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  #16  
Old 06-19-2007, 03:49 PM
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Default Re: Lycoming O-235 C/G

Quote:
Originally Posted by Marc Zeitlin View Post
How are you determining where your new CG range should be, now that you've changed the aerodynamic center of the aircraft? You have to know where it SHOULD be before you can put it there...
That's correct Marc. I can't figure that without the information pretaining to the moment calcs. I am working with some folks who have a background in this design at a manufacturing level so I don't want to appear as though I'm just dreaming this up as I go.

Sorry if I gave that impression.
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  #17  
Old 06-19-2007, 04:46 PM
Marc Zeitlin Marc Zeitlin is offline
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Default Re: Lycoming O-235 C/G

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Originally Posted by TMann View Post
That's correct Marc. I can't figure that without the information pretaining to the moment calcs.
I'm not sure what you mean here. You're asking about engine weights and CG's, and that will certainly allow you to calculate where your CG will be, but it gives you no information about where it SHOULD be.

Quote:
Originally Posted by TMann View Post
I am working with some folks who have a background in this design at a manufacturing level ...
What do you mean by "this design"? The Long-EZ? The engine?

I'm referring to the aerodynamics of the Long-EZ aircraft. With the canard 10" further forward, the aerodynamic center of the aircraft is no longer where it was. Therefore, the approved CG range (97" to 103", IIRC) is now meaningless. Knowing the weight/CG of the engine (or anything else in the plane), while important for calculating the CG position, does not tell you what the new CG range of the new aircraft is.

I was just wondering how you were going to determine the new CG range, and it has nothing to do with what the engine weights, and all to do with the changed aerodynamics of the new airframe.
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  #18  
Old 06-20-2007, 12:11 AM
chasingmars chasingmars is offline
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Default Re: Lycoming O-235 C/G

I read once before about your fuselage stretch, and almost chimed in then, but it wasn't clear there that you meant adding distance between the wings and canard, as opposed to adding a 10" longer nose ahead of the canard...

I'd thought you meant adding length in front of the canard. If you change the wing-canard separation, Marc's right, that more than just moment calculations, it;s quite likely to affect the entire dynamic and static stability characteristics of the aircraft, and have structural considerations to boot on fuselage bending and torsional stiffness. You're redesigning the airplane if you change this dimension.
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Last edited by chasingmars : 06-20-2007 at 12:25 AM.
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  #19  
Old 06-20-2007, 08:36 AM
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Default Re: Lycoming O-235 C/G

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Originally Posted by Marc Zeitlin View Post
Therefore, the approved CG range (97" to 103", IIRC) is now meaningless.
The Berkut has a stretch at the point mentioned of 12".
The passenger was moved 6" forward.

Does anyone want to venture a guess as to what the approved CG range is?
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  #20  
Old 06-20-2007, 10:08 AM
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Default Re: Lycoming O-235 C/G

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Does anyone want to venture a guess as to what the approved CG range is?
See, now this is inviting trouble. A couple of really smart people in this space have taken the time to think about what you're doing, have noted a potential safety issue and asked for clarification to get you to think about it a bit .... this practice should be encouraged. I am quite certain that the aforementioned smart people are asking only because there's a warning sign ahead; If you have and answer, then I would think a "yup, got it ... here's my logic" or even "yup, got it ... don't want to talk about it" would be sufficient!

BTW, not an aircraft/aero engineer, so not qualified to judge validity of your answer about the Berkut, but aren't there other design mods on the Berkut that might come into play as well? Extrapolating from one plane to another w/out some math to back it up seems another warning sign.
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  #21  
Old 06-20-2007, 10:15 AM
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Default Re: Lycoming O-235 C/G

yup, got it ... don't want to talk about it in this forum. Already have the answer and don't want to drag others into the post as they have been most helpful.
Thanks for the PR lesson Satch

BTW......the Datum in the Berkut has been recalculated and the CG limits remain the same.
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Last edited by TMann : 06-20-2007 at 11:00 AM.
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  #22  
Old 06-20-2007, 11:27 AM
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Default Re: Lycoming O-235 C/G

if the long has 6" of movement around the cg then will it hold true that the new plane "20b-stretcher-test plane-ez" will need 6" of movement around the cg point ?
1" forward to start with ?
do the plans give us a 0 point ? is it 100 ?
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  #23  
Old 06-20-2007, 11:32 AM
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Default Re: Lycoming O-235 C/G

TMann, give me a call on my cell phone

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  #24  
Old 06-21-2007, 12:55 AM
chasingmars chasingmars is offline
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Default Re: Lycoming O-235 C/G

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Originally Posted by TMann View Post
yup, got it ... don't want to talk about it in this forum. Already have the answer and don't want to drag others into the post as they have been most helpful.
That's a fair answer, but I should note that when you change this sort of parameter, it's not only a question of where the new safe C.G. range lies for adequate stability, but also IF such a range still exists.

Now, likely, it does, and I haven't done the math. I'm not saying the aircraft won't be stable, only that the work needs to be done to assure oneself that it is. But the data to do the math isn't readily available, so if someone has already done that math for you, great, but if I were you, I'd be spending some time learning how aircraft stability is worked out and ask your engineering friend to step you through what he did to assure you that the aircraft won't land you in the obituaries section (And I'm not being melodramatic, it's happened before, and this level of mod can bite, c.g. mods are lethal if not done right).

Quote:
Originally Posted by TMann View Post
The Berkut has a stretch at the point mentioned of 12".
As I mentioned, a change like this and you've no longer got a Long-EZ. You've not got a Berkut either, you've got a new airplane, how's your design aero engineering and test pilot training?
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  #25  
Old 06-21-2007, 10:37 AM
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Default Re: Lycoming O-235 C/G

Good topic
Lots of very interesting posts and questions.

As yet no clear answer. But, the cautions are intelligent. The coupling arrangement between the canard and main wing have a relationship that appear to be a departure from convention and has interested me for a long time.

Nevertheless it is there on the original design platforms. As Marc has said moving the canard forward by elongating the fuse put the flight behavior
into a "real" experimental catagory.

I would be more comfortable with knowns.

Extending the fuse by

1" = ?
2" = ?
10" = ?

The Berkut did it is only a partial answer. What else did they do. Is the canard
the same shape and size? Etc. W&B could be easily handled by moving the engine further away from the firewall but this is more than just a CG or W&B issue.

Anyhow, TMann I like your enthusiam. My suggestion is to make your mods with safety in mind.
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