing area in the fuselage calculations, all it does is subtract that area from the wings without redoing the rest of the wing calculations as though that section were removed. In other words, it will change the wing area value, but it won't change the aero center, lift curve slope, etc. for the wing. So I think it is better to manually remove those sections from the wings as you've shown in the first picture.
Oct 22, 2019, 11:46 PM
Purdue Engineering
Rocketman1092's Avatar
Thread OP
Quote:
Originally Posted by Montag DP
It's been so long that I wrote it that I had to look it up. It looks like when you tell it to remove the intersecting wing area in the fuselage calculations, all it does is subtract that area from the wings without redoing the rest of the wing calculations as though that section were removed. In other words, it will change the wing area value, but it won't change the aero center, lift curve slope, etc. for the wing. So I think it is better to manually remove those sections from the wings as you've shown in the first picture.
Thanks, that makes sense. Nice work by the way!
Oct 23, 2019, 12:25 AM
B for Bruce
BMatthews's Avatar
It's fair to say that on a design like yours there would be SOME contribution from the fuselage. But it would not add up to being the total visible area as seen in the top down view. Instead there would be some reduction factor that would apply to the fuselage area that would be based on the cross section shape.

And while the nose is quite long some of it is canceled out by the rear portion. So it's not ALL about the nose.

Between the reduction factor based on cross section and the cancelation of some of the nose area by the tail area I'd suggest that you could use the CG calculator but fudge things by going for a generous stability margin value like 10 or 12. That should be in the ball park for the first flight or at least be survivable.

The other option would be to make a scale model of your design. Something about 1/4 to 1/3 size with a pusher prop in the tail and test it for the CG position. You'd want a reasonably faithful fuselage cross section but it could be fairly crude. Like carved and sanded to shape from some pink or blue Styrofoam just for this testing. Something quick and dirty just to prove the calculation without a lot of risk for time and cost.
Oct 23, 2019, 05:21 AM
Registered User
Though the pusher prop itself can affect the stability margin somewhat. What's the next step, build a wind tunnel scale model for the scale model and strap it to a measuring rig on the car roof rack?
Oct 23, 2019, 07:30 AM
Sink stinks
Montag DP's Avatar
Quote:
Originally Posted by BMatthews
It's fair to say that on a design like yours there would be SOME contribution from the fuselage. But it would not add up to being the total visible area as seen in the top down view. Instead there would be some reduction factor that would apply to the fuselage area that would be based on the cross section shape.

And while the nose is quite long some of it is canceled out by the rear portion. So it's not ALL about the nose.

Between the reduction factor based on cross section and the cancelation of some of the nose area by the tail area I'd suggest that you could use the CG calculator but fudge things by going for a generous stability margin value like 10 or 12. That should be in the ball park for the first flight or at least be survivable.
Why would you ignore the fuselage and then add a fudge factor (i.e., total guess) when there are better options available? The calculator I posted does not treat the fuselage as a wing, it uses slender body theory to estimate its contribution to longitudinal stability. It already assumes a rounded cross section like his model has. Of course it's not perfect, but I would argue that it is better than guessing.
Oct 23, 2019, 01:37 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by Montag DP
Why would you ignore the fuselage and then add a fudge factor (i.e., total guess) when there are better options available? The calculator I posted does not treat the fuselage as a wing, it uses slender body theory to estimate its contribution to longitudinal stability. It already assumes a rounded cross section like his model has. Of course it's not perfect, but I would argue that it is better than guessing.
And I'd say you're right and it is better to use a known slender body theory option to include the fuselage effect....

I missed where you said earlier that your application covered that option.... Sorry.
Oct 23, 2019, 02:03 PM
An itch?. Scratch build.
eflightray's Avatar


Kratos XQ-58A Valkyrie ?


.
Oct 23, 2019, 02:40 PM
AndyKunz's Avatar
If your gear is in the scale location, the CG is 15 degrees forward at the wing, measured from the bottom of the tire. That will ballpark you pretty easily.

Andy
Oct 23, 2019, 04:07 PM
Purdue Engineering
Rocketman1092's Avatar
Thread OP
Quote:
Originally Posted by AndyKunz
If your gear is in the scale location, the CG is 15 degrees forward at the wing, measured from the bottom of the tire. That will ballpark you pretty easily.

Andy
I wish it were that simple! But the airplane this is inspired by (which is indeed the XQ-58) doesn't even have landing gear. I placed it ~15deg behind my best approximation for CG location at the time.
Oct 24, 2019, 10:53 AM
AndyKunz's Avatar
What's holding it up in this photo?

Andy
Oct 24, 2019, 12:37 PM
Purdue Engineering
Rocketman1092's Avatar
Thread OP
The imagination of an artist who perhaps made too many assumptions

XQ-58A is catapult launched, descends with a chute, and lands on airbags.
Oct 24, 2019, 01:28 PM
An itch?. Scratch build.
eflightray's Avatar
Rail launched using rockets with parachute and airbag recovery during testing.

Kratos XQ-58 Valkyrie

.


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